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Complex polarity pattern at the former Plio–Pleistocene global stratotype section at Vrica (Italy): Remagnetization by magnetic iron sulphides
Abstract
The Vrica section in Calabria, southern Italy, was the global stratotype for the Pliocene–Pleistocene boundary until this boundary was redefined in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly defined Pliocene–Pleistocene boundary, which was a key calibration point for the astronomical polarity timescale (APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed, neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We confirm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth significantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments.
... The magnetostratigraphy of the Vrica section was investigated by Haq et al. (1977), Tauxe et al. (1983), Zijderveld et al. (1991) and, most recently, by Roberts et al. (2010) who showed that the complex polarity pattern at the top of the Olduvai Subchron is the result of authigenic growth of iron sulfide minerals such as greigite and pyrrhotite. Accordingly, the GSSP is now considered to occur B8 m (5B20 kyr) below the observed top of the Olduvai Subchron (Roberts et al., 2010;Head, 2019). ...
... The magnetostratigraphy of the Vrica section was investigated by Haq et al. (1977), Tauxe et al. (1983), Zijderveld et al. (1991) and, most recently, by Roberts et al. (2010) who showed that the complex polarity pattern at the top of the Olduvai Subchron is the result of authigenic growth of iron sulfide minerals such as greigite and pyrrhotite. Accordingly, the GSSP is now considered to occur B8 m (5B20 kyr) below the observed top of the Olduvai Subchron (Roberts et al., 2010;Head, 2019). ...
... The GSSP is dated to 1.80 Ma and coincides with the transition from MIS 65À64 (Cita et al., 2012). It occurs B8 m below the observed top of the Olduvai Subchron, but as noted earlier, diagenetic overprinting obscures the precise position of this polarity reversal (Roberts et al., 2010). Remagnetization may have been caused by sulfidic fluids generated during tectonism along the Calabrian arc (Roberts et al., 2010). ...
The Quaternary System/Period, comprising the Holocene and Pleistocene series/epochs, encompasses the last ~2.6 Myr during which time Earth’s climate was strongly influenced by bipolar glaciation, and the genus Homo first appeared and diversified. The base of the Quaternary System and Pleistocene Series is defined by the Global Boundary Stratotype Section and Point (GSSP) for the Gelasian Stage at Monte San Nicola in Italy with an age of 2.58 Ma. The Calabrian Stage of the Lower Pleistocene Subseries is defined by a GSSP at the Vrica section, also in Italy, with an age of 1.80 Ma. The Chibanian Stage of the Middle Pleistocene Subseries is defined by a GSSP at the Chiba section in Japan with an age of 0.774 Ma. The Upper Pleistocene subseries is defined in name only with an age of ~129 ka. The base of the Holocene Series is defined in the NGRIP2 Greenland ice core with an age of 11 700 years b2k (before CE 2000). The Holocene is subdivided into lower, middle, and upper subseries and their corresponding Greenlandian, Northgrippian, and Meghalayan stages. The Northgrippian Stage is defined by a GSSP in the NGRIP1 Greenland ice core with an age of 8.2 ka, and the Meghalayan Stage by a GSSP in a speleothem from India with an age of 4.2 ka. These are the only GSSPs ever to be defined in an ice core or a speleothem.
... The GSSP occurs ∼8 m below the observed top of the Olduvai Subchron. The paleomagnetic record has been affected by diagenetic overprinting that prevents a more precise estimate of this polarity reversal (Roberts et al., 2010). Indeed, a brief interval of reversed polarity earlier recorded near the top of the Olduvai at Vrica (Zijderveld et al., 1991) has not been replicated either in high-resolution marine records (Channell et al., 2016) or from a high-resolution unweathered subsurface section in east Africa (Sier et al., 2017). ...
... Indeed, a brief interval of reversed polarity earlier recorded near the top of the Olduvai at Vrica (Zijderveld et al., 1991) has not been replicated either in high-resolution marine records (Channell et al., 2016) or from a high-resolution unweathered subsurface section in east Africa (Sier et al., 2017). This would appear to confirm later remagnetization in the Vrica record, caused perhaps by sulphidic fluids generated during tectonism along the Calabrian arc (Roberts et al., 2010). Elsewhere, the top of the Olduvai Subchron is consistently placed within MIS 63, and at North Atlantic IODP Site 1308 it has an age of 1.780 Ma on the Lisiecki and Raymo (2005) time scale (Channell et al., 2016). ...
... This estimate is consistent with its observed position ∼8 m above the GSSP, based on a sediment rate of 36 cm/kyr immediately below sapropelic bed 'e' using the sapropel chronology of Lourens et al. (1996b). Further paleomagnetic analysis of the Vrica section is probably not profitable (Roberts et al., 2010), but the acquisition of a 10 Be record as a proxy for the position of the top of the Olduvai should be considered given that 10 Be is not subject to remagnetization or to lock-in depth. ...
The Quaternary System/Period and Pleistocene Series/Epoch were defined in 2009 by the Global boundary Stratotype Section and Point (GSSP) for the Gelasian Stage/Age (2.58 Ma), which aligns with Marine Isotope Stage (MIS) 103 and approximates the Gauss–Matuyama Chron boundary, contrary to earlier reports. The Vrica GSSP (1.80 Ma) was repurposed in 2011 to define the Calabrian Stage, effectively completing the Lower Pleistocene Subseries/Subepoch. The candidate for the Middle Pleistocene Subseries (and proposed Chibanian Stage) GSSP (∼774 ka) is the Chiba section, Japan. It aligns with MIS 19 and approximates the Matuyama–Brunhes Chron boundary (∼773 ka). The Upper Pleistocene Subseries, with a base traditionally marked by the onset of the Last Interglacial, is not yet defined by GSSP. The Holocene Series/Epoch was formally defined in 2008 by a GSSP in the NGRIP2 Greenland ice core with an age of 11,700 yr b2k (before 2000 CE) and in 2018 was subdivided, using climatic events at 8.2 and 4.2 ka, into the Greenlandian, Northgrippian and Meghalayan stages/ages and their corresponding Lower/Early, Middle, Upper/Late subseries/subepochs. The Northgrippian GSSP (8236 yr b2k) is defined in the NGRIP1 Greenland ice core, and the Meghalayan GSSP (4250 yr b2k) in a speleothem from Meghalaya, India. This subdivision formally introduces the rank of subseries/subepoch, and incorporates by far the briefest of all stages into the geological time scale. Using ice cores and a speleothem for GSSPs is unique to the Holocene. The presently undefined term Anthropocene is already used extensively and, like Holocene subdivisional terms, its functionality will be enhanced by formal definition. The Anthropocene should not be confused with anthropogenic: it reflects a tipping point in the Earth System response to the marked intensification of human impacts, not simply the fact of human impact. The geological Anthropocene, as currently envisioned, would start in the mid-twentieth century, holding the rank of series, and terminating the Holocene but not interfering with its subdivision other than to terminate the Meghalayan Stage.
... EM2 is a strongly magnetostatically interacting SD greigite component (Figure 8b), while EM3 corresponds to the authigenic SP/SD component that is seen in all examples above (Figure 8c). EM4 (Figure 8d) is typical of methanic environments and is due to authigenic pyrrhotite (e.g., Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002). Relationships among the EMs for this data set are shown in Figures 8e and 8f. ...
... The presence of negative peaks such as feature 2 in Figure 5f in greigitebearing samples and EMs as documented here (Figure 5a) confirms that greigite has cubic magnetocrystalline anisotropy (Roberts, 1995;Roberts et al., 2011). This type of negative peak is also seen systematically in FORC diagrams for authigenic pyrrhotite (Figures 8d and 8g) that forms in methanic environments (e.g., Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002). This is due to triaxial anisotropy in the basal plane of pyrrhotite crystals (Martín-Hernández et al., 2008). ...
... Journal of Geophysical Research: Solid Earth negative slope (Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002;Wehland et al., 2005) suggest that pyrrhotite can be identified readily from FORC distributions (Figures 8d and 8g). This negative region is sometimes not evident because of the scaling of FORC diagrams, but it can be made more visible through manual adjustment of the color scale. ...
Diagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first-order reversal curve unmixing method on well-studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate-methane transition. Below the sulfate-methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.
... FORC diagrams (Figure 6) also indicate that different magnetic properties exist among the studied metamorphic and sedimentary pyrrhotite, and goethite and greigite samples. All pyrrhotite samples, regardless of their metamorphic or sedimentary origins (Figures 6a-6f), have kidney-shaped FORC distributions with negative slopes, as have been reported before for pyrrhotite (Larrasoaña et al., 2007;Roberts et al., 2006Roberts et al., , 2010Roberts et al., , 2014Weaver et al., 2002;Wehland et al., 2005), that lie largely below the B i = 0 axis. The sedimentary pyrrhotite samples (Figures 6d-6f) have higher coercivities than the metamorphic pyrrhotites (Figures 6a-6c). ...
... Pyrrhotite and greigite have contrasting magnetic characteristics at low temperatures. Pyrrhotite clearly exhibits the Besnus magnetic transition at~34 K in low-temperature SIRM warming curves, whereas greigite lacks a transition (Besnus & Meyer, 1964;Chang et al., 2009;Dekkers et al., 1989;Fillion & Rochette, 1988;Moskowitz et al., 1993;Roberts, 1995;Roberts et al., 2010;Rochette et al., 1990;Torii et al., 1996). The metamorphic pyrrhotite and sedimentary greigite samples studied here behave as expected with a Besnus transition and no transition, respectively (Figures 7d, 7e, 7g, and 7h). ...
... The results shown in Figure 7 demonstrate that the Besnus transition at~34 K (Besnus & Meyer, 1964;Dekkers et al., 1989;Fillion & Rochette, 1988;Rochette et al., 1990) is observed only for monoclinic pyrrhotite. Hexagonal pyrrhotite demonstrably lacks a low-temperature magnetic transition, as does greigite (Chang et al., 2009;Moskowitz et al., 1993;Roberts, 1995;Roberts et al., 2010;Torii et al., 1996). Lack of a Besnus transition in hexagonal pyrrhotite provides one way of avoiding confusion with monoclinic pyrrhotite, but lack of a phenomenon is not a diagnostic property. ...
... In the top of the Olduvai Subchron dated to 1.778 ± 0.003 Ma (Hilgen et al., 2012), an interval of reversed polarity has been inconsistently observed, creating an extra but short normal Subchron, the so called 'Vrica Subchron' (Zijderveld et al., 1991). The interval of reversed polarity has been argued by Roberts et al. (2010) to be the result of postdepositional remagnetization through formation of late diagenetic greigite at the type locality, based on observations from a drill core at Vrica, Italy. Several studies predating the definition of the Vrica Subchron found mixed polarities near the top of the Olduvai in the Turkana Basin (Brock and Isaac, 1974;Hillhouse et al., 1977), while some recent studies in the basin specifically argue for the existence of the Vrica Subchron Kent, 2015, 2010;Lepre et al., 2011). ...
... Considering the low temperature at which the remanence rapidly decays, this could indicate that the greigite is of biogenic origin and very fine-grained (e.g. Roberts et al., 2010). ...
... The authors rejected this option, however, and placed the top of the Olduvai at the top of the Vrica Subchron. Later, Roberts et al. (2010) drilled a core at the same site as Zijderveld et al. (1991) and identified a complex polarity pattern that they attributed to diagenetic remagnetization processes by magnetic iron sulfides. They rejected the interpretation of the reversed zone in the top of the Olduvai at the Vrica section as being a geomagnetic feature, and thus concluded that the Vrica (normal) Subchron was an artefact. ...
... In these cases, remagnetisations are typically detected using paleomagnetic field tests such as the fold test Roberts, 2006, 2008) or by detecting contradictory paleomagnetic polarities (e.g., Jiang et al., 2001;Sagnotti et al., 2005a). The difficulty in conclusively identifying the timing of authigenic magnetic mineral growth in poorly lithified sedimentary rocks has led to attribution of remagnetisations as occurring during "later" diagenesis (e.g., Jiang et al., 2001;Roberts and Weaver, 2005;Roberts et al., , 2010Rowan and Roberts, 2006). This term is vague, but it recognises the reality that the diagenetic magnetic mineral transformations of interest fall within neither of the traditionally recognised early or late diagenetic stages. ...
... Individual pyrite framboids are evident in (f), which have subsequently been completely overgrown by pyrite that has filled and replaced the remaining vascular structures. These images are from a sample from early Pleistocene marine sediments, Vrica, Italy (Roberts et al., 2010). (g, h) Elongated framboids and non-framboidal pyrite aggregates that probably represent remineralisation of plant cellular material (from early Pleistocene marine sediments, Crostolo River, Italy ). ...
... If greigite is preserved within sheet silicate layers as a result of slow reaction of iron with dissolved sulphide, it will have major implications for the paleomagnetic record of such sediments. Pyrite, greigite and monoclinic pyrrhotite have been observed commonly within silicate sheets (e.g., Fig. 10) in remagnetised sediments (Jiang et al., 2001;Roberts and Weaver, 2005;Rowan and Roberts, 2006;Roberts et al., 2010). In the examples shown in Fig. 10a-c, the sheet silicates remain only partially sulphidised, which suggests that sulphidisation has not continued for many half-lives. ...
Reduction-oxidation (redox) reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components. Diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium, and control the long-term stability of sedimentary iron-bearing minerals. Magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time, with diagenetic effects ranging from subtle to pervasive. Despite the importance of magnetic mineral diagenesis in paleomagnetism, rock magnetism, and environmental magnetism, and the usefulness of these subjects in the Earth and environmental sciences, there is no systematic single published treatment of magnetic mineral diagenesis. This paper is an attempt to provide such a treatment for the full range of diagenetic environments. Magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic, where the free energy yielded by different oxidants decreases progressively in each environment. In oxic environments, the most important diagenetic processes involve surface oxidation of detrital minerals, and precipitation of Fe3+-bearing minerals from solution. In ferruginous environments, the most reactive detrital and authigenic iron oxides undergo dissolution, often mediated by dissimilatory iron-reducing bacteria, which releases Fe2+ that becomes available for other reactions. The Fe2+ in solution can diffuse upward where it is oxidised to form new authigenic iron (oxyhydr-)oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite, generally at the base of the overlying nitrogenous zone. Alternatively, dissimilatory iron-reducing bacteria can produce extracellular magnetite within ferruginous environments. In sulphidic environments, iron-bearing detrital mineral assemblages undergo more radical alteration. Hydrogen sulphide, which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane, reacts with the Fe2+ released from iron mineral dissolution or directly with solid iron (oxyhydr-)oxide minerals to form iron sulphide minerals (mackinawite, greigite, and pyrite). Authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording. Secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane. Methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments. Most of the above-described diagenetic processes occur below 50°C. With continuing burial above 50°C, but at sub-metamorphic temperatures, magnetic minerals can undergo further thermally-induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment. These changes include remagnetisations. Magnetic analysis can provide much valuable information concerning diagenesis in environmental processes. The range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component.
... In addition, the heating curves of 22DL06 have a slight increase at 400 and 500 °C. The transformation from clay minerals or iron sulfides into magnetite might cause susceptibility to increase during the temperature increase [36,37]. (3) There is a distance between the cooling curve and heating curve before 550 °C in the 22DL09, indicating that new minerals with higher susceptibility were generated during the heating process. ...
... This may be due to the dehydration of lepidocrocite (γ-FeO(OH), homogeneous and polymorphic with goethite) and its transformation into maghemite [34,35]. In addition, the heating curves of 22DL06 have a slight increase at 400 and 500 • C. The transformation from clay minerals or iron sulfides into magnetite might cause susceptibility to increase during the temperature increase [36,37]. (3) There is a distance between the cooling curve and heating curve before 550 • C in the 22DL09, indicating that new minerals with higher susceptibility were generated during the heating process. ...
The Qaidam Block, located at the northern Qinghai–Tibet Plateau, is a pivotal area in unraveling the closure time of the Kunlun Ocean basin which might have recorded the transformation process between the Proto-Tethys and Paleo-Tethys Ocean basins. However, the late Triassic position of the Qaidam Block remains enigmatic, largely due to the scarcity of paleomagnetic data essential for quantitatively determining its paleolatitude. The widespread presence of the Elashan formation, particularly along the southern periphery of the Qaidam block, presents good material for conducting paleomagnetic work. Nevertheless, the primary magnetic carriers preserved within the Elashan formation might be influenced by multiple tectonic thermal events, particularly those associated with collisions between southern blocks and the Qaidam Block. Here we present rock magnetism and magnetic fabrics studies to identify the content and composition of magnetic minerals within the Elashan formation. The rock magnetic and petrologic results show that the magnetic carriers in the samples from the Elashan formation are dominated by magnetite with a small amount of goethite, pyrrhotite, and hematite. The results of Anisotropy in Magnetic Susceptibility indicate that the south of the Longwalangku section might not be obviously influenced by the tectonic events. Our results also provided guidance for future paleomagnetic research, emphasizing the importance of conducting further sampling away from adjacent faults, particularly in the southern Longwalangku area.
