Area-wide hydroacoustic mapping in coastal environments is a time-consuming and cumbersome task due to the limited swath width of most devices, especially in shallow waters. At the same time, these environments hold important functions for coastal ecosystems, are subject to intensive anthropogenic use, and are characterized by dynamic and complex geomorphological interactions of waves, currents, and tides. We presently investigate the seafloor geomorphology of a marine protected area in the eastern North Sea based on a combination of own archived hydroacoustic data, hydrographic single-beam survey data, and fishing vessel position data. The research area is located within 12-nautical miles from the coast, covers approx. 1,200km², and is characterized by water depths between 12 and 18 m. The topography of the seafloor is relatively flat and dominated by mobile sands although gravel and hard substrate (boulder reef) environments commonly occur throughout the area and are protected under the EU Habitats Directive. The properties and spatial distribution of these habitats remain currently unknown, despite the fact that the area hosts intensive fisheries with bottom-contact gears and one of Europe’s largest marine sand extraction sites. Our results show that the integration of different data sources allows an effective assessment of essential habitat parameters, natural seafloor processes, and anthropogenic stressors. Against this background, a strategy to more closely survey and/or monitor specific areas can be devised in order to better protect seafloor habitats and to mitigate human impacts on coastal ecosystems.  

How to cite: Sander, L., Yapa, T., Hoffmann, J., and Saathoff, M.: Ship-based mapping of protected seafloor habitats and anthropogenic stressors in a very shallow coastal environment: Spatial data integration in a marine protected area offshore Sylt (North Sea), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9300, https://doi.org/10.5194/egusphere-egu24-9300, 2024.

Our understanding and knowledge about Earth´s surface and processes benefitted greatly from the increasing availability of open accessible, large-scale remote sensing topographic data, including GTOPO30 (c. 1 km, 1996), ASTER (c. 30 m, 2009), SRTM 3.0 (c. 30 m, 2014) and TanDEM-X (c. 12 m, 2014). In comparison, the documentation of the sea floor at higher resolutions and/or its accessibility is scarce (e.g. https://emodnet.ec.europa.eu/en/bathymetry, https://www.ngdc.noaa.gov/mgg/bathymetry/lidar.html, Otero & Mytilineou 2022) and vast submarine areas are still a terra incognita. We have carried out a preliminary multi-beam bathymetric survey during 29-31 May 2023 with the 13.4 m research vessel “Alkyon” of the Hellenic Centre for Marine Research, funded by the Cheops Privatstiftung Wien and the Walter Munk Foundation for the Oceans (https://www.waltermunkfoundation.org). The Alkyon was equipped with a hull-mount Teledyne Reson T-50R multibeam echosounder. In total, an area of 26.8 km² was surveyed west, south and east of Strongylo Island, south of Despotiko Island up to Cape Petalida at the southern tip of Antiparos and especially the Bay of Despotiko, between Despotiko and Antiparos islands with measured depth ranging between -6.7m to -105.7 m below sea-level. The survey aims include the (i) high-resolution documentation of this previously unknown sea-floor, (ii) information concerning local sea-level rise (see Lykousis 2009, Kolaiti & Mourtzas 2020, 2023), (iii) the possible continuation of tectonic features as well as coastal mass-movements investigated above sea-level. Processing and thorough geomorphological analysis of the high resolution bathymetric data provide valuable information on the extend of posidonia meadows on the seafloor  (e.g. Despotiko Bay), evidence for possible palaeo-sealevel indicators (palaeo-coastlines, wave-cut terraces) at various depths, palaeo-valleys and other geomorphological features belonging to the terrestrial landscape that was drowned during the post-glacial sea-level rise, as well as several deposits associated with the mass movements mapped on the adjacent rocky slopes of Strongylo, Despotiko and South Antiparos islands. Marine geological-geophysical research will be continued and complimented with high resolution sub-bottom profiling data and visual observation to unravel the recent geomorphological evolution of the survey area.

Kolaiti, E. & Mourtzas, N. 2020. New insights on the relative sea level changes during the Late Holocene along the coast of Paros Island and the northern Cyclades (Greece). Annals of Geophysics, 63(6), https://doi.org/10.4401/ag-8504

Kolaiti, E. & Mourtzas, N. 2023. Late Holocene relative sea-level changes and coastal landscape readings in the island group of Mykonos, Delos, and Rheneia (Cyclades, Greece). Mediterranean Geoscience Reviews, 5, 99-128. https://doi.org/10.1007/s42990-023-00104-4

Lykousis, V. 2009. Sea-level changes and shelf break prograding sequences during the last 400 ka in the Aegean margins: Subsidence rates and palaeogeographic implications. Continental Shelf Research, 29(16), 2037-2044.

Otero, M. & Mytilineou, C. (eds.) 2022. Deep-sea Atlas of the Eastern Mediterranean Sea: Current knowledge. IUCN-HCMR DeepEastMed Project. IUCN Gland, Málaga, 371 p. https://uicnmed.org/docs/deep-sea-eastern-med/DEEP-SEA-EASTERN-MEDITERRANEAN.pdf

How to cite: Heine, E., Draganits, E., Sakellariou, D., and Morfis, I.: A glimpse below the wine-dark sea: Multi-beam bathymetric survey around Despotiko and Strongylo islands (Cyclades, Greece), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17529, https://doi.org/10.5194/egusphere-egu24-17529, 2024.

