Cooling and ice growth across the Eocene-Oligocene transition

The Eocene-Oligocene (E-O) climate transition (ca. 34 Ma) marks a period of Antarctic ice growth and a major step from early Cenozoic greenhouse conditions toward today's glaciated climate state. The transition is represented by an increase in deep-sea benthic foraminiferal oxygen isotope (delta O-18) values occurring in two main steps that reflect the temperature and delta O-18 of seawater. Existing benthic Mg/Ca paleotemperature records do not display a cooling across the transition, possibly reflecting a saturation state effect on benthic foraminiferal Mg/Ca ratios at deep-water sites. Here we present data from exceptionally well preserved foraminifera deposited well above the calcite compensation depth that provide the first proxy evidence for an similar to 2.5 degrees C ocean cooling associated with the ice growth. This permits interpretation of E-O delta O-18 records without invoking Northern Hemisphere continental-scale ice.