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 "whip" found floating in the ocean offshore Point Conception in August 2005.
Natural tar seep offshore Gaviota in approximately 60-m water depth. Photograph by Donna Schroeder, MMS.
