Subarctic weathering of mineral wastes provides a sink for atmospheric CO₂

The mineral waste from some mines has the capacity to trap and store CO₂ within secondary carbonate minerals via the process of silicate weathering. Nesquehonite [MgCO₃•3H₂O] forms by weathering of Mg-silicate minerals in kimberlitic mine tailings at the Diavik Diamond Mine, Northwest Territories, Canada. Less abundant Na- and Ca-carbonate minerals precipitate from sewage treatment effluent deposited in the tailings storage facility. Radiocarbon and stable carbon and oxygen isotopes are used to assess the ability of mine tailings to trap and store modern CO₂ within these minerals in the arid, subarctic climate at Diavik. Stable isotopic data cannot always uniquely identify the source of carbon stored within minerals in this setting; however, radiocarbon isotopic data provide a reliable quantitative estimate for sequestration of modern carbon. At least 89% of the carbon trapped within secondary carbonate minerals at Diavik is derived from a modern source, either by direct uptake of atmospheric CO₂ or indirect uptake though the biosphere. Silicate weathering at Diavik is trapping 102-114 g C/m²/y within nesquehonite, which corresponds to a 2 orders of magnitude increase over the background rate of CO₂ uptake predicted from arctic and subarctic river catchment data.