Walvis Ridge

The Walvis Ridge (walvis means whale in Dutch and Afrikaans) is an aseismic ocean ridge in the southern Atlantic Ocean. More than 3,000 km (1,900 mi) in length, it extends from the Mid-Atlantic Ridge, near Tristan da Cunha and the Gough Islands, to the African coast (at 18°S in northern Namibia).[1] The Walvis Ridge is one of few examples of a hotspot seamount chain that links a flood basalt province to an active hotspot. It is also considered one of the most important hotspot tracks because the Tristan Hotspot is one of few primary or deep mantle hotspots.[2]

WalvisRidge
The Walvis Ridge stretches some 3,000 km (1,900 mi) from the African continental shelf to the Tristan da Cunha hotspot, separating the Angola and Cape Basins.

Geology

Apart from the Mid-Atlantic Ridge, the Walvis Ridge and the Rio Grande Rise are the most distinctive feature of the South Atlantic sea floor. They originated from hotspot volcanism and together they form a mirrored symmetry across the Mid-Atlantic Ridge, with the Tristan Hotspot at its centre. Two of the distinct sections in the Walvis Ridge have similar mirrored regions in the Rio Grande Rise; for example, the eastern section of the Walvis Ridge evolved in conjunction with the Torres Arch (the western end of the Rio Grande Rise, off the Brazilian coast) and, as the South Atlantic gradually opened, these structures became separated. The complex of seamounts in the western end of the Walvis Ridge, however, does not have a similar structure on the American side, but there is a Zapiola Seamount Complex south of the eastern end of the Rio Grande Rise.[3] The formation of this mirrored structure is the result of the opening of the South Atlantic some 120 Mya and the Paraná and Etendeka continental flood basalts, the lateral-most parts of the structure, formed at the beginning of this process in areas that are now located in Brazil and Namibia.[2]

The Walvis Ridge is divided into three main sections:[1]

  1. A first 600 km (370 mi) long segment stretching from Africa to approximately longitude 6°E and varying in width between 90–200 km (56–124 mi).
  2. A second section, 500 km (310 mi) long, stretching north-south, and narrower than the first section.
  3. A third more discontinuous section, which is marked by seamounts and connects the Walvis Ridge to the Mid-Atlantic Ridge.

Cretaceous kimberlites in the central Democratic Republic of Congo and Angola align with the Walvis Ridge.[4]

The Tristan-Gough hotspot track first formed over the mantle plume that formed the Etendeka-Paraná continental flood basalts some 135 to 132 Ma.[5] The eastern section of the ridge is thought to have been created in the Middle Cretaceous period, between 120 to 80 Ma.[6][7] While the mantle plume remained large and stable, the eastern Walvis Ridge formed along with the Rio Grande Rise over the Mid-Atlantic Ridge.[5] During the Maastrichtian 60 million years ago, the orientation of spreading changed, which is still visible in the orientation of the various sections of the Walvis Ridge.[2] The mantle plume then gradually became unstable and bifurcated 60 to 70 Ma to produce the two separate Tristan and Gough hotspot tracks. It finally disintegrated 35 to 45 Ma and formed the guyot province in the western end of the ridge.[5]

Hundreds of volcanic explosions were recorded on the Walvis Ridge in 2001 and 2002. These explosions seemed to come from an unnamed seamount on the northern side of the ridge and are thought to be unrelated to the Tristan hotspot.[8]

The Ewing Seamount is part of the ridge.

