Downstream MET-IRSL single-grain distributions in the Mojave River, southern California: Testing assumptions of a virtual velocity model

Abstract

We use samples from a prior study (McGuire and Rhodes, 2015) to investigate the bleaching trend of Mojave River sand in more detail. We present new single grain data which provides insight into how previously presented multiple-grain luminescence signals decrease downriver. The single grain dose distributions allow for a test of the assumption that multiple-grain equivalent dose (De) is representative of fluvial transport down the Mojave River. For samples at the Forks, Victorville and Barstow, with laboratory luminescence sample codes J0262, J0267 and J0265, respectively, inspection of the kernel density estimate (KDE)-generated probability density function supports the assumption, though is does not prove it directly. Implementing a Kolmogorov–Smirnov (K–S) test shows that the downriver samples are statistically different from each other, suggesting bleaching as the primary mechanism for changes is dose distribution downriver. Single-grain dose distributions show that the De of the Afton Canyon sample (J0260) is not representative of grain travel from source to sink, but instead likely a result of local mixing of sediment populations. This result is confirmed by visual inspection of the KDE, and quantitatively using the K–S test. As has been noted by several authors, the single-grain dose distribution in active channels of rivers may represent a worst case in terms of poor bleaching, due to mixing of older-age populations (Cunningham et al., 2014; Jain et al., 2004). This observation holds for our data set and presents an opportunity to test and develop luminescence techniques to determine provenance. In particular, future sampling in the vicinity of the Afton Canyon site has the potential to identify the entry point of a poorly-bleached population.