『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 Ca2+ and Mg2+ 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 CO2 both shows a seasonal trend,
with concentration maxima occurring in September and minima in
April, suggesting that soil water Ca2+ and Mg2+
concentrations are under equilibrium control with carbonate solubility
limited by temperature-dependent pCO2 rather
than by direct effects of temperatures. Importantly, monolith
soil water Mg2+/Ca2+ 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 (Ca2+, Mg2+, 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 H4SiO40 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 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 (Ca0.5Mg0.5CO3), 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.
Keywords: Carbonates; Aluminosilicates; Soil water; Reaction kinetics;
Carbon dioxide; Hydrological tracers』
1. Introduction
2. Field site description and methodology
2.1. Soils and geology
2.2. Climate and precipitation chemistry
2.3. Monoliths and tracer experiments
2.4. Collection and analyses of soils, soil gas and soil water
samples
3. Results
3.1. Soil mineralogy
3.2. Water budget
3.3. Conservative solute tracer experiment
3.4. Soil water chemistry and soil zone pCO2
4. Discussion
4.1. Seasonal variation in solute concentrations
4.1.1. N+ and Cl- variation: atmospheric
contributions and evapoconcentration
4.1.2. Soil water Ca2+ and Mg2+, carbonate-mineral
dissolution, and soil zone pCO2
4.1.3. Sources and reactivity of NO3-
and SO42-
4.2. Controls on chemical weathering of carbonates and silicates
4.2.1. Soil exchangeable cation pool and its equilibrium with
soil solutions
4.2.2. Dolomite versus calcite dissolution
4.2.3. Residence time of water in the soil column
4.2.4. Silica, Na* and silicate weathering
4.3. Mineral weathering rates from solute fluxes and from soil
mass balance
4.3.1. Short-term weathering rates from solute fluxes
4.3.2. Long-term plagioclase-weathering rates from soil elemental
mass balances
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References