The composition and geochemical significance of organic matters in surface sediments from the Southwest Sub-basin of the South China Sea

Abstract

Information about ocean evolution and material-energy exchange between the ocean and surrounding continents can be preserved in marine sediments. The Southwest Sub-basin, located among the Xisha Islands, the Zhongsha Islands, and the Nansha Islands, is an ideal tectonic unit to analyze the sedimentary environments of the South China Sea. In this paper, the distribution patterns of lipid biomarkers and their compound-specific stable carbon isotopes in surface sediments from the Southwest Sub-basin of the South China Sea were analyzed. Lipid biomarkers are composed of different proportions of aliphatic hydrocarbons, carboxylic acids, alcohols and series of sterols, indicating that the organic matter of the Southwest Sub-basin was derived from marine bacteria, algae and terrestrial higher plants. The average concentration of total organic carbon (TOC) in the study samples was 0.5 ± 0.16%. TOC, total n-alkanes, and total carboxylic acids (TFA) decreased gradually from the margins to the center of the sub-basin, whereas the content of total n-alkanols increased. The spatial distribution trends resulted from varying water depths and the contribution of terrestrial organic matter. The concentration of TFA and the TFA/TOC revealed that the accumulation of organic matter in this area was generally low. The strong correlation between TOC and TFA of surface sediments in the study area may be related to a balance between the ocean production and ocean deposition rates.