Potential glacial-interglacial changes in stable carbon isotope ratios of methane sources and sink fractionation

Abstract : Past atmospheric methane emissions can be constrained by δ13CH4 records from ice cores only if changes to source δ13CH4 signatures and sink isotope effects with varying environmental and climatic conditions are accurately known. We present reconstructions of such changes based on paleodata and recent systems observations. The results are specific for budget scenarios and are reported here for two alternative types of budgets, one including aerobic methane emissions (AMP) from plants and the other type without AMP. Shifting atmospheric δ13CO2 potentially led to 13CH4 enrichment by 0.8‰ in the preindustrial Holocene (PIH) (~150–11,000 years (a) B.P.) and ~0.3–0.6‰ at the Last Glacial Maximum (LGM) (~18,000 a B.P.) relative to today. Differing distribution of C3 and C4 plant precursor material may account for 13CH4 enrichment of ~0.4‰ (PIH) and ~0.6–1.1‰ (LGM). Temperature-dependent fractionation and varying methanogenic pathways in wetlands may lead to atmospheric 13CH4 depletion by ~0.1–1.2‰. Sink fractionation today (7.4‰) is higher than during the PIH (~7.0‰) and the LGM (~5.7‰). The cumulative effect of all processes is ~0.8‰ 13CH4 enrichment in the PIH and ~1–1.2‰ 13CH4 depletion at the LGM. Budget reconstructions will be inaccurate if these changes are not included.