Wadden Sea Research Articles

The effects of gas extraction under intertidal mudflats on sediment and macrozoobenthic communities

Abstract Land subsidence in intertidal environments may change the flooding regime and sediment composition, two drivers of the macrozoobenthic community. In the Dutch Wadden Sea, a UNESCO world heritage site, gas extraction has resulted in an average subsidence of intertidal mudflats of 2 mm year−1. These mudflats support an abundant macrozoobenthic community that offers important resources for birds and fish. The area is managed through the ‘hand on the tap’ principle, meaning that human activities should be halted if they affect the natural values. To what extent land subsidence affects sediment and macrozoobenthos remains unknown and is increasingly important given sea level rise. Taking advantage of a large‐scale monitoring program, we evaluated the effect of anthropogenically caused subsidence on sediment composition and macrozoobenthos. Nearly 4600 points were sampled yearly (2008–2020) across the Dutch Wadden Sea, allowing us to compare sediment composition and macrozoobenthos biomass within and outside the subsidence area while controlling for the main drivers of these variables. We also compared population trends within and outside the subsidence area for 31 species with different habitat use. Mud fraction was 3% higher within the subsided area and median grain size decreased at 1 μm year−1 while remaining constant in other mudflats. This had no effect on the total biomass of macrozoobenthos. Within the subsidence area, however, the biomass of species that use deeper areas increased compared to outside, and the opposite was true for species using shallower habitat. Policy implications: Land subsidence is related to changes in median grain size and macrozoobenthic community composition. However, because thresholds have not been defined, it is not clear if this requires management actions. For a successful implementation of the ‘hand on the tap’ principle in the Wadden Sea, it is necessary to define beforehand the relevant variables that represent the natural values, implement proper monitoring and define thresholds above which effects are not acceptable. We propose median grain size, mud fraction and macrozoobenthic composition as good measures of the natural values of the Wadden Sea, and the methods used here as a way for identifying anthropogenic effects on them.

The seafloor from a trait perspective. A comprehensive life history dataset of soft sediment macrozoobenthos

Biological trait analysis (BTA) is a valuable tool for evaluating changes in community diversity and its link to ecosystem processes as well as environmental and anthropogenic perturbations. Trait-based analytical techniques like BTA rely on standardised datasets of species traits. However, there are currently only a limited number of datasets available for marine macrobenthos that contain trait data across multiple taxonomic groups. Here, we present an open-access dataset of 16 traits for 235 macrozoobenthic species recorded throughout multiple sampling campaigns of the Dutch Wadden Sea; a dynamic soft bottom system where humans have long played a substantial role in shaping the coastal environment. The trait categories included in this dataset cover a variety of life history strategies that are tightly linked to ecosystem functioning and the resilience of communities to (anthropogenic) perturbations and can advance our understanding of environmental changes and human impacts on the functioning of soft bottom systems.

Temporal change in phytoplankton diversity and functional group composition

One of the key challenges in managing eutrophication in coastal marine ecosystems is the harmonized cross-border assessment of phytoplankton. Some general understanding of the consequences of shifting nutrient regimes can be derived from the detailed investigation of the phytoplankton community and its biodiversity. Here, we combined long-term monitoring datasets of German and Dutch coastal stations and amended these with additional information on species biomass. Across the integrated and harmonized dataset, we used multiple biodiversity descriptors to analyse temporal trends in the Wadden Sea phytoplankton. Biodiversity, measured as the number of species (S) and the effective number of species (ENS), has decreased in the Dutch stations over the last 20 years, while biomass has increased, indicating that fewer species are becoming more dominant in the system. However, biodiversity and biomass did not show substantial changes in the German stations. Although there were some differences in trends between countries, shifts in community composition and relative abundance were consistent across stations and time. We emphasise the importance of continuous and harmonized monitoring programmes and multi-metric approaches that can detect changes in the communities that are indicative of changes in the environment.

