minor incident, page-15

TAR BAlls

Tar Balls Washed Onto Central California Beaches by Storms
By Fran Hostettler
May 2007 in this issue:
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Above: Map of California, showing some of the areas where USGS researchers have collected tar balls for geochemical analysis. [larger version]


Above: This tar ball, collected at Moss Landing State Beach on February 14, 2007, by volunteers John Hostettler and Bob Seese, may have originated from a seep in the offshore Santa Maria basin west of Casmalia (see map). [larger version]


Above: Tar ball collected at Asilomar State Beach on February 14, 2007, by volunteers John Hostettler and Bob Seese. This sample may have originated from a seep in the offshore Santa Maria basin, but its chemistry differs somewhat from that of the Moss Landing State Beach sample and links it to "wander-prone" samples the USGS group collected from 1997 to 2003, from Surf Beach near Casmalia north to Drakes Beach, Point Reyes, and Angel Island in San Francisco Bay. [larger version]
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In February 2007, unusually large numbers of mystery tar balls washed up on beaches in central California, from Monterey Bay north to Half Moon Bay and San Francisco. Calls came in to State officials asking where these sticky globs of tar might have come from and whether they posed a threat to wildlife or the affected beaches. The California Department of Fish and Game (CDFG) was asked to analyze samples of the tar balls to determine whether they were of natural origin or possibly from an oil spill from a passing tanker. CDFG concluded that the tar balls most probably originated from natural offshore seeps and were disbursed by recent storms.

Geochemists at the U.S. Geological Survey (USGS) in Menlo Park, California, received several telephone calls from local reporters asking whether they had any additional information about the origin of the mystery tar balls. In conjunction with the U.S. Minerals Management Service (MMS), the USGS has conducted a 10-year study, initiated by Keith Kvenvolden and now supervised by Tom Lorenson, on tar and oil seeps along the southern California coastline. (To learn about some of this work, read Sound Waves article, "Beginning the Search for Offshore Oil Seeps Near Point Conception, California.") The USGS research group was asked by MMS to collect and analyze samples from the recently tarred beaches in central California to determine whether the new tar balls fit into the group's database.

Fran Hostettler and Bob Rosenbauer collected tar residues from Moss Landing, Asilomar, and Half Moon Bay beaches. Chemical analysis by Hostettler and chemometric mathematical matching by Ken Peters indicated that the mystery tar balls were indeed from natural sources offshore California. The tar balls likely originated in the Santa Barbara Channel or the offshore Santa Maria basin. Analyses showed that although these recent tar samples are similar to each other—that is, all are from the Miocene Monterey Formation—they are not all identical but rather came from several different seeps. Therefore, the USGS scientists could exclude a single-seep source or a human-caused spill. Apparently, winds associated with the large storm systems that had recently swept through the area helped blow the tar balls to shore. Ocean currents like the Davison Current from southern California are known to flow northward in the winter and so are a natural transport system for the floating tar balls. USGS work in past years identified tar balls washed up on beaches from the Monterey Bay National Marine Sanctuary northward to Point Reyes as originating from the same southern California seeps. No natural seeps are known to occur within or north of the Monterey Bay National Marine Sanctuary, and so when tar shows up on central or northern California beaches, it is suspect in terms of possible human-caused oil spills. However, the tar balls that washed up in February appear to be attributable entirely to natural processes.