Weathering rates studies

The first studies, dealing with calculation of weathering rates from radioactive disequilibria, used and Mainly for illustrating the potential of the method, Osmond and Cowart (1976) used early estimates of ratios in the average... 

So far we have discussed weathering rates and rate laws from laboratory experiments on pure minerals. These laboratory studies are meant to provide insight for natural systems (rates and variables that affect these rates). We may first try to compare laboratory and field results. 

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. 

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

The primary objectives of mass-balance studies are (i) quantify the mass fluxes into and out of watershed systems (ii) interpret the reactions and processes occurring in the watershed that cause the observed changes in composition and flux (iii) determine weathering rates of the various minerals constituting the bedrock, regolith, and soils of the watershed and (iv) evaluate which mineral phases are critically involved in controlling water chemistry to help develop models of more general applicability (i.e., transfer value). 

Mineral composition and structure are the primary intrinsic factors controlling weathering rates. Based on early weathering studies, Goldich (1938) observed that the weathering sequence for common igneous rocks in the field was the reverse of Bowen s reaction series that ranked minerals in the order of crystalhzation from magma. Amphi-boles and pyroxenes are expected to weather faster than feldspars which weather faster than... 

The ability to isolate climate effects decreases as the scale of the weathering process increases. For example, a number of studies comparing weathering rates in soils and small catchments have found a significant climate effect (Velbel, 1993 White and Blum, 1995 Dessert et al., 2001). In contrast, comparison of solute concentrations and fluxes originating from large scale river systems commonly fail to detect a climate signature (Edmond et al., 1995 Huh et al., 1998). 

The majority of rate studies are based on watershed solute fluxes normalized to catchment area and are equivalent to rates of chemical denudation. Previous efforts have tabulated these rates, which have proved to be valuable in evaluating the importance of a number of environmental controls on chemical weathering including precipitation, temperature, vegetation, and rock type. The present chapter summarizes... 

Studies of modern ecosystems reviewed in Section 6.01.2.3.5 indicate that there is no consensus over their relative effects on weathering rates. The data in Table 2 show a total range... 

The occurrence of highly reactive minerals, such as evaporitic minerals, pyrite and even calcite, in low proportions—a percent or less— in a given rock, e.g., calcareous sandstone, pyritic shale, marl with traces of anhydrite, granite with traces of calcite, may determine the chemical character of stream water (Miller, 1961 Drever, 1988). In a study of 200 streams from monolithologic catchments underlain by various rock types under similar climatic conditions in France, the relative weathering rate based on the cation sum (Meybeck, 1986) ranges from 1 for quartz sandstone to 160 for gypsiferous marl. 

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