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

Abstract

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 (210Pb, 137Cs, 241Am, 14C) 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 Tümlau, 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.