ocean basins

ocean basins The enormous depths of the ocean floor have been known since the days of the Challenger expedition in the nineteenth century. However, the methods used to determine its depth (i.e. sending a weight attached to a line down to the ocean floor 8 km or more below) meant that only a few random measurements could be made. When these spot heights were used to construct contour maps, the ocean floor looked extremely smooth. It was not until echo sounding could be used to give a continuous profile that the ruggedness of the ocean floor was appreciated. These echo sounders showed that not only were some parts of the ocean floor, such as the mid-oceanic ridges, extremely rugged, but, equally surprising, that other parts, such as some basins, were extremely smooth. Another noticeable feature was that much of the ocean floor reaches a depth of about 5–6 km, and, outside the narrow, elongate deep-sea trenches (see island arcs), is no deeper. Now that we know that oceans are formed by the creation of new lithosphere at mid-ocean ridges and that this lithosphere progressively cools and sinks, it is clear that the depth of the ocean floor is limited, unless some other factor is involved. This may be a subduction zone that produces very deep trenches or intraplate volcanism that forms oceanic plateaux and raises the level of the ocean floor significantly above its ‘normal’ (5–6 km) depth by the creation of thickened crust.

Ocean basins are defined as those basins that are underlain by oceanic crust. They have many different forms, large or small, rugged or smooth, and, as with their complementary ridges, hills, and plateaux, have quite diverse origins.

The most spectacular ocean basins are the abyssal plains, a specific type of basin plain located on true ocean floor where any original abyssal hills have now been submerged by the infllling of the deeper areas by sediment. They range in size from very small ones, such as the Alboran in the Mediterranean (2600 km²) through the Madeira (54 000 km²) in the North Atlantic and the Angola in the South Atlantic (1 000 000 km²) to the Enderby in the Antarctic (3 703 000 km²), some thousand times larger than the smallest ones. Because the supply of sediment from the land is an essential condition of their formation, abyssal plains are abundant in the Atlantic and generally absent from the Pacific, where subduction-related trenches and marginal (back-arc) basins entrap most terrigenous sediment except in its north-east corner, adjacent to the North American continent. Their importance around the Antarctic is a reminder that the Antarctic continent supplies enormous volumes of sediment to the oceans, because ice is such an efficient agent of erosion.

Since the principal control on their formation is the blanketing of an original rugged topography by sediment, abyssal plains are distinguished on the basis of:(1) sediment composition (e.g. terrigenous vs. carbonate);(2) whether they are open or closed (i.e. whether sediment-supplying turbidity currents bank up within them or pass out from an outlet;(3) depth, i.e. distance above or below the carbonate compensation depth (CCD);(4) the volume of sediment supplied compared with the area of the basin; and(5) the size of the surrounding drainage area as compared with the basin area.The most intensively studied abyssal plain in the world is the Madeira Abyssal Plain. (It was once considered as a possible site for the disposal of radioactive waste.) It is oversupplied because the area from which sediments are drained (which includes part of the north-west African continent) is 50 times larger than the basin area. This contrasts with the large under-supplied abyssal plains in the western North Atlantic, where the ratio is an order of magnitude less. The basin is fed by turbidity currents from three distinct sources, emplaced about every 20  000 to 30  000 years. Turbidites make up about 90 per cent of the basin sediment (600 km³ over 300 000 years), the largest having a volume of 190 km³. Pelagic sediments comprise only 10 per cent of the total sediment volume.

Dominating the Pacific Ocean are the rugged ocean basins still displaying their original topography of lows and abyssal hills. In these ocean basins, pelagic sediments dominate and, instead of just filling the lows, as in abyssal plains, tend to drape the existing topography with a blanket of sediment. The rate of sedimentation and thickness of the blanket depend primarily on the productivity of the overlying waters. Since most of the basins are below the CCD, the sediments comprise red clay, radiolarian ooze, and diatomaceous ooze. Ferromanganese nodules are common, as are whales' earbones and sharks' teeth. The smooth blanket is often disturbed by local resedimentation from topographic highs into lows. Where the ocean floor is not below the CCD the calcareous skeletal matter is dominated by foraminifera and nannofossils. One of the interesting features of deep-sea drilling is the discovery of a marked cyclicity in carbonate productivity, terrigenous dilution, or carbonate dissolution. The cycles have been matched to the Milankovich timescale, suggesting that there were cyclic changes in CCD and the glacial supply of terrigenous material.

In addition to the relatively large basins in the principal oceans, small ocean basis also occur where incipient or localized ocean-floor spreading takes place. Examples are the Red Sea, the Gulf of California, and some Mediterranean basins; others occur in back-arc settings, particularly around the Pacific, such as the Japan Sea. These small ocean basins are characterized by having a large supply of terrigenous sediment and being above the CCD. Pelagic sediments occur only on the highs and in parts of the basins shielded from the clastic sediments. Hemipelagic sediments (mixed terrigenous and pelagic sediments) abound.

Harold G. Reading

Bibliography

Weaver, P. P. E. and Thomson, J. (eds) (1987) Geology and geochemistry of abyssal plains. Geological Society of London Special Publication No. 15.
Weaver, P. P. E.,, Rothwell R. G.,, Ebbing, J.,, Gunn, D.,, and and Hunter, P. M. (1992) Marine Geology, 109, 1–20.