Redox-sensitive Elements Research Articles

Selenium Uptake from Livestock Pasture Extremely Enriched in Selenium, Molybdenum and Uranium: A Field and X-ray Absorption Study

The agricultural soils of West Limerick, Ireland, contain very localised, extremely high natural Se concentrations that reach levels that are very toxic to grazing livestock. The Carboniferous shales that formed in anoxic deep-water marine environments are the source of the selenium, which, along with the other redox-sensitive elements of molybdenum, uranium, arsenic and vanadium, were mobilised and reprecipitated in post-glacial anoxic marshes. The result has been a history of selenosis and molybdenosis in livestock in this important dairy province. Soils collected at 10–20 cm from five different agricultural sites were analysed, and all yielded concentrations greatly in excess of the safe Se limits of 3–10 mg kg−1; the highest value recorded was 1265.8 mg kg−1 Se. The highest recorded value for Mo in these soils was 1627.5 mg kg−1, and for U, 658.8 mg kg−1. There was a positive correlation between Se, Mo U and organic matter in the soils. Analysis of non-accumulator pasture grasses (Lolium perenne (perennial ryegrass), Festuca arundinacea (tall fescue), Dactylis glomerata (cocksfoot) and Phleum pretense (timothy grass)) revealed the shoot/leaf to contain up to 78.05 mg kg−1 Se while Trifolium repens (white clover) leaves contained 296.15 mg kg−1 Se. An in situ growing experiment using the Se accumulator species Brassica oleracea revealed 971.2 mg kg−1 Se in the leaves of premier kale, which also contained 1000.4 mg kg−1 Mo. Translocation factors (TFs) were generally higher for Mo than Se across all plant species. Combined X-ray absorption near edge spectroscopy (XANES) with micro-X-ray fluorescence (μ-XRF) showed the Se was present in the soil predominantly as the reduced immobile phase, elemental Se (Se0), but also as bioavailable organoselenium species, mainly selenomethionine (SeMet). SeMet was also the main species identified within both the Se non-accumulator and Se accumulator plants. The Se soil–plant system in West Limerick is dominated by SeMet, and uptake into the cattle pasture results in selenosis in the grazing dairy herds. The hyperaccumulating Brassica oleracea species could be used to extract both the Se and Mo to reduce the toxicity of the blighted fields.

Geochemical factors associated with deposition of lower Aptian organic-rich sediments during OAE1a in the Basque-Cantabrian Basin, northern Spain

Introduction: The Early Aptian Oceanic Anoxic Event 1a (OAE1a) is characterized by organic matter (OM) bearing rocks identified worldwide, which also occur in the hemipelagic succession of the Cuchía section, North Basque-Cantabrian Basin (NBCB) in Northern Spain. Previous works identified OAE1a using δ13C calibrated with biostratigraphy of planktonic foraminifera, calcareous nannoplankton, and ammonites. However, the geochemical characteristics that controlled OM sources and the redox conditions during OAE1a are yet to be determined. Therefore, here we present the results of a high-resolution complementary study along 67 m of the succession at Playa de los Caballos beach that highlights how local factors in the basin modulated the effects of increased precipitation during enhanced global hydrological cycles.Methods: Samples were analyzed for total inorganic carbon (TIC), total organic carbon (TOC), δ13C, major elements (Al, Ti, Si), redox sensitive trace elements (RSTEs), clay mineralogy, bulk mineralogy, and biomarkers.Results and Discussion: Carbon isotope segments C2 to C6 were recognized to determine the age of the outcrop. Lipid biomarkers indicated that OM is predominantly marine with significant terrestrial contribution. Terrigenous OM was carried to the basin through pulses of fluvial input as attested by the major elements and by high relative proportions of quartz, feldspar, and clay minerals. Increased terrestrial inputs also enhanced primary production and facilitated OM preservation. Periods with hightened RSTE content correspond with increases in the major elements, thus implying their common provenance. High sedimentation rates together with the presence of less labile OM and the interaction with clay minerals most likely played a major role in preserving the OM. This study further highlights how enhanced hydrological cycles significantly influenced the marine conditions that controlled the expression of OAE1a in the NBCB.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.

