Elemental and Isotopic Signatures of Bulk Sedimentary Organic Matter in Shenhu Area, Northern South China Sea

Hydrate-bearing sediments provide excellent materials for studying the primary sources and diagenetic alterations of organic matter. In this study, the elemental and isotopic signatures of total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), and total sulfur (TS) are systematically investigated in three hydrate-bearing sediment cores (similar to 240 m) retrieved from the Shenhu area, South China Sea. All sediment layers from three sites are with low TOC content (average 0.35%) with marine and terrestrial mixed sources (-23.6 parts per thousand < delta C-13(org) < -21.4 parts per thousand). However, the generally low delta N-15 (2.49-5.31 parts per thousand) and C/N ratios (4.35-8.2) and their variation with depth cannot be explained by the terrestrial sources (Pearl River) and marine sources, binary end-member mixing processes. Contribution from lateral allochthonous organic matter from the mountainous river is considered after excluding other possible factors and ingeniously elucidating the organic matter origins. Furthermore, specific layers in W01B and W02B exhibit elevated S/C ratios (up to 2.39), positive bias of delta S-34-TS (up to +29.7 parts per thousand), and negative excursion of delta C-13-TIC (up to -8.29 parts per thousand), which are the characteristics of sustained occurrence of sulfate-driven anaerobic oxidation of methane. The occurrence of coupled carbon-sulfur anomaly may be accompanied by deep hydrocarbon leakage and the formation of hydrate with high saturation.

Automated summary

This study systematically investigates the elemental and isotopic signatures of organic matter in three hydrate-bearing sediment cores from the Shenhu area in the South China Sea. The results reveal that the sediment layers have a mixed marine and terrestrial origin, with specific layers showing characteristics of sustained occurrence of sulfate-driven anaerobic oxidation of methane, suggesting the presence of deep hydrocarbon leakage and the formation of hydrate with high saturation.