SOURCES OF ATMOSPHERIC CH4 IN EARLY POSTGLACIAL TIME

At the terminations that marked the ends of the Oldest and Younger Dryas, at the close of the last major glaciation, atmospheric methane levels rose sharply. Biological sources may have played a part in this increase, but it is unlikely that they could have accounted for the suddenness of the rise in methane production. Some fossil methane stores, such as marine and continental hydrates, may have become unstable at the time of the terminations, and there is much geological evidence that many catastrophic releases of methane occurred. It is possible that part of the increase in global atmospheric methane may have been the result of these releases, which may also have played a partial role in recharging the inventory of carbon in the atmosphere-biosphere system. The Younger Dryas cooling may represent a transient response to a later reduction in greenhouse forcing when the CH4 mixing ratio in the atmosphere dropped. This transient response would have been more marked in the northern hemisphere than in the southern. The termination of the Younger Dryas may have occurred after a further catastrophic release of fossil methane.