Wōdejebato (formerly known as Sylvania) is a

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .tablemount in the northern

Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Inte ...

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

. Wōdejebato is probably a

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more vi ...

and is connected through a submarine ridge to the smaller Pikinni Atoll southeast of the guyot; unlike Wōdejebato, Pikinni rises above sea level. The seamount rises for to depth and is formed by

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

ic rocks. The name Wōdejebato refers to a sea god of Pikinni. It was probably formed by a hotspot in what is present-day

French Polynesia )Territorial motto: ( en, "Great Tahiti of the Golden Haze") , anthem = , song_type = Regional anthem , song = "Ia Ora 'O Tahiti Nui" , image_map = French Polynesia on the globe (French Polynesia centered).svg , map_alt = Location of French ...

before

plate tectonic Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

s moved it to its present-day location. The Macdonald,

Rarotonga Rarotonga is the largest and most populous of the Cook Islands. The island is volcanic, with an area of , and is home to almost 75% of the country's population, with 13,007 of a total population of 17,434. The Cook Islands' Parliament buildings a ...

Rurutu Rūrutu is the northernmost island in the Austral archipelago of French Polynesia, and the name of a commune consisting solely of that island. It is situated south of Tahiti. Its land area is .Society A society is a group of individuals involved in persistent social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same political authority and dominant cultural expectations. Soc ...

hotspots may have been involved in its formation. The first volcanic phase took place in the

Cenomanian 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 ...

and was followed by the formation of a

carbonate platform A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually m ...

that quickly disappeared below the sea. A second volcanic episode between 85 and 78.4 million years ago (in the

Campanian 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. Campani ...

) led to the formation of an island. This island was eventually eroded and rudist

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock ...

s generated an

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

or atoll-like structure, covering the former island with carbonates and thus a second carbonate platform. The second carbonate platform drowned about 68 million years ago (in the

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the inte ...

), perhaps because at that time it was moving through the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

ial area which may have been too hot or too nutrient-rich to support the growth of a coral reef.

Thermal subsidence In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools a ...

lowered the drowned seamount to its present depth. After a hiatus, sedimentation commenced on the seamount and led to the deposition of manganese crusts and

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

sediments, some of which were later modified by

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

Name and research history

Wōdejebato is also written as Wodejebato. The name of the seamount comes from Wōdejebato, the name of the most feared and respected sea god of Pikinni Atoll. Wōdejebato was formerly called Sylvania, after the , a ship which was involved in its first mapping in 1946. The seamount was discovered in 1944, and was first investigated, using mainly

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

data, during

Operation Crossroads Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the ...

(a nuclear bomb test). Later, several times rocks were dredged from the seamount and drill cores were taken; cores 873–877 of the Ocean Drilling Program are from Wōdejebato.

Geography and geology

Local setting

Wōdejebato lies within the

Ralik Chain The Ralik Chain ( Marshallese: , ) is a chain of islands within the island nation of the Marshall Islands. Ralik means "sunset". It is west of the Ratak Chain. In 1999 the total population of the Ralik islands was 19,915. Christopher Loeak, who b ...

of islands and seamounts in the northern

, which consist of about three northwest-trending groups of islands of volcanic origin. Pikinni Atoll (formerly named Bikini) is located about southeast of the seamount. The seamount lies at a depth of and is about long with a flat top that narrows southeastward from over to less than . The surface of the flat top slopes inward and is covered by small depressions and knobs with an average

relief Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term '' relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that th ...

of about as well as

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''u ...

. The flat top is surrounded by a ridge, which has a width of and an average height of . On its northern and northeastern side, this ridge is in turn surrounded by another wide slightly raised ridge. The flat top has been interpreted as a

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons ...

surrounded by

s which form the inner ridge; the outer ridge appears to be a pile of skeletal sand rather than a reef and may be a spit formed by reworked material. Small mounds, probably of biological origin, are found at the margins of the seamount. The seamount is high above the sea floor and has an irregular shape, with spurs projecting from its circumference. These spurs have widths of and surface features that are distinct from those on the main flat top. The spurs appear to be

rift zone A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed t ...

