Mineral weathering was studied on a 71-ha, arid watershed comprised of soil developed from mixed basaltic-felsic parent material. The composition of spring water draining the watershed was used to derive a weathering scheme for the principal soil minerals. The composition of the perennial spring water draining the arid watershed exhibited a high Mg concentration and low Na:Ca ratio. These appeared to be controlled by pyroxene and basaltic plagioclase weathering processes, respectively, and were relatively unaffected by felsic mineral weathering processes. The mean partial pressure of carbon dioxide that served as a source of acid for weathering in the arid watershed system was greater than that of the atmosphere. This likely resulted from the respiration of soil organisms and plant roots. Thus, respiration may ultimately control the weathering dynamics in such a system.
A theoretical reconstitution of primary rock-forming minerals in the arid watershed from spring water and soil clay mineral compositions demonstrated that soluble SiO2 resulted from the weathering of plagioclase, pyroxene, and biotite, and not from abundant quartz; and that Fe and Al were largely retained in solid phases within the watershed. Weathering reactions were relatively rapid and probably occurred largely within the soil profile, as indicated by a high soil pH in the B-horizons and soil parent material.
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