Co-created by the Wadden Sea

In Anna-Rosja Haveman’s essay, the largely forgotten work of the Groningen artist Han Jansen is examined. Her analysis unpacks the artistic oeuvre, within a broader perspective of the nature-culture debates in art and society. The public discussion that emerged around the Waddenprojects brings to bear contrasting ideas about nature in society. By embedding the project in the historical context of emerging environmental activism, the tensions inherent in art that aims to ‘raise awareness of nature’ become clear. As illustrated, the position of Han Jansen remains paradoxical: while he certainly intended to contribute to the awareness of nature and reconfigured hierarchies between humans and nature during the making process, the project created an image of a human, the artist, as polluter without further reflection on the ethics of his own actions.

A rockling's choice: The trade-off between thermal preference and physical structure in the five bearded rockling, Ciliata mustela

Changes in the environment can alter the suitability of habitats for organisms. In marine systems, fish species have their own specific requirements in terms of temperature and other habitat features. Behavioral responses such as thermoregulatory behavior in ectothermic species allow mobile organisms to respond to detrimental changes and search for more suitable habitats. However, for many species, limited information exists on the ecological requirements to help explain species abundance in a changing habitat. An example of a quickly changing habitat is the Wadden Sea, where five bearded rockling (Ciliata mustela) abundance has increased, unlike other Wadden Sea species. The increasing abundance of rockling has coincided with increasing average sea water temperatures and the recovery of mussel and Pacific oyster beds. Warming waters and increased structural habitat may have provided rockling with a more desirable habitat. Therefore, to better understand why rockling abundance is increasing within a changing Wadden Sea, a water temperature preference chamber was used to determine rockling's preferred temperature range. In addition, rockling's affinity for structural habitat and the trade-off between preferred temperature was examined by following their response to the systematic removal of artificial physical structures within the preferred temperature conditions. The preferred temperature range of rockling was found to be 10.4–15.7 °C. Following structure removals, rockling repeatedly moved away from their chosen temperatures to adjacent compartments with different temperatures but containing physical structure, indicating that the presence of physical structure was more important than preferred temperature until 18.6 °C. These novel findings provide insight and experimental support for the hypothesis explaining rockling's increase in the Wadden Sea: 1) mean annual temperatures have been steadily increasing towards rockling's preferred thermal range and 2) increasing mussel and Pacific oyster beds are plausibly providing structural habitat, an important habitat requirement for rockling. When fish display a strong association with physical structure it is necessary to link physiological and habitat preferences to better understand climate change related responses.

Analysis of the potential of full-waveform stacking techniques applied to coastal airborne LiDAR bathymetry data of the German Wadden Sea National Park

As a result of natural processes and human activities, water bodies and in particular the seabed are in a constant state of change. Collecting data on the topography of the seabed for monitoring tasks, coastal protection or to ensure safe navigation is a major challenge. Airborne LiDAR bathymetry is an efficient area-wide method for acquiring seabed topography. However, this measurement method is limited in water depth penetration due to the attenuation of the measurement signal in the water column and water turbidity. Therefore, it is only suitable for bottom detection in shallow water areas. However, recent developments in full-waveform processing techniques allow an increase in the usable portion of the signal waveform, resulting in an improved representation of the seabed. In this contribution, two novel full-waveform processing techniques are evaluated for the first time on a dataset from the German Wadden Sea National Park. In addition, an enhanced water surface correction method is introduced, which accounts for the local sea surface topography with the goal of improving the accuracy potential of the full-waveform stacking processing. The study demonstrates an increase in the analyzable water depth on the order of 26 %. This results in an improved coverage of the seabed in terms of point density and area covered (+ 14.6 %). A comprehensive analysis of the results shows that the additional seabed points represent the seabed well.

Alkalinity and nitrate dynamics reveal dominance of anammox in a hyper-turbid estuary

