Tracking the long-term limnological impacts of silver mining near Keno City (Yukon, subarctic Canada)

Mining in northern Canada has been known to cause major environmental problems; however, historical monitoring data are scarce or non-existent. Here, we use a multi-proxy (metals, bioindicators, pigments) paleolimnological approach to track the impacts of mining activity near Keno City, on the traditional land of the First Nation of Na-Cho Nyäk Dun, in central Yukon (Canada). Silver was discovered in the early 1900s, primarily on or between two hills (Galena Hill and Keno Hill). Intensive mining has taken place ever since, with brief hiatuses dependent on ore prices and ownership of the claims. Christal Lake, a shallow site located in the valley between both hills, lies near many historical and current mines, and was once the site of a processing mill. Geochemical data show elevated background concentrations of many metals and faithfully track known mining activity. Interestingly, background (pre-mining) sediment concentrations of arsenic, cadmium, and zinc were all elevated above the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life, reflecting the natural weathering of elements in high concentrations from the local catchment. These, and other metal(loid)s, increased and peaked in sedimentary concentration after ca. 1920s, when intensive mining began. Sedimentary chlorophyll-a concentrations declined with the rise of metal concentrations, although values increased again slightly in more recent sediments, perhaps reflecting the decline in recent metal inputs and reclamation of historic mine sites. Meanwhile, subfossil diatom assemblages were dominated by small benthic Fragilaria sensu lato taxa, whose assemblage composition only changed subtly with mining (similar to other shallow, non-acidified sites in the highly metal-impacted area of Norilsk, Siberia). There was no biological evidence of acidification, likely due to the neutralizing effect of the carbonate-rich catchment. Cladoceran subfossils were only present in very low numbers throughout the core, reflecting both the shallow nature and high background metal concentrations in the lake. Collectively, these data show the long-term impacts of silver mining in this subarctic environment.