Agulhas Basin

The Agulhas Basin is an oceanic basin located south of South Africa where the South Atlantic Ocean and south-western Indian Ocean meet. Part of the African Plate, it is bounded by the Agulhas Ridge (part of the Agulhas-Falkland Fracture Zone) to the north and the Southwest Indian Ridge to the south; by the Meteor Rise to the west and the Agulhas Plateau to the east. Numerous bathymetric anomalies hint at the basin's dynamic tectonic history.[1]

Agulhas Basin NOAA
The Agulhas Basin and some of the bathymetric structures mentioned in the text

Geology

In a Late Paleocene (59-56 Ma) reconstruction of the opening of the South Atlantic Ocean (i.e. the separation of South America and Africa during the Gondwana break-up) the Meteor Rise lies conjugate to the Islas Orcadas Rise (east of the Falkland Plateau).[1] The separation of the Meteor Rise and the Islas Orcadas Rise marks the beginning of the formation of the Agulhas Basin.

The Agulhas Ridge extends from the northern tip of the Meteor Rise towards the Agulhas Bank south of South Africa. The ridge, however, ends abruptly in a small plateau at 40°S 15°E / 40°S 15°E where it intersects a northeastward-trending spreading centre (the Agulhas Rift) that was abandoned during the Early Paleocene (61 Ma).[1] The presence of a short-lived tectonic plate between these structures was first proposed by LaBrecque & Hayes 1979. They named it the Malvinas Plate and proposed that it was active from 90 Ma until the spreading ceased in the Agulhas Basin at 65 Ma. The plate is located at a proto-Bouvet Triple Junction.[2]

Marks & Stock 2001 found that Late Cretaceous (100-66 Ma) fracture zones generated on the Agulhas Rift do not align with those north of the Agulhas Fracture Zone and cannot therefore have been formed by the spreading of South America and Africa. Furthermore, magnetic anomalies on the Malvinas Plate do not align with their conjugates on the African Plate if the spreading rates and directions of South America and Africa are used as a guide. They also noted that the Agulhas Fracture Zone do not lay perpendicular to traces of the South America-Africa spreading north of it and cannot, therefore, have been generate by this spreading.[3]

The Agulhas Rift is the abandoned Malvinas-Africa ridge crest. 97 Ma the plate boundary in the Agulhas Basin was reorganised when the Mid-Atlantic Ridge made an eastward jump. This brought the boundary towards the Agulhas Plateau where excessive volcanism was building a large igneous province. The inception of the Malvinas Plate accompanied this shortening of the Agulas Fracture Zone. 61 Ma, the Malvinas Plate was finally incorporated into the African Plate when the Malvinas-Africa ridge was abandoned as the result of a westward ridge jump along the Agulhas-Falkland Fracture Zone.[4] This second ridge jump reduced one of the most spectacular fracture zones in Earth's history — 1,200 km (750 mi) in length — to 180 km (110 mi).[5]

Oceanography

The Agulhas Current flows south along the African east coast. When it reaches the southern tip of Africa, it retroflects back into the Indian Ocean. At this retroflection it leakes warm core eddies known as Agulhas rings into the South Atlantic. This mesoscale anti-cyclonic rings feed the Atlantic meridional overturning circulation (AMOC) and are therefore believed to affect the global climate, although the connection between the leakage, AMOC, and climate change is still poorly understood.[6] In the Agulhas basin, half of these rings are subdivided one or several times. A majority of the subdivided rings split at or near the bathymetric obstacles on the western side of the Agulhas Basin but almost a quarter of them also merge there either because of the obstacles or because of intense interaction with other rings.[7]

Antarctic Bottom Water (AABW) originated in the Oligocene with the opening of the Drake Passage and the Tasmanian Seaway and resulted in the thermal insulation of Antarctica. AABW mixes with other masses in the Antarctic Circumpolar Current (ACC) to form the Circumpolar Deep Water (CDW). In the Agulhas Basin CDW flows northward and is deflect mostly westward by the Agulhas Ridge. A branch of CDW, however, enters the Cape Basin west of the ridge from where it flows west along the northern side of the ridge before being deflected north-east (along the Walvis Ridge) at the eastern end of the ridge. This long detour in the Cape Basin and mixing with North Atlantic Deep Water (NADW) results in warmer water than other CDW masses. Antarctic Intermediate Water (AAIW) flows above the CDW in the Agulhas Basin in an anti-cyclonic path (in contrast to the cyclonic path followed by CDW.)[8]

References

Notes

  1. ^ a b c Raymond & LaBrecque 1988, Introduction, p. 27
  2. ^ Raymond & LaBrecque 1988, Agulhas Fracture Zone Ridge, pp. 28-29
  3. ^ Marks & Stock 2001, Abstract; Marks 2001
  4. ^ Pérez-Díaz & Eagles 2014, 4.5 The Short Life of a Small Plate in the South Atlantic (80 Ma to Present), p. 19; Figs. 13-14, pp. 16-17
  5. ^ Bird 2001, Agulhas-Falkland Fracture Zone, p. 152
  6. ^ Beal et al. 2011, p. 1
  7. ^ Dencausse, Arhan & Speich 2010, "Initial rings" and "sub-rings", pp. 6-7
  8. ^ Schut, Uenzelmann-Neben & Gersonde 2002, Oceanography, p. 187

