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A luminescence age from a sample is typically calculated from a distribution of De values measured from multiple individual grains or multi-grain aliquots from that sample. The overdispersion of the De distribution (commonly abbreviated “OD” or “σ” and presented in percent) represents the “spread” in the distribution that remains after all measurement uncertainties have been taken into account (Galbraith and Roberts, 2012).
Below, we show two De distributions obtained from river sediments along the Colorado River, USA (Fig. 1). These are plotted as KDE plots and radial plots, and show different levels of OD. The OD value of Sample 1 is 27% while that of Sample 2 is much higher (101%). This suggests that most grains in Sample 1 had adequate sun exposure to deplete their luminescence signal while being transported by the river prior to burial, while Sample 2 was not sufficiently exposed.
You can see that the KDE plot from Sample 2 is heavily skewed with a tail of high De values plotting to the right. This pattern is typical of partially bleached sediments. By contrast, the KDE plot from Sample 1 is more symmetrical, and is typical of well-bleached samples. The radial plot from Sample 1 shows most De values plotting within the ± 2 σ grey shaded region and are consistent within error with the weighted mean central value of 1.04 Gy. The radial plot from Sample 2, however, shows most De values lie outside of the shaded region, and are not consistent with this sample’s weighted mean. These two samples from the Colorado River show how the level of bleaching of sediments can vary significantly within a single depositional environment.
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Figure 1. KDE plots (left) and radial plots (right) of samples from the Colorado River, USA. Sample 1 has a lower OD value than Sample 2. The radial plots are centered on the sample weighted mean De value.
Similar variations in OD were observed in very young sediments measured from the Buffalo River, Arkansas. Langston et al. (2024) observed significant changes in sample OD depending on where they sampled in the river channel (Figs 2 & 3).
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Figure 2. Sampling sites in the Buffalo River channel, Arkansas. Reproduced from Langston et al. (2024).
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Figure 3. De distributions from sediments sampled from a channel bar (BR003, BR004) and from floodplain sediments (BR002, BR005) in the Buffalo River, Arkansas. The blue lines in the KDE plots mark the MAM De value, while the red dashed lines mark the CAM weighted mean De value.
After scrutinising De distributions and their overdispersion, as well as taking into consideration the geomorphic and stratigraphic context of the samples, luminescence practitioners will choose the best statistical model with which to calculate the final sample De.