David R Montgomery Darlene Zabowski Fiorenzo C Ugolini Rolf O Hallberg and Henri Spaltenstein

8.1 Introduction

The surface of the Earth is a dynamic place. Over geologic time, rocks uplifted above sea level break down and are converted into soils by weathering processes. Soils release soluble components into rivers and can be eroded and transported across landmasses until both soluble and particulate components are eventually deposited in marine sedimentary basins. Once buried, high pressures and temperatures gradually convert the sediment back to rock. Tectonic processes can then uplift the new rock and expose it again at the Earth's surface, resulting in a cycle of uplift, erosion, deposition, burial, and renewed uplift called the rock cycle. The rock cycle continually modifies the Earth's surface (Fig. 8-1). Soils are an especially reactive component of the Earth's surface. They not only provide nutrients and water for terrestrial ecosystems, but they also store and exchange gases with the atmosphere, and affect the movement of surface and groundwater. Thus, soils affect and are affected by the biosphere, atmosphere, and hydrosphere.

Soil is a key component of the rock cycle because weathering and soil formation processes transform rock into more readily erodible material. Rates of soil formation may even limit the overall erosion rate of a landscape. Erosion processes are also a key linkage in the rock cycle between soil production and the filling of sedimentary basins. When water falls onto the Earth's surface it can seep into the ground and percolate down to the water table or it can run off downslope to collect into streams that ultimately combine to form rivers. Flowing water transports eroded material until it is either deposited in local depositional environments or delivered to the oceans. If there was no sink for these sediments, the oceans would fill up in less than 100 Myr, and if there was no source for uplift of rocks on the continents they would be

Fig. 8-1 Generalized cycle of sediments and sedimentary rocks.

Earth System Science ISBN 0-12-379370-X

Copyright (' 2000 Academic Press Limited All rights of reproduction in any form reserved degraded to ocean level in less than 50 Myr (Holland, 1978, p. 146). Global tectonic activity prevents these scenarios through rock uplift and the return of sediments accumulated on the ocean floor back to the continents either by accretion or subduction at plate margins.

The material transformations and interactions that occur between soil, sediment, rock, water and the atmosphere during geological cycles of uplift and erosion are important in global bio-geochemical cycles. Over geologic time, for example, material incorporated into sedimentary rocks can be sequestered in long-term storage in marine basins. In contrast, weathering products are vented to the atmosphere over much shorter time scales. The development of soils, erosional processes, and deposition in marine basins all play key roles in global bio-geochemical cycles.

8.2 Weathering

Weathering occurs because rocks and minerals become exposed to physical and chemical conditions that differ from conditions under which they formed. Rocks form at higher temperatures and pressures than that of the surficial environment so they are unstable at the temperatures and pressure of the Earth's surface and are thus susceptible to weathering. The inorganic solid phase of any soil consists of a number of minerals displaying different degrees of weathering susceptibility. The extent of weathering of these minerals depends on the stabilities of the minerals and the physical and chemical environment to which the minerals are exposed in the soil or at surface conditions, including the supply of water and the removal or transport of weathering products (Garrels and Christ, 1965; Rai and Kittrick, 1989; Colman and Dethier, 1986).

Weathering can be separated into two types: physical and chemical. Physical weathering involves changes in the degree of consolidation with little or no chemical and mineralogical changes of rocks and minerals. Chemical weathering involves changes in chemical and mineralogical composition that generally act on the surfaces of rocks or minerals. Physical weathering increases the surface area of rocks and minerals such that chemical weathering can proceed at a faster rate. In nature these two processes occur concurrently and are difficult to separate (Jackson and Sherman, 1953; Birkeland, 1999).

8.2.1 Physical Weathering

Rocks and minerals break when stressed above their tensile strength. Commonly, rocks fracture along joints, fissures, or planes that have developed during cooling, tectonism, and sedimentary processes or along lines of weakness at the boundaries between mineral grains. When previously buried rock masses are exposed at the Earth's surface, the lowering of the overburden pressure, or unloading, allows the rocks to expand. This expansion induces fracturing that aids in the conversion of rock to soil. Physical weathering processes expand these fractures or cause the development of new ones.

Frost wedging is the prying apart of materials by expansion of water when it freezes. The pressure produced by freezing water is well above the tensile strength of many rocks; however, this pressure may not be commonly attained in nature because rocks are not completely saturated but contain air gaps. Hydration shattering, the ordering and disordering of water molecules adsorbed at the surface of rocks, may be responsible for processes ascribed to frost wedging (Dunn and Hudec, 1972; Hudec, 1974). Nonetheless, the presence of shattered bedrock, and generally angular rock debris in cold environments provides sufficient evidence that frost wedging is at work. Laboratory experiments suggest that repeated freeze-thaw cycles can even produce clay-sized particles (Lautridou and Ozouf, 1982).