Abstract. Total alkalinity (TA) regulates the oceanic storage capacity of atmospheric CO2. In heterotrophic temperate estuaries, anaerobic respiration of organic matter, e.g., by denitrification, can be an important source of TA. Denitrification is the anaerobic reduction of nitrate (NO3-) to elemental nitrogen (N2). By contrast, anammox yields N2 as its terminal product via comproportionation of ammonium (NH4+) and nitrite (NO2-); however, this occurs without release of TA as a byproduct. In order to investigate these two nitrate and nitrite respiration pathways and their resulting impact on TA generation, we sampled the highly turbid estuary of the Ems River, discharging into the North Sea in June 2020. During ebb tide, a transect was sampled from the Wadden Sea to the upper tidal estuary, where we additionally sampled fluid mud for incubation experiments and five vertical profiles in the hyper-turbid tidal river. The data reveal a strong increase of TA and dissolved inorganic carbon (DIC) in the tidal river, where stable nitrate isotopes indicate water column denitrification as the dominant pathway. However, in the fluid mud of the tidal river, the measured TA and the N2 incubation experiments imply only low denitrification rates, with the majority of the N2 being produced by anammox (>90 %). The relative abundances of anammox and denitrification, respectively, thus exert a major control on the CO2 storage capacity of adjacent coastal waters.

Evaluation of seagrass as a nature-based solution for coastal protection in the German Wadden Sea

Global climate change increases the overall risks for coastal flooding and erosion. Meanwhile, nature-based solutions (NbS) are increasingly becoming a focus of coastal protection measures to improve the climate adaptability. In this study, the present and potential future role of seagrass in coastal risk reduction strategies were explored for the highly energetic Wadden Sea area of the German Bight. The methodology in this study combined seagrass coverage data (Zostera marina and Zostera noltei) obtained by field surveys and what-if scenario simulations using the SCHISM unstructured grid model framework, coupling hydrodynamics, waves, sediments, and a seagrass module. The results suggest that the introduction of seagrass meadows locally can reduce both current velocities and significant wave heights in the order of up to 30% in the deeper areas and above 90% in the shallow areas. Reduction in bottom shear stress of a similar relative magnitude significantly reduced sediment mobilisation on the order of 2 g/L in the 95th quantile of bottom layer sediment concentrations. Effectively altering hydromorphodynamic conditions favouring sediment accumulation, seagrass expansion could help tidal flats height growths to keep up with SLR, thus further maintaining the bathymetry-induced tidal dampening and lowering flooding and erosion risks as well the amount of energy at dike infrastructure. The accumulated effect of seagrass under calm weather conditions is considered more important than the increased attenuation in absolute values it provides during extreme conditions. The overall conclusion is that seagrass expansion could be a useful addition to engineered coastal protection measures.

Resolving the variability in habitat use by juvenile small pelagic fish in a major tidal system by continuous echosounder measurements

Shallow coastal areas are important nurseries for larvae and juveniles of many fish species. However, empirical data on small pelagic fish (SPF) in such regions are lacking, and temporal variability in abundance and habitat use by SPF are unknown. Given the critical role of SPF as a trophic link, their commercial value and their sensitivity to climate variability, there is a need for a quantitative and high-resolution monitoring method. We deployed a bottom-mounted echosounder, combined with a water current profiler, to examine the temporal variation in density, vertical distribution and movement behaviour of SPF in the Marsdiep, a dynamic tidal inlet between the North Sea and the Wadden Sea. The acoustic setup provided year-round records of fish density distribution in the water column every 90 min. Monthly fish samples were collected to help interpret acoustic observations. Our analyses indicated that the Marsdiep is a migration gateway for SPF from the North Sea into the Wadden Sea, particularly for juvenile herring and sprat. We observed clear seasonality with high fish densities from mid-spring to early autumn. Fish typically used the upper half of the water column. Swimming behaviour was primarily driven by currents. Yet, during outgoing tide, SPF resisted the outflowing current, suggesting efforts to remain in the Wadden Sea, supporting the hypothesis that the Wadden Sea serves as a preferred nursery area for SPF. Our high frequency, multi-seasonal and vertically resolved observations provide unique insight into the use of the Marsdiep by SPF. Furthermore, potential applications of autonomous echosounder moorings are discussed.

