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For millennia, humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectiv

Dates of Cruise: 09.07.2023 – 20.07.2023 Areas of Research: Marine Geophysics Port Calls: Kiel – Helgoland – Kiel Institute: Institute of Geoscience (CAU), University Malta Chief Scientist: Dr. Jens Schneider von Deimling Number of Scientists: 4 Projects: CDRmare sea4soCiety, Marispace-X

Dates of Cruise: 17.02.2023 – 24.02.2023 Areas of Research: Marine Geophysics, Baltic Sea Port Calls: Kiel – WISMAR – Kiel Institutes: Institute of Geoscience (CAU), German Aerospace Centre (Deutsches Luft- und Raumfahrtzentrum DLR) Chief Scientist: Dr. Jens Schneider von Deimling Number of Scientists: 2+6 Projects: CDRmare sea4soCiety, Marispace-X

Shallow, semi-enclosed coastal systems are particularly prone to eutrophication. Depending on local site conditions and historical nutrient legacies, sea-based measures might be necessary in addition to land-based nutrient removal. In this study, C:N:P ratios were combined with open-source bathymetric information and linked with the prevailing geomorphological and sedimentological regimes to gain insights into nutrient hotspots and understand their sources and fate in coastal waters. Land-based sediment samples were taken behind outlets at three sites in Eckernförde Bay (Baltic Sea), and complemented with ship-based sampling at locations approximately 8 m and 12 m water depth. The total carbon, nitrogen and phosphorus concentrations in surface sediments increased at deeper sites. This suggests that an increased downslope particle transport and deposition regime, based on local geomorphology, might influence nutrient hotspots to a larger extent than proximity to sources (e.g., outlets). Overall, the recorded C:N ratios (mean = 28.12) were closer to the ratio of terrestrial plants than those of marine phytoplankton, indicating allochthonous sources of organic matter.

ALKOR cruise AL600 served as a marine geophysical field course for ‘Physics of the Earth System’ bachelor students at Kiel University. Beside taking an active role in planning and realization of the individual geophysical measurements, the students also performed some first processing and interpretation of the obtained data. This work had to be documented in form of a scientific presentation as well as writing of the respective chapter in this cruise report. For the following chapters, we (Sebastian Krastel, Jens Schneider von Deimling) decided to only slightly modify the text and figures provided by the students. This should emphasize the student’s achievements, and underline the overarching aim of the cruise to train the students in acquisition, processing, and documentation of marine geophysical data.

For millennia humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspective

In 1964, exploration drilling in the German Sector of the North Sea hit a gas pocket at ∼2900 m depth below the seafloor and triggered a blowout, which formed a 550 m-wide and up to 38 m deep seafloor crater now known as Figge Maar. Although seafloor craters formed by fluid flow are very common structures, little is known about their formation dynamics. Here, we present 2D reflection seismic, sediment echosounder, and multibeam echosounder data from three geoscientific surveys of the Figge Maar blowout crater, which are used to reconstruct its formation. Reflection seismic data support a scenario in which overpressured gas ascended first through the lower part of the borehole and then migrated along steeply inclined strata and faults towards the seafloor. The focused discharge of gas at the seafloor removed up to 4.8 Mt of sediments in the following weeks of vigorous venting. Eyewitness accounts document that the initial phase of crater formation was characterized by the eruptive expulsion of fluids and sediments cutting deep into the substrate. This was followed by a prolonged phase of sediment fluidization and redistribution widening the crater. After fluid discharge ceased, the Figge Maar acted as a sediment trap reducing the crater depth to ∼12 m relative to the surrounding seafloor in 2018, which corresponds to an average sedimentation rate of ∼22,000 m3/yr between 1995 and 2018. Hydroacoustic and geochemical data indicate that the Figge Maar nowadays emits primarily biogenic methane, predominantly during low tide. The formation of Figge Maar illustrates hazards related to the formation of secondary fluid pathways, which can bypass safety measures at the wellhead and are thus difficult to control., QC 20220908

