Allison Guyot

Coordinates: 18°16′N 179°20′E / 18.26°N 179.33°E / 18.26; 179.33
Source: Wikipedia, the free encyclopedia.

Allison Guyot
Allison is located in Pacific Ocean
Allison
Allison
Height1.5 kilometres (4,900 ft)
Summit area35 by 70 kilometres (22 mi × 43 mi)
Location
GroupMid-Pacific Mountains
Coordinates18°16′N 179°20′E / 18.26°N 179.33°E / 18.26; 179.33[1]
Geology
TypeGuyot

Allison Guyot (formerly known as Navoceano Guyot) is a

tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres (4,900 ft) above the seafloor to a depth of less than 1,500 metres (4,900 ft), with a summit platform 35 by 70 kilometres (22 by 43 mi) wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere
.

The tablemount was probably formed by a

crocodilians
lived on Allison Guyot.

The platform emerged above sea level during the

pelagic sedimentation commenced on the seamount and led to the deposition of sediments including limestone, ooze and sand
, which bear traces of climatic events and ocean currents.

Name and research history

Allison Guyot is named after E.C. Allison, an oceanographer and paleontologist at the

drill core 865A,[8] which was bored into the summit platform of Allison Guyot[9] in 1992[10] but did not reach the volcanic structure of the underwater mountain.[11] Two other cores 865C and 865B were obtained during the same operation; Allison Guyot is Site 865 Ocean Drilling Program.[8] These drill cores were part of a larger project to investigate and clarify the history of the flat-topped submarine mountains in the Pacific Ocean.[12]

Geography and geology

Local setting

Allison Guyot is located in the equatorial Pacific Ocean,[1] part of the western Mid-Pacific Mountains.[13] The Mid-Pacific Mountains contain seamounts which were covered by limestones during the Barremian and Albian (circa 129.4 – circa 125 million years ago and circa 113–100.5 million years ago, respectively[14]).[15] Hawaii lies due east and the Marshall Islands southwest;[16] Resolution Guyot is 716 kilometres northwest.[17]

The

tablemount[18]) has an outline resembling a trapezoid[13] and consists of two connected volcanic ridges facing north-northwest to east-northeast.[19] Its western parts may be a distinct volcano.[20] The surface platform has dimensions of 35 by 70 kilometres,[21] with an upwards-doming form 0.3–0.5 kilometres high,[22] and is covered by large sediment mounds;[23] the rim surrounding the platform lies at a depth of about 1,650 metres and there is evidence of former reefs.[21] The structure appears to consist of lagoonal sediments surrounded by a reef,[24] and the shallowest point of Allison Guyot lies at less than 1,500 metres (4,900 ft) depth below sea level.[25] Volcanic cones dot the eastern side of the summit plateau.[26] The seamount bears traces of slumping,[27] which on the southeastern side of Allison Guyot has removed part of the platform perimeter.[28]

The seamount rises 1.5 kilometres[29] above the seafloor. Underneath Allison Guyot, the seafloor is about 130–119 million years old,[15] and a 128-million year-old magnetic lineation is located nearby.[30] The Molokai Fracture Zone forms a ridge which passes close to Allison Guyot and intersects with another ridge at the seamount.[31] Tectonically the seamount is part of the Pacific Plate.[4]

Regional setting

Diagram of how an active volcano is accompanied by decaying inactive volcanoes that were formerly located on the hotspot but have been moved away
Diagram showing a cross section through the Earth's lithosphere (in yellow) with magma rising from the mantle (in red)

The west central and south central Pacific Ocean

tidal flats.[38] Continued subsidence offset by growth of the reefs led to the formation of thick carbonate platforms.[39] Sometimes volcanic activity continued 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[b] developed.[41] Eventually, these platforms drowned for reasons that are often unclear.[33]

The formation of many such seamounts has been explained with the

plate moves the volcano is moved away from the heat source and volcanic activity ceases, producing a chain of volcanoes that become older away from the currently active one.[43]

The "South Pacific Superswell" is a region in the

Southern Pacific at the present-day Austral Islands, Cook Islands and Society Islands, where intense volcanic activity was underway during the Cretaceous, and is where the Cretaceous seamounts of the Mid-Pacific Mountains originated. The Easter hotspot, Marquesas hotspot and the Society hotspot may have been involved in the formation of the Mid-Pacific Mountains. After the Mountains had formed, plate tectonics shifted them northwards to their present-day position.[15] Allison Guyot appears to have formed in the same region.[11]

Composition

One drill core on Allison Guyot has found a 136-metre-thick layer of

phosphatisation, and manganese has accumulated in the upper layers.[50]

hydrothermal fluids when the sills formed.[61] The formation of the sills led to the hardening and hydrothermal alteration of the surrounding sediments.[54]

Clays are found both within the limestones

claystones[d] have been found in some places.[46] Mudstones with evidence of animal burrows[69] and containing amber, glauconite, organic material including plant debris and pyrite have also been encountered;[53] pyrite indicates that anoxic environments existed on Allison Guyot.[9]

algae.[73] Cells and tracheids can be found in the plant remnants.[74]

Geological history

Eniwetok Atoll
today. Allison Guyot may have resembled Eniwetok in the past.

