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Discovery Seamounts

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Coordinates: 42°00′S 0°12′E / 42°S 0.2°E / -42; 0.2[1]
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Discovery Seamounts
Location
LocationSouthern Atlantic Ocean
Coordinates42°00′S 0°12′E / 42°S 0.2°E / -42; 0.2[1]

The Discovery Seamounts are a chain of seamounts in the Southern Atlantic Ocean, including Discovery Seamount. The seamounts are 850 kilometres (530 mi) east of Gough Island and once formed islands. Various volcanic rocks as well as glacial dropstones and sediments have been dredged from the Discovery Seamounts.

The Discovery Seamounts appear to be a volcanic seamount chain produced by the Discovery hotspot, whose earliest eruptions occurred 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 rocks from the Mid-Atlantic Ridge that may reflect the presence of a neighbouring hotspot.

Name and discovery

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Discovery Seamount was discovered in 1936 by the research ship RRS Discovery II.[2] It was named Discovery Bank[3] by the crew of a German research ship, RV Schwabenland. Another name, Discovery Tablemount, was coined in 1963. In 1993 the name "Discovery Bank" was transferred by the General Bathymetric Chart of the Oceans to another seamount at Kerguelen, leaving the name "Discovery Seamounts" for the seamount group.[4]

Geography and geomorphology

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The Discovery Seamounts are a group of 12 seamounts[5] 850 kilometres (530 mi) east of Gough Island[2] and southwest from Cape Town.[6] The seamounts are more than 4 kilometres (2.5 mi) high[7] and reach a minimum depth of 426 metres (1,398 ft)[8] or 389 metres (1,276 ft),[9] typically 394–400 metres (1,293–1,312 ft)[10] or 400–500 metres (1,300–1,600 ft). They are guyots,[11] former islands[12][13] that were eroded to a flat plateau and submerged through thermal subsidence of the lithosphere.[14] These seamounts are also referred to as the Discovery Rise and subdivided into a northwestern and a southeastern trend.[15] The group extends over an east-west region of more than 611 kilometres (380 mi) length.[3]

The largest of these seamounts is named Discovery Seamount.[1] It is covered with ice-rafted debris[16] and fossil-containing sediments,[3] which have been used to infer paleoclimate conditions in the region during the Pleistocene.[17] Other evidence has been used to postulate that the seamount subsided by about 0.5 kilometres (0.31 mi) during the late Pleistocene.[18] Other named seamounts are Shannon Seamount southeast and Heardman Seamount due south from Discovery.[19] The seafloor is covered by ponded sediments, sand waves,[20] rocks, rubble and biogenic deposits; sediment covers most of the ground.[21]

The crust underneath Discovery Seamount is about 67 million years (late Cretaceous) old.[12] A fracture zone (a site of crustal weakness) is located nearby.[22]

Geology

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The Southern Atlantic Ocean contains a number of volcanic systems such as the Discovery Seamounts, the Rio Grande Rise, the Shona Ridge and the Walvis Ridge. Their existence is commonly attributed to hotspots,[23] although this interpretation has been challenged.[1] The hotspot origin of Discovery and the Walvis–Tristan da Cunha seamount chains was proposed first in 1972.[2] In the case of the Shona Ridge and the Discovery Seamounts, the theory postulates that they formed as the African Plate moved over the Shona hotspot and the Discovery hotspot, respectively.[24]

The Discovery hotspot, if it exists,[1] would be located southwest of the Discovery Seamounts,[15] off the Mid-Atlantic Ridge.[25] The seamounts wane out in that direction, but the Little Ridge close to the Mid-Atlantic Ridge may be their continuation[26] after the hotspot crossed the Agulhas Fracture Zone.[27] The Discovery Ridge close to the Mid-Atlantic Ridge may come from the hotspot as well.[28] Low seismic velocity anomalies have been detected in the mantle southwest of the Discovery Seamounts and may constitute the Discovery hotspot.[29] Deeper in the mantle, the Discovery hotspot appears to connect with the Shona and Tristan hotspots to a single plume, which in turn emanates from the African superplume[30] and might form a "curtain" of hotspots at the edge of the superplume.[31] Material from the Discovery hotspot reached as far as Patagonia in South America, where it appears in volcanoes.[32]

