Abstract
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 (R2 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.
Highlights
Reconstructing the landscape evolution of desert dunefields over the Late Quaternary helps to elucidate the mechanisms driving changes in key climate variables, such as precipitation, moisture balance and the wind
Because the MBa samples have very low infra-red stimulation (IRSL) signals, we present the blue light stimulation (BLSL) from the portable optically stimulated luminescence reader (POSL) (Fig. 2) (IRSL signals plots are available in Supp Info 3)
POSL signals from 144 sand dune samples across southern Africa for which there are established published OSL ages reveal excellent relative age information, as shown by POSL signals plotted by depth (Fig. 2), and the strength of regressions between POSL signals and established, published ages confirms that signals reflect sample age (Fig. 3)
Summary
Reconstructing the landscape evolution of desert dunefields over the Late Quaternary helps to elucidate the mechanisms driving changes in key climate variables, such as precipitation, moisture balance and the wind This understanding, coupled with using the Quaternary proxy datasets to test the robustness of numerical climate model simulations of dune-field dynamics, facilitates better model simulations that can be used to predict future changes and remobilisations Using chronostratigraphies to reconstruct past environmental, and possibly climatic, response at the dunefield scale is both extremely time consuming and resource intensive when undertaking laboratory-based luminescence dating protocols, due to the large number of samples needed to overcome noise inherent in the preservational record (Stone and Thomas, 2008; Telfer et al, 2010; Bailey and Thomas, 2014). The SGC approach has the benefit of chemical sample refinement and a test dose to correct for differences in natural signal intensity arising from differences in sample mass or intrinsic luminescence brightness, but doesn't offer information in the field. Munyikwa and Brown (2014) combined use of the POSL and SGC approach, gamma-irradiating bulk sample material in the laboratory to produce a POSL-based SGC
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