Anaerobic and aerobic oxidation of methane at Late Cretaceous seeps in the Western Interior Seaway, USA

Abstract

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 methaneseeps was confirmed by this study. delta(13)C values as low as - 50 parts per thousand 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 (13)C-depleted archaeal lipids ( PMI, biphytane; delta(13)C: - 118 and - 102 parts per thousand), derived from anaerobic methanotrophs. A suite of (13)C-depleted bacterial biomarkers ( branched fatty acids; - 73 to - 51 parts per thousand) 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 C(34)/C(35)-8,14-secohexahydrobenzohopanes with low delta(13)C values (- 110 and - 107 parts per thousand) points to the presence of aerobic methanotrophic bacteria at the ancient seep site.