Seawater Os Research Articles

Re-Os geochronology of the Middle to Upper Jurassic marine black shales in the Agardhfjellet Formation, Central Spitsbergen, Svalbard: A cornerstone for global faunal correlation and Os isotopic change

Ammonite provincialism makes it difficult to link the Boreal and Tethyan realms during the Late Jurassic to Early Cretaceous time. Absolute ages through radiometric dating offer new age information that is independent and complementary to relative ages based on biostratigraphy. In this study, we report seven new Re-Os ages for black shales from the Agardhfjellet Formation, Svalbard. We also report pyrolysis results, carbon and sulfur stable isotopic data, and initial 187Os/188Os ratios to evaluate the paleoenvironment during organic-rich shale deposition. Re-Os ages are derived from three Boreal ammonite zones: (1) 159.2 and 160.1 Ma from the shale intervals containing Cadoceras sp.; (2) 149.6, 149.9, 150.8, and 151.9 Ma from the Rasenia cymodoce Zone in the Early Kimmeridgian, which is the equivalent of the Sutneria platynota and Ataxioceras hypselocyclum zones in the Tethyan realm; (3) 146.8 Ma from an upper Volgian interval above the occurrence of Laugeites sp.. Within this 14.5 Myr interval, the initial Os profile shows a gradually increasing trend from 0.335 (one of the lowest shale values throughout the Phanerozoic) to 0.529, consistent with previous initial Os studies and Sr isotopic ratio chemostratigraphic studies. The trend of increasing initial Os ratios indicates an increase in continental runoff relative to unradiogenic Os input from mantle sources, such as seafloor hydrothermal activity or ultramafic source rocks. We provide new Re-Os geochronological anchors in the Late Jurassic with new Re-Os ages and propose correlations of specific Late Jurassic ammonite zones between the Tethyan and Boreal realms. In turn, the gradual increase in seawater Os isotopic ratios may signal climate change in the Late Jurassic, indicating the increase of radiogenic Os or decrease of unradiogenic Os, induced by high continental runoff or less hydrothermal/volcanic activity.

Marine osmium isotopic composition reconstructed from the early Permian umber deposit in the Japanese accretionary complex

• Re-Os isotopes of early Permian umbers in Japan were analyzed. • Oxidative Mn-oxides prevented Re fixation into the Kunimiyama umbers. • 187 Os/ 188 Os values of early Permian seawater ranged from 0.523 to 0.676. 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 ( 187 Os/ 188 Os) 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 ( 187 Os/ 188 Os) 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.

Tracking magmatism and oceanic change through the early Aptian Anoxic Event (OAE 1a) to the late Aptian: Insights from osmium isotopes from the westernmost Tethys (SE Spain) Cau Core

Some of the major Carbon cycle perturbations of the Phanerozoic occurred during the Aptian, in relation to magmatism. The highest temperatures reconstructed for the Cretaceous Period correspond to the Oceanic Anoxic Event of the early Aptian (OAE 1a), an episode of accelerated global change. Here we present a chemostratigraphic study based on osmium isotopes integrated with high-resolution Carbon-Oxygen stable isotope data from the Cau Core (Western Tethys, SE Spain), including a 6.4 Ma record from the early to the late Aptian. This high-resolution study of the continuous and expanded Cau section permits a thorough understanding of the duration of the Aptian events, as well as an evaluation of the mechanisms triggering the abrupt changes of the global carbon and osmium cycles and their interdependence. Here we show that the Large Igneous Province (LIP) Aptian magmatism initiated 550–750 kyr prior to the OAE 1a, and persisted for 1.4 Myr after the event, influencing the composition of seawater for 2.8 Myr. We show a continuous Os isotope record encompassing the OAE 1a and the late Aptian for the first time, and demonstrate that the recovery from the exceptionally unradiogenic composition of seawater Os produced by the dominance of the Ontong Java Plateau volcanism, was slow. Our results demonstrate the different time duration of some events, and the asynchronous relationship between the carbon and osmium cycles

Seawater Chemistry and Hydrothermal Controls on the Cenozoic Osmium Cycle

AbstractOsmium isotope ratios are a key tool to track changes in global weathering and carbon cycle evolution through time. Long‐term changes in seawater Os isotope records over the Cenozoic have been used to argue for changes in weathering from increased uplift, leading to long‐term global cooling. Sulfide mineral precipitation during hydrothermal circulation traps the majority of mantle‐derived Os within mid‐ocean ridge (MOR) hydrothermal systems. Building from evidence that the amount of sulfide mineral precipitation in the subseafloor of MOR systems is related to the amount of sulfate and calcium in seawater, we show that as seawater chemistry changed through time, so too has the extent of hydrothermal sulfide formation and the global Os cycle. With currently estimated changes in seawater Ca/SO4 ratios, the observed progressive increase in seawater Os isotope values through the Cenozoic can be linked to changes in seawater chemistry—instead of a major shift in continental weathering processes.

