The Trendermarsch polder (North Frisia, Germany) - Geophysical and geoarchaeological investigations of an anthropogenic medieval coastal landscape and its vulnerability against natural hazards

Abstract

Since medieval times, North Frisia (Schleswig-Holstein, Germany) has experienced large-scale natural and man-made geomorphological changes. First, Frisian settlers turned the coastal marshes and fenlands into arable land but also increased the region's vulnerability to flooding. Then major storm surges drowned wide areas in 1362 CE and 1634 CE and many (geo-)archives that could provide knowledge about medieval man-environment interactions and the overall appearance of the coastal landscape at that time were irretrievably lost.To better understand the natural palaeogeographical evolution and human intervention with the coastal environment, our research focused on the Trendermarsch polder (Nordstrand), that is one of few sites still reflecting the cultivated marsh landscape of the 13th to 14th cent. AD.Fieldwork comprised DEM analysis, geophysical prospection by electrical resistivity tomography (ERT), seismic reflection measurements, Direct Push (DP)-EC logging and coring to obtain high-resolution stratigraphic data. Sedimentary, geochemical and microfaunal palaeoenvironmental parameter (PEP) analyses of sediment samples allowed to calibrate geophysical and DP-EC results. Radiocarbon dating, archaeological age estimations and historical reports provided a geochronological framework.Results confirm the 12th cent. AD age of the Trendermarsch polder but also reveal distinct phases of channel incisions, that are likely related to extreme events. These tidal channels appeared to be a distinct gateway for storm surges and considerably influenced medieval to early modern land reclamation and cultivation measures. The Trendermarsch therefore sets an impressive example for the interplay of natural processes and man-made geomorphological changes and their consequences for a coastal landscape.