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Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East G
... After more than three decades of chemostratigraphic studies, the secular 87 Sr/ 86 Sr and δ 13 C trends in Phanerozoic seawater are now well-established, and our understanding of Precambrian seawater isotopic patterns and its significance has substantially increased. Large negative δ 13 C excursions are remarkable features of the Neoproterozoic Era (Knoll et al. 1986, Kaufman et al. 1991, Shields & Veizer 2002, Halverson et al. 2010 ) that have been considered as a consequence of multiple glaciations marked by rapid precipitation of overlying cap carbonates (Hoffman et al. 1998, Figueiredo 2010). Globally, at least three major glacial events exhibit these features: Sturtian (c. ...
... 580 Ma). On the other hand, positive δ 13 C excursions are most explained by an increase in the fraction of organic carbon fraction during burial (Knoll et al. 1986 ), sometimes associated with tectonic events (Santos et al. 2000). In east-central Brazil, the Bambuí Basin covers most of the São Francisco Craton, showing some glacial deposits in the Carrancas Formation, which is covered by a thick carbonate unit known as Sete Lagoas Formation. ...
... Several hypotheses have been suggested to explain the expressive positive δ 13 C excursions. Besides a dramatic increase of bioproductivity, such features could have been caused by alterations in the balance between inorganic and organic C reservoir, due to changes in the burial rates of organic matter (Knoll et al. 1986, Iyer et al. 1995 ). Other possibilities may be the deposition in evaporitic environment (Mees et al. 1998), the reduction of CO 2 and sulphides, and changes in the basin paleogeography possibly caused by tectonics (Santos et al. 2000). ...
The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ¹³C (c. -4‰), and a large oscillation of the δ¹⁸O (-6 to -15‰). The Interval II shows a striking homogeneity in δ¹³C and δ¹⁸O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ¹³C (up to 8‰) and δ¹⁸O (-8 to -3‰) values. Unaltered ⁸⁶Sr/⁸⁷Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the ⁸⁶Sr/⁸⁷Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.
... The final theme is on biotic innovations and responses, with foci on evidence for early animal evolution, biotic survival of panglaciations , and the search for links between glacial and evolutionary events. CHEMICAL SEDIMENTOLOGY: ATMOSPHERIC AND HYDROSPHERIC SIGNALS Much of the intellectual excitement about evolution of Neoproterozoic environments has centred on the chemical evolution of atmosphere and oceans and their impact on global climates and evolution (Knoll et al., 1986; Canfield, 1998; Hoffman et al., 1998; Lenton et al., 2014). Such activity builds on the essential and hard-won foundation of global palaeogeographic reconstructions (Dalziel, 1997; Evans, 2000; Li et al., 2008) and geochronology (Hoffmann et al., 2004; Condon et al., 2005; MacDonald et al., 2010a; Rooney et al., 2015 ). ...
... Because of the usual problems with chronology , extension of the Phanerozoic database of chemostratigraphic parameters to the Precambrian relied on the generation of substantial datasets from individual sedimentary basins where the age-relationships were at least internally consistent. The most influential of these was generated by Knoll et al. (1986) who combined a huge field sample collection effort in the correlated successions of eastern Greenland and north-east Svalbard with both organic and carbonate carbon analyses. These authors pointed to the usefulness of these parameters for chronostratigraphic purposes and the coincidence (although not exclusive) of relatively light carbon-isotope compositions with glacial events. ...
... These authors pointed to the usefulness of these parameters for chronostratigraphic purposes and the coincidence (although not exclusive) of relatively light carbon-isotope compositions with glacial events. Knoll et al. (1986) also pointed to the sustained period of sedimentation of (Tonian) carbonates with high d 13 C values as evidence for enhanced burial of organic carbon with associated release of oxidants, although it is now known that this did not require an increase in free oxygen (Hayes & Waldbauer, 2006; Fairchild & Kennedy, 2007). The first foray into Neoproterozoic Sr-isotope chemostratigraphy was also based on this sample set (Derry et al., 1989), but curiously made no mention at all of the primary carbonate mineralogy. ...
Sedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post-glacial facies, including their age constraints; III) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep-sea oxidation through the Ediacaran Period. Techniques such as Fe-speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea-level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma and plausibly indicate long durations for each (55 Ma for Sturtian and 5 to 15 My for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr-isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and age and synchroneity of individual negative δ13C anomalies. Sturtian deposits are typically thick, rift-related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice-free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO2 was high as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold-climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea-levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative model of slower formation with clastic sediment starvation during transgression may prove to be consistent with new data and models showing extensive glacier terminations on land. For all facies, but especially caps, the use of microanalytical techniques and holistic studies of petrogenesis are future priorities. There are important discrepancies between molecular clock predictions of early metazoan origination and the hard evidence from the sedimentary records which largely depend on local exceptional preservation by early diagenesis. A variety of life survived panglaciation, and there is little evidence that glaciations directly caused oxygenation or stimulated evolution. This article is protected by copyright. All rights reserved.
... Because dolomitization has been considered a potential agent for carbon isotopic alteration, the chemical signature of dolomites relative to limestones in the same succession is important when evaluating diagenetic trends. In Neoproterozoic and Early Cambrian successions that have been extensively sampled for isotopic analysis, empirical observation shows that δ 13 C variation of both limestones and dolomites accurately trace the same pattern of secular variation (Knoll et al. 1986; Narbonne et al. 1994; Kaufman and Knoll 1995; Pelechaty et al. 1996), although minor variation in isotopic composition may be observed (Pelechaty et al. 1996). Possible factors for observed variation in lithologic and isotopic composition are discussed in depth by Pelechaty et al. (1996) and include postdepositional dolomitization from fluids of differing isotopic composition, equilibrium fraction between limestone and dolostone phases (Sheppard and Schwarcz 1970), and depositional variation in fluid composition reflected in synsedimentary dolomite formation. ...
... Exchange between carbonate carbon and organic carbon reservoirs can be evaluated by examining both the composition of organic carbon (δ 13 C org ) and the difference between carbonate carbon and organic carbon isotopic signals (∆δ = δ 13 C carb – δ 13 C org ). If biological fractionation does not change through a stratigraphic section, δ 13 C and δ 13 C org should track together (i.e., a constant ∆δ; Knoll et al. 1986; Hayes et al. 1989). Variation in ∆δ arises with exchange between inorganic and organic carbon reservoirs. ...
... Except at the lowest TOC contents , ∆δ is 29 ± 1‰. These values compare favorably to those commonly observed in Proterozoic successions (Knoll et al. 1986 1995; Des Marais et al. 1992) and suggest that isotopic compositions of carbonate and organic carbon have not been strongly affected by post-depositional processes. Paired carbonate and organic carbon analyses, however, show minor variation that may elucidate local depositional variation in δ 13 C. Microbial dolostone from White Bay has carbon isotopic values generally -1‰, and as much as nearly 2‰, lighter than corresponding oolitic dolostone from Milne Inlet (Tables 1, 2; Fig. 4). ...
The Bylot Supergroup, northern Baffin Island, contains >1500 m of platform, shelf, and slope carbonates deposited between ~1270 and ~723 Ma. Limited chronostratigraphic data have led to the broad correlation of the Bylot Supergroup with predominantly Neoproterozoic successions in northern and western Laurentia; yet, detailed correlation has been impossible given biostratigraphic and lithostratigraphic limitations. Carbon-isotope chemostratigraphy represents a potential dataset to constrain such interregional correlations. Carbon isotopic data from the Bylot Supergroup and broadly coeval successions from Somerset Island and northwest Greenland reveal distinct stratigraphic trends in δ13C, with intervals of moderate 13C enrichment (+3.5 ± 1‰) punctuated by excursions to slightly negative values (-1.0 ± 1‰). Although the scale of the observed variation is muted relative to Neoproterozoic standards, the dissimilarity of values to those recorded in northwestern Laurentia suggests that these strata delineate a discrete depositional interval. Comparison of isotopic values with published data indicates that δ13C values between approximately -1.0 and +4.0‰ are characteristic of the interval between ~1300 and ~800 Ma. This pattern is distinct from that of younger Neoproterozoic successions, which typically record values >+5‰, and older Mesoproterozoic successions, which record values near 0‰, and suggests that these moderately positive values may be useful for broad time correlation. Compilation of new and published data permits the tentative reconstruction of a global Mesoproterozoic carbon isotopic curve.
... esis on the isotope record, important isotope events were demonstrated on a global scale (Knoll et al., 1986; Magaritz et al., 1986; Holser, 1997 V ...
... tool for providing a timeline compensating for poor biostratigraphic resolution of the Precambrian fossils (Veizer et al., 1980; Magaritz et al., 1986; Knoll et al., 1986; Knoll and Walter, 1992; Kaufman et al., 1997; Corsetti and Kaufman, 2003; Halverson et al., 2005hi w ...
... AE V U W b E carbonates are a good proxy of ancient SW carbon isotope composition. Measurements of wellpreserved , and even metamorphosed, carbonates have provided valuable insights on the biogeochemical cycling of carbon in the geological past (e.g., Schidlowski et al., 1976a,b; Knoll et al., 1986; Veizer and Hoefs, 1976; Derry et al., 1992; Kah et al., 1999; Maheshwari et al., 1999; Sreenivas et al., 2001; Bekker et al., 2003 Carbon isotope chemostratigraphy has become an important tool in the Precambrian geochronology (e.g., Knoll et al., 1986hi However, the oscillatory nature due to a comparatively short residence time of carbon makes dating by means of carbon isotopes alone impossible. Instead, the use of 87 Sr/ 86 Sr ratios in combination with carbon isotopes on marine carbonates has been successful in many cases, especially in the Neoproterozoic (Halverson et al., 2007Halverson et al., , 2010a Kaufman et al., 2009; Frei et al., 2011h V with highly positive carbon isotope values (e.g., Schidlowski et al., 1976b; Knoll et al., 1986; Veizer and Hoefs, 1976; Derry et al., 1992; Kah et al., 1999; Maheshwari et al., 1999; Sreenivas et al., 2001; Bekker et al., 2003hi C E δ 13 C chemostratigraphic curve for marine carbonates starting at 2.5 Ga (Fig. 1h G C H u G a widespread positive δ 13 C anomaly in the age interval of 2.33–2.06 ...
In this chapter we report results of some of the latest cutting-edge research on traditional (C, O, S, N, Sr) isotope stratigraphy and some newer isotope systems (e.g. B, Ca, Mo, Fe, Cr, N), and elemental or elemental ratio (e.g. V, Mo, Hg, Rb/K, V/Cr, Zr/Ti, Mg/Ca, Mn/Sr, I/Ca) stratigraphy. The latter, in parallel with isotope stratigraphy, could be used as potential tracer for glacial events, buildup of volcanic gases (e.g. CO2) during glaciations, role of volcanism in mass extinction, salinity variation, redox state of the ocean and atmosphere and provenance, among other applications. Moreover, this chapter reviews studies that are broad in both space and time, highlighting chemical events from the Archean to the Cenozoic. Special attention is given to Paleoproterozoic and Neoproterozoic isotope chemostratigraphy and problems including the reliability of the use of carbon isotopes for Neoproterozoic stratigraphic correlation. Some attention is also devoted to the early Paleozoic, K–T boundary (KTB) and Paleogene climatic and biogeochemical changes in the light of isotope chemostratigraphic studies.
... 3), although values cluster more tightly around the line of best fit for the more negative part of the dataset, with a greater spread in data for the samples with the most positive 13 C carb values. In the traditional framework of interpretation, this would indicate that the C org was derived and fractionated from the same contemporaneous dissolved inorganic carbon captured in the carbonate carbon isotope values (e.g., Knoll et al., 1986; Kump et al., 2011; Swanson-Hysell et al., 2010). However, there are other scenarios that could produce a similar correlation. ...
... Further, the 13 C carb and 13 C org trends are coupled, which would not be expected if the carbonate C isotope values were diagenetically reset at the burial stage (Jiang et al., 2012). Close co-variation between the 13 C carb and 13 C org values is commonly used as the strongest argument against diagenetic alteration (e.g., Knoll et al., 1986; Sansjofre et al., 2011). ...
