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Nitrogen storage capacity and partitioning between main N hosts have been investigated in N-bearing Al-rich natural pelite at 3.0–7.8 GPa, 825–1070 °C and oxygen fugacity (f O 2) from NNO (Ni-NiO buffer)+0.3 to NNO-4.1 log units. Under the conditions of hot subduction, pelite converts into a phase assemblage typical of ultrahigh-pressure (UHP) eclogites. Phengitic muscovite in this assemblage is in equilibrium with a volatile-rich granite-like melt at 3.0 GPa and with supercritical fluid at 5.5–7.8 GPa, and contains, respectively, 115–135 and 759–962 wt ppm N H 4 + at f O 2 values close to NNO. Both melt and supercritical fluid have H 2 O and CO 2 as predominant volatiles and the NH 3 /(NH 3 + N 2) ratio from 0.04 to 0.12. The pattern of nitrogen partitioning between its main hosts in pelite (D NH 4 Ms - M e l t = 0.41–0.58 and D NH 4 Ms - F l u i d = 0.63–2.4) proves that the ammonium exhibits moderately incompatible to compatible behavior within a hot oxidized slab in the pressure range from 3.0 GPa to 7.8 GPa. Therefore, even relatively oxidized sediments containing phengitic muscovite can efficiently transport nitrogen to the mantle at sub-arc depths under the hot subduction conditions. At the same time, the changeover of the N host from biotite to muscovite at the arc depths in combination with subsequent pelite melting and an abrupt decrease in phengitic muscovite abundance in pelite may lead to an avalanche-like N outgassing of the slab. The incorporation of an additional nitrogen source of (NH 3 + N 2) into pelite reduces f O 2 to NNO-3.1 – NNO-4.1 log units and increases both the NH 3 /(NH 3 + N 2) ratio in the fluid (up to 0.44–0.86) and the N H 4 + concentration in phengitic muscovite (up to 3750–3820 wt ppm). K-cymrite was produced in pelite at P ≥ 6.3 GPa and the bulk nitrogen content 3.2–5.9 wt%. K-cymrite possesses exceptional nitrogen storage capacity: it hosts 1.4–1.6 wt% N H 4 + , up to 0.5 wt% NH 3 , and 4–6 wt% N 2. The D N - NH 4 Cym - M s coefficient is as high as 20. Being stable in sediments subducted to mantle depths, K-cymrite, with its extremely high storage capacity, can act as a huge hidden redox-insensitive nitrogen reservoir in the mantle and thus can be involved in the deep nitrogen cycle. [ABSTRACT FROM AUTHOR]

The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13C org values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis. [ABSTRACT FROM AUTHOR]

Throughout the Proterozoic Era, sedimentary organic carbon burial helped set the pace of global oxygenation and acted as a major modulator of atmospheric CO 2 and climate. Although Proterozoic rocks generally contain low concentrations of organic matter (OM), there are key exceptions to this rule, including the relatively OM-rich Arctic Bay shales from Baffin Island, Canada (Bylot Supergroup, Borden Basin, ∼1.05 Ga). The mechanisms driving elevated OM concentrations in these and other Proterozoic shales remain poorly understood. In the Mesozoic and Cenozoic, organic matter sulfurization can be a major driver of enhanced OM burial across a range of redox conditions comparable to those inferred for many Proterozoic environments. Therefore, in this study, we evaluate the role of sulfurization in driving OM preservation in the Mesoproterozoic Borden Basin and discuss its relevance to Proterozoic systems in general. We present the first evidence for syngenetic-to-early-diagenetic OM sulfurization in a Proterozoic basin, which begins to fill a several-billion-year gap in our record of organic S across Earth history. We find that OM sulfurization was particularly extensive in shales from a relatively shallow-water section (Alpha River) but less extensive in shales deposited in deeper water (Shale Valley), which is consistent with models that infer sulfidic 'wedges' or O 2 -minimum-zone-type structures on shelf margins at least intermittently at this time. At the shallower site, organic S and pyrite are similarly 34S-depleted and thus likely formed at roughly the same time near the sediment–water interface under conditions previously interpreted to have been ferruginous to intermittently sulfidic. In contrast, at the deeper-water site, large S-isotope differences between pyrite and organic S along with low apparent OM sulfurization intensities indicate that pyrite formation was favored over OM sulfurization during early sedimentation under variable but primarily ferruginous conditions. Although Mesoproterozoic biomass can be substantially sulfurized, indicators of sulfurization intensity are not correlated with OM concentrations, and therefore sulfurization does not appear to have been the primary driver of enhanced OM concentrations in Arctic Bay Formation shales. The link between sulfurization and total OM preservation may have been modulated during the deposition of Arctic Bay Formation shales by differences in iron availability, nutrient cycling, and particle dynamics in the Mesoproterozoic. [ABSTRACT FROM AUTHOR]

