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 k1 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 k2 (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%.

Cycling and fates of iron (Fe) and sulfur (S) in marine sediments are influenced by depositional settings to differential extents. The information is crucial for addressing the responses of their benthic diagenesis to changing climates and environments and also for reconstructing paleo-depositional conditions, but has not been well constrained. Detailed chemical speciation was utilized to characterize geochemistry of Fe and S, and then to reveal the impacts of depositional settings on their diagenesis at three locations, representing contrasting depositional environments: (i) highly dynamic Yangtze River estuary (YRE), (ii) the depocenter of South Yellow Sea (SYS), which is only remotely impacted by large river, and (iii) the middle Okinawa trough (OT), a back-arc deep basin along the outer edge of the East China Sea (ECS) slope. Results show that the YRE sediments favor accumulation of total highly reactive Fe (FeHR), while the SYS sediments are poor in FeHR due to low FeHR in their source material and/or preferential trapping of FeHR during sediment transport through the semi-enclosed Bohai Sea. Ferruginous sediment regimes in the highly dynamic YRE system facilitate Fe(III) reduction and burial of unsulfidized Fe(II), while the SYS and OT sediments favor sulfate reduction and pyritization of Fe(II). Pyrite is always the main sink of reduced S that escapes reoxidation in the entire continental margin, regardless of depositional environments. Abundant reactive Fe(III) but low total reduced inorganic sulfide (TRIS) contents in the three sites suggest that TRIS burial is largely controlled by the availability of degradable OC and/or dynamic regimes of sediments. The applicability of two widely used Fe- and S-based proxies to distinguish bottom-water conditions, that is, OC/TRIS ratio and FeHR to total Fe (FeHR/FeT) ratio, were examined in the three contrasting environments, and caveats were given for future applications of the two proxies.