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Miocene seawater 187 Os/ 188 Os ratios inferred from metalliferous carbonates
Abstract
Seawater 187 Os/ 188 Os ratios for the Middle Miocene were reconstructed by measuring the 187 Os/ 188 Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187 Os/ 188 Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187 Os/ 188 Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187 Os/ 188 Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87 Sr/ 86 Sr ratios at 16 Ma. A large increase of 1 in benthic foraminiferal 18 O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate 13 C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187 Os/ 188 Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187 Os/ 188 Os ratios and also nearly constant 87 Sr/ 86 Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO 2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187 Os/ 188 Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187 Os/ 188 Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187 Os/ 188 Os variations may correlate inversely with short-term benthic foraminiferal 18 O and bulk carbonate 13 C variations that reflect glacioeustatic events.
... Seafloor magnetic records indicate high crustal production rates between 19 and 14 Ma 25,26 (Fig. 1d). Substantial amounts of CO 2 2.5 36,38 , and 598 36,38 ). Atmospheric pCO 2 was estimated based on the density of stomata in fossil leaves 15 , δ 11 B of planktonic foraminifer 16,17,19 , δ 13 C of alkenones 14,20 , and δ 13 C of paleosols 18 could accumulate in the atmosphere due to this volcanism, although the reconstructed crustal production rates do not show an increasing trend associated with the MCO 22,25,26 . ...
... Seafloor magnetic records indicate high crustal production rates between 19 and 14 Ma 25,26 (Fig. 1d). Substantial amounts of CO 2 2.5 36,38 , and 598 36,38 ). Atmospheric pCO 2 was estimated based on the density of stomata in fossil leaves 15 , δ 11 B of planktonic foraminifer 16,17,19 , δ 13 C of alkenones 14,20 , and δ 13 C of paleosols 18 could accumulate in the atmosphere due to this volcanism, although the reconstructed crustal production rates do not show an increasing trend associated with the MCO 22,25,26 . ...
... Miocene seawater 187 Os/ 188 Os records derived from contemporaneous pelagic sediments have been previously reported [36][37][38][39] (Fig. 1e, IODP Sites U1336, 1337, and 1338 in the eastern equatorial Pacific Ocean were drilled to collect Neogene sediments 40 . Continuous carbonate-rich successions were recovered from these sites. ...
The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum. The Miocene climatic optimum could have been triggered by coeval increase in magmatism, suggests global carbon cycle simulations constrained by seawater osmium isotope data from Pacific Ocean sediments.
... Paleoproterozoic sedimentary successions record dramatic paleoenvironmental changes in the exosphere between 2.45 and 2. 22 Ga [1,2], including: (1) the rise of atmospheric oxygen, (2) compressional tectonism on the margins of the oldest recognized supercontinent, Kenorland, and an accompanying superplume event in the interior and on the periphery of the supercontinent at 2.48-2. 42 Ga, and (3) a glacial epoch with three documented glaciations extending to low latitudes, two of which are recognized in South Africa. ...
... Modern seawater has a 187 Os/ 188 Os ratio of about 1 [13,14], slightly below the average composition of currently eroding upper continental crust [15]. Several studies document spatial or temporal excursions in seawater 187 Os/ 188 Os resulting from local input of low 187 Os/ 188 Os hydrothermal fluid [13,16,17], large impacts of extraterrestrial material [18], and variable rates of chemical weathering [5,[19][20][21][22]. Most important is oxidative weathering of sulfidic, carbonaceous sedimentary rocks, a key reservoir for crustal Re, and therefore of radiogenic 187 Os [6,18,23,24]. ...
... Similar negative d 13 C values are found in carbonates above the middle of three glacial diamictites of the Huronian and Snowy Pass supergroups, ON and WY [33], but a positive d 13 C excursion has not yet been recognized in any other succession at the same stratigraphic level. The age of both excursions is constrained only between 2.45 and 2. 22 Ga. ...
Re-Os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the Rooihoogte and Timeball Hill formations, Transvaal Supergroup, South Africa, provides a precise isochron age of 2316 +/- 7 Ma ( +/-4 statistical uncertainty if error on decay constant is excluded) and a chondritic initial (187)Os/(188)Os ratio of 0.1121 +/- 0.0012. These units were deposited between what are interpreted as the second and third of three Paleoproterozoic global glacial events, and thus provide minimum and maximum ages, respectively, for these events. The Rooihoogte Formation is correlative with the Duitschland Formation, which records previously undated carbon isotope excursions. Because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative delta(34)S values, the rise of atmospheric oxygen most likely began prior to 2.32 Ga. The chondritic initial (187)OS/(188)Os ratio requires that primitive hydrothermal sources of Os dominated the Os budget of seawater at 2.32 Ga. Os is mobile only under oxidizing conditions. Therefore, the chondritic initial (187)Os/(188)Os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic Os to offset the primitive hydrothermal/magmatic flux. Even if atmospheric oxygen levels had increased significantly by 2.32 Ga, anoxic conditions throughout the Archean most likely limited Os enrichment in Archean marine shales, so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic Os to Paleoproterozoic seawater.
... Models of the sedimentary Os budget that describe Os isotope composition of bulk sediment as a three-component mixture of hydrogenous, lithogenic and extraterrestrial Os (Esser and Turekian, 1988) were used to argue that the vast majority of Os in these metalliferous sediments was hydrogenous in origin (Ravizza, 1993). The similarity in 187 Os/ 188 Os of recently deposited metalliferous sediments (Ravizza and McMurtry, 1993; Ravizza et al., 1996) with the 187 Os/ 188 Os of seawater, and of the similarity of bulk sediment and leach Os isotope data from the same sample (Peucker-Ehrenbrink et al., 1995; Reusch et al., 1998) support this conclusion. Several independent approaches have suggested that the marine residence time of Os was longer than the mixing time of the modern ocean (Palmer et al., 1988), but substantially shorter than that of Sr (strontium) (Richter and Turekian, 1993; Peucker-Ehrenbrink and Ravizza, 1996; Levasseur et al., 1999a). ...
... Several independent approaches have suggested that the marine residence time of Os was longer than the mixing time of the modern ocean (Palmer et al., 1988), but substantially shorter than that of Sr (strontium) (Richter and Turekian, 1993; Peucker-Ehrenbrink and Ravizza, 1996; Levasseur et al., 1999a). The rapid accumulation rate of metalliferous carbonates, approximately 1 cm/kyr, allowed investigation of the marine Os isotope record on shorter time scales from 10 4 to 10 5 yrs (Oxburgh, 1996Oxburgh, , 1998 Reusch et al., 1998). These studies provided evidence of high frequency variations in seawater 187 Os/ 188 Os, suggesting that the marine residence time of Os was poised within the range of 10 000 to 50 000 years; i.e. sufficiently long that Os is well mixed in the global ocean but short enough to record the globally integrated change in oceanic inputs associated with glacialeinterglacial cycles. ...
... Analytical uncertainties are smaller than the symbols. are less radiogenic than contemporaneous seawater (Ravizza, 1993; Reusch et al., 1998). These carbonates show 3 He enrichments that may indicate enhanced delivery of extraterrestrial 3 He by a meteor shower in the late Miocene (Farley et al. 2006). ...
Osmium, an ultra-trace element in seawater, is subject to strong authigenic enrichment in reducing marine sediments, and to a lesser degree of enrichment in oxic marine deposits. Temporal variations in sedimentary 187 Os/ 188 Os ratio preserve a rich archive from several types of deposi-tional environments that reflect changes on the Earth's surface. This chapter presents a compilation of marine Os isotope records from the Pleistocene to the Precambrian, and reviews the interpretations of these temporal variations, focusing on the most highly resolved events in the Cenozoic and Mesozoic. Chapter Outline 8.
... Investigation of the Margi umbers, metalliferous sedimentary rocks associated with the Troodos ophiolite in Cyprus, lays a foundation for extending the marine Os isotope record beyond the Cenozoic into the Mesozoic and Paleozoic. These middle Cretaceous deposits, now subaerially exposed, are known to result from the accumulation of metal-rich hydrothermal precipitates from the water column (Robertson, 1975) and are analogous to metalliferous sediments recovered from the present-day ocean (Boyle, 1990) that have been used to reconstruct the Cenozoic seawater 187 Os/ 188 Os record (Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Reusch et al., 1998). Compositional similarity between the Margi umbers and other metalliferous sediments known to accurately record the Os isotope composition of present-day and Cenozoic seawater provides evidence that depositional signatures carried in metalliferous sediments can survive both burial diagenesis and emplacement on land. ...
... Pacific Plume particles (Sherrell et al. 1999 Pelagic clay leach ( Pegram et al., 1992;Turekian and Pegram, 1997) Metalliferous sediment (Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Reusch et al., 1998) Initial ratios from black shale isochrons (Cohen et al., 1999) 0 marine Sr and Os isotope records. Specifically, the Cretaceous and Jurassic Os data shown are similar to or more radiogenic than Paleogene Os data. ...
... The opposite is true of the marine Sr isotope record. This observation augments the growing body of data showing decoupling of the marine Os and Sr isotope records (Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Turekian and Pegram, 1997;Reusch et al., 1998), and points to differences in the underlying processes that control them. ...
The major element, rare earth element, and platinum-group element
chemistry of the Margi umbers have been investigated in order to
evaluate these sediments as recorders of past variations in the Os
isotopic composition of seawater. Because of the compositional
similarity of the Margi umbers to metalliferous sediments and
hydrothermal-plume particles from the modern ocean, we conclude that
these supraophiolite sediments record the Os isotope composition of
Cretaceous seawater. This conclusion requires that the Margi umbers have
in large part retained their compositional integrity throughout burial
diagenesis and subsequent uplift. That the
187Os/188Os ratio (0.52 0.55) of ca. 90 Ma
seawater is similar to, or more radiogenic than, seawater throughout the
Paleogene provides evidence of decoupling of the marine Sr and Os
isotope records. Beyond constraining the
187Os/188Os of Cretaceous seawater, these results
are significant in that they provide a set of chemical criteria that can
be used to evaluate the potential of other metalliferous sediment
sequences as recorders of the 187Os/188Os ratio of
ancient seawater. Establishing these criteria represents an important
step toward extending the marine Os isotope record into the Mesozoic and
Paleozoic.
