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Geochemistry of the sediments deposited during the Archaean Era is potentially useful to decipher the Earth’s pristine seawater chemistry and the depositional environment. This study highlights the geochemical evolution, redox conditions, and the sedimentary environment based on carbonate sediments of the Tanwan group rocks, Bhilwara supergroup, Rajasthan, India, deposited during 2.9–2.5 Ga. The petrographic features and variation in major elements of these carbonate sediments indicate extensive dolomitization during this time. The elemental ratios such as Mg/Ca, Fe/Al, Fe/Sr, and Mn/Sr ratios and their mutual relationships suggest anoxic conditions during precipitation of the carbonates. The anoxia is well supported by the redox-sensitive element ratios such as Ni/Co, V/Cr, V/V + Ni, V/V + Cr, and Th/U, which show reducing/anoxic sedimentary environments during 2.9–2.5 Ga. The post-Archaean Australian Shale (PAAS) normalized anomalies, such as Ce anomaly, Gd anomaly, and Eu anomaly, showing average 1–1.48 values, indicate minor positive anomalies in the Archaean seawater. The rare earth elements and yttrium (REY) content in these sediments show no significant contamination, mixing, and alteration because Dissolution II (carbonate leachate) is characterized by Zr from 0.015 ppm to below detection limit (BDL), Th < 1 ppm, Sc < 1.5 ppm, and Al2O3 < 1%. The Y/Ho ratio > 26 represents insignificant detrital input during the precipitation of the carbonates. The Y/Ho ratio versus Nd also supports this contention. The positive anomalies for Y (i.e. Y/Ho > 26), Gd anomaly (Gd/Gd*), and La anomaly (La/La*) confirmed the preservation of anoxic pristine marine seawater signatures in these carbonates of the Bhilwara supergroup that were mainly deposited during the Neoarchaean Era. [ABSTRACT FROM AUTHOR]

Intertidal flats are highly productive coastal marine ecosystems which are affected by fast changes in environmental conditions and host dynamic biogeochemical cycles in their sediments. Bioturbation by burrowing organisms and roots of plants strongly affects speciation and cycling of redox-sensitive elements in intertidal sediments. In this work, we have studied the impact of sediment type and vegetation on the cycling of redox-sensitive elements including sulfur, iron, and manganese in sandy and muddy tidal flats sediments in the Jade Bay (North Sea) and adjacent area. The redox speciation of these elements was analyzed in the pore-waters and the total sediment. The isotopic compositions of sulfur species were measured in non-vegetated sediments and in sediments which are inhabited by various plants. In the cohesive sediments, which are not affected by vegetation, a decrease in sulfate concentration, coupled with the presence of relatively high concentrations of hydrogen sulfide in the pore-waters and the presence of sulfide minerals as well the isotopic compositions of sulfur species are consistent with fast rates of sulfate reduction in the sediments. In the cohesive sediments affected by roots of Salicornia stricta and sediments desiccation, a cryptic sulfur cycle, which is characterized by microbial sulfate reduction coupled to fast reoxidation of hydrogen sulfide by Fe(III) (hydr)oxides and, possibly, by oxygen is present. Below the roots penetration depth, speciation of redox-sensitive elements is similar to those in the baren sediments. In the cohesive sediments affected by the roots of Spartina anglica and Triglochin maritima , which have longer roots, a cryptic sulfur cycle was detected in the upper 30 cm of sediments. At the sites that are characterized by permeable surface sediments and alternating permeable and cohesive layers in the deeper sediments, the composition of the sediment has a similar or even more significant impact on the speciation of the redox-sensitive elements than penetration of relatively weak roots of Spartina anglica. These sediments are characterized by the formation of hydrogen sulfide and sulfide oxidation intermediates in the cohesive layers and their diffusion to (and oxidation at) the boundaries between cohesive and permeable sediments. We conclude that in the cohesive sediments, the penetration of roots and desiccation leads to the formation of overall oxidized conditions, while in the sediments with alternating layers, permeability may provide a more significant control for speciation of redox-sensitive elements. • In cohesive sediments the redox conditions are strongly affected by vegetation. • In September concentration of H 2 S in the cohesive sediments with roots is low. • In the mixed sediments particle sizes control the speciation of S, Fe and Mn. • Thiocyanate is formed in the sediments from polysulfides and hydrogen cyanide. [ABSTRACT FROM AUTHOR]

Early diagenesis processes and the geochemistry of redox-sensitive elements (RSEs) in four sediment cores in an active hydrothermal field in the southern Okinawa Trough (OT) was investigated. Dissolved Fe, Mn, ΣHS–, and several other RSEs (Mo, U, and V) in pore water were measured. We also studied metal distribution in sediments using sequential extraction methods. Very high dissolved Fe concentrations (∼140 μmol L–1) but insignificant dissolved Mn were observed in surficial pore waters in the station adjacent to the hydrothermal vent, where highly reactive Fe, Mo, U in the sediments were also measured. Such an atypical diagenetic sequence found in those cores could be driven by the overwhelmingly high reactive Fe fraction (mostly Fe oxides) delivered from the vents. Consequently, significant upward benthic fluxes of Fe and Mo were estimated for the studied stations. In addition, we performed a principal component analysis (PCA), together with relative ratios of carbonate-related elements (Sr, Ca, Mg), to identify particles’ origins in the hydrothermal field; two endmembers being the hydrothermal source and hydrogenous processes. This comprehensive study on a unique set of samples collected by advanced technology provided valuable data to demonstrate distinctive geochemical features that occur in hydrothermal sediments.

• A redox shift from ferruginous to suboxic to oxic conditions in the Nanhua Basin shallow waters. • The inner-shelf environments record a change from high productivity to oligotrophic environments in the Nanhua Basin. • Marine nitrogen cycles were associated with changes in redox conditions and productivity. • Water-column anoxia caused the extinction of small shelly faunas. • Oligotrophic environments delayed the recovery of small shelly faunas. Widespread oceanic anoxia has been frequently invoked for metazoan extinctions during the Cambrian. However, this hypothesis remains to be debated. In this study, we measured total organic carbon (TOC), total bulk nitrogen (TN bulk), iron speciation, major and trace elements, and isotopic compositions of sedimentary bulk nitrogen (δ15N bulk) for the Shiyantou Formation in the Meishucun section. Our results suggest ferruginous conditions and high productivity in the lower member (LM: 0–16.0 m), dominantly suboxic conditions and low productivity in the middle member (MM: 16.0–23.7 m), and oxic conditions and low productivity in the upper member (UM: 23.7–28.0 m). Low δ15N bulk values (average 1.0 ‰) in the LM suggest quantitative assimilation of NH 4 + and strong nitrogen fixation as a consequence of high primary productivity and widespread water-column reducing conditions in the Nanhua Basin. δ15N bulk values close to 0 ‰ in the MM suggest intense nitrogen fixation owing to quantitative denitrification and anammox in an oligotrophic environment. Low δ15N bulk values (average 1.2 ‰) in the UM are indicative of marine nitrogen limitations in an oligotrophic environment. These findings suggest a similar range of low δ15N bulk values might result from different environments. In comparison with marine redox conditions and δ15N bulk data from the other coeval sections in the Nanhua Basin, we propose that widespread anoxic conditions might cause the extinction of small shelly faunas. More importantly, the oligotrophic environment and anoxic conditions might delay the recovery of small shelly faunas in the shallow and deep waters of the Nanhua Basin, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • First iron speciation data of the Ediacaran/Cambrian Corumbá Group. • Ediacaran biomineralizing organisms lived under a shallow oxic layer. • Deep waters were ferruginous in this time frame, in a stratified basin. • A redox shift led to deepening of the redoxcline and generated oxic bottom waters. • Early Cambrian bioturbators thrived in the oxic bottom waters. The Ediacaran–Cambrian transition witnessed some of the most important biological, tectonic, climatic and geochemical changes in Earth's history. Of utmost importance for early animal evolution is the likely shift in redox conditions of bottom waters, which might have taken place in distinct pulses during the late Ediacaran and early Paleozoic. To track redox changes during this transition, we present new trace element, total organic carbon and both inorganic and organic carbon isotopes, and the first iron speciation data on the Tamengo and Guaicurus formations of the Corumbá Group in western Brazil, which record important paleobiological changes between 555 Ma to < 541 Ma. The stratigraphically older Tamengo Formation is composed mainly of limestone with interbedded marls and mudrocks, and bears fragments of upper Ediacaran biomineralized fossils such as Cloudina lucianoi and Corumbella werneri. The younger Guaicurus Formation represents a regional transgression of the shallow carbonate platform and is composed of a homogeneous fine-grained siliciclastic succession, bearing meiofaunal bilateral burrows. The new iron speciation data reveal predominantly anoxic and ferruginous (non-sulfidic) bottom water conditions during deposition of the Tamengo Formation, with Fe HR /Fe T around 0.8 and Fe Py /Fe HR below 0.7. The transition from the Tamengo to the Guaicurus Formation is marked by a stratigraphically rapid drop in Fe HR /Fe T to below 0.2, recording a shift to likely oxic bottom waters, which persist upsection. Redox-sensitive element (RSE) concentrations are muted in both formations, but consistent with non-sulfidic bottom water conditions throughout. We interpret the collected data to reflect a transition between two distinct paleoenvironmental settings. The Tamengo Formation represents an environment with anoxic bottom waters, with fragments of biomineralized organisms that lived on shallower, probably mildly oxygenated surficial waters, and that were then transported down-slope. Similar to coeval successions (e.g., the Nama Group in Namibia), our data support the hypothesis that late Ediacaran biomineralized organisms lived in a thin oxygenated surface layer above a relatively shallow chemocline. The Guaicurus Formation, on the other hand, records the expansion of oxic conditions to deeper waters during a sea level rise. Although the relationship between global biogeochemical changes and the activities of early bioturbators remains complex, these results demonstrate an unequivocal synchronous relationship between oxygenation of the Corumbá basin and the local appearance of meiofaunal bioturbators. [ABSTRACT FROM AUTHOR]

