Search

Iron (Fe), sulfur (S), and molybdenum (Mo) geochemistry in marine sediments impacted by hydrothermal plumes and/or cold seeps is complex and has not been systematically documented. Here we characterize Fe, S, and Mo diagenesis in sediments between the Minami‐Ensei Knoll hydrothermal field and a cold‐seep site of the middle Okinawa Trough. Results show that distances away from the hydrothermal field and the steep trough slope may significantly affect the transport of hydrothermal Fe. The transformation of hydrothermal reactive Fe to poorly reactive or unreactive Fe‐bearing phyllosilicates decreased the relative fractions of highly reactive Fe (FeHR) in total Fe (FeHR/FeT). Despite this, the standing stocks of Fe oxides in the methane‐free sediments have not been dampened, indicating no net impacts of hydrothermal Fe inputs on the size of Fe oxides. In the methane‐free sediments, low ratios of total reduced inorganic sulfide (TRIS) to total organic carbon (TOC) (TRIS/TOC), highly 34S‐depleted pyrite, and low Mo contents suggest that organoclastic sulfate reduction is at low rates and plays a limited role in carbon cycle. In the cold‐seep sediments, however, intense sulfate reduction coupled to anaerobic methane oxidation significantly elevate TRIS/TOC ratios, Mo enrichment, and isotope compositions of Mo and pyrite‐S. This pathway is expected to be important in carbon cycle in the basin due to the wide occurrence of cold seeps. Our results highlight the important controls of the local extreme depositional/diagenetic conditions on sedimentary S and Mo records, with implications for the reconstruction of paleoredox states of the past earth's surface. Plain Language Summary: Submarine methane‐rich cold seeps and hydrothermal venting exert important influences on the geochemistry of iron, sulfur, and molybdenum. However, the influences of the two extreme environments on the three interlinked elements in seafloor have not been well documented. Here, we used solid‐phase chemistry of four sediment cores located between a hydrothermal field and a cold‐seep site in the middle Okinawa Trough to document the impacts of the two extreme environments on the geochemistry of iron, sulfur, and molybdenum. Results show that hydrothermal Fe signature in the sediments is largely controlled by the distances away from the hydrothermal field and the steep slope of the trough; the hydrothermal Fe inputs have no net impacts on sedimentary iron oxide contents due to simultaneous inactivation of reactive Fe. In the methane‐free sediments, sulfate reduction (SR) is coupled mainly to oxidation of sedimentary organic carbon. In the cold‐seep site, however, SR coupled to anaerobic methane oxidation is the main pathway of carbon cycle, leading to significant enrichments of molybdenum and isotopically heavy pyrite. The differences highlight the important control of local extreme depositional/diagenetic conditions on geochemical records of iron, sulfur, and molybdenum, with implications for the reconstruction of paleoredox states in the past. Key Points: Distances away from the vent, steep basin slope and Fe inactivation control the signature of hydrothermal Fe in the basin sedimentsLow lability of organics limits sulfate reduction (SR) in CH4‐free sediments, while CH4 largely facilitates SR in cold‐seep sedimentsLocal extreme conditions exert key controls on S‐Fe‐Mo diagenesis in the basin [ABSTRACT FROM AUTHOR]

Abstract Diagenesis of iron (Fe) and sulfur (S) in river-dominated marine systems is complex and has not been well documented. In this study, sediments from the Yellow River-dominated Bohai Sea (BHS) were used to examine the diagenesis of S and Fe and interpret it in light of anthropogenic disturbances. Organic carbon (OC) source characterization indicates that the bulk OC in the sediments is dominated by marine source (62.8 ± 9.4%). Based on low total reduced inorganic sulfide (TRIS) contents and abundant unsulfidized Fe(II), it is inferred that dissimilatory iron reduction (DIR) dominates over sulfate reduction (SR) in the sediments. DIR-dominated regimes and active physical reworking of the sediments are limiting SR and TRIS accumulations, resulting in a significant deviation of total OC (TOC) to TRIS ratio from that for typical marine sediments. Speciation of Fe and S indicates that, besides carbonate-associated Fe(II), other phases are also an important sink of unsulfidized Fe(II), and that reactive Fe is capable of quick buffering towards episodically produced dissolved sulfide on one month timescale. Pulsed sedimentation, seasonal sediment erosion, translocation, and sorting have exerted significant influences on the cycling of S and Fe, resulting in atypical patterns of Fe and S diagenesis. Nevertheless, there is no a discernable impact of eutrophication and algal blooms on geochemistry of the two elements in the sediments, which is ascribable to active physical reworking of sediments as well as the dilution of marine OC by rapid accumulation of terrestrial material. Highlights • Sedimentary organic carbonin the Bohai Sea is dominated by marine source • Dissimilatory iron reduction (DIR) exceeds iron oxide reduction by dissolved sulfide • DIR-dominated regimes and sediment resuspension limit sulfide accumulation • Sediment resuspension and pulsed input lead to atypical diagenesis of sulfur and iron • Terrestrial input and resuspension offset the impact of eutrophication on diagenesis [ABSTRACT FROM AUTHOR]

Natural processes and human activities exert important impacts on elemental cycling in coastal sediments, which has not been well documented. Sediments in the Bohai Sea and North Yellow Sea were investigated to assess the impacts of the Yellow River inputs and/or anthropogenic perturbations on diagenesis of iron and sulfur. Labile iron (0.5 M HCl-extractable iron) in the sediments is low due to iron-poor nature of source materials. Dynamic regimes and low availability of labile organic carbon (OC) result in relatively low sulfide contents in deltaic sediments. However, low but continuous supply of labile OC exported from an anthropogenically impacted bay could substantially elevate sulfide burial in sediments near the bay. Neither offshore oil exploitations nor frequent algal blooms in the seas have detectable influences on iron and sulfur diagenesis in the sediments. The sediments are capable of quickly consuming porewater sulfide by reaction with reactive iron under the current conditions. [Display omitted] • Availability of labile organic carbon is driving factor of sulfate reduction. • Labile iron is low in the sediment due to iron-poor nature of source materials. • Human perturbations have no significant impact on sulfur and iron diagenesis. • The Yellow River estuary exerts important influences on sulfur cycling. [ABSTRACT FROM AUTHOR]