Magma may flow from the Discovery hotspot to the Mid-Atlantic Ridge,[33] feeding the production of excess crustal material[34] at its intersection with the Agulhas-Falklands Fracture Zone,[35] one of the largest transform faults of Earth.[36] There is a region on the Mid-Atlantic Ridge southwest of the seamounts where there are fewer earthquakes than elsewhere along the ridge, the central valley of the ridge is absent,[37] and where dredged rocks share geochemical traits with the Discovery Seamount.[15] Petrological anomalies at spreading ridges have been often attributed to the presence of mantle plumes close to the ridge, and such has been proposed for the Discovery hotspot as well.[38] Alternatively, the Discovery hotspot may have interacted with the ridge in the past, and the present-day mantle temperature and neodymium isotope anomalies next to the ridge could be left from this past interaction.[39]

The Agulhas-Falkland fracture zone has an unusual structure on the African Plate, where it displays the Agulhas Ridge, two over 2-kilometre (1.2 mi) high ridge segments which are parallel to each other.[40] This unusual structure may be due to magma from the Discovery hotspot, which would have been channelled to the Agulhas Ridge.[41]

Whether there is a link between the Discovery hotspot and Gough Island[1] or the Tristan hotspot is unclear.[42] An alternative hypothesis is that the Discovery Seamounts formed when magma rose along a fracture zone or other crustal weakness.[43]

Composition

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Rocks dredged from the seamounts include lavas, pillow lavas and volcaniclastic rocks.[44] Geochemically they are classified as alkali basalt, basalt, phonolite, tephriphonolite,[11] trachyandesite, trachybasalt and trachyte.[45] Minerals contained in the rocks include alkali feldspar, apatite, biotite, clinopyroxene, iron and titanium oxides, olivine, plagioclase, sphene and spinel.[11] Other rocks are continental crust rocks, probably glacial dropstones,[46] and manganese.[44]

The Discovery hotspot appears to have erupted two separate sets of magmas with distinct compositions[47] in a north-south pattern,[48] similar to the Tristan da Cunha-Gough Island hotspot.[47] The composition of the Discovery Seamounts rocks has been compared to Gough Island.[23] The more felsic rocks at Discovery appear to have formed in magma chambers, similar to felsic rocks at other Atlantic Ocean islands.[49]

Biology

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Seamounts tend to concentrate food sources from seawater and thus draw numerous animal species.[5] In the Discovery Seamounts they include bamboo corals, brachiopods, cephalopods, cirripedes, sea fans, sea urchins and sea whips.[50][51][52] There are c. 150 fish species at Discovery Seamount,[53] including the pygmy flounder;[54] the deep-sea hatchetfish Maurolicus inventionis[55] and the codling Guttigadus nudirostre are endemic to Discovery Seamount.[56] Fossil corals have been recovered in dredges,[57] while no stone coral colonies were reported during a 2019 investigation.[51]

Both Japanese and Soviet fishers trawled the seamounts during the 1970s and 1980s, but there was no commercial exploitation of the resources.[58] Observations in 2019[50] detected changes in the Discovery Seamount ecosystems that may be due to fishing or sea urchin outbreaks.[59]

Eruption history

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A number of dates ranging from 41 to 35 million years ago have been obtained on dredged samples from the seamounts on the basis of argon-argon dating.[15] The age of the seamounts decreases in southwest direction, similar to the Walvis Ridge, and at a similar rate.[13] It is possible that Discovery Seamount split into a northern and southern part about 20 million years ago.[60] Activity there may have continued until 7-6.5 million years ago.[18]

Unlike the Walvis Ridge, which is connected to the Etendeka flood basalts, the Discovery Seamounts do not link with onshore volcanic features.[23] However, it has been proposed that the 70- to 80-million-year-old Blue Hills, Gibeon and Gross Brukkaros kimberlite fields in southern Namibia may have been formed by the Discovery hotspot,[61] and some plate reconstructions place it underneath the Karoo-Ferrar large igneous province at the time at which it was emplaced.[62] Kimberlites in South Africa[31] and Greater Cederberg-False Bay large igneous province has been associated with the Discovery hotspot.[63] The latter large igneous province may have formed at a triple junction around the nascent South Atlantic Ocean,[31] and, together with hotspots farther north, precipitated the rifting of the South Atlantic.[64] Between 60 and 40 million years ago the hotspot was located close to the spreading ridge of the South Atlantic.[60]

