Biogeographic patterns of Pacific planktonic foraminifera have been quantitativelymapped for two time-slices in the early Miocene (22 and 16 Ma) and one in the lateMiocene (8 Ma). Important differences are apparent between the early and late Miocenethat resulted from changes in surface water circulation within the Pacific Ocean andbetween the tropical Pacific and Indian Oceans.In the early Miocene, tropical Pacific planktonic foraminifera were dominated bydifferent taxa in the eastern and western areas, but by the late Miocene the assemblageswere similar across the entire tropical Pacific. East to west faunal differences wereprobably due to differences in the surficial water-mass structure and temperature. It islikely that a deeper thermocline existed in the west favoring shallow water dwellers suchas Globigerinoides and Globigerina angustiumbilicata, and a shallower thermocline inthe east favoring slightly deeper-dwelling forms, especially Globorotalia siakensis andG. mayeri. During the late Miocene a trans-equatorial assemblage developed, dominatedby Globorotalia menardii-G. limbata and Globigerinoides groups. These faunalchanges are interpreted to reflect both the development, during the middle Miocene, ofthe Equatorial Undercurrent system when the Indonesian Seaway effectively closed andthe general strengthening of the gyral circulation and Equatorial Countercurrent thatresulted from increased Antarctic glaciation and high-latitude cooling during the middleMiocene.The trans-equatorial planktonic foraminiferal distribution patterns typical of thelate Miocene did not persist to the present-day oceans when east-west differences areagain evident. However, these differences in modern assemblages are exhibited withinforms that usually inhabit deeper waters. There is a successive changing dominancefrom west to east of Pulleniatina obliquiloculata to Globorotalia tumida to Neogloboquadrinadutertrei. The modern west to east differences in these deeper-dwelling formsreflect an intensification of the Equatorial Undercurrent system and its shallowing towardsthe east to depths well within the photic zone. Shallow-water forms, such asGlobigerinoides, maintain trans-tropical distribution patterns in the modern ocean un-like the early Miocene that lacked an effective equatorial countercurrent system in thePacific.The distribution of faunas in the North Pacific indicates that the gyral circulationsystem was only weakly developed in the early Miocene, but was strong by the lateMiocene. In the northwest Pacific, temperate faunas were displaced northward as theKuroshio Current intensified in the late Miocene. In the South Pacific, more distinctlatitudinal faunal provinces appeared during the middle to late Miocene along with anorthward expansion of the polar-subpolar provinces and contraction of the tropicalprovince. These faunal changes resulted from the continued areal expansion of the polarand subpolar water masses as Australia drifted northward from Antarctica and from thesteepening of pole to equator thermal gradients related to increased Antarctic glaciation.
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