Английская Википедия:Horizon Guyot

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Шаблон:Short description Шаблон:EngvarB Шаблон:Infobox seamountШаблон:Location map

Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over Шаблон:Convert long and Шаблон:Convert high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of Шаблон:Convert. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.

It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants.

Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks.

Name and research history

The seamount is named after the research vessel Шаблон:ShipШаблон:Sfn and is also known as Horizon Ridge,Шаблон:Sfn Horizon Tablemount, Gora Khorayzn and Шаблон:Lang.[1] During the Deep Sea Drilling Project, the drill cores called Site 44 and Site 171 were taken on Horizon Guyot in 1969 and 1971, respectively;[2]Шаблон:Sfn a further drill core was obtained north of the seamount at Site 313Шаблон:Sfn in 1973.[3] This seamount is the best studied seamount of the Mid-Pacific MountainsШаблон:Sfn and more is known about its morphology than any other seamount of the Mid-Pacific Mountains.Шаблон:Sfn

Geography and geology

Local setting

Horizon Guyot lies west of HawaiiШаблон:Sfn and is part of the Mid-Pacific Mountains.Шаблон:Sfn Unlike conventional island chains in the Pacific Ocean, the Mid-Pacific Mountains feature an oceanic plateauШаблон:Sfn with guyots[4] (also known as tablemounts[5]) which become progressively younger towards the east.Шаблон:Sfn Other guyots in the Mid-Pacific Mountains are Sio South, Darwin, Thomas, Heezen, Allen, Caprina, Jacqueline, Allison[6] and Resolution.[4] South of Horizon Guyot, deep water in the "Horizon passage" leads into the Line IslandsШаблон:Sfn[7] and Horizon Guyot is sometimes considered to be a member of that chain.Шаблон:Sfn

The seamount rises Шаблон:ConvertШаблон:Sfn-Шаблон:ConvertШаблон:Sfn to a minimum depth of Шаблон:ConvertШаблон:Sfn–Шаблон:Convert, and is a ridge Шаблон:Convert wide and over Шаблон:Convert long;Шаблон:SfnШаблон:Sfn Horizon Guyot is the largest seamount in the Mid-Pacific Mountains.Шаблон:Sfn It trends in a southwest-northeast directionШаблон:Sfn with an orientation matching that of other structures in the regionШаблон:Sfn such as fracture zones on the seafloor.Шаблон:Sfn Faulting has been observed on the western side of the seamount.Шаблон:Sfn

Two summit platforms lie on the ridge.Шаблон:Sfn The eastern one is the larger of these platformsШаблон:Sfn and the western oval-shaped platform lies close to the western end of the ridge.Шаблон:Sfn These platforms are relatively flat and are surrounded by a slope break beyond which the guyot falls off steeply to the surrounding abyssal plain.Шаблон:Sfn This appearance characterises Horizon Guyot as a guyotШаблон:Sfn although the elongated shape is unlike that of most guyots in the region which have one circular summit platform.Шаблон:Sfn At the margin of the platform, lie terraces which are up to Шаблон:Convert wide and up to Шаблон:Convert high and that discontinuously surround the summit platform;Шаблон:Sfn the flat surfaces of the terraces may be former fringing reefs.Шаблон:Sfn On the eastern summit platform there are buried terraces beneath the sediment cover.Шаблон:Sfn

Sediment layers cover almost the entire summit of Horizon Guyot,Шаблон:Sfn and consist mainly of sand, with clay and silt making up a minor part.Шаблон:Sfn Features on the sedimented seafloor are flat areas, hummocks, ripples and sediment waves.Шаблон:Sfn Seismic transects have revealedШаблон:Sfn a relief of about Шаблон:Convert in the underlying basementШаблон:Sfn and the presence of a central peak.Шаблон:Sfn Material obtained in drill cores includes chalk, chert, hyaloclastite, limestone, ooze and sandstone;Шаблон:Sfn basalt and chert outcrop in some places.Шаблон:Sfn In certain areas boulders and cobbles cover the seafloor;[8] ferromanganese crusts cover exposed rocks.Шаблон:Sfn

