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The University of Arizona’s Landscape Evolution Observatory (LEO), at Biosphere 2, is a series of three large artificial hillslopes containing basaltic tephra soil. A scaled-down analog of LEO (Mini-LEO) with dimensions of 2 m (L) x 0.5 m (W) x 1 m (H) and a 10o slope is uniformly packed with tephra basalt (<2mm in particle size) and used to examine hydrologic response of the hillslope to rainfall during incipient soil formation. In this study, solute chemistry of water samples was examined to infer geochemical weathering patterns and relate them to fluctuations in discharge rates in Mini-LEO. Solute chemistry analysis was performed on two types of water samples: 1) discharge water samples from the toe-end of the slope (seepage samples), and 2) pore water samples taken from Prenart samplers, distributed throughout the slope at fifteen locations. Mini-LEO was subjected to a rain cycle every-other day for 36-days; each cycle consisted of two 3-hour rains at a rate of 13 mm/hr with a 2-hour break in between. Samples for geochemical analysis were collected at the beginning (1st cycle), middle (10th cycle), and the end (16th cycle) of the experiment, and analyzed for major cations and anions, inorganic carbon, and trace elements. Concentrations for seepage samples were plotted as a function of measured discharge rates from the Mini-LEO slope at the time the sample was taken. Results indicated that flux of most major cations was limited by formation of secondary minerals. At the 16th cycle, secondary mineral formation was inhibited, resulting in chemostatic weathering patterns. When limited by rates of secondary mineral formation, cations showed greater congruence (greater percentage of elements remaining in solution after initial dissolution of primary mineral) during higher discharge rates. Pore water samples were used to resolve secondary mineral formation along hydrologic flow paths. These findings have implications for understanding secondary mineral formation during incipient soil weathering, which affects carbon sequestration in soils and nutrient cycling within watersheds.
Hitzelberger, M., Wang, Y., Dontsova, K., Hunt, E., Chorover, J., and Troch, P.A. (2018): Coupling Hydrologic Processes and Geochemical Weathering Patterns in a Fully Controlled Basaltic Soil Lysimeter . Abstract ED13E-0792 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.