Impacts of hillslope thaw slumps on the geochemistry of permafrost catchments (Stony Creek watershed, NWT, Canada)

Retrogressive thaw slumps are one of the most dramatic thermokarst landforms in periglacial regions. This study investigates the impacts of one stable and two active thaw slumps on the geochemistry of streams in the Stony Creek watershed (Peel Plateau, NWT, Canada). The objective of this study is to elucidate the geochemical processes associated with ground ice ablation in retrogressive thaw slumps and the geochemical evolution of slump runoff to streams. This is accomplished by describing the geochemical composition of runoff across active mega-slumps, impacted and pristine tundra streams, as well as that of the ice-rich permafrost exposed in the slump headwalls. In the Stony Creek watershed, runoff derived from active and stable thaw slumps is characterized by a Ca(Mg)–SO4 geochemical facies with conductivity and solute concentrations approximately one order of magnitude higher than in pristine streams. The elevated solute concentrations in the slump runoff are directly related to thawing of highly weatherable Late Pleistocene age ice-rich and solute-rich permafrost exposed in the headwalls of slumps, which has solute concentrations nearly 100 times higher than those measured in the uppermost 1–2 m (i.e., above the early Holocene thaw unconformity). An examination of ionic relations revealed a strong relation between Ca2 + and SO42 − and (Ca2 ++Mg2 +)–SO42 −, suggestive that sulfate dissolution is the main process responsible for the geochemical composition of slump impacted streams. Thaw slumps significantly impact the geochemistry of streams, by increasing their solute load well above that of pristine streams along any reach of impacted streams. Unlike shallow active layer disturbances, the thaw slumps can degrade permafrost to depths of 10 m or more and the impacts of abundant slump activity on stream geochemistry can be detected at the 10^2 km2 watershed-scale.