Radiocarbon evidence of fossil-carbon cycling in sediments of a nearshore hydrocarbon seep

Abstract

WIDESPREAD seepage of petroleum and natural gas in the southern California continental borderland1–4 provides an opportunity to study the long-term fate and biogeochemical effects of hydrocarbons in nearshore sedimentary environments. The hydrocarbons that enrich seep sediments have been hypothesized to serve as a carbon and energy source for sediment metabolism and in-faunal populations5–9. Here we present 14C natural abundances in sediment total organic carbon (TOC), pore-water dissolved inorganic carbon (DIG) and infauna from in and around a hydrocarbon seep off southern California which were measured to help test this hypothesis. Concentrations of 14C in each pool reflect the admixture of fossil (14C-depleted) seep-derived carbon with carbon from the euphotic zone. The 14C depletion in TOC and DIC increased with proximity to the seepage zone and with sediment depth; 14C abundances differed between meiofauna and macrofauna, suggesting that the two groups incorporate fossil carbon in different ways. The results indicate that fossil carbon can indeed comprise a major component of these carbon pools in nearshore seep sediments.