Changes of carbon fluxes and major carbon sources after palsa thawed into pond

English Abstract:

Permafrost thaw is pronounced in peatlands across the discontinuous permafrost zone. Our research assesses the carbon (C) fluxes before and after palsa peatlands thaw. The terminal point of the thaw sequence is often thawed ponds, which is believed to be a major source of C gases to the atmosphere. At our study site in northern Quebec near Kuujjuarapik, the thawed ponds (~3 m in depth) were strongly stratified with thick anoxic hypolimnias from 0.5 m to the pond bottoms. The CO₂ flux and most the CH₄ fluxes from the ponds were positive to the atmosphere, while the palsas took up CO₂ and CH₄ for most of the growing season. The average seasonal C losses from the ponds were 9.46 mmol CO₂ m^-2 h^-1 and 1.69 to 2.00 µmol CH₄ m^-2 d^-1. Over the summer larger pond had significantly higher CO₂ efflux, while the smallest pond had significantly higher CH₄ efflux. Bubble emission accounted a large part of pond CH₄ emission, with greater ebullition at the pond edges. The dissolved CO₂ and CH₄ concentrations increased ~ 10 fold from 10 to 50 cm deep, and more than 100 fold at 230 cm depth. The δ¹³CH₄ indicated CH₄ production shifted from acetoclastic methanogenesis dominated at the surface to CO₂ reduction dominated at the bottom. DOC mass and biodegradability (SUVA/FI) strongly correlated with pond C gas effluxes, suggesting pond surface DOC may be a C source supporting pond C gas effluxes. Comparing δ¹³C of surface dissolved CO₂ and CH₄, DOC, peat, sediments, and plant litter & exudates, we determined that plant exudates was the main source of the labile DOC that decomposed into C gas fluxes. Palsa thaw to ponds leads to large CO₂ and CH₄ emissions, supported by newly produced labile DOC from the plant communities surrounding the ponds rather than the peat and pond sediments.