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Bloom-induced macroalgal enrichment on the seafloor can substantially facilitate dissolved sulfide (DS) production through sulfate reduction. The reaction of DS with sedimentary reactive iron (Fe) is the main mechanism of DS consumption, which however usually could not effectively prevent DS accumulation caused by pulsed macroalgal enrichment. Here we used incubations to investigate the performance of Fe-rich red soil for buffering of DS produced from macroalgae (Ulva prolifera)-enriched sediment. Based on our results, a combination of red soil additions (6.8 kg/m 2 ) before and immediately after pulsed macroalgal deposition (455 g/m 2 ) can effectively cap DS within the red soil layer. The effective DS buffering is mainly due to ample Fe-oxide surface sites available for reaction with DS. Only a small loss (4 %) of buffering capacity after 18-d incubation suggests that the red soil is capable of prolonged DS buffering in macroalgae-enriched sediments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Solid-phase speciation and porewater chemistry measured by the diffusive gradients in thin films (DGT) technique were used to understand the diagenesis of sulfur (S), iron (Fe), and phosphorus (P) in sediments of Jiaozhou Bay (China), which has been impacted by multiple anthropogenic perturbations. Despite water eutrophication, sediments of the bay are low in organic carbon and sulfide, but high in unsulfidized Fe(II). Dissimilatory iron reduction (DIR) prevails in sediments of the bay, and there is no evidence for responses of S and Fe diagenesis to the water eutrophication, which is largely attributable to unique depositional and diagenetic regimes in association with multiple anthropogenic perturbations. Good coupling of porewater Fe 2+ and P in the porewaters suggests that P mobilization is driven mainly by DIR. Low Fe 2+ /P ratios in porewaters imply that oxidative regeneration of Fe oxides within the upper sediments is incapable of efficiently scavenging upward diffusing P., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Dissolved sulfide, iron (Fe), and phosphorus (P) concentrations in sediments of the East China Sea were simultaneously measured in situ by diffusive gradients in thin films (DGT) technique. The results, by combination with solid-phase Fe speciation, were used to characterize the interplays of Fe, S and P. Diverse distributions of dissolved sulfide among the sites are attributable to highly dynamic diagenetic regimes and varying availability of labile organic carbon (OC). The DGT technique provided high-resolution evidence for coexistence of microbial iron reduction (MIR) and sulfate reduction in localized zones, and for Fe-coupled P mobilization. Measured Fe 2+ /P ratios suggest that Fe 2+ reoxidatiion at the oxic zones can serve as an efficient scavenger of P. Empirical estimation indicates that MIR plays an important role in anaerobic OC mineralization in the sediments, which is a combined result of low availability of labile OC, high reactive Fe content, and unsteady diagenetic regimes., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Reactive iron (Fe) oxides in marine sediments play a critical role in removal of free sulfide. In this study, 0.5 and 6 N HCl-extractable Fe, acid volatile sulfide (AVS), and pyrite were examined in sediments at three sites of eutrophic Jiaozhou Bay to investigate the interactions of sulfur and Fe and possible influences of eutrophication on free sulfide removal. The results indicate that formation and accumulation of AVS and pyrite are limited by low availability of labile organic matter, despite eutrophication of the bay water. Quick buffering of free sulfide proceeded mainly via consumption of 0.5 N HCl-extractable Fe (labile Fe), however, the consumption did not result in a depletion of the Fe pool. High residual buffering capacity enables a quick removal of free sulfide in porewater, and thereby it is difficult for sulfide to accumulate and to cause detrimental effects on benthic organisms at the present steady state. Significant effects of eutrophication on Fe and sulfur geochemistry is restricted only to the estuarine sediments which were subject to direct wastewater discharges, whereas no such effects were observed in other sediments of the bay., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Recent advances in high-field MRI have dramatically improved the visualization of human brain anatomy in vivo. Most notably, in cortical gray matter, strong contrast variations have been observed that appear to reflect the local laminar architecture. This contrast has been attributed to subtle variations in the magnetic properties of brain tissue, possibly reflecting varying iron and myelin content. To establish the origin of this contrast, MRI data from postmortem brain samples were compared with electron microscopy and histological staining for iron and myelin. The results show that iron is distributed over laminae in a pattern that is suggestive of each region's myeloarchitecture and forms the dominant source of the observed MRI contrast.

