Novel use of unique minerals to reveal an intensified methane seep during the last glacial period in the South China Sea

Abstract

Globally, research aiming to reveal the unique biogeochemical processes and gas hydrate potential in methane seep areas is well established. Furthermore, methane seeps discovered in the South China Sea (SCS) have been widely studied and have achieved significant results. Herein, by systematically analyzing sedimentary samples from the Shenhu area, SCS, we discovered a methane seep that was once active during the last glacial period, particularly the Last Glacial Maximum (LGM) period, and fixed at the seafloor for ∼10 ka. This discovery was achieved by integrating the features of the glauconite within the foraminifera chamber, different morphologies of pyrites, and unique framboid goethites (pyrite pseudomorphs). In addition, this study accounted for the high chromium-reducible sulfur content and extremely negative δ34S values (from −50.78 to −46.92%) between 121 and 231 cmbsf, the Mo content and its correlation with total sulfur (TS) and total organic carbon (TOC), the TS/TOC ratios of sediments, the results of AMS 14C dating, and the results of planktonic foraminifera analysis in the case of carbon and oxygen isotopes. Glauconite, which is usually considered a product associated with transgressive deposits and condensed sections, is widely found in warm climatic intervals. However, these intervals are generally associated with methane emission events. Thus, our results demonstrate another mechanism for the formation of glauconite in methane seeps during the LGM. Furthermore, the existence of framboid goethites implies that this methane seep has been subsiding since the end of the LGM and that few pyrites that formed previously were reoxidized. Notably, this study is the first to report a systematic investigation of methane seep activity during the last glacial period by the indication of authigenic minerals, such as glauconite and framboid goethite, in the SCS. Overall, our results show a direct relationship between methane seep activity and climatic variation.