Weathering rate

Clayton, J. L. (1986). An estimate of plagioclase weathering rate in the Idaho batholith based upon geochemical transport rates. In "Rates of Chemical Weathering of Rocks and Minerals" (S. M. Coleman and D. P. Dethier, eds). Chap. 19, pp. 453-466. Academic Press, New York. 

Contributions made by physical weathering are much greater in steep terrains (i.e., more primary minerals remain), and overall weathering rates are higher. 

Weathering rates are most sensitive to the throughput of water. In soils, this is a decidedly discontinuous process. Typically, water flows through soil following rainfall or snowmelt. Once saturated, the flux of water is largely dependent on the physical properties of the soil and not on the rate of supply. Water that cannot be accommodated by flow through the soil. 

Groundwater environments can be represented as a simple flow-through system. For the situation where chemical weathering of mineral grains is transport controlled, the weathering rate of a mineral should be directly dependent on the rate of throughput of water. For the situation where rates are controlled by surface... 

The influence that variations of temperature and levels of atmospheric CO2 and O2 have on chemical weathering are more subtle. Temperature appears to have a direct effect on weathering rate (White and Blum, 1995). The silica concentration of rivers (Meybeck, 1979, 1987) and the alkalinity of ground waters in carbonate terrains (Harmon et al., 1975) are both positively correlated with temperature variations. It is not clear, however, whether temperature-related variations in weathering rates are largely due to variations in vegetational activity that parallel temperature variations. 

In contrast, under transport-limited conditions, weathering rates are ultimately limited by the formation of soils that are sufficiently thick or impermeable to restrict free access by water to unweathered material. Erosion rates... 

For soil profiles that are less than the optimum thickness, there is a destabilizing feedback between soil thickness and weathering rate. Assume that a thin soil is in a dynamic equilibrium such that weathering inputs balance transport losses (A in Fig. 9-3). Weathering rate... 

In regions where the erosion regime is weathering limited, susceptibility of the bedrock to chemical and physical weathering controls erosion rates. This susceptibility relates directly to the chemical and physical properties of the rock. Susceptibility also depends on local climate. Moreover, weathering rates are affected by the... 

In regions where erosion is transport limited, weathering rates are controlled by the supply of reactive fluids to unstable minerals. This is controlled by soil properties, regional base level, and ultimately, sea level. 

Describe how vegetation can increase and decrease the weathering rate. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

Blum, J. D., Erel, Y., and Brown, K. (1994). Sr/ Sr ratios of Sierra Nevada stream waters Implications for relative mineral weathering rates. Geochim. Cosmochim. Acta 58,5019-5025. 

Assuming that ( " U/ U) fractionation variations, Boulad et al. (1977) proposed (230Th/238u) variations with depth in a laterite from Cameroon. These authors assumed the occurrence of two main U-Th fractionation fronts one at the base of the weathering profile with U loss, and one in the upper part of the profile where U is released and redeposited deeper in the profile (Fig. 15). By fitting theoretical curves to the data, they estimated weathering rates ranging from 50 to 70 mm/ka. An estimate of 50 mm/ka was also obtained with a similar approach by Mathieu et al. (1995) for a Brazilian laterite. 

Physical separation of granulometric fractions by sedimentation and ultracentrifugation could also help in constraining weathering rates at local scale. The fine fractions are useful when they exclusively contain secondary minerals, that is, when they are not polluted by... 

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