Horizon Guyot

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Horizon Guyot
Main Line Islands, NOAA bathymetric map with lineations (Horizon Guyot).jpg
Location
Coordinates 19°07.9′N169°27.6′W / 19.1317°N 169.4600°W / 19.1317; -169.4600 [1]

[2]

North Pacific location map.svg
Red triangle with thick white border.svg
Horizon
Location in the North Pacific Ocean

Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over 300 kilometres (190 mi) long and 4.3 kilometres (2.7 mi) high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of 1,443 metres (4,730 ft). The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.

The Cretaceous is a geologic period and system that spans from the end of the Jurassic Period 145 million years ago (mya) to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide.

Guyot An isolated, flat-topped underwater volcano mountain

In marine geology, a guyot, also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.

Mid-Pacific Mountains An underwater mountain range from the southern tier of the Japan Trench to the Hawaiian Islands

The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.

Contents

It was probably formed by volcanic activity occurring during the Turonian-Cenomanian eras 100.5–89.8 million years ago. 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.

The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma. The Turonian is preceded by the Cenomanian stage and underlies the Coniacian stage.

The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding age. Both age and stage bear the same name.

Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.

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.

The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma. The Coniacian is preceded by the Turonian and followed by the Santonian.

The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous series. Campanian spans the time from 83.6 to 72.1 million years ago. It is preceded by the Santonian and it is followed by the Maastrichtian.

Ocean current Directional mass flow of oceanic water generated by external or internal forces

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

Name and research history

The seamount is named after the research vessel RV Horizon [3] and is also known as Horizon Ridge, [4] Horizon Tablemount, Gora Khorayzn and Гора Хорайзн. [2] 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; [5] [6] a further drill core was obtained north of the seamount at Site 313 [7] in 1973. [8] This seamount is the best studied seamount of the Mid-Pacific Mountains [9] and more is known about its morphology than at any other seamount of the Mid-Pacific Mountains. [10]

RV <i>Horizon</i>

RV Horizon, ex Auxiliary Fleet Tug ATA-180, was a Scripps Institution of Oceanography research vessel from 1949 through 1968. During that time she made 267 cruises and logging 610,522 miles (982,540 km) spending 4,207 days at sea.

Deep Sea Drilling Project Ocean drilling research program between 1966–1983

The Deep Sea Drilling Project (DSDP) was an ocean drilling project operated from 1968 to 1983. The program was a success, as evidenced by the data and publications that have resulted from it. The data are now hosted by Texas A&M University, although the program was coordinated by the Scripps Institution of Oceanography at the University of California, San Diego. DSDP provided crucial data to support the seafloor spreading hypothesis and helped to prove the theory of plate tectonics. DSDP was the first of three international scientific ocean drilling programs that have operated over more than 40 years.

Geography and geology

Local setting

Horizon Guyot lies west of Hawaii [1] and is part of the Mid-Pacific Mountains. [11] Unlike conventional island chains in the Pacific Ocean, the Mid-Pacific Mountains feature an oceanic plateau [12] with guyots [13] (also known as tablemounts [14] ) that become progressively younger towards the east. [15] Other guyots in the Mid-Pacific Mountains are Sio South, Darwin, Thomas, Heezen, Allen, Caprina, Jacqueline, Allison [16] and Resolution. [13] South of Horizon Guyot, deep water in the "Horizon passage" leads into the Line Islands [17] [18] and Horizon Guyot is sometimes considered to be a member of that chain. [19]

Hawaii U.S. state in the United States

Hawaii is a state in the Pacific United States. It is the most recent state to join the United States, on August 21, 1959. Hawaii is the only U.S. state geographically located in Oceania, although it is governed as a part of North America, and the only one composed entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean.

Oceanic plateau Relatively flat submarine region that rises well above the level of the ambient seabed

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.

Darwin Guyot

Darwin Guyot is a volcanic underwater mountain top, or guyot, in the Mid-Pacific Mountains between the Marshall Islands and Hawaii. Named after Charles Darwin, it rose above sea level more than 118 million years ago during the early Cretaceous period to become an atoll, developed rudist reefs, and then drowned, perhaps as a consequence of sea level rise. The flat top of Darwin Guyot now rests 1,266 metres (4,154 ft) below sea level.

