Legacy permafrost conditions limit deep carbon decomposition in thermokarst peatlands and ponds

Abstract Thermokarst fens and ponds form following thaw in permafrost peatland complexes and can release large but poorly constrained quantities of permafrost carbon to the atmosphere. However, the extent of deep peat decomposition and resulting carbon dioxide and methane emissions remains uncertain. Here, we compared decomposition potential and greenhouse gas production across 12 thermokarst fen and pond sites at four locations with similar past ecological conditions in northern Fennoscandia and investigated drivers of degradation. Using oxic and anoxic incubations, elemental analysis, and quantification of energy availability, we demonstrate that deep peat decomposition is limited in thermokarst fens and ponds. While thermokarst ponds strongly influence regional carbon balances, greenhouse gas production was similar in fens and ponds, and decomposition was thermodynamically inhibited at depth in both ecosystems. These results suggest that the release of aged carbon from deep peat is similarly limited in thermokarst landscapes where peat deposits share similar ecological histories.