... These irreversible features for magnetic susceptibility are interpreted as the conversion of magnetic minerals, for instance, other Fe-bearing minerals with low magnetization, like maghemite or hematite, changed into magnetite with high magnetization intensity or produced new magnetite particles with relatively minor size during the heating process [51][52][53][54]. Furthermore, the heating curves of all samples slightly increased between 400 and 500 °C, which might be caused by iron-containing clay minerals or iron sulfides (e.g., pyrrhotite, melnikovite) transformed into magnetite [55][56][57]. The magnetic minerals appear to contain magnetite and hematite since the heating curves decay to near zero at about 560 °C or 660 °C. ...
... These irreversible features for magnetic susceptibility are interpreted as the conversion of magnetic minerals, for instance, other Fe-bearing minerals with low magnetization, like maghemite or hematite, changed into magnetite with high magnetization intensity or produced new magnetite particles with relatively minor size during the heating process [51][52][53][54]. Furthermore, the heating curves of all samples slightly increased between 400 and 500 • C, which might be caused by iron-containing clay minerals or iron sulfides (e.g., pyrrhotite, melnikovite) transformed into magnetite [55][56][57]. The magnetic minerals appear to contain magnetite and hematite since the heating curves decay to near zero at about 560 • C or 660 • C. ...
The early Ordovician location of the Qaidam Block remains unclear, mainly due to the sparse paleomagnetic data to quantitatively determine the coeval position. The early Ordovician limestone is widespread in the northern Qaidam Block providing an ideal window to obtain a paleomagnetic result. The results derived from the limestone are, however, frequently constrained by the hydrothermal fluid activities after the strata formed and complex magnetic minerals in the samples. Therefore, systematic petrological and rock magnetic studies were conducted to identify the content and composition of magnetic minerals in the limestone samples from the early Ordovician Duoquanshan Formation in the northern Qaidam Block. The rock magnetic and petrologic results show that the magnetic carriers in the limestone samples are dominated by magnetite with a small amount of pyrrhotite, goethite, and hematite. These results are consistent with the characteristics of stepwise demagnetization. Combined with the results published previously, the secondary remanence component carried by pyrrhotite might be produced by thermal fluid or magma active by the multiple Tethys evolution.
... In addition to conventional interpretation in terms of coercivity, magnetostatic interactions, and domain state (e.g., Roberts et al., 2014), recent work demonstrates that FORC diagrams contain valuable information about the controlling magnetic anisotropy type in magnetic materials (Harrison and Lascu, 2014;Valdez-Grijalva et al., 2018;Valdez-Grijalva and Muxworthy, 2019;Harrison et al., 2019;Egli, 2021). For example, experimental FORC distributions with "kidney", "banana", or "boomerang" shapes have been reported widely for monoclinic 4C pyrrhotite (e.g., Fig. 8a, b) (Wehland et al., 2005a;Horng, 2018), and authigenic pyrrhotite (Weaver et al., 2002;Larrasoaña et al., 2007;Roberts et al., 2010;Kodama, 2015a, 2015b;Horng, 2018;Roberts et al., 2018b), which is a ferrimagnetic hexagonal 3C pyrrhotite (Horng and Roberts, 2018), and hematite ( Fig. 8c-8h) (e.g., Liu et al., 2010;Brownlee et al., 2011;Jovane et al., 2011;Martín-Hernández and Guerrero-Suárez, 2012;Church et al., 2016;Zhao et al., 2017;Roberts et al., 2021). Roberts et al. (2010) attributed the kidney shaped FORC distributions for hematite and pyrrhotite to the fact that their spontaneous magnetizations are confined to the basal crystallographic plane. ...
... For example, experimental FORC distributions with "kidney", "banana", or "boomerang" shapes have been reported widely for monoclinic 4C pyrrhotite (e.g., Fig. 8a, b) (Wehland et al., 2005a;Horng, 2018), and authigenic pyrrhotite (Weaver et al., 2002;Larrasoaña et al., 2007;Roberts et al., 2010;Kodama, 2015a, 2015b;Horng, 2018;Roberts et al., 2018b), which is a ferrimagnetic hexagonal 3C pyrrhotite (Horng and Roberts, 2018), and hematite ( Fig. 8c-8h) (e.g., Liu et al., 2010;Brownlee et al., 2011;Jovane et al., 2011;Martín-Hernández and Guerrero-Suárez, 2012;Church et al., 2016;Zhao et al., 2017;Roberts et al., 2021). Roberts et al. (2010) attributed the kidney shaped FORC distributions for hematite and pyrrhotite to the fact that their spontaneous magnetizations are confined to the basal crystallographic plane. ...
The magnetic domain state of a material determines its magnetic recording capability and magnetic properties. Constraining the domain state of magnetic components within complexly mixed natural magnetic mineral assemblages is challenging because other bulk magnetic methods do not enable component-specific domain state identification. First-order reversal curve (FORC) diagrams are the most diagnostic tool for this important endeavour. Over the last 20+ years, an extensive framework has been developed for FORC diagram interpretation. Recent years have been fertile and key developments are highlighted here. New FORC measurement types provide enhanced domain state diagnosis, including recognition of vortex state signatures and their importance in rock magnetism. FORC diagrams are also indicative of the dominant magnetic anisotropy type in a material, with multi-axial, in addition to uniaxial anisotropy, signatures recognised increasingly. A fundamental challenge in FORC data processing is to avoid emphasizing noise at the expense of signal or distorting a FORC distribution by excessive smoothing. Selection of an optimal FORC distribution that avoids over- or under-smoothing is now possible with machine learning approaches. A further new FORC measurement protocol enables identification of magnetically viscous particles and can assist in separating signals due to magnetic mineral mixtures. Furthermore, FORC unmixing for large sample sets now enables quantitative separation of magnetic mineral mixtures. Splitting of the FORC signal into remanent, induced, and transient magnetization components, each of which provides information about magnetic domain state fractions in a sample, holds potential for future single sample unmixing.
... In the Mediterranean record, the stratigraphic position of these biohorizons with respect to the base of the Olduvai Subchron (ca. 1.945 Ma) is not precisely established, because the available paleomagnetic records are ambiguous (e.g., Tauxe et al., 1983;Zijderveld et al., 1991;Roberts et al., 2010). At ODP Site 652 (Western Mediterranean), D. brouweri disappears within the Olduvai (Channell et al., 1990;Glaçon et al., 1990a). ...
... In general, documentation on the Olduvai Subchron in the central Mediterranean region is sparse and inconsistent. In the key Vrica section, paleomagnetic investigations carried out over the years by several Authors across this interval have been demonstrated to provide conflicting results (see discussion in Roberts et al., 2010). Correlation between our paleomagnetic and biostratigraphic records confirms that, other than poorly defined, the base of the Olduvai at Monte San Nicola is older than expected. ...
The Monte San Nicola area (Southern Sicily) offers a spectacular exposure of open-marine sediments that were employed in 1998 for defining the Global Stratotype Section and Point (GSSP) of the Gelasian Stage (Upper Pliocene). After the lowering of the Pliocene/Pleistocene boundary to ca. 2.6 Ma in 2010, the Gelasian GSSP has been redefined as the base of both the Pleistocene Series and the Quaternary Period, which increased its importance and visibility within the scientific community. However, documentation on the Monte San Nicola reference section is still sparse. In the light of its renewed status, we decided to undertake a complete revision of the Gelasian Stage in its type area, in order to evaluate whether the succession of bio- and magnetostratigraphic events that are expected to occur in the interval of relevance are represented adequately in the local record. The results of our investigation demonstrate that the Monte San Nicola succession spans continuously from the upper Piacenzian to the lower Calabrian, and is therefore suitable to host the Unit Stratotype, or even the Astronomical Unit Stratotype, of the Gelasian Stage.
... Ridge-type signatures are readily explained as a manifestation of uniaxial SD magnetic behavior (Egli et al., 2010;Muxworthy et al., 2004;Newell, 2005;Pike et al., 1999). Kidney-shaped FORC distributions are less well explained and have also been observed in monoclinic 4C (Horng, 2018;Wehland et al., 2005) and authigenic pyrrhotite (Horng, 2018;Kars & Kodama, 2015a, b;Larrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002), which is a ferrimagnetic hexagonal 3C pyrrhotite (Horng & Roberts, 2018). Roberts et al. (2010) attributed the similar kidney-shaped features for hematite and pyrrhotite to the fact that their spontaneous magnetizations are confined to the basal crystallographic plane. ...
... Kidney-shaped FORC distributions are less well explained and have also been observed in monoclinic 4C (Horng, 2018;Wehland et al., 2005) and authigenic pyrrhotite (Horng, 2018;Kars & Kodama, 2015a, b;Larrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002), which is a ferrimagnetic hexagonal 3C pyrrhotite (Horng & Roberts, 2018). Roberts et al. (2010) attributed the similar kidney-shaped features for hematite and pyrrhotite to the fact that their spontaneous magnetizations are confined to the basal crystallographic plane. Harrison et al. (2019) varied the balance between uniaxial and triaxial (hexagonal) anisotropies to simulate and explain the observed ridge-type and kidney-shaped FORC distributions for hematite. ...
Hematite carries magnetic signals of interest in tectonic, paleoclimatic, paleomagnetic, and planetary studies. First‐order reversal curve (FORC) diagrams have become an important tool for assessing the domain state of, and magnetostatic interactions among, magnetic particles in such studies. We present here FORC diagrams for diverse hematite samples, which provide a catalog for comparison with other studies and explain key features observed for hematite. Ridge‐type signatures typical of uniaxial single‐domain particle assemblages and “kidney‐shaped” FORC signatures, and combinations of these responses, occur commonly in natural and synthetic hematite. Asymmetric features that arise from the triaxial basal plane anisotropy of hematite contribute to vertical spreading in kidney‐shaped FORC distributions and are intrinsic responses even for magnetostatically noninteracting particles. The dominant FORC distribution type in a sample (ridge, kidney‐shaped, or mixture) depends on the balance between uniaxial/triaxial switching. The identified signals explain magnetization switching and anisotropy features that are intrinsic to the magnetic properties of hematite and other materials with multiaxial magnetic anisotropy.
... Further studies showed that samples containing greigite (Fe 3 S 4 ) can acquire a GRM and/or RRM, which seriously contaminate paleomagnetic signals upon the AF demagnetization (Fu et al., 2008;Horng et al., 1998;Hu et al., 1998Hu et al., , 2002Liu et al., 2014;Nilsson et al., 2013;Oda & Torii, 2004;Roberts et al., 2010;Ron et al., 2007;Sagnotti et al., 2010;Sagnotti & Winkler, 1999;Snowball, 1997aSnowball, , 1997bStephenson & Snowball, 2001). Some studies verified that the GRM is ascribed to the irreversible moment flip of SD greigite, unrelated to the external bias fields (Hu et al., 1998). ...
... A significant acquisition of GRM is obtained in single-axis AF demagnetization process of anhysteretic remanent magnetization (ARM) (Ron et al., 2007). GRM acquisition is frequently reported in the AF demagnetization process of the lake (e.g., Ron et al., 2007) and marine sediments (e.g., Roberts et al., 2010) and even in the lunar rock (Garrick-Bethell et al., 2009). However, the origin and effects of GRM are not well understood yet. ...
Gyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite.
... The negative slope of the FORC distribution with a negative trough immediately beneath the main positive peak (Figures 7a and 7b) is an indicator of multiaxial anisotropy (Harrison et al., 2019;Harrison & Lascu, 2014). Features that are now recognized to be diagnostic of multiaxial anisotropy have been referred to as "kidney," "banana," or "boomerang" shapes and are reported widely for hematite (e.g., Brownlee et al., 2011;Church et al., 2016;Jovane et al., 2011;Liu et al., 2010;Martín-Hernández & Guerrero-Suárez, 2012;Muxworthy et al., 2005), monoclinic 4C pyrrhotite (Horng, 2018;Wehland et al., 2005), and what is now known to be 3C pyrrhotite (Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002). Hematite and 3C and 4C pyrrhotite have a similar relationship between their magnetization and crystal structure, with their magnetization confined to the basal plane. ...
... The magnetization of smythite is controlled by multiaxial anisotropy, as is evident in the negative slope of FORC diagrams (Figure 7) with a smaller negative peak below the major positive peak (Harrison & Lascu, 2014). Multiaxial behavior is also evident in FORC diagrams for 4C (Horng, 2018;Wehland et al., 2005) and 3C pyrrhotite (Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002). Simulated FORC diagrams for the cubic anisotropy of greigite also have kidney shapes (Valdez-Grijalva et al., 2018;Valdez-Grijalva & Muxworthy, 2019). ...
Smythite (Fe9S11) is an occasionally reported magnetic iron sulfide mineral that occurs in varied geological settings and co‐occurs commonly with other magnetic iron sulfide minerals. Determining the magnetic properties of smythite is important for understanding its geological distribution and paleomagnetic and environmental magnetic significance. We report sedimentary smythite occurrences from three locations in Taiwan (one terrestrial and two marine), which suggest that smythite forms in methanic diagenetic environments into which sulfide has been reintroduced. We report the magnetic properties of our purest smythite sample, which also contains greigite, and compare them with those of other magnetic iron sulfide minerals. The magnetization of smythite is controlled by multiaxial anisotropy, with magnetic easy axes that lie within the crystallographic basal plane. Smythite is magnetically stable with no low‐temperature magnetic transition. Hysteresis and coercivity properties of stable single domain smythite are similar to those of greigite at and below room temperature. The magnetic properties of smythite at elevated temperatures are dominated by thermal alteration, which precludes Curie temperature determination. In contrast to greigite and like pyrrhotite polytypes, smythite crystals occur as hexagonal plates. This morphological contrast facilitates discrimination of smythite from greigite in electron microscope observations, but it does not assist discrimination from pyrrhotite. Similar magnetic and morphological properties between smythite and other magnetic iron sulfides means that diagnostic mineralogical analyses (e.g., X‐ray diffraction) are needed to identify these minerals.
... The presence of hematite in these intervals is supported by the tentative observation of Morin transitions in samples from 427.6 mbsf in Unit I ( Figure 7d) and from 1,087.9 mbsf in Unit IV (Figure 7f). None of the S −0.3T values are low enough to provide conclusive evidence for hematite; however, and S −0.3T decrease may alternatively be attributed to higher-coercivity ferrimagnets, which could include greigite or pyrrhotite (Roberts et al., 2010), or elongated or maghemitized magnetite. Bacterial magnetite magnetosomes, which could contribute a higher-coercivity component (Kruiver & Passier, 2001), can be largely ruled out as a source of low S −0.3T by the FORC evidence. ...
... Site U1437 demonstrates the potential for sulfate inflow to locally stimulate greigite authigenesis, even in a sequence that has already experienced an initial phase of sulfide reduction in the near-seafloor sulfidic zone. Methane migration and accumulation is also a common feature of convergent margin settings, but most existing studies of the influence of methane on late diagenesis have focused either on modern gas hydrate settings (Horng & Chen, 2006;Housen & Musgrave, 1996;Kars & Kodama, 2015;Larrasoaña et al., 2007;Musgrave et al., 2006) or have inferred the likelihood of past gas hydrate accumulations (Roberts et al., 2010;Rowan & Roberts, 2008;van Dongen et al., 2007). Gas accumulations at Site U1437 lie far below the base of potential gas hydrate stability and appear to be controlled by migration along faults and accumulation in porous reservoirs; our results extend the record of AOM to deep methane at relatively high temperatures. ...
International Ocean Discovery Program (IODP) Site U1437 recovered an 1,800‐m‐long sediment sequence in a volcano‐bounded basin on the Izu rear arc. Pore fluid studies revealed a pattern of repeated fluid inputs, fluid diffusion, and methane and ethane accumulations, which represent “fluid anomalies” that disturb the fluid profiles. First‐order reversal curve analysis, magnetic hysteresis, saturation isothermal remanent magnetization, and low‐temperature remanence cycling reveal a detrital input dominated by vortex‐state and multidomain magnetite, which passes through an initial stage of partial magnetite dissolution and greigite authigenesis in the upper few tens of meters. Progressive magnetic mineral diagenesis comprises the continued loss of fine‐grained magnetite and gradual pyritization of greigite and produces a background logarithmic decrease in saturation isothermal remanent magnetization normalized by magnetic susceptibility. This process implies a continuing slow supply of S²⁻ to depths >1,500 m. Thermally driven diagenesis, which would cause extensive loss of greigite at these depths, does not appear to be significant here. Multidomain magnetite grains dominate the magnetic mineralogy in the deepest part of the sequence, but some single‐domain magnetite survives as inclusions in silicates. Fluid anomalies representing sulfate influx drive locally renewed greigite authigenesis, as do methane and ethane accumulations. In some cases, where methane is accompanied by H2S (“sour gas”), fine‐grained greigite is converted to pyrite. We term these multiple episodes of enhanced magnetic mineral alteration “punctuated magnetic mineral diagenesis.” Despite both progressive and punctuated magnetic mineral diagenesis, enough depositional remanence survives to allow recognition of the magnetostratigraphy to 1,320 m below seafloor.
