Probing Organomineral Associations across a Chronosequence of Yedoma Permafrost Deposits in Fox, AK.

Projected increases in polar temperatures will decrease the stability of terrestrial carbon (C) sinks, which promotes the mobilization of C to the atmosphere in the form of CO₂ and CH₄. Aged C, stored in deep deposits, represents a substantial component of permafrost C currently stabilized below thermodynamic and metabolic thresholds, but the mechanisms governing the fate of deep permafrost C under changing climate conditions remains unknown. Mineral association, one of the most important C stabilization mechanisms in soils, could contribute to stability within deep permafrost. Using a suite of high-resolution analytical techniques, we characterized organomineral associations across a chronosequence of Yedoma permafrost from Fox, Alaska. Core samples were taken from the Cold Regions Research and Engineering Laboratory Permafrost Tunnel, spanning 19, 27, and 36 kya, an active freeze-thaw surface horizon, and a thawed former-permafrost site. Scanning electron microscopic imaging and STXM spectroscopy revealed strong correlations between C and iron (Fe) across all samples and co-precipitation of C-calcium-Fe in some systems. Elemental analysis revealed high C:Fe ratios in soils, and high C concentrations were detected in thaw from the freeze-thaw and 19 kya tunnel soils; both declining with increasing age. Thaw samples were analyzed via FTICR-MS, which displayed a drastic disappearance of proteins and aliphatic groups as well as a steady decrease in lignin-like compounds with increasing sample age. Sequential selective mineral dissolutions were conducted to probe the role of Fe speciation in organomineral stabilization. Across all samples, a majority of the abundant C detected by elemental analysis was sorbed to crystalline Fe phases, whereas very little C was associated with short-range ordered phases. Multi-approach characterization of these soils revealed abundant sorption between C and Fe phases that may play protective roles under unstable conditions. Changes in freeze-thaw regimes can destabilize organomineral associations by oscillating the E_h of the system and solubilizing Fe(III) oxide phases. Understanding the stability of these organomineral linkages in aged, deep permafrost stores under such climate pressures will be important for predicting the rate of C mobilization.