... Hayes et al., 1999 ), which has already been proposed as a mechanism for sustained positive 13 C carb values through the Neoproterozoic (e.g. Holland, 1984; Knoll et al., 1986 ). In a deviation from this traditional view, some authors have attributed the markedly negative excursions in C isotope values of the Neoproterozoic carbonates to the oxidation of a large DOC pool (e.g. ...
A new δ13Ccarb curve combined with δ13Corg values is presented for the upper Shaler Supergroup (∼900 to ∼720 Ma), Amundsen Basin, northwestern Canada. The dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust. There is a pronounced negative δ13C excursion in the Wynniatt Formation that can be correlated with a putative worldwide negative carbon isotope excursion, namely the Bitter Springs stage. However, in the Amundsen Basin, the δ13Ccarb excursion drops to anomalously negative values (-14‰), which we attribute to local overprints wherein isotopically light carbon in pore waters, released by oxidation of methane and organic matter during sulphate and iron reduction, was incorporated into authigenic carbonate cement. We document basin euxinia and anoxia during the same time interval using a multi-proxy approach; specifically, Fe-speciation and redox-sensitive trace metal data. Patterns of pronounced enrichment in Mo, V, and U concentrations in euxinic black shales suggest that the Bitter Springs stage was a transitional period in Earth's redox evolution, from the more reduced global oceans during the mid-Proterozoic to the more oxygenated oceans during the Phanerozoic.
... Owing to the lack of significant carbon in meteoric and diagenetic fluids compared to concentrations in carbonate rocks or sediments, δ 13 C carb and δ 13 C org are considered to be less susceptible to diagenetic alteration in comparison with 87 Sr/ 86 Sr or δ 18 O. This is often argued on empirical grounds to be the case even in diagenetically altered or dolomitized carbonates (Tucker, 1983; Tucker, 1985; Knoll et al., 1986; Fairchild and Spiro, 1987; Burdett et al., 1990; Kaufman et al., 1991; Narbonne et al., 1994;), despite the fact that dolomite in isotopic equilibrium with calcite as been shown to be enriched by ~2‰ (Sheppard and Schwarcz, 1970). Overprinting of the original carbon isotopic composition of carbonates occurs by reequilibration with fluids of contrasting isotopic composition. ...
... ragraph) may indicate such an effect. Hayes et al. (1999) compiled estimated averages for isotopic compositions of Neoproterozoic marine carbonates (δ 13 C carb ) and sedimentary organic material (δ 13 C org ), and the absolute difference between these primary marine products (ε TOC ). A consistent offset in ε TOC (typically in the range of ~28.5‰, Knoll et al., 1986) is often considered to be a good indication of preservation of marine δ 13 C carb . However, the δ 13 C org record, particularly in near-shore facies, contains a lot of locally controlled signals (Hayes, personal communication), and significant local stratigraphic variations in δ 13 C org are not uncommon. We report this index but, due ...
... Optimal sequences in terms of data density currently derive from NW Canada and Svalbard, but these lack internal radiogenic age control and require chemostratigraphic correlations to localities having, or inferred to have, better age control. Due to the lack of internal age control, previous models of seawater δ 13 C carb and 87 Sr/ 86 Sr evolution have a rather schitzophrenic history in terms of how glacial intervals among reference sequences have been respectively binned as " Sturtian " versus " Marinoan " (Knoll et al., 1986; Asmerom et al., 1991; Derry et al., 1992; Kaufman and Knoll, 1995; Jacobsen and Kaufman, 1999; Hayes et al., 1999; Halverson et al., 2005; Halverson et al., 2006a in press). These limited Sr-isotope results have also been used to support mutually incompatible correlations, for example that the low compositions in the Shaler Group (Asmerom et al., 1991) support a correlation of a lower negative δ 13 C carb excursion in the lower Akademikerbreen group of Svalbard to both the c. 750-700 Ma " Sturtian " (Kaufman and Knoll, 1995; Jacobsen and Kaufman, 1999; Hayes et al., 1999) glacial and the c. 800-815 Ma Bitter Springs anomaly of Australia (Halverson et al., 2005Halverson et al., , 2006a). ...
... Some of the major changes that occurred during the Neoproterozoic include profound changes in redox states of surface environments in conjunction with global climatic and other biogeochemical changes as observed from carbon isotope ratios ( 13 C/ 12 C) of carbonate and organic matter in sedimentary rocks of marine origin (Halverson, 2006; Halverson et al., 2005 Halverson et al., , 2007 Halverson et al., , 2010 Johnston et al., 2012; Swanson-Hysell et al., 2008 MacDonald et al., 2010). These global perturbations in the carbon cycle have been used to build chemo-stratigraphic and biogeochemical models for the Neoproterozoic (Halverson et al., 2005Halverson et al., , 2010 Des Marais et al., 1992; Kaufman et al., 1991; Kaufman and Knoll, 1995; Knoll et al., 1986). Both positive and negative values of carbon isotopes have been recorded from the Neoproterozoic carbonates globally. ...
... Both positive and negative values of carbon isotopes have been recorded from the Neoproterozoic carbonates globally. The positive carbon isotope values have been linked to organic carbon burial coupled with increase in oxygen levels (Kaufman and Knoll, 1995; Knoll et al., 1986; Des Marais, 1994) whereas negative values have been linked with phases of glaciation during the Neoproterozoic (Kennedy et al., 1998; Fike et al., 2006). The later observation is based on the fact that negative carbon isotope values occur in cap carbonates that followed glaciation events and also in marine sediments that preceded glacial sediments in several parts of the world (Frimmel et al., 1996; Knoll et al., 1986; Kaufman and Knoll, 1995; Kaufman et al., 1991 Kaufman et al., , 1997 Hoffman et al., 2004; Halverson et al., 2005 Halverson et al., , 2010). ...
... The positive carbon isotope values have been linked to organic carbon burial coupled with increase in oxygen levels (Kaufman and Knoll, 1995; Knoll et al., 1986; Des Marais, 1994) whereas negative values have been linked with phases of glaciation during the Neoproterozoic (Kennedy et al., 1998; Fike et al., 2006). The later observation is based on the fact that negative carbon isotope values occur in cap carbonates that followed glaciation events and also in marine sediments that preceded glacial sediments in several parts of the world (Frimmel et al., 1996; Knoll et al., 1986; Kaufman and Knoll, 1995; Kaufman et al., 1991 Kaufman et al., , 1997 Hoffman et al., 2004; Halverson et al., 2005 Halverson et al., , 2010). Carbon isotope studies have also been conducted on Neoproterozoic rocks of northwest India (Pandit et al., 2001Pandit et al., , 2009). ...
The early Neoproterozoic carbonates of the Sirohi Group, northwestern India, were studied for
geochronology as well as for oxygen and carbon isotopes across three different sections to understand
their relationship with contemporaneous global events. New single zircon Pb–Pb evaporation data constrain
the age of the Sirohi Group between ca. 920 and ca. 820 Ma, though actual span for deposition
of the carbonates may be little shorter taking into allowance the time for peneplanation. Narrow range
in variation of �18O values is generally observed in the carbonate which is consistent with generally
well-preserved original compositions. Two sections (SA and SS) out of the three that were studied had
similarly small variations in �18O (about 2‰), whereas the third section (SN) had a slightly wider range
of �18O (around 5‰). The �18O values of the SN section varied between
−11.9 and
−17.6‰, which can be
attributed to marginal modifications in primary composition caused by contact metamorphism which is
evident by the occurrence of wollastonite in the carbonates.
The �13Ccarb values of these carbonates varied between
−4.1 and +4.7‰ in all sections. Positive and
near-zero �13Ccarb values are from SA and SS sections which are least affected by the metamorphism. The
�13Ccarb values of these two sections are interpreted as primary and can be correlated with contemporary
sea-water compositions. The SN section showed negative �13Ccarb values which might be marginally
affected by contact metamorphism, though we avoided sampling from the contact aureole. However,
the metamorphic equilibration with 13C-depleted fluids leading to insignificant alteration in isotope
compositions cannot be totally ruled out for the analyzed samples.
Positive and near-zero carbon isotope values of Sirohi carbonates (SA and SS sections) closely match
the global trends of carbonate dated between 900 and 850 Ma but the negative carbon isotope anomaly
(SN section) shown by the Sirohi Group carbonates is unmatched. Since the SN section is stratigraphically
lower to SA and SS sections we suggest that the negative carbon isotope values of the Sirohi Group as one
of the oldest Neoproterozoic negative carbon anomaly possibly close to 900 Ma and not associated with
glaciation. The carbon isotopic compositions of carbonates from the Sirohi Group suggest a variable degree
of preservation of original compositions that represent yet another locality with moderately heavy values
between 850 and 900 Ma and an unmatchable example of the associated negative values. This reflects a
unique biogeochemical signature of changing carbon cycles prior to the onset of Snowball Earth Events
in the Neoproterozoic.
... Early studies of carbonate and organic carbon geochemistry were carried out by Knoll et al. (1986). Samples from Bedgroup 7 through 20 (Eleonore Bay Supergroup) as well as from the Canyon and Spiral Creek formations (Tillite Group) revealed negative d 13 C org values between –20 and – 35‰PDB. ...
... 0 to –5‰PDB) in the Canyon and Spiral Creek formations . Both geochemical data sets followed very similar profiles suggesting a primary signature and an unusual secular variation in the Neoproterozoic carbon cycle (Knoll et al. 1986). Total carbon (TC), total organic carbon (TOC) and total sulphur (TS) have been recorded through an interval ranging from the upper part of the Andrée Land Group and up to and including the top of Tillite Group; the results will be presented elsewhere. ...
... This similarity is also strengthened by comparable and substantial thicknesses of the lithological units in the two regions (Harland & Geyer 1972; Hambrey 1989; Fairchild & Hambrey 1995; Harland 1997). The chemostratigraphy shows similarities (Knoll et al. 1986) suggesting close affinities between the two areas. Halverson et al. (2005) identified the distinctive pre-Marinoan negative d 13 C anomaly or the 'Trezona anomaly' beneath the lowest diamictite in northeastern Svalbard. ...
The Neoproterozoic succession of East and North-East Greenland (over 14 000 m thick) includes the Eleonore Bay Supergroup
(?Tonian–Cryogenian) and the Tillite Group (Cryogenian–Ediacaran). The upper units of the Eleonore Bay Supergroup
consist of shallow to deeper-water carbonates, succeeded by siliciclastic fine-grained sediments (Bedgroup 19) that characterize the
top unit of the supergroup.
The Tillite Group includes two diamictite-bearing units (Ulvesø and Storeelv formations) of glaciogenic origin and two upper,
upwards-shallowing strata (Canyon and Spiral Creek formations) that were deposited during semiarid conditions and concluded the
Neoproterozoic depositional cycle. Diamictite is preserved on the craton and compares with the Storeelv Formation (Fm.) of the
Tillite Group.
Detailed investigations of the diamictite-bearing units (i.e. Ulvesø and Storeelv formations) demonstrate that the lower of the two formations
is mainly of marine origin, whereas the upper one has both marine and terrestrial origins. Chemostratigraphic data include analyses
on total carbon (TC), total organic carbon (TOC), total sulphur (TS) and d13C. The data set for d13C shows a substantial and abrupt
shift towards negative values of �10%, from below Bedgroup 19. Low-diversity acritarch assemblages (Cryogenian) are recorded from
the Andre´e Land and Tillite groups; a thin cherty dolostone unit present above the Storeelv Fm. suggests that the diamictite units are of
late Cryogenian age and the upper part of the Tillite Group is Ediacaran.
Bedgroup 19 disconformably overlies older carbonates and the unit is a prelude to the succeeding (upper Cryogenian–lower
Ediacaran) diamictite sediments of the Tillite Group. A disconformity separates the Tillite Group from the overlying Lower
Palaeozoic sediments. Both disconformities are, according to palaeomagnetic data, related to rift–drift episodes that occurred during
the late Neoproterozoic. Alternatively, the isotope data suggest that the diamictites were deposited during the late Cryogenian glaciation
and the older disconformity may be interpreted as a significant gap developed by the lowering of sea level during an early
Cryogenian glaciation.