High-precision Re[sbnd]Os dating of Lower Jurassic shale packages from the Western Canadian Sedimentary Basin

The Lower Jurassic rock record contains abundant organic-rich mudrocks suitable for refining Early Jurassic time using ReOs organic-rich shale (ORS) geochronometry, such as those associated with the Sinemurian–Pliensbachian boundary event or the Toarcian Ocean Anoxic Event (T-OAE). Despite this, most studies to date have failed to obtain high precision, geologically meaningful, ages for such Lower Jurassic rocks. The cause of imprecision in ReOs ORS age dates, at least for the Toarcian, has been linked to subtle chemostratigraphic variations in 187Os/188Osi values. Here, we report the first high-precision ReOs ages of organic-rich shale packages of the Gordondale and basal Poker Chip Members of the Lower Jurassic Fernie Formation of Northwestern Alberta, Canada from the Western Canadian Sedimentary Basin (WCSB). Our ReOs age results bracket sediment deposition between the late Sinemurian-early Pliensbachian (192.11 ± 0.77 Ma [2σ] and 193.83 ± 1.34 Ma [2σ]) and late Toarcian (180.10 ± 2.68 Ma [2σ]). When coupled with chemo- (87Sr/86Sr and 187Os/188Os isotopes) and bio-stratigraphy (ammonite and coccolith), our ReOs ages correlate with regional and global stratigraphic tie points corresponding to pivotal points of Early Jurassic time, such as the Sinemurian–Pliensbachian boundary event, T-OAE, and late Toarcian, that are otherwise weakly constrained by isotopic methods. This integrative approach helps to place these events in a more robust temporal framework. Based on seawater Os isotope data, we also speculate that a previously unidentified positive Os isotope excursion exists at the Sinemurian–Pliensbachian boundary - perhaps triggered by late-stage Central Atlantic Magmatic Province (CAMP) volcanism.

Re-Os geochronology and isotope systematics, and organic and sulfur geochemistry of the middle–late Paleocene Waipawa Formation, New Zealand: Insights into early Paleogene seawater Os isotope composition

In the middle–late Paleocene, a marine, organic-rich sedimentary unit (Waipawa Formation [Fm]) in which the organic matter was derived mainly from terrestrial plants was deposited in many of New Zealand's sedimentary basins. The unique organofacies of this formation has not been identified in any other time interval within the geological history of the Southwest Pacific, indicating that unusual climatic and oceanographic conditions likely prevailed during this time. It has, therefore, attracted wide scientific interest due to its significance for regional and global reconstruction of the early Paleogene transitional climate as well as potential for oil and gas production. Scarcity of age-diagnostic fossils, presence of unconformities and lack of volcanic interbeds have, however, hindered precise dating and correlations of all the known occurrences of the formation. Here, rhenium‑osmium (Re-Os) geochronology has yielded the first radiometric age for the formation (57.5 ± 3.5 Ma), which is consistent with available biostratigraphic age determinations (59.4–58.7 Ma). Further, a comparison of Re-Os, bulk pyrolysis, sulfur and palynofacies data for the Waipawa Fm with those of more typical marine sediments such as the underlying Whangai Fm supports the interpretation that the chelating precursors or fundamental binding sites responsible for uptake of Re and Os are present in all types of organic matter, and that these elements have a greater affinity for organic chelating sites than for sulfides. The results also indicate that sedimentation rate may not play a dominant role in enhanced uptake of Re and Os by organic-rich sedimentary rocks.The initial 187Os/188Os values for the Waipawa (~0.28) and Whangai (~0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine 187Os/188Os record of the Paleocene. We present a compilation of 187Os/188Os values from organic-rich sedimentary rocks spanning the period between 70 and 50 Ma which shows that seawater Os gradually became less radiogenic from the latest Cretaceous, reaching a minimum value in the earliest late Paleocene (~59 Ma) during the deposition of Waipawa Fm, and then increased through the later Paleocene and into the early Eocene. The composite Os isotope record broadly correlates with global temperature (δ18O and TEX86) and carbon isotope (δ13C) records from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns.

