Optical Dating of Dune Ridges on Rømø, a Barrier Island in the Wadden Sea, Denmark
Abstract
The application of optically stimulated luminescence (OSL) to the dating of recent aeolian sand ridges on Rømø, an island off the southwest coast of Denmark, is tested. These sand ridges began to form approximately 300 years ago, and estimates of the ages are available from historical records. Samples for OSL dating were taken ∼0.5 m below the crests of four different dune ridges; at least five samples were recovered from each ridge to test the internal consistency of the ages. Additional samples were recovered from the low lying areas in the swales and from the scattered dune formations in a broad hummocky area landward of the well-defined ridges. Despite low luminescence signals, we were able to obtain a mean age for the youngest ridge of 17 ± 2 years, consistent with the known age of >28 years. Optical ages of individual samples in our study ranged between 10 ± 3 years and 690 ± 50 years, and all ages were broadly consistent with those expected from historical information. The oldest aeolian surface on Rømø appears to be 370 ± 30 years. This is built on what we interpret as a marine sandbank, whose surface is ∼700 years old. The sand ridges seaward of the hummocky dune field have well-defined building phases separated by inactive periods and the first major ridge formed ∼235 years ago. This study demonstrates that optical dating can be successfully applied to these young aeolian sand deposits, and we conclude that OSL dating is a powerful chronological tool in studies of coastal change.
Map showing the Frisian Islands and the Wadden Sea, stretching from the Netherlands in the south to Denmark in the north. The study site of Rømø, and the southernmost island of Texel, are both marked.
Elevation map of Rømø indicating the parallel ridge structure. The expected age ranges of the ridges on the basis of historical maps are also shown.
Sample positions at Rømø. All samples were taken ∼50 cm or more below the present surface.
Typical stimulation decay curves of sample A1, B4, and B20.
The growth curve of (a) sample A1 and (b) sample A4. Filled circles represent dose responses to different regeneration doses, and open circles are the recycling points. The open triangle is the recuperation signal.
Dependence on preheat temperature of the equivalent dose (filled circles), recuperation (filled triangles), and recycling ratio (open circles) for sample A1.
Dependence of apparent De on preheat temperature after bleaching with blue light in samples A1 and A4. Thermal transfer increases with higher preheat temperatures.
Results of a dose recovery test with samples B16, B9, and A1, showing that preheat temperatures below 260°C give an accurate measurement of the known dose.
A summary of the optical ages as related to individual topographic units.
Testing the internal consistency in the OSL data set. The ages from ridges 1 and 3 are presented with the expected age ranges. Unit 4 has a similar restricted age range, with an average of 17 ± 2 years (n = 5) consistent with the expected range of <28 years.
Storm surge record from Clemmensen et al. (2001c, after Gram-Jensen, 1991).
