Asphalt volcanism off the coast of southern California may have caused the high levels of methane in ocean waters recorded several thousand years ago, suggests a paper published online this week in Nature Geoscience. Unlike other submarine volcanoes, that emit lava, asphalt volcanoes release degraded petroleum, along with substantial amounts of methane.

David Valentine and colleagues discovered seven extinct submarine asphalt volcanoes offshore from Santa Barbara, California. Using deep submergence and automated underwater vehicles, they studied the geometry and composition of the mounds. Sampling revealed that the mounds were composed of asphalt, sourced from underground stores, which erupted onto the ocean floor between 44,000 and 31,000 years ago. The volumes of oil and gas emitted during the eruptions accounts for the previously unexplained elevated levels of methane and tar thought to be present in Santa Barbara's coastal waters at that time.