Biogeographic patterns of Pacific planktonic foraminifera have been quantitatively mapped for two time-slices in the early Miocene (22 and 16 Ma) and one in the late Miocene (8 Ma). Important differences are apparent between the early and late Miocene that resulted from changes in surface water circulation within the Pacific Ocean and between the tropical Pacific and Indian Oceans.

In the early Miocene, tropical Pacific planktonic foraminifera were dominated by different taxa in the eastern and western areas, but by the late Miocene the assemblages were similar across the entire tropical Pacific. East to west faunal differences were probably due to differences in the surficial water-mass structure and temperature. It is likely that a deeper thermocline existed in the west favoring shallow water dwellers such as Globigerinoides and Globigerina angustiumbilicata, and a shallower thermocline in the east favoring slightly deeper-dwelling forms, especially Globorotalia siakensis and G. mayeri. During the late Miocene a trans-equatorial assemblage developed, dominated by Globorotalia menardii-G. limbata and Globigerinoides groups. These faunal changes are interpreted to reflect both the development, during the middle Miocene, of the Equatorial Undercurrent system when the Indonesian Seaway effectively closed and the general strengthening of the gyral circulation and Equatorial Countercurrent that resulted from increased Antarctic glaciation and high-latitude cooling during the middle Miocene.

The trans-equatorial planktonic foraminiferal distribution patterns typical of the late Miocene did not persist to the present-day oceans when east-west differences are again evident. However, these differences in modern assemblages are exhibited within forms that usually inhabit deeper waters. There is a successive changing dominance from west to east of Pulleniatina obliquiloculata to Globorotalia tumida to Neoglobo-quadrina dutertrei. The modern west to east differences in these deeper-dwelling forms reflect an intensification of the Equatorial Undercurrent system and its shallowing towards the east to depths well within the photic zone. Shallow-water forms, such as Globigerinoides, maintain trans-tropical distribution patterns in the modern ocean unlike the early Miocene that lacked an effective equatorial countercurrent system in the Pacific.

The distribution of faunas in the North Pacific indicates that the gyral circulation system was only weakly developed in the early Miocene, but was strong by the late Miocene. In the northwest Pacific, temperate faunas were displaced northward as the Kuroshio Current intensified in the late Miocene. In the South Pacific, more distinct latitudinal faunal provinces appeared during the middle to late Miocene along with a northward expansion of the polar-subpolar provinces and contraction of the tropical province. These faunal changes resulted from the continued areal expansion of the polar and subpolar water masses as Australia drifted northward from Antarctica and from the steepening of pole to equator thermal gradients related to increased Antarctic glaciation.  PDF