Miocene deep-sea benthic foraminifera, analyzed from numerous Pacific DSDP sites, are found to respond to climatically induced oceanic variations by: (1) changes in depth distribution with time; (2) changes in species proportion within assemblages; and (3) becoming extinct. Because benthic species are long-ranging, many species occurring today were present in the Miocene and provide a basis for studying Miocene paleo-oceanographic changes. Analyses of ^dgrO¹⁸ and ^dgrC¹³ compositions of benthic foraminifera which record fluctuations in paleotemperatures and in the marine HCO₃ pool reveal major changes between the early and late Miocene.

Shifts in benthic foraminiferal populations and isotopic compositions during the Miocene imply the following water mass changes: (1) early Miocene deep waters appear to have been warmer with older, light ^dgrC¹³; and (2) a sharp middle Miocene increase in ^dgrO¹⁸ which we interpret to be a major bottom water cooling concomitant with Antarctic glacial buildup and thickening of the Antarctic bottom waters, restructuring the Miocene ocean and increasing the equatorial thermal gradient. Benthic fauna species dominance, species assemblage, and water depth indicate that by the late Miocene, both modern benthic foraminiferal assemblages and modern oceanographic conditions were approached. Intensification of the oxygen minimum zone in the late Miocene is supported by the dominance of Uvigerina fauna in the west Pacific.

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