Henrik Grob

Offshore freshened groundwater (OFG) is a significant yet underexplored component of the global water cycle, and its spatial distribution, particularly its links to past glacial processes, remains poorly constrained. The southeastern North Sea, characterized by extensive tunnel valley networks, provides an ideal setting to test how Pleistocene meltwater shaped offshore groundwater systems. Using newly acquired time-domain controlled-source electromagnetic data integrated with high-resolution seismic reflection surveys, we present a resistivity-based mapping of OFG in this region. The models reveal a laterally continuous OFG body with resistivities of 8–14 Ω·m, extending across multiple stratigraphic units and well beyond individual subglacial landforms. This configuration is best explained by subglacial meltwater flushing of sediment porewaters during Pleistocene glaciations. Subsequent preservation was aided by density contrasts that limit vertical mixing and by fine-grained glaciomarine deposits sealing the system from above. The connection between resistivity anomalies and mapped subglacial landforms shows that the latter can serve as predictive proxies for OFG occurrence. These findings shift the perspective from OFG as isolated anomalies to regionally extensive systems linked to glacial processes, establishing subglacial landforms as a testable screening proxy for locating OFG in formerly glaciated margins worldwide.

The widespread presence of submarine permafrost in the southern Canadian Beaufort Sea provides necessary stability conditions for gas hydrates on the shallow shelf. However, the submarine permafrost is still relatively underexplored. Representing mostly relict terrestrial permafrost, submarine permafrost was formed by subaerial exposure to ∼−20°C in the Pliocene and Pleistocene. With the end of the last glaciation, the former terrestrial permafrost became inundated as a result of a marine transgression that caused a significant change in thermal conditions. The submarine permafrost is still responding to this thermal change and continues to degrade. The degradation of permafrost can cause greenhouse gas emissions having an important impact on the global climate. In this study, we use marine multichannel seismic data to calculate the base of permafrost using the seismically detected base of the gas hydrate stability zone. From the depth of the base of the gas hydrate stability zone, we estimate the theoretical gas hydrate dissociation temperature, which allows us to calculate the depth of the remotely inaccessible thermal base of permafrost (0°C isotherm). This calculated base of permafrost correlates with the lower boundary of a diffuse zone of high diffraction strength in seismic data indicating the presence of ice‐bearing permafrost. Our study provides a new approach to access the current depth and extent of submarine permafrost on the outermost Canadian Beaufort Shelf and indicates less thawing of permafrost than previously thought.

Gravitational volcanic flank collapses are recurrent processes producing large-scale landslides, commonly associated with secondary instabilities and destructive tsunamis. Ocean island volcanoes are particularly prone to such failures, yet the dynamics, emplacement mechanisms, and tsunamigenic potential of gravitational collapses remain poorly constrained due to the scarcity of direct observations. The ~ 68 ka Monte Amarelo flank collapse at Fogo, Cabo Verde, is one of the prominent examples of tsunamigenic gravitational collapses globally. But key parameters, including the collapse volume and the number of collapse stages, remain debated. We identify an extensive (914 km ² ) debris avalanche deposit associated with the Monte Amarelo volcanic flank collapse using high-resolution multibeam bathymetry and multichannel seismic reflection data. In addition, we find a smaller, previously unknown debris avalanche deposit superimposed on the northern half of the main deposit. Our results indicate that the Monte Amarelo collapse involved up to ~ 98 km ³ of material—the first volume estimate constrained by direct imaging of the submarine debris avalanche deposit through seismic data. The local morphology and internal structure of the main deposit suggest rapid emplacement and high tsunamigenic potential. The presence of a second deposit indicates multiple stages of collapse. Our results provide new constraints on the dynamics, volume, and sequence of events of one of the most recent gravitational collapses of a volcanic island in the Atlantic. These findings have important implications for tsunami generation and modelling and contribute to an improved understanding of hazards associated with gravitational collapses of volcanic islands.

