Chemical weathering approach

Lasaga, A. C. (1995). Fundamental approaches in describing mineral dissolution and precipitation rates. In Chemical Weathering Rates of Silicafe Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America, Washington, DC, Reviews in Mineralogy 31, 23-86. 

The on-line implementation of environmental processes in the GEM model allows running in global uniform, global variable, and limited area configurations, allowing for multiscale chemical weather forecasting (CWF) modelling. This approach provides access to all required dynamics and physics fields for chemistry at every time step. The on-line implementation of chemistry and aerosol processes... 

Attempts to model chemical weathering of catchments have used a variety of approaches and were originally designed to understand acidification processes. The BIRKENES code (Christophersen et al., 1982) was one of the first developed to model catchment stream chemistry. It used cation-anion charge balance, a gibbsite equilibrium solubility control for aluminum concentrations, a Gapon ion exchange for metals sorption, and rates for sulfate adsorption/ desorption in a two-reservoir model. The model was calibrated by input mass fluxes and output mass fluxes for the Birkenes catchment in Norway to provide the water flux information and to fit empirical parameters. 

The measurement of solute fluxes in surface-water discharge is an indirect approach to estimating chemical weathering rates. Due to low mineral-to-fluid ratios and short residence times, minimal silicate weathering occurs in the streambed and the hyporheic environment. Rather, surface-water solutes represent discharges from other weathering environments that are spatially and temporally integrated by the watershed flux. 

A particularly valuable approach to investigating the effect of time on chemical weathering has been the use of soil chronosequences which are defined as a group of soils that differ in age and, therefore, in duration of weathering. These soils have similar parent materials and have formed under similar climatic and geomorphic conditions (Jenny, 1941). Individual soils, therefore, provide snapshots of the progressive nature of chemical weathering with time. 

Another class of models relevant to chemical weathering is based on the reaction path approach originally developed by Helgeson et al. (1970). EQ3/EQ6 (Wolery et al. (1990), PHREEQC (Parkhurst and Plummer, 1993), and PATHARC.94 (Gunter et al., 2000) are some codes currently used to describe the progressive reaction of primary silicates and the precipitation of secondary phases as a function of time and mass. These codes are discussed in Nordstrom (see Chapter 5.02). They commonly permit the introduction of user-defined silicate reaction rates. Such models also commonly consider solubility controls on reaction kinetics as defined by... 

Dom R. (1995) Digital processing of back-scatter electron imagery a microscopic approach to quantifying chemical weathering. Geol. Soc. Am. Bull. 107, 725 -741. 

The rate of chemical weathering has been evaluated using historic, mass-balance, and theoretical-empirical mineral dissolution rate approaches, often with different results. What are these approaches and why may their results differ ... 

The lichen prefer rock surfaces that face north because they are warmed by solar radiation, as demonstrated by Miotke (1979a) in Taylor Valley. His results indicated that the temperature at a depth of 10 cm in marble and dolerite fluctuated daily and approached +10°C in the afternoon, provided that the sun was shining. Liquid water that forms by melting of snow can infiltrate the underlying rock where it supports the metabolic activity of the endolithic organisms (Friedmann 1978) and promotes chemical weathering (Miotke 1980). When the weathering rind flakes off, the endolithic lichen are exposed at the surface where they cannot survive. 

Nesbitt, H.W. and Jambor, J.L. (1998). Role of malic minerals in neutralizing ARD, demonstrated using a chemical weathering methodology. In L.J. Cabri and D. J. Vaughan (eds). Modem approaches to ore and environmental mineralogy, Mineralogical Association of Canada. 

In our approach, three types of minerals accumulate as weath ing proceeds and serve as indicators of regional chemical weathering intensity ... 

Alteration is always a cause for concern in geochemical investigations and the best approach will always be to avoid samples with visual or chemical evidence for alteration. The differential fluid mobility of U, Th, Pa and Ra undoubtedly provides the potential for weathering or hydrothermal circulation to disturb the U-series signatures of arc lavas. In a study of lavas from Mt. Pelee on Martinique, Villemant et al. (1996) found that domeforming lavas were in U-Th equilibrium whereas plinian deposits from the same eruptions had small U-excesses which they interpreted to reflect hydrothermal alteration. However, whilst the addition of U could be due to hydrothermal alteration, the plinian deposits were also displaced to lower °Th/ Th ratios which cannot. Instead, the two rock types may just be from separate magma batches. 

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