Funding from the DFG between 1997 and 2001
Salt marsh horizon_Juist_1
Salt marsh horizons on the seaward beach of Juist (Photo: Freund)


Introduction

Cyclical climatic fluctuations during the Quaternary have left behind a sequence of strata in the southern North Sea and the adjacent mainland areas, characterised by a large number of marine transgressions and regressions. Phases of falling temperatures and extreme glacial periods are characterised by regressions – that is, the retreat of the sea or low sea levels – whilst phases of rising temperatures and interglacial periods are characterised by the advance of the sea or high sea levels. For example, between 22,000 and 18,000 years ago, at the peak of Europe’s most recent glacial period – the Weichselian glaciation – the North Sea level was approximately 110–130 metres lower than today, and the coastline had retreated far to the north, to a point approximately 350 km north of the Dogger Bank. As the climate warmed again, sea levels rose, and the coastline shifted rapidly inland and higher across former Ice Age landscapes. Initially, the floodwaters entered the present-day North Sea basin from the north; it was not until around 7000 BC that the English Channel opened up as a further link to the Atlantic, turning Great Britain into an island.

Holocene sea-level rise was initially relatively rapid and continuous, with an average rate of rise of approximately 2.1 m per 100 14C years. It was not until 5000 BC that the rise slowed significantly, with phases of advance and retreat alternating. These phases are characterised in the geological structure of the coastal Holocene by an alternating sequence of organic and clastic deposits, with peat and soil formations corresponding to regression phases and clastic sediments to transgression phases. We are well informed about the earlier transgressive and regressive phases of the Holocene thanks to numerous studies; however, for the period from the turn of the era until the start of historical water-level measurements in the mid-19th century, there is a gap in our knowledge, with only a few isolated pieces of information available. By investigating older salt marsh and mudflat horizons that formed in the area of the East Frisian Islands, it is possible to close this gap. As the subsoil of the islands consists mainly of sandy sediments, subsidence – such as that which occurs on the mainland, for example due to interbedded peat – is negligible here.

Older salt marsh and mudflat horizons on the East Frisian Islands as natural water level markers

It is well established through geological investigations and current observations that the East Frisian Islands are not geostatically stable. Due to marine morphodynamics, the position, shape and alignment of the island chain have changed steadily over the last few centuries. For example, due to a north–south displacement, the dune front of the islands has advanced by at least 500 m onto the respective salt marshes over the last 2,000 years. As a result, these have been covered by the island dunes, and their original state of development has been preserved. Today, such older salt marshes or mudflat deposits are being eroded and exposed on the beach. In places, several such horizons, stepped in relation to one another, are exposed on the beach (Fig. 1).

Salzwiesenhorizont_Juist_2
Fig. 1: Salt marsh horizons on Juist

Scientific investigations allow the depositional conditions of these older horizons to be reconstructed. In addition to geological methods (geochemistry, sedimentology), this involves, above all, palaeoecological analyses, such as pollen analysis, as well as the identification of diatoms (Fig. 2) and plant macroremains (fruits, seeds, leaves, flowers, etc.).

Nitzschia sigma
Fig. 2: Nitzschia sigma

The vegetation and diatom assemblages of the salt marshes and mudflats typically change along a transect from the eulittoral to the supralittoral. They therefore show a relatively close relationship to the level of the mean high tide (MThw). Thus, if the facies of the preserved older deposits are reconstructed using palaeoecological analyses, a relative reference of the deposits to the MThw level at that time is obtained. The elevation of the sampling points is determined using GPS, whilst the age of the samples is established by measuring their 14C content. The datasets obtained on MThw levels, which have been compiled from profiles on various islands, are ultimately incorporated into a time–depth diagram showing the evolution of the MThw over the last 2,000 years for the East Frisian Islands region (Fig. 3).

Sea-level rise
Fig. 3: Curve showing sea-level changes for the area around the islands of Borkum, Juist and Mellum over the last 2,000 years (from Freund et al. 2004).




References:

Freund, H. & Streif, H. (1999): Natural sea-level markers for sea-level fluctuations over the last 2,000 years, using the island of Juist as an example. – 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 Juist beach. – Archaeology in Lower Saxony, 4, 45–48.

Freund, H. (2002): Climate change and landscape genesis of the barrier islands in the north-western German coastal region. – Oldenburg Geoecological Studies, 5, 85–105.

Freund, H., Petersen, J. & Pott, R. (2003): Investigations into 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 macrofossils 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 changes as an indicator of the island’s development since the Early Middle Ages. – Problems of Coastal Research in the Southern North Sea Region, 28 [in press].

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


Contact: holger.freund@uol.de