... The age model constructed here is based on Os isotope data and follows a probabilistic approach. As the data compilation of 187 Os/ 188 Os seawater ratios over the Cenozoic and Late Cretaceous ( Fig. 5A) (Klemm et al., 2005;Pegram and Turekian, 1999;Peucker-Ehrenbrink and Ravizza, 2012;Ravizza and Peucker-Ehrenbrink, 2003;Reusch et al., 1998) lacks the temporal resolution for a precise, continuous record, due to Os residence time in seawater, an envelope was constructed based on an approximation of the average Os isotopic composition of seawater at a given time. As a result, one 187 Os/ 188 Os data point can fit many positions within the envelope and additional data is required to improve the 187 Os/ 188 Os seawater record. ...
... While the methodology and accuracy of age-depth models derived Reusch et al. (1998), Pegram and Turekian (1999), Ravizza and Peucker-Ehrenbrink (2003), Klemm et al. (2005), Peucker-Ehrenbrink and Ravizza (2012), defining the seawater 187 Os/ 188 Os evolution over the Cenozoic and Late Cretaceous. (B) 187 Os/ 188 Os vs. depth in core 085_004 with hiatuses and macro layers boundaries identified in Fig. 2. Note that the uncertainty of the 187 Os/ 188 Os data is smaller than the symbol. ...
Accurate age models for marine ferromanganese (Fe-Mn) crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity. However, no absolute method exists for dating these deposits beyond the age of 10 Myr, which requires the combination of a number of approaches. Here, we present a composite age model for a 15 cm thick Fe-Mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m, on the summit of Tropic Seamount, in the north-east Atlantic. The age model is based on cross-validation of laser-ablation U-Pb dating, Co-chronometry and Os isotopes. These enable robust calibration of the age-depth model using the Bayesian statistical modelling of Markov Chain Monte Carlo (MCMC) simulations. The results show that this Fe-Mn crust commenced growth in the Late Cretaceous between 73 and 77 Ma, and grew at a rate between 1 and 24 mm/Myr, averaging 4 mm/Myr. The phosphatised carbonate substrate, capping Tropic Seamount and underlying most of the Fe-Mn crusts, yields a U-Pb age of 84 ± 4 Myr, and provides the upper age limit for the model. Less radiogenic excursions of ¹⁸⁸ Os/ ¹⁸⁷ Os in the vertical profile through the crust permit the identification of key inflection points in the Os isotope seawater curve at the Eocene-Oligocene and Cretaceous-Paleogene transitions. Growth rates estimated from the empirical Co-chronometer are combined with the age envelope defined by the Os data and used to validate the MCMC simulations. The model identifies five hiatuses that occurred during the Pliocene (2.5 ± 1.9–5.3 ± 1.7 Ma), Early Miocene (16 ± 1–27 ± 2 Ma), Oligocene (29 ± 2–32 ± 1 Ma), Eocene (41 ± 2–52 ± 0.6 Ma), and the Late Paleocene (55 ± 1–59 ± 1.4 Ma). A major phosphatisation event affecting the Fe-Mn core can be dated to the Late Eocene (38 ± 1.2 Ma), which coincides with a recorded change in the global oceanic system, from warm and sluggish circulation to cold and vigorous thermohaline-driven meridional overturn at the onset of Antarctic glaciation. © 2019 United Kingdom Research and Innovation, as represented by its component body, the British Geological Survey
... -des sédiments métallifères (Oxburgh, 1998 ;Reusch et al., 1998), -des argiles pélagiques , ...
... Les teneurs élevées en Re et en Os de certains carbonates des TSS et une tendance positive entre la [Os] et 1870S/1880S et la teneur en carbonates des sédiments de rivière (Figure 5) nous a conduit à considérer l'hypothèse que les carbonates pourraient jouer un rôle significatif dans le budget d'Os et de Re. Si certains auteurs s'accordent à dire que les carbonates ne sont pas une source majeure d'Os ou de Re(Reusch et al., 1998),Levasseur (1999) montre que les carbonates sont une source significative de l'Os dissous. Les analyses de carbonates provenant des trois formations montrent que cette lithologie doit être prise en compte dans le budget de Re et d'Os continental. ...
Cette étude a permis de déterminer la composition isotopique d'Os des trois principales formations himalayennes (TSS, HHC et LH) à partir de roches sources, ainsi que celle de sédiments de rivières qui représentent le produit d'érosion de la chaîne. Les formations du Haut Himalaya ont un rapport 187 0s/188 0s comparable à celui de la croûte continentale moyenne. Par contre, le LH se distingue par des rapports radio géniques rencontrés dans deux lithologies distinctes: les schistes noirs et les marbres impurs du LH. Un mélange de trois pôles (silicates du Haut Himalaya + schistes noirs du LH + carbonates impurs du LH) permet d'expliquer les signatures des sédiments de rivières à la sortie de la chaîne ou dans le Gange. Les sédiments du Brahmapoutre montrent un rapport moins radio génique que ceux du Gange, expliqué d'une part par la présence d'affleurement d'ophiolites dans le bassin versant du Brahmapoutre et d'autre part par l'absence de carbonates résiduels. L'origine des carbonates radiogéniques dans le LH est à relier au métamorphisme himalayen qui a permis la mobilisation d'Os et de Re à partir des schistes noirs radio géniques du LH. L'altération à basse température étudiée à partir de sols a montré que les compositions isotopiques et les concentrations d'Os et de Re ne varient pas avec la profondeur. Cependant une perte importante d'Os et de Re a probablement lieu dans les premiers stades de l'altération, lors du passage de la roche saine au saprolite. Les sédiments récents du Cône du Bengale montrent que l'érosion de la chaîne himalayenne est la source majeure des sédiments, surtout dans le cône actif. Cependant, dans le reste de la Baie du Bengale, cette source himalayenne peut être masquée par des sources secondaires, comme l'érosion de la chaîne indo-birmane ou l'érosion du Sri Lanka. Il n'y a aucune preuve qu'une perte significative d'Os lors du transport ou après le dépôt dans le Cône se produise. Ceci implique que la contribution de l'érosion himalayenne au budget de l'Os marin est limitée à la phase dissoute, et donc insuffisante pour expliquer l'augmentation du rapport 187 0s/188 0s marin durant les 16 derniers Ma.
... Here, we present a combined tectonic-geochemical model that links earlier uplift of the LH to the Neogene paleoseawater record of 187 Os/ 188 Os and 87 Sr/ 86 Sr (Ravizza, 1993;Oslick et al., 1994;Peucker-Ehrenbrink et al., 1995;and Reusch et al., 1998). The 187 Os/ 188 Os record shows a period of stability from about 29 Ma to 16 Ma (Fig. 4) followed by a monotonic rise starting at approximately 16 Ma. ...
... Based on discussions The strontium isotope data can similarly be fit with two linear segments, from 22.2 to 15.8 (dR/dt = 6.86 × 10 −5 Ma −1 ) and from 15.5 to 9.2 Ma (dR/dt = 2.67 × 10 −5 Ma −1 ). Data sources: Ravizza (1993), Oslick et al. (1994), Peucker-Ehrenbrink et al. (1995, and Reusch et al. (1998). above, we postulate that initial uplift and exhumation of upper Lesser Himalayan strata (possibly the oLH) took place at ∼16 Ma, and that it is recorded in marked inflections in the marine Os and Sr records at that time. ...
... In the former case, the initial ratio of 0.621 ± 0.013 is somewhat smaller than the Miocene seawater values. It is now well established that the Os isotopic compositions of seawater has changed considerably through the last 60 Ma (Peucker-Ehrenbrink et al., 1995;Reusch et al., 1998). Miocene metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) have 187 Os/ 188 Os that range from about 0.73 to 0.78 (Reusch et al., 1998), and the 187 Os/ 188 Os ratios for the middle Miocene metalliferous sediments from the Pacific are 0.732 and 0.847 (Peucker-Ehrenbrink et al., 1995). ...
... It is now well established that the Os isotopic compositions of seawater has changed considerably through the last 60 Ma (Peucker-Ehrenbrink et al., 1995;Reusch et al., 1998). Miocene metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) have 187 Os/ 188 Os that range from about 0.73 to 0.78 (Reusch et al., 1998), and the 187 Os/ 188 Os ratios for the middle Miocene metalliferous sediments from the Pacific are 0.732 and 0.847 (Peucker-Ehrenbrink et al., 1995). These studies suggest that the 187 Os/ 188 Os ratio of seawater at 15 Ma was approximately 0.73 to 0.85. ...
Re-Os dating method was applied to the Kuroko ores from of the Wanibuchi Mine, southwestern Japan. Two whole-rock Kuroko ores and two yellow color fragments separated from one Kuroko ore sample were analyzed. Their Re and Os concentrations range from 3.52 to 50.6 ppb and from 20.1 to 92.9 ppt, respectively, and their 187Os/188Os ratios vary from 0.911 to 1.609. These data yield an isochron age of 18.44 ± 0.60 Ma with initial 187Os/188Os of 0.621 ± 0.013. The age is older than the inferred Kuroko formation age of northeastern Kuroko deposits, and also about three Ma older than the time of the clockwise rotation of southwestern Japan (ca. 15 Ma). The Wanibuchi Kuroko deposits are considered to have formed at the waning stage of the first phase (28-18 Ma) of the rifting and seafloor spreading in the Japan Sea area. The initial 187Os/188Os ratio might reflect the contemporaneous local seawater ratio that was more affected by mantle-derived Os resulted from the volcanic activities of Japan Sea formation than the typical oceanic Os ratio.
