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• The diatom assemblages are closely correlated with the environmental variables. • Three assemblages are distinguished with different hydrological characteristics. • Important for palaeoceanographic studies in the northern and middle Okinawa Trough. Determining the relationship between diatoms in surface sediments and the local sedimentary environment is important for paleoenvironmental reconstruction, including in marine settings. In this study, we used canonical correspondence analysis of 41 surface sediment samples from the northern and middle Okinawa Trough to analyze the relationship between diatom species and environmental variables. The principal results are as follows. (i) The diatom assemblages are closely correlated with the environmental variables, specifically, July sea surface salinity (SSS-Jul), February sea surface temperature (SST-Feb), and June sea surface temperature (SST-Jun). (ii) Three diatom assemblages are distinguished, representing environments with different hydrological characteristics. The high-temperature diatom assemblage may indicate a warm-water mass (the Kuroshio Current), and the polyhaline diatom assemblage can be used as an indicator of a high salinity water mass. The mixed diatom assemblage is associated with the warm-water masses of the Kuroshio Current, and shelf water masses with a low salinity and low temperature. (iii) The high-temperature and mixed diatom assemblages occur on the boundary of the continental shelf and Okinawa Trough, and are temperature sensitive; the high-temperature assemblage is located further upstream of the Kuroshio Current, indicating higher temperatures. Changes in the distribution of the polyhaline diatom assemblage within sediment cores can be regarded as an indicator of changes in water masses, and of possible salinity variations, in this area. These three assemblages are potentially important for future palaeoceanographic studies. [ABSTRACT FROM AUTHOR]

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]

Deciphering sedimentary provenance in the Okinawa Trough (OT) is vital for understanding the dispersal system from source to sink and the paleoenvironmental evolution in the East China Sea (ECS). Sources include the Huanghe, Changjiang, and Taiwan rivers, as well as the ECS shelf; however, there is controversy over which source has dominated sediment supply since the last deglaciation, and how the tidal currents, shelf circulation, and Kuroshio Current (KC) has controlled the sediment transport. Here, we present a high-resolution record of grain size and clay minerals over the last 14.3 kyrs in an AMS 14C dated piston core from the middle OT. We use these data to reconstruct changes in the KC, identify the provenance of fine-grained sediment, and unravel the impact of paleoenvironmental factors on these provenance changes. A grain size component of 15.6–31.3 μm was extracted to reconstruct the KC changes. These data show that the KC strengthened sharply until 10.2 ka, then kept strong and stable during the Holocene. Clay mineral assemblages indicate that the sediment was mainly derived from the ECS shelf between 14.3 and 8.6 ka, then from the Taiwan rivers after 8.6 ka. The shelf sediment was eroded and transported by intense tidal currents. Taiwan river sediment became the dominant source due to the development of Taiwan Warm Current rather than the strengthening of KC. The ratio of smectite and kaolinite to illite and chlorite content indicates that the sediment contribution from the Huanghe (old Huanghe delta) and Changjiang rivers increased from 7.6 to 5.0 ka and from 3.2 ka to the present. This increase is attributed to the strengthening of East Asian winter monsoon and the consequent intensification of cross-shelf circulation. • Sediment source changed from East China Sea shelf to Taiwan rivers at 8.6 ka. • Tidal reworking controls shelf sediment supply during the last deglaciation. • Taiwan Warm Current induces early Holocene provenance change rather than Kuroshio. • Winter monsoon enhances Huanghe and Changjiang supply at 7.6–5.0 ka and 3.2–0 ka. [ABSTRACT FROM AUTHOR]

Geochemistry of iron and trace metals in seep carbonates and its dependence on anaerobic oxidation of methane (AOM)-driven diagenesis in cold seeps are not well documented. Here we characterize geochemistry of Fe and trace metals in carbonate nodules collected at an active cold seep in the middle Okinawa Trough (OT) impacted by hydrothermal plumes. The carbonate nodules in the studied core are dominated by aragonite even at sulfate-low core bottom. Most aragonite at the core bottom may have formed much earlier close to the seafloor under high sulfate concentration. Additionally, porewater at the depth may not be developed to a condition for the formation of calcite dominating over aragonite due to constant downward replenishment of sulfate across through shallow SMTZ (40 cm below sediment surface). As indicated by δ13C and δ18O of the carbonates, anaerobic oxidation of thermogenic methane from gas hydrate dissociation is the dominant fluid source for carbonate precipitation. It can be inferred from the existence of vivianite that Fe reduction coupled to AOM (Fe-AOM) may have ever been prevalent at the hydrothermal Fe-impacted seep site. Enhanced inputs of Mo, Co, Cu, Ni, and Zn associated with hydrothermal Fe-oxyhydroxides-organic colloids is the main source for their high enrichment in the carbonate nodules, and clay minerals and AOM-related microbial organics may have played an important role in their partitioning in the nodules. Uranium enrichment in the carbonate nodules is mainly through enhanced U delivery associated with Fe-oxyhydroxides-scavenged organic colloids and additional U enrichment induced by U(VI) reduction. U(IV) incorporation into carbonate lattices dominates U partitioning in the carbonate nodules over the upper 60 cm of the seep sediments, but the incorporation is substantially dampened at depth due to an increase in concentration of dissolved inorganic carbon, which consequently may have rendered U(IV) adsorption on clay minerals as the important U sink in the nodules. Our results suggest that the exceptional enrichments of Co, Cu, Ni, Zn, Mo, and U in seep carbonates may be used as a paleo-proxy to differentiate cold-seep carbonates whose formation has been co-impacted by hydrothermal plumes. • Aragonite is the dominant carbonate mineral even at sulfate-poor core bottom. • Carbonate formation is related to AOM and hydrate dissociation. • Hydrothermal Fe input enhances trace metal enrichment in carbonate nodules. • Clays and AOM-microbial organics play an important role in trace metal partitioning. [ABSTRACT FROM AUTHOR]

