Ita Mai Tai

Bathymetry of Ita Mai Tai Guyot. The smaller guyot in the lower left corner is Gelendzhik Guyot

Coordinates: 12°54′N 156°54′E / 12.9°N 156.9°E / 12.9; 156.9[1]Ita Mai Tai is a Cretaceous-early Cenozoic seamount northwest of the Marshall Islands and north of Micronesia.[2] One among a number of seamounts in the Pacific Ocean, it is part of the Magellan Seamounts which may have a hotspot origin although Ita Mai Tai itself may not have formed on a hotspot.

The seamount is formed by volcanic rocks which form two adjacent volcanic centres that erupted between the Aptian-Albian and possibly as late as the Pliocene. Reef systems developed on the seamount after its formation and led to the deposition of limestones. Especially during the Oligocene the seamount subsided and lies now at 1,402 metres (4,600 ft) depth below sea level. Ferromanganese crusts as well as pelagic oozes were deposited on the submerged rocks.

Name and research history

The name Ita Mai Tai comes from the Tahitian language and means "no damn good". The name was coined by Bruce C. Heezen and is probably a reference to unsuccessful attempts to obtain drill cores during the early research history of the seamount.[3] The seamount has also been named OSM1[2] and Ita Matai.[4] The Deep Sea Drilling Project drill cores 202,[5] 201 and 200 were taken at Ita Mai Tai, a drill site selection that was motivated in part by technical problems in the drilling equipment.[6]

Geography and geology

Regional

The Pacific Ocean seafloor is characterized by a striking contrast between the relatively flat floor of the Eastern Pacific and the Western Pacific whose seafloor is dotted by oceanic plateaus and seamounts. These structures may have formed on top of large Cretaceous uplift episodes, moving hotspots, mid-ocean ridges and transform faults.[3]

Ita Mai Tai is considered to be part of the Magellan Seamounts,[7] a chain of seamounts that extends northwest away from this seamount,[2] and one of their best studied members.[8] The activity of the Magellan Seamounts has been attributed to a hotspot in the South Pacific,[9] but attributing Ita Mai Tai to such a hotspot is difficult as Ita Mai Tai appears to be too old in comparison to the other Magellan Seamounts to be a product of the same hotspot.[10] The Rarotonga hotspot, Samoa hotspot and Society hotspot appear to coincide with the reconstructed location of the Magellan Seamounts hotspot; one of these may have formed the Magellan Seamounts.[11]

Local

Ita Mai Tai has a flat summit with a surface area of 650 square kilometres (250 sq mi),[12] and a slope break at about 2,200 metres (7,200 ft) depth.[13] Unconsolidated sediments cover the summit platform.[14] There is evidence that the flat summit was a lagoon surrounded by a coral reef,[15] and the volcanic basement forms an uplift in the central section of the flat summit.[16] Volcanic cones form swells on the western part of the summit plateau of Ita Mai Tai,[17] and structures such as domes, ridges, scarps, steps and terraces are dispersed all over the seamount.[14]

The seamount is about 4.6 kilometres (2.9 mi) high and reaches a depth of 1,402 metres (4,600 ft) below sea level. On the seafloor, it occupies a surface of 6,400 square kilometres (2,500 sq mi), making it much larger than other Pacific seamounts, and is surrounded by a shallow moat on the northern and southeastern side.[13] The outer slopes of the seamount have a step-like appearance[18] and feature radial grabens formed presumably by subsidence.[14] At their foot, sediments descending from the seamount has formed talus deposits.[19]

The seamount has several rift zones crosscut by dykes and sills[12] and features an "L"-shaped ridge to the west[13] with a width of 10–15 kilometres (6.2–9.3 mi).[20] South of the "L"-shaped ridge lies another seamount which is also considered to be part of Ita Mai Tai; it is uneroded and features parasitic vents. The ridge that connects the two may be the western edge of a collapse caldera.[21] This 13 kilometres (8.1 mi) wide and 2,525 metres (8,284 ft) deep[20] southern seamount is also known as Gelendzhik Seamount[22] after a research ship of the same name[23] and forms a volcano-tectonic massif with Ita Mai Tai;[24] thus it consists of two separate volcanoes.[21]

