Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene

High-resolution records of atmospheric methane over the last 11,500 years have been obtained from two Antarctic ice cores (D47 and Byrd) and a Greenland core (Greenland Ice Core Project). These cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution; this together with a good relative chronology, provided by unequivocal CH4 features, allows a direct comparison of the synchronized Greenland and Antarctic records, and it reveals significant changes in the interpolar difference of CH4 mixing ratio with time. On the average, over the full Holocene records, we find an interpolar difference of 44±7 ppbv. A minimum difference of 33±7 ppbv is observed from 7 to 5 kyr B.P. whereas the maximum gradient (50±3 ppbv) took place from 5 to 2.5 kyr B.P. A gradient of 44±4 ppbv is observed during the early Holocene (11.5 to 9.5 kyr B.P). We use a three-box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere. The model results support the previous interpretation that past natural CH4 sources mainly lay in tropical regions, but it also suggests that boreal regions provided a significant contribution to the CH4 budget especially at the start of the Holocene. The growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the Holocene. Finally, our model results suggest a large source increase in tropical regions from the late Holocene to the last millennium, which may partly be caused by anthropogenic emissions

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 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

over the Last Glacial Termination

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