Paleoredox Proxy Research Articles

Controls on organic matter accumulation on the Late-Ordovician shales in Awati Sag in northwestern Tarim Basin, NW China

The black shales of the Upper Ordovician Yingan Formation (Fm.) in the northwestern margin of the Tarim Basin was previously considered to be an important source rock. However, the total organic carbon (TOC) values of Yingan black shales was low, averaging 0.51%. Therefore, it remains debated whether they can be served as high-quality source rock. Here, we investigated the sedimentary environment (including terrigenous input, chemical weathering intensity, paleoproductivity, paleoredox conditions) of the Yingan shales and the factors affecting organic matter (OM) enrichment through organic and inorganic geochemical methods. The proxies of terrigenous flux (e.g., Ti/Al and Zr/Al) and sedimentation rate flux [e.g., (La/Yb)P] shows that the terrigenous inputs has no effect on OM enrichment. The low primary productivity (e.g., low values of P/Al, Ba/Al and Ni/Al) of the Yingan shales also exhibits a little effect on OM enrichment. However, the paleoredox proxies (e.g., U/Al, V/Al, U/Th and V/Cr) show that the black shale was deposited in a partial oxidation environment of bottom waters, which is unfavorable for the preservation and enrichment of OM evidenced. The oxidation environment was caused by the enhanced thermohaline circulation and ventilation of bottom waters due to the increasing cooling global climate during Late Ordovician. Therefore, the Yingan shales cannot be served as a source rock or shale gas reservoir. Additionally, the poor OM enrichment model provides new insights for the evaluation of shales linked to cold climatic conditions.

Aluminium-normalised trace-element paleoredox proxies and their application to the study of the conditions of Burgess Shale-type preservation

The paleo-ecological settings of Burgess Shale-type (BST) fossils have been the subject of extensive investigations, particularly with regard to the role of anoxia in their exceptional preservation. Our understanding of the complexity of both paleontological and geochemical environmental proxies has evolved with increased knowledge of niche communities and marine geochemical and biogeochemical processes. Accordingly, dated approaches should be reevaluated. The commonly applied paleoredox proxies V/Cr, Ni/Co, and V/(V + Ni), as well as the redox proxy V/Al2O3 and productivity proxy Ba/Al2O3, were all tested on the Wheeler Formation (Utah, USA) for which a fluctuating oxycline environment has been well established based on paleontological features. Subsequently, these proxies were applied to three additional BST fossil-bearing deposits: the Emigsville Member of the Kinzers Formation (Pennsylvania, USA), the Burgess Shale (British Columbia, CA) and the Emu Bay Shale (South Australia, AU). Their host shales lack perceptible V of marine origin, indicative of oxic conditions, in contrast to the Wheeler and brine-related strata from the Burgess Shale which include additional marine input of V in some beds, consistent with varying redox conditions. The black shales of the Longs Park Member (Kinzers) have elevated V/Al2O3 indicative of anoxic conditions. Molybdenum concentrations are relatively low in most samples, but minor variations tend to correlate with V/Al2O3 patterns. Ba/Al2O3 indicates high productivity with deposition of BST fossils in the Wheeler Formation, and diagenetic remobilisation of Ba owing to sediment anoxia in the Burgess Shale. Our findings show that redox proxy V/Al2O3 can be effectively applied to BST samples, whereas Ba/Al is a reliable proxy for low to moderate paleoproductivity levels and may be useful as a secondary or tangential proxy for redox conditions. Taphonomic processes leading to BST preservation occur in a variety of environments characterised by oxyclines, matgrounds, variable sedimentation rates, hypersalinity and/or anoxia. KEY POINTS V/Al2O3 and Ba/Al2O3 are effective proxies for anoxia and productivity in BST deposits. BST preservation can occur in a variety of depositional environments. Marine anoxia is not required for BST preservation.

