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Dune ridge progradation resulting from updrift coastal reconfiguration and increased littoral drift

Fruergaard, Mikkel, Kirkegaard, Lasse, Østergaard, Anni T., Murray, Andrew S., Andersen, Thorbjørn J.
Geomorphology 2019 v.330 pp. 69-80
climatic factors, coasts, islands, littoral zone, luminescence, marine sediments, shorelines, storms, Denmark, North Sea
Coastal dune ridges are prominent morphological features of many barrier islands. To investigate the environmental controls on coastal dune ridge formation 38 samples from prominent coastal dune ridges on the barrier island of Fanø in the Wadden Sea were dated by the optically stimulated luminescence (OSL) technique. Multiple samples were recovered from each dune ridge to test the internal consistency of the ages and to establish the last time the ridges were active. The OSL ages range from 4 ± 1 yr to 318 ± 27 yr and the average age of the dune ridges from 255 ± 30 to 6 ± 2 yr. The results show that Fanø prograded between 1000 and 1500 m in the last 250 to 300 years with an average long-term progradation rate of 4 to 5 m yr−1. We argue that dune ridge formation was favoured by normal regressive conditions resulting from increased sediment supply after an extreme storm surge event in 1634, induced by a combination of regional-scale morphological changes in the updrift coastal configuration and erosion and deposition resulting from reorientation of the barrier shoreline. Increased storminess during the Little Ice Age does not appear to be directly responsible for dune building at Fanø or in the Danish Wadden Sea but may have contributed to an overall increased sediment supply along the coast. In a regional context, dune ridge building was controlled by the inherited morphology of the shoreline that forms a large-scale embayment favouring sediment surplus and regressive conditions in the inner part to the embayment and sediment deficit and transgressive conditions along protruding parts of the embayment. It is further argued that increased marine sediment supply due to changes in local to regional scale coastal morphology and changes in climatic drivers can be equally important controlling dune ridge formation.