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.

How to cite: Englert, R., Boettner, C., Brandt, P., Cartigny, M., Huang, H., Krahmann, G., Puig, P., Schoenke, M., Stevenson, C., Talling, P., and Krastel, S.: Deciphering the origin of sediment waves along the Northwest African margin through multidisciplinary analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12684, https://doi.org/10.5194/egusphere-egu24-12684, 2024.

Bedforms are widely distributed within deep-water submarine canyons, which are usually documented by vessel-mounted sensors. Yet, fine-scale geomorphology and shallow structures of bedforms in deep-water submarine canyons remain poorly documented, and understood, because of the insufficient resolution of vessel-based data. This study utilizes high-resolution autonomous underwater vehicle (AUV) dataset combined with intermediate seismic reflection profile and sediment cores to analyze bedform sets along a slope-confined submarine canyon (canyon C14) from the northern South China Sea. A train of crescent-shaped to inverted U-shaped axial steps in plan view are aligned downstream along the canyon thalweg from upper course to lower course. Based on comprehensive analysis of morphologic features, subsurface structures, flow estimates, and potential origins, these steps are likely to be cyclic steps created by supercritical turbidity currents. Sediment cores mainly comprised by silt with minor sand contents further suggesting the shallow canyon sediments probably deposited by diluted turbidity currents. Axial steps (S1-S4) with lower asymmetry and wavelengths in the upper course show an erosional truncation and horizontal to sub-horizontal reflectors draping on the lee side and stoss side, respectively, illustrating the erosional-depositional cyclic steps formed by more confined flow with higher erosion capability due to the narrow canyon (average width of 3.5 km) and steep slope gradient (average of 2.36°). Leaving transition segment, the less confined flow passing through lower course can be subject to wider canyon (average width of 5.5 km) and gently slope gradient (average of 1.2°) that increases the asymmetry and wavelengths of axial steps (S5-S7) and leave backset bed deposits on the stoss sides, probably pointing to the depositional cyclic steps with higher aggradation. Sediment filling, almost padding each cyclic step-associated scour, indicate that the previous-formed bedforms can be reworked by subsequent gravity flows deposits which mainly consist of slope failures-associated mass-transport deposits and turbidity currents deposits. Near the lower end of the canyon, reduction in flow velocity caused by further decrease of slope gradient (average of 1.05°) as the key factor leading to the shift from cyclic steps to furrows, but always under supercritical flow conditions. In this context, a sector of axial channel probably promotes the re-convergence of turbidity currents, resulting in the erosion of fine-grained cohesive deposits on the canyon floor, to form linear furrows within the axial channel. This work provides a good opportunity to investigate the fine-scale morphological features and shallow structures of bedforms in deep-water submarine canyon, and understand their evolution under the influence of canyon topography.

How to cite: Sun, Y., Wang, D., Canals, M., Alves, T. M., and Zeng, F.: Fine-scale seafloor bedform morphology along a slope-confined submarine canyon in the Northern South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2937, https://doi.org/10.5194/egusphere-egu24-2937, 2024.

How to cite: Ottesen, D. and Diesing, M.: Automatic pockmark detection in the Norwegian Channel , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5760, https://doi.org/10.5194/egusphere-egu24-5760, 2024.

Lake Sapanca is located in the Eastern Marmara region of Turkiye and formed as a tectonic lake by the right-lateral strike slip North Anatolian Fault (NAF). Located 33 m above sea level, this tectonic lake has a length of 16 km in the east-west direction and 5 km in the north-south direction. The area in which the lake is located is on the Izmit-Sapanca Corridor, which is bordered by the segments of the northern branch of the NAF, between the Samanlı Mountains to the south and the Kocaeli Peneplain to the north. Multibeam bathymetry and high-resolution seismic data were acquired in the lake as a part of a TUBITAK project (Project No: 117Y130) in August 2018 to determine the structural and stratigraphic elements of the lake. Many pockmark structures, as well as lineaments related to the NAF, were observed on multibeam maps giving cm-scale resolution on the lake bottom. Thus, the effects of NAF in the lake can be better determined depending on the geometrical properties of the pockmarks, more than 300 in number, which are formed due to gas or fluid outflows from the lake-bottom, and their distribution in certain parts of the lake. In order to determine these features of pockmarks, semi-automatic approaches of QGIS and ARCGIS software programs were used and it was observed that the pockmark distribution increased along the lineament direction of the NAF and in the northeast of the lake. Moreover, we conclude that, the consistent orientation of the individual pockmarks may indicate that all pockmarks were formed in a relatively short period of time or that the bottom current regime in the lake has been effective for a long time. 

How to cite: Sönmez, E. and Kurt, H.: An Example of Determining Fault Properties from Morphological Analysis of Pockmarks is Sapanca Lake, Located on the North Anatolian Fault, Turkiye, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-997, https://doi.org/10.5194/egusphere-egu24-997, 2024.