The seawater osmium isotope record of South China Sea: Implications on its history and evolution

The South China Sea is the largest marginal basin in the world, and is located in the confluence of three major tectonic plates (Pacific-Philippine Sea, Indo-Australia, Eurasia). Its tectonic history and evolution throughout the Cenozoic has been the subject of much discussion with regards to its mechanism, timing, and relationship to neighboring geological features. We conducted the first Re-Os isotope study of the South China Sea cores recovered from ocean drilling (ODP-Ocean Drilling Program and IODP-International Ocean Discovery Program) and from Philippine oil exploration wells in order to investigate whether or not the basin's tectonic evolution will be reflected in the seawater Os isotope record. These cores represent the northern and southern conjugate margins of the South China Sea, and should have complementary records of the tectonic events in the basin. We found three important negative anomalies in the South China Sea's seawater Os isotope record at 35–33Ma, 26–23Ma, and ~13–6Ma, which are temporally related to tectono-magmatic events. At the northern margin, the record starts with the lowest (187Os/188Os)i values of 0.37 and 0.32 for the oldest samples, which then progresses to more radiogenic values for younger sediments. The trend is consistent with the start of seafloor spreading at ~33Ma that saw the peak in the flux of mantle-derived material, which later slowed down until the second peak between 26 and 23Ma. However, these unradiogenic values also mark the Eocene-Oligocene Transition and more likely implies South China Sea connectivity to global ocean circulation since pre-Oligocene. In both margins, slight to marked decrease in (187Os/188Os)i values deviate from the global trend and coincides with the ridge jump at ~23Ma. In the southern conjugate margin, (187Os/188Os)i show a general decreasing tendency opposite that of the northern margin and the global trend starting from Mid- to Late Miocene (~13–6Ma). Mass balance modeling suggests that the two younger negative excursions may be accounted for by mantle and hydrothermal Os inputs of ~372mol/my at ~23Ma and post ~13Ma. We suggest that the post ~13Ma protracted decrease in (187Os/188Os)i values for the southern margin reflects localized input of mantle-derived lithogenic Os from the Luzon Arc combined with post-spreading magmatic intrusions, and possibly gradual changes in paleoceanographic condition at the southern East Sub-basin.

Temporal record of osmium concentrations and 187Os/188Os in organic-rich mudrocks: Implications for the osmium geochemical cycle and the use of osmium as a paleoceanographic tracer

We present a compilation of 192Os concentrations (representing non-radiogenic Os) and initial 187Os/188Os isotope ratios from organic-rich mudrocks (ORM) to explore the evolution of the Os geochemical cycle during the past three billion years. The initial 187Os/188Os isotope ratio of a Re-Os isochron regression for ORM constrains the local paleo-seawater 187Os/188Os, which is governed by the relative magnitudes of radiogenic Os (old continental crust) and unradiogenic Os (mantle, extraterrestrial, and juvenile/mafic/ultramafic crust) fluxes to seawater. A first-order increase in seawater 187Os/188Os ratios occurs from the Archean to the Phanerozoic, and may reflect a combination of increasing atmosphere-ocean oxygenation and weathering of progressively more radiogenic continental crust due to in-growth of 187Os from radioactive decay of 187Re. Superimposed on this long-term trend are shorter-term fluctuations in seawater 187Os/188Os ratios as a result of climate change, emplacement of large igneous provinces, bolide impacts, tectonic events, changes in seafloor spreading rates, and lithological changes in crustal terranes proximal to sites of ORM deposition. Ediacaran-Phanerozoic ORM have mildly higher 192Os concentrations overall compared with pre-Ediacaran Proterozoic ORM based on the mean and 95% confidence interval of 10,000 median values derived using a bootstrap analysis for each time bin (insufficient Archean data exist for robust statistical comparisons). However, there are two groups with anomalously high 192Os concentrations that are distinguished by their initial 187Os/188Os isotope ratios. Ediacaran-Cambrian ORM from South China have radiogenic initial 187Os/188Os, suggesting their high 192Os concentrations reflect proximal Os-rich crustal source(s), ultraslow sedimentation rates, and/or other unusual depositional conditions. In contrast, the unradiogenic initial 187Os/188Os and high 192Os concentrations of some Mesozoic ORM can be tied to emplacement of large igneous provinces. Excluding these two anomalous groups and repeating the bootstrap analysis, we find that, overall, the 192Os concentrations for the Ediacaran-Phanerozoic and pre-Ediacaran Proterozoic time bins are not significantly different. An improved understanding of Os geochemical behavior in modern environments is required before our compilation can be fully used to constrain the temporal evolution of the seawater Os reservoir.