... Greigite can form in association with early diagenetic processes (Roberts, 2015) such as sulfate reduction (Liu et al., 2004), anaerobic oxidation of methane Kasten et al., 1998;Neretin et al., 2004), and formation of methane hydrates (Housen & Musgrave, 1996;Kars & Kodama, 2015a;Larrasoaña et al., 2007). The kinetics of monoclinic pyrrhotite formation preclude its formation during earliest burial; its presence as an authigenic mineral is indicative of formation over longer periods (e.g., Horng & Roberts, 2006), often in relation to anaerobic oxidation of methane and formation of gas hydrates (Horng & Chen, 2006;Kars & Kodama, 2015a;Roberts et al., 2010;Shi et al., 2017;van Dongen et al., 2007;Weaver et al., 2002). The formation of these magnetic iron sulfide minerals (particularly pyrrhotite) has been associated with increasingly methane seepage in marine sediments Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Neretin et al., 2004;van Dongen et al., 2007). ...
... CO 2 and sulfide-saturated fluids appear to have penetrated interstitial spaces and Geochemistry, Geophysics, Geosystems 10.1002/2017GC007208 microcracks in detrital grains (Figure 5g), which gave rise to siderite and pyrrhotite formation around and inside detrital mineral grains. Pyrrhotite and greigite have been described in other marine sediments where concentration gradients of methane occur near disseminated gas hydrates Kars & Kodama, 2015a, 2015bNeretin et al., 2004;Roberts et al., 2010). Formation of sphalerite in the studied Paleocene oolitic ironstones may also have been associated with localized bacterial sulfate reduction and HS 2 release. ...
Authigenesis of ferrimagnetic iron sulfide minerals (greigite and monoclinic pyrrhotite) occurred across the Paleocene-Eocene Thermal Maximum (PETM) within the Bakchar oolitic ironstone in southeastern Western Siberia. Co-occurrence of these minerals is associated with diagenetic environments that support anaerobic oxidation of methane, which has been validated by methane fluid inclusion analysis in the studied sediments. In modern settings, such ferrimagnetic iron sulfide formation is linked to upward methane diffusion in the presence of minor dissolved sulfide ions. The PETM was the most extreme Cenozoic global warming event and massive methane mobilization has been proposed as a major contributor to the globally observed warming and carbon isotope excursion associated with the PETM. The studied sediments provide rare direct evidence for methane mobilization during the PETM. Magnetic iron sulfide formation associated with methanogenesis in the studied sediments can be explained by enhanced local carbon burial across the PETM. While there is no strong evidence to link local methane venting with more widespread methane mobilization and global warming, the magnetic, petrographic, and geochemical approach used here is applicable to identifying authigenic minerals that provide telltale signatures of methane mobility that can be used to assess methane formation and mobilization through the PETM and other hyperthermal climatic events.
... Greigite can form in association with early diagenetic processes (Roberts, 2015) such as sulfate reduction (Liu et al., 2004), anaerobic oxidation of methane Kasten et al., 1998;Neretin et al., 2004), and formation of methane hydrates (Housen & Musgrave, 1996;Kars & Kodama, 2015a;Larrasoaña et al., 2007). The kinetics of monoclinic pyrrhotite formation preclude its formation during earliest burial; its presence as an authigenic mineral is indicative of formation over longer periods (e.g., Horng & Roberts, 2006), often in relation to anaerobic oxidation of methane and formation of gas hydrates (Horng & Chen, 2006;Kars & Kodama, 2015a;Roberts et al., 2010;Shi et al., 2017;van Dongen et al., 2007;Weaver et al., 2002). The formation of these magnetic iron sulfide minerals (particularly pyrrhotite) has been associated with increasingly methane seepage in marine sediments Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Neretin et al., 2004;van Dongen et al., 2007). ...
... CO 2 and sulfide-saturated fluids appear to have penetrated interstitial spaces and Geochemistry, Geophysics, Geosystems 10.1002/2017GC007208 microcracks in detrital grains (Figure 5g), which gave rise to siderite and pyrrhotite formation around and inside detrital mineral grains. Pyrrhotite and greigite have been described in other marine sediments where concentration gradients of methane occur near disseminated gas hydrates Kars & Kodama, 2015a, 2015bNeretin et al., 2004;Roberts et al., 2010). Formation of sphalerite in the studied Paleocene oolitic ironstones may also have been associated with localized bacterial sulfate reduction and HS 2 release. ...
Recently the formation of pyrrhotite and greigite is more often linked with sulfate-methane transit zones of modern sediments. The paper considers methane generations as one of the possible ways of forming ferromagnetic iron sulfides in sedimentary rocks of Bakchar deposit. The main aim: of the study is to reconstruct the environmental conditions for formation of ferromagnetic sulphides in rocks of the Bakchar deposit for disclosure processes, which occurred in sediments of the ancient epicontinental West Siberian Sea at the Paleocene-Eocene boundary and also for interpretation of the high magnetization of these rocks. The methods used in the work: mineralogical (scanning electron microscopy, X-ray diffraction analysis), geophysical (kappametry) and geochemical (X-Ray analysis, ICP-MS) studies of sediments (samples from core) which contain pyrrhotite and greigite to reconstruct the evolution of specific postdepositional processes within the shelf of ancient West-Siberian Sea at Paleocene-Eocene boundary. As a result of the investigation the authors advance the theory of ferromagnetic iron sulfides formation in Bakchar deposit sediments with the upward diffusing methane and limited amount of sulfide ion. This environment was enriched with paleoredox proxies (Mo, U, V) and paleoproductivity proxies (Babio and P). Sedimentary rocks containing pyrrhotite and greigite are characterized by high values of magnetic susceptibility (more than 80-10⁻⁵SI) that is a useful property to identify analogical rocks in drill cores. Presence of ferromagnetic iron sulfides within sedimentary rocks might be used to detect ancient gas hydrate systems in the Paleocene-Eocene sequences of Western Siberia. Methane emissions within the ancient West Siberian Sea might probably triggering large submarine landslides and affecting the global climate change at the Paleocene-Eocene boundary.
... The magnetostratigraphy of the Vrica section has been investigated by Haq et al. (1977), Tauxe et al. (1983), Zijderveld et al. (1991) and, most recently, by Roberts et al. (2010), who showed that the complex polarity pattern at the top of the Olduvai Subchron is the result of authigenic growth of iron sulphide minerals such as greigite and pyrrhotite. However, uncertainties arising from the complex magnetostratigraphic record at Vrica result in maximum age differences of less than 50 ka for the upper Olduvai reversal (Roberts et al., 2010) and hence a similar uncertainty in the boundary may be assumed. ...
... The magnetostratigraphy of the Vrica section has been investigated by Haq et al. (1977), Tauxe et al. (1983), Zijderveld et al. (1991) and, most recently, by Roberts et al. (2010), who showed that the complex polarity pattern at the top of the Olduvai Subchron is the result of authigenic growth of iron sulphide minerals such as greigite and pyrrhotite. However, uncertainties arising from the complex magnetostratigraphic record at Vrica result in maximum age differences of less than 50 ka for the upper Olduvai reversal (Roberts et al., 2010) and hence a similar uncertainty in the boundary may be assumed. ...
The Quaternary Period, comprising the Holocene and Pleistocene Epochs, encompasses the last ~2.6 Ma during which time Earth's climate was strongly influenced by bi-polar glaciation and the genus Homo first appeared and evolved.
... However, doubts have been raised over the possibility that authigenic monoclinic pyrrhotite can form soon after deposition [Wilkin and Barnes, 1996;Horng and Roberts, 2006]. Many studies have found that apparent magnetic reversals were artifacts introduced by the presence of authigenic pyrrhotite and greigite that formed during later diagenesis [e.g., Jiang et al., 2001;Weaver et al., 2002;Roberts et al., 2010], especially in sediments that have undergone tectonic deformation after deposition. Similarly, authigenic magnetic mineral formation may obscure any climate signal derived from the concentration and mineralogy of the detrital magnetic fraction [e.g., Blanchet et al., 2009]. ...
... Authigenic pyrrhotite is most frequently found where tectonic deformation events or sulfide fluid/gas circulation have occurred [e.g., Urbat et al., 2000;Weaver et al., 2002;Larrasoaña et al., 2007;Roberts et al., 2010]. ...
Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51 ± 0.24 Ma and 23.38 ±. 24 Ma (2σ). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long-term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene. This article is protected by copyright. All rights reserved.
... The source and primacy of the magnetic minerals in the sedimentary deposits are crucial for paleomagnetic studies. Magnetostratigraphic dating of continental sediments is often more complex than for marine sequences due to a highly variable lithology causing varying rock magnetic properties, alteration and remagnetization processes (Hounslow and Nawrocki, 2008;Roberts et al., 2010;Kodama, 2012). The parameters of magnetic minerals, which are controlled by the concentration, particle size, and type of magnetic minerals present, have been widely utilized in sedimentary stratigraphy and paleoenvironmental reconstructions (Liu et al., 2012;Razik et al., 2014;Raj et al., 2016;Su et al., 2013;Wu et al., 2010). ...
Paleomagnetic records from the sediments provide the foundation for studying variations in the geomagnetic field across the geological time sacle. Here, we report a new paleomagnetic data from Quaternary sediments of Ramganga Paleolake, NW Himalaya, India, to provide more insight into the global geomagnetic excursions and paleoenvironmental implications. Paleomagnetic and rock magnetic analyses were performed on a 5 m thick vertical profile of unconsolidated sediments from the Ramganga paleolake (29°46′867″N; 79°14′043″E), located in the Lesser Himalaya at a height of 798 m. One hundred fifty oriented samples from 50 sites (10 cm intervals) were obtained from the vertical section. Extrapolating the optically stimulated luminescence (OSL) dates from literature indicates that the entire section was probably deposited between 38 and 0.8 ka. The rock magnetic data shows that the remanence is carried by a low coercivity mineral and a significant amount of a soft magnetic mineral, most likely magnetite. Virtual Geomagnetic Pole (VGP) latitudes were calculated. Plot of the VGP latitudes against the sample locations from the litho-column allows us for the construction of Magnetic Polarity Stratigraphy (MPS) column. The MPS of the Ramganga paleolake has detected three geomagnetic excursions that are correlated with the global events: the Mono Lake Event (∼28.4-25.8 ka), the Gulf of Mexico Event (∼12.5-17 ka), and the Bagwalipokar Excursion Events (∼16-14 ka and 8-2.5 ka) respectively. The lower and middle portion of the paleoklake represents the low and high energy depositional environment during the sediment deposition due to wet and arid climatic conditions.
... Therefore, the combination of above-mentioned mineral magnetic properties, paleomagnetic demagnetization behavior, microscopic observations, and chemical composition data provides compelling evidence that all of the studied sediments are dominated magnetically by greigite with variable size and distinct morphology, with minor magnetite in the host sediments. It is also worth mentioning that lath-shaped magnetic iron sulfide particles (Figures 5e and 5g) are identified here for the first time as greigite; such morphologies have previously been misinterpreted as monoclinic pyrrhotite and are likely to be a different authigenic phase (e.g., Roberts, 2015;Roberts et al., 2010Roberts et al., , 2011Weaver et al., 2002). Lath-shaped 3T pyrrhotite polytype has also been reported (Horng, 2018;Horng & Roberts, 2018). ...
Authigenic greigite may form at any time within a sediment during diagenesis. Its formation pathway, timing of formation, and geological preservation potential are key to resolving the fidelity of (paleo‐)magnetic signals in greigite‐bearing sediments. In the cored sequence of the International Ocean Discovery Program Expedition 362 (Sumatra Subduction Margin), multiple organic‐rich mudstone horizons have high magnetic susceptibilities. The high‐susceptibility horizons occur immediately below the most bioturbated intervals at the top of muddy turbidite beds. Combined mineral magnetic, microscopic, and chemical analyses on both thin sections and magnetic mineral extracts of sediments from a typical interval (∼1,103.80–1,108.80 m below seafloor) reveal the presence of coarse‐grained greigite aggregates (particles up to 50–75 μm in size). The greigite formed under nonsteady state conditions caused by the successive turbidites. Organic matter, iron (oxy)(hydr)oxides, Fe²⁺, and sulfides and/or sulfate were enriched in these intensively bioturbated horizons. This facilitated greigite formation and preservation within a closed diagenetic system created by the ensuing turbidite pulse, where pyritization was arrested due to insufficient sulfate supply relative to Fe (oxy)(hydr)oxide. This may represent a novel greigite formation pathway under conditions modulated by turbidites and bioturbation. Paleomagnetic analyses indicate that the early diagenetic greigite preserves primary (quasi‐)syn‐sedimentary magnetic records. The extremely high greigite content (0.06–1.30 wt% with an average of 0.50 wt% estimated from their saturation magnetization) implies that the bioturbated turbiditic deposits are an important sink for iron and sulfur. Mineral magnetic methods, thus, may offer a window to better understand the marine Fe–S–C cycle.
... Besides, AF demagnetization studies of natural rock samples are many times affected by an abnormal remanent magnetization called GRM, which is not well explored until date and was initially reported by Doell and Cox (1967), with few recent studies like Duan et al. (2020), Roberts et al. (2010), and Hu et al. (2002). The samples with GRM acquisition are not suitable for tectonics-related palaeomagnetic studies. ...
The present study deals with the alternating field (AF) demagnetization of the volcanic rocks and meta-sediments from the
North Singhbhum Mobile Belt (NSMB) within a field of 2.5 to 100 mT. All the three rock types encountered were subjected to AF demagnetization and the behavior during demagnetization was noted. As the rocks of the NSMB have variable
potential to record remanences, the samples to demagnetize were sorted based on their feasibility to record characteristic
remanent magnetization (ChRM). A gradual decrease in the magnetic intensity of the quartzites during AF demagnetization
yielded two stable palaeomagnetic vectors recorded in them. Two palaeomagnetic poles are determined from these stable
vectors. The palaeomagnetic poles thus obtained from the quartzites lie on a hairpin bend in the Proterozoic apparent polar
wandering path (APWP) of the Indian subcontinent. This is significant towards the ~ 1850 Ma initiated Satpura orogeny
which developed the regional mesoscopic as well as the magnetic fabrics in the NSMB. Thus, these palaeomagnetic poles
are useful in relating the regional tectonics and magneto-tectonics of the NSMB. However, the magnetic intensity of the
schists and volcanic rocks continues to decrease with increasing applied AF up to a certain extent, and then again continues
to increase, which proves that these samples are affected by the acquisition of gyro remanent magnetization (GRM), which is
a magnetic vector component acquired by certain rocks during AF demagnetization of their natural remanent magnetization
(NRM). As GRM is an imperfection commonly associated with AF demagnetization of the rocks, the samples with such
features are not considered for tectonics-related palaeomagnetic studies in the NSMB.
... Strongly interacting greigite (a) and pyrrhotite (b) SD particles modified fromRoberts et al. (2014) afterRoberts et al. (2010). The FORC diagrams reveal strong interaction features, namely a spreading along B u , accompanied by a downward shift of the central maximum, and negative distributions for negative B u values (see text for details). ...
... In contrast, mixed-polarity zones that are not consistent with the expected polarity and extend across a few to tens of metres in thickness must have been caused by remagnetization (e.g. Elston & Bressler 1977;Kirschvink 1980;Horng et al. 1998;Sagnotti et al. 2005b;Roberts et al. 2010). In many cases, normal-polarity components are interpreted to represent a recent magnetic overprint (e.g. ...
Remagnetization is an important issue in palaeomagnetism. Here, we discuss an extraordinarily thick (∼74 m) dual-polarity transition zone between the Gauss and Matuyama Chrons. The studied succession is from a drill core through lacustrine sediments of palaeo-Lake Idaho (Snake River Plain, NW United States of America) that are intercalated with basalt units. We identified detrital Ti-rich titanomagnetite and magnetite in lamellar exsolutions as the main carriers of a primary remanence, likely derived from the basalts that erupted in the Snake River Plain. Stepwise thermal demagnetization revealed a single-component remanent magnetization with reversed and normal polarities above and below the transition zone, respectively. Based on rock-magnetic results, microscopic observations, and previously known events in the evolution of palaeo-Lake Idaho, the reversed-polarity component in the transition zone represents a secondary chemical remanent magnetization caused by magnetic mineral alteration or partial neo-formation of magnetite, in association with strong depletion of the primary detrital magnetic minerals that affected a wide depth range below the level where the remagnetization event occurred. This remagnetization event was most likely related to lake-level lowering and partial desiccation of palaeo-Lake Idaho. Understanding the nature and origin of the remagnetization allows to identify the polarity boundary in the unusual case of a secondary magnetization with reversed polarity produced downward in a sequence to an extraordinary large depth. Based on available age information, the observed reversal represents the Gauss/Matuyama boundary, which provides an important age constraint for palaeoclimatic interpretation of the succession.
... Greigite, on the other hand, typically grows in sedimentary environments as equidimensional crystals with cuboctahedral symmetry (Roberts et al., 2011), and commonly gives rise to the asymmetric combination of positive and negative background features predicted here for hexagonal particles and by Valdez-Grijalva and Muxworthy (2019) for cubic particles. Similar arguments apply to pyrrhotite, which also has highly asymmetric FORCs (Horng, 2018;Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002;Wehland et al., 2005). ...