The Ordovician karst fracture-cave reservoir in Tahe oilfield has strong heterogeneity, and palaeogeomorphology, fault and fracture play important roles in the development of the complexity of the fracture-cave reservoir. Based on rich geological and geophysical data, the influence of faults on the karst reservoir development in carbonatite under different palaeogeomorphic conditions are analyzed through the interpretation of faults and the activity of internal faults in carbonate rocks in different periods, combined with palaeokarst environment and karst products. 
The results show that there are not only strike-slip faults but also an NNE-thrust fault in the sixth and seventh districts of Tahe Oilfield, which control the direction of long-axis anticline in the center of the study area. The anticline becomes an important watershed and most of the surface gullies develop along suitable faults from the top of the anticline to lower areas. According to the karst geomorphology, water system and fracture-cave distribution, landforms are divided into three types: hoodoo-upland, karst depression and karst basin. In the hoodoo-upland, the fracture networks around the faults are dissolved and small and medium-sized fractures develop, and the reservoirs have low filling degree and good performance. In the karst depression, the landforms are transformed by strong water erosion and karst dissolution. The underground rivers and the palaeogeomorphic gullies controlled by high-angle strike-slip faults are relatively straight, while the others controlled by low-angle faults are tortuous. Unfilled caves and intergranular pores in cave fillings are the main reservoir spaces. In karst basin, the Ordovician soluble limestone is covered by stucco deposits, which greatly weakens the karstification. The fractures and caves can develop only along the faults and fractures at a very deep depth. The spatial structure, connectivity, porosity and permeability are complicated. The main reservoir types are fractures, fracture-cave and isolated caves. The filling types are fault karst breccia, giant crystal chemical filling or no filling. Therefore, faults affect the development of reservoir types and fillings under different geomorphology and karst water conditions, which has important guiding significance for the accurate exploration and development of carbonate fracture-cave reservoirs. 

How to cite: Zhang, X. and Jin, Q.: The influence of faults on the development of canbonate karst reservoir in main area of tahe oilfield and its significance in petroleum geology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-67, https://doi.org/10.5194/egusphere-egu24-67, 2024.

Diamantina Trench in the southeast Indian Ocean is one of the less unexplored hadal trenches (> 6000 m) of our planet, which develops the second deepest point (Dordrecht Deep, ca. 7019 m depth) in the Indian Ocean. Humans did not visit its ocean floor until the Chinese submersible Fendouzhe reached its deepest point in January 2023. This expedition collected high-resolution multibeam sonar bathymetry data covering about 3000 nautical miles and conducted 28 scientific dives with high-resolution videos and push core sediments of the upper seafloor (max. 40 cm) at a wide range of submarine geomorphology. This study combines these materials to fully assess the morphological variability of the trench and the causative factors and processes determining such characteristics.

Bathymetry data indicate a rugged and complex landscape with various seamounts and debris deposits in the Diamantina Trench which could be classified into three sections. Bounded by the Broken Ridge to the north, the western section contains a series of basins and gorges, as well as parallel intruded ridges (WNW striking). The eastern section shows deeper and steeper slopes compared to the western section. The transitional area of the two sections (the Dordrecht Deep area, 270 km²) is the deepest part of the trench.

Four push core sediment profiles were analyzed from the most west and east locations, the Dordrecht Deep area, and the western trend with foraminifera oozes. Layers of foraminifera and calcareous nannofossil oozes occur at the western section, whereas brownish pelagic sediments with occasionally coarse-grained Fe-Mn nodules develop at the eastern section. The preliminary results of total carbon (TC) and total nitrogen (TN) suggest distinct differences among and within profiles. TC values reach 12% in foraminifera oozes and less than 1.2% in the pelagic sediments. TC values decrease rapidly at the upper 10 cm and remain low (0.1–0.2%) at the lower part in the profiles from the eastern section and Dordrecht Deep area. An analogous trend applies to the TN graphs. The sediment profile from the western section, however, shows decreasing TC and TN values within depth.

This research provides the first knowledge of the highly spatial heterogeneity of submarine geomorphological characteristics and sediment dynamics in the Diamantina Trench. The ongoing measurements of organic matter content, carbon isotope, and grain size from different topographic locations with the potential of dating methods (e.g., ¹⁴C and paleontological data) will further aid in reconstructing the spatial variations of paleoenvironmental changes and organic cycling process, as well as in understanding the relationship with tectonic activities and catastrophic events in hadal zones.

How to cite: Yang, X., Huang, X., Zhou, P., and Peng, X.: Submarine geomorphology of the Diamantina Trench (SE Indian Ocean) based on high-resolution multibeam sonar bathymetry and push core sediments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5882, https://doi.org/10.5194/egusphere-egu24-5882, 2024.

 This study presents a detailed analysis of grain-size distributions of closely spaced surface sediment samples from the eastern continental margin of the Korean Peninsula off Gangneung and Donghae. This region is characterized by a wave-dominated beach, narrow shelf less than 10 km wide, and slopes with varying gradients ranging from 0.7 to 6.3 degrees. Spatial variations in dominant sediment transport modes were identified using end-member analysis (EMA) of the grain-size distributions.

