Funded by the DFG in the years 1997 - 2001
Salzwiesenhorizont_Juist_1
Salt marsh horizons on the seaward beach of Juist (photo Freund)


Introduction

Cyclical Quaternary climate fluctuations have left a sequence of layers in the southern North Sea and the neighbouring mainland areas that is structured by a large number of marine transgressions and regressions. Phases of falling temperatures and extreme cold periods are characterised by regressions, i.e. sea retreat or sea level lows, while phases of rising temperatures and warm periods are characterised by an advance of the sea or sea level highs. For example, between 22,000 and 18,000 years ago, at the peak of the last cold period in Europe, the Weichselian cold period, the North Sea level was approx. 110 - 130 metres lower than it is today, and the coastline had retreated far to the north to approx. 350 km north of the Dogger Bank. With the rewarming, the sea level rose and the coastline rapidly moved inland and higher over former glacial landscapes. Initially, the floods penetrated from the north into what is now the North Sea basin; it was not until around 7000 BC that the English Channel opened up as a further connection to the Atlantic and turned Great Britain into an island.

The Holocene sea level rise was initially relatively rapid and continuous, with an average rate of rise of around 2.1 metres per 100 14C years. It was only from 5000 BC onwards that the rise slowed down significantly and phases of advance and retreat alternated. These phases are characterised in the geological structure of the coastal Holocene by an alternating sequence of organogenic and clastic deposits, whereby peat and soil formations correspond to regression phases and the clastic sediments to transgression phases. The older transgressive and regressive phases in the Holocene are well known from numerous studies, but there is a gap in the information available for the period after the turn of the millennium up to the beginning of historical tide gauge measurements in the middle of the 19th century. The investigation of older salt marsh and mudflat horizons, which formed in the area of the East Frisian Islands, offers the possibility of closing this gap. As the subsoil of the islands is mainly made up of sandy sediments, subsidence, such as that which occurs in the continental area, e.g. due to intercalated peat, can be neglected here.

Older salt marsh and tidal flat horizons on the East Frisian Islands as natural level markers

The fact that the East Frisian Islands are not stable is well known from geological investigations and current observations. Due to marine morphodynamics, the position, shape and course of the island chain has constantly changed over the last few centuries. Due to a north-south shift, for example, the dune front of the islands has moved forward by at least 500 metres onto the respective salt marshes of the islands over the last 2000 years. As a result, these were covered by the island dunes and the original state of development was preserved. Today, such older salt marshes or mudflat deposits are exposed erosively on the beach. In places, several of these horizons are exposed on the beach, stepped against each other (Fig. 1).

Salt marsh horizon_Juist_2
Fig. 1: Salt marsh horizons on Juist

The deposition conditions of these older horizons can be reconstructed through scientific investigations. In addition to geological methods (geochemistry, sedimentology), palaeoecological investigations such as pollen analysis and the identification of diatoms (Fig. 2) and plant macroremains (fruits, seeds, leaves, flowers, etc.) are used for this purpose.

Nitzschia sigma
Fig. 2: Nitzschia sigma

The vegetation and diatom assemblages of salt marshes and mudflats typically change along a transect from the eulittoral to the supralittoral. They thus indicate a relatively close relationship to the position of the mean high tide (MThw). If the facies of the preserved older deposits are reconstructed by means of palaeoecological investigations, a relative relationship of the deposits to the MThw level at that time is obtained. The altitude of the survey points is determined using GPS, the age of the analysed samples is obtained by determining the 14C content. The data sets on MThw levels obtained from profiles on various islands are ultimately incorporated into a time/depth diagram that shows the development of the MThw over the last 2000 years for the area of the East Frisian Islands (Fig. 3).

Sea level rise
Fig. 3: Curve of sea level changes for the area of the islands of Borkum, Juist and Mellum over the last 2000 years (from Freund et al. 2004).




Literature:

Freund, H. & Streif, H. (1999): Natural tide gauges for sea level fluctuations over the last 2000 years using the example of the island of Juist. - Petermanns Geographische Mitteilungen, 143, 34-45.

Freund, H. & Streif, H. (2000): Natural sea-level indicators recording the fluctuations of the mean high tide level of the Southern North Sea. - Wadden Sea Newsletter, 2, 16-18. (download pdf)

Freund, H. & Streif, H. (2001): Wells and other finds on Juister beach - Archaeology in Lower Saxony, 4, 45-48.

Freund, H. (2002): Climate change and landscape genesis of the barrier islands in the north-west German coastal area. - Oldenburger Geoökologische Studien, 5, 85-105.

Freund, H., Petersen, J. & Pott, R. (2003): Investigations on recent and subfossil salt-marsh vegetation of the East Frisian barrier islands in the southern North Sea (Germany). - Phytocoenologia, 33, 349-375.

Freund, H., Gerdes, G., Streif, H., Dellwig, O. & Watermann, F. (2004): The indicator value of diatoms, pollen and botanical macro fossils for the reconstruction of sea-level fluctuations along the coast of Lower Saxony; Germany. - Quaternary International, 112, 71-97. (download pdf)

Freund, H. (2004): The dunes and salt marsh vegetation on Juist and their change as an indicator for the development of the island since the Early Middle Ages. - Probleme der Küstenforschung im südlichen Nordseegebiet, 28 [in press].

Watermann, F., Freund, H. & Gerdes, G. (2004): Distribution and palaeoecological importance of diatoms in Holocene coastal deposits of NW Germany. - Eiszeitalter und Gegenwart, 54, [in press].


Contact: holger.freund@uol.de