Anomalous chlorofluorocarbons in the Southern California Borderland Basins

Abstract

During the past two decades, unexpectedly high concentrations of chlorofluorocarbons (CFCs) have been observed in the bottom waters of the Southern California Borderland Basins (SCBB), with relatively constant spatial distribution patterns. In contrast to offshore waters in this region, CFC concentrations below the oxygen minimum layer (OML) in the deep SCBB increase with depth. The uniformity of the bottom‐enhanced CFC signals and the near‐zero levels of tritium suggest that this feature is likely maintained by release of CFCs from sediments and vertical mixing, and not by dumped CFC‐bearing materials or an intrusion of recently ventilated waters. We hypothesize that CFC scavenging processes, either on particulate organic matter or hydrocarbon residues from the adjacent natural seeps, occur in these high‐productivity near‐surface coastal waters. The subsequent release of CFCs at the bottom boundary layer during the degradation of particulate material may cause the anomalous CFC distributions in the SCBB.