Lipid biomarkers and their specific carbon isotopic compositions of cold seep carbonates from the South China Sea

Abstract

Cold seep carbonates were studied from two locations (Dongsha and Chiasian) in the northern South China Sea and on Taiwan island. Lipid biomarkers and their stable carbon isotopic compositions extracted from these seep carbonate samples were analyzed in this study and the results pointed at the anaerobic oxidation of methane (AOM) as responsible for carbonate formation. Distinct AOM communities were indicated by isotopically highly depleted archaeal biomarkers (with δ13C values as low as −140.8‰ VPDB) and bacterial biomarkers (with δ13C values as low as −128.6‰ VPDB), which confirmed that biogenic methane was the dominant carbon source. Lipid biomarker distributions indicated that ANME-2 archaea were prevalent in sample TVG3-C2g from Dongsha and sample JX8 from Chiasian. In addition, differences of the AOM communities between the carbonates appeared to be caused by the respective biogeochemical environments. ANME-1 archaea tended to be abundant in high-Mg calcite carbonates, and their formation might be linked to low methane fluxes. In contrast, ANME-2 consortia were prevalent in aragonite carbonates and appeared to have prospered at sites with high methane supply. Our data imply that the mineralogy of the seep carbonates precipitated in the northern South China Sea appears to be influenced by the distinct ANME consortia, which are induced by methane flux.