Chemical Weathering of Granitic Soils In the Santa Catalina Mountains, Arizona: Effects of Climate and Landscape Position.

Precipitation, topography, vegetation distribution and physical erosion rates demonstrate significant control over soil chemical weathering and soil development in landscape evolution. This study examined the interactive effects of climate and landscape position on soil chemical weathering processes along the Santa Catalina Mountain environmental gradient of southern Arizona. Soil profiles were sampled from divergent-convergent landscape unit pairs within five vegetation communities ranging from desert scrub to mixed conifer forest. Primary and secondary mineral compositions of soils and representative rocks were determined using electron microprobe, quantitative and semi-quantitative x-ray diffraction (XRD) and x-ray fluorescence (XRF) analyses. The mineral compositions of the granitic rocks consisted of quartz, biotite, orthoclase and calcium/sodium-rich plagioclase feldspars as prominent primary minerals at the low elevation sites and quartz, muscovite, orthoclase and sodium-rich plagioclase feldspars at high elevation regions. These data reflect differences in rock composition and age of pluton intrusion.  Depth to paralithic contact increased with elevation, systematically in divergent positions (28 cm/1000 m) and less consistently at convergent sites. Relative differences in depth to paralithic contact between convergent and divergent landscape positions was greatest at the low and high elevation sites and is hypothesized to be a product of changes in physical erosion rates across the gradient. A Quartz/Plagioclase (Q/P) ratio was used as a general proxy of chemical weathering in bulk soils.  The Q/P ratios were lowest in the desert scrub environments and greater in mixed conifer systems, which is indicative of increased soil chemical weathering at the high elevation sites. Secondary mineral assemblage of desert scrub and grassland sites was dominated by smectite and partially dehydrated halloysite whereas vermiculite, chlorite and kaolinite were predominate in the mixed conifer systems. Increases in depth to paralithic contact, degree of chemical weathering and secondary mineral transformations across the gradient correspond to increased water availability. Degree of chemical weathering changed as a function of topographic position, demonstrating changes in water and solute distribution across the landscape. The results of this work emphasize the combined importance of climate, landscape position and erosion on chemical weathering processes and soil development.