Geochemical evidence of methane seepage in the sediments of the Qiongdongnan Basin, South China Sea

In this paper, the QS-1 sediments (3.64 m long; at 1500 m water depth) collected from the Qiongdongnan Basin were used to analyze the relationship between sedimentary geochemical anomalies and methane seepage. Mo-U covariation, Ce anomaly, and Mg/Ca-Sr/Ca end-member model were analyzed to identify the variations of redox environment and authigenic carbonate precipitation, finally constructing the methane release events in this study area. These observations suggest that: 1) Significant Mo/U enrichments and Ba front indicate the presence of high-flux active methane seepage with SMI of 260 cmbsf in this study area. The depositional environment evolved as oxic-anoxic-reducing-sulfide environment from bottom to top, as evidenced by Mo-U covariation and Ce anomaly; 2) Based on Sr/Ca and Mg/Ca end-member model proposed by Bayon et al. (2007), the high-Mg calcite was the dominant authigenic carbonate currently which usually precipitated behind the methane seep events; and 3) The active cold seep can be divided into three methane release events (MREs): MRE I, the most intense methane seepage occurred at 22.1 ka BP; MRE II, three low-intensity methane seepage occurred at 19.3, 16.5, and 11.3 ka BP, respectively; and MRE III, the continuous methane seep began at 9.3 ka BP until now. The synthesis of geochemical indicators has significant implications for tracing methane seepage of varying intensity and duration.