Oil shale deposits of the mid-Eocene Wadi Shallala Formation in northern Jordan consist of organic-rich limestones and chalks. These organic-rich deposits are exposed at the surface near Wadi Ash Shajara, in a 12-m-thick outcrop. For this study, a total of 21 oil shale samples were analysed for bulk elemental geochemistry, inorganic geochemistry, organic geochemistry and microfacies analysis. Three microfacies types were recognised: pelagic mudstone/wackestone, foraminiferal packstone and bioclastic packstone/wackestone. The observed cyclicity of these microfacies in the study area indicates fluctuations in depositional conditions in Neo-Tethys during the mid-Eocene. A shallow-water, calm, restricted marine environment with limited water circulation and low energy currents evolved into a deeper-water environment with open circulation at or just below the wave base. Wadi Shallala Formation in Wadi Ash Shajara composes mainly of chalk (CaCO3 = ~92 wt%) with a TOC of up to 5 wt % and comprises abundant calcareous nannofossils, foraminifera and ostracods as well as non-clastic components like calcitic fragments. The sulphur content of the analysed samples is significant (1.24 wt %) and is associated with the organic matter. Redox-sensitive elements (Fe, Ni, Zn, S and Zr) are enriched in the oil shale samples, indicating an anoxic depositional environment fluctuating between euxinic and sub-euxinic. These fluctuations in depositional conditions of the basin are a result of many factors such as climate, and tectonisms affected deposition of oil shales during Middle Eocene time. [ABSTRACT FROM AUTHOR]

The decay of organic matter of anthropogenic origin in a highly contaminated shallow groundwater system occurs permanently regardless of the availability of oxygen. Oxidation of organic matter smoothly changes from aerobic to anaerobic and vice versa. Hydrogeochemical transformations occurring in the interior of the contaminant plume are conditioned by the position in the 3D zone of the so-called "redox reactor" and its edge. The primary reaction initiating the decay of organic matter (TOC max 1620 mg/L, phenol max 613 mg/L) is its aerobic oxidation. In the case of the consumption of free oxygen, the decay undergoes anaerobic oxidation, where the source of electrons are oxides and hydroxides (MnO2, Fe(OH)3). As a result of these reactions, mobile ions Mn2+ and Fe2+ pass into the aqueous environment creating a concentration anomaly (max 15 mg/L for Mn2+, 673 mg/L for Fe2+). The presence of Fe2+ in groundwater is crucial. A strong correlation between the organic matter decay processes and concentration of the Fe2+ showed that "iron index" may be a preliminary marker for the hydrogeochemical recognition of aquifer and allows to diagnose zones with an intense organic matter decay, especially by anaerobic oxidation through redox reactions. At the edge of the "redox reactor" redox sensitive metals (Fe2+, Mn2+ and also Cu2+, Cr3+, Hg2+) undergo aerobic oxidation due to the access of oxygen as a result of mixing of contaminated groundwater and oxygenated pure Quaternary water. These transformations produce oxides and hydroxides (MnO2, Fe(OH)3)—new reaction products, however, are used for anaerobic oxidation of organic matter. Organic matter decay is an cyclic system of redox processes up to the full decay of pollutants and generation of the anomalously high concentrations of redox sensitive metals in the ground. [ABSTRACT FROM AUTHOR]

Metalliferous black slates consisting of black slates and interbedded coaly slates in the Okcheon Metamorphic Belt of South Korea were analyzed for redox-sensitive trace elements and rare earth elements (REEs) to examine their depositional conditions. Our data show that the coaly slates have elevated concentrations of redox-sensitive trace metals (U, V, Mo, and Cr), low Mn contents, high V/(V + Ni), V/Cr, and U/Th ratios, and high total organic carbon (TOC) contents. A general tendency of positive correlation between TOC and trace metals was established. The results suggest that the coaly slates were developed under a strongly reducing environment, while the black slates were deposited under a suboxic-oxic condition. The REE patterns of the coaly slates typically show positive Eu and negative Ce anomalies compared to the North American Shale Composite reference and they are essentially inherited from submarine hydrothermal fluids under high temperature reducing condition. The enrichment of the redox-sensitive elements including V (up to 3,564 ppm) and Mo (up to 358 ppm) may have been controlled by direct hydrothermal input of metals into the rift basin from hydrothermal vents, proximal from the vent sites, as supported by textural evidences, and the metals could have been fixed from seawater by means of scavenging process via exhalative oxide particles in a hydrothermal plume. The sorption of metals from seawater under anoxic conditions into organic-rich sediments occurred as well. The metal enrichments in the black slates seem to have been achieved in somewhat similar way to black shales in South China. [ABSTRACT FROM AUTHOR]

The Eureka deposit in Castell-estaó in the Catalan Pyrenees is a Cu-U-V deposit, hosted by Triassic red-bed sandstones, and classified here as a low-temperature, sandstone-hosted stratabound metamorphite U deposit. The main mineralisation is stratabound, related to coal-bearing units and produced during the Alpine deformation by migration of hydrothermal fluids. In this stage, the original sedimentary and diagenetic components (quartz and calcite, micas, hematite and locally apatite) were replaced by a complex sequence of roscoelite, fine-grained REE phosphates, sulphides and Ni-Co arsenides and sulpharsenides, Ag-Pb selenides, bismuth phases, sulphosalts and uraninite. The black shales of the Silurian sediments underlying the deposit and the nearby Carboniferous volcanoclastic rocks are interpreted as the source of the redox-sensitive elements concentrated in Eureka. The sulphur source is related to leaching of the evaporitic Keuper facies. The REE transport would be facilitated by SO4-rich solutions. The reduction of these solutions by interaction with organic matter resulted in the widespread precipitation of REE and redox-sensitive elements, including many critical metals (V, Bi, Sb, Co), whereas barite precipitated in the oxidized domains. The occurrence of similar enrichments in critical elements can be expected in other similar large uranium deposits, which could be a source of these elements as by-products. [ABSTRACT FROM AUTHOR]

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Redox-sensitive elements (uranium, molybdenum and cadmium) and productivity proxies (total organic carbon, calcium carbonate and phosphorous) were analysed to assess the importance of productivity on redox variation for the last ∼18 kyr in a sediment core off Cochin at a water depth of 280 m in the southeastern Arabian Sea. These proxies document a combination of higher productivity and intense suboxic conditions during the last deglaciation (∼18–13 kyr) and late Holocene (5.5 kyr to Present) versus lower productivity and less suboxic conditions between these intervals (∼13–5.5 kyr). The productivity and redox proxies behave similar to one another suggesting that the variations in productivity controlled the redox conditions. This relationship is supported by patterns during the last deglaciation, when a decrease in productivity at ∼16 kyr preceded improved deep water oxygenation by ∼14 kyr. Therefore, productivity changes were responsible for the development of reducing conditions in shallow waters in the southeastern Arabian Sea. [ABSTRACT FROM AUTHOR]

Climate variability has major implications for marine geochemical cycles and biogenic carbonate production. Therefore, past climate-driven changes in marine environments are often inferred from geochemical data of the marine carbonate archive. Proxy calibration studies are essential for the reconstruction of such past environmental changes. Here, we use the geochemical composition of living specimens of the benthic foraminifer Ammonia tepida at three sites in a seasonally hypoxic (oxygen concentration < 63 μmol/L) marine coastal system (Lake Grevelingen, the Netherlands) to explore the use of Mn/Ca as a proxy for coastal hypoxia. The study is based on samples from three stations along a depth transect, that show contrasts in the seasonal cycle of Mn2+ concentrations in the pore water of the surface sediment. In general, the sediment and pore water geochemistry of the three stations in Lake Grevelingen show increased Mn2+ concentrations in late winter/spring, combined with increased Mn refluxing in summer, which are due to cable bacteria activity and bottom water hypoxia/anoxia, respectively. Laser Ablation-ICP-MS (LA-ICP-MS) allowed a comparison of Mn/Ca ratios of different parts of the benthic foraminiferal test. Our results show that higher Mn/Ca ratios are registered at the deepest station, which experiences the longest and most severe seasonal periods of hypoxia/anoxia. Additionally, the signal preserved in the central part of the benthic foraminiferal tests, which is thought to reflect the entire calcification history of the analysed specimen, appears to be driven by high pore water Mn2+ concentrations due to cable bacteria activity in late winter/spring. Conversely, high Mn/Ca ratios in the last chambers reflect increased Mn refluxing in the surface sediment due to summer hypoxia/anoxia. Thus, Mn/Ca ratios of A. tepida give insight into the complex spatial and temporal variability of pore water manganese. • Inter-site differences in Mn/Ca reflect contrasts in seasonal bottom water hypoxia. • Main processes causing high Mn/Ca are cable bacteria activity and seasonal hypoxia. • Mn/Ca of the central part reliably displays inter-site environmental differences. [ABSTRACT FROM AUTHOR]

