Geochemical response and benthic recolonization following a sensitive-clay landslide in a subarctic river

On April 22, 2021, a large sensitive-clay landslide buried sediments of the Great Whale River (Nunavik, Canada) under millions of cubic metres of debris and liquefied postglacial clay, disrupting sediment geochemistry and benthic communities. In the summer of 2021, and at three subsequent time points over 28 months post-disturbance, we documented biogeochemical and ecological responses along a 12-km transect from the landslide origin to the river mouth at Hudson Bay, measuring major and trace elements in sediment cores, dissolved species in porewater, and benthic macroinvertebrate community composition. Within the first year, approximately 0.7 kg of Hg and 340 kg of Pb accumulated at the sediment surface across affected sites. Near the river mouth, dissolved porewater Fe and Mn increased up to 5-fold relative to upstream sites, coinciding with sediment Hg concentrations reaching 45.9 ng g^-1 in deeper layers. By 11-16 months, these surface enrichments were leached or eroded downstream, indicating transient retention of landslide-derived trace elements. At 28 months, porewater Mn, Fe, sulfate and sulfide profiles exhibited redox zonation characteristic of reestablished diagenetic processes, coinciding with the reestablishment of a diverse benthic macroinvertebrate community. This study provides quantitative recovery timescales for a subarctic river system following landslide disturbance: <12 months for surface trace element export and 28 months for benthic recolonization. These recovery timescales are urgently needed as climate-driven increases in permafrost thaw and extreme precipitation intensify disturbance frequency across Arctic watersheds.