Palaeoclimatic role

The Eocene Layer of Mysterious Origin (Elmo) is a period of global warming that occurred 53.7 Ma, about two million years after the Paleocene–Eocene Thermal Maximum. This period manifests as a carbonate-poor red clay layer unique to the Walvis Ridge and is similar to the PETM, but of smaller magnitude.[9][10]

Oceanography

The Walvis Ridge is a natural obstacle for the Agulhas rings, mesoscale warm core rings that are shed from the Agulhas Current south of the Agulhas Bank. In average, five such rings are shed each year, a number that varies considerably between years.[11] The rings tend cross the Walvis Ridge at its deepest part, but they still lose transitional speed and many rings decay rapidly.[12] Their transitional speed drop from 5.2±3.6 km/day to 4.6±3.1 km/day, but it is not clear how much the Walvis Ridge is responsible for this drop, since the rings' speed drop to 4.3±2.2 km/day between the Walvis Ridge and the Mid-Atlantic Ridge.[13] The rings can cross the South Atlantic in 2.5–3 years but only two thirds make it farther than the Walvis Ridge.[11] When the rings pass over the Cap Basin south of the Walvis Ridge they are frequently disturbed by the Benguela Current, interaction between rings, and bottom topography such as the Vema Seamount, but there are fewer obstacles and disturbances west of the Walvis Ridge were the rings tend stabilise. [14] The Agulhas rings transport an estimated 1-5 Sv (millions m3/s) of water from the Indian Ocean to the South Atlantic.[15]

Originating around Antarctica, Antarctic Bottom Water (AABW) enters the Cape Basin between the Agulhas Bank and the Agulhas Ridge after which it flows west north of the Agulhas Ridge. AABW then retroflects at the south-western end of the Walvis Ridge, flows north-east along the ridge before being retroflected south by North Atlantic Deep Water, with which it exits the Cape Basin and flows into the Indian Ocean.[16]

References

Notes

  1. ^ a b Goslin et al. 1974, Introduction, p. 469
  2. ^ a b c Sager 2014, pp. 2–5
  3. ^ O'Connor & Duncan 1990, Introduction, p. 17475
  4. ^ de Wit 2007, Fig. 7, p. 380; Fig. 9, p. 385
  5. ^ a b c Rohde et al. 2013, Conclusions, pp. 69-70
  6. ^ Pastouret & Goslin 1974
  7. ^ Müller, Royer & Lawver 1993
  8. ^ Haxel & Dziak 2005, Abstract
  9. ^ Lourens et al. 2005, Abstract
  10. ^ "Eocene Layer of Mysterious Origin". JOIDES Resolution. Retrieved May 2015. Check date values in: |accessdate= (help)
  11. ^ a b Schouten et al. 2000, Discussion and Conclusions, p. 21933
  12. ^ Schouten et al. 2000, Abstract, Introduction, pp. 21913-21914
  13. ^ Schouten et al. 2000, Rings paths, pp. 21916-21918
  14. ^ Schouten et al. 2000, Ring Decay, pp. 21918-21919
  15. ^ Ruijter et al. 2003, p. 46
  16. ^ Gruetzner & Uenzelmann-Neben 2014, Fig 1.A

Sources

Coordinates: 26°S 6°E / 26°S 6°E

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.

Alkenone

Alkenones are long-chain unsaturated methyl and ethyl n-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. Alkenones have been observed containing between 35 and 41 carbon atoms and with between one and four double bonds. Uniquely for biolipids, alkenones have a spacing of five methylene groups between double bonds, which are of the less common E configuration. The biological function of alkenones remains under debate although it is likely that they are storage lipids. Alkenones were first described in ocean sediments recovered from Walvis Ridge and then shortly afterwards in cultures of the marine coccolithophore Emiliania huxleyi. The earliest known occurrence of alkenones is during the Aptian 120 million years ago. They are used in organic geochemistry as a proxy for past sea surface temperature.

Alkenone-producing species respond to changes in their environment — including to changes in water temperature — by altering the relative proportions of the different alkenones they produce. At higher temperatures more saturated alkenones are produced proportionally. This means that the relative degree of unsaturation of alkenones can be used to estimate the temperature of the water in which the alkenone-producing organisms grew. The relative degree of unsaturation is typically described as an Unsaturation Index of di- versus tri- unsaturated C37 alkenones according to:

UK′37 = C37:2/(C37:2 + C37:3) The UK′37 can then be used to estimate sea surface temperature according to an empirical relationship determined from core-top calibrations. The most commonly used calibration is that of Müller et al., 1998:

UK′37 = 0.033T [°C] + 0.044 The Müller et al. (1998) calibration is not suitable for all environments and, in particular, different calibrations are required for high latitudes and lacustrine settings.

