Title: Ice core records of the evolution of atmospheric methane in the Holocene
Author: Brook, E.J., Mitchell, L., Severinghaus, J. and Harder, S.
Periodical: American Geophysical Union, Fall Meeting 2008, abstract #U33B-02
Abstract: Atmospheric methane mixing ratios declined from peak values of ~680-730 ppb in the early Holocene, to a broad minimum of ~560-610 ppb in the mid-Holocene, then rose toward values of 690-725 ppb in the late pre-Industrial Holocene. The mid-Holocene minimum is unusual relative to the long-term ice core record, leading to suggestions that the post 5 ka increase may have been due to early human sources. We generated two new high-precision Holocene ice core methane records, from Greenland and Antarctica, analyzing over 380 new samples in duplicate. This allows an accurate comparison of absolute methane levels in the high latitude northern and southern hemispheres, providing clues to changes in source locations. The late Holocene rise is associated with a decreasing inter-polar methane gradient, indicating a dominating increase in low latitude sources. Could the rise be due to natural changes in emissions from wetlands? Northern hemisphere summer isolation decreased over this time period, which should have decreased temperature and precipitation in many methane-producing regions. One important observation, though, is that over 60% of the late pre-industrial Holocene rise happened after 2,000 years ago. Asian monsoon proxies from Chinese cave deposits, and the isotopic composition of atmospheric oxygen (Severinghaus et al., this meeting), suggest increased rainfall in the tropics after 2,000 years B.P. In addition, southern hemisphere wetland methane sources must have increased as southern summer insolation increased through the Holocene, though the contribution to the global budget is unclear. Another puzzling aspect of the data is that mid-Holocene results seem to require a source 'flip-flop'. The mid- Holocene minimum is associated with a high gradient, indicating relatively larger northern sources and smaller tropical sources. This may reflect the drying of a tropical region, perhaps in Africa (which is also consistent with the atmospheric oxygen isotope data), and increases in boreal emissions as the Laurentide ice sheet disappeared. In the early Holocene, a slow decline of methane concentrations and inter-polar gradient from 11,000 years B.P. to ~6,000 years B.P. reflects decreasing sources globally, but a relatively larger decline in the high latitude north. This trend is plausibly related to declining northern hemisphere insolation. We also produced a record with decadal resolution for the last 1000 years from the WAIS Divide ice core in Antarctica. The data show obvious methane maxima at about 1150, 1300, 1550, and 1700 AD, consistent with the one other record (Law Dome) that covers this time period in detail (MacFarling Meure et al., 2006; Geophysical Research Letters, 33). Preliminary high-resolution data from Greenland match these patterns extremely well. A direct relationship with population trends or climate patterns is not yet obvious.
Year: 2008