Re-assessment of methane cycle dynamics from the preindustrial to present-day 

Ice core reconstructions of atmospheric methane (CH4) and its stable carbon isotope ratio (δ13CH4) provide important constraints for understanding the links between human activity, methane and climate. However, uncertainties in existing δ13CH4 records since the preindustrial (~1850 CE), reconstructed from measurements of polar firn air and a small number of high-accumulation ice core sites, limit the precise determination of the timing and rate of recent changes in source/sink evolution. To re-assess methane dynamics over the last two centuries, we present continuous multi-core records of atmospheric CH4 and carbon monoxide (CO) between 1824 and 1994 CE reconstructed from high snow accumulation Antarctic sites and supplement these data with new bubble ice measurements of δ13CH4 spanning 50-years from 1938 to 1988 CE at a < 5-year resolution. Across the 50-year record, atmospheric CH4 mixing ratios increase by > 580 ppb and each δ13CH4 measurement therefore requires a considerable correction for diffusive fractionation resulting from a sustained growth in the overlying atmospheric methane burden during firn transport. An overlap with direct atmospheric observations is used to validate corrections for this phenomenon. Source/sink dynamics necessary to drive the simultaneous temporal trends observed in CH4, CO and δ13CH4 since 1850 CE are then inferred using a 6-troposphere, multi-tracer box model. Isotopic corrections, their implications and subsequent modelling results will be discussed.