Chromium (Cr) isotopes represent a powerful tool for tracing the redox conditions during planetary magmatic evolution. However, so far, the systematic investigation of Cr isotope variation has been only performed on ocean island basalts (OIBs) in terrestrial magmatic rocks. Therefore, the Cr stable isotope compositions (expressed as δ53Cr relative to NIST SRM 979) of other magmatic rocks, formed under different oxygen fugacity, have remained unconstrained. In this paper, we present the first Cr stable isotopic data of mid-ocean ridge basalts (MORBs) from the Eastern Pacific Ocean ridge, the Indian ridge, and the Atlantic Ocean ridge. The oxygen fugacity of such basalts is different from that of the above-mentioned OIBs. The Rhyolite-MELTS model shows that the chemical composition variations in the studied basalts are induced by varying extents of fractional crystallization of olivine, clinopyroxene, plagioclase, and spinel. The δ53Cr values of the MORBs range from −0.27 ± 0.03‰ to −0.07 ± 0.02‰ (n = 28), and two distinct groups of basalts are identified based on the correlation between δ53Cr and MgO. On the one hand, the δ53Cr of group I basalts (−0.27 ± 0.03‰ to −0.14 ± 0.03‰; n = 24) are systematically lower than the established average value of the Bulk Silicate Earth (BSE) (δ53Cr = −0.12 ± 0.04‰; 2SD), which are positively correlated with their MgO and Cr concentrations, indicating that Cr isotopes are fractionated during magmatic differentiation. Moreover, Rayleigh fractionation modelling suggests that the crystallization of olivine, clinopyroxene, and spinel gives rise to the Cr depletion and thereby decreases δ53Cr values. On the other hand, group II basalts (−0.10 ± 0.03‰ to −0.07 ± 0.02‰; n = 4) exhibit higher δ53Cr values than group I with identical MgO and Cr concentrations. This is possibly associated with the crystallization of clinopyroxene under low pressure. The average Cr isotope composition (δ53Cr = −0.16 ± 0.02‰, n = 3) of the primitive basalts (MgO > 9%) represents that of the primary MORB melt, which is lighter than the average value of BSE. Using the non-modal melting equations, the δ53Cr of the MORB mantle source is estimated to be −0.12 ± 0.02‰ (2σ), which is consistent with that of BSE. Compared with the Cr isotopic data of OIBs from Fangataufa island, we find that the equilibrium fractionation factors (Δ53Cr crystal-melt = +0.04‰ to +0.13‰) of MORBs during fractional crystallization are larger than that of Fangataufa island lavas (Δ53Cr crystal-melt = 0.010 ± 0.005‰ for low-K suite, and 0.020 ± 0.010‰ for high-K suite; Bonnand et al., 2020a), indicating that the basalts from Fangataufa island have higher oxygen fugacity than those of MORBs analyzed in this study, which is strongly supported by their higher V/Sc ratios. [ABSTRACT FROM AUTHOR]