... New C-isotope data (both d 13 from carbonate units in the Kwagunt Forma- tion at Nanko weap Butte range from -2.74‰ to +2.91‰, with d 18 O carb of +0.3‰ to -7.0 ‰ ( Fig. 2; supplementary data S4 [see footnote 1]), similar to those values interpreted to indicate pri- mary or near-primary values ). The updated Dd 13 C carb-org values have an average value of 26.65‰, which falls into the range of what is considered representative of mid-Neo- proterozoic seawater ( Knoll et al., 1986). ...
... ChUMP data sets provide a calibrated proxy for ocean conditions between 780 and 740 Ma and can be used to explore middle Neoprotero- zoic Earth systems and possible mechanisms leading to onset of the Cryogenian glaciations. There has been great emphasis on the relation- ship between negative C-isotope excursions and global glaciation (e.g., Knoll et al., 1986;Hoffman et al., 1998;Tziperman et al., 2011) but less emphasis on how positive excursions might relate to the onset of glaciation. The APCIE in the Chuar record and coeval positive C-isotope trends elsewhere indicate significant burial and sequestration of organic carbon (relative to car- bonate carbon), which would increase atmo- spheric and shallow-ocean-water O 2 (e.g., Cox et al., 2016). ...
The upper Tonian Chuar, Uinta Mountain, and middle Pahrump (ChUMP) groups of present-day western Laurentia collectively record the early breakup of Rodinia, largescale perturbations in the carbon cycle, and eukaryotic evolution, all of which preceded the onset of global glaciation by tens of millions of years. The spectacularly preserved and shale-rich Chuar Group of the Grand Canyon Supergroup stands out as one of the best global records of this time period, particularly for paleobiology. A new U-Pb age of 782 Ma on detrital zircons (n = 14 young grains) from the underlying Nankoweap Formation refines the Chuar Group's maximum depositional age to younger than 782 Ma. A new ⁴⁰Ar/³⁹Ar age of 764 ± 16 Ma (2σ) from K-feldspar within early diagenetic marcasite nodules from the upper Chuar Group (Awatubi Member) helps calibrate the rich Chuar microfossil record and constrain the large-magnitude shift in δ¹³Corg (up to 18‰; referred to here as the Awatubi positive carbon- isotope excursion or APCIE) to between ca. 764 and ca. 742 Ma, the date of an ash near the top of the Chuar Group. In addition to the maximum depositional age of ca. 782 Ma, U-Pb detrital zircon analyses (n = 826 grains) on sandstone beds from the underlying Nankoweap Formation indicate the presence of multiple older Laurentian age peaks. The similarity of detrital zircon populations and sedimentary character to that of the overlying Chuar Group (n = 764 grains) suggests that the Nankoweap Formation should be included as the lowermost unit in the Chuar Group. This revised geochronological framework indicates a 300 Ma unconformity between the Chuar Group (including the Nankoweap Formation) and the underlying 1.1 Ga Cardenas Basalt of the Unkar Group. Chuar Group detrital zircon populations share similarities with those of the Uinta Mountain Group and especially the middle Pahrump Group, including ca. 780 Ma grains. Biostratigraphic correlation using microfossils enhances the ChUMP connection and shows a trend of higher acritarch diversity in the lower Chuar and Uinta Mountain groups, and the presence of vase-shaped microfossils in the upper intervals of all three ChUMP units. Comparisons of δ¹³Corg and δ13Ccarb among ChUMP successions suggest a combination of local and regional controls. Thus, ChUMP successions are coeval within the 780-740 Ma range, show similar fossil and C-isotope trends, and derived sediments from similar Laurentian sources or source types. In light of recent age constraints and compiled paleontology in other Neoproterozoic basins, our high-resolution correlation of ChUMP successions can be extended to the Callison Lake dolostone of NW Canada and the Akademikerbreen-Polarisbreen groups of Svalbard. Biostratigraphic correlation with poorly age-constrained strata such as the Akademikerbreen- Polarisbreen groups and, farther afield, the Visingsö Group of Baltica suggests that ChUMP units record continentwide- and perhaps global-evolutionary patterns. The δ¹³Corg and δ¹³Ccarb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that δ¹³Corg values from organic-rich shale record variations in the C-isotope composition of late Tonian oceans. Intracratonic basins and contiguous rift margins of ChUMP age are inferred to have been important locations for microbial productivity and significant organic carbon burial that induced large positive shifts in δ¹³C and changes in global carbon balance prior to the onset of snowball Earth.
... An O 2 -free Archean has been explained by more reducing volcanism (Holland, 1984) or an absent biological O 2 source (Kirschvink and Kopp, 2008), but the long-term maintenance of an intermediate Proterozoic plateau in atmospheric pO 2 ( Fig. 1) is difficult to explain. Carbon isotopic measurements of Proterozoic marine carbonates (Bartley and Kah, 2004;Knoll et al., 1986;Halverson et al., 2005) imply sustained organic carbon burial rates that were ∼1-2× Phanerozoic levels, assuming exogenic carbon mass fluxes and their isotopic compositions were approximately equivalent (Hayes and Waldbauer, 2006). Such high net O 2 production rates are potentially incongruent with low pO 2 in the Proterozoic (Johnston et al., 2009;Schrag et al., 2013;Laakso and Schrag, 2014;Derry, 2015). ...
... The latter stands in contrast to isotope-based models of the Proterozoic carbon cycle. Although previous work has noted isotopic evidence for transient increases in fractional organic carbon burial near the end-Archean and end-Proterozoic boundaries (Knoll et al., 1986;Des Marais et al., 1992), background Proterozoic organic carbon burial fluxes have been interpreted as comparable to the Phanerozoic average (Krissansen-Totton et al., 2015) despite putatively low Proterozoic pO 2 (Fig. 1). This potential inconsistency has to led to challenges against canonical models that invert carbon isotopic time series for organic carbon burial histories (Schrag et al., 2013). ...
Atmospheric oxygen concentration has increased over Earth history, from ∼0 before 2.5 billion years ago to its present-day concentration of 21%. The initial rise in pO2 approximately 2.3 billion years ago required oxygenic photosynthesis, but the evolution of this key metabolic pathway was not sufficient to propel atmospheric oxygen to modern levels, which were not sustained until approximately two billion years later. The protracted lag between the origin of oxygenic photosynthesis and abundant O2 in the surface environment has many implications for the evolution of animals, but the reasons for the delay remain unknown. Here we show that the history of sediment accumulation on continental crust covaries with the history of atmospheric oxygen concentration. A forward model based on the empirical record of net organic carbon burial and oxidative weathering of the crust predicts two significant rises in pO2 separated by three comparatively stable plateaus, a pattern that reproduces major biological transitions and proxy-based pO2 records. These results suggest that the two-phased oxygenation of Earth's surface environment, and the long delays between the origin of life, the evolution of metazoans, and their subsequent diversification during the Cambrian Explosion, was caused by step-wise shifts in the ability of the continents to accumulate and store sedimentary organic carbon. The geodynamic mechanisms that promote and inhibit sediment accumulation on continental crust have, therefore, exerted a first-order control on the evolution of Earth's life and environment.
... Palaeomagnetic constraints suggest that north-east Svalbard lay in the subtropics throughout the Cryogenian (Li et al., 2013) as part of Laurentia, which spanned the equator in the centre of the fragmenting Rodinia supercontinent. The closely similar stratigraphy in central East Greenland (now officially redesignated as Northeast Greenland; Hambrey & Spencer, 1987; Moncrieff & Hambrey, 1990) indicates that it represents a basin continuation (Hambrey, 1983; Knoll et al., 1986; Fairchild & Hambrey, 1995), subsequently offset to the south by left-lateral strike-slip faulting (Harland, 1997). The distinct Neoproterozoic succession in western Svalbard cannot easily be correlated with its counterpart in north-east Svalbard and was probably deposited in a separate basin (Harland et al., 1993). ...
... Snowball Earth theory stimulated data-collection and modelling which have significantly clarified the criteria for attribution of a suite of phenomena to Snowball Earth conditions. Neoproterozoic glaciations are associated with negative carbon isotope anomalies (Knoll et al., 1986; Kaufman et al., 1997) and Hoffman et al. (1998) argued that the anomaly apparently encompassing the Marinoan glacial period reflected low organic productivity as a result of glacial climate and an ocean that became isolated from the atmosphere. Additionally , the post-glacial cap carbonates and the associated negative carbon isotope anomalies were regarded as a positive test of the Snowball Earth hypothesis, reflecting rapid meltdown and re-equilibration of oceanic and atmospheric CO 2 during a post-Snowball, ultra-greenhouse environment . ...
The Marinoan panglaciation (ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite-cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave-rippled or aeolian siliciclastic facies); and Calcareous Lake (slump-folded and locally re-sedimented rhythmic/stromatolitic limestones and dolomites associated with ice-rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (-12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of -6 to -7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (-8 to -15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica.
... However, it is also highly debated whether carbon isotope ratios of marine carbonates really reflect original seawater composition or not (Bristow and Kennedy, 2008; Knauth and Kennedy, 2009; Frimmel, 2010). Nevertheless, the large volume of reported data indicates that the Neoproterozoic carbonate rocks have positive δ 13 C values in general, and large negative anomalies during glacial events (Figure 3) (Knoll et al., 1986; Derry et al., 1989; Kaufman et al., 1991; Des Marais et al., 1992; Shields and Veizer, 2002; Halverson et al., 2005; Halverson et al., 2007; Halverson et al., 2010). Carbon is a proxy for a variety of environmental factors that controls the biogenic-abiogenic carbon balance in the Earth surface processes (Knoll et al., 1986; Kaufman et al., 1997). ...
... Nevertheless, the large volume of reported data indicates that the Neoproterozoic carbonate rocks have positive δ 13 C values in general, and large negative anomalies during glacial events (Figure 3) (Knoll et al., 1986; Derry et al., 1989; Kaufman et al., 1991; Des Marais et al., 1992; Shields and Veizer, 2002; Halverson et al., 2005; Halverson et al., 2007; Halverson et al., 2010). Carbon is a proxy for a variety of environmental factors that controls the biogenic-abiogenic carbon balance in the Earth surface processes (Knoll et al., 1986; Kaufman et al., 1997). Although secular trends in carbon isotope ratios can be applied for chemostratigraphic purposes for shorter time spans (few tens of millions of years) in the Neoproterozoic, recent studies have also indicated decoupling from a simple biologically driven isotopic excursions (Rothman et al., 2003; Knauth and Kennedy, 2009; Derry, 2010). ...
Chemostratigraphy or chemical stratigraphy deals with the correlation of sedimentary strata based on systematic variation of a particular chemical composition with time in the history of the Earth. In particular, this method is widely used in decoding the temporal variations seen in marine sediments that are deposited uninterruptedly in deep sea or in shallow marine carbonate depositional environments. In this review, the application of strontium and carbon isotope based chemostratigraphy in marine carbonate sediments is discussed. Irrespective of post depositional geological events such as metamorphism or tectonic displacements to form Mountain chains during continental collision, chemostratigraphy helps in determining the apparent depositional ages of sedimentary rocks in the Precambrian time, where biostratigraphy is not applicable. Although carbonate rocks are vulnerable to post depositional alterations, a systematic geochemical screening can guide in identifying the best chemically preserved carbonate rocks. A novel method of using Mn/Sr ratios for determining the best estimate of strontium initial ratio for carbonate rocks is presented, on the basis of a worked example of metamorphosed carbonate rocks from the Sør Rondane Mountains in the Dronning Maud Land, East Antarctica. The future potential of chemostratigraphy in decoding the early history of the Earth is also discussed.
