HR: 09:15h
AN: U21F-06    [Abstracts]
TI: Timing and Trends in Northern and Southern Hemisphere Atmospheric Methane During the Holocene: new Results from Antarctic and Greenlandic ice Cores
AU: * Brook, E J
EM: brooke@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States
AU: Mitchell, L
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States
AB: A striking feature of the ice core greenhouse gas record during the Holocene is the mid Holocene minimum in atmospheric methane, widely believed to result from changes in methane emissions from terrestrial ecosystems. High mixing ratios in the early Holocene give way to low values at about 5-6 kyr, followed by a slow rise until the rapid increase associated with the industrial revolution. The slow rise is of particular interest because of suggestions that it may have been due to early human influence. Ice cores in Greenland and Antarctica record these trends, with a small difference between the records due to dominance of northern hemisphere methane sources. This difference changes with time, and can be exploited as a tracer of methane source location. Previous efforts to do so over the Holocene have met with some success, but suffer from limitations of existing samples and problems of comparing data sets generated in different laboratories. We created completely new methane records from the GISP2 and Siple Dome ice cores, analyzing 280 samples in duplicate, with measurements of samples of the same age from both hemispheres on the same day. We used a new high precision method (Grachev and Brook, in press, Geophysical Research Letters), obtaining a mean 1 sigma precision of 1.9 ppb for individual samples, and a mean difference between the 280 duplicates of 3.1 ppb. We smoothed the records using a 500-year running mean, and examined the difference between the smoothed records to determine trends in the interpolar methane difference, expressed as ([Cn/Cs] – 1). This difference was ~7 per cent in the early Holocene, but declined to minimum values of ~ 4 per cent by 6 ka, as methane mixing ratios also declined. At ~ 5.5 ka methane mixing ratios started to rise, and this rise is associated with a dramatic increase in the interpolar difference, with values reaching greater than 10 per cent by 4.8 kyr. As methane rose in the late Holocene the gradient shrank, reaching values of ~ 5 per cent by 0.5 ka. The decrease in mixing ratio and gradient in the early Holocene is plausibly attributed to decline in emissions from both northern wetlands and tropical wetlands, with a greater decline in the north. The gradient expansion at about 5.5 ka may be related to opening up new wetland regions in North America, perhaps associated with the demise of the Laurentide ice sheet. Expansion of methane sources post 4.8 ka appears to have been driven by changes tropical emissions. Shorter-term variability is apparent and will also be discussed.
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0330 Geochemical cycles (1030)
DE: 0724 Ice cores (4932)
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 4930 Greenhouse gases
SC: Union [U]
MN: 2007 Fall Meeting