 The EMA revealed five distinct end-member distributions (EMD) with mean grain sizes of 221.2 μm (EMD1), 89.2 μm (EMD2), 52.4 μm (EMD3), 22.0 μm (EMD4), and 4.5 μm (EMD5), respectively. EMD1, is significant only at two shallow sample sites near the coast, adjacent to the exposed rocky seafloor, indicating an origin from a relict sand during the post-glacial transgression. EMD2 predominates on the shelf and upper slope (40 to 150 m water depths) along the margin, particularly near a local river mouth, suggesting bedload transport of riverine sand by longshore drifts or episodic storm surges. EMD3, potentially representing coarser suspended load, dominates the upper middle slope (200 to 400 m water depths) where the slope gradient is relatively constant, and the isobaths generally run parallel to the shoreline about 15 km apart. EMD4, potentially representing finer suspended load, prevails in the deeper middle slope (400 to 800 m water depths) characterized by varying morphology: narrower and relatively steeper in the northern part, and wider and gentler in the southern part of the margin. The higher proportions of EMD4 extend far offshore in the wider and gentler southern part more than about 35 km, whereas they are limited to within 30 km from the shoreline at the base of the steeper and narrower slope in the northern part. The morphologically controlled EMD4 distributions suggest that a density current was responsible for the offshore fine-grained sediment transport in this margin, rather than diffusion or advection by ocean currents. Finally, the finest EMD5, predominates in the deepest part of the study area, showing no significant further offshore variations, and is interpreted to represent aeolian dust from the Asian inland.

How to cite: Sim, G., Bahk, J.-J., Jang, J., Kim, H., Jeong, J., and Um, I.-K.: Distinguishing sediment transport modes in the eastern continental margin of the Korean Peninsula through end-member analysis of surface sediment grain-size distributions., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7149, https://doi.org/10.5194/egusphere-egu24-7149, 2024.

Plio-Pleistocene records of ice-rafted detritus suggest that northwest European ice sheets regularly advanced across palaeo-coastlines. However, while these records are important, they provide only a limited insight on the frequency, extent, and dynamics of the ice sheets that were delivering the detritus. Three-dimensional reflection seismic data of the northwest European glaciated margin have previously documented buried glacial landforms that inform us on these uncertainties. This work combines existing landform records with new seismic geomorphological observations to catalogue landform occurrence along the European glaciated margin and considers how these features relate to ice sheet history. The compilation shows that Early Pleistocene ice sheets regularly advanced onto and across the continental shelves. This is important because Early Pleistocene sea level reconstructions show lower magnitude fluctuations between glacial-interglacial cycles than when compared to the Middle-Late Pleistocene. The potential for more extensive and more frequent Early Pleistocene glaciation provides a possible mismatch with these sea level reconstructions. This evidence is considered with global records of glaciation to contemplate the possible impacts on our wider understanding of Plio-Pleistocene climate changes, in particular how well Early Pleistocene sea level records capture ice sheet volume changes and how quickly large ice sheets waxed and waned. Resolving such issues relies on how well landforms are dated, whether they can be correlated with other proxy datasets of environmental change, and how accurately these proxies reconstruct the magnitudes of past climatic changes. The results leave many more questions about Pleistocene glaciation in Europe unresolved, with significant impacts on our global understanding of how sea level evolved through the Pleistocene and its association with ice sheet development.

How to cite: Newton, A., Montelli, A., Batchelor, C., Bellwald, B., Harding, R., Huuse, M., Dowdeswell, J., Ottesen, D., Johansen, S., and Planke, S.: Glacial seismic geomorphology offshore northwest Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10932, https://doi.org/10.5194/egusphere-egu24-10932, 2024.

Antarctica’s continental slopes hold invaluable insights for understanding past climate, ice-sheet dynamics, ocean circulation, erosional and depositional processes, and submarine geohazards over millennial timescales. We present a multidisciplinary dataset from the Ross Sea continental margin, Antarctica from the EUROFLEETS-funded ANTSSS expedition and International Ocean Discovery Program Expedition 374, including core records spanning ~3 Ma, multibeam echosounder and single-channel seismic data and legacy seismic data available through the Antarctic Seismic Data Library System. Here, gully and channel systems occur at the head of the Hillary Canyon, with palaeo-gullies evident in seismic data. New sediment core-seismic correlations show that palaeo-gullies evolved on the Ross Sea continental margin over multiple glacial cycles, filling and reforming associated with glacial advances, cold dense water cascading and other processes. We show multidisciplinary datasets that constrain the signature of down and along-slope processes and examine factors driving their timing, frequency, and impact on gully evolution. We discuss the implications of these findings in relation to Neogene and Quaternary West Antarctic Ice Sheet expansions to the shelf edge.

How to cite: Gales, J., McKay, R., De Santis, L., Rebesco, M., Laberg, J. S., Kulhanek, D., patterson, M., King, M., and Kim, S.: Assessing the influence of climate on Antarctic submarine gully evolution  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8780, https://doi.org/10.5194/egusphere-egu24-8780, 2024.