Nano-scale zero-valent iron particles (NZVI) are increasingly being used to treat sites contaminated with chlorinated solvents. This study investigated the effect of NZVI on dechlorinating microorganisms that participate in the anaerobic bioremediation of such sites. NZVI can have a biostimulatory effect associated with water-derived cathodic H(2) production during its anaerobic corrosion (730+/-30 micromol H(2) was produced in 166 h in abiotic controls with 1 g/L NZVI) or an inhibitory effect upon contact with cell surfaces (assessed by transmission electron microscopy). Methanogens, which are known to compete for H(2) with dechlorinators, were significantly biostimulated by NZVI and methane production increased relative to NZVI-free controls from 58+/-5 to 275+/-2 micromol. In contrast, bacteria dechlorinating TCE were inhibited by NZVI, and the first-order degradation rate coefficient decreased from 0.115+/-0.005 h(-1) (R(2)=0.99) for controls to 0.053+/-0.003 h(-1) (R(2)=0.98) for treatments with 1 g/L NZVI. Ethene production from TCE was initially inhibited by NZVI, but after 331 h increased to levels observed for an NZVI-free system (7.6+/-0.3 micromol ethene produced in 502 h compared to 11.6+/-0.5 mmol in the NZVI-free system and 3.8+/-0.3 micromol ethene for NZVI alone). Apparently, cathodic H(2) was utilized as electron donor by dechlorinating bacteria, which recovered following the partial oxidation and presumably passivation of the NZVI. Overall, these results suggest that reductive treatment of chlorinated solvent sites with NZVI might be enhanced by the concurrent or subsequent participation of bacteria that exploit cathodic depolarization and reductive dechlorination as metabolic niches.

A trichloroethylene (TCE) dechlorinating enrichment (Dehalococcoides spp.), which was isolated from soil of chlorinated ethene contaminated site, was used to investigate whether nano-scale zero valent iron (NZVI) could serve as electron donor for this consortium via cathodic H2 production during anaerobic corrosion. The results show that in the presence of methanol serving as electron donor, dechlorinating culture of 25 fold dilution [(2.0 +/- 0.44) x 10(5) cell/mL] degraded 20 mg/L TCE completely in 96 h, which was accompanied by the production of 2.706 micromol ethene in 190 h. Methanol-free control caused partial degradation of TCE to primarily cis-DCE in 96 h, with only 0.159 micromol ethene produced in 190 h. This indicates bacteria cannot reduce TCE to ethene without electron donor. But when 4 g/L NZVI was added as sole electron donor, this dechlorinating culture degraded 20 mg/L TCE into ethene and vinyl chloride (VC) in 131 h at a speed higher than that by NZVI alone. Compared to 2.706 micromol ethene produced by Dehalococcoides spp. with methanol added as the electron donor, there was only 1.187 micromol ethene produced by bacteria with NZVI serving as the electron donor, which means NZVI has a potential toxicity on Dehalococcoides spp.. At the meantime, 0.109 micromol acetylene was produced in 190 h, which was relatively lower than 0.161 micromol produced by NZVI alone, indicating bacteria competed with NZVI under electron deficient condition. In conclusion, NZVI could serve as electron donor and support dechlorination activity for Dehalococcoides spp. which could enhance the application of NZVI and usage of dechlorinating culture as a polishing strategy in future ground water remediation.

