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

Abstract

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ∼200 m 2 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 CH 4 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% CO 2 , which should act to dissolve carbonates. The average δ 13 C of the carbonates was −29.2±2.8‰ VPDB, compared to a range of −1.0 to +7.8‰ for CO 2 in the seep gas, indicating that CO 2 from the seep gas is quantitatively not as important as 13 Cdepleted bicarbonate derived from methane oxidation. Methane, with a δ 13 C of approximately −43‰, is oxidized and the resulting inorganic carbon precipitates as highmagnesium calcite and other carbonate minerals. This finding is supported by 13 C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ 13 C ranged from −84 to −25‰). The inconsistency in δ 13 C between the carbonates and the seeping CO 2 was resolved by discovering pockets of gas trapped near the base of the sediment column with δ 13 C-CO 2 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.

Methane oxidation in permeable sediments at hydrocarbon seeps in the Santa Barbara Channel, California

“…The overall low methane concentrations observed in the sediments surrounding the vents would be explained if only a minor fraction of the gas venting through the sandy seabed actually dissolves into the adjacent porewater. This hypothesis is supported by the relatively high methane content (91% Kinnaman et al, 2010) of gas entering the water column. If major parts of methane would dissolve into the porewater during its movement through the sediment, we would expect a stronger replacement of methane in the free gas phase by other components such as N 2 , CO 2 , or H 2 S, (McGinnis et al, 2006).…”

“…Samples were taken in April, June and November of 2006. In April, a horizontal transect was sampled at 0, 20, 50, 100, 200, and 400 cm distances from a gas vent, which showed a continuous release of gas visible as streams of bubbles (methane content ∼91%, Kinnaman et al, 2010). At the 400 cm distance, a new gas bubble stream emerged from the sediment, when core liners were forced into the sediment.…”

“…The present study focused on methanotrophic activity and biogeochemical processes in the sediments of a study area known as "Brian Seep", which is located off the coast of Coal Oil Point at water depths around 10 m (Kinnaman et al, 2010;Kinnaman et al, 2007). In this coastal area, sediments consist of permeable sand (permeability ∼10 −11 m 2 , see results).…”

Bubble Transport Mechanism: Indications for a gas bubble-mediated inoculation of benthic methanotrophs into the water column

“…Using biomolecular methods and biogeochemical techniques, the process of microbial anaerobic and aerobic CH 4 oxidation has been identified clearly in sediments and in the overlying water column of the Coal Oil Point seep field (e.g., Kinnaman et al, 2010;Mau et al, 2012;Treude and Ziebis, 2010). Seep gas composition varies between different regions of the Coal Oil Point seep field.…”

“…Seep gas composition varies between different regions of the Coal Oil Point seep field. At the Brian Seep site (10 m water depth, 34° 24.109' N; 119° 49.917' W) captured gas consisted of 91% methane, 7% CO 2 , 1% ethane, and 0.2% propane (Kinnaman et al, 2010). Fig.…”