Angola Basin

The Angola Basin is located along the West African South Atlantic Margin which extends from Cameroon to Angola.

It is characterized as a passive margin that began spreading in the south and then continued upwards throughout the basin.

This basin formed during the initial breakup of the supercontinent Pangaea during the early Cretaceous, creating the Atlantic Ocean and causing the formation of the Angola, Cape, and Argentine basins.

It is often separated into two units: the Lower Congo Basin, which lies in the northern region and the Kwanza Basin which is in the southern part of the Angola margin.

The Angola Basin is famous for its "Aptian Salt Basins," a thick layer of evaporites that has influenced topography of the basin since its deposition and acts as an important petroleum reservoir.

Atlantic Ocean

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers (41,100,000 square miles). It covers approximately 20 percent of Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and the Americas to the west. As one component of the interconnected World Ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Equatorial Counter Current subdivides it into the North(ern) Atlantic Ocean and the South(ern) Atlantic Ocean at about 8°N.Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office.

Discovery Seamounts

Discovery Seamounts are a chain of seamounts in the Southern Atlantic Ocean, which include the Discovery Seamount. The seamounts lie 850 kilometres (530 mi) east of Gough Island and once rose above sea level. Various volcanic rocks as well as glacial dropstones and sediments have been dredged from the seamounts.

The Discovery Seamounts appear to be a volcanic seamount chain controlled by the Discovery hotspot, which had its starting point either in the ocean, Cretaceous kimberlite fields in southern Namibia or the Karoo-Ferrar large igneous province. The seamounts formed between 41 and 35 million years ago; presently the hotspot is thought to lie southwest of the seamounts, where there are geological anomalies in the Mid-Atlantic Ridge that may reflect the presence of a neighbouring hotspot.

Eocene Thermal Maximum 2

Eocene Thermal Maximum 2 (ETM-2), also called H-1 or the Elmo (Eocene Layer of Mysterious Origin) event, was a transient period of global warming that occurred approximately 53.7 million years ago (Ma). It appears to be the second major hyperthermal that punctuated the long-term warming trend from the Late Paleocene through the early Eocene (58 to 50 Ma).The hyperthermals were geologically brief time intervals (<200,000 years) of global warming and massive carbon input. The most extreme and best-studied event, the Paleocene-Eocene Thermal Maximum (PETM or ETM-1), occurred about 1.8 million years before ETM-2, at approximately 55.5 Ma. Other hyperthermals likely followed ETM-2 at nominally 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3). The number, nomenclature, absolute ages and relative global impact of the Eocene hyperthermals are the source of much current research.

In any case, the hyperthermals appear to have ushered in the Early Eocene Climatic Optimum, the warmest interval of the Cenozoic Era. They also definitely precede the Azolla event at about 49 Ma.

ETM-2 is clearly recognized in sediment sequences by analyzing the stable carbon isotope composition of carbon-bearing material. The 13C/12C ratio of calcium carbonate or organic matter drops significantly across the event. This is similar to what happens when one examines sediment across the PETM, although the magnitude of the negative carbon isotope excursion is not as large. The timing of Earth system perturbations during ETM-2 and the PETM also appear different. Specifically, the onset of ETM-2 may have been longer (perhaps 30,000 years) while the recovery seems to have been shorter (perhaps <50,000 years). (Note, however, that the timing of short-term carbon cycle perturbations during both events remains difficult to constrain).

