Paper No. 143-7
Presentation Time: 3:15 PM
USING FALLOUT AND IN SITU RADIONUCLIDES TO UNDERSTAND THE EFFECTS OF A LARGE TROPICAL CYCLONE ON EROSION PROCESSES ON CARIBBEAN ISLANDS (Invited Presentation)
SCHMIDT, Amanda H.1, QUOCK, Melinda2, GRANDE, Alexandra3, CORBETT, Lee B.4, BIERMAN, Paul5, HIDY, Alan J.6, CAFFEE, Marc W.7, LÓPEZ-LLOREDA, Carla8, WILLENBRING, Jane9, MCDOWELL, William H.10, HILL, Marcus11, MONYAK, Paige N.12, AMBER, Eliza12, LEWIS, Amelia11 and BORDT, Ely Addison11, (1)Geology Department, Oberlin College, 403 Carnegie Building, Oberlin, OH 44074, (2)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (3)Department of Geology, Oberlin College, 258 N Professor, Oberlin, OH 44074, (4)Department of Geology, The University of Vermont, Burlington, VT 05405, (5)Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT 05405, (6)Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, (7)Department of Physics and Astronomy and Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, (8)Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824, (9)Department of Geological Sciences, Stanford University, Stanford, CA 94305, (10)Department of Natural Resources and the Environment, Department of Natural Resources and the Environment, 56 College Rd. James Hall 114, Durham, NH 03824-2601, (11)Geology, Oberlin College, Geology Department, Rm. 403, 52 W. Lorain St, Oberlin, OH 44074, (12)Geology, Oberlin College, 52 West Lorain Street, Oberlin, OH 44074-1044
Tropical islands are exposed to recurring disturbances from extreme storms such as tropical cyclones. To test whether such storms bias isotopic indicators of long-term erosion, we quantified temporal variability in meteoric and
in situ 10Be (
10Be
m,
10Be
i),
210Pb
ex, and
137Cs. We measured isotopes in river sediment samples from Dominica, an andesitic bedrock island in the Caribbean, before and after category five Hurricane María (2017). We also analyzed 20 river sand samples collected over 18 months after Hurricane María from two nested granitic watersheds in Puerto Rico to quantify recovery time after a major disturbance.
Populations of before- and after-storm isotopic concentrations on Dominica are statistically indistinguishable (n=7 for in situ 10Be, n=11 for meteoric 10Be). Individual before-after pairs have 10Be concentrations that vary from -138% to +73% relative to the mean of the before and after samples. These new data suggest that processes controlling the depth and amount of near-surface erosion on Dominica during extreme storms are spatially variable, but related to landslide frequency for meteoric 10Be in coarse grain (250-850 mm) sediments. Our data support theoretical assertions of Niemi et al. (2005) and Yanites et al. (2009) that basin-by-basin comparisons of erosion rates should be approached with caution in small watersheds affected by landslides and extreme storms. Erosion rates determined from in situ 10Be in Dominica (geometric mean=0.102 mm/yr, n=12) are low compared to similarly steep and wet areas globally and appear to be controlled by landslide frequency.
In Puerto Rico we sampled the Icacos (3.14 km2, 0.09% landslide area after Hurricane María) and Guabá basins (0.11 km2, 1.23% landslide area after Hurricane María). 10Bei concentrations in Icacos basin sediment remained steady over time whereas concentrations in Guabá basin sediment were initially half those in the Icacos basin and increased linearly over 18 months, constraining recovery time. 10Bem concentrations in both drainages varied randomly; 210Pbex and 137Cs were below detection limits in all samples. Our data suggest that 10Bei concentrations could be biased low (and thus erosion rates biased high) in disturbance-affected watersheds if stream sediment is sampled soon after large or extensive mass movements.
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