Using a portable luminescence reader for rapid age assessment of aeolian sediments for reconstructing dunefield landscape evolution in southern Africa

Analysis of sedimentary materials using a portable luminescence reader (or portable optically stimulated luminescence reader POSL), is a useful, rapid (a few minutes per sample), cost-effective and safe (not requiring exposure to chemicals) way to establish relative sample age. Moving beyond information that guides initial field interpretations, or develops targeted sampling strategies for full laboratory-based dating protocols, toward rapid age assessment has been more challenging. This study is the first demonstration of a simple, elegant and practical calibration of POSL signals into sample age estimates. This involved measuring the POSL signals from 144 samples with established published ages from across southern Africa, and a regression analysis. The data show that a regional-specific approach to calibration is needed, with regional patterns in POSL signals that are supported by 148 further undated samples. Four broad regions are defined: the Namib Sand Sea (NSS), the northern Kalahari (barchan dunes on the floor of Makgadigadi) (Nnk-MBa), the western Kalahari (WK) and the southern Kalahari (SK). Sample composition data, such as quartz-to-feldspar ratios (Q/F) appears to account for the largest contrasts within the dataset, whilst inherent POSL signal brightness and grain coloured-coatings (iron and clay) may also influence signals. The strength of the regressions (R-2 of 0.99, 0.93, 0.81 and one moderate at 0.52 for the NSS, SK, WK and NnK-MBa respectively) between POSL signals and sample age, (for ages back to 118, 104 74 and 5 ka for the NSS, SK, WK and NnK-MBa respectively), demonstrates the practicality and huge value of this simple approach. The implication is that region-specific calibrations must be built prior to using the POSL reader for rapid age assessments. This approach is a cost and time-effective method for inter-dunefield landscape-scale analyses, which will cast light on the key climatic variables driving landscape change in sand-rich drylands during the Late Quaternary, and also has the potential for large-scale analysis within other geomorphic settings.