Negative feedback? Documenting hydrological, geomorphological, biogeochemical, and botanical aspects of climate-change-driven palsa deterioration

Abrupt global warming poses threats to hydrological cycles and peatland ecosystem development. Northern peatlands, such as palsa mires, experience prompt degradation due to disappearing ice cores caused by increasing air temperatures and changes in precipitation patterns that induce the development of thicker snow covers insulating existing ice cores that impair their development. Disappearing palsa mires are experiencing subsidence, which in turn is flattening their topography and changing their hydration. What's more, changes in the shape and position of the ice cores cause local changes in water flow, and the lowering surface of the peatlands approaches the groundwater table, increasing their saturation. Increasing the moisture content of the topsoil within the degrading palsa mire system, in turn, causes changes in biogeochemical processes manifested in changes in water balance, carbon balance, and plant species composition. It, therefore, seems that the ongoing decomposition of palsa mires results in the development of novel peatland ecosystems, which, despite not being affected by the thermokarst processes, are suspected to become effective carbon sinks capable to sequestrate massive amounts of carbon that, in turn, may decrease greenhouse gas emissions.

We conducted a comprehensive field-research-based study on Šuoššjávri palsa mire located in Northern Norway (Finnmark). We documented the water balance of the peatland. We described the structure of the palsa mire with the use of electrical resistivity imaging. We modeled the directions of groundwater flow. We applied an Interferometric Satellite Radar approach to quantify the speed of peatland subsidence. We used automated chambers to measure greenhouse gas emissions in a gradient of palsa peatland deterioration and a thermokarst lake. We also applied a novel approach to document vertical profiles of dCO₂ and dCH₄ content in groundwater at different levels of the peatland with the use of newly developed piezometers to check whether palsa-deterioration-driven groundwater flow patterns can affect carbon sequestration.

We documented that subsidence of palsa peatland occurs at a rate of about 2 mm/year while peatlands formed in place of disappearing palsa peatland grow steadily, most likely due to the persistence of stable moisture content and the maintenance of a proper peat-forming process. We revealed that the degradation of palsa mire can be expressed by a range of hydrological indicators representing the duration of groundwater levels at specific depths: inundation time at matured peatlands that remain one of the last steps of palsa mire degradation is shortening, which, in turn, results in limiting methane emissions, yet keeping the carbon dioxide emissions at levels twice as low as the ones documented in a thermokarst lake. Botanical analyses allowed us to describe the development of peatlands that formed in the place of degraded palsa mires and to quantify biomass production and peat accumulation.

In the light of results of our study, we hypothesize that degradation of palsa peatlands due to climatic change results in the development of peatland ecosystems that are likely to prevent global warming due to stable and high topsoil saturation followed by an efficient carbon sequestration in the peat-forming process and novel peatland development.