The diagnostic power of first‐order reversal curve (FORC) diagrams has recently been enhanced by an extended measurement protocol that yields three additional FORC‐like diagrams: the remanent (remFORC), induced (iFORC), and transient (tFORC) diagrams. Here, we present micromagnetic simulations using this extended protocol, including numerical predictions of remFORC, iFORC, and tFORC signatures for particle ensembles relevant to rock magnetism. Simulations are presented for randomly packed single‐domain (SD) particles with uniaxial, cubic, and hexagonal anisotropy, and for chains of uniaxial SD particles. Noninteracting particles have zero tFORC, but distinct remFORC and iFORC signals, that provide enhanced discrimination between uniaxial, cubic, and hexagonal anisotropy types. Increasing interactions lessen the ability to discriminate between uniaxial and cubic anisotropy but reproduces a change in the pattern of positive and negative iFORC signals observed for SD‐dominated versus vortex‐dominated samples. Interactions in SD particles lead to the emergence of a bi‐lobate tFORC distribution, which is related to formation of flux‐closure in super‐vortex states. A predicted iFORC signal associated with collapsed chains is observed in experimental data and may aid magnetofossil identification in sediments. Asymmetric FORC and FORC‐like distributions for hexagonal anisotropy are explained by the availability of multiple easy axes within the basal plane. A transition to uniaxial switching occurs below a critical value of the out‐of‐plane/in‐plane anisotropy ratio, which may allow FORC diagrams to provide insight into the stress state of hexagonal minerals, such as hematite.
... High sedimentation rate records (10 to 20 cm/ka) reveal complex reversal dynamics but are limited to a few records in the North Atlantic (e.g., Channell, 2017). In addition, the rare reversal records from sites with higher sedimentation rates (>20 cm/ka) are often associated with variable sedimentation patterns or complex diagenetic processes (e.g., complete or partial remagnetization, chemical transformations of magnetic minerals) that call into question their ability to properly record geomagnetic field changes over transitional periods characterized by weak fields (e.g., Quidelleur et al., 1992;Roberts et al., 2010;Sagnotti et al., 2010Sagnotti et al., , 2016Evans and Muxworthy, 2018). (Simon et al., 2017;Nomade et al., 2019); MD95-2016, MD95-0949, and MD98-2183(Simon et al., 2018bValet et al., 2019); MD90-0961 (Valet et al., 2014); Epica Dome C (EDC) (Raisbeck et al., 2006); MD97-2143 (Suganuma et al., 2010;Simon et al., 2018a); MD05-2930 (Ménabréaz et al., 2014;Simon et al., 2016a); and Heqing paleolake (Du et al., 2018). ...
We present new magnetic (direction and relative paleointensity) and beryllium isotope (10 Be, 9 Be and 10 Be/ 9 Be ratio) results covering the last geomagnetic reversal, i.e., the Matuyama-Brunhes transition (MBT), from the Chiba composite section (CbCS), east-central Japan. The very high sedimentation rates (>90 cm/ka) of the studied site, a candidate site for the global boundary stratotype section and point (GSSP) of the Lower-Middle Pleistocene boundary, allow the acquisition of a dataset of exceptional resolution. Coupled cosmogenic 10 Be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features, allowing accurate paleomagnetic interpretations. A polarity switch (PS) capturing most of the angular deviation occurring between 771.9 and 773.9 ka was followed by a directional instability rebound (IC1) until 768.5 ka. This sequence with 5.4 ka duration was characterized by a weak dipole field as low as 2.3 ± 0.3 × 10 22 Am 2. Four rapid episodes of directional instabilities preceding and following the PS-IC1 phase completed the reversal sequence. The asymmetry observed between the long-term dipole decay and sharp recovery, and rapid oscillations (<1 ka) demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state. Our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action. Although this poses problems for understanding the underlying physical processes that produce reversals, it does not hamper stratigraphic correlations among most geologic records.
... 2b), which may have been helpful to correlate the two logs using lithologic markers. In the case of the Arda section, this discrepancy may be explained by making reference to the short reversed polarity interval found in the Vrica section at the top of the Olduvai subchron (the ß interval in Zijderveld et al., 1991;Roberts et al., 2010;Cita et al., 2012), which may be tentatively correlated with the reversed interval occurring between 50 and 90 m in the Arda section (Monesi et al., 2016). Another possibility to consider would be microfossil reworking; this can be taken into account to explain the discrepancies at the top of the Arda section, but not the ones concerning the base of the Olduvai subchron. ...
The Arda and Stirone marine successions (Italy) represent key sections for the early Pleistocene; they were deposited continuously within a frame of climate change, recording the Calabrian cooling as testified by the occurrence of the “northern guests,” such as the bivalve Arctica islandica . In addition, although the first occurrence of A. islandica in the Mediterranean Sea was used as the main criterion to mark the former Pliocene-Pleistocene boundary, the age of this bioevent was never well constrained. Here, we describe the Stirone depositional environment and constrain for the first time the section age using calcareous nannofossil and foraminifera biostratigraphy. We also correlate the Arda and Stirone sections using complementary biostratigraphic and magnetostratigraphic data. Our results indicate that A. islandica first occurred in both the successions slightly below the top of the CNPL7 biozone (dated at 1.71 Ma). Comparisons with other lower Pleistocene Mediterranean marine successions indicate that the stratigraphically lowest level where A. islandica first occurred in the Mediterranean Sea is in the Arda and Stirone sections; these environments satisfied the ecological requirements for the establishment and the proliferation of the species, which only subsequently (late Calabrian) has been retrieved in southern Italy and other areas of the Mediterranean Sea.
... The presence of pyrrhotite has been reported widely in methane-rich sedimentary environments where high concentration of methane occur near disseminated gas hydrates Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Neretin et al., 2004;Roberts et al., 2010). Hexagonal pyrrhotite (Fe 9 S 10 or Fe 11 S 12 ) often forms authigenically in methanic sedimentary environments (Horng, 2018), while monoclinic pyrrhotite (ferrimagnetic; Fe 7 S 8 ) is found widely in igneous and metamorphic rocks (Horng & Roberts, 2006;Horng et al., 2012;Horng & Huh, 2011;O'Reilly et al., 2000;Rochette et al., 2001Rochette et al., , 2003 and can be eroded and transported through river systems to be deposited in marine environments. ...
We evaluate environmental magnetic, geochemical, and sedimentological records from three sediment cores from potential methane-hydrate bearing sites to unravel linkages between sedimentation, shale-tectonics, magnetite enrichment, diagenesis, and gas hydrate formation in the Krishna-Godavari (K-G) basin. Based on down-core rock magnetic variations, four sedimentary magnetic property zones (I-IV) are demarcated. A uniform band of enhanced magnetic susceptibility (zone-III) appears to reflect a period of high sedimentation events in the K-G basin. Highly pressurized sedimentary strata developed as a result of increased sedimentation that triggered development of a fault system that provided conduits for upward methane migration to enter the gas hydrate stability zone (GHSZ) leading to formation of gas hydrate deposits and potentially seal the fault system. Magnetic susceptibility fluctuations and the presence of iron sulfides in a magnetically enhanced zone suggests that fault system growth facilitated episodic methane venting from deeper sources that led to multiple methane seepage events. Pyrite formation along sediment fractures resulted in diagenetic depletion of magnetic signals and potentially indicates paleo sulfate-methane transition zone positions. We demonstrate that a close correlation between magnetic susceptibility and chromium reducible sulfur concentration can be used as a proxy to constrain paleo-methane seepage events. Our findings suggest that the interplay between higher sedimentation events and shale-tectonism facilitated fluid/gas migration and trapping, and development of the gas hydrate system in the K-G basin. The proposed magnetic mineralogical approach has wider scope to constrain understanding of gas hydrate systems in marine sediments.
... The presence of pyrrhotite has been reported widely in methane-rich sedimentary environments where high concentration of methane occur near disseminated gas hydrates Kars & Kodama, 2015a, 2015bLarrasoaña et al., 2007;Neretin et al., 2004;Roberts et al., 2010). Hexagonal pyrrhotite (Fe 9 S 10 or Fe 11 S 12 ) often forms authigenically in methanic sedimentary environments (Horng, 2018), while monoclinic pyrrhotite (ferrimagnetic; Fe 7 S 8 ) is found widely in igneous and metamorphic rocks (Horng & Roberts, 2006;Horng et al., 2012;Horng & Huh, 2011;O'Reilly et al., 2000;Rochette et al., 2001Rochette et al., , 2003 and can be eroded and transported through river systems to be deposited in marine environments. ...
We evaluate the environmental magnetic, geochemical, and sedimentological records from three sediment cores from potential methane‐hydrate bearing sites to unravel linkages between sedimentation, shale tectonics, magnetite enrichment, diagenesis, and gas hydrate formation in the Krishna‐Godavari basin. Based on downcore rock magnetic variations, four sedimentary magnetic property zones (I–IV) are demarcated. A uniform band of enhanced magnetic susceptibility (zone III) appears to reflect a period of high‐sedimentation events in the Krishna‐Godavari basin. Highly pressurized sedimentary strata developed as a result of increased sedimentation that triggered the development of a fault system that provided conduits for upward methane migration to enter the gas hydrate stability zone, leading to the formation of gas hydrate deposits that potentially seal the fault system. Magnetic susceptibility fluctuations and the presence of iron sulfides in a magnetically enhanced zone suggest that fault system growth facilitated episodic methane venting from deeper sources that led to multiple methane seepage events. Pyrite formation along sediment fractures resulted in diagenetic depletion of magnetic signals and potentially indicates paleo sulfate‐methane transition zone positions. We demonstrate that a close correlation between magnetic susceptibility and chromium reducible sulfur concentration can be used as a proxy to constrain paleomethane seepage events. Our findings suggest that the interplay between higher sedimentation events and shale tectonism facilitated fluid/gas migration and trapping and the development of the gas hydrate system in the Krishna‐Godavari basin. The proposed magnetic mineralogical approach has wider scope to constrain the understanding of gas hydrate systems in marine sediments.
... The sediments were sampled for both stepwise progressive Alternating Field (AF) and THermal (TH) demagnetisation because the potential presence of greigite (Maher and Hallam, 2005;Parfitt et al., 2010) may seriously interfere with AF demagnetisation as a gyroremanent magnetisation (GRM) often develops (e.g. Snowball, 1997;Roberts et al., 2010). ...
The timing, environmental setting and archaeological signatures of an early human presence in northern Europe have been longstanding themes of Palaeolithic research. In the space of 20 years, the earliest record of human occupation in Britain has been pushed back from 500 ka (Boxgrove) to 700 ka (Pakefield) and then to >800 ka (Happisburgh Site 3). Other sites also contribute to this record of human occupation; a second locality at Happisburgh, referred to as Site 1, attests to human presence at around 500 ka (MIS 13). This paper provides the first comprehensive account of research undertaken at Happisburgh Site 1 since 2000. The early human landscape and depositional environment was that of a river floodplain, where an active river channel, in which a grey sand was deposited, was abandoned, forming a floodplain lake, with marginal marsh/swamp environments, which was infilled with organic mud. This succession is sealed by Middle Pleistocene glacial deposits. An assemblage of 199 flint flakes, flake tools and cores was recovered from the grey sand and organic mud. The evidence from Happisburgh Site 1 is placed in the context of the wider British and European MIS 13 record. The growing evidence for a significant dispersal of humans into northern Europe around 500 ka raises critical questions concerning the environmental conditions under which this took place. We also consider the evolutionary and behavioural changes in human populations that might have enabled the more widespread and persistent period of human presence in northern Europe at this time.
... Secondary magnetic overprints due to viscous remanences, new formation of magnetic minerals and magneto-mineralogical alteration associated with burial, diagenesis, exhumation and exposure may complicate the analysis of characteristic remanences. Magnetostratigraphic dating of continental sediments is often more complex than for marine sequences due to a highly variable lithology causing varying rock magnetic properties, alteration and remagnetization processes (Hounslow and Nawrocki, 2008;Roberts et al., 2010;Kodama, 2012). However, under favorable conditions, one can link lithologic variations to cyclic variations in the depositional environment, i.e., precession, obliquity and eccentricity (Hays et al., 1976;Olsen and Kent, 1996;Kodama and Hinnov, 2015). ...
The Neogene succession of the Aktau Mountains in the Ili Basin, southeast Kazakhstan, is a terrestrial archive well suited for researching the role of Central Asia in Miocene climate evolution. We present an integrated approach for dating the well-exposed Bastau Formation, based on magnetostratigraphy and constraints from cyclostratigraphy and biostratigraphy. Stepwise demagnetization yielded characteristic remanence directions that are consistent with those expected for the Miocene in Central Asia. The reddish-colored alluvial floodplain deposits and gray lacustrine deposits show partly complex magnetic behavior with magnetite and hematite as the main magnetic carriers, with variable demagnetization behavior and non-dipolar normal and reverse polarity directions. The observed magnetic properties are best explained by depositional variability and magneto-mineralogical alteration effects of both dissolution and neo-formation of magnetite, including significant secondary magnetization. The mean of reverse polarity directions is flatter than the expected Middle Miocene Earth magnetic field, which is an indicator for the existence of inclination shallowing that supports a primary origin. Detailed rock magnetic analyses are used to analyze the nature of the characteristic remanent magnetization and to discriminate primary and secondary remanence directions in order to obtain a reliable magnetostratigraphic result. The proposed age of 15.3–13.9 Ma for the Bastau Formation corresponds to the known biostratigraphic setting, matches with typical sedimentation rates of foreland basins in Central Asia, and coincides with spectral analysis of geochemical proxies of that section. The resulting age model serves as a robust framework for paleoclimate reconstruction of Neogene climate dynamics in Central Asia.
... For example, Lanci (2010) concluded that both uniaxial and multiaxial anisotropies exist in igneous rocks. FORC diagrams can be used to identify multiaxial anisotropy (Harrison & Lascu, 2014), which explains features observed for hematite (Brownlee et al., 2011;Carvallo et al., 2006;Jovane et al., 2011;, monoclinic pyrrhotite (Larrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002;Wehland et al., 2005), and monoclinic magnetite below the Verwey transition temperature at low applied fields (Smirnov, 2007). Variable magnetocrystalline anisotropy type means that the limits for different domain states will vary in the Day diagram. ...
The “Day” diagram (Day et al., 1977, https://doi.org/10.1016/0031-9201(77)90108-X) is used widely to make inferences about the domain state of magnetic mineral assemblages. Based on theoretical and empirical arguments, the Day diagram is demarcated into stable “single domain” (SD), “pseudo single domain” (“PSD”), and “multidomain” (MD) zones. It is straightforward to make the necessary measurements for a sample and to plot results within the “domain state” framework based on the boundaries defined by Day et al. (1977, https://doi.org/10.1016/0031-9201(77)90108-X). We discuss 10 issues that limit Day diagram interpretation, including (1) magnetic mineralogy, (2) the associated magnetocrystalline anisotropy type, (3) mineral stoichiometry, (4) stress state, (5) surface oxidation, (6) magnetostatic interactions, (7) particle shape, (8) thermal relaxation, (9) magnetic particle mixtures, and (10) definitional/measurement issues. In most studies, these variables are unknowns and cannot be controlled for, so that hysteresis parameters for single bulk samples are nonunique and any data point in a Day diagram could result from infinite combinations of relevant variables. From this critical appraisal, we argue that the Day diagram is fundamentally ambiguous for domain state diagnosis. Widespread use of the Day diagram has also contributed significantly to prevalent but questionable views, including underrecognition of the importance of stable SD particles in the geological record and reinforcement of the unhelpful PSD concept and of its geological importance. Adoption of approaches that enable correct domain state diagnosis should be an urgent priority for component-specific understanding of magnetic mineral assemblages and for quantitative rock magnetic interpretation.
... To the best of our knowledge, the only available documentations of the M-B transition in the region are the paleomagnetic record reconstructed for the Sulmona paleolake (Sagnotti et al., 2014(Sagnotti et al., , 2016 and the 10 Be/ 9 Be stratigraphy obtained for the Montalbano Jonico section (Simon et al., 2017). In particular, magnetostratigraphic investigations performed in the Crotone Basin (CB hereafter) yielded contrasting outcomes, showing that sapropel-bearing sections usually provide ambiguous results (e.g., the Vrica section: Roberts et al., 2010), while massive blue clays, like those in the VdM section, are generally amenable to magnetostratigraphic studies Consolaro et al., 2013;Macrì et al., 2014). Therefore, the VdM record provides a unique opportunity to gauge the chronology and geometry of the M-B transition in the Central Mediterranean. ...
High-resolution palaeomagnetic and rock magnetic investigations on the Valle di Manche section (Crotone Basin, Calabria, Southern Italy) provide a detailed record of the Matuyama-Brunhes (M-B) reversal that, to our best knowledge, is the only available record of the last geomagnetic reversal for the Mediterranean on-land marine stratigraphy. The M-B transition can be pinpointed precisely, as it develops within a 3-cm-thick interval located just above a prominent tephra layer (the “Pitagora ash”) where the sedimentation rates are about 27 cm/kyr. Demagnetization analyses indicate a stable palaeomagnetic behaviour throughout the section for both normal and reversed polarity directions, with demagnetization vectors aligned toward the origin of Zijderveld diagrams after the removal of a small viscous low-coercivity remanence component. In the lower part of the studied interval, some samples acquired a spurious gyromagnetic remanent magnetization (GRM) during AF demagnetization in high fields. Rock magnetic analyses confirm that magnetite is the main magnetic carrier for all measured specimens, which also have an abundant paramagnetic fraction. Only the lower part of the record, well below the M-B boundary, is characterized by a downward-increasing presence of iron sulphides (greigite). According to our chronology, which is based on a robust, cross-validated age model, the final reverse-to-normal directional change of the M-B transition occurred at ca. 786.9 ± 5 ka (error includes uncertainty in orbital tuning) and was very rapid, of the order of 100 years or less.
