Christoph Böttner

Iceberg scours are created by drifting icebergs that plough into the seafloor with their keels. These prominent geomorphic features are widespread in the Arctic and provide critical insights into past ice-sheet dynamics and ocean circulation. At local scales, iceberg scours influence benthic ecosystems, pose risks to offshore seafloor infrastructure, and can trigger submarine landslides. Here, we report on multibeam echosounder and subbottom profiler data from the 2025 AOC cruise on R/V Dana that document iceberg scours from the newly discovered Dana IV seamout (71°40’N, 15°W) in more than 975 m of water depth. Semi-automated mapping of 212 iceberg scours shows that they are predominantly oriented in northeast-southwest direction. Iceberg scours occur in two clusters around 810 m and 860 m water depth and are typically ~20 m deep, 330 m wide, and >2 km long. The longest iceberg scour is more than 10 km long and more than 60 m deep, crossing the entire seamount. Some iceberg scours trend parallel indicating multi-keeled or tabular icebergs. One iceberg scour terminates in a landslide scar, documenting that icebergs can be geohazards hundreds of kilometers away from their source. Sediment core data from the top of the seamount indicate that the timing of scouring is older than the Last Glacial Maximum. Given the large water depths in which we find iceberg scours and evidence for multi-keeled icebergs, we attribute them to giant or tabular paleo-icebergs that were more than 1 km thick. The absence of parallel lineations speak against a grounded iceshelf this far south in the North Atlantic. We conclude that these scours are formed by individual icebergs that probably came from an ice sheet calving front at the Northeast Greenland shelf edge or migrated from the Arctic Ocean southward through the Fram Strait during past glacial maxima.

In this study, we integrate marine geophysical datasets and analyses of four marine sediment cores to reconstruct the deglaciation and paleoenvironmental development of the Ardencaple Fjord and the adjacent cross-shelf trough in Northeast Greenland. Although recent studies have presented isochron-based reconstructions of the circum-Greenland ice margin since the last deglaciation, crucial knowledge gaps regarding the timing and dynamics of ice retreat still exist, particularly in offshore Northeast Greenland, where former glaciated trough systems hosted fast-flowing ice.Our preliminary results indicate fast retreat dynamics of the ice based on observations of glacial lineation morphologies on the seabed, while sedimentological data enable spatiotemporal reconstructions of grounding line positions and floating ice margins. A preliminary chronological framework constraining the ice retreat across the core sites is based on radiocarbon dates, supplemented by paleomagnetic secular variation records. Our reconstruction further allows us to assess benthic ecosystem responses to deglaciation, contributing to the current evaluation of benthic foraminifera as a proxy for identifying stages of deglaciation in marine sediments around Greenland.

This study is part of the KOMSO project that focusses on standardised measurement methods for the long term carbon storage potentials in the Baltic Sea while the contemporaneous methane release of the seafloor works as an antagonistic player. We want to understand how pockmarks are formed and how stable these structures are. Another open question is whether vessel traffic affects the shape and degassing amount of the structures.The Baltic Sea was formed during the last Weichselian glaciation and underwent a multi-phase development. It was finally flooded during the Littorina transgression, while the basins and bays of the southern Baltic Sea contain thick glacial and post-glacial sediments. In particular, the Littorina and late Holocene deposits contain sediments rich in organic matter. Depending on the varying depth of the sulphate-methane transition zone in the different basins, which is partly modified through submarine groundwater discharge, methane is produced below this zone. This results in the accumulation of free gas within the sediments. The migration of the shallow biogenic gas forms gas fronts and leakage zones in the form of pockmarks at the seafloor.In a first step, we mapped the pockmarks in the various basins using a bathymetric grid with a general resolution of up to 10 m, in smaller areas also up to 1 m. We catalogued the mapped pockmarks according to the following criteria: (i) their location (near the coast or in the central basin, and water depth), (ii) their lateral and vertical dimensions (diameter and depth), and (iii) their form (elongate or round, single structure or clustered, depression/negative or upbending/positive). Additional parametric sediment echo sounder (SES) profiles (vertical 2D sections) allow further conclusions to be drawn, such as the stratigraphic affiliation of the gas escaping from the pockmarks and the depth of the underlying gas front.Adjacent to some existing pockmarks, the seafloor forms upward bulges above gas chimneys that may indicate the build-up of methane overpressure in the shallow subsurface. Whether these doming structures develop into new pockmarks needs to be evaluated with future differential bathymetric surveys. During the course of the projects, we will repeat SES profiles in order to determine any possible temporal or seasonal variation in the size of the pockmarks.