Modeling the impacts of diagenesis on carbonate paleoredox proxies

The geochemistry of the carbonate sedimentary record is invaluable for understanding the ancient seawater evolution of carbon and redox-sensitive elements. However, the application of carbonate geochemical paleoredox proxies can be limited by our ability to recognize records impacted by syn- and post-depositional diagenesis with pore fluids that have geochemical compositions which are distinct from the overlying seawater—resulting from variable redox conditions, fluid sources, and sediment- vs seawater-buffered conditions. We extend a numerical framework, established for the application of Ca isotopes, for recognizing sediment- vs fluid-buffered alterations during aragonite-to-calcite recrystallization for widely applied carbonate paleoredox proxies: iodine ratios (I/Ca), cerium anomaly (Ce/Ce*), and carbonate-associated Cr, U, and S isotopes. We model endmember reducing and oxidizing fluid diagenetic scenarios, as opposed to incorporating biogeochemical evolution of pore fluids within the model. The results reveal that early, fluid-buffered diagenesis of relatively unevolved seawater (“seawater-buffered”) represents the ideal scenario for preservation for all proxies, including C isotopes. Conversely, fluid-buffered alteration in pore fluids with redox conditions highly evolved from the overlying water column is likely to alter the primary carbonate geochemistry. Model calibration against records from the Bahamas Clino and Unda cores suggests each proxy is uniquely sensitive to a given style of diagenetic alteration, with Ce/Ce* the most robust and I/Ca ratios most sensitive to early diagenetic alteration. We recommend multiple strategies, including relationships with C, O, and Ca isotope data, that can be leveraged to identify the preservation of primary seawater geochemistry for a given proxy. Last, we compare the diagenetic model predictions with published records from the Permian/Triassic boundary to demonstrate how incorporating stratigraphic geochemical patterns alongside interpretation of diagenetic model results can support interpretation of secular seawater signals. To improve our understanding of the effects of diagenesis on carbonate paleoredox proxies, additional experimental constraints, field-based observations, and multi-proxy datasets are needed.

Paleoceanographic Conditions of Neo-Tethys Deposits in Northeast Iraq (Shiranish Formation) by Geochemical Proxies

Geochemical proxies provide detailed information on depositional environment and diagenetic processes. The main objectives of the present study are the evaluation of the paleoenvironment and diagenetic conditions of selected three sections of the Shiranish Formation (Late Campanian-Maastrichtian) in Dokan-Sulaymaniyah, Kurdistan Region, northeast of Iraq. The major and some trace elements were analysed by X-Ray Fluoresces. These geochemical results showed a positive correlation of SiO2, Al2O3, Fe2O3, MgO and TiO2 between each other due to the influence of detrital influx from the active oceanic margins and thrust belts of the northeast Arabian Plate. The terrigenous supply of silicates (K-feldspar) and clay minerals (Illite) caused a positive correlation between K2O values and Al2O3. In general, there is a low abundance of SiO2 and Al2O3 contents, referring to the deepening of the Neo-Tythus ocean basin in the formation's lower part. The Mn* factor of the lower and middle part of the formation exhibited anoxic conditions while oxic- suboxic conditions at the upper part. The Mn/Sr ratios were less than one, which means that Shiranish marly limestone was well preserved out of diagenetic processes. The paleoredox proxies {Th/U, V/Cr &V/(V+Ni)} confirmed that the lower part of the Formation was deposited in a relatively anoxic deep-outer shelf environment with a rising in paleoproductivity. In contrast, the upper part was characterised by dyoxic-oxic redox. Due to the inner shelf environment, this redox fluctuation reflects a regression phase of sea level, which is compatible with the closure events of the Neo-Tethys in the Eocene epoch.

Pentavalent Uranium Incorporated in the Structure of Proterozoic Hematite.

Characterizing the chemical state and physical disposition of uranium that has persisted over geologic time scales is key for modeling the long-term geologic sequestration of nuclear waste, accurate uranium-lead dating, and the use of uranium isotopes as paleo redox proxies. X-ray absorption spectroscopy coupled with molecular dynamics modeling demonstrated that pentavalent uranium is incorporated in the structure of 1.6 billion year old hematite (α-Fe2O3), attesting to the robustness of Fe oxides as waste forms and revealing the reason for the great success in using hematite for petrogenic dating. The extreme antiquity of this specimen suggests that the pentavalent state of uranium, considered a transient, is stable when incorporated into hematite, a ubiquitous phase that spans the crustal continuum. Thus, it would appear overly simplistic to assume that only the tetravalent and hexavalent states are relevant when interpreting the uranium isotopic record from ancient crust and contained ore systems.

