Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating

A sediment core taken from a tidal mudflat in Ho Bugt in the northernmost part of the Wadden Sea in Denmark is used to explore the application of optically stimulated luminescence (OSL) dating to young fine-grained estuarine sediments, using 210Pb dating and the 137Cs Sellafield peak from 1980 as independent age control. Despite the anticipated difficulties of weak luminescence signals and incomplete resetting of residual radiation dose prior to deposition, the OSL ages from the sediment core ranged from 7.0±1.5 at the surface to 305±16 years (68 cm depth). OSL- and 210Pb-dates were in good agreement back to ∼1975, and even as far back as ∼1945 using the CRS-model. The average OSL age of 9±3 years for the surface mixing zone indicates that the OSL signal of the quartz grains was well zeroed at deposition. The OSL ages give new information on past deposition regimes on the Ho Bugt mudflat: fine-grained sediment started to deposit at ∼0.5 mm year−1 approximately 300 years ago, when a terrestrial reed-swamp was transgressed. About 100 years ago, the sedimentation rate increased to ∼4 mm year−1. Approximately 40 years ago, the sediment became more fine-grained, perhaps because of emergence of the tidal flat; the accretion rate at that time was already well above the local rate of sea-level rise of ∼1 mm year−1. The post-1980 accretion rate was found to be ∼16 mm year−1 with OSL dating and ∼9-12 mm year−1 with 210Pb and 137Cs dating. Advantages of the OSL method include applicability over a much larger time-span (∼125 ka compared to ∼120 years for 210Pb) and a lack of significant dependence on changes in mean grain size or 210Pb supply. With this method, it is also possible to date sediment cores from sand flats, providing a new approach to the problem of evaluation of stability and calculation of sediment budgets for estuaries and coastal lagoons.