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FIGURE 2.7 Dissolution rates of silicates and carbonates as a function of pH. Band bracketing dissolution rates for silicates are from data in Table 2.3. For carbonates, data of Plummer et al. (1978) are used for calcite, and of Chou et al. (1989) for calcite and dolomite. Chemical weathering rates shown for comparison with the experimental values are from Table 2.2. Adopted from Lerman (1979).
having reportedly the lowest rates at 25°C. The band of dissolution rates of silicates is plotted in Figure 2.7.
For carbonates, the experimentally determined rates of dissolution for calcite and dolomite (Plummer et al., 1978; Chou et al., 1989), shown in Figure 2.7, are significantly higher than those for silicates; this feature finds its reflection in the weathering rates of carbonate rocks, which are generally higher than those of crystalline silicate rocks.
HYDROGEN-ION AND TEMPERATURE DEPENDENCE OF DISSOLUTION RATES
The pH and Natural Acidity
The rate of dissolution of a mineral is generally some function of the environmental conditions, such as the composition of the aqueous solution and temperature. The H+ concentration (a characteristic of the aqueous solution composition) and the temperature are two of the environmental variables addressed in this section.
The silicate and carbonate mineral dissolution rates are plotted in Figure 2.7 as bands bracketing the higher and lower values of the dissolution rates of the individual minerals. For the silicates it should be noted that in acidic solutions of pH ≤ 5.5, the dissolution rates increase in proportion to a power of the hydrogen-ion concentration as [H+]0.5 to [H+]1.0. The solution pH at which an increase in the dissolution rate begins has been reported to vary from mineral to mineral, occurring between a pH of about 5.5 and lower.
In the near-neutral pH range from approximately 5.5 to 7.5 or 8, the dissolution rates are about constant near the low end of each range shown in Table 2.3. In alkaline solutions, above pH 7.5 to 8, the dissolution rates increase in proportion to [H+]-0.3. Theoretical interpretations of the dependence of silicate dissolution rates on a power of [H+] have been advanced through the mechanisms of surface reactions and transition-state theory (Wollast and Chou, 1988; Schott and Petit, 1987).
Carbon dioxide is the main atmospheric gas that determines the background acidity of atmospheric precipitation and fresh waters exposed to the atmosphere. Variations in the reported CO2 content of the atmosphere during the past 18,000 yr indicate an increase from about 180 parts per million by volume (ppmv) or a PCO2 of 1.8 x 10-4 atm, through the preindustrial-age value of about 280 ppmv, to the present concentration of 345 ppmv (Barnola et al., 1987; Siegenthaler and Oeschger, 1987). Such variations in the carbon dioxide partial pressures could have had only very minor effects on the pH of pure rainwater, which indicates a decrease of about 0.14 pH unit:
PCO2 = 1.8 x 10-4 atm: pH = 5.73 (5°C) and 5.79 (25°C),
PCO2 = 3.4 x 10-4 atm: pH = 5.59 (5°C) and 5.65 (25°C).
For ocean water, in anticipation of a continued anthropogenic CO2 increase in the atmosphere, Whitfield (1974) estimated that a rise from 313 to 453 ppmv would lower the pH of surface ocean water by about 0.08 pH unit, from 8.24 to 8.16.
Additional sources of background natural acidity in atmospheric precipitation are the oxidation products of reduced nitrogen and sulfur emissions, which yield H2SO4 and HNO3, and some HCl emissions from active volcanoes. On the land surface, the primary sources of acidity are the oxidation of pyrite (FeS2) to produce sulfuric acid; the release of organic acids by living plants and humus; and the oxidation of organic matter, which mostly contributes CO2 to groundwaters. The importance of the latter process to weathering releases is discussed later.
For purposes of visualization in Figure 2.7, where an increase in the silicate dissolution rate is shown to begin at pH ≈ 5.5, a decrease in the pH of a solution by 1 pH unit may result in a three- to tenfold increase in the dissolution rate.
Temperature Effects on Dissolution Rates
The warming trend since the glaciation peak about 18,000 yr ago translates into a rise in mean global temperature from about 10 to 16°C at present (Lorius et al., 1985; Jouzel et al., 1987). Considerably greater variations in temperature char-