Porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate-bearing system from the Taixinan Basin, the South China Sea

Anaerobic oxidation of methane (AOM) is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling. Though AOM has been extensively investigated, the variations of its microorganisms and lipid biomarkers are still not fully understood. In this study, the porewater geochemical and lipid biomarker evidence for AOM was systematically investigated in the hydrate-bearing system from the Taixinan Basin, the South China Sea, aiming to better expound the vertical variations of AOM-related biomarker signatures and causes of the weak lipid biomarker patterns. Accordingly, the sulfate-methane interface (SMI) depth of 1,000 cmbsf and onset of AOM were determined on the basis of the vertical profiles of porewater parameters. The occurrence of C-13-depleted 2,6,10,15,19-pentamethylicosane (PMI) with the delta C-13 ratios of -77.7 parts per thousand to -131.4 parts per thousand near the SMI are in accord with the porewater parameters, despite the absence of archaeol and sn2-hydroxyarchaeol and low abundances of PMI. Meanwhile, the strongly C-13-enriched iso-C-15:0 and anteiso-C-15:0 fatty acids suggest that non-methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate-reducing bacteria due to the presence of unresolved complex mixtures (UCMs) and odd-even predominance (OEP) values (about 1.0). The weak in situ signal of AOM-related biomarkers may be attributed to the low-flux methane supply, and dilution effect from the high input of allochthonous and autochthonous organic matters. The great difference in biomarkers between the Taixinan Basin and previously reported sites should be further clarified on the basis of more extensive studies in various AOM-related settings.