Holocene relative sea-level curves for the German North sea coast
The German North Sea coast is not only recently characterized by different geomorphological, hydrographical and geological situations that have to be taken into account when working with sea-level data. Today’s sea-level research asks for high resolution and accuracy. The question arises whether this can be achieved by the field data being available and whether potential sea-level fluctuations are masked or not by the error ranges to be assumed (cf. Gehrels 1999). This holds for any coastal region around the North Sea, the Irish Sea and the English Channel. The present German North Sea coast may be subdivided into different landscapes such as estuaries, barrier islands, embayments and marshlands. These landscapes all have their own specific sedimentation processes in terms of changing rates of accommodation space to sediment supply and also flood basin effects. These in turn influence sedimentation and sedentation rates. Therefore, progradation and retrogradation of the coastline are not necessarily connected to sea-level changes alone. Also, the tidal situation along the German North Sea coast at present varies from approximately microtidal in the North at the island Sylt to macrotidal conditions in the Jade Bay. The mean high water level differs up to more than 1 m along the German North Sea coast, which is caused by the present palaeogeographical setting in relation to the amphidromic point in the North Sea. The construction heights of the local dwelling mounds on the marshland islands in the North of the German Bight differ up to 0.75 m due to differences in the storm wave surges in that area. Furthermore, regionally different glacio-isostatic movements during the Holocene due to the position to the collapsing glacial forebulge have to be regarded. Following the most recent geophysical models, the differences in isostatic subsidence along the German North Sea coast during the Holocene are in the order of several metres (i.e. Busschers et al. 2007). Therefore, comparing data from regions with different tidal situations and glacio-isostatic movements by amalgamation of data of the entire German North Sea coast in one single curve without quoted vertical error may lead to data artefacts showing fluctuations in a decimetre scale up to more than 1 m within 200–500 years as in the curve published by Behre (2003, 2007). Baeteman et al. (2011) examined the sea-level index points of the dataset used by Behre (2003, 2007) by setting them in their stratigraphic and also geographical context. These re-evaluated data show a range of age and altitude not supporting any interpretation of fluctuations any more. Consequently, several coastal sections have to be defined to reduce the error resulting from the above-mentioned influences and to reduce the probability of upscaling local effects. In our publication in 2010 (Bungenstock 2010), we present five coastal sections (Fig. 1) as a ‘‘first approach’’ and as ‘‘... a first step to show that several curves are needed to demonstrate the Holocene sea-level history of such a big area as the entire German North Sea coast ...’’. The suggestion we made in our 2010 paper is to construct several new regional sea-level curves for the German North Sea coast based on the assumption that the vertical error ranges to be adopted may be reduced when collecting data from smaller defined areas mirroring the F. Bungenstock (&) Lower Saxony Institute for Historical Coastal Research, Wilhelmshaven, Germany e-mail: bungenstock@nihk.de