CONTROLS ON RATES OF SILICATE MINERAL WEATHERING

Both experimental and field-based approaches have been used to quantify silicate weathering kinetics contributing to the solute compositions of many aquifers. The advantages of the experimental approach is that chemical, physical and biological conditions are tightly constrained whereas the disadvantage is that such studies commonly react freshly prepared minerals over short reaction times. The alternate approach is to determine long-term silicate reaction rates from field-based fluxes in soil pore waters, ground waters and watershed discharge. Uncertainties in natural weathering rates involve estimating fluid residence time, surface areas of complex mineral assemblages and past variations in climate, solute composition and biogeochemical interactions. Comparison of results indicates the silicate rates are commonly 2 to 6 orders of magnitude faster than field-derived rates. In addition, the rates at which different silicate minerals weather in the laboratory are sometimes very different than the relative rates observed in the field. The difference in silicate weathering rates is dependent on both intrinsic characteristics of mineral such as physical surface area and reactive site distributions as well as extrinsic features of the weathering environment including reaction affinity with the coexisting solute species. Specific examples of these controls will be discussed.