EVALUATING CONTROL OF SLOPE-ASPECT ON DEEP GEOCHEMICAL WEATHERING WITHIN THE BOULDER CREEK CRITICAL ZONE OBSERVATORY

Geochemical weathering in the Critical Zone (CZ) is an important source of mobile regolith and solutes that drive landscape evolution and nutrient cycling. An in-depth understanding of geochemical processes is integral to prediction and evaluation of Earth surface response to natural and anthropogenic perturbations, such as global climate change and land use. In part due to the difficulty of collecting deep samples, previous CZ investigations focused primarily on shallow (<3 m) weathering. This study uniquely evaluates chemical and mineralogical changes from the surface to depths of up to 15 m, utilizing the Gordon Gulch watershed of the Boulder Creek Critical Zone Observatory (BcCZO) as a site investigation of hillslope aspect control on weathering processes. In the shallow subsurface, weathering intensity is higher for south-facing hillslopes than for north-facing slopes. However, the thickness of weathered material is deeper on north-facing hillslopes based on exchangeable cation concentration and clay fraction trends. Geologic heterogeneity complicates interpretations of parent material for the weathering profile, and commonly used methods for calculating weathering rates (e.g., mass-balance models) do not accurately assess the complex variability of protolith composition and mineral reactions. Ultimately, this study finds that further constraints, such as depth-dependent quantitative mineralogy, are needed in order to comprehensively evaluate weathering processes in areas with highly heterogeneous bedrock, such as the BcCZO.