Ironstone as a proxy of Paleozoic ocean oxygenation

Marine ironstone is a Phanerozoic biochemical sedimentary rock that contains abundant primary iron. Although rare, ironstone is conspicuous in the Paleozoic sedimentary record. Its iron source remains contentious, with traditional models invoking a continentally derived source. Increasing sedimentologic evidence suggests that many Paleozoic ironstones formed along favourably oriented continental margins where coastal upwelling delivered ferruginous waters, with the postulated source of iron being deep-ocean hydrothermal fluids. Thus, deep-water redox conditions are potentially a critical control on the concentration of iron in early Phanerozoic seawater and the accumulation of ironstone. We report novel in situ strontium isotopic and europium anomaly data from five geographically and temporally distinct Ordovician-Silurian ironstones from the Rheic and Iapetus oceans to test the emerging ferruginous upwelling model. These data provide robust evidence that ironstone formed from marine fluids with a component of mid-ocean ridge hydrothermal waters that remained largely unmixed during transport to the locations of ironstone accumulation. These results imply that the deep oceans must have been oxygen deficient to allow local or regional enrichment in hydrothermal iron. Such conditions are interpreted to have been associated with a spatiotemporally variable redox-stratified global ocean with anoxia beneath an oxic surface layer. Thus, sedimentologic and geochemical data presented herein agree with other paleoredox proxies that suggest rising oxygen levels in the Neoproterozoic gave way to early Paleozoic oceans with dynamic redox conditions. The temporal record of Paleozoic ironstone suggests redox stratification, poor deep-ocean ventilation, and upwelling-related ironstone genesis persisted until the Devonian but not into the Carboniferous.

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

Redox conditions in Late Permian seawater based on trace metal ratios in fluid inclusions in halite from the Polish Zechstein Basin

Previous studies have confirmed that primary fluid inclusions in halite record and preserve information concerning the major ion chemistry of seawater. Here, we determine ratios of redox sensitive trace metals (Fe, Mn, V, Mo, U) in paleo-seawater trapped as primary fluid inclusions in halite from two boreholes (Przyborów IG3, Gorzów Wielkopolski IG1) from the Polish sector of the European Southern Permian Basin and estimate the original seawater concentrations of the redox sensitive trace metals.Our results show generally higher ratios of Fe/U, Fe/Mo, and Fe/V in fluid inclusions in halites associated with sulfate-depleted (calcium-rich) brines, compared to inclusions in halites formed in association with sulfate-rich brines in the Przyborów IG3 borehole. This behavior is consistent with the interpretation of García-Veigas et al. (2011) to reflect changes in redox conditions from oxidizing (represented by sulfate-rich brines) to reducing (represented by sulfate-depleted and calcium-rich brines) in the Zechstein epicontinental sea during the Late Permian. The correlation is more complex in halites from the Gorzów Wielkopolski IG1 borehole, likely reflecting local controls on redox state.Results suggest that ratios of Mn/U, Mn/V, and Mn/Mo in fluid inclusions may be used to refine redox conditions in the water column when these data are evaluated in combination with Fe/U, Fe/Mo, Fe/V, and Mn/Fe ratios because reduction of Mn(III, IV) to Mn(II) occurs at a higher redox potential compared to that required to reduce Fe(III) to Fe(II).The estimated original Mn and Mo concentrations in the seawater in the Late Permian Zechstein Basin are generally within the concentration range observed in modern anoxic basins while concentrations of V and Fe are generally higher and U is generally lower. Our results suggest that trace metal abundances in fluid inclusions in halite vary in response to redox changes in seawater and provide a potential redox proxy. To better understand and apply results obtained from fluid inclusions in halite as a paleoredox proxy, however, further work is required. These include the study of fluid inclusions in halite from more locations worldwide and the more detailed characterization of redox sensitive trace metal behavior during seawater evaporation.

