• NEANIAS Underwater Research Community

Abstract Over the years, hurricane track forecasts and storm surge models, as well the digital terrain and bathymetry data they depend on, have improved significantly. Strides have also been made in the knowledge of the detailed variation of the surface wind field driving the surge. The area of least improvement has been in obtaining data on the temporal/spatial evolution of the mound of water that the hurricane wind and waves push against the shore to evaluate the performance of the numerical models. Tide gauges in the vicinity of the landfall are frequently destroyed by the surge. Survey crews dispatched after the event provide no temporal information and only indirect indications of the maximum water level over land. The landfall of Hurricane Bonnie on 26 August 1998, with a surge less than 2 m, provided an excellent opportunity to demonstrate the potential benefits of direct airborne measurement of the temporal/spatial evolution of the water level over a large area. Despite a 160-m variation in aircraft altitude, an 11.5-m variation in the elevation of the mean sea surface relative to the ellipsoid over the flight track, and the tidal variation over the 5-h data acquisition interval, a survey-quality global positioning system (GPS) aircraft trajectory allowed the NASA scanning radar altimeter carried by a NOAA hurricane research aircraft to demonstrate that an airborne wide-swath radar altimeter could produce targeted measurements of storm surge that would provide an absolute standard for assessing the accuracy of numerical storm surge models.

Lake Afdera is a hypersaline endorheic lake situated at 112 m below sea-level in the Danakil Depression. The Danakil Depression is located in the northern part of the Ethiopian Afar and features an advanced stage of continental rifting. The remoteness and inhospitable environment explain the limited scientific research and knowledge about this lake. Bathymetric data were acquired during 2 weeks expeditions in January/February 2016 and 2017 using an easily deployable echosounder system mounted on an inflatable motorized boat. This study presents the first complete bathymetric map of the lake Afdera. Bathymetric results show that the lake has an average depth of 20.9 m and a total volume of 2.4 km3. The maximum measured depth is 80 m, making Lake Afdera the deepest known lake in Afar and the lowest elevation of the Danakil Depression. Comparison with historical reports shows that the lake level did not fluctuate significantly during the last 50 years. Two distinct tectonic basins to the north and the south are recognized. Faults of different orientations control the morphology of the northern basin. In contrast, the southern basin is affected by volcano-tectonic processes, unveiling a large submerged caldera. Comparison between the orientation of faults throughout the lake with the regional fault pattern indicates that the lake is part of two transfer zones: the major Alayta–Afdera Transfer Zone and the smaller Erta Ale–Tat’Ali Transfer Zone. The interaction between these Transfer Zones and the rift axis forms the equivalent of a developing nodal basin which explains the lake’s position as the deepest point of the depression. This study provides evidence for the development of an incipient transform fault on the floor of the Afar depression.

Abstract Carbonate sedimentary successions that developed on isolated oceanic islands typically comprise a series of unconformity-bounded packages of strata that reflect eustatic sea level changes superimposed on local tectonic movements. Resolving the subsidence and/or uplift of these islands, which are often assumed to have simple tectonic histories, is challenging because the tectonic movements are commonly of similar magnitudes to the eustatic oscillations. The uncertainty associated with each of the components involved in the construction of subsidence diagrams (e.g., age constraints, decompaction, eustatic sea level curves, paleobathymetry), therefore, introduces significant error margins when assessing the tectonic histories of isolated carbonate platforms. By using two end-member subsidence diagrams for the Paleogene to Neogene successions on Grand Cayman and Cayman Brac, it can be shown that their subsidence rates were heterogeneous over time and that the evolution of these islands vary significantly even though they are situated in the same basin. Although these islands, located 150 km apart, were subject to uniform changes in eustatic sea level, they have different stratigraphic architectures owing to their independent tectonic histories. From the Oligocene to the late Pliocene, the tectonic histories of Grand Cayman and Cayman Brac were analogous, and they subsided at a rate of 5.6 to 9.9 m/Myr. From the late Pliocene to ~400 ka, however, northeast Cayman Brac was uplifted by 165 m and tilted with a rotational axis offshore from the southwest end of the island, whereas Grand Cayman was uplifted by ~10 m with no rotational component. The results of this study challenge the assumption that isolated carbonate platforms have simple tectonic histories, while exploring and highlighting the common problems that are encountered with the construction of subsidence diagrams.

