Collaborative Research: Testing Critical Zone Controls on Mountain-Scale Relief in a Tropical Climate

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The Critical Zone is the thin layer of Earth that extends from the base of the soil layer to treetops and supports most terrestrial life on our planet. This project examines how differences in Critical-Zone processes influence topography through a comparative experiment of two different bedrock units on the tropical island of Puerto Rico. On the island, ancient volcanic rocks form steep, high peaks and have clay-rich soils, whereas granitic rocks underlie relatively more subdued, rolling hills covered by sandier soils. This study will evaluate the impact of these differences on the occurrence of landslides, the size of sediment grains that populate river channels, and the size and frequency of floods. The principal hypothesis is that landslides cut more deeply in the clay-rich soils and, as a result, deliver coarser sediment to rivers. The coarser sediment, in turn, better shields the river channel from erosion such that larger floods are needed to carve the underlying bedrock. In addition, the clay-rich volcanic soils retain more rainwater, resulting in fewer floods than areas covered by sandy granitic soils. The net result is that the volcanic rocks maintain a higher elevation and steeper topography than the granitic terrain. This project will test these ideas through detailed landslide mapping, grain-size measurements, statistical analyses of streamflow gaging data, and measurements of long-term landscape erosion rates. This project will form the basis for graduate and undergraduate research experiences and also engage additional students at the University of Puerto Rico, Mayagüez and Colorado State University in an experiential learning, research, and cultural exchange program centered on comparing and contrasting the landscapes in Puerto Rico and Colorado.Variations in topographic relief, steepness, and height are often observed to correlate with differences in bedrock geology. Most often, these changes are interpreted to result from differences in the erodibility of the underlying bedrock. On the tropical island of Puerto Rico intermediate-to-mafic volcaniclastic units underlie high relief, steep mountains, whereas felsic-rich granitic-to-granodioritic rocks are characterized by gentler, rolling hills. Preliminary results indicate that unweathered bedrock strength and fracture density between these two rock units is similar, implying the differences in primary rock strength or erodibility cannot explain the stark contrast in topography. Previous studies show that the soils developed atop each of these rock units are distinct, with the more mafic units comprised of clay-rich soils and sandier soils developed in the more felsic units. These differences in soil characteristics and associated Critical Zone architecture promote relief production in the volcaniclastic units relative to the granitic units. Specifically, differences in the Critical Zone affect the magnitude of incision thresholds by controlling bedload grain size distributions and the frequency of floods that breach these thresholds by modulating shallow, subsurface hydrology. These hypotheses will be tested by mapping landslide locations, measuring grain size distributions in sediment deposits, analyzing flood statistics from existing stream gaging stations, and calculating long-term erosion rates from cosmogenic radionuclides. Datasets will be synthesized into a modeling framework that predicts fluvial landscape relief as a function of rock strength, the magnitude of incision thresholds, and the frequency of threshold breaching floods. This project is jointly funded by the Geomorphology & Land-use Dynamics program in Earth Sciences and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.