Oxygen isotopic evidence for the development of the psychrosphere 38 Myr ago

THREE major elements were involved in the evolution of oceanic circulation during the Cainozoic. The first of these was the diminishing role of oceanic meridional circulation in equatorial areas, the second was the development of circum-Antarctic circulation and the third involves the development of the present-day system of bottom waters of the world ocean, the ‘psychrosphere’1,2. Because deep bottom waters are mostly derived from the polar regions, their characteristics, such as temperature, reflect surface conditions at high latitudes3. Past temperature changes of deep oceanic waters, and thus of polar surface waters, can be determined from the oxygen isotopic composition of deep-dwelling benthonic microfossils3,4. A deep-sea palaeo-temperature record has been established4 for the past 54 Myr based on DSDP sites in the sub-Antarctic; detailed arguments4 give evidence that the Antarctic ice sheet developed during the Middle Miocene. Previous to this, however, a significant 5 °C decline in bottom-water temperature is recorded close to the Eocene–Oligocene boundary and is considered4 to represent the onset of Antarctic bottom-water formation at temperatures close to freezing. This heralds the beginning of the modern thermo-haline circulation and is thus one of the most important palaeo-oceanographic events in the Cainozoic. We examine here in more detail the palaeotemperature event near the Eocene-Oligocene boundary, the latter having an age of 38 Myr (ref. 5).