References

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  1. ^ a b c d e Jokat & Reents 2017, p. 78.
  2. ^ a b c Kempe & Schilling 1974, p. 101.
  3. ^ a b c Buckley 1976, p. 937.
  4. ^ Summerhayes, Colin; Lüdecke, Cornelia (2012). "A German Contribution to South Atlantic Seabed Studies, 1938-39" (PDF). Polarforschung. 82 (2): 100. doi:10.2312/polarforschung.82.2.93. S2CID 56109064. Archived (PDF) from the original on 1 November 2018. Retrieved 19 March 2018.
  5. ^ a b Perez et al. 2022, p. 3.
  6. ^ Werner & Hauff 2011, p. 6.
  7. ^ Werner & Hauff 2011, p. 4.
  8. ^ Richardson, Philip L. (August 2007). "Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters" (PDF). Deep Sea Research Part I: Oceanographic Research Papers. 54 (8): 1378. Bibcode:2007DSRI...54.1361R. doi:10.1016/j.dsr.2007.04.010. hdl:1912/2579. ISSN 0967-0637. S2CID 140704046. Archived from the original on 2024-03-16. Retrieved 2019-09-30.
  9. ^ 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): 133–152. doi:10.2205/2004ES000146. Archived from the original on 19 March 2018. Retrieved 19 March 2018.
  10. ^ Perez et al. 2022, p. 6.
  11. ^ a b c Schwindrofska et al. 2016, p. 169.
  12. ^ a b Kempe & Schilling 1974, p. 102.
  13. ^ a b Schwindrofska et al. 2016, p. 170.
  14. ^ Werner & Hauff 2011, p. 20.
  15. ^ a b c d Schwindrofska et al. 2016, p. 168.
  16. ^ Buckley 1976, p. 945.
  17. ^ Buckley 1976, pp. 943–944.
  18. ^ a b Buckley 1976, p. 947.
  19. ^ Perez et al. 2022, p. 2.
  20. ^ Perez et al. 2022, p. 4.
  21. ^ Perez et al. 2022, p. 7.
  22. ^ Jokat & Reents 2017, p. 84.
  23. ^ a b c Jokat & Reents 2017, p. 77.
  24. ^ le Roex et al. 2010, p. 2090.
  25. ^ Parnell-Turner 2024, p. 8.
  26. ^ Schwindrofska et al. 2016, p. 171.
  27. ^ Parnell-Turner 2024, p. 3.
  28. ^ Gibson & Richards 2018, p. 209.
  29. ^ Homrighausen et al. 2018, p. 6.
  30. ^ Zhou, H.; Hoernle, K.; Geldmacher, J.; Hauff, F.; Homrighausen, S.; Garbe-Schönberg, D.; Jung, S.; Bindeman, I. (November 2022). "A HIMU volcanic belt along the SW African coast (~83–49 Ma): New geochemical clues to deep mantle dynamics from carbonatite and silica-undersaturated complexes in Namibia". Lithos. 430–431: 14. Bibcode:2022Litho.43006839Z. doi:10.1016/j.lithos.2022.106839. S2CID 251659165.
  31. ^ a b c Kingsbury et al. 2023, p. 482.
  32. ^ Søager et al. 2021, p. 54.
  33. ^ Gibson & Richards 2018, p. 205.
  34. ^ Gibson & Richards 2018, p. 216.
  35. ^ Tu et al. 2023, p. 1.
  36. ^ Uenzelmann-Neben & Gohl 2004, p. 305.
  37. ^ de Alteriis, G.; Gilg-Capar, L.; Olivet, J.L. (July 1998). "Matching satellite-derived gravity signatures and seismicity patterns along mid-ocean ridges". Terra Nova. 10 (4): 181. Bibcode:1998TeNov..10..177D. doi:10.1046/j.1365-3121.1998.00190.x. S2CID 129097438.
  38. ^ Douglass et al. 1995, p. 2893.
  39. ^ Tu et al. 2023, p. 11.
