Anaerobic and Aerobic Oxidation of Methane at Late Cretaceous Seeps in the Western Interior Seaway, USA

The Late Cretaceous (Campanian) Tepee Buttes represent a series of conical, fossiliferous limestone deposits embedded in marine shales that deposited in the Western Interior Seaway. The previously suggested origin of the Tepee Buttes at methane-seeps was confirmed by this study. δ13C values as low as −50‰ of early diagenetic carbonate phases of two Tepee Buttes near Pueblo (Colorado) reveal that methane was the major carbon source. Molecular fossils released from a methane-seep limestone contain abundant 13C-depleted archaeal lipids (PMI, biphytane; δ 13C: −118 and −102‰), derived from anaerobic methanotrophs. A suite of 13C-depleted bacterial biomarkers (branched fatty acids; −73 to −51‰) reflects the former presence of sulfate-reducing bacteria, corroborating that a syntrophic consortium of archaea and bacteria mediating anaerobic oxidation of methane already existed in Cretaceous times. Molecular fossils also suggest that methane was not exclusively oxidized in an anaerobic process. A series of unusual C34/C35-8,14-secohexahydrobenzohopanes with low δ13C values (−110 and −107‰) points to the presence of aerobic methanotrophic bacteria at the ancient seep site.