... Consequently, the carbon isotopic compositions of carbonate rocks resist alteration. Beginning in the mid-1980s, carbon isotopes were increasingly used for stratigraphic correlation of Neoproterozoic carbonates and to a lesser extent for organic-rich shales (Knoll et al. 1986; Knoll & Walter 1992; Kaufman et al. 1997). Studies show that Neoproterozoic glaciogenic formations are typically bracketed by negative isotopic excursions. ...
... The second (organisms unspecified) supposedly occurred between 1.2 and 0.7 Ga, and the third in the middle Phanerozoic in response to the rise of vascular plants. The observed enrichment in 13 C of most pre-Ediacaran carbonates (e.g. Knoll et al. 1986) is cited in support of the middle (and largest) step in weathering efficiency (Carver & Vardavas 1994), although the isotopic pattern requires an increase in fractional organic burial as opposed to overall carbon burial. Increased Neoproterozoic biocatalysed weathering receives some support from micropalaeontology (Horodyski & Knauth 1994), molecular divergence analysis (Heckman et al. 2001) and clay mineralogy (Kennedy et al. 2006), but remains controversial. ...
Neoproterozoic glacial records have been discovered on 23 palaeocontinents, their rate of discovery changing little since 1871. Yet, half of all the resulting publications appeared since 2000. The history of research before 1998 is described in five stages defined by publication spikes; subsequent work is not covered because historical perspective is lacking. In stage 1 (1871-1907), 'Cambrian' (now Neoproterozoic) glaciation was recognized successively in Scotland, Australia, India, Norway, Svalbard and China. Criteria for recognition included faceted and striated pebbles in matrix-supported conglomerates resting on ice-worn bedrock pavements. In stage 2 (1908-1940), Neoproterozoic glaciation was shown to have been widespread in Africa, Asia and the Americas. Major textbooks summarized these findings, but the rejection of continental drift (to account for late Palaeozoic glacial dynamics) put a chill on research. In stage 3 (1942-1964), the occurrence of glacial deposits within carbonate successions, as well as nascent palaeomagnetic observations, suggested that Neoproterozoic glaciers reached sea-level at low palaeolatitudes, but the belated recognition of sediment gravity flowage caused glacial interpretations to be prematurely abandoned in key areas. In stage 4 (1965-1981), the extent of Neoproterozoic glaciation was rethought in light of plate tectonics. Distinctive chemical sediments (iron±manganese formations and cap carbonates) were identified. In basic climate models, modest lowering of solar luminosity resulted in global glaciation due to ice-albedo feedback, and deglaciation due to greenhouse forcing resulted from silicate-weathering feedback in the carbon cycle. Neoproterozoic glacial geologists were blind to these ideas. In stage 5 (1982-1997), reliable palaeomagnetic data combined with glacial marine sedimentation models confirmed that Neoproterozoic ice sheets reached sea level close to the palaeoequator.
... The isotope profile of the Beaver Meadows core recovered from eastern New York (Fig.1) illustrates a negative carbon isotope excursion corresponding with the most organicerich interval of the Marcellus in this core, the lower part of the Union Springs Member (Fig. 10). The correlation of the lowest d 13 C org values of the studied interval with that horizon delimiting the period of highest organic carbon burial is inconsistent with a model relating amplified organic matter flux to elevated surface productivity (Knoll et al., 1986; Holser, 1997). Elevated organic matter flux to the deep ocean seafloor during an oceanic anoxic event is accompanied by the sequestration of 12 C-enriched CO 2 in bottom water (Knoll et al., 1986; Kimura et al.,1997). ...
... The correlation of the lowest d 13 C org values of the studied interval with that horizon delimiting the period of highest organic carbon burial is inconsistent with a model relating amplified organic matter flux to elevated surface productivity (Knoll et al., 1986; Holser, 1997). Elevated organic matter flux to the deep ocean seafloor during an oceanic anoxic event is accompanied by the sequestration of 12 C-enriched CO 2 in bottom water (Knoll et al., 1986; Kimura et al.,1997). Overturn of such a water mass, perhaps related to the expansion of an oxygen minimum zone, could have emplaced isotopically light CO 2 onto shallow shelf areas such as the Marcellus basin (e.g., Kimura et al., 1997; Schr€ oder and Grotzinger, 2007; Chen et al., 2013). ...
Variations in the concentration of redox sensitive elements combined with pyrite framboid size data documented from a Marcellus Formation (Middle Devonian) core recovered from southwestern Pennsylvania elucidate the redox, organic matter accumulation, and diagenetic history of these deposits in this region of the basin. Uranium and Mo enrichment and Fe/Al display sharp increases coincident with diminishing Th/U upward through the initial 3rd order trangressive systems tract (lower Union Springs Member). These data as well as abundant small (<6 μm) pyrite framboids record establishment of strongly reducing benthic conditions, perhaps related to the expansion of an oxygen minimum zone induced by increased surface productivity. Strongly anoxic, even euxinic, conditions were interrupted by episodes of dysoxia, perhaps seasonal or longer term. Overlying regressive systems tract (RST) deposits record modestly improved redox conditions, likely a reflection of a receding oxygen minimum zone as base level dropped. A subsequent 3rd order base level rise and renewed expansion of the oxygen minimum zone triggered by increased surface productivity resulted in the accumulation of the organic-rich lower Oatka Creek Member. Still, the mix of abundant small and subordinate large (>10 μm) framboids preserves the record of oxygen deficient to sulfidic bottom conditions frequently interrupted by episodes of (dys)oxia. Improving redox conditions through the overlying RST were accompanied by a two-fold increase in Al and consequent dilution of the organic matter flux and authigenic trace metal uptake at the sediment–water interface. The upper half of the Oatka Creek comprises a depositional sequence not obvious from core inspection or gamma-ray signature but revealed by Mo enrichment and Al concentration profiles. Diagenetic modification of the Marcellus includes several horizons of authigenic calcium carbonate concretions and marked Ba enrichment. Both features reflect the effects of non-steady state microbial diagenesis within a methane-rich sedimentary column.
... The employment of carbon isotope chemostratigraphy for global correlations of Proterozoic sedimentary successions was first docu- mented by Knoll et al. (1986) on this same succession in Svalbard, and it has since served as one of the most widely used tools to correlate Precambrian strata worldwide (e.g., Halverson et al., 2005;Macdonald et al., 2010;Shields et al., 2021). The Tonian succession in the Eastern basement province of Svalbard records two pronounced and globally identifiable negative CIEs. ...
... Covariant d 13 C carb and d 13 C org records evidence that both the carbonate and organic matter were originally produced in the surface waters of the ocean and have retained their original d 13 C composition (e.g. Korte and Kozur, 2010; Meyer et al., 2013) as it is believed that no secondary processes are able to shift d 13 C carb and d 13 C org in the same direction at the same rate (Knoll et al., 1986). Decoupled d 13 C carb and d 13 C org records indicate diagenetic alteration (e.g. ...
We investigate the use of Hg as a proxy for volcanism by studying four distal and two proximal sections in relation to the Deccan volcanic center, straddling the CretaceousePaleogene (KPg) boundary at (a) Højerup (Denmark), Bottaccione and Padriciano (Italy), (b) Meghalaya and Jhilmili (India), and (c) Bajada del Jagüel (Argentina). Hg sequestration by organic matter results in constant Hg/TOC ratio and linear correlation between Hg content of the sediments and total organic carbon (TOC). Elevated Hg concentrations that deviate from this linear relationship represent most likely true Hg anomalies and these notable Hg/TOC spikes (all TOC <1%) are found in the Meghalaya, Bottaccione and Højerup sections within the CF2 planktic foraminiferal biozone (spike I), at the KPg boundary (spike II), and within the P1a planktic foraminiferal subzone (spike III). Spike III occurs also in the Jhilmili section. No clear correlation between Hg/TOC and Al 2 O 3 exists in any of the studied sections. The Hg anomalies probably result from strong volcanic episodes of the Deccan phase-2 (started 250 kyr before the KPg boundary and lasted for 750 kyr) that exhaled sulfuric aerosols, carbon dioxide and other toxic agents which reached a critical threshold, represented in true Hg enrichments in the paleoenviron-ments. The possibility that Hg enrichments resulted from anoxia scavenging on the seafloor and penetration downward into sediments is not supported in the stratigraphic record of Mo/Al ratios redox proxy. Hg isotopes were analyzed in samples from all KPg boundary sections in this study and from Bidart, France, the latter for comparison. Hg isotopes yielded d 202 Hg values ranging from À1 to À2‰ and D 201 Hg signatures from 0 to 0.05‰ (spike II in Højerup, Bottaccione and Meghalaya KPg boundary layers) consistent with volcanic emission of Hg (0 to À2‰). The d 202 Hg in spike I in Meghalaya and Padriciano and spike III in Jhilmili is consistent with volcanic emission of Hg. Two samples from Bajada del Jagüel and four from Bidart, however, display isotope signals compatible with volcanic emission/chondrite Hg. The results of three other samples are characteristic for reworked sediment, soil and/or peat. Most of the data show small positive D 201 Hg, in favor of long-term atmospheric Cretaceous Research 66 (2016) 60e81 transport prior to deposition, supporting a volcanic origin for the Hg. The present study broadens, therefore, the potential use of Hg as stratigraphic marker and, moreover, confirms that in the critical KPg transition, Hg was enriched in paleoenvironments at three distinct stages during the Deccan phase-2.
... However, semi-continuous Cryogenian successions display a clear stratigraphic motif in which two glacial units bound an unambiguously non-glacial interval, representing a mid-Cryogenian interlude of unknown duration ($5–27 My; Rooney et al., 2014). The Neoproterozoic succession of NE Svalbard, formerly contiguous with present-day Northeast Greenland (formerly known as East Greenland; Knoll et al., 1986; Fairchild and Hambrey, 1995; Hoffman et al., 2012), preserves this Cryogenian non-glacial interval. Chronological and chemostratigraphic constraints on the mid- Cryogenian in general, although improving (Halverson et al., 2010; Rooney et al., 2014 Rooney et al., , 2015), are currently more fluid than the base and top of the Cryogenian (Spence et al., 2016). ...
The Late Cryogenian Warm Interval (LCWI) refers to a non-glacial interval that separates presumed representatives of the Sturtian and Marinoan panglaciations. Its duration is poorly constrained radiometrically and its deposits are relatively poorly known in most geographic regions. This paper aims to constrain the duration, palaeoenvironments and petrogenesis of such deposits in the classic region of NE Spitsbergen, Svalbard. The succession comprises a 200-205 m dolomitic shale (Macdonaldryggen Member, known as E3, of the Elbobreen Formation) overlain by oolitic dolomite Slangen Member (E4), 15-25 m thick, with limestone developed at top and base of E3 in the south of the area. The assumed age context of the succession has been confirmed by the presence of a typical Sturtian cap carbonate profile of negative to positive δ13C, and primary Sr isotope compositions of basal E3 limestones <0.7072 and of upper E3 limestones of 0.7076.
... Covariant δ 13 C carb and δ 13 C org records evidence that both 556 the carbonate and organic matter were originally produced in the surface waters of the 557 ocean and have retained their original δ 13 C composition (e.g. Korte and Kozur, 2010; 558 Meyer et al., 2013) as it is believed that no secondary processes are able to shift δ 13 C carb 559 and δ 13 C org in the same direction at the same rate (Knoll et al., 1986). Decoupled 560 δ 13 C carb and δ 13 C org records indicate diagenetic alteration (e.g. ...
... For isotopic analyses of total organic carbon (TOC), 5 to 10 mg of powdered, decalcified sample were combusted in a sealed quartz combustion tube at 850°C for at least 4 hours (Knoll et al., 1986). Kerogen for Sl3C, Sl~ and m measurements, was isolated using the methods described by Wedeking et al., (1983). ...