Abstract Redox-sensitive elements (Mo, V, U, Re, Cd, Co, As, Sb, Tl, Ni, Cr, Mn, Fe, Pb, Cu, Zn, Se) and proxies for detrital clastic sedimentation (Al, Ti, Sc, Th) along with total organic carbon (TOC) and total sulfur (TS) were analysed to constrain the paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile (Abay) Basin, Ethiopia. These samples were collected from the deep marine Antalo Limestone (Oxfordian-Kimmeridgian), Gohatsion Formation (Bathonian) and glauconitic shale-mudstone unit (Aalenian-Toarcian). Significant variations in redox-sensitive elements concentration within and between these shales were evident for variable redox conditions. In general, the concentration of proxies for detrital clastic sedimentation increases from the Antalo Limestone to Gohatsion Formation and further to glauconitic shale-mudstone unit shales, respectively. The Antalo Limestone shales show slightly higher enrichment in redox-sensitive elements than the Gohatsion Formation and glauconitic shale-mudstone unit shales. The Antalo Limestones shales have higher TOC content than the glauconitic shale-mudstone unit shales and become very low in the Gohatsion Formation shales. The TS content in contrast, is relatively lower in the Antalo Limestone shales, intermediate in the Gohatsion Formation shales and becomes higher in the glauconitic shale-mudstone unit shales. The Antalo Limestone shales were deposited under anoxic-suboxic conditions whereas the Gohatsion Formation and glauconitic shale-mudstone unit shales were deposited under suboxic-oxic conditions. The TOC content and redox conditions of the Antalo Limestone shales implies favourable organic matter preservation and future source rock explorations in the basin should be targeted there. Highlights • Redox sensitive elements were used to examine the paleoredox conditions. • The Antalo Limestones shales have higher TOC content. • The Antalo Limestone shales show higher redox-sensitive elements enrichment. • The Antalo Limestone shales were deposited under suboxic to anoxic conditions. [ABSTRACT FROM AUTHOR]

Abstract To constrain the redox conditions and related nitrogen cycles during the Middle Permian (Guadalupian) to latest Late Permian (Lopingian) deep mid-Panthalassa, we determined the abundances of major, trace, and rare earth elements along with the carbon and nitrogen isotope ratios in shales interbedded with deep-sea cherts that are well-exposed at the Gujo-Hachiman section in the Mino-Tanba belt, SW Japan. The positive Ce anomalies together with low Mo and U enrichment factors (Mo EF < 1.0; 0.7 < U EF < 2.5) during the Guadalupian and the most of the Lopingian indicate that the deep mid-Panthalassa was under oxic condition. On the other hand, the slightly higher Mo EF (2.0 < Mo EF < 9.2) and U EF (0.9 < U EF < 2.5) values in some of the middle-late Lopingian shales suggest deposition under intermittent suboxic condition. These new findings indicate that the redox condition in the deep mid-Panthalassa never reached an anoxic condition during the Guadalupian and Lopingian until the Permian-Triassic boundary (P-TB) interval. In view of the redox state from the Guadalupian to latest Lopingian, our newly obtained δ15N TN values (−0.12‰ to +2.57‰) indicate nitrate-rich conditions in the mid-Panthalassa along with large isotopic fractionation during nitrate assimilation. However, unlike the oxic and nitrate-rich deep-Panthalassa, we speculate that oxygen-depleted (i.e., anoxic/euxinic) and bioavailable nitrogen-poor conditions developed in the deep Tethys immediately before the Guadalupian-Lopingian boundary (G-LB). These environmental stresses were potentially driven by a global cooling episode (Kamura event) together with the unique paleogeography, i.e., no contact with polar ice caps in the Tethyan Ocean. Upwelling of the anoxic watermass accumulated in the deep Tethys during the global cooling episode likely triggered oceanic anoxia in the shallow-marine regions around the G-LB, which eventually resulted in the G-LB mass extinction. Graphical abstract Unlabelled Image Highlights • We collected the Middle-Late Permian shales at the Gujo-Hachiman, SW Japan. • δ13C, δ15N, and redox-sensitive elements data are presented from the shales. • The Middle-Late Permian deep-Panthalassa was oxic and nitrate-rich. • Global cooling likely caused the redox contrast between the Panthalassa and Tethys. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Rhaetian to Hettangian shales interbedded in chert in the Katsuyama, SW Japan. • δ 13 C org and major, trace, and rare earth elements data obtained from the shales. • Three negative δ 13 C org excursions (NCIEs) are recognized in the mid-Panthalassa. • Multiple episodes of volcanic outgassing is a plausible cause for three NCIEs. • Deep Panthalassa remained under oxic conditions. Abstract To examine environmental changes in the biosphere during the Triassic–Jurassic transition, with a particular focus on the global carbon cycle related to Central Atlantic Magmatic Provinces (CAMP) volcanism in the mid-Panthalassa, we established stratigraphic δ 13 C org variations using Rhaetian (Late Triassic) to Hettangian (Early Jurassic) shales interbedded within deep-sea cherts in the Katsuyama section in the Mino-Tanba belt, SW Japan. High-resolution record of Rhaetian to Hettangian δ 13 C org values in the mid-Panthalassa contain three distinct negative carbon isotopic excursions (NCIEs) before and across the Triassic–Jurassic boundary (TJB): the Rhaetian NCIE1 and NCIE2 show a deviation of 5.0‰ from ca. −24.0‰ to ca. −29.0‰, whereas NCIE3 across the TJB shows a 3.5‰ deviation from ca. −23.5‰ to ca. −27.0‰. Our newly obtained NCIEs in the deep mid-Panthalassa can be correlated with the δ 13 C org records in the shallow-marine Tethyan regions (i.e., precursor, initial, and main CIEs), suggesting that three NCIEs in the Tethys and mid-Panthalassa likely reflected the global perturbations of the carbon cycle. Three NCIEs before and across the TJB can be interpreted as the consequence of the multiple CAMP volcanic episodes; i.e., the release of thermogenic methane from organic-rich sediments by CAMP intrusive rocks for NCIE1 and large-scale volcanically derived carbon species for NCIE2 and NCIE3. In addition, progressive increase of atmospheric p CO 2 throughout three NCIEs was possibly attributed to accumulation of volcanically derived CO 2 from multiple CAMP eruptions, which resulted in the development of ocean acidification across the TJB. On the other hand, in view of the oxic conditions in the deep mid-Panthalassa during three NCIEs, the development of coeval oceanic anoxic–euxinic conditions was restricted solely to shallow-marine regions. Therefore, ocean acidification together with localized shallow-marine anoxia acted as environmental stresses on the biosphere, which eventually resulted in the severe biotic crisis at the end of the Triassic. [ABSTRACT FROM AUTHOR]

The Cretaceous black shales of Colombia are among the most important successions in the north of South America and have attracted the attention of many geoscientists and exploration companies over the last few decades, because of their high hydrocarbon potential and the presence of emerald deposits. However, many uncertainties still remain with regard to their tectonic setting, sequence stratigraphy, depositional environments, palaeoxygenation conditions, and organic matter preservation. In order to develop a more integrated picture of these different processes and conditions, we conducted a detailed sedimentological, inorganic geochemical (U, V, Ni, Zn, Mn, Fe, Ti, Mo, Cu, Cr, Cd, Ba) and sequence stratigraphic analysis of the Cretaceous black shales in the Magdalena-Tablazo Sub-Basin (Eastern Cordillera Basin) of Colombia. Eleven lithofacies and five lithofacies associations of a storm-dominated, siliciclastic-carbonate ramp were identified, which range from basin to shallow inner ramp environments. These facies were grouped into six third-order stratigraphic sequences showing high-order cycles of marine transgression with constrained regressive pulses during the late Valanginian to early Coniacian. The black shales succession represents deposition under anoxic bottom water with some intervals representing dysoxic-suboxic conditions. The evolution of the sedimentary environments and their palaeoxygenation history reflect tectonic and eustatic sea-level controls that 1) produced a variable orientation and position of the coastline throughout the Cretaceous; 2) conditioned the low-gradient ramp geometry (<0.3°) and 3) modified the oxygenation conditions in the Magdalena-Tablazo Sub-Basin. An improved understanding of the sedimentary setting during deposition of the Cretaceous black shales in the Magdalena-Tablazo Sub-Basin assists in highlighting the interplay between the mechanism of sedimentation and continuum anoxic conditions prevailing in a basin, as well the important role of nutrient input from continental runoff as a trigger of high productivity and extended anoxia conditions. [ABSTRACT FROM AUTHOR]