The seamount lies on the eastern margin of the Mariana Basin. The lack of magnetic lineations on the seafloor surrounding Ita Mai Tai[3] makes it difficult to tell how old the ocean crust is. However, during the Aptian neighbouring volcanic islands deposited volcanic rocks on the seafloor[13] and the crust is now considered to be of Jurassic age.[16] The Ogasawara Fracture Zone passes just north of Ita Mai Tai;[25] seamounts in the neighbourhood are Butakov in the south, Arirang in the southeast, Zatonskii east, Gramberg northeast and Fedorov north-northwest.[26]

Composition

Among the rocks found at Ita Mai Tai are alkali basalts,[27] basalts,[28] clays,[19] hawaiites,[29] limestone, muds,[12] picrites,[19] tholeiites, trachytes and trachybasalt;[19] volcanic rocks contain potassium feldspar and plagioclase.[30]

The volcanic rocks have been subdivided into a lower tholeiitic subunit and an upper more trachytic unit; there are also compositional differences between various parts of the seamount.[19] Some of the volcanic rocks take the form of breccia,[24] lava, tuffs and tuffites.[18] The limestone takes the form of siltstone, sandstone, gravelstone and coquina.[31] In drill cores of the summit region the limestone reaches a thickness of 35 metres (115 ft) and the mud of 45 metres (148 ft); the mud formed in lagoonal settings.[32] Terrigenous rocks have also been encountered within the limestones.[24]

Guyots such as Ita Mai Tai often accumulate ferromanganese crusts. These are generated by the oxidative precipitation of manganese salts which also include iron[33] and absorb trace elements such as cobalt, copper, molybdenum, nickel, platin, rare earth elements and zinc from the water through as-yet unknown processes.[21] In the case of Ita Mai Tai these crusts have been found all over the seamount and sometimes reach thicknesses of over 20 centimetres (7.9 in),[7] with geochemical differences between the various sectors of the seamount.[34] These ferromanganese crusts have aroused scientific interest in the seamount.[20] Some evidence of hydrothermal alteration has been found in the form of barite deposits within the ferromanganese crusts.[35]

Geologic history

Ita Mai Tai erupted first during the Albian and Aptian periods.[36] Another episode of volcanic activity may constitute late stage volcanism; it might be represented by Campanian volcaniclastic rocks,[37] an Eocene dome[36][38] and Pliocene cones on Gelendzhik seamount.[37] such late volcanism has been observed in other neighbouring seamounts as well.[38] An uplift episode took place during the Cretaceous.[39] Radiometric dating has yielded ages of about 118-120 million years ago.[40]

At least during the Paleocene, Ita Mai Tai emerged above sea level.[41] From the Aptian to the Miocene, carbonates were deposited on the seamount[8] and reached an eventual thickness of about 525 metres (1,722 ft).[41] Additionally, the seamount has subsided by about 2,090 metres (6,860 ft), albeit with time periods where this subsidence was interrupted by the growth of coral reefs.[42] Most of the subsidence occurred during the Oligocene when sedimentation rates were depressed,[28] but the carbonate platform drowned no later than the Eocene,[43] with oolithes forming underwater.[44]

During three different episodes in the Aptian-Turonian, Santonian-Maastrichtian and Paleocene-Eocene,[45] oolithic limestones were deposited on Ita Mai Tai, presumably by reefs and living organisms in shallow water. Lifeforms that inhabited the seamount included algae, belemnites, bivalves, bryozoans, corals, decapods, echinoderms, foraminifera, gastropods,[46][24] ostracods,[47] pogonophora, rudists[24][48] and sea urchins;[19] their fossils have been recovered in the limestone from Ita Mai Tai[24][48] and the rudists are the most commonly encountered reef builders on this seamount.[49][50] The oolith-containing limestone[51] was formed by a coral reef. The reef was affected by wave activity[52] and there were lagoonal environments as well.[53] Bioherms developed on the Gelendzhik seamount as well,[24] where cephalopods including belemnites have been found.[54]

