GREENHOUSE GASES GENERATED FROM THE BIODEGRADATION OF NATURAL SEEP OILS OFFSHORE CALIFORNIA

Natural hydrocarbon seeps are common in the shallow offshore Santa Barbara Channel and are significant sources of greenhouse gases. Recent attention has focused on gas emissions associated with oil near Coal Oil Point, California. A less studied but significant contribution is made by gases generated by oil degradation resulting in impressive asphaltic seeps that occur offshore of Point Conception, California. Here, asphalt mounds as much as 18 m thick cover the seafloor associated with gas seepage over an area estimated at 8.4 km² . The asphalt volume estimate is about 27 million m³, or the volumetric equivalent of 170 million barrels of oil. The largest contiguous area covers about 2 km² and is as much as 10 m thick and occurs in water depths of 50 to 70 m. The mounds are typically 10 to 100 m in diameter with concentric flows of asphalt extruding from the center. Measuring the rates of gas and asphalt emission is still an ongoing technical challenge, however the composition of the gases sampled from a variety of seeps strongly suggests oil in the shallow subsurface has been anaerobic degraded resulting in the generation of carbon dioxide. Methanogenic microbial processes resulting in secondary methane production in turn can consume carbon dioxide. This mineralization of oil can result in the consumption of up to 60 wt% of the original oil with concurrent ¹³C depletion of methane and ¹³C enrichment of carbon dioxide. Analysis of gas emitted from asphaltic accumulations on the seafloor are up to 11% carbon dioxide with carbon isotopes as enriched as +24.0 ppt. Methane concentrations range from less than 1% up to 97% with isotopic compositions of between -34.9 to -66.1 ppt. Higher molecular weight hydrocarbon gases are present in highly varying concentrations reflecting both the thermogenic origin of the oil-associated gas as well as biodegradation. The loss of 60% of the original oil converted by microbes into methane or carbon dioxide results in an estimated release of up to 15 billion kg of methane or 41 billion kg over the current lifetime of seepage. The age of the seeps are likely no older than the Holocene based on geologic relationships. Thus, this area may contribute fluxes on the order of 1.5 and 4.1 million kg per year of methane or carbon dioxide respectively, to the coastal ocean environment.