The microbial ecology and, hence, related aspects of the paleoenvironment, of the Green River Lake system (Eocene, Wyoming, Utah, and Colorado) have been reconstructed through geochemical analyses. Specifically, concentrations and isotopic compositions of bulk carbon, sulfur, and nitrogen, as well as carbon-isotopic compositions and abundances of numerous individual hydrocarbons ("biomarkers") have been examined. The Green River Formation was deposited in two large alkaline, saline lakes occupying hydrologically closed basins. Utilizing core material from the Piceance Creek Basin, this study has focused on (1) the sources and cycling of nutrients for the producer community and (2) the pathways of carbon flow in
the ecosystem.
Estimates of annual accumulation of organic carbon in the Green River lakes are low compared to measurements of productivity in modem saline lakes, suggesting that a great deal of the organic material was reworked prior to preservation. Carbon sulfur relationships indicate sulfate reduction was limited by available sulfate. Organic material enriched in 15N indicates a complex and active nitrogen cycle in which nitrogenous intermediates were cycled through microbial redox reactions. Enrichment of 13C in carbonates suggests extensive methanogenic activity in the sediments.
In order to examine effects of environmental changes, samples for isotopic analysis were taken at intervals of 60 m through three different "sulfur cycles", intervals in which concentrations of total sedimentary S varied between -0.2 and -3.5%. Stratigraphic variations of 13C abundance in individual n-alkanes indicated derivation from at least five distinct biological sources. Isotopic variations in numerous materials derived from polyisoprenoids and from di-, tri- and tetraterpenoids indicate mixing of three components: products of primary photoautotrophs and their immediate consumers, products of methanotrophic bacteria, and products of unknown bacteria. Independent variations in 13C contents of a variety of photosynthetic products indicated the presence of a diverse producer community. Hopanoids produced at times of low sulfur concentration were particularly depleted in 13C. indicating significant inputs from methanotrophic bacteria. and suggesting competition between methanogenic bacteria and sulfate-reducing bacteria for metabolic intemediates.
This study has been the first to combine detailed stratigraphic sampling with compound-specific isotopic analyses. and indicates that this approach can significantly enhance process-oriented detail in the reconstruction of ancient environments.
... Consequently, temporal variations of carbon isotope compositions in sedimentary rocks are commonly attributed to changes in the fractional burial of organic matter (Goldberg et al., 2007). The burial of organic matter is mainly controlled by primary productivity (Knoll et al., 1986) and redox conditions in the water column (Calvert and Pedersen, 1993). The carbon isotope composition of the primary biomass is also dependent on sources of CO 2 in the ocean. ...
Characteristics of redox-sensitive elements and organic carbon isotopes of a drilling section covering the Ediacaran-Cambrian (E-C) boundary at Anhui Province, South China were studied. Total organic carbon (TOC), organic carbon isotope values of organic compounds, abundances of Mo, V, U, and Ni, and ratios of V/(V+Ni) and Th/U co-vary across the section, suggesting that the redox condition is the main factor controlling carbon isotope values of organic matter, and sequestration of trace elements. The E-C boundary is characterized by the lowest organic carbon isotope and the highest ratios of Mo/TOC, V/TOC, and U/TOC in the section, which are correlated with reported carbon isotope excursions in other regions. This is the first time that signatures of Mo, V, and U in the Hetang Formation at the Lower Yangtze Platform are used for stratigraphic correlations. The results highlight abnormally high ratios of Mo/TOC, V/TOC, and U/TOC near the E-C boundary. Petrological and geochemical characteristics in this section suggests that intense upwelling activities during the E-C transition may be the main contributor to 13C-depleted CO2 and enrichment of trace metals. The co-existing pervasive transgression led to ocean anoxia and facilitated the preservation of 13C-depleted organic matter and trace metal sequestration. Furthermore, oxidation of the bottom ocean water may have completed during the early Cambrian. A combination of these factors may be among the reasons for significant evolutionary changes during the E-C transition.
... These deposits are systematically capped by carbonate sequences with anomalous facies, and are interpreted to have been deposited immediately after the glacial sediments, revealing rapid ice-house to green-house transitions (Kennedy, 1996; Nogueira et al., 2003). Variation in C-isotopic composition in these " cap carbonates " is a major record of such dramatic environmental changes, and has been the main criteria in establishing global correlations between Neoproterozoic sedimentary successions (Knoll et al., 1986; Kaufman and Knoll, 1995; Kaufman et al., 1997). Also, since C and Sr variations follow different time-scales, together they can be used to constrain the age of a sedimentary succession if their variation is tied to reliable radiometric data in a reference curve. ...
... Two major glacigenic units have been described; an older, Petrovbreen Member (E2) of the Elbobreen Formation and the younger Wilsonbreen Formation (Fairchild & Hambrey, 1984). These units are closely correlated with diamictite pairs in Northeast Greenland (Kulling, 1934; Knoll et al., 1986; Fairchild & Hambrey, 1995) and are very similar to other diamictite pairs in the North Atlantic region, although geochronological evidence is commonly lacking which precludes accurate correlation (Hambrey, 1983). The Wilsonbreen Formation is the younger and thicker glacial interval within the Polarisbreen Group, consisting of 130 to 180 m of a grey to maroon, massive to weakly bedded diamictite-dominated unit overlying the Slangen Member in the south and the Br avika Member in the north (which thins and becomes absent in the south) and underlying the Dracoisen Formation (Halverson et al., 2004). ...
Palaeoenvironmental reconstruction of Neoproterozoic successions has been the subject of long-standing debate, particularly concerning the interpretation of diamictites. The Wilsonbreen Formation of north-east Svalbard is a 130 to 180 m thick diamictite-dominated glacigenic succession deposited during a late Cryogenian (Marinoan) glaciation. Previous research has highlighted a complex sedimentary architecture with evidence of subaqueous, subglacial and non-glacial conditions. This study combines well-established sedimentological techniques with the first sedimentological application of the anisotropy of magnetic susceptibility technique in Neoproterozoic glacial sediments, to investigate the origin and palaeoenvironmental significance of glacigenic sediments within the Wilsonbreen Formation. A range of lithofacies occurs within the succession, dominated by massive diamictites, sandstones and conglomerates. Some of these facies display evidence of primary deformation and can be grouped into a Deformed Facies Association; these are interpreted to have been formed through glacitectonic deformation in a subglacial environment. Fabric investigation reveals that this deformation was associated with glacier flow towards the north. In addition, an Undeformed Facies Association records deposition in ice-proximal and ice-distal subaqueous environments. Taken together with intervening non-glacial facies, the glacigenic sediments record a series of advance-retreat cycles, with ice flow involving sliding and sediment shearing below wet-based ice.
... Carbon isotope values have long been used to reconstruct paleoenvironmental conditions ranging from organic carbon burial (e.g., Arthur et al., 1988; Sackett, 1991; Hayes et al., 1999) to partial pressure of CO 2 (Popp et al., 1989; Freeman and Hayes, 1992) to, more recently, an approximation of the sink of carbonate carbon precipitated on the seafloor through remineralization processes (Schrag et al., 2013). Despite the wide use of carbon isotopes as proxies for ancient environments and their utility in providing chemostratigraphic linkages between ancient strata (e.g., Knoll et al., 1986; Hayes et al., 1999), substantial debate still surrounds the meaning of these values and the timing of their origins (e.g., Grotzinger et al., 2011). A δ 13 C signal in the marine dissolved inorganic carbon (DIC) reservoir is often interpreted as reflecting the balance between the flux and isotopic composition of the major sources to and from the ocean (Kump and Arthur, 1999), but ascertaining the timing of the incorporation of δ 13 C signals in carbonate sediments and sedimentary rocks – and the local processes that may affect their composition (e.g., Oehlert and Swart, 2014) – is critical before any paleoenvironmental interpretation can be made. ...
Carbonates of western Newfoundland span the Cambro-Ordovician interval and preserve a record of slope-basinal deposition in the Cow Head Group near Cow Head. This unit consists of conglomerates and ribbon and laminated limestone interbedded with shale that is well exposed in sea cliffs at Cow Head Peninsula. These conglomerates, although prevalent throughout the section, vary in thickness and abundance stratigraphically and record both local disruption and large-scale episodic sedimentation events. Microfacies drilled for carbon isotope (δ13Ccarb) analysis of conglomerates reveal isotopic heterogeneity within individual samples, in some cases more than 1‰. While this might be an expected outcome of drilling multiple areas of a heterogeneous conglomerate hand sample, permil-level variability was observed both between individual clasts in a sample, between different parts of the same matrix, and between a clast and its surrounding matrix. No associated variation in δ18Ocarb or trace element distributions exists to suggest that this δ13Ccarb variability is the result of later-stage meteoric diagenesis. The δ13Ccarb variability suggests multiple sources of dissolved inorganic carbon (DIC) associated with carbonate precipitation for phases within these individual samples. These data indicate that processes such as local organic matter remineralization and early authigenic carbonate precipitation during lithification at the sediment–water interface (SWI) are either contributing to or controlling δ13Ccarb values in Cambrian carbonates, perhaps more so than at other intervals in Earth history.
... Geochemistry has played a central role in unraveling the origin of BIFs, and many proxies such as rare earth element (REE) concentrations, have been widely used (e.g., Kato et al., 2006Ad However, chemostratigraphic studies are scarce, probably because traditional stable and radiogenic isotope studies such as δ 13 C, δ 18 O, δ 34 S and 87 Sr/ 86 Sr are impossible to apply for the more widespread oxide-facies BIF. Isotope chemostratigraphy has played a significant role in the study of Neoproterozoic successions since the pioneer reports by Knoll et al. (1986) V E 9 Kaufman et al. (1991 ...
Neoproterozoic banded iron formations (BIFs) are not restricted to the middle Cryogenian, c. 715. Ma glaciation, occurring in Tonian, Cryogenian, and Ediacaran successions. Many Neoproterozoic BIFs were deposited in glacially influenced settings, such as the Rapitan Group (Canada), Jacadigo Group (W Brazil), Chuos, and Numees Formations (Namibia) and Holowilena Ironstone (Australia). However, many occurrences are not related to glacial processes and can be assigned to the Algoma and Lake Superior types. Neoproterozoic Algoma-type BIF includes the Wadi Karim and Um Anab (Egypt), the correlative Sawawin BIF (Saudi Arabia), and the Jucurutu Formation of the Seridó Belt (NE Brazil). Lake Superior type BIFs are represented by the Tonian Shilu Group (South China) and the late Ediacaran Arroyo del Soldado Group (Yerbal and Cerro Espuelitas formations, Uruguay).Useful chemostratigraphic tools for the study of BIFs include rare earth element distribution, especially Eu and Ce normalized concentrations, iron speciation, and Nd and Cr isotopes (δ53Cr). Whereas Rapitan type BIFs exhibit no Eu or Ce anomalies, the Algoma-type Neoproterozoic BIFs show both. Positive δ53Cr values characterize glacially influenced BIFs, and differentiates them from nonfractionated, mantle-like δ53Cr values of Algoma type BIF. The most positive δ53Cr values occur in the open-shelf, Lake Superior type BIF, especially that of Ediacaran age.We suggest that glaciation was not a prerequisite for deposition of BIF in the Neoproterozoic, and that hydrothermal activity related to Rodinia rifting played the key role in their reappearance after the Mesoproterozoic gap. Their deposition has more to do with the return to ferruginous conditions in the oceans than with near-global glaciation.
... Powis (2006), Misi et al. (2007) and Vieira et al. (2007) report similar isotopic variation in the carbonate successions of the Bambuí Group, in the São Francisco Basin. Negative anomalies like that found in Unit C are characteristic of many carbonate successions immediately above glaciogenic lithotypes around the world (Knoll et al. 1986; Kaufman et al. 1991; Halverson et al. 2005; Shields 2005). ...
The Bebedouro Formation (Fm.) is a Neoproterozoic glaciogenic succession at the base of the Una Group on the São Francisco Craton. The glaciogenic sequence is composed of diamictites, pelites and sandstones with a variety of lithofacies that are grouped into four associations: (i) ice-contact, (ii) pro-glacial, (iii) ice-rafted and (iv) aeolian (extraglacial). Thus, the Bebedouro Fm. is interpreted to have been deposited in a shelf marine environment where glacio-proximal sedimentation dominated. It variably overlies Palaeoproterozoic/Archaean basement and Mesoproterozoic metasedimentary rocks of the Chapada Diamantina Group.
