Geochemical evidence of rapid hydrocarbon venting from a seafloor-piercing mud diapir, Gulf of Mexico continental shelf

A research submersible was employed to collect sediments from a previously undescribed diapiric mud mound on the Gulf of Mexico continental shelf. The sediments contain high concentrations of C1–C6+ hydrocarbon gases and crude oil. The mud mound hydrocarbons are relatively unaffected by biodegradation, in contrast to the heavily biodegraded hydrocarbons that characterize the sediments of some other Gulf of Mexico seep sites, including those colonized by chemosynthetic communities. The molecular and isotopic properties of the gas and oil suggest rapid hydrocarbon transport from the mound sediments to the water column. The mud mound is an episodic point source of an oil slick on the sea surface. Gas venting is observed on the seafloor, and bubble trains recorded close to the sea surface suggest that greenhouse thermogenic gases (mainly methane) may escape to the atmosphere. Improved understanding of the fate of C1–C6+ gases and crude oil in shallow marine sediments will contribute to better assessment of the impact of seep hydrocarbons on the global inventory of atmospheric sources.