Abstract

Understanding the linkages among climate, erosion and weathering is central to quantifying pedogenesis and critical zone evolution. We approach these linkages through a combination of regional scale synthesis of climate, erosion and regolith geochemical data for upland terrain, in addition to detailed studies on climate and landscape position controls on local- and pedon-scale regolith weathering patterns across the steep semiarid climate gradient encompassed by the Santa Catalina Mountain (SCM) Critical Zone Observatory in southern Arizona, USA. The regional analysis indicated strong correlation between regolith plagioclase depletion and total Na mass loss with both total precipitation and the annual water balance as quantified using a humidity index (precipitiaton/potential evapotranspiration). Plagioclase depletion increased exponentially with water availability for sites with annual temperature greater than 5°C and erosion rates greater than 10 g/m2/yr, suggesting second order control of temperature and erosion on plagioclase weathering. The local- and pedon-scale SCM study sites span a steep environmental gradient on granitic and metamorphic parent materials: mean annual air temperature decreases (20-10°C) and mean annual precipitation increases (25-95 cm) with elevation, with concomitant changes in vegetation from desert-scrub to mixed conifer forest. We sampled full regolith profiles from each vegetation community across the climate gradient on spatially coupled divergent and convergent landscape positions. Pedon geochemical and mineralogical data, when coupled with cosmogenic nuclide derived denudation rates, indicated strong linkages among water availability, regolith mass loss and mineral weathering reactions. Specifically, divergent landscape positions demonstrated a pattern similar to that noted in the regional scale synthesis of increasing weatherable mineral mass loss with greater water availability. In contrast, convergent landscape positions demonstrated minimal mineral mass loss and relatively greater content of neogenic secondary mineral phases across the entire SCM, despite having localized conditions of greater water availability. Solution chemistry data suggest the convergent positions concentrate soluble weathering products from adjacent divergent positions, thus resulting in locally reduced mineral-solution weathering gradients and promotion of neogenic mineral precipitation. Across all scales, timing and amount of water available for weathering reactions appears as a central control on regolith weathering. This relation is strongly modified by local-scale solution chemistry whereby detailed understanding of landscape position controls on redistribution of water and solutes is critical to accurate prediction of regolith weathering reactions at the pedon-scale.

Citation

Rasmussen, C., Lybrand, R., Jardine, A.B., Heidbuechel, I., Troch, P.A., Chorover, J. (2010): Climate and landscape controls on chemical weathering - regional to pedon-scale analysis. AGU Fall Meeting (Invited) Abstract EP42A-02..