Multiple ocean oxygenation events during the Ediacaran Period: Mo isotope evidence from the Nanhua Basin, South China

The Ediacaran Period (ca. 635–539 Ma) was an eventful interval in Earth history, during which a succession of biological and environmental changes, including episodic ocean oxygenation events (OOEs), paved the way for the Cambrian radiations of animal life. To better understand the evolution of ocean redox conditions and to estimate the extent of seafloor oxygenation during this period, we analysed molybdenum (Mo) isotope compositions and redox sensitive element (RSE) concentrations from a continuous, mid-slope section of the Doushantuo Formation (ca. 635–560 Ma) of the Nanhua Basin on the South China Craton. Alongside an updated compilation of published Mo isotope and RSE data, our new data show that three OOEs occurred within a generally anoxic Ediacaran ocean, with the last, particularly extensive event occurring during deposition of Doushantuo Member IV. Here we show how the global balance of redox-related Mo sinks shifted dynamically in response to these transient OOEs, which correlate well with the first appearance of Ediacaran fossil groups and so may have triggered or stimulated biotic innovations and radiations. Moreover, the spatial distribution of the Mo data from multiple sections supports episodic expansion of a euxinic wedge on the slopes of the Nanhua Basin, consistent with pyrite burial as a potential cause of the OOEs.

Mantle wedge oxidation from deserpentinization modulated by sediment-derived fluids

High-pressure dehydration of serpentinite during subduction generates fluids that flux and melt the overlying mantle wedge, forming primary arc basalts. These basalts are substantially more oxidized than their mid-ocean ridge counterparts. At the slab surface of current subduction zones, these deserpentinization fluids are intrinsically oxidized, but, owing to the low sulfur content of subducted serpentinite, they only result in a low mantle wedge oxidation rate, which cannot account for the oxidized source of arc basalts. Here we show that infiltration of sediment-derived fluids modulates and can drastically change the oxidation capacity of deserpentinization slab fluids. The modulation of the deserpentinization oxidation capacity mostly depends on the stability and abundance of dissolved oxidized aqueous species of redox-sensitive elements—notably sulfate—and not solely on the oxidation state of the sediment. Infiltration of CH4-bearing fluids derived from graphite-bearing sediment reduces the intrinsically high oxidant capacity of deserpentinization fluids, explaining the relatively low fO2 observed in natural metaperidotite. Infiltration of sulfate-CO2-bearing, sediment-derived fluids—prevalent in modern subduction zones—generates deserpentinization fluids with a high oxidation capacity in cold and hot subduction zones, resulting in a global mantle wedge oxidation rate of 3.5 km3 yr−1. Such slab fluids will oxidize the mantle wedge at a rate similar to that of arc-basalt generation and thus account for the oxidized nature of arc volcanism. The source of highly oxidized arc magmas may rely on the infiltration of sediment-derived fluids that contain oxidized aqueous species—notably sulfate—into deserpentinization fluids, according to thermodynamic modelling.

Modelling the reaction of uranium with carboxylic groups on surfaces through mono- and multi- dentate surface complexes on the basis of pH and redox potential

An analytical expression is proposed to simulate effects of pH and redox potential (E) on the sorption of uranium onto bioorganic model particles in saline or other aquatic environments. The elaborated expression is intended to avoid use of the classical approach of sorption which relies on experimental data and empirical models. The goal is to produce an expression that provides a distribution coefficient (Kd e.g. mL g−1) as function of pH, E and ligand concentration (through complex formation in solution) by applying a surface complexation model on one type of mono-dentate surface sites> (SuOH) as well as utilizing multi-dentate surface sites> (SuOH)c. The formulation of the worked out expression makes use of correlations between the surface complexation and hydrolysis constants for all species and sorption sites. The model was applied to the sorption of uranium onto bioorganic sites with and without carbonates in solution e.g. Log Kd: + 2.75 at pH 8 for 2 sites per nm2. The calculated distribution coefficients were found very sensitive to the presence of carbonates, e.g. Log Kd: − 7.0 at pH 8 for 2 × 10−3 M total carbonate. The potential reduction of uranium U(VI) and its complexes (carbonates) which are the primary stable species in surface waters, to U(IV) during sorption was simulated in association with a decrease in the redox potential and was found generally below the redox stability limits of water. The calculated distribution coefficient values were validated by the values reported in literature for the sorption of uranium onto specific adsorbents. The investigated simulations are also applicable to the sorption of other redox sensitive elements.

