Excess methane, ethane, and propane production in Greenland ice core samples and a first characterization of the δ13C-CH4 and δD-CH4 signature

Air trapped in polar ice provides unique records of the past atmospheric composition ranging from key greenhouse gases such as methane (CH₄) to short-lived trace gases like ethane (C₂H₆) and propane (C₃H₈). Interpreting these data in terms of atmospheric changes requires that the analyzed species accurately reflect the past atmospheric composition.

Recent comparisons of Greenland CH₄ records obtained using different extraction techniques revealed discrepancies in the CH₄ concentration for the last glacial. Elevated methane levels (excess methane or CH_4(exs)) were detected in dust rich ice core sections measured by discrete melt extraction techniques pointing to an artefact sensitive to the measurement technique. To shed light on the underlying mechanism, we analyzed Greenland ice core samples for methane and other short-chain alkanes (ethane and propane) covering the time interval from 12 to 42 kyr using a classic wet extraction technique. The artefact production happens during the melting and extraction step (in extractu) and reaches 14 to 91 ppb CH_4(exs) in dusty ice samples. For the first time in ice core analyses, we document a co-production of excess methane, ethane, and propane (excess alkanes) with the observed concentrations for ethane and propane exceeding, at least by a factor of 10, their past atmospheric concentration. Independent of the produced amounts, excess alkanes were produced in a fixed molar ratio of approximately 14:2:1, indicating a common production. We also discovered that the amount of excess alkanes scales generally with the amount of mineral dust (or Ca²⁺ as a proxy for mineral dust) within the ice samples. Moreover, applying the Keeling-plot approach we are able to isotopically characterize CH_4(exs) revealing a relatively heavy carbon isotopic signature of (-46.4 ± 2.4) ‰ and a light deuterium isotopic signature of (-326 ± 57) ‰ of the excess methane in the samples analyzed.

The co-production ratios of excess alkanes and the isotopic composition of excess methane allows us to confine potential formation processes. We discovered that this specific alkane pattern is not in line with an anaerobic methanogenic origin but indicative for abiotic decomposition of organic matter as also found in sediments, soils, and plant leaves. From the present-day state of research little is known about this process and there is urgent need to improve our understanding for future ice core measurements. Moreover, the already existing discrete records of atmospheric CH₄ in Greenland ice need to be corrected for excess CH₄ contribution (CH_4(exs), δ¹³C-CH_4(exs), δD-CH_4(exs)) in dust-rich intervals.

While the size of the excess methane production has little effect on reconstructed radiative forcing changes of CH₄ in the past, it is in the same range as the Inter-Polar Difference (IPD) for CH₄. Knowing the empirical relation of excess CH_4(exs)/ Ca²⁺ and CH_4(exs)/ C₂H₆ allows us to derive a first-order correction of existing CH₄ data sets to revise previous interpretations of the relative contribution of high latitude northern hemispheric CH₄ sources based on the IPD.