[Display omitted] Molybdenum (Mo) isotopes can be fractionated during redox-related hydrothermal and Earth's surface processes. Distinct Mo isotope features of different reservoirs, thus, make Mo isotopes as a potential tracer for the recycling of crustal materials, particularly subduction sediments. Here, we report the Mo isotope compositions of 61 early Cretaceous high-K calc-alkaline rocks (defined as adakitic rocks) with distinct source affinities from central-eastern China, including low-Mg adakitic rocks from the Dabie orogen, ore-barren high-Mg adakitic rocks from the South Tan-Lu Fault and ore-bearing high-Mg adakitic rocks from the Lower Yangtze River Belt. Low-Mg adakitic rocks have the low δ98Mo NIST3134 of −0.48‰ to −0.03‰, and both ore-barren and ore-bearing high-Mg adakitic rocks display heavier isotope compositions and larger variations: for example, −0.38‰ to 0.41‰ and −0.58‰ to 1.39‰, respectively. Magma evolutions (e.g., hornblende crystallizations) appear to dominate the Mo isotope variation in low-Mg adakitic rocks but do not account for those in high-Mg adakitic rocks. Some ore-bearing high-Mg adakitic rocks characterized by high S contents and hydrothermal alteration minerals, display positive correlations between S and Mo, Cu contents, δ98Mo values, reflecting the effects of isotopically heavy hydrothermal fluid. Having eliminated the effects of magmatic evolution and hydrothermal alteration, both ore-barren and ore-bearing high-Mg adakitic rocks display multiple-end-member mixing trends according to the correlations of Ce/Mo, Th/Yb, and Ba/Th ratios to δ98Mo. First, a common end-member with extremely low Ce/Mo, low Th/Yb, high Ba/Th and mantle-like δ98Mo values was identified, representing an enriched mantle origin metasomatized by fluids from subduction oxidized sediments. The other end-member of ore-barren high-Mg adakitic rocks displays high Ce/Mo, Ba/Th ratios and low δ98Mo values similar to low-Mg adakitic rocks, thus, it represents the lower continental crust derived melt component. By contrast, the other end-member of ore-bearing high-Mg adakitic rocks displays extremely high δ98Mo (>1.5‰), implying that initial adakitic melts involve reduced sediments in addition to the oceanic mafic crust. Notably, the common enriched mantle end-member for both ore-barren and ore-bearing adakites is characterized by high Mo and Cu concentrations, implying that the incorporation of oxidized sediments into the mantle likely plays a key role in the initial enrichment of the chalcophile metals in the mantle. This work demonstrates that Mo isotopes can be useful for understanding adakitic source affinities and associated Cu-Au-Mo ore-forming processes. [ABSTRACT FROM AUTHOR]

Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr2+, Cr3+ and Cr6+) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 < log f O 2 < −4), this study presents the first experimental petro-isotopic investigation of Cr isotope fractionation and reveals clear systematics between Cr concentration, isotopic compositions and f O 2. Two series of experiments were performed to study (a) the dissolution of a natural magnesiochromite into Cr-free silicate melts (series A) and (ii) the crystallisation of magnesiochromite from Cr-doped silicate melts (series B). In agreement with previous studies, the Cr solubility in the silicate melts at equilibrium with magnesiochromite is strongly controlled by oxygen fugacity. Melts produced at low f O 2 are enriched in Cr compared to more oxidised melts. In series A experiments, the Cr isotopic composition of silicate melts are lighter than the initial chromite starting material. The experiments, performed at −12 < log f O 2 < −6, reveal that Cr isotopic compositions of the silicate melts are correlated with f O 2. This demonstrates that, as for the Cr solubility, Cr isotopes are sensitive to f O 2 and could be used to track changes in redox conditions in high-temperature processes. Furthermore, the Cr isotopic compositions of silicate melts that are reacted under more oxidising conditions (log f O 2 > −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions. [ABSTRACT FROM AUTHOR]

