Thursday, October 11, 2018
4:00 pm

Geology Club Seminar

Landscape evolution and the critical zone in the Luquillo Experimental Forest, Puerto Rico
Emma Harrison, Scripps Institution of Oceanography

The Luquillo Experimental Forest is a long-studied tropical ecosystem in northeastern Puerto Rico. Recent research constraining the tectonic history of the island has shown that there is a strong legacy of that tectonism in the evolving topography of the Luquillo Mountains. Using cosmogenic radionuclides, we have constrained the timing and sequence of topographic evolution to reveal a patchwork of differentially eroding domains within the greater watershed. This setting provides a natural experiment for understanding the influence of geomorphic change on critical zone processes. In the most stable portions of this landscape, thick nutrient-poor soil profiles develop. Here, mineral residence time is apparently grain-size dependent, implying the importance of kinetic sorting in the subsurface. As hillslope morphology evolves in response to the fluvial incision rate, soil profiles on ridgelines become thinner and less mature. Using multiple geochronological toolkits, I investigate the rate of chemical weathering in the adjusting and stable soil profiles and evaluate the timescale on which these processes are sensitive to geomorphic change. The duration and intensity of chemical weathering impacts the average size of the river bedload, and in turn, the relative rates of landscape evolution along different tributary streams within the watershed. These complex feedbacks determine the architecture of the critical zone and have a significant impact on the structure of the forest's biomass.

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