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Suzanne Maclachlan, National Oceanography Centre, Southampton (United Kingdom)
Gavin Elliott, National Oceanography Centre, Southampton (United Kingdom)
Alan Evans, National Oceanography Centre, Southampton (United Kingdom)
Lindsay Parson, National Oceanography Centre, Southampton (United Kingdom)
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A key component of delineating juridical continental shelf beyond 200M, according to UNCLOS Article 76, is the identification of the foot of the (continental) slope (FOS). Once the FOS positions are determined either (or both) of the two formulae i.e. FOS+60M or the 1 % sediment thickness criteria, can be applied to identify outer limit positions. In order to determine the precise position of the FOS it is necessary to understand the geometry of the distal part of the geological continental margin. On many margins the morphological boundary between the slope and the rise is easily located, and the identification of the point of maximum change in gradient is a useful procedure to generate FOS positions. However, along many margins where the complex geometries preclude such analysis, the extent of the continental shelf may have to be derived more accurately from other information.
The transition between the continental margin and the deep ocean can be characterised by a large variety of seafloor features. In some cases it is an abrupt boundary where the stratified surface sediments of the abyssal plain onlap a well-defined lower slope. In others it is a morphologically complex transition zone where the low gradient ocean floor merges into an ill-defined lower slope region. The sedimentary drape at the foot of the slope normally consists of material derived from the shelf and upper slopes. However, whilst downslope derived sediment dominates, alongslope contour current deposition and accumulation of pelagic oozes are significant components along many of the world's continental margins. These deposits frequently produce morphologically complex transition zones which can complicate FOS determination.
The interplay between the various processes described above can be illustrated using the NW Hatton Bank margin as a case study area. Here a complex slope morphology has resulted from the interaction of contour current sedimentation with a structurally controlled seabed topography. The foot of the slope region is characterised by both depositional (sediment waves, drifts) and erosional (moats, furrows) features created by bottom currents, all of which may contribute to multiple or anomalous FOS positions. In addition, a deep sea sedimentary channel system, the Maury Channel is located adjacent to the foot of the slope region. Large-scale turbidity currents derived from Iceland have incised a channel adjacent to the Hatton slope, which can produce a maximum change in gradient not representative of the ?true' FOS positions. Similarly, the relief of seamounts and other basement structures has influenced the sediment deposition and local slope gradient.
In summary, the correct interpretation of seafloor bathymetry and geological processes down and across the margin is a critical preliminary phase for the correct identification of the foot of the slope, in the pursuit of an accurate delineation of the outer limit of the continental shelf.
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