Magnetic susceptibility provides an important contrast mechanism for MRI. Increasingly, susceptibility-based contrast is being exploited to investigate brain tissue microstructure and to detect abnormal levels of brain iron as these have been implicated in a variety of neuro-degenerative diseases. However, it remains unclear to what extent magnetic susceptibility-related contrast at high field relates to actual brain iron concentrations. In this study, we performed susceptibility weighted imaging as a function of field strength on healthy brains in vivo and post-mortem brain tissues at 1.5 T, 3 T and 7 T. Iron histology was performed on the tissue samples for comparison. The calculated susceptibility-related parameters R(2)(*) and signal frequency shift in four iron-rich regions (putamen, globus pallidus, caudate, and thalamus) showed an almost linear dependence (r>or=0.90 for R(2)(*); r>or=0.83 for phase, p<0.01) on field strength, suggesting that potential ferritin saturation effects are not relevant to susceptibility-weighted contrast for field strengths up to 7 T. The R(2)(*) dependence on the putative (literature-based) iron concentration was 0.048 Hz/T/ppm. The histological data from brain samples confirmed the linear dependence of R(2)(*) on field strength and showed a slope against iron concentration of 0.0099 Hz/T/ppm dry-weight, which is equivalent to 0.05 Hz/T/ppm wet-weight and closely matched the calculated value in vivo. These results confirm the validity of using susceptibility-weighted contrast as an indicator of iron content in iron-rich brain regions. The absence of saturation effects opens the way to exploit the benefits of MRI at high field strengths for the detection of iron distributions with high sensitivity and resolution.

Nanoscale alpha-Fe particles with size of about 80 nm were prepared with a microemulsion-coated method. The results demonstrated that this kind of iron particles could exist stably in the air for 7 d compared with nanoscale iron particles prepared by liquid-phase and microemulsion methods. The removal rate of trichloroethylene with an initial concentration of 10 mg x L(-1) can reach 90% in 700 h. The reduction kinetics was studied under room temperature, neutral, and anaerobic conditions. Experiments show that the reduction process of TCE by nanoscale iron particles conforms to pseudo first order reaction law. The apparent rate constant (k(obs)) is proportional to concentration of nanoscale iron particles. The k(obs), values 6.49 x 10(-40, 6.64 x 10(-4), 7.10 x 10(-4), 7.43 x 10(-4) min(-1), are corresponding to concentrations of 87.5, 175, 262.5, 350 mg x L(-1) respectively. In the reaction, nanoscale iron particles provides electrons and forms an inner film of Fe3O4, on the surface of which an outer film of Fe2O3 is formed together with water. TCE is degraded by electrons. The principal degradation products were ethene and ethane, and smaller amounts of other chlorinated degradation products were also founded.

In this study, a higher surface area spherical nanoscale zero valent iron (HNZVI) cluster (80 nm, 54.25 m(2)g(-1)) was synthesized in ethanol-water mixed solvent in the presence of dispersion agent of polyglycol (PEG). At the same time, a lower surface area nanoscale zero valent iron (LNZVI) particle (80 nm, 8.08 m(2)g(-1)) was also prepared with only de-ioned water as reaction media. Their structures, compositions and physical properties were characterized by transmission electron microscope (TEM), X-ray diffractometer (XRD), inductively coupled plasma atomic emission spectrophotometer (ICP-AES), and Brunauer-Emmett-Teller (BET) surface area analyzer and the results obtained for these two kinds of nanoscale iron were compared with each other and also with those reported in the literatures. The HNZVI clusters seemed to be accumulated by smaller iron particles (<10 nm). At the same time, whiskers were formed in the final produce. Reactivity of the HNZVI was affirmed via denitrification of nitrate. The factors controlling the reduction of nitrate, such as pH, dissolved oxygen (DO), iron content as well as the initial nitrate concentration were also discussed. Finally, kinetic analysis revealed that chemical reduction of nitrate by HNZVI could not be described by the first- or pseudo-first-order kinetic model.

The muddy sediments of the central South Yellow Sea (SYS) are not only an important depocenter, but also an important area for early diagenesis. A systematic study of the interlinked sulfur (S), iron (Fe), and phosphorus (P) in sediments of the sea could provide important information on the diagenetic cycle of these elements; however, their interactions have not been well documented. In this study, an in situ, high-resolution diffusive gradient in thin films (DGT) technique, together with solid-phase speciation was used to reveal the diagenesis of S, Fe, and P in muddy sediments, and to quantify the benthic flux of dissolved inorganic phosphate (DIP). Solid-phase and porewater chemistry indicate that ferruginous conditions prevail in the sediments, dissimilatory iron reduction (DIR) dominated Fe reduction, and the sulfate reduction rate is generally low due to the low lability of sedimentary organic carbon. Two-dimensional porewater distributions provide direct evidence of high spatial heterogeneity of sulfide and DIP on a microscale, and spatial coexistence of DIR and sulfate reduction. A positive linear correlation (R2 > 0.75) between porewater Fe and DIP measured by the DGT over the depth interval of rapid increase in DGT-labile Fe suggests that DIP release is primarily driven by DIR. Overall, the muddy sediments are a weak source of DIP, with an average benthic flux of 0.422 ± 1.004 × 10−3 mmol/m2/day, which accounts for only 0.02−0.04% of DIP required for the primary production of the SYS, and thus has only minor impact on P budget in the sea. [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]

