METHANE OXIDATION ASSOCIATED WITH MID-DEPTH METHANE MAXIMA IN THE SOUTHERN CALIFORNIA BIGHT

Methane concentration in the water column of the Southern California B ight exhibits two or more maxima. In the upper water column, a subsurf ace concentration maximum is often observed, usually near the bottom o f the euphotic zone. Deeper maxima are often detected, well below the euphotic zone and separated from the sediments, which suggests an adve ctive source from continental slope sediments. We measured methane con centrations and oxidation rates in an attempt to quantify the biologic al loss tem for methane throughout the water column in the Santa Monic a Basin, one of the semi-enclosed basins in the Southern California Bo rderland. The study site was in the central basin at a water depth of 900 m, which was sampled several times over 4 years, and an offshore t ransect of three stations, which was sampled once. Layers of methane r ich water were detected in the mid water column (500-800 m depth) at t he deeper stations, with concentrations exceeding those found in the s ubsurface maximum. Oxidation rates in these layers greatly exceeded ra tes associated with the subsurface maximum and represented turnover ti mes on the order of a few months. The source of the methane at mid-dep th is probably advection from petroliferous sediments in the margin of the borderland, rather than diffusion from underlying anoxic sediment s in the basin. Relatively rapid oxidation rates and resulting turnove r rates at depth imply the presence of a community adapted for the uti lization of methane as a carbon source, a community which is not found in surface waters.