... In this case, greigite will carry a significantly younger CRM that can lead to a complex polarity pattern. For example, the complex magnetic polarity pattern at the Vrica section in Calabria results from the fact that greigite growths occur significantly later than deposition (Roberts et al., 2010). Besides, it is difficult to ascertain the time at which the CRM (greigite) was acquired (e.g. ...
A detailed magnetic analysis has been done on sedimentary core of the International Ocean Discovery Program (IODP) Site U1433A during Leg 349 in the South China Sea (SCS). Results show that dominant carriers of the natural remanent magnetization are greigite and (titano) magnetite. The major shift in both declination and inclination at ~ 185 mbsf is assigned to the Matuyama-Brunhes reversal boundary (~ 0.773 Ma). Constrained by biostratigraphic ages, variations in magnetic parameters of the core can be well correlated to the marine oxygen isotope record at glacial/interglacial cycles. Low values of concentration-dependent magnetic parameters correspond to the interglacials, and vice versa. During the interglacial periods, the dominant magnetic minerals are detrital (titano) magnetite and have relatively coarser grain sizes, while fine-grained greigites dominate the glacial periods. This indicates that during the glacials, greigite prevails at the anoxic condition with amount of terrigenous iron oxide caused by the disconnection between the SCS and the Indian Ocean and the exposure of shelf, but digenesis is suppressed at the opposite environment by the high sea level (interglacials). Thus, the preservation/sulfide process of (titano) magnetite is intimately related to the transformation of sea level changes with the monsoon-related rainfall caused by the glacial/interglacial variation.
... Based on the study of the most complete natural sections in western Georgia and boreholes from the Kertch Peninsula it has been shown that the upper boundary of the Kujalnikian is situated between the Olduvai and Gilsa Chrons (Pevzner, 1989). In modern terms (Roberts et al., 2010;Ogg, 2012) it can be interpreted as placed between the main part of the Olduvai Chron (C2n) and its upper normal subchron ("Vrika subchron"). Thus the upper limit of the Kujalnikian is drawn inside the Olduvai. ...
The discovery of the Early Pleistocene sites of Bogatyri/Sinyaya Balka and Rodniki 1 on the Taman
Peninsula in western Ciscaucasia led to the recognition of a distinctive “Tamanian industry”, with a timerange of 1.6e1.2 Ma, and with characteristics that are transitional between Oldowan and Acheulean (“Archaic Acheulean”). The site of Kermek was discovered by V.E. Shchelinsky in 2008 during investigation of the older Lower Pleistocene sediments exposed in the coastal cliff of the Sea of Azov in the vicinity of previously studied sites.
In this paper, we present preliminary data from a multidisciplinary study of the Early Pleistocene site
of Kermek, situated near to the sites of Bogatyri/Sinyaya Balka and Rodniki 1, but which is significantly older. This site is connected with a well-studied Early Pleistocene (late Kujalnikian) fluviatile-shallow marine sequence. These reverse magnetized deposits are characterized by freshwater and brackish water mollusks (with Dreissena theodori), and by a small mammal fauna (with Allophaiomys deucalion), and are dated to the latest Gelasian or early Calabrian ca. 2.1e1.8 Ma. The lithic industry from the site can be attributed to the Classic Oldowan but with distinctive local features that include indications of “advanced technologies” such as the manufacture of large flakes and picks. In this respect, this industry is assumed to have been a genetic precursor of the later Early Pleistocene Tamanian industry, which has a well pronounced Acheulean component.
... при этом на основе изучения наиболее полных разрезов в западной грузии и материалов бурения на керченском полуострове было установлено, что верхняя граница куяльника располагается между хронами олдувей и гилза (там же). по современным представлениям о последовательности палеомагнитных инверсий (Roberts et al., 2010; Ogg, 2012), она может быть проведена между основной частью хрона олдувей (c2n) и его верхним обратно намагниченным субхроном (врика). таким образом, верхний предел куяльника приходится на олдувей. ...
Новая раннепалеолитическая стоянка Кермек, расположенная в Запад-
ном Предкавказье, связана с хорошо изученной толщей аллювиально-прибрежно-
морских отложений раннего плейстоцена (позднего куяльника). Эти отложения
имеют обратную намагниченность и характеризуются пресноводными и солонова-
то-водными моллюсками (с Dreissena theodori) и фауной мелких млекопитающих
(с Allophaiomys deucalion), что позволяет датировать стоянку поздним гелазием или
ранним калабрием в диапазоне 2,1–1,8 млн л. н. Каменную индустрию стоянки мож-
но определить как классический олдован. Вместе с тем она имеет местные отличи-
тельные особенности, которые проявляются прежде всего в наличие в ней призна-
ков «продвинутой» технологии ашеля, таких как изготовление крупных отщепов
и пиков. Вероятно, мы имеем дело с переходной индустрией от олдована (mode 1)
к ашелю. Стоянка Кермек является самой древней раннепалеолитической стоянкой
в Западной Евразии за пределами Кавказа.
... Based on the study of the most complete natural sections in western Georgia and boreholes from the Kertch Peninsula it has been shown that the upper boundary of the Kujalnikian is situated between the Olduvai and Gilsa Chrons (Pevzner, 1989). In modern terms (Roberts et al., 2010;Ogg, 2012) it can be interpreted as placed between the main part of the Olduvai Chron (C2n) and its upper normal subchron ("Vrika subchron"). Thus the upper limit of the Kujalnikian is drawn inside the Olduvai. ...
The discovery of the Early Pleistocene sites of Bogatyri/Sinyaya Balka and Rodniki 1 on the Taman Peninsula in western Ciscaucasia led to the recognition of a distinctive “Tamanian industry”, with a time-range of 1.6–1.2 Ma, and with characteristics that are transitional between Oldowan and Acheulean (“Archaic Acheulean”). The site of Kermek was discovered by V.E. Shchelinsky in 2008 during investigation of the older Lower Pleistocene sediments exposed in the coastal cliff of the Sea of Azov in the vicinity of previously studied sites.
... The GSSP coincides with the transition from Marine Isotope Stage 65 to 64. The top of the Olduvai Subchron is identified 8 m above the GSSP, but diagenetic overprinting has obstructed efforts to locate the boundary more precisely (Roberts et al., 2010). Primary guides to global correlation are therefore marine isotope stratigraphy and magnetostratigraphy, with sapropel stratigraphy available for correlation throughout the eastern Mediterranean. ...
The Quaternary System/Period represents the past 2.58 million years and is officially subdivided into the Pleistocene and Holocene series/epochs, with the base of the Holocene assigned an age of 11,700 calendar years before AD 2000. The two lowest stages of the Pleistocene, the Gelasian (base 2.58 Ma) and the Calabrian (base 1.80 Ma), have been ratified and these effectively constitute the Lower Pleistocene Subseries. All other official subdivisions are pending. For the Middle Pleistocene Subseries, three candidate global boundary stratotype sections and points (GSSPs) are under consideration: the Valle di Manche in Calabria and Montalbano Jonico in Basilicata, both in southern Italy, and the Chiba section in Japan. The Matuyama-Brunhes Chron boundary (~773 ka) serves as the principal guide for the base of the Middle Pleistocene. The base of the Upper Pleistocene Subseries is generally agreed to coincide approximately with that of the last interglacial (Marine Isotope Substage 5e ~130 ka): the Fronte section near Taranto in southern Italy represents a possible candidate GSSP, but an Antarctic ice core might also serve this purpose. A tripartite subdivision of the Holocene, with subseries/stage boundaries at 8200 and 4200 years B.P., is also under consideration. Additional fine-scale formal subdivision of the Quaternary is being explored, with the Last Glacial Maximum serving as a test case. The "Anthropocene" is both an informal and undefined interval of time that includes the present day. Its duration, formal/informal status, rank, and method of definition are all under debate, with one suggestion that it be defined as a formal unit beginning with the world's first nuclear bomb explosion, on July 16th 1945. Suggested and proposed GSSPs are compared and critiqued. The history leading to ratification of the Quaternary Period in 2009 is examined drawing upon published and unpublished material.
... Contrary to this mineralogical interpretation, Hawkes et al. [2003] identified in sediment cores from drifts 3, 4, 4a and 5 only magnetite. Understanding the magnetic mineral responsible for the remanence is important because diagenetic formation of an iron sulfide component may have delayed the magnetic field recording process to some time after deposition [e.g., Florindo and Sagnotti, 1995;Roberts et al., 2005;Sagnotti et al., 2005;Florindo et al., 2007;Roberts et al., 2010]. ...
Three piston cores (SED-02, SED-04 and SED-06 of Pudsey and Camerlenghi, 1998) from a sediment drift (Drift 7) on the continental rise of the Pacific margin of the Antarctic Peninsula, were sampled for a high-resolution paleomagnetic and rock magnetic study. Measurements were conducted at 1 cm intervals on u-channel samples with a 2-G Enterprises DC SQUID cryogenic magnetometer system at the ING, in Rome. All the cores span the time interval between the present-day and the last interglacial. One of the cores (SED-06) extends to about 150 kyr. Magnetic properties were used to confirm and improve correlation between cores, and to investigate variations induced by paleoclimatic changes. Paleomagnetic studies were conducted to search for possible characteristic paleomagnetic features (e.g., geomagnetic excursions and relative paleointensity variations). Results indicate that the main rock magnetic variations occur in correspondence with decimetric dark-colored layers and not at the interglacial-glacial transition. Anomalously low paleomagnetic inclinations (up to about -35) are observed in a c. 30 cm thick interval of the SED-06 core, which may represent the Blake excursion. Normalized remanence records from the cores match fairly well, but appear rather different from the Sint-200 paleointensity reference curve.
... The SEM observations provide a straightforward way to distinguish different iron sulfide minerals. Pyrrhotite has a platy morphology, while greigite has an octahedral morphology within larger, often framboidal, aggregates (Larrasoaña et al., 2007;Roberts et al., 2010;Weaver et al., 2002). The SEM images from the magnetic extracts at 345 cm show the presence of abundant fine-grained aggregates (Fig. 8a, b). ...
High-resolution proxy records from Tibetan Plateau are essential to understand the past climatic and environmental changes. In this work, we conducted systematic environmental magnetic studies on lacustrine sediments from Lake Pumoyum Co, southern Tibet, spanning from the last deglaciation (similar to 19 ka) to present. We correlated the magnetic proxies in the lake core with environmental changes. The magnetic proxies from the 3 units were studied. Both Unit 3 (19-13.2 ka) and Unit 2 (13.2-8.4 Ica) contain evidences for an anoxic or sub-anoxic environment. Fine-grained greigite dominates the bulk magnetic properties of Unit 3, while both magnetite and greigite coexist in Unit 2. Overall, this reveals a transition from an anoxic environment to an oxic environment. In contrast, Unit 1 (8.4 ka-present) spans the Holocene when the lake environment was oxic. There is an increasing trend in the concentration of magnetic minerals, which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post-glacial melt-water flow. Although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units, our results suggest that the magnetic proxies of the local lake environment on southern Tibet are sensitive to environmental changes.
... Acquisition of magnetization after the formation of a rock, termed remagnetization, can obscure or remove the primary magnetizations and thus limit the utility of paleomagnetism in global (plate) tectonic reconstructions [Elmore et al., 2012;Font et al., 2012]. Widespread remagnetization events are usually linked to large-scale tectonic processes [McCabe et al., 1983;Miller and Kent, 1988;Chen and Courtillot, 1989;Appel et al., 1995;Molina Garza and Zijderveld, 1996;Weil and Van der Voo, 2002;Font et al., 2006;Rapalini and Bettucci, 2008;Font et al., 2012] and can affect both sedimentary rocks [Appel et al., 1991;Gong et al., 2009a;Roberts et al., 2010;Liu et al., 2011; and igneous rocks [Harlan et al., 1996;Edel and Aifa, 2001;Geissman and Harlan, 2002;Otofuji et al., 2003;Borradaile et al., 2008;Preeden et al., 2009]. Suspected mechanisms for remagnetization include elevated temperature, chemical alteration, and secondary mineral growth. ...
components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by ~52 Ma at ~20°N.
... These processes can act alone or in concert, and they frequently result in changes in the magnetic mineralogy, in the NRM, or both [Jackson and Swanson-Hysell, 2012]. The severity of the problem is illustrated by widespread remagnetization events linked to large-scale tectonic processes, which are common in orogens in North America [e.g., McCabe et al., 1983;Miller and Kent, 1988;McCabe and Elmore, 1989;Elmore and McCabe, 1991;Elmore et al., 2001;Geissman and Harlan, 2002;Evans et al., 2012;Zechmeister et al., 2012], South America [e.g., D' Agrella-Filho et al., 2000;Trindade et al., 2004;Rapalini and Bettucci, 2008;Font et al., 2011Font et al., , 2012Tomezzoli et al., 2013], Europe [e.g., Molina Garza and Zijderveld, 1996; Van der Voo et al., 1997;Dinarès-Turell and Garcia-Senz, 2000;Weil and Van der Voo, 2002;Zegers et al., 2003;Gong et al., 2009a;Zwing et al., 2009;Roberts et al., 2010], and Asia [e.g., Chen and Courtillot, 1989;Appel et al., 1991Appel et al., , 1995Otofuji et al., 2003;Torsvik et al., 2005;Kim et al., 2009;Liu et al., 2011;Appel et al., 2012;Kirscher et al., 2013]. Therefore, proper identification of "remagnetized" and "nonremagnetized" rocks is critical to paleomagnetic research, particularly for paleogeographic reconstructions of continental blocks in convergent orogenic belts. ...
Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to constrasting paleogeographic interpretations. Here, we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of post-tilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° [35.7°, 40.5°] (95% significance) and a secondary remanence with inclination of 42.9° [41.5°,44.4°] (95% significance). The paleolatitude defined by the modeled primary remanence—21°N [19.8°N, 23.1°N]— is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination-shallowing. The start of the Tibetan Himalaya-Asia collision was situated at ~20°N, and took place by ~50 Ma.
... At a later stage, magnetite can be partly or completely dissolved, leaving greigite as the dominant remanence carrier. Unlike the authentic self-reversed magnetization, e.g., the highly oxidized titanomagnetite [e.g., Schult, 1964;Heller and Petersen, 1982;Krása et al., 2005], the term "magnetic interaction-induced contradictory magnetization" is suggested to describe the reversed polarity, which is linked to a specific process within particles [e.g., Jiang et al., 2001;Roberts and Weaver, 2005;Sagnotti et al., 2005;Roberts et al., 2010]. This model predicts that the greigite-bearing layer can record both normal and "reversed" polarity depending on the exact spatial interaction between SD greigite and PSD/MD magnetite. ...
Sediments from the continental shelf are sensitive to sea-level, climatic changes as well as local tectonic history. In this study, we carried out a high resolution magnetostratigraphic investigation on the longest core (NHH01, 125.64 m) recovered from the South Yellow Sea (SYS). An abnormal interval dominated by negative inclinations was discovered by applying alternating field demagnetization (AFD) on samples from a greigite-bearing layer (44.90 − 51.80 m). In contrast, the inclinations of most greigite-bearing samples changed from negative to positive when heated to ~360 °C. This strongly indicates that this inclination anomaly revealed by the AFD alone is not a true negative subchron. After neglecting the effects of greigite-bearing layers, the straightforward correlation of the interpreted magnetostratigraphy defines the Matuyama − Brunhes boundary (MBB, 781 ka) and the Jaramillo top (990 ka) at 68.64 m and 101.54 m, respectively. The linearly-extrapolated basal age of the core is ~1.10 Ma. In addition, several short-lived inclination anomalies can be tentatively assigned to magnetic excursions, which indicates that the sedimentation could be continuous even at the millennial time scale at depth intervals bracketing these fast geomagnetic events. Moreover, the excellent correspondence between clay content variations of the core and the marine oxygen isotope record indicates the potential of clay content as a paleoclimatic proxy in the studied region in the past ~1 Ma. In brief, our study not only provides a robust age model in the SYS but also a methodological guide for paleomagnetic studies in continental shelf region.
The Quaternary System/Period represents the past 2.58 Ma on the Geological Time Scale, during which time distinctive biotic associations arose, the genus Homo diversified, and climates were strongly influenced by Northern Hemisphere glaciation. Although derived from a seventeenth century geological subdivision of northern Italy and having a long history of use, the term Quaternary was formalized only in 2009. It is defined by a Global boundary Stratotype Section and Point (GSSP) at Monte San Nicola in Sicily, Italy and is subdivided into the Pleistocene and Holocene series/epochs. All subdivisions of the Quaternary are defined using paleomagnetic reversals and/or distinctive climatic changes. The Holocene Series/Epoch began 11,700 years b2k (before 2000 CE) and extends to the present day. If the Anthropocene is officially recognised as a series/epoch, then it will terminate the Holocene in the mid-twentieth century thereby adding a third series/epoch to the Quaternary.