The submarine channel, also known as submarine canyon, is a narrow and long negative terrain 
that cuts to the shelf or slope and widely developed on the global continental margins. There are 
numerous factors in forming channels, including climate change, topography, sediment sources and 
grain size, and sea level change. However, for high latitudes, especially in the Antarctic region, the 
controlling factors of the formation and evolution of the channel are still poorly understood. In this 
study, we conduct a systematic analysis of the channels in various regions of the Antarctic 
continental margin with the aim of identifying the differences of the channels between the East and 
West Antarctic continental margins and associated controlling factors. We identified 2126 channels 
on the Antarctic continental margin based on IBCSO V2 data (International Bathymetric Chart of 
the Southern Ocean Version 2). The submarine channels and their possible factors in six regions 
(Weddell Sea, Amundsen Sea, Ross Sea, Wilkes Land, Prydz Bay and Dronning Maud Land) are 
statistically analyzed. Quantitative analysis shows that there are obvious differences in the 
geomorphology of submarine channels between the East and West Antarctic continental margin. 
First, consider the differences in the landscape. The shelf is narrower on the east and wider on the 
west. There are prominent troughs running across the broad shelf. West Antarctica has a gentler 
slope gradient than the East Antarctic continental margin, and the ice velocity is much faster. Second, 
submarine channels on the West Antarctica continental margin are longer and wider in cross section, 
with most large-scale channels extending beyond the slope foot, whereas submarine channels on the 
East Antarctica continental margin are deeper but shorter, with fewer channels. We consider that 
shelf width, slope gradient, trough and ice velocity can control sediment transportation and thus 
affect the size of channels. Channels are longer and wider on the margin with wide shelf, prominent 
trough and fast ice velocity, while they are shorter and deeper on the margin with steep slope.

How to cite: Huang, H. and Huang, X.: Quantitative analysis of Antarctic channel distribution and the role played by continental geomorphology in channel evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22420, https://doi.org/10.5194/egusphere-egu24-22420, 2024.

During the last 500 000 years, ice sheets occupied parts of the North Sea during three major glaciations. The existence of these ice sheets was accompanied by a large fall in sea level, causing the Southern North Sea to emerge and become isolated from the Atlantic. In this area a complex drainage system was created by river water of the West-European rivers (e.g., Thames, Rhine, Meuse and Scheldt) and glacial meltwater. Furthermore, most offshore studies support the idea of the formation of large proglacial lakes in front of these ice sheets, which may have caused high-magnitude outburst floods at the end of each glacial period. The existence of such a proglacial lake is used in the argument that glacial outburst floods during the Elsterian (500-450 ka) created erosional features still preserved nowadays in the Dover Strait.

A remnant of this large, complex fluvial and glacial drainage system is the (North) Axial Channel, a prominent geomorphological feature seen on the present-day sea floor of the Southern North Sea. Its formation and evolution, however, are still uncertain. Previous studies state that the Axial Channel forms the northern extension of the Lobourg Channel, located in the Strait of Dover, which was formed during Middle Miocene times. Further erosion is assumed to have occurred during the Pliocene and Pleistocene, as sediments within the Murray Pit (located in the Axial Channel, about 100 km northeast of the Lobourg Channel) are assumed to be Early Pliocene, and no Quaternary infilled sediments have been identified. A series of NE-SW oriented scarps are identifiable from bathymetric and seismic reflection data and have been attributed to different Pleistocene incisional events. However, currently only a relative chronology of potential events has been established, with large uncertainties. Understanding the paleogeographic changes that affected the region also increases the knowledge on how early humans may have settled in and/or migrated through the region. 

In the framework of the WALDO project (“Where are All the (proglacial) Lake seDiments in the NOrth Sea Basin?”), a survey has been conducted in October 2023 during which high-resolution geophysical data (multibeam bathymetry and backscatter, acoustic and seismic data) combined with ground-truth data (vibrocores) have been acquired. One of the reflection-seismic grids was conducted ~40 km east of the East of England coast, over the western edge of the North Axial Channel, where also four sediment cores were taken. Here, we present the first interpretation of these new data, which allow us to evaluate, update and improve the relative chronology of the formation of the (North) Axial Channel.  

How to cite: Vervoort, M., Kyriakoudi, D., Plets, R., Mestdagh, T., Missiaen, T., and De Batist, M.: Understanding the paleogeographic evolution of the North Axial Channel, Southern North Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9993, https://doi.org/10.5194/egusphere-egu24-9993, 2024.

Understanding the course and dynamics of ancient river systems, such as the pre-Odra, is crucial for unravelling the geological history of regions like the southern Baltic Sea, providing valuable insights into the post-glacial evolution of landscapes and riverine processes. We will present marine reflection seismic and acoustic data from three research cruises on the research vessel ALKOR that allow us to investigate the pre-Odra river system in the southern Baltic Sea. Our analysis focuses on the region off the east coast of the island Rügen. This region corresponds to the suspected location of the pre-Odra river system, which was situated during the post-glacial phase approximately 9,000 - 14,500 years BP off Rügen. The seismic reflection data indicate that the sediment infill of the pre-Odra is charged by shallow gas of presumably biogenic origin. Since the seismic gas indicators correspond with the pre-Odra where its location has been determined by previous geological studies, we use gas lineaments as a proxy for the braided paleo-river bed. This study refines and extends the known fluvial extent of the Odra river system by 60 km, tracing it north towards the Tromper Wiek, indicating its terminus in the Baltic Sea close to eastern to Rügen.

