Spatial distribution of organic matter in surface sediments from the Nansha sea area of the South China Sea and its implications for marine productivity and monsoon

The knowledge of modern hydrologic process provides important clues for marine palaeoenvironmental and palaeoclimatic evolution. Twenty-three surface sediment samples collected throughout the Nansha sea area of the South China Sea (SCS) were analyzed for sedimentary bulk parameters, including total organic carbon (TOC), total nitrogen (TN) and their isotopes (δ¹³C_TOC and δ¹⁵N), in order to understand the controlling factors on the spatiotemporal variations of sedimentary organic matter (OM). Results show that the TOC/TN (5.5～7.9 in range and 6.5±0.6 on average) and δ¹³C_TOC (–21.9‰ ～ –18.7‰ in range and –21.0‰±0.7‰ on average) reflect the dominance of marine OM input to surface sediments in the Nansha sea area. Correlation analyses of δ¹³C_TOC vs TOC and vs TOC/TN indicate that sedimentary TOC was not significantly affected by early diagenesis, thereby TOC can be used as an effective tracer for surface productivity. The values of TOC (0.32%～0.97% in range and 0.67%±0.17% on average) show a descending trend from the western to eastern parts of the Nansha sea area, indicating clearly the importance of southwesterly summer monsoon in delivering rich nutrients from the productive Vietnam upwelling and Mekong delta. Moreover, strong correlation between TOC and TN (R²=0.95) and clear spatial distribution of δ¹⁵N manifested the ability of bulk δ¹⁵N to reflect upper water δ¹⁵N signal due to the minimal influence of riverine terrigenous inorganic N, thereby indicating the relevant nitrogen cycle process.