Elemental vanadium (V), an essentially redox-sensitive metal in seawater, has had a significant impact on the understanding of the evolution of the atmosphere-ocean system throughout the history of the Earth. In fact, the geochemical cycle of V in early Cambrian seawater may have had an influence on the Chengjiang Biota in South China; however, it has not yet been well established. Given the authigenic vanadium accumulation is sensitive to the redox conditions of seawater, here, to constrain the geochemical cycle of V in seawater during the early Cambrian, the Mo, U and total organic carbon (TOC) distributions with high-resolution samples from both the outer shelf and slope facies (e.g., the Duoding and Longbizui sections), are applied to evaluate the redox conditions of ambient seawater. The Mo-U relationships indicate that the redox conditions of the mid-depth seawater evolved in a systematic way in South China, transitioning from an Fe-Mn reduction zone to anoxic/intermittently euxinic states and then to oxic conditions during the early Cambrian. As a consequence, the authigenic V enrichment, constrained by the marine redox conditions, was mainly controlled by the Fe-Mn particulate shuttle and the reduction and adsorption of organic matter in anoxic/euxinic conditions. However, the decoupling among V, Mo, U and TOC under anoxic/euxinic conditions suggests a dwindling vanadium concentration in the early Cambrian seawater of South China. The scavenging efficiency of V from seawater is much higher than those of Mo and U under anoxic/euxinic conditions. Ultimately, these trace elements (e.g., Mo, U, and especially V) in seawater could effectively be regulated and adjusted to a reasonable level under the widespread anoxic/euxinic conditions. The drawdown of trace elements in seawater might provide an early-stage preparation of the marine environment for the subsequent Chengjiang Biota. [ABSTRACT FROM AUTHOR]

The mobility of heavy metalloids including As, Sb, Mo, W, and Cr in soil was investigated under both reducing and oxidizing conditions. The effects of soil mineralogy and the presence of competitive anions were studied as important factors affecting the mobility of these contaminants. Batch experiments conducted with the addition of oxidized and fresh FeS exhibited enhanced sorption rates for As and W under oxidizing conditions, and for Mo under reducing conditions. The inhibitory effect of phosphate on the sorption rates was most apparent for As and Mo under both oxidizing and reducing conditions, while only a small phosphate effect was observed for Sb and W. For Sb and W mobility, pH was determined to be the most important controlling factor. The results of long-term batch experiments revealed that differences in the mobility of metalloids, particularly As, were also influenced by microbial activity in the oxidizing and reducing conditions. [ABSTRACT FROM AUTHOR]

By assessing U geochemistry in marine sediments of Toulon Bay (NW Mediterranean Sea), authigenic U accumulation was found to be tightly linked to that of Mo and V with slight differences in accumulation rate depending on sediment redox conditions. In sediments collected on a transect along a river plume, the authigenic accumulation of these redox-sensitive elements appears to be linked to sediment grain size which probably drives the redox status of the sediments. A typical U loss in re-oxidized sediments was observed in sediments that had been previously disturbed. However, the dissolved U profiles do not show a simple and typical depletion trend but rather a depletion in the top 10 cm followed by release in deeper pore waters that could be linked to a potential reoxidation/mobilization of authigenic U(IV). The released U could be further scavenged as a U-P precipitate. The U isotopic data (δ 238 U), which are the first reported for the coastal hypoxic sediments of Toulon Bay, average −0.12 ± 0.12‰ in surface sediments. However, sediments situated in front of river mouths show higher values of δ 238 U (up to +0.75‰); these high values are reported for the first time in sediments below an oxic water column. [ABSTRACT FROM AUTHOR]

We apply trace element analysis of syn -sedimentary pyrite to track the redox and nutrient variations in the Ediacaran-Cambrian Bambuí Group of eastern Brazil. Sedimentary pyrite from black shales interleaved within the basal cap carbonate of the 635–600 Ma Pedro Leopoldo Member, deposited in an open marine setting connected to the global sea, preserve high Mo up to >200 ppm, erratic enrichment of micronutrients such as Cu, Ni, Ba, Zn and uniform enrichment of Se. This indicates the importance of oxidative continental weathering and development of an enhanced Redox Sensitive Elements (RSE) dissolved reservoir on the aftermath of the Marinoan glaciation, with atmospheric O 2 levels, calculated from Se/Co ratios of pyrites, around 2.5–3.5 wt%. A shallow oxic layer over deep euxinic realms made possible by high concentrations of dissolved sulfate was probably maintained up to the late Ediacaran, during deposition of the Cloudina-bearing Lagoa Santa Member (585–540 Ma). Upsection, however, radical shifts to muted Mo concentrations and overall low micronutrient concentrations are recorded in the Serra de Santa Helena Formation, with O 2 levels plunging below 1 wt%. This indicates the development of ferruginous bottom water conditions in a sulfate-poor setting, due to restriction of the basin by the encircling Brasiliano mountains that formed Gondwana. The restricted, oxygen-poor and nutrient-depleted conditions caused the demise of complex life forms and hampered the development of typical Ediacaran-Cambrian ecosystems. Basin re-connection might have occurred during deposition of the Lagoa do Jacaré Formation, with independent proxies such as local tidal facies indicating a return to more ventilated conditions in the upper Bambuí Group. Our results highlight the importance of basin restriction and connection over coupled redox-nutrients variations during the development of Ediacaran-Cambrian ecosystems. • Sedimentary pyrite trace element concentrations of the Bambuí Group. • Basin restriction and connection controls coupled redox-nutrients variations. • Radical shifts in nutrient and oxygen levels in the Ediacaran-Cambrian transition. • Redox-nutrient variations control the development of complex life forms. [ABSTRACT FROM AUTHOR]

The high-resolution study of vertical geochemical variability of shallow-water Tremadocian black shales of the Türisalu Formation targeted two drill core sections from Suur-Pakri Island, NW Estonia. Altogether 374 samples from 4.6 m thick shale were analysed by XRF. The metalliferous and organic-rich black shales revealed significant centimetre-scale variation in the concentration of redox-sensitive trace metals – U, Mo and V. The V profiles show cyclic variations in half a metre- to metre-scale and the strongest correlation with loss on ignition (LOI) 500 °C (interpreted to reflect organic matter abundance). The abundance of Mo presents high values near the lower and upper contacts of black shale and otherwise moderate covariance with LOI. The distribution of U is not coupled with LOI, being characterized by irregular local enrichment anomalies in the profiles of both sections. This suggests that sequestration of U may have been time-dependent and possibly favoured by dissimilatory U-reduction at the sediment–water interface under iron-reducing conditions. Significant depositional variability of the studied organic-rich muds apparently supported dynamic physicochemical and biological microenvironments at the sediment–water interface and thus temporally and spatially diversified the paths and efficiency of synsedimentary redox-sensitive trace element enrichment.

To get more data on the geochemistry of Black Sea euxinic sediments, a 50-cm core was collected at a depth of 600 m on a Western Black Sea Continental Platform slope. The core contained unconsolidated sediments rich in cocoolithic ooze and mud. Epithermal Neutron and Prompt Gamma Activation Analysis were used to determine the content of nine major (Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe as oxides) and 32 trace elements (Cl, Sc, V, Cr, Co, Ni, Zn, As, Se, Br, Rb, Sr, Zr, Mo, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, Hf, Ta, W, Th, and U) with a precision varying between 3 and 9%. The core contained unconsolidated sediment rich in coccolithic ooze and mud. Previous 210 Pb geochronology suggests an age of ∼1 ky of considered sediments. Major components distribution showed that, except for Cl and Ca, the contents of all other elements are similar to Upper Continental Crust (UCC) and North American Shale Composite (NASC). The distribution of the 32 trace elements showed similarities to the UCC, except for redox-sensitive metals Fe, Se, Mo, and U, of which the significantly higher content reflects the presence of euxinic conditions during deposition. A chondrite normalized plot of nine rare earth elements indicated a similarity to UCC and NASC, suggesting a continental origin of sedimentary material.