The overlying carbonate succession (Salitre Fm.) is informally subdivided into five mappable units. Unit C, at the base, is composed of red argillaceous dolostone with typical negative δ ¹³ C signatures averaging –5.1‰ (VPDB). Unit B consists of grey laminated limestones that grade upward into grey dolostone with tepee structures of Unit B1; both have δ ¹³ C values around 0‰. The overlying Unit A consists of interbeds of greyish marl, shales and siltstones grading upward into massive black organic-rich limestone with oolitic and pisolitic beds of Unit A1. Positive δ ¹³ C signatures averaging +8.5‰ VPDB characterize the black limestones of Unit A1. ⁸⁷ Sr/ ⁸⁶ Sr ratios of well-preserved samples of these carbonate units show values of 0.70745 and 0.70765. These units can be correlated litho- and chemostratigraphically with formations of the Bambuí Group (São Francisco Basin) to the west.
... Geochemistry has played a central role in unraveling the origin of BIFs, and many proxies such as rare earth element (REE) concentrations, have been widely used (e.g., Kato et al., 2006Ad However, chemostratigraphic studies are scarce, probably because traditional stable and radiogenic isotope studies such as δ 13 C, δ 18 O, δ 34 S and 87 Sr/ 86 Sr are impossible to apply for the more widespread oxide-facies BIF. Isotope chemostratigraphy has played a significant role in the study of Neoproterozoic successions since the pioneer reports by Knoll et al. (1986) V E 9 Kaufman et al. (1991 ...
Neoproterozoic BIF are not restricted to the middle Cryogenian, ca. 715 Ma glaciation, occurring in Tonian, Cryogenian and Ediacaran successions. Many Neoproterozoic BIFs were deposited in glacially influenced settings, such as the Rapitan Group (Canada), Jacadigo Group (W Brazil), Chuos and Numees formations (Namibia) and Holowilena Ironstone (Australia). However, many occurrences are not related to glacial processes, and can be assigned to the Algoma and Lake Superior types. Neoproterozoic Algoma-type BIF include the Wadi Karim and Um Anab (Egypt) the correlative Sawawin BIF (Saudi Arabia), and the Jucurutu Formation of the Seridó Belt (NE Brazil). Lake Superior type BIF are represented by the Tonian Shilu Group (South China) and the late Ediacaran Arroyo del Soldado Group (Yerbal and Cerro Espuelitas formations, Uruguay).
Useful chemostratigraphic tools include rare earth element distribution, especially Eu and Ce normalized concentrations, iron speciation, Nd and Cr isotopes (δ53Cr). Whereas Rapitan type BIF exhibit no Eu or Ce anomalies, Algoma type Neoproterozoic BIF show both. Positive δ53Cr values characterize glacially-influenced BIF, and differentiates them from non-fractionated, mantle-like δ53Cr values of Algoma type BIF. The most positive δ53Cr values occur in the open-shelf, Lake Superior type BIF, especially those of Ediacaran age.
We suggest that glaciation was not a pre-requisite for deposition of BIF in the Neoproterozoic, and that hydrothermal activity related to Rodinia rifting played the key role in their re-appearance after the Mesoproterozoic gap. Their deposition has more to do with the return to ferruginous conditions in the oceans than with near-global glaciation.
... 730 Ma for Sierras Bayas Group. This means that the Cerro Negro Formation was deposited during or later Sturtian glacial epoch, established as around 780 Ma by Knoll et al. (1986). The micritic black limestones sampled in Olavarría and Barker area, at Loma Negra and El Infierno quarries, respectively, are exhibiting 87 Sr/ 86 Sr in a wide range, between 0.70584 and 0.71409 (Figure 4). ...
... Diagenetic stabilization of carbonates therefore results in transposition of carbon from a precursor to a successor mineral phase. The above oscillations in the d 13 C trend carbonate are being increasingly utilized for correlation and Ž isotope stratigraphy e.g., Knoll et al., 1986; Derry et . al., 1992 . ...
A total of 2128 calcitic and phosphatic shells, mainly brachiopods with some conodonts and belemnites, were measured for their , and values. The dataset covers the Cambrian to Cretaceous time interval. Where possible, these samples were collected at high temporal resolution, up to 0.7 Ma (one biozone), from the stratotype sections of all continents but Antarctica and from many sedimentary basins. Paleogeographically, the samples are mostly from paleotropical domains. The scanning electron microscopy (SEM), petrography, cathodoluminescence and trace element results of the studied calcitic shells and the conodont alteration index (CAI) data of the phosphatic shells are consistent with an excellent preservation of the ultrastructure of the analyzed material. These datasets are complemented by extensive literature compilations of Phanerozoic low-Mg calcitic, aragonitic and phosphatic isotope data for analogous skeletons. The oxygen isotope signal exhibits a long-term increase of from a mean value of about −8‰ (PDB) in the Cambrian to a present mean value of about 0‰ (PDB). Superimposed on the general trend are shorter-term oscillations with their apexes coincident with cold episodes and glaciations. The carbon isotope signal shows a similar climb during the Paleozoic, an inflexion in the Permian, followed by an abrupt drop and subsequent fluctuations around the modern value. The ratios differ from the earlier published curves in their greater detail and in less dispersion of the data. The means of the observed isotope signals for , , and the less complete (sulfate) are strongly interrelated at any geologically reasonable (1 to 40 Ma) time resolution. All correlations are valid at the 95% level of confidence, with the most valid at the 99% level. Factor analysis indicates that the , , and isotope systems are driven by three factors. The first factor links oxygen and strontium isotopic evolution, the second and , and the third one the and . These three factors explain up to 79% of the total variance. We tentatively identify the first two factors as tectonic, and the third one as a (biologically mediated) redox linkage of the sulfur and carbon cycles. On geological timescales (≥1 Ma), we are therefore dealing with a unified exogenic (litho-, hydro-, atmo-, biosphere) system driven by tectonics via its control of (bio)geochemical cycles.
... Kaufman et al. 1993Kaufman et al. , 1997 Halverson et al. 2007). These dramatic climatic changes, global carbon cycling and atmospheric oxygen budget (Knoll et al. 1986; Derry et al. 1992; Knoll 1992; Des Marais 1994; Strauss 1997; Hoffman et al. 1998; Canfield 1999) are recorded globally by chemostratigraphic fluctuations of C, Sr and S isotopic compositions in the worldwide late Mesoproterozoic (Stenian, i.e. 1200–1000 Ma; Fairchild et al. 1990; Des Marais et al. 1992; Knoll et al. 1995; Podkovyrov et al. 1998; Canfield 1999; Kah et al. 1999; Bartley et al. 2001), and mid-(Cryogenian, i.e. 850–635 Ma) to late (Ediacaran, i.e. 635–541 Ma) Neoproterozoic series (Derry et al. 1989; Kaufman and Knoll 1995; Asmeron et al. 1991; Halverson et al. 2005 Halverson et al. , 2007 Kaufman et al. 2006; Tewari and Sial 2007). The origin of these large global isotope fluctuations is still being debated. ...
The late Mesoproterozoic - middle Neoproterozoic carbonate succession (1155 Ma - 800 Ma) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) is poorly investigated with respect to sedimentology including climate, tectonics and sea level changes. In this paper, our data including microfacies analysis, sequence stratigraphy, Fischer plots coupled with C and O isotopes, are combined to understand the paleoecology and sea level variations of the carbonate series of the Mbuji-Mayi Supergroup, as well as to establish a stratigraphic hierarchical approach in the interpretation of the stratigraphic framework and the comprehension of the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin evolution. Microfacies analysis showed that the carbonate succession consists of strata accumulated on a ramp and sequence stratigraphy indicated that sedimentation was cyclic on the inner ramp, as archived by plurimetric ‘thin’ peritidal cycles (± 4 m-thick on average) recording relative sea level of a maximum of 4 m, with fluctuations in the range around 1 m to 4 m. This shallow-water depth and the abundance of cyanobacteria suggest that water column was oxygenated. The subtidal outer/middle ramp facies are also cyclic with ‘thick’ subtidal sequences characterized by an average thickness of ± 17 m. Accurate relative sea level fluctuations are difficult to assess in this ‘deeper’ environment since the facies could have been deposited in a wide range of shallow water that did not completely fill the accomodation space. A probable magnitude for such fluctuations is around 10 m to 20 m. These data are the first ones that put quantitative constraints on the late Mesoproterozoic – middle Neoproterozoic carbonate deposits that have covered much of the Congo Shield at the end of the Precambrian. This area is therefore an important type section for Central Africa.
... At the top of the water column, photoautotrophic organisms fixed carbon which was in equilibrium with the atmosphere. They carried an isotopic signature similar to that of the organic matter preserved in other co-eval sediments which defines the general secular trend in 13 C org for this period (Knoll et al., 1986; Kaufman et al., 1993; Calver, 1995 ). This rain of 'planktonic' organic matter would have been more or less continuous, as shown by the uniformity of biomarkers, such as phytane and steranes ascribed to planktonic sources. ...
A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centralian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from 'planktonic' and 'benthic microbial-mat' populations contributed to the sediment. The 'planktonic' source provided a suite of n-alkanes with C 20 predominance and the odd n-alkanes C 20 , whereas, the 'benthic' source contributed an overlay of n-alkanes C 20 with a strong even preference, together with mid-chain methyl alkanes. Kerogen and biomarkers derived from the microbial mat were found to be depleted in 13 C relative to planktonic material. Pyrite in the micorbial mats was also found to be depleted in 34 S compared to surrounding facies. The combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria. These organisms have specific environmental tolerances that set limits on palaeo-environment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the results and age determinations reported here, it would appear that mid-shelf environments of the Centralian Superbasin of Australia were seeing significant levels of oxygen through the Ediacarian.
... Cloud and Glaessner (1982) proposed that the " Ediacarian " instead be defined as a period with its base at the top of the Marinoan tillites and their equivalents worldwide. Neoproterozoic strata are characterized by major, apparently synchronous isotopic excursions in carbon (Knoll et al., 1986; Kaufman and Knoll, 1995; Halverson et al., 2005 Halverson et al., , 2010), strontium (Kaufman et al., 1993; Halverson et al., , 2010), and sulfur (Halverson et al., 2010), that provide a third criterion for recognition and correlation of a terminal Neoproterozoic system worldwide. Neoproterozoic isotope excursions are not unique in either shape or magnitude, but in conjunction with biostratigraphy (Kaufman et al., 1991; Narbonne et al., 1994), climatic indicators (Kaufman et al., 1997), and precise U-Pb dates (Grotzinger et al., 1995), provide an exceptionally useful tool for global correlation of Neoproterozoic strata. ...
The Ediacaran System and Period was ratified in 2004, the first period-level addition to the geologic time scale in more than a century. The GSSP at the base of the Ediacaran marks the end of the Marinoan glaciation, the last of the truly massive global glaciations that had wracked the middle Neoproterozoic world, and can be further recognized world-wide by perturbations in C-isotopes and the occurrence of a unique "cap carbonate" precipitated as a consequence of this glaciation. At least three extremely negative isotope excursions and steeply rising seawater 87 Sr/ 86 Sr values characterize the Ediacaran Period along with geochemical evidence for increasing oxygenation of the deep ocean envi-ronment. The Ediacaran Period (635e541 Ma) marks a pivotal position in the history of life, between the micro-scopic, largely prokaryotic assemblages that had dominated the classic "Precambrian" and the large, complex, and commonly shelly animals that dominated the Cambrian and younger Phanerozoic periods. Diverse large spiny acritarchs and simple animal embryos occur immediately above the base of the Ediacaran and range through at least the lower half of the Ediacaran. The mid-Ediacaran Gaskiers glaciation (584e582 Ma) was immediately followed by the appearance of the Avalon assemblage of the largely soft-bodied Ediacara biota (579 Ma). The earliest abundant bilaterian burrows and impressions (555 Ma) and calcified animals (550 Ma) appear towards the end of the Ediacaran Period.
