Greenhouse gases generated from the anaerobic biodegradation of natural offshore asphalt seepages in southern California

Significant offshore asphaltic deposits with active seepage occur in the Santa Barbara Channel offshore southern California. The composition and isotopic signatures of gases sampled from the oil and gas seeps reveal that the coexisting oil in the shallow subsurface is anaerobically biodegraded, generating CO 2 with secondary CH 4 production. Biomineralization can result in the consumption of as much as 60% by weight of the original oil, with 13 C enrichment of CO 2 . Analyses of gas emitted from asphaltic accumulations or seeps on the seafloor indicate up to 11% CO 2 with 13 C enrichment reaching +24.8‰. Methane concentrations range from less than 30% up to 98% with isotopic compositions of –34.9 to –66.1‰. Higher molecular weight hydrocarbon gases are present in strongly varying concentrations reflecting both oil-associated gas and biodegradation; propane is preferentially biodegraded, resulting in an enriched 13 C isotopic composition as enriched as –19.5‰. Assuming the 132 million barrels of asphaltic residues on the seafloor represent ~40% of the original oil volume and mass, the estimated gas generated is 5.0×10 10 kg (~76×10 9 m 3 ) CH 4 and/or 1.4×10 11 kg CO 2 over the lifetime of seepage needed to produce the volume of these deposits. Geologic relationships and oil weathering inferences suggest the deposits are of early Holocene age or even younger. Assuming an age of ~1,000 years, annual fluxes are on the order of 5.0×10 7 kg (~76×10 6 m 3 ) and/or 1.4×10 8 kg for CH 4 and CO 2 , respectively. The daily volumetric emission rate (2.1×10 5 m 3 ) is comparable to current CH 4 emission from Coal Oil Point seeps (1.5×10 5 m 3 /day), and may be a significant source of both CH 4 and CO 2 to the atmosphere provided that the gas can be transported through the water column.