Assessing the reproducibility and accuracy of optical dating of fluvial deposits

Optical dating is increasingly being applied to fluvial deposits to derive rates of geomorphological and sedimentary processes. Partial bleaching of the water-transported sediment grains is a common occurrence, however, and creates problems in deriving burial ages. This paper presents results from optically stimulated luminescence (OSL) dating performed on samples collected from discrete reaches of palaeochannels of the Klip River, South Africa, in order to determine when bedload transport last occurred. The samples were analysed using the single-aliquot regenerative-dose protocol on aliquots made up of ∼30 quartz grains. OSL ages were calculated using both the finite mixture model and minimum age model; comparison of the ages for samples with radiocarbon ages for overlying organic-rich sediment indicates that the finite mixture model gives more accurate results. Samples collected from along the same reach of palaeochannel do not always give consistent burial ages, and careful interpretation of the results is needed. This study demonstrates the importance of the selection of sample locations, and the need for appropriate analysis of the equivalent dose distributions to derive the burial dose and hence the age of a sample.