Dual sources and consumption of methane in shallow sediments of southern SCS: Insight from porewater composition and numerical modeling

Abstract

Methane seepages have significant impacts on marine environments and ecosystems. Deep understanding of that is critical for the resources exploration and the climate change. Numerous methane seepages have been reported worldwide, and mostly investigated on the slope and margin, with less attention paid to the basin. Here presents the determination of three sediment cores (H59, H79, and H55Y) retrieved in the Beikang Basin in the southern of the South China Sea (SCS), an important hydrocarbons and gas hydrates potential domain.A preliminary evaluation of the methane-related geochemical processes was obtained based on the compositions of porewater, and a reactive transport model was further developed to quantitively investigate the corresponding processes. The model results matched well the measurements. Both porewater compositions and model results indicated that anaerobic oxidation of methane dominantly consumed the sulfate at sites H55Y and H59, while sulfate was mainly consumed by organoclastic sulfate reduction at site H79. The shallow depths of SMI (3.8–7.8 mbsf), benthic methane fluxes (0.5–1.5 mmol m−2 yr−1) and high AOM rates (37–103 mmol m−2 yr−1) indicated the methane seepages at H55Y and H59. Model and sensitivity tests revealed further that the consumed methane was mainly supplied by external source, which ascended from deep reservoir, while in-situ methanogenesis contributed only a negligible fraction. The deduced δ13C of the external methane of H55Y demonstrated it was also microbial origin, which was probably contributed by the biogenic source rocks and/or dissociation of gas hydrates.The comprehensive regional research of the source and consumption of methane determined different seepage scenarios in the basin of southern SCS and enhanced our understanding of methane cycling in the marine basins.