Table 3. The geomorphic features of the Southern Ocean classified according to the attributes of the seabed surface substratum.

Geomorphic class	Name (map code)	Definition adapted from O’Brien et al. [64]
Continental Shelf and related features	Bank (2)	Broad shallow regions typically at depths of 100–200 m. The boundary between the shelf bank and shelf depressions is set at a depth of around 500. Banks within both the South Georgia and Kerguelen Plateau ecoregions were classed as Oceanic Shallow features.
	Coastal (Rugged)Terrane (5)	Region of varying seafloor type and depth ranges along rugged coastlines
	Cross Shelf Valley (7)	Shelf depressions that are connected to the shelf edge via valleys.
	Shelf (22)	Unclassified regions within the continental shelf region.
	ShelfDeeps-Depressions (23)	Shelf region generally deeper than 550 m with closed contours.
	Volcano (28)	Distinguished from other islands and seamounts, where volcanic processes directly impact the marine environment. Mapped volcanoes within the Southern Ocean all occur on the shelf.
Oceanic Shallow features	Island Arc (9)	Islands formed from bow-shaped volcanic ridges adjacent to subduction zones.
	Island Coastal Terrane (10)	Similar to coastal (rugged) terrane representing a zone of high variability around islands.
	Margin Ridge (13)	Ridges formed from igneous or basement intrusions along the continental margin and protruding hundreds of meters above the (abyssal) sediment plain.
	Marginal Plateau (14)	Areas of relatively level sea floor at mid depth extending from continental margins and separated from the shelf by a saddle.
	Plateau (16)	Relatively flat regions elevated above the surrounding sea floor by more than a few hundred meters.
	Ridge (18)	Elongate ridges that may extend from a plateau or other feature.
	Seamount Ridges (20)	Elongate ridges that protrude hundreds to thousands of meters above the surrounding sea floor. Their shape has the potential to influence deep current activity.
	Seamounts (21)	Roughly circular areas which rise above the surrounding sea floor by at least 1000 m.
Slope and related features	Canyon (3)	A relatively narrow, deep gully with steep sides. Axes were traced along landward contour inflection points, particularly in the shelf edge region.
	Lower Slope (11)	Region on the continental slope of reduced gradient with a lower limit where slope canyons are no longer obvious (around 2500–3500 m below sea level).
	Plateau Slope (17)	Broad regions sloping from the margins of large plateaus to the surrounding deep ocean floor.
	Structural SlopeRegion (24)	Low relief topographic features formed from underlying structures, such as basement protrusions, that extend beyond the lower slope.
	Trough Mouth Fans (26)	Broad aprons of smooth to slightly gullied sediment on the Upper Slope extending from the shelf break to 2500–3000 m water depth.
	Upper Slope (27)	Seaward dipping slope extending from the continental shelf break which is defined as the position at which the rate of change in slope gradient is at a maximum.
Abyss and related features	Abyssal Plain (1)	Extensive, flat, gently sloping or nearly level region of sediment covered seafloor at abyssal depths.
	Cliff (4)	Very steep or near vertical features normally occurring at major crustal fractures or on the sides of glacial valleys on the shelf and are likely to expose hard substrates.
	Contourite Drift (6)	Sediment mounds constructed by strong bottom currents that rise gently above the surrounding sea floor.
	Fracture Zone (8)	Major oceanic crustal fracture zones.
	Mid-Ocean Ridge Rift Valley (12)	Elongate troughs created by seafloor spreading, extending several hundred meters below the rift shoulders and containing hydrothermal vents.
	Ocean Trough (15)	Closed elongate depressions (in the ocean floor) more than 4500 m deep and hundreds of kilometres long, generally associated with fracture zones.
	Rugose Ocean Floor (19)	Relatively young oceanic crust with rugged features protruding through the sediment.
	Trench (25)	Arcuate depressions, typically at depths of more than 5000 m and reaching 6000 m in places, formed by subduction of oceanic crust.