Enrichment in 13C of atmospheric CH4 during the Younger Dryas termination

The abrupt warming across the Younger Dryas termination (similar to 11 600 yr before present) was marked by a large increase in the global atmospheric methane mixing ratio. The debate over sources responsible for the rise in methane centers on the roles of global wetlands, marine gas hydrates, and thermokarst lakes. We present a new, higher-precision methane stable carbon isotope ratio (delta(CH4)-C-13) dataset from ice sampled at Pakitsoq, Greenland that shows distinct C-13-enrichment associated with this rise. We investigate the validity of this finding in face of known anomalous methane concentrations that occur at Pakitsoq. Comparison with previously published datasets to determine the robustness of our results indicates a similar trend in ice from both an Antarctic ice core and previously published Pakitsoq data measured using four different extraction and analytical techniques. The delta(CH4)-C-13 trend suggests that C-13-enriched CH4 sources played an important role in the concentration increase. In a first attempt at quantifying the various contributions from our data, we apply a methane triple mass balance of stable carbon and hydrogen isotope ratios and radiocarbon. The mass balance results suggest biomass burning (42-66% of total methane flux increase) and thermokarst lakes (27-59%) as the dominant contributing sources. Given the high uncertainty and low temporal resolution of the (CH4)-C-14 dataset used in the triple mass balance, we also performed a mass balance test using just delta C-13 and delta D. These results further support biomass burning as a dominant source, but do not allow distinguishing of thermokarst lake contributions from boreal wetlands, aerobic plant methane, or termites. Our results in both mass balance tests do not suggest as large a role for tropical wetlands or marine gas hydrates as commonly proposed.