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Plot of 187 Os/ 188 Os vs. Pt/Ir in the Rainbow samples, showing the mixing relationship between a high Pt/Ir, high 187 Os/ 188 Os hydrothermal component and an ultrama¢c component with low Pt/Ir and 187 Os/ 188 Os. Calculated physical mixing curves are shown using each of the three rock fragments analyzed from Cores 343 (serpentinite) and 316 (serpentinite^sul¢de amalgams). Note that several samples with very high Pt/Ir exhibit 187 Os/ 188 Os slightly lower than seawater. This is consistent with an additional source of unradiogenic Os associated with the hydrothermal end-member.
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Osmium and other platinum-group elements (PGEs) have been measured in sediments accumulating directly under the Rainbow hydrothermal plume at 36°N on the Mid-Atlantic Ridge. 187Os/188Os and PGE concentrations in the sediments of four cores taken 2-25 km from the active vent site reveal evidence of both radiogenic Os scavenged from seawater and, imp...
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... plot of 187 Os/ 188 Os vs. Pt/Ir (Fig. 5) provides a means of resolving the relative importance of ultrama¢c and hydrothermal contributions more clearly because all PGE and Os isotope data are measured from the same individual fusions. This allows the in£uence of PGE-rich phases to be represented accurately. The Rainbow plume com- ponent is characterized by high Pt/Ir ratios, ...
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1] Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extru-sive basalts have d 18 O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (<200-250 C). Bulk...
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... Terrigenous input does not exceed 5%, while cosmogenic source is estimated as 1 Pt, 5 Os, and 10% Ir (Dalai and Ravizza, 2006). It was shown that sediments from the area of the hydrothermal plume influence in the Rainbow field contain Os from seawater and nonradiogenic Os from basalts (Cave et al., 2003). Some samples are characterized by PGE composition close to mantle source, and indicates a hydrothermal input of PGE in seawater, and then in sediments. ...
... Some samples are characterized by PGE composition close to mantle source, and indicates a hydrothermal input of PGE in seawater, and then in sediments. However its influence is lower than that of hydrogenous source (Cave et al., 2003). The high contents of Ir, Pt, and Pd relative to pelagic clays were found in the metalliferous sediments of the western flank of the East Pacific Rise (EPR, Table 4, Fig. 3). ...
... It was found that platinum in sulfides of the Rainbow hydrothermal field occur mainly in Fe,Cu-sulfides, but not in sphalerite (Lein et al., 2003). Factor analysis (1) and sediments near the Rainbow hydrothermal field (2) (average contents in cores 23#12 and 316, Cave et al., 2003), reduced sediments (3), and pelagic red clays (4) (Bertine et al., 1993;Koide et al., 1991;Terashima et al., 2002), metalliferous sediments of EPR (5, our unpublished data), as compared to the PGE compositions in ferromanganese crusts of the World Ocean (6) (average values from data of Banakar et al., 2007;Hein et al., 2005;Koschinsky et al., 2020). Hereinafter, PGE in plots are arranged in order of decreasing condensation temperature in magmatic melts. ...
The paper considers the current state of Platinum Group Elements (PGEs) geochemistry in the
ocean. The behavior of PGEs in the aquatic environment is defined by their oxidation state, the ability to
change it, and complexation. The difference in chemical properties leads to PGEs fractionation in the
ocean. This is their characteristic feature, along with their ultra-low contents. The paper describes the
sources of PGEs supply to the ocean, PGEs behavior in the river–sea mixing zone, and their distribution
in seawater. The processes of PGE accumulation in sediments, seafloor sulfides, and ferromanganese
deposits of the ocean are reviewed. Possible mechanisms of PGE accumulation on ferromanganese oxyhydroxides
are discussed.
... In addition, Os-bearing alloys and sulfides can form during hydrothermal alteration (e.g., 350-800°C; Petrou and Economou-Eliopoulos 2009), sequestering the Os and further restricting its mobility in these environments. Numerous Re-Os isotopic studies of active or recent ocean-floor hydrothermal systems have demonstrated mixing between seawater and crustal components, with seawater typically dominating the Os isotopic composition of the sulfides Brügmann et al. 1998;Cave et al. 2003;Zeng et al. 2014). Osmium and Re concentrations also reflect mixing between oxidised seawater and reduced hydrothermal fluid, as well as variations in redox conditions within the system, creating variations in 187 Re/ 188 Os that can be useful for geochronology (Zeng et al. 2014). ...