... The calculated 187 Os/ 188 Os initial ratios for the manganese ores from the Lucifer deposit range from 0.43 to 0.51, whereas those for the Boléo district range from 0.70 to 0.74 (Fig. 8). -Ehrenbrink et al. 1995;Woodhouse et al. 1999), average rivers (Peucker-Ehrenbrink and Ravizza, 2000), mean continental crust (Saal et al. 1998), metalliferous sediments from the Atlantic Ocean (Reusch et al. 1998), high-temperature hydrothermal fluids from the Juan de la Fuca ridge (Sharma et al. 1999), mid-ocean ridge basalts (Snow and Reisberg 1995;Roy-Barman et al. 1998;Hart et al. 1999;Escrig et al. 2005), volcanic rocks from the Michoacan Guanajuato Volcanic Field in the Trans-Mexican volcanic belt (Chesley et al. 2002), and Arizona Upper Crust (Saal et al. 1998). Figure 8 illustrates that the 187 Os/ 188 Os ratios of the manganese deposits are less radiogenic than seawater at 7 Ma. ...
... The mineralizing solutions ascended through these structures, as indicated by the presence of veinlets containing manganese and copper minerals along many of the faults around the Santa Rosalía district. The close spatial association of the manganese mantos with the brecciated jasper and Fe-oxide mineral zones and with the NW-SE structures suggests that the mineralization was emplaced in Ocean (Reusch et al. 1998), high-temperature hydrothermal fluids from Juan de la Fuca ridge (Sharma et al. 1999), mid-ocean ridge basalts (Snow and Reisberg 1995;Roy-Barman et al. 1998;Hart et al. 1999;Escrig et al. 2005), volcanic rocks from Michoacan Guanajuato volcanic field in the Trans-Mexican volcanic belt (Chesley et al. 2002), and Arizona Upper Crust (Saal et al. 1998) structurally controlled isolated basins by structurally controlled feeder zones. The manganese oxide mineralogy and geochemistry supports a hydrothermal origin. ...
The manganese ores in the Santa Rosalía region, western Mexico, are mainly stratiform horizons or mantos, constrained to the
initial stages of sedimentary cycles of the Miocene Boléo Formation. The manganese mineralization is generally restricted
to isolated paleo-basins and related to NW–SE faults formed during the early stages of the opening of the Gulf of California.
Jasper, Fe, and Mn oxides associated to the NW–SE structures may represent feeder zones for the mineralized system. The manganese
oxide minerals include pyrolusite, cryptomelane, todorokite, hollandite, jacobsite, and pyrochroite. Trace elements in the
manganese ores indicate a hydrothermal origin for the deposits of the Santa Rosalía area. Rare earth elements (REE) patterns
obtained for manganese minerals from the Lucifer and El Gavilán deposits also support a hydrothermal origin, whereas the middle
REE enrichment observed in samples from the Boléo district indicates mixing between hydrothermal and hydrogenous sources.
Osmium and rhenium concentrations of the manganese minerals range between 33–173ppt and 0.14–89ppb, respectively. The initial
187Os/188Os ratios in the manganese oxides from Lucifer and the Boléo district range between 0.43 to 0.51 and 0.70 to 0.74, respectively.
These ratios are different from seawater at 7Ma (0.84–0.89), which suggests important contributions of osmium from underlying
rocks such as the Miocene volcanic rocks and the Cretaceous quartz–monzonite basement. Field evidence, manganese oxide mineralogy
coupled with major and trace element geochemistry and Re–Os systematics support a hydrothermal origin for the manganese deposits
from the Santa Rosalía region. The ore deposition style indicates an exhalative-intraformational environment restricted to
isolated basins in a diagenetic stage related to the initial evolution of the Gulf of California.
... The pattern of change we find in δ 11 B sw is broadly similar to temporal trends in a number of other related biogeochemical signals, such as oceanic 87/86 Sr, 187/188 Os, and δ 7 Li. Here we take published 87/86 Sr and δ 7 Li from Misra and Froelich (2012) Ravizza (1993); Ravizza and Turekian (1992); Ravizza andPeucker-Ehrenbrink (2003b, 2003a); Reusch et al. (1998); Robinson et al. (2009). For each signal, we assume that data constraints have Gaussian uncertainty (with magnitude given with original estimates, except in the case of 87/86 Sr where an illustrative uncertainty is used), which allows us to perform a straightforward Gaussian Process reconstruction, with the residence time of each element informing the length scale used ( 87/86 Sr-5.1 Myr (Broecker & Peng, 1982), δ 7 Li-2.8 ...
The boron isotope ratio of seawater (δ¹¹Bsw) is a parameter which must be known to reconstruct palaeo pH and CO2 from boron isotope measurements of marine carbonates. Beyond a few million years ago, δ¹¹Bsw is likely to have been different to modern. Palaeo δ¹¹Bsw can be estimated by simultaneously constraining the vertical gradients in foraminiferal δ¹¹B (Δδ¹¹B) and pH (ΔpH). A number of subtly different techniques have been used to estimate ΔpH in the past, all broadly based on assumptions about vertical gradients in oxygen, and/or carbon, or other carbonate system constraints. In this work we pull together existing data from previous studies, alongside a constraint on the rate of change of δ¹¹Bsw from modeling. We combine this information in an overarching statistical framework called a Gaussian Process. The Gaussian Process technique allows us to bring together data and constraints on the rate of change in δ¹¹Bsw to generate random plausible evolutions of δ¹¹Bsw. We reconstruct δ¹¹Bsw, and by extension palaeo pH, across the last 65Myr using this novel methodology. Reconstructed δ¹¹Bsw is compared to other seawater isotope ratios, namely Sr87/86 ${}^{87/86}\mathrm{S}\mathrm{r}$, Os187/188 ${}^{187/188}\mathrm{O}\mathrm{s}$, and δ⁷Li, which we also reconstruct with Gaussian Processes. Our method provides a template for incorporation of future δ¹¹Bsw constraints, and a mechanism for propagation of uncertainty in δ¹¹Bsw into future studies.
... Other applications of Os isotopic chemostratigraphy. In addition to organic-rich sediments, Os isotopes are measured in Fe-Mn crusts and nodules to reconstruct Cenozoic seawater trends (Koide et al., 1991;Reusch et al., 1998;Burton et al., 1999;Klemm et al., 2005Klemm et al., , 2008Meng et al., 2008;Goto et al., 2023). Although slow growth rates (millimeters per millions of years) result in low-resolution records, aligning the Os isotopic profiles of Fe-Mn crusts with the global marine reference record of Cenozoic 187 Os/ 188 Os generated from sedimentary rocks enables the generation of growth curves for Fe-Mn nodules, which are important archives of paleo-seawater chemistry (Josso et al., 2019;Peucker-Ehrenbrink and Ravizza, 2020). ...
The rhenium-osmium (187Re-187Os) system is a highly versatile chronometer that is regularly applied to a wide range of geological and extraterrestrial materials. In addition to providing geo- or cosmo-chronological information, the Re-Os system can also be used as a tracer of processes across a range of temporal (millennial to gigayear) and spatial scales (lower mantle to cryosphere). An increasing number of sulfide minerals are now routinely dated, which further expands the ability of this system to refine mineral exploration models as society moves toward a new, green economy with related technological needs. An expanding range of natural materials amenable to Re-Os geochronology brings additional complexities in data interpretation and the resultant translation of measured isotopic ratios to a properly contextualized age. Herein, we provide an overview of the 187Re-187Os system as applied to sedimentary rocks, sulfides, and other crustal materials and highlight further innovations on the horizon. Additionally, we outline next steps and best practices required to improve the precision of the chronometer and establish community-wide data reduction procedures, such as the decay constant, regression technique, and software packages to use. These best practices will expand the utility and viability of published results and essential metadata to ensure that such data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR).
... Os isotopic ages of sample 6K#1207R01 obtained by fitting the 187 Os/ 188 Os ratios to the paleo-seawater 187 Os/ 188 Os curve. Data sources: Ravizza (1993), Oxburgh (1998), Reusch et al. (1998), Pegram and Turekian (1999), Ravizza et al. (2001), Ravizza andPeucker-Ehrenbrink (2003a, 2003b), Dalai et al., (2005Dalai et al., ( ,2006, Paquay et al., (2008Paquay et al., ( ,2014, Nielsen et al. (2009), Robinson et al. (2009), Peucker-Ehrenbrink and Ravizza (2012, Ravizza and VonderHaar (2012), van der Ploeg et al. (2018), Kuwahara et al. (2021), and Tanaka et al. (2022). ...
... ( 187 Os/ 188 Os) i values of early Permian seawater reconstructed from the Kunimiyama umber (red box plot), compared with the long-term trend of paleoseawater 187 Os/ 188 Os records since 400 Ma (light blue). Gray filled circles (Cenozoic and latest Cretaceous, 0-75 Ma):Ravizza (1993);Oxburgh (1998);Reusch et al. (1998);Pegram and Turekian (1999);Ravizza and Peucker-Ehrenbrink (2003a);Ravizza and Peucker-Ehrenbrink (2003b);Dalai et al. (2005Dalai et al. ( , 2006); Paquay et al. (2008); Nielsen et al. (2009); Robinson et al. (2009); Paquay at al. (2014); van der Ploeg et al. (2018); Kuwahara et al. (2021); and Tanaka et al. (2022). Gray open triangles (early Aptian to early Turonian, Early to Late Cretaceous, ~122-92 Ma): Matsumoto et al. (2022). ...