In arid environments, where the soluble products of weathering are not completely removed from the soil, saline solutions may circulate in the soil as well as in rock fractures. If upon evaporation the salt concentration increases above its saturation point, salt crystals form and grow (Goudie et al., 1970). The growth of salt crystals in crevices can force open fractures. Salt weathering occurs in cold or hot deserts or areas where salts accumulate. Boulders, blocks, and cliffs affected by salt weathering display cavities and holes and sometimes acquire grotesque forms, as observed in the cold desert of Antarctica (Ugolini, 1986). Frost and salt weathering combined have a synergistic effect that could be more effective at breaking down rocks than salt or frost alone (Williams and Robinson, 1981).

Thermal expansion induced by insolation may be important in desert areas where rocky outcrops and soil surfaces are barren. In a desert, daily temperature excursions are wide and rocks are heated and cooled rapidly. Each type of mineral in a rock has a different coefficient of thermal expansion. Consequently, when a rock is heated or cooled, its minerals differentially expand and contract, thereby inducing stresses and strains in the rock and causing fractures. Oilier (1969) discussed examples of rock weathering due to insolation. Fire can develop temperatures far in excess of insolation and be quite effective in fracturing rocks (Black-welder, 1927).

Plants and animals disrupt and disaggregate rocks and fracture or abrade individual grains or minerals. Endolithic algae growing in deserts may be capable of disintegrating rocks through shrinking and swelling (Friedman, 1971). Lichens are effective agents in physical weathering by extending fungal hyphae into rocks and by expansion and contraction of the thalli (Syers and Iskandar, 1973). Higher plants grow roots in rock crevices and eventually the increased pressure breaks and disrupts the substratum. In addition, the physical mixing of rock and soil that occurs from tree throw is a primary process in the conversion of bedrock into soil in forested regions. Earthworms, as discussed by Darwin (1896), digest and abrade a considerable amount of soil. Mammals, such as moles, gophers, and ground squirrels tunnel and excavate a substantial amount of soil when they build dens (Black and Montgomery, 1991; Butler, 1995). Similarly, rodents break down rocks and create fine particles (Ugolini and Edmonds, 1983).

8.2.2 Chemical Weathering

Chemical weathering involves chemical changes of rocks and minerals under near-surface conditions. Mineral grains in soils (see Table 8-1) are bathed in a film of water and the dissolution of these minerals depends on a number of factors. First, the solubility of the mineral affects the potential of a mineral to be weathered; this is determined largely by the number and strength of chemical bonds within the crystal lattice. Second, temperature affects the rate of weathering reactions. Third, the composition of the soil solution surrounding the mineral grains will determine weathering rates; solution pH, organic acids, carbonic acid, concentration of other ions already in solution, redox, and com-plexing ligands can all affect how readily the ions released by weathering can go into solution. And last, water; water is not only the universal solvent in the weathering environment, but it is also the vehicle for the redistribution of products of weathering. The amount of contact between the soil solution and the mineral surface in conjunction with the frequency of removal of soil solution containing ions released by weathering (and its replacement with new soil solution) will all determine how readily a mineral weathers. Taking these factors into consideration it is possible to determine the thermodynamic stability of minerals, and predict the weathering sequence of minerals in an environment (Garrels and Christ, 1965). There are six fundamental processes that chemically weather minerals. These are dissolution, hydration, hydrolysis, acidolysis, chelation, and oxidation/reduction.

8.2.2.1 Dissolution

Dissolution of a mineral occurs when the crystal lattice breaks down and it separates into its component ions in water. Minerals most affected are salts, sulfates, and carbonates. For example, calcite dissolution is described by

In this case the two ions, Ca2+ and CO2-, are released into the soil solution and are able to react with water (to form bicarbonate or carbonic acid) or other solution components, or be removed from the soil by leaching. The

Table 8-1 Primary and secondary minerals commonly found in soils

Primary minerals

Approximate composition

Weatherability

Quartz

Si02

—

K-Feldspar

KAlSi3Og

+

Ca, Na-plagioclase

CaAl2Si208 to NaAlSi308

+ to (+)

Muscovite

KAl3Si3O10(OH)2

+(+)

Amphibole

Ca2Al2Mg2Fe3Si5022(0H)2

+(+)

Biotite

KAl(Mg,Fe)3Si3O10(OH)2

++

Pyroxene

Ca2(Al,Fe)4(Mg,Fe)4Si6024

++

Apatite

[3Ca3(P04)2]-Ca0

++

Volcanic glass

Variable

++

Calcite

CaC03

+++

Dolomite

(Ca,Mg)C03

+++

Gypsum

CaS04-2H20

+++

Secondary minerals

Approximate composition

Type

Kaolinite

Al2Si205(0H)4

1:1 layer-silicate

Vermiculite

(Al1.7Mg0.3)Si3.6Al0.4O10(OH)2

2:1 layer-silicate

Montmorillonite

(Ali.7Mgo.3)Si3.9Alo.iO,o(OH)2

2:1 layer-silicate

Chlorite

(Mg2.6Feo.4)Si2.5(Al/Fe)1.501o(OH)2

2:1:1 layer-silicate

Allophane

(Si02)^2Al205-2.5-3(H20)

