Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Last Glacial and the transition to the Holocene

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Abstract
Significant variations in the interpolar difference of atmospheric CH4 concentration over the Holocene period were observed by Chappellaz et al., [1997]. Here we extend this study to the Last Glacial and the transition to the Holocene. We observe a gradient of −3±4 parts per billion by volume (ppbv) during the Last Glacial Maximum. It increases to 26±10 ppbv during the Bølling/Allerød and remains at 26±9 ppbv during the Younger Dryas cold period. On average, we find an interpolar difference of 14±4 ppbv during the cold phases and of 37±10 ppbv during the warm periods of the Last Glacial. With a three‐box model we derive from the measured gradients the contributions of methane from the Tropics and the mid‐to‐high latitudes of the northern hemisphere. The Tropics have been the largest source in all glacial epochs. The contribution by the northern latitudes have been very small during the last glacial maximum but surprisingly large during the earlier part of the glacial epoch. The model result suggests completely unexpected, that the higher atmospheric CH4 concentration during the warm Dansgaard/Oeschger events are caused by a higher source strength of the northern latitudes and not of the Tropics.