Geochemical signature related to lipid biomarkers of ANMEs in gas hydrate-bearing sediments in the Ulleung Basin, East Sea (Korea)

Abstract

The emission of methane as a greenhouse gas is controlled by the anaerobic oxidation of methane (AOM), which plays an important role in the biogeochemical methane cycle. During the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2), the distribution of lipid biomarkers and their compound-specific stable carbon isotope ratios related to methane were investigated in venting and non-venting sites (UBGH2-3, UBGH2-10) of gas-hydrate-bearing sediments in the Ulleung Basin. The objective of this study was to understand the microbial signatures related to methane cycling in organic-rich sediment in a marginal sea (East Sea/Japan Sea) of the western North Pacific. The concentrations of methane-related specific biomarkers (archaeol and sn-2-hydroxyarchaeol) at the sulphate–methane transition zone (SMTZ; sediment depth in UBGH2-3: 1–2 mbsf, in UBGH2-10: 6.8 mbsf) are typically higher than in other sediment sections and their δ 13C valuesare apparently depleted (−73.3‰ to −102.7‰) in the UBGH2-3 and UBGH2-10 study sites. However, the δ13C values of archaeol and sn-2-hydroxyarchaeol (between −59.6‰ and −66.5‰) are not depleted with the increased methane concentration in the sediments below the SMTZ in UBGH2-3, compared to the δ 13C values (about −60‰) of in situ methane. This suggests that methane production processes should be dominant in the deeper sediment sections (2.7–3.8 mbsf) rather than methane consumption by anaerobic methanotrophs (ANMEs) at the corresponding sediment depths. There were also higher δ 13C values (−47‰ to −32‰) for archaeol and sn-2-hydroxyarchaeol in the 3–6 mbsf sections at UBGH2-10, suggesting the prevalence of methanogenic activities. However, the δ13C values (−89.0‰ to −92.2‰) of archaeol and sn-2-hydroxyarchaeol were unexpectedly depleted in the deeper sediment section (5.2 mbsf) of the venting site (UBGH2-3), indicating that the past AOM occurred under low sulphate concentrations in the corresponding pore water. This study used the biomarker ratio (sn-2-hydroxyarchaeol/archaeol) of Archaea as a tool to demonstrate the different ANME communities, which was supported by 16S rRNA analysis in the sediments of venting and non-venting sites (UBGH2-3, UBGH2-10). Consequently, the biochemical signatures of methanotrophic and methanogenic activity were found at varying sediment depths at both sites.