The Sulu orogenic belt is an uplift zone that was formed in the Late Triassic. Several Jurassic to Cretaceous sedimentary successions has been recognized within the Sulu orogenic belt in recent studies, including outcrops that are considered to be related to the newly discovered Riqingwei Basin. This basin has been the focus of extensive study due to its continuous Cretaceous rock sequence, geological location and petroleum resource potential. However, the lack of a consolidated chronology for the strata has precluded a better understanding of stratigraphy, tectonic evolution and resource potential of the Riqingwei Basin. Here, we present the results of a new magnetostratigraphic study of the continental scientific drilling borehole LK-1, which is located on Lingshan Island, offshore Shandong province, eastern China. The goals of this study are to (1) refine the Late Jurassic to Early Cretaceous chronostratigraphic framework of the Riqingwei Basin, and (2) investigate the location of the J/K boundary in the Borehole Core LK-1. The observed patterns of the paleomagnetic polarity zone in the LK-1 borehole correlate well with the geomagnetic polarity time scale (GPTS), and the continuous magnetostratigraphy profile defined in this core indicates an age ranging from 146.5 to 125.8 Ma for the samples interval. The sediment accumulation rates (SAR) of LK-1 show one period of high SAR (∼10.5 cm kyr−1) at 135.3–130.6 Ma and two periods of low SAR (∼4.8 and ∼2.2 cm kyr−1) at 145.7–135.3 and 130.6–125.8 Ma, respectively. In addition, the magnetostratigraphic results suggest that the Jurassic-Cretaceous (J/K) boundary of the LK-1 is located within the magnetozone N21.2n (∼1254 m). This comprehensive geochronologic framework provides a good correlation of the marine Upper Jurassic to Lower Cretaceous strata in the Riqingwei Basin to other marine strata and continental sequences, in addition to providing a foundation for the study of the structural evolution of eastern China.
We review the ages of Quaternary magnetic polarity reversals and magnetic excursions. Improvements in radioisotopic ⁴⁰Ar/³⁹Ar dating of reversals and excursions reflect the use of multi-collector mass spectrometers, along with ultra-low backgrounds, laser-based incremental heating methods, and adoption of 28.201 Ma for the Fish Canyon sanidine (FCs) standard age (Kuiper et al., 2008). These improvements have brought ⁴⁰Ar/³⁹Ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of δ¹⁸O data to LR04 (Lisiecki and Raymo, 2005) and, in a few cases, correlation to ice-core chronologies. Ages of two polarity reversals, the Matuyama-Brunhes boundary and base Olduvai, are at odds with ages given in popular geomagnetic polarity timescales. Sedimentary records of the Laschamp and Iceland Basin excursion are numerous and widespread, and attest to global manifestation and global synchronicity of these excursions. Records of other Quaternary excursions are less numerous. The Blake excursion (∼115 ka) has been recorded in North Atlantic sediments, Chinese loess, Indian Ocean volcanics, and Spanish speleothems. The Santa Rosa excursion (∼932 ka) has been recorded in North Atlantic and Phillipine Basin sediments, as well as in volcanic rocks from New Mexico and Galapagos. The Punaruu excursion (∼1115 ka) has been observed in sediments from the North Atlantic and California Margin as well as in volcanic rocks on Tahiti. Uniformity of individual excursion ages, using astrochronological, ⁴⁰Ar/³⁹Ar and U–Th methods, argues for synchronous global manifestation of excursions, strengthening the usefulness of magnetic excursions (and reversals) in Quaternary stratigraphy.
Natural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques. Here a non-linear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent Preisach maps. Using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved. The method is applied to synthetic and natural standard samples containing magnetite and hematite, as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed. It is shown that certain offset high coercivity patterns in remanent Preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite, and that in some magnetite-bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range. Experimental results and theoretical considerations indicate a minimal coercivity of about 10 mT for SD magnetite, such that observation of lower coercivities implies PSD or MD grain sizes. A diagnostic hematite pattern with a peak downwards offset of 17 ± 2% of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities, and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field.
In Part II of our ongoing series on interpretation of low-temperature data, we take a look at the nature of the hematite Morin transition. This is the first of several magnetic phase transitions that will be highlighted, including the magnetite Verwey transition and the spin glass transition in the hemo-ilmenite solid solution. Hematite (αFe2O3) is a common mineral in many natural samples, and its presence is often easy to recognize via its high coercivity and high unblocking temperatures (just below 675°C). However, in some cases a non-destructive method may be preferred, in which case identification of the Morin transition [Morin, 1 1950] at ~262K (-1111°C) is diagnostic of hematite. Natural samples at high elevations or latitudes may cycle many times through the Morin transition, so an understanding of what happens to magnetic properties at this temperature is important for paleomagnetic interpretations.
Integrated Ocean Drilling Program (IODP) Site U1308 (49°53'N, 24°14'W; water depth 3871 m) provides a record of relative paleointensity (RPI) and benthic stable isotope stratigraphy back to 3.2 Ma. The record since 1.5 Ma was published previously, and here we present the interval from 1.5 Ma to 3.2 Ma (Early Pleistocene-Late Pliocene). The benthic oxygen isotope record in this interval is correlated to Marine Isotope Stage (MIS) 51 to KM2, with an apparent hiatus that removed part of the interval spanning MIS 104-G2 (2.6-2.65 Ma), at the Gauss-Matuyama boundary. The mean sedimentation rate for the 1.5-3.2 Ma period is 8.5 cm/kyr. The age model was built by correlation of the benthic oxygen isotope record to a global stack (LR04). Apart from the expected polarity reversals, three magnetic excursions are recorded: Punaruu in MIS 31/32 at 1092 ka, Gilsa in MIS 54/55 at 1584 ka, and a newly recognized excursion labeled Porcupine (after the nearby Porcupine Abyssal Plain) in MIS G6/G7 at 2737 ka. The ages of polarity reversals at Site U1308, on the LR04 time scale, are consistent with the current geomagnetic polarity timescale (GPTS) with the exception of the base of the Olduvai Subchron that occurred in MIS 73, corresponding to 1925 ka on the LR04 time scale, 25 kyr younger than in the current GPTS. The RPI record at Site U1308 is calibrated using the oxygen isotope age model, and combined with four other North Atlantic records to obtain a North Atlantic RPI stack for 1.2-2.2 Ma (NARPI-2200) that is compared with published RPI stacks: Epapis, Sint-2000 and PADM2M. For 2.2-3.2 Ma, the Site U1308 RPI record is compared with a RPI record from North Atlantic IODP Site U1314, and with the Pacific Epapis stack. The mean sedimentation rates of the North Atlantic sites in NARPI-2200 are greater (by about an order of magnitude) than most of the records incorporated in other stacks. The comparison of Epapis and NARPI-2200 yields an apparent lag for NARPI-2200 relative to Epapis, attributed the Epapis age model constructed by correlation of magnetic concentration parameters (a proxy for carbonate percentage) to a calibrated oxygen isotope record. The long RPI record from Site U1308 yields a very similar mean value for the Brunhes and Matuyama virtual axial dipole moments (7.05 × 1022 Am2), implying no polarity bias in the strength of the main geomagnetic dipole, in contrast to interpretations from Sint-2000 and PADM2M. The results strengthen the case that RPI can be used to improve global stratigraphic correlation for sites with mean sedimentation rates up to several decimeters/kyr.
Three new profiles (LCM, PMD and CMD) through the lower Cochiti polarity zone enable comparison of the effects of weathering and diagenesis on the magnetic properties of the Early Pliocene Trubi marls These marls have been well dated, by means of astronomical calibration, and are of particular interest in terms of polarity zonation and regional tectonics The freshest exposures (LCM profile) carry a remanence associated with pyrrhotite, which is particularly enhanced in some grey layers This is the first time that occurence of pyrrhotite as the mam remanence carrier in the Trubi marls is documented (SD magnetite was previously recognised as the fundamental ferromagnetic phase) The mean ChRM direction from the LCM record does not show the expected 30-35° clockwise rotation found in other studies, and does not record the normal Cochiti interval either (i. e. is reverse throughout the sampled interval), suggesting a relatively late origin for the NRM Standard rock magnetic techniques are used to investigate the nature of the magnetization in the three laterally equivalent profiles at Punta di Maiata The actual data set seems to be better explained by a complex diagenetic history involving a multistage redox history that invokes a late' anoxia event The previously postulated iron migration model (van Hoof et al. 1993) was put forward for the 'suboxic situation' and does not take into account the presence of magnetic sulfides, which are clearly demonstrated in the zone under study The 'extended diagenetic model' inferred here proposes a pathway that accounts for most of the new observations The salient feature is the postulated existence of a diagenetic phase that has altered the original NRM in some areas (dissolution of magnetite) resulting in a later stage acquisition of chemical remanence that does not date to the timing of deposition of the sediments The extension and the detailed mechanisms by which such a process has taken part deserve further research Nonetheless, the present investigation accounts for previously unrecognised features in the Trubi marls from Sicily and outlines how intricate the nature of the mechanisms contributing to the blocking of remanence in sediments can be
We demonstrate a powerful and practical method of characterizing interactions in fine magnetic particle systems utilizing a class of hysteresis curves known as first order reversal curves. This method is tested on samples of highly dispersed magnetic particles, where it leads to a more detailed understanding of interactions than has previously been possible. In a quantitative comparison between this method and the δM method, which is based on the Wohlfarth relation, our method provides a more precise measure of the strength of the interactions. Our method also has the advantage that it can be used to decouple the effects of the mean interaction field from the effects of local interaction field variance.
Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first-order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first-order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two-dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single-domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.
Paleomagnetic, rock magnetic and micro-textural analyses from a Middle Pleistocene lacustrine sequence in the Southern Apennines (Italy) indicate the presence of greigite and magnetite as main magnetic minerals at different stratigraphic levels. In all cases a normal polarity characteristic remanent magnetization (ChRM) was observed, in agreement with an 40Ar-39Ar age of 0.712±0.018 Ma. After correction for bedding tilt, the normal polarity directions, which are carried by greigite, do not coincide with the expected geocentric axial dipole field direction at the site latitude, whereas the magnetite ChRMs directions do. The data indicate that the greigite magnetization was acquired after tilting and after lock-in of the magnetite remanence. Scanning electron microscope analyses indicate that alteration of detrital volcanic minerals has occurred and that authigenic iron sulfides phase formed. Greigite is generally present in agglomerates and around volcanic grains. This observation is consistent with a late diagenetic origin of greigite due to the presence of anoxic conditions and availability of dissolved sulfide associated with decomposition of organic matter in the paleolake. Documentation of a late diagenetic magnetization suggests that care should be taken when using greigite-bearing sediments for magnetostratigraphic and tectonic studies.
Two alternative age models are presented for the Pleistocene part of the
Pliocene-Pleistocene boundary stratotype section at Vrica (Calabria,
southern Italy). Both age models, or chronologies, are based on the
correlation of the characteristic sapropel pattern to the summer
insolation time series of astronomical solution La90. The first age
model is based on the assumption that the succession is continuous,
whereas a global near synchroneity of the base of large-sized
Gephyrocapsa in oxygen isotope stages 48-49 is assumed in the second
model. This model implies that a major hiatus with exactly the same age
and duration is present in both the Vrica and Singa sections.
Cross-spectral analysis between sea surface temperature and
δ18O time series and the summer insolation target
yields highly significant coherencies in the precession and obliquity
frequency bands of the spectrum in cases where the continuous age model
is applied but results in less significant coherencies in cases where
the alternative age model is applied. According to the continuous model,
the position of almost all calcareous plankton datum planes relative to
the isotope stages is consistent with that found in the adjacent
northern Atlantic. The only marked exception is the base of large
Gephyrocapsa, which is found in stage 55 in the Mediterranean, whereas
it occurs in stage 48 (or top 49) in the open ocean. The alternative age
model solves the large Gephyrocapsa discrepancy but leads to significant
discrepancies in the position of other calcareous plankton events. On
the basis of these data sets, it is not justified to conclude which age
model is correct, although most arguments are in favor of the continuous
age model. This model is also supported by shipboard results from Ocean
Drilling Program Legs 154 and 160 sites.
Sediment from the FOAM (Friends of Anoxic Mud) site in Long Island Sound, Connecticut, was partitioned into several different size fractions (and magnetite), and the concentration and isotopic composition of sulfur in each fraction was determined. Size fractions were chosen to represent different associations of pyrite with iron. Demonstrates that calculated values of sediment DOP (degree of pyritization) cannot be used to demonstrate uniquely iron or sulfur limitation in pyrite formation. Pyrite formation depends on the relative rates of sulfate reduction compared to rates of sulfide reaction with iron minerals; DOP does not yield information on these factors. -from Authors
Previously proposed models for the evolution of the Tyrrhenian basin-Apenninic arc system do not seem to satisfactorily explain the dynamic relationship between extension in the Tyrrhenian and compression in the Apennines. The most important regional plate kinematic constraints that any model has to satisfy in this case are: (1) the timing of extension in the Tyrrhenian and compression in the Apennines, (2) the amount of shortening in the Apennines, (3) the amount of extension in the Tyrrhenian, and (4) Africa-Europe relative motion. The estimated contemporaneous (post-middle Miocene) amounts of extension in the Tyrrhenian and of shortening in the Apennines appear to be very similar. The extension in the Tyrrhenian Sea is mostly accomplished in an E-W direction, and cannot be straightforwardly related to the calculated N-S Africa-Europe convergence. A model of outward arc migration fits all these constraints. In a subducting system, the subduction zone is expected to migrate outward due to the sinking of the underthrusting plate into the mantle. The formation of a back-arc or internal basin, i.e. of a basin internal to the surrounding belt of compression, (in this case the Tyrrhenian Sea) is then expected to take place if the motion of the overriding plate does not compensate for the retreat of the subduction zone. The sediment cover will be stripped from the underthrusting plate by the outward migrating arc of the overriding plate, and will accumulate to form an accretionary wedge. This accretionary body will grow outward in time, and will eventually become an orogenic belt, (in this case the present Apennines) when the migrating arc collides with the stable continental foreland on the subducting plate. An arc migration model satisfactorily accounts for the basic features of the Tyrrhenian-Apennine system and for its evolution from 17 Ma to the present, and appears to be analogous to the tectonic evolution of other back-arc settings both inside and outside the Mediterranean region. An interesting implication of the proposed accretionary origin of the Apennines is that the problematic “Argille Scagliose” (scaly clays) melange units might have been emplaced as overpressured mud diapirs, as observed in other accretionary prisms, and not by gravity slides from the internal zones.
Because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material, much can be learned by monitoring that response through what is known as a ``magnetic hysteresis loop.'' The measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research. Yet the interpretation of hysteresis loops is not simple. It has become apparent that although classic ``single-domain,'' ``pseudo-single-domain'' and ``multidomain'' loops described in textbooks occur in natural samples, loops are frequently distorted, having constricted middles (wasp-waisted loops) or spreading middles and slouching shoulders (potbellies). Such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions. The physics of the problem have been understood for nearly half a century, yet numerical simulations appropriate to geological materials are almost unavailable. In this paper we discuss results of numerical simulations using the simplest of systems, the single-domain/superparamagnetic (SD/SP) system. Examination of the synthetic hysteresis loops leads to the following observations: (1) Wasp-waisting and potbellies can easily be generated from populations of SD and SP grains. (2) Wasp-waisting requires an SP contribution that saturates quickly resulting in a steep initial slope, and potbellies require low initial slopes (the SP contribution approaching saturation at higher fields). The approach to saturation is dependent on volume hence the cube of grain diameter. Therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size. (3) We were unable to generate potbellies using a SP/SD threshold size as large as 30 nm, and wasp waists cannot be generated using a threshold size as small as 8 nm. The occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm (+/-5). (4) Simulations using a threshold size of 15-20 nm with populations dominated by SP grain sizes, that is with a small number of SD grains, produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass. (5) Simulations and measurements using two populations with distinct coercivity spectra can also generate wasp-waisted loops. A relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp-waisting resulting from SP/SD behavior.
On a global scale, the Crotone basin preserves one of the best-developed and most complete Pleistocene marine records available in outcrop, as important as those in California, New Zealand, and Japan. A deformed, markedly cyclothemic, lower to middle Pleistocene succession is present in the territory of San Mauro Marchesato (Crotone area, southern Italy), showing an overall shallowing trend from slope mudstones to marginal marine and continental deposits. Preservation and high resolution of cyclothems occurred through the interaction between high-amplitude relative sea-level fluctuations, a particular pattern of differential subsidence due to interaction between high-amplitude relative sea-level fluctuations, a particular pattern of differential subsidence due to intrabasinal tectonics, and high rates of sediment supply. The studied succession was laid down in the Crotone basin under an extensional tectonic regime, following a major, middle Pliocene contractional phase probably of transpressional nature. Two major unconformities, locally accompanied by angular discordances, occur within the succession. The former, centered at ca. 1.2 Ma, is thought to reflect the opening of the San Mauro subbasin within the Crotone basin in the early Pleistocene, following dextral transtensional motion along north- to north-northeast-trending faults. The latter, with a hiatus lasting from ca. 0.65 to 0.45 Ma, may reflect the decoupling of the Calabrian block with respect to Adria and Sicily, allowing further advancing of the Calabrian arc in the Ionian area, where subduction could continue until the present time. The lower part of the succession (the H. sellii and "large Gephyrocapsa" Zones, from ca. 1.67 to ca. 1.23 Ma) consists of slope to outer-shelf monotonous mudstones and is bounded at the top by the first unconformity, whose gap suppresses the upper part of to locally the entire "large Gephyrocapsa" Zone (1.608-1.235 Ma) and the lower part of the "small Gephyrocapsa" Zone (1.235-0.96 Ma). A number of cyclothems developed in an outer- to innershelf environment within the "small Gephyrocapsa" Zone. Biomagnetostratigraphic constraints strongly support a correlation between the condensed sections of cyclothems and MIS (marine isotope stage) 33 to MIS 25. From the base of the P. lacunosa Zone (at ca. 0.96 Ma) upward, the succession rapidly becomes sand dominated, a change that can be confidently correlated with the major climatic shift associated with MIS 24 to MIS 22. In the following succession, two tephra layers, named "Pitagora ash" and "Parmenide ash," provide mappable isochronous surfaces across the subbasin. The sedimentary record is remarkably cyclothemic, characterized by a stack of simple or composite, seaward-prograding, sand-dominated tongues and intervening aggradational deposits related to transgressive-deepening episodes. The cyclothems can be confidently correlated with the oxygen isotope record up to the Matuyama-Brunhes inversion, i.e., up to MIS 19, whereas the stratigraphic record postdating MIS 19 has poorer chronological constraints. Dating is provided by tracing the Parmenide ash in the deeper-water coeval succession of the southern part of the Crotone basin, where the deposits including the ash can be correlated by means of nannofossil biostratigraphy with termination V (transition from MIS 12 to MIS 11). The second unconformity marks an abrupt increase in the proximal character of the sedimentary deposits forming the cyclothems, which incorporate increasing amounts of marginal-marine to continental deposits in the upper part of the subbasin infill. Several lines of direct and indirect evidence indicate that, in spite of the dramatic role of tectonics in shaping stratigraphic architecture, the roles of tectonics and eustasy can be disentangled, owing to the different time scales of the tectonic events and the high-frequency, high-amplitude glacio-eustatic Pleistocene cycles. Interaction between intrabasinal tectonics and high rates of sediment supply allowed forced regressive and possibly also lowstand systems tracts to be preserved in some cyclothems, particularly in the lower part of the succession, an unusual fact in shelf deposits. Considering the far younger age of marine terraces on the Ionian side of Calabria when compared to the Tyrrhenian side, it is thought that, during ongoing subduction of the Ionian crust, a wave of uplift and related extensional tectonics migrated southeastward in the rear of the frontal accretionary wedge.
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SMYTHITE, the rhombohedral Fe3S4 first reported in a natural occurrence in 1957 (ref. 1), has been synthesized by the reaction between siderite and sodium sulphide in aqueous solution at 25° C and 1 atm. pressure.
We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ 18 O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ 18 O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Réunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Réunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.
In this paper, we present the results of down core variations in mag-netic susceptibility and magnetic mineral composition of fifteen piston cores retrieved from the active continental margin offshore of southwestern Taiwan, where a wide distribution of bottom simulating reflectors (BSRs) related to gas hydrate layers has been detected. X-ray analysis on magnetic extracts from the cores indicates that detrital magnetite and authigenic greigite, in various proportions, are the dominant magnetic minerals in sediments. Non-magnetic, authigenic pyrite is generally associated with greigite, but it may co-exist with magnetite or may solely occur in sediments. Consequently, the sediment sequences of the fifteen piston cores have com-plicated magnetic mineral assemblages that result in various magnetic sus-ceptibility profiles. For core segments containing detrital magnetite as the only magnetic mineral, values of magnetic susceptibility are moderate with small variations (8 -15 × 10 -6 SI), which can be regarded as the susceptibil-ity backgrounds for the initially deposited sediments. However, high mag-netic susceptibilities relative to the backgrounds were found in core seg-ments where magnetic mineral is enriched in greigite. Magnetic suscepti-bilities lower than the backgrounds were found in segments where neither magnetite nor greigite were detected. The complicated occurrence of mag-netic minerals reveals that their host sediments at different levels have suf-fered various degrees of early diagenetic process ranging from oxic to an-oxic conditions. Due to very low sedimentary organic matter content in the study area, detrital magnetite persisted in sediments that were subjected to Terr. Atmos. Ocean. Sci., Vol. 17, No. 4, December 2006 1010 initially oxic (or sub-oxic) conditions. However, the sedimentary organic matter is apparently insufficient for providing reducing environments for the formation or enrichment of authigenic iron sulfides. Instead, we pro-pose that gaseous methane derived from gas hydrates in deeper sedimen-tary layers should be the most likely source of extra organic matter. The different flux intensities of this gas either through slow diffusion or rapid venting resulted in various anoxic conditions, which caused the dissolution or survival of magnetite and the formation or enrichment of greigite and pyrite.
The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field (Bg) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDFRRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and yFe3O4 tape particles, acquired Bg values between -3 and -14 μT, and MDFRRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDFRRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDFRRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.
A new occurrence of ferrimagnetic greigite (Fe3S4) is reported in freshwater Holocene sediments from Gaskkamus Gorsajávri in Swedish Lappland. The presence of greigite was inferred from the mineral magnetic analysis of sediment cores. The greigite-bearing sediments possess a high saturation isothermal remanent magnetisation to susceptibility (SIRM/χ) ratio, a characteristic shared by other natural sediments known to contain greigite. The hysteresis properties (measured using a vibrating sample magnetometer) of natural greigite extracted from lake sediments show much higher saturation coercivity ((B0)C) and ratio values (approximately 50 mT and 0.5 respectively) than synthetic samples of greigite (approximately 12 mT and 0.2). The results suggest that synthetic greigite samples contain crystals ranging from small non-remanence carrying superparamagnetic grains to large multi-domain grains. In contrast, the natural samples probably consist predominantly of pseudo-single-domain and single-domain crystals capable of carrying a chemical remanent magnetisation. However, the unstable nature of greigite under laboratory conditions creates difficulty in characterising its physical attributes.
The Santernian is the basal chronostratigraphic unit of the Pleistocene, according to the Quaternary chronostratigraphic scale proposed in 1991 by the "Working Group for the Quaternary Stratigraphy' of the Italian Geological Society. In this paper the authors propose to locate in the Vrica section (Calabria) the Santernian/Emilian boundary-stratotype, in coincidence with the first appearance of Hyalinea baltica, proposed by Ruggieri & Sprovieri 1977a as marker of this boundary. H. baltica appears in the Vrica section 71.1 meters above the Pliocene/Pleistocene boundary-stratotype, in coincidence or in proximity of first or last appearances of species belonging to calcareous plankton, which allow global scale correlations of this boundary. In the Vrica section is is possible to locate also the Santernian stratotype because: 1) in this section the Santernian sediments are continuous (unlike in the Santerno Section, previously proposed as Santernian stratotype by Ruggieri & Sprovieri 1977a); 2) the Vrica seciton shows all the requisites requested by the International Stratigraphic Guide (Hedberg, 1976) for the definition of a stratotype. In this present paper the new age of 1.795 ± 0.030 Ma is proposed for the Pliocene/Pleistocene (or Neogene/Quaternary), boundary, and new information on the climatic change occurred in coincidence with this boundary is given. -Authors
We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9-2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr-1, respectively. For the older part of the record (> 1 .2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30-60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190-1215 (Cobb Mountain Subchron), 1255, 1472-1480, 1567-1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.
A paragenetic sequence of framboidal and euhedral pyrites is developed from a study of models defining different pathways of reaction, depending on the form of Fe and the reaction, kinetics as determined by rate of sulphate reduction (sulphide generation) and/or Fe availability. The study is based on analysis of pyrite textures, isotopic compositions and reactive iron content of the host sediment of pyritiferous carbonate concretions from a Jurassic outcrop on the NE coast of Yorkshire, England.-M.S.
This study evaluates the processes that form smythite, a mineral associated normally either with siderite in sedimentary iron formations and hydrothermal carbonate veins, or with pyrrhotite in nickel sulfide ore deposits. Smythite was found to form via either of two processes, the replacement of siderite in low temperature aqueous environments as proposed by Rickard (1968) or exsolution from high-temperature, S-rich pyrrhotite during cooling as proposed by Fleet (1982).In solutions with pH > 6 at temperatures below 53°C, surface-bound Fe2+ reacts with aqueous sulfide on the siderite surface to form smythite. On the other hand, in low pH sulfide solutions with little dissolved CO2, siderite dissolves rapidly yielding free Fe2+ and saturates the bulk solution with amorphous Fe(HS)2. Amorphous Fe(HS)2 is subsequently replaced by mackinawite and then by pyrite but not smythite.When S-rich (less than ∼46 Fe atomic %) hexagonal pyrrhotite 1C is cooled slowly from 500°C or higher, it re-equilibrates to an assemblage that is stable at low temperature (S-poor pyrrhotite + pyrite). However, if cooling is rapid (several hundred °C/s), in the absence of local pyrite grains, pyrite does not exsolve owing to its slow nucleation. Because smythite is structurally similar to pyrrhotite, its nucleation is possible during such rapid cooling thereby balancing the bulk composition of the system without pyrite exsolution. Smythite exsolution occurs only in a pyrite-free environment because pyrite overgrowths can occur if pre-existing pyrite is present for nucleation. This eliminates the need for metastable exsolution of smythite to achieve a mass balance for sulfur. Pyrrhotite containing some Ni (1.6 ∼ 2.9 atomic %) exsolves smythite during slower cooling because Ni retards the equilibrium exsolution of pyrite.
We present a magnetostratigraphic record from an IMAGES piston core in
the western Philippine Sea that is tied to a marine oxygen isotope
record for the past 2.14 million years. Using the Australasian
microtektite layer as an independent marker, the lock-in depth of the
Matuyama/Brunhes reversal was evaluated to be shallow, which suggests
that the stratigraphic offsets between the observed and true geomagnetic
reversals in this core are relatively small. The ages of geomagnetic
reversals were astronomically calibrated by tuning the oxygen isotopic
stratigraphy, yielding a chronology for the various subchrons of the
Matuyama: Matuyama/Brunhes boundary, 781+/-3 ka (slightly above oxygen
isotope Stage 19.3); top of the Kamikatsura polarity interval, 920+/-2
ka (Stage 23/24); base of the Kamikatsura polarity interval, 925+/-1 ka
(Stage 24); top of the Jaramillo subchron, 988+/-3 ka (Stage 27); base
of the Jaramillo subchron, 1072+/-2 ka (Stage 31); top of the Cobb
Mountain subchron, 1173+/-4 ka (Stage 35/36); base of the Cobb Mountain
subchron, 1185+/-5 ka (Stage 36); top of the Olduvai subchron, 1778+/-3
ka (Stage 63/64); base of the Olduvai subchron, 1945+/-4 ka (Stage
71/72); top of the Reunion II subchron, 2118+/-3 ka (Stage 80/81); and
base of the Reunion II subchron, 2133+/-5 ka (Stage 81). This
astronomically calibrated chronology provides accurate ages for short
polarity intervals in the Matuyama chron, which were previously not well
dated.
New Zealand contains many sequences of tectonically uplifted Miocene/Pliocene marine sediments, which have been central to the development of a biostratigraphic framework for temperate southern latitudes. Chronological control is almost invariably dependent on magnetostratigraphy. Interpretation of the natural remanent magnetization (NRM) of these sediments is, however, notoriously difficult because of the weak magnetization, secondary components that resist demagnetization, and thermal instability of the clay minerals. A new paleomagnetic study of Wanganui Basin sediments has yielded unusually well defined demagnetization data from which systematic characterization, explanation, and interpretation of the multicomponent nature of the NRM have been possible. In general, the magnetization consists of three components: (1) a low (250°C) Tb component that is considered to be diagenetic in origin. This interpretation of the intermediate and high Tb components is supported by rock magnetic data which show that specimens carrying strong high Tb components have a distinctly different spectrum of ferrimagnetic grains, specifically a population with much higher coercivities than is seen in specimens which lack a high Tb component. It is argued that these high-coercivity, high Tb grains are secondary in origin, leading to the conclusion that the intermediate Tb component is the primary detrital magnetization. Consistency of the paleomagnetic data within and between sites also supports this conclusion. It is suggested that failure to recognize these three components has, in some cases in the past, led to misidentification of the primary magnetization. Using this hypothesis of the multi-component magnetization, the Turakina River data of McGuire [1989] have been reinterpreted, yielding a high-resolution magnetostratigraphy from the upper Gilbert, Chron 3n, (circa 4 Ma) to the lower Matuyama, Chron 2r. The correlation of this magnetostratigraphy with the goemagnetic polarity timescale, is supported by biostratigraphic constraints.
Because of the relative constancy of pH in most subaqueous sediments and the general lack of measurability of Eh, these parameters are not practically useful for classifying sedimentary environments. In their place a new classification is offered which is based on the presence or absence of dissolved oxygen and dissolved sulfide in at the time of authigenic mineral formation. Sedimentary environments are divided into oxic and anoxic depending upon the presence of measurable dissolved oxygen. Anoxic environments, in turn, are divided into sulfidic and non-sulfidic depending upon the presence of measurable dissolved sulfide. Anoxic-nonsulfidic environments are further divided into post- oxic, or resulting from oxygen removal without sulfate reduction (weakly reducing), and methanic, or resulting from complete sulfate reduction with consequent methane formation (strongly reducing). The environments are shown to succeed one another during early diagenesis in the order: oxic, post-oxic, sulfidic, methanic. Iron and manganese minerals characteristic and stable in each environment are listed and discussed and that they may be used to distinguish environments in the geological record. -Author
We present an integrated geochronology for late Neogene time (Pliocene, Pleisto- cene, and Holocene Epochs) based on an analysis of data from stable isotopes, mag- netostratigraphy, radiochronology, and cal- careous plankton biostratigraphy. Dis- crepancies between recently formulated astronomical chronologies and magneto- chronologies for the past 6 m.y. have been resolved on the basis of new, high-precision Ar/Ar ages in the younger part of this in- terval, the so-called Brunhes, Matuyama, and Gauss Epochs ( Chrons C1n–C2An; 0–3.58 Ma), and revised analysis of sea floor anomalies in the Pacific Ocean in the older part, the so-called Gilbert Epoch ( Chron C2Ar–C3r; 3.58 –5.89 Ma). The magneto- and astrochronologies are now concordant back to the Chron C3r/C3An boundary at 5.89 Ma.
The Neogene (Miocene, Pliocene, Pleisto- cene, and Holocene) and Paleogene are treated here as period/system subdivisions of the Cenozoic Era/Erathem, replacements for the antiquated terms Tertiary and Qua- ternary. The boundary between the Miocene and Pliocene Series (Messinian/Zanclean Stages), whose global stratotype section and point (GSSP) is currently proposed to be in Sicily, is located within the reversed interval just below the Thvera (C3n.4n) Magnetic Polarity Subchronozone with an estimated age of 5.32 Ma. The Pliocene/Pleistocene boundary, whose GSSP is located at Vrica (Calabria, Italy), is located near the top of the Olduvai (C2n) Magnetic Polarity Sub- chronozone with an estimated age of 1.81 Ma. The 13 calcareous nannoplankton and 48 planktonic foraminiferal datum events for the Pliocene, and 12 calcareous nannoplankton and 10 planktonic foram- iniferal datum events for the Pleistocene, are calibrated to the newly revised late Neogene astronomical/geomagnetic polar- ity time scale.
Recently reported radioisotopic dates and magnetic anomaly spacings have made it
evident that modification is required for the age calibrations for the geomagnetic polarity timescale
of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted
geo-qtagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is
consistent with astrochronology in the Pleistocene anq Pliocene and with a new timescale for the
Mesozoic.
Large, clockwise, vertical axis tectonic rotations of the Hikurangi margin, East Coast, New Zealand, have been inferred over both geological and contemporary timescales, from paleomagnetic and geodetic data, respectively. Previous interpretations of paleomagnetic data have laterally divided the margin into independently rotating domains; this is not a feature of the short-term velocity field, and it is also difficult to reconcile with the large-scale boundary forces driving the rotation. New paleomagnetic results, rigorously constrained by field tests, demonstrate that late diagenetic growth of the iron sulfide greigite has remagnetized up to 65% of sampled localities on the Hikurangi margin. When these remagnetizations are accounted for, similar rates, magnitudes, and timings of tectonic rotation can be inferred for the entire Hikurangi margin south of the Raukumara Peninsula in the last 7-10 Ma. Numerous large (50-80°) declination anomalies from magnetizations acquired in the late Miocene require much greater rates of rotation (8-14° Ma-1) than the presently observed rate of 3-4° Ma-1, which is only likely to be characteristic of the tectonic regime established since 1-2 Ma. These new results are consistent with both long- and short-term deformation on the Hikurangi margin being driven by realignment of the subducting Pacific plate, with collision of the Hikurangi Plateau in the late Miocene potentially being key to both the initiation of tectonic rotations and the widespread remagnetization of Neogene sediments. However, accommodating faster, more coherent rotation of the Hikurangi margin in Neogene reconstructions of the New Zealand plate boundary region, particularly in the late Miocene, remains a challenge.
This concise handbook presents a summary of Earth's history over the past 4.5 billion years as well as a brief overview of contemporaneous events on the Moon, Mars and Venus. The authors have been at the forefront of chronostratigraphic research and initiatives to create an international geologic time scale for many years, and the charts in this book present the most up to date, international standard, as ratified by the International Commission on Stratigraphy and the International Union of Geological Sciences. This book is an essential reference for all geoscientists, including researchers, students, and petroleum and mining professionals. The presentation is non-technical and illustrated with numerous colour charts, maps and photographs. The book also includes a detachable laminated card of the complete time scale for use as a handy reference in the office, laboratory or field.