This paper presents an adaptive bathymetric estimation algorithm (ABE) for use with forward-looking hull mounted sonar in shallow water environments. In addition to providing improved positional estimates of ocean bottom contacts in front of the host vehicle, it will automatically estimate and adapt to changes in the local sound speed. Preliminary simulation results are given for a simple two-dimensional encounter of a single bottom contact using an isovelocity, straight line ray model.

The topographic structure of the ocean bottom is investigated at different scales of resolution to answer the question: Can the seafloor be described as a fractal process? Methods from geostatistics, the theory of regionalized variables, are used to analyze the spatial structure of the ocean floor at different scales of resolution. The key to the analysis is the variogram criterion: Self-similarity of a stochastic process implies self-similarity of its variogram. The criterion is derived and proved here: it also is valid for special cases of self-affinity (in a sense adequate for topography). It has been proposed that seafloor topography can be simulated as a fractal (an object of Hausdorff dimension strictly larger than its topological dimension), having scaling properties (self-similarity or self-affinity). The objective of this study is to compare the implications of these concepts with observations of the seafloor. The analyses are based on SEABEAM bathymetric data from the East Pacific Rise at 13°N/104°W and at 9°N/104°W and use tracks that run both across the ridge crest and along the ridge flank. In the geostatistical evaluation, the data are considered as a stochastic process. The spatial continuity of this process is described by variograms that are calculated for different scales and directions. Applications of the variogram criterion to scale-dependent variogram models yields the following results: Although the seafloor may be a fractal in the sense of the definition involving the Hausdorff dimension, it is not self-similar, nor self-affine (in the given sense). Mathematical models of scale-dependent spatial structures are presented, and their relationship to geologic processes such as ridge evolution, crust formation, and sedimentation is discussed.

Currently, due to the global change and biodiversity loss, the knowledge of the species taxonomy and distribution is more than ever crucial. Especially in areas as the Avilés canyon systems (ACS), which is a Site of Community Importance (SCI) included in the Natura 2000 Network. A check-list of the echinoderm species belonging to the class Holothuroidea (a group that has been less studied and with scarce literature available) from the ACS was carried out, as well as a characterization of the holothurian fauna and its distribution. Samples were collected from both the continental shelf, slope and bathyal zones of the Asturian central coast (COCACE cruise, 1987–88), and the slope, bathyal and abyssal areas of the Avilés canyon systems (BIOCANT cruises, 2012–13). The identification of the species was based on morphological traits as well as on the observation of the microscopic calcareous ossicles extracted from different parts of the holothurian body. One hundred and seventy four specimens, belonging to 35 species of the five orders of the class Holothuroidea, were identified. Multivariate analysis allowed the differentiation of four main assemblages, which corresponded to abyssal plain, lower continental slope, upper continental slope, and continental shelf. Depth was the main structuring agent. Holothurian species richness was higher in deep-sea areas, the abyssal plain being the area with the highest number of species found.

International audience; A unique late Glacial–Preboreal record of changes in sea-level and sediment fluxes originating from the Alps is recorded in the Rhône subaqueous delta in the Western Mediterranean Sea. The compilation of detailed bathymetric charts, together with high-resolution seismic profiles and long cores, reveals the detailed architecture of several sediment lobes, related to periods of decreased sea-level rise and/or increased sediment flux. They are situated along the retreat path of the Rhône distributaries, from the shelf edge and canyon heads up to the modern coastline. They form transgressive backstepping parasequences across the shelf, the late Holocene (highstand) deltas being confined to the inner shelf. The most prominent feature is an elongated paleo-shoreface/deltaic system, with an uppermost sandy fraction remolded into subaqueous dunes. A long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system. In seismic data, it displays aggradation, starting at not, vert, similar 15 cal kyr BP, followed by progradation initiated during the first phase of the Younger Dryas, a period of reduced sea-level rise or stillstand. The delta kept pace with resumed sea-level rise during the Preboreal (which is estimated at about 1 cm/yr), as a result of increased sediment supply from the Alps (melting of glaciers and more humid climate “flushing” the sediment down to the sea). Abandonment of the delta occurred around 10,500 cal yr BP, that is to say about 1000 yr after the end of the Younger Dryas, probably because of decreased sediment flux.

