Resolving Deep Critical Zone Architecture in Complex Volcanic Terrain

Abstract

Critical zone (CZ) structure, including the spatial distribution of minerals, elements, and fluid-filled pores, evolves on geologic time scales resulting from both top-down climatic forcing and bottom-up geologic controls. Climate and lithology may be imprinted in subsurface structure as depth-dependent trends in geophysical, geochemical, mineralogical, and biological datasets. As the weathering profile is as much (or more) a product of past environmental conditions, development of predictive models requires understanding the relative roles of climatic forcing and the geologic template on which CZ processes evolve. Doing so in complex volcanic terrains with high initial bedrock porosity and distinct depositional and hydrothermal alteration histories is particularly challenging. To resolve CZ structure in a rhyolitic catchment in the Valles Caldera National Preserve (NM, USA), this study combined geophysics, drilling, and laboratory analyses to produce depth-resolved porosity, geochemistry, and mineralogy datasets to >40 m in depth. Quantitative X-ray diffraction analysis showed that local mineral transformations control complex chemical enrichment/depletion (tau) patterns. Using linear discriminant analysis, key variables enabled separation of complex-layered geology into discrete zones. Contemporary, matrix-dominated weathering processes and modern hydrologic fluxes occur dominantly within the top 15 m of the weathering profile. This zone is convoluted by incomplete primary mineral weathering and overprinted by post-eruption weathering and metasomatism. Matrix weathering transitions to fracture surface weathering driven by deep percolation of slower moving, longer residence time meteoric waters at depth. By altering initial conditions and weathering trajectory, geologic legacy is a critical factor in how this subsurface landscape evolved and functions.