Formation and evolution of soils from an acidified watershed: Plastic Lake, Ontario, Canada

Abstract

The Plastic Lake watershed contains podzols developed on glacial tills deposited 12,000 years ago. Present-day, cationic fluxes from the soils are greater by a factor of 2 than long-term fluxes averaged over the age of the tills. The high rates of present-day chemical weathering may be a result of increased input of anthropogenic acids into the Plastic Lake watershed. Time-averaged proportions of cations leached from the soils are strikingly different from the proportions of cations now being leached, indicating that the character of chemical weathering has changed over time. Weathering was and is dominated by mineral dissolution, but cation exchange has become increasingly important as the soils have matured. The large amount of Ca now released to the soil solutions probably is derived from exchange substrates, promoted by preferential uptake of Al onto the substrates. Aluminum is derived primarily through dissolution of feldspars. Bulk compositional analyses of soil profiles demonstrate that feldspars of the AE horizon release base cations (Na, K, Ca) and Al to solution in near-stoichiometric proportions, just as is observed experimentally for feldspar dissolution in acidic solutions. Furthermore, surface area normalized dissolution rates of plagioclase, K-feldspar, quartz, and micas of Plastic Lake soils are similar to rates obtained experimentally. The favourable comparison between natural and experimental dissolution rates, and near-stoichiometric release of base cations and Al from feldspars to acidic solutions in both natural and experimental settings, suggests that recent laboratory release rates can be applied to dissolution of feldspars within the AE horizon of the Plastic Lake Podzols. Surface area-normalized, time-averaged, dissolution rates of primary minerals of Plastic Lake soils are significantly greater than present-day rates measured for mature soil profiles. Rapid release from Plastic Lake soils probably results from reaction of acidic soil solutions with highly reactive biotite and ultrafine grains of feldspars produced by comminution during glaciation. Exhaustion of these reactive phases may well explain the low leach rates observed for mature profiles. After just 12,000 years of weathering, biotite has been effectively exhausted in the Plastic Lake soils and much of the fine-grained feldspar probably has been dissolved. Time-averaged concentrations of elements removed from the soil, recast into essential mineralogy, indicate that vermiculite weathers most rapidly, followed closely by plagioclase. K-feldspar weathers less rapidly than plagioclase but more rapidly than quartz or hornblende. Al-silicate, possibly imogolite, and goethite are added to the soils during weathering. Plagioclase dissolution consumes more acid than all other reactions, accounting for 60% of the total acid consumed. Combined with K-feldspar, they account for more than 80% of the total acid neutralized. The dissolution rate of quartz is one-half that of plagioclase; furthermore, the rate is consistent with the experimentally determined rates. The combination of high abundance and dissolution rate results in large amounts of quartz being dissolved from the soils of Plastic Lake; it may also contribute significantly to total elemental fluxes from other catchments.