Methane oxidation associated with mid-depth methane maxima in the Southern California Bight

Abstract

Methane concentration in the water column of the Southern California Bight exhibits two or more maxima. In the upper water column, a subsurface concentration maximum is often observed, usually near the bottom of the euphotic zone. Deeper maxima are often detected, well below the euphotic zone and separated from the sediments, which suggests an advective source from continental slope sediments. We measured methane concentrations and oxidation rates in an attempt to quantify the biological loss term for methane throughout the water column in the Santa Monica Basin, one of the semi-enclosed basins in the Southern California Borderland. 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 transect of three stations, which was sampled once. Layers of methane rich water were detected in the mid water column (500–800 m depth) at the deeper stations, with concentrations exceeding those found in the subsurface maximum. Oxidation rates in these layers greatly exceeded rates associated with the subsurface maximum and represented turnover times on the order of a few months. The source of the methane at mid-depth is probably advection from petroliferous sediments in the margin of the borderland, rather than diffusion from underlying anoxic sediments in the basin. Relatively rapid oxidation rates and resulting turnover rates at depth imply the presence of a community adapted for the utilization of methane as a carbon source, a community which is not found in surface waters.