Mineral weathering rates in glacial drift soils (SW Michigan, USA): New constraints from seasonal sampling of waters and gases at soil monoliths

Abstract

Soil solutions and gases were sampled along 200 cm deep soil profiles from four instrumented soil monoliths in southwest Michigan, established on coarse-grained glacial drift deposits. Seasonal sampling enabled evaluation of thermodynamic versus kinetic controls on carbonate- and silicate-mineral weathering rates, allowing better integration with past field hydrogeochemical studies of Michigan soil and surface water systems. Silicate-weathering products dominate water chemistry in the upper soil zones. Carbonate minerals, comprised of subequal amounts of calcite and dolomite, are only present at depths below 150 cm. When present, carbonate dissolution is rapid and soil water Ca 2+ and Mg 2+ concentrations increase dramatically as observed in other natural soil study sites in southern Michigan. Soil water saturation states are near equilibrium with respect to calcite and slightly less saturated with respect to dolomite. The divalent cations of soil waters and soil CO 2 both show a seasonal trend, with concentration maxima occurring in September and minima in April, suggesting that soil water Ca 2+ and Mg 2+ concentrations are under equilibrium control with carbonate solubility limited by temperature-dependent pCO 2 rather than by direct effects of temperatures. Importantly, monolith soil water Mg 2+/Ca 2+ and calcite and dolomite saturation states are lower than those of streams in the same watershed and also lower than those of soil waters in other Michigan watersheds. Because carbonate weight percentages and chemical compositions in these sites are similar, this difference likely reflects the short exposure path (thus short residence time) of soil waters to carbonate-rich horizons in the monoliths. The dissolution reactions of primary aluminosilicate minerals are incongruent with respect to Al and Si due to kaolinite formation. However, major cations (Ca 2+, Mg 2+, K + and Na +) are stoichiometrically released from silicate dissolution. Na ⁎ (soil water Na + after correction for atmospheric input and derived primarily from plagioclase weathering) exhibits much less seasonality than divalent cations, with only slight elevations observed in the summer months. Soil water H 4SiO 4 0 concentrations show seasonal variations similar to the divalent cations, but are determined by the balance between production (silicate-mineral dissolution) and consumption (kaolinite precipitation). Plagioclase and amphibole are below saturation, and these dissolution reactions must be kinetically controlled. Through a conservative tracer study, about 15% to 45% of applied Br passed out of the monolith profiles in 40–160 days and this long mineral–water contact time is especially important for slow reactions such as silicate dissolution. Based on water chemistry and discharge, bulk reaction rates of calcite, dolomite (Ca 0.5Mg 0.5CO 3), K-feldspar and plagioclase are calculated to be at 3400, 3100, 220, and 320 mol ha − 1 yr − 1 , respectively. Based on mass balance of soil composition, long-term plagioclase-weathering rates (over the past 12,500 years) are calculated at about 2400 mol ha − 1 yr − 1 , much higher than the current rates. This agrees with previous conclusions that weathering rates decrease with time, due to loss of reactive mineral surfaces. Furthermore, both long-term and short-term plagioclase dissolution rates in Michigan are relatively high compared to those in other watersheds with similar age, possibly due to fresh surfaces produced by glaciation, in combination with the high discharge and high plagioclase abundances.