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

Across the Eocene-Oligocene transition, the oxygen isotopic composition (O-18) 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 O-18, we measured the clumped isotopic composition of planktonic foraminifera tests from Ocean Drilling Program Site 689 in the Southern Ocean. Near-surface temperatures were similar to 12 degrees C in the intervals 0-1.5Myr before and 1-2Myr after the major (Oi-1) transition, in agreement with estimates made using other proxies at nearby sites. Temperatures cooled by 0.41.1 degrees C between these intervals, indicating that the long-term change in O-18 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 (O-18(sw)) is 0.750.23 parts per thousand, which is within the range of previous estimates, and represents global ice growth equivalent to roughly similar to 110-120% of the volume of the modern Antarctic ice sheet or similar to 80-90m of eustatic sea level change.