Strata-bound ferromanganese (Fe–Mn) deposits (umbers) are thought to have formed by precipitation of Fe–Mn oxyhydroxides derived from seafloor hydrothermal activity at a mid-oceanic ridge. Fe–Mn oxyhydroxides effectively adsorb various elements (e.g., P, V, As, rare earth elements, and Os) dissolved in seawater. Of these elements, osmium (Os) is particularly important for reconstructing the Earth’s surface environment during formation of the deposits. Here we report the Os and rhenium (Re) contents and marine Os isotopic ratio (¹⁸⁷Os/¹⁸⁸Os)i of the early Permian reconstructed from the Kunimiyama umber deposit in the Northern Chichibu Belt, central Shikoku, southwest Japan. The Kunimiyama umbers exhibit comparable to slightly lower Os (42–183 ppt) and much lower Re (1.3–49 ppt) contents than other umbers in Japanese accretionary complexes and modern seafloor metalliferous sediments, the latter of which are regarded as umber precursors. The (¹⁸⁷Os/¹⁸⁸Os)i ratio of the Kunimiyama umbers is 0.523–0.676, which is comparable to or slightly higher than the values of the middle to Late Cretaceous and early Paleogene. This work is the first report of seawater Os isotope ratios in the Panthalassa Ocean of the early Permian, representing an important achievement in understanding the marine Os isotopic record of the Paleozoic Era.
... The REY composition of the Kawame umber supports the hypothesis described above. The less negative Ce anomalies in the Kawame umber and red chert than in younger umbers and present-day hydrothermal ferromanganese sediments (Fig. 12b) indicate that conditions in Panthalassa during the Late Devonian were less oxidative than in the Late Ravizza et al. (1999Ravizza et al. ( , 2001a; deep-sea sediments, Dalai et al. (2005Dalai et al. ( , 2006, Kuwahara et al. (2021), Nozaki et al. (2019b), Ohta et al. (2020), Oxburgh (1998), Paquay et al. (2008Paquay et al. ( , 2014, Ravizza (1993), Ravizza et al. (2001b), Ravizza andPeucker-Ehrenbrink (2003a, 2003b), Reusch et al. (1998), Robinson et al. (2009) Cretaceous to Cenozoic ocean. In contrast, pelagic chert of the Upper Kellwasser Event at Bancheng, southern China, shows almost no to slightly positive Ce anomalies (i.e., Ce/Ce* ≈ 1; Ning et al., 2008;Fig. ...
The Late Devonian, during which one of the “Big Five” Phanerozoic mass extinction events occurred, was one of the most important time intervals in Earth history. Nevertheless, the paucity of deep-sea records due to subduction has hampered elucidation of the pelagic environment during the Late Devonian in Panthalassa. However, ancient hydrothermal ferromanganese sediments, which were deposited on the abyssal seafloor and then accreted onto continental margins, are preserved as umber deposits and exposed in accretionary prisms. These sediments can provide key information to characterize the paleo-ocean. An Upper Devonian (Famennian) umber deposit is exposed in Kawame Quarry, Nedamo Belt, northeastern Japan. We investigated the bulk chemical composition and osmium isotopic ratio (¹⁸⁷Os/¹⁸⁸Os) of the Kawame umber to discuss the pelagic environment of the Panthalassa Ocean during the Late Devonian. The Kawame umber shows positive correlations between Fe content and the levels of P, V, Os, and rare earth elements, suggesting that hydrothermal Fe-oxyhydroxide particles adsorbed these elements from ambient seawater in the same way as hydrothermal ferromanganese sediments in the present-day ocean. Newly obtained initial ¹⁸⁷Os/¹⁸⁸Os values of Late Devonian seawater are 0.49–0.57, lower than that of present-day seawater (~1.06) and comparable to Paleogene and Late Cretaceous values. One of the most remarkable characteristics of the Kawame umber is its markedly high Fe levels (up to 90 wt%) compared to present-day seafloor hydrothermal ferromanganese sediments and younger umber deposits in Japanese accretionary complexes. Because seawater ¹⁸⁷Os/¹⁸⁸Os values were not notably different from those during the Paleogene and Late Cretaceous, anomalously vigorous hydrothermal activity in the Late Devonian is unlikely to have caused the increased Fe deposition. The rare earth element patterns of the Kawame umber (normalized to post-Archean average Australian shale) show weaker negative Ce anomalies than those of younger umbers and present-day hydrothermal ferromanganese sediments, suggesting that the Kawame umbers were deposited under less oxidizing conditions. Considering that many red beds were deposited at ~370 Ma, during recovery from the oceanic anoxic events (OAEs) of the Late Devonian (early Famennian), the anomalous Fe enrichment in the Kawame umber likely reflects enhanced oxidization and precipitation of abundant dissolved Fe during the post-OAE transition from anoxic to oxic conditions.
... Recent studies of platinum group elements (PGE) in marine sediments have tended to concentrate on Os and the evolution of the seawater ^^^Os/^^^Os ratio through time. Studies of metalliferous sediments have examined the long-term (lO^yr) (Peucker- Ehrenbrink and Blum, 1998;Feucker-Ehrenbrink et al, 1995;Ravizza, 1993;Reusch aZ., 1998) and medium-term (lO^yr) records (Oxburgh, 1998; Ravizza a/., 2001a), while anoxic sediments have been used to construct medium and short-term records (Oxburgh, 2001;Feucker-Ehrenbrink, 1996;Ravizza et al, 1996). Studies have also patterns (e.g. ...
p>The geochemistry of a suite of cores accumulating at increasing distance down-stream of a major hydrothermal vent site has been characterized for the first time, in order to examine the changing chemistry of particles being deposited from the plume with distance from the vent, to try to establish the different processes operating in the plume, particularly with regard to scavenging of elements from seawater, and to quantify the fluxes from seawater to the sediments as a result of venting, for a wide range of major and trace elements (Mg, Fe, V, Mn, Co, Cu, Zn, REE, PGE, Os isotopes).
Sediment cores were collected at 2-25km downplume of the Rainbow hydrothermal vent site at 36<sup>o</sup>14'N on the Mid-Atlantic Ridge, a known site of vigorous high-temperature venting. Samples from long-term sediment traps located 0.5-2 km from this vent were also studied. All samples showed similar bulk chemistry, with an overprint of hydrothermally derived material. Sedimentation rates of 40-60 mg.m<sup>2</sup>d<sup>-1</sup> for the cores were established using bulk radiocarbon dating of the carbonate material, and were found to be of the same order as the sediment traps 11-25 mg.m<sup>2</sup>.d<sup>-1</sup>. The maximum age of the core samples ranged from 8-26 x 10<sup>3</sup> radiocarbon years.
For many elements, particularly the PGEs, the element to iron ratios in plume derived material were found to be up to an order of magnitude higher than those found globally in metalliferous sediments of hydrogenous origin. Burial fluxes to the Rainbow sediments have been calculated for hydrothermally derived elements - Mn (16-37 μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Fe (368-718μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Co(0.3-0.5μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Cu(3.5-12μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Zn(0.2-0.6μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), - and elements scavenged from seawater by hydrothermal Fe-oxyhydroxides - P (36-64μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), V (3-5μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), As (0.6-1.4μmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Pd(20-53pmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Os (0.1-2.3pmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Ir(0.2-0.6pmol.cm<sup>-2</sup>.kyr<sup>-1</sup>), Pt(6.5 - 18.5pmol.cm<sup>-2</sup>.kyr<sup>-1</sup>). Magnesium from seawater appears to co-precipitate with Fe in the vent plume at this site, and Mg burial fluxes are calculated to be 73-146μmol.cm<sup>-2</sup>.kyr<sup>-1 </sup>. Burial fluxes of the hydrothermally derived elements represent only a fraction of the estimated vent output, implying that vent products are dispersed over long distances. Element/Fe ratios for scavenged elements are high compared to other metalliferous sediments, indicating enhanced scavenging efficiency at this site.</p
... Marine 187 os/ 188 Os curve since 40 Ma. To determine the age of our samples, we compiled a marine 187 Os/ 188 Os curve from 40 Ma to the present (Supplementary Fig. S4) based on several published 187 Os/ 188 Os data obtained from deep-sea sediment cores [53][54][55][56][57][58][59][60][61] . The ages of the cores used in these previous studies were tightly constrained by biostratigraphy and magnetostratigraphy. ...
The deep-sea clay that covers wide areas of the pelagic ocean bottom provides key information about open-ocean environments but lacks age-diagnostic calcareous or siliceous microfossils. The marine osmium isotope record has varied in response to environmental changes and can therefore be a useful stratigraphic marker. In this study, we used osmium isotope ratios to determine the depositional ages of pelagic clays extraordinarily rich in fish debris. Much fish debris was deposited in the western North and central South Pacific sites roughly 34.4 million years ago, concurrent with a late Eocene event, a temporal expansion of Antarctic ice preceding the Eocene–Oligocene climate transition. The enhanced northward flow of bottom water formed around Antarctica probably caused upwelling of deep-ocean nutrients at topographic highs and stimulated biological productivity that resulted in the proliferation of fish in pelagic realms. The abundant fish debris is now a highly concentrated source of industrially critical rare-earth elements.
... In the interval above the manganese layer, the depositional ages of the uppermost sample (PC11-1_4-6) and the two lowermost samples (PC11-5_4-6 and PC11-5_14-16) were dated at 2.0, 12.0 and 14.8 Ma, respectively, by fitting the seawater Os isotope curve. When fitting the secular change curve, we relied first on the seawater Os isotope curve based on pelagic sediment 18,[48][49][50][51][52][53][54][55] . In intervals without Os isotope data from pelagic sediment, we used Os isotope compositions of Fe-Mn crusts in the Pacific Ocean 56,57 . ...
Meteorite impacts have caused catastrophic perturbations to the global environment and mass extinctions throughout the Earth’s history. Here, we present petrographic and geochemical evidence of a possible impact ejecta layer, dating from about 11 Ma, in deep-sea clayey sediment in the Northwest Pacific. This clay layer has high platinum group element (PGE) concentrations and features a conspicuous negative Os isotope anomaly (¹⁸⁷Os/¹⁸⁸Os as low as ~0.2), indicating an influx of extraterrestrial material. It also contains abundant spherules that include pseudomorphs suggestive of porphyritic olivine as well as spinel grains with euhedral, dendritic and spherical forms and NiO contents as great as 23.3 wt%, consistent with impact ejecta. Osmium isotope stratigraphy suggests a most plausible depositional age of ~11 Ma (Miocene) for this layer, as determined by fitting with the seawater evolution curve. No large impact crater of this age is known on land, even within the relatively large uncertainty range of the relative Os age. Thus, we suggest that an unrecognised impact event in the middle or late Miocene produced the impact ejecta layer of the Northwest Pacific.
