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Van Tuyl Lecture: Daniella Rempe, University of Texas at Austin
April 19, 2018 @ 4:00 pm - 5:00 pm
Daniella Rempe, Jackson School of Geosciences, University of Texas at Austin
Van Tuyl Lecture, BE 241, 4 PM
Topic: Probing Weathered Bedrock in Earth’s Critical Zone: Rock Moisture and its Implications for Weathering and Biogeochemical Processes
Abstract: Many mountainous landscapes are mantled with thin soils that overlie meters to tens of meters of weathered bedrock. This transition zone between soil and bedrock often controls how infiltrating water is partitioned between evapotranspiration and runoff, as well as the rate and pattern of chemical denudation in a landscape. Despite this region’s significance to hydrologic and geomorphic processes, it is largely invisible to observation and the geochemical reactions and hydraulic properties that govern its structural evolution over time are poorly constrained. In this talk, I will present results from an intensive effort to directly quantify the complex fluid and solute pathways in the weathered bedrock region at the Eel River Critical Zone Observatory (ERCZO). We have instrumented an 18 m thick variably saturated argillite weathering profile in the uplifting Northern California Coast Ranges with a series of flexible sensors and samplers that span the continuum from soil to fresh bedrock. In the seasonally dry setting, we observe significant hydrologic dynamics within the weathered bedrock region. Wet-season rainfall passes through unsaturated fractured bedrock before reaching the groundwater table and running off to the adjacent stream. The water that is seasonally stored in the weathered bedrock (rock moisture) is used by plants for transpiration. Using our sampling system, we have collected nearly two years of bi-weekly major and trace ion chemistry from samplers distributed within the weathered bedrock region that separately collect freely draining and tension-held water. We compare water chemistry to in-situ water content and gas phase O2 and CO2 , as well as rock characterization data such as bulk density and elemental composition, to identify controls on the observed solute concentrations. Our preliminary results reveal that both the freely-draining and tension-held waters are seasonally geochemically dynamic, while these same dynamics are dampened or absent in groundwater and streamflow. The tension-held samples show consistently higher concentrations relative to freely draining fluid, suggesting that the dual-domain nature (i.e. flow through matrix and fracture components) of the weathered bedrock system significantly impacts solute generation and weathering of bedrock. We compare our field observations to preliminary 1D reactive transport simulations to explore the role of dual-domain transport on weathering reactions. Our results highlight the coupled nature of geochemical and hydrologic processes in weathered bedrock and the importance of studying this region in Earth’s Critical Zone.