Glacially derived, marine sediments preserve a record of the timing, extent, and dynamics of shelf glaciation. In addition, these deposits can provide constraints on glacial erosion rates and offer insights into landscape evolution. However, in Greenland, the total offshore volume of glacially derived sediments remains poorly constrained due to an uneven distribution of offshore seismic surveys and a lack of dating constraints. To address this, we present a first quantification of Quaternary glacial sediment thicknesses around Greenland, combining interpretations of available marine seismic data with age constraints where scientific boreholes are available and a neural network approach. We train the neural network using the seismic-derived thicknesses, along with several parameters related to glacial and geomorphological features. This approach allows us to predict Quaternary sediment thicknesses in regions with sparse data coverage, thereby constraining the total volumes of deposition. Our estimates reveal regional variations in glacial deposition volumes and sediment thicknesses around Greenland. On the southern and parts of the northern Greenlandic continental slope, Quaternary sediments are thin, whereas in west and east Greenland, larger sediment deposits have led to a greater shelf progradation throughout the Quaternary. These patterns demonstrate a diverse influence of (paleo-)climatic, oceanographic, and orographic processes on glacial dynamics and the source-to-sink sediment transport. Finally, we compare our estimates of Quaternary offshore deposition with estimates of onshore glacial erosion inferred from paleo-topographic reconstructions and erosion potentials of the present ice sheet, based on ice sliding velocities. This provides insights into the temporal and spatial variability of erosion around Greenland, advancing our understanding of the long-term landscape evolution in glaciated regions.

In the Arctic, substantial amounts of methane are stored in marine sediments. Yet, the distribution and stability of methane reservoirs is poorly understood, especially offshore Greenland. As Arctic warming accelerates, understanding natural methane seepage is crucial for assessing its role in the carbon cycle and climate impact. Here, we present a unique interdisciplinary geoscientific data set that spans the ice-covered Northeast Greenland shelf and captures entire fluid flow systems from source to sink. Our data documents widespread natural seepage of oil and gas from the seafloor through the water column, locally reaching the sea surface. The released hydrocarbons are sourced from deep petroleum systems and primarily migrate along permeable beds towards the seafloor. Glacial erosion during the Quaternary has effectively removed the sealing units, creating geologic windows that allow widespread natural seepage. Our first-order estimates suggest that minimum 677–1460 Mt of thermogenic gas ( ~ 0.5–1.1 Gt of carbon) has been released into the ocean since grounded ice retreated from the shelf around ~15 ka BP. These findings provide new insight into the impact of active natural seepage in a rapidly changing climate and have substantial implications for the Arctic Ocean’s carbon cycle and marine ecosystems.

Magma emplacement in the top unconsolidated sediments of rift basins is poorly understood. We compare two shallow sills from the Guaymas Basin (Gulf of California) using core data and analyses from IODP Expedition 385, and high‐resolution 2D seismic data. We show that magma stalling in the top uncemented sediment layer is controlled by the transition from siliceous claystone to uncemented silica‐rich sediment, favoring flat sill formation. Space is created through a combination of viscous indentation, magma‐sediment mingling and fluidization processes. We show that sills emplace above the opal‐A/CT diagenetic barrier. Our model suggests that in low magma input regions sills emplace at constant depth from the seafloor, while high magma input leads to upward stacking of sills, culminating in a funnel‐shaped intrusions. Our petrophysical, petrographic, and textural analyses show that magma‐sediment mingling creates significant porosity (up to 20%) through thermal cracking of the assimilated sediment. Stable isotope data suggest carbonate formation at 70–90°C, consistent with background geothermal gradient at 250–325 m depth. The unconsolidated, water‐rich host sediments produce little thermogenic gas through contact metamorphism, but deep diagenetically formed gas bypasses the low‐permeability top sediments via hydrothermal fluids flowing through the magma plumbing system. This hydrothermal system provides a steady supply of hydrocarbons at temperatures amendable for microbial life, serving as an incubator that may be abundant in magma‐rich young rift basins and play a key role in sustaining subseafloor ecosystems.

