Agulhas Bank

The Agulhas Bank (, from Portuguese for Cape Agulhas, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to south of Cape Agulhas before falling steeply 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 and characteristics

South African marine ecoregions from the 2011 classification

The Agulhas Bank stretches approximately along the African coast, from off Cape Peninsula (18°E) to Port Alfred (26°E), and up to from it. The bank slopes down relatively steeply from the coast to about deep and reaches before dropping steeply to on its southern edge.
The shelf spans an area of with a mean depth slightly over . It is entirely within the exclusive economic zone of South Africa.

The National Spatial Biodiversity Assessment 2004 recognised 34 biozones nested within 9 bioregions (of which four were offshore). The National Biodiversity Assessment 2011 replaced these ecozones and biozones with the terms ecoregions and ecozones. In 2011, the Agulhas Ecoregion was divided into four distinct ecozones: Agulhas inshore, Agulhas inner shelf, Agulhas outer shelf, and Agulhas shelf edge. 33 different benthic habitats types were identified on the Agulhas Bank.

There are dozens of warm temperate reefs along the coast of the Agulhas Ecoregion spanning from below sea level. Many rocky sub-tidal reefs are of aeolianite or sandstone origin, but granite, quartzite and siltstone reefs are also present. The Agulhas reefs are very heterogeneous and include several possible different sub-types. Some of the reefs are within protected areas, but only a few of those protected areas include protection from fishing.

Oceanography

The Agulhas Bank is a natural boundary between ocean currents from the Atlantic Ocean, Indian Ocean, and Southern Ocean, resulting in one of the most turbulent waters of the world oceans.

Agulhas Current

The Agulhas Current flows south along the African east-coast and along the south-eastern edge of the bank. It then retroflects back into the Indian Ocean south-west of the bank. This retroflection results in intense eddy activities such as meanders, eddies, and filaments.
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.

Upwellings

Cyclonic eddies is another source of edge upwelling west of Port Elisabeth. Plumes of warm surface water migrate onto the bank along its eastern edge, providing subtropical surface water from the Indian Ocean.
In summer, easterly winds can intermittently drive coastal upwelling along the South African south coast.
The Agulhas Bank is dominated by westerly winds and most of the upwelling on the bank is related to the interaction of the Agulhas Current on the eastern edge, but easterly winds do occur, especially in summer and fall, and can generate local upwelling cells.

As the current is diverged away from the coast, dynamic processes draws an onshore Ekman layer of cold water from below the warm shelf-edge flow. In spring and summer, at a depth of , a semi-permanent ridge of cold water is present on the eastern and central shelf.

In summer, there is mixture of subtropical water separated by thermoclines from cool waters, but there is a considerable seasonal variation. On the shelf, bottom waters exhibit characteristics of the central Indian Ocean in the east and central Atlantic Ocean waters in the west.

Agulhas meanders and Natal pulses

As the Agulhas Current flows south along the African east coast, it tends to bulge inshore frequently, a deviation from the current's normal path known as Agulhas Current meanders (ACM). These bulges are occasionally (1-7 times per year) followed by a much larger offshore bulge, known as Natal pulses (NP). Natal pulses move along the coast at per day. An ACM can bulge up to and a NP up to from the current's mean position. The AC passes offshore and an ACM can reach offshore. When the AC meanders, its width broadens from to and its velocity weakens from to . An ACM induces a strong inshore counter-current.

Large-scale cyclonic meanders known as Natal pulses are formed as the Agulhas Current reaches the continental shelf on the South African east-coast (i.e. the eastern Agulhas Bank off Natal). As these pulses move 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.

Agulhas leakage and rings

Agulhas rings are large anticyclonic eddies or warm core rings of ocean water that are pinched off the Agulhas Current along the eastern edge of the Agulhas Bank from where they move into the South Atlantic. As the Agulhas Current reaches the east coast of South Africa, large solitary meanders known as Natal pulses form at irregular intervals. 165 days after the appearance of a Natal pulse, an Agulhas ring is formed off Durban. The Agulhas rings are among the largest eddies in the world and play an important role in the Agulhas Leakage, the transport of warm water from the Indian Ocean to the Atlantic Ocean, which affects the global climate.

