RESOLVING CRITICAL ZONE STRUCTURE AND WEATHERING PROFILES ACROSS A GEOLOGICALLY COMPLEX SUB-ALPINE WATERSHED

Critical zone (CZ) structure, including the spatial distribution of minerals, chemical elements, and fluid-filled pores, evolves on geologic time scales as a result of both top-down climatic forcing and bottom-up geologic controls such as lithology, fracture density, and other parent material structural properties. Signatures of climate and lithology may be imprinted in subsurface structure as depth-dependent trends in geophysical, geochemical, mineralogical, and biological datasets. One goal of CZ science is to predict the evolution of CZ structure from measurements of current hydro-biogeochemical processes. However, to the extent that the weathering profile is as much (or more) a product of past environmental conditions, development of predictive models requires the ability to understand and tease apart the relative roles of climatic forcing and the geologic template (or stage) on which CZ processes evolve. Doing so is particularly challenging in complex volcanic terrains with high initial bedrock porosity and a past history of distinct depositional and hydrothermal mineral weathering reactions. To resolve CZ structure in a rhyolitic catchment in the Valles Caldera National Preserve (NM, USA), this study combined surface borehole geophysical logging, drilling and subsequent comprehensive laboratory analyses to produce depth-resolved quantitative data on porosity, geochemistry, and mineralogy down to >40 m. The catchment was subject to metasomatic alteration under an alkaline lake prior to uplift of the resurgent Redondo Mountain volcanic dome. Using linear discriminant analysis, key variables enabled separation of complex layered geology into discrete zones. Zeolites, magnesite, talc, and crystalline smectite were associated with post-eruption weathering and metasomatism. Lower seismic velocities and natural gamma radiation, combined with quantitative geochemical analyses indicate contemporary, matrix-dominated weathering processes, suggesting modern hydrologic fluxes occur dominantly within the top 15 m of the weathering profile. By combining a complementary set of techniques to analyze CZ profile structure, we were able to separate structural features of CZ architecture and contextualize the weathering profiles at three slope positions in differing geologies.