... Notably, the carbon isotope records of these successions are characterized by relatively little variation, predominantly ranging between −4‰ and +4‰, with shifts generally not exceeding 5‰ (Kah et al., 1999; Bartley et al., 2001 Bartley et al., , 2007 Kah et al., 2012; Gilleaudeau and Kah, 2013 ). This record stands in stark contrast to the succeeding Neoproterozoic when carbon isotope compositions of marine proxies reveal significant overall 13 C enrichments and high amplitude variations (Knoll et al., 1986; Kaufman and Knoll, 1995; Kaufman et al., 1997; Prave et al., 2009; Halverson et al., 2010). As mentioned above, carbon isotope compositions of marine carbonates in the Vazante Group reveal a moderate spread in 13 C values, largely falling within the range of −4 to +3‰ (Azmy et al., 2006). ...
A new Re–Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time. Field observations of a reverse fault juxtaposing older Mesoproterozoic sedimentary rocks above younger Neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of U–Pb detrital zircon ages through the succession. Combined, the geochronological, geochemical and stratigraphic evidence suggest that the Vazante Group sediments accumulated along a passive margin of the São Francisco craton and are correlative with the neighboring Paranoá Group. Biomarker, sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post-glacial transgression and deposition of one of the bituminous shale horizons. The relationship of the glaciogenic Vazante Group to other late Mesoproterozoic successions, such as the non-glacial Atar Group in West Africa and Bylot Supergroup in arctic Canada, however, remains enigmatic.
... Although many biostratigraphic attempts have been made to define the exact level of the SDB in West Qinling (Rong et al., 1987; XIGMR & NIGPAS, 1987a), the SDB in the area remains contentious. Carbon isotope as well as oxygen and strontium isotopes has been intensely used to study secular isotopic variations of ancient ocean water, and some variations in the carbon isotopic composition of marine carbonate rocks and components (fossils, marine cements) have been used for chemostratigraphic correlations and paleoenvironmental interpretations (Veizer & Hoefs, 1976; Fischer & Arthur, 1977; Scholle & Arthur, 1980; Arthur et al., 1985; Dean et al., 1986; Knoll et al., 1986; Popp et al., 1986; Hayes et al., 1989; Kaufman et al., 1991; Popp et al., 1997; Geldern et al., 2006). An appreciation of the possibilities of variations in carbon isotopes in ancient carbonates was first deduced by Garrels & Perry in 1974, but reliable δ 13 C curves of bulk carbonate samples were only available for the SDB during the last 12 years (Hladikova et al., 1997; Veizer et al., 1999; Saltzman, 2002; Buggish & Mann, 2004; Buggisch & Joachimski, 2006; Gill et al., 2007;), and evidenced also in Podolian well preserved brachiopod shell calcites (Azmy et al., 1998; Veizer et al., 1999; Prokoph et al., 2008). ...
Carbon isotope (δ13Corg) analyses of neritic carbonates and black shales spanning the Silurian-Devonian transition are compared from two richly fossiliferous sequences in Zoige County (West Qinling Region) of Sichuan Province, China. The two sections, Putonggou and Yanglugou sections in West Qinling, reveal positive δ13Corg shifts happening in the upper Pridoli and lower Devonian and reaching peak values as heavy as -19.9‰ (Putonggou) and -19.7‰ (Yanglugou) in the lowermost Lochkovian following the first occurrence of the Protathyris-Lanceomyonia brachiopod fauna and the conodont Icriodus woschmidti woschmidti. These results replicate a globally known positive shift in δ13Corg from the uppermost Silurian to the lowermost Devonian. The isotopic trend for organic carbon across the
Silurian-Devonian Boundary (SDB) offers an approach to a potential correlation of the SDB from different sedimentary facies. The δ13Corg variations across the Silurian-Devonian transition at the two sections in West Qinling exhibit a similar shift in their carbon isotopic composition
as the detailed SDB curves from the borehole Klonk-1 drilled at top of the Klonk GSSP in the Prague Basin, Czech Republic. This suggests that the SDB in West Qinling is located at the upper part of the Yanglugou Formation (between ZY-07 and ZY-06) in the Yanglugou Section and the lower part of the Xiaputonggou Formation (between ZP-09 and ZP-10) in the Putonggou Section. More data will help to reveal individual
causes for the isotopic shift, such as enhanced carbon burial due to anoxic conditions and/or increased productivity, alternating arid and humid periods, or changes in the composition of primary producers.
... The age and global character of the Sturtian glaciation is at present the subject of much debate (Hoffman et al., 1996; Frimmel et al., 1996; Brasier et al., 2000; Lund et al., 2003; Fanning and Link, 2004; Halverson et al., 2005; Kendall et al., 2006; Babinski et al., 2007; Macdonald et al., 2010a). The Sturtian and Marinoan glaciations are both preceded and followed by negative isotope anomalies, and the less severe Gaskiers (Ediacaran) glaciation appears to overlap the Shuram (or Wonoka) 13 C anomaly — the most extreme in the Neoproterozoic (Knoll et al., 1986; Hayes et al., 1999; Halverson et al., 2005). In the case of the Sturtian and Marinoan events, 13 C dropped to as low as −6‰ (Halverson, 2006; Prave et al., 2009) in shallow water carbonates deposited prior to onset of glaciation: the Islay and Trezona anomalies, respectively. ...
Remnants of a Neoproterozoic glaciation in east central Brazil are represented by thin diamictite layers (Jequitaí Formation and correlative units), locally overlying striated pavements on the São Francisco craton. The diamictites are covered by the Sete Lagoas Formation of the basal Bambuí Group, which is generally accepted to be a typical cap carbonate sequence. Although most authors have preferred a mid-Cryogenian (post-Sturtian) age for it, based mainly on Pb-Pb whole rock data, the Sete Lagoas Formation bears lithostratigraphic and isotopic characteristics that are identical to early Ediacaran cap carbonates worldwide, including a basal thin (0–10 m) pale and flinty cap dolostone, preserving a drop in δ13C values from around −3.2‰ to −4.5‰ with associated δ18O around −5‰, and crystal-fan facies interpreted as aragonite pseudomorphs. Ediacaran zircons have been recovered from the middle of the Sete Lagoas Formation, constraining the deposition of its upper half to be younger than 610 Ma (Rodrigues, 2008). Although there is an unconformity below the point where the zircons were collected, it is short-lived, as suggested by the identical, typically Ediacaran 87Sr/86Sr values above and below (0.7074–0.7076). Carbonate clasts from the Jequitaí Formation and correlative diamictite-bearing units in the fold belts that surround the São Francisco craton (Canabravinha and Serra do Catuni formations) display similar ranges in δ13C (−6.7 to +2.6‰), suggesting the erosion of a pre-glacial carbonate platform with negative δ13C values (i.e. the Islay and/or Trezona anomalies). The cratonic Carrancas Formation, on the other hand, yielded pale dolostone clasts with δ13C in a small range between −4.2 and −3.4‰, and δ18O values around −6.5‰. These clasts could be derived from the cap dolostone unit itself, in which case the Carrancas Formation would represent resedimented basal Sete Lagoas Formation and imply that sections of the Sete Lagoas Formation sitting atop the Carrancas Formation are incomplete. The base-truncated sections have confused previous attempts to correlate the Sete Lagoas Formation with other cap carbonate successions. In light of the available lithostratigraphic, isotopic and U-Pb zircon data, we propose that the Sete Lagoas Formation represents a basal Ediacaran cap carbonate sequence (∼635–610 Ma) deposited after the Marinoan glaciation in east central Brazil.
... 6). 13 C values reflect changes in the biogeochemical redox cycling of carbon and a large negative shift in 13 C (i.e. an increased proportion of lighter 12 C) before break-up is a possible indicator of reduction in ocean productivity (e.g. Knoll et al., 1986; Magaritz et al., 1986) because organic matter preferentially incorporates 12 C. Stagnant ocean conditions during warm climates following Neoproterozoic glaciation are interpreted to have led to dyoxic deep waters enriched in nutrients (Kimura et al., 1997; Ripperdan, 1994 ). Negative 13 C excursions are associated with organic remineralisation and could be produced by either transgression or post-Marinoan deglaciation; however, the subsequent massive upwelling of deepwater accompanying the break-up of Palaeopangaea to the archaeocyathids; the lower line applies to metazoans with these excluded. ...
... A monotonic$ 8% rise in d 13 C carb through the Vapol' Formation may suggest an increase in organic carbon burial (Hayes et al., 1999), but a tight, parallel change in d 13 C org is lacking. This lack of isotopic covariance is not uncommon in Neoproterozoic carbonates (Fike et al., 2006; Swanson-Hysell et al., 2010), with recent work pointing to complexities associated with d 13 C org as masking classic carbon isotope behavior (Johnston et al., 2012; Knoll et al., 1986). In the case of the Cryogenian from Russia, the data reported here support a stratigraphic link to the Ural Mountains and provide yet another example of a pre-Sturtian carbon cycle with a large degree of variability. ...
The Neoproterozoic arrival of animals fundamentally changed Earth's biological and geochemical trajectory. Since the early description of Ediacaran and Cambrian animal fossils, a vigorous debate has emerged about the drivers underpinning their seemingly rapid radiation. Some argue for predation and ecology as central to diversification, whereas others point to a changing chemical environment as the trigger. In both cases, questions of timing and feedbacks remain unresolved. Through these debates, the last fifty years of work has largely converged on the concept that a change in atmospheric oxygen levels, perhaps manifested indirectly as an oxygenation of the deep ocean, was causally linked to the initial diversification of large animals. What has largely been absent, but is provided in this study, is a multi-proxy stratigraphic test of this hypothesis. Here, we describe a coupled geochemical and paleontological investigation of Neoproterozoic sedimentary rocks from northern Russia. In detail, we provide iron speciation data, carbon and sulfur isotope compositions, and major element abundances from a predominantly siliciclastic succession (spanning>1000 m) sampled by the Kel'tminskaya-1 drillcore. Our interpretation of these data is consistent with the hypothesis that the pO2 threshold required for diversification of animals with high metabolic oxygen demands was crossed prior to or during the Ediacaran Period. Redox stabilization of shallow marine environments was, however, also critical and only occurred about 560 million years ago (Ma), when large motile bilaterians first enter the regional stratigraphic record. In contrast, neither fossils nor geochemistry lend support to the hypothesis that ecological interactions altered the course of evolution in the absence of environmental change. Together, the geochemical and paleontological records suggest a coordinated transition from low oxygen oceans sometime before the Marinoan (∼635 Ma) ice age, through better oxygenated but still redox-unstable shelves of the early Ediacaran Period, to the fully and persistently oxygenated marine environments characteristic of later Ediacaran successions that preserve the first bilaterian macrofossils and trace fossils.
... 3 The carbon isotope data determined on carbonates show a depletion in 13 C toward the base of the sequence, characterizing a negative excursion. Ž According to many authors Knoll et al., 1986; Kaufman et al., 1991; Derry et al., 1992; Knoll and . Walter, 1992; Smith et al., 1994 the negative excursion indicates that the deposition took place during or soon after a glacial event. ...
A U–Pb study was carried out on carbonate rocks from the Neoproterozoic Bambuı́ Group in the southern part of the São Francisco basin, Brazil. Pb isotopic compositions and U and Pb concentrations were determined on more than 90 samples from different parts of the basin. These samples were found to contain four distinct types of Pb, here called Types I, II, III and IV. Type I Pb was found in samples with low Pb concentrations and relatively high U concentrations; it represents in situ growth of radiogenic Pb and has a large enough variation to define Pb/Pb isochron ages. Type II Pb is present in samples with relatively high Pb concentrations and low U concentrations; it is nonradiogenic crustal Pb that could either represent average crustal Pb at the time of deposition or, as interpreted here, at the time of deformation of these rocks. Type III Pb is also found in samples with high Pb concentrations and low U concentrations, but it apparently is radiogenic crustal Pb from the Archean or Paleoproterozoic basement that was incorporated into the carbonates during the 600 Ma Brasiliano orogeny. Type IV Pb is intermediate in composition between Type III and Type I Pb; it represents a mixture of these two types.
