Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH, CO_2, and oxalate
Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO_2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m^(−2) h^(−1) at pH 2.78 in the batch experiments to 0.009 μmol Si m^(−2) h^(−1) at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H^+ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO_2 demonstrated that dissolution rates were independent of CO_2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO_2 have been elevated over the last decade. This release of magmatic CO_2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO_2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO_2 can be excluded.