Determining fluvial sediment virtual velocity on the Mojave River using K-feldspar IRSL: Initial assessment

The Mojave River of Southern California was chosen as a field site to investigate the applicability of luminescence dating to sediment transport rate problems. Grains in the active channel of the river are expected to show signs of partial bleaching and this makes it difficult to determine time since deposition accurately. A modification of the multiple elevated temperature post-IR IRSL (METpIRIR) procedure, (Buylaert et al., 2009; Li and Li, 2011), was used for K-feldspar grains (175–200 μm) at temperature increments of 50, 95, 140, 185, and 230 °C in order to provide more information about relative signal bleaching among samples. The measurements show an exponential decrease in equivalent dose (De) with distance down the Mojave River. Higher temperature pIRIR signals are bleached more slowly than lower temperature ones (Buylaert et al., 2009). The De for samples at 50 °C is roughly constant along the river. These results suggest cyclical bleaching and burial as grains are transported downriver and higher energy (deeper) traps are vacated. The pattern of De values for the Mojave River can be used to constrain the sediment transport rate for this river by building a model of growth and bleach for each temperature increment. A bleaching experiment was run with multiple aliquot samples for direct sunlight exposure times of 0, 10, 30, 300, 1000, 3000, 10,000, and 30,000 s. The MET-pIRIR procedure was applied at each temperature increment for each exposure time aliquot and the results for all exposure times were fit to the general order kinetics equation using a non-linear regression. The bleaching parameters were used in conjunction with the SAR growth curves to build a model of partial bleaching of grains during transport that is fitted with a χ2χ2 test to the pIRIR data from the Mojave River. This model is not a unique solution, but can be used to assess the likelihood of various sediment transport regimes.