A thin clay-rich horizon marks ETM-2 in marine sediment from widely separated locations. In sections recovered from the deep-sea (for example those recovered by Ocean Drilling Program Leg 208 on Walvis Ridge), this layer is caused by dissolution of calcium carbonate. However, in sections deposited along continental margins (for example those now exposed along the Clarence River, New Zealand), the clay-rich horizon represents dilution by excess accumulation of terrestrial material entering into the ocean. Similar changes in sediment accumulation are found across the PETM. In sediment from Lomonosov Ridge in the Arctic Ocean, intervals across both ETM-2 and the PETM shows signs of higher temperature, lower salinity and lower dissolved oxygen.The PETM and ETM-2 are thought to have a similar generic origin, although this idea is at the edge of current research. During both events, a tremendous amount of 13C-depleted carbon rapidly entered the ocean and atmosphere. This decreased the 13C/12C ratio of carbon-bearing sedimentary components, and dissolved carbonate in the deep ocean. Somehow the carbon input was coupled to an increase in Earth surface temperature and a greater seasonality in precipitation, which explains the excess terrestrial sediment discharge along continental margins. Possible explanations for changes during ETM-2 are the same as those for the PETM, and are discussed under the latter entry.

The H-2 event appears to be a "minor" hyperthermal that follows ETM-2 (H-1) by about 100,000 years. This has led to speculation that the two events are somehow coupled and paced by changes in orbital eccentricity.As in the case of the PETM, reversible dwarfing of mammals has been noted during the ETM-2.

Epigonus telescopus

Epigonus telescopus, the black cardinal fish, is a species of deepwater cardinalfish found in most temperate oceans worldwide, at depths of between 75 and 1,200 metres (246 and 3,937 ft) though mostly between 300 and 800 metres (980 and 2,620 ft). It can reach a length of 75 centimetres (30 in) TL though most specimens do not exceed 55 centimetres (22 in) TL. It has been reported that this species can reach an age of 104 years.

Ewing Seamount

The Ewing Seamount is a seamount in the southern Atlantic Ocean, which lies on the Tropic of Capricorn. Ewing is part of the Walvis Ridge having a mean depth of 4,500 metres and a summit depth of 700 metres.

Gymnothorax walvisensis

Gymnothorax walvisensis is an eel in the family Muraenidae (moray eels). It was described by Artem Mikhailovich Prokofiev in 2009. It is a subtropical, marine eel which is known from Walvis Ridge, in the southeastern Atlantic Ocean (from which its species epithet is derived). Males are known to reach a maximum total length of 41.3 cm.

Hotspot (geology)

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Longnose pygmy shark

The longnose pygmy shark (Heteroscymnoides marleyi) is a rare species of squaliform shark in the family Dalatiidae and the only member its genus. It is known only from a handful of specimens collected from the cold oceanic waters of the Southern Hemisphere, between the surface and a depth of 502 m (1,647 ft). Reaching 37 cm (15 in) in length, this diminutive shark is characterized by a slender, dark brown body with a very long, bulbous snout. In addition, it has two spineless dorsal fins of nearly equal size, with the origin of the first lying over the pectoral fin bases. The longnose pygmy shark does not appear substantially threatened by fisheries, and has been assessed as Least Concern by the International Union for Conservation of Nature (IUCN).

Paraná and Etendeka traps

The Paraná-Etendeka traps (or Paraná and Etendeka Plateau; or Paraná and Etendeka Province) comprise a large igneous province that includes both the main Paraná traps (in Paraná Basin, a South American geological basin) as well as the smaller severed portions of the flood basalts at the Etendeka traps (in northwest Namibia and southwest Angola). The original basalt flows occurred 128 to 138 million years ago. The province had a post-flow surface area of 1.5 x 106 km² (580,000 miles²) and an original volume projected to be in excess of 2.3 x 106 km³.

Razorback scabbardfish

The razorback scabbardfish, Assurger anzac, is a rare species of cutlassfish, family Trichiuridae, and the only member of its genus. It has been caught from scattered locations worldwide: in the Atlantic Ocean it is known from off Puerto Rico, Uruguay, and the Walvis Ridge, in the Indian Ocean it is known from off western Australia, and in the Pacific Ocean it is known from off New Guinea, southern Japan, Midway Island, California, and the Nazca and Sala y Gomez Ridges. Adults are thought to be benthopelagic, occurring at a depth of 150–400 meters (490–1,310 ft), while juveniles are found near the surface or in midwater.

