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.