The subsurface methane maximum in the Southern California Bight

Abstract

The distribution of dissolved methane in the water column of the Santa Monica Basin was investigated on five cruises in the Southern California Bight. The cruises included summer, fall and winter sampling dates. A subsurface methane maximum was usually observed but it was not a consistent feature. Its absolute depth, depth relative to other features and its magnitude all varied on the scale of hours, as well as among seasons. The surface water was always supersaturated with respect to the atmosphere, but the steady state diffusive sea-air flux was small due to the low aqueous concentrations. A single wind event could change the surface concentrations drastically and enhance the air-sea flux by several fold. Methane oxidation rates were negligible in the depth interval of the subsurface concentration maximum, implying that methane is not an active component of the surface ocean carbon cycle. Internal tides and waves are implicated as major factors controlling the methane distribution, due to the apparent semidiurnal periodicity in depth and magnitude of the methane maximum.