Low-cost UAV monitoring: insights into seasonal volumetric changes of an oyster reef in the German Wadden Sea

This study aims to quantify the dimensions of an oyster reef over two years via low-cost unoccupied aerial vehicle (UAV) monitoring and to examine the seasonal volumetric changes. No current study investigated via UAV monitoring the seasonal changes of the reef-building Pacific oyster (Magallana gigas) in the German Wadden Sea, considering the uncertainty of measurements and processing. Previous studies have concentrated on classifying and mapping smaller oyster reefs using terrestrial laser scanning (TLS) or hyperspectral remote sensing data recorded by UAVs or satellites. This study employed a consumer-grade UAV with a low spectral resolution to semi-annually record the reef dimensions for generating digital elevation models (DEM) and orthomosaics via structure from motion (SfM), enabling identifying oysters. The machine learning algorithm Random Forest (RF) proved to be an accurate classifier to identify oysters in low-spectral UAV data. Based on the classified data, the reef was spatially analysed, and digital elevation models of difference (DoDs) were used to estimate the volumetric changes. The introduction of propagation errors supported determining the uncertainty of the vertical and volumetric changes with a confidence level of 68% and 95%, highlighting the significant change detection. The results indicate a volume increase of 22 m³ and a loss of 2 m³ in the study period, considering a confidence level of 95%. In particular, the reef lost an area between September 2020 and March 2021, when the reef was exposed to air for more than ten hours. The reef top elevation increased from -15.5 ± 3.6 cm NHN in March 2020 to -14.8 ± 3.9 cm NHN in March 2022, but the study could not determine a consistent annual growth rate. As long as the environmental and hydrodynamic conditions are given, the reef is expected to continue growing on higher elevations of tidal flats, only limited by air exposure. The growth rates suggest a further reef expansion, resulting in an increased roughness surface area that contributes to flow damping and altering sedimentation processes. Further studies are proposed to investigate the volumetric changes and limiting stressors, providing robust evidence regarding the influence of air exposure on reef loss.

Braving the extremes: foraminifera document changes in climate-induced and anthropogenic stress in Wadden Sea salt marshes

Tidal wetlands are highly dynamic ecosystems that are susceptible to changes in sea level and flooding from storm surges. Among them, salt marshes play a key role in coastal protection as they contribute to wave attenuation through their regulating ecosystem services, thereby promoting sediment deposition and shoreline stabilization. However, the resilience of salt marshes, particularly those that have been modified and cultivated for centuries, is questionable in the face of accelerated sea-level rise (SLR) and increasing run-up heights of storm surges. In this context, this study aims to investigate the historical foraminiferal records of two sedimentary salt marsh archives from the Wadden Sea area (Dithmarschen and North Frisia, Germany) that have been modified to varying degrees by human management activities over the last century. The foraminiferal records document how physico-chemical traits of salt marshes of the central Wadden Sea have responded to storm tide inundation over the last century, providing information about salt marsh stability and vulnerability. Abnormally grown tests of the salt marsh indicator speciesEntzia macrescensincreased in number between 1950 CE and the late 1980s, indicating the concurrent increase of environmental stress caused by the effects of times of increased salt marsh flooding. These trends can be linked to observations of amplified North Sea storm surges, corroborating that salt marsh ecosystems respond to changing climate conditions. Differences in the number of abnormal foraminifera between the studied salt marshes suggest a particularly high vulnerability of intensively human-modified coastal wetland ecosystems to amplified storm climate conditions.

Diel and seasonal methane dynamics in the shallow and turbulent Wadden Sea

Abstract. The Wadden Sea is a coastal system along the fringe of the land–sea borders of Denmark, Germany and the Netherlands. The Wadden Sea is extremely productive and influenced by strong variations in physical and biological forcing factors that act on timescales of hours to seasons. Productive coastal seas are known to dominate the ocean's methane emission to the atmosphere, but knowledge of controls and temporal variations in methane dynamics in these vastly dynamic systems is scarce. Here we address this knowledge gap by measuring methane inventories and methanotrophic activity at a temporal resolution of 1 h over a period of 2 d, repeatedly during four successive seasons in the central Dutch Wadden Sea. We found that methane dynamics varied between colder and warmer seasons, with generally higher water column methane concentrations and methanotrophic activity in the warmer seasons. The efflux of methane to the atmosphere was, on the other hand, lower in the warmer seasons because of lower wind speeds. On a diel scale, tides controlled methanotrophic activity, which increased ∼40 % at low tide compared to high tide. We estimate that methane oxidizing bacteria reduce the methane budget of the Dutch Wadden Sea by only 2 %, while ∼1/3 escapes to the atmosphere and ∼2/3 are flushed out into the open North Sea at ebb tide. Our findings indicate that tides play a key role in controlling methane dynamics and methanotrophic activity and highlight the importance of high-resolution and repeated sampling strategies to resolve methane dynamics in fast-changing coastal systems.

