# Oligocene deep ocean oxygen isotope variations primarily driven by temperature

## TL;DR

Oligocene deep ocean isotope changes were mainly due to temperature, not ice sheet size, according to new clumped isotope data.

## Contribution

First Oligocene deep ocean temperature record using clumped isotope thermometry, independent of seawater chemistry.

## Key findings

- Large temperature variations of up to 4°C were driven by eccentricity-paced orbital cycles.
- δ18Ocalcite fluctuations can be explained by temperature changes without requiring major ice volume shifts.
- Results align with simulated stability of the Antarctic ice sheet during the Oligocene.

## Abstract

Our understanding of the long-term behaviour of global climate and the Antarctic ice sheet relies heavily on the oxygen isotopic composition of marine calcite (δ18Ocalcite), which reflects a combination of ocean temperature and the amount of water stored in ice sheets. On the basis of δ18Ocalcite, the Antarctic ice sheet has been interpreted as extremely dynamic in the Oligocene, 34–23 million years ago. Yet, the proposed continental-scale ice volume changes are challenging to reproduce with models and may be overestimated owing to a larger influence of temperature on the deep-sea δ18Ocalcite than previously assumed. Here we present the first Oligocene record of orbital variability in deep ocean temperature based on benthic foraminiferal clumped isotope thermometry, a method affected only by temperature and independent of seawater chemistry. We find large, eccentricity-paced temperature variations of up to 4 °C, sufficient to explain the δ18Ocalcite cycles without requiring continental-scale ice volume changes. This finding is consistent with the simulated stability of the Antarctic ice sheet, highlighting the importance of robust independent temperature reconstructions. Our results show that the temperature in the deep Southern Ocean, and possibly globally, is highly sensitive to the seasonal distribution of insolation in an Oligocene-like climate state warmer than today.

Large benthic oxygen isotope fluctuations in the Oligocene Southern Ocean primarily represent deep water temperature changes, suggesting the Antarctic ice sheet volume was relatively stable, according to a clumped isotope record.