James I. Drever

The effect of land plants on weathering rates of silicate minerals

Abstract Land plants and their associated microbiota directly affect silicate mineral weathering in several ways: by generation of chelating ligands, by modifying pH through production of CO2 or organic acids, and by altering the physical properties of a soil, particularly the exposed surface areas of minerals and the residence time of water. In laboratory experiments far from equilibrium, 1 mM oxalate (a strong chelator of Al) has a negligible effect on the dissolution rate of alkali feldspars, but some effect on calcic feldspars and olivine. By analogy to oxalate, the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small, except perhaps in microenvironments adjacent to roots and fungal hyphae. The effect of pH on silicate mineral dissolution rate depends on pH: below pH 4–5, the rate increases with decreasing pH, in the circumneutral region the rate is pH-independent, and at pH values above around 8 the rate increases with increasing pH. Vegetation should thus cause an increase in weathering rate through the pH effect only where the pH is below 4–5. As an overall generalization, the effect of plants on weathering rate through changes in soil-solution chemistry is probably small for granitic rocks; it may be greater for more mafic rocks. It is the release of Ca and Mg from mafic rocks that has the greatest influence on the global CO2 budget. The effect of changes in soil physical properties on weathering rate can be major. By binding fine particles, plants can greatly increase weathering rates in areas of high physical erosion. Where erosion rates are lower, the effect of plants is less clear. On long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water. A major unknown in estimating the effect of the advent of land plants on weathering rates is the nature (thickness, particle size distribution, permeability) of the regolith on the pre-Silurian continents. The indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects.

The role of organic acids in mineral weathering

Organic acids and their anions (for brevity we shall use the term “acids” to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in solution of ions such as Al3+ that themselves affect mineral dissolution rate. In this paper we review the effects of organic acids on the dissolution rates of silicate minerals, particularly feldspars, under conditions approximating the natural weathering environment −25°C, pH 4–7 and with concentrations of organic acids comparable to those measured in soil solutions. Feldspar dissolution rates far from equilibrium increase with decreasing pH below pH 4–5. They appear to be independent of pH between pH 4–5 and about 8, and above pH 8 feldspar dissolution rates increase with increasing pH. Small chelating ligands such as oxalate appear to accelerate feldspar dissolution through complexation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enhancement to enhancements of a factor of 15, depending upon the data set, pH, and aluminum content of the mineral: there is a great deal of scatter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic acids do not appear to increase feldspar dissolution rates significantly. Dissolution rates must decrease as the solution approaches saturation with respect to the primary phase (the chemical affinity effect). Organic acids will influence chemical affinity by complexing Al (and possibly other elements) in solution and hence decreasing the chemical activity of Al3+. There are essentially no data on the effect of chemical affinity on feldspar dissolution rate at 25°C and mildly acid pH, so it is hard to evaluate the importance of organic acids in accelerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an important determinant of silicate dissolution rates in nature. Observed rates of silicate weathering in the field are typically much slower than predicted from laboratory experiments far from equilibrium, suggesting control by transport of solutes between “micropores” and “macropores” (“micropores” include fractures and crystal defects within grains). If such transport is rate-controlling, analysis of the effect of organic acids on weathering rates in nature in terms of dissolution rates far from equilibrium may be misleading.

The Chemistry of Weathering

Preface.- Chemical Models of Weathering in Soils.- Multicomponent Solid Solutions for Clay Minerals and Computer Modeling of Weathering Processes.- Dissolution Mechanisms of Pyroxenes and Olivines During Weathering.- The Effects of Complex-Forming Ligands on The Dissolution of Oxides and Aluminosilicates.- Kinetic Study of The Dissolution of Albite With a Continuous Flow-Through Fluidized Bed Reactor.- Interstratified Clay Minerals and Weathering Processes.- Formation of Secondary Iron Oxides in Various Environments.- Physical Conditions in Alunite Precipitation as a Secondary Mineral.- This Planet is Alive-Weathering and Biology, A Multi-Facetted Problem.- Solubilization, Transport, and Deposition of Mineral Cations by Microorganisms - Efficient Rock Weathering Agents.- Chemical Weathering and Solution Chemistry in Acid Forest Soils: Differential Influence of Soil Type, Biotic Processes, and H+ Deposition.- Proton Consumption Rates in Holocene and Present-Day Weathering of Acid Forest Soils.- Equilibrium and Disequilibrium Between Pore Waters and Minerals in the Weathering Environment.- Hydrogeochemical Constraints on Mass Balances in Forested Watersheds of the Southern Appalachians.- Past and Present Serpentinisation of Ultramafic Rocks An Example from the Semail Ophiolite Nappe of Northern Oman.- Manganese Concentration Through Chemical Weathering of Metamorphic Rocks Under Lateritic Conditions.- River Chemistry, Geology, Geomorphology, and Soils in the Amazon and Orinoco Basins.- List of Workshop Participants.

