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Vema Seamount

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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) northwest 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.

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

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

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Vema Seamount was discovered by the research ship RV Vema of the Lamont–Doherty Earth Observatory in 1959.[1][2] Vema was 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] It lies in international waters[6] and its summit is so shallow that it is a navigation hazard to ships.[7]

Geomorphology and geography

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Vema Seamount lies in the South Atlantic Ocean, 1,600 kilometres (1,000 mi) from Tristan da Cunha.[1] The cities of Cape Town and Lüderitz lie east-southeast and northeast of Vema, respectively;[8] the distance from Cape Town is about 1,000 kilometres (620 mi).[9] An unrelated seamount with the same name lies in the North Atlantic, close to the Mid-Atlantic Ridge.[10]

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[1][2] - later determined to be 21.5 metres (71 ft) deep -[11] and is called Collins Point.[12] At least one source gives a minimum depth of 11 metres (36 ft) for the seamount,[13] while recent bathymetric surveys have found a minimum depth of 21.5 metres (71 ft).[14] The flat top is a summit plateau with a width of 8.0 kilometres (5 mi)[2] and as more recently determined 11 by 8.5 kilometres (6.8 mi × 5.3 mi)[14] at a mean depth of 73 metres (40 fathoms)[3] 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).[12] The summit plateau mostly consists of hard rock, like the upper slopes,[11] with rocky outcrops separated by sandy plains.[15] The plateau appears to be a wave-cut platform of Pleistocene age, when sea levels were lower,[3] and is swept by strong ocean currents.[16]

The seamount rises from a depth of 4,600 metres (2,500 fathoms), where it occupies a breadth of 56 kilometres (35 mi)[2] and forms an isolated conical feature.[14] The seafloor from which Vema rises is part of the abyssal plain of the Cape Basin.[1] From there, the slopes of Vema first rise steeply and feature subsidiary summits; above 130 metres (70 fathoms) depth the slopes flatten.[3]

Geology

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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-containing basalt has also been found.[3] A minimum age of 11.0 ± 0.3 million years has been obtained from samples taken at Collins Point by potassium-argon dating,[3] with another age estimate being 15 million years.[17] 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.[18] Light-coloured rocks on the summit platform may constitute a former carbonate platform.[19]

Vema is an intraplate volcano.[17] It is considered to be the present-day location of a hotspot, the Vema hotspot,[20] although the hotspot itself may have moved farther south[21] or west (by about 200 kilometres (120 mi)) since when it created the Vema Seamount[22] and may not be active anymore.[23] Earlier volcanism[a] caused by the Vema hotspot may have manifested itself in southern Namibia in the form of alkaline volcanics[29] that define a HIMU[b] suite.[24]

Seismic tomography has shown what may be a mantle plume underneath Vema,[30] another theory considers the Vema hotspot is a consequence of the Tristan hotspot shedding a secondary diapir.[31] The hotspot origin of the Vema Seamount is not universally agreed upon.[32]

Water conditions

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Water temperatures at Vema range between 18–21 °C (64–70 °F),[33] and decrease with increasing depth.[14] The cold Benguela Current does not reach the seamount,[5] which is instead influenced by the South Atlantic oceanic gyre.[34] The movement and strength of ocean eddies are altered when they interact with Vema Seamount,[35] with Agulhas eddies often splitting apart at the seamount.[36] During the ice ages, sea level drop may have exposed part of the summit platform.[37]

Biology

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The seamount features both shallow-water and deep-water life.[38] Its summit is shallow enough that sunlight can reach it, resulting in the growth of various types of algae and seaweeds such as Ecklonia kelp.[3] Such kelp covers large parts of the seamount,[39] and a coral framework makes up much of the summit platform.[11]

A number of animals inhabit Vema, usually hidden or encrusting animals.[40] Ascidians,[39][3] black corals,[41] non-reef building corals[42] including gorgonia and scleractinia,[41] crayfish such as the rock lobster (Jasus tristani),[34] and other decapods,[43] holothurians, hydroids, polyzoa,[39][3] sea fans[44] and sponges live on the seamount.[39][3] Other animals such as bryozoans, echinoderms, gastropods, oysters, pelecypods, serpulids and other worms have also left their traces on Vema.[3] Rock lobsters propagate from Gough Island and Tristan da Cunha to Vema Seamount,[16] while other species appear to originate from South Africa.[45]

Several species appear to be endemic to Vema Seamount, including the sea snail species Austromitra rosenbergi discovered in 2015[46] and the sponge Strongylodesma areolata described in 1969;[47] 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.[48] The holothurian Holothuria vemae is named after the seamount, where it was discovered in 1965-1966[49] as is the sea snail Trivia vemacola.[50] Vema Seamount is the type locality for the deepwater sponge Desmacidon clavata.[51]

