of Hydrocarbon Gas Plumes from Seafloor
Seeps in the Northern Santa Barbara Channel, CA
L Washburn, (Inst. Computational Earth System Science, and Dept. of Geography, Univ. of Calif., Santa Barbara, CA 93106-4060) J S Hornafius, (Inst. for Crustal Studies, Univ. of Calif., Santa Barbara, CA 93106-1100); B P Luyendyk, (Inst. Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-1100); J F Clark, (Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-9630); D Quigley, (Inst. Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-1100); R D Francis, (Dept. of Geology, Calif. State Univ., Long Beach CA 90840-3902)
Prolific natural hydrocarbon seepage occurs offshore from Coal Oil Point, near Santa Barbara, CA. Most of the seepage occurs in the form of gas bubble plumes which rise through the water column and vent into the atmosphere. The gas bubbles contain 90% methane and 10% non-methane hydrocarbons. We estimate a total gas flux of 1.4x10^5 m3/day. During gas bubble rise, the hydrocarbons partially dissolve in ambient seawater to form neutrally buoyant dissolved hydrocarbon plumes. Oil contained in the rising bubbles is left on the sea surface as slicks. The gas seeps emit 4.5 metric tons per day of reactive organic gases (nonmethane hydrocarbons) and are a significant air pollution source for Santa Barbara County. The locations and relative flux of seep gases are determined using 3.5 and 50 kHz sonar surveys controlled by differential GPS. In situ gas chromatograph observations are used to map the dissolved hydrocarbon plume which extends several km down-current from the vents. Oceanographic conditions are determined from high resolution CTD surveys, and moored vector-measuring and acoustic Doppler current meters. Preliminary analysis indicates that 1) the dissolved hydrocarbon concentration is proportional to the gas seepage rate and inversely proportional to current velocity, 2) the seep flux has decreased ca. 80% over 22 years within one km of oil platform Holly, possibly correlating with a decrease in subsurface reservoir pressure due to oil production, 3) aqueous CO2 and H2 anomalies 2x and 100x above background, respectively, are associated with the dissolved hydrocarbon plumes and are interpreted to be due to oxidation of hydrocarbons by bacteria in the ocean, 4) He-4 anomalies with concentrations 500x ambient levels, but associated with low dissolved methane content, are interpreted to be due to submarine water vents associated with the nearshore oil seeps. Future analysis will focus on determining sub-seafloor structure, understanding the source(s) of the He-4 anomalies, improving seepage rate estimates, and determining the evolution of the hydrocarbon plumes in the water column. We are also establishing a baseline for long-term studies of variations in seep chemistry, intensity, and location.