University of California Energy Institute

The relationship between surface bubble composition and gas flux to the atmosphere was examined at Coal Oil Point seep field, which is located about 3 km offshore of Santa Barbara County, CA in the Santa Barbara Channel. The field research was conducted using a spar buoy designed to simultaneously measure the surface gas flux, the buoy’s position with differential GPS, and collect gas samples. Results show that the gas composition varies by 10-20% at sub-seeps within seep areas. The nitrogen mole fraction correlated directly with oxygen mole fraction (R2 = 0.94) and inversely with methane mole fraction (R2 = 0.97). These data demonstrate that the bubble composition is modified by gas exchange during ascent from the seafloor: dissolved air enters and hydrocarbon gases leave the bubbles. While compositional differences were observed at sub-seeps, there was no relationship between flux and composition. Factors other than seep intensity controls the amount of gas transfer between the ocean water and bubbles. Therefore, when calculating the atmospheric source function of specific gases such as methane or ROGs from marine seeps, it is best to use mean compositional values determined for bubbles collected near the sea surface.