Chemical weathering in glacial environments

Do glaciers enhance or inhibit chemical weathering rates relative to other environments? The importance of glaciers in the global carbon cycle and climate change hinges on the answer. We show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge. Silica denudation rates are distinctly lower in glacier-covered catchments than in their nonglacial counterparts. Because sediment yields are high from glaciers, this suggests that water flux, rather than physical erosion, exerts the primary control on chemical erosion by glaciers. Potassium and calcium concentrations are high relative to other cations in glacial water, probably due to dissolution of soluble trace phases, such as carbonates, exposed by comminution, and cation leaching from biotite. Preferential weathering of biotite may result in higher 87 Sr/ 86 Sr in glacial runoff than expected from whole-rock compositions. Thus, although glaciers do not influence total chemical denudation rates at a given runoff, they may yield compositionally distinctive chemical fluxes to the oceans. Disruption of mineral lattices by grinding increases dissolution rates; this and high surface area should make glacial sediments exceptionally weatherable. Weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles.

CHEMICAL WEATHERING OF SILICATE ROCKS AS A FUNCTION OF ELEVATION IN THE SOUTHERN SWISS ALPS

Surface water and soil samples were collected from a series of small catchments on granitic gneiss in the Canton of Ticino in southern Switzerland. Elevations of the sampling points ranged from 220 to 2400 m; vegetation varied correspondingly from deciduous forest through coniferous forest to alpine pasture and essentially unvegetated rock and talus. Annual precipitation averaged 1.9 to 2.4 m. The concentrations of the major cations and silica in surface waters decreased more or less exponentially with elevation. The cationic denudation rate decreased from about 500 meq/m2 · y at the lower elevations to about 20 meq/m2 · y at the highest. Alkalinity decreased from 250 to about −7 μeq/1. Although total concentrations decreased with elevation, there were no clear systematic trends in the ratios of the concentrations of the major cations and silica. This suggests that the nature of the secondary minerals formed during weathering in the area does not change with elevation, despite great changes in soil type and environmental conditions. The clay mineralogy of the soils is dominated by unweathered and slightly weathered bedrock minerals: mica and chlorite, hydrobiotite, and poorly characterized mixed-layer material. Small amounts of kaolinite and smectite were observed in a few samples, but there do not appear to be any systematic trends in clay mineralogy with elevation. Mass-balance arguments suggest that the major (in terms of solute generation) weathering product is either kaolinite or a mixture of A1(OH)3 and 2:1 clays. The lack of dependence of weathering stoichiometry on elevation (a surrogate for several environmental variables) or solute concentrations perhaps reflects the importance of local relief, which did not vary systematically with elevation.

Chemical weathering in the foreland of a retreating glacier

Chemical denudation rates and strontium isotope ratios in streams vary substantially and systematically in the foreland of the retreating Bench Glacier in south-central Alaska. To study weathering of young glacier sediments, we sampled 12 streams draining a chronosequence of till and moraine soils derived from Cretaceous metagraywacke–metapelite bedrock. Both sediment age and vegetation cover increase with distance from the glacier. Cation denudation rates decline with increasing distance from the glacier, whereas silica denudation rates increase. Carbonate dissolution and sulfide oxidation account for roughly 90% of the solute flux from the youngest sediments. Biotite alteration accounts for 5–11% of the solute flux; its peak contribution is found in the glacier outlet stream. Silicate weathering is the dominant reaction only in the oldest sediments. In a laboratory dissolution experiment using fresh glacial sediment, carbonate dissolution dominated the solute flux during the first 700 hours, paralleling the behavior of young sediments in the field. In contrast to trends in the field, the silica flux did not increase after the carbonate was exhausted from the reactor. A possible reason for this difference is that establishment of vegetation causes an increase in silicate weathering. The 87Sr/86Sr ratio in the glacier outlet stream is greater than that in proglacial streams and in bulk rock, due to a greater contribution of biotite weathering in the outlet than in proglacial streams. Strontium isotope ratios decline with sediment exposure age in the proglacial streams, and are consistent with a carbonate source. Because the dominant weathering reactions in the young sediments are of carbonate and sulfide rather than silicate minerals, weathering at the glacier margin is not an important long-term sink for atmospheric CO2.