... The increase of 87 Sr/ 86 Sr could have resulted from terrestrial weathering and/or enhanced orogeny, but there is generally a lack of widespread diagenetic-resistant materials available for Sr isotope analysis (Blum and Erel 1995;Derry and France-Lanord 1996;Quade and Ojha 1997). Os isotopic composition of seawater provides an alternative signature to trace the continental flux relative to hydrothermal flux, particularly for organic-rich sediments (Pegram et al. 1992;Ravizza 1993;Peucker-Ehrenbrink et al. 1995;Oxburgh 1998;Reusch et al. 1998;Pegram and Turekian 1999). In general, the Os isotopic ratio is controlled by the redox conditions of the medium and source of Os. ...
The black shale series that formed in the Ediacaran–Cambrian transition are important stratigraphic records of the co-evolution of the paleo-ocean, -climate, and -biology. In this study, we measured Re–Os isotopic compositions of the black shale in the Niutitang Formation from the Gezhongwu section in Zhijin, Guizhou Province. The samples had high Re and Os contents, with Re ranging from 21.27 to 312.78 ng/g and Os ranging from 0.455 to 7.789 ng/g. The Re–Os isotope isochron age of 522.9 ± 8.6 Ma implies deposition of the Niutitang black shale predated the Chengjiang Fauna, providing an age constraint for the expansion of oceanic anoxia in the study area. The initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.826 ± 0.026 indicates that enhanced continental weathering might have triggered the expansion of the oceanic anoxia.
... The seawater Os isotopes reflect the balance of these various inputs (Burton et al., 1999), and previous research had established the seawater 187 Os/ 188 Os evolution curve (Peucker-Ehrenbrink et al., 1995;Klemm et al., 2005) and the modern seawater 187 Os/ 188 Os values (1.010 and 1.065; Ravizza, 1993;Levasseur et al., 1998), which are close to those of terrigenous weathering materials. Therefore, the present seawater 187 Os/ 188 Os features are considered to be derived from substantial terrigenous detrital input caused by the Cenozoic Himalayan orogeny (Reusch et al., 1998). ...
... Analyses of Os isotopic compositions in pelagic sediments have revealed that seawater 187 Os/ 188 Os ratio increased throughout the Cenozoic, from ~0.2 to ~1.06, with several negative excursions, such as at ~65.5 Ma and ~35 Ma (Os isotope curve; e.g., Peucker-Ehrenbrink and Ravizza, 2000. The long-term increasing trend, especially from the early Oligocene to the present day, is likely due to changes in continental weathering regime (Peucker-Ehrenbrink et al., 1995;Ravizza, 1993;Reusch et al., 1998). The Os isotope anomaly at ~65.5 Ma is pronounced ( 187 Os/ 188 Os = ~0.2) and reflects a bolide impact that caused the Cretaceous-Tertiary mass extinction event (Paquay et al., 2008) and preceding eruption and weathering of the Deccan flood basalt (Ravizza and Peucker-Ehrenbrink, 2003a). ...
Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn–Fe–10 × (Ni + Co + Cu) and rare-earth element plus Yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained ¹⁸⁷Os/¹⁸⁸Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over ~30 Myr with growth rates of 0.5–3 mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past ~30 Myr. The Pb isotopic compositions show very limited ranges (e.g., ²⁰⁶Pb/²⁰⁴Pb = 18.80–18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since ~30 Ma.
... In the context of studying climate-weathering feedbacks (Walker et al., 1981;Raymo et al., 1992), marine Os isotope records serve as a valuable tool. Reconstructing seawater 187 Os/ 188 Os relies on measurements of Os isotopes in marine sediment records such as pelagic clays (Pegram et al., 1992), metalliferrous carbonates and calcareous oozes (Ravizza, 1993;Ravizza and McMurtry, 1993;Peucker-Ehrenbrink, 1996;Reusch et al., 1998;Oxburgh, 1998;Ravizza and Peucker-Ehrenbrink, 2003;Dalai et al., 2006), reducing sediments (Peucker-Ehrenbrink and Ravizza, 2000a;Cohen et al., 1999, Williams andTurekian, 2004;Dalai et al., 2005) and ferromanganese crusts (Burton et al., 1999;Klemm et al., 2005). Down-core variations of 187 Os/ 188 Os and d 18 O of benthic foraminifer C. wuellerstorfi at Site 806 are shown in Fig. 6. ...
New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published 3He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30pgcm−2kyr−1, respectively. Average Ir/3He and Ir/xs230Thinitial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, 3He and xs230Thinitial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured 187Os/188Os of the OJP sediments are systematically lower than the inferred 187Os/188Os of contemporaneous seawater and a clear glacial–interglacial 187Os/188Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed 187Os/188Os variations. The difference between the 187Os/188Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of 187Os/188Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, 187Os/188Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low 187Os/188Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core 3He variations.
... Leaching techniques have been widely used in Os isotope geochemistry to extract hydrogenous Os from Ž marine and freshwater sediments e.g., Pegram et al., 1992Pegram et al., , 1994Peucker-Ehrenbrink and Ravizza, 1996;Peucker-Ehrenbrink et al., 1995;Oxburgh, 1998; . Reusch et al., 1998 and to extract labile Os from Ž rocks and soils e.g., Peucker-Ehrenbrink and Blum, . 1998 . ...
We present a method for the rapid determination of Os isotopic compositions and platinum group element (PGE) concentrations using ICP-MS on the same sample split. The method makes use of transfer of volatile OsO4 by an Ar gas stream into the torch of a magnetic sector ICP-MS for analysis. A variety of partial or complete sample dissolution methods can be used, namely (1) microwave oven digestion of samples in pressurized Teflon vessels under oxidizing acidic conditions, (2) NiS fire assay for complete sample dissolution and PGE preconcentration, (3) acid leaching techniques to extract labile PGE fractions, and (4) Carius tube digestion. Microwave oven sample digestion with HF–HNO3–HCl in an all-Teflon system as well as Carius tube digestion also allows the determination of Os isotopic composition and Os and Re concentrations on the same sample split. Spike–sample equilibration can be monitored on-line during sparging into the ICP-MS. The elimination of a nebulizer for Os isotope ratio measurements minimizes memory problems that are often associated with the determination of Os isotope ratios in liquid samples by ICP-MS. The external reproducibility of measurements of an in-house Os standard solution using a single-collector, magnetic sector ICP-MS (Finnigan ELEMENT) is 0.78% (1 S.D., n=13) for analyte amounts ranging from 81 pg to 1.22 ng total Os. The ion yield for 7-min data acquisitions is ∼5×10−5. Compared to negative thermal ionization mass spectrometry (N-TIMS), sample throughput is about on an order of magnitude higher. If NiS fire assay is used for PGE preconcentration, complementary PGE can be determined in the liquid residue by ICP-MS using a microconcentric nebulizer after the extraction of OsO4.
... In the river sediments from the LH, the carbonate fraction can represent up to 25% [13]. Although in the past carbonate-rich rocks have not been considered to be major sources of Os or Re [14], recent results from Levasseur et al. [4] suggest that carbonates may indeed provide important quantities of dissolved Os. Levasseur et al. [4] showed that while the Os of Ganges river water is mostly derived from erosion of black shales (70%), carbonates also play an important role (25%). ...
The marked rise of the (187)Os/(188)Os ratio of seawater over the Cenozoic Era is generally attributed to increased continental input, and particularly to increased erosion of black shales. Thus Os isotopes are thought to trace the weathering of organic carbon. Here we present evidence that impure marbles, lacking organic carbon, provide an important source of radiogenic Os to the bedload of Himalayan rivers. These marbles provide a critical component necessary for balancing the Os isotopic budget of the bedloads. The analyzed marbles have (187)Os/(188)Os ratios as high as 194 and often contain as much (187)Os as typical black shales. Most of this radiogenic Os is not contained in the carbonate itself, but rather in associated phases. These phases are often of hydrothermal origin, and their Os may ultimately be provided by fluids circulating through black shales. While these results complicate the relationship between the Os isotopic ratio of seawater and erosion of organic-rich rocks, they also provide new information necessary for understanding the marine Os isotopic record.
... This has been attributed by some to increased rates of uplift and denudation in the Alpine-Himalayan mountain belt and the American Cordillera, although the same trend can be produced by changes in the relative contributions of chemically dissimilar sources (e.g., Pegram et al., 1992;Raymo and Ruddiman, 1992;Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Blum et al., 1998;Derry and France-Lanord, 1996;Quade et al., 1997). The latter alternative may be most consistent with the observation that Os and Sr isotopic compositions appear to have been decoupled between ϳ60 and 40 Ma, when 87 Sr/ 86 Sr remained nearly constant but 187 Os/ 188 Os increased substantially, and between ϳ30 and 15 Ma, when 87 Sr/ 86 Sr increased rapidly, but 187 Os/ 188 Os remained nearly constant (Pegram et al., 1992;Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Reusch et al., 1998;MacArthur et al., 2001). ...
High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187Os/188Os, it has 186Os/188Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic 186Os/188Os, as well as radiogenic 187Os/188Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have 187Os/188Os ratios that are much higher than average UCC, but 186Os/188Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower 186Os/188Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the 187Os/188Os of seawater can change as sources and weathering conditions change, yet seawater 186Os/188Os varies only minimally.