A unique suit of Wuchiapingian marine red beds in central China representing a specific paleogeography and paleoclimate association

Marine red beds (MRBs) as a special geological phenomenon have been considered relevant to specific palaeogeographic or paleoclimatic conditions. The Wuchiapingian Yundoutan Formation in Xikou, Shaanxi, Central China is characterized by a huge thick succession of red limestones, which is a unique deposition among marine strata of China during the Wuchiapingian. The lithology, mineralogy, and geochemistry features of the Yundoutan Formation MRBs were analyzed by using techniques of field measurement, polarized optical microscope, field emission scanning electron microscope (FESEM), diffuse absorbance spectroscopy (DAS), X-ray diffraction (XRD), and geochemical analysis, which were used to discuss their origin and palaeogeographic and paleoclimatic responses. The results show that the stain mineral for the MRBs is mainly weakly-crystalline hematite which can not be observed in the FESEM but was detected from the DAS first derivative spectra with featured peaks around 570 nm. The similar trend of Fe with refractory elements and the strong positive correlation between Fe and Al suggest Fe is mainly a terrestrial source. Limestones of the Yundoutan Formation contain different colors including red, light gray, and dark gray. Red limestones have comparable contents of Fe with dark gray limestones, which are larger than that of light gray limestones, but they are depleted in redox-sensitive elements (Cr, Ni, Cu, Zn, Mo, U) and riched in Fe(III) compared with dark gray limestones. Those indicate that the formation of red limestones needs both enough terrestrial Fe and strong oxic conditions. A mechanism proposed for the formation of red limestones was that Fe was transported from the highlands, which were then deposited under a strong oxic environment and transformed into hematite. We suggest the Wuchiapingian MRBs formed in Central China represent a unique paleogeography and palaeoclimate association. During that time, the study area attached to the northern margin of the United North China-Qinling Block received terrestrial materials from its northward highlands, which provided Fe under a warm climate. Meanwhile, the study area located between the South and North China blocks was on the narrow seaway connecting the Palaeotethys and Panthalassa, which produced relatively fast exchanging rates of seawater and thus a high content of oxygen that can easily transform terrestrial-sourced Fe into hematite. The gradual transition among color-different limestones may be driven by the changes in rainfall that influenced river transportation on terrestrial materials or sea-level changes that influenced the redox state of the water-sediment interface.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

Abstract Here we report the composition of biotite from the peraluminous Devonian South Mountain Batholith (SMB) of southwestern Nova Scotia (Canada), the largest intrusive body within the Appalachian orogen. The batholith was emplaced in two phases: an early (379–375 Ma) granodiorite-monzogranite suite (Stage 1) and a later (375–372 Ma) more-evolved monzogranite-leucogranite suite (Stage 2). Biotite analyses (major and minor elements) were obtained on 55 unmineralized samples representing 11 plutons. Regardless of the stage of pluton emplacement, biotite is commonly interstitial to alkali feldspar, quartz and plagioclase, indicating similar timing of biotite saturation. This suggests that biotite chemistry records conditions at similar extents of magma evolution for the chosen suite of samples. Biotite compositions are Fe-rich, with Fe/(Fe+Mg) ranging from 0.6 to 0.98, and Al-rich, with IVAl ranging from 2.2 to 2.9 atoms per formula unit (apfu; 22 oxygen basis), the latter reflecting the coexistence of other Al-rich phases, such as muscovite, garnet, aluminosilicates, and cordierite. Biotite anion sites are dominated by OH (>3 apfu), followed by F (~0.3 apfu) and Cl (≤0.02 apfu), with a general trend of decreasing OH, increasing F and a marked decrease in Cl, with increasing differentiation. Pressure (P) is estimated from the Al content of biotite to be between 280–430 MPa, consistent with a range of 240 to <470 MPa derived from phase equilibria and fluid inclusion microthermometry combined with mineral thermobarometry. Temperature (T) calculated from the Ti content of biotite ranges from 603–722 °C. Comparison of P-T estimates with water-saturated granite phase relations suggest minimum water contents of 6–7 wt% for the SMB magmas. The redox state of the SMB was estimated by comparing biotite Fe#-Ti relations with compositions calculated using the MELTS thermodynamic model, as experiments have shown that biotite Fe# increases with decreasing fO2 at a given extent of crystallization. Results of MELTS modeling for the most primitive magmas of the SMB sample suite indicate that the observed biotite Fe#-Ti variation is consistent with crystallization at FMQ to FMQ-1, with more oxidizing conditions suggested for the most strongly differentiated samples. To constrain the origin of the biotite anion site variation, a quantitative model using biotite-melt exchange coefficients (KD) derived from existing experimental data was used to track the change in biotite OH-F-Cl abundances as a function of crystallization, with or without an extant magmatic vapor phase (MVP). The model reproduces the relative OH, F, and Cl abundances in biotite, and suggests that SMB crystallization occurred in the presence of a MVP. The relatively reduced redox state of the SMB, similar to other peraluminous granitoid occurrences worldwide, aligns with other measures of fO2 for the SMB, including the occurrence of primary ilmenite. The observed correspondence between the estimated fO2 and that imposed by graphite-gas equilibrium suggests a role for reduced carbon in the generation and evolution of the SMB. This is consistent with evidence for SMB interaction with graphite-bearing felsic granulites of the underthrust Avalon terrane, and assimilation of carbonaceous and sulfidic metasediments during pluton ascent and emplacement. Reducing conditions and development of a MVP have implications for granophile element concentration processes in the SMB magmatic system. Low fO2 during crystallization affects the mineral/melt partitioning and solubility of the redox-sensitive elements Sn, W, U, and Mo, serving to suppress early SnO2 precipitation, and cause both an increase in W/Mo and an overall buildup of all four elements in evolving SMB liquids. Available experimental data indicate that reducing conditions also shifts DMVP/melt to favor partitioning into the melt phase. Therefore, early vapor exsolution under reducing conditions also lessens the extraction efficiency of these redox-sensitive elements to the MVP, further underscoring the role of extensive crystallization as an important metal enrichment process.

