The late Cenozoic was a time of major palaeoclimatic and palaeoceanographic events (Fig. 10.1). The latest Miocene was the most critical time: there was a global cooling of ocean surface waters at middle and high latitudes; a northward migration of cold Antarctic surface waters; an expansion of the Antarctic ice sheet; a major sea-level fall; isolation and desiccation of the Mediterranean Basin; and a lightening in carbon isotopes. In the early Pliocene, there was a warming and a major sea-level rise, followed by late Pliocene climate deterioration and gradual intensification of northern hemisphere glaciation leading to late Pleistocene climate changes visible in the prominent 100 k.y. cycle. The late Cenozoic thus has the most complete record available for defining global change during development of a Cool Mode, especially as related to orbitally driven glacial cyclicity.

Summary of marine climates from oxygen isotopes

Oxygen isotopic ratios in foraminifera from late Cenozoic deep-sea sediments are primarily a function of the δ18O composition and temperature of the ambient sea water. The abyssal waters of the ocean experience relatively small changes in temperature, thus the δ18O in benthic foraminifera is mainly a function of the isotopic composition of the bottom water, which in turn is controlled largely by global ice volume. However, the effect of diagenesis was emphasized by Killingley (1983) who noted that the longterm decrease in ocean temperatures from the Palaeocene to the Miocene, derived from oxygen isotopes, might reflect progressive recrystallization of calcite and therefore all palaeotemperature estimations for the early Tertiary remain speculative.