Glide planes, the basal surface or failure surface upon which submarine landslides initiate, commonly develop along weak, distinctive stratigraphic horizons but their lithological/mechanical characteristics and genetic mechanisms remain largely unknown. We use 2-D multi-channel seismic reflection data, integrated with multibeam bathymetry and deep drilling data from the Ulleung Basin margins, East (Japan) Sea, to: (1) identify and characterize the nature of glide planes associated with submarine landslides; (2) understand the influence of climate-modulated factors in preconditioning slope failures; and (3) document the post-failure evolution of the landslides. 24 glide planes were identified among 38 submarine slides (SL1 – SL38), which correspond to regionally continuous, positive-polarity high-amplitude seismic reflections. Well-seismic integration support ca. 340 ka – 1,200 ka ages of formation of the major glide planes in the southwestern and western margins of the basin. These glide planes developed at the interface between clay-rich sediment deposited during glacial periods and biogenic diatom-rich sediments deposited during interglacial periods. Physical, mineralogical and geochemical properties determined by density, porosity, gamma-ray, shear strength, X-ray diffraction, and X-ray fluorescence data reveal significant lithological and mechanical changes at the interface between these two lithologies. We therefore infer that these interfaces dictate the position of failure surfaces, with the diatom-rich layers acting as a weak layer. Excess pore pressure in these layers is likely due to initial high-water contents (up to 75%) and high compressibility; this is considered an important pre-condition for failure. In contrast, the glide planes along the northwestern margin of the Ulleung Basin (SL34 – 37) are older (ca. 1,200 ka – 2,140 ka). Seismic data further reveal three distinct contrasting styles of landslide post-failure behavior throughout the margins: (1) evacuated slide scars with areas of smooth seafloor; (2) slide scars with residual debris consisting of blocky sediments; and (3) slide scars with buried intact sediment blocks in front of the headwalls. Lateral variability of fluid flow, sediment composition, and mechanical properties of basal ‘weak’ layer(s), or the magnitude of earthquakes may have contributed to forming different types of mass-transport deposits (MTDs). Overall, these results show that landslide formation in the East (Japan) Sea result from a complex climatic, volcanic and tectonic interplay that controlled the formation of weak layers. Some of these layers extend regionally and can be identified and mapped by remote geophysical methods and targeted drilling This study was supported by ‘Geological survey in the Korean Peninsula and publication of the geological maps’ Project (GP2020-009) funded by the Ministry of Knowledge Economy (MKE; currently Ministry of Trade, Industry and Energy: MOTIE), Korea, and the research fund of the Chungnam National University. D. Cukur was supported by the KIGAM project (research fund number: 22-3111-2). S.H. Lee is supported by the KIOST Basic Project (PE99941). R.U. is supported by project PID2020-114856RB-100 / AEI / 10.13039/501100011033 24 pages, 18 figures, 2 tables, supplementary material https://doi.org/10.1016/j.margeo.2022.106956.-- Data availability statement: Supporting of the data were provided by the Korea Institute of Geoscience and Mineral Resources (KIGAM) under confidential status and the restrictions do not allow open sharing of the proprietary data used in this research. The data can be available upon reasonable request made to the authors with permission from the KIGAM With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) Peer reviewed

Waves at sea limit both the resolution and accuracy of bathymetric surveys. Vessel roll introduces errors difficult to correct. By limiting operations to calm days according to criteria given, these errors can be controlled. Heave and roll appear as wiggles on the record, masking bottom irregularities. Speed limitations are given to assure seeing irregularities of a given size. Three tidal datum plan are defined, and indirect methods of obtaining offshore tidal data are described.

Abstract A three-dimensional Princeton Ocean Model has been configured for the east coast of India to study the circulation and coastal upwelling during the pre-monsoon season of 2000. The model uses an orthogonal curvilinear grid and a terrain-following sigma coordinate in the vertical with monthly data fields of temperature and salinity from “Levitus94.” The model is applied to study the coastal ocean processes and its variability on weekly timescales off the east coast of India. The forcing in the model is the real-time weekly averaged wind stress obtained from Quik Scattero-meter/National Center for Environmental Prediction blended wind of May 2000. The discernable variability in the winds of May 2000 over the weekly timescale influenced the surface layers reflected in the 3-day mean satellite images. To simulate this feature and to understand the dynamics, numerical experiments are carried out with the associated real-time winds. Both diagnostic and prognostic computations are used for this, and the ...