Records of the 13C/12C (δ13CH4) and the D/H (δ DCH4) 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 δ13CH4 and 65 adjacent samples for δ DCH4 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 δ13CH4 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‰ δ13CH4 drop is likely to be a combination of increased CH4 emissions from Arctic lake ecosystems and an increase in the ratio of C3/C4 plants in wetlands where CH4 is emitted. The C3/C4 ratio increase is the result of increasing CO2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C3 type. The δ DCH4 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 1 ka, δ DCH4 values increase by ∼20‰ while δ13CH4 values remain effectively constant. There are at least two plausible explanations for this 20‰ δ DCH4 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 δ13CH4 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched δ DCH4 values explains both the δ DCH4 and δ13CH4 records over this period.
All Science Journal Classification (ASJC) codes
- Global and Planetary Change
- Ecology, Evolution, Behavior and Systematics