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

Significant variations in the interpolar difference of atmospheric CH4 conc entration over the Holocene period were observed by Chappellaz et al., [199 7]. 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 (ppb v) during the Last Glacial Maximum. It increases to 26+/-10 ppbv during the Bolling/Allerod and remains at 26+/-9 ppbv during the Younger Dryas cold p eriod. On average, we find an interpolar difference of 14+/-4 ppbv during t he cold phases and of 37+/-10 ppbv during the warm periods of the Last Glac ial. With a three-box model we derive from the measured gradients the contr ibutions of methane from the Tropics and the mid-to-high latitudes of the n orthern hemisphere. The Tropics have been the largest source in all glacial epochs. The contribution by the northern latitudes have been very small du ring the last glacial maximum but surprisingly large during the earlier par t of the glacial epoch. The model result suggests completely unexpected, th at the higher atmospheric CH4 concentration during the warm Dansgaard/Oesch ger events are caused by a higher source strength of the northern latitudes and not of the Tropics.