... Many believe these environmental perturbations to be responsible for the changing conditions in the biosphere that facilitated the radiation of metazoan life (animals) towards the end of the Neoproterozoic (Fike et al., 2006; Kennedy et al., 2006; Knoll and Caroll, 1999; Peltier et al., 2007) (Fig. 1). The carbon isotopic record of ancient ocean water preserved in marine carbonates serves as an important means to recognize changes in the exogenic cycle coincident with these evolutionary events (Knoll et al., 1986; Magaritz, 1991; Peltier et al., 2007; Schidlowski and Aharon, 1992). The Neoproterozoic d 13 C record shows several unprecedented fluctuations (Halverson et al., 2005; Le Guerroué et al., 2006c; Shields and Veizer, 2002) (Fig. 1 ) with implications especially interesting in the Ediacaran interval (635–542 Ma), during which atmospheric oxygen concentrations were likely to have been particularly dynamic (Bartley and Kah, 2004; Rothman et al., 2003) (Fig. 1), ultimately stabilizing closer to modern levels (Canfield and Teske, 1996; Kah et al., 2004). ...
Carbonate δ 13C values provide a useful monitor of
changes in the global carbon cycle because they can record the burial
ratio of organic to carbonate carbon. The most pronounced isotope
excursions in the geologic record occur during the Neoproterozoic and
have assumed a central role in the interpretation of biogeochemical
events preceding the Ediacaran and Cambrian radiations. The most
profound negative carbon isotope excursion is best recorded in the
Ediacaran-aged Shuram Formation of Oman and has potential equivalents
worldwide including the Wonoka Formation of South Australia and other
sections in China, India, Siberia, Canada, Scandinavia and Brazil. All
these excursions are less well understood than those in the Phanerozoic
because of their unusual magnitude, long duration (> 1 Ma) and the
difficulty in correlating Neoproterozoic basins to confirm independently
that they do indeed record global change in the mixed ocean reservoir.
Alternatively, these δ 13C anomalies could reflect
diachronous diagenetic processes. Currently none of these excursion are
firmly time constrained and critical to their interpretation is a
coherent reproducibility and synchroneity at the global ocean scale.
Here we use available strontium isotope record as an independent
chronometer to test the timing and synchroneity of the Shuram δ
13C and its potential equivalents. The use of the
86Sr/ 87Sr ratio allows the reconstruction of a
coherent, global δ 13C record calibrated independently
against time. The calibrated δ 13C curve indicates that
the Shuram negative anomaly spans several tens of millions of years and
reaches values below -10‰. This carbon isotopic anomaly therefore
represents a meaningful oceanographic event that fundamentally
challenges our understanding of the carbon cycle as defined in the
Phanerozoic.
Reconstruction of the global biological events in the Precambrian requires a multidisciplinary approach. Data from the poor fossil record of the prokaryotes has to be supplemented by the interpretation of some sedimentological, geochemical and paleoclimatic phenomena as the biologically controlled processes. Biochemical and ecological conservatism of the prokaryotic ecosystems makes it possible to decode their signals documented in the Precambrian geological history. Expansion of the eukaryotes was the cause of the strong transformation of the fossil record as a whole. In addition to the biological innovation represented by the growth and change in the taxonomic diversity and cell (or body) size of the organisms, there were other global bio-events connected with the restructuring of the ecosystems, colonization of new environments, and rise of new physiologies which strongly affected the sedimentological and taphonomic processes.
More than 60 global “events” have been identified in the isotopic records of ∂
13C, ∂
18O, ∂
34S and 87Sr/86Sr. Over half of these are carbon isotopic events, about equally divided between positive excursions (or a simple rise) and negative excursions (or a fall). The positive excursions generally have been ascribed to “oceanic anoxic events” or similar incidents of gross storage of organic carbon, and negative isotopic events have been related to a catastrophic reduction of primary productivity, but many records suggest a complex origin. Likewise strontium isotopic shifts are generally related to changes in the balance of inputs to the ocean of light strontium from reaction with MOR basalts and of heavy strontium from uplift and erosion of old cratonic terranes. Despite this variety of origins, most of the isotopic events of these elements have potential for worldwide stratigraphic correlation, with resolution that may equal or exceed that of biostratigraphy. Diagenetic distortion of isotopic profiles should be minimized by screening samples with appropriate textural and trace element criteria.
Sedimentary successions across the Permian–Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C–O isotopes and Mn–Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C–δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of − 1.2(± 0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‘Boundary Clay’ in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by − 2.1(± 0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~ 50–80 °C and water/rock ratios of < 10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon — at a global scale — that has received relatively little attention to date.
The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ13Ccarb and δ18Ocarb as well as organic carbon δ13Corg values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ13C trends have not been studied.
The SiO2/Al2O} and K2OM2O 3 ratios are used as proxies for grain size and sea depth changes in a more than 400 m thick succession of Telychian to Ludfordian shale and marlstone of the Priekule core. Good correlation of the increased SiO 2/Al2O3 and K2O/Al2O 3 values in the Priekule section with positive δ13C excursions recognized worldwide suggests that these sea depth changes correspond to global sea level changes. The curves composed indicate a moderate long-range sea level low stand from the Sheinwoodian to the middle Ludfordian, superimposed with larger sea level falls in the lower Sheinwoodian, upper Homerian and middle Ludfordian. The Telychian and upper Ludfordian are characterized by sea level high stands. Higher sulphur contents over 1% and higher contents of carbonates occur close to the intervals interpreted as sea level low stands.
With sedimentary organic carbon (in the form of kerogen and its graphitic derivatives) dating back to 3.8 Ga ago, and the isotopic signature of autotrophic carbon fixation persisting over the same time span if allowance is made for a metamorphic overprint of the oldest (> 3.5 Ga) record, we may state with fair confidence that biological modulation of the terrestrial carbon cycle had commenced very early in geological history. Ever since then, carbon transformations in the Earth’s exogenic system have proceeded in the form of a biogeochemical cycle comprising an organic (reduced) and an inorganic (oxidized) carbon branch. Since the partitioning of the element between the reduced (kerogen) and oxidized (carbonate) moiety is coupled with the isotopic compositions of both carbon species by an isotope mass balance, fluctuations in the 13C/12C record, notably of marine carbonates, reflect changing ratios Corg/Ccarb in the total carbon flux from the surficial exchange reservoir to the sedimentary shell that has subsequently come to store the far bulk (≈6× 1022 g) of carbon residing in and above the crust. Relying on the sensor function of δ 13Ccarb, it is possible to identify numerous imbalances in the past operation of the carbon cycle which, however, appear to be demonstrably tethered to a time-invariant average state. On time scales approaching the length of currently documented Earth history, these oscillations are largely smoothed out, with the sedimentary carbon isotope record as a whole constituting a long-term index line of autotrophic carbon fixation that primarily reflects the isotopediscriminating properties of ribulose-l,5-bisphos-phate (RuBP) carboxylase, the principal CO2-fixing enzyme of the Calvin cycle.
Abstract New Re–Os ages from black shales of the Neoproterozoic Shaler Supergroup from the Minto Inlier, Victoria Island, Arctic Canada, constrain the late Tonian to early Cryogenian depositional history of the Amundsen Basin. The Re–Os ages of 761 ± 41 Ma and 848 ± 49 Ma for the Wynniatt Fm., and 892 ± 13 Ma for the Boot Inlet Fm., along with recently reported U-Pb ages, provide anchor points for correlation of stable isotope stratigraphy, most notably the Bitter Springs isotopic anomaly within the Wynniatt Fm. across the Tonian–Cryogenian transition in NW Canada. These data suggest that deposition of lower units of the Shaler Supergroup began earlier than previously estimated. Initial 187Os/188Os for all three sample suites is around 0.6, lower than modern seawater, but significantly higher than unradiogenic Archean/Paleoproterozoic seawater composition, supporting a Proterozoic transition from mantle-dominated Os input into the global ocean towards a crust-dominated flux.
Many negative δ(13)C excursions in marine carbonates from the geological record are interpreted to record significant biogeochemical events in early Earth history. The assumption that no post-depositional processes can simultaneously alter carbonate and organic δ(13)C values towards more negative values is the cornerstone of this approach. However, the effects of post-depositional alteration on the relationship between carbonate and organic δ(13)C values have not been directly evaluated. Here we present paired carbonate and organic δ(13)C records that exhibit a coupled negative excursion resulting from multiple periods of meteoric alteration of the carbonate δ(13)C record, and consequent contributions of isotopically negative terrestrial organic matter to the sedimentary record. The possibility that carbonate and organic δ(13)C records can be simultaneously shifted towards lower δ(13)C values during periods of subaerial exposure may necessitate the reappraisal of some of the δ(13)C anomalies associated with noteworthy biogeochemical events throughout Earth history.
Sediments are reliable records of changes in physical, chemical, and biological conditions that take place before, during, and after their deposition and express the changes through constituent mineralogical and thus geochemical compositions. Individual sedimentary events create more or less homogeneous bulk chemistry of sediments at varying temporal and spatial scales. Distinguishing these homogeneities and for classification of stratigraphic records and correlation of the strata at varying spatiotemporal scales is emerging to be a reliable method of stratigraphy and is termed as chemostratigraphy a la chemical stratigraphy. This method helps stratigraphic correlation with ease where other formal stratigraphic methods have limitations or fail to achieve required spatiotemporal resolution.The study of geochemical variations in stratigraphic context has gained importance since the 1980s. Chemostratigraphy is, thus relatively a younger branch of geosciences. Attempts on distinguishing depositional units at varying spatiotemporal scales (from local to global and from tidal cycles to few tens of millions of years) have been influenced to a larger extent by the sequence stratigraphic concepts. Contemporaneous developments in sophisticated instrumentation for fast, accurate, and less expensive geochemical analyses have also contributed to the popularity and applications of chemostratigraphy. From a humble beginning of identification of similar geochemical values and similar pattern of geochemical profile, chemostratigraphy has traveled a long way. Currently, a wide variety of techniques and data from other subdisciplines of geosciences are used for distinction/recognition and correlation chemozones/geochemically distinguishable depositional units.Yet, chemostratigraphy consists of vaguely defined and often misleading and/or overlapping terminologies. Through an extensive review of published literature, this chapter attempts to enlist these terminologies namely, chemostratigraphy, chemical stratigraphy, geochemical fingerprinting, geochemical signature, geochemical fingerprint, geochemical marker, geochemical proxy, excursion, shift, fluctuation, perturbation, anomaly, trend, chemostratigraphic index, chemozone, chemochron, resolution, and scale of correlation and provides definitions/explanations. This attempt is made for initiating discussion among the practitioners that may lead to consensus on definitions and standardized usage.Despite, fulfilling the criteria required for any standard stratigraphic method and finding its applications in many different fields, this method/tool remains to be formally given its due. Elucidation of the traits and enlisting the terminologies of chemostratigraphy with the criteria for formal recognition prescribed by International Stratigraphic Commission suggests that chemostratigraphy deserves to be formalized as an independent stratigraphic method.
Carbonate rocks from the Middle Ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in Earth history. This finding suggests a dramatic reorganization of Earth's carbon cycle. Named the Shuram excursion for its original discovery in the Shuram Formation, Oman, the anomaly closely precedes impressive events in evolution, including the rise of large metazoans and the origin of biomineralization in animals. Instead of a true record of the carbon cycle at the time of sedimentation, the carbon isotope signature recorded in the Shuram excursion could be caused by alteration following deposition of the carbonate sediments, a scenario supported by several geochemical indicators. However, such secondary processes are intrinsically local, which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe. Both possibilities are intriguing: if the Shuram excursion preserves a genuine record of ancient seawater chemistry, it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern Earth. If, however, it represents secondary alteration during burial of sediments, then marine sediments must have been globally preconditioned in a unique way, to allow ordinary and local processes to produce an extraordinary and widespread response.
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