Copper and Zinc isotopes trace the evolution of the Ediacara-Early Cambrian paleo-ocean redox condition in the Tarim Basin, China

The Ediacaran-early Cambrian transition is a critical period in the Earth's evolutionary history. A series of important geological and biological events are likely related to environmental change. Ocean oxygenation levels during the early Cambrian are still controversial and barely constrained although abundant geochemical studies have been conducted. The black shale widely developed in the Early Cambrian can record paleo-environmental information of contemporaneous seawater. The early Cambrian Yuertusi Formation black shale in the Luntan 3 well from the Tarim Basin was studied in terms of Cu and Zn isotopic composition and redox sensitive trace element to improve the knowledge of the extent of ocean oxygenation during the Ediacaran-Early Cambrian. The Yuertusi Formation is divided into four stages according to lithology variation and records of Cu and Zn isotopes and redox sensitive elements, indicating the fluctuation of ocean redox condition. The total organic carbon (TOC) varied widely from 0.10% to 18.25% with an average of 6.34%. Enrichment of Mo, U and V elements which have a synergistic relationship with TOC and redox proxies (ratios of U/Th and V/Cr, EF(Mo)-EF(U)) of the four stages in the Yuertusi Formation, indicate an anoxic even euxinic deposition environment, especially in stages Ⅰ and Ⅲ. Most of the Cu isotope values, lower than the modern seawater values, also indicate that the ocean state was anoxic and did not reach the current ocean oxidation level, which shows the same characteristics as the other profiles in different areas. The variation in the Cu isotope ranges from −3.10‰ to +1.23‰, and the Zn isotope ranges from 0.09‰ to 1.46‰. Relatively large Cu and Zn isotopic fractionation in stages Ⅰ and Ⅲ occur with an intense reduction environment, which is attributed to the combination of sulfide deposits that are eventually immobilized in sediments. Under the overall background of an anoxic environment, the four stages show a cycle of relative anoxic-weak oxidation-anoxic-weak oxidation, which is inferred to be connected with the transgressive-regressive cycle and weathering enhancement variation based on Ce anomaly, Mo/Toc ratios and C stable isotope data. The Early Cambrian transgression provided an anoxic even euxinic environment conducive to organic matter preservation for the development of black shale during stage Ⅰ. With the sea level fluctuation and weathering enhancement, the sea retreated gradually, productivity gradually increased, and the marine environment was relatively oxidized. The increasing trend of the whole Cu and Zn isotope values from bottom to top in the Yuertusi Formation is interpreted to be gradual relative oxidizing.

