Methylmercury concentrations in a degrading lithalsa field: Effects of thermokarst development and revegetation

Organic matter sequestered in permafrost environments contains mercury (Hg) which can be exposed via permafrost thaw to conditions that favour its microbial conversion into its organic and neurotoxic form, methylmercury (MeHg). Permafrost thaw affects landscapes unevenly, creating a patchwork of conditions controlled by ground ice distribution and time since degradation. However, little is known about the relationship between the evolution of thaw features and net MeHg production in thawing landscapes. Near Kangiqsualujjuaq, Nunavik (Québec), the degradation of permafrost mounds, such as lithalsas, creates evolving mosaics of hydrological and ecological conditions with well-drained elevated surfaces, rim ridges, and thaw ponds where ecological succession leads to terrestrialisation. This research investigated whether the geomorphic and ecological evolution of a degrading lithalsa field, including the revegetation of thaw ponds, creates environmental conditions conducive to net methylation of inorganic mercury (IHg). Hydrological and ecological conditions, as well as total mercury (THg), MeHg, percent carbon (C), nitrogen (N), and sulfur (S), were assessed for 175 soil samples collected along a chronosequence of lithalsa degradation and thaw pond revegetation. The degradation of lithalsas increased net Hg methylation potential, with %MeHg increasing from 0.57 % on intact mounds to 4.6 % in thermokarst depressions. Among the latter, the highest %MeHg values were associated with the earliest phase of thaw pond evolution, where newly submerged organic matter can favour high microbial activity in a reduced environment. %MeHg decreased with colonisation of the ponds by Cyperaceae (4.8 %) and further with terrestrialisation by Sphagnum spp. (2.6 %), which contain recalcitrant organic compounds and may further inhibit MeHg production by raising the surface above the water table. Air photo analysis reveals rapid progression towards complete permafrost disappearance from the lithalsa field and full revegetation of thaw ponds. In such a state, net methylation potential is expected to stabilise at levels comparable to non-permafrost bogs.