A re‐evaluation of the effects of mechanical cockle dredging in the Dutch Wadden Sea

Abstract Marine bivalves provide an important seafood resource, but conflicts between bivalve fisheries and conservation goals may arise. A scientific debate about cockle dredging in the Dutch Wadden Sea resulted in an apparent consensus among scientists of a severe and long‐lasting impact of the mechanical fisheries, and the fisheries were banned. Specifically, two highly cited papers concluded that the fisheries caused a strong reduction in bivalve recruitment for a period of 8 years and that the fisheries led to the loss of red knots from the Dutch Wadden Sea and a decline of the European wintering population. Here, we show that these papers had severe shortcomings in terms of data selection and data analysis and we challenge their conclusions.

Marine Stations Helgoland and Sylt operated by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research

The Marine Stations Helgoland and Sylt are permanent coastal stations in the German Bight operated as one joint research infrastructure by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI). Using both stations, the south-west region of the North Sea and its ecosystem features are tightly monitored via ecological time series, which are recorded and made available to government offices, professional associations, and research institutes world-wide. The stations are a hub for national and transnational access for guest researchers and visiting scientists in the German Bight. For over fifty years, the stations have served as centres of student education by providing facilities for university courses. The stations operate the coastal research vessels Mya II, Uthörn and Aade. The Biologische Anstalt Helgoland (BAH) is home to the AWI Centre for Scientific Diving, which conducts, promotes, and supports diver-related underwater science within the framework of all AWI research themes. The BAH is also home to the school laboratory OPENSEA, which offers high school students a scientific environment to explore marine science. The Wadden Sea Station on the island of Sylt offers 24 seawater mesocosms for ecological studies of future climate scenarios. Public outreach centres are associated with both locations (BLUEHOUSE and Erlebniszentrum Naturgewalten).

Tracking wintering areas and post-breeding migration of a declining farmland bird – An indispensable basis for successful conservation

Many farmland birds, such as the Eurasian Curlew (Numenius arquata; here in after Curlew), are in steep decline. So far, decreased reproduction and, hence, an insufficient number of offspring to compensate for adult mortality has been considered the main driver of the recent population collapse. However, despite extensive conservation measures in most breeding areas, there are no signs of a general reversal of the population trend. Accordingly, conservation has to focus on decreasing adult mortality outside the breeding areas as well. For Curlews breeding in NW Germany, ring recoveries suggest that the main wintering areas are the coastlines of western France and southern England. However, such data are often biased in space and time. Here, we used GPS tracking to investigate the wintering areas and post-breeding migration of Curlews of the main German breeding population based on large sample size. Altogether, we tagged 86 adult breeding birds at 23 subareas across a transect of about 250 km (in length) in NW Germany. Curlews started post-breeding migration right after finishing breeding or attempting to breed, which was mainly in mid-June. We identified the coastlines of Great Britain/Ireland, western France and the Netherlands, in particular the Rhine-Meuse-Delta and the Wadden Sea, as the main wintering areas of Curlews breeding in NW Germany. We found a latitudinal structuration of migration, with birds nesting further north using more northerly wintering areas. However, in some cases, wintering areas of birds of the same subpopulation were located more than a thousand kilometres apart from each other. All three main stopover areas identified by our study (German/Dutch Wadden Sea, Dutch Rhine-Meuse-Delta, French Normandy) overlap in large parts with the main wintering areas. Since Curlews spend most of the year outside their breeding areas, and wintering and stopover areas also suffer from rapid environmental change, conservation has to additionally focus on these areas. Nearly one third (29%) of the tracked birds wintered in France or used French stopover areas. Accordingly, they might potentially be affected by a resumption of hunting, which is currently in discussion in France. Our study provides an important basis for the protection of wintering and stopover areas of the main German breeding population but also for other wader species migrating along the East Atlantic Flyway.