The protracted oxygenation of the ocean-atmosphere system is one of the most fundamental changes to the Earth system through its history. The uranium isotopic composition (238U/235U, denoted as δ238U) of marine carbonates has been developed as a proxy to quantitatively track the timing, duration, and extent of global marine redox chemistry changes. This proxy has been applied to many critical evolutionary intervals in the last decade, significantly advancing our understanding of how life on Earth and its environment have co-evolved through geological history. Successful application of the uranium isotope paleoredox proxy requires a thorough understanding of the marine uranium budget, the processes by which seawater U-isotope signatures are recorded in marine carbonates, and the potential for alteration of these primary signatures by syn- and post-depositional diagenetic processes. Here, we provide a critical review of the U isotope proxy in marine carbonates with a focus on the current problems and areas where future work is needed to further develop this proxy. We also use a recently developed global C-P-U cycle model to illustrate that when the carbon cycle is perturbed by volcanic carbon injections, the ensuing transient relationship between seafloor anoxic area and δ238U can be complex and sometimes counter-intuitive. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Era (1.6–1.0 Ga), long regarded as an interval of sluggish biotic evolution and persistently low atmospheric-oceanic oxygen levels, has become the subject of recent controversy regarding putative large-scale oxygenation events. In this study, we conducted a comprehensive investigation of redox, productivity, seawater sulfate concentrations, and hydrographic conditions for the ∼1.4–1.32-Ga Xiamaling Formation in the shallow Hougou and mid-depth Huangtugui sections in the Yanshan Basin (North China). Integrated Fe-trace metal data suggest that bottom-water redox conditions mainly underwent a three-stage secular evolution: (1) suboxic (-oxic), (2) predominantly anoxic, and (3) oxic. Integrated productivity (TOC, P/Al, and Cu-Zn-Ni enrichment factors) and depositional system (major and trace elemental enrichment patterns) data suggest varying productivity levels during deposition of the Xiamaling Formation, with high productivity fueled mainly by intense open-marine upwelling. In combination with published data from the deep Xiahuayuan and Chicheng sections, a refined stratigraphic correlation framework was developed for the Xiamaling Formation based on the basinal upwelling event and existing lithostratigraphic framework. Within this new framework, sulfur concentration and pyrite δ34S (δ34S py) data suggest a rise in marine sulfate concentrations at ∼1.36 Ga. Furthermore, strong spatiotemporal redox heterogeneity was observed within the ∼1.4–1.32-Ga Yanshan Basin, i.e., strong bottom-water anoxia (or euxinia) in deep sections and less anoxic (even oxic) conditions in shallow sections, although all sections experienced high productivity owing to oceanic upwelling. We propose a productivity-driven shelf oxygenation model to explain this spatiotemporal redox heterogeneity within the context of eustatic changes, an upwelling event, and the rise of oceanic sulfate concentrations at ∼1.36 Ga. In this model, from initially low atmospheric oxygen levels, increasing surface productivity in shelf areas due to elevated nutrient supplies from upwelling not only intensified bottom-water anoxia through organic matter export but also stimulated expansion of the ocean-surface oxic layer through photosynthetic O 2 release, contributing to the development of contrasting bottom-water redox conditions between shallow and deep sections during the upwelling event identified in this study. Our model helps reconcile conflicting observations concerning Mesoproterozoic marine redox conditions, and provides new insights into putative Mesoproterozoic oceanic oxygenation events and their relationship to the evolution of early eukaryotes. [ABSTRACT FROM AUTHOR]

Exfiltration of anoxic Fe-rich groundwater into surface water and the concomitant oxidative precipitation of Fe are important processes controlling the transport of phosphate (PO 4 ) from agricultural areas to aquatic systems. Here, we explored the relationship between solution composition, reaction kinetics, and the characteristics of the produced Fe hydroxyphosphate precipitates in a series of aeration experiments with anoxic synthetic water and natural groundwater. A pH stat device was used to maintain constant pH and to record the H + production during Fe(II) oxidation in the aeration experiments in which the initial aqueous P/Fe ratios ((P/Fe) ini ), oxygen concentration and pH were varied. In general, Fe(II) oxidation proceeded slower in the presence of PO 4 but the decrease of the PO 4 concentration during Fe(II) oxidation due to the formation of Fe hydroxyphosphates caused additional deceleration of the reaction rate. The progress of the reaction could be described using a pseudo-second-order rate law with first-order dependencies on PO 4 and Fe(II) concentrations. After PO 4 depletion, the Fe(II) oxidation rates increased again and the kinetics followed a pseudo-first-order rate law. The first-order rate constants after PO 4 depletion, however, were lower compared to the Fe(II) oxidation in a PO 4 -free solution. Hence, the initially formed Fe hydroxyphosphates also affect the kinetics of continuing Fe(II) oxidation after PO 4 depletion. Presence of aqueous PO 4 during oxidation of Fe(II) led to the formation of Fe hydroxyphosphates. The P/Fe ratios of the precipitates ((P/Fe) ppt ) and the recorded ratio of H + production over decrease in dissolved Fe(II) did not change detectably throughout the reaction despite a changing P/Fe ratio in the solution. When (P/Fe) ini was 0.9, precipitates with a (P/Fe) ppt ratio of about 0.6 were formed. In experiments with (P/Fe) ini ratios below 0.6, the (P/Fe) ppt decreased with decreasing (P/Fe) ini and pH value. Aeration experiments with natural groundwater showed no principal differences in Fe(II) oxidation kinetics and in PO 4 immobilisation dynamics compared with synthetic solutions with corresponding P/Fe ratio, pH and oxygen pressure. However, aeration of groundwater with relative high DOC concentrations and a low salinity lead to P-rich Fe colloids that were colloidally stable. The formation of a Fe hydroxyphosphate phase with a molar P/Fe ratio of 0.6 can be used for predictive modelling of PO 4 immobilisation upon aeration of pH-neutral natural groundwater with an (P/Fe) ini ratio up to 1.5. These findings provide a solid basis for further studies on transport and bioavailability of phosphorus in streams, ditches and channels that receive anoxic Fe-rich groundwater. [ABSTRACT FROM AUTHOR]