How to cite: Schmidt, M. C., Hübscher, C., Seidel, E., Preine, J., and Haimerl, B.: Exploring the Outline of the Pre-Odra River System Through Seismic Reflection Imaging Offshore Rügen Island, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10335, https://doi.org/10.5194/egusphere-egu24-10335, 2024.

This study examines subaqueous dunes located on the southern periphery of the flat top of Dokdo Seamount at water depths from 120 to 170 meters, where the present ocean currents are incapable of generating such large bedforms. To determine their origin, we conducted a comprehensive analysis of the geomorphic characteristics using high-resolution multi-beam bathymetry and the grain-size characteristics of seafloor sediments. The analysis of the dune spacing in relation to height, as well as their migration and growth pattern, indicates that the Dokdo subaqueous dunes (DSDs) originally formed as aeolian dunes. These were shaped by northerly winter winds that carried sands from the wave erosion surfaces on the northern part of the flat top. The DSDs are believed to have transitioned to their current submerged state without experiencing significant erosion or reactivation. Considering the variations in the Plio-Pleistocene global mean sea level, we estimate the possible subsidence rate of the flat top to be approximately 130 m/myrs, aligning with the conditions required for the formation of DSDs. This study highlights how relict features such as submerged aeolian dunes on seamount summits can be utilized to precisely estimate the subsidence rate of oceanic volcanoes.

How to cite: Bahk, J.-J., Kim, S.-J., Kim, C. H., Sohn, Y. K., and Park, C. H.: Submerged aeolian dunes on the flat-topped Dokdo seamount in the East (Japan) Sea, Korea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7144, https://doi.org/10.5194/egusphere-egu24-7144, 2024.

The Josephine submarine seamount is located in the NE Atlantic Ocean around 470 km west of the coast of mainland Portugal and 500 km northeast of Madeira Island on the vicinity of the Africa-Eurasia plate boundary. With 47 km in length and 10 km in width, it rises 2500 meters above the adjacent abyssal plains, with its top standing at a depth of 170 m. It is a basaltic submarine seamount that yielded ages of 16.3 ± 0.9Ma to 11.7±0.7Ma (Geldmacher et al. 2006).

Multibeam and backscatter data were acquired to increase our knowledge about the seamount. The backscatter data shows the presence of fine sediment on the eroded top of the seamount, not present in the northern part. The top of the seamount dips gently (~1º) to the northwest allowing the deposition of fine grained sediment down to ~500 m where the seafloor is irregular with slopes dipping between 10⁰ and 15⁰ and the igneous rocks crop out.

Morphologic analysis suggests that the very flat and smooth surface of the southernmost portion of Josephine Seamount has been above sea level and subjected to near coastal erosive processes of areas lying at ~420 m depth. Since the sea level of the last 15 Ma has not been lower than 160 m of the present-day levels (Miller et al. 2020), tectonic and/or erosional processes must have lowered the seamount’s height by at least 260 meters.

Sedimentary rocks were dredged from depths from 480 to 347 m, on the southeastern part of the Josephine seamount and were analyzed for their shape, composition and sedimentary facies. The dredged samples are calciclastic limestone blocks that have a half horn torus shape (donut). Most of the specimens are trespassed by a 3 to 5 cm long conical cavity that developed from the base to the top of each sample.

The rocks consist of 96.6% of foraminifera tests (82.4% planktic and 17.6% benthic), with a very low mineral content. The grains are diagenetically cemented by a calcite matrix (confirmed by EDS analysis). The presence of the planktonic foraminifera species Globorotalia truncatulinoides, whose first occurrence dates of 1.93 Ma (Wade et al., 2011), provides a maximum age of formation, making these sedimentary rocks at least 10 Ma younger than the volcanic rocks that constitute the Josephine Seamount basement. The presence of the benthic foraminifera Lobatula lobatula and Discanomalina semipunctata indicate strong currents that could have contributed to the erosion of the seamount’s top.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds through the project LISA (https://doi.org/10.54499/PTDC/CTA-GEF/1666/2020).

How to cite: Carvalho, V., Terrinha, P., Neres, M., Voelker, A., Batista, L., and Rosa, M.: Josephine Submarine Seamount: New Insights from multibeam data and seabed sampling for environmental conditions in the Early Quaternary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10619, https://doi.org/10.5194/egusphere-egu24-10619, 2024.