This fish has a very long and thin body, measuring 25 to 28 times as long as deep and 12 to 14 times as long as the head. The dorsal profile of the head is straight or slightly convex, with a prominent sagittal crest. The eye diameter is about one-eighth the head length. The lower jaw juts out past the upper, and both jaws are tipped with a short dermal process. There are three pairs of fangs at the front of the jaws. The dorsal fin is very long, containing a few weak spines at the front followed by 116-123 soft rays. The origin of the anal fin lies beneath the 42nd to 44th dorsal fin ray; the anal fin contains 74-87 elements but only 14-17 have external rays, led by 2 small spines. The pelvic fins are reduced to only 1 scale-like spine and 1 soft ray, placed beneath the 8th or 9th dorsal fin ray. A tiny forked caudal fin is present. The coloration is silver, with the front of the dorsal fin membrane black until the 3rd or 4th soft ray.The razorback scabbardfish attains a maximum length of 2.5 meters (8.2 feet). Its diet consists of fishes, such as the Californian anchovy (Engraulis mordax) and the North Pacific hake (Merluccius productus), and squid. The only known parasite of the razorback scabbardfish is the copepod Avitocaligus assurgericola. Reproduction is presumably oviparous with pelagic eggs and larvae as with other members of the family, though eggs and newly hatched larvae have not been described. Flexion (the bending of the notochord as part of caudal fin formation) occurs at around a length of 14 mm (0.55 in). Post-flexion larvae have slender bodies and pointed heads, with the pelvic and first dorsal spines elongated. They can be distinguished by the presence of pigment on the top of the head, snout, and opercles. This species is not targeted by fisheries; it is sometimes hooked or netted, or found washed ashore.

Rio Grande Rise

The Rio Grande Rise, also called the Rio Grande Elevation, is an aseismic ocean ridge in the southern Atlantic Ocean off the coast of Brazil. Together with the Walvis Ridge off Africa, the Rio Grande Rise forms a V-shaped structure of mirrored hotspot tracks or seamount chains across the northern South Atlantic.

In 2013, Brazilian scientists announced that they found granite boulders on the Rio Grande Rise and speculated that it could be the remains of a submerged continent, which they called the "Brazilian Atlantis". Other researchers, however, noted that such boulders can end-up on the ocean floor by less speculative means.

Scaly stargazer

The scaly stargazer, Pleuroscopus pseudodorsalis, is a stargazer of the family Uranoscopidae. It is a demersal species which has been recorded at depths between 40 and 800 metres (130 and 2,620 ft), although it is most commonly recorded at 200–400 metres (660–1,310 ft). It has a maximum length of 33 centimetres (13 in). It is found in the South Atlantic, southern Indian Ocean and south western Pacific Ocean; in the South Atlantic it has been recorded from the Rio Grande Rise and the Walvis Ridge to the coasts of Namibia around southern Africa to the Madagascar Ridge, south of the island of Madagascar, in the south western Indian Ocean; in Australia it occurs from Western Australia and northern New South Wales, around New Zealand, and it also occurs on ocean ridge near Norfolk Island. It is the only species in the monotypic genus Pleuroscopus.

Shewanella benthica

Shewanella benthica is a obligately piezophilic bacterium from the genus of Shewanella which has been isolated from the sea cucumber Psychropotes longicauda from the Walvis Ridge.

Tristan hotspot

The Tristan hotspot is a volcanic hotspot which is responsible for the volcanic activity which forms the volcanoes in the southern Atlantic Ocean. It is thought to have formed the island of Tristan da Cunha and the Walvis Ridge on the African Plate.

Walvisteuthis virilis

Walvisteuthis virilis is a species of squid from the family Onychoteuthidae, it is the type species of the genus Walvisteuthis. It may be synonymous with Walvisteuthis rancureli. The type specimen was collected a mature male with a mantle length of 71 mm collected near the Walvis Ridge in the eastern South Atlantic Ocean at a depth of 1000m. A second specimen was subsequently collected on the other side of the South Atlantic, also a mature male with a total length of 71mm.

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