Mineral dissolution rates in plot-scale field and laboratory experiments

Abstract Mineral dissolution rates were measured on identical mineral material in field and laboratory experiments. Field dissolution rates were measured in 6 small (2 m2) plots on a spodosol in eastern Maine, U.S.A. The plots were irrigated with HCl at pHs 2, 2.5 and 3; soil solutions were collected by tension lysimeters at 25-cm depth. The composition of the soil solutions, together with the grain-size distribution and mineralogy of the soil, were used to calculate mineral dissolution rates. Laboratory dissolution experiments were performed on the 75–150-μm size fraction of soil from the site in flow-through reactors at pH-values corresponding to the pH of the bulk soil solution. The use of small plots and “untreated” minerals from the same plots eliminates many of the uncertainties encountered in previous field-laboratory comparisons. Dissolution rates observed in the field, normalized on the basis of geometrical mineral surface area, were smaller than laboratory rates by a factor of ∼200–400. The discrepancy can be explained by a number of factors. The major reason is probably imperfect contact between soil minerals and percolating solution. Macropore flow or limited exchange between water in micropores and the bulk soil solution could reduce the mineral surface area participating in the dissolution process. Retention of water in micropores could result in elevated pH-values and solute concentrations (particularly Al) near the mineral surface, resulting in lower dissolution rates. Differences in mineral surface characteristics between field and laboratory experiments can be largely excluded as a reason for the observed discrepancy since identical mineral material was used in both sets of experiments. However, part of the discrepancy might be explained by the dissolution rate not scaling with geometrical surface area.

Neutralization of atmospheric acidity by chemical weathering in an alpine drainage basin in the North Cascade mountains

The most important weathering reaction that neutralizes incoming atmospheric acidity in the South Cascade Lake basin is weathering of calcite, which occurs in trace amounts in veins, on joint surfaces, and as a subglacial surficial deposit. Although the basin is underlain by igneous and high-grade metamorphic rocks, weathering of plagioclase is quantitatively negligible; the principal silicate weathering reaction is alteration of biotite to vermiculite. These conclusions are based on mass-balance calculations involving runoff compositions and on mineralogical observations. For predictive modeling of the effects of increased acid deposition, it is essential to identify the relevant weathering reactions. Feldspar weathering is commonly not an important source of solutes in alpine basins underlain by granitic rocks. 30 references, 2 figures, 1 table.

The effect of oxalate on the dissolution rates of oligoclase and tremolite

Abstract The effect of oxalate, a strong chelator for Al and other cations, on the dissolution rates of oligoclase feldspar and tremolite amphibole was investigated in a flow-through reactor at 22°C. Oxalate at concentrations of 0.5 and 1 mM has essentially no effect on the dissolution rate of tremolite, nor on the steady-state rate of release of Si from oligoclase. The fact that oxalate has no effect on dissolution rate suggests that detachment of Si rather than Al or Mg is the rate-limiting step. At pH 4 and 9, oxalate has no effect on the steady-state rate of release of Al, and dissolution is congruent. At pH 5 and 7, oligoclase dissolution is congruent in the presence of oxalate, but in the absence of oxalate Al is preferentially retained in the solid relative to Si. Large transient “spikes” of Al or Si are observed when oxalate is added to or removed from the system. The cause of the spikes is unknown; we suggest adsorption on feldspar surfaces away from sites of active dissolution as a possibility. Solutions in the reactors are undersaturated with respect to both gibbsite and kaolinite, so neither the spikes nor the incongruent dissolution can be explained by formation of a secondary precipitate. The rate of dissolution of tremolite is independent of pH over the pH range 2–5, and decreases at higher pH. The rate of dissolution of oligoclase in our experiments was independent of pH over the pH range 4–9. Since the dissolution rate of these minerals is independent of pH and organic ligand concentration, the effect of acid deposition from the atmosphere on the rate of supply of cations from weathering of granitic rocks should be minor.

Carbon isotope systematics of monoaromatic hydrocarbons: vaporization and adsorption experiments

Abstract This study investigates the carbon isotope systematics of benzene, toluene, ethylbenzene, and xylene monoaromatic hydrocarbons (BTEX) with regard to an improved understanding of the behavior of these compounds in the subsurface, particularly during remediation processes. We found that fractionation effects due to vaporization are small and positive for all compounds studied (Δ 13 C vapor–liquid ≌+0.2‰), and that fractionation effects due to soil adsorption are also likely to be small ( 13 C-labeled compounds cannot be resolved from unlabeled compounds by HPLC). We also evaluated use of the isotopic composition of contaminants as tracers of source and migration in the subsurface by performing a survey of the bulk isotopic composition of commercially available sources of BTEX. The results indicate that a wide range of δ 13 C values exist (e.g., benzene, −23.87‰ to −29.40‰). Our work suggests that stable C isotope analysis has great potential for qualifying, and possibly quantifying, the subsurface processes affecting contaminant concentrations. In particular, stable isotope analysis may be especially beneficial for monitoring the efficacy of abiological and biological remediation efforts.