  40. ^ Uenzelmann-Neben & Gohl 2004, p. 306.
  41. ^ Uenzelmann-Neben & Gohl 2004, p. 316.
  42. ^ Sushchevskaya et al. 2019, p. 129.
  43. ^ Jokat & Reents 2017, p. 89.
  44. ^ a b Werner & Hauff 2011, p. 11.
  45. ^ le Roex et al. 2010, p. 2094.
  46. ^ le Roex et al. 2010, p. 2093.
  47. ^ a b Schwindrofska et al. 2016, p. 175.
  48. ^ Søager et al. 2021, p. 42.
  49. ^ le Roex et al. 2010, p. 2109.
  50. ^ a b Buhl-Mortensen, Braga-Henriques & Stevenson 2022, p. 1.
  51. ^ a b Perez et al. 2022, p. 10.
  52. ^ Perez et al. 2022, p. 5.
  53. ^ Balushkin, A. V.; Prirodina, V. P. (1 March 2010). "Findings of Andriashevs eel cod Muraenolepis andriashevi (Gadiformes: Muraenolepididae) at Discovery Seamount (South Atlantic)". Russian Journal of Marine Biology. 36 (2): 133. Bibcode:2010RuJMB..36..133B. doi:10.1134/S1063074010020082. ISSN 1063-0740. S2CID 45673975.
  54. ^ Voronina, Elena P.; Sideleva, Valentina G.; Hughes, Dianne R. (21 September 2019). "Lateral line system of flatfishes (Pleuronectiformes): Diversity and taxonomic distribution of its characters". Acta Zoologica. 102: 24. doi:10.1111/azo.12311. S2CID 203877922.
  55. ^ Rees, David J.; Poulsen, Jan Y.; Sutton, Tracey T.; Costa, Paulo A. S.; Landaeta, Mauricio F. (25 November 2020). "Global phylogeography suggests extensive eucosmopolitanism in Mesopelagic Fishes (Maurolicus: Sternoptychidae)". Scientific Reports. 10 (1): 9. doi:10.1038/s41598-020-77528-7. ISSN 2045-2322. PMC 7689477. PMID 33239750. S2CID 227175592.
  56. ^ Meléndez, Roberto C.; Markle, Douglas F. (1 November 1997). "Phylogeny and Zoogeography of Laemonema and Guttigadus (Pisces; Gadiformes; Moridae)". Bulletin of Marine Science. 61 (3): 663. Archived from the original on 18 July 2018. Retrieved 19 March 2018.
  57. ^ Werner & Hauff 2011, p. 24.
  58. ^ Tony J. PitcherTelmo MoratoPaul J. B. HartMalcolm R. ClarkNigel HagganRicardo S. Santos (2007). Pitcher, Tony J; Morato, Telmo; Hart, Paul J. B; Clark, Malcolm R; Haggan, Nigel; Santos, Ricardo S (eds.). Seamounts ecology, fisheries & conservation. Oxford: Blackwell Publishing. pp. 384–385. doi:10.1002/9780470691953. ISBN 9780470691953.
  59. ^ Buhl-Mortensen, Braga-Henriques & Stevenson 2022, p. 4.
  60. ^ a b Sushchevskaya et al. 2019, p. 130.
  61. ^ Reid, D. L.; Cooper, A. F.; Rex, D. C.; Harmer, R. E. (2009). "Timing of post–Karoo alkaline volcanism in southern Namibia". Geological Magazine. 127 (5): 430. doi:10.1017/S001675680001517X. ISSN 1469-5081. S2CID 128395266.
  62. ^ Storey, Bryan C.; Leat, Philip T.; Ferris, Julie K. (2001). Special Paper 352: Mantle plumes: Their identification through time. Vol. 352. p. 77. doi:10.1130/0-8137-2352-3.71. ISBN 978-0-8137-2352-5.
  63. ^ Kingsbury et al. 2023, p. 481.
  64. ^ Kingsbury et al. 2023, p. 483.

Sources

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