INTRODUCTION

Planktonic foraminifera are microscopic, free-floating protists that are widely distributed throughout the surface waters of the world's oceans. Unlike other members of the marine micro-zooplankton, planktonic foraminifera secrete a calcareous test during the course of their life cycle and, as a result, are a major contributor to the marine sediments that accumulate at the sea floor. Substantial deposits of this ‘Globigerina ooze’, more correctly termed foraminiferal ooze, are preserved within the geological record, often reaching thicknesses of several hundred metres. The excellent representation of planktonic foraminifera within Cenozoic marine sediments, whilst being disproportionate to their actual abundance levels within the total biomass of the marine zooplankton in the surface waters, is one of the prime reasons for the extensive usage of this particular microfossil group in both biostratigraphical and palaeoecological investigations.

Wei and Kennett (1986) documented an overall decline in the diversity of planktonic foraminiferal faunas over the last 5 million years, with species diversity levels in the modern ocean being approximately two-thirds those of the Neogene maximum. The most significant climatic event during this period of time was the rapid expansion of the Northern Hemisphere glaciations around 2.5 million years ago (Shackleton et al., 1984). It has been shown that ocean circulation patterns were significantly affected by the environmental changes that accompanied the Late Pliocene build-up of the Northern Hemisphere ice-sheets (Loubere & Moss, 1986; Raymo et al., 1989).