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Atmospheric methane isotope records covering the Holocene period

Authors
Journal
Quaternary Science Reviews
0277-3791
Publisher
Elsevier
Publication Date
Volume
29
Identifiers
DOI: 10.1016/j.quascirev.2009.05.023
Disciplines
  • Biology
  • Ecology

Abstract

Abstract Records of the 13C/ 12C (δ 13CH 4) and the D/H ( δ D CH 4 ) 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 δ 13CH 4 and 65 adjacent samples for δ D CH 4 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 δ 13CH 4 record exhibits a decreasing trend throughout the Holocene beginning at −46.4‰ at 11,000 years BP (BP defined as 1950 AD = 11 ka), and decreasing to −48.4‰ at 1 ka. The 2‰ δ 13CH 4 drop is likely to be a combination of increased CH 4 emissions from Arctic lake ecosystems and an increase in the ratio of C 3/C 4 plants in wetlands where CH 4 is emitted. The C 3/C 4 ratio increase is the result of increasing CO 2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C 3 type. The δ D CH 4 record over the early-mid Holocene shows a slightly decreasing trend that would be predicted by increased CH 4 emissions from Arctic lakes. Between 4 ka and 1 ka, δ D CH 4 values increase by ∼20‰ while δ 13CH 4 values remain effectively constant. There are at least two plausible explanations for this 20‰ δ D CH 4 shift. First, a dramatic shift in CH 4 emissions from higher latitudes to the tropics could account for the observed shift though the lack of a corresponding δ 13CH 4 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched δ D CH 4 values explains both the δ D CH 4 and δ 13CH 4 records over this period.

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