The Sea of the Hebrides is an island-studded region of complex bathymetry on the UK continental shelf, west of the Scottish mainland. An extensive area (2200 km2) of recently collected multibeam bathymetry data, combined with seismic reflection profiles reveal this part of the shelf to have been extensively modified by both glacial and modern processes. Our new geomorphological evidence strongly supports the contention that an ice stream drained ice from western Scotland and the Inner Hebrides towards the Barra Fan at the continental shelf break at the height of the last glaciation (Marine Isotope Stage 2-3). Convergent seabed glacial lineations and other subglacially streamlined features eroded in bedrock around the Islands of Canna and Rum preserve the direction of ice sheet movement, and strongly suggest the onset of ice streaming in a southwesterly direction on the continental shelf in the Sea of the Hebrides region. This fast-flow zone formed part of a larger convergent ice stream system draining much of western Scotland and the north of Ireland-the southern part of which has been postulated already. A number of rock basins, linear troughs and deeps west and south of Muck, kept sediment-free by the dominant modern tidal regime, are interpreted as being at least partly subglacial in origin. Similarly, a large complex of buried deeps between Eigg and Arisaig are interpreted as an overdeepened glacial drainage network. It is suggested that intense bedrock erosion by focused subglacial abrasion and subglacial meltwater discharge over multiple glacial advance and decay cycles led to significant modifications of the pre-Quaternary bedrock surface. Other features such as moraines are only found in shallower water (typically < 50 m), west of Canna, west of Rum and south of Skye. Their relative paucity indicates that the modern tidally dominated environment of the area may have removed much of the localised geomorphological evidence of offshore glacial limits within the Sea of the Hebrides, or alternatively that the ice sheet margin was floating and retreated predominantly by calving leaving little geomorphological evidence of its retreat
Cold-water corals form substantial biogenic habitats on continental shelves and in deep-sea areas with topographic highs, such as banks and seamounts. In the Atlantic, many reef and mound complexes are engineered by Lophelia pertusa, the dominant framework-forming coral. In this study, a variety of mapping approaches were used at a range of scales to map the distribution of both cold-water coral habitats and individual coral colonies at the Mingulay Reef Complex (west Scotland). The new ArcGIS-based British Geological Survey (BGS) seabed mapping toolbox semi-automatically delineated over 500 Lophelia reef 'minimounds' from bathymetry data with 2-m resolution. The morphometric and acoustic characteristics of the minimounds were also automatically quantified and captured using this toolbox. Coral presence data were derived from high-definition remotely operated vehicle (ROV) records and high-resolution microbathymetry collected by a ROVmounted multibeam echosounder. With a resolution of 0.35 9 0.35 m, the microbathymetry covers 0.6 km 2 in the centre of the study area and allowed identification of individual live coral colonies in acoustic data for the first time. Maximum water depth, maximum rugosity, mean rugosity, bathymetric positioning index and maximum current speed were identified as the environmental variables that contributed most to the prediction of live coral presence. These variables were used to create a predictive map of the likelihood of presence of live cold-water coral colonies in the area of the Mingulay Reef Complex covered by the 2-m resolution data set. Predictive maps of live corals across the reef will be especially valuable for future long-term monitoring surveys, including those needed to understand the impacts of global climate change. This is the first study using the newly developed BGS seabed mapping toolbox and an ROV-based microbathymetric grid to explore the environmental variables that control coral growth on cold-water coral reefs.
Pockmarks are indicators of focused fluid seepage, most notably gas such as methane, and can occur in vast numbers in many marine and even in lacustrine environments. The presence and distribution of pockmarks need to be considered in the development of any infrastructure at the seabed. However, manual mapping of these features can be extremely timeconsuming and it is implicitly subjective. An extensive area in the central North Sea, where the seabed comprises a thick sequence of muds and sandy muds of the late glacial Witch Ground Formation, shows numerous inactive pockmarks, typically 20-100 m diameter and 3-4 m deep. Within this area a few larger, active pockmarks, 500 m diameter and up to 17 m deep are known. Modern site investigations in this area regularly include multibeam sonar mapping, a technique that collects large volumes of bathymetric data that can be used to produce digital depth models of the seafloor with sufficient resolution to characterize individual pockmarks. This paper presents a semi-automated method to recognize, spatially delineate and characterise morphometrically pockmarks at the seabed. The method comprises two scripts, Pockmark Mapping and Pockmark Characterization, that allow the systematic application of a sequence of well defined tools available within the ESRI ArcGIS toolbox. Almost 4150 pockmarks were mapped applying this method to 18 selected site surveys across the central North Sea. The mapping and morphometric characterization of such vast number of pockmarks allows the identification of certain trends reflecting the hydrodynamic regime, whereas the pockmarks density and spatial distribution appears to be attributable to differences in shallow gas availability and deeper geology controlling fluid migration pathways.
Pockmarks are seabed depressions developed by fluid flow processes that can be found in vast numbers in many marine and lacustrine environments. Manual mapping of these features based on geophysical data is, however, extremely time-consuming and subjective. Here, we present results from a semi-automated mapping toolbox developed to allow more efficient and objective mapping of pockmarks. This ArcGIS-based toolbox recognizes, spatially delineates, and morphometrically describes pockmarks. Since it was first developed, the toolbox has helped to map and characterize several thousands of pockmarks on the UK continental shelf, especially within the central North Sea. This paper presents the latest developments in the functionality of the toolbox and its adaptability for application to other geographic areas (Barents Sea, Norway, and Malin Deep, Ireland) with varied pockmark and seabed morphologies, and in different geological settings. The morphometric characterization of vast numbers of pockmarks allows an unprecedented statistical analysis of their morphology. The outputs from the toolbox provide an objective, quantitative baseline for combining this information with the geological and oceanographical knowledge of individual areas, which can provide further insights into the processes responsible for their development and their influence on local seabed conditions and habitats.The variability of size, spatial distribution, and geometry results from their development depending on a variety of parameters such as fluid type, flow fluxes, thickness and nature of near bottom sediments, underlying structure, and lithology.Most of the initial descriptions of these features were based on low penetration seismic profiles (mainly Boomer system) and/or from sidescan sonar data (e.g., [1,4,8]). Presently, pockmarks are predominantly mapped manually using seabed digital terrain models (DTM) created from MBES data (e.g., [6]), generally a time-consuming task. In addition, delineating their individual boundaries is subjective and consistency of criterion is hard to achieve. To address this, the British Geological Survey (BGS) developed a semi-automated mapping toolbox. This ArcGIS-based BGS Seabed Mapping Toolbox recognizes, spatially delineates, and morphometrically describes seabed features including pockmarks [9], coral mounds [10], and other confined features. The toolbox is embedded in ESRI ® ArcGIS, a geographic information system (GIS) widely used in the field of marine geology, allowing users to work in a familiar and integrated mapping environment. Furthermore, the scripts developed use standard ESRI ® algorithms that increase the clarity of the steps taken during delineation and characterization processes. With this approach, human interaction and expert knowledge is still part of the mapping process but is limited to restricting criteria for feature mapping. This allows multiple mapping exercises to be performed with the same criterion, improving comparisons across different areas or quantification of seabed changes ove...
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