The average diameter of the Agulhas rings is , but they can reach 500 km. They extend down to the ocean floor; circulate at ; and move into the South Atlantic at /day. Only half of the Agulhas eddies that leave the Cape Basin manage to cross the Walvis Ridge and those that do tend to lose half their energy before reaching the ridge within six months. The Agulhas rings transport an estimated 1-5 Sv (millions m²/s) of water from the Indian Ocean to the South Atlantic.

The Agulhas rings 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 degrees, 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.
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.

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.

Benguela Current

Compared to the Agulhas Current, the Benguela Current on the west and south-west coast of Africa is more intense and steadier. Its dynamic southern upwelling system is driven by the prevailing northward winds that produce an intense off-shore Ekman transport. Most of this upwelling is concentrated to a few upwelling cells in the southern region: Namaqua (30°S), Cape Columbine (32.5°S), and Cape Peninsula (34°S). The wind is most intense from October to February, and the contrast in sea surface temperature between the open sea and the shelf is most prominent during summer.

Coastal upwelling is also common on the western bank, but the more stable atmospheric condition results in larger cold water plumes that sometimes merge to form a continuous upwelling regime along the South African south-west coast. This upwelling zone is the southernmost extension of the Benguela Current Large Maritime Ecosystem. The Agulhas Current regularly flows around the southern tip of the bank and brings warm water to the western bank along the bank's western edge.
Regularly, the mesoscale eddies from the east interact with the Benguela upwelling system on the African west coast.

Deep water eddies

Flowing south along the South American continental slope, the Deep Western Boundary Current (DWBC) carries North Atlantic Deep Water (NADW) into the South Atlantic. At about 8°S and at a depth of , the DWBC breaks into anticyclonic eddies during periods of strong meridional overturning circulation. One such NADW eddy was observed in 2003 and the researchers speculated that a deeply penetrating Agulhas ring pinched it off the NADW slope current. Spinning at , these deep-water eddies move around the southern tip of the Agulhas Bank and into the Indian Ocean. Most of the NADW flow (more than 7 Sv) meanders east around the Agulhas Plateau together with the surface Agulhas Return Current, but a smaller portion (3 Sv) continues north along the African east-coast as the Agulhas Undercurrent.
Of 89.5 Sv released from the North Atlantic, 3.6 Sv leaves the South Atlantic south of the Agulhas Bank. However, 0.9 Sv recirculate in the basin north of the Walvis Ridge for centuries, of which 50-90% end up flowing south of the Agulhas Bank within 300 years, increasing the net inter-oceanic exchange with 4.1-4.5 Sv.

Alphard Banks

The Alphard Banks are a small group of long extinct volcanic seamounts on the Agulhas Bank south of Cape Agulhas. They rise from the bottom at about 80 m to about 14 m on the top. They are the southernmost recreational diving sites on the African continental shelf, and are seldom dived due to the distance offshore. The Alpard banks are in the Alphard Banks Controlled-Pelagic Linefish Zone of the Agulhas Bank Complex Marine Protected Area. The habitats are depth and profile related, with the shallower, flatter, areas of the pinnacle tops dominated by spiny kelp ('' Ecklonia radiata''), and encrusting invertebrates, and the deeper, steeper, areas below 30 m having little kelp and a more upright invertebrate community.

Dalgleish Bank

Dalgleish Bank is a relatively shallow area of rocky reef about 11 km offshore of Buffelsbaai, or 15.5 km, (about 8.3 nautical miles) bearing 218°T from Knysna Heads.
Maximum depth in the immediate vicinity is about 80 m, and the shallowest part of the reef is about 29 m.

Geology

The oldest rock found along the coastline of the Agulhas Bank are eugeosynclinal sediments of the up to thick Kaaimans Group deposited during continental rifting some 900 million years ago (Mya). The proto-South Atlantic closed during the Saldanian orogeny to form part of the supercontinent Gondwana (700-600 Mya). The Cape granites were emplaced and the Kaaimans Group rocks were folded and thermally metamorphosed during this period. The formation of the main basin in the Cape Province commenced 570 Mya and lasted for 200 My. The Table Mountain Group is thick and an erosional unconformity marking its base is composed of both terrestrial and marine sediments. Synclines along the coast of the southern Cape contains sediments from the Bokkveld Group.