The seamount shows evidence of repeated mass failures; including hummocky terrain, scarps and slump blocks,Шаблон:Sfn which are on average Шаблон:Convert thick.Шаблон:Sfn Landsliding is probably triggered by earthquakes;Шаблон:Sfn after the failure the landslides either stay coherent and do not travel far but some advance quickly and far.Шаблон:Sfn Talus blocks up to Шаблон:Convert in size cover the seafloorШаблон:Sfn around Horizon Guyot.Шаблон:Sfn

Regional setting

The West Pacific Ocean seafloor contains many guyots of Mesozoic age (251.902 ± 0.024Шаблон:Efn – 66 million years ago[9]) that developed in unusually shallow seas.[6] These are submarine mountains which are characterised by a flat top and usually the presence of carbonate platforms that rose above the sea surface during the middle Cretaceous (c. 145–66 million years ago[9]).[10] During the Second World War, it was discovered that the seafloor of the Western Pacific Ocean was dotted with numerous flat-topped seamounts. These were promptly identified as sunken islands; at first, it was believed that they had sunk below the water in the Precambrian (over 541 ± 1 million years ago[9]), before the presence of Cretaceous reefs on many of them was noticed.Шаблон:Sfn About 6% of the Pacific seafloor is covered with almost a million seamounts.Шаблон:Sfn

While there are some differences to present-day reef systems,Шаблон:Efn[11]Шаблон:Sfn many of these seamounts were formerly atolls. All these structures originally formed as volcanoes in the Mesozoic ocean. Fringing reefs may have developed on the volcanoes, which then became barrier reefs as the volcano subsided and turned into an atoll; the barrier reefsШаблон:Sfn in turn surround a lagoon or tidal flat.Шаблон:Sfn The crust underneath these seamounts tends to subside as it cools, and thus the islands and seamounts sink.Шаблон:Sfn Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms.[12] Sometimes volcanic activity continued even after the formation of the atoll or atoll-like structure, and during episodes where the platforms rose above sea level erosional features such as channels and blue holesШаблон:Efn developed.Шаблон:Sfn

The formation of many such seamounts has been explained by the hotspot theory.Шаблон:Sfn According to this theory, an active volcano lies on a spot of the lithosphere heated from below; as the plate above this hotspot moves, the volcano is moved away from the heat source and volcanic activity ceases. The hotspot will then heat the area of the plate now above it, producing another active volcano. In this way, a chain of volcanoes that get progressively older away from the currently active one is generated.[13] With some exceptions, radiometric dating of the Mid-Pacific Mountains has yielded evidence of an eastward movement of volcanism which is consistent with the hotspot theory;Шаблон:Sfn in the case of Horizon Guyot, volcanism may have migrated southwestward which is not entirely consistent with the hotspot theory.Шаблон:Efn[14] When it formed, Horizon Guyot may have been located close to a spreading centre.Шаблон:Sfn

Composition

Volcanic rocks dredged from Horizon Guyot are of basaltic composition and define a tholeiitic suite. Augite,Шаблон:Sfn labradorite,Шаблон:Sfn olivine, plagioclase and pyroxene form phenocrysts while pigeonite is found in the groundmass.Шаблон:Sfn Other guyots and samples from the Mid-Pacific Mountains have similar compositions to these found on Horizon Guyot.Шаблон:Sfn Some volcanic rocks occur in the form of hyaloclastite which contains palagonite and sideromelane.Шаблон:Sfn Dredged volcanic rocks are heavily altered;Шаблон:Sfn this has given rise to analcime, augite, calcite, clay, clinoptilolite,Шаблон:SfnШаблон:Sfn iddingsite,Шаблон:Sfn ilmenite, labradorite, magnetiteШаблон:Sfn iron oxides and talc.Шаблон:Sfn

Carbonates are found as limestone and siltstone;Шаблон:Sfn some limestones were formed by living beings.[15] At one point in the drill core, carbonates were found mixed with volcanic rocks; presumably this is a place where hyaloclastite accumulated and was reworked by sea currents.Шаблон:Sfn The limestone contains fossils of algae,Шаблон:Sfn bryozoans, echinoids, foraminiferans, molluscs and ostracods;Шаблон:Sfn dinoflagellates, pollen and scolecodonts are also found.Шаблон:Sfn Some limestones have been modified by silicification and phosphatisation.Шаблон:Sfn