Quantifying Molybdenum Isotopic Speciation in Sulfidic Water: Implications for the Paleoredox Proxy

Molybdenum (Mo) isotopic signature and concentrations can be a geologic indicator of changes in molecular oxygen concentrations in Earth’s oceans and atmosphere. However, in natural waters that are weakly to strongly sulfidic, few studies have been able to determine the exact mechanisms controlling partitioning and speciation related to isotopic fractionation of Mo. To understand the isotopic contribution of each Mo species to the overall bulk Mo isotope signature, we report a method that measures both the concentration and the isotopic composition of each Mo-bearing species during thiolation using synthesized sulfidic water. We accomplish this by using reversed-phase ion chromatography to isolate individual Mo species and then measure the isotopic ratio of each species with a multicollector inductively coupled plasma mass spectrometer. Our findings show that progressively thiolated species contribute to an increasingly negative isotopic value (i.e., −3.16 to +3.14) compared to the precursor stock solution. Additionally, these results corroborate Tossell’s theoretical predictions, where lower Mo isotope values correlate to increasing sulfide presence. Furthermore, the formation of thiolated species and evolution of Mo isotopic signatures in our experiments do not occur via a stepwise formation or a quantitative conversion model for molybdate to tetrathiomolybdate. Overall, our study improves our understanding of Mo systematics, shows the potential of measuring specific Mo species, and thereby strengthens the application of Mo paleoproxy through consideration of all Mo species.

A critical review of molybdenum sequestration mechanisms under euxinic conditions: Implications for the precision of molybdenum paleoredox proxies

The observed difference in molybdenum (Mo) mobility and isotopic fractionation under oxic versus euxinic (i.e., anoxic and sulfidic) aqueous conditions provides a sound operational basis for the use of Mo geochemical signatures in ancient sedimentary records to infer palaeoceanographic redox conditions . While Mo is known to exist predominantly as molybdate (MoO 4 2− ) in oxic waters and convert into thiomolybdate species (MoO x S 4-x 2− ) under euxinic conditions, the pathways that lead to Mo sequestration are highly debated. As mechanistic understanding of Mo sequestration is crucial for accurately reconstructing the chemistry of ancient oceans and constraining the timing and intensity of oxygenation events through Earth's history using Mo paleoproxies, we have closely examined the current proposed mechanisms for Mo sequestration across a wide range of euxinic conditions. Through compilation and comparison of such information, we aim to provide an integrated view of Mo sequestration processes, to identify the current controversies as well as the roots of such controversies, and importantly, to propose avenues for future research. Sequestration of Mo may occur through complexation with organic matter (OM), reactions with oxyhydroxide and sulfide mineral surfaces, incorporation into iron sulfide crystal structures, formation of Mo-sulfide, and cellular assimilation. However, several major questions remains unresolved, including whether OM complexation plays a significant role in overall sedimentary Mo accumulation, whether the reduction of Mo(VI) occurs during interaction with Fe-S clusters/precipitates and/or with OM, whether microbial enzymatic reduction of Mo(VI) is important in Mo sequestration, whether synergistic effects exist between the biological and abiotic processes in Mo sequestration, and whether differences in reactivity among the various thiomolybdate intermediate species and tetrathiomolybdate influence the Mo sequestration process. At the end of the review, the reliability of Mo paleoproxies is discussed and reevaluated.

Paleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 1 – Lithogeochemistry of proximal and distal shales

ABSTRACT The MacMillan Pass District in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Bulk geochemical paleoredox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate anoxic–dysoxic water column and sulfidic porewater conditions persisted during the Late Devonian deposition of the Lower Earn Group host rocks. Positive Eu/Eu* anomalies (up to 3.31) in sulfide mineralization at the Tom deposit are consistent with relatively high temperature (probably &amp;gt;250 °C), reducing, acidic hydrothermal fluids that infiltrated laterally through unconsolidated sediments proximal to the hydrothermal upflow zone and/or exhaled at the seafloor as moderate- to high-density brines. Molybdenum and U enrichment factors (relative to upper continental crust) and Mo/organic C values are consistent with a moderately restricted basin; Mo/C values fall between those of the Black Sea (highly restricted) and the Framvaren Inlet (moderately restricted). A Ba-rich shale was identified in rocks that are distal and time-equivalent units to the Pb-Zn mineralization; based on the bulk chemical compositions and on previous S and Sr isotope studies, we interpret the baryte in this unit to be largely hydrothermal in origin and perhaps remobilized and reprecipitated during hydrothermal base-metal mineralization.

A multi-proxy approach to constrain reducing conditions in the Baltic Basin during the late Silurian Lau carbon isotope excursion

The Silurian was a dynamic time characterized by significant climatic and sea level changes, biotic crises, and carbon cycle volatility. The largest magnitude perturbation to the Silurian global carbon cycle was the mid-Ludfordian carbon isotope excursion, termed the Lau CIE, which was coincident with the Lau/Kozlowskii extinction (LKE). Much of the published research on the late Silurian has described changes in the biotic record and global marine redox conditions. Limited work has been done, however, to elucidate the variability in local paleo-redox conditions. Here, we use a suite of paleo-redox proxies to examine a shallow shelf carbonate succession from Gotland, Sweden, and a deep shelf clastic sequence from Latvia. Low iodine-to‑calcium ratios from the carbonate succession suggest an anoxic water column, or significant exchange with nearby reducing water masses. Iron speciation data and moderate trace metal enrichments suggest that the sediments of the deeper shelf were deposited in an oxygen minimum zone with a denitrifying water column before and during the LKE with sulfidic conditions limited to the sediment porewaters. After the extinction, a relative depletion in trace metal concentrations (V, Hg, U, Mo) suggest that water column conditions were locally less reducing. The depletion in trace metal concentrations is coeval with previously reported global increases in anoxia and euxinia after the LKE and, thus, potentially could have resulted from a global drawdown in trace metals overprinted upon the local record of paleo-redox change.

Anoxic depositional overprinting of 238U/235U in calcite: When do carbonates tell black shale tales?

Abstract The fidelity of uranium isotopes (δ238U) in marine carbonates as a paleoredox proxy relies on whether carbonates can record and preserve seawater δ238U. Although modern carbonate sediments deposited under oxic conditions have been shown to track seawater δ238U, it remains unknown whether this is true for carbonates deposited under anoxic conditions. This is a crucial question because many ancient carbonates were likely deposited or reworked under anoxic bottom waters. To better understand the behavior of uranium isotopes under this scenario, we investigated U isotope geochemistry in the meromictic Fayetteville Green Lake (FGL; New York, USA), where primary calcite is precipitated from oxic surface waters, sinks past the chemocline, and is deposited under anoxic bottom waters. We observed significant depletions of dissolved U concentration (from 2.7 to 0.9 ppb) and δ238U (from –0.55‰ to –0.96‰) below the chemocline in FGL. Parallel with these depletions, δ238U of sediment traps increased progressively from –0.51‰ to –0.16‰, suggesting that U(VI) reduction was occurring in the anoxic water column. Carbonate sediments deposited under anoxic bottom waters were enriched in U by 6–18× compared to primary calcite. Our data suggest that such significant authigenic U enrichments resulted from U(VI) reduction in the anoxic water column and below the sediment-water interface. The δ238U value in the top 0.25 cm of sediments was –0.29‰ ± 0.10‰, overprinting original δ238U in primary calcite (–0.51‰ ± 0.02‰). Future applications of carbonate δ238U as a paleoredox proxy should consider depositional environments (oxic vs. anoxic) of carbonates.