Dates of Cruise: 06.02.2022 – 17.02.2022 Port Calls: Kiel – Heligoland – Kiel Areas of Research: Marine Geophysics Institutes: Institute of Geoscience (CAU), Institute of Geography (CAU), AWI Sylt Chief Scientist: Dr. Jens Schneider von Deimling Number of Scientists: 4 Projects: CDRmare sea4soCiety

(Berichtszeitraum 05.02 – 12.02.2022)

Seagrass meadows are one of the most important benthic habitats in the Baltic Sea. Nevertheless, spatially continuous mapping data of Zostera marina, the predominant seagrass species in the Baltic Sea, are lacking in the shallow coastal waters. Sentinel-2 turned out to be valuable for mapping coastal benthic habitats in clear waters, whereas knowledge in turbid waters is rare. Here, we transfer a clear water mapping approach to turbid waters to assess how Sentinel-2 can contribute to seagrass mapping in the Western Baltic Sea. Sentinel-2 data were atmospherically corrected using ACOLITE and subsequently corrected for water column effects. To generate a data basis for training and validating random forest classification models, we developed an upscaling approach using video transect data and aerial imagery. We were able to map five coastal benthic habitats: bare sand (25 km²), sand dominated (16 km²), seagrass dominated (7 km²), dense seagrass (25 km²) and mixed substrates with red/ brown algae (3.5 km²) in a study area along the northern German coastline. Validation with independent data pointed out that water column correction does not significantly improve classification results compared to solely atmospherically corrected data (balanced overall accuracies ˜0.92). Within optically shallow waters (0–4 m), per class and overall balanced accuracies (>0.82) differed marginally depending on the water depth. Overall balanced accuracy became worse (<0.8) approaching the border to optically deep water (˜ 5 m). The spatial resolution of Sentinel-2 (10–20 m) allowed delineating detailed spatial patterns of seagrass habitats, which may serve as a basis to retrieve spatially continuous data for ecologically relevant metrics such as patchiness. Thus, Sentinel-2 can contribute unprecedented information for seagrass mapping between 0 and around 5 m water depths in the Western Baltic Sea.

The cruise AL542 took place in the Western Baltic Sea in the period 14. – 21.08.2020. The cruise was carried out as a marine geophysical field course of Kiel University. Starting and ending point of the cruise was Kiel. One stopover in Kiel took place during the cruise due to an exchange of parts of the scientific party (18.08.2020). The main aim of the cruise was to introduce marine geophysical acquisition to the students including hands-on experience in collecting marine geophysical data. This approach also included a first processing and interpretation of the data as well as the presentation of the first results. The main survey area of the first leg of the cruise AL 542 was the Bay of Mecklenburg. In the eastern part of the bay seismic and acoustic data were collected with the aim to identify historical coastlines and buried glacial structures. Further, the central part of the bay was mapped with the multibeam echosounder to find the Blinkerhügel, a small mound with reported accumulation of manganese nodules, investigated in 2002 by Hlawatsch et al. The Blinkerhügel was clearly identified as an outcropping ground moraine. Seafloor samples at eight locations were collected with a grab from the area of the Blinkerhügel. At one location stones with manganese crusts were successfully retrieved. The four survey areas of the second leg of the cruise were Mittelgrund, Noer and Damp which are located in the Eckernförde Bay and an area near Fehmarn which is located in the northwest of the island in the Fehmarn Sund. In the region Mittelgrund in the Eckernförde Bay a wellknown, developing pockmark field was surveyed with hydroacoustic and seismic methods. Furthermore, a known pockmark near Noer was surveyed with hydroacoustic methods. From the third survey area Damp Laminaria agitate algae have been reported. The aim in this area was to check, if it is possible to detect the algae with the hydroacoustic systems. Additionally, some video transects and seafloor samples were

Submarine groundwater discharge (SGD) into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and SGD on high frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intra‐pockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped, pockmarks. The small pockmarks, which we refer to as “intra‐pockmarks”, have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of ‘eyed’ pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.