Clinoptilolite, pyrite, radiolarian fossils and yellow glass shards are found in the ooze,Шаблон:Sfn and some volcanic rocks and manganese rocks are cemented by indurated ooze.Шаблон:Sfn FerromanganeseШаблон:Sfn and phosphorite crusts coat rocks.Шаблон:Sfn These ferromanganese crusts consist of iron oxides and manganese oxides and are related to manganese nodulesШаблон:Sfn and might become targets for future mining efforts.[8] Other materials found on Horizon Guyot are analcime,Шаблон:SfnШаблон:Sfn barite,[16] calcite, celadonite,Шаблон:SfnШаблон:Sfn cristobalite,[16] glauconite,Шаблон:Sfn gypsum,[17] ironstone,Шаблон:Sfn kaolinite, mica, montmorillonite, mudstone, quartz,[16] sapropel,Шаблон:Sfn smectite and zeolite.Шаблон:SfnШаблон:Sfn

Geologic history

Horizon Guyot is at least of Albian (c. 113–100.5 million years ago[9]) age and is perhaps as much as 120 million years old.Шаблон:Sfn Radiometric dating has yielded ages of 88.1 ± 0.4 million years and more recently of 82.5 ± 0.4 million years; this may reflect either prolonged volcanism or that the older date is incorrect.Шаблон:Sfn About 100 and 80 million years ago a pulse of volcanism occurred in the Pacific Ocean; the formation of Horizon Guyot may have coincided with this pulse.Шаблон:Sfn

Volcanism

Basalt lava flows were emplaced on Horizon Guyot during the Cretaceous,Шаблон:Sfn before or during the Albian.Шаблон:Sfn A second volcanic phase occurred during the Turonian (93.9 – 89.8 ± 0.3 million years ago[9]) and Cenomanian (100.5 – 93.9 million years ago[9]);[18] thus volcanic activity on Horizon Guyot was recurrent.[19] The basalts include both typical ocean island basalts and basalts resembling mid-ocean ridge basalts, with the former found deeper in drill cores.Шаблон:Sfn Hyaloclastites which outcrop at the margin of the summit platformШаблон:Sfn indicate the occurrence of submarine eruptions.Шаблон:Sfn

Eruptions probably occurred on aligned vents, explaining the elongated shape of Horizon Guyot.Шаблон:Sfn At first the formation of the terraces was also attributed to volcanic activity;Шаблон:Sfn an origin as wave cut terraces was considered to be unlikelyШаблон:Sfn but when it was found that Horizon Guyot had risen above sea level in the Cretaceous a wave cut origin was reproposed.Шаблон:Sfn

Carbonate island phase and renewed volcanism

During the Cretaceous, carbonates accumulated on Horizon GuyotШаблон:Sfn while it subsided, forming a carbonate deposit which in one drill core is Шаблон:Convert thick.Шаблон:Sfn The carbonates accumulated directly on the previous volcanoШаблон:Sfn and reefs started growing when volcanic activity was still underway;Шаблон:Sfn Horizon Guyot featured lagoonal environments with algal reefs.Шаблон:Sfn Prior to 1973 there was no evidence that Horizon Guyot had ever formed an islandШаблон:Sfn but later a stage of emergence was postulated.Шаблон:Sfn The seamount was an island for at least 6 million years.[20]

During the late Cretaceous, a second volcanic episode took place on Horizon Guyot[21] and produced volcanites and volcanic sedimentsШаблон:Sfn which buried older limestones.[18] At that time, volcanic activity was underway not only on this seamount but also in the Line Islands;[21] on Horizon Guyot this phase occurred perhaps as much as 30 million years after the previous volcanic stage.Шаблон:Sfn

Before this volcanic phase, Horizon Guyot had emerged from the sea and erosion had reworked some older rocks;Шаблон:Sfn also, plants grew on the now exposed island.Шаблон:Sfn Shallow water deposition in Coniacian (89.8 ± 0.3 – 86.3 ± 0.5 million years ago[9]) or Santonian (86.3 ± 0.5 – 83.6 ± 0.2 million years ago[9]) to Maastrichtian (72.1 ± 0.2 – 66 million years ago[9]) time has been inferred from the presence of unstableШаблон:Efn coccoliths of such age in drill cores.Шаблон:Sfn