• Cold seeps were linked to fault scarps and dome-structures in the Okinawa Trough. • Numerical models were used to quantify CH 4 fluxes and biogeochemical processes. • Cold seeps are C sources to the seawater, and Seep-wide budgets were constrained. The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH 4 , dissolved inorganic carbon (DIC), Cl−, Br−, SO 4 2−, Na+, Mg2+, Ca2+, Sr2+, and NH 4 +, and the δ13C DIC , δ13C CH4 , and δD CH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13C CH4 ~ –70‰ V-PDB) and thermogenic (δ13C CH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2 yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5 mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2 yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH 4 and DIC fluxes are ~30 and 20 mmol m−2 yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems. [ABSTRACT FROM AUTHOR]

The diagenetic interplays of organic carbon (OC), sulfur (S), and iron (Fe) in marine sediments and their responses to changes in depositional and climatic conditions are poorly characterized. In this study, chemical speciation and isotopes were combined to characterize diagenesis of OC, S and Fe in sediments of the middle Okinawa Trough (OT), a back-arc deep basin along the edge of the East China Sea outer shelf, since the Last Glacial Maximum (LGM). Two key geochemical proxies, i.e., OC/pyrite-S ratios and the degrees of pyritization, indicate anoxic bottom-water conditions during the last Glacial/Deglacial, which promoted burial of OC and pyrite. However, a shift to oxic conditions during the Deglacial-Holocene transition facilitated aerobic respiration, resulting in lower burial of OC and pyrite. In contrast to previous studies, the environmental changes induced by large sea-level rises since the LGM have not triggered significant variations in pyrite-S isotopic compositions, probably due to persistently low sedimentation rates. Variable extents of Fe enrichment are identified in the sediments since the LGM, with lower enrichment during the Holocene probably due to increased distance of Fe shuttling and concomitant Fe ageing. Anoxic bottom water during the Glacial/Deglacial did not induce an expected intensification of OC sulfurization, probably because sedimentary Fe enrichment favored pyrite formation but competitively dampened OC sulfurization. Lines of geochemical evidence hint at a discontinuous deposition at ~200 cm depth probably caused by mass wasting. • Bottom water redox regimes exert major controls on diagenesis in the Okinawa Trough. • Anoxic bottom water is critical for high burial of organics during the last Glacier. • Iron shuttled from the East China Sea shelf is enriched in the Okinawa Trough. [ABSTRACT FROM AUTHOR]

Cold seep carbonate represents a faithful record of the ancient methane seepage and provides a nonnegligible contribution to the global carbon reservoir. On the western slope of the Northern Okinawa Trough (NOT), a recent seafloor visualized survey has discovered widespread crust of cold seep carbonate. Here we study mineralogy and geochemistry of these authigenic carbonate to investigate source origin and reconstruct its growth history. Mineralogically, the carbonate crusts are mainly composed of micritic aragonite, with botryoidal aragonite, framboidal pyrite, and microcrystalline authigenic gypsum. Petrographic characteristic unambiguously indicates that this carbonate precipitates in relatively open systems due to a considerable rate of sulfate-dependent anaerobic oxidation of methane (AOM). Regarding geochemistry, strongly 13C-depleted carbon isotope values (as low as −56.1‰, V-PDB) demonstrate that the carbon in the carbonate crusts is mainly derived from biogenic methane coupled with AOM. In contrast, the δ18O enrichment (up to +2.7‰, V-PDB) suggests that the fluid flow from which carbonate precipitated is sourced from dissociation of underlying natural gas hydrates. The U–Th ages of authigenic carbonates fall in the timescale of 22.8–55.7 ka BP, consistent with the period of sea-level lowstand in the late Pleistocene. Overall, several lines of evidence of this study indicate that extensive methane was released by gas hydrate decomposition during sea level fall, consequently resulting in the precipitations of carbonate crust in the NOT. Furthermore, the obviously episodic methane seepages led to the constant accretion from the interior to the exterior within the preformed crust, ultimately inducing the carbonate blocks, slabs and crusts to be exposed on the seafloor. The existence of large-scale carbonate crusts represents a good trapper of the later released carbon especially the isotopically light methane from the deep. • Aragonite is the dominated mineral phase in carbonate crust of Okinawa Trough. • Carbonate is sourced from gas hydrate and carbon mainly from biogenic methane. • U–Th age of carbonate crust is within the periods of sea-level lowstand. [ABSTRACT FROM AUTHOR]