Agulhas Bank

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Agulhas Bank
Map of the Agulhas Bank
Map of the Agulhas Bank centred on the Outeniqua Basin
Ecology
Ecozone Temperate Southern Africa
Geography
Area 116,000 km2 (45,000 sq mi)
Country South Africa
Elevation -50 to -200 m
Coordinates 34°42′33.1″S 22°28′12.4″E / 34.709194°S 22.470111°E / -34.709194; 22.470111Coordinates: 34°42′33.1″S 22°28′12.4″E / 34.709194°S 22.470111°E / -34.709194; 22.470111
Oceans or seas Atlantic Ocean, Indian Ocean

The Agulhas Bank is a broad, shallow part of the southern African continental shelf which extends 250 kilometres (160 mi) south of Cape Agulhas before falling away steeply away to the abyssal plain.

It is the ocean region where the warm Indian Ocean and the cold Atlantic Ocean meet. This convergence leads to treacherous sailing conditions, accounting for numerous wrecked ships in the area over the years. However the meeting of the oceans here also fuels the nutrient cycle for marine life, making it one of the best fishing grounds in South Africa.

Extent[edit]

The Agulhas Bank stretches approximately 800 km (500 mi) along the African coast,[1] from off Cape Peninsula (18°E) to Port Alfred (26°E),[2] and up to 250 km (160 mi) from it. The bank is 50 m (160 ft) deep near the coast and reaches 200 m (660 ft) before dropping steeply to 1,000 m (3,300 ft) on its southern edge.[1] Triangukar in shape, the shelf spans an area of 116,000 km2 (45,000 sq mi) with a mean depth slightly over 100 m.[3]

Oceanography[edit]

The Agulhas Bank is a natural boundary between the Atlantic Benguela Current and the Indian Ocean Agulhas Current, and upwelling forces large volumes of nutrient-rich cold water onto the Agulhas Bank, making it a major spawning ground for pelagic fish.[1]

It is bounded along its southeastern edge by the Agulhas Current, which retroflects south of the bank, resulting in intense eddy activities. This large-scale current brings in vast amounts of water from the Indian Ocean which interact with the dynamic southern Benguela upwelling system on the Atlantic side. The Agulhas Bank is dominated by westerly winds and most of the upwelling phenomena are associated with the Agulhas Current, though easterly winds do occur seasonally and can generate local upwelling cells.[2]

In summer, there is mixture of subtropical water separated by thermoclines from cool waters. Bottom waters exhibit characteristics of Indian Ocean waters in the east and Atlantic Ocean waters in the west.[3]

On the eastern bank, the Agulhas Current is diverged away from the coast by the bank's shelf, and dynamic processes produce a semi-permanent Ekman layer of cold water along the bank's eastern edge during spring and summer. Meanders in the Agulhas Current produce cyclonic eddies, and on the eastern shelf-side it results in upwelling that brings subtropical Indian Ocean water onto the bank. In summer, occasional easterly winds can drive upwelling at promontories along the coast. [3]

On the western bank, coastal upwelling is also common, but, because of lower atmospheric variability, large plumes of cold water are brought up to the western bank, occasionally forming a system of upwelling along Africa's southwestern coast — the southern extension of the Benguela Current. The Agulhas Current regularly brings warm water to the western bank along the bank's western edge. [3]

Agulhas rings[edit]

Light blue plankton in a 150 km (93 mi) wide anti-cyclonic (counter-clockwise) Agulhas ring in the South Atlantic. Such eddies, among the largest in the world, are peeled off The Agulhas Current on the Agulhas Bank.

