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Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Ancient Greek πᾶν]] "all" and θάλασσα]] "sea"), was the vast superocean that encompassed planet Earth and surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During the Paleozoic–Mesozoic transition ( 250 ), the ocean occupied almost 70% of Earth's surface, with the supercontinent Pangaea taking up the remaining one third. The original, ancient ocean floor has now completely disappeared because of the continuous subduction along the continental margins on its circumference. Panthalassa is also referred to as the Paleo-Pacific ("old Pacific") or Proto-Pacific because the Pacific Ocean is a direct continuation of Panthalassa.
Most of the continental fragments, , and Oceanic basin added to Laurentia this way contained faunas of Tethyan or Asian affinity. Similar terranes added to the northern Laurentia, in contrast, have affinities with Baltica, Siberia, and the northern Caledonies. The latter terranes were probably accreted along the eastern Panthalassa margin by a Caribbean Plate–Scotia Plate-style subduction system.
During the Permian, developed near the Equator on the mid-Panthalassic seamounts. As Panthalassa subducted along its western margin during the Triassic and Early Jurassic, those seamounts and palaeo-atolls were accreted as allochthonous limestone blocks and fragments along the Asian margin. One such migrating atoll complex now form a and body of limestone in central Kyushu, south-west Japan.
Fusulinida foraminifera, a now extinct order of single-celled organisms, diversified extensively and developed gigantism—the genus Eopolydiexodina, for example, reached up to in size—and structural sophistication, including symbiont relationships with photosynthesising algae, during the Late Carboniferous and Permian, in what is known as the Carboniferous-Earliest Permian Biodiversification Event. The Capitanian mass extinction event 260 , however, put an end to that development, with only dwarf taxa persisting throughout the Permian until the final fusuline extinction in the Great Dying 252 . Permian fusulines also developed a remarkable provincialism by which fusulines can be grouped into six domains. Because of the large size of Panthalassa, a hundred million years could separate the accretion of different groups of fusulines. Assuming a minimum accretion rate of , the seamount chains on which those groups evolved would be separated by at least . Those groups apparently evolved in completely different environments.
A significant sea-level drop at the end of the Permian led to the end-Capitanian extinction event. The cause for the extinction is disputed, but a likely candidate is an episode of global cooling, which transformed a large amount of sea-water into continental ice.
Seamounts accreted in eastern Australia as parts of the New England orogen reveal the hotspot history of Panthalassa. From the Late Devonian to the Carboniferous, Gondwana and Panthalassa converged along the eastern margin of Australia along a west-dipping subduction system, which produced (west to east) a magmatic arc, a forearc basin, and an accretionary wedge. Subduction ceased along that margin in the Late Carboniferous and jumped eastward. From the Late Carboniferous to the Cisuralian the New England orogen was dominated by an extensional setting related to a subduction to strike-slip transition. Subduction was re-initiated in the Permian and the granitic rocks of the New England Batholith were produced by a magmatic arc, indicating the presence of an active plate margin along most of the Orogeny. Permian to Cretaceous remains of the convergent margin, preserved as fragments in Zealandia (New Zealand, New Caledonia, and the Lord Howe Rise), were rifted off Australia during the Late Cretaceous to Early Tertiary break-up of eastern Gondwana and the opening of the Tasman Sea.
The Cretaceous Junction Plate, located north of Australia, separated the eastern Tethys Ocean from Panthalassa.
In northern Panthalassa, there were mid-latitude westerlies north of 60°N with easterlies between 60°N and the Equator. Atmospheric circulation north of 30°N is associated with the North Panthalassa High, which created Ekman transport between 15°N and 50°N and Ekman divergence between 5°N and 10°N. A pattern developed that resulted in Sverdrup balance that went northward in divergence regions and southward in convergence regions. Western boundary currents resulted in an anti-cyclonic subtropical North Panthalassa gyre at mid-latitudes and a meridional anti-cyclonic circulation centred on 20°N.
In tropical northern Panthalassa, trade winds created westward flows while equatorward flows were created by westerlies at higher latitudes. Consequently, trade winds moved water away from Gondwana towards Laurasia in the northern Panthalassa Equatorial Current. When the western margins of Panthalassa were reached, intense western boundary currents would form the Eastern Laurasia Current. At mid-latitudes, the North Panthalassa Current would bring the water back east where a weak Northwestern Gondwana Current would finally close the gyre. The accumulation of water along the western margin, coupled with the Coriolis force, would have created a Panthalassa Equatorial Counter Current.
In the southern Panthalassa, the four currents of the subtropical gyre, the South Panthalassa Gyre, rotated counterclockwise. The South Equatorial Panthalassa Current flowed westward between the Equator and 10°S into the western, intense South Panthalassa Current. The South Polar Current then completed the gyre as the Southwestern Gondwana Current. Near the poles easterlies created a subpolar gyre that rotated clockwise.
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