In-depth characterisation of organic matter thermal lability and composition from Arctic Permafrost thaw slumps

Abstract. The rapid warming of the Arctic is accelerating permafrost thaw and mobilising large, previously frozen organic-carbon reservoirs. Retrogressive thaw slumps (RTS) are dynamic hotspots of abrupt permafrost disturbance that expose deep, millennial-aged material to erosion and transport. To assess the fate of slump-derived organic matter (OM), we analysed samples from (i) the seasonally thawed active layer, (ii) Holocene and Pleistocene permafrost, (iii) freshly thawed debris, and (iv) runoff across four RTS of contrasting sizes and ecological settings on the Peel Plateau, north-western Canada. We specifically quantified OM abundance, thermal stability, and radiocarbon content, complemented by thermally-sliced pyrolysis–gas chromatography–mass spectrometry (Ts-Py-GCMS) for molecular fingerprints. Our results show that OM age and stability primarily reflect geomorphic feature type. Permafrost, debris, and runoff contain radiocarbon-depleted, thermally stable carbon, whereas active-layer OM is younger and more labile, with minor contributions of stabilised, higher-energy fractions. Ts-Py-GCMS shows that low-temperature fractions are dominated by carbohydrate- and cellulose-derived pyrolysates, while higher-temperature fractions contain aromatic and long-chain aliphatic compounds consistent with more processed or mineral-associated OM. The close similarity between permafrost, debris, and runoff indicates that RTS predominantly export ancient, thermally stable OM with limited early-stage alteration. These findings highlight that a substantial portion of thaw-mobilised particulate carbon likely remains stable during initial transport, with important implications for Arctic carbon-climate feedbacks.