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

Geo-Marine Letters

By:
, , ,
DOI: 10.1007/s00367-014-0359-1

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Abstract

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 13C enrichment of CO2. Analyses of gas emitted from asphaltic accumulations or seeps on the seafloor indicate up to 11% CO2 with 13C 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 13C 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 5.0×1010 kg (~76×109 m3) CH4 and/or 1.4×1011 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 ~1,000 years, annual fluxes are on the order of 5.0×107 kg (~76×106 m3) and/or 1.4×108 kg for CH4 and CO2, respectively. The daily volumetric emission rate (2.1×105 m3) is comparable to current CH4 emission from Coal Oil Point seeps (1.5×105 m3/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.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Greenhouse gases generated from the anaerobic biodegradation of natural offshore asphalt seepages in southern California
Series title:
Geo-Marine Letters
DOI:
10.1007/s00367-014-0359-1
Volume
34
Issue:
2-3
Year Published:
2014
Language:
English
Publisher:
Springer
Contributing office(s):
Pacific Coastal and Marine Science Center
Description:
15 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geo-Marine Letters
First page:
281
Last page:
295
Number of Pages:
15
Country:
United States
State:
California
Other Geospatial:
Santa Barbara Channel