Frozen stocks and hidden fluxes: quantifying mercury release from coastal erosion along the Yukon coast, Canada

Ongoing climate warming accelerates permafrost thaw and coastal erosion in the Arctic, mobilizing sediment-bound mercury (Hg) that was previously locked in frozen ground. Its neurotoxic and bioaccumulating form, methylmercury (MeHg), poses serious risks for ecosystems, marine biota, and Arctic Indigenous Peoples. To better understand this risk, this study aimed to quantify the present Hg stock in terrestrial sediments of the Yukon Coastal Plain (YCP), Canada, to estimate the annual Hg flux from coastal erosion, and to evaluate the likely fate of released Hg in the marine environment.

Hg concentrations were measured for 158 terrestrial and marine sediment samples. Based on previous studies, the YCP was divided into 44 terrain units, consisting of several uniform depth layers. Layer-specific Hg contents were upscaled from the concentrations measured in terrestrial samples of the respective terrain unit and layer. Where no measurement was available, the Hg concentration was predicted using a random forest model. Calculated stocks per terrain unit were combined with erosion rates to derive Hg fluxes. Marine sediment Hg concentrations and MeHg measurements provided insights into offshore transport and transformation pathways.

The total Hg stock of the YCP was estimated at 381,080 kg (271,540 to 501,930 kg), of which about 113 kg (87 to 163 kg) are released annually into the Beaufort Sea. Highest Hg stocks and fluxes occurred in high bluffs at Herschel Island–Qikiqtaruk (HIQ) and the eastern part of the mainland coast. Marine sediments exhibited generally lower Hg concentrations than terrestrial sediments, implying that a substantial fraction of the released Hg is transported offshore, re-emitted into the atmosphere, or incorporated into biogeochemical cycling in the nearshore zone. Sedimentary MeHg concentrations were low, but substantial methylation might occur in the water column, where it can effectively be taken up by phytoplankton, entering the Arctic food web. Since sediment data alone cannot resolve exposure risk, future work should target Hg speciation in water and key biota to understand risks for ecosystems and local communities.