Large-scale cyclonic meanders known as Natal pulses form as the Agulhas Current reaches the continental shelf on the South African east-coast (i.e. the eastern Agulhas Bank off the Natal Province). As these pulses moves along the coast on the Agulhas Bank, they tend to pinch off Agulhas rings from the Agulhas Current. Such a ring shedding can be triggered by a Natal pulse alone, but sometimes meanders on the Agulhas Return Current merge to contribute to the shedding of an Agulhas ring.[4] Meanders on the Agulhas Current are much more frequent than the Natal pulses and a Natal pulse is almost always precceded by an Agulhas meander. Natal pulses occur 1-2 times per year and propagate at a speed of 20 km (12 mi) per day.[5]

The Agulhas rings, warm core rings of ocean water southwest off South Africa, are thought to be of global climatic importance. Their delivery of warm water from the Indian to the Atlantic ocean can control the rate of thermohaline overturning of the entire Atlantic. Other factors contribute to various extent to the inter-ocean exchanges in the region, including filaments from the Agulhas Current and intrusions of water from Antarctica. Cold, cyclonic eddies have been observed in the southwestern Atlantic.[6] Based on model simulations, researchers have found that the interaction of the Agulhas Current and the eastern edge of the bank can result in the Agulhas rings.[7]

Currents from the Atlantic Ocean, Indian Ocean, and Southern Ocean meet south of Africa, resulting in one of the most turbulent waters of the world oceans. In upper layer water, the Agulhas rings and eddies move warm and salty water into the large South Atlantic gyre, which exports it to the tropics. In the lower ocean layers water is transported in the opposite direction. The average diameter of the Agulhas rings is 320 kilometres (200 mi), but they can reach 500 km. They extend down to the ocean floor; circulate at between 0.3–1.5 metres per second (0.98–4.92 ft/s); and move into the South Atlantic at 4–8 kilometres (2.5–5.0 mi)/day. Only half of the Agulhas eddies originating in the Cape Basin cross the Walvis Ridge and in average they lose half their energy before reaching that ridge within six month.[8]

The provenance of ocean sediments can be determined by analysing terrigenous strontium isotope ratios in deep ocean cores. Sediments underlying the Agulhas Current and Return Current have significantly higher ratios than surrounding sediments. Analyses of cores in the South Atlantic deposited during the Last Glacial Maximum (LGM, 20 000 years ago), show that the Agulhas leakage (shedding of Agulhas rings) was significantly reduced. It has been hypothesised that the reason for this was that the Agulhas Current was stronger which resulted in a more eastward retroflection and therefore less leakage. However, analyses of such cores south of Africa show that the trajectory of the current was the same during the LGM and that the reduced leakage must be explained by a weaker current. Consequently, it can be predicted that a stronger Agulhas Current will result in its retroflection occurring more eastward and an increased Agulhas leakage.[9]

Geology[edit]

The Agulhas Bank relative to the Agulhas Ridge, Basin, and Plateau
As Gondwana formed 500 mya, a rift appeared which eventually developed into the Agulhas Sea — the beginning of the Atlantic and Indian Oceans.

The Agulhas-Falkland Fracture Zone (AFFZ) stretches 1,200 kilometres (750 mi) across the South Atlantic. It is one of the largest and most spectacular fracture zones on Earth. It developed during the Early Cretaceous as West Gondwana (=South America) broke up from Africa. The AFFZ is characterized by a pronounced topographic anomaly, the Agulhas Ridge (41°S,16°E-43°S,9°E) which rises more than 2 km above the surrounding sea floor. The only equivalent in size are the neighbouring Diaz Ridge and the Falkand Escarpment. The Agulhas Ridge is unique because it was not formed during the continental breakup during the Cretaceous and because it separates oceanic crusts of different age, and not oceanic crust (~14 km thick) from continental crust (25 km thick).[10][11]

The Outeniqua Basin is located on the Agulhas Bank's eastern side, north of the AFFZ. It consists of smaller subbasins in the north and a much deeper subbasin in the south, the Southern Outeniqua Basin. These basins can be underlain by the Cape Fold Belt but can also be products of the shear process during the Gondwana breakup. Reconstructions of sedimentary basins on the Falkland Plateau show that they are conjugate to the Outeniqua Basin before the breakup.[12]