... However, the Os isotopic ratio of seawater has varied through geological time. Reush et al. (1998) ; NMLi, BLi, and CLi from Li and Gao (2000); NMLuo, BLuo, and PLuo from Luo et al. (2003); BWu and PWu from Wu et al. (2001); PYi from Yi et al. (2004). value of seawater during Early Cambrian time is poorly constrained. ...
Extreme metal enrichments occur in the Lower Cambrian Niutitang Formation, a thick black shale sequence deposited on the Yangtze Platform of South China, and form a regionally distributed conformable polymetallic Ni–Mo–PGE–Au sulfide horizon. The origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of Re–Os isotopes and platinum-group elements. The black shales of the Lower Cambrian Niutitang Formation in Guizhou province yielded an Re–Os isochron age of 535±11 Ma with an initial 187Os/188Os ratio of 0.80±0.04, which is similar to the initial 187Os/188Os ratio (0.78±0.19) of the Ni–Mo sulfide ores, but slightly lower than the initial 187Os/188Os ratio (1.18±0.02) of Lower Cambrian black shales from the Lesser Himalaya, India. A comparison of Os isotope data among the Chinese Ni–Mo sulfide ores, the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits. The PGE concentrations, Pt anomaly (Pt/Pt⁎), Pt/Pd, Ir/Pd, Au/Pd ratios of the Chinese Ni–Mo sulfide ores are similar to those metal-rich black shales of submarine volcanogenic hydrothermal origin from Czech Republic, Namibia and Canada, but are different from those data of the ocean floor Fe–Mn crusts that have scavenged their PGE and other metals directly from seawater. It is therefore suggested that the Ni–Mo sulfide ores from the Niutitang black shale sequence in South China had a submarine-hydrothermal origin. The anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments. The discharge of hydrothermal fluids into the Cambrian Ocean may have had a great effect on life during the Cambrian Explosion.
... for several Kuroko ores from the Wanibuch Mine, southeastern Japan. The former age is consistent with the previously estimated age of about 15 Ma (Kaneoka 1983;Tanimura et al. 1984;Horikoshi 1990), and its initial ratio is almost identical to the contemporaneous seawater ratio (Reusch et al., 1998;Fig. 1 Geological sketch map showing sample localities and Neogene volcanic fields. Numbers refer to the sample sites indicated in Table 1. ...
Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, 87Sr/86Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (206Pb/204Pb=18.35–18.84, 207Pb/204Pb=15.59–15.97 and 208Pb/204Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.
... The majority of Os concentrations fall between 10 and 50 pg/g, similar to values reported for other pelagic carbonates (Reusch et al., 1998;Ravizza et al., 2001) but lower than those reported for pelagic clays (Esser and Turekian, 1988;Ravizza, 2007). Ir concentrations remain low through C30n at all sites, not increasing above background levels (~8 to 20 pg/g) until just below the KPB. ...
A composite late Maastrichtian (65.5 to 68.5 Ma) marine osmium (Os) isotope record, based on samples from the Southern Ocean (ODP Site 690), the Tropical Pacific Ocean (DSDP Site 577), the South Atlantic (DSDP Site 525) and the paleo-Tethys Ocean demonstrates that subaerially exposed pelagic carbonates can record seawater Os isotope variations with a fidelity comparable to sediments recovered from the seafloor. New results provide robust evidence of a 20% decline in seawater 187Os/188Os over a period of about 200 kyr early in magnetochron C29r well below the Cretaceous–Paleogene Boundary (KPB), confirming previously reported low-resolution data from the South Atlantic Ocean. New results also confirm a second more rapid decline in 187Os/188Os associated with the KPB that is accompanied by a significant increase in Os concentrations. Complementary platinum (Pt) and iridium (Ir) concentration data indicate that the length scale of diagenetic remobilization of platinum group elements from the KPB is less than 1 m and does not obscure the pre-KPB decline in 187Os/188Os. Increases in bulk sediment Ir concentrations and decreases in bulk carbonate content that coincide with the Os isotope shift suggest that carbonate burial flux may have been lower during the initial decline in 187Os/188Os. We speculate that diminished carbonate burial rate may have been the result of ocean acidification caused by Deccan volcanism.
The Cretaceous provides us with an excellent case history of ocean-climate-biota system perturbations. Such perturbations occurred several times during the Cretaceous, such as oceanic anoxic events and the end-Cretaceous mass extinction, which have been the subject of an abundant literature. Other perturbations, such as the mid-Maastrichtian Event (MME) remain poorly understood. The MME was associated with global sea-level rise, changes in climate and deep-water circulation that were accompanied by biotic extinctions including “true” inoceramids and the demise of the Caribbean-Tethyan rudist reef ecosystems. So far, the context and causes behind the MME remain poorly studied. We conducted high-resolution integrated biotic, petrological and geochemical studies in order to fill this knowledge gap. We studied, in particular, carbonate Nd and Os isotopes, whole-rock Hg, C and N content, C and N isotopes in organic matter, SCAS isotopes, along with C and O isotopes from foraminifera from the European Chalk Sea: the Polanówka UW-1 core from Poland and the Stevns-1 core from Denmark. Our data showed that sea-level rise of ∼50-100 m lasted around ∼2 Ma and co-occurred with anomalously high mercury concentration. Along with previously published data, our results strongly suggest that the MME was driven by intense volcanic–tectonic activity, likely related to the production of vast oceanic plateaus (LIP, Large Igneous Province). The collapse of reef ecosystems could have been the consequence of LIP-related environmental stress factors, including climate warming, presumably caused by emission of greenhouse gases, modification of the oceanic circulation, oceanic acidification and/or toxic metal input. The disappearance of the foraminifer Stensioeina lineage on the European shelf was likely caused by the collapse of primary production triggered by sea-level rise and limited amount of nutrient input. Nd isotopes and foraminiferal assemblages attest for changes in sea-water circulation in the European Shelf and the increasing contribution of North Atlantic water masses.
深海氧同位素记录揭示新生代以来全球气候呈整体变冷趋势，南北两极先后发育冰盖，地球由温室气候变为冰室气候，但是其变冷机制仍不明确。大气CO2浓度降低和大洋环流模式改变均被认为与新生代气候变冷密切相关，但目前对两者的作用还未达成统一的认识，由此存在各种假说，如BLAG假说、高原隆升-风化假说、构造隆升-碳埋藏假说、火山铁肥效应和岛弧隆升-风化假说及海道开合假说等，用以解释新生代全球变冷。围绕新生代气候变冷机制方面的争论，评述了过去近几十年来相关研究的进展和存在的问题，讨论了不同机制对新生代气候变化的影响，并提出未来需要加强的研究重点：建立准确的新生代大气CO2浓度演变序列、建立更加准确的地球内部排气和青藏高原隆升及海道开合时刻表、建立完善的风化指标体系、加强火山作用及其大洋生物地球化学效应的研究。
We report geochemical and isotopic data for umber and related greenstone samples from the Aki umber deposit in the Northern Shimanto Belt, central Shikoku, southwest Japan. The greenstone underlying the umber deposit shows a geochemical signature typical for mid-ocean ridge basalts. Umber samples are characterized by notable enrichments in Fe, Mn, P, V, Co, Ni, and Zn relative to post-Archean average Australian Shale (PAAS). In addition, rare-earth elements and yttrium (REY) are markedly enriched in the umber samples (up to 1120 ppm total REY), and their PAAS-normalized REY patterns are almost identical to those of hydrothermal ferromanganese plume fall-out precipitates (suspended particulates) emanating from a mid-oceanic ridge. These geochemical features suggest that the Aki umber represents a hydrothermal iron oxyhydroxide-type REY-rich mud deposited originally in a pelagic deep-sea setting during the Middle Cretaceous (113.2–100.5 Ma), which was subsequently accreted onto the proto-Japanese Island arc. The initial ¹⁸⁷Os/¹⁸⁸Os ratios of the Aki umber samples range from 0.554 to 0.668. Because the umbers preserve the primary geochemical signature of hydrothermal Fe-oxyhydroxide-type REY-rich mud deposited near a mid-oceanic ridge, their Os isotopic ratios can be regarded as the marine ¹⁸⁷Os/¹⁸⁸Os ratios at the time of deposition (Middle Cretaceous). Our results indicate that the Cretaceous marine ¹⁸⁷Os/¹⁸⁸Os ratios were lower than the present day ratio, probably reflecting more intense hydrothermal activity and/or a lower continental weathering flux during that period. We also suggest that marine Os isotopic compositions during the Middle to Late Cretaceous were relatively constant (0.4–0.6), except during ocean anoxic events.
This chapter presents the currently available ¹⁸⁷Os/¹⁸⁸Os seawater data and emphasizes some of the challenges involved in reconstructing the seawater record from the present to the Precambrian. Radioactive decay of long-lived ¹⁸⁷Re to ¹⁸⁷Os, large differences in the compatibilities of rhenium (mildly incompatible) and osmium (very compatible) during magmatic differentiation, and various processes that mobilize and transport Os into the ocean shape this seawater record. Differences in magmatic compatibilities of Re and Os have created large isotope contrasts between osmium delivered to seawater from the radiogenic Earth’s crust (e.g. rivers, submarine groundwater discharge, aeolian dust) compared to osmium from the unradiogenic mantle (submarine hydrothermal vents) and undifferentiated extraterrestrial matter. Together with the relatively short (10³–10⁴ yr) residence time of osmium in seawater these factors are responsible for the very dynamic isotope evolution of seawater that offers great potential for chemostratigraphic reconstructions. A full assessment of the chemostratigraphic utility of the marine ¹⁸⁷Os/¹⁸⁸Os record requires that it be reconstructed in multiple ocean basins at a temporal resolution approaching the marine residence time of Os. Presently, large time intervals are undersampled. Where high temporal resolution studies have been conducted at widely separated sites, observed ¹⁸⁷Os/¹⁸⁸Os offsets between sites exceed analytical uncertain. It is therefore a possibility that subtle offsets between the ¹⁸⁷Os/¹⁸⁸Os of seawater in different ocean basins could have existed in the past.
