Soil Mineral Weathering Research Articles

Hematite and calcite coatings on fossil vertebrates

ABSTRACT Hematite coatings are common on vertebrate fossils from Paleocene/Eocene paleosol deposits in the Bighorn Basin, Wyoming. In general, hematite coatings are found only on fossils and are limited to soils exhibiting hydromorphic features and moderate maturity. Petrographic and isotopic evidence suggests that hematite and micritic calcite formed at nearly the same time in a pedogenic environment, whereas sparry calcite formed later at greater burial depths. The parent material of paleosols is rich in iron, supplying an ample source of iron for hematite formation. Decomposition of animal tissues around bones may enhance the weathering of iron-bearing minerals in soils surrounding carcasses, while the bones might provide favorable sites for iron accumulation. The predominance of discrete smectite, together with regional geothermal history, suggests that burial temperatures have not exceeded 70°C. Hematite coatings on fossils can serve as a substrate for geochemical analysis in continental paleoclimatic research, owing to their pedogenic origin, abundance, and resistance to diagenetic alteration.

Determination of mineral surface area in relation to the calculation of weathering rates

The release of base cations by the weathering of soil minerals provides the primary buffer against the detrimental impact of acid deposition on the soil. The rate of release of base cations will be governed by mineral composition and the exposed surface area of mineral grains. In theory it is possible to calculate the surface area of a mixture of grains on the basis of particle size and geometry. An alternative is the use of correlations. It has been suggested (Sverdrup et al. 1990, Nordic Council of Ministers 98) that mineral surface area can be related to soil particle size distribution via a simple empirical equation. This contribution compares the surface areas of soils as calculated by geometric equations and Sverdrup's equation with actual BET measurements. The equations tested do not appear to be applicable to the soils used in the current work. Modified versions of the equation suggested by Sverdrup, applied to subsets of our data, based on sites and soil types, yield better results for the soils used in the current work. Statistically significant improved fits are obtained when soil mineralogy as well as soil texture is taken into account.

Fluid flow through clayey soils: stable isotope and mineralogical evidence

AbstractThe evaluation of clays as barriers to fluid movement can be improved by geochemical methods that provide ways to examine the reactivity and weathering of minerals in soils and sediments. X-ray radiography, X-ray powder diffraction, and stable isotope geochemistry provide new data from field locations in the Mississippi River Delta of Louisiana indicating that the clays are not effective barriers to the vertical migration of fluids in the shallow subsurface. Systematic changes in the mineral assemblages, the soil structure and the δD and δ18O values of time clay fractions can best be explained by an alteration sequence produced as the originally smectiterich clay mineral assemblage was kaolinized by percolating groundwater.

Soil acidification and cycling of metal elements: cause–effect relationships with regard to forestry practices and climatic changes

Anthropogenic changes in atmospheric deposition have led to accelerated soil acidification in many sites, especially on soils and/or bedrocks low in acid neutralizing capacity. This paper presents data from field studies in forest sites in temperate and tropical regions, which are suitable to discuss cause–effect relationships and give evidence of toxic effects caused by aluminum and heavy metal ions in soil solution. The discussion includes metals of anthropogenic origin, brought to the site by atmospheric transport or by direct deposition, and also metals originating from weathering of bedrocks and soil minerals. Possible restrictions of forestry practice to counteract negative effects of soil chemical changes and destabilization of forest ecosystems are discussed.

