Light carbon isotope events of foraminifera attributed to methane release from gas hydrates on the continental slope, northeastern South China Sea

Abstract

In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea (SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L−1. The hydrocarbon δ13C values, ranging from −69.4‰ to −72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results (−183‰ to −185‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO2 reduction. By isotopic measurement, five light δ13C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ13C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ13C values of benthic Uvigerina peregrina are extremely depleted (as low as −15.85‰ PDB), while those of planktonic Globigerinoides ruber reach −5.68‰ PDB. Scanning electron micrograph (SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ13C composition of benthic foraminifera. This means that the anomalous δ13C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane (AOM). Furthermore, the negative δ13C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.