Acyclic hydrocarbons and ketones in cold-seep carbonates from central Hokkaido, northern Japan

Limestones associated with fossil chemosynthetic biological communities are found in Tappu Kanajirizawa and Teshionakagawa Abeshinaigawa, central Hokkaido, northern Japan. They were studied to ascertain their lipid distributions and δ13C values of lipid in order to investigate molecular records of archaea and bacteria associated with the anaerobic oxidation of methane (AOM). The limestones contain the tail-to-tail linked irregular isoprenoid hydrocarbons, 2,6,11,15-tetramethylhexadecane (crocetane), and its C25-homologue 2,6,10,15,19-pentamethylicosane (PMI). Furthermore, C30-homologue 2,6,10,15,19,23-hexamethyltetracosane (squalane) was detected. The abundance of these compounds indicates a pronounced role of particular archaea in the cold-seep. The δ13C values of PMI and crocetane were depleted, ranging from -129‰ to -116‰DB. New biomarkers found were C13 and C18 isoprenoid ketones, which could potentially be from anaerobic methanotrophic archaea. Small amounts of C14 and C19 isoprenoid ketones were also detected. The C13, C14 and C18 isoprenoid ketones also had extremely low δ13C values ranging from -115‰ to -104‰. δ13C values suggest that the origin of the isoprenoid ketones are lipids of anaerobic methanotrophioc archaea, but the diagenetic pathway leading to them is uncertain. This study shows that the limestones are cold seep carbonates and that isoprenoid ketones are potentially useful markers for anaerobic methanotrophic archaea.