Constraining sources and sinks of subglacial methane from the Greenland ice sheet using clumped isotopes

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The subglacial environment under the Greenland Ice Sheet (GrIS) is an active zone of methane (CH4) production and consumption (1). Recent studies have shown that the meltwaters are a net source of CH4 to the atmosphere (2), although its global significance remains unquantified. It is unknown how CH4 cycling and net emission is linked to the melting of the GrIS, which is expected to increase (3) as the Artic is warming four times faster than the global average. Evaluating the importance of this poorly known source for the atmospheric CH4 budget and its drivers requires a fundamental understanding of the amounts released, the sources and sinks and its age.

Traditionally, measurements of the isotopic composition (13CH4 and 12CH3D) are used as fingerprints to identify sources and sinks of CH4. However, this method is limited due to the overlap of source signatures. For example, microbial methanogenesis in some environments can produce stable isotope compositions resembling thermogenic methane (4). Furthermore, substrate isotopic composition, substrate limitation, the kinetics of methane production, transport, and oxidation substantially impact the stable isotope composition of microbially produced CH4. This complicates the interpretation of CH4 cycling and its physicochemical drivers.

Clumped isotopes of methane, i.e. molecules of CH4 with two rare isotopes, (13CH3D and 12CH2DD), and its clumping anomaly (the relative difference between the measured value of 13CH3D and 12CH2DD and its stochastic distribution) provide additional insight to constrain CH4 sources and sinks. From the clumping anomaly, it is possible to calculate the formation temperature of methane (i.e. source of methane) if CH4 was formed in thermodynamic equilibrium. In the case of disequilibrium, the clumped signatures can be used to identify various kinetic gas formation and fractionation processes that are impossible to reconstruct from the bulk isotopic composition alone.

In this study, we present for the first-time isotopic data of clumped CH4 and traditional isotopes of subglacial CH4 together with radiocarbon measurements (14CH4). These data are related to the isotopic composition of subglacial CO2 and mole fractions of the gases in the air and meltwater. Based on this data set, we will discuss the production and consumption pathways of CH4 in the subglacial environment and how it relates to diurnal and seasonal cycles of meltwater discharge.

Christiansen et al. (2021). DOI: 10.1029/2021JG006308
Christiansen, J. R., & Jørgensen, C. J. (2018). DOI: 10.1038/s41598-018-35054-7
Ranlanen, et al. (2022), Commun Earth Environ, 2022. DOI: 10.1038/s43247-022-00498-3
Valentine et al. (2004). DOI: 10.1016/j.gca.2003.10.012
Original languageEnglish
Publication date2023
Number of pages2
Publication statusPublished - 2023
EventEGU General Assembly 2023: Vienna, Austria & Online - Vienna, Austria
Duration: 24 Apr 202328 Apr 2023


ConferenceEGU General Assembly 2023

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