Rhenium, Os, and Pt are redox sensitive elements that are concentrated in highly reducing environments such as those associated with black shales but mobile under more oxidizing conditions such as those associated with arc volcanism. They are chalcophile in many terrestrial ore-forming environments, and their isotopic systematics provide unique opportunities to date the formation of sulfide ore deposits and understand their petrogenesis. Fractional crystallization of magmatic sulfide ores generates primary variations in Re/Os and Pt/Os that allow mineral and whole rock isochron ages to be determined and discrimination of crustal and mantle sources based on initial Os isotopic compositions. Molybdenite is especially well suited for geochronology due to its high Re/Os and resistance to resetting. Rhenium concentrations in molybdenite tend to reflect the composition or provenance of the ore-forming fluids, with higher concentrations associated with more primitive sources or more oxidized fluids and lower concentrations with more evolved and/or reduced conditions, although local and regional factors also have a significant influence. Many studies have used pyrite for dating but its typically low Re concentration, variable initial Os isotopic composition (reflecting fluid mxing), and susceptibility to re-equilibration makes its use as a geochronometer problematic in many cases. Other sulfide minerals such as bornite and arsenopyrite have shown promise for Re–Os isotope geochronology but additional studies are needed to evaluate their broader applicability for dating of ore deposits. The isobaric beta decay of parent isotope ¹⁸⁷ Re to ¹⁸⁷ Os has restricted investigation of this system by microbeam techniques such as ion microprobe or laser ablation mass spectrometry, especially for geochronology. This requires either chemically processing the sample to separate the elements or novel techniques such as collision-cells that preferentially ionize the Re and Os during the analysis. Thermal ionization mass spectrometry (TIMS) and inductively-coupled plasma mass spectrometry (ICPMS) are the most widely applied techniques for Re-Pt-Os isotopic analyses. Specialized techniques for sample digestion to ensure redox equilibrium between Os in the sample and the isotopically enriched spikes used for isotope dilution measurements are typically required. This chapter briefly reviews development of the ¹⁸⁷ Re- ¹⁸⁷ Os and ¹⁹⁰ Pt- ¹⁸⁶ Os isotopic systems for earth science, physico-chemical controls on their behavior in ore-forming environments, and applications to metallogenic systems.
... Seafloor hydrothermal ferromanganese (Fe-Mn) sediments have been utilized as records of pelagic ocean chemistry and environments because Fe -Mn oxyhydroxide particulates, which are exhaled by hydrothermal activity at mid-ocean ridges (MORs), scavenge various elements including P, V, As, Y, Mo, Os, and rare earth elements (REE) from ambient seawater (Trefry and Metz, 1989;German et al., 1990German et al., , 1991Feely et al., 1991;Ravizza and McMurtry, 1993;Ravizza et al., 1999;Sherrell et al., 1999;Cave et al., 2003;Kato et al., 2005aKato et al., , 2005bKato et al., , 2011aKato et al., , 2011bJosso, 2016;Kuwahara et al., 2021). Due to the movement of the oceanic lithosphere, ancient submarine hydrothermal ferromanganese deposits have been subducted to the Earth's interior, making it difficult to directly obtain information about the ocean prior to ~180 million years ago (Ma). ...
... However, contemporaneous seawater 187 Os/ 188 Os values from pelagic areas have not yet been reported. Because hydrothermal Feoxyhydroxides scavenge Os efficiently from the ambient seawater, ancient ferromanganese sediments record the 187 Os/ 188 Os value of seawater with only a negligible effect from the hydrothermal fluid itself (e.g., Ravizza and McMurtry, 1993;Cave et al., 2003;German and Seyfried, 2014). This feature renders ferromanganese sediments ideal for recording 187 Os/ 188 Os values of past pelagic seawater (e.g., Ravizza, 1993;Peucker-Ehrenbrink et al., 1995;Oxburgh, 1998;Kuwahara et al., 2021). ...
