Jasper Hoffmann

Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal under-consolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.

Electrical resistivity tomography (ERT) is one of the most widely used geophysical techniques for hydrogeological questions, such as investigating the groundwater transition zone between land and sea. The interpretation of 2D coast‐parallel ERT profiles is challenging because most of the electrical current flows through the highly conductive seawater and not the ground beneath the profile. We study this effect using synthetic data and a field data example and propose an approach for improved evaluation of coast‐parallel geoelectric profiles. The synthetic study shows that the impact of the seawater on apparent resistivities is negligible (errors below 1.5%) only if the distance between current injection electrodes ( a ) is equal to or smaller than the distance ( d ) between ERT profile and shoreline (). Ignoring this criterion, standard 2D geoelectric inversion results may diverge by 50% from the actual resistivity distribution at depths of . To improve the inversion also for profiles with we tested an approach to consider the seawater effect through a correction factor applied to the measured apparent resistivity values prior to the inversion. The correction factor depends on , the seafloor slope and the resistivity ratio of seawater and average underground. We present an analytical function approximating the correction factor well for seafloor slopes up to 10° and seawater‐to‐average‐land resistivity ratios of 0.040–0.002. We found that this approach is effective at removing errors in apparent resistivities for . Inversion results then show more reliable resistivities up to relative depths of . We applied the correction approach to field data from a coast‐parallel profile at Konyaaltı Beach (Antalya, Türkiye). The field study demonstrates the importance of considering the seawater effect to interpret possible locations of fresh submarine groundwater discharges and saltwater intrusions. It also shows the practical applicability of the new correction approach.

Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.

Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.

Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.

Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.

After the retreat of the Weichselian glaciers, Northern Europe was populated by highly mobile hunter-gatherer groups. Traces of these societies are difficult to find, hampering our understanding of their life. Some remote basins of the western Baltic Sea, however, only drowned in the Holocene, and it has recently been postulated, that they preserve architectures from the Stone Age, that did not survive on land. In 2021 we documented the Blinkerwall, a stonewall megastructure located in 21 m water depth in the Bay of Mecklenburg, Germany. Shipborne and autonomous underwater vehicle hydroacoustic data as well as optical images show that the wall is composed of about 1700 stones, predominantly less than 1 m in height, placed side by side over 971 m in a way that argues against a natural origin by glacial transport or ice push ridges. Combining this information with sedimentological samples, radiocarbon dates, and a geophysical reconstruction of the paleo-landscape, we suggest that the wall was likely used as a drive lane for hunting during the late Pleistocene or earliest Holocene. Ranging among the oldest hunting structures on Earth and the largest Stone Age structures in Europe, the Blinkerwall will become important for understanding subsistence strategies, mobility patterns, and inspire discussions concerning the territorial development in the Western Baltic Sea region.

Groundwater acts as a critical link between onshore and offshore environments, connecting freshwater systems to saline oceans. With 40% of the world's population residing along coastlines, understanding coastal groundwater reserves is paramount. One open question involves the vital role of submarine groundwater springs in global hydrology, and how the distribution and groundwater flux can be better constrained across the coastline to better predict both groundwater discharge into the ocean and saltwater inflow into coastal aquifers. Especially urban areas pose unique challenges where water demand is high and groundwater exploration problematic since geophysical remote sensing techniques often interfere with surface and subsurface constructions (e.g. cables, pipelines etc.), making innovative approaches for groundwater exploration crucial for sustainable groundwater management.In this study, we aim to address the complex dynamics of coastal karstic groundwater systems in urban regions, where meteoric waters discharge into the ocean through coastal and submarine freshwater springs, while concurrently facing the risk of saltwater intrusions. Our investigations in the bay of Antalya (Turkey) aim to provide a comprehensive understanding of the land-ocean transition zone in the karstic groundwater systems and provide new tools for future groundwater monitoring in coastal regions.We employ advanced hydroacoustic and resistivity methods, combining onshore and offshore electrical resistivity tomography with electromagnetic measurements to bridge the gap between onshore and offshore domains. This integration of geophysical datasets enables us to (1) delineate karstic groundwater flow pathways from land to ocean, (2) identify coastal and submarine freshwater springs, and (3) assess the risk of saltwater intrusions along the coastline.The study showcases the potential of offshore geoelectric measurements as a tool for groundwater investigations in urbanized coastal regions. The proposed approach will facilitate exploration efforts for groundwater in urbanised karstic areas, but much more importantly will facilitate monitoring strategies to avoid intrusions of saltwater into freshwater aquifers. Our findings contribute valuable insights for water management strategies in Antalya, with implications for safeguarding todays and future freshwater resources.