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.[43] Fishing operations have attracted seabirds to Vema Seamount.[52] Humpback whales have been detected through their sounds,[53] and baleen whales feed at Vema.[54] Euphausiids and copepods are also found in the waters,[3] including at least one copepod that parasitizes fish.[55] Among the fish species encountered at Vema Seamount are:

Fish on the seamount were commercially fished.[3][c] Rock lobsters in particular were heavily exploited;[43] they disappeared from Vema Seamount after overfishing in the 1960s, briefly recovered and then disappeared again by 1981 due to renewed overfishing.[16][9] The collapse of this fishery is one of the first instances of a seamount fishery collapsing,[64] and has been cited as an example of how fisheries outside of exclusive economic zones end up ungoverned and abused.[65] Vema Seamount was closed to fishery in 2007[38] by the South East Atlantic Fisheries Organisation,[66] and man-made debris such as crab traps[67] and ropes can be found there.[68] The ecosystem is classified as "threatened" by the Food and Agriculture Organization.[38]

Notes

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  1. ^ Including the Gibeon kimberlites and carbonatites[24] (79–72 million years old), the 77 ± 2 million years old Gross Brukkaros, the 49 ± 1 million years old Dicker Willem carbonatites[25] (and associated Tsirub nephelinites[26]), the 46 million years old Klinghardt phonolites, the 37 million years old Swartkop phonolite and the 36 million years old Schwarzeberg nephelinite.[25] Other potential correlates are the melilites close to the mouth of the Orange River, which are 37 million years old,[27] or even the Karoo-Ferrar large igneous province.[28]
  2. ^ HIMU is a suite of mantle melts with high uranium-non-wiktionary:radiogenic lead ratios and a high ratio of radiogenic to non-radiogenic lead.[21]
  3. ^ In the late 1970s, Mackerel scad fishing commenced at Vema, followed in the 1980s[56] by tuna.[63]

