Greenhouse gas balance over thaw‐freeze cycles in discontinuous zone permafrost

Abstract

Abstract Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost‐thaw cycles since the end of the mid‐Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO 2 and CH 4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO 2 and CH 4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost‐thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH 4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO 2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO 2 uptake that is partially offset by increasing CH 4 emissions. In the short‐term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long‐term (centuries) net C deposition provides a negative feedback to climate warming.