Atmospheric methane isotope records covering the Holocene period

Records of the ¹³C/¹²C (δ¹³CH₄) and the D/H (δ 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 δ¹³CH₄ and 65 adjacent samples for δ 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 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 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 1 ka, δ D_CH4 values increase by ∼20‰ while δ¹³CH₄ values remain effectively constant. There are at least two plausible explanations for this 20‰ δ 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 δ¹³CH₄ shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched δ D_CH4 values explains both the δ D_CH4 and δ¹³CH₄ records over this period.