s, similar to these formed on

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

by dyke injection although some of the ridges at Wōdejebato may have a different origin. Wōdejebato appears to have four such ridges, which is more than is observed at Hawaii. One explanation is that the northwestern ridge is another seamount; another that Wōdejebato consists of more than one volcano although the relatively small size of the seamount would argue against this view. Wōdejebato's slopes descend rather steeply until, at depth, where they become more gentle, they are decorated with forms resembling cones and channels. Part of its southern flank, where there is a downdropped terrace, seems to have collapsed in the past. Another satellite volcanic cone lies north of Wōdejebato at a depth of . Wōdejebato contains a volcanic structure within a superficial sediment cap, and a

free-air gravity anomaly In geophysics, the free-air gravity anomaly, often simply called the free-air anomaly, is the measured gravity anomaly after a free-air correction is applied to account for the elevation at which a measurement is made. It does so by adjusting these ...

has been observed on the seamount. Wōdejebato is connected to Pikinni by a wide, long and high submarine

ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...

and both volcanoes share a pedestal; Wōdejebato is the bigger of the two and its flat top has a larger surface than Pikinni's.

Magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particl ...

anomalies are also found on both volcanoes, with Wōdejebato featuring the more extensive one. Debris from these two volcanoes has formed an apron on their southwestern foot that is up to thick. The seafloor beneath Wōdejebato was formed during the Jurassic Quiet Zone over 156.9 million years ago. Farther north from Wōdejebato lies Lōjabōn-Bar seamount, and Look Guyot is due east. Wōdejebato appears to be one source of

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites wer ...

s in the Nauru Basin.

Regional setting

The

seafloor, especially the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

seafloor, contains most of the world's

corals Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

and

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

platforms. While there are some differences to present-day reef systems, many of these seamounts were formerly

s. Some atolls still exist, for example at Pikinni. All these structures originally formed as volcanoes in the Mesozoic ocean.

Fringing reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows dire ...

s may have developed on the volcanoes, which then became

barrier reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. ...

s as the volcano subsided and turned into an atoll. The crust underneath these seamounts tends to subside as it cools, and thus the islands and seamounts sink. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms. Sometimes volcanic activity continued even after the formation of the atoll or atoll-like structure, and during episodes where the carbonate platforms rose above sea level, erosional features such as channels and blue holes developed. The formation of many such seamounts has been explained with the hotspot theory, which describes the formation of chains of volcanoes which get progressively older along the length of the chain, with an active volcano only at one end of the system. Seamounts and islands in the Marshall Islands do not appear to have originated from such simple age-progressive hotspot volcanism as the age progressions in the individual island and seamount chains are often inconsistent with a hotspot origin. One explanation for this contradiction may be that more than one hotspot passed through the Marshall Islands, and it is also possible that hotspot volcanism was affected by extensional deformation of the

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

. In the case of Wōdejebato, candidate present-day hotspots are the

Macdonald hotspot The Macdonald hotspot (also known as "Tubuai" or "Old Rurutu") is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of the Macdonald Seamount, and possibly the Austral-Cook Islands chain. It probabl ...

which passed close to the seamount during the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

and

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

ages, between 115 and 94 million years ago in the early

, and the Society hotspot and

Rarotonga hotspot The Rarotonga hotspot is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of Rarotonga and some volcanics of Aitutaki. In addition to these volcanoes in the Cook Islands, the composition of volcanic ...

which approached the seamount in the late Cretaceous 85-80 million years ago, both time periods where volcanism occurred on Wōdejebato. A third hotspot which interacted with Wōdejebato is the

Rurutu hotspot Arago hotspot is a hotspot in the Pacific Ocean, presently located below the Arago seamount close to the island of Rurutu, French Polynesia. Arago is part of a family of hotspots in the southern Pacific, which include the Society hotspot and th ...

. The last two are the hotspots most likely to be long-lived, while many others, such as the

Marquesas hotspot The Marquesas hotspot is a volcanic hotspot in the southern Pacific Ocean. It is responsible for the creation of the Marquesas Islands – a group of eight main islands and several smaller ones – and a few seamounts. The islands and seamounts for ...

, were probably active discontinuously or only for brief time intervals. Based on plate motion reconstructions, the region of the Marshall Islands was located in the region of present-day

during the time of active volcanism. Both regions have numerous island chains, anomalously shallow ocean floors and the presence of volcanoes. About eight hotspots have generated a large number of islands and seamounts in that region, with disparate geochemistries.