Nitrogen isotopes as paleoenvironmental proxies in marginal-marine shales, Bohai Bay Basin, NE China

Nitrogen isotopes have been widely used as indicators of depositional conditions in studies of ancient sediments. Bohai Bay Basin is a major continental hydrocarbon province in NE China, but nitrogen isotopic signatures of this lacustrine system (connected with the open ocean occasionally) have barely been investigated. We report a study of paleoenvironmental information recorded by N isotopic variations in the lower third member (Es3L) and upper fourth member (Es4U) of the Eocene Shahejie Formation in the Jiyang Sub-basin, southern Bohai Bay Basin. Paleosalinity (B/Ga, Sr/Ba, and S/TOC) and paleoredox (Corg/P, DOP, UEF, and Fe/Al) proxies and δ13Corg values were used to elucidate paleoenvironmental factors that affect N isotopic signals in the study core. Results indicate that δ15N values are positively correlated with paleosalinity proxies in the Luo-69 and Niu-1 drill-cores. Two possibilities may account for this: (1) a paleosalinity surge caused by marine transgression events led to stratification of the water column, creating an anoxic–suboxic deep-water layer and promoting denitrification in sediments; or (2) strong stratification of the water column impeded the upwelling process that brought nutrients to the upper water layer, weakening nitrogen fixation there. In marginal-marine intervals such as the Eocene Shahejie Formation, water-column salinity may play a critical role in regulating the δ15N signal. This study indicates that nitrogen isotopic compositions may aid paleoenvironmental and paleoclimatic interpretations of marginal-marine shale, particularly when applied together with multiple paleoenvironmental proxies to improve the understanding of ancient sedimentary conditions in marginal-environment.

Temporally and spatially dynamic redox conditions on an upwelling margin: The impact on coupled sedimentary Mo and U isotope systematics, and implications for the Mo-U paleoredox proxy

The abundances and isotope compositions of molybdenum (Mo) and uranium (U) in ancient sediments are promising tracers of the redox state of the past ocean, whose basis lies in the environmentally dependent Mo and U isotope signatures in modern oceanic settings. Despite their dominance in oceanic budgets, the controls on the Mo-U systematics of upwelling margin sediments remain to be fully understood. Here we present a comprehensive sediment-porewater Mo and U isotope study in the Benguela upwelling system off Namibia, including the first dataset incorporating coupled Mo-U abundance and isotope analysis of both solid authigenic phases and porewaters.The investigated stations represent shelf-to-slope settings, which lie on the upper edge, within and below the oxygen minimum zone (OMZ) of the South Atlantic. The sediments across all stations share similar characteristics: both Mo and U show increasing authigenic enrichment with depth, coupled to an overall decrease in δ98Moauth (from ~2.0‰ to 1.3‰ and from ~2.0‰ to 0.3‰) and increase in δ238Uauth (from −0.18‰ to 0.05‰ and from −0.34‰ to −0.21‰). Nevertheless, the extents of Mo and U enrichment and associated isotopic fractionations display spatial variability across the OMZ, reflecting variations in local sedimentary redox conditions. Porewater Mo and U concentration patterns are more complex, exhibiting peaks in Mo and U abundance well in excess of seawater (up to 8 times seawater for Mo) associated with correlated shifts in isotope composition. As a result, porewaters exhibit a wide range in isotope compositions, between 0.90‰ and 2.79‰ for δ98Mo and between −1.74‰ and 0.26‰ for δ238U.Porewater gradients at the time of sampling are inconsistent with diffusion downwards across the sediment–seawater interface as a means of enrichment of the sediment-porewater system. Though these sampled conditions may represent only a snapshot, so that periodically more reducing conditions could lead to concentration gradients that do permit downward diffusion, the data are also readily explained by addition of Mo and U to the sediment-porewater system in particulate form, also under more reducing conditions than at the time of sampling. For example, sequestration of Mo and U to particulate matter as a result of the presence of intermittent sulfide, either in bottom water or in porewater right at the sediment–water interface, explains much of the geochemical and isotope data.The data thus suggest that the early diagenetic enrichment of Mo and U in sediments of upwelling margins is strongly governed by temporal redox fluctuations. Early diagenesis under these dynamic redox conditions on the Namibian upwelling margin are strongly reflected in Mo-U co-variation patterns, as well as anti-correlations between authigenic δ98Mo and δ238U in sediments. Overall, our new data demonstrate that early diagenetic processes on open-marine continental margins reproduce patterns previously observed for coupled Mo-U isotope systematics in restricted and semi-restricted basins, but via a different set of processes and with important implications for the use of such a coupled approach in the study of ancient marine anoxia.