Chemical weathering and cation loss in a base-poor watershed

Weathering of minerals in soils provides important plant nutrients and consumes acidity, yet mineral weathering is difficult to observe and quantify. We derived present-day weathering flux estimates for soil minerals in a forested watershed in central New Hampshire using a geochemical mass balance. The Cone Pond watershed is characterized by low-alkalinity surface waters and acidic Spodosols developed on a thin mantle of locally derived till, making it susceptible to continued inputs of acid precipitation. Weathering reactions were developed on the basis of observed patterns of mineral abundance in the soil profile and measured mineral chemistry. The dissolution of approximately 171 mol ha −1 yr −1 of plagioclase feldspar is the predominant weathering reaction at Cone Pond. Weathering fluxes of hornblende, biotite, chlorite, and potassium feldspar ranged from 3 to 20 mol ha −1 yr −1 , an order of magnitude lower. When normalized to their abundances in the soil, however, hornblende and chlorite had the fastest weathering rates, as measured by cation release. Chemical weathering, mainly of plagioclase, resulted in the neutralization of only 52% of incoming acidity. Furthermore, silicate weathering could only account for the release of about 53 mol ha −1 yr −1 of Ca, compared to the observed net output (stream loss minus bulk precipitation input) of 75 mol ha −1 yr −1 . On the basis of these observations and a companion study, we conclude that current acid inputs exceed the ability of Cone Pond soils to neutralize hydrogen ion, and that depletion of approximately 22–53 mol ha −1 yr −1 of Ca from labile soil pools is occurring in this area. The rate of Ca depletion is one-third to one-eighth of the rate estimated for a more base-rich watershed nearby.

Estimation of weathering rates for critical load calculations in Finland

The formation of base cations through mineral weathering in forest soils is one of the key parameters in calculating critical loads. Weathering rates in Finland have been estimated using a variety of methods. In the first approach, three weathering rate categories were assigned to soils according to the bedrock type. The second approach was based on an empirical relationship obtained from Swedish field studies. Changes in zirconium content through the soil profile were used to estimate element losses in soil after deglaciation. These calculated losses correlated well with the total calcium and magnesium concentrations in till C-horizons and the effective temperature sum (ETS). Comprehensive geochemical data for the parent till fine fraction (<0.06 mm) was available through the reconnaissance scale till geochemical mapping program of the Geological Survey of Finland (GSF). The equations obtained from Swedish studies were based on the use of the coarse (<2.0 mm) till fraction, and the differences in element concentrations between the fine and coarse size fractions remained a potential source of uncertainty estimating overall weathering rates. In the third approach, new geochemical data from the <2.0 mm till fraction from southern Finland were used to make new weathering rate estimates. The use of soil geochemistry instead of bedrock map classification clearly led to an improvement in the estimates of soil weathering rates in glacied terrains. The use of the fine till fraction (<0.06 mm) in the zirconium approach generally resulted in overestimations of the weathering rate. The new geochemical data for the coarse till fraction (<2 mm) are now consistent with the input requirements of the zirconium method, although the results still require further evaluation. Finnish soil profiles have a shorter weathering history than most of the Swedish ones and the uppermost layer in Finnish podsols has in some cases developed in a different till layer than the C-horizon.

Forest soil mineral weathering rates: use of multiple approaches

Knowledge of rates of release of base cations from mineral dissolution (weathering) is essential to understand ecosystem elemental cycling. Although much studied, rates remain enigmatic. We compared the results of four methods to determine cation (Ca + Mg + K) release rates at five forested soils/sites in the northcentral U.S.A. Our premise was that multiple approaches, each with their own specific strengths and weaknesses, would yield a “best” overall estimate. We used (1) a cation input-output budget on a pedon scale; (2) trends in elemental and mineral depletion in silt-size particles; (3) a laboratory batch dissolution technique, with the results adjusted for field conditions; and (4) a steady-state soil chemistry model, PROFILE. The soils included a loamy sand Typic Udipsamment, a sandy loam Spodic Udipsamment, a fine sandy loam Typic Dystrochrept, a very fine sandy loam Glossic Eutroboralf, and a clayey Glossic Eutroboralf. Weathering rates varied among both soils and methods, and neither methods nor soils could easily be grouped; the data spanned a continuum with overlapping ranges of least significant differences. Although the assumptions necessary for some methods were better suited to specific soils, we rejected only one method-soil combination as being inappropriate (input-output budget for the clay). Mean release rates for the sum of cations ranged from 470 eq ha −1 yr −1 for the clayey soil, to 460 for the fine sandy loam soil, to 430 for the very fine sandy loam soil, to 375 for the sandy loam soil, to 195 for the loamy sand soil. These rates are lower than those reported for similar soils in the literature because most reported rates are based on watershed studies. Our low rates of cation release within soil pedons, the ultimate source of nutrient ions for plant growth, has implications for estimated nutrient budgets and long-term soil sustainability.