Sediment waves are widely observed on the seafloor in a variety of marine environments (e.g., open slope, submarine channels, levees). They are important for understanding marine hazards because they can influence slope stability and be indicators of currents capable of damaging seafloor infrastructure (e.g., telecommunication cables). However, sediment wave dynamics may vary in different settings and several mechanisms have been invoked to explain their formation including gravity-driven (sediment failure, turbidity currents) and oceanographic (bottom currents, internal tides) processes. In this study, we investigate the generation of large unconfined sediment wave fields along the continental slope of the Northwest African margin using an integrated dataset acquired on the R/V Maria S. Merian cruise MSM113. Data collection included direct monitoring of ocean currents and water column properties over sediment wave fields by CTD casts, acoustic water column profiling, and deployment of short-term moorings equipped with velocity (ADCPs), temperature, salinity, and turbidity sensors. Additional datasets such as shallow and multichannel 2D seismic profiles, multibeam bathymetry, gravity cores, and box cores capture the geomorphic, subsurface, and sedimentary characteristics of the seafloor features. Sediment wave fields occur on the mid-lower slope between 600 – 1900 m water depths and are intersected by straight channels up to 2 km wide and 300 m deep. Individual waves have slope-parallel crests, wavelengths between 400 – 2000 m, and wave heights between 6 – 56 m. In subsurface seismic profiles, sediment waves are composed of upslope-stacking reflectors that indicate preferential deposition on their stoss slopes and upslope crest migration. Sediment cores from sediment waves are predominantly composed of bioturbated gradational sequences of mud, sandy mud, muddy sand, and sand that vary depending on location, suggesting a progressive process of differential sedimentation. Intermittent chaotic muddy deposits and sharp-based sand layers represent occasional punctuated flow events. Time series from moored instruments are dominated by strong semidiurnal tidal fluctuations with current velocities up to 0.3 m/s. Water column measurements and acoustic images reveal a stratified water column with wavy interfaces and small-scale fluctuations caused by the passage of internal waves. Collectively, these findings suggest that downslope gravity flows, along-slope currents, and internal tides contribute to sediment transport along the Northwest African margin; although, tide-topographic interactions are the most likely candidate for maintaining sediment waves. Our integrated analysis provides insight into oceanographic processes, which shape the seafloor and transport sediment along ocean margins.

The North Sea Basin has been covered by ice sheets originating from both the British Isles and Scandinavia at multiple times during the Pleistocene. The Witch Ground Basin (WGB) in the central northern North Sea is a critical location in terms of interpreting Late Pleistocene glacial to glacimarine history of the North Sea since it was the location of the Witch Ground Ice Stream that was active on multiple occasions during the Mid to Late Pleistocene. We map five mega‐scale glacial lineation flowsets corresponding to the changing ice flow direction of the Witch Ground Ice Stream and investigate the sedimentological fingerprint and corresponding subglacial depositional processes of this palaeo‐ice stream. We show that sorted sand layers within a subglacial traction till represent periodic hydraulic jacking and ice–bed decoupling at the base of the Witch Ground Ice Stream. In contrast to previous studies that have described glacitectonites deposited below the most recent grounded ice in the WGB, we present analysis of sediment cores that recovered primarily massive diamictons without any obvious deformation structures. The most recent ice cover in the WGB (~18–16 ka) was thought to have been sourced from a localized ice cap over Orkney and Shetland. The presence of chalk clasts sourced from NW of the WGB described in this study from the stratigraphically youngest till confirms this interpretation. The transition from subglacial to glacimarine deposition, while acoustically well defined (from opaque to laminated acoustic units), appears surprisingly uniform in the recovered sediment cores, but can be differentiated based on a change in colour including mottling and banding, presence of whole intact shells, and the increased number of silt and sand lenses. ¹⁴C dating of glacimarine muds indicate high sedimentation rates of between 80 and 260 cm ka⁻¹. The transition from glacimarine to marine deposition is represented by a comparative decrease in sedimentation rate and deposition of Holocene age sandy mud. This study demonstrates a highly dynamic Witch Ground Ice Stream in the northern North Sea during the Late Pleistocene with evolving subglacial hydrology and depositional processes at the ice stream bed that left a distinct geomorphological and sedimentological fingerprint within the WGB.

Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre‐conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long‐lived structures, while sandy lithologies lead to short‐lived, small‐scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation.

Magma emplacement in the top unconsolidated sediments of rift basins is poorly constrained in terms of mechanics and associated hydrothermal activity. Our study compares two shallow sills from the Guaymas Basin (Gulf of California) using core data and analyses from IODP Expedition 385, and high-resolution 2D seismic data. We show that magma stalling in the top uncemented sediment layer is controlled by the transition from siliceous claystone to uncemented silica-rich sediment, promoting flat sill formation. Space is created through a combination of viscous indentation, magma-sediment mingling and fluidization processes. In low magma input regions, sills form above the opal-A/CT diagenetic barrier, while high magma input leads to upward stacking of sills, forming funnel-shaped intrusions near the seafloor. Our petrophysical, petrographic, and textural analyses show that magma-sediment mingling creates significant porosity (up to 20%) through thermal cracking of the assimilated sediment. Stable isotope data of carbonate precipitates indicate formation temperatures of 70−90°C, consistent with the current background geothermal gradient at 250−325 m depth. The unconsolidated, water-rich host sediments produce little thermogenic gas through contact metamorphism, but deep diagenetically formed gas bypasses the low-permeability top sediments via hydrothermal fluids flowing through the magma plumbing system. This hydrothermal system provides a steady supply of hydrocarbons at temperatures amendable for microbial life, acting as a major microbial incubator. Similar hydrothermal systems are expected to be abundant in magma-rich young rift basins and play a key role in sustaining subseafloor ecosystems.