Pseudocrystalline, spherical

Imogolite

Si02Al203-2.5H20

Pseudocrystalline, strands

Hallyosite

Al2Si205(0H)4 -2H20

Pseudocrystalline, tubular

Gibbsite

Al(OH)3

Hydroxide

Goethite

FeOOH

Oxyhydroxide

Hematite

Fe203

Oxide

Ferrihydrite

5Fe20s-9H20

Oxide

dissolution of CaC03 is regulated by the following reactions:

Dissolution of CaC03 is a congruent reaction; the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of C02 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach out over time, this residue becomes the substratum upon which soils develop in karst terrain found in areas of readily dissolved limestone. This terrain is characterized predominantly by underground drainage and marked by numerous abrupt ridges, fissures, sinkholes, and caverns.

8.2.2.2 Hydration and hydrolysis

Hydration is the incorporation of water molecule^) into a mineral, which results in a structural as well as chemical change. This can drastically weaken the stability of a mineral, and make it very susceptible to other forms of chemical weathering. For example, hydration of anhydrite results in the formation of gypsum:

CaS04 + 2H20oCaS04-2H20 (6)

(anhydrite) (gypsum)

Gypsum is a relatively soluble mineral and can undergo dissolution whereas anhydrite is less soluble.

Hydrolysis is the incorporation of either H+ or OH", the components of water, into a mineral. Although water has a low dissociation constant, it is abundant in most environments. Even though little H+ or OH~ may be provided by dissociation of water, the sheer volume of water moving through a soil over time makes hydrolysis an extremely important reaction.

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by H+ or OH~ may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H'+:

(feldspar)

(goethite)

In Equation (7), an altered solid phase is produced by the weathering of feldspar with a K+ ion released - an example of incongruent weathering (not everything is weathered into solution). In Equation (8), the goethite goes completely into solution—another example of congruent weathering. Both of these examples demonstrate a critical property of weathering, namely that almost all weathering reactions consume H+. Thus, as long as weatherable minerals are present, weathering reactions can help counteract the natural tendency of soils to become acidic or neutralize the effects of acid rain.

8.2.2.3 Acidolysis

Acidolysis is a similar weathering reaction to hydrolysis in that H+ is used to weather minerals, but in this case the source of H+ is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H+ to weather minerals. All of these acids occur naturally in soils; in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the upper soil where they cause intense weathering. Carbonic acid and bicarbonate are more important to weathering in young soils, or deep in the soil profile where organic acids are not prevalent.

8.2.2.4 Chelation

Besides attacking minerals by providing H+, organic acids can also cause weathering by chelation. A chelator is a ligand capable of forming multiple bonds with a metal ion such as Fe, Al, or Ca, resulting in a ring-type structure with the metal incorporated into the complex. The large, complex organic acids formed in soils can act as chelators, and are capable of stripping metal ions from some primary minerals (Huang, 1989). Artificial chelators such as EDTA (ethylene diamine tetraacetic acid) are often used to test soils for the availability of micronutrients. Some low-molecular-weight organic acids are also capable of chelating metals.

8.2.2.5 Oxidation and reduction

Oxidation and reduction reactions weather minerals by the transfer of electrons. Minerals containing elements that can have multiple valence states such as Fe, Mn, S, or even N, are susceptible to redox reactions. A common reaction that occurs in soil involves both the oxidation and reduction of iron, which when present in a mineral is usually in the Fe(II) form. Fe(II) in the parent material oxidizes slowly in well-drained and aerated soils (Bohn et al., 1985; Birkeland, 1999). In this oxidizing environment an electron may be removed from Fe2+ at mineral edges causing disruption in the crystal due to charge imbalance, triggering disintegration of mineral edges or making the mineral more susceptible to other forms of weathering. The Fe3+ released by weathering is very insoluble and readily combines with oxygen and water to form goethite

(goethite)

This occurs in well-drained temperate soils, and is the reverse of Equation (8). In very well-drained warm soils, hematite can form. For example, a primary Fe-bearing mineral such as a pyroxene or amphibole weathers through oxidation to release Fe3+ into solution, which then precipitates out as goethite or hematite, depending on the environment. An important aspect of this process is that ultimately H+ is released and able to weather other minerals. Minerals containing oxidized elements can undergo reduction reactions in anaerobic soils causing them to weather. Weathering of goethite in an anaerobic soil will release Fe2+ into solution.

Overall, weathering controls the chemistry of material that is transported into the sediment and that which stays behind in the soil. As an example, consider a general weathering reaction for an aluminosilicate (Stumm and Morgan, 1995):

cation-Al-silicate + H2C03 + H20 -> HCO3 + cation + H2Si04 + layer-silicate clay

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