What drove the methane cycle in the past - evidence from carbon isotopic data of methane enclosed in polar ice cores

Abstract

During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. This thesis assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition d13C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. Major findings are that variations in the d13C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO2 concentrations. It is interpreted to reflect a climatic and CO2-related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable d13C values occurred during intervals of large changes in the atmospheric loading of methane. The findings suggest that most methane sources - most notably tropical wetlands - must have responded simultaneously to climate changes across these periods.