Tracing the paleo-methane seepage activity over the past 20,000 years in the sediments of Qiongdongnan Basin, northwestern South China Sea

The seepage activity of paleo-methane has been proven to exist in geological history and play an important role in the evolution of the Earth's environment. This study reports grain size, major/trace elements, total nitrogen (TN), total carbon (TC), total organic carbon (TOC), total sulfur (TS), magnetic susceptibility and AMS-14C measurements for sediment samples in a drill core (~282 cm long; Site Q6 at ~1400 m water depth) from the ‘Haima seep’ sedimentary area in the Qiongdongnan Basin, along the northern slope of the South China Sea (SCS). The alternative index system for paleo-methane seepage identification was established through testing and analysis, and three methane release events (MREs) were successfully identified: 254–282 cm (MRE1: 19.3–17.8 ka), 144–162 cm (MRE2: 11. 9–10.8 ka) and 90–124 cm (MRE3: 10.0–8.5 ka). When methane seepage occurred, the enrichment of authigenic carbonate minerals (high Mg-calcite or aragonite) led to the increase in the content of CaO and ‘dilution’ in the total rare earth element content (∑REE). However, the occurrence of authigenic pyrite resulted in a significant increase in TS and the total sulfur/total organic carbon ratio (S/C) in the sediments, and a synchronous decrease in the magnetic susceptibility. Meanwhile, the change of redox environment also induced the enrichment of Mo, U, and the positive anomaly of δCe. Based on the records of methane release events, the control mechanisms for the different time scales and different periods, such as sea level change, sea floor temperature change, and turbidity current activity were preliminarily discussed. Finally, the history of methane seepage activity was reconstructed, and conceptual diagrams were drawn of the study area encompassing the last 20 ka.