Carbon isotopic constraints on the degradation and sequestration of organic matter in river-influenced marginal sea sediments

Organic matter degradation and sequestration in marine sediments are important processes involved in carbon cycling in the ocean. Here, we present the results of carbon isotope (C-14 and C-13) and concentration measurements of sedimentary organic carbon (SOC), pore-water dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in sediments collected from the East China Sea, Yellow/Bohai Sea, and South China Sea. Our results indicated that selective degradation and preservation of organic matter occurred in these sediments, and marine-derived young organic carbon degraded preferentially and rapidly, resulting in high concentrations of pore-water DOC and DIC with distinct carbon isotopic signatures. The average C-14 age of pore-water DOC was thousands of years younger than that of SOC in the sediment, suggesting that DOC was newly produced and cycled much faster than SOC. Aged SOC was refractory and preserved in sediments. Using a dual-isotope three-end-member model, the contributions of potential sources to SOC, DOC, and DIC were estimated. Marine-derived biomass organic carbon contributed the most to DOC, and dissolution of biogenic carbonate contributed the most to DIC. Riverine inputs of pre-aged soil and fossil organic carbon dominated the SOC pool. Our study demonstrated that marginal sea sediments are important sites of young DOC and DIC fluxes into the water column, thus acting as a major pathway for carbon and nutrient cycling in the ocean.