During the Eocene to Quaternary, foraminiferal ooze accumulated on the guyot[6] at a rate of 6.7 millimetres per millennium (0.26 in/ka)[55] but with occasional erosional periods which show up as hiatuses in the sedimentary record,[28] the ooze also contains fish teeth and radiolarian fossils.[56] This sediment layer is unusually thick by the standard of other Pacific Ocean seamounts,[57] its thickness reaching 150 metres (490 ft)[58]-170 metres (560 ft).[48] Presently, scleractinian corals without zooxanthelles form "meadows" and "patches" on the surface of Ita Mai Tai.[59]

See also

References

  1. ↑ Lee et al. 2003, p. 359.
  2. 1 2 3 Lee et al. 2003, p. 356.
  3. 1 2 3 Wedgeworth & Kellogg 1987, p. 73.
  4. ↑ "Diagenesis of a seamount oolite from the West Pacific, Leg 20, DSDP". Scientific Ocean Drilling Database. Retrieved 30 September 2018.
  5. ↑ The Shipboard Scientific Party 1973b, p. 87.
  6. 1 2 The Shipboard Scientific Party 1973, p. 87.
  7. 1 2 Asavin et al. 2010, p. 426.
  8. 1 2 Mel'nikov et al. 2012, p. 217.
  9. ↑ Koppers et al. 1998, p. 54.
  10. ↑ Koppers et al. 1998, p. 61.
  11. ↑ Koppers et al. 1998, p. 66.
  12. 1 2 3 Lee et al. 2005, p. 1935.
  13. 1 2 3 4 Wedgeworth & Kellogg 1987, p. 74.
  14. 1 2 3 Mel'nikov et al. 2012, p. 219.
  15. ↑ Wedgeworth & Kellogg 1987, p. 79.
  16. 1 2 Mel'nikov, Tugolesov & Pletnev 2010, p. 583.
  17. ↑ Mel'nikov et al. 2016, p. 440.
  18. 1 2 Mel'nikov, Tugolesov & Pletnev 2010, p. 586.
  19. 1 2 3 4 5 6 Mel'nikov et al. 2012, p. 220.
  20. 1 2 3 Mel'nikov et al. 2012, p. 218.
  21. 1 2 3 Asavin et al. 2010, p. 424.
  22. ↑ Mel'nikov, Tugolesov & Pletnev 2010, p. 584.
  23. ↑ "GEBCO Gazetteer of Undersea Feature Names". GEBCO. Retrieved 16 September 2018.
  24. 1 2 3 4 5 6 7 Zakharov et al. 2007, p. 36.
  25. ↑ Koppers et al. 1998, p. 56.
  26. ↑ Mel'nikov, M. E.; Avdonin, V. V.; Pletnev, S. P.; Sedysheva, T. E. (January 2016). "Buried ferromanganese nodules of the Magellan Seamounts". Lithology and Mineral Resources. 51 (1): 4. doi:10.1134/s0024490215060073. ISSN 0024-4902.
  27. ↑ Prokof'ev, V. Yu.; Avdonin, V. V.; Mel'nikov, M. E. (31 August 2008). "Physicochemical parameters of the crystallization of plagioclases in basaltic rocks from guyots of the Magellan Seamounts (Pacific Ocean)". Doklady Earth Sciences. 421 (2): 996. doi:10.1134/S1028334X08060305.
  28. 1 2 3 Mel'nikov, Tugolesov & Pletnev 2010, p. 588.
  29. ↑ Koppers et al. 1998, p. 55.
  30. ↑ Koppers, Anthony A.P; Staudigel, Hubert; Wijbrans, Jan R (May 2000). "Dating crystalline groundmass separates of altered Cretaceous seamount basalts by the 40Ar/39Ar incremental heating technique". Chemical Geology. 166 (1–2): 145. doi:10.1016/S0009-2541(99)00188-6. ISSN 0009-2541.
