Paleotemperatures and ice volume of the past 27 Myr revisited with paired Mg/Ca and 18O/16O measurements on benthic foraminifera

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Abstract

[1] We explore the applicability of paired Mg/Ca and 18O/16O 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 δ18O values of seawater (δ18Osw) 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 δ18O 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 δ18O 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 δ18Osw curve to the Haq et al. [1987] sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18O/16O ratios. At ∼12–11 Ma in particular, when benthic foraminiferal δ18O values do not support a further increase in ice volume, the δ18Osw 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 δ18Osw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the δ18Osw 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 δ18O record.

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