Digging Deeper: Bioturbation increases the preserved sulfur isotope fractionation

Bioturbation enhances mixing between the seafloor and overlying ocean due to changes the redox state of the sediment and influences the biogeochemical cycling of redox-sensitive elements such as sulfur. Before the widespread appearance of burrowing fauna over the Proterozoic-Phanerozoic transition, marine sediments were largely undisturbed and transport of material across the sediment-water interface was diffusion-dominated. Through both a microcosm experiment and numerical model, we show that the effect of bioturbation on marine sediments is to enhance the drawdown of sulfate from the water column into the sediment and thus “open-up” the sedimentary system. The key finding is that bioturbation increases the difference between the isotopic signature of seawater sulfate and pore water sulfide, the latter of which is preserved in sedimentary sulfide minerals. Our study empirically demonstrates a long-held assumption and helps identify the isotopic impact of bioturbation in the geological record and its environmental effects in modern marine systems.

Elevated CO2 increases soil redox potential by promoting root radial oxygen loss in paddy field

Soil redox potential (Eh) plays an important role in the biogeochemical cycling of soil nutrients. Whereas its effect soil process and nutrients’ availability under elevated atmospheric CO2 concentration and warming has seldom been investigated. Thus, in this study, a field experiment was used to elucidate the effect of elevated CO2 concentration and warming on soil Eh, redox-sensitive elements and root radial oxygen loss (ROL). We hypothesized elevated CO2 and warming could alter soil Eh by promoting or inhibiting ROL. We found that soil Eh in the rhizosphere was significantly higher than that of non-rhizosphere. Elevated CO2 enhanced soil Eh by 11.5%, which corresponded to a significant decrease in soil Fe2+ and Mn2+concentration. Under elevated CO2, the concentration of Fe2+ and Mn2+ decreased by 14.7% and 13.7%, respectively. We also found that elevated CO2 altered rice root aerenchyma structure and promoted rice root ROL. Under elevated CO2, rice root ROL increased by 79.5% and 112.2% for Yangdao 6 and Changyou 5, respectively. Warming had no effect on soil Eh and rice root ROL. While warming increased the concentration of Mn2+ and SO42− by 4.9% and 19.3%, respectively. There was a significant interaction between elevated CO2 and warming on Fe2+ and Mn2+. Under elevated CO2, warming had no effect on the concentration of Fe2+ but decreased Mn2+ concentration significantly. Our study demonstrated that elevated atmospheric CO2 in the future could increase soil Eh by promoting rice root ROL, which will alter some soil nutrients’ availability, such as Fe2+ and Mn2+.

Geochemical characteristics of the Rajmahal coals in Dhulia North Block, Eastern India: implication to their utilization and environment.

The borehole coal samples of Dhulia North Block from the Rajmahal Basin, Eastern India, were systematically analyzed based on the chemical composition and concentration of major and trace elements (including rare earth elements, REEs) to assess the distribution of REEs and their environmental implications with utilization potential. TheDhulia North Block coals are characterized by the predominant majoroxides of SiO2, Al2O3, and Fe2O3, accounting for 94% of the total ash composition, indicating the presence of quartz, clay-rich minerals, and pyrite. Compared with the average world coal ash, the total REE content in the analyzed samples ranged from 341.0 to 810.4ppm, which is substantially higher. Hot humid climate conditions with intermediate igneous source rocks of the basin were demonstrated by the major oxide ratios (Al2O3/TiO2 < 20) and plots of TiO2 with Al2O3 and Zr. The redox-sensitive elements such as V, Ni, Cr, and Co found in the Dhulia North Block coal indicate that an oxic sedimentary environment existed in the basin when coal was formed. The low sulfur content (1% in most samples) indicates freshwater conditions in the basin at the time of organic matter deposition. The outlook coefficient (Coutl) varies between0.7 and 1.6, indicating that the Dhulia North Block coals are a prospective source of REEs. The Dhulia North Block coals are characterized by low H/C and O/C atomic ratios ranging from 0.56 to 0.90 and 0.10 to 0.22, respectively, and contain type-III kerogens, indicating gas-prone source rock. Further, the basic-to-acid oxide ratio suggested that Dhulia North Block coals were suitable for utilization during combustion processes.

