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Understanding the interaction between molybdenum (Mo) and organic matter during microbial sulfate reduction is critical for the use of Mo to reconstruct marine redox conditions throughout Earth's history. However, little is known about Mo isotope fractionation and how it relates to organic matter remineralization during microbial sulfate reduction. Here, we report Mo abundances and isotopic (δ98Mo) compositions for bulk‐rock, non‐lithogenic and sequentially extracted fractions, including carbonate (carb), pyrite, and organic matter (OM), of seep carbonate rocks. Our data indicate that the difference between δ98Mocarb and δ98MoOM (Δ98Mocarb‐OM) displays significant variability in the studied samples, ranging between 0.72 and 1.01‰. Remarkably, the obtained Δ98Mocarb‐OM values indicate correlative trends with stable carbon isotope ratios and bulk abundances of (a) total organic carbon, (b) Mo, and (c) pyrite in seep carbonates, which we interpret as reflecting sustained adsorption of isotopically light Mo onto organic matter during enhanced sulfate reduction. On this basis, we put forward the concept that Δ98Mocarb‐OM of authigenic carbonate rocks can be used as a measure of the intensity of sulfate reduction and for reconstructing past interactions between Mo and organic matter in marine sediments. Plain Language Summary: Molybdenum (Mo) is a useful element for reconstructing marine redox conditions throughout the Earth's history. The sequestration of Mo through sulfate‐reducing bacterial activity acts as a significant pathway for Mo burial in the ocean. However, the impact of microbial sulfate reduction in Mo isotope fractionation remains unclear, preventing the understanding of the interaction between Mo and organic matter. We report Mo abundances and isotopic (δ98Mo) compositions for different phases extracted from seep carbonate rock fractions, including carbonate (carb), pyrite, and organic matter (OM). Our findings indicate that organic matter tends to preferentially adsorb isotopically light Mo. The observed δ98Mo difference between carbonate and organic matter fractions (Δ98Mocarb‐OM from 0.72 to 1.01‰) represents the first report of the extent of Mo isotope fractionation during Mo adsorption to organic matter in marine environments. We attribute greater Δ98Mocarb‐OM offsets to the preferential adsorption of isotopically light Mo onto organic matter during enhanced sulfate reduction, and in turn put forward the idea that such offsets can be possibly used in the sedimentary record to trace the intensity of sulfate reduction and to reconstruct the past interaction between Mo and organic matter in marine sediments. Key Points: Organic matter tends to adsorb isotopically light Mo, resulting in low δ98Mo values of organic matter in marine environmentsIsotopically light Mo is inferred to be preferentially adsorbed onto organic matter during enhanced sulfate reductionδ98Mo offset between carbonate and organic phases in authigenic carbonates is promising to trace the past intensity of sulfate reduction [ABSTRACT FROM AUTHOR]

Authigenic molybdenum (Moauth) in marine sediments holds great potential to archive the Mo isotopic composition of seawater and biogeochemical processes. However, the factors that control authigenic Mo isotope (δ98Moauth) distribution patterns remain poorly constrained. Here, we report Mo abundances and δ98Mo compositions for bulk‐rock (bulk) and sequentially extracted fractions—including total authigenic (auth; i.e., non‐lithogenic fraction), carbonate (carb), iron and manganese oxyhydroxides, pyrite (py), and organic fractions (OM)—of authigenic carbonates recovered from various hydrocarbon seep sites in the Gulf of Mexico and the South China Sea. Extracted pyrite fractions exhibit Mo contents varying from 0.1 to 23.4 μg/g and generally dominate the Mo budget of seep carbonate rocks. Our data indicate large ranges of δ98Mobulk and δ98Moauth values relative to NIST 3134 (0.25‰), varying from 1.02 to 1.98‰ (n = 4) and from 0.15 to 3.07‰ (n = 34), respectively. The difference in δ98Mo values between carbonate and pyrite fractions of seep carbonate rocks formed under sulfidic conditions increases with higher Moauth contents, suggesting a control of dissolved hydrogen sulfide concentrations on Mo isotope fractionation during carbonate precipitation. Compared with δ98Moauth and δ98Mopy, δ98Mocarb of seep carbonate rocks formed under sulfidic conditions shows a relatively narrow range with an average of 1.98 ± 0.31‰ (1 SD; n = 10), providing constraints on the δ98Mo composition of seawater in the course of Earth history. Overall, our findings show that the δ98Mo composition of sequentially extracted phases of carbonate‐rich sedimentary rocks can provide insights into seawater‐sediment interactions and biogeochemical pathways of Mo during early diagenesis. Plain Language Summary: Sulfate reduction has created widespread euxinic conditions in the water column of ancient oceans and prevalent sulfidic conditions in subseafloor pore‐water environments of the modern ocean. Ancient seawater δ98Mo compositions have been considered to reflect the overall extent of oxic versus euxinic conditions of the global ocean, highlighting the importance of a robust reconstruction of the marine Mo isotope inventory largely unaffected by diagenesis. Under sulfidic conditions, the seawater δ98Mo value can be archived by authigenic Mo in marine carbonate sediments formed during early diagenesis. However, the conditions of capturing the seawater δ98Mo signature remain poorly constrained. Using a sequentially chemical leaching procedure, we investigated Mo contents and isotopic compositions of different fractions of modern seafloor seep carbonate rocks. Our results show that a δ98Mo average value of 1.98 ± 0.31‰ (1 SD; n = 10) of the carbonate mineral phase results from small or no Mo isotope fractionation during near‐quantitative authigenic Mo sequestration under sulfidic conditions, potentially capturing seawater δ98Mo. Our study shows that δ98Mo of sequentially extracted phases of sedimentary carbonate rocks is a robust tool to constrain seawater and pore fluid properties. Key Points: δ98Mo of sequentially extracted phases of carbonates is useful to trace Mo isotope fractionation during early diagenesisDegree of Mo isotope fractionation during carbonate precipitation is controlled by hydrogen sulfide concentrationsAuthigenic Mo sequestration into the carbonate phase under sulfidic conditions generates small Mo isotope fractionation [ABSTRACT FROM AUTHOR]