Composition

The rocks at Wōdejebato include

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

, carbonates,

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

manganese Manganese is a chemical element with the Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of ...

, manganese phosphate,

peloid Peloid is mud, or clay used therapeutically, as part of balneotherapy, or therapeutic bathing. Peloids consist of humus and minerals formed over many years by geological and biological, chemical and physical processes. Numerous peloids are ava ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

; with an unusually large amount of

pyroclastic rock Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyrocl ...

s present. Organic material such as

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

peat Peat (), also known as turf (), is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. The peatland ecosystem covers and ...

and

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...

y material has also been found. Ferromanganese crusts have been found on the seamount. The crusts are composed of

asbolane Asbolane is a manganese (IV) oxy-hydroxide mineral containing also cobalt, nickel, magnesium, and calcium ions. It crystallizes in the hexagonal crystal system. Its chemical formula is . Naming It is named after the Greek Greek may refer to ...

, birnessite and buserite and contain

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

and

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...

. Wōdejebato has been evaluated as a possible

mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the econom ...

site for its mineral deposits. The limestones appear in several forms such as floatstone, grainstone,

micrite Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( Amer ...

, packstone, peloid and wackestone. Some grainstones and rudstones appear to be derived from algal and animal

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s. Many carbonate rocks have been altered, for example by cementation and leaching of their components and the dissolution of

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

; in some samples up to half of all the rock has been altered. These processes are collectively known as

diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

. Basalts at Wōdejebato mostly form an

alkali basalt Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...

suite but also include

ankaramite Ankaramite is volcanic rock type of mafic composition. It is a dark porphyritic variety of basanite containing abundant pyroxene and olivine phenocrysts.https://dialnet.unirioja.es/descarga/articulo/281059.pdf Luis Enrique Ortiz Hernández, ''An A ...

and

hawaiite Hawaiite is an olivine basalt with a composition between alkali basalt and mugearite. It was first used as a name for some lavas found on the island of Hawaii. It occurs during the later stages of volcanic activity on oceanic islands such as Ha ...

. The rocks contain clinopyroxene,

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...

and

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s. Alteration has led to the formation of

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

, chabazite,

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorou ...

, hydromica,

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

, serpentine and smectite, and gaps and cavities in the rock have been filled by sediments. The element geochemistry of lavas from Wōdejebato resembles that of South Central Pacific islands such as

Marotiri Marotiri is a group of four uninhabited volcanic rocks protruding from the sea (and several submerged rocks), forming the southeastern end of the Austral Islands of French Polynesia. Marotiri is also known as Bass Rocks (''Îlots de Bass'' in Fre ...

and

and is consistent with magma sources of

intraplate In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny (mountain building). Anorogenic magmatism occurs, for example, at mid-ocean ridges, hotspots and continental rifts. This contrasts ...

volcanism.

Isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s show affinities to those of volcanic rocks from the Macdonald, Rurutu, Rarotonga and Society hotspots; differences between isotope ratios of various stages of volcanism may reflect the passage of Wōdejebato over more than one " plumelet".

Geologic history

Wōdejebato formed either before or during the

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. ...

age (86.3 ± 0.5 – 83.6 ± 0.2 million years ago), with the Albian age (about 113 to 100.5 million years ago) being a likely candidate. Wōdejebato originated in the Southern Hemisphere and was moved by plate tectonics into the Northern Hemisphere, and

paleomagnetism Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

indicates that the seamount was located at 10 degrees southern latitude when the most recent lavas erupted. It subsequently underwent several episodes of uplift and subsidence and eventually drowned, forming the present-day

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...

. Ruwitūntūn is another seamount in the Marshall Islands with a similar history.

Volcanism and first biotic phenomena

Volcanism at Wōdejebato appears to have occurred during two phases over a timespan of about 20 million years. The first phase took place during the

(100.5 – 93.9 million years ago); it was characterized by

explosive eruption In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma su ...

s and may be the source of 93.9–96.3 million year old volcanic debris found in the surroundings of Wōdejebato. The second phase occurred during the

between 78.4 and 85 million years ago during

chron Chron may refer to: Science * Chronozone or chron, a term used for a time interval in chronostratigraphy * Polarity chron or chron, in magnetostratigraphy, the time interval between polarity reversals of the Earth's magnetic field Other * ...