Submarine permafrost in the Canadian Beaufort Sea is relict terrestrial permafrost, which is continuously degrading since the change of thermal conditions induced by a marine transgression that followed the last glaciation. Permafrost degradation has a crucial socio‐ecological significance because its thawing can result in geohazards like landslides or an increase in greenhouse gas emissions. These consequences are mostly regulated by the state of ice in permafrost. In this study, we use marine multichannel seismic data to apply a diving wave tomographic inversion on the outer 50 km of the Canadian Beaufort Shelf. Due to the close relationship between seismic velocity and ice content, we are able to infer detailed information about the present submarine permafrost condition. We find a clear variability of permafrost occurrences between the inner and outer Canadian Beaufort Shelf. At the inner shelf, discontinuous ice‐bonding permafrost occurs extensively close to the seafloor but is interrupted by taliks. Within the outer ∼27 km of the shelf, ice‐bonding permafrost is absent in the upper sediments and its top has plunged to >200 m below sea level. These findings add new details to the current state of the degrading permafrost. In addition, we observe seismic anisotropy in the frozen permafrost sediments.

During the last 1 Ma in the Canadian Arctic, permafrost and permafrost-associated gas hydrates formed extensively due to mean annual subaerial temperatures of approximately -20°C. Following the last glaciation, a marine transgression occurred and former terrestrially exposed shelves became inundated, resulting in present submarine bottom water temperatures around -1°C. Relict submarine permafrost and gas hydrates in the Canadian Beaufort Sea are still responding to this thermal change resulting in their ongoing degradation. Thawing of permafrost and destabilisation of permafrost-associated gas hydrates can release previously trapped greenhouse gases and can lead to even further gas hydrate dissociation with important implications for the global climate. However, both the extent of the submarine permafrost and the permafrost-associated gas hydrates are still not well known. In this study, we use marine multichannel seismic data to model the base of permafrost from the depth of the base of the gas hydrate stability zone. From this depth, we estimate the theoretical gas hydrate dissociation temperature, which allows us to model the depth of the thermal base of permafrost (0°C isotherm). The base of permafrost we modelled correlates with the lower boundary of a diffuse zone of high diffractivity in seismic data suggesting the presence of ice-bearing permafrost. These results combined show that the base of permafrost still extends close to the shelf edge indicating less permafrost retreat than previously suggested. Our study provides a different approach to accessing the current depth and extent of submarine permafrost on the outermost Canadian Beaufort Shelf.

Within the ICDP-funded project COSC (Collisional Orogeny in the Scandinavian Caledonides), mountain building processes are investigated with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. Drilled in 2014, borehole COSC-1 near Åre studied the emplacement of the high-grade metamorphic allochthons and obtained a section through the Lower Seve Nappe as well as the underlying mylonite zone. The second borehole COSC-2, drilled in 2020 near Järpen/Mörsil, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.An extended walkaway VSP survey at the COSC-2 drill site was performed in September-October 2021. This study aims to support the geological interpretation with a high-resolution 3D image of the subsurface in the direct vicinity of the borehole. This allows the determination of the origin of the basement reflections and reveals the nature of the main décollement as well as the degree of basement thrusting. Two 2D surface seismic lines approximately perpendicular to each other (North to South, West to East) and centered around the COSC-2 drill site were acquired using single (1C) and three-component (3C) geophones at 5-30m intervals. Furthermore, the West-East line was extended by 30 geophones at 100m intervals on each line end to allow the registration of wide-angle shots. A 32 t Vibroseis source operated along both lines with source point distances of 100 m within the central part of the line and 500 m at the wide-angle stations, respectively. Ocean bottom seismometers (OBS) were deployed on the bottom of a lake north of the borehole along a ~1.5 km portion of the North-South line. An airgun source was activated on this part of the profile. Along the entire borehole down to a depth of 2.26 km a 3C geophone chain recorded the seismic wavefield from all source points with a geophone spacing of 10 m, complemented by the recording from one single zero-offset source point with a geophone spacing of 2 m.The obtained surface seismic and VSP data set exhibits exceptionally good quality and shows many pronounced and clear reflections in the raw gathers. They are observed even at the largest source-receiver offsets (~11 km) and are visible at two-way-traveltimes up to 3-4 s, corresponding to structures at a depth of approximately 11 km. We present results of the ongoing surface seismic data processing and analysis, including a P-wave velocity model obtained from first arrival traveltime tomography, an analysis of seismic anisotropy related to the geological structures in the area and a first imaging result from the surface seismic data.

The Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross‐cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models.

Large Pleistocene ice sheets have produced glacial structures both at and below the surface in northern Europe. Some of the largest and most erosive structures are so-called tunnel valleys (TVs): large and deep channels (typically up to 5 km wide and up to 400 m deep, with lengths up to 100 km), which formed below ice sheets. Although the subject of many studies, the details of their formation and fill are still not well understood. Here, we present an update on the distribution of TVs in the southeastern North Sea between Amrum and Heligoland based on a very dense grid of high-resolution 2D multi-channel reflection seismic data (400 m line spacing). The known tunnel valleys (TV1–TV3) in that area can now be traced in greater detail and further westwards, which results in an increased resolution and coverage of their distribution. Additionally, we were able to identify an even deeper and older tunnel valley, TV0, whose orientation parallels the thrust direction of the Heligoland Glacitectonic Complex (HGC). This observation implies a formation of TV0 before the HGC during an early-Elsterian or pre-Elsterian ice advance. For the first time, we acquired high-resolution longitudinal seismic profiles following the thalweg of known TVs. These longitudinal profiles offer clear indications of an incision during high-pressure bank-full conditions. The fill indicates sedimentation in an early high-energy environment for the lower part and a subsequent low-energy environment for the upper part. Our results demonstrate that a very dense profile spacing is required to decipher the complex incisions of TVs during multiple ice advances in a specific region. We also demonstrate that the time- and cost-effective acquisition of high-resolution 2D reflection seismic data holds the potential to further our understanding of the incision and filling mechanisms as well as of the distribution, complexity and incision depths of TVs in different geological settings.

In the Canadian Arctic, permafrost and permafrost-associated gas hydrates formed extensively during the last 1 Ma. After the last glaciation, a marine transgression followed and former terrestrially exposed shelf areas became submerged. Subaerial mean annual temperatures of -20°C or even less changed to present submarine bottom water temperatures near -1°C. The relict submarine permafrost and gas hydrates present in the Beaufort Sea still react to this ongoing thermal change which results in their continued degradation. Thawing permafrost and destabilisation of permafrost-associated gas hydrates may release previously trapped greenhouse gases and can lead to even further gas hydrate dissociation. Moreover, thawing permafrost poses a geohazard in form of landslides and ground collapses. Yet, both the extent of the submarine permafrost and the permafrost-associated gas hydrates are still not well known. Here, we present three different approaches using marine 2D multichannel seismic data to improve the current knowledge of the distribution of offshore permafrost and gas hydrates occurrences in the southern Canadian Beaufort Sea. The acoustic properties of permafrost are determined by the content of ice and unfrozen pore fluids. Changing permafrost conditions affect the elasticity of the medium making seismic methods appropriate for permafrost detection. First, we identify direct and indirect seismic reflection indicators from permafrost and gas hydrates by the presence of cross-cutting, polarity-reversed, and upward-bend reflections as well as velocity pull-ups and shallow pronounced high-amplitude reflections. Second, using diving-wave tomography provides insights into the near-surface permafrost structure by imaging the velocity structure in greater detail than achievable by standard velocity analyses.  And third, diffractions separated from the reflected wavefield yield insights into the sub-wavelength architecture of the permafrost realm on the southern Canadian Beaufort Shelf that may add information about weak phase-boundaries and small-scale heterogeneities. All methods are applied to seismic lines crossing the outer continental margin, where a maximum thermal effect of the transgression is expected, and thus a maximum lateral variation in permafrost and permafrost-associated gas hydrate phase boundaries is expected to be present.