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

Past atmospheric methane emissions can be constrained by delta(CH4)-C-13 records from ice cores only if changes to source delta(CH4)-C-13 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 delta(CO2)-C-13 potentially led to (CH4)-C-13 enrichment by 0.8% in the preindustrial Holocene (PIH) (similar to 150-11,000 years (a) B.P.) and similar to 0.3-0.6 parts per thousand at the Last Glacial Maximum (LGM) (similar to 18,000 a B.P.) relative to today. Differing distribution of C-3 and C-4 plant precursor material may account for (CH4)-C-13 enrichment of similar to 0.4 parts per thousand (PIH) and similar to 0.6-1.1 parts per thousand (LGM). Temperature-dependent fractionation and varying methanogenic pathways in wetlands may lead to atmospheric (CH4)-C-13 depletion by similar to 0.1-1.2 parts per thousand. Sink fractionation today (7.4 parts per thousand) is higher than during the PIH (similar to 7.0 parts per thousand) and the LGM (similar to 5.7 parts per thousand). The cumulative effect of all processes is similar to 0.8 parts per thousand (CH4)-C-13 enrichment in the PIH and similar to 1-1.2 parts per thousand (CH4)-C-13 depletion at the LGM. Budget reconstructions will be inaccurate if these changes are not included.