Radiocarbon in Marine Methane Reveals Patchy Impact of Seeps on Surface Waters

Geological sources of methane (CH 4 ), such as hydrocarbon seeps, are significant yet poorly constrained sources of CH 4 to seawater and the overlying atmosphere . We investigate the radiocarbon content ( 14 C) and concentrations of dissolved CH 4 in surface waters from the Coal Oil Point seep field to test the hypothesis that geological sources can dominate the regional background signal of CH 4 . We find that surface waters with elevated CH 4 concentration were populated with seep‐CH 4 and that lower concentrations of CH 4 were well explained by mixing with the regional background of nongeological CH 4 . Substantial differences in concentration and 14 C‐CH 4 were observed over distances <5 km, demonstrating that surface currents mix background‐CH 4 into the seep field. These results indicate that even a prolific seep region like the Santa Barbara Basin exerts limited influence on the regional background of CH 4 in the surface layer but is a significant driver of patchiness in oceanic CH 4 biogeochemistry.