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

<p>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.</p> <p>Traditionally, measurements of the isotopic composition (¹³CH₄and 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). &#160;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.</p> <p>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.</p> <p>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.</p> <p>Reference:</p> <ul> <li>Christiansen et al. (2021). DOI: 10.1029/2021JG006308</li> <li>Christiansen, J. R., & J&#248;rgensen, C. J. (2018). DOI: 10.1038/s41598-018-35054-7</li> <li>Ranlanen, et al. (2022), Commun Earth Environ, 2022. DOI: 10.1038/s43247-022-00498-3</li> <li>Valentine et al. (2004). DOI: 10.1016/j.gca.2003.10.012</li> </ul> <p>&#160;</p>