Marine black shale beds in the Duwi Formation of NE Egypt are part of the Late Cretaceous-Palaeogene Middle East-North African phosphogenic province. These black shales were analyzed for their redox-sensitive trace elements (V, Ni, Mo, U, Cu, Cr, Re, Cd, Sb, Tl, and Mn) to examine their depositional conditions. The data show that the black shales have elevated concentrations of redox-sensitive trace metals, low Mn contents, low Th/U and V/Mo ratios, and high V/Ni, Ni/Co, V/(V + Ni) and V/(V + Cr) ratios as well as a positive correlation of metal Mo to V concentrations. These geochemical data suggest that the Campanian-Maastrichtian Tethys was stratified and stagnant, with reducing bottom water conditions. The high V/Ni ratios in the study shales indicate their dominantly marine origin and suggest that the high-productivity upwelling system that persisted over ∼20 m.y. at the southern margins of the Tethys Ocean is the main source of the organic matter. A combination of elevated primary productivity, remineralization and reducing depositional conditions is the main control of enrichment of redox-sensitive trace elements in the black shales of the Duwi Formation. [ABSTRACT FROM AUTHOR]

Two sediment cores, ABP-32/GC-01R and ABP-32/GC-03 were collected at a water depth of 642 m and 1086 m off Goa from the present day Oxygen Minimum Zone (OMZ) of eastern Arabian Sea (EAS) cover time span of last 18 ka and 32 ka respectively were analysed for multi-proxy redox-sensitive elements to understand the variation in the redox conditions and factors responsible for its development. Redox-sensitive elements concentration and their normalized ratios (Mn/Al, U/Th, Mo/Al and Ce/Ce*) suggest that sediment core ABP-32/GC-01R is under more reducing conditions due to its location within the centre of OMZ compared to core ABP-32/GC-03 which is at the base of OMZ. Sediments from the EAS are of non-euxinic environments where dissolved sulfide is present but restricted to the sediment pore-waters. Lack of significant correlation (r=< 0.1) of organic carbon with U and Mo suggest that productivity may not have control on the development of reducing conditions. The lowest Mn/Al ratio, strong negative Ce/Ce* anomaly and remarkable enrichment of U/Th and Mo/Al ratios during the last deglaciation, and Heinrich events (H1, H2, H3) indicate intense reducing conditions probably due to poor ventilation by oxygen depleted bottom waters from Subantarctic Mode Waters (SAMW) - Antarctic Intermediate waters (AAIW). There is a distinct lathanide fractionation in the sediment cores where, La (n) /Yb (n) ratio is <1, ≈1 and >1 during the last 10 ka (Holocene), 14–10 ka (includes-Younger Dryas and Bǿlling-Allerǿd), 18–14 ka (last deglaciation) and Heinrich events suggesting less reducing, terrigenous dominated and intense reducing condition respectively. [ABSTRACT FROM AUTHOR]

Concentrations of redox-sensitive elements (S, Mn, Mo, U, Cd, Re) were analyzed in a set of 27 sediment cores collected along the North American Arctic margin (NAAM) from the North Bering Sea to Davis Strait via the Canadian Archipelago. Sedimentary distributions and accumulation rates of the elements were used to evaluate early diagenesis in sediments along this section and to estimate the importance of this margin as a sink for key elements in the polar and global oceans. Distributions of Mn, total S and reduced inorganic S demonstrated that diagenetic conditions and thus sedimentary carbon turnover in the NAAM is organized regionally: undetectable or very thin layers (<0.5 cm) of surface Mn enrichment occurred in the Bering–Chukchi shelves; thin layers (1–5 cm) of surface Mn enrichment occurred in Barrow Canyon and Lancaster Sound; and thick layers (5–20 cm) of surface Mn enrichment occurred in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Inventories of authigenic S below the Mn-rich layer decreased about fivefold from Bering–Chukchi shelf and Barrow Canyon to Lancaster Sound and more than ten-fold from Bering–Chukchi shelf to Beaufort Shelf, Canadian Archipelago and Davis Strait. The Mn, total S and reduced inorganic S distributions imply strong organic carbon (OC) flux and metabolism in the Bering–Chukchi shelves, lower aerobic OC metabolism in Barrow Canyon and Lancaster Sound, and deep O 2 penetration and much lower OC metabolism in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Accumulation rates of authigenic S, Mo, Cd, Re, and U displayed marked spatial variability along the NAAM reflecting the range in sedimentary redox conditions. Strong relationships between the accumulation rates and vertical carbon flux, estimated from regional primary production values and water depth at the coring sites, indicate that the primary driver in the regional patterns is the supply of labile carbon to the seabed. Thus, high primary production combined with a shallow water column (average 64 m) leads to high rates of authigenic trace element accumulation in sediments from the Bering–Chukchi shelves. High to moderate primary production combined with deep water (average 610 m) leads to moderate rates of authigenic trace element accumulation in sediments from Lancaster Sound. Low to very low primary production combined with moderate water depths (average 380 m) leads to low rates of authigenic trace element accumulation in sediments in the Beaufort Shelf, Davis Strait and Canadian Archipelago. Authigenic Mo accumulation rates show a significant relationship with vascular plant input to the sediments, implying that terrestrial organic matter contributes significantly to metabolism in Arctic margin sediments. Our results suggest that the broad and shallow shelf of the Chukchi Sea, which has high productivity sustained by imported nutrients, contributes disproportionately to global biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Con el objetivo de evaluar la relación entre la preservación de escamas de peces y las variaciones en las condiciones redox en sedimentos del margen continental de Chile, este trabajo presenta recuentos de escamas de peces y concentraciones normalizadas de elementos redox sensibles (Mo/Al, S/Al, Fe/Al) en ocho testigos de sedimento recolectados en la zona de minimo oxígeno frente a Chile (Iquique: 20°15' S, bahía de Mejillones: 23° S y Concepción: 36° S). En el norte de Chile (Iquique y Mejillones) predominan las escamas de anchoveta (Engraulis ringens) y de peces de la familia Myctophidae (media = 90 y 120 escamas 1.000 cm-3, respectivamente) mientras que en Chile centro-sur (Concepción) son más abundantes las escamas de jurel (Trachurus murphy; media = 140 escamas 1.000 cm-3). La abundancia de escamas de merluza común (Merluccius gayi gayi) y de sardina (Sardinops sagax) es aproximadamente un orden de magnitud más baja que la de anchoveta o jurel. En general, los valores más altos y los rangos más amplios de variación en las razones Mo/Al, S/Al y Fe/Al se encuentran en Mejillones (Mo/Al ~0,8-12 mg g-1, S/Al 0,2-4,6 g g-1, Fe/Al 0,3-0,7 g g-1) seguidos por Iquique (Mo/Al -0,2-1,8; S/Al 0,2-0,7, Fe/Al 0,5-0,8), mientras que Concepción revela valores más bajos y poco variables (Mo/Al ~0,07, S/Al ~0,15 y Fe/Al ~0,5). La razón Mo/Al, utilizada como indicador de paleo-oxigenación, permite inferir condiciones reductoras relativamente más intensas en los sedimentos de la Bahía de Mejillones e Iquique que en Concepción. En las tres localidades de muestreo se evidencia una asociación estadísticamente significativa entre la abundancia de escamas de anchoveta y el logaritmo de la razón Mo/Al (r²= 0,46; P < 0,0001), apuntando a una dependencia entre estas variables. En especial, para razones Mo/Al < 1 mg g-1, pequeños cambios en las condiciones redox pueden afectar considerablemente la preservación de escamas en el sedimento, mientras que esta dependencia tiende a minimizarse para razones Mo/Al > 1 mg g-1.The relationship between fish scale preservation and variations in the sediment redox conditions on Chile's continental shelf are evaluated herein through fish scale counts and normalized concentrations of redox sensitive elements (Mo/Al, S/Al, Fe/Al) using eight sediment cores taken from sites under the influence of the oxygen minimum zone off Chile (Iquique: 20°15' S, Mejillones Bay: 23° S, Concepción: 36° S). Off northern Chile (Iquique and Mejillones), fish scales from anchovy (Engraulis ringens) and the Myctophidae family are dominant (mean = 90 and 120 scales 1,000 cm-3, respectively), whereas off central-southern Chile (Concepción), jack mackerel (Trachurus murphy) scales are the most abundant (mean = 140 scales 1,000 cm-3). The abundance of hake (Merluccius gayi gayi) and sardine (Sardinops sagax) scales is approximately one order of magnitude lower than that of anchovy or jack mackerel. In general, the highest valúes and widest ranges of variation in the Mo/Al, S/Al, and Fe/Al ratios are found in Mejillones (Mo/Al ~ 0.8-12 mg g-1, S/Al 0.2-4.6 g g-1, Fe/Al 0.3-0.7 g g-1), followed by Iquique (Mo/Al ~ 0.2-1.8, S/Al 0.2-0.7, Fe/Al 0.5-0.8); Concepción has lower, less variable valúes (Mo/Al ~ 0.07, S/Al ~ 0.15, Fe/Al ~ 0.5). According to the Mo/Al ratio, used as an indicator of paleo-oxygenation, reducing conditions in the Mejillones Bay and Iquique sediments are relatively more intense than in those off Concepción. At all three sampling sites, the relationship between the abundance of anchovy scales and the logarithm of the Mo/Al ratio is statistically significant (r² = 0.46, P < 0.0001), indicating a dependence between these two variables. In particular, for Mo/Al < 1 mg g-1, small changes in the redox conditions can affect considerably scale preservation in the sediment, whereas this dependence decreases with Mo/Al > 1 mg g-1.

