Natural Marine Hydrocarbon Seep Source of Dissolved Methane to California Coastal Waters

J F Clark, (Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-9630); L Washburn (ICESS, 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. for Crustal Studies, and Dept. of Geological Sciences, Univ. of Calif., Santa Barbara, CA 93106-1100)

An important regional source of dissolved methane to the southern California Bight is examined. One of the most intense areas of natural marine hydrocarbon seepage is found near Coal Oil Point along the northern margin of the Santa Barbara Channel in these waters. The dispersal and source strength of the injected methane from this field was determined using a variety of oceanographic and geochemical techniques. The results shows that methane is injected into stratified coastal waters creating a plume which extends for at least 12 km. The plume structure is complex because of the large geographical distribution of seep vents and because of the chaotic nature of advection and mixing near to the seeps. The complexity of advection and mixing is clearly illustrated by the results of a sulfur hexafluoride tracer experiment which was performed in conjunction with the methane surveys. The methane was injected onto density surfaces between st = 24.5 - 26.0. Earlier work has shown that subsurface methane maxima in the upper waters of the southern California Bight are typically found on these density surfaces. The total flux of methane into the water column above the Coal Oil Point seeps was 2.1 ± 0.9 x 10^10 g yr-1 and is approximately equal to the total flux of dissolved methane to the atmosphere estimated for the entire southern California Bight. These observations strongly support the inference of others that coastal sources which includes some of the worlds largest marine hydrocarbon seeps maintain the methane maximum observed off shore. Estimates of the global methane flux from coastal waters derived by extrapolating the flux from coastal California may be too large because of the anomalous amount of marine hydrocarbon seepage in these waters.

Clark, Jordan F., Washburn, Libe, Hornafius, J. Scott., Luyendyk, Bruce P. 2000. "Dissolved hydrocarbon flux from natural marine seeps to the southern California Bight" Journal of Geophysical Research, vol. 105, no. C5, pp. 11509-11522