Uranium Isotope Fractionation during the Anoxic Mobilization of Noncrystalline U(IV) by Ligand Complexation.

Uranium (U) isotopes are suggested as a tool to trace U reduction. However, noncrystalline U(IV), formed predominantly in near-surface environments, may be complexed and remobilized using ligands under anoxic conditions. This may cause additional U isotope fractionation and alter the signatures generated by U reduction. Here, we investigate the efficacy of noncrystalline U(IV) mobilization by ligand complexation and the associated U isotope fractionation. Noncrystalline U(IV) was produced via the reduction of U(VI) (400 μM) by Shewanella oneidensis MR-1 and was subsequently mobilized with EDTA (1 mM), citrate (1 mM), or bicarbonate (500 mM) in batch experiments. Complexation with all investigated ligands resulted in significant mobilization of U(IV) and led to an enrichment of 238U in the mobilized fraction (δ238U = 0.4-0.7 ‰ for EDTA; 0.3 ‰ for citrate; 0.2-0.3 ‰ for bicarbonate). For mobilization with bicarbonate, a Rayleigh approach was the most suitable isotope fractionation model, yielding a fractionation factor α of 1.00026-1.00036. Mobilization with EDTA could be modeled with equilibrium isotope fractionation (α: 1.00039-1.00049). The results show that U isotope fractionation associated with U(IV) mobilization under anoxic conditions is significant and needs to be considered when applying U isotopes in remediation monitoring or as a paleo-redox proxy.

Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42−) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state.We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere–ocean system. We analyzed the δ186/184W of Precambrian igneous rocks to investigate the detrital background δ186/184W signature. This Precambrian igneous inventory (PII) shows δ186/184W values between −0.007 and +0.097‰, identical to the range of modern igneous crustal rocks, which indicates that the average Earth’s crust δ186/184W remained constant over billions of years. Furthermore, we present δ186/184W values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with δ186/184W data of Archean and Proterozoic black shales (3.47–2.3 Ga). Modern Black Sea sapropels reveal crustal-like δ186/184W values between +0.050 and +0.071‰ suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like δ186/184W values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated δ186/184W values up to +0.246‰, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales δ186/184W values can help to distinguish between anoxic-ferruginous and euxinic depositional environments.Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the δ186/184W and δ98/95Mo values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between −0.4 V and −0.25 V, enabling the persistence of soluble WO42− but not MoO42− and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above −0.25 V, as indicated by δ98/95Mo values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below −0.25 V indicated by the lack of authigenic Mo enrichment and fractionated δ98/95Mo values in deep-sea shales. (3) After 2.7 Ga elevated δ98/95Mo and δ186/184W values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above −0.25 V. We therefore suggest that the combination of δ98/95Mo and δ186/184W values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean.

Geochemistry of iron and sulfur in the Holocene marine sediments under contrasting depositional settings, with caveats for applications of paleoredox proxies

