Evolution of atmospheric methane clumped isotope anomalies since the 1990s reconstructed from firn air measurements

Abstract

The global atmospheric methane mole fractions have risen since the pre-industrial times, primarily attributed to anthropogenic emissions, overlayed by significant multi-annual variability. Atmospheric methane is influenced by different methane sources, variations in the atmospheric OH concentration and other sink reactions. Understanding the contribution of each of these factors is crucial for a comprehensive understanding of the methane cycle. Recent modelling studies have suggested that measurements of clumped isotopologues (13CH3D and 12CH2D2) can help constrain the global methane budget [1,2]. The first measurements of present ambient air (2022-23) show that the clumping anomalies of atmospheric methane have distinct signatures of about 1 ± 0.3 ‰ for Δ13CH3D and 44 ± 3 ‰ for Δ12CH2D2, strongly enriched in Δ12CH2D2 compared to all known sources [3,4]. We have measured the bulk and clumped isotope composition of methane from firn air samples (~ 500 L volume) collected at the East Greenland Ice core Project (EGRIP) site in high-pressure cylinders. At this location, open porosity allows the collection of air samples from firn down to a depth of 70 m, dating back to the 1990s. These are the first-ever measurements of the clumped isotopic composition of atmospheric methane from the past. Results of bulk isotope measurements are in line with the known temporal evolution, supporting the integrity of the sampling and analysis procedure. The clumped isotope results reveal a clear increase of 10 ± 2 ‰ for Δ12CH2D2 over the last 30 years (~1993 to 2018), while Δ13CH3D remains constant within the experimental uncertainty. We use a 2-box atmospheric model to investigate source and sink scenarios that are consistent with this trend in the clumped isotope anomalies as well as the bulk isotopic composition of methane.