The Agulhas Plateau is located southeast of the shelf, separated from it by the Agulhas Passage (through which the Agulhas Current flows.)[13]

The Bouvet hotspot track stretches south-east from the African continent near the South Africa-Mozambique border and stretches east of the AFFZ down to Bouvet Island/Bouvet Triple Junction.[14] The hotspot was located in present-day South Africa from the late Triassic 220 mya and until the Africa-Antarctica breakup 120 mya.[15]

Human evolution[edit]

Anatomically modern humans evolved around 200 kya. The genetic diversity in the human lineage is relatively low, which indicate one or several population bottlenecks late in our lineage. It has been estimated that the population was limited to maybe 600 individuals during the MIS 6 glacial stage (195-125 kya), one of the longest cold periods in the Quaternary of Africa. A technological and behavioural revolution that occurred globally about 50 kya led to a cultural complexity which happened in South Africa around 120-70 kya.[16]

The Cape Floral Region, a thin coastal strip and a botanic hotspot, which developed at the confluence of the Benguela Upwelling and Agulhas Current. According to what Marean 2011 calls the "Cape Floral Region – South Coast Model" for the origins of modern humans, the early hunter-gatherers survived on shellfish, as well as geophytes, fur seal, fish, seabirds, and wash-ups found along the South African south coast. The Agulhas Bank slopes into the sea and a reconstruction of how the coastline has changed over 440 000 years shows that the coast during the Pleistocene was located as far as 90 kilometres (56 mi) from the present coast.[17]

The present South African southern coastal plain (SCP) is still separated from the rest of Africa by the Cape Fold Belt. During glacial maxima the sea-level dropped 120 metres (390 ft). This not only left large parts of the Agulhas Bank exposed, which greatly expanded the area of the SCP, but it also reconnected the SCP to the rest of Africa by the shallow water shelves, which broke the isolation of the SCP. Modern humans evolved on the SCP and the fluctuation in sea-levels would have resulted in a significant variation in selective pressure. No fossil records are known from the now submerged shelf, but a series of key fossil sites along the costal margin of the present SCP provide earliest traces of anatomically modern humans and the use of marine resources.[18]

See also: Pinnacle Point

Commercial importance[edit]

The Agulhas Bioregion in green

South Africa began oil exploration on the Agulhas Bank in the 1980s. Of more than 200 offshore wells in South Africa, most are found on Bredasdorp Basin on the Agulhas Bank.[19]

Fishery[edit]

The Agulhas Bank is also significant for fisheries who use demersal trawling, demersal longline fishing, and midwater trawl fishing on the bank. Squid and small pelagic fishes are also caught. Before the introduction of the EEZ, foreign fisheries used roch-hopper gear trawling on the bank.[19]

Most of the catches are short-lived shelf-zone pelagic species and more long-lived deep-water species. The large populations of sardine and anchovy also present on the shelf follow an annual cycle. Anchovy spawn on the western Agulhas Bank in early summer while the sardines span over a broader season and area — eggs are transported by currents to the nursery area in the St Helena Bay on the South African west coast from where juvenile then migrate back to the Agulhas Bank to spawn.[20]

South Africa has a relatively large fishing industry mostly catching pelagic pilchard and anchovy and demersal Hake on the south and western coasts. Though the east coast has fewer commercial fisheries, the large human population along there has resulted in overexploitation of coastal fish and invertebrate stocks by recreational and subsistence fishers. A small aquaculture industry produces mussles and oysters offshore.[21]

See also: Sardine run

Biodiversity[edit]

There are at least 12,914 marine species in South Africa, but small bodied species are poorly documented and the abyssal zone is almost completely unexplored. Almost a quarter of South Africa's coast line is protected, excluding deeper water.[21] A third of the marine species are endemic to South Africa (though poor levels of taxonomic research in adjoining countries probably affects the apparent endemism.) The degree of endemism varies considerably among taxa: Bryozoa 64%, Mollusca 56%, Echinodermata 3.6%, Porifera 8.8%, Amphipoda 33%, Isopoda 85%, or Cumacea 71%.[22]