This study was conducted to investigate the effects of different levels of dietary partial MEs and coated cysteamine (CC) supplementation on gut microbiota in finishing pigs. Results showed that whittling down dietary partial MEs (Cu, Fe, Zn, Mn) by 20% and 40% had little effect on the microbial diversity, community structure, and bacterial relative abundance in the ileum of finishing pigs. Supplementation with 1,600 mg/kg CC also had no obvious effect on the microbial diversity, community structure, and bacterial relative abundance in the finishing pig ileum when fed diets with a normal MEs level. However, the abundance of Peptostreptococcaceae, Pasteurella, and Pasteurella_aerogenes was higher, and the abundance of Actinobacillus_minor was lower in the 20% ME reduction diet treatment than that in the 20% ME reduction with 1,600 mg/kg CC diet group (p < 0.05). In conclusion, our results suggested that there is no obvious effect on gut microbiota when dietary partial MEs are reduced by 20% or 40%, which indicates the feasibility of reducing dietary partial MEs by 20% or 40% in finishing pigs. Supplementation with CC changed the relative abundance of some bacteria related to opportunistic pathogenicity in the finishing pig ileum when were fed a 20% ME reduction diet. [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]

The influence of differential chemical weathering intensity on the contents and spatial distribution of highly reactive iron (Fe HR) in the marginal sea sediments is not well documented. Here, we used sequential extractions to quantify various Fe pools in offshore surface sediments of the northern South China Sea (nSCS), which receives multiple fluvial sediments with distinct chemical weathering intensity, and then to unravel whether and if so, to what extent, the contents and spatial distributions of Fe HR are linked to chemical index of alternation (CIA) of the sediments. Our results show that, despite the inputs of Fe HR -rich, highly chemically weathered fluvial sediments, Fe HR contents in offshore sediments of the nSCS are almost independent of the CIA values, and Fe HR was not enriched relative to the global average of Fe HR in oxic sediments, probably due to effective trapping of Fe HR in the estuary and dilution by moderately chemically weathered Taiwan-sourced sediments. Our results indicate that the signature of Fe HR enrichment previously observed in the offshore sediments, which are discharged from a highly chemically weathered mountainous island (Hainan Island) of the nSCS, becomes lost beyond the local zone around the island. Relative to the marginal sediments of the nSCS (water depth <1000 m), the higher Fe HR contents in the deep-water sediments (water depth larger than 1000 m) can be attributed to higher smectite contents, lower inorganic carbon contents, and low dilution by Taiwan-sourced fluvial material in the deep-water area. A comparison of the subtropical nSCS and the temperate East China Sea indicates that chemical weathering intensity is not the overarching factor governing Fe HR /Fe T ratios in the two climatically different regions, though chemical weathering intensity along with depositional/diagenetic regimes is an important factor controlling the sizes and transformation of some specific Fe HR pools. • Reactive Fe (Fe HR) enriched by chemical weathering is restricted to local zone. • Chemical weathering intensity has minor effect on Fe HR content in offshore sediment. • Sediment sources, sorting, and depositional/diagenetic regimes impact Fe HR and CIA. [ABSTRACT FROM AUTHOR]

