Paleo-cold seep activity in the southern South China Sea: Evidence from the geochemical and geophysical records of sediments

摘要

Over the past decade more than 30 cold seep sites have been reported along the continental slope of the South China Sea (SCS). These seep sites have all been located in the northern SCS, and no methane seepage has been discovered in the southern SCS. Here, we report on the analysis results of the geochemical and geophysical of the marine core sediments (58S) collected from the southern continental slope of the SCS. We suggest that the sedimentary total sulfur (TS), total inorganic carbon (TIC), delta S-34(CRs) values of chromium reducible sulfur (CRS) and carbonate delta C-13(TIC) values of the bulk sediments, coupled with the ratio of (NRMAF60mT-NRMAF80mT)/NRM can be used to trace the methane release events in a methane-rich environment. At least two methane release events were identified at depths of 5.5-6.2 m and 6.8-7.2 m. Moreover, according to the characteristic of the TS/TOC and (NRMAF60mT-NRMAF80mT)/NFtM ratio of sediments, we suggest the methane flux has changed episodically. This inference is supported by the occurrence of the strongly negative carbonate delta C-13(TiC) values of bulk sediments and the positive delta S-34(CRS) values, which indicate the occurrence of anaerobic oxidation of methane (AOM) and the location of a past or present sulfate methane transition zone (SMTZ), respectively. Ultimately, these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment.