Quantifying groundwater exchanges in the water balance of small thermokarst ponds: implications for carbon cycling
Reginald Somera, Jean-Michel Lemieux, S. Jean Birks, Philippe Fortier, Alexandra Bélanger, Aaron A. Mohammed
2025
À retenir
Dans les régions de pergélisol discontinu, des monticules de sol gelé fondent avec le réchauffement climatique, ce qui crée des dépressions entourées de talus et forme des étangs thermokarstiques. Ces étangs, alimentés par la pluie et la fonte du pergélisol, jouent un rôle important dans les échanges de carbone et les émissions de gaz à effet de serre. Autrefois isolés des eaux souterraines à cause du sol gelé, ces systèmes devraient devenir de plus en plus interconnectés à mesure que le dégel progresse. Une étude menée dans la vallée de Tasiapik (Nunavik) a montré que certains étangs perdent de l’eau vers le sol, tandis que d’autres réagissent aux variations de la nappe selon la nature du sol (sable ou silt). Ces interactions influencent la circulation de l’eau et du carbone, ce qui est crucial pour comprendre l’impact du dégel sur les écosystèmes nordiques.
Résumé
Permafrost mounds, formed in discontinuous and sporadic permafrost regions, are subject to thawing due to climate warming. Thaw-induced ground subsidence creates depressions encircled by high ramparts, leading to the formation of thermokarst ponds. These water bodies, accumulating water from precipitation and thawed permafrost, play key roles in biogeochemical cycling and greenhouse gas (GHG) emissions. Traditionally viewed as hydrogeologically isolated due to permafrost impeding groundwater flow and the low hydraulic conductivity of soil underneath these ponds, these systems are expected to become more interconnected as thaw progresses. This study uses several methods to examine groundwater connections in the Tasiapik Valley, Nunavik, Québec, Canada, where differing stages of lithalsa permafrost mound degradation result in a series of varying pond conditions. Four thermokarst ponds were monitored using direct hydrological measurements, water balance, and stable water isotopic tracer approaches to quantify groundwater contributions. Findings reveal varying degrees of pond connectivity to the subsurface, with ponds in silt experiencing net groundwater loss and those in sandy substrates showing dynamic responses to groundwater fluctuations. These varied interactions will have implications for how surface and subsurface carbon cycling occurs. The results of this study underscore the complex interactions between thermokarst ponds and groundwater systems and the importance of these dynamics in understanding hydrological and carbon cycling in permafrost regions.
mots clés:
Thermokarst lakes, Water balance, Groundwater, Water isotope tracers, Carbon, Nunavik