The spatial scales, distribution, and intensity of natural marine hydrocarbon seeps near Coal Oil Point, California

Abstract

Natural hydrocarbon seepage from marine environments is an important source of methane and other gases to the atmosphere. Quantifying this flux is necessary for constraining global budgets and understanding local air pollution sources. A field of strong hydrocarbon seepage offshore of Coal Oil Point near Santa Barbara, California produces extensive areas of bursting bubbles at the sea surface. An instrumented buoy was deployed in the Coal Oil Point field to measure directly the atmospheric gas flux from three seeps of varying size and intensity. Spatial scales of continuity of the seeps, quantified by semivariograms, are small, ranging from <1 to about 9 m such that flux values on larger scales are uncorrelated. These de-correlation scales are comparable to the horizontal extents of individual bubble plumes estimated visually at the sea surface. Semivariograms for each seep are used in an ordinary Kriging procedure to interpolate flux measurements onto a regular grid and produce objective maps of the spatial distribution of flux. Spatial integrals yield total flux estimates from the three seeps of ∼7400 m 3 day −1 which amount to 4–13% of the total flux from the Coal Oil Point field based on estimates from previous studies.