  31. ↑ Mel'nikov et al. 2012, p. 221.
  32. ↑ Wedgeworth & Kellogg 1987, p. 77,79.
  33. ↑ Asavin et al. 2010, p. 423.
  34. ↑ Asavin et al. 2010, p. 444.
  35. ↑ Asavin et al. 2010, p. 430.
  36. 1 2 Wedgeworth & Kellogg 1987, p. 83.
  37. 1 2 Mel'nikov et al. 2016, p. 439.
  38. 1 2 Mel'nikov et al. 2012, p. 228.
  39. ↑ Petersen, L.D.; Duennebier, F.K.; Shipley, T.H. (September 1986), "Site Surveys in the Western Pacific Conducted aboard the Kana Keoki, Cruise KK810626 Leg 4" (PDF), Initial Reports of the Deep Sea Drilling Project, U.S. Government Printing Office, p. 606, doi:10.2973/dsdp.proc.89.126.1986, retrieved 2018-10-06
  40. ↑ Koppers et al. 1998, p. 60.
  41. 1 2 Schlanger, Seymour O. (1981), "Shallow-Water Limestones in Oceanic Basins as Tectonic and Paleoceanographic Indicators", The Deep Sea Drilling Project: A Decade of Progress, SEPM (Society for Sedimentary Geology), p. 220, doi:10.2110/pec.81.32.0209, ISBN 9781565761629, retrieved 2018-09-18, (Subscription required (help))
  42. ↑ Wedgeworth & Kellogg 1987, p. 81.
  43. ↑ Hesse 1973, p. 363.
  44. ↑ Hesse 1973, p. 367.
  45. ↑ Mel'nikov et al. 2012, p. 227.
  46. ↑ Zakharov, Mel'nikov & Khudik 2003, p. 44.
  47. ↑ Mel'nikov et al. 2012, p. 222.
  48. 1 2 3 Mel'nikov, Tugolesov & Pletnev 2010, p. 587.
  49. ↑ Zakharov, Mel'nikov & Khudik 2003, p. 46.
  50. ↑ Zakharov et al. 2007, p. 38.
  51. ↑ The Shipboard Scientific Party 1973b, p. 98.
  52. ↑ The Shipboard Scientific Party 1973b, p. 101.
  53. ↑ Haggerty, J.A.; Premoli Silva, I. (September 1986), "Ooids and Shallow-Water Debris in Aptian-Albian Sediments from the East Mariana Basin, Deep Sea Drilling Project Site 585: Implications for the Environment of Deposition of the Ooids", Ooids and Shallow-Water Debris in Aptian-Albian Sediments from the East Mariana Basin, Deep Sea Drilling Project Site 585: Implications for the Environment of Deposition of the Ooids (PDF), Initial Reports of the Deep Sea Drilling Project, 89, U.S. Government Printing Office, p. 399, doi:10.2973/dsdp.proc.89.112.1986, retrieved 2018-09-16
  54. ↑ Zakharov, Mel'nikov & Khudik 2003, p. 38.
  55. ↑ The Shipboard Scientific Party 1973, p. 89.
  56. ↑ The Shipboard Scientific Party 1973, p. 88.
  57. ↑ The Shipboard Scientific Party 1973, p. 93.
  58. ↑ Lee et al. 2005, p. 1947.
  59. ↑ Keller, N. B.; Shcherba, I. G. (March 2006). "Features of the distribution of azooxanthellata scleractinia (Anthozoa) on mid-pacific guyots". Oceanology. 46 (2): 238. doi:10.1134/s000143700602010x. ISSN 0001-4370.

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