Lipid biomarker patterns reflect seepage activity and variable geochemical processes in sediments from the Haima cold seeps, South China Sea

Abstract

The anaerobic oxidation of methane (AOM), aerobic methane oxidation and the oxidation of non-methane hydrocarbons have been demonstrated to occur in seepage ecosystems, but when they occur during seepage evolutionary is lack of constraints. Here, lipid biomarker patterns combined with total organic carbon (TOC), δ13Corg, δ13CTIC and δ18OTIC of a sediment core (~ 3 m long) from the active Haima cold seep were analyzed. The observed lipid inventories reflected the prevailing of anaerobic methane oxidizing archaea (ANME)/Desulfosarcina/Desulfococcus (DSS) and ANME-2/DSS dominated over the whole sediments. Seepage fluids originated from local gas hydrate dissociation as indicated by the pronounced δ13CTIC and δ18OTIC values, while a mixture of microbial and thermogenic methane (−65‰ to −55‰) was corroborated by back-calculation based on the predominant consortia and δ13C values of ANME-specific biomarkers. AOM occurred over the whole sediments and was strengthened between depths 40 cm and 120 cm as revealed by the extremely low δ13CTIC, δ13Corg values, and abundant lipid biomarkers of ANME such as crocetane, 2,6,10,15,19-pentamethylicosane (PMI), archaeol and sn2-OH-archaeol with strong 13C depletions (δ13C values as low as −118‰). For sediment below the depth of 120 cm, molecular fossils of aerobic methanotrophs 4α-methylcholesta-2,8(14),24-triene and C34-3β-methyl tetracyclic-ketone occurred and yielded their δ13C values of −64‰ to −61‰, indicating the prevailing of oxic condition and aerobic methane oxidation. Compared to the sediment between 40 cm and 120 cm, the abundant iso-/anteiso-C15:0 (i-/ai-C15:0) fatty acids with relatively enriched δ13C values in the bottom sediment were better explained by contribution of sulfate-reducing bacteria (SRB)-driven oxidation of non-methane hydrocarbons. The distinct biomarker patterns and the respective geochemical processes at various sediment depths revealed the microbial community succession sequence in seepage evolutionary, since the different growth rates may have led to a successive colonization of aerobic methanotrophs, SRB, and ANME in an emerging cold seep ecosystem.