Neogene ice volume and ocean temperatures: Insights from infaunal foraminiferal Mg/Ca paleothermometry

Antarctic continental-scale glaciation is generally assumed to have initiated at the Eocene-Oligocene Transition, yet its subsequent evolution is poorly constrained. We reconstruct changes in bottom water temperature and global ice volume from 0 to 17 Ma using delta O-18 in conjunction with Mg/Ca records of the infaunal benthic foraminifer, O. umbonatus from Ocean Drilling Program (ODP) Site 806 (equatorial Pacific; similar to 2500m). Considering uncertainties in core top calibrations and sensitivity to seawater Mg/Ca (Mg/Ca)(sw), we produce a range of Mg/Ca-temperature-Mg/Ca-sw calibrations. Our favored exponential temperature calibration is Mg/Ca = 0.66 +/- 0.08 x Mg/Ca-sw(0.27 +/- 0.06) x e((0.114 +/- 0.02 x BWT)) and our favored linear temperature calibration is Mg/Ca = (1.21 +/- 0.04 + 0.12 +/- 0.004 x BWT (bottom water temperature)) x (Mg/Ca-sw -0.003 +/- 0.02) (stated errors are 2 s.e.). The equations are obtained by comparing O. umbonatus Mg/Ca for a Paleocene-Eocene section from Ocean Drilling Program (ODP) Site 690 (Weddell Sea) to delta O-18 temperatures, calculated assuming ice-free conditions during this peak warmth period of the Cenozoic. This procedure suggests negligible effect of Mg/Ca-sw on the Mg distribution coefficient (D-Mg). Application of the new equations to the Site 806 record leads to the suggestion that global ice volume was greater than today after the Middle Miocene Climate Transition (similar to 14Ma). ODP Site 806 bottom waters cooled and freshened as the Pacific zonal sea surface temperature gradient increased, and climate cooled through the Pliocene, prior to the Plio-Pleistocene glaciation of the Northern Hemisphere. The records indicate a decoupling of deep water temperatures and global ice volume, demonstrating the importance of thresholds in the evolution of the Antarctic ice sheet.