Improving the past methane budget using dual-isotope methane records over the last glacial cycle

Ice core derived records of the past atmospheric methane concentration ([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 a strong sensitivity of NH low-latitude CH₄ sources to the position of the Inter-Tropical Convergence Zone.

Past [CH₄] are well recorded by the measurements of Antarctic and Greenland ice cores, however, large parts of existing Greenland records over the last glacial period suffered from excess methane production during analysis (Mühl et al., 2023). The individual contributions of various sources and sinks to the global methane budget are still a matter of debate and a quantitative assessment is still missing for many time periods in the past. Synchronized ice core records from both polar regions allow to derive the Inter-Polar Difference (IPD) in [CH₄] reflecting latitudinal emission variability and are used to distinguish low and high latitude CH₄ sources. Another powerful tool to separate emissions from different sources are measurements of the stable hydrogen and carbon isotopic signature of CH₄ (δ²H-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, providing additional constraints on past CH₄ source variability and top-down quantifications of the CH₄ budget.

In this study we present the first complete δ²H-CH₄ record over the last glacial cycle complementing our existing δ¹³C-CH₄ record (Möller et al., 2013). The record shows only relatively small variations in δ²H-CH₄ over the last glacial cycle, while δ¹³C-CH₄ showed pronounced millennial variability, which are correlated to concurrent CO₂ changes but not to stadial/interstadial climate variability. With additional measurements of Greenland ice core samples (GRIP) in the time interval 73-105 kyr, we can derive for the first time an IPD in both the methane concentration and the methane dual-isotopic signature during glacial times. We concentrate our CH₄ budget reconstruction on selected time intervals during Heinrich Stadials 7b and 9, and DO events 21-23, where excess CH₄ production does not affect our results.