Improving the CH4 budget using new dual-isotope CH4 records over the last glacial cycle

Ice core derived records of the past atmospheric methane concentration ([CH₄]) and its isotopic composition (δ¹³C-CH₄ and δD-CH₄) allow us to reconstruct its past variability and its link to changes in the climate system.  During the last glacial cycle, [CH₄] showed pronounced increases from glacial to interglacial conditions, but [CH₄] also closely followed large and rapid millennial-scale warming events in the Northern Hemisphere associated with Dansgaard-Oeschger (DO) events, indicating the strong sensitivity of terrestrial biogeochemistry to (hydro-) climatic changes.

To better understand the climate-greenhouse gas feedback cycle and what controlled past atmospheric methane variability it is essential to quantify the response of the CH₄ budget and terrestrial biogeochemistry to such abrupt climate variations. Such a budget provides a framework to infer the strength and temporal dynamics of individual CH₄ sources (e.g. wetlands, biomass burning, geologic emissions). However, for most parts of the last glacial cycle a quantitative source attribution is missing or still a matter of debate.

Synchronized ice core records from both polar regions allow us to derive the Inter-Polar Difference in [CH₄] reflecting latitudinal emission difference and are used to distinguish low and high latitude CH₄ emissions.  Another powerful tool to uncover source contribution to the global CH₄ budget is provided by records of methane’s stable isotopic composition (δD-CH₄, δ¹³C-CH₄) as CH₄ released by the various sources are associated with characteristic isotopic signatures and different sinks are connected to systematic isotope fractionations.

Here we present new δD-CH₄ data from bi-polar ice cores (EDC, EDML and GRIP ice core samples) covering large parts of the last glacial cycle complementing our existing δ¹³C-CH₄ record (Möller et al., 2013).  We use δ¹³-CH₄ and δD-CH₄ as quantitative tracers of changes in the CH₄ budget and interpret atmospheric signals in a simple CH₄ stable isotope-enabled one-box model of the global CH₄ cycle concentrating on the prominent DO-21interval between 86 kyr and 76 kyr. We derive quantitative estimates of plausible global CH₄ source mix scenarios but also review in this context uncertainties arising from poorly constrained assumptions in the past. Limited knowledge of past isotopic source signatures of biogenic CH₄ sources (wetlands) and their latitudinal distribution introduces substantial uncertainty into reconstructions of the past methane budget. Because drivers of past changes remain poorly understood, uncertainties in these assumptions propagate into estimated CH₄ emissions. For the first time, dual-isotopic CH₄ records enable an evaluation of temporal changes. Building on the new dual-isotopic constraints, we go beyond previous studies and present a new Monte Carlo approach that simulates realistic past isotopic source signatures and assesses their impact on the inferred CH₄ budget.