Atmospheric methane isotope records covering the Holocene period

Records of the 13C/12C (δ13CH4) and the D/H (δDCH4δDCH4) 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 δDCH4δDCH4 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 δDCH4δDCH4 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, δDCH4δDCH4 values increase by ∼20‰ while δ13CH4 values remain effectively constant. There are at least two plausible explanations for this 20‰ δDCH4δDCH4 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 δDCH4δDCH4 values explains both the δDCH4δDCH4 and δ13CH4 records over this period.