In the late Miocene the Mediterranean Sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins. In this study we report Os isotopic records from Deep Sea Drilling Project and Ocean Drilling Project cores in the Mediterranean: the Balearic Sea (Site 372), the Tyrrhenian Sea (Site 654), the Ionian Basin (Site 374), and the Florence Rise (Sites 375–376), as well as Integrated Ocean Drilling Project Site U1387 in Gulf of Cadiz, North Atlantic. Pliocene-Pleistocene sediments at all sites show 187Os/188Os values close to that of the coeval ocean water, indicating that the Mediterranean was connected to the North Atlantic. Evaporitic sediments deposited during the latest Miocene, however, have 187Os/188Os values significantly lower than coeval ocean water values. The offset of the Mediterranean evaporite 187Os/188Os is attributed to limited exchange with the North Atlantic during the Messinian salinity crisis. The source of unradiogenic Os is likely to be weathering of ultramafic rocks (ophiolites) cropping out in the Mediterranean's drainage basins. Based on a box model we estimated the amount of unradiogenic Os and the Atlantic-Mediterranean exchange rate to explain this offset. Os isotopic ratios of the pre-evaporite sediments in the western Mediterranean are almost identical to that of the coeval ocean water. In contrast, equivalent sediments from the Florence Rise have significantly lower 187Os/188Os values. The offset in the Os isotopic ratio on the Florence Rise is attributed either to limited water exchange between eastern and western Mediterranean or to local effects associated with exhumation of the Troodos ophiolites (Cyprus).
Platinum Group Elements (PGE) have been considered as being inert in surficial condition for a long time. However, the study of mobility of PGE during weathering of ultramafic-mafic complex and black shale indicates that PGE, released from the primary rocks due to weathering, are abnormally concentrated in soil, plant and water. The secondary enrichment of PGE, especially of Pt and Pd, in surficial media around complex could be used as a potential tool for exploring PGE deposits. And osmium in black shale, transferring during weathering, could contribute main importance to the increase of seawater 187 Os/188 Os since Cenozoic. While PGE emitted from automobile catalysis converters would have a negative effect on ecological environment and human health by their mobility.
Weathering of uplifted continental rocks consumes carbon dioxide and transports cations to the oceans, thereby playing a critical
role in controlling both seawater chemistry and climate. However, there are few archives of seawater chemical change that
reveal shifts in global tectonic forces connecting Earth ocean-climate processes. We present a 68-million-year record of lithium
isotopes in seawater (δ7LiSW) reconstructed from planktonic foraminifera. From the Paleocene (60 million years ago) to the present, δ7LiSW rose by 9 per mil (‰), requiring large changes in continental weathering and seafloor reverse weathering that are consistent
with increased tectonic uplift, more rapid continental denudation, increasingly incongruent continental weathering (lower
chemical weathering intensity), and more rapid CO2 drawdown. A 5‰ drop in δ7LiSW across the Cretaceous-Paleogene boundary cannot be produced by an impactor or by Deccan trap volcanism, suggesting large-scale
continental denudation.
Platinum group elements (PGE), Au contents and Os isotopic compositions in ferromanganese crusts and their basaltic substrates from the central and west Pacific Ocean were analyzed. The results show that their PGE and Au contents are quite high and greatly varied, the ∑PGE is (70.09 ∼ 629.26) × 10-9 and the total Au is (0.60 ∼ 26900) × 10-9 respectively. In the three-layered (outer, porous and compact layers) crusts, the ∑PGE contents of porous ((339.37 ∼ 545.82) × 10-9) and compact ((280.09 ∼ 629.26) × 10-9) layers are significantly higher than the outer ((70.09 ∼ 133.27) × 10-9) layers. The single-layered crusts are also enriched in PGE (∑PGE = (83.94 ∼ 479.75) × 10-9), and their Au contents are usually higher than the three-layered ones. In addation, ∑PGE and Au contents of ferromanganese crusts are much higher than that in the nodules (∑PGE = (101.57 ∼ 155.83) × 10-9, Au = (1 - 4) × 10-9). Different sedimentary water depths and oxygen fugacity of the ambient sea water of the crust and nodules may be the major reason for their different PGE and Au contents. ∑PGE and Pt are positively correlated with Mn % but inversely correlated with Fe %, which is just contrary to that in nodules, suggesting that PGE in the crusts may mainly occur in manganese mineral phases such as vernadites (δ-MnO2), while the PGE in the nodules may predominantly occur in iron mineral phases such as goethites(FeOOH · nH2O). Chondrite-normalized patters and various geochemical parameters of PGE demonstrate that PGE and Au in the ferromanganese crusts were sourced from water/rock reaction between the basaltic substrates and seawater, and partly from extraterrestrial materials such as iron meteorite dusts, but not related to submarine hydrothermal venting. Calculation based on cobalt contents show that ferromanganese crusts in the west Pacific Ocean began to grow at about 42.5Ma, and underwent two growth hiatuses corresponding to the geological ages of 21.8Ma and 8.0Ma respectively. The growth rates of the outer, porous and compact layers are 2.64mm/Ma, 1.45mm/Ma and 1.06mm/Ma, and the 187Os/188Os of the corresponding ambient sea water are 0.948 ∼ 0.953, 0.599 ∼ 0.673 and 0.425 ∼ 0.536 separately, implying that the outer layers contained more continental eroded dusts, while the sediments in the porous and compact layers were mainly sourced from submarine basalt alteration and extraterrestrial materials such as interplanetary dust particles (IDPs).
187Os/188Os ratios in all sub-layers of the Co-rich ferromanganese crust MP5D17 from Line Seamount in Central Pacific have been systematically determined. The 187Os/188Os values in this crust are highly variable, ranging from 0.4262 to 1.0155. From the bottom to the top, the osmium isotopic ratios of the crust display a regular variation trend, which correlates well with its changes in structure and texture in the vertical profile. In comparison with the evolution curve of Os isotopic composition in seawater over the past 80 Ma, a similar evolution tendency can be found with respect to the Os isotopic composition of the crust in this study. It is preliminarily concluded that the crust MP5D17 began to grow at about 72 Ma, and underwent three growth hiatuses, corresponding to the geological ages of 64, 53 and 37 Ma, respectively.
The Fly and Sepik are two of the largest rivers draining the island of New Guinea and among the world's large rivers in discharge and sediment yield. Sediment from the upper Fly River and its delta have low 187Os/188Os of 0.511-0.560, and sediments from the upper and lower Sepik River have even lower ratios of 0.294-0.418. Filtered waters (1.2). Burial of Os of both seawater and riverine derivation occurs in conjunction with organic carbon burial in the Gulf of Papua. There is a negative correlation between the Os isotopic composition of the bulk sediment and the fraction of organic matter of terrestrial origin based on C isotopes. The systematic relationship between C and Os isotopic compositions in these samples indicates that organic matter is the major host of Os in tropical estuarine sediments. The Os/Corg of Gulf of Papua surface sediments is intermediate between that of other modern and ancient sediments. Approximately 1011 mol/yr of Corg and 20-57 mol/yr of Os are buried in the inner shelf (
Platinum group elements, specifically iridium and osmium, are the most sensitive non-volatile elemental tracers of extraterrestrial (meteoritic) matter in marine sediments. The budget of these elements in sediments can be considered a mixture of extraterrestrial, eolian (i.e., wind-blown dust), and hydrogenous (i.e., seawater-derived) sources. Differences in the osmium isotopic composition between these three sources allow quantification of the amount of extraterrestrial osmium in sediments. Osmium isotope data for about 30 pelagic sediments from the Atlantic and Pacific oceans, spanning the past 80 Myr, yield an average annual flux of extraterrestrial matter of 30,000 ± 15,000 metric tons. The only clear exception is the large impact at the Cretaceous-Tertiary boundary. Temporal resolution of these flux estimates is limited to at least several thousand years by the need to obtain statistically representative samples. The scatter in the flux estimates is large and secular variations in the flux of less than a factor of about three cannot be excluded. However, samples from individual cores indicate that the flux of extraterrestrial matter has varied by less than a factor of two, if the flux is averaged over the scale of temporal resolution in marine sediment samples.
The Mineoka umbers occur within the Tertiary accretionary complex of central Japan, immediately overlying ~53.7Ma tholeiitic basalts. The associated tholeiitic basalts exhibit typical normal mid-ocean ridge basalt (N-MORB)-type geochemical signatures and are interpreted to be a remnant of Tertiary oceanic crust. Centimeter-resolution bulk-rock chemical analyses were performed to cover the entire section of the ~35cm thick umber deposit. There are striking similarities in major element, trace element, Re, and Os contents between the umbers and modern metalliferous sediments along the East Pacific Rise. These results demonstrate that the Mineoka umbers are ancient counterparts of MOR metalliferous sediments that are plume fall-out ferromanganese precipitates scavenging various seawater-derived elements, including P, V, Y, REEs, and Os, and confirm that the umbers subaerially exposed in the accretionary complex generally preserve their primary geochemical signatures at the time of deposition. The marine 187Os/188Os ratios inferred from the Mineoka umbers gradually increase up-section with moderate fluctuations. The 187Os/188Os composition, ranging from 0.47 to 0.54, is significantly higher than that of the prior period (post-Paleocene–Eocene thermal maximum (PETM); 187Os/188Os~0.38). This was probably the result of an increase in the chemical weathering rate of continental crust caused by the long duration of noticeably warm climate during the early Eocene greenhouse interval. Several fluctuations observed in the marine 187Os/188Os record during this interval are most likely related to PETM-like, brief hyperthermals, supporting the suggestion that the intensity of continental crustal weathering controlled by climatic changes can fluctuate on timescales of ~105 yr.