The cruise AL527 took place in the Western Baltic Sea in the period 6. – 14.09.2019. The cruise was carried out as a marine geophysical field course of Kiel University, supported by BONUS ECOMAP project. Starting and ending point of the cruise was Kiel. One stopover in Kiel took place during the cruise due to an exchange of parts of the scientific party (10.09.2019). The main aim of the cruise was to introduce marine geophysical acquisition to the students including hands-on experience in collecting marine geophysical data. This approach also included a first processing and interpretation of the data as well as the presentation of the first results. Two areas in the Western Baltic Sea were the main working areas of AL527. The first survey area was at Boknis Eck, a part of the Eckernförde Bay. The main objective in this area was to search for an underwater observatory from the Coastal Observing System for Northern and Arctic Seas Project (COSYNA), which was operated by GEOMAR and disappeared end of August 2019. For this purpose, a survey with a bathymetric multibeam system from the ”Marine Geophysics and Hydroacoustics” working group (Kiel University) was carried out. Furthermore, an underwater camera system was used for visual inspections. The second survey area was in the Mecklenburger Bay. The main objective was a pre-investigation of a buried beach for an upcoming cruise within the EU-funded project ACT-SENSE. Therefore, 2D reflections seismic, sediment echo sounder, and multibeam data were acquired. Additionally, 7 gravity cores were taken for ground trothing and sampling of the buried beach. In order to analyze major tectonic structures in the Fehmarn Belt and the Mecklenburger Bay, 12 additional seismic profiles were collected when transiting between the survey areas. Our investigations showed that a buried beach is located in the Mecklenburger Bay beneath a layer of mud. The sand deposits have an estimated variable thickness between 1m and 9m in the survey ar

Backscatter mosaics based on a multi-frequency multibeam echosounder survey in the continental shelf setting of the North Sea were compared. The uncalibrated backscatter data were recorded with frequencies of 200, 400 and 600 kHz. The results showed that the seafloor appears mostly featureless in acoustic backscatter mosaics derived from 600 kHz data. The same area surveyed with 200 kHz reveals numerous backscatter anomalies with diameters of 10–70 m deviating between −2 dB and +4 dB from the background sediment. Backscatter anomalies were further subdivided based on their frequency-specific texture and were attributed to bioturbation within the sediment and the presence of polychaetes on the seafloor. While low frequencies show the highest overall contrast between different seafloor types, a consideration of all frequencies permits an improved interpretation of subtle seafloor features.

Highlights • Improved position accuracy of up to 0.08 cm using SAPOS®-corrected DGNSS • High-resolution snippet backscatter to detect and monitor UXO and its environment • Combined evaluation of bathymetry, backscatter intensities, and sediments Abstract The present study reports the evaluation of snippet backscatter information gathered with a high-frequency multibeam echosounder system (200–400 kHz) due to their usability to detect ammunition of different sizes in shallow coastal waters. Besides the feasibility of the snippet backscatter data, it was focused on the attainable horizontal accuracy in comparison to side-scan sonar and autonomous underwater vehicle (AUV) surveys. The data was collected in shallow coastal waters of up to 18 m water depth (Baltic Sea) close to an ammunition dumping site characterized by an almost flat seafloor covered with sand and silt sediments. The analysis of the multibeam compared to sidescan data indicates the snippet backscatter to be a promising prospective method for ammunition detection and being able to improve horizontal position accuracy of up to 0.08 m.

Submarine currents are a principal factor in controlling seafloor geomorphology. Herein, we investigate the role of dynamic current systems associated with the Subtropical Front in the formation and modification of seafloor depressions off the east coast of New Zealand’s South Island. Seafloor depressions are widespread in this region, with a diverse range of morphologies and sizes. We focus on two ‘end-member’ classes of depressions; densely spaced decametre-scale structures and more isolated ‘giant’ depressions of up to 12 km in diameter. Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions. We present a model to illustrate the role of submarine currents in shaping the morphology of these enigmatic seafloor depressions. This model demonstrates how contour currents, and potentially eddy currents, have extensively modified seafloor structures, resulting in elongate, asymmetrical depressions, partially infilled by sediment drift deposits.