Carbon isotope ratios suggest no additional methane from boreal wetlands during the rapid Greenland Interstadial 21.2

Samples from two Greenland ice cores (NEEM and NGRIP) have been measured for methane carbon isotope ratios ( δ 13 C‐CH 4 ) to investigate the CH 4 mixing ratio anomaly during Greenland Interstadial (GI) 21.2 (85,000 years before present). This extraordinarily rapid event occurred within 150 years, comprising a CH 4 mixing ratio pulse of 150 ppb (∼25%). Our new measurements disclose a concomitant shift in δ 13 C‐CH 4 of 1‰. Keeling plot analyses reveal the δ 13 C of the additional CH 4 source constituting the CH 4 anomaly as −56.8 ± 2.8‰, which we confirm by means of a previously published box model. We propose tropical wetlands as the most probable additional CH 4 source during GI‐21.2 and present independent evidence that suggests that tropical wetlands in South America and Asia have played a key role. We find no evidence that boreal CH 4 sources, such as permafrost degradation, contributed significantly to the atmospheric CH 4 increase, despite the pronounced warming in the Northern Hemisphere during GI‐21.2.