The Cape Fold Belt (CFB) rocks and the Karoo Basin were deposited 450 Mya; the Cape Supergroup 450-300 Mya during a series of transgression- regression cycles.
Pan-African thrusts were reactivated 270-215 Mya to form the CFB which was then part of a continuous fold belt that developed during the Gondwanide orogeny together with Sierra de la Ventana (Argentina), Pensacola Mountains (East Antarctica), and Ellsworth Mountains (West Antarctica). In the late Carboniferous and early Jurassic, the Karoo Supergroup was deposited in the Karoo Basin north of where the CFB is located today, and covering nearly two-thirds of present-day South Africa.

Gondwana breakup

Basaltic lavas were extruded 183 Mya to form the Karoo large igneous province; a volcanism caused by the Bouvet hotspot which is linked to the Gondwana break-up. The Bouvet hotspot was located in or near present-day South Africa from the late Triassic 220 mya and until the Africa-Antarctica breakup 120 mya. The Bouvet hotspot track stretches south-east from the African continent, near the South Africa-Mozambique border, and east of the AFFZ down to Bouvet Island/ Bouvet triple junction in the South Atlantic. 100 Mya, the region where the triple junction was located passed over the hotspot, resulting in a continuous eruption that lasted to about 94 Mya and the seafloor spreading that still separates Antarctica, Africa, and South America.

The Agulhas-Falkland Fracture Zone (AFFZ) stretches 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 Falkland 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).

North of the AFFZ is the Outeniqua Basin which is a complex system of sub-basins separated from each other by faults and basement arches; there are several smaller fault-bounded sub-basins in the north (Bredasdorp, Infanta, Pletmos, Gamtoos, and Algoa) and a distinctively deeper sub-basin in the south, the South Outeniqua Basin. The sedimentary fill of these basins developed as the northern edge of the Falkland Plateau separated from the South African southern margin during the early Cretaceous.

The Diaz Marginal Ridge (DMR) separates these basins from the AFFZ. The DMR is buried under of sediments and sedimentary rocks and of this sedimentary material is undisturbed Cretaceous sediments younger than the oldest Cretaceous sedimentary rocks in the Southern Outeniqua Basin. The DMR must therefore have formed after the initial West Gondwana breakup 130-90 Mya. The DMR probably formed when new, hot oceanic crust slid past old, cold continental crust and the contrast in temperatures induced a thermal uplift.

As West Gondwana drifted away from Africa roughly 125 Myr, the South Atlantic seafloor formed between them and magnetic anomalies north of the AFFZ reflects phase of the seafloor spreading. South of the AFFZ traces can be found of how the Falkland Plateau and the Agulhas Bank moved relative to each other. On a modern map, the Falkland Plateau can still be rotated and fitted into the Natal Valley in the Indian Ocean east of South Africa. The Agulhas Plateau is located southeast of the shelf, separated from it by the Agulhas Passage (through which the Agulhas Current flows.)

Pliocene

The Alphard Tertiary Igneous Province includes Palaeocene tuffs, trachybasalts, aegirine–augite trachytes and aegirine–augite phonolitic trachytes, which have been radiometrically dated at about 58 million years old. The intrusions appear to be tectonic effects.

One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank in the Plioceneous sediment masses separated by a large glide plane scar. In the western part, the sediments are dammed by basement ridges, but, in the eastern part, they have spread into the Transkei Basin. A series of slump scarps along the western edge of the shelf are 18–2 Mya, but covered by younger sediments brought there by the Benguela upwelling.

Human evolution

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 perhaps 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.

The Cape Floral Region is a thin coastal strip and a botanic hotspot which developed at the confluence of the Benguela Upwelling and Agulhas Current. According to what professor Curtis Marean 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 on the exposed Agulhas Bank. The bank slopes into the sea and a reconstruction of how the coastline has changed over 440 kya shows that the coast during the Pleistocene was located as far as from the present coast.

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 . 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 coastal margin of the present SCP provide earliest traces of anatomically modern humans and the use of marine resources.

Commercial importance

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.

Fishery

The Agulhas Bank is also significant for fisheries who use demersal trawling, demersal longline fishing

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