Novel use of unique minerals to reveal an intensified methane seep during the Last Glacial Period in the South China Sea
Globally, research aiming to reveal the unique biogeochemical processes and gas hydrate potential in methane-seep areas is well established. Furthermore, the methane seeps discovered in the South China Sea (SCS) are widely studied and have achieved considerable results. In this study, we discovered a methane seep that was once active during the Last Glacial Period, particularly during the Last Glacial Maximum (LGM), and fixed at the seafloor for approximately 10 ka. This study integrated the features of glauconite fillings in foraminifera chamber, different morphologies of pyrites, unique framboid goethites (pyrite pseudomorphs), high chromium reducible sulfur content and extremely negative δ34S values between 121 and 231 cmbsf, molybdenum content and its correlation between TS and TOC, TS/TOC ratio of sediment, and results of AMS14C dating and carbon as well as oxygen isotopes of planktonic foraminifera analysis. The glauconite, usually regarded as a product associated with transgressive deposits and condensed sections, is widely found in the warm climatic intervals, such as the Paleocene Eocene Thermal Maximum. However, these intervals are generally associated with the methane emission events. Thus, our results demonstrated another mechanism for the formation of glauconite in methane seeps even during the LGM. Furthermore, the existence of framboid goethites implies that this methane seep has been subsiding since the end of LGM and few pyrites formed previously were reoxidized. Moreover, the crucial factor responsible for the aforementioned phenomenon is that the elevated hydrostatic pressure caused by the rising sea level, thereby promoting the gas hydrate potential to become stable again. Thus, the methane fluxes reduced and seep activity subsequently decreased. Notably, this study is the first to report a systematic investigation of methane-seep activity during the Last Glacial Period by the presence of authigenic minerals, such as glauconite and framboid goethite in SCS. Overall, our results showed a direct relation between methane-seep activity and climatic variation.