Modification of Snowmelt Chemistry by Forest Floor and Mineral Soil, Northern Michigan

AbstractPrecipitation, snowpack, snowmelt, forest floor percolate, soil water, and streamwater chemistry were studied throughout winter for 7 yr in a small (176 ha) Northern Michigan watershed vegetated by northern hardwoods. The objective was to see how precipitation and snowmelt chemistry is modified before it reaches the stream. Intermittent snowmelt occurred throughout winter, and 65 to 78% of total snowpack loss of H+, NH+4, NO−3, and SO2−4 occurred before peak snowpack and snowmelt. From 70 to 95% of snowmelt H+, NH6+4, and NO−3 was retained in the forest floor and shallow mineral soil by exchange, adsorption, or biological uptake. Snowmelt SO2−4 concentration was enriched in forest floor percolate and shallow soil water probably by organic mineralization, low adsorption, and perhaps desorption. The significant relationship between dissolved organic carbon (DOC) and SO2−4 concentration in forest floor percolate and shallow soil water indicates organics may contribute to reduced SO2−4 adsorption. Sulfate and organic anions were major contributors to base cation (CB) flux in the forest floor while SO2−4 was related to CB flux in shallow soils. A decline in mineral soil and streamwater DOC concentration and increase in streamwater HCO−3 relative to concentrations in forest floor percolate suggests the relative importance of organic decomposition decreased and soil mineral weathering increased with soil depth. High streamwater Ca2+, Mg2+, and HCO−3 concentrations indicated that soil mineral weathering was a major process altering snowmelt chemistry at the watershed level.

The calculation of base cation release from mineral weathering in soils derived from Palaeozoic greywackes and shales in upland UK

Weathering rates for soil profiles and catchments developed on greywacke and shale parent materials have been calculated using four common methods cited in the literature (conservative element depletion, the PROFILE model, catchment input-output mass balance (budgets) and the MAGIC model). A comparison of the results suggests that for any single method the results are consistent. However, comparison between methods suggests a greater variation and systematic difference between weathering rates calculated from soil profiles compared to those calculated on a catchment basis. The reason for these differences and the implications for acidification and critical load work are discussed.

Phyllosilicate Distribution and Origin in Aridisols on a Granitic Pediment, Western Mojave Desert

AbstractThere is considerable uncertainty about the extent of mineral weathering and neosynthesis in arid soils. Although researchers have speculated that smectite neosynthesis can occur in soils containing opaline silica cement, no evidence has been presented to support this hypothesis. We investigated the mineralogy of two Aridisols in the western Mojave Desert, California, to study phyllosilicate distribution and origin in soils with opaline silica. The clay fraction of both soils is dominated by Al‐rich, dioctahedral smectite, characterized by a Mg‐saturated d(001) spacing of 1.52 nm and a d(060) spacing of 0.149 to 0.150 nm. This smectite is also present in silt and sand fractions of deeper horizons, where it exists mainly as microagglomerates of clay‐sized crystals. Biotite is most abundant in silt and sand fractions of near‐surface horizons of both pedons, where physical weathering is greatest. Deep, alkaline, silica‐rich horizons of both pedons contain more silt‐ and sand‐sized vermiculite than biotite, probably due to rapid chemical weathering of biotite to trioctahedral vermiculite. Clay‐ and silt‐sized hydroxy‐interlayered 2:1 minerals are present in the upper horizons of these soils. Given the lack of gibbsite and substantial amounts of kaolinite and Al hydroxy‐interlayered 2:1 phyllosilicates, we propose that neosynthetic dioctahedral smectite was the dominant sink for Al released by feldspar weathering in these Aridisols. Silica not consumed in smectite neosynthesis can cement microagglomerates. We further speculate that minimal physical weathering in the alkaline, high‐silica environment of deep horizons favors aggregation of smectites to silt‐ and sand‐sized microagglomerates.