OYSTER REEF AND MUSSEL BED SURROGATES SUBJECTED TO WAVES

Benthic bivalve organisms are important eco-engineers for coastal ecosystems. They create habitats for many other species, attenuate waves and currents, enhance the water quality through filter-feeding, and stabilize seabed sediments. In shallow marine environments, they form bio-encrusted surfaces with large horizontal expanses. In the German Wadden Sea, these bio-encrusted surfaces account for roughly 6 percent of the tidal basins. A common feature of bio-encrusted surfaces by different species is the formation of a complex three-dimensional (3D) relief and increased hydraulic roughness compared to the surrounding sea floor. The potential for dissipating wave energy due to bed friction has been recognized in several studies (e.g., Donker et al. 2013; Manis et al. 2015) and applied as a nature-based solution (NbS) for coastal protection, e.g., on the East Coast of the USA (Morris et al. 2021). However, systematic studies on wave-biota interactions are still missing, which are needed to derive comprehensive recommendations for their use as NbS in costal protection.

BED PROTECTION FOR EXTREME STORM SURGE INFLOW THROUGH FAILING SLUICEGATES

At the northern part of The Netherlands the closure dam Afsluitdijk is situated. It is the primary flood defense that closes off Lake IJssel from the Wadden Sea to protect the northern provinces against storm surges. This structure was built in the 1930’s and needed to be improved to withstand the foreseen increase in storm surge conditions and sea level rise. It consists of a 32 km long dam with five discharge sluices for regulating the water level at Lake IJssel. Three of these sluices are located in the west end of the dam and two in the east end. Due to climate change larger discharges are foreseen. Therefore two new sluices were built in between the three old sluices at Den Oever in the west end of the dam to increase the capacity. A new bed protection was designed at Lake IJssel to ensure the stability of the old and new sluices. This bed protection needs to withstand the inflow during the worst case scenario that one sluice does not close during a 45 hour storm surge. This event has a probability of occurrence of 1/1,000,000 per year. During this extreme load case, water flows from the Wadden Sea through the open sluice gates into Lake IJssel. The water level difference is up to 6.6 m. This causes discharges up to 3600 m3/s and currents that are supercritical with speeds up to 9 m/s. Erosion of the bed cannot be prevented entirely with these high velocities. The length of the bed protection is designed large enough to keep the scour hole away from the sluices. The bed protection consists of a structural and a natural part. The natural part was introduced to minimize the length of the structural protection, reducing the construction costs and CO2 footprint. For this design approach the erosion resistant boulder clay layers in the subsoil were taken into account as natural bed protection, ‘Building with Nature’. To achieve the custom made design was a challenge. To predict the flow behavior in combination with the erosion process was extremely complex. This could not be predicted with the conventional formulas and methods. Therefore multiple approaches were used and combined. This resulted in the design being based on the combination of computer simulations, 2D and 3D model tests and calculations with conventional formulas. The structural part of the bed protection has a length of 24 m. The first 19 m is a concrete slab with a sheet pile at the end. The last 5 m consists of grouted rock. The thickness of the concrete slab and grouted rock is 1-1.5 m. This large thickness is required to prevent uplift due to the supercritical flow. Further downstream of this structural bed protection a scour hole will develop. At this location the boulder clay forms the natural part of the bed protection. The expected development of the flow field and geometry of the scour hole during the 45 hour storm surge was determined. The scour resistance of the boulder clay was based on laboratory tests with samples taken on site. This lead to the conclusion that the boulder clay reduces the development of the scour hole enough to ensure the stability of the structural part of the bed protection and the stability of the sluices.

