Integrated stratigraphy of foreland salt-marsh sediments of the south-eastern North Sea region

Depositional processes in coastal wetlands respond to a changing climate as documented in the sediment sequences of salt marshes. In this context, robust chronologies are crucial for the reconstruction of salt-marsh depositional processes in the past. However, salt-marsh sediments from the highly dynamic North Sea coast often lack a reliable stratigraphy due to the combined influences of natural processes and human activities, causing a reworking and re-deposition of the sediments. Here, a combination of absolute and relative dating methods has been applied in order to establish an integrated stratigraphic framework for active foreland salt marshes along the south-eastern North Sea coast. This stratigraphic framework is based on radionuclides (Pb-210, Cs-137, Am-241, C-14) and mercury (Hg) contaminations, together with ln(Zr/Rb) as a grain-size proxy for additional inter-correlation between the four studied sites. The studied salt marshes encompass different environmental settings concerning the inundation frequency and intensity, and anthropogenic influences. As a result, the reconstructed mean sediment-accretion rates range from 1.16 cm yr(-1) in the anthropogenically modified and grazed coastal salt marsh at Friedrichskoog, to 1.31 cm yr(-1) in the more sheltered and semi-enclosed salt marsh in the Bay of Tihnlau, and up to 1.75 cm yr(-1) in the dynamic open coastal salt-marsh at Kaiser-Wilhelm-Koog. Similar mean high accretion rates of 1.72 cm yr(-1) are documented for the Eider estuary until AD 1965, before they dropped to 0.72 cm yr(-1) after completion of the Eider tidal barrier in AD 1973. The results highlight the advantage of combining independent dating methods for the establishment of salt-marsh chronologies, which proves to be essential to compensate for absence or blurring of distinct stratigraphic signals in highly dynamic coastal depositional settings, such as the salt-marsh systems at the south-eastern North Sea coastal region. The reconstructed sediment-accretion rates suggest a high resilience of salt-marsh systems to ongoing sea-level rise as long as sediment availability and natural flooding dynamics are maintained.