The interactions of organic carbon (OC) with ferric iron (Fe) oxides play an important role in OC stabilization, but reductive dissolution of Fe(III) oxides under anoxic conditions potentially compromises the stabilization of Fe-bound OC (Fe-OC). Up to date, there are very limited studies on the fate of Fe-OC in sulfidic conditions commonly encountered in OC-rich marine sediments. Herein, multi-pronged studies were implemented to quantitatively characterize the capacity of metastable iron sulfide (FeS) to sequester OC extracted from marine sediments and to unravel the fate of ferrihydrite-bound OC complexes (Fh-OCs) when exposed to dissolved sulfide. Results show that, for a given mechanism of OC association (adsorption or coprecipitation), FeS has DOC retention capacity generally comparable to Fh, implying that the metastable phase could serve as an important OC sink in anoxic marine sediments. During reduction of Fh-OCs by dissolved sulfide, Fh reduction was progressively retarded with increasing OC pre-adsorption, while Fh reduction was promoted after DOC pre-coprecipitation. Despite high extents of Fe reduction by dissolved sulfide, OC release from Fh-OC coprecipitates was low due to efficient re-sequestration of released DOC by coprecipitation with FeS, and an increase in OC pre-loading could facilitate Fe2+ sulfidization and thus re-sequestration of released DOC by FeS. When Fh-OC adsorption complexes were exposed to dissolved sulfide, substantial OC release was restricted to the complexes with lower OC pre-adsorption (C/Fe ≤ 1.5). Overall, reactive Fe reduction by dissolved sulfide may not lead to substantial release of associated OC as long as reduced Fe could be sulfidized to efficiently re-sequester concomitantly released DOC. Results of carbon-1 s near-edge X-ray absorption fine structure spectroscopy indicate that, upon reduction of Fh-OCs by dissolved sulfide and subsequent FeS formation, carboxyl-C was selectively released into the solution but aromatic-C selectively remained on the solid matrixes after Fe reduction, suggesting that OC sink switching from Fe oxides to Fe sulfides could result in molecular fractionations of porewater DOC to some extent, with potential implications for OC biogeochemical cycling. Ferrihydrite reduction by dissolved sulfide and concomitant release of bound DOC, FeS formation, and re-sequestration of DOC by coprecipitation with FeS. [Display omitted] [ABSTRACT FROM AUTHOR]

In marine sediments, microbial mineralization of organic carbon (OC) proceeds through a cascade of pathways using different electron acceptors. Rates and relative importance of individual pathways can provide quantitative constraints on biogeochemical cycling of a number of elements, such as carbon, nitrogen, sulfur, iron, and manganese. However, such information is hitherto very scarce in the China marginal seas. To fill the gap, a multicomponent-coupled reaction-transport model CANDI was applied to quantify the rates and relative contributions of individual pathways of OC mineralization at three sites in the South Yellow Sea (SYS) sediments, based on which OC budgets were constructed. Model results at steady state generally well-reproduced the profiles of measured porewater solutes. The fast degrading OC pools G1 become exhausted over the upper 4 cm at the studied sites with the degradation rate constants k 1 ranging between 0.6 and 1.65 yr−1, while the slow degrading OC pool G2 remains almost undegradable on the timescales of the studied depth (∼25 cm) due to the low degradation rate constants k 2 (10−5‒10−4 yr−1) The depth-integrated rates of OC mineralization at the studied sites are in the range of 5.1–18.3 mmol C m−2 d−1, and aerobic respiration stands out as the dominant pathway, with its average contribution of 62%, followed by sulfate reduction (22.6%), dissimilatory manganese/iron reduction (8.4%), and denitrification (6.1%). Methanogenesis is a negligible process of OC mineralization within the studied depths. The contributions of aerobic respiration in the SYS sediments are much higher than the global average for marginal seas, but the contributions of sulfate reduction and iron dissimilatory reduction are much lower than their global averages, which could be ascribed to the low availability of degradable OC for anaerobic respiration. Budget of OC indicates that the mineralization and burial of OC in the muddy sediments account for 7.3 and 1.7%, respectively, of the total OC supplied to the sea, with the net OC accumulation efficiency of ∼19%. • Rates of organic carbon (OC) oxidation pathways are quantified by diagenetic modeling. • Aerobic respiration accounts for 62% of OC oxidation, being the dominant pathway. • Low availability of labile OC limits the anaerobic respiration pathways. • OC oxidization is oxidized at a rate of 9.8 mmol C m−2 d−1 in the muddy sediments. [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]