Holocene coastal lowland evolution: reconstruction of land-sea transitions in response to sea-level changes (Jade Bay, southern North Sea, Germany)

The climatic changes in the Late Quaternary have led to a series of environmental changes in the coastal area of the southern North Sea. The Pleistocene landscape around the Jade Bay was gradually inundated and covered with Holocene coastal deposits during sea-level rise after the Last Glacial Maximum (LGM). In this context, a data base of 827 core logs and 8 piston cores was interpreted with regard to lithological indicators of environmental changes south of the modern Jade Bay. By the sedimentological and stratigraphical analysis of ancient extended salt marsh belts, silting-up zones and tidal flat / tidal channel complexes, the position, internal structure and extension of the land-sea interface was reconstructed in detail giving insight into principle processes and interactions taking place in this highly sensitive zone during sea-level changes. The Holocene coastal sediment sequence in the Jade area consists of four distinct lithological units representing semiterrestrial, brackish-lagoonal and shallow marine sub-environments. With regard to small scale sea-level fluctuations in the latest phase of the Holocene transgression, distinct periods of stagnation or even regression occurred. In contrast to the typical internal composition of the Holocene deposits of the backbarrier coast with a multiple alternation of landward and seaward facies shifts, indicating repeated shoreline displacements, the Holocene sequence of the Jade Bay is characterised by only one landward-seaward facies shift. However, the sedimentary record is displayed in complex sequences of clastic deposits of tidal flat and brackish environments and intercalated beds of autochthonous fen and raised bog peat. From 11th century, onward increasing human activity has significantly influenced the development of the coastal landscape by dike construction, dewatering and land reclamation along the entire coastline. Consequently, the former swamptype characteristic of the extended lower supratidal zone has significantly changed into a narrow belt of salt marshes restricted by an artificially fixed coastline and the decoupling of marine-terrestrial processes. In the light of recent regional sea-level curve discussions, the presence of sea-level index points is critically discussed, due to the widespread existence of thick peat beds.