Paleoceanographic conditions on the São Paulo Ridge, SW Atlantic Ocean, for the past 30 million years inferred from Os and Pb isotopes of a hydrogenous ferromanganese crust

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 ~30Myr with growth rates of 0.5–3mm/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 ~30Myr. 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 ~30Ma.

Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet

High-resolution Os isotope stratigraphy can aid in reconstructing Pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine Os residence time. This paper presents new Os isotope data from ocean cores adjacent to the West Greenland ice sheet that have excellent chronological controls. Cores MSM-520 and DA00-06 represent distal to proximal sites adjacent to two West Greenland ice streams. Core MSM-520 has a steadily decreasing Os signal over the last 10 kyr (187Os/188Os = 1.35–0.81). In contrast, Os isotopes from core DA00-06 (proximal to the calving front of Jakobshavn Isbræ) highlight four stages of ice stream retreat and advance over the past 10 kyr (187Os/188Os = 2.31; 1.68; 2.09; 1.47). Our high-resolution chemostratigraphic records provide vital benchmarks for ice-sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change. Variations in Os isotope composition from sediment and macro-algae (seaweed) sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater Os isotope composition. Further, these findings demonstrate that the residence time of Os is shorter than previous estimates of ∼104 yr.

Secular change of the Triassic seawater Os isotope composition reconstructed from bedded cherts in the Sakahogi Section, Mino Belt, Japan

Secular change of the Triassic seawater Os isotope composition reconstructed from bedded cherts in the Sakahogi Section, Mino Belt, Japan

Rhenium–osmium abundance and isotopic compositions of massive sulfides from modern deep-sea hydrothermal systems: Implications for vent associated ore forming processes

Studies of rhenium (Re) and osmium (Os) concentrations and isotopic compositions in seafloor hydrothermal sulfides are an important tool for understanding the evolution of hydrothermal systems, allowing the determination of both metal sources and reconstructing the physicochemical conditions of their deposition. The Re–Os concentrations and isotopic compositions of 38 massive sulfide samples have been studied in different hydrothermal fields from the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and Back-Arc Basin (BAB). The majority of the sulfides possess 187Os/188Os that span a narrow range (1.004 to 1.209), which is most easily explained as a seawater-derived component. This may suggest that those initial 187Os/188Os isotope compositions of ancient seafloor hydrothermal sulfide deposits which were formed by the mixing process between seawater and hydrothermal fluid, are possible for analysing ancient seawater Os components. Only two of samples (MAR05-TVG1-10-2 and MAR05-TVG1-21 from the Logatchev hydrothermal field (LHF), MAR) possess moderately less radiogenic 187Os/188Os (0.645 to 0.730), which may reflect the lesser extent of hydrothermal fluid–seawater mixing during hydrothermal ore-forming process.The Re and Os concentrations and 187Re/188Os ratios of pyrite and Fe–Cu sulfide mineral aggregates (avg Re 11.46 ppb; avg Os 17.76 ppt; avg 187Re/188Os 11,980.4; n=24) are usually higher than those of sphalerite or Zn-enriched sulfide mineral aggregate samples (avg Re 0.31 ppb; avg Os 7.09 ppt; avg 187Re/188Os 206.99; n=9), suggesting that Re and Os are more likely to be incorporated into Fe- and Fe–Cu sulfide mineral facies. A significant positive correlation is observed between Os/Re ratios and Pb concentrations in the sulfide samples from the Edmond hydrothermal field (EHF), whereas Pb-enriched sulfide is characteristic of low-temperature paragenetic association, suggesting that Os enriched under low-temperature (<200 °C) condition. In addition, our Re–Os data are used to estimate that modern seafloor sulfide deposits contain roughly 0.6–44 t (avg 4 t, n=38) of Re, and 1 to 48 kg (avg 8 kg, n=38) of Os. The Os flux of global low-temperature hydrothermal fluids to vents is about 11 kg/yr, and the excess Os (i.e. Os not incorporated in sulfides) may be carried and become bound in metalliferous sediment, Fe–Mn crusts and nodules distal to the hydrothermal vents.

