FOOT OF THE SLOPE DETERMINED AS THE THE POINT OF MAXIMUM CHANGE OF GRADIENT (CHAPTER 5) Harald Brekke

Foot of the continental slope ”In the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change of gradient at its base” (Paragraph 4 (b)) The CLCS regards the determination of the foot of the continental slope by means of the point of maximum change in the gradient at its base, as the general rule (See 5.1.3)

Fundamental requirements The identification of the region defined as the base of the continental slope The determination of the location of the point of maximum change in gradient at the base of the continental slope (See 5.1.3)

Continental slope definition The CLCS defines the continental slope as the outer portion of the continental margin that extends from the shelf edge to the upper part of the rise or to the deep ocean floor where a rise is not developed. The rise is the wedge-shaped sedimentary body having a smaller gradient than the continental slope (See 5.4.4)

RL /5 EROSJONS- DAL MHe 6-15 Km Sketch of geological principles OUTER PLATEAU Determination of foot of slope Oceanic crust Mid - oceanic ridge Continental Shelf Continental slope Melted mantle Earth's mantle Foot of slope Deep ocean floor Continental margin Crystalline continental crust Sediments Km Continental rise

The base of the continental slope The CLCS defines the base of the continental slope as a region where the lower part of the slope merges into the top of the continental rise, or into the top of the deep ocean floor where a continental rise does not exist (See 5.4.5)

Relevant data types To identify the base of the slope: –Bathymetric data –Geomorphological data –Geological data –Geophysical data To determine the point of maximum change in the gradient: –Bathymetric data (See 5.2.1)

Relevant bathymetric data Single-beam echo soundings Multi-beam echo soundings Hybrid side-scan sonar measurements Interferometric side-scan sonar measurements Seismic reflection-derived bathymetric measurements (See 5.2.2)

Relevant geological and geophysical data In situ samples and measurements Geochemical and radiometric data Reflection and refraction seismic data Grav/mag potential field data Side-scan imagery (See )

Mathematical determination of the foot of the slope The point of maximum change in gradient to be determined by means of mathematical analysis of: –Two-dimensional profiles –Three-dimensional bathymetric models Second derivative-based analyses may be most relevant, but the CLCS will not prescribe any particular method Methods based on a purely visual perception of bathymetric data will not be accepted by the CLCS (See )

Filtering and smoothing Mathematical analyses for determining the point of maximum change in the gradient may require filtering and smoothing of the bathymetric data in order not to enhance noise and/or irrelevant features The CLCS may request full disclosure of all original data and the details of the filtering and smoothing algorithms (See 5.3)

Selection of foot of slope point Among several points of change in gradient at the base of the slope, the point of maximum change is to be selected The selection of any other point (other than the maximum) will be regarded as an exception and must be supported by evidence to the contrary to the general rule (See )