Estimates of osmium residence time in the oceans that are based on
oceanic mass balance calculations (35-50 kyr) appear irreconcilable with
those inferred from the recent evolution of the osmium isotope
composition of seawater (3-4 kyr). It is argued that the osmium budget
of the oceans is currently close to steady state and thus that the
estimates made by the two methods should agree. As the inventory of
osmium in the oceans is relatively well constrained, these disparate
residence time estimates imply wildly different osmium input fluxes to
the oceans. An osmium residence time of 8-10 kyr is proposed by
evaluating the uncertainties and limitations of both methods, and it is
argued that osmium inputs to the ocean are currently underestimated by a
factor of ˜3. This reflects in part the underestimation of the
river input of osmium to the oceans owing to a bias within the existing
data set and in part the probable existence of sources of osmium to the
oceans that have not yet been identified. The very short residence time
of 3-4 kyr inferred from the postglacial change in seawater composition
(assuming a single step change in input flux) is rejected as it implies
unreasonably high osmium input fluxes to the oceans. It is concluded
that a postglacial spike in osmium flux, associated with a meltwater
event, must have driven part of the change in seawater composition.
However, it is also shown that such a spike cannot be the dominant cause
of the most recent shift in seawater 187Os/188Os.
Based on its microstructure, Co-rich crust A1-1 from seamount Allison, central Pacific, was scraped at averaged interval of
1.3 mm to measure osmium isotopic composition, and subsequently to establish the 187Os/188Os profile of scraping section of the crust. By observing the variation of 187Os/188Os under 10Be chronology and matching it to the well-known seawater Os isotope evolution of the past 40 Ma, two growth hiatuses
(H1 and H2) occurring in the periods respectively between 13.6 and 29.6 Ma and between 8 and 9.8 Ma in the crust were recognized.
According to the two hiatuses, the dating scheme for each scraped layer of the crust was suggested. For the upper layers younger
than 6.8Ma, their growth ages were calibrated under 10Be chronology; for the lower layers older than 6.8Ma, their growth ages were obtained from 187Os/188Os evaluation curve by linear interpolation. Hereby, the age for the most inner layer of the crust was determined to be 39.5
Ma. H1 and H2 exactly correspond to the boundary between phosphatization and non-phosphatization and volcanic ash layer in
the crust, respectively.
In the latest Paleocene an abrupt shift to more negative delta13C values has been documented at numerous marine and terrestrial sites [Bralower et al., 1997; Cramer et al., 1999; Kaiho et al., 1996; Kennett and Stott, 1991; Koch et al., 1992; Stott et al., 1996; Thomas and Shackleton, 1996; Zachos et al., 1993]. This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4°-6°C [Kennett and Stott, 1991] and more moderate warming in the subtropics [Thomas et al., 1999]. Here we report 187Os/188Os isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the 187Os/188Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187Os/188Os ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate [Walker et al., 1981]. Estimates of the duration of late Paleocene CIE [Bains et al., 1999; Bralower et al., 1997; Norris and Röhl, 1999; Röhl et al., 2000] in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 104-105years. This interpretation requires that the late Palcocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher 187Os/188Os ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the 187Os/188Os ratio of rivers.
The Lower Cambrian Niutitang Formation consists of a thick black shale sequence with a regionally distributed conformable Ni-Mo polymetallic sulfide horizon and a chert bed at its basal strata on the Yangtze Platform, South China. In this paper, we discuss all available data on Re-Os isotopes and Platinum Group Element (PGE) distribution pattern of the Ni-Mo polymetatlic sulfide ore and its host rocks (black shales, cherts, and phosphorites) from Guizhou and Hunan provinces. Our results show that the black shales and the Ni-Mo sulfide ore have a high initial (OS)-O-187/Os-188 ratio of 0. 78 similar to 0. 86, indicating that the Early Cambrian ocean across the Yangtze Platform had a highly radiogenic Os value. This ratio is slightly lower than but still similar to present-day seawater, possibly as a result of high continental weathering at that time. The Ni-Mo sulfide ore yields a Re-Os isochron of 537 +/- 10 Ma (MSWD = 11. 9), possibly representing the depositional age of the Niutitang Formation. The chondrite-normalized PGE pattern, Pt anomaly (Pt/Pt-*), Pt/Pd, Ir/Pd, Au/Ir and Re/Mo ratios of the Ni-Mo sulfide ore and its host rocks from South China indicate a varying source contribution of the PGE and other metals for different rocks. It is suggested that the cherts and Ni-Mo sulfide ore may have a significant proportion of PGE and probably other metals deriving from submarine-hydrothermal fluids with a mantle signature.
The Cenozoic marine osmium isotope record is largely driven by changes in the continental input. We aid its interpretation by supplying direct measurements of present day riverine Os in known geological and environmental settings. We analyzed Os concentrations and isotopic ratios in the dissolved, suspended, and bed materials of the Mackenzie River in Canada and smaller rivers draining the Western Cordillera into the Pacific Ocean. Rhenium content was measured for suspended and bed materials to provide further constraint. Dissolved 187Os/188Os ratios range from 0.74 to 4.5 and are controlled to first order by age and lithology of the bedrock. The rivers draining Jurassic volcanic rocks are unradiogenic, as low as 0.74, and those draining the Precambrian Canadian Shield are radiogenic, as high as 3.5. The headwaters of the Mackenzie left bank tributaries draining metamorphic carbonates in the Eastern Cordillera and Paleozoic black and gray shales in the Transition zone and the Interior Platform are especially radiogenic (3.0–4.5) with relatively high concentrations of Os. These latter rivers are responsible for the Mackenzie being above world average in Os concentration and yield (mol Os/km2/year) as well as in 187Os/188Os. The dissolved load is more radiogenic than the suspended or bed loads and constitutes a significant fraction of total fluvial Os.
In this review emphasis is placed on isotopic records of ocean chemistry. In general terms, a conscious decision was made to emphasize those records that document long-term changes in the chemical and physical properties of the global ocean over the course of the Cenozoic. For example, while reconstructions of burial fluxes of barium or phosphorus may place valuable constraints on paleo-productivity in a specific setting, making extrapolations to infer globally integrated trends from these data sets is very difficult because of sparse data coverage in space and time. Similarly, we have also chosen to exclude discussion of short-residence-time tracers like lead and neodymium isotopes that can yield important information about changing patterns of ocean circulation and regional shifts in oceanic inputs.
Re and Os abundances and Os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines, Maldeota and Durmala, in the Lesser Himalaya. The Re and Os concentrations in these samples exhibit wide range, 0.2 to 264 ng/g and 0.02 to 13.5 ng/g, respectively, with a mean Re/Os of ∼25. The 187Os/188Os of these samples also show a broad range 1.02 to 11.6, with many of them far more radiogenic than the currently eroding continental crust. The black shales from the Maldeota and Durmala mines, collected a few meters above the Krol-Tal (Pc-C) boundary and occurring in the same stratigraphic horizon, yield 187Re–187Os isochron ages of 554 ± 16 and 552 ± 22 Ma, respectively, consistent with fossil evidences and those assigned for the Pc-C boundary at various other locations. These results indicate closed system behaviour of Re and Os in these mine samples and reaffirm the validity of 187Re–187Os pair to date organic-rich sediments. The age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary Re-Os age of 839 ± 138 Ma for black shales from Theog, collected from the inner belt. These initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt; however, more analyses of black shales are needed to confirm this. The role of weathering black shales from the Himalaya in contributing to the Os isotope evolution of seawater over the past ∼25 Ma was assessed using a simple budget model for seawater Os and assuming values for Os fluxes and 187Os/188Os based on the available limited data on Os concentration and its 187Os/188Os in rivers and oceans. These calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187Os/188Os over the past ∼16 Ma is difficult to satisfy by varying only the Os flux from HTP rivers (187Os/188Os kept constant at 1.324) or their 187Os/188Os (188Os flux constant at today’s value, ∼131 moles/yr). The requirements to balance Os isotope budget in ocean through weathering of black shales, however, is easier to accommodate if both the Os flux from HTP (Himalayan-Tibetan Plateau) rivers and their 187Os/188Os are allowed to vary. Measurements of Os abundance in HTP rivers and its isotopic composition are needed to test the validity of the model results.
Umbers from 8 of 11 localities studied yield 187Os/188Os ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. 187Os/188Os measured in samples from the remaining three localities fall above and below this range and are believed to result from post-depositional redistribution of Os. Extremely large 187Os/188Os ratios occur in hydrothermal sulfides (>200) and in their oxidation products, ochres (3.6–13.2). Measured Re/Os ratios in these samples require open system behavior. Model age calculations indicate that ochre formation occurred over a 1–5 million year interval after sulfide precipitation. In most samples >99% Re loss occurred during sulfide oxidation. Comparison of element ratios in Cyprus umbers to those of modern seawater suggest the following sequence of scavenging efficiency by hydrothermal Fe-oxides: Ir>Pt>Os>Re. Cyprus sulfides exhibit the opposite pattern of element enrichment. These patterns of aqueous platinum-group element (PGE) fractionation are potentially relevant to study of recycled Os in mantle-derived rocks because they give rise to the following phenomena: (1) ephemeral Re enrichment within the basal sediments of the oceanic crust, (2) PGE concentrations in metalliferous sediments (Os up to 1 ppb and Pt up to 22 ppb) that are comparable to those in the mantle, and (3) Pt/Re ratios in Cyprus umbers that exceed those required to explain the coupled 186Os–187Os variations in Hawaiian basalts.
Over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research. Several analytical developments, particularly in the past few years, have significantly increased our ability to study the behaviour of osmium in the surficial environment. The ¹⁸⁷Os/¹⁸⁸Os and osmium concentration of seawater, river water, rain, and hydrothermal vent fluids have been measured directly. Recently, the behaviour of osmium in estuaries–critical for estimating the marine residence time of osmium–has been studied. Our knowledge of the surficial osmium cycle has thus significantly improved. In addition, reconstructions of past variations in the marine ¹⁸⁷Os/¹⁸⁸Os recently have been extended back into the Mesozoic. This review attempts to summarize our current understanding of the marine osmium system–present and past.