Streamlined islands and the English Channel megaflood hypothesis

Recognising ice-age catastrophic megafloods is important because they had significant impact on large-scale drainage evolution and patterns of water and sediment movement to the oceans, and likely induced very rapid, short-term effects on climate. It has been previously proposed that a drainage system on the floor of the English Channel was initiated by catastrophic flooding in the Pleistocene but this suggestion has remained controversial. Here we examine this hypothesis through an analysis of key landform features. We use a new compilation of multi- and single-beam bathymetry together with sub-bottom profiler data to establish the internal structure, planform geometry and hence origin of a set of 36 mid-channel islands. Whilst there is evidence of modern-day surficial sediment processes, the majority of the islands can be clearly demonstrated to be formed of bedrock, and are hence erosional remnants rather than depositional features. The islands display classic lemniscate or tear-drop outlines, with elongated tips pointing downstream, typical of streamlined islands formed during high-magnitude water flow. The length-to-width ratio for the entire island population is 3.4 ± 1.3 and the degree-of-elongation or k-value is 3.7 ± 1.4. These values are comparable to streamlined islands in other proven Pleistocene catastrophic flood terrains and are distinctly different to values found in modern-day rivers. The island geometries show a correlation with bedrock type: with those carved from Upper Cretaceous chalk having larger length-to-width ratios (3.2 ± 1.3) than those carved into more mixed Paleogene terrigenous sandstones, siltstones and mudstones (3.0 ± 1.5). We attribute these differences to the former rock unit having a lower skin friction which allowed longer island growth to achieve minimum drag. The Paleogene islands, although less numerous than the Chalk islands, also assume more perfect lemniscate shapes. These lithologies therefore reached island equilibrium shape more quickly but were also susceptible to total erosion. Our observations support the hypothesis that the islands were initially carved by high-water volume flows via a unique catastrophic drainage of a pro-glacial lake in the southern North Sea at the Dover Strait rather than by fluvial erosion throughout the Pleistocene

Seismic sequence stratigraphy of the Palaeogene offshore of Belgium, southern North Sea

A fine-scale seismic stratigraphic model has been developed for the Palaeogene of the southern North Sea on the basis of interpretation of a dense high-resolution reflection seismic grid, covering the Belgian sector of the continental shelf and the adjacent parts of the Dutch, French and UK sectors. Classical seismic stratigraphic criteria have allowed up to 13 major units to be defined; the geometry and seismic facies characteristics of each have been analysed in detail. The seismic stratigraphy has been compared with the results of four offshore boreholes. 'Events and trends' identified on seismic sections and in outcrops in northern Belgium have been correlated, and offshore seismic facies have been tentatively matched with onshore lithofacies. The geological history of the study area is discussed in terms of eustatic sea level changes and regional tectonic events, and the main characteristics of the offshore Palaeogene deposits are evaluated in a sequence stratigraphic context

On how X-ray (micro) computed tomography on turbidites can help us unravel paleoflow successions, directions and dynamics

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New evidence for important lake-level changes in Lake Baikal during the Last Glaciation

In recent years, a number of estimates have been proposed of fluctuations of the Baikal lake level caused by climate changes. They were mainly based on the interpretation of reflection seismic data from the Selenga delta area (eastern coast of Lake Baikal). These estimates range between 2 m [Colman, 1998] and 600 m [Romashkin et al., 1997]. Better-constrained values of lake-level changes during the last 100 ky were presented by Urabe et al. [2004]. According to their reflection seismic data from the Selenga delta area, the level of Lake Baikal was significantly lower than the present-day level during the two last cold stages (i.e. -45 m during MIS2 and -73 m during MIS4). To precise and verify these values, we carried out an additional high-resolution reflection seismic study in the area of Olkhon Gate (western shore of Lake Baikal). The maximum water depth in this area does not exceed 40 m. The seismic data were collected using two different types of seismic sources: i) a multi-electrode CENTIPEDE sparker with a frequency range of 350-1400 Hz, and ii) the “Sonic-2” seismic system with a frequency range of 2-5 kHz. They allow investigation of the sedimentary record with a resolution of about 1 m (to 300 m depth) and 15-20 cm (to 30 m depth), respectively.Interpretation of these new data allowed distinguishing several seismic units separated by unconformities (erosion surfaces) in the upper part of the seismic profiles. These unconformities could be traced across the entire study area. The uppermost two erosion surfaces are more sharply defined. In the deepest parts of the channel (at 37-40 m water depth) the uppermost unconformity occurs at 5-10 ms below the lake floor, and the second unconformity at 15-20 ms below the lake floor. Both unconformities are interpreted as subaerial erosion surfaces and thus mark a lowstand of the lake level during a prolonged time. For calculation of the thickness of these two units, we used the acoustic logging data from the BDP-98 borehole [BDP Members, 2000]. According these data p-wave velocities vary from 1.6 to 1.8 km/s. The thickness of our upper two seismic units can thus be converted to 4-8 m and 12-16 m, respectively. This implies that the uppermost unconformity occurs at 41-48 m, and the second unconformity at 52-64 m below present-day lake level, which is approximately at the same depth as the two unconformities in the Selenga delta area that were studied by Urabe et al. [2004] and attributed with the MIS2 and MIS4 cold periods, respectively.Our new data thus support the growing amount of evidence of a lowering of the Lake Baikal water level by 40-65 m during glacial/cold periods. The lowstands are probably caused by water redistribution in the Lake Baikal watershed due to climate changes (i.e. glaciation and atmospheric circulation). These data also allow making quantitative assessments of water balance and paleoclimate parameters in the past

A new methodology for quantifying bubble flow rates in deep water using splitbeam echosounders: Examples from the Arctic offshore NW-Svalbard

Quantifying marine methane fluxes of free gas (bubbles) from the seafloor into the water column is of importance for climate related studies, for example, in the Arctic, reliable methodologies are also of interest for studying man-made gas and oil leakage systems at hydrocarbon production sites. Hydroacoustic surveys with singlebeam and nowadays also multibeam systems have been proven to be a successful approach to detect bubble release from the seabed. A number of publications used singlebeam echosounder data to indirectly quantify free gas fluxes via empirical correlations between gas fluxes observed at the seafloor and the hydroacoustic response. Others utilize the hydroacoustic information in an inverse modeling approach to derive bubble fluxes. Here, we present an advanced methodology using data from splitbeam echosounder systems for analyzing gas release water depth (> 100m). We introduce a new MATLAB-based software for processing and interactively editing data and we present how bubble-size distribution, bubble rising speed and the model used for calculating the backscatter response of single bubbles influence the final gas flow rate calculations. As a result, we highlight the need for further investigations on how large, wobbly bubbles, bubble clouds, and multi-scattering influence target strength. The results emphasize that detailed studies of bubble-size distributions and rising speeds need to be performed in parallel to hydroacoustic surveys to achieve realistic mediated methane flow rate and flux quantifications