An Infrared Stimulated Luminescence (IRSL) Procedure for Estimating the Transport Rate of Potassium-Feldspar Grains in a Fluvial Setting

The Mojave River and Santa Clara River of Southern California were chosen as field sites to assess the feasibility of implementing infrared stimulated luminescence (IRSL) techniques to determine sediment transport rate. Feldspar sand grains in the active channel of these rivers are expected to be incompletely (partially) bleached by sunlight exposure during transport, causing the grains to have inherited charge at the time of deposition. A modification of the Post-IR IRSL procedure developed by Buylaert et al. (2009) was used for K-Feldspar grains (175-200 μm) at temperature increments of 50, 95, 140, 185, 230 °C over multiple bleach and artificial dose cycles, providing 5 signals of different sensitivity to light exposure. The measurements show an exponential decrease in equivalent dose (De) with distance down the Mojave River, with relatively less bleaching downriver for higher temperature measurements. The equivalent dose for samples at 50 °C is roughly constant along the river, at a low value of approximately 0.7 Grays. The results for higher temperature measurements suggest cyclical bleaching and burial as grains are transported downriver and higher energy (deeper) traps are gradually vacated. However, this interpretation cannot be applied to the Santa Clara River, as no simple relationship exists between the location of samples and their equivalent dose. Possible explanations for this observation include significant sediment flux from catchments with different mineralogy and recent geologic history. For the Mojave River, the relationship between De and distance downriver can be used to constrain transport rate. A bleaching experiment was designed for the Mojave River samples to assess the rate of signal loss as a function of daylight exposure time for each of the different IRSL signal components. The results for each exposure time were fit to the general order kinetics equation, a function used to fit IRSL read-out, using a non-linear regression (Levenberg-Marquardt). The fitted bleaching parameters were used in addition to the single aliquot regenerative-dose (SAR) protocol growth curves to build a model of bleach and growth of IRSL during transport, deposition, and burial. While this model does not provide a unique solution for the sediment transport rate, it can be used to assess the likelihood of a range of transport rates. References: Buylaert, J.P., A.S. Murray, K.J. Johnson, and M. Jain. 'Testing the potential of an elevated temperature IRSL signal from K-feldspar.' Radiation Measurements. 44 (2009), 560-565. Critelli, Salvatore, Emilia le Pera and Raymond V. Ingersoll. 'The effects of source lithology, transport, deposition and sampling scale on the composition of southern California sand.' Sedimentology, 44 (1997), 653-671. Enzel, Y. and S.G. Wells. 'Extracting Holocene Paleohyrology and Paleoclimatology from modern extreme flood events: an example from Southern California.' Geomorphology. 19 (1997), 203-226