Burning shales and extreme acidity - Toxic stew in the Smoking HillsWednesday, November 20, 2019 - 8:20am to 8:40am Theatre Two
The Smoking Hills, east side of Cape Bathurst, 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, mineral deposits. The burning has been shown to be related to combustion of organic matter and is associated with oxidation of disseminated pyrite within the shales. 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. Auto combustion of the Smoking Hills Formation is not restricted to sea cliffs of Franklin Bay as thought by earlier workers. We observed that both bocannes and clinker sites occur over a wide area of Cape Bathurst, between Franklin Bay and the Anderson River, that occur in clear association with outcrops of the bituminous Smoking Hills Formation. Actively burning sites all occurred in areas with evidence of recent slumping that has exposed fresh surfaces of the bituminous shale, either by sea or stream erosion of outcrops. Also, bocannes form not only directly below the failure faces, but within debris flow lobes extending well down-slope of the initial failure.
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. Along with low pH, these waters have extremely high trace metal content, creating some of the most toxic naturally occurring waters known.