Biogeochemical controls on aluminum chemistry in the O horizon of a red spruce (Picea rubens Sarg.) stand in central Maine, USA

This study examined the biotic and abiotic processes controlling solution chemistry and cycling of aluminum (Al) in the organic horizons of a northern coniferous forest ecosystem. A mass balance budget indicated that aboveground inputs of Al to the O horizon averaged 0.9 kg ha−1 1 yr−1, with major inputs accounted for by litterfall (69%), followed by precipitation (21%), and net canopy throughfall plus stemflow (10%). Estimated leaching losses of Al from the O horizon averaged 2.1 kg Al ha-1 yr1. We hypothesize that the difference between measured Al inputs and outputs can be accounted for by Al release from weathering of soil minerals admixed into the O horizon. Variations in O horizon solution Al chemistry were influenced by a number of factors, including pH, Al equilibria with different solid-phase organic exchange sites, and Al complexation with humic ligands in soil solution.

Rates and processes of mineral weathering in soils developed on greywackes and shales in the Southern Uplands of Scotland

Critical loads of acid deposition are exceeded in parts of the Southern Uplands of Scotland where base saturation in the topmost mineral horizons in many soils developed on greywackes and shales is 4.3. The base cation least depleted is Ca and this is in sharp contrast to current weathering rates calculated from input-output budgets where Ca is the main base cation being exported. This discrepancy may be due to a contribution to the output from easily soluble Ca-bearing minerals (e.g. calcite) in narrow veins and fractures in the bedrock.

Soil nutrient supply in natural and managed forests

Soils differ in their ability to supply the nutrients necessary to sustain forest productivity. Nutrients are added through natural processes such as weathering of primary and secondary soil minerals, mineralization of soil organic matter including the forest floor layer, fixation of nitrogen primarily through symbiotic microorganisms, and natural or induced atmospheric deposition. Nutrients become unavailable for plant uptake through immobilization by soil microorganisms and through chemical and mineralogical reactions including precipitation and adsorption reactions and ionic fixation within lattice structures of clay minerals. Losses of nutrients can take place through soil leaching and erosional processes. Nutrients can also be added or lost through human activities such as fertilization and harvesting. Nutrient supply continually shifts with the rate and direction dependent on the prevailing processes in the soil system, but subject to overriding human influence. Over relatively short periods of time, the soil nutrient supply can be subject to seasonal fluctuations. Factors affecting long-term nutrient availability are functions of soil mineralogy, the rate of mineralization of the organic matter of the soil and forest floor layer, and plant-soil relationships of the species occupying the site (deciduous vs. coniferous species, deep vs. shallow rooting, symbiotic nitrogen fixation). The long-term stability of the soil nutrient supply is of increasing concern in the face of a diminished forest land base, increased demand for forest products, and reluctance to apply nutrients to many forest areas because of environmental or economic constraints. There are questions to consider in evaluating the nutrient sustainability of forest areas if we expect to maintain the long-term nutrient stability of natural and managed forest ecosystems.

Aluminium and sulphate in acidic soils and groundwaters on the Swedish west coast

Acidic groundwaters and soils in Halland County (Hailands lan), southwest Sweden, have been investigated with respect to conditions of soluble aluminium (Al) and sulphate (SO4 2−. Basic Al-sulphate, Fe-oxide, Al-oxide, Al-hydroxide and clay minerals, are discussed and evaluated in their roles for governing Al and SO4 2− in the groundwaters. Based on this investigation, it is suggested that Al3+ solubility is controlled by amorphous Al-hydroxide. The SO4 2− in the groundwaters will depend primarily on the H2SO4 input. The H2SO4 load enhances soil mineral weathering which enhances the production of Fe-oxides, i.e. anion exchange surface sites, to which groundwater SO4 2− attain adsorption equilibra. The factors that control solubility of Al and SO4 2− are both influenced by the acidity in the soil catenas which in the area largely depend on the H2SO4 input. Clay minerals such as illite, smectites, halloysite, and variable composition Al-silicates do not exert strong control on Al in the groundwaters investigated.

