Microbial methane and ethane from gas hydrate nodules of the Makassar Strait, Indonesia

Intact, white nodules of gas hydrate were recovered in piston cores ( 0–6.4 m sediment depth) from the deep water region of the Makassar Strait, between the islands of Borneo and Sulawesi, offshore Indonesia. Consistent with structure I gas hydrate, the dominant hydrocarbon is methane ( 99.96%). The hydrate-bound methane is relatively depleted in 13C (mean δ13C = −70.6‰; mean δD = −189.9‰). Microbial ethane, also present in the hydrate, is more highly depleted in 13C (mean = −52.6‰) than nearly all previously discovered gas hydrates, and is accompanied by trace volumes of microbial ethylene and propylene. Detrital higher-plant material is the likely sedimentary carbon source. The source of microbial methane and ethane appears to be in situ reduction of CO2 by extremophile bacteria adapted to high pressure. The hydrate occurs several hundred meters above the base of the gas hydrate stability zone. Nodular hydrate is associated with seafloor authigenic carbonate rock and chemosynthetic clams characteristic of deep cold vent sites.