First-order reversal curve (FORC) diagrams have become a standard tool in rock magnetism, yet magnetite is the only magnetic mineral that is well characterized using FORC diagrams. We present FORC diagrams for predominantly single-domain (SD) synthetic aluminous hematite (α-Fe2-xAlxO3) and goethite (α-(FeAl)OOH) and natural greigite (Fe3S4) and pyrrhotite (Fe7S8) to constrain interpretation of FORC diagrams from natural samples. Hematite and goethite have low spontaneous magnetizations and negligible magnetic interactions, while greigite and pyrrhotite have higher spontaneous magnetizations and can have strong magnetic interactions. The coercivity of hematite systematically increases with Al content only for samples produced using the same synthesis method, but it is variable for samples produced with different methods even for similar Al content. This precludes use of magnetic coercivity alone to quantify the Al content of natural hematites. Goethite has much higher coercivity than hematite for all measured samples. SD and superparamagnetic (SP) behavior is common in natural greigite samples, with peak coercivities ranging from ∼70 mT (SD) to zero (SP). This range overlaps with that of lower-coercivity minerals, which can make greigite identification ambiguous at room temperature. Fine-grained SD pyrrhotite has slightly higher coercivities than greigite, which progressively decreases with increasing grain size within the SD size range and overlaps the range for greigite. While FORC diagrams are useful for magnetic characterization, care is needed in interpretation because of overlaps in the broad range of magnetic properties, which result from variations in domain state, for any magnetic mineral with respect to other minerals.
New palaeomagnetic data from the proposed Plio-Pleistocene boundary
stratotype section (Vrica section, southern Italy) are presented here in
order to improve the correlation of these sediments to the magnetic
polarity time scale. If the interpretation preferred here is correct,
the boundary falls above the Olduvai normal subchron at approximately
1.6 Myr.
We present the results of a micropaleontological study performed on the sapropel sequence associated with insolation cycle 180 from the Plio–Pleistocene Vrica sequence (Calabria, Italy). We performed a high-resolution study on the 3.38-m-thick layer c from a core drilled close to the classical outcrop section in which we analyze fluctuations in the abundance and composition of calcareous nannofossils and planktic and benthic foraminifera. Changes in the fossil assemblages reveal at least three major paleoenvironmental phases in layer c. The base of the sapropel contains an abrupt decrease in benthic fauna that continues through all of layer c. It also has an increase of the coccolithophorids species Coccolithus pelagicus. Planktic foraminifera show at the same depth a peak of the cold species Globorotalia scitula. These changes are followed by decreases in the carbonate preservation index and in abundances of Globigerinita glutinata, Globigerinita uvula and Neogloboquadrina pachyderma (sinistral), which suggest cold and highly productive upwelling waters. A short interval in the middle of the sapropel is characterized by low values of C. pelagicus, a fluctuating increase of Pseudoemiliania lacunosa and among the foraminifera an increase of Globigerinoides ruber together with the presence (although decreased) of G. glutinata, G. uvula and N. pachyderma (sinistral). We interpret these features as suggesting high seasonality with warm stratified and probably oligotrophic waters during summer and relatively cold conditions during winter. Finally, the topmost interval of the Vrica layer c exhibits the re-appearance of P. lacunosa together with abundant siliceous phytoplankton. Planktic microfauna show the disappearance of the cold species G. glutinata, G. uvula and N. pachyderma (sinistral). Thus this interval appears to be characterized by warmer temperature. The transition from the laminated to the massive sediment displays a sequence of events, including a decrease of the carbonate preservation index and peaks of Globorotalia inflata and G. scitula, suggesting again upwelling and mixing of the whole water column and, thus, transition to the oxygenated conditions characterizing the massive layer. Neither increased productivity nor stratification appear to characterize the whole sapropel interval, which is, however, always dysoxic.
A mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe3S4). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO3) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H2S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms).We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies. Published 89-100 2.2. Laboratorio di paleomagnetismo JCR Journal
Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.
Eastern Mediterranean sapropels are organic-rich sediments whose formation is related to variations in the Earth's orbit. They are therefore important for reconstructing past climatic variations and for producing astronomically tuned geological timescales. Previous studies have suggested that the distinctive magnetic properties of sapropels, which result from non-steady-state diagenetic reactions related to degradation of organic matter, might be used for identifying sapropels that have escaped visual identification after being completely erased during post-depositional oxidation. We present a high-resolution multi-proxy magnetic, geochemical and paleontological data set from selected intervals of Ocean Drilling Program Sites 966 and 967. Our results demonstrate that magnetic properties can be unambiguously used for identifying oxidized sapropels, and also for determining whether suspected intervals actually correspond to oxidized sapropels, because they enable detection of the former presence of organic matter and of climatic and oceanographic conditions suitable for sapropel formation. Systematic application of high-resolution magnetic analyses to future coring efforts in the eastern Mediterranean should allow determination of the original distribution of sapropels through long sedimentary sequences, which will improve our knowledge of paleoceanographic and paleoclimatic conditions that led to their formation.
1] First-order reversal curve (FORC) diagrams for mixtures of different magnetic phases and bimodal distributions have been measured to examine the efficiency of the FORC method at unraveling complex magnetic signals. The FORC distributions for various magnetic minerals, including magnetite, maghemite, hematite, and goethite, and their linear additivity are assessed. Mixtures containing only hard magnetic minerals like hematite or goethite, which have relatively small spontaneous magnetizations (M S) and large magnetocrystalline anisotropies, can be adequately described by a linear addition of the two end-members, because there are virtually no magnetostatic interactions between the phases. Mixtures dominated by softer minerals like magnetite and maghemite are more susceptible to interactions and exhibit nonlinear behavior. When a hard phase with low M S like hematite is mixed with a softer phase with high M S like magnetite, it can still be identified using the FORC technique, whereas it is impossible to do so using standard magnetic hysteresis measurements. When the weaker phase can be identified, then weak-strong mixes add linearly; however, beyond a certain critical concentration the mineral with high M S swamps the magnetic signal and linearity breaks down. It is suggested that the FORC method is highly suitable for identifying small traces of hard magnetic minerals like hematite and goethite in the presence of minerals with high M S such as magnetite.
1] Goethite (a-FeOOH) is an antiferromagnetic iron oxyhydroxide that forms as a weathering product of iron-bearing minerals. We systematically investigated the low-and room temperature properties of well-defined aluminous goethites [a-(Fe, Al)OOH] with varying grain size and Al content. A marked decrease in the Néel temperature with increasing Al content for goethite lowers the blocking temperature distribution, which produces an increase in the remanent magnetization on cooling. The zero-field-cooled (ZFC) and field-cooled (FC) curves are irreversible. This is due to an additional partial thermal remanent magnetization (pTRM), which is acquired at low temperatures during the FC process because the initial 300 K remanence is far from being saturated due to the extremely high saturation field of goethite. This pTRM can be thermally demagnetized when reheating a sample back to 300 K. Finally, a sharp decrease in the bulk coercivity at 11–13 mol % Al is mostly caused by a broad coercivity distribution due to nonuniformity of Al substitution, which becomes more significant with increasing Al content. The positive correlation between pTRM acquisition during the FC process and the bulk coercivity strongly indicates that the ZFC/FC behavior is controlled by the bulk coercivity, which is, in turn, determined by Al substitution. This explanation of the characteristic low-temperature magnetic properties of Al goethite provides important constraints for identifying natural Al goethite bearing samples and quantitatively estimating the contributions of this mineral to the bulk magnetic properties of such samples.
We have recently developed a technique for characterizing the magnetic components within natural particle assemblages. This technique is based on the transformation of magnetization data from first-order reversal curves (FORCs) into contour plots of a 2-D distribution function (FORC diagrams). FORC diagrams are useful for obtaining information about switching fields and interactions in magnetic particle systems. Here, we examine experimental data and a theoretical model in order to provide a rigorous framework for interpreting FORC diagrams for samples that contain superparamagnetic particles. We have found four distinct manifestations of thermal relaxation on FORC diagrams. First, thermal relaxation will shift the FORC distribution to lower coercivities. Second, at intermediate temperatures, thermal relaxation can generate a secondary peak about the origin of a FORC diagram. This secondary peak indicates that part of a single-domain particle assemblage has become superparamagnetic. At high enough temperatures, the primary peak of the FORC distribution will be located about the origin of a FORC diagram. Third, thermal relaxation can produce a small, but systematic, upward shift of a FORC distribution. Fourth, thermal relaxation will produce contours that lie near and parallel to the vertical axis in the lower quadrant of a FORC diagram. These manifestations make FORC diagrams a powerful tool for studying the effects of thermal relaxation (superparamagnetism) in bulk natural samples, particularly when the samples contain mixed magnetic particle assemblages.
The classic, multivariate technique of principal component analysis can be used to find and estimate the directions of lines and planes of best least-squares fit along the demagnetization path of a palaeomagnetic specimen, thereby replacing vector subtraction, remagnetization circles and difference vector paths with one procedure. Eigenvalues from the analysis are the variance of the data along each principal axis, and provide a relative measure of collinearity or coplanarity which may be used to define a general palaeomagnetic precision index. Demagnetization planes found with principal component analysis may be used in place of difference vector paths for locating Hoffman—Day directions, avoiding unnecessary vector subtraction and intensity truncation steps. Two methods are discussed for jointly estimating an average remanence direction from demagnetization lines and planes.
The reliability of primary natural remanent magnetization (NRM) signals in greigite-bearing sediments has been frequently questioned. Here, we show that the stable NRM in the deep marine Middle to Late Miocene sediments at Monte dei Corvi, northern Italy, is mainly carried by greigite. Combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two greigite populations. One fine-grained and relatively well-dispersed greigite population (grain size between 60 and 200 nm) is most likely of magnetotactic origin. The second greigite population with larger grain sizes (typically 700 nm to 1 μm) is most likely of authigenic (bacterially mediated) origin, and is related to post-depositional sulphidization processes. Greigite is the main magnetic remanence carrier in the older part of the section (12.8 to 8.7 Ma), whereas greigite and fine-grained (presumably magnetotactic) magnetite are present in the younger part of the section (8.7 to 6.9 Ma). Similar remanent magnetization directions of the magnetite and greigite components, and the likelihood of a magnetotactic origin, suggests that the NRM is of syn-depositional age. We suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine greigite. This corroborates the reliability of the previously established magnetostratigraphy at Monte dei Corvi.
Magnetic measurements were carried out on a collection of samples from different geological settings throughout the Italian peninsula. The samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals. Paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments (thermal, static and tumbling alternating field (AF) demagnetization). Greigite (Fe3S4) was positively identified in most samples. In agreement with previous studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230°C, resulting in decreased magnetic susceptibility values and maximum unblocking temperatures (Tub) mostly in the range 320–350°C; high saturation isothermal remanent magnetization (SIRM) to low-field magnetic susceptibility (k) ratios (SIRM/k > 10 kA/m); hysteresis ratios that are typical for single domain (SD) grains (Mrs/Ms>0.5 and (B0)cr/(B0)c<1.5, where Mrs is the saturation remanence, Ms is the saturation magnetization, (B0)c is the coercive force and (B0)cr is the coercivity of remanence); moderate coercivity, with (B0)cr values that range between 52 and 81 mT and a tendency to acquire a significant rotational remanent magnetization (RRM). We also found that greigite-bearing sediments are particularly sensitive to field impressed anisotropy. We propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite, which relies on its SD behaviour in field impressed anisotropy experiments. In greigite-bearing sediments, the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field (0.9 T in our experiments) at right angles to this direction. The susceptibility increase is directly correlated to the concentration of greigite in the sediment. The comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization, in close steps up to 350–380°C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF demagnetization treatments display significant acquisition of unwanted magnetic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT.
Detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur > 1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies.
A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mössbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.
Sedimentary greigite (Fe3S4) is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine-grained marine and terrestrial sediments. We have conducted detailed scanning electron microscope observations on polished sections from such sediments, coupled with elemental microanalysis, to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases. This detailed observational work, in conjunction with recently published work, has enabled identification of several mechanisms for remagnetization involving greigite: (1) neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite; (2) greigite growth within cleavages of detrital sheet silicate grains; (3) neoformation of greigite on the surfaces of authigenic clays (smectite, illite); (4) greigite nucleation on the surfaces of siderite; (5) greigite growth on the surface of gypsum that resulted from earlier oxidation of nodular pyrite. Many other mechanisms for greigite neoformation are conceivable. The key variables are appropriate redox conditions for generation of sulphide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron-bearing minerals. Documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization, which will complicate or compromise studies of environmental magnetism and geomagnetic field behaviour.
In several recent published studies, paleomagnetic results from greigite-bearing sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observations have important implications for the reliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the Lower Gutingkeng Formation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng greigite has three modes of occurrence, including nodular, framboidal and matrix greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolved edges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detrital phyllosilicates, indicate that all three types of greigite have a diagenetic origin that post-dates early diagenetic pyrite. In addition, paleomagnetic data yield contradictory polarities even for greigite-bearing sister samples from the same stratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite-bearing sediments.
In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4′N, 60°08.0′E) and northern California (38°24.8′N, 123°58.2′W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis.
The biomarker compositions of iron sulfide nodules (ISNs; upper Pliocene Valle Ricca section near Rome, Italy) that contain the ferrimagnetic mineral greigite (Fe3S4) were examined. In addition to the presence of specific terrestrial and marine biomarkers, consistent with formation in coastal marine sediments, these ISNs contain compounds thought to originate from sulfate reducing bacteria (SRB). These compounds include a variety of low-molecular-weight and branched alkanols and several non-isoprenoidal dialkyl glycerol diethers (DGDs). In addition, archaeal biomarkers, including archaeol, macrocyclic isoprenoidal DGDs and isoprenoidal glycerol dialkyl glycerol tetraethers are also present. Both SRB and archaeal lipid δ13C values are depleted in 13C (δ13C values are typically less than −50‰), which suggests that the SRB and archaea consumed 13C depleted methane. These biomarker and isotopic signatures are similar to those found in cold seeps and marine sediments where anaerobic oxidation of methane (AOM) occurs with sulfate serving as the terminal electron acceptor. Association of AOM with formation of greigite-containing ISNs could provide an explanation for documented remagnetization of the Valle Ricca sediments. Upward migration of methane, subsequent AOM and associated authigenic greigite formation are widespread processes in the geological record that have considerable potential to compromise paleomagnetic records.
Monoclinic pyrrhotite (Fe7S8) is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments. In contrast, geochemical literature indicates that pyrrhotite formation is extremely slow below 180 °C, which makes it a highly unlikely carrier of early diagenetic remanences in sediments. This view is confirmed by the occurrence of late diagenetic Fe7S8 in ancient sediments and the general lack of Fe7S8 in modern sediments. Horng et al. [C.S. Horng, M. Torii, K.S. Shea, S.J. Kao, Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan, Earth Planet. Sci. Lett. 164 (1998) 467–481.] documented the presence of Fe7S8 that carries a magnetic signal indistinguishable from that of detrital magnetite in Pleistocene marine sediments from Taiwan. We tested the possibility that the Fe7S8 could have a detrital origin by conducting a source-to-sink study and found Fe7S8 in metamorphic rocks of the Taiwan Central Range and in material eroded from these rocks in the transportation pathway and in the depositional sink. This confirms that the Fe7S8 has a detrital origin. Rapid transportation from source to sink (e.g., by typhoon-associated flood events) probably assists preservation of the Fe7S8, which might otherwise oxidize during transportation. The widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of Fe7S8 in marginal sedimentary basins. Detrital Fe7S8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance.
Narrow-access long-core cryogenic magnetometers enable measurement of a range of magnetic parameters at a speed and resolution that cannot be matched by other techniques. Despite the revolutionary impact that these instruments have had on paleomagnetic and environmental magnetic investigations, some fundamental constraints limit their usefulness. First, the pick-up coils have different response functions for the transverse and axial measurement axes. Transverse coils typically have regions of negative response on either side of the main response peak, whereas the axial coil usually lacks the negative response lobes. Zones of negative response affect the measured remanence intensity, for which corrections can be made by normalizing the measured magnetic moment by the area under each respective response curve. This correction works adequately for homogeneously magnetized cores. Second, in cores with significant changes in remanence intensity, the ratio of axial to transverse moment varies with intensity change, which can introduce spurious artefacts into the paleomagnetic directional record. Deconvolution is required to remove such effects. Third, measurements of non-centred samples with irregular cross-section (e.g., split core measurements), cause geometric effects that can introduce small but paleomagnetically important artefacts. Corrections for such effects are only possible if spatial variability of the magnetometer response is known throughout the entire measurement volume rather than solely along the centre-line of the magnetometer. Fourth, analysis of cores deposited at rates >10 cm/ky is desirable to minimize the effects of measurement smoothing. Finally, measurements of magnetic susceptibility should be conducted using loop sensors with a similar response function as a u-channel magnetometer to ensure comparability of data. Routine adoption of these five strategies should help to maximize the value of long-core magnetic measurements.