Changes in the Stable Carbon Isotope Composition of Methane at the end of the Younger Dryas

The abrupt warming at the end of the Younger Dryas is associated with a significant rise in the atmospheric concentration of methane. A high-resolution record of δ ¹³C of atmospheric methane covering this time interval was measured using samples from the ice margin at Pakitsoq, Western Greenland. Our CF-IRMS measurements show a prominent shift of almost 2 ‰ to heavier (¹³C-enriched) values accompanying the concentration increase. The methane carbon isotope ratios then abruptly return to the initial values present prior to the concentration increase. This pattern suggests that the rising methane concentration was caused by a short perturbation to the system, rather than a gradual reorganization between two states. The most likely explanation for the observed carbon isotope shift is a temporary, enhanced input of methane from an isotopically heavier source. The source remains unclear, but possible candidates include increased emissions from wildfires, thermogenic reservoirs, wetlands or permafrost soils, etc. Secondary processes such as microbial methane oxidation may also contribute to the ¹³C-enriched methane. Our initial results point away from marine, microbial methane sources.