Soluble aluminium- and iron-organic complexes and carbon cycle in Hapludaffs and Haplorthods under forest and cultivation

In loamy Hapludalfs and sandy Haplorthods under forest and cultivation, the annual C, Al and Fe balances are presented in order to indicate element transfer into the subsoil. Aluminium and iron in the soil solution are separated into inorganic and organic fractions. In the Typic Hapludalf under forest initial podzolization is documented by an increase in Al release, its translocation into the subsoil and soil acidification. The Al3+ concentration is 2.7 mg l−1 and probably phytotoxic. The Hapludalfs under cultivation are influenced by stagnant water and the main part of iron is translocated in iron-organic complexes from the Bt into the subsoil. The aluminium release and transfer is minimal. In one Hapludalf erosion hinders an accumulation of humus, in spite of a C surplus. Extensive soil management in the forest and intensive cultivation without application of manure are incompatible with soil protection. In a Typic Haplorthod under spruce, podzolization is documented by the translocation of carbon and especially aluminium. A low pH induces weathering of soil minerals with a high release of Al3+ and its translocation into the subsoil. Its concentration in the topsoil is extremely phytotoxic. In a similar Typic Haplorthod under cultivation the carbon and metal sink (Bh/Bhs) is being decomposed and has become a source of carbon and iron-organic complexes; these are translocated into the subsoil. The natural accumulation process has been reversed by man.

Calculating field weathering rates using a mechanistic geochemical model PROFILE

A new model for base cation release due to chemical weathering of soil minerals has been developed based on transition state theory, and included in the integrated soil chemistry model PROFILE. The data required for model application can be operationally determined on soil samples, making the model generally applicable and independent of any type of calibration. The model considers the contribution to the weathering rate from 12 groups of the most common primary and secondary minerals of soils, reacting in separate reactions with H +-ion, H 2O, CO 2 and organic acids expressed as dissolved organic carbon (DOC). The weathering rate sub-model couples the effects of dissolved Al and base cations on the reaction mechanisms. The model takes into account changes in soil temperature, different chemical conditions, the effect of vegetation interactions with the soil and N transformations. The kinetic coefficients and reaction orders are based on a complete re-evaluation of weathering data available in the literature, and additional kinetic data determined by the authors. Data from 23 different independent determinations of the field weathering rate from 15 sites in Scandinavia, Central Europe and North America were compiled and used to verify the model. The model is capable of estimating the release rate of base cations due to chemical weathering from information on soil mineralogy, texture and geochemical properties of the order of ±20% of the rate determined by independent methods. The results indicate that small amounts of dark minerals like epidote and hornblende, and the plagioclase content, largely determine the field weathering rate.

Transport of soluble salts in a large semiarid basin: River Murray, Australia

Median annual concentrations of Cl, SO 4, HCO 3, Na, K, Mg and Ca increase greatly downstream in the River Murray along its 2500 km route to the sea. Chloride increases from less than 2 mg l −1 near the headwaters to about 170 mg l −1 near the downstream end of the main channel. Water near the head of the river has HCO 3 balanced by similar equivalents of Na, Ca and Mg, while higher salinity water in the downstream half of the river is dominated by Na and Cl. Sources of these dissolved ions include: (a) sea-derived airborne salts, contributing essentially all of the Cl, Br and 80% of the Na, and (b) rock-derived ions from weathering of sedimentary rock and soil minerals, contributing most of the observed HCO 3, 80% of the Ca, and 20–25% of the Mg and SO 4. Much of the change in chemistry along the axis of the Murray is due to influx of saline groundwater which has a major ion composition similar to sea water. Inflows of irrigation drainage water also enhance downstream transport of salts, especially HCO 3. The stable isotope composition of the River Murray ( δ 2H and δ 18O) begins with relatively depleted heavy isotope abundances (−47‰, −8‰) in the uplands of the southeastern margin of the Murray-Darling basin, increasing steadily by + 30‰ and + 6‰, respectively, due to evaporative enrichment from river surfaces and influx of drainage returned from irrigation diversions. Despite the similarity of groundwater major ion chemistry to that of sea water, stable isotope compositions of groundwaters clearly demonstrate that water molecules from past marine incursions have been completely flushed from the system. Similarly, the Cl/Br ratios of groundwaters are not appreciably different from seawater values, indicating that dissolution of fossil evaporites does not contribute significant salt to the groundwaters or the river.

