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EROSION RATE ANALYSIS OF TWO WATERSHEDS IN SOUTHEASTERN PUERTO RICO USING 10BE
We collected river channel sand from two adjacent, but topographically distinct, watersheds within the Humacao district; the Río Guayanés (68.4 km²) and the Río Maunabo (13.9 km²). Both drainage basins are underlain by the San Lorenzo granodiorite-quartz diorite but the degree and scale of landscape dissection differs. We collected 15 samples of sand-sized, quartz-rich sediment and then isolated quartz and extracted pure beryllium from these samples at the University of Vermont. We analyzed the samples for cosmogenic 10Be content using Accelerator Mass Spectrometry at PRIME. To calculate erosion rates for our samples, we used the CRONUS erosion rate calculator (v3) to interpret measured isotope concentrations.
The average rate of erosion determined from the 10 samples for which we have measured 10Be is 53 m/My. The Río Maunabo is eroding at a rate of 67+/-11 (average, 1SD) m/My and the Río Guayanés is eroding at a rate of 40+/-25 m/My (average, 1SD). Erosion rates for Río Maunabo range from 56 to 81 m/My. Erosion rates for the Río Guayanés range from 15 to 74 m/My.
The erosion rates we measured in this study are consistent with others measured on the island of Puerto Rico. For example, Brocard et al. collected samples about 70 km of where we worked in a similar granitic pluton; they report rates ranging from 15+/-1 to 72+/-8 /My. The erosion rates we measured in Puerto Rico (mean = 53 m/My; n = 10) in basins underlain by granitic rocks are more rapid than most in Cuba (mean = 23 m/My; n = 25, Campbell et al., 2022) where most basins are underlain by a mix of sedimentary and metamorphic rocks. 10Be-determined erosion rates in Puerto Rico are much slower than those measured by Quock et al. (2022) in Dominica, where steep basins are underlain by volcanic rocks (mean = 102 m/My; n = 7).