Methane seep in the Shenhu area of the South China sea using geochemical and mineralogical features

Abstract

Extensive studies on methane seeps have facilitated the exploration of potential gas hydrates and have revealed biogeochemical processes on the seafloor. In this study, a short core, GG00, was collected from the Shenhu area, South China Sea. By combining various geochemical and mineralogical methods—including planktonic foraminiferal AMS14C dating, as well as their carbon and oxygen isotopes, chromium-reducible sulfur (CRS) content and δ34S values, and hand-picked pyrite and other minerals, we identified a methane seeps that has been stable since the end of the Last Glacial Period. Extremely negative values of δ34S for CRS (as low as −49.7‰) indicated that only organoclastic sulfate reduction and its coupled sulfur intermediate disproportionation were present in the shallow sediments (from 0 to 209 cmbsf). Conversely, the increasing TS and Mo content and the progressively more positive δ34S values (up to −34.76‰) of the CRS indicated enhanced sulfate reduction coupled with the anaerobic oxidation of methane (SR-AOM) in deeper sediments (between 209 and 370 cmbsf). In addition, a large number of hypautomorphic-euhedral elemental sulfur (ES) microcrystals was found at 237 cmbsf using scanning electron microscopy, and energy-dispersive X-ray spectroscopy identified the upper edge of the sulfate–methane transition zone (SMTZ), which was close to the position that was determined from other data (e.g., major and trace elements as well as CRS and its δ34S value). The construction of a stratigraphic framework based on planktonic foraminiferal AMS14C dating and carbon and oxygen isotopes, combined with the large number of framboid pyrites consisting of cubic microcrystals found further below the SMTZ, suggests that this SMTZ has been relatively stable since the end of the Last Glacial Period. In addition, discrepancies in the aggregation morphology of cubic pyrite at different intervals are indicative of variations in Fe and S supply and growth space. Therefore, this study demonstrates the significance of detailed geochemical and mineralogical signatures in revealing the correlation between methane seep and climate.