A depth stratification of planktonic foraminifers based on oxygen isotopic ranking is proposed for the Miocene. Species are grouped into surface, intermediate, and deep dwellers based upon oxygen isotopic composition of individual species. The depth stratification is applied to planktonic foraminiferal populations in three Miocene time-slices (21 Ma, 16 Ma, and 8 Ma) in the equatorial, north, west, and east Pacific. The late Miocene time-slice is compared with modern Pacific GEOSECS transect water-mass profiles of temperature and salinity in order to illustrate the similarities between the depth ranking of planktonic foraminifers and temperature and salinity conditions. The geographic distribution of inferred surface, intermediate, and deep water dwellers was found to be very similar to modern temperature profiles: surface dwellers appear to be associated with warmest temperatures (>20°C), upper intermediate water dwellers with temperatures between 10 and 20°C, and lower intermediate and deep water dwellers with temperatures below 10°C. Tropical high-salinity water appears to be associated with the upper intermediate Globorotalia menardii group in the modern ocean.
Depth stratification applied to two Miocene time-series analyses in the equatorial Pacific (Sites 77B and 289) indicates increased vertical and latitudinal provincialism between early, middle, and late Miocene time. The early and middle Miocene equatorial Pacific was dominated by the warm surface water group, which shows distinct east-west provincialism. This provincialism is interpreted as the periodic strengthening of the equatorial surface circulation during polar cooling phases. During the late Miocene the upper intermediate group increased and the surface group declines. At the same time the east-west provincialism disappeared. This faunal change may have been associated with the major Antarctic glaciation and resultant strengthening of the general gyral circulation and the strengthening of the Equatorial Countercurrent due to the closing of the Indonesian Seaway at that time.