The South Atlantic Oxygen Isotope Record of Planktic Foraminifera

This paper reviews the recording of oxygen isotope ratios in planktic foraminifera and summarizes recent results of the application of oxygen isotopes in paleoceanographic studies of the South Atlantic. The most important factors controlling the δ18O of planktic foraminifera are temperature, the δ18O and the pH of ambient seawater. Seasonal and vertical calcification weight the mean δ18O of a foraminiferal population towards the hydrographic conditions in the preferred ecological niche. After deposition, the δ18O signal is affected by bioturbation and dissolution. Despite many influence factors, the composition of oxygen isotopes in fossil tests of planktic foraminifera provides important constraints on variations of the surface water hydrography of the South Atlantic and the Southern Ocean throughout the past 20,000 years. During the last glacial maximum, the Polar Front remained close to its modem position or shifted only slightly towards the north. In the tropics, oxygen isotopes indicate only a moderate glacial cooling of 2–3°C. During deglaciation, oxygen isotope ratios in the eastern boundary currents of the subtropical South Atlantic decreased asynchronously relative to those in the eastern North Atlantic, with the highest interhemispheric contrasts during the Younger Dryas and the Heinrich Event 1. This pattern is consistent with a redistribution of heat within the Atlantic Ocean in response to a weakening of the thermohaline circulation. The slowdown of deglacial overturning was associated with a southward displacement of the thermal equator and the Intertropical Convergence.