Burning shales, acid ponds, and toxic waters – the long term impact of Cretaceous volcanism on the Canadian Arctic


Stephen E. Grasby

From Geological Survey of Canada

Hosted by Henrik Svensen

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    The Smoking Hills is an area of naturally burning Late Cretaceous organic- and metal-rich mudstones (Smoking Hills Formation) in the Canadian Arctic. Auto-combustion is related to oxidation of disseminated pyrite in areas with recent slumping that has exposed fresh surfaces to oxygen and moisture that initiates combustion. Actively burning sites, known as bocannes, are characterised by emissions of hot sulphuric acid gas. Ground temperatures at bocannes could not be measured, as the thermometer melted, but the presence of paralavas suggest > 1000 °C. Extinct bocanne sites are characterised by vividly coloured brick red to yellow deposits of thermally altered mudstone known as klinker deposits. Outcrops of the Smoking Hills Formation are also characterised by hyper-acidic ponds with some of the lowest pH waters in the world, as low as negative 1.44 (–1.44), related to processes similar to acid mine drainage. Streams cutting down into the Smoking Hills Formation likewise rapidly drop in pH, forming acid-streams. Along with low pH, these waters have extremely high metal content, creating some of the most toxic naturally occurring waters known (e.g. Cd is 7000x above drinking water guidelines). This metal flux further contaminates larger river systems that flow into the nearby Arctic Ocean and potentially impacts marine mammals. Climate warming, permafrost degradation, and thaw slumping is enhancing this metal flux to arctic environments.
    How did this all start? Paleo-environmental studies show that the Smoking Hills Formation mudstone was deposited under euxinic conditions in a stratified water column. Surface waters were oxic and highly productive. Geochemical data and mineralogy support that high productivity was related to a period of enhanced volcanism that likely supplied nutrients to the water column driving productivity and in turn bottom water euxinia and metal drawdown. These volcanic released metals are being recycled into the modern otherwise pristine arctic environment, causing widespread environmental impacts millions of years after the eruption. Potential volcanic sources include High Arctic Large Igneous Province (HALIP) eruptions or a period of enhanced arc magmatism activity.
Published Dec. 15, 2021 9:58 AM - Last modified Jan. 24, 2022 8:15 AM