First biomarker evidence for methane oxidation at cold seeps in the Southeast Atlantic (REGAB pockmark)

Sediment cores from the REGAB pockmark, an active cold seep area in the southeast Atlantic, were analysed for their lipid biomarker distribution and associated stable carbon isotopic composition. Substantial amounts of diagnostic archaeal lipids were found, consisting mainly of archaeol, sn-2 hydroxyarchaeol and crocetane. All archaeal lipids were profoundly depleted in 13C with δ13C values as low as −133‰. Concurrently, abundant monoalkylglycerolethers (MAGE), assigned to sulphate-reducing bacteria, were identified and showed strong 13C-depletions (δ13C between −86‰ and −95‰). The structural and isotopic patterns of these microbial lipids provided compelling evidence for anaerobic oxidation of methane (AOM) occurring in REGAB sediments, mediated by archaea and sulphate reducing bacteria. Lipid fingerprints indicated that anaerobic methanotrophic archaea (ANME-2) and sulphate-reducing bacteria from the Desulfosarcina/Desulfococcus cluster are the dominant AOM assemblages. Depth profiles implied that highest AOM takes place below the upper 2 cm, mainly in the 6–12 cm depth interval. Significant abundances of 13C-depleted diploptene and 4α-methylsterols were found as well, inferring that aerobic methanotrophy occurs in the surface sediment interval. This first biomarker study at the recently investigated cold seeps in the SE Atlantic expand on existing work on AOM settings and add new evidence for aerobic and anaerobic methanotrophic communities occurring in close vicinity.