Etch-pit size and shape distribution on orthoclase and pyriboles in a loess catena

Etch pits on 50–100 μm orthoclase and pyribole grains from the E, B, and C horizons of a catena or drainage sequence of well-drained, somewhat poorly drained, and poorly drained loessal soils were analyzed using SEM and digital image analysis. The objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces. Etch-pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals. This indicated low growth rates for etch pits in the poorly drained soil. Analysis of etch-pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil, possibly resulting from a greater growth rate; whereas etch pits on orthoclase grains were more elongated and irregular in the well-drained soil. The larger, more elongated and irregularly shaped etch pits on orthoclase from the well-drained soil, indicative of a greater pit growth rate, is consistent with a flushing model of mineral weathering in soils. The etch-pit size distribution on pyriboles is consistent with the flushing concept, but the shape data indicates that other mechanisms of weathering may be involved.

A new method for determining mineral weathering rates in soils

Abstract A new procedure based on measuring the amounts of H+ consumed when a pulse of acidic artificial soil solution is passed through a soil column was used to determine mineral weathering rates of a soil (Humo‐Ferric Podzol) from Roundtop Mountain near Sutton, Quebec. The method allows for quantification of H+ consumption due to weathering and H+ consumption due to cation exchange. Initial experiments yield a soil mineral weathering rate of 0.263 kmol ha‐1 yr‐1. This value is well within the range of results obtained by other workers in similar ecosystems. Multiple soil samples can be analysed in a relatively short time, allowing for better quantification of vertical and horizontal variation of H+ consumption within a site.

High‐Elevation Forest Soils of the Southern Appalachians: II. Geomorphology, Pedogenesis, and Clay Mineralogy

AbstractWe investigated soil mineral weathering sequences in three geologically distinct high‐elevation areas of southern Appalachian spruce‐fir (Picea rubens Sarg.‐Abies fraseri [Pursh.] Poir) forests in order to elucidate the pedogenic factors that distinguish these unique soils from their low‐elevation counterparts. Four pedons representing a developmental sequence ranging from soils having distinct spodic field morphology (E plus Bhs horizons) to those having either minimal (weak E horizons) or no spodic character (umbric epipedons/cambic horizons) were selected for further study from a total of 35 excavated pits. Whereas the mineralogy of sand and silt fractions reflected inheritance from parent materials, clay mineral suites of all soils were very similar, indicating biotic and climatic controls on weathering. Surface‐horizon clay fractions were dominated by regularly interstratified mica/vermiculite and high‐charge smectite, which decreased with depth in all profiles. Subsoil clay fractions were principally composed of gibbsite and hydroxy‐interlayered vermiculite. Genesis of secondary 2:1 layer silicates in these soils occurs initially by the pseudomorphic transformation of biotite to both hydrobiotite and vermiculite. However, direct conversion of biotite to kaolinite is of little importance in these soils. The demonstrated mobility of Al‐organic complexes out of surface horizons and the inability of hydroxy‐Al interlayers to form in vermiculite in the presence of organic acids suggest that these soils are undergoing early stages of podzolization and that they are more similar to northern Appalachian Spodosols than they are to adjacent low‐elevation soils of the southern mountains.