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 (delta C-13-CH4) to investigate the CH4 mixing ratio anomaly during Greenland Interstadial (GI) 21.2 (85,000 years before present). This extraordinarily rapid event occurred within 150 years, comprising a CH4 mixing ratio pulse of 150 ppb (similar to 25%). Our new measurements disclose a concomitant shift in delta C-13-CH4 of 1 parts per thousand. Keeling plot analyses reveal the delta C-13 of the additional CH4 source constituting the CH4 anomaly as -56.8 +/- 2.8 parts per thousand, which we confirm by means of a previously published box model. We propose tropical wetlands as the most probable additional CH4 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 CH4 sources, such as permafrost degradation, contributed significantly to the atmospheric CH4 increase, despite the pronounced warming in the Northern Hemisphere during GI-21.2.