Weathering rates mass-balance calculations

Mass-balance studies are widely considered to be the most reliable means of making quanta-tive determinations of elemental transfer rates in natural systems. Garrels (1967) and Garrels and Mackenzie (1967) pioneered the use of mass-balance calculations for mineral weathering in their classic study of Sierra Nevada springwaters. These waters were chosen because a careful set of water analyses and associated primary igneous rock minerals and the soil mineral alteration products were known. Since the actual compositions of the minerals were not known, Garrels and Mackenzie used the theoretical formulas for the minerals. 

This method may be used for soils, and for groundwater and surface runoff water from catchments. It identifies the mean, long-term sources of acidity and alkalinity in the system and then determines the maximum acid input that will bring about a balance that is biologically safe. Weathering rates, biomass acidity input, acid inputs from nitrogen transformations and alkalinity outflux are estimted. Models like MACAL referred to above can be used or more simple mass balance calculation performed. 

Many mass balance studies which report weathering rates as a function of unit area of landscape surface do not permit comparison of those rates with laboratory dissolution rates, and cannot, therefore, contribute to the objectives of this paper. Only two published studies have thus far attempted to renormalize such calculated rates to mineral surface area. Discussion of these studies therefore forms the basis for comparisons of laboratory rates with natural weathering rates. 

Rates estimated in the above studies are shown in Table I. Watershed-scale geochemical mass balance studies yield calculated feldspar weathering rates one to three orders of magnitude slower than rates determined in laboratory experiments. 

Sverdrup (1990) and Sverdrup and Warfvinge (1995) developed Ae PROFILE model to calculate mineral weathering rates by means of a geochemical mass-balance procedure. This model differs from the others in that it uses dissolution constants, which were, for the most part, determined in the laboratory. Empirical fitting parameters, such as surface area of mineral exposed, are used to adjust the model to the real system being described. The model appears to work satisfactorily in many catchments if the fitting parameters are chosen judiciously. This requires a considerable amount of knowledge... 

In the mass-balance approach an attempt is made to determine field weathering rates from element flux calculations, usually focusing on the plant-nutrient important base cations (Ca- Mg ", K, and Na+) or on silica (cf. Velbel 1985). In some studies balances are also computed for Al (Swoboda-Colberg and Drever 1992) and/or the the major anions and nitrogen species (Likens et al. 1977 Mast et al. 1990). A general mass-balance equation for the base cations (BC) might be... 

Figure 7,14 The weathering rate of 15 European and American soil sites computed with the PROFILE model with and without correction for the effect of DOC, compared to the weathering rate estimated using historic and mass-balance methods. Reprinted from Applied Geochemistry, 8, H. Sverdrup and P. Warfvinge, Calculating field weathering rates with a mechanistic geochemical model PROFILE, 273-83, with permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, U.K.

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