... As described above, the precursor materials of Kawame umber are likely to have been Fe-oxyhydroxide particles deposited as seafloor ferromanganese sediments. Because Os in seawater, which exists as an oxyanion (dominantly H 3 OsO 6 − ; Palmer et al., 1988), is scavenged by Fe-oxyhydroxide particulates, the Os in seafloor ferromanganese sediments is considered to be mostly derived from seawater (Ravizza and McMurtry, 1993;Cave et al., 2003). Therefore, seafloor ferromanganese sediments have been utilized as an effective recorder of the 187 Os/ 188 Os values of paleo-seawater (e.g., Ravizza, 1993;Ravizza and Peucker-Ehrenbrink, 2003a;Oxburgh, 1998;Kuwahara et al., 2021). ...
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.
... We assign basalt-hosted systems with an initial Os concentration, [Os]Initial-bas = 1.5 pmol kg -1 , and ultramafic-hosted systems with a range of [Os]Initialultra = 1.5 -1500 pmol kg -1 , consistent with peridotite having Os concentrations at least 10 3 times greater than basalt (Cave et al., 2003;. We assume that the global hightemperature hydrothermal fluid flux is partitioned at a 75:25 proportion between basalt-hosted and ultramafic-hosted hydrothermal systems, respectively (Baker & German, 2004;Coogan & Dosso, 2012). ...
Plain Language Summary
Osmium (Os) isotopes have been used to track continental weathering, which plays a significant role in controlling climate and shifts in the global carbon cycle throughout Earth history. Tectonic drivers, such as increased uplift, are often invoked to explain the long‐term rise in Os isotope composition recorded from 65 million years (Myr) to present. Here, we offer a contrasting perspective by providing evidence that the increase in Os isotope composition may be controlled by variations in the flux of Os coming from high‐temperature fluids venting from mid‐ocean ridge hydrothermal systems, which can be directly tied to changes in global ocean chemistry, such as the amount of calcium and sulfate in seawater. A steady decrease in this hydrothermal flux from 65 Myr to present provides a simple explanation for the evolution of the seawater Os curve over the past 65 Myr without requiring changes in continental weathering fluxes, which has been the traditional interpretation. This unconsidered player in the Os system has major implications for how we interpret Os data throughout Earth’s history and the conclusions drawn from them.
... Several studies have proposed that oxidation on δ-MnO 2 surfaces is responsible for coincident precipitation of PGE in the crusts and nodules 3,7 . However, some studies have postulated that absorption or adsorption followed by oxidation of platinum (Pt(II) → Pt(IV)) on manganese oxide also fixed Pt in the crusts and nodules [31][32][33][34][35] . Platinum hydroxo-chloro (II) and (IV) complexes react with freshly precipitated Fe-Mn oxyhydroxides over a long time and enriched in PGE 12 . ...
... PGE in seawater are either derived from terrestrial or from cosmogenic inputs 7,31 . PGE sources in the ocean are from continental-derived rivers, hydrothermal activities, cosmic source and halmyrolysis of oceanic basalts [32][33][34] Due to the occurrence of PGE in dissolved and particulate forms, deviation of PGE concentration in crusts and nodules points towards the provenance of metal supply 7,8 . Hence, a comparative study of the different ratios of PGE in the Andaman Sea crusts and nodules with probable source materials was used to understand the sources of PGE. ...
Present study explores the processes influencing the concentration, mechanism of incorporation and sources of Platinum Group of Elements (PGE) in hydrogenous ferromanganese crusts and nodules (hereafter crusts and nodules) from the West Sewell
Ridge and Sewell Rise in the Andaman Sea. ∑PGE content is lower in nodules (633 ppb) as compared to crusts (average = 1284 ppb) due to the diagenetic influence. The chondrite-normalized PGE patterns of crusts and nodules demonstrate consistent Platinum (Pt) enrichment and Palladium (Pd) depletion. We
suggest that Pt, Iridium (Ir), and Ruthenium (Ru) are scavenged from seawater by sorption and oxidation on δ-MnO2 surfaces of suspended ferromanganese oxy-hydroxides. However, non-correlation of Pd with other trace elements revealed its nonaccumulation in
crusts and nodules. Different PGE ratios reveled that Ir and Pd originated largely from a cosmogenic source and Ru, Rh, and Pt were probably sourced from ophiolites in the Andaman Accretionary Prism.
... They show no significant platinum and palladium fractionation. The PGE composition in hydrothermal crusts and metalliferous sediments inherits the ratios of PGE characteristic of seawater and MORB (mid-ocean ridge basalts) [16,40]. This PGE composition is formed due to rapid bonding of platinoids with iron and manganese oxyhydroxides. ...
