Basins

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.

Ocean	All basins Area km2	All basins Area%	All basins number	Major ocean basins km2	Major ocean basins%	Major ocean basins number
Arctic Ocean	3,809,710	29.3	1998	0	0
Indian Ocean	33,051,130	46.4	2595	27,943,290	39.2	7
Mediterranean and Black Sea	1,648,220	54.5	516	0	0
North Atlantic Ocean	17,955,140	40.1	5153	13,618,790	30.4	3
North Pacific Ocean	34,175,490	41.7	3417	27,034,090	33.0	4
South Atlantic Ocean	18,056,480	44.6	1581	16,858,500	41.7	7
South Pacific Ocean	39,533,570	45.4	2395	30,971,330	35.5	7
Southern Ocean	10,299,940	50.7	1369	9,267,020	45.6	5
All Oceans	158,529,660	43.8	18930	125,693,000	34.7	33
Ocean	Large basins of seas and oceans km2	Large basins of seas and oceans%	Large basins number	Small basins of seas and oceans km2	Small basins of seas and ocean%	Small basins number
Arctic Ocean Slope/Shelf	2,948,760	22.7	48	11,080	0.0853	49
Indian Ocean Slope/Shelf	4,187,270	5.87	388	212,290	0.298	872
Mediterranean and Black Sea - Slope/Shelf	1,605,680	53.1	21	4,660	0.154	95
North Atlantic Ocean Slope/Shelf	2,820,330	6.30	255	194,240	0.434	848
North Pacific Ocean Slope/Shelf	6,439,030	7.85	370	205,310	0.250	814
South Atlantic Ocean Slope/Shelf	1,419,840	3.51	181	130,120	0.322	541
South Pacific Ocean Slope/Shelf	8,037,700	9.22	559	293,760	0.337	1,002
Southern Ocean Slope/Shelf	204,660	1.01	95	78,410	0.386	299
All Oceans Slope/Shelf	27,663,260	7.64	1,917	1,129,870	0.312	4,520
Ocean	Basins perched on slope km2	Basins perched on slope%	Basins on slope number	Basins perched on shelf km2	Basins perched on shelf%	Basins on shelf number
Arctic Ocean Slope/Shelf	3,830	0.0295 (0.419)	22	846,780	6.52 (12.6)	1,879
Indian Ocean Slope/Shelf	47,400	0.0665 (1.13)	402	150,800	0.212 (3.72)	927
Mediterranean and Black Sea - Slope/Shelf	12,220	0.404 (1.35)	228	25,660	0.849 (3.61)	172
North Atlantic Ocean Slope/Shelf	307,270	0.686 (8.72)	337	1,017,810	2.27 (13.9)	3,712
North Pacific Ocean Slope/Shelf	147,540	0.180 (3.04)	248	343,810	0.419 (5.60)	1,981
South Atlantic Ocean Slope/Shelf	8,340	0.0206 (0.523)	77	153,010	0.378 (7.47)	777
South Pacific Ocean Slope/Shelf	46,390	0.0532 (1.43)	152	182,550	0.209 (7.21)	676
Southern Ocean Slope/Shelf	5,060	0.0249 (0.822)	41	744,800	3.66 (27.4)	930
All Oceans Slope/Shelf	578,050	0.160 (2.91)	1,507	3,465,220	0.957 (10.7)	11,054

References

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.