Antarctic ice growth before and after the Eocene-Oligocene transition: New estimates from clumped isotope paleothermometry

Abstract

Across the Eocene-Oligocene transition, the oxygen isotopic composition (δ18O) of benthic and planktonic foraminifera increased by over 1‰. This shift is thought to represent a combination of global cooling and the growth of a large ice sheet on the Antarctic continent. To determine the contribution of each of these factors to the total change in δ18O, we measured the clumped isotopic composition of planktonic foraminifera tests from Ocean Drilling Program Site 689 in the Southern Ocean. Near-surface temperatures were ∼12C in the intervals 0-1.5 Myr before and 1-2 Myr after the major (Oi-1) transition, in agreement with estimates made using other proxies at nearby sites. Temperatures cooled by 0.4 ± 1.1C between these intervals, indicating that the long-term change in δ18O seen in planktonic foraminifera at this site is predominantly due to changes in ice volume. A larger instantaneous cooling may have occurred during Oi-1 but is not captured in this study due to sampling resolution. The corresponding change in the isotopic composition of seawater (δ18Osw) is 0.75 ± 0.23‰, which is within the range of previous estimates, and represents global ice growth equivalent to roughly ∼110-120% of the volume of the modern Antarctic ice sheet or ∼80-90 m of eustatic sea level change.