There are concerns over the increasing concentrations of trace metals being found in the environment. Deltas are essentially integrators of watershed contamination as they are the repositories of sediment transported from and through the watershed. In order to assess changes in trace metal concentrations transported by the Mississippi River–Atchafalaya River systems, vibracores were collected from three coastal freshwater marsh sites under the influence of the Atchafalaya River and emerging Wax Lake Delta (WLD). The cores extended to a depth of 4 m which included deposits of an earlier Mississippi River Delta lobe. C-14 dating showed an age at the lower depth corresponding to approximately 3500 years ago. Vertical profile distribution of metals and metalloids were measured and comparisons between older deposits and concentrations in recent deposits were made. Concentrations of As, Cr, Cu, Ni, Zn, Mo, V, Co, and Hg metals were measured in the profiles along with Fe, Mn, and Al. There was no significant increase in heavy metals or metalloids in recent years as compared to more than 3000-year-old sediment associated with an earlier Mississippi River Delta lobe. Results show that sediment diversion through the Wax Lake Outlet did not increase concentrations of these metals in surface marsh soils. The metal concentrations in the marsh profile were compared to ERL (effects range low) and ERM (effects range medium) values to allow an ecotoxicological assessment. Arsenic was below the ERM values but was greater than the ERL which indicate a potential toxicity under certain conditions. All other metal/metalloids measured were below ERL limits. This study suggests that planned Mississippi River sediment diversions designed to slow the rate of coastal land loss are not likely to lead to trace metals contamination.. [ABSTRACT FROM AUTHOR]

This study investigates the conditions of occurrence and petrographic characteristics of low-Mg calcite ( LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The 13C-depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca2+-enriched and Mg2+-depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high-Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H2S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record. [ABSTRACT FROM AUTHOR]

The Mississippi delta region of south Louisiana houses a wealth of resources within a dynamic, subsiding landscape. Foundations for the delta region reflect Neogene evolution of the depocenter, whereas details of the modern landscape reflect late Pleistocene to Holocene evolution of the alluvial-deltaic plain. The Holocene delta plain was constructed by cyclical growth of deltaic headlands, followed by avulsion and relocation of the fluvial sediment source. Abandoned headlands were then submerged and reworked while a new headland was created at the site of active fluvial sediment input. Historic-period levees have decoupled the delta plain from its fluvial sediment source at the same time global sea-level rise was accelerating, which has accelerated delta-plain submergence. Diversions of Mississippi River water and sediment are necessary to achieve delta plain sustainability, but upstream dams trap ∼50% of the total sediment load, and there is not enough supply to keep pace with subsidence and accelerated sea-level rise. The future delta region will not resemble the recent past, and significant drowning is inevitable. [ABSTRACT FROM AUTHOR]

Abstract: Although less common than the occurrence of authigenic carbonate, barite has been observed frequently at cold seeps on continental margins worldwide. It is understood that barite forms by the interaction of barium-rich and sulfate-free seeping fluids with dissolved sulfate of pore water near the seafloor, but questions remain about the geochemical processes and mode(s) of the barite formation. Here, we report geochemical characteristics of barite deposits at 11 cold seep locations from the northern Gulf of Mexico continental slope. Samples from these sites of fluid and gas expulsion provide environmental information on barite formation. Seafloor observations and samples acquired indicate that barites occur as chimneys, cones, crusts, irregular mound-like buildups up to 2-meters high, and as a material disseminated in host sediment. Most barite samples are white-to-gray and usually have a porous fabric and layered internal structure. Mineralogically, samples of barite may contain a significant amounts of carbonate minerals, such as calcite and dolomite, but aragonite is absent in all samples analyzed in this study. Negative δ13C values (as low as −46.4‰ V-PDB) of the associated carbonates strongly suggests that methane is the primary carbon source. The δ34S and δ18O values of the barites have large variations, ranging from 18‰ to 80.4‰ V-CDT, and 7.5‰ to 26.7‰ V-SMOW, respectively. On δ34S versus δ18O plots, many barite deposits show a linear trend that projects down toward the isotopic composition of seawater sulfate. The trend suggests that barite formed from seawater sulfate that has been isotopically modified to varying degrees by biological sulfate reduction. The δ34S/δ18O ratios vary between 2.4 and 4.1. The variations are interpreted to reflect local controls on the flux of barium-rich seep fluids, changes in the rate of bacterial sulfate reduction, and/or the openness of pore fluid system. The 87Sr/86Sr values of the barites indicate that within-site variation is small (<0.00026) although there is a considerable range of Sr isotopic variations across multiple geographic sites (from 0.70782 to 0.71005). The observed variations probably reflect local controls on the source(s) and diagenetic evolution of seeping fluids. Strong deviation of the Sr isotope ratios of barites from coeval seawater (87Sr/86Sr=0.70917) is interpreted as the modification of the strontium from less radiogenic sources like older marine sediments or more radiogenic terrigenous material such as basinal brine and/or meteoric water. The new results further offer a better understanding of the origin and geochemical history of barite deposits that occur in geological record on the basis of δ34S and δ18O compositions. [Copyright &y& Elsevier]