33R; it appears to be part of a volcanic event that affected a number of other islands and seamounts in the Marshall Islands and at Wōdejebato lasted for at least four million years. The second stage appears to have been a secondary volcanic episode. Volcanic rocks sampled at Wōdejebato all belong to the second stage, probably due to

sampling bias In statistics, sampling bias is a bias in which a sample is collected in such a way that some members of the intended population have a lower or higher sampling probability than others. It results in a biased sample of a population (or non-human f ...

as the samples all come from the summit region. Tectonic evidence indicates that Pikinni formed at the same time as Wōdejebato, while the northern parasitic cone may be less than 80 million years old and reefs have been covered by volcanic rocks of

(80 - 70 million years ago) age. An earlier proposal by Schlanger ''et al.'' 1987 envisaged

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

(56 – 33.9 million years ago) eruptions at Wōdejebato but today the older ages are considered to be correct. The volcanic activity produced breccia and

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

, probably first generating a

. Volcanic activity occurred both in shallow water and

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

forming hyaloclastite and highly vesicular rocks during

phreatomagmatic Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...

eruptions, and above sea level as indicated by the presence of basaltic pebbles. Some early volcanic deposits were buried by later activity. There are conflicting reports about whether

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

activity took place. Vegetation including

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

s and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

grew on the exposed island during the Campanian, leaving abundant wood remnants.

Weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

of basaltic rocks produced clay sediments and

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

s thick have been obtained in drill cores.

Platform carbonates and reefs

After volcanic activity ceased, environmental processes transformed Wōdejebato into a flat-topped platform, equivalent to a present-day atoll, as the crust beneath Wōdejebato seamount subsided. Erosion and subsidence lowered the volcanic pile until seawater flooded it and marine sedimentation commenced. This platform phase lasted only about 10 million years and took place in at least two stages, in line with the generally short duration of such platform phases; they do not generally last longer than 20 million years. The growth of the platform was not continuous and was probably interrupted by one drowning event between the Albian and Campanian ages, similar to other seamounts in the Pacific Ocean which also drowned during this time. Cycle émersif Maroc

Limestones and carbonates forming a platform accumulated on Wōdejebato, with drill cores showing total thicknesses of . Compositionally, it consists mainly of sandy carbonates that are often leached and cemented by calcitic material. These deposits eventually covered the entire upper area of the volcanic high and formed the inner ridge. Variations in sea level occasionally led to parts of the platform either emerging above sea level or submerging, leading to erosion that generated the outer ridge and to the development of characteristic sequences within the deposits. Such carbonate platforms look like present-day atolls but unlike the biogenic frameworks of modern atolls they were formed by biogenic sediments; at Wōdejebato sandy shoals appear to have been a principal component. These carbonate deposits would then have been surrounded by a barrier reef and the redeposition, followed by stabilization, of eroded material had a role in the development of the surrounding rim. Reef mounds grew to several tens of metres in height.

Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

l fossil data imply that lagoonal environments existed on Wōdejebato. The central part of the guyot surface and its margins feature different platform structures, and the platform has been subdivided into several different assemblages on the basis of foraminifera stages. Environmental conditions on the platform were characterized by

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

influences. Wōdejebato was probably located in equatorial waters with temperatures likely exceeding , with temperature ranges of during the

. The platform was sometimes affected by

storm A storm is any disturbed state of the natural environment or the atmosphere of an astronomical body. It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm), ...

s that reworked the rock material. Soil properties imply that

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

on Wōdejebato was less than , but erosion by precipitation water and dissolution of parts of the carbonate platform have been inferred from dissolution traces in the rocks. Sea level variations induced the formation of step-like reef tracts on Wōdejebato's carbonate platform. Much of the reefbuilding was carried out by

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and se ...

s, rudists and

stromatoporoid Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Ordovician through the Devonian. They were especially abundant and important reef-formers in the Silurian and most of the Devonian.Stock, C.W. 2001, Stromat ...

s. Unlike present-day coral reefs, reef building in the Cretaceous was carried out mainly by rudists which probably started appearing at Wōdejebato in the Albian; rudist taxa active at Wōdejebato included caprinids and radiolitids, such as '' Antillocaprina'', '' Coralliochama'', '' Distefanella'', '' Mitrocaprina'' and '' Plagioptychus''. Furthermore,