Drowning and sedimentation

Horizon Guyot reached above sea level at least until the Cenomanian,Шаблон:Sfn unlike other Mid-Pacific Mountains guyots which sank below sea level during the Albian.Шаблон:Sfn Plant remnants are found in rocks of Turonian and Coniacian age,[18] implying that Horizon Guyot was still emergent at that time;Шаблон:Sfn but by the Coniacian, Horizon Guyot was submerging.Шаблон:Sfn It is not known why Horizon Guyot drowned but the burial of the reefs by volcanic activity may have played a role.[22]

Pelagic sedimentation commenced in the Campanian[18] (83.6 ± 0.2 – 72.1 ± 0.2 million years ago[9]) when Horizon Guyot had already sunk to a depth of Шаблон:Convert.Шаблон:Sfn Since the Miocene (23.03 – 5.333 million years ago[9]), sedimentation rates appear to have decreased as the guyot moved away from waters with high biological productivity and at some point in the last 10 million years erosion increased due to bottom currentsШаблон:Sfn linked to the glaciation of Antarctica.Шаблон:Sfn

Pelagic sediments accumulate on some guyots after they have drowned. A dome-shapedШаблон:Sfn cap of pelagic ooze accumulated on top of Horizon Guyot during the Tertiary,[23] reaching a maximum thickness of Шаблон:ConvertШаблон:Sfn-Шаблон:Convert in some places.Шаблон:Sfn In the saddle between the summit platforms it is about Шаблон:Convert thick;Шаблон:Sfn an unconformity separates it from Cretaceous deposits.Шаблон:Sfn The sediment layers span a timespan encompassing the Eocene (56 – 33.9 million years ago[9]) to the Quaternary (last 2.58 million years[9]) with gaps in the sediment sequence between the Cretaceous and the Eocene and between the Eocene and the Oligocene (33.9 – 23.03 million years ago[9]).Шаблон:Sfn During the Eocene and Oligocene, older foraminifera were redeposited;Шаблон:Sfn there is evidence that sediments were actively eroded.Шаблон:Sfn During TertiaryШаблон:Sfn phases of low sea level, sea currents swept sediments off the surface of Horizon Guyot,Шаблон:Sfn with fine sediments being particularly affected.[23]

Present state

The topШаблон:Sfn and almost all the upper slopes of Horizon Guyot are covered by sediments.Шаблон:Sfn Chert and chalk are found within the sediments;Шаблон:SfnШаблон:Sfn chert forms seismically reflective layers within the sediment cap.Шаблон:Sfn These layers crop out at the margin of the sediment platform.Шаблон:Sfn The seamount lies in a region of the Pacific with nutrient poor surface waters.Шаблон:Sfn

Sea currents are unusually strong on the top of Horizon Guyot,Шаблон:Sfn probably due to the interaction of the slopes of Horizon Guyot with tidal currents.Шаблон:Sfn The seamount induces its own semidiurnal tide and the sea currents reach their maximum at the margin of the summit platform where Шаблон:Convert have been measured.Шаблон:Sfn Scour marks have been observed.Шаблон:Sfn The currents sweep down the seamount slopes and may act to remove sediment from the seamount surface;Шаблон:Sfn this also results in sediments accumulating to form steep slopes that undergo landsliding.Шаблон:Sfn Most of the sediments however are transported upslope;Шаблон:Sfn those which do end up at the bottom of the seamount form talus deposits around Horizon Guyot.Шаблон:Sfn

Ecology

A video of animal life on the dark seafloor, with tree like animals, animals looking like large hooks and animals looking like white sacs attached by thin stalks to the seafloor
Video of animal life on Horizon Guyot

