Abstract

Forest rings are large circular features common in boreal forests in Ontario, Canada, characterized by ring-shaped topographic depressions in carbonate-rich soil. This paper documents the compositional variation of soil and of the peat that commonly fills the depression, along transects across two representative rings: one centred on an accumulation of CH4 in glacial sediments and the other on H2S in both glacial sediments and bedrock. Clayey mineral soil at the ring edge (annulus) shows low pH, oxidation-reduction potential (ORP), Ca and carbonate, and high Al, Fe and Mn by both aqua regia digestion and a 0.25M hydroxylamine hydrochloride leach (0.25M NH2OH.HCl at 60°C). Antithetic responses occur in the overlying peat, including elevated carbonate and pH over the areas with low pH, ORP and carbonate in the mineral soil. The observed relationships suggest vertical migration of carbonate species from mineral soil into peat at the annulus, and lateral migration from the annulus to adjacent areas in the mineral soil. The geochemical data support the hypothesis that forest rings are the surface expression of reduced chimneys, similar to those observed over metallic mineral deposits, despite both these sites being known to be barren. Strongly negative ORP values in shallow soils in the annulus suggest autotrophic microbiological activity contributes to the sharp change in redox conditions at the ring boundaries. The similar geochemical responses at forest rings and soils over mineral deposits show that these features can be used to understand the variation in redox, pH, metals and soil hydrocarbons over mineral deposits and to help differentiate ore-related from secondary geochemical features due to the presence of a reduced chimney.

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