Carbon Accumulation, Flux, and Fate in Stordalen Mire, a Permafrost Peatland in Transition

Stordalen Mire is a peatland in the discontinuous permafrost zone in arctic Sweden that exhibits a habitat gradient from permafrost palsa, to Sphagnum bog underlain by permafrost, to Eriophorum-dominated fully thawed fen. We used three independent approaches to evaluate the annual, multi-decadal, and millennial apparent carbon accumulation rates (aCAR) across this gradient: seven years of direct semi-continuous measurement of CO2 and CH4 exchange, and 21 core profiles for Pb-210 and C-14 peat dating. Year-round chamber measurements indicated net carbon balance of -13 +/- 8, -49 +/- 15, and -91 +/- 43 g C m(-2) y(-1) for the years 2012-2018 in palsa, bog, and fen, respectively. Methane emission offset 2%, 7%, and 17% of the CO2 uptake rate across this gradient. Recent aCAR indicates higher C accumulation rates in surface peats in the palsa and bog compared to current CO2 fluxes, but these assessments are more similar in the fen. aCAR increased from low millennial-scale levels (17-29 g C m(-2) y(-1)) to moderate aCAR of the past century (72-81 g C m(-2) y(-1)) to higher recent aCAR of 90-147 g C m(-2) y(-1). Recent permafrost collapse, greater inundation and vegetation response has made the landscape a stronger CO2 sink, but this CO2 sink is increasingly offset by rising CH4 emissions, dominated by modern carbon as determined by C-14. The higher CH4 emissions result in higher net CO2-equivalent emissions, indicating that radiative forcing of this mire and similar permafrost ecosystems will exert a warming influence on future climate.