Autumn destabilization of deep porewater CO₂ store in a northern peatland driven by turbulent diffusion

The deep porewater of northern peatlands stores large amounts of carbon dioxide (CO₂). This store is viewed as a stable feature in the peatland CO₂ cycle. Here, we report large and rapid fluctuations in deep porewater CO₂ concentration recurring every autumn over four consecutive years in a boreal peatland. Estimates of the vertical diffusion of heat indicate that CO₂ diffusion occurs at the turbulent rather than molecular rate. The weakening of porewater thermal stratification in autumn likely increases turbulent diffusion, thus fostering a rapid diffusion of deeper porewater CO₂ towards the surface where net losses occur. This phenomenon periodically decreases the peat porewater CO₂ store by between 29 and 90 g C m^-2 throughout autumn, which is comparable to the peatland's annual C-sink. Our results establish the need to consider the role of turbulent diffusion in regularly destabilizing the CO₂ store in peat porewater.