4 Radionuclides: Biogeochemistry, Transport, and Geological Applications I

Friday, 20 March 2020
8:00 AM - 12:00 PM
Regency Ballroom A

TEMPORAL VARIANCE OF EROSION IN LUQUILLO, PUERTO RICO POST-HURRICANE MARIA DETECTED USING TIME SERIES OF ISOTOPIC MEASUREMENTS IN DETRITAL SEDIMENT

Paper number: 4-4
Program: Symposia Sessions
Time: 8:45 AM – 9:05 AM

Authors

Abstract

Hurricane Maria was one of the strongest hurricanes ever recorded. With climate change, more strong storms are likely and with them, increased erosion. This study quantifies temporal variability in sediment sourcing using isotope concentration in two small (≤3.4 km2) watersheds in eastern Puerto Rico: the Guabá and Icacos. By quantifying the temporal variability in meteoric and in situ 10Be and analyzing the fallout radionuclides 210Pbex and 137Cs, we test the assumption of temporal invariance of detrital sediment isotope concentration.

We extracted meteoric and in situ 10Be from 7 sediment samples collected at both watershed outlets over 18 months after Hurricane Maria. Extractions were done at the University of Vermont, and isotope ratios measured at Purdue. The presence of 137Cs and 210Pbex was determined in 20 samples from each watershed at Oberlin College.

In samples from the Guabá, we measured low in situ 10Be concentration after Maria; about 3 months after the event, in situ 10Be concentrations were 5.7x104 10Be atoms/g (similar to landslide sediments in Brown et al., 1995) and 1.45x105 10Be atoms/g in the Icacos (similar to river sediment concentrations measured there by Brown et al. 20 years earlier). During the 18 months after Maria, 10Be concentrations in Guabá sediment steadily increased to 1.47x105 10Be atoms/g. After Maria, in situ 10Be concentrations in the Icacos were steady with a final value of 1.53x105 10Be atoms/g, statistically indistinguishable from the first sample, reflecting the lack of landslides upstream. Meteoric 10Be had no systematic changes over time for either watershed but varied more than in situ 10Be. 137Cs and 210Pb were below detection limit, indicating erosion deeper than 30 cm in both watersheds.

Our data are consistent with landslide erosion at least several meters deep directly after Hurricane Maria in the Guabá watershed, with a gradual return to baseline concentrations over 18 months. This indicates temporal variation in sediment sourcing and demonstrates in the uplands of Puerto Rico, that there is an 18-month response in the erosional system to landsliding - a quantification of landscape recovery in steep, tropical regions after large storms like Hurricane Maria.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-345011

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