HR: 10:50h
AN: U32A-03    [Abstracts]
TI: Probability Distribution Function Of Cosmogenic ³He In Olivine From Hawaiian River Sediments: New Insights Into Erosional Processes
AU: * Gayer, E
EM: egayer@eps.harvard.edu
AF: Harvard University, Dept of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Mukhopadhyay, S
U32A-03 AF: Harvard University, Dept of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138 United States
AB: Understanding mechanisms that modify landscapes requires quantification of the rates at which landscapes respond to tectonic and climatic signals. Because measurements of long-term erosion rates are critical for understanding landform evolution, the use of cosmogenic isotopes in river sediments to estimate average erosion rates of drainage areas has grown rapidly in the recent years. In order to extract additional information about erosional processes, sediment transport, and sediment storage time, we have constructed a probability distribution function by measuring the cosmogenic ³He concentrations ([³He_c]) of multiple aliquots of olivine rich sands from Hawaiian river sediments. The olivine rich sands come from the Waimea River in the western part of the island of Kauai. He concentrations and isotopic ratios have so far been measured in 26 aliquots of olivine grains (1-2 mm) each weighing 100 mg. The [³He_c] have been calculated using: (i) the ³He/⁴He ratio measured by crushing and (ii) the ³He and ⁴He concentrations measured by melting the resulting powder. Initial results, from one sampling site, indicate a quasi bi-modal distribution of the [³He_c] and suggest erosion rates of ~0.07 mm/yr and ~0.03 mm/yr. The difference between these values and the average erosion rate of ~0.04 mm/yr, obtained from the average [³He_c] of the 26 aliquots, suggests that the statistical approach employed here should give new information about erosion rates in the drainage area. Interestingly, not only do we find evidence of two distinct populations but also the standard deviations of the distributions are quite distinct. This could be related to the different erosional processes (landslides vs. soil creep), storage/transportation time of sediments, or can represent the erosion of different sub-basins (main stream vs. tributary). Additional measurements on different grain size aliquots from the main stream and the tributaries will allow us to distinguish the processes responsible for these distributions. Nevertheless, our initial results are promising and the statistical method is a feasible approach given the low cost and the short time (< 2 weeks) within which the He isotopic measurements have been completed.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1815 Erosion
DE: 4918 Cosmogenic isotopes (1150)
SC: Union [U]
MN: Fall Meeting 2005