The paleoceanography of the North Pacific Ocean at 8 Ma has been determined using radiolarian biogeographic patterns that were defined through Q-mode factor analysis. Forty-four late Miocene radiolarian species categories were grouped into assemblages exhibiting latitudinal zonality and distinct ecological preferences. The assemblages represent three major water mass types: (1) tropical, shallow water; (2) subarctic, deep to intermediate water; and (3) transitional, intermediate water. Several extant species that characterize the subarctic and transitional regions have survived for the past 8 Ma without apparent morphologic change. These species may have developed the ability to withstand large changes in depth habitat, allowing them to live deeper in water masses at lower latitudes when climatic variations made it necessary.
Comparison of late Miocene and Recent biogeographic patterns using extant species indicates important differences between the 8 Ma time-slice and the Recent, which can be interpreted as changes in surface-water circulation. At 8 Ma, flow of Atlantic water into the equatorial Pacific (through the Central American Seaway) disrupted the flow pattern of the Equatorial Countercurrent, causing warm water, which is usually returned to the eastern tropical Pacific, to be diverted into the northward-flowing western limb of the subtropical gyre. This resulted in increased transport of warmer waters by the subtropical gyre and warmer sea-surface temperatures in the North Pacific.
As the Panama Isthmus became a barrier to surface water flow between the Atlantic and Pacific, transport of warm tropical water into the middle and high latitudes of the Pacific decreased, resulting in progressive cooling of the subpolar regions. The absence of a late Miocene equivalent to the Modern Eastern Tropical assemblage suggests that changes in the character of water masses in the eastern equatorial Pacific occurred at some time after 8 Ma. Decreased equatorial sea-surface temperatures in response to increased advection of cold subpolar water (via eastern boundary currents), an increase in colder, upwelled waters at the equatorial divergence, and an increased importance in surfacing of the Equatorial Undercurrent in the eastern tropical Pacific are likely oceanographic responses to the creation of a geographic barrier to inter-ocean exchange. The gradual cessation of Atlantic-Pacific surface water communication accelerated the global cooling trend that began in the middle Miocene with the development of the East Antarctic ice sheet. Termination of this inter-ocean flow in the middle Pliocene marked the beginning of a new chapter in Earth history in which climate was characterized by glacial-interglacial extremes due to cyclic variations in continental ice volume.