This research will develop a record of the isotopic composition of atmospheric methane from the Greenland Ice Sheet Project Two (GISP II) ice core for time intervals implicated in the "clathrate gun hypothesis" over the past 40,000 years spanning the deglaciation sequence, the Bolling warm period, and the Younger Dryas cool phase.
The "clathrate gun hypothesis" suggests that the primary factor governing abrupt methane increases in the atmosphere is the destabilization of marine clathrate in ocean sediments as opposed to increased methane emissions from terrestrial wetlands. Evidence from the geologic records suggest that slight ocean temperature increases or oceanic slumping events (caused by instabilities in the continental shelf) may have caused catastrophic destabilization of clathrate deposits in the past and a consequent release of methane into the atmosphere. For example, throughout the last 40,000 years, a record of clathrate destabilization events in the Santa Barbara basin off the California coast closely resembles the record of abrupt increases in atmospheric methane recorded in ice cores.
The strong similarity between these two records has prompted scientists to consider the origin of the abrupt methane increases in terms of their impact on climate. Because methane is a potent greenhouse gas, clathrate destabilization events may have contributed to abrupt global temperature increases in the past. This is an important idea because it identifies a previously unknown mechanism for abrupt warming of the atmosphere over geologic time.
Along many continental margins and deep below the Arctic permafrost, methanogenic bacteria produce large quantities of methane as a metabolic byproduct. The liberated methane diffuses up towards the surface until it reaches a critical temperature and pressure where the gas is concentrated into an icy substance know as a clathrate or gas hydrate. Clathrate deposits are very susceptible to small changes in environmental pressure and temperature in that higher temperatures and lower pressures tend to melt the clathrate deposits leading to the release of methane.
Fortuitously, marine clathrate methane is isotopically distinct from terrestrially derived methane from wetlands, so it should be possible to determine whether or not the cause of rapid methane concentration in the atmosphere was due to degassing from marine clathrates.
