Excess methane, ethane and propane production in Greenland ice core samples and a first isotopic characterization of excess methane
Authors: Mühl Michaela , Schmitt Jochen , Seth Barbara , Lee James Edward , Edwards Jon Shelley , Brook Edward J. , Blunier Thomas , Fischer Hubertus 
Published: November 2022

Abstract. 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). Provided that the analyzed species concentrations and their isotopic fingerprints accurately reflect the past atmospheric composition, biogeochemical cycles can be reconstructed. Recently, the comparison of CH4 records obtained using different extraction methods revealed disagreements in the CH4 concentration for the last glacial in Greenland ice. Elevated methane levels were detected in dust-rich ice core sections measured discretely pointing to a process sensitive to the melt extraction technique. To shed light on the underlying mechanism, we performed targeted experiments and analyzed samples for methane and other short-chain alkanes ethane and propane covering the time interval from 12 to 42 kyr. Here, we report our findings of these elevated alkane concentrations occurring in dust-rich sections of Greenland ice cores. The alkane production happens during the melt extraction step (in extractu) of the classic wet extraction technique and reaches 14 to 91 ppb for CH4 excess in dusty ice samples. We document for the first time a co-production of excess methane, ethane, and propane (excess alkanes) with the observed concentrations for ethane and propane exceeding their past atmospheric background at least by a factor of 10. Independent of the produced amounts, excess alkanes were produced in a fixed molar ratio of approximately 14:2:1, indicating a shared origin. The amount of excess alkanes scales linearly with the amount of mineral dust within the ice samples. The isotopic characterization of excess CH4 reveals a relatively heavy carbon isotopic signature of -46.4 ‰ (± 2.4 ‰) and a light deuterium isotopic signature of -318 (± 53 ‰) in the samples analyzed. With the co-production ratios of excess alkanes and the isotopic composition of excess methane we established a fingerprint that allows us to confine potential formation processes. This fingerprint is not in line with a microbial origin, rather such an alkane pattern is indicative of abiotic decomposition of organic matter as found in sediments, soils and plant leaves. This study provides first indications for an abiotic reaction producing excess alkanes during ice core analyses and discusses potential mechanisms. We see an urgent need to correct the already existing discrete CH4 records for excess CH4 contribution (CH4(xs), δ13C-CH4(xs), δD-CH4(xs)) in dust-rich intervals in Greenland ice. Specifically, excess CH4 has a significant effect on the assessments of the hemispheric CH4 source distribution. As we observe that in some intervals excess CH4 is in the same range as the Inter-Polar Difference (IPD), previous interpretations of relative contribution of high latitude northern hemispheric CH4 sources need to be revised.