Cryogenic Fractionation and Thaw-Gradient Reorganization of Carbon, Nutrient and Trace Element Pools in Permafrost Peatlands

Abstract. Permafrost peatlands are highly vulnerable to climate warming because active layer deepening can mobilize previously frozen solutes into suprapermafrost flow paths and regional hydrological networks. Yet the composition of the frozen porewater reservoir, i.e., pore ice, and its role in regulating C, nutrient and trace-element release during thaw, remain poorly constrained. Here, we characterized dissolved organic matter (DOM), nutrients, and major and trace solutes across the active layer–permafrost interface in a continuous (> 90 % ice) polygonal peatland of the subarctic tundra. Four microtopes representing a thaw gradient (convex polygon → concave polygon→ peat transitional fen → peat-mineral fen) were sampled, with porewater and pore ice (< 0.45 µm) analyzed in active and frozen layers. Pore ice DOM showed a predominantly microbial and aliphatic signature, with elevated dissolved organic carbon (DOC) concentrations, indicating selective preservation of microbially processed, low-aromatic compounds during freezing. Freeze–thaw cycling homogenized active layer porewater chemistry, whereas pore ice retained stronger site-specific geochemical signatures. Overall, DOC, dissolved N and P, and exchangeable cations (Ca, Mn, Ba, Sr) preferentially accumulated in pore ice, while lithogenic elements (Zr, Ga, Hf, Ge, Nb, Cr, Y and REE) were more concentrated in active layer porewaters. Frozen horizons also exhibited lower C:N:P ratios than active layers, highlighting nutrient-enriched stoichiometry in the permafrost compartment. A coupled stoichiometric and decay-rate model suggests that phosphorus is largely consumed in situ during progressive thaw, thereby constraining further carbon and nitrogen processing, whereas substantial fractions of DOC and dissolved N remain available for lateral export. Viewed as a space-for-time thaw sequence, the transition from polygonal bog to fen indicates that permafrost degradation may initially enhance mobilization of labile carbon and nutrients before hydrological redistribution dominates. These results identify pore ice as both a cryogenic archive of past biogeochemical processing and a reactive reservoir capable of amplifying Arctic carbon–nutrient fluxes under continued warming.