Sills – Sills are “a sea floor barrier of relatively shallow depth restricting water movement between basins” (IHO, 2008). Thus every basin has a sill, over which fluid would escape if the basin were filled to overflowing. The identification of sills in this study is based on selecting contours at a specified interval of 10 m on the shelf (shelf except for Antarctica), 50 m (on the Antarctic shelf) or 100 m (all other areas). Selecting the most shoal, closed contour defines the basin; one contour interval above this typically identifies a discrete location where contours “escape” from the basin and join into the regional bathymetry. This location is mapped as the sill. Sills were mapped for all of the major ocean basins and seas and for the larger basins perched on the continental shelf; sills were not mapped for the smaller basins perched on the slope or shelf or for the smaller abyssal basins.
Sills are geomorphic features of particular significance for oceanography and biogeography. From an oceanographic perspective, sills separate basins that may contain particular water masses of different properties. Sills have already been mentioned in relation to fjords and glaciated shelves, but they also occur on nonglaciated shelves. Ivanov et al (2004) have documented over 60 locations in the ocean associated with cascades of bottom water over sills. Once dense bottom water has accumulated within basins up to the depth of the sill it will eventually overflow the sill. If the basin is perched on the shelf the water may drain down-slope through canyons towards the deep sea. Basins at abyssal depths will simply overflow via a sill into adjacent basins that occur at a greater water depth.
Sills can be a biogeographic barrier for species located in deep basins on either side. The basin environments are vastly different from the shallow water depths, high current energy, and warmer water temperature of the sill environment. The Strait of Gibraltar is a type-case for geomorphic sills. Here, ocean circulation is characterized by a two-layer system: a surface eastward Atlantic water inflow and a deep westward outflow of saline Mediterranean water, with variable interface depth of around 100 m and a sill depth of around 200 m. De Mol et al. (2012) document reef-forming cold-water coral deposits up to 40 m in thickness in the deepest part of the Strait of Gibraltar between 180 and 330 m water depth, which have developed to exploit the unique oceanographic conditions. Coral buildups on the tops of N-S orientated rocky crests are the most common form in the Strait of Gibraltar.