T143 Surface Process Response to Extreme Events

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

USING FALLOUT AND IN SITU RADIONUCLIDES TO UNDERSTAND THE EFFECTS OF A LARGE TROPICAL CYCLONE ON EROSION PROCESSES ON CARIBBEAN ISLANDS

Paper number: T143-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

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 (10Bem, 10Bei), 210Pbex, 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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355936

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