>5 billion tons of karstic bauxites were formed during the Late Carboniferous in the North China Craton (NCC). However, the metallogenic mechanism of bauxite is still obscure due to the unclear formation process of the main ore mineral, diaspore. We conducted geological, mineralogical, δ34S pyrite , and in-situ diaspore trace element analyses on the Baode-Xingxian large bauxite deposit in the northern NCC, aiming to clarify the diaspore-forming mechanism and explore the genesis of bauxite. Generally, bauxites in northern NCC contains three layers from bottom to top: Fe-bearing claystone, bauxite ore, and claystone. The bauxite ore is mainly diaspore, kaolinite, and illite; the Fe-bearing claystone is illite and berthierine; and the claystone is primarily kaolinite. The broadly distributed high positive δ34S pyrite values (10‰–40‰) indicate that bauxites in the northern NCC formed in shallow-water environments. Total 216 diaspore grains have similar trace element compositions, and are obviously enriched in Cr, V, Re and U, but not in Mo, indicating that diaspore was formed in the subreduced facies. Diaspore with Fe/Al up to 4 mol% has slightly large cells, indicated that small-scale larger ions, Fe, entered the diaspore lattice to form Fe-diaspore. The favorable temperature and water activity of Fe-diaspore precipitation is similar to that of Al-hematite+ boehmite. We thus propose a probable diaspore formation mechanism: the small scale of Fe for Al substitution in the diaspore lattice and stabilize it to induce Fe-diaspore formation. Fe-diaspore is metastable with respect to diaspore and tends to convert into it during the diagenesis process. [Display omitted] • The high positive δ34S values revealed that bauxites formed in the shallow water. • The elemental proxies revealed that diaspore formed in the subreduced facies. • The Fe3+for Al3+ substitution induced to form Fe-diaspore. • Metastable Fe-diaspore tends to convert into diaspore during diagenesis. [ABSTRACT FROM AUTHOR]

The variation in chemical compositions of regolith along a weathering profile developed on a granite substrate in Jiangxi province, in southern China, was investigated in this paper, with the aim to characterize the speciation of redox-sensitive elements and to evaluate their mobility and redistribution during chemical weathering. Mass balance calculations indicate titanium (Ti) is the most immobile element in this weathering profile. A new method, X-ray absorption fine structure (XAFS) spectroscopy, was used to determine the speciation of Fe and Mn along the profile. Fe K-edge X-ray absorption near edge structure (XANES) spectra show Fe in saprolith is stabilized mainly in the state of Fe(III), suggesting Fe is as immobile as conservative elements during granite weathering. Mn K-edge XANES spectra show Mn (III/IV) oxides are reduced to Mn(II) in surface soil, where soil organic matter (SOM) acts as an important reductant. Although Ce, Co and V were unable to be analysed by XAFS, their concentrations have significant correlations with that of Mn, indicating that the mobilization and redistribution of Ce, Co and V may also be governed by redox condition. All in all, the results suggest that redox process impacts significantly on the redistribution of Mn, Ce, Co and V along the profile. The successful application of XAFS in the study on migration of redox-sensitive elements during granite chemical weathering has provided valuable information for the understanding and evaluating the geochemical behavior of elements in the environment. [ABSTRACT FROM AUTHOR]

Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus , Dicellograptus complexus , and Paraorthograptus pacificus , were deposited under euxinic conditions at both localities, as shown by high Mo EF , U EF , and δ 98 Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high Mo EF , U EF , and δ 98 Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct. The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ 13 C carb , δ 13 C org , and δ 34 S py records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ 13 C carb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system. [ABSTRACT FROM AUTHOR]

The high-resolution study of vertical geochemical variability of shallow-water Tremadocian black shales of the Türisalu Formation targeted two drill core sections from Suur-Pakri Island, NW Estonia. Altogether 374 samples from 4.6 m thick shale were analysed by XRF. The metalliferous and organic-rich black shales revealed significant centimetre-scale variation in the concentration of redox-sensitive trace metals--U, Mo and V. The V profiles show cyclic variations in half a metre- to metre-scale and the strongest correlation with loss on ignition (LOI) 500 °C matter abundance). The abundance of Mo presents high values near the lower and upper contacts of black shale and otherwise moderate covariance with LOI. The distribution of U is not coupled with LOI, being characterized by irregular local enrichment anomalies in the profiles of both sections. This suggests that sequestration of U may have been time-dependent and possibly favoured by dissimilatory U-reduction at the sediment-water interface under iron-reducing conditions. Significant depositional variability of the studied organic-rich muds apparently supported dynamic physicochemical and biological microenvironments at the sediment-water interface and thus temporally and spatially diversified the paths and efficiency of synsedimentary redox-sensitive trace element enrichment. [ABSTRACT FROM AUTHOR]

The distribution of redox-sensitive elements (V, U, Mo, Mn) was studied in a sedimentary archive of the Pierre-Blanche lagoon encompassing 3500 years of sedimentation. Between 240 and 255 cm (i.e. between 0 and 350 cal. ad) in the core, there was a dark layer with no mollusc shells. Enrichments of V, U and Mo and depletion of Mn were observed in this layer compared with overlying and underlying sediments. This was attributed to the occurrence of anoxia in bottom water or at the sediment/water interface during the Roman period. This episode was concomitant with the formation of a sandy barrier, which now isolates the lagoon from the sea. Our results suggest that the isolation of the lagoon, which was deeper in the period concerned than at present, favoured stagnation of the water. Combined with the excess nutrient inputs because of human activities in Lattara harbour, stagnation may have caused anoxia in bottom water at least in some parts of the Palavasian lagoon system. Bottom conditions nevertheless remained sufficiently reducing for the accumulation of U and Mo to persist until around 800 cal. ad. Although the lagoon has been hyper-eutrophic since at least the middle of the 20th century and although anoxia occurs frequently in summer, redox proxies show no evidence for their occurrence. The shallow water and associated resuspension and reoxidation events explain the lack of V, Mo and U enrichment in recent sediments. [ABSTRACT FROM AUTHOR]

Abstract: Reconstruction of paleo-redox conditions in a radiocarbon (14C) dated sediment core (SK-218/1), covering the past 45ka (thousand calendar years), collected from the western Bay of Bengal (Lat: 14° 02′N; Long: 82° 00′E) at a water depth of 3307m, has been made based on redox-sensitive element geochemistry. The high U/Th ratio, Mo enrichment, Mo/U enrichment factor ratio, Ce/Ce*<1 and lower Mn/Al and Fe/Al ratios, compared to upper continental crust are all indicative of prevalence of suboxic condition in the benthic environment from 15.2ka to 4.5ka, peaking around 9.5ka. The suboxic condition around 9.5ka corresponds to the previously recorded southwest (SW) monsoon intensification in response to the increase in northern hemisphere summer insulation. However, productivity proxies – organic carbon and nitrogen contents – do not indicate marked increase in productivity at this time. It is proposed that as a result of large increase in lithogenic material supplied from land due to SW monsoon intensification, which is evident by the very high concentration of Al, Zr and Hf, the flux of fresh labile organic matter and these might have formed dense mineral matter — biogenic aggregates which sinks rapidly to the seafloor, and the degradation of labile organic matter might have led to the development of suboxic condition in the benthic environment. There exists a strong positive correlation (r=0.98) between Mo and Zr during 15.2ka to 4.5ka suggesting a coupling between suboxic condition and lithogenic flux supply by the intensified SW monsoon. Our results suggest that temporal variability of the ballasting effect of the terrestrially-derived material could play a key role in benthic biogeochemistry and ecology of the Bay of Bengal. We also provide the first record of the nitrogen isotopic composition (δ15N) of sedimentary organic matter in the western Bay of Bengal, a region where the mesopelagic oxygen minimum zone (OMZ) is just short of being suboxic (denitrifying) today. The sedimentary δ15N fluctuated considerably in the past, especially during the Marine Isotope Stage 3. Oscillations in δ15N were apparently in concert with those in organic carbon and nitrogen contents and could be related to climatic changes (Heinrich and Dansgaard–Oeschger events) in the North Atlantic. The Dansgaard–Oeschger 12 event appears to have exerted the most intense effect on Bay of Bengal biogeochemistry when surface productivity, as inferred from the organic carbon and nitrogen contents, was the highest recorded in the core, and the δ15N reached up to 6.3‰. Considering the probable dilution by isotopically light terrigenous organic matter, it would appear that OMZ of the Bay of Bengal had turned denitrifying. However, the absence of suboxic conditions in the sediments at this time suggests a decoupling of the benthic processes with those in the mesopelagic water column. [Copyright &y& Elsevier]

