NATURAL OIL SEEPS AND COASTAL TAR IN CALIFORNIA--AN EXTENSION OF THE KAPLAN LEGACY

About 600,000 metric tons of crude oil enters the marine environment worldwide each year through natural seepage. Much of this oil is rapidly assimilated by the environment, but some of the oil impacts the coastal zone as tar residues, often in the form of tar balls. In North America, the principal regions of natural crude oil seepage are the Gulf of Mexico, where the oil-seepage rate is estimated to be about 150,000 metric tons/yr, and the California Borderland, where the rate is estimated to be about 20,000 metric tons/yr. It was 1977 when Ian Kaplan first turned his attention to the chemistry of marine-petroleum (oil and gas) seeps in the California Borderland. As an extension of his early work, we have examined the chemistry of coastal tar residues along the California coast with recent emphasis on the islands of the Santa Barbara Channel at the northern end of the Borderland. The tar residues were fingerprinted using a wide variety of biomarkers and also stable carbon isotopes and then organized into groups based on these parameters. Sterane biomarker parameters were found to be particularly useful discriminants. All coastal residues examined were derived from the Miocene Monterey Formation and likely came from shallow offshore seeps. Geochemical signatures of some tar residues, collected at Point Reyes ~500 km north of the Santa Barbara Channel, correlate with the signatures of tar residues from the Channel Islands, suggesting long distance transport of tar along the California coastline by offshore ocean currents.