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Tar Balls from Southern California Seeps Appear on Central California Beaches
When tar balls appeared on California beaches south of San Francisco in late January 2008, beachgoers wondered whether the sticky black globs were residues of oil spilled nearly 3 months earlier by the container ship Cosco Busan in San Francisco Bay. On November 7, 2007, the Cosco Busan struck the San Francisco-Oakland Bay Bridge in heavy fog, tearing a 100-ft-long gash in the port side of the ship that punctured one ballast tank and two fuel tanks. Within 10 to 15 seconds, an estimated 58,000 gallons of oil (about the volume of two backyard swimming pools) spilled into the bay. In response to that spill, a Unified Command composed of the U.S. Coast Guard, the California Department of Fish and Game (CDFG), and a contractor hired to represent the ship's owner was established to coordinate and manage cleanup operations. Upon the appearance of tar balls on Pacific coast beaches on January 28, 2008, the Unified Command responded quickly, mobilizing more than 75 personnel to clean the affected shoreline over a 3-day period. At the same time, the Coast Guard collected samples of the tar balls for chemical analysis by the CDFG's Office of Spill Prevention and Response (OSPR).
The chemical analysis by OSPR showed that the tar balls were not residues of the Cosco Busan spill but had a natural origin in the Miocene Monterey Formation, an oil-bearing rock that is the source of many natural oil and tar seeps along the California coast, as well as much of the oil produced by California's onshore and offshore oil wells. This result was confirmed by geochemists from the U.S. Geological Survey (USGS), who have been "fingerprinting" tars and oils from natural seeps, offshore oil and gas platforms, and California shorelines for more than 10 years. Their studies—conducted in cooperation with the Minerals Management Service (MMS)— have shown that virtually all the tar balls that wash up on the California coast come from natural seeps of oil and tar derived from the Monterey Formation. Natural seeps occur both onshore (the La Brea Tar Pits are a famous example) and offshore. Most of the known sea-floor seeps are in the Santa Barbara Channel in southern California, where tar balls (to the surprise of unsuspecting tourists) are common year-round on beaches nearest the seeps.
Tar balls that appear on central California shores during the winter months mostly originate in southern California seeps, as evidenced by their chemical fingerprints. These tar balls are believed to be carried northward by the Davidson Current, which periodically flows northward along the California coast, often aided by winter storms that bring southwesterly winds to the region. Unusually large numbers of tar balls sometimes appear on central California beaches after a series of storms, as occurred in January 2008 and a year earlier, in February 2007 (see "Tar Balls Washed onto Central California Beaches by Storms" in Sound Waves, May 2007.
Although natural seeps have long been a part of the California landscape, the appearance of tar on beaches where it is not commonly seen arouses much curiosity. USGS research geologist Tom Lorenson, who leads a cooperative USGS-MMS effort to chemically fingerprint tar and oil seeps along the southern California coast, fielded several inquiries about the likely origin of the tar balls that appeared in late January. He was interviewed by newspaper reporters from the San Francisco Examiner and the TriValley Herald, as well as a television reporter from the San Francisco NBC affiliate (NBC 11). On January 31 and February 1, Jackson Currie of the USGS Pacific Science Center in Santa Cruz, California, and Leticia Diaz, a work-study student from nearby Cabrillo College, collected tar-ball samples from affected beaches for analysis in the USGS organic geochemistry laboratory in Menlo Park, California; resulting data will become part of the natural-oil-seep fingerprint library being compiled in cooperation with MMS. Having determined that the tar balls were a natural phenomenon and that they were weathered and posed no significant threat to the environment, the Unified Command suspended cleanup operations and sent a fact sheet to agencies in the affected region, offering the following tips:
For additional information, visit these USGS Web sites: "Offshore Hydrocarbon Seeps in Southern California: A U.S. Geological Survey—Minerals Management Service Cooperative Project," and "Natural Oil and Gas Seeps in California."
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in this issue:
Tar Balls Appear on California Beaches
USGS Participates in Judging Science Fair
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U. S. Department of the Interior | U.S. Geological Survey
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