Cycling and fates of iron (Fe) and sulfur (S) in marine sediments are influenced by depositional settings to differential extents. The information is crucial for addressing the responses of their benthic diagenesis to changing climates and environments and also for reconstructing paleo-depositional conditions, but has not been well constrained. Detailed chemical speciation was utilized to characterize geochemistry of Fe and S, and then to reveal the impacts of depositional settings on their diagenesis at three locations, representing contrasting depositional environments: (i) highly dynamic Yangtze River estuary (YRE), (ii) the depocenter of South Yellow Sea (SYS), which is only remotely impacted by large river, and (iii) the middle Okinawa trough (OT), a back-arc deep basin along the outer edge of the East China Sea (ECS) slope. Results show that the YRE sediments favor accumulation of total highly reactive Fe (FeHR), while the SYS sediments are poor in FeHR due to low FeHR in their source material and/or preferential trapping of FeHR during sediment transport through the semi-enclosed Bohai Sea. Ferruginous sediment regimes in the highly dynamic YRE system facilitate Fe(III) reduction and burial of unsulfidized Fe(II), while the SYS and OT sediments favor sulfate reduction and pyritization of Fe(II). Pyrite is always the main sink of reduced S that escapes reoxidation in the entire continental margin, regardless of depositional environments. Abundant reactive Fe(III) but low total reduced inorganic sulfide (TRIS) contents in the three sites suggest that TRIS burial is largely controlled by the availability of degradable OC and/or dynamic regimes of sediments. The applicability of two widely used Fe- and S-based proxies to distinguish bottom-water conditions, that is, OC/TRIS ratio and FeHR to total Fe (FeHR/FeT) ratio, were examined in the three contrasting environments, and caveats were given for future applications of the two proxies.

Solid bitumen in shales from the Middle to Upper Jurassic Sargelu and Naokelekan Formations of northernmost Iraq: implication for reservoir characterization

Petrographic, organic, and inorganic geochemical analysis of the solid bitumen and host shales from the Middle and Late Jurassic-age Sargelu and Naokelekan Formations of the Banik section, northernmost Iraq, was undertaken. The aim was to understand their derivation and preservation, as well as examine the carbon and oxygen isotopes, and paleoredox proxies under which the solid bitumen and host sediments were deposited. Petrographic analysis of both formations revealed the presence of solid bitumen high reflectance (first phase) and solid bitumen low reflectance (second phase). The equivalent vitrinite reflectance indicates that the solid bitumen of the two formations probably accumulated within the shale reservoirs following oil migration from source rocks located within the same formations. Mineralogical study (XRD and SEM - EDX) revealed that the shales hosting the solid bitumen also contain clay minerals (illite, rectorite, chlorite, montmorillonite, and kaolinite) as well as carbonate minerals, quartz, alkali feldspar, and pyrite. Carbon and oxygen isotope data along with paleoredox indicators suggest that both the solid bitumen sources and host shales in both formations formed within a shallow-marine setting, most probably under anoxic conditions where water circulation was restricted.

Climate-driven fluxes of organic-bound uranium to an alpine lake over the Holocene

Uranium (U) isotopic signatures and concentration in sediments are widely used as paleo-redox proxies, as the behavior of U is often controlled by bottom water oxygenation. Here, we investigated the processes controlling U accumulation in the sediments of Lake Nègre (Mediterranean Alps, South-East France) over the past 9200 years. Exceptionally high natural U concentrations (350–1250 μg·g−1) allowed the measurement of U along with other elements by high-resolution X-Ray Fluorescence core-scanning. Weathering and erosion proxies (Ti content, Zr/Al and K/Ti ratios) indicate that sedimentary inputs were controlled by Holocene climatic variations. After a period of low erosion during the Holocene Climatic Optimum, a major regime shift was recorded at 4.2 kyr BP when terrigenous fluxes consistently increased until present with high sensitivity to centennial-scale climatic events. Sedimentary organic matter (OM) inputs were dominated by terrigenous OM from the catchment soils until 2.4 kyr BP, as attested by carbon to nitrogen (C/N) and bromine to organic carbon (Br/TOC) ratios. From 2.4 kyr BP to present, lake primary production and soils equally contributed to sedimentary OM. Uranium fluxes to the sediments were well correlated to terrigenous OM fluxes from 7 kyr BP to present, showing that U supply to the lake was controlled by U scavenging in the soils of the watershed followed by transport of U bound to detrital organic particles. Higher U/OM ratios before 7 kyr BP likely reflect the development of the upstream wetland. The fluctuations of U sedimentary inputs appear to be independent of bottom water oxygenation, as estimated from constant Fe/Mn ratios and δ238U isotopic signatures, and rather controlled by the production, erosion and sedimentation of terrigenous OM. This finding confirms that the use of U (and potentially other metals with high affinity to OM) concentrations alone should be used with caution for paleo-redox reconstructions.