Alastair C. Cunningham

Expectations of scatter in equivalent-dose distributions when using multi-grain aliquots for OSL dating

In the OSL dating of sediment, the scatter in equivalent dose (De) between grains is almost always larger than would be expected due to counting statistics alone. Some scatter may be caused by insufficient (partial) bleaching of some of the grains prior to deposition. In order to date partially bleached sediment, it is essential to estimate the amount of scatter caused by other processes (e.g. grain-to-grain variability in the natural dose rate). Measurements of such scatter are performed at the single-grain level; by contrast, most OSL dating is performed on multi-grain subsamples, for which grain-to-grain scatter is reduced through averaging.Here we provide a model for estimating the expected scatter (i.e. excluding that caused by partial bleaching) for multi-grain aliquots. The model requires as input the single-grain sensitivity distribution, the number of grains in the sub-samples, and the expected scatter at the single-grain level, all of which can be estimated to an adequate degree. The model compares well with measured values of scatter in De, determined using aliquots of various sizes, and can be used to help produce a minimum-age De from multi-grain subsamples that is consistent with single-grain data.

Extracting storm-surge data from coastal dunes for improved assessment of flood risk

Future changes in climate and sea level are likely to increase the threat from storm surges in many coastal regions. Mitigation of this threat requires an understanding of storm surge magnitude and frequency, and the relationship of these variables to climate parameters. This understanding is currently limited by the brevity of instrumental records, which rarely predate the twentieth century. However, evidence of former storm surges can be recorded in coastal dunes, because the dune topography may trap high-magnitude deposits at elevated locations. Here we combine a range of techniques to extract storm-surge data from coastal dune sediment. The sediment is tracked in the subsurface with ground-penetrating radar to assess its height and extent, and its age is determined with good precision through optically stimulated luminescence dating. The probable age of the sediment (A.D. 1775 or 1776) is within a period of increased storminess in northwest Europe, and the local magnitude of the event is likely to be greater than any on instrumental record. By utilizing coastal dunes for storm surge analysis, our approach provides a valuable new source of information for understanding storm surge risk, which is vital for the protection of coastal regions.