Vema Seamount

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Vema Seamount
Location of the seamount off southwestern Africa
Location of the seamount off southwestern Africa
Vema Seamount (Africa)
Summit depth26 metres (14 fathoms)
Summit area13 square kilometres (5 sq mi)[1]
Location
Coordinates31°38′S 8°20′E / 31.633°S 8.333°E / -31.633; 8.333[2][3]Coordinates: 31°38′S 8°20′E / 31.633°S 8.333°E / -31.633; 8.333[2][3]
Geology
TypeGuyot
Age of rock11.00 ± 0.3 million years old
History
Discovery date1959
Discovered byRV Vema

Vema Seamount is a seamount in the South Atlantic Ocean. Discovered in 1959 by a ship with the same name, it lies 1,600 kilometres (1,000 mi) from Tristan da Cunha and 1,000 kilometres (620 mi) northeast of Cape Town. The seamount has a flat top at a mean depth of 73 metres (40 fathoms) which was eroded into the seamount at a time when sea levels were lower; the shallowest point lies at 26 metres (14 fathoms) depth. The seamount was formed between 15-11 million years ago, possibly by a hotspot.

The seamount rises high enough that its summit is at shallow depth, allowing sunlight to reach it and thus permitting the growth of kelp and algae. A number of sea animals and fish are encountered on the seamount; active fisheries existed at Vema Seamount and caused the disappearance of some animal species.

History[edit]

Vema Seamount was discovered by the research ship RV Vema of the Lamont–Doherty Earth Observatory in 1959.[2][3] Vema is one of the first seamounts to be the subject of scientific study,[4] and the first seamount investigated by scuba divers without special equipment.[5] Vema lies in international waters[6] and its summit is so shallow that it is a navigation hazard to ships.[7]

Geomorphology and geography[edit]

Vema Seamount lies in the South Atlantic Ocean, 1,600 kilometres (1,000 mi) away from Tristan da Cunha.[2] The cities of Cape Town and Lüderitz lie east-southeast and northeast of Vema, respectively,[8] with Cape Town about 1,000 kilometres (620 mi) distant.[9]

The seamount has a conical shape with a flat top; the shallowest point rises to an elevation of 26 metres (14 fathoms) below sea level[2][3] and is called Collins Point.[10] At least one source gives a minimum depth of 11 metres (36 ft) for the seamount.[11] The flat top is a summit plateau has a width of 8.0 kilometres (5 mi)[3] at a mean depth of 73 metres (40 fathoms)[1] and has been named Emerson Plateau; it has a vaguely triangular shape pointing west, and Collins Point lies close to the western margin of the Emerson Plateau. Other points on the Plateau also rise to depths of less than 55 metres (30 fathoms).[10] The plateau appears to be a wave-cut platform of Pleistocene age, when sea levels were lower,[1] and is swept by strong ocean currents.[12]

The seamount rises from a depth of 4,600 metres (2,500 fathoms), where it occupies a breadth of 56 kilometres (35 mi).[3] The seafloor from which Vema rises belongs to the abyssal plain of the Cape Basin.[2] From there, the slopes of Vema first rise steeply and feature subsidiary summits; above 130 metres (70 fathoms) depth the slopes flatten.[1]

Geology[edit]

Volcanic rocks such as tuff as well as calcareous aggregates are found on the plateau. Collins Point is composed of phonolite, which contains aegirine, alkali feldspar, augite and nepheline. Olivine basalt has also been found.[1] A minimum age of 11.0 ± 0.3 has been obtained from samples taken at Collins Point by potassium-argon dating,[1] with another age being 15 million years.[13] Older ages have been obtained deeper on the seamount; a sample from 3,000 metres (9,800 ft) depth gave an age of 18 million years.[14]

Theorized locations of hotspots; Vema is #43

Vema is an intraplate volcano[13] and is considered to be the present-day location of a hotspot, the Vema hotspot,[15] although the hotspot itself may have moved farther west (by about 200 kilometres (120 mi)) since when it created the Vema Seamount.[16] Seismic tomography has shown what may be a mantle plume underneath Vema,[17] another theory considers the Vema hotspot is a consequence of the Tristan hotspot shedding a secondary diapir.[18] The hotspot origin of the Vema Seamount is not universally agreed upon.[19] Earlier volcanism caused by the Vema hotspot may have manifested itself in southern Namibia in the form of alkaline volcanics, such as the Klinghardt phonolite, the 49 million years old Dicker Willem volcanic complex[20] or melilites close to the mouth of the Orange River, which are 37 million years old,[21] or even the Karoo-Ferrar large igneous province.[22]

Water temperatures at Vema range between 18–21 °C (64–70 °F),[23] and the cold Benguela Current does not reach the seamount.[5] The movement and strength of ocean eddies are altered when they interact with Vema Seamount,[24] with Agulhas eddies often splitting apart at the seamount.[25]

Biology[edit]

The summit of Vema Seamount is shallow enough that sunlight can reach it, resulting in the growth of various types of algae and seaweeds such as Ecklonia kelp.[1] Such kelp covers large parts of the seamount.[26]

A number of animals inhabit Vema, usually cryptic or encrusting animals.[27] Ascidians,[26][1] non-reef building corals,[28] decapods,[29] holothurians, hydroids, polyzoa, rock lobsters (Jasus tristani) and sponges live on the seamount.[26][1] Other animals such as bryozoans, echinoderms, gastropods, oysters, pelecypods, serpulids and other worms have also left their traces on Vema.[1] Rock lobsters propagate from Gough Island and Tristan da Cunha to Vema Seamount,[12] while other species appear to originate from South Africa.[30]

