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 CO2 with secondary CH4 production. Biomineralization can result in the consumption of as much as 60{%} by weight of the original oil, with C-13 enrichment of CO2. Analyses of gas emitted from asphaltic accumulations or seeps on the seafloor indicate up to 11{%} CO2 with C-13 enrichment reaching +24.8aEuro degrees. Methane concentrations range from less than 30{%} up to 98{%} with isotopic compositions of -34.9 to -66.1aEuro degrees. 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 C-13 isotopic composition as enriched as -19.5aEuro degrees. Assuming the 132 million barrels of asphaltic residues on the seafloor represent similar to 40{%} of the original oil volume and mass, the estimated gas generated is 5.0x10(10) kg (similar to 76x10(9) m(3)) CH4 and/or 1.4x10(11) kg CO2 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 similar to 1,000 years, annual fluxes are on the order of 5.0x10(7) kg (similar to 76x10(6) m(3)) and/or 1.4x10(8) kg for CH4 and CO2, respectively. The daily volumetric emission rate (2.1x10(5) m(3)) is comparable to current CH4 emission from Coal Oil Point seeps (1.5x10(5) m(3)/day), and may be a significant source of both CH4 and CO2 to the atmosphere provided that the gas can be transported through the water column.