Based on the morpho–bathymetric data coming from the MaGIC project (Marine Geohazards along the Italian Coast) and the high-resolution seismic reflection profiles acquired in 2010 by OGS-Explora, we depict the complex geomorphology of the Gulf of Cagliari and the evolution of the most striking morphosedimentary features developed during the PQ. The seafloor is shaped by the canyon system, the Sarroch and S. Elia-Foxy canyons. The obtained results point that their onset and location do not coincide with paleoincisions formed by the Messinian erosion. The main pathway changes of the Sarroch canyon are conditioned by extensional tectonics of the Campidano Graben and are controlled by the Banghittu High. Cut-and-fill features and infill deposits indicate that retrogressive erosive processes affect the canyon heads and produce landslides. transport deposits in the basin. In fact, four large MTD's have been recognized and analysed within the PQ sequence. They show different seismic facies, from transparent to chaotic, and are locally affected by internal deformational structures which allow us to distinguish the translational and compressional domains. The interplay between the morphosedimentary evolution of the systems canyons and the MTDs are useful to understanding the role played by the downslope channelized and non-channelized sedimentary processes over time and to explore the factors, local and/or global, controlling their occurrence and/or predominance. This analysis of the submarine canyon morphologies and occurrence of MTDs can help evaluate the potential geo-hazard implications of the region.

How to cite: Caradonna, M. C., Del Ben, A., Geletti, R., Pini, G. A., Frisicchio, V., Ercilla, G., and Estrada, F.: Imaging the Plio-Quaternary submarine geomorphological evolution of the Gulf of Cagliari, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10732, https://doi.org/10.5194/egusphere-egu24-10732, 2024.

The Plio-Quaternary sedimentary deposition in the Western Mediterranean Sea was strongly influenced by the Messinian Salinity Crisis (MSC) and by the consequences of the oceanic opening that produced regional fault systems and the following thermal subsidence, still largely active during the Plio-Quaternary (PQ). We analyse the PQ events that occurred in the West Sardinian margin and in the adjacent deep basin by integrating vintage and more recently acquired seismic data, obtaining the currently most complete regional seismic grid in the study area. The base of the PQ (“Ms” reflector) represents the top of the Messinian evaporites in the lower slope and deep basin and the Messinian erosional truncation in the continental upper slope and shelf. Two units have been recognized within the PQ sequence: the low amplitude lower Plio-Quaternary unit (l-PQ) and the high amplitude upper Plio-Quaternary unit (u-PQ), separated by the “A₀” reflector, for which we assume an age of 2.6 My (near Quaternary base), through the correlation with the published ECORS profile.

The thermal subsidence, related to the Oligo-Miocene (OM) oceanic opening, produced the increased inclination of the slope and, coupled with the halokinetics of Messinian evaporites, triggered most of the geological processes in the study area. In the lower continental slope, rollover structures are produced by salt sliding, which is related to the increased deepening of the slope, while in the deep basin typical sub-vertical faults developed above the salt diapirs: these processes, that continued throughout the entire PQ slowing down in the Quaternary, influence the thickness and distribution of the PQ sequence. Faults usually act as a preferential path for magma upwelling and gas rising: fault systems developed during the OM produced some large volcanoes at the boundary between slope and deep basin, while on the continental shelf and upper slope the main volcanic buildings are ascribed to the later Pliocene magmatic phase and are related to fault reactivation caused by the PQ thermal subsidence. On the tilted continental outer shelf, OM faults reactivation led to gas rising phenomena and related pockmarks, generated from the Early Pliocene until Present. During the Quaternary, the accentuated tilting of the continental slope triggered erosional processes that led to the formation of three new canyon systems, not inherited by the Messinian erosion as often hypothesized; at the same time, erosion of the onshore area led to a high sediment supply, responsible for the widening of the clinoforms on the inner shelf. In this study we analyse the evolution of the different PQ process that affected the West Sardinian margin and their relationships with previous regional events occurred in all the West Mediterranean Sea: the objective is to create basic information to subsequently compare with other passive margins of the sea.

How to cite: Frisicchio, V., Del Ben, A., Geletti, R., Caradonna, M. C., Rebesco, M., and Bellucci, M.: Plio-Quaternary geological events in the Western Mediterranean Sea: focus on the West Sardinian margin and adjacent oceanic basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11305, https://doi.org/10.5194/egusphere-egu24-11305, 2024.

Terminal lobes constitute the endmembers of siliciclastic systems. They are of great interest to marine geologists and constitute high-quality reservoirs actively sought out and exploited by the oil and gas industry. The sizes and shapes of lobes vary depending on the type of sedimentary system and the nature of associated gravity flows. Sea bottom topography -induced either by preexisting lobes or mass transport deposition or tectonic deformation- is another important factor controlling lobe morphology. Previous studies carried out on recent systems (based on multibeam bathymetry and 2D/3D seismic data) show different shapes, classically characterized as lobate, but also radial or elongated. Currently, much remains to be known on the relative influences of autocyclic and allocyclic forcings on the internal architecture of lobes.

This study focuses on a better definition of lobe shapes in natural systems to build a ruled -based model of lobes that will later be incorporated into the FLUMY© software. To this end, a database is built based on cases from the literature in various systems (e.g., the Congo, Amazon, Indonesia and East Corsica). We use the classical shape ratios defined by Prélat et al. (2010) as well as a new metric, called the progradation factor (PF), defined as the length ratio between the upstream and downstream segments of lobes (i.e., with respect to their centroids).

