Empirical Parameterization of Organic Matter Reactivity in Subsea Permafrost and Implications for Greenhouse Gas Fluxes From a Warming Arctic Shelf

Abstract Warming bottom waters on the Arctic shelf are thawing subsea permafrost, unlocking large amounts of old organic carbon. This thaw is expected to accelerate with continued sea‐ice loss and ocean warming. However, the rate at which thawed subsea permafrost organic matter (OM) is degraded into CO 2 and CH 4 remains uncertain. Here, we use data from 156 subsea and terrestrial permafrost incubation experiments, combined with a reactive continuum model, to estimate permafrost OM reactivity (i.e. parameters a , ν ) and quantify OM degradation rates after thaw. Our results show that the reactivity of subsea permafrost OM is similar to terrestrial permafrost OM degraded under anoxic conditions, underscoring that the terrestrial data set provides a strong empirical basis for constraining subsea permafrost OM reactivity. (Subsea) permafrost OM is, on average, less reactive ( a mean = 7.39 × 10 −4 yr; ν mean = 1.85 × 10 −3 ) than terrestrial or marine OM but retains a small, highly reactive fraction driving high initial degradation rates. These initially high rates decline rapidly over years to decades and most OM degrades slowly under anoxic conditions. Using a 1,000‐member ensemble of thaw and degradation scenarios, we estimate cumulative subsea permafrost OC loss of up to 96 Pg C (18–126 Pg C) over 300 years, with mean annual degradation rates of ∼350 Tg C yr −1 (60–450 Tg C yr −1 ) under moderate thawing. If fully converted by methanogens, CH 4 production could exceed current global ocean CH 4 emissions by tenfold. This study provides the first quantitative framework for informing subsea permafrost degradation models over long timescales and can help improve estimates of greenhouse gas emissions and their uncertainties under future warming scenarios.