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

Abstract

AbstractGeological sources of methane (CH4), such as hydrocarbon seeps, are significant yet poorly constrained sources of CH4 to seawater and the overlying atmosphere. We investigate the radiocarbon content (14C) and concentrations of dissolved CH4 in surface waters from the Coal Oil Point seep field to test the hypothesis that geological sources can dominate the regional background signal of CH4. We find that surface waters with elevated CH4 concentration were populated with seep‐CH4 and that lower concentrations of CH4 were well explained by mixing with the regional background of nongeological CH4. Substantial differences in concentration and 14C‐CH4 were observed over distances <5 km, demonstrating that surface currents mix background‐CH4 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 CH4 in the surface layer but is a significant driver of patchiness in oceanic CH4 biogeochemistry.