O/O measurements on benthic foraminifera

[1] We explore the applicability of paired Mg/Ca and O/O measurements on benthic foraminifera from Southern Ocean site 747 to paleoceanographic reconstructions on pre-Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27–0 Ma) history of paleotemperatures and the evolution of the dO values of seawater (dOsw) at a temporal resolution of 100–200 kyr. Absolute paleotemperature estimates depend on assumptions of how Mg/Ca ratios of seawater have changed over the past 27 Myr, but relative changes that occur on geologically brief timescales are robust. Results indicate that at the Oligocene to Miocene boundary (23.8 Ma), temperatures lag the increase in global ice-volume deduced from benthic foraminiferal dO values, but the smaller-scale Miocene glaciations are accompanied by ocean cooling of 1 C. During the mid-Miocene phase of Antarctic ice sheet growth ( 15–13 Ma), water temperatures cool by 3 C. Unlike the benthic foraminiferal dO values, which remain relatively constant thereafter, temperatures vary (by 3 C) and reach maxima at 12 and 8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an 4 C cooling at site 747. A comparison of our dOsw curve to the Haq et al. [1987] sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater O/O ratios. At 12–11 Ma in particular, when benthic foraminiferal dO values do not support a further increase in ice volume, the dOsw curve comes to a maximum that corresponds to a major mid-Miocene sea level regression. The agreement between the character of our Mg/Ca-based dOsw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the dOsw on pre-Pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater Mg/Ca ratios. The Mg/Ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes, which can lead to misinterpretations of the dO record.