Archaeal lipid biomarkers in near-surface sediments at a giant colony of the bivalve Calyptogena: Molecular records of a massive methane release event associated with methane hydrate dissociation

Abstract

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ¹³C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ¹³C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ¹³C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ¹³C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ¹³C values, and the relationships between diethers and methane δ¹³C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ¹³C values of the ANME source methane predicted from those regression lines and the diether δ¹³C values agreed well with methane δ¹³C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ¹³C value, measured in a mixture with phytane and estimated from the correlation of the δ¹³C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ¹³C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.