Measures of PF were applied at different scales from the bed element to the lobe complex. Independently of the sedimentary system type, three different shapes of lobate bodies were identified: (i) a “classical” lobate shape -wider downstream- when PF > 1.15, (ii) lobes that are wider upstream when PF < 0.85 and, (iii) an elliptical shape when 0.85 ≤ PF ≤1.15. The classical lobate shape is interpreted as marking the absence of topographic confinement. Elliptical lobes occur mainly during maximum and minimum of progradation/retrogradation cycles. Lobes that are wider upstream result from topographic confinement and are mainly deposited at the end of retrogradation cycles. Finally, plotting thickness vs area allows identifying semi-confined lobes as a third category located between confined and unconfined lobes. This category follows a linear trend and exhibits a minimum thickness of 20 m.

How to cite: Grimaud, J.-L., Mercier, L., Ors, F., and Huyghe, D.: Assessing the dynamics of turbiditic terminal lobes: a geometrical approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10857, https://doi.org/10.5194/egusphere-egu24-10857, 2024.

Submarine braided channels, driven by turbidity currents, have been revealed on several deep-sea fans, displaying similar morphological features to fluvial braided rivers. Past experimental studies on submarine braided channels have shown that active braiding intensity (BI_A) is proportional to fixed confinement width, dimensionless stream power (ω^*) and dimensionless sediment-stream power (ω^**). However, the field-scale submarine braided channels may not restrict to a fixed confinement width (B); instead, the confinement shape often exhibits gradual widening or narrowing. In this study, we use physical experiments to investigate the influence of confinement shapes and inflow-to-sediment discharge ratios (Q_in/Q_s) on the evolution of submarine braided channels. In the experiments, three confinement shapes were simulated: diamond, hourglass, and reversed trapezoid. The experimental results show that the BI_A is strongly proportional to the varying confinement width, i.e., increasing confinement width facilitates the degree of braiding; decreasing confinement width suppresses the degree of braiding. The measured BI_A is proportional to both the ω^* and ω^**. Additionally, increasing Q_in/Q_s causes a slightly decrease of BI_A. The measured active width (W_a) is proportional to the bulk change (V_bulk). These relations all agree with the published trends of both fluvial and submarine braided channels. For the geometric properties of sandbars, the measured sandbar aspect ratio and sandbar compactness ratio remain constant regardless the change of confinement shape or Q_in/Q_s. Finally, the experimental results may aid our understanding to the morphological evolution of submarine braided channels and provide insights to the stacking patterns of hydrocarbon reservoirs.

Keywords: submarine braided channels, turbidity current, physical experiment, confinement shape, active braiding intensity

How to cite: Tsai, Y.-T. and Lai, S. Y. J.: Submarine braided channels in response to channel confinement shapes and inflow-to-sediment discharge ratios: Insights from physical experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6154, https://doi.org/10.5194/egusphere-egu24-6154, 2024.

We document for the first time the extensive occurrence of “feather-like channels” in the 19000-year-old glacigenic submarine strata in the North Sea Fan (NSF). We describe these features in the uppermost deposits of the NSF, predominantly on the surface that marks the end of the period of shelf-edge glaciation, using over 14000 km² of high-resolution 3D seismic reflection data (vertical resolution of 2m and bin size of 6.25 x 18.75m). 

These channels are a few 10’s of meters wide and depths on the limit of seismic resolution (~2 m). They lack clear cross sections, mostly presenting as disruptions in the otherwise readily traceable reflections, which make them easier to identify in amplitude maps rather than structure maps and seismic profiles. These “feather” channels occur exclusively in association with larger, deeper channels. The “feathers” diverge from the margins of the main channel, forming an obtuse angle with the flow direction of the main channel, becoming progressively sub-parallel further downstream, similar to a bird’s feather, with the divergence from the main channel axis in the downstream direction. They run for varied distances, as short as a few 100’s of meters and up to 7 kilometers. It’s also important to highlight that they occur extensively throughout the surface, with a small spacing of 10’s of meters between each other.

Similar features have been described by others as lineations formed at the base of debris flows. This was credited to circular depressions found at the end of such lineations and the fan shape that those features would create at the end of a main channel body. This description is clearly different from what we have described in this study, where both circular depressions and fan-shaped terminations were absent. Here, we interpret them as the record of overbank flows from the main channel, due to their geometry and dimensions, representing the large pulses meltwater coming from the shelf.

Our investigation into the “feather-like channels’’ reveals a unique seismic geomorphology, in a well understood palaeogeographical setting. The exclusive association of these channels with larger, deeper counterparts, their small spacing, and varied distances emphasize their pervasive nature. This research not only refines our understanding of submarine sedimentary dynamics, but also highlights the indispensable role of high-resolution 3D seismic data in understating the subsurface.

How to cite: Machado Garcia, A., Bellwald, B., Planke, S., Anell, I., Myklebust, R., and Midtkandal, I.: Feather-like submarine channels: a unique imprint of overbank flows imaged by 3D seismic data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22150, https://doi.org/10.5194/egusphere-egu24-22150, 2024.