Surficial and deeper processes couple to control the complex evolution of hills. These processes simultaneously create the landscape we see as well as subsurface geochemical landscapes. Specifically, both vertical and lateral flows of rock and water occur within hills experiencing uplift and erosion. When considered over geological timeframes, rock advects vertically upward under such hilltops. Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion. At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table. The depths of these deep and shallow water tables in hills fluctuate over time and drive biogeochemical reactions between water, CO2, O2, and minerals and these in turn can drive fracturing. The depth intervals of water table fluctuation are thus reaction fronts that are characterized by changes in composition, fracture density, porosity, and permeability. The shallow and deep reaction zones can act like valves that reorient downward water flow into lateral flow. As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate approaches the erosion rate. It is possible that a permeability architecture emerges to partition each evolving hill into dissolved and particulate material fluxes to allow the hill to approach a steady state.
About the Speaker – Susan Brantley
Susan Brantley is Distinguished Professor of Geosciences in the College of Earth and Mineral Sciences at the Pennsylvania State University. She also serves as director of the Earth and Environmental Systems Institute (EESI). She has been on the faculty at Penn State since 1986. Dr. Brantley’s career as a geochemist focuses on the chemistry of natural waters both at the surface of the earth and deeper in the crust. Much of her research is an attempt to understand what controls the chemistry of natural water, and how water interacts with the rocks through which it flows. Dr. Brantley and her research group investigate chemical, biological, and physical processes associated with the circulation of aqueous fluids in shallow hydrogeologic settings. Investigations incorporate field and laboratory work, and theoretical modelling of observations. Of particular interest are questions concerning the measurement and prediction of the rates of natural processes, including chemical weathering with and without micro-organisms. Recent work has focused on measuring and modeling how rock turns into regolith and water quality issues in areas with hydraulic fracturing.
Brantley is a member of the National Academy of Sciences and Fellow of the American Geophysical Union, the Geological Society of America, the Geochemical Society, the European Association for Geochemistry, and the International Association of GeoChemistry. She is a recipient of the Wollaston Medal of the Geological Society, the Arthur L. Day Medal of the Geological Society of America, and the Presidential Award of the Soil Science Society of America. She holds honorary doctorates from the Paul Sabatier University (Toulouse III) in France and the University of Lausanne in Switzerland. In addition, she has been appointed to the U.S. Nuclear Waste Technical Review Board.
Brantley earned her B.A. in chemistry (magna cum laude) and her M.A. and Ph.D. degrees in Geological and Geophysical Sciences, all from Princeton University.
Host: Bhavna Arora