Paleotemperatures and ice volume of the past 27 Myr revisited with paired Mg/Ca and 18O/16O measurements on benthic foraminifera -: art. no. 1003

[1] We explore the applicability of paired Mg/Ca and O-18/O-16 measurements on benthic foraminifera from Southern Ocean site 747 to palcoceanographic reconstructions on pre-Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27-0 Ma) history of palcotemperatures and the evolution of the WO values of seawater (delta(18)O(sw)) at a temporal resolution of similar to100-200 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 delta(18)O values, but the smaller-scale Miocene glaciations are accompanied by ocean cooling of similar to1degreesC. During the mid-Miocene phase of Antarctic ice sheet growth (similar to15-13 Ma), water temperatures cool by similar to3degreesC. Unlike the benthic foraminiferal delta(18)O values, which remain relatively constant thereafter. temperatures vary, (by 3degreesC) and reach maxima at similar to12 and similar to8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an similar to4degreesC cooling at site 747. A comparison of our delta(18)O(sw) curve to the Haq et al. [1987] sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater O-18/O-16 ratios. At similar to12-11 Ma in particular, when benthic foraminiferal delta(18)O values do not support a further increase in ice volume, the delta(18)O(sw) 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 delta(18)O(sw) curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the 8180, 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 delta(18)O record.