Agulhas Basin

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Location of the Agulhas Basin south of Africa

The Agulhas Basin is a sea ​​basin south of the southern tip of Africa at approximately 42 degrees south. It is located in the southwest Indian Ocean , just on the border with the South Atlantic . The marine ecosystem manifests itself through a great biodiversity as well as some endemic species. The dominant ocean current in the Agulhas Basin is the Agulhas Current . The masses of water, which are quickly transported to the surface, dive to greater depths southeast of the African continent.

geology

Bathymetric map of the Indian Ocean south of South Africa

Geologists at the beginning of the 20th century were already speculating about the reasons for the steep continental slope on the continental margin south of South Africa . One of the earliest publications on this was published in 1938 by LE Kent . In this publication he summarizes previous theories, later publications then mainly referred to Kent's hypothesis. The reason for the rejection in this undersea area was hitherto unknown, but one was horizontal displacement ( strike-slip fault ) or expulsion ( normal faulting ) suspects. With the advancement of the theory of continental drift and the application of this concept to the formation of the South Atlantic , it was above all the French Francheteau and Le Pichon who made significant contributions to today's level of knowledge at the beginning of the 1970s . They justified the steep drop of the continental slope by a horizontal shift in connection with a fracture zone as a result of the Cretaceous opening of the South Atlantic . This shift took place between South Africa and the Falkland Plateau (as part of the South American plate ). This fault zone was established in 1972 by the research vessel RV Atlantis II as part of the International Decade for Marine Research (Idö) ( International Decade of Ocean Exploration ) repeatedly explored, and it was KO Emery , in 1972 the name Agulhas Fracture Zone ( Agulhas Fracture Zone awarded).

The bathymetry south of Africa shows a strikingly sloping continental margin between the flat Agulhas bank and the city of Durban . On the southern continental slope there is a shelf edge that drops abruptly by 100 to 200 meters as well as steeply adjacent slopes (up to 15 °). The base of the continental slope almost immediately goes into the deep over 4000 meters ocean basins of Agulhasbeckens and Transkeibeckens over. The Agulhas Plateau is a rugged oceanic plateau at a depth of approx. 3000 meters that separates the two deep-sea basins and is separated from the continental slope itself by a depression 4700 meters deep .

Scrutton (1972) and Labrecque & Hayes (1979) provide a detailed presentation including explanatory illustrations of the evolution of the Agulhas Basin and adjacent structures .

Agulhas current

The Agulhas Current is an ocean current along the east coast of Africa at approximately 30 degrees south. The Agulhas Current flows in a narrow band (approx. 10 to 15 kilometers) southwards along the steeply sloping continental slope . The stream flows south through the Mozambique Strait . At times larger meandering movements (the so-called natal pulses ) occur. These meanders have an extension of an average of 170 kilometers with downward water mass displacement rates of approx. 21 cm per second. This rate is reduced to 2 inches per second when the continental shelf spreads at about 34 degrees south. From here the Agulhas Current moves away from the coast and moves south along the Agulhas bank . The maximum transport rate of the Agulhas stream occurs near the Agulhasbank, where values ​​from 95 to 136 Sverdrup are achieved.

Marine ecosystem

The mangroves , seagrass and coral reefs in and around the Agulhas Basin reflect a high degree of biodiversity and endemism (e.g. the Zanzibar rockfish , the East African angelfish or the banded parrotfish ). The South African De Hoop Marine Protected Area is an important habitat for numerous dolphins and the southern right whale . There are also numerous species of turtles , such as porpoises , African penguins , great white sharks and abalones .

literature

Individual evidence

  1. ^ LE Kent: The geology of a portion of Victoria County, Natal. In: Trans. Geol. Soc. S. Africa. Volume 41, 1938, pp. 1-36.
  2. ^ J. Francheteau, X. Le Pichon: Marginal fracture zones as structural framework of continental margins in south Atlantic Ocean. In: Bull. Am. Assoc. Petrol. Geol. Vol. 56, 1972, pp. 991-1007. open access
  3. ^ RA Scrutton: Structure and evolution of the sea floor south of South Africa. In: Earth and Planetary Science Letters. Volume 19, No. 2, 1973, p. 250. doi: 10.1016 / 0012-821X (73) 90125-8
  4. ^ RA Scrutton: Structure and evolution of the sea floor south of South Africa. In: Earth and Planetary Science Letters. Volume 19, No. 2, 1973, pp. 250-256. doi: 10.1016 / 0012-821X (73) 90125-8
  5. ^ JL Labrecque, DE Hayes: Seafloor spreading history of the Agulhas Basin. In: Earth and Planetary Sci. Let. Volume 45, 1979, pp. 411-428. doi: 10.1016 / 0012-821X (79) 90140-7

Coordinates: 42 ° 0 ′ 0 ″  S , 21 ° 0 ′ 0 ″  E