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

Abstract

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 ( 14 C and 13 C) 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 14 C 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.

Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments

“…The concentrations of SOC, TN, DOC, and carbon isotope compositions of SOC and DOC have been reported in our recent publication, and these results are referenced and described for our BC results in this study. The contents of SOC and TN were 0.46 ± 0.07 and 0.05 ± 0.01% (dry wt) on average in the surface sediments of the Yangtze River Estuary and from 0.13 to 0.53% and 0.01 to 0.14% in the surface sediments of the Yellow River Estuary, respectively (Supporting Information Table S2).…”

“…We applied a one-sided F test and a two-sided T test, and the gray areas mark the 95% confidence interval of the linear regression with the p values shown. (The DOC isotope data are cited from our recent work).…”

“…The color bar shows the elevation ( m ) on the land (positive values) and the water depth ( m ) of the sea (negative values). This figure is from our recent publication …”

“…The resultant CO 2 was purified and flame-sealed in a 6 mm OD Pyrex tube for isotope analysis. Blanks associated with the tube oxidation method contained little contamination (∼2 μg C) …”

Carbon sequestration in aquatic ecosystems: Recent advances and challenges

“…Saltmarsh alkalinity production followed by porewater‐derived exports to the ocean was comparable to carbon burial in local sediments. Fu et al (2022) quantified the contribution of microbial respiration of organic carbon and carbonate dissolution as sources of both DIC and DOC to porewaters. These porewater‐derived carbon fluxes represented a major carbon source to surface waters in the coastal ocean, potentially releasing some of the carbon previously stored in sediments.…”