Coupled δ15NTN and δ13CTOC Insights into Methane Seepage Activities in Bulk Marine Sediments of the Qiongdongnan Basin, South China Sea

Recently, methane seepage related to the dissociation of natural gas hydrates has attracted much attention, which has a significant impact on the study of the global carbon and nitrogen cycles. Based on the detailed geochemical analyses of sediments (core Q6) from the Qiongdongnan Basin, South China Sea, three methane seepage activities were identified and the exact horizons of anaerobic oxidation of methane (AOM) were defined. Furthermore, organic carbon isotopic (δ 13 C TOC ) levels ranged from −23.696‰−−20.696‰ PDB; nitrogen isotopes (δ 15 N TN ) of the same sedimentary samples ranged from 1.899‰–5.399‰. We also found obvious simultaneous negative excursions of organic carbon isotopes (δ 13 C TOC ) and nitrogen isotopes (δ 15 N TN ) in the horizons of methane seepages. Compared with the normal sediments, their maximum negative excursions were 2.699‰ and 2.599‰, respectively. We discuss in detail the various characteristics of δ 15 N TN and δ 13 C TOC levels in sediments and their coupling responses to methane seepage activities. We believe that the methane seepage events changed the evolution trajectory of δ 15 N TN and δ 13 C TOC levels in sediment records, which resulted in the simultaneous negative excursions. This phenomenon is of great significance to reveal the historical dissociation of natural gas hydrates and their influence on the deep-sea carbon and nitrogen pool.