References

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  1. ^ a b c d e Berrisford 1969, p. 387.
  2. ^ a b c d e Simpson & Heydorn 1965, p. 249.
  3. ^ a b c d e f g h i j k l m n o p q Simpson & Heydorn 1965, p. 251.
  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. ^ Maxwell, Jamie; Taboada, Sergi; Taylor, Michelle L. (1 July 2022). "Gorgoniapolynoe caeciliae revisited: The discovery of new species and molecular connectivity in deep-sea commensal polynoids from the Central Atlantic". Deep Sea Research Part I: Oceanographic Research Papers. 185: 3. Bibcode:2022DSRI..18503804M. doi:10.1016/j.dsr.2022.103804. ISSN 0967-0637.
  11. ^ a b c Bergstad et al. 2019, p. 15.
  12. ^ a b Simpson & Heydorn 1965, p. 250.
  13. ^ 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.
  14. ^ a b c d Bergstad et al. 2019, p. 39.
  15. ^ Bergstad et al. 2019, p. 16.
  16. ^ 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. Bibcode:1981DSRA...28.1237L. doi:10.1016/0198-0149(81)90060-1. ISSN 0198-0149.
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  18. ^ O'Connor & le Roex 1992, p. 347.
  19. ^ Bergstad et al. 2019, p. 22.
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  22. ^ Morgan, W. Jason; Morgan, Jason Phipps (2007). "Plate velocities in hotspot reference frame: electronic supplement". geosociety.org: 19. doi:10.1130/2007090.
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  24. ^ a b Zhou et al. 2022, p. 1.
  25. ^ a b Zhou et al. 2022, p. 12.
  26. ^ Nakashole, Albertina; le Roex, Anton; Reid, David (1 February 2020). "Geochemistry and petrogenesis of the Tsirub nephelinite intrusions, southern Namibia". Journal of African Earth Sciences. 162: 18. Bibcode:2020JAfES.16203701N. doi:10.1016/j.jafrearsci.2019.103701. ISSN 1464-343X. S2CID 210281752.
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  31. ^ O'Neill, Craig; Müller, Dietmar; Steinberger, Bernhard (April 2005). "On the uncertainties in hot spot reconstructions and the significance of moving hot spot reference frames". Geochemistry, Geophysics, Geosystems. 6 (4): 23. Bibcode:2005GGG.....6.4003O. doi:10.1029/2004GC000784.
  32. ^ Mark A. Richards, Richard G. Gordon, Rob D. van der Hilst 2000, p. 343.
  33. ^ Kensley 1980, p. 31.
  34. ^ a b Elwen et al. 2023, p. 2.
  35. ^ Matano, R. P.; Beier, E. J. (1 December 2002). "On the mesoscale dynamics of the Indian/Atlantic interocean exchange". AGU Fall Meeting Abstracts. 22: OS22E–03. Bibcode:2002AGUFMOS22E..03M.
  36. ^ Giulivi, Claudia F.; Gordon, Arnold L. (August 2006). "Isopycnal displacements within the Cape Basin thermocline as revealed by the Hydrographic Data Archive". Deep Sea Research Part I: Oceanographic Research Papers. 53 (8): 1292. Bibcode:2006DSRI...53.1285G. doi:10.1016/j.dsr.2006.05.011. ISSN 0967-0637.
  37. ^ Bergstad et al. 2019, p. 23.
  38. ^ a b c Ross-Marsh et al. 2022, p. 1.
  39. ^ a b c d Berrisford 1969, p. 396.
  40. ^ Kensley 1980, p. 29.
  41. ^ a b Bergstad et al. 2019, p. 37.
  42. ^ 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. Bibcode:1985MGeol..65...73F. doi:10.1016/0025-3227(85)90047-7. ISSN 0025-3227.
  43. ^ a b c Kensley 1980, p. 30.
  44. ^ Bergstad et al. 2019, p. 40.
  45. ^ Berrisford 1969, p. 397.
  46. ^ 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. S2CID 128660650.
  47. ^ Samaai, Toufiek; Gibbons, Mark J.; Kelly, Michelle (2009). "A revision of the genus Strongylodesma Lévi (Porifera: Demospongiae: Latrunculiidae) with descriptions of four new species" (PDF). Journal of the Marine Biological Association of the United Kingdom. 89 (8): 1692. Bibcode:2009JMBUK..89.1689S. doi:10.1017/S0025315409000101. hdl:10566/248. ISSN 1469-7769. S2CID 46616991.
  48. ^ Richer de Forges, Bertrand; Koslow, J. Anthony; Poore, G. C. B. (2000). "Diversity and endemism of the benthic seamount fauna in the southwest Pacific". Nature. 405 (6789): 944–7. Bibcode:2000Natur.405..944R. doi:10.1038/35016066. ISSN 1476-4687. PMID 10879534. S2CID 4382477.
  49. ^ 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.
  50. ^ Liltved, W.R. (1987). "A new Trivia (Triviidae) and Primovula (Ovulidae)(Gastropoda: Prosobranchia) from the South Atlantic and Southwest Indian Oceans". The Veliger. 29: 417. ISSN 0042-3211.
  51. ^ Soest, Rob W. M. Van; Hooper, John N. A.; Butler, Peter J. (28 February 2020). "Every sponge its own name: removing Porifera homonyms". Zootaxa. 4745 (1): 17. doi:10.11646/zootaxa.4745.1.1. ISSN 1175-5334. PMID 32230307. S2CID 214748168 – via ResearchGate.
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  54. ^ Elwen et al. 2023, p. 9.
  55. ^ 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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  58. ^ Anderson, William D.; Murdy, Edward O. (October 2006). "Meganthias carpenteri, new species of fish from the eastern Atlantic Ocean, with a key to eastern Atlantic Anthiinae (Perciformes: Serranidae)". Proceedings of the Biological Society of Washington. 119 (3): 411. doi:10.2988/0006-324X(2006)119[404:MCNSOF]2.0.CO;2. S2CID 85822828.
  59. ^ Andrew, Timothy G.; Buxton, Colin D.; Hecht, Thomas (1 June 1996). "Aspects of the reproductive biology of the concha wrasse, Nelabrichthys ornatus, at Tristan da Cunha". Environmental Biology of Fishes. 46 (2): 139. Bibcode:1996EnvBF..46..139A. doi:10.1007/BF00005215. ISSN 0378-1909. S2CID 39535647.
  60. ^ 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.
  61. ^ a b Edwards 2007, p. 501.
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  63. ^ FAO (2006). Deep Sea 2003: Conference poster papers and workshop papers. FAO. p. 208. ISBN 9789251054574.
  64. ^ Tony J. Pitcher 2007, p. 52.
  65. ^ Vousden, D.; Scott, L. E. P.; Sauer, W.; Bornman, T. G.; Ngoile, M.; Stapley, J.; Lutjeharms, J. R. E. (2008). "Establishing a basis for ecosystem management in the western Indian Ocean". South African Journal of Science. 104 (11–12): 417–420. ISSN 0038-2353.
  66. ^ Bergstad et al. 2019, p. 30.
  67. ^ Bergstad et al. 2019, p. 41.
  68. ^ Bergstad et al. 2019, p. 49.

Sources

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Further reading

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