Copepods comprise 90% of the zooplankton carbon on the Agulhas Bank, and are thus an important source of food for pelagic fish and juvenile squids. The population of Calanus agulhensis, a large species that dominates the copepod community in terms of biomass, has a center of distribution on the central Agulhas Bank. Since 1997 the copepod biomass on the central Agulhas Bank has declined significantly while the biomass of pelagic fish has increased significantly. While it is likely that predation has played an important role in the copepod decline, global warming (sea surface temperature and Cholorphyll A abundance) is believed to have contributed to a smaller population.[23]

The bank is an important spawning ground for anchovy. Their eggs and larvae are transported via the Good Hope Jet to Africa's southwestern coast where they mature. Young anchovies then return to the Agulhas Bank to spawn.[2]

A vulnerable population of fish-eating killer whales are present offshore on the Agulhas Bank. Observations peak in January while few are sited in April and May. The killer whales move in pods of 1-4 individuals and are mostly sited over the shelf edge off the south-east coast.[24]

A vagrant Commerson's dolphin — a species with two isolated populations, one along the southern coast of Argentina and the other around the Kerguelen Islands — was sighted on the Agulhas Bank in 2004. It is not known from which population the sighted individual stems. The Kerguelen Islands are located 4,200 km (2,600 mi) and South America 6,300 km (3,900 mi) from the Agulhas Bank, but the west-ward direction of the Antarctic Circumpolar Current would force the dolphin to swim against the current from the Kerguelen Islands.[25]

Fossil beaked whales have been recovered by trawling from the seafloor off South Africa.[26] Stranded pygmy sperm whales have been recorded on both the east and west coasts of South Africa.[27]

57 species of sharks have been reported off the western coast of South Africa, of which 21 are squaloid sharks.[28]

References[edit]

Notes[edit]

  1. ^ a b c "Sea Atlas - Agulhas Bank". Bayworld Centre For Research & Education. Retrieved January 2015. 
  2. ^ a b c Blanke et al. 2009, Introduction, pp. 1-2
  3. ^ a b c d Whittle 2012, Introduction
  4. ^ Leeuwen, Ruijter & Lutjeharms 2000, Abstract
  5. ^ Jackson et al. 2012
  6. ^ Penven et al. 2001, Introduction, p. 1055
  7. ^ Penven et al. 2001, Conclusion, p. 1057
  8. ^ Ruijter et al. 2003, pp. 45-47
  9. ^ Franzese, Goldstein & Skrivanek 2012
  10. ^ Uenzelmann-Neben & Gohl 2003, Abstract
  11. ^ Bird 2001, p. 152
  12. ^ Parsiegla et al. 2009, Introduction, p. 2
  13. ^ Parsiegla et al. 2009, Fig. 1
  14. ^ Gohl & Uenzelmann-Neben 2012, Fig. 1
  15. ^ Golonka & Bocharova 2000, Figs. 3-8
  16. ^ Marean 2011, pp. 421-423
  17. ^ Marean 2011, pp. 423-425
  18. ^ Compton 2011, p. 508
  19. ^ a b "Assessment of Offshore Benthic Biodiversity on the Agulhas Bank and the Potential Role of Petroleum". WWF. November 2008. Retrieved January 2015. 
  20. ^ Jury 2011, p. 1-2
  21. ^ a b Griffith et al. 2010, p. 1
  22. ^ Griffith et al. 2010, pp. 6, 8
  23. ^ Huggett et al. 2012
  24. ^ Williams et al. 2009, Abstract
  25. ^ Bruyn, Hofmeyr & Villiers 2006
  26. ^ Bianucci, Lambert & Post 2007, Abstract
  27. ^ Elwen et al. 2013, Introduction
  28. ^ Ebert, Compagno & Cowley 1992, Introduction

Sources[edit]