How a barrier island may react on a sea-level rise: The Holocene to Recent Rømø barrier island, Danish Wadden Sea

• Main Authors: Johannessen, Peter N., Nielsen, Lars H., Møller, Ingelise, Nielsen, Lars, Pejrup, Morten, Andersen, Thorbjørn Joest, Madsen, Anni Tindahl, Clemmensen, Lars B, Murray, Andrew S.
• Format: Conference Object
• Language: English
• Published: 2010
• Subjects: Barrier Island; Barrier Islands; Greenland; Johannessen; Swash
• Online Access: https://curis.ku.dk/portal/da/publications/how-a-barrier-island-may-react-on-a-sealevel-rise-the-holocene-to-recent-roemoe-barrier-island-danish-wadden-sea(f43e06dc-26d5-4e31-aabc-d7ba582f7851).html

The Rømø barrier island is situated in the northern part of the European Wadden Sea. It has been intensively studied on the basis of recent depositional systems and morphology, seven 25 m long sediment cores, 35 km ground penetrating radar (GPR) reflection profiles with a maximum signal penetration of c. 15 m and a resolution of c. 20–30 cm (Nielsen et al., 2009), and dating of 70 core samples using optically stimulated luminescence (OSL). The area has experienced a relative sea-level rise of c. 15 m during the last c. 8000 years. The Recent tidal amplitude reaches c. 1.8 m. During strong wind set up the water level increases considerably and the highest measured water level is 4.9 m above mean sea level. The barrier island is c. 14 km long and c. 4 km wide and is separated from the mainland by a c. 8 km wide lagoon. At the northern and southern parts of the island, tidal inlets occur with a width of 400–1000 m and depths of 7–30 m. Salt marsh areas, up to 2 km wide, are fringing the lagoonal coast of the island. Active eastward migrating aeolian dunes cover large parts of the island. The Rømø barrier island system is a very sand rich system as it receives coast parallel transported sand from north and south along the shoreface and is resting on fluvial sand. The combination of cores, GPR and studies of the Recent morphology and depositional processes is a powerful tool to identify palaeo-sedimentary environments (Johannessen et al., 2008). On GPR sections from the central part of the island a series of beach ridges, up to 2.5 m high, often with swales in between are underlying the modern aeolian dune sands. Washover fans, up to 2.5 m thick, are often seen in the GPR sections immediately east of the beach ridges and can be followed c. 250 m eastwards and may have steep slipfaces. Shoreface sands may be followed eastward to beach ridges and show westward progradation. Swash bars are occasionally seen on the shoreface close to the beach ridges. In the northernmost area of the island the GPR sections are dominated by co-sets with westerly and easterly dipping foresets, indicating bipolar current directions (Møller et al., 2008). These sediments were probably deposited in a deep, broad tidal inlet north of the initial barrier island similar to what is observed at the island today. Facies analysis on the cores and correlations between wells show that barrier island sediments and related shoreface sand and lagoonal sediments are up to 20 m thick and overlie Weichselian fluvial sand. The first 5000 years the barrier island aggraded and the last 3000 years it prograded despite the relative rising sea level rise of c. 15 m during the last c. 8000 years. This shows, that if there is a surplus of sand in a tidal area, barrier islands may aggrade even if there is a rise in sea level. If the rate of sea level rise decreases then the barrier island may prograded. With this unique dataset with extremely large amounts of OSL datings from core sediments it has been possible to construct detailed palaeogeographic maps of the barrier island development through time. Johannessen, P.N., Nielsen, L.H., Nielsen, L., Møller, I., Pejrup. M., Andersen, J.T., Korshøj, J.S., Larsen, B. and Piasecki, S. (2008) Sedimentary facies and architecture of the Holocene to Recent Rømø barrier island in the Danish Wadden Sea. Geological Survey of Denmark and Greenland Bulletin, 15, 49-52. Møller, I., Nielsen, L., Johannessen, P.N., Nielsen, L.H., Pejrup, M., Andersen, T.J., and Korshøj, J.S. (2008) Creating the framework of sedimentary architecture of a barrier island in the Danish Wadden Sea. Proceedings of 12th International Conference on Ground Penetrating Radar, Birmingham, UK, 8 pp. Nielsen, L., Møller, I., Nielsen, L.H., Johannessen, P.N., Pejrup, M., Korshøj, J.S. and Andersen, T.J. (2009) Integrating ground-penetrating radar and borehole data from a Wadden Sea barrier island. Journal of Applied Geophysics, 68, 47-59.