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

foraminifers were active from the Campanian to the Maastrichtian; they include '' Asterorbis'', '' Pseudorbitoides trechmanni'', '' Omphalocyclus macroporus'' and '' Sulcoperculina'' as well as other discorbids, lituolids, miliolids, opthalmiids, orbitoids, peneroplids, placopsilinids, rotaliids and textulariids. Other lifeforms that were fossilized in the carbonate reefs were

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

e including

green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

e ( codiaceans and dasycladaceans) and

red alga Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

e ( corallinaceans, peyseonneliaceans and solenoporaceans); some algae formed

rhodolith Rhodoliths (from Greek for ''red rocks'') are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic gr ...

s. In addition there were

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

s ( inoceramids and pycnodonts), bryozoans, corals,

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...

s, echinoids,

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

s and

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...

Drowning and post-drowning evolution

It is likely that Wōdejebato drowned during the Maastrichtian age around 68 million years ago, probably accompanied by a sea level rise of about . Before the terminal drowning, Wōdejebato's carbonate platform emerged from the sea, leading to the development of

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, Dolomite (rock), dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathe ...

features; two separate emersion events took place 68 and 71 million years ago. Sea level rise on its own probably does not explain the drowning. Various paleoenvironmental stressors have been invoked to explain the drowning such as short-term climate fluctuations during the Maastrichtian and the passage of the seamount through the equatorial

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...

zone. The water in this region may have been too hot for the reef to survive: Other guyots in the Pacific Ocean such as Limalok, Lo-En and Takuyo-Daisan also drowned when they were within ten degrees from the equator on the Southern Hemisphere, implying that this region of the Pacific Ocean was in some way harmful to shallow water reefs. The

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

that occurred after Wōdejebato moved away from the influence of the Rurutu hotspot may have also played a role. Pikinni was probably higher than Wōdejebato at this time and hence escaped drowning. After the drowning had taken place, thermal subsidence of the crust beneath Wōdejebato occurring at a rate of lowered the platform of Wōdejebato to a depth of about below sea level. Between the Maastrichtian and the Eocene, manganese crusts formed on the exposed limestones and

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

s formed by erosion; in turn they were subject to alteration processes such as

phosphatization Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the s ...

during three different episodes in the Eocene. Marine-microfossils-major hg

Approximately 40 million years passed between the drowning and subsequent deposition events.

Pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

sedimentation took place, which formed an

ooze Ooze may refer to: * Pelagic sediments, fine-grained sediments on the ocean floor, containing at least 30% biogenous material Games * Ooze (''Dungeons & Dragons''), a type of monster in the ''Dungeons & Dragons'' role-playing game * '' The Ooze ...

consisting of foraminiferal and nannofossil deposits between the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

and

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

, with a Miocene

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

. In one drill core, this sediment layer is thick.

Current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

s affected mid- to late Pleistocene sedimentation. Among the foraminifera deposited here are '' Florisphaera'', ''

Gephyrocapsa ''Gephyrocapsa'' is a genus of haptophyte The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for ''Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used ins ...

'', '' Globigerina'', '' Globorotalia'', '' Helicosphaera'', '' Pseudoemiliania'' and potentially '' Sphaeroidinella'' species. Foraminifera taken from Wōdejebato usually belong to pelagic species. Ostracods have also been identified; common taxa are cytherurids as well as '' Bradleya'', '' Cytheralison'' and '' Krithe'' species. Presently, Wōdejebato lies below the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

and the temperature of the water washing over the seamount is about . Circumstantial evidence indicates that deep seawater dissolved large amounts of carbonate rocks including aragonite after Wōdejebato was submerged; the seamount is located below the aragonite saturation depth and that causes the aragonite to dissolve. Some of the dissolved aragonite has precipitated again in the form of calcite, and sediments have partially filled cavities within the carbonate rocks.

Notes

References

Sources

* * * * * * * * * * * * * * * * * * * * * * * * {{Marshall Islands topics Seamounts of the Pacific Ocean Extinct volcanoes Landforms of the Marshall Islands Mesozoic volcanoes Ralik Chain