Atmospheric methane isotope records covering the Holocene period

Records of the C-13/C-12 (delta(CH4)-C-13) and the D/H (delta D-CH4) ratio of atmospheric methane were recovered from the GISP II ice core covering the last 11,000 years. All totaled, 76 samples were analyzed for delta(CH4)-C-13 and 65 adjacent samples for delta D-CH4 between 86 and 1696 m below surface (mbs) providing a temporal resolution that is better than one pair of isotope samples every 200 years. The delta(CH4)-C-13 record exhibits a decreasing trend throughout the Holocene beginning at -46.4 parts per thousand at 11,000 years BP (BP defined as 1950 AD = 11 ka), and decreasing to -48.4 parts per thousand at 1 ka. The 2 parts per thousand delta(CH4)-C-13 drop is likely to be a combination of increased CH4 emissions from Arctic lake ecosystems and an increase in the ratio Of C-3/C-4 plants in wetlands where CH4 is emitted. The C-3/C-4 ratio increase is the result of increasing CO2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C-3 type. The delta D-CH4 record over the early-mid Holocene shows a slightly decreasing trend that would be predicted by increased CH4 emissions from Arctic lakes. Between 4 ka and I ka, delta D-CH4 values increase by similar to 20 parts per thousand while delta(CH4)-C-13 values remain effectively constant. There are at least two plausible explanations for this 20 parts per thousand delta D-CH4 shift. First, a dramatic shift in CH4 emissions from higher latitudes to the tropics could account for the observed shift though the lack of a corresponding delta(CH4)-C-13 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched delta D-CH4 values explains both the delta D-CH4 and delta(CH4)-C-13 records over this period. (C) 2009 Elsevier Ltd. All rights reserved.