The seamount rises 3.4 kilometres (2.1 mi) [20] -3.5 kilometres (2.2 mi) [21] to a minimum depth of 1,443 metres (4,734 ft) [20] 1,440 metres (4,720 ft), and is a ridge 75 kilometres (47 mi) wide and over 300 kilometres (190 mi) long; [4] [21] Horizon Guyot is the largest seamount in the Mid-Pacific Mountains. [22] It trends in a southwest-northeast direction [23] with an orientation matching that of other structures in the region [24] such as fracture zones on the seafloor. [25] Faulting has been observed on the western side of the seamount. [26]

Fracture zone A junction between oceanic crustal regions of different ages on the same plate left by a transform fault

A fracture zone is a linear oceanic feature—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on either side of an active transform fault move in opposite directions; here, strike-slip activity occurs. Fracture zones extend past the transform faults, away from the ridge axis; seismically inactive, they display evidence of past transform fault activity, primarily in the different ages of the crust on opposite sides of the zone.

In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

Two summit platforms lie on the ridge. [4] The eastern one is the larger of these platforms [27] and the western oval-shaped platform lies close to the western end of the ridge. [3] These platforms are relatively flat and are surrounded by a slope break beyond which the guyot falls off steeply to the surrounding abyssal plain. [4] This appearance characterises Horizon Guyot as a guyot [28] although the elongated shape is unlike that of most guyots in the region which have one circular summit platform. [29] At the margin of the platform, lie terraces which are up to 3 kilometres (1.9 mi) wide and up to 100 metres (330 ft) high and that discontinuously surround the summit platform; [30] the flat surfaces of the terraces may be former fringing reefs. [31] On the eastern summit platform there are buried terraces beneath the sediment cover. [32]

Sediment layers cover almost the entire summit of Horizon Guyot, [23] and consist mainly of sand, with clay and silt making up a minor part. [33] Features on the sedimented seafloor are flat areas, hummocks, ripples and sediment waves. [34] Seismic transects have revealed [32] a relief of about 150 metres (490 ft) in the underlying basement [31] and the presence of a central peak. [32] Material obtained in drill cores includes chalk, chert, hyaloclastite, limestone, ooze and sandstone; [35] basalt and chert outcrop in some places. [36] In certain areas boulders and cobbles cover the seafloor; [37] ferromanganese crusts cover exposed rocks. [38]

The seamount shows evidence of repeated mass failures; including hummocky terrain, scarps and slump blocks, [39] which are on average 30 metres (98 ft) thick. [40] Landsliding is probably triggered by earthquakes; [39] after the failure the landslides either stay coherent and do not travel far but some advance quickly and far. [41] Talus blocks up to 5 metres (16 ft) in size cover the seafloor [38] around Horizon Guyot. [21]

Regional setting

The West Pacific Ocean seafloor contains many guyots of Mesozoic age (251.902 ± 0.024 [lower-alpha 1] – 66 million years ago [42] ) that developed in unusually shallow seas. [16] 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 [42] ). [44] 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 [42] ), before the presence of Cretaceous reefs on many of them was noticed. [45] About 6% of the Pacific seafloor is covered with almost a million seamounts. [46]

While there are some differences to present-day reef systems, [lower-alpha 2] [47] [48] 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 reefs [49] in turn surround a lagoon or tidal flat. [50] The crust underneath these seamounts tends to subside as it cools, and thus the islands and seamounts sink. [51] Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms. [52] 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 holes [lower-alpha 3] developed. [54]

The formation of many such seamounts has been explained by the hotspot theory. [55] 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. [56] 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; [57] in the case of Horizon Guyot, volcanism may have migrated southwestward which is not entirely consistent with the hotspot theory. [lower-alpha 4] [58] When it formed, Horizon Guyot may have been located close to a spreading centre. [59]

Composition

Volcanic rocks dredged from Horizon Guyot are of basaltic composition and define a tholeiitic suite. Augite, [4] labradorite, [27] olivine, plagioclase and pyroxene form phenocrysts while pigeonite is found in the groundmass. [60] Other guyots and samples from the Mid-Pacific Mountains have similar compositions to these found on Horizon Guyot. [61] Some volcanic rocks occur in the form of hyaloclastite which contains palagonite and sideromelane. [27] Dredged volcanic rocks are heavily altered; [3] this has given rise to analcime, augite, calcite, clay, clinoptilolite, [62] [63] iddingsite, [64] ilmenite, labradorite, magnetite [62] iron oxides and talc. [27]

