Modification of local environment by natural marine hydrocarbon seeps

Abstract

The bubble gas partial pressures, dissolved gas and oil, and fluid motions within the rising bubble streams of three shallow (< 70 m) natural hydrocarbon seeps in the Santa Barbara Channel were measured. Aqueous methane concentrations near the surface in the bubble plumes were substantially greater (> 10 8 times) than atmospheric equilibrium values: at two of the seeps it was slightly supersaturated with respect to the bubble's partial pressure. This indicates that the rate limiting step for methane. CH 4 transfer into the water column was not bubble gas transfer, but rather turbulence transfer of water out of the saturated bubble stream to the bulk ocean. Strong upwelling flows were observed, as well as elevated fluorescence indicative of dissolved oils. At the bottom, bubbles are circa 90% CH 4, but at the surface they were circa 60% CH 4, 30% air, and 10% higher hydrocarbons.