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Abstract. Air trapped in polar ice provides unique records of the pastatmospheric composition ranging from key greenhouse gases such as methane(CH4) to short-lived trace gases like ethane (C2H6) andpropane (C3H8). Recently, the comparison of CH4 recordsobtained using different extraction methods revealed disagreements in theCH4 concentration for the last glacial in Greenland ice. Elevatedmethane levels were detected in dust-rich ice core sections measureddiscretely, 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 the 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, which scale linearly with the amount of mineral dust within the ice samples. The alkane production happens during the melt extraction step of the classic wet-extraction technique and reaches 14 to 91 ppb of CH4 excess in dusty ice samples. We document for the first time a co-production of excess methane, ethane, and propane, 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 measured carbon isotopic signature of excess methane is (-47.0±2.9) ‰ and its deuterium isotopic signature is (-326±57) ‰. With the co-production ratios of excess alkanesand the isotopic composition of excess methane we established a fingerprintthat allows us to constrain potential formation processes. This fingerprintis not in line with a microbial origin. Moreover, an adsorption–desorptionprocess of thermogenic gas on dust particles transported to Greenlanddoes not appear very likely. Instead, the alkane pattern appears to beindicative of abiotic decomposition of organic matter as found in soils andplant leaves. 

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.