Osmium isotope evidence for a large Late Triassic impact event

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (187Os/188Osi) of ∼0.477 to unradiogenic values of ∼0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous–Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3–7.8 km in diameter) produced a global decrease in seawater 187Os/188Os ratios in the Late Triassic.

Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a

The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of ∼880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (∼100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.

Osmium concentrations and 187Os/188Os ratios of three sediment reference materials

187Os/188Os ratios and osmium concentrations were analyzed in sedimentary reference materials JSd-3, JMS-1, and JMS-2, issued by the Geological Survey of Japan, using the isotope dilution method with negative thermal ionization mass spectrometry (ID-N-TIMS). The chemical procedures used in this study are based on Carius tube digestion combined with carbon tetrachloride extraction to separate Os. The obtained 187Os/188Os ratios of JSd-3, JMS-1, and JMS-2 were 0.941-1.149, 0.3894-0.465, and 0.747-0.848, respectively, and their Os abundances were 27.07-39.32 ppt, 80.8-106.6 ppt, and 281-317.2 ppt, respectively. Variations among the samples presumably reflect differences in sources and depositional environments. The Os isotopic ratios and abundances were variable within individual samples. The relative 95% confidence intervals of Os concentrations and isotopic ratios were respectively 13% and 6.2% for JSd-3, 15% and 9.8% for JMS-1, and 4.3% and 4.2% for JMS-2. These variations probably reflect the heterogeneity of the original rock of each reference material. Low concentration and high isotope ratio of JSd-3 probably results from the derivation of the original rocks from continental components with low Os abundance and elevated 187Os/188Os. Inputs of anthropogenic Os were inferred in JMS-1, a nearshore sediment from an urbanized locality, which had high Os contents and relatively low Os isotopic ratios. Greatly elevated Os concentrations and high 187Os/188Os in JMS-2, a pelagic clay, may reflect the scavenging of Os in seawater into Mn-Fe oxyhydroxides deposited on the seafloor.

Behavior of osmium at the freshwater-saltwater interface based on Ganga derived sediments from the estuarine zone

[1] Leaching experiments performed on several sediments of the Ganga river suggest that between 10 and 20% of the osmium is in an easily exchangeable position. Analyses of sediments from two estuarine rivers (the Pussur and the lower Meghna) reveal no enrichment of Os in the saltwater–freshwater mixing zone relative to Ganga sediments of similar Al2O3/SiO2 ratio, suggesting that osmium is not significantly trapped in this estuary. However, a significant decrease of the 187Os/188Os ratio is observed in the Pussur sediments relative to the Ganga composition. These latter are derived entirely from the Ganga or from erosion of the Ganga paleodelta, and thus would be expected to have similar Os isotopic compositions. Nd isotopic results from the Pussur are indistinguishable from those of the Ganga, while the Sr isotopic results are at the lower end of the Ganga range, confirming the absence of a major source difference between Ganga and Pussur sediments. It thus seems unlikely that the difference in Os isotopic signature can be entirely explained by a change in provenance, suggesting instead that the Os compositions have been modified. Our results show that the less radiogenic Os compositions of the Pussur sediments cannot result simply from desorption of radiogenic Os or from scavenging of river or seawater Os. Instead, the decrease of the 187Os/188Os ratio could imply a complex exchange between dissolved Os, derived partly from seawater, and Os in the leachable fraction of sediments. This mechanism could therefore constitute both a source and a sink for seawater osmium and may significantly influence the osmium marine budget.

Lead isotopic record of Barremian–Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis

We present initial isotopic ratios of lead for Early Cretaceous (Barremian–Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

Climate driven glacial–interglacial variations in the osmium isotope composition of seawater recorded by planktic foraminifera

