Oceanic methane seepage during warming

Atmospheric levels of methane, a potent greenhouse gas, vary greatly in the climate record and are usually thought to be due to emissions from continental wetlands. However, Tessa Hill et al. report that methane was released into the atmosphere from the oceans during the period of global warming associated with the most recent deglaciation. The results suggest that, at times, much more methane was released from ocean sediments than previously thought, and the ocean may be a significant contributor to changing atmospheric methane concentrations. The authors studied tar concentrations in ocean sediment cores off the California coast. While oil persistently seeps from ocean f loor cracks, lighter hydrocarbons, such as methane, evaporate, leaving behind tar preserved in the ocean sediments. Hill et al. dated the sediments and used tar as a proxy to determine how much methane seeped from the ocean f loor. Methane seepage was found to have peaked during the two most dramatic phases of warming, 16,000–14,000 and 11,000–10,000 years ago, when the earth was changing from a glacial to an interglacial state. Hill et al. say that methane from such natural seeps may increase positive climate feedback during global warming episodes. — P.D.