Water, Energy, and Biogeochemical Budgets (WEBB) Program
U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, USA
Soil can be defined as "a natural body consisting of generally unconsolidated layers or horizons of mineral and/or organic constituents of variable thickness which differ from parent rock; in morphological, physical, chemical and mineralogical properties" (Joffe, 1949). As such, the study of soils have long been tied to the nature and extent of chemical weathering. The literature on soil chemical weathering related to secondary mineralogy, chemical equilibrium and ion exchange, in addition to the effects on geomorphology and other physical processes, has been reviewed in a number of books (Lindsay, 1979; Nahon, 1991; Sparks and Suarez, 1991; and Sposito, 1994). The present review discusses the rates of chemical weathering of primary silicate minerals in soil environments. Although soils represent one of the most accessible natural environments, relatively few studies have quantitatively addressed weathering rates of feldspars, amphiboles, pyroxenes, micas and other silicate minerals under such conditions. Estimating weathering rates of silicate minerals in soils is important because such reactions ultimately control rates of soil development and secondary mineral formation. Weathering rates also influence soil buffering capacities related to acidic deposition in watersheds and control the cycling of many inorganic nutrients which are important in soil fertility and carbon cycling.
The chemical weathering of a primary mineral j in a soil can be defined in simplest terms as
|Mj = kj S delta t||(1)|
where Mj is the mass loss (mol), kj is the rate constant (mol/cm2/s), S is the surface area (cm2 and delta t the time period (s) during which weathering occurred. These terms, as well others used throughout the paper, are tabulated in Table 1. In weathering studies of natural systems, Equation (I) can be employed as a predictive tool to determine any one of the above parameters provided that the other terms are known. The present paper will discuss methods used to determine mass losses, surface areas and duration of weathering, which in turn, will permit calculation of weathering rate constants for soil environments.
Chemical weathering of primary silicates in soils will be addressed in the present paper from two perspectives. These will be (a) rates determined from solid state element and mineral losses relative to initial or parent material and (b) rates determined from solute fluxes through the soil profile. These parallel approaches require the determination of mass loss Me based on either mineral or solute compositions. Soil mineralogy represents the residual product of chemical reactions which integrate the weathering rate over the entire period of soil development. In contrast, solute chemistry and fluxes reflect present day weathering under currrent chemical and hydrologic conditions. Both approaches require the determination of the duration of weathering. In the ease of mineral or elemental loses, duration of weathering equates to the age of the soil profile. For solutes, the resident time is equivalent to fluid transit time through the soil. Both approaches normalize chemical fluxes to the unit surface area of the mineral phase. A detailed discussion will follow on the definition and role of surface area in weathering rate calculations. Finally, the derived rate constants k, based on Equation (1), will be compared for various natural soils and experimental studies. Reasons for observed variations will then be addressed.