Sources

  • Beal, L. M.; De Ruijter, W. P. M.; Biastoch, A.; Zahn, R.; SCOR/WCRP/IAPSO Working Group 136 (2011). "On the role of the Agulhas system in ocean circulation and climate". Nature. 472: 429–436. doi:10.1038/nature09983. PMID 21525925.
  • Bird, D. (2001). "Shear margins: Continent-ocean transform and fracture zone boundaries" (PDF). The Leading Edge. 20 (2): 150–159. doi:10.1190/1.1438894. Retrieved 4 January 2015.
  • Dencausse, G.; Arhan, M.; Speich, S. (2010). "Routes of Agulhas Rings in the southeastern Cape Basin" (PDF). Deep-Sea Research Part I: Oceanographic Research Papers. 57 (11): 1406–1421. doi:10.1016/j.dsr.2010.07.008. Retrieved 10 July 2015.
  • LaBrecque, J. L.; Hayes, D. E. (1979). "Seafloor spreading history of the Agulhas Basin". Earth and Planetary Science Letters. 45 (2): 411–428. Bibcode:1979E&PSL..45..411L. doi:10.1016/0012-821X(79)90140-7.
  • Marks (2001). "Malvinas plate controversy resolved using altimetry" (PDF). Earth System Monitor. 11 (4): 1–2, 4. Retrieved 10 July 2015.
  • Marks, K. M.; Stock, J. M. (2001). "Evolution of the Malvinas Plate South of Africa". Marine Geophysical Researches. 22 (4): 289–302. doi:10.1023/A:1014638325616.
  • Pérez-Díaz, L.; Eagles, G. (2014). "Constraining South Atlantic growth with seafloor spreading data" (PDF). Tectonics. 33: 1848–1873. doi:10.1002/2014TC003644. Retrieved 10 July 2015.
  • Raymond, C. A.; LaBrecque, J. L. (1988). "Geophysical signatures of the Agulhas Fracture Zone Ridge and Meteor Rise, Indo-Atlantic basin" (PDF). Proceedings of the Ocean Drilling Program, Initial Reports. 114: 27–33. Retrieved 10 July 2015.
  • Schut, E. W.; Uenzelmann-Neben, G.; Gersonde, R. (2002). "Seismic evidence for bottom current activity at the Agulhas Ridge" (PDF). Global and Planetary Change. 34: 185–198. doi:10.1016/s0921-8181(02)00114-5. Retrieved 10 July 2015.

Coordinates: 45°S 20°E / 45°S 20°E

Agulhas Current

The Agulhas Current is the western boundary current of the southwest Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary current in the world ocean, with an estimated net transport of 70 Sverdrups (Sv, millions m3/s), as western boundary currents at comparable latitudes transport less — Brazil Current (16.2 Sv), Gulf Stream (34 Sv), Kuroshio (42 Sv).

Agulhas Passage

The Agulhas Passage is an abyssal channel located south of South Africa between the Agulhas Bank and Agulhas Plateau. About 50 km (31 mi) wide, it connects the Natal Valley and Transkei Basin in the north to the Agulhas Basin in the south and is the only near-shore connection between the south-western Indian Ocean and South Atlantic Ocean.

Agulhas Plateau

The Agulhas Plateau is an oceanic plateau located in the south-western Indian Ocean about 500 km (310 mi) south of South Africa. It is a remainder of a large igneous province (LIP), the Southeast African LIP, that formed 140 to 95 million years ago (Ma) at or near the triple junction where Gondwana broke-up into Antarctica, South America, and Africa. The plateau formed 100 to 94 Ma together with Northeast Georgia Rise and Maud Rise (now located near the Falkland Island and Antarctica respectively) when the region passed over the Bouvet hotspot.

Agulhas Return Current

The Agulhas Return Current (ARC) is an ocean current in the South Indian Ocean. The ARC contributes to the water exchange between oceans by forming a link between the South Atlantic Current and the South Indian Ocean Current. It can reach velocities of up to 4 knots (7.4 km/h; 4.6 mph) and is therefore popular among participants in trans-oceanic sailing races.

Antarctic bottom water

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F), salinities from 34.6 to 34.7 psu. Being the densest water mass of the oceans, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level.The major significance of Antarctic bottom water is that it is the coldest bottom water, giving it a significant influence on the movement of the world's oceans. Antarctic bottom water also has a high oxygen content relative to the rest of the oceans' deep waters. This is due to the oxidation of deteriorating organic content in the rest of the deep oceans. Antarctic bottom water has thus been considered the ventilation of the deep ocean.

Indian Ocean

The Indian Ocean is the third-largest of the world's oceanic divisions, covering 70,560,000 km2 (27,240,000 sq mi) (19.8% of the water on the Earth's surface). It is bounded by Asia on the north, on the west by Africa, on the east by Australia, and on the south by the Southern Ocean or, depending on definition, by Antarctica.

List of marine bony fishes of South Africa

This is a sublist of the List of marine fishes of South Africa for bony fishes recorded from the oceans bordering South Africa.

This list comprises locally used common names, scientific names with author citation and recorded ranges. Ranges specified may not be the entire known range for the species, but should include the known range within the waters surrounding the Republic of South Africa.

List ordering and taxonomy complies where possible with the current usage in Wikispecies, and may differ from the cited source, as listed citations are primarily for range or existence of records for the region.

Sub-taxa within any given taxon are arranged alphabetically as a general rule.

Details of each species may be available through the relevant internal links. Synonyms may be listed where useful.

Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic group of fish that have skeletons primarily composed of bone tissue, as opposed to cartilage. The vast majority of fish are members of Osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, and over 435 families and 28,000 species. It is the largest class of vertebrates in existence today.

The group Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years old, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.

List of submarine topographical features

This is a list of submarine topographical features, oceanic landforms and topographic elements.

Southwest Indian Ridge

The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the African Plate to the north from the Antarctic Plate to the south, the SWIR is characterised by ultra-slow spreading rates (only exceeding those of the Gakkel Ridge in the Arctic) combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues (20°30′S 70°00′E) in the Indian Ocean and Bouvet (54°17′S 1°5′W) in the Atlantic Ocean.

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