... Such sediments contain large amounts of Fe-oxyhydroxide, which efficiently absorbs Os from seawater and is rapidly deposited without significant contamination with continental detrital materials 20 . The Os isotope ratio in hydrothermal metalliferous carbonates captures the signature of ambient seawater, and it does not reflect the signal of the hydrothermal fluid 21 . We reconstructed the seawater Os isotope record, together with the planktonic foraminiferal radiocarbon ( 14 C) and stable oxygen isotope (δ 18 O) records, used to determine depositional age, using samples collected from Ocean Drilling Program (ODP) Site 834A in the Lau Basin in the South Pacific Ocean (Fig. 1), where the sedimentary sequence covers the past 300 kyr, from marine isotope stage (MIS) 8 to the present (MIS 1) (Supplementary Fig. S3). Figure 2 shows high-resolution records of δ 18 O (Fig. 2a) and initial Os isotopic ratios ( 187 Os/ 188 Os) i (Fig. 2b, red line) of sediments from ODP Site 834A. ...
The solid earth plays a major role in controlling Earth’s surface climate. Volcanic degassing of carbon dioxide (CO 2 ) and silicate chemical weathering are known to regulate the evolution of climate on a geologic timescale (> 10 ⁶ yr), but the relationship between the solid earth and the shorter (< 10 ⁵ yr) fluctuations of Quaternary glacial–interglacial cycles is still under debate. Here we show that the seawater osmium isotope composition ( ¹⁸⁷ Os/ ¹⁸⁸ Os), a proxy for the solid earth’s response to climate change, has varied during the past 300,000 years in association with glacial–interglacial cycles. Our marine Os isotope mass-balance simulation reveals that the observed ¹⁸⁷ Os/ ¹⁸⁸ Os fluctuation cannot be explained solely by global chemical weathering rate changes corresponding to glacial–interglacial climate changes, but the fluctuation can be reproduced by taking account of short-term inputs of (1) radiogenic Os derived from intense weathering of glacial till during deglacial periods and (2) unradiogenic Os derived from enhanced seafloor hydrothermalism triggered by sea-level falls associated with increases of ice sheet volume. Our results constitute the first evidence that ice sheet recession and expansion during the Quaternary systematically and repetitively caused short-term (< 10 ⁵ yr) solid earth responses via chemical weathering of glacial till and seafloor magmatism. This finding implies that climatic changes on < 10 ⁵ yr timescales can provoke rapid feedbacks from the solid earth, a causal relationship that is the reverse of the longer-term (> 10 ⁶ yr) causality that has been conventionally considered.
... No other high temperature hydrothermal fluids have been sampled from active MOR environments specifically for Os concentration and 187 Os/ 188 Os analysis. Other estimates have been made by the utilization of Os concentration and isotope systematics of sulfide minerals and metalliferous sediments to predict the Os composition of hydrothermal fluids from which they are formed upon mixing with seawater (Brü gmann et al., 1998;Cave et al., 2003) (Fig. 6(a)). Specifically for the basalt-hosted TAG hydrothermal system, Brü gmann et al. (1998) estimated the hydrothermal fluid composition from the Os composition of massive sulfides sampled from the interior of an active sulfide mound, resulting in an Os concentration and 187 Os/ 188 Os composition of approximately 160 fmol/ kg and 0.37, respectively. ...
... Together, the basalt-hosted high temperature hydrothermal fluids have an average Os concentration and 187 Os/ 188 Os composition of 70 fmol/kg and 0.24, respectively. Alternatively, elevated Os concentration and the intermediate 187 Os/ 188 Os isotopic composition of metalliferous sediment surrounding the ultramafic-hosted Rainbow hydrothermal system along the Mid-Atlantic Ridge have collectively been used to predict the Os concentration of high temperature hydrothermal fluid venting from the seafloor into the overlying seawater column (Cave et al., 2003). In particular, this study predicts high concentrations of unradiogenic Os, approximately between 500 and 1400 fmol/kg, in hydrothermal fluids venting from the seafloor at Rainbow, much greater in concentration relative to measured concentration Os of vent fluids from the JdFR basalt hosted hydrothermal systems. ...
