Basins – A basin is “a depression, in the sea floor, more or less equidimensional in plan and of variable extent” (IHO, 2008).  In this study basins are restricted to seafloor depressions that are defined by closed bathymetric contours.  Basins were mapped based on the identification of the most shoal, closed, bathymetric contours, examined regionally for the major ocean basins and shelf seas.  Basins of the major oceans are nominally bounded by the foot of slope and by the mid-ocean spreading ridges (Wright and Rothery, 1998; Gille et al., 2004).  However, numerous smaller basins of the bathyal and hadal zones, located outside of the major ocean basin areas, were mapped separately, again by identification of the most shoal, closed, 100 m, bathymetric contours.  At abyssal depths we distinguish between major ocean basins, which are large basins (>800 km2), and small basins (<800 km2).  The depths of major ocean basins (defined by the shoalest closed contour that they contain) illustrates that the major ocean basins in the Northwest Pacific are the deepest, at 5,300 m.


Basins mapped by Harris et al., (2014).  The numbers indicate depths of major ocean basins based on the most shallow, closed, bathymetric contour that defines the basin outline, illustrating that the deepest basins are located in the northwest Pacific. 

We also identified basins perched on the slope, again mapped by identification of the most shoal, closed, 100 m, bathymetric contours that defined a discrete basin.   Basins perched on the Antarctic shelf were mapped by identification of the most shoal, closed, 50 m, bathymetric contours that defined a discrete basin (see glacial troughs).  Basins perched on the rest of the world’s shelf areas were mapped by identification of the most shoal, closed, 10 m, bathymetric contours that defined a discrete basin.  These included the basins within shelf seas, glacial troughs and fjord basins found in the higher latitudes.  A key point about basins is that they overlay not only the basal layers, but also other features (i.e. other individual features may occur partly or wholly within a basin or basins).  Basins cover the greatest area across all feature layers, equal to 158,529,660 km2, or 43.8% of the oceans.


Map of basins (cross-hatched) overlain with bottom water dissolved oxygen (DO; taken from NOAA World Ocean Atlas), showing how basins connect zones of high bottom water DO adjacent to source areas in Antarctica and low DO in the northeast Pacific.  Arrows suggest possible pathways of bottom water flow.

Basins and bottom water

The formation of cold, dense water that occupies the abyssal layer of the global ocean occurs in the polar regions of the earth and is driven by two major processes: 1) the cooling of Gulf Stream waters around Iceland to form North Atlantic Deep Water; and 2) the formation of bottom water by sea ice formation and brine rejection.  North Atlantic Deep Water (NADW) is formed as cold surface waters during the winter in the Greenland and Norwegian Seas. This water sinks to fill the basin north of a ridge spanning the distance from Greenland to Scotland.

The coldest bottom water, Antarctic Bottom Water (AABW), is created in winter on the continental shelf of Antarctica by sea ice formation and it is about -1 oC; this highly oxygenated water fills basins in the Pacific Ocean. As the water moves northwards it gets warmer and is gradually depleted of oxygen until it reaches the northeast Pacific.  At this point, at the end of the global ocean conveyor, it contains less than 2 ml/l dissolved oxygen.

Basin statistics (from Harris et al., 2014).  The percentage areas refer to the percentage of ocean region that is basins, except for basins perched on the slope, where the second number (in brackets) refers to the percentage of slope area that is occupied by basins, or for basins perched on the shelf, where the second number (in brackets) refers to the percentage of shelf area that is occupied by basins.

OceanAll basins Area km2All basins Area%All basins numberMajor ocean basins km2Major ocean basins%Major ocean basins number
Arctic Ocean3,809,71029.3199800 
Indian Ocean33,051,13046.4259527,943,29039.27
Mediterranean and Black Sea 1,648,22054.551600 
North Atlantic Ocean17,955,14040.1515313,618,79030.43
North Pacific Ocean34,175,49041.7341727,034,09033.04
South Atlantic Ocean18,056,48044.6158116,858,50041.77
South Pacific Ocean39,533,57045.4239530,971,33035.57
Southern Ocean10,299,94050.713699,267,02045.65
All Oceans158,529,66043.818930125,693,00034.733
OceanLarge basins of seas and oceans km2Large basins of seas and oceans%Large basins numberSmall basins of seas and oceans km2Small basins of seas and ocean%Small basins number
Arctic Ocean Slope/Shelf2,948,76022.74811,0800.085349
Indian Ocean Slope/Shelf4,187,2705.87388212,2900.298872
Mediterranean and Black Sea - Slope/Shelf1,605,68053.1214,6600.15495
North Atlantic Ocean Slope/Shelf2,820,3306.30255194,2400.434848
North Pacific Ocean Slope/Shelf6,439,0307.85370205,3100.250814
South Atlantic Ocean Slope/Shelf1,419,8403.51181130,1200.322541
South Pacific Ocean Slope/Shelf8,037,7009.22559293,7600.3371,002
Southern Ocean Slope/Shelf204,6601.019578,4100.386299
All Oceans Slope/Shelf27,663,2607.641,9171,129,8700.3124,520
OceanBasins perched on slope km2Basins perched on slope%Basins on slope numberBasins perched on shelf km2Basins perched on shelf%Basins on shelf number
Arctic Ocean Slope/Shelf3,8300.0295 (0.419)22846,7806.52 (12.6)1,879
Indian Ocean Slope/Shelf47,4000.0665 (1.13)402150,8000.212 (3.72)927
Mediterranean and Black Sea - Slope/Shelf12,2200.404 (1.35)22825,6600.849 (3.61)172
North Atlantic Ocean Slope/Shelf307,2700.686 (8.72)3371,017,8102.27 (13.9)3,712
North Pacific Ocean Slope/Shelf147,5400.180 (3.04)248343,8100.419 (5.60)1,981
South Atlantic Ocean Slope/Shelf8,3400.0206 (0.523)77153,0100.378 (7.47)777
South Pacific Ocean Slope/Shelf46,3900.0532 (1.43)152182,5500.209 (7.21)676
Southern Ocean Slope/Shelf5,0600.0249 (0.822)41744,8003.66 (27.4)930
All Oceans Slope/Shelf578,0500.160 (2.91)1,5073,465,2200.957 (10.7)11,054

Gille, S.T., Metzger, E.J., Tokmakian, R., 2004. Seafloor topography and ocean circulation. Oceanography 17, 47-54.

Harris, P.T., MacMillan-Lawler, M., Rupp, J., Baker, E.K., 2014. Geomorphology of the oceans. Marine Geology 352, 4-24.

IHO, 2008. Standardization of Undersea Feature Names: Guidelines Proposal form Terminology, 4th ed. International Hydrographic Organisation and Intergovernmental Oceanographic Commission, Monaco, p. 32.

Wright, J., Rothery, D.A., 1998. The Ocean Basins: Their Structure and Evolution, 2 ed. Elsevier Ltd., Open University, Walton Hall, Milton Keynes, MK7 6AA, UK.