The distribution of methane in the upper waters of the southern California Bight

Profiles of dissolved methane in the upper 300 m of a 190,000 km 2 area of southern Californian waters were determined during November 1989 and March 1990. Methane in near‐surface waters ranged from 1 to 1416 nmol L −1 . The lowest concentrations were at depths >200 m. The highest levels were found along the coast, especially in the region extending from Point Conception to Santa Barbara. Vertical profiles of methane are characterized by maxima in supersaturation within a narrow density band located within the pycnocline; this layer is continuous over a large geographic area. Offshore stations exhibiting elevated levels of methane are spatially coincident with mesoscale eddies of the California Current system. Interpretation of hydrographic and chemical data suggests that CH 4 ‐rich waters are upwelled from relatively shallow areas that are characterized by numerous hydrocarbon seeps. The data are consistent with a nearshore source of methane, presumably submarine seeps. Subsurface maxima in offshore waters are likely maintained by lateral transport of allochthonous CH 4 along density surfaces from coastal waters and autochthonous microbial production.