Fraction modern: molecular carbon dynamics across a Yedoma permafrost thaw chronosequence

Permafrost soils contain vast stocks of organic carbon (C) stabilized during Pleistocene glaciation that are increasingly susceptible to mineralization and export as dissolved organic matter (DOM) under ongoing climate warming. Over 25% of permafrost C is stored in deep Yedoma deposits preserved below the oxic threshold, vulnerable to anaerobic decomposition but not well characterized. Thawing of these ancient C stocks poses a great threat to climate forcing, yet the molecular composition, bioavailability and chemical reactivity of C in deep permafrost soils remains unknown. Deep permafrost thaw may also simultaneously alter the speciation of abundant redox-active iron (Fe) phases, documented to stabilize C in surface active-layer horizons, and solubilize mineral-associated DOM. To explore the coupling between C speciation, organomineral association and temporal persistence in permafrost systems, thaw- and mineral-associated DOM samples collected from a thaw chronosequence in Fox, Alaska were characterized via FTICR-MS, NEXAFS spectroscopy, and bulk analyses. The chronosequence encompasses modern fully-thawed, actively freeze-thawing and cemented Yedoma deposits from the Cold Regions and Research and Engineering Laboratory (CRREL) tunnel dating to 19, 27 and 33 ka. Distinct molecular signatures were detected across thaw and age gradients. In the oldest permafrost DOM characterized, 33 ka samples were comprised of a highly microbial signature of amino acids, lipids, and lignin-like moeities. With decreasing age in tunnel deposits, aliphatic structures increased in abundance. In contrast, actively freeze-thawing permafrost was dominated by lignin-like and condensed aromatic compounds, suggesting dynamic oscillations are reducing DOM complexity. Congruently, fully-thawed soils contained a diverse assemblage of hydrocarbons, lipids, proteins, and carbohydrate compounds. Throughout the chronosequence, mineral-associated compounds were preferentially aromatic, and decreased in aromaticity and saturation with age and thaw frequency. Observed variance in bound and unbound organic speciation across this thaw chronosequence suggests the extent of thaw may mobilize discrete DOM populations, fractionated in lability and reactivity, throughout the warming Arctic.