Redox-sensitive elements in sediments, such as manganese (Mn), vanadium (V), molybdenum (Mo), and uranium (U), are promising indicators of past redox conditions during sedimentation and early diagenesis. However, in the Ontong Java Plateau, west equatorial Pacific, there are sparse datasets of redox-sensitive elements in sediment cores. Here, we present a 250 ka record of redox sensitive elements from a 460 cm gravity core at site WP7 (3°56′S, 156°E, water depth 1 800 m), which was recovered from the southwest Ontong Java Plateau during the 1993 cruise of R/V Science I of the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). Relative to the Post-Archean Australian Shale (PAAS), authigenic Mn, cobalt (Co), nickel (Ni), Mo, V, U, and cadmium (Cd) were found at constantly low levels except when peaks occurred at several depth intervals. Manganese, Co, Ni, and Mo concentrations were elevated at 25-35 cm due to Mn redox cycling. The core was divided into three distinct sections, the top 0-25 cm being oxic, a suboxic section at 25-35 cm and from 35-460 cm which was anoxic. Differential authigenic enrichments of Co, Ni, Mo, V, U, and Cd at the same depth intervals were observed indicating that the enrichments happened during sedimentation or diagenesis and suffered no post settlement redox changes. Therefore, no significant changes in redox conditions during sedimentation must have happened. The water at depth on the Ontong Java Plateau during past 250 ka must have been well oxygenated, possibly resulted from the more or less continuous presence of oxygen-rich deep water like the modern Antarctic Intermediate Water (AAIW) and Antarctic Circumpolar Water (ACW); while it's slightly less oxygenated in glacial intervals, possibly due to ventilation weakening and/or the surface productivity increase. [ABSTRACT FROM AUTHOR]

Geochemical and rock-magnetic investigations were carried out on a sediment core collected from the SE Arabian Sea at 1420 m depth in oxygenated waters below the present-day oxygen minimum zone. The top 250 cm of the core sediments represent the last 35 kaBP. The ·18O values of Globigerinoides ruber are heaviest during the Last Glacial Maximum (LGM) and appear unaffected by low-saline waters transported from the Bay of Bengal by the strong northeast monsoon and West Indian coastal current. The signatures of Bølling-Allerød and Younger Dryas events are distinct in the records of magnetic susceptibility, organic carbon (OC) and ·18O. Glacial sediments show higher OC, CaCO3, Ba, Mo, U and Cd, while the early-to-late Holocene sediments show increasing concentrations of OC, CaCO3, Ba, Cu, Ni and Zn and decreasing concentrations of Mo, U and Cd. Productivity induced low-oxygenated bottom waters and reducing sedimentary conditions during glaciation, and productivity and oxygenated bottom waters in the Holocene are responsible for their variation. The core exhibits different stages of diagenesis at different sediment intervals. The occurrence of fine-grained, low-coercivity, ferrimagnetic mineral during glacial periods is indicative of its formation in organic-rich, anoxic sediments, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea and Japan Sea. The glacial sediments exhibiting reductive diagenesis with anoxic sedimentary environment in this core correspond to reductive diagenesis and intermittent bioturbation (oxygenation) reported in another core in the vicinity. This suggests that the poorly oxygenated bottom water conditions during glacial times should not be generalized, but are influenced locally by productivity, sedimentation rates and sediment reworking. [ABSTRACT FROM AUTHOR]

Geochemical analysis of the 23 sediment samples collected from a short (0.6 m long) core retrieved from the coastal creek that was previously connecting the northern and southern Al-Shuaiba Lagoons, Red Sea, Saudi Arabia, was accomplished to assess the elemental enrichment levels and the natural and anthropogenic driving forces for this enrichment. Statistical analysis and upcore variation in elemental concentrations enabled subdivision of the core formally into three units, lower, middle, and upper. The enriched elements in the lower and middle units display poor to negative correlations with the enriched elements in the upper unit. The lower unit is enriched in elements (Mo, As, U, and Re) suggesting deposition under anoxic conditions, possibly related to the Medieval Climate Anomaly. The middle unit is enriched in the carbonate-related constituents (CaCO3, Ca, and Sr). The upper unit is enriched in elements that co-vary significantly with Al suggesting increased terrigenous supply associated with the construction of the road between the two lagoons. The enrichment of elements in the lower and middle units is naturally driven, whereas the enrichment of lithogenic elements in the upper unit, though of geogenic origin, is induced after the road construction. [ABSTRACT FROM AUTHOR]

Redox-sensitive element (RSE) compositions in marine sedimentary archives have been important in improving our understanding of redox history, especially in the Meso-Neoproterozoic ocean, where early biological innovations took place before the Cambrian Explosion. However, despite increasing availability of RSE data that represent this long time interval, temporal comparisons are difficult, and thus provide poor constraints on the dramatic changes in atmosphere-ocean chemical settings. Here we analyzed carbonate rocks and black shales from the North China and Yangtze platforms (eastern China) for trace elements and isotopic compositions (δ13C, δ34S and 87Sr/86Sr). These data were combined with previously published RSE data to allow establishment of successive elemental chemostratigraphies from sequences of both black shale and carbonate RSE archives, thus yielding direct insights into the role of marine RSE cycling. Generally low Th/RSE ratios in relatively pure carbonate rocks (Al < 0.35%, Th < 0.5 μg/g) and black shales likely indicate enriched RSEs in oxidized shallow conditions, consistent with a more oxidized Neoproterozoic ocean or confined water body. The chemostratigraphy of RSEs not only shows enrichment in Neoproterozoic marine sediments, but temporally variable trends in the Meso-Neoproterozoic. A mass balance model based on modern marine budgets demonstrates that even a small oxidation event within the dominantly anoxic setting could be an important factor for increasing dissolved RSE reservoirs, and that observed heterogeneous elemental records may represent the amplified results of a fluctuating palaeoredox (oxic-anoxic) structure with pulsed marine oxidation. Meso-Neoproterozoic marine redox heterogeneity in eastern China suggests that increases in free oxygen were potentially related to: 1) supercontinent cycles, 2) enhanced weathering inputs with high 87Sr/86Sr ratios (> 0.705 at ca. <1.4–1.1 Ga), and 3) previously reported biological processes as emphasized by biological C-S cycles. Thus, pulsed oxic conditions may have provided sequential opportunities for biological innovation. [Display omitted] • Meso-Neoproterozoic sediments record the marine redox heterogeneity. • A oxidation event within anoxic oceans account for the greatest RSE fluctuation. • Pulsed oxic conditions promote biological innovation in ancient oceans. [ABSTRACT FROM AUTHOR]

The seepage activity of paleo-methane has been proven to exist in geological history and play an important role in the evolution of the Earth's environment. This study reports grain size, major/trace elements, total nitrogen (TN), total carbon (TC), total organic carbon (TOC), total sulfur (TS), magnetic susceptibility and AMS-14C measurements for sediment samples in a drill core (~282 cm long; Site Q6 at ~1400 m water depth) from the 'Haima seep' sedimentary area in the Qiongdongnan Basin, along the northern slope of the South China Sea (SCS). The alternative index system for paleo-methane seepage identification was established through testing and analysis, and three methane release events (MREs) were successfully identified: 254–282 cm (MRE1: 19.3–17.8 ka), 144–162 cm (MRE2: 11. 9–10.8 ka) and 90–124 cm (MRE3: 10.0–8.5 ka). When methane seepage occurred, the enrichment of authigenic carbonate minerals (high Mg-calcite or aragonite) led to the increase in the content of CaO and 'dilution' in the total rare earth element content (∑REE). However, the occurrence of authigenic pyrite resulted in a significant increase in TS and the total sulfur/total organic carbon ratio (S/C) in the sediments, and a synchronous decrease in the magnetic susceptibility. Meanwhile, the change of redox environment also induced the enrichment of Mo, U, and the positive anomaly of δCe. Based on the records of methane release events, the control mechanisms for the different time scales and different periods, such as sea level change, sea floor temperature change, and turbidity current activity were preliminarily discussed. Finally, the history of methane seepage activity was reconstructed, and conceptual diagrams were drawn of the study area encompassing the last 20 ka. [ABSTRACT FROM AUTHOR]

Rocks of Darriwilian and Sandbian age (Middle–Late Ordovician) record the extraordinary peak of the Great Ordovician Biodiversification Event (GOBE) and significant global environmental changes. To track paleo-environmental changes through this critical time, we have analyzed iron speciation, trace elements and total organic carbon in samples of the Miaopo Formation from an intra-shelf basin in South China. The results reveal moderate primary productivity and possible fluctuations in benthic redox. Upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters, sustaining the biological pump. Enhanced export of organic matter would have consumed dissolved oxygen at depth, driving the system toward anoxia. However, it is also possible for deep-water renewal events to transfer more dissolved oxygen into the bottom water. Those proxies suggests high primary productivity and varying sea level (global and local), not benthic anoxia, as the driver of accumulated "black shale". Thus, the local marine ecosystem thrived in this oxic intra-shelf basin, challenging the preexisting paradox that the high-diversity community flourished in an oxygen-depleted environment. • Ba xs and TOC suggest moderate-high primary productivity yet no major increase. • Enhanced anoxia occurred in porewaters rather than in the water column. • High productivity and sea level rise prompted the organic-rich shale to form. • Our study solve previous debates on O 2 -depleted waters and thrived community. [ABSTRACT FROM AUTHOR]