Several species appear to be endemic to Vema Seamount, including the sea snail species Austromitra rosenbergi discovered in 2015[31] and the sponge Strongylodesma areolata described in 1969;[32] it is estimated that about 22-36% of all species at Vema are endemic, similar to the proportion of endemic species at other seamounts of the world.[33] The holothurian Holothuria vemae is named after the seamount, where it was discovered in 1965-1966[34] as is the sea snail Trivia vemacola.[35]

A number of fish have been encountered at Vema, although most fish species appear to be pelagic species that are not directly bound to the seamount environment.[29] Fishing operations have attracted seabirds to Vema Seamount.[36] Euphausiids and copepods are also found in the waters,[1] including at least one copepod that parasitizes fish.[37] Among the fish species encountered at Vema Seamount are:

Fish on the seamount are commercially fished,[1] with the late 1970s and 1980s seeing the initiation of Mackerel scad[38] and tuna fishing, respectively.[45] Rock lobsters in particular were heavily used;[29] they disappeared from Vema Seamount after overfishing in the 1960s, briefly recovered and then disappeared again by 1981 due to renewed overfishing.[12][9] The collapse of this fishery is one of the first instances of a seamount fishery collapsing,[46] and has been cited as an example of how fisheries outside of exclusive economic zones end up ungoverned and abused.[47]

References[edit]

  1. ^ a b c d e f g h i j k l m n o p Simpson & Heydorn 1965, p. 251.
  2. ^ a b c d e Berrisford 1969, p. 387.
  3. ^ a b c d e Simpson & Heydorn 1965, p. 249.
  4. ^ Tony J. Pitcher 2007, p. 42.
  5. ^ a b Berrisford 1969, p. 389.
  6. ^ Bergh, M. O.; Johnston, S. J. (June 1992). "A size-structured model for renewable resource management, with application to resources of rock lobster in the South-East Atlantic". South African Journal of Marine Science. 12 (1): 1006. doi:10.2989/02577619209504758.
  7. ^ Russell, Ian (29 April 2015). "No Solace from SOLAS The Effects that Deeper Draughts and the ENC are having on Safe Navigation". The International Hydrographic Review (11): 12. ISSN 0020-6946.
  8. ^ Armesto et al. 2003, p. 779.
  9. ^ a b c d Shillington, F. A. (1986). "Oceanography of the Southern African region". Smiths' Sea Fishes. Springer, Berlin, Heidelberg. p. 23. doi:10.1007/978-3-642-82858-4_7. ISBN 9783642828607.
  10. ^ a b Simpson & Heydorn 1965, p. 250.
  11. ^ Pushcharovskii, Yu. M. (April 2004). "Deep-sea basins of the Atlantic ocean: The structure, time and mechanisms of their formation". Russian Journal of Earth Sciences. 6 (2): 133. doi:10.2205/2004ES000146. Retrieved 19 March 2018.
  12. ^ a b c Lutjeharms, J.R.E.; Heydorn, A.E.F. (October 1981). "Recruitment of rock lobster on Vema Seamount from the islands of Tristan da Cunha". Deep Sea Research Part A. Oceanographic Research Papers. 28 (10): 1237. doi:10.1016/0198-0149(81)90060-1. ISSN 0198-0149.
  13. ^ a b O'Connor & le Roex 1992, p. 362.
  14. ^ O'Connor & le Roex 1992, p. 347.
  15. ^ Duncan, R. A.; Hargraves, R. B.; Brey, G. P. (2009). "Age, palaeomagnetism and chemistry of melilite basalts in the Southern Cape, South Africa". Geological Magazine. 115 (5): 325. doi:10.1017/S001675680003733X. ISSN 1469-5081.
  16. ^ Morgan, W. Jason; Morgan, Jason Phipps (2007). "Plate velocities in hotspot reference frame: electronic supplement" (PDF). geosociety.org. p. 19.
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  27. ^ Kensley 1980, p. 29.
  28. ^ Farrow, George E.; Durant, Graham P. (May 1985). "Carbonate-basaltic sediments from Cobb Seamount, Northeast Pacific: Zonation, bioerosion and petrology". Marine Geology. 65 (1–2): 98. doi:10.1016/0025-3227(85)90047-7. ISSN 0025-3227.
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  30. ^ Berrisford 1969, p. 397.
  31. ^ Salisbury, Richard (11 August 2015). "A new (Gastropoda: Costellariidae) from the Vema Seamount in the South Atlantic Ocean". Proceedings of the Academy of Natural Sciences of Philadelphia. 164 (1): 17. doi:10.1635/053.164.0104.
  32. ^ Samaai, Toufiek; Gibbons, Mark J.; Kelly, Michelle (2009). "A revision of the genus Strongylodesma Lévi (Porifera: Demospongiae: Latrunculiidae) with descriptions of four new species". Journal of the Marine Biological Association of the United Kingdom. 89 (8): 1692. doi:10.1017/S0025315409000101. hdl:10566/248. ISSN 1469-7769.
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  34. ^ Thandar, Ahmed S. (May 1988). "A new subgenus of Holothuria with a description of a new species from the south-east Atlantic Ocean". Journal of Zoology. 215 (1): 50. doi:10.1111/j.1469-7998.1988.tb04884.x.
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  37. ^ Oldewage, W.H. (2 October 2015). "Three species of piscine parasitic copepods from southern African coastal waters". South African Journal of Zoology. 28 (2): 113. doi:10.1080/02541858.1993.11448303.
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  42. ^ Khalaf, Maroof; Zajonz, Uwe (2007). "Fourteen additional fish species recorded from below 150 m depth in the Gulf of Aqaba, including Liopropoma lunulatum (Pisces: Serranidae), new record for the Red Sea". Fauna of Arabia (23): 428. Retrieved 20 March 2018.
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Sources[edit]