Accumulation and release of methane from clathrates below the Laurentide and Cordilleran ice sheets

Ice-age cycles are associated with large fluctuations in the concentration of atmospheric methane. These fluctuations are commonly attributed to changes in wetlands, although clathrates have also been proposed as a potential source. We examine the possibility that methane clathrates accumulate below continental ice sheets during an ice age. The source of methane is due to microbial decomposition of organic material below the ice sheet. Methane is stored in clathrate when the pressure and temperature conditions permit thermodynamic stability. Deglaciation releases methane from clathrate into the atmosphere. We use a numerical model for the Laurentide–Cordilleran ice sheet [Marshall, S.J., Tarasov, L., Clarke, G.K.C., Peltier, W.R., 2000. Glaciological reconstruction of the Laurentide ice sheet: physical processes and modeling challenges, Can. J. Earth Sci. 37, 769–793.] to assess the aerial extent, thickness, and the thermal conditions at the base of the ice sheet as a function of time. Both low and high inventories of the organic carbon below the ice sheet are considered, based on soil carbon estimates for tundra and for the present potential vegetation. We model the spatial distribution of clathrate as the ice sheet grows and quantify the amplitude and timing of methane releases as the ice sheet retreats. The predicted fluctuations in atmospheric methane are 80–200 ppbv, which are comparable to fluctuations recorded in ice cores from Greenland and Antarctica. However, clathrates cannot explain the entire atmospheric methane record because there is insufficient methane in clathrate to sustain the elevated atmospheric concentration for more than 1 kyr.