Cruise Report ALKOR 438 [AL438] - Foraminiferal biomonitoring in the North Sea, May 29 - May 31, 2014, Kiel (Germany) - Kiel (Germany)

The aim of R/V Alkor cruise AL438 was to investigate the assemblage composition, population density and distribution of Recent benthic foraminifera in the Elbe estuary and southern North Sea. We focused on an assessment of the present state of the ecosystems and a comparison with data from former decades. Our work resumed investigations of the Geologisch-Paläontologisches Institut of Kiel University and the Federal Hydrographic Office from 1960s till the 1980s, as well as topical studies to the west of Helgoland. We revisited the same stations and took sediment samples with historical and modern grab samplers, box corer, Minicorer and a gravity corer. These samples were accomplished with samples from new stations in order to describe the relationships of foraminiferal faunas living in the Helgoland mud area with those inhabiting the surrounding sands. The sampling was accompanied with hydrographical measurements and water sampling to document relevant environmental parameters. The foraminiferal sampling was carried out for the first time following of the FOraminiferal BIoMOnitoring group (FOBIMO) recommendations as much as possible, in order to test the practicability of the guidelines. In total, 109 deployments at 37 stations were successfully accomplished

Organic carbon, XRF scans (Br, Cl, Rb, Zr), and relative grain size distribution, obtained from sediment sequences TB13-1 and GeoHH-FK (south-eastern North Sea)

The sedimentological and geochemical proxy data shown in here were used to evaluate the response of coastal wetlands to 20th-century storm-climate variability at the south-eastern North Sea coastal area. The identification and evaluation of storm-surge layers within well-stratified salt-marsh archives and associated marine influences on the salt marshes was based on mean grain sizes, organic carbon (Corg) and XRF scanning datasets (Br, Cl, Rb and Zr). Local changes in the sedimentary organic matter supply were illustrated by the ln(Br/Cl) ratio, while the Br/Corg ratio represented the marine versus terrestrial organic matter supply, and the ln(Zr/Rb) ratio was used as a proxy for the relative grain-size distribution. It was found that abrupt drops in the ln(Br/Cl) ratio mainly coincided with siliciclastic sand layers, whereas the Br/Corg and ln(Zr/Rb) records revealed an increasing long term trend starting from the mid-century towards today, resembling the observed strengthening in North Sea storminess. Here, we focused on sedimentary salt-marsh sequences from two different salt-marsh sites at the German North Sea coast, both of which were influenced by natural processes but exposed to human activities to different degrees. These sites comprise: I) a more natural developed and un-grazed salt marsh (Bay of Tümlau, Eiderstedt Peninsula), and II) an intensely human-modified and grazed salt marsh (Friedrichskoog, Dithmarschen). Field work was carried out in August 2013 in the Bay of Tümlau (Müller-Navarra et al., 2019; DOI: 10.1016/j.ecss.2018.12.022) and in November 2016 in Friedrichskoog. The erosional cliffs that fringe both salt marshes at their seaward edges were chosen for sediment recovery. For this purpose, 10–15 plastic U-channels open at one long side were pressed vertically into the previously cleaned erosional cliff face. The recovered sediment sequences are TB13-1 (Bay of Tümlau; Müller-Navarra et al., 2019; DOI: 10.1016/j.ecss.2018.12.022) and GeoHH-FK (Friedrichskoog). As coastal wetlands are increasingly exposed to rising sea level and associated inundation by storm surges, this study helped to better understand how and to what extent changes in storm-surge climate are translated into the sediment archives and how human-modified salt marshes have so far been able to copy to changing climate conditions over the last century