In response to unprecedented 2011 floods in southern Africa, this RAPID project will sample isotope concentrations in recently transported fluvial sediments in Namibia before these sediments are disturbed in the subsequent rainy season. With the 2011 'flood of record' occurring over much of the area that has been sampled on three previous occasions in the past 14 years, this project seizes a unique opportunity to test a fundamental (and virtually untested) assumption underlying the inference of basin-scale erosion rates from in situ Berilium-10 concentrations--that nuclide concentrations at a given location are consistent over time, even after major flood events. The PIs will also test inferences made about sediment sources from other sediment-associated isotopes (meteoric 10Be, 210Pb and 137Cs).
Understanding the temporal variability of in situ 10Be and meteoric 10Be, 210Pb, and 137Cs concentrations in river sediment, as well as how each isotopic system responds to massive storm events, is a prerequisite to applying these isotopes responsibly as sediment tracers and erosion rate monitors. Sediment-associated isotopes of 10Be, 210Pb, and 137Cs are popular and widely applied for such purposes, yet their interpretation depends on whether or not nuclide concentrations are consistent over time at a given location. This project will provide a test of this key assumption.
Project Outcome Report Award: EAR 1143775 PI: Kyle Nichols Skidmore College RAPID: Effect of massive, exceptional floods on sediment-associated isotope concentrations - implications for sediment source identification and erosion rate estimation During the austral winter of 2010-2011 the African country of Namibia experienced the largest and longest flood season on record. Water flowed in Namibian Rivers, including those that cross the hyper-arid Namib Desert, for weeks - a very unusual situation as most of these rivers carry flow at most once every several years. Photographs and videos of the flood as well as eye-witness reports indicate that suspended sediment concentrations were quite high, suggesting large amounts of sediment were moved off the landscape. The large amount of sediment transported during these floods deposited new sediment at sites that we sampled previously three times since 1997. By having up to four temporal sample replicates, the last of which was collected after the flood of record, we can test both the impact of large floods on the source of sediment to arid-region channels as well as the consistency of isotopic (meteoric 10Be, in situ 10Be, 210Pb and 137Cs) concentrations over time. Such temporal consistency of in situ 10Be is fundamental to understanding geologic rates of erosion, while consistency of meteoric 10Be, 210Pb and 137Cs are prerequisite to applying them responsibly as sediment tracers. Such research is important to compare contemporary erosion rates to reliable background, geological rates of landscape change in order to understand the magnitude present-day processes compared to natural process rates. Furthermore, knowing from where sediment is derived, and how quickly it moves, is important to determine if sediment sources are time invariant. To date, the samples are still being processed. We resampled 68 sediment samples and have chosen a subset to compare to the 1997, 2001, and 2010 data. Due to a backlog of samples we have been unable to process all of the samples. However, preliminary results show that 8 of 11 resampled sites in 2011 have 210Pd activities that are within analytical uncertainty of the 2010 samples, while 3 of the 2011 samples have significantly less 210Pb than the 2010 samples. This project is providing undergraduate research experience for one Skidmore College undergraduate student. The student is being trained on the measurement of fallout radionuclides, will analyzing the funded samples, and write a senior thesis during spring 2013. The student and PI will present the data at a national geological conference in 2013.
