Periods of warming temperatures during the last ice age triggered the release of methane from beneath the ocean, according to U.S. and French researchers. Once in the atmosphere, the methane would have acted as a heat-trapping greenhouse gas.
"This is a new source of methane which has not been looked at before," said Tessa Hill, now assistant professor of geology at UC Davis and at the university's Bodega Marine Laboratory.
Off the California coast -- and elsewhere around the world -- natural petroleum seeps release oil, tar and gas into the bottom of the ocean. Some methane gas finds its way to the surface, while the tar sinks back to the bottom.
Methane is also generated in marine sediments by bacteria and other organisms. Much of the biological methane remains at the sea floor in a chemically "frozen" form.
During 2002, Hill, then a graduate student at UC Santa Barbara, and colleagues sampled ocean sediments off California from a French government research vessel, the R/V Marion Dufresne.