SATELLITE DERIVED BATHYMETRY FOR MONITORING NEARSHORE DYNAMICS

Research on the physical, ecological, and environmental processes in the coastal zone traditionally rely on the availability of accurate seafloor topography (bathymetry) information. This information becomes increasingly important as coastal environments are under pressure by climate change and anthropogenic developments. Nonetheless, the bathymetry of shallow nearshore waters is only marginally monitored by cost- and time-intensive survey techniques that are dependent on in-field environmental conditions. Here, Satellite Derived Bathymetry (SDB) offers a valuable alternative to enrich available bathymetric data in both data sparse and data rich environments, reducing the need for in-situ measurements in the future. The high spatial and temporal resolution of satellite imagery yields a more comprehensive understanding of our coast and its dynamics. Yet, consistently mapping temporal change is still challenging, as the majority of currently available empirical SDB algorithms are heavily dependent on in-situ data for calibration purposes. In this study we present a methodology for assessing nearshore horizontal morphodynamics using the uncalibrated reflectance signal from satellite sensors while focusing on the Friese Zeegat, a tidal inlet of the Dutch Wadden Sea.

Transgression Related Holocene Coastal Glendonites from Historic Sites

This study examines the occurrence of glendonite along coastlines since 1825, which have been previously referred to under different names such as Pseudogaylussite, Fundylite, and Kool Hoot across eleven sites. By utilising element ratios and 14C radiometric dating techniques, we establish a more accurate chronology for these varied sites ranging from 10 to 1 thousand years before the present (Ky BP). Sites include tidal flats, coastal barrier islands, and Wadden Sea environments. While some sites still exist, others are only known through publications and museum collections. Our research expands upon previous findings by presenting petrographic evidence that correlates with glendonite formation. Through the examination of the Olenitsa site on the Kola Peninsula, we demonstrate that marine bioclasts enclosed within concretions surrounding glendonites provide temporal context, suggesting that these outcrops were formed during a single event under changing conditions. Notably, certain sediment structures at selected sites indicate the occurrence of cold-water ice-raft storm events and the presence of drop stones. Furthermore, our paper explores the association of historic coastal sites with the formation of ikaite, highlighting the limitations of relying solely on geochemistry and isotopic analysis for interpretation. Intriguingly, we observe that pseudomorphs are abundant in specific areas but absent in adjacent regions with similar environmental, physical, and chemical conditions. No apparent connection is found between volcanic dust cloud-induced cold spells and glendonite. The distribution of coastal glendonites is more likely related to periods of climatic cooling through other means. We show that radiometric dating with 14C provides an indication of age, but the results can be erroneous due to the inclusion of older carbon sources in the analysis. The oldest locations discussed in this study are Kool Hoot (Alaska) and the river Clyde (Scotland), and the youngest glendonites discussed are from the Bay of Fundy in Canada. Occurrences from the Wadden Sea are intermediate in age and sit between the other two groups. The age of the Olenitsa site on the Russian Kola Peninsula is uncertain and still debated. We show that measuring the ratio of Mg/Ca can indicate how much the recrystallised ikaite preserved as calcite is influenced by diagenetic pore waters.

A novel seismic full waveform inversion approach for assessing the internal structure of a medieval sea dike

AbstractCoastal protection in the form of dike constructions has a long history at the German North Frisian coast dating back to the High Middle Ages. As the vast majority of the dikes built prior to the devastating storm surges of the Middle Ages have been irretrievably destroyed, mostly sparse remains and only a few well preserved of these medieval dikes are found along the German North Frisian coast and within the Wadden Sea. Not all details of their construction and dimensions are yet understood. In the present case study, we investigate the historical Schardeich on the island of Pellworm in the German North Sea in a noninvasive way using shear waves (SH‐waves). For the data interpretation, we applied a combination of seismic full waveform inversion and classical seismic reflection imaging to determine the interior structure of the dike and its underlying layers at the highest possible resolution. The results obtained on land are compared with dike remains found in the tidal flats. These remains show up in marine seismic sections as characteristic reflections, which probably represent a compaction layer caused by the load of the former dike. For ground truthing, we compare the seismic results with internal dike structures found in nearby excavations. The comparison highlights that FWI is a reliable tool for near‐surface archaeological prospecting. We find that SH‐wave FWI provides decimetre‐scale velocity and density models that allow, together with the seismic reflection section, to determine distinct construction phases of the dike. The investigated dike further shows a depression at base level of about 0.75 m, which is of the same order as observed for the dike base reflections in the tidal flats. Transferring these findings to the dike remains mapped in the tidal flats, we derive a height of the former dike from 2.2 to 4.4 m.