Potassium–argon dating on the sills has yielded ages of 102 ± 6 million years ago and 87 ± 3 million years ago, while argon–argon dating also on the sills produced ages of 111.1 ± 2.6 million years,[54] 111.2 ± 1.2 million years ago and 104.8 ± 0.8 million years ago.[9] Other ages from the sills are about 110.7 ± 1.2 million years ago and 104.9 ± 2.0 million years ago.[75] Rocks dredged from the slopes of Allison Guyot have yielded ages of 101.2 ± 0.8 million years,[9] 102.7 ± 2.7 million years and 85.6 ± 1.3 million years ago.[75] Overall, the volcano is considered to be at least 111 million years old[75] and volcanic activity probably spanned 30[26]–25 million years and several stages.[29]

Both the sills and the dredged rocks were probably erupted after the main

Paleomagnetic data taken from limestones show that Allison Guyot developed in the Southern Hemisphere, at a latitude of about 11.2° ± 2.0° south.[76]

Emergent phase

Dark green land with the deep blue sea behind and a lighter blue lagoon in the foreground; the lagoon has isolated reefs visible through its water
Aerial image of the rim of Bikini Atoll. Allison Guyot during its carbonate platform stage may have resembled Bikini.

Allison Guyot began as a

humid and runoff was intense.[80]

The platform contains lagoon and

Eniwetok atolls in terms of morphology when Allison Guyot was emergent.[82]

The carbonate deposits indicate sea level changes following orbital cycles[83] consistent with Milankovitch forcing;[77] parts of the platform occasionally rose above sea level.[84] At some point, karst environments existed on Allison Guyot and are probably the reason for the irregular surface of the summit platform[85] and the presence of sinkholes; there are clear indications of about 200 metres of emergence.[86]

crocodilians have been found on the seamount.[53] Its 110 million years old[90] remnants are the oldest known crocodilians in the region of the Pacific Ocean. They indicate that such species lived within the lagoon of Allison Guyot and may give clues about the history of Pacific animals and their dispersal.[53]

Drowning and post-drowning evolution

A carbonate platform is said to 'drown' when sedimentation can no longer keep up with relative rises in sea level.[91] Carbonate sedimentation on Allison Guyot ended during late Albian times,[85] about 99 ± 2 million years ago, at the same time as at Resolution Guyot.[92] By Turonian times (93.9 – 89.8 ± 0.3 million years ago[14]), pelagic sedimentation was prevailing on Allison Guyot.[93] On both Allison and Resolution Guyots, the drowning was preceded by an episode where the platform rose above the sea;[94] possibly it was this emergence and the following submergence which terminated carbonate deposition and prevented it from beginning again.[95] Such emergence and drowning has been recorded at carbonate platforms of that age around the world and may be the consequence of tectonic events across the Pacific Ocean,[86] culminating in the uplift of a part thereof.[77] At that time, a last phase of volcanic activity on Allison Guyot generated several cones on its eastern part.[96] The evidence for this theory is not conclusive,[97] and another theory holds that the drowning of Allison Guyot occurred when it moved through equatorial waters, where upwelling increased the amount of nutrients available,[98] hampering the growth of platforms.[e][99] The waters might also have been too hot to support the survival of reef builders, as happens in present-day coral bleaching events.[100]

About 160 metres[f] of pelagic sediment[17] in the form of sand, ooze[101] and pelagic limestone accumulated on Allison Guyot; pelagic limestone is of Turonian to Campanian (83.6 ± 0.2 − 72.1 ± 0.2 million years ago[14]) age while the oozes and sands were deposited starting in the early Paleocene (66–56 million years ago[14]).[84] In drill cores, the ooze has a sandy, watery habitus owing to the prevalence of fossil foraminifera in the sediment.[13] The pelagic sediments have been bioturbated[g] in some places[103] and modified by sea currents, which have formed the large mound of pelagic sediment.[23] In drill cores, the ooze overlies Cretaceous shallow-water limestones,[104] which were modified by phosphatisation and manganese accumulation.[50] As plate tectonics moved Allison Guyot northward, its surrounding water masses changed, as did the properties of the pelagic cap.[85] Slumping of the platform occurred during the Cenozoic (the last 66 million years).[14][27]

The pelagic ooze bears evidence of the

isotope ratios of carbon in sediments on Allison Guyot[106] and changes in foraminifera[107] and ostracod fossils found in the ooze. The latter underwent a major extinction during the Paleocene–Eocene Thermal Maximum on the seamount and took a long time to recover.[108]

Sea currents have altered the pelagic deposits by removing smaller particles. In particular deposits from warmer periods have been altered in this way on Allison Guyot, perhaps because warmer climates increased

hurricane activity and thus the energy available in sea currents or deep-sea circulations shifted.[109] Furthermore, pauses in the sedimentation or episodes of slowdown have been identified.[110]

Notes

  1. drill cores from the oceans[6] and lasted from 1983 to 2003.[7]
  2. ^ Pit-like depressions within carbonate rocks that are filled with water.[40]
  3. ^ Between circa 125 and 113 million years ago[14]
  4. ^ Clays that became solid rocks.[68]
  5. symbiotic organisms in the platform builders.[99]
  6. ^ Some of which was later probably eroded away.[17]
  7. ^ Animals have stirred, mixed and otherwise modified the sediments.[102]
  8. ^ The Paleocene–Eocene Thermal Maximum was an episode of extreme global warmth about 55.5 million years ago, during which temperatures rose by about 5–8 °C.[105]

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Sources