Geochemistry of Precambrian black shales from Ossa-Morena Zone (Portugal): depositional environment and possible source of metals

The paleodepositional environments related to Ediacaran black shales from the Série Negra succession in Ossa-Morena tectonostratigraphic zone were investigated using their geochemical composition to identify the sedimentary sources and depositional conditions. Their potential for assisting the genesis of metal deposits was also discussed. The sedimentary sources of the black shales are related to the break up of a Cadomian magmatic-arc developed in the North Gondwana realm, where the Ossa-Morena Zone was located during late Ediacaran times, showing acidic to mixed signatures. Some ratios were calculated based on redox sensitive elements, indicating both anoxic and oxic paleoenvironmental conditions. The anoxic conditions were predominant and they were identified on samples with higher organic carbon content, emphasizing that the redox conditions favored organic matter preservation and the accumulation of selected metals. Considering the mode of occurrence, the elements Ag, Cd, Cu, Pb and Se are assumed to be preferentially associated with sulfides, whereas Mo and U are preferentially organically bound. The results reveal that the black shales from the Série Negra succession in Ossa-Morena Zone might represent a source of metals, which were probably remobilized during Variscan events.

Formation and implication of cold-seep carbonates in the southern South China Sea

Southern South China Sea (SCS), rich in oil and gas resources, can provide a favorable condition for the formation and development of cold seeps. However, the geochemical effect of cold seep activities and the carbon cycle in the SCS are not well understood. Cold-seep carbonates as a window to exploration of gas hydrate are of great significance to investigate the material circulation in cold-seep system and to reconstruct the paleo-climate environment in the SCS. In this research, eight cold-seep carbonates collected from the Beikang Basin, southern SCS, are generally presented as yellow-black irregular column, crust and block, and mainly composed of high magnesium calcite (HMC), indicating they probably formed inside the deep fluid migration pathway. The middle rare earth element (MREE) enrichment with no or positive Ce anomaly in the carbonates from BH-CL37A suggests that they precipitated in anoxic pore water, while heavy rare earth element (HREE) enrichment in BH-CL32A carbonate shows great effect of seawater and they were formed in shallower depth. Different enrichments of redox sensitive trace elements and (Mo/U) EF indicate the strong anoxic environment. It is concluded that the difference of fluid environment could affect the enrichment degree and precipitation rate of trace elements by the relations between trace elements and Al. Extremely depleted 13C, slightly enriched 18O and the variation of Sr isotopes have archived the main biogenic methane source and the co-effect of seawater and seep fluid driven from a deep source (possibly containing the water from hydrate dissociation). The unique and complex geological conditions in the southern SCS will lead to the distinct regional specificity of the cold seep, which is different from the cold seep system in the northern SCS. This study plays an essential role in fulfilling the understanding of the activities of cold seeps in the SCS, revealing the element migration and transformation in cold seeps and their influencing factors.

Threshold Calibration of Redox Conditions in Lacustrine Fine-Grained Sedimentary Rocks Based on the Internal Cross-Calibration of Multiple Redox Proxies: A Case Study of the Upper Triassic Chang 8–Chang 7 Members in the Southern Ordos Basin, China

Redox conditions of lacustrine strata assessed by redox proxies with unified thresholds were conflicted in the Ordos Basin. The internal cross-calibration approach of multiple redox proxies, based on the enrichment sequence model of authigenic redox-sensitive element (RSE) governed by redox potential, has recently been proposed to calibrate redox thresholds in marine depositional systems. However, this assessment model often induces an overlapped threshold between anoxic–ferruginous and anoxic–euxinic conditions. Except for the redox potential, the enrichment of RSE is also controlled by its host phase. Thus, we introduced the enrichment degree and occurrence state model controlled by the host phase content into this approach to identify anoxic–euxinic conditions by an enrichment threshold of Mo (or U). Based on this method including double redox assessment models, we calibrated the thresholds of various redox conditions of the Chang 8–Chang 7 Members in the southern Ordos Basin. Subsequently, the effectiveness of previous bi-element proxies, the evolution of redox conditions, and their influencing factors were analyzed based on the calibrated thresholds. Results show that this method is applicable in the lacustrine strata. The cross plot of DOPT (degree of pyritization based on the total Fe and S content) (or SEF)–MoEF (or UEF) (enrichment factor) is suggested for calibrating thresholds of different redox conditions. Previous bi-element proxies are not suggested to be used again. There is a positive redox evolution sequence from oxic–suboxic (suboxidized) to anoxic–euxinic conditions in Chang 8–Chang 7 Members. Hydrothermal activity and paleoproductivity play different roles in the formation of redox conditions.

