Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas
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Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

  • Author(s): Kinnaman, Franklin S.
  • Kimball, Justine B.
  • Busso, Luis
  • Birgel, Daniel
  • Ding, Haibing
  • Hinrichs, Kai-Uwe
  • Valentine, David L.
  • et al.
Abstract

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ∼200 m2 area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ∼450 moles of CH4 per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO2, which should act to dissolve carbonates. The average δ13C of the carbonates was −29.2 ± 2.8‰ VPDB, compared to a range of −1.0 to +7.8‰ for CO2 in the seep gas, indicating that CO2 from the seep gas is quantitatively not as important as 13C-depleted bicarbonate derived from methane oxidation. Methane, with a δ13C of approximately −43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by 13C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ13C ranged from −84 to −25‰). The inconsistency in δ13C between the carbonates and the seeping CO2 was resolved by discovering pockets of gas trapped near the base of the sediment column with δ13C-CO2 values ranging from −26.9 to −11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation. Figure Processes modulating carbonate formation at Brian Seep (California) during times of high and low sediment burden

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