The surfaces of Horizon Guyot are inhabited by many organisms.Шаблон:Sfn Fish found on Horizon Guyot include batfish, bathypteroids, chimeras, morids, sharks and synaphobranchid eels.Шаблон:Sfn Brittle stars, chaetognatha (arrow worms), copepods, corals, crustaceans, hydroids, loricifera, molluscs, nematodes, nemertinea, ophiuroids, ostracods, polychaetes, sipuncula, squat lobsters, vermes[8]Шаблон:Sfn and xenophyophorans make up the bulk of the fauna on Horizon Guyot today.Шаблон:Sfn At least 29 macrofaunal species have been found.Шаблон:Sfn Other lifeforms presently active on the seamount are barnacles, crinoids,Шаблон:Sfn echiurids, enteropneusts (acorn worms),Шаблон:Sfn gorgonians,Шаблон:Sfn holothuroids (sea cucumbers), pennatulids (sea pens),Шаблон:Sfn spongesШаблон:Sfn and starfish.Шаблон:Sfn Unidentified stalk or twig-like creatures have also been observed on the platform, which are among the most common lifeforms there.Шаблон:Sfn Bacteria are also found in the sediment.Шаблон:Sfn

Biological activity has left traces in the sediments such as mounds, pits, and trails on the surface.Шаблон:Sfn There is a certain zonation in the ecology of Horizon Guyot; for example suspension feeders live on the margin of the summit platform.Шаблон:Sfn Genetic differences between individuals of a given species which live on the top and these which live at the foot of the guyot have been noted.[24] Some ostracods found on Horizon Guyot such as Cytherelloidea appear to have evolved from Cretaceous shallow water species as the seamount sank into colder waters.[25]

Notes

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References

Шаблон:Reflist

Sources

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Шаблон:Refend

External links

Шаблон:Mid-Pacific Mountains Шаблон:Featured article

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  1. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Etymology не указан текст
  2. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Winterer1973 не указан текст
  3. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Site313 не указан текст
  4. ↑ 4,0 4,1 Ошибка цитирования Неверный тег <ref>; для сносок BakerCastillo1995 не указан текст
  5. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Bouma1990 не указан текст
  6. ↑ 6,0 6,1 Ошибка цитирования Неверный тег <ref>; для сносок McNuttWinterer1990 не указан текст
  7. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Edmond1971 не указан текст
  8. ↑ 8,0 8,1 8,2 Ошибка цитирования Неверный тег <ref>; для сносок KelleyMashkoor2017 не указан текст
  9. ↑ 9,00 9,01 9,02 9,03 9,04 9,05 9,06 9,07 9,08 9,09 9,10 9,11 9,12 9,13 Ошибка цитирования Неверный тег <ref>; для сносок ICC2018 не указан текст
  10. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Winterer1995 не указан текст
  11. ↑ Ошибка цитирования Неверный тег <ref>; для сносок IryuYamada1999 не указан текст
  12. ↑ Ошибка цитирования Неверный тег <ref>; для сносок StrasserArnaud1995 не указан текст
  13. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Sleep1992 не указан текст
  14. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Moberly1975 не указан текст
  15. ↑ Ошибка цитирования Неверный тег <ref>; для сносок McKenzie1980 не указан текст
  16. ↑ 16,0 16,1 16,2 Ошибка цитирования Неверный тег <ref>; для сносок RexEklund1971 не указан текст
  17. ↑ Ошибка цитирования Неверный тег <ref>; для сносок ZemmelsCook1973 не указан текст
  18. ↑ 18,0 18,1 18,2 18,3 Ошибка цитирования Неверный тег <ref>; для сносок Schlanger1981 не указан текст
  19. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Lancelot1975 не указан текст
  20. ↑ Ошибка цитирования Неверный тег <ref>; для сносок TSSP1981 не указан текст
  21. ↑ 21,0 21,1 Ошибка цитирования Неверный тег <ref>; для сносок Vallier1981 не указан текст
  22. ↑ Ошибка цитирования Неверный тег <ref>; для сносок RougerieFagerstrom1994 не указан текст
  23. ↑ 23,0 23,1 Ошибка цитирования Неверный тег <ref>; для сносок Israelson1995 не указан текст
  24. ↑ Ошибка цитирования Неверный тег <ref>; для сносок Shank2010 не указан текст
  25. ↑ Ошибка цитирования Неверный тег <ref>; для сносок BoomerWhatley1995 не указан текст
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