Organic Carbon Sources in Surface Sediments on the Northern South China Sea

The burial of organic carbon (OC) in marine sediments regulates CO2 content in the atmosphere. However, the OC sources and their effect on the OC preservation in sediments of the continental marginal sea remain elusive. Here, we survey the abundance, stable carbon and radiocarbon isotopes of OC, as well as mineral surface area and grain size in surface sediments from the shelf to the abyssal plain of the South China Sea. We found that the marine OC comprises the 63 +/- 9% and 78 +/- 14% of sedimentary OC off South China and Luzon, respectively. The petrogenic OC contributing to sedimentary OC in these sites is generally higher than that of soil OC (22 +/- 10% vs. 12 +/- 6%). The sedimentary OC off Taiwan is predominantly derived from petrogenic OC, accounting for 57 +/- 10%, with remaining consisting of marine OC (36 +/- 10%) and soil OC (6 +/- 1%). High soil OC contents are found in fine sediments on the inner shelf off South China, and high petrogenic OC contents occur in fine sediments off Taiwan. Marine OC contents are high in fine sediments on the middle shelf and upper slope off South China, but low in coarse sediments on the outer shelf and upper slope, and fine sediments on the lower slope and abyssal plain. The OC sources, mineral surface area, and oxygen exposure time of OC together control the preservation of OC in sediments with their relative importance differing on varying depositional settings of this sea. The organic carbon preserved in marine sediments controls the atmospheric CO2 content. However, the sources of organic carbon in marine sediments and the factors affecting their preservation are currently not fully understood. Here, we analyze the content, stable isotope, and radioactive isotope of total organic carbon, as well as the mineral surface area and grain size of surface sediments in the South China Sea. We found that the sedimentary organic carbon is mainly marine derived organic carbon off South China and Luzon, while off Taiwan it is mainly rock derived organic carbon. Fine-grained sediments in the South China Sea contain high contents of soil derived organic carbon on the inner shelf and high contents of marine derived organic carbon on the middle shelf and upper slope, while on the lower slope and abyssal plain, they contain low contents of marine derived organic carbon off South China and high contents of rock derived organic carbon off Taiwan. Coarse-grained sediments in the South China Sea all contain low contents of organic carbon. Our research reveals that mineral surface area, organic carbon source, and oxygen exposure time jointly modulate the preservation of sedimentary organic carbon in marine sediments. Marine organic carbon made up a majority of surface sediments off South China and Luzon Petrogenic organic carbon significantly contributes to the surface sediments off Taiwan Mineral surface area, sources and oxygen exposure time of organic carbon control the preservation of organic carbon on surface sediments