Carbonates are found as limestone and siltstone; [35] some limestones were formed by living beings. [65] 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. [63] The limestone contains fossils of algae, [63] bryozoans, echinoids, foraminiferans, molluscs and ostracods; [66] dinoflagellates, pollen and scolecodonts are also found. [35] Some limestones have been modified by silicification and phosphatisation. [67]

Clinoptilolite, pyrite, radiolarian fossils and yellow glass shards are found in the ooze, [35] and some volcanic rocks and manganese rocks are cemented by indurated ooze. [68] Ferromanganese [69] and phosphorite crusts coat rocks. [27] These ferromanganese crusts consist of iron oxides and manganese oxides and are related to manganese nodules [70] and might become targets for future mining efforts. [37] Other materials found on Horizon Guyot are analcime, [63] [11] barite, [71] calcite, celadonite, [11] [63] cristobalite, [71] glauconite, [72] gypsum, [73] ironstone, [74] kaolinite, mica, montmorillonite, mudstone, quartz, [71] sapropel, [35] smectite and zeolite. [11] [63]

Geologic history

Horizon Guyot is at least of Albian (c. 113–100.5 million years ago [42] ) age and is perhaps as much as 120 million years old. [27] 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. [75] 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. [76]

Volcanism

Basalt lava flows were emplaced on Horizon Guyot during the Cretaceous, [1] before or during the Albian. [77] A second volcanic phase occurred during the Turonian (93.9 – 89.8 ± 0.3 million years ago [42] ) and Cenomanian (100.5 – 93.9 million years ago [42] ); [78] thus volcanic activity on Horizon Guyot was recurrent. [79] The basalts include both typical ocean island basalts and basalts resembling mid-ocean ridge basalts, with the former found deeper in drill cores. [63] Hyaloclastites which outcrop at the margin of the summit platform [29] indicate the occurrence of submarine eruptions. [27]

Eruptions probably occurred on aligned vents, explaining the elongated shape of Horizon Guyot. [80] At first the formation of the terraces was also attributed to volcanic activity; [81] an origin as wave cut terraces was considered to be unlikely [82] but when it was found that Horizon Guyot had risen above sea level in the Cretaceous a wave cut origin was reproposed. [83]

Carbonate island phase and renewed volcanism

During the Cretaceous, carbonates accumulated on Horizon Guyot [66] while it subsided, forming a carbonate deposit which in one drill core is 134 metres (440 ft) thick. [84] The carbonates accumulated directly on the previous volcano [57] and reefs started growing when volcanic activity was still underway; [85] Horizon Guyot featured lagoonal environments with algal reefs. [84] Prior to 1973 there was no evidence that Horizon Guyot had ever formed an island [86] but later a stage of emergence was postulated. [87] The seamount was an island for at least 6 million years. [88]

During the late Cretaceous, a second volcanic episode took place on Horizon Guyot [89] and produced volcanites and volcanic sediments [66] which buried older limestones. [78] At that time, volcanic activity was underway not only on this seamount but also in the Line Islands; [89] on Horizon Guyot this phase occurred perhaps as much as 30 million years after the previous volcanic stage. [77]

Before this volcanic phase, Horizon Guyot had emerged from the sea and erosion had reworked some older rocks; [87] also, plants grew on the now exposed island. [27] Shallow water deposition in Coniacian (89.8 ± 0.3 – 86.3 ± 0.5 million years ago [42] ) or Santonian (86.3 ± 0.5 – 83.6 ± 0.2 million years ago [42] ) to Maastrichtian (72.1 ± 0.2 – 66 million years ago [42] ) time has been inferred from the presence of unstable [lower-alpha 5] coccoliths of such age in drill cores. [90]

Drowning and sedimentation

Horizon Guyot reached above sea level at least until the Cenomanian, [91] unlike other Mid-Pacific Mountains guyots which sank below sea level during the Albian. [92] Plant remnants are found in rocks of Turonian and Coniacian age, [78] implying that Horizon Guyot was still emergent at that time; [87] but by the Coniacian, Horizon Guyot was submerging. [28] It is not known why Horizon Guyot drowned but the burial of the reefs by volcanic activity may have played a role. [93]

Pelagic sedimentation commenced in the Campanian [78] (83.6 ± 0.2 – 72.1 ± 0.2 million years ago [42] ) when Horizon Guyot had already sunk to a depth of 1,500 metres (4,900 ft). [94] Since the Miocene (23.03 – 5.333 million years ago [42] ), 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 currents [28] linked to the glaciation of Antarctica. [95]