Organic- and sulfide-rich sediments have formed in oxygen-depleted environments throughout Earth's history. The fact that they are generally enriched in redox-sensitive elements reflects the sedimentary environment at the time of deposition. Although the modern ocean is well oxidized, oxygen depletion occurs in certain areas such as restricted basins and high-productivity zones. We measured bulk chemical compositions (major and trace elements, total organic carbon, and total sulfur) of organic- and sulfide-rich sediments collected from eight areas having oxygen-depleted water to discuss relationships between geochemical features and sedimentary environments. Major elemental compositions generally show mixtures of terrigenous detritus and biogenic carbonate. Some redox-sensitive elements might be controlled by organic matter content, whereas others could be contained in sulfide minerals in sediments. In particular, Mo and U show a characteristic trend; areas with higher Mo and U—at least partially owing to a depositional process called the "particulate shuttle"—generally correspond to regions influenced by the open ocean. In contrast, areas with lower Mo and U are more restricted marine environments. This suggests that the degree of Mo and U enrichment reflects the geography in terms of proximity to the open ocean, or the degree of the supply of these elements from the open ocean. [ABSTRACT FROM AUTHOR]

The Lower Jurassic Água de Madeiros (AM Fm) and Vale das Fontes (VF Fm) Formations, and the Upper Jurassic Cabaços Formation (Cab Fm) are considered the most important units with petroleum source-rock potential in the Lusitanian Basin, and the likely sources for its widespread hydrocarbon occurrences. Nonetheless, the bottom-water redox conditions during their deposition remain debatable. To resolve this controversy, we investigated whole-rock inorganic geochemical data of the most significant organic-rich levels of the AM, VF and Cab Fms cropping out at basin scale. Significant variations in TOC, TS and redox-sensitive elements contents and their ratios are evident within and between the organic-rich levels of the studied units. A positive covariation between primary productivity proxies, TOC enrichment and paleo-redox proxies for the Lower Jurassic units clearly suggest an interdependence between enhanced primary productivity, organic matter accumulation and bottom-water redox conditions. For the AM Fm, the paleo-redox proxies indicate that the black shales deposition took place beneath bottom-waters that ranged from suboxic to strongly euxinic conditions, with a clear predominance of the latter. A TOC threshold of ~6–7 wt% clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions, and is interpreted as the minimum amount of organic matter (OM) accumulation within sediments required to sustain H 2 S production at rates that outpaced its consumption through precipitation of sulphides and/or OM sulphurization, allowing the diffusion of H 2 S to the bottom-waters. For the Vale das Fontes Fm, the paleo-redox proxies suggest a prevalence of suboxic to anoxic bottom-waters, despite all black shales displaying TOC contents greater than the above threshold. The less reducing conditions relative to the AM Fm euxinic black shales are attributed to: i) a larger proportion of continental OM; ii) lower reactivity of the OM reaching the sediment-water interface (decreasing oxygen consumption); and iii) higher availability of reactive iron (increasing the buffer effect to H 2 S produced through OM remineralization by sulphate-reducing bacteria). Both units display Mo-EF vs U-EF covariation patterns that deviate from the field of sediments deposited under unrestricted open-marine conditions, towards higher Mo enrichment relative to that of U, indicating an accelerated transport of aqueous Mo to the sediments, consistent with the operation of a metal-oxyhydroxide particulate shuttle linked to Mn and Fe redox cycling within the water column in a weakly-restricted basin. The Mo-COT regression-line slope indicates a moderate degree of bottom-water mass restriction, with [Mo] aq concentration of ~60–70% of the present-day seawater value and bottom-water renewal times of ~10–20 years, akin to the present-day Cariaco Basin. The redox conditions of the Cab Fm organic-rich levels are dependent on the location of the depositional environment in the basin, with deposition taking place beneath oxygenated bottom-waters at Cabo Mondego, and under suboxic conditions at Vale de Ventos and Pedrógão, the latter denoting a larger degree of oxygen depletion than the former. In this context, the overall potential for OM preservation should increase from Cabo Mondego to Vale de Ventos and Pedrógão. • Redox sensitive elements elucidate the redox conditions of Jurassic organic-rich levels. • Água de Madeiros Fm black shales were deposited predominantly under euxinic conditions. • Vale das Fontes Fm black shales were deposited predominantly under suboxic to anoxic conditions. • Cabaços Fm organic-rich rocks were deposited under oxic to suboxic conditions. [ABSTRACT FROM AUTHOR]

A multiproxy approach, including stable nitrogen isotopes and redox sensitive trace elements, was employed in order to evaluate the impact of depositional redox conditions on organic matter accumulations within the Mississippian Limestone interval in the Anadarko Shelf of Oklahoma and also to investigate hydrocarbon source rock potential of the Mississippian Limestone in the study area. Nitrogen isotopes (δ15N), total organic carbon (TOC), and concentrations of redox-sensitive trace elements (Cu, Mn, Mo, Ni, U, V) were measured on samples from a core drilled in Canadian County, Oklahoma. Values of TOC are between 0 and 2.5 wt. % and average 0.8 wt. % in the studied interval of the core. The δ15N data (3‰–14‰, average 8 ± 1‰) indicate sediment deposition under oxic to suboxic water conditions. This interpretation of depositional redox conditions is supported by elemental redox proxies which show no enrichment relative to average carbonate. Our geochemical data suggest that the Mississippian Limestone is unlikely to be an oil source rock due to pervasively oxic to suboxic depositional redox conditions which would have resulted in oxidation of organic matter. • Water column and benthic redox conditions at time of deposition were oxic-suboxic. • Depositional redox does not influence organic matter accumulation in study site. • Mississippian Limestone sediments have poor oil source rock potential due to oxygenated conditions during deposition. [ABSTRACT FROM AUTHOR]

Past millennial-scale changes in atmospheric CO 2 (CO 2,atm) concentrations have often been attributed to variations in the overturning timescale of the ocean that result in changes in the marine carbon inventory. Yet, there remains a paucity of proxy evidence that documents changes in marine carbon storage globally, and that links them to abrupt climate variability in the northern hemisphere associated with perturbations of the Atlantic Meridional Overturning Circulation (AMOC). The last two glacial periods were suggested to differ in the spatial extent of the AMOC and its sensitivity to perturbations. This provides an opportunity to compare the nature of marine carbon cycle-climate feedbacks between them. Here, we reconstruct variations in respired carbon storage (via oxygenation) and the AMOC "geometry" (via carbonate ion saturation) in the deep South Atlantic. We infer decreases in deep South Atlantic respired carbon levels at times of weakened AMOC and rising CO 2,atm concentrations during both glacial periods. These findings suggest a consistent pattern of increased Southern Ocean convection and/or air-sea CO 2 fluxes during northern-hemisphere stadials accompanying AMOC perturbations and promoting a rise in CO 2,atm levels. We find that net ocean carbon loss, and hence the magnitude of CO 2,atm rise, is largely determined by the stadial duration. North Atlantic climate anomalies therefore affect Southern Ocean carbon cycling in a consistent manner, through oceanic (e.g., ventilation seesaw) and/or atmospheric processes (e.g., Ekman pumping). • Comparison of South Atlantic carbon cycle dynamics during the last two glacials. • Observed respired carbon changes parallel millennial-scale CO 2,atm variability. • Possible links to northern-hemisphere stadials and AMOC changes are identified. • Ocean carbon loss during millennial-scale events is proportional to stadial durations. • Proxy data-model comparison highlights a strong role of Southern Ocean ventilation. [ABSTRACT FROM AUTHOR]

To get more data on the geochemistry of Black Sea euxinic sediments, a 50-cm core was collected at a depth of 600 m on a Western Black Sea Continental Platform slope. The core contained unconsolidated sediments rich in cocoolithic ooze and mud. Epithermal Neutron and Prompt Gamma Activation Analysis were used to determine the content of nine major (Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe as oxides) and 32 trace elements (Cl, Sc, V, Cr, Co, Ni, Zn, As, Se, Br, Rb, Sr, Zr, Mo, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, Hf, Ta, W, Th, and U) with a precision varying between 3 and 9%. The core contained unconsolidated sediment rich in coccolithic ooze and mud. Previous 210 Pb geochronology suggests an age of ∼1 ky of considered sediments. Major components distribution showed that, except for Cl and Ca, the contents of all other elements are similar to Upper Continental Crust (UCC) and North American Shale Composite (NASC). The distribution of the 32 trace elements showed similarities to the UCC, except for redox-sensitive metals Fe, Se, Mo, and U, of which the significantly higher content reflects the presence of euxinic conditions during deposition. A chondrite normalized plot of nine rare earth elements indicated a similarity to UCC and NASC, suggesting a continental origin of sedimentary material. [ABSTRACT FROM AUTHOR]