Organic matter accumulation of the Upper Triassic Ma'antang shales in the Longmenshan Foreland Basin (western Sichuan, China)

The Upper Triassic Ma'antang Formation is not only an important set of source rocks in the Sichuan Basin but may also be a successor for shale gas in China. In this study, the provenance, tectonic setting, chemical weathering, environment and productivity of the middle–late Carnian (Tuvalian) are reconstructed from petrological and geochemical data to determine the origin of the Ma'antang shales. The brachiopod-dominated fossil composition, type II-dominated organic matter, and fluctuating water salinity reflect that the shale formed in a marine environment influenced by terrestrial influx. TiO2 vs. Zr and La/Th vs. Hf diagrams, as well as the combination of major elements indicate that the Sichuan Basin was a collisional setting in the Tuvalian, and the provenance was felsic igneous source rocks. Redox-sensitive elements indicate dysoxic–anoxic conditions. Samples with Cd/Mo < 0.1 and Co*Mn > 0.4 show normal levels of Cd and relative enrichment of Mn and Co, which supports that the shale is more likely to be a product of anoxic preservation mode associated with the restricted basin. In contrast, the moderate productivity conditions reflected by Babio and P/Ti are also positive factors for organic matter accumulation. Based on the reconstruction of these environmental parameters, several driving factors that promote the formation of high TOC shales are explored. It is believed that the rapid tectonic subsidence in the early stage of foreland basin evolution (undercompensation period) created accommodation space for the shale, and the deep water also promoted anoxic bottom water, which facilitated the preservation of organic matter. The Carnian Pluvial Episode (CPE) increased global weathering and rainfall, allowing the oceans to receive more terrestrial nutrients, and freshwater influx caused stratification due to differences in salinity between the surface water and bottom water. Volcanic ash from the Wrangellia Large Igneous Province (W-LIP) and volcanic ash from local volcanoes were transported over long distances in the Late Triassic megamonsoons. Both were able to stimulate marine productivity, maintain and expand oceanic anoxia, and were beneficial for organic matter accumulation.

Trace element enrichment mechanisms in black shales during the early cambrian (ca. 521–514 Ma), South China

It is widely accepted that significant perturbations in ocean redox states are closely related to biological evolution during the early Cambrian. However, the host and geochemical cycle of redox-sensitive elements (V and Zn) of the Niutitang Formation shale in South China have not been well-constrained. This study reports lithofacies, mineralogy and geochemical data of high-resolution samples from slope and basin settings (Xa1 and Xb1 wells, Hunan Province), to evaluate the significance of redox condition and hydrothermal contribution to trace element accumulation during the early Cambrian. Our results reveal the Zn-rich host is primarily sphalerite, while the V-rich host includes organic matter, illite, and anatase minerals in the Niutitang shale. Illite and anatase minerals have significant V concentration (14.8% and 18.3%), whereas organic matter contains relatively lower V concentration (1.4%). Trace elements can be scavenged by dissolved organic matter in seawater, and then taken up by clay minerals or form sulfide during deposition/diagenesis of organic-rich shales. Although seawater restriction of the lower member (LM) shale was stronger than in the upper member (UM) shale, euxinic conditions were more conducive to V and Zn enrichment than ferruginous conditions. Specifically, dwindling concentrations of V and Zn in seawater existed in South China during deposition of the early LM shale. However, the ocean experienced episodic hydrothermal activity, which may have supplied abundant trace elements to supplement seawater content. Overall, we propose that the V and Zn geochemical cycles in the early Cambrian paleo-ocean were controlled by redox condition, organic matter, and the trace element inventory of seawater.

Manganese Oxidation States in Volcanic Soils across Annual Rainfall Gradients.

Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils, and the Mn oxidation state controls the roles of Mn in numerous environmental processes. However, the variations of Mn oxidation states with climate remain unknown. We determined the Mn oxidation states in highly weathered bulk volcanic soils (primary minerals free) across two rainfall gradients covering mean annual precipitation (MAP) of 0.25-5 m in the Hawaiian Islands. With increasing MAP, the soil redox conditions generally shifted from oxic to suboxic and to anoxic despite fluctuating at each site; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP, but accumulated substantially, up to 80% of total Mn, in soils with prevalent suboxic conditions at intermediate MAP. Mn(III) was likely hosted in Mn(III,IV) and iron(III) oxides or complexed with organic matter, and its distribution among these hosts varied with soil redox potentials and soil pH. Soil redox conditions and rainfall-driven leaching jointly controlled exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The Mn redox chemistry was at disequilibrium, with the oxidation states correlating with long-term average soil redox potentials better than with soil pH. The soil redox conditions likely fluctuated between oxic and anoxic conditions more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-sensitive elements may be actively cycled.

Trace Element Assemblages of Pseudomorphic Iron Oxyhydroxides of the Pobeda-1 Hydrothermal Field, 17°08.7′ N, Mid-Atlantic Ridge: The Development of a Halmyrolysis Model from LA-ICP-MS Data

Halmyrolysis, as one of the global processes of alteration of seafloor hydrothermal sediments, needs to be recognized in terms of mineral and trace element evolution to elaborate new criteria for metallogenic and geoecological forecasts with respect to ocean exploration. The purpose of this paper is to explain trace elements’ behavior during the halmyrolysis of sulfide deposits. This task is resolved using an LA-ICP-MS analysis of iron oxyhydroxides (IOHs) on examples of oxidized pyrrhotite-rich diffusers of the ultramafic-hosted Pobeda-1 hydrothermal field (Mid-Atlantic Ridge). The IOHs formed after the sulfides were enriched in seawater-derived trace elements (Na, K, Mg, Ca, Sr, P, U, Mo, V, REE, Cr). Six trace element assemblages (TEAs) are statistically recognized for the IOHs. TEA-I (Cu, In, Sn, Bi, Se, Te) is inherited from chalcopyrite, isocubanine and bornite microinclusions. TEA-II is typical of Zn sulfides (Zn, Cd, Sb, Tl, Ag) interacted with seawater (Mg, U, Mo, Ni, Na, K) and hydrothermal fluid (Eu). TEA-III (Ca, Sr, Cu, Si, Se, P, As) reflects the inclusions of aragonite, opal, atacamite and possibly native selenium, while P and As occur as absorbed oxyanion groups on IOHs or Ca–Fe hydroxyphosphates. TEA-IV (Al, Ga, Ge, Tl, W, Ti ± Mn, Co, Ba) indicates the presence of minor clays, Co-rich Mn oxyhydroxides and barite. TEA-V with Pb and V is closely related to TEA-VI with REEs except for Eu. The halmyrolysis of sulfides includes two stages: (i) oxidation of S(II) of primary sulfides and the formation of supergene sulfides, which scavenge the redox-sensitive elements (e.g., U, Mo, Ni, Eu), and (ii) oxidation of Fe (II) to Fe (III) and absorption of most elements of TEAs III, IV, V and VI by IOHs.

Dynamic mobilization of redox sensitive elements Mo, U and V in seasonal hypoxic sediments off the Changjiang Estuary

The coastal waters off the Changjiang Estuary are a seasonal hypoxic region, offering an ideal system for studying the geochemical cycles of redox sensitive elements (RSEs) at the sediment-water interface (SWI) and their response potential to redox changes. The profiles of porewater and solid RSEs and their chemical speciation and diffusive fluxes at five sites in two seasons were studied to evaluate the migration and removal of RSEs at the SWI off the Changjiang Estuary. Results showed that porewater and solid RSEs displayed the corresponding profile variations. The porewater Mo and U concentrations generally decreased with depth, attributed to the reductive removal. Particularly, the removal of Mo may be mediated by the Fe–S phase, and the removal of U appears to be via the microbially-mediated reduction. Porewater V, U and Mo were removed successively with depth, corresponding to the peaks of dissolved nitrate, Fe2+ and acid volatile sulfur (AVS). However, porewater V increased again in the deep layers due to its complexation with dissolved organic matter. The upward shift of RSE peaks in porewater and transformation depths of their oxidation states based on the model from spring to summer reflected the occurrence of hypoxia in summer. The downward diffusive fluxes of RSEs at hypoxic site in summer indicated their hypoxia-induced enrichment. The Mo–U covariation suggested that the seasonal hypoxic sites off the Changjiang Estuary were more favorable for the accumulation of authigenic U.