Methane seepage inferred from the porewater geochemistry of shallow sediments in the Beikang Basin of the southern South China Sea

Abstract

Several tens of methane (CH4) seepage sites have been discovered in the northern South China Sea (SCS) during the investigation and exploration of gas hydrates and cold seeps over the past decade. However, our knowledge about methane seepage in the southern SCS remains scarce, especially in the Beikang Basin on the southern slope of the SCS. Four piston cores (BH-CL18, BH-CL23, BH-CL29 and BH-CL35) were collected in the Beikang Basin to understand CH4 cycling in shallow sediments. The extrapolated depths of the sulfate–methane transition zones (SMTZ) of the four cores are between 5.3 and 8.8 mbsf. By simulating the porewater profiles of SO42−, CH4, DIC and PO43− using the reaction-transport model, we estimated that the dissolved SO42− was predominantly consumed by the anaerobic oxidation of methane (AOM) at rates of 27.5 mmol m−2 yr−1 in BH-CL18, 28.1 mmol m−2 yr−1 in BH-CL23, 43.1 mmol m−2 yr−1 in BH-CL29 and 39.4 mmol m−2 yr−1 in BH-CL35, respectively. Due to intense AOM, almost all the methane sourced from subsurface sediments was depleted within the SMTZ, resulting in negligible benthic methane fluxes. The kink-type depth SO42− profile of BH-CL23 was apparently attributable to an enhanced methane flux, which was not captured by the steady-state modelling. Furthermore, highly depleted δ13C values of the dissolved inorganic carbon (DIC) in BH-CL18 (−43.6‰), BH-CL23 (−44‰) and BH-CL29 (−51.2‰) also indicated a methane-derived DIC. In contrast, relatively higher δ13C values in BH-CL35 suggested a mixture of 13C-enriched DIC produced by methanogenesis, as shown by the reversal toward more 13C-enriched DIC below 4.2 mbsf in BH-CL29. To our knowledge, this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern SCS and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern SCS.