HYPER-ACIDITY AND METAL LOADING RELATED TO SHALE WEATHERING IN ARCTIC CANADA- TOXIC STEW OF THE SMOKING HILLS

The Smoking Hills, west side of Franklin Bay, Canadian Arctic, are characterised by emissions of smoke composed of water vapour and hot sulphuric acid gases from vent holes (termed bocannes) that are surrounded by variable, but often brightly coloured, metal-sulphate mineral deposits. Auto-combustion of organic rich mudstones of the Smoking Hills Formation, related to oxidation of disseminated pyrite, drives the natural burning process. Actively burning sites all occurred in areas with evidence of recent slumping that has exposed fresh surfaces of the bituminous shale. Bocannes tend to form within debris flow lobes that can extend well down-slope of the initial failure face. Extinct sites that are no longer venting gas are characterised by vividly coloured brick red to yellow deposits of thermally altered shale known as clinker deposits. Previous work in this area noted ponds with pH values as low as 3. Workers had suggested that ponds were acidified in a process similar to modern anthropogenic acid rain acidification of lakes, whereby clouds of acid gas from bocannes blown inland acidify ponds in the downwind direction. This previous work though did not examine the ponds relative to the bedrock geology. We show that acid ponds are restricted to outcrop of the Smoking Hills Formation. Some ponds show extreme acidity with pH as low as negative 1.44 (–1.44). In contrast, ponds that are being actively fumigated by burning shales, but lie on the Mason Formation, or glacial materials, are near neutral pH, showing that pH is moderated by the strong buffering capacity of the underlying bedrock. We hypothesize that the occurrence of acid ponds is related to a process more similar to natural acid mine drainage rather than acid rain. We tested this through lab experiments, whereby samples of the Smoking Hills Formation are shown to acidify waters within hours of exposure. As well, we document streams cutting down into the Smoking Hills Formation that rapidly drop in pH, in the absence of any fumigation. Along with low pH, these waters have extremely high trace metal content, creating some of the most toxic naturally occurring waters known (e.g. Cd is 7000x above drinking water limits). The weathering of the Smoking Hills Formation thus adds significant metal flux to river systems that flow into the nearby Arctic Ocean. Impact of climate warming, permafrost degradation and thaw slumping on enhancing this metal flux to arctic environments is uncertain.