This study presents osmium (Os) isotope and elemental data for cleaned planktic foraminifera, authigenic Fe–Mn oxyhydroxides and pelagic carbonate host sediments from ODP site 758 in the southernmost reaches of the Bay of Bengal. The Os in the bulk sediments appears to be dominantly hydrogeneous (sourced by carbonate and Fe–Mn oxyhydroxide), but variations in this particular core are controlled by the presence of volcanic ash. Fe–Mn oxyhydroxide leachates (of the bulk sediments) from Holocene samples also yield an Os isotope composition close to that of seawater, but the record diverges from that of foraminifera at a depth corresponding to the oxic/post-oxic boundary, suggesting diagenetic mobilization of Os at depths below this. Holocene planktic foraminifera, cleaned using oxidative-reductive techniques, also give Os isotope compositions indistinguishable from modern seawater, but the record obtained for the past 150kyr shows strong covaraitions of 187Os/188Os with both the local and global oxygen isotope record, with less radiogenic Os isotope compositions during glacial intervals. These results indicate that foraminifera provide a robust record of seawater Os isotope compositions, and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187Os/188Os over this time interval, while the covariation with oxygen isotopes suggest a process controlling the Os isotope composition that is in phase with global climate cycles. Global excursions to relatively unradiogenic 187Os/188Os during glacial intervals are consistent with decreased input of radiogenic continental material, reflecting cooler temperatures and reduced continental runoff. Modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an ∼30% decrease in the global river flux, with an ∼5% change in river composition. If the residence time of Os in the oceans is ∼5ka then the post-glacial recovery to present-day seawater values is consistent with a corresponding increase in the river flux of around 30%. However, if the residence time of Os is closer to 40ka, as is suggested by the global river flux, then this demands either significant changes in both the riverine Os flux and composition of around 40% and 30%, respectively, that closely follow the oxygen isotope record, or else a short-lived post-glacial pulse of weathering some 75% greater than the steady-state flux. In either case, these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial–interglacial timescales.

Re–Os geochronology of Arctic black shales to evaluate the Anisian–Ladinian boundary and global faunal correlations

Boreal and Tethyan faunas differ throughout the Triassic, limiting the use of biostratigraphy for global correlation. Rhenium–osmium (Re–Os) dating of organic-rich black shales, calibrated with existing biostratigraphy, provides a new means to establish correlations of global fauna using absolute time. Here we present Re–Os radiometric ages for Middle Triassic organic-rich shales from two biostratigraphically defined sections at Svalbard and the Svalis Dome in the Barents Sea. Mature black shales from Svalbard, inferred to be Middle to Late Anisian, define a 241.2 ± 2.2 Ma isochron with an initial 187Os/ 188Os ratio of 0.831 ± 0.025, the highest seawater Os isotope ratio yet recorded between the Cambrian and Lower Jurassic. Svalis Dome shales in the uppermost Anisian yield a 239.3 ± 2.7 Ma age and initial 187Os/ 188Os ratio of 0.679 ± 0.020. The higher initial 187Os/ 188Os ratio for Svalbard shales most likely reflects global seawater at the time of deposition, whereas the lower initial 187Os/ 188Os ratio for the Svalis Dome may represent seawater with limited communication to the open ocean, as suggested by the ratio of trace metal abundances and total organic carbon contents. These age data indicate a maximum age of 239.3 ± 2.7 Ma for the Anisian–Ladinian boundary in the Arctic Boreal realm. This age agrees with the proposed boundary in the Tethyan realm, 240 to 243 Ma, and thus confirms the correlations between Boreal and Tethys based on biostratigraphy and magnetostratigraphy. Our study affirms that Re–Os geochronology of black shale is a reliable method for obtaining depositional ages and establishing biostratigraphic correlations between paleogeographically separated regions.

High Precision and High Sensitivity Measurements of Osmium in Seawater

We present improved chemical separation and mass spectrometric procedures that permit precise and accurate determination of Os concentration and isotopic composition in seawater samples. Water samples (50 mL) with added (190)Os tracer are heated to 300 degrees C at 128 bar in quartz carius tubes with Cr(VI)O(3). This allows tracer-sample equilibration via complete oxidation of all Os species to OsO(4), which is then separated and purified using distillation. Samples are measured using negative thermal ionization mass spectrometry. A new loading technique using face-to-face double-filament geometry is developed that reduces organic interference and improves effective ionization efficiency, which is about 5% for 200 fg of Os (1 fg of Os = 3 x 10(6) atoms). The total yield of the chemical procedure is approximately 90%, and the total procedural blank is 3.6 fg. We applied the new procedure on seawater depth profiles from the Atlantic, Pacific, and Southern Oceans and found that (1) the improved procedure results in an increase in Os concentration of 25-35% over previous methods, (2) Os in seawater does not appear to behave conservatively, and (3) surface seawater has lower (187)Os/(188)Os values than deep waters.