CRITICAL ZONE BIOGEOCHEMISTRY: LINKING STRUCTURE AND DYNAMICS (Invited Presentation)

An overarching goal of critical zone (CZ) science is to unravel how contemporary climatic forcing - modulated by biological, physical, and chemical processes occurring in the landscape - gives rise to matter and energy fluxes that then drive the evolution, over much longer time scales, of CZ structure. Our team has investigated links between CZ structure and function at several water-limited sites in the southwestern U.S., including rhyolitic sites in the Jemez Mountains (NM, USA) and granite or schist sites in the Catalina Mountains (AZ, USA). Using sensor/sampler network arrays, we have documented tight couplings between seasonal variation in water/energy availability and ecosystem processes (net ecosystem exchange of water and carbon). These seasonal and event-based dynamics of land-atmosphere exchange are transmitted into the CZ and reflected in patterns of behavior in subsurface hydrology and biogeochemistry. Such behaviors, associated with the routing of water and the location or timing of chemical reactions (e.g., redox, adsorption-desorption, or mineral transformation), have been correlated with structural (geochemical and physical) patterns (obtained using a variety of methods) imprinted in the regolith across scale. For example, hillslope water and carbon flux is correlated with chemical depletion profiles at the meter scale, and element molecular speciation patterns reflect variation in reactivity at the micrometer scale, e.g., in soil aggregates or across fracture surfaces. Using a variety of tools, subsurface biogeochemical transformations, observed in real-time, are being linked to structure evolution of the CZ, with feedbacks to routing of water and weathering products. The evolution of subsurface porosity exerts control over saturation-dependent shifts in hydrologic flow paths and associated geochemical reactions at the catchment scale – changes that are reflected in the concentration-discharge behavior of stream water effluents.