... For example, in high-temperature hydrothermal fluids at subseafloor MOR physiochemical conditions, dissolved Os is predicted to exist as Os 2+ and complexed with dissolved Cl À (Xiong and Wood, 2000). Upon venting into oxidizing seawater, hydrothermally derived dissolved Os is expected to become oxidized and exist predominantly as an oxyanion species, similar to dissolved V and Mo in modern seawater, and may complex to dissolved organic ligands and become scavenged onto Fe 3+ -oxide minerals and/or organic matter within the hydrothermal plume overlying the seafloor hydrothermal system (Palmer et al., 1988;Ravizza and McMurtry, 1993;Woodhouse et al., 1999;Ravizza et al., 2001a;Cave et al., 2003). On a much broader scale, changes in atmospheric and seawater chemistry throughout Earth's past may have influenced the physiochemical processes within high-temperature hydrothermal systems and thus the amount of sulfide precipitation in the subseafloor, which could potentially alter the hydrothermal Os flux (Kump and Seyfried, 2005;Antonelli et al., 2017). ...
This study presents experimental, geochemical modeling, and field data focusing on the partitioning of osmium (Os) between pyrite and fluid upon precipitation at conditions representative of mid-ocean ridge hydrothermal environments. Dissolved Os partitions strongly into pyrite upon precipitation at experimental conditions, 350 °C and 50 MPa, with a representative relative Os/Fe partition coefficient, DPyrite-FluidOs/Fe, between 10 and 15. Integrating the experimentally determined DPyrite-FluidOs/Fe into a geochemical model indicates that a significant amount of Os is retained within the subseafloor due to sulfide precipitation induced by mixing of conductively heated seawater with pristine high temperature hydrothermal fluids that are enriched in dissolved metals, such as Os and Fe. Comparison with existing Os concentration and isotopic data of hydrothermal fluids and sulfide minerals from a wide range of hydrothermal systems with the experimental and modeling constraints suggest that modern high temperature hydrothermal systems are a minor source of unradiogenic Os to the modern global ocean dissolved Os budget due to the majority of Os being sequestered into sulfide minerals formed within and on the seafloor.
... Ravizza and Turekian, 1989;Ravizza et al., 1991;Ravizza and Turekian, 1992;Pegram et al., 1992;Peucker-Ehrenbrink et al., 1995;Pegram and Turekian, 1999;Ravizza et al., 2001;Cohen andCoe, 2002, 2007;Ravizza and Peucker-Ehrenbrink, 2003;Cohen, 2004). Major sources of Os in seawater include volcanism (Alves et al., 1999), weathering of young subduction arc-related crust (Martin et al., 2000), hydrothermal metasomatism at mid-ocean spreading centers (Ravizza et al., 1996;Sharma et al., 2000;Pierson-Wickmann et al., 2002;Cave et al., 2003), dissolution of cosmic dust (Peucker-Ehrenbrink, 1996), and riverine input from oxidative weathering and erosion of upper Os/ 188 Os may result from major changes in the proportion of radiogenic Os and unradiogenic Os delivered to the oceans (Peucker-Ehrenbrink et al., 1995;Levasseur et al., 1999;Pegram and Turekian, 1999;Ravizza et al., 2001;Cohen andCoe, 2002, 2007;Ravizza and Peucker-Ehrenbrink, 2003;Cohen, 2004;Dubin and Peucker-Ehrenbrink, 2015). ...
The Early Cambrian was a key period in Earth's history, and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period. New Early Cambrian data for Re–Os and Mo isotopic compositions and redox-sensitive elements (RSE) are reported here for the Niutitang Formation, SE Chongqin, SW China. A Re–Os age of 520 ± 30 Myr (Model 3, 2σ, n = 21, MSWD = 62) and a (¹⁸⁷Os/¹⁸⁸Os)i ratio of 0.79 ± 0.11 were obtained, consistent with previously reported data for equivalent strata from other areas. Separate regression of samples with (¹⁸⁷Os/¹⁸⁸Os)i of 0.72–0.79 and 0.81–0.89 produce more precise ages of 520.1 ± 9.5 Myr (Model1, 2σ, n = 7, MSWD = 1.0) and 513 ± 10 Myr (Model 1, 2σ, n = 8 MSWD = 0.96) respectively, suggesting heterogeneity in the primary source of Os as the dominant cause of scatter and uncertainty. High initial ¹⁸⁷Os/¹⁸⁸Os of 0.79 ± 0.11 may indicate a high Os input from oxidative weathering of upper continental crust for Early Cambrian ocean. Based on RSE data such as Mo, U, V enrichment and Ni/Co, V/Cr, V/(V + Ni), MoEF/UEF, Re/Mo ratios, two geochemically distinct zones (upper and lower) can be delineated in the sedimentary sequence, as reflected in drillcore sections. The upper section (n = 9) is characterized by low total organic carbon (TOC) contents (mean 1.8 ± 1.5 wt%), low RSE contents (Mo (0.008 ± 0.006) × 10⁻³ g/g; V (0.22 ± 0.15) × 10⁻³ g/g; U (0.008 ± 0.004) × 10⁻³ g/g), and low V/Cr (2.0 ± 1.7), Ni/Co (5.9 ± 3.5), MoEF/UEF (2.6 ± 1.8), and Mo/TOC (6.0 ± 3.9) ratios; in contrast, the lower core section (n = 13) is characterized by high TOC contents (4.4 ± 2.2 wt%), high RSE contents (Mo (0.10 ± 0.08) × 10⁻³ g/g; V (1.0 ± 1.0) × 10⁻³ g/g; U (0.05 ± 0.04) × 10⁻³ g/g), high V/Cr (6 ± 5), Ni/Co (24 ± 14), MoEF/UEF (6.3 ± 2.2), and Mo/TOC (21 ± 9) ratios (uncertainties are ±1SD here). Significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic/euxinic to oxic condition. All samples display extremely low Re/Mo ratios of 0.2 × 10⁻³–1.9 × 10⁻³, indicating the absence of intermediate reducing conditions, possibly reflecting rapid ocean oxygenation. The Mo isotopic composition in the sedimentary succession of the YC9 core exhibits a large variation, with δ98/95Mo values of 0.04–2.00‰. Upper section δ98/95Mo values display an opposite trend to Mn content and negative correlation with Mo/TOC ratios, indicating Mo fractionation associated with adsorption on Mn oxides or oxyhydroxides. Lower section δ98/95Mo values are positively correlated with Mo/TOC ratios and negatively with Re/Mo ratios, suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions. High δ98/95Mo of 2.00‰ and high average Mo/TOC ratios of 21 (10⁻⁶ g/g)/wt% were observed in euxinic shales in the Niutitang Formation, suggesting extensive ocean oxygenation may have taken place at ca. 520 Ma.
... Possible sources of platinum group elements in the ocean can be riverine fluxes, hydrothermal processes, cosmic dust flux, and halmyrolysis of oceanic basalts. If riverine and cosmic inputs have been estimated in some works [59,60], the influence of hydrothermal process and basalt halmyrolysis are significantly less studied [61]. Ferromanganese deposits in the ocean are a significant sink of metals entering the ocean and indicate the source of the metal supply. ...
Distribution of platinum group elements (Ru, Pd, Pt, and Ir) and gold in hydrogenous ferromanganese deposits from the southern part of the Atlantic Ocean has been studied. The presented samples were the surface and buried Fe–Mn hydrogenous nodules, biomorphous nodules containing predatory fish teeth in their nuclei, and crusts. Platinum content varied from 47 to 247 ng/g, Ru from 5 to 26 ng/g, Pd from 1.1 to 2.8 ng/g, Ir from 1.2 to 4.6 ng/g, and Au from less than 0.2 to 1.2 ng/g. In the studied Fe–Mn crusts and nodules, Pt, Ir, and Ru are significantly correlated with some redox-sensitive trace metals (Co, Ce, and Tl). Similar to cobalt and cerium behaviour, ruthenium, platinum, and iridium are scavenged from seawater by suspended ferromanganese oxyhydroxides. The most likely mechanism of Platinum Group Elements (PGE) accumulation can be sorption and oxidation on δ-MnO2 surfaces. The obtained platinum fluxes to ferromanganese crusts and to nodules are close and vary from 35 to 65 ng∙cm−2∙Ma−1. Palladium and gold do not accumulate in hydrogenous ferromanganese deposits relative to the Earth’s crust. No correlation of Pd and Au content with major and trace elements in nodules and crusts have been identified.
