The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons

• Published in: Climate of the Past
• Main Authors: Hutchinson, David K., Coxall, Helen K., Lunt, Daniel J., Steinthorsdottir, Margret, Boer, Agatha M., Baatsen, Michiel, Heydt, Anna, Huber, Matthew, Kennedy-Asser, Alan T., Kunzmann, Lutz, Ladant, Jean-Baptiste, Lear, Caroline H., Moraweck, Karolin, Pearson, Paul N., Piga, Emanuela, Pound, Matthew J., Salzmann, Ulrich, Scher, Howie D., Sijp, Willem P., Śliwińska, Kasia K., Wilson, Paul A., Zhang, Zhongshi
• Format: Text
• Language: English
• Published: 2021
• Subjects: Antarc*; