Pathways of organic carbon mineralization and benthic fluxes of alkalinity and dissolved inorganic carbon in sediments of large inland seas: The Bohai Sea and North Yellow Sea

Abstract

Shallow inland seas are sensitive to benthic-pelagic coupling and susceptible to anthropogenic perturbations. In the sediments, organic carbon (OC) mineralization-driven diagenetic cycling and benthic-pelagic coupling are impacted by numerous intricate factors, many of which have not been well documented. In this study, conventional geochemical measurements and multicomponent-coupled diagenetic modeling are combined to quantitatively investigate the partitioning of OC mineralization among individual electron acceptors in sediments of two large inland seas: the Bohai Sea (BHS) and the North Yellow Sea (NYS). We then estimate OC transfer efficiency in the sediments and benthic fluxes of dissolved inorganic carbon (DIC) and total alkalinity (TA). Modeling results suggest that the reactivity of sedimentary OC is 1–3 orders of magnitude lower than the most commonly observed values, indicating overall low degradability of OC. Depth-integrated OC mineralization rates over a 20-cm depth are 3.4 and 2.4 mmol m⁻² d⁻¹ on average in the BHS and NYS sites, respectively, with aerobic respiration as the most important pathways (mean: 54 %), followed by sulfate reduction (mean: 24 %). Dissimilatory iron reduction (DIR), dissimilatory manganese reduction (DMR), and denitrification combined contribute only 16 %–33 % of OC mineralization. Sedimentation rate, rather than OC mineralization rate, is the primary control of OC transfer efficiency in the sediments. Consequently, high sedimentation rates in the BHS result in much higher OC transfer efficiency (mean: 78 %) over the 20-cm depth of the sediment compared to that in the NYS (mean: 34 %). Albeit covering a relatively small area, these two inland seas represent hotspots of OC burial. Our estimates of benthic fluxes suggest that benthic exports of DIC and TA may play an important role in benthic-pelagic coupling in these seas. Contrasting TA/DIC flux ratios imply that benthic TA export may mitigate bottom-water acidification in the BHS but exacerbate the acidification in the NYS.