Pelagic sediments accumulate on some guyots after they have drowned. A dome-shaped [20] cap of pelagic ooze accumulated on top of Horizon Guyot during the Tertiary, [96] reaching a maximum thickness of 110 metres (360 ft) [20] -160 metres (520 ft) in some places. [34] In the saddle between the summit platforms it is about 500 metres (1,600 ft) thick; [21] an unconformity separates it from Cretaceous deposits. [97] The sediment layers span a timespan encompassing the Eocene (56 – 33.9 million years ago [42] ) to the Quaternary (last 2.58 million years [42] ) 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 [42] ). [72] During the Eocene and Oligocene, older foraminifera were redeposited; [97] there is evidence that sediments were actively eroded. [72] During Tertiary [98] phases of low sea level, sea currents swept sediments off the surface of Horizon Guyot, [99] with fine sediments being particularly affected. [96]

Present state

The top [100] and almost all the upper slopes of Horizon Guyot are covered by sediments. [101] Chert and chalk are found within the sediments; [1] [97] chert forms seismically reflective layers within the sediment cap. [21] These layers crop out at the margin of the sediment platform. [29] The seamount lies in a region of the Pacific with nutrient poor surface waters. [102]

Sea currents are unusually strong on the top of Horizon Guyot, [34] probably due to the interaction of the slopes of Horizon Guyot with tidal currents. [103] The seamount induces its own semidiurnal tide and the sea currents reach their maximum at the margin of the summit platform where 20 centimetres per second (7.9 in/s) have been measured. [104] Scour marks have been observed. [39] The currents sweep down the seamount slopes and may act to remove sediment from the seamount surface; [105] this also results in sediments accumulating to form steep slopes that undergo landsliding. [106] Most of the sediments however are transported upslope; [29] those which do end up at the bottom of the seamount form talus deposits around Horizon Guyot. [40]

Ecology

Video of animal life on Horizon Guyot

The surfaces of Horizon Guyot are inhabited by many organisms. [101] Fish found on Horizon Guyot include batfish, bathypteroids, chimeras, morids, sharks and synaphobranchid eels. [107] Brittle stars, chaetognatha (arrow worms), copepods, corals, crustaceans, hydroids, loricifera, molluscs, nematodes, nemertinea, ophiuroids, ostracods, polychaetes, sipuncula, squat lobsters, vermes [37] [108] and xenophyophorans make up the bulk of the fauna on Horizon Guyot today. [109] At least 29 macrofaunal species have been found. [69] Other lifeforms presently active on the seamount are barnacles, crinoids, [110] echiurids, enteropneusts (acorn worms), [111] gorgonians, [110] holothuroids (sea cucumbers), pennatulids (sea pens), [111] sponges [110] and starfish. [111] Unidentified stalk or twig-like creatures have also been observed on the platform, which are among the most common lifeforms there. [110] Bacteria are also found in the sediment. [112]

Biological activity has left traces in the sediments such as mounds, pits, and trails on the surface. [113] There is a certain zonation in the ecology of Horizon Guyot; for example suspension feeders live on the margin of the summit platform. [114] 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. [115] 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. [116]

Notes

  1. The beginning of the Mesozoic coincides with the end of the Permian, the beginning of the Triassic [42] and the Permian-Triassic extinction event, the largest mass extinction in over half a billion years; in order to determine its cause the chronology of the Permian-Triassic transition has been measured to a high precision. [43]
  2. Carbonate precipitates and grains or sediments are common in Cretaceous reefs, while in Cenozoic reefs reef-building organisms deposited carbonates within their bodies. [47]
  3. Pit-like depressions within carbonate rocks that are filled with water. [53]
  4. There is also an eastward movement of volcanism, which together with the southwestward movement implies a movement into two opposite directions [58] rather than one chain as in the normal hotspot theory. [56]
  5. Some fossils dissolve in deep water and thus are found only in shallow water sediments. [90]

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References

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  2. 1 2 "Horizon Tablemount". Marine Regions. Flanders Marine Institute . Retrieved 17 August 2019.
  3. 1 2 3 Hamilton 1956, p. 5.
  4. 1 2 3 4 5 Heezen et al. 1973, p. 667.
  5. Winterer, E.L. (1973). "Introduction" (PDF). Initial Reports of the Deep Sea Drilling Project, 17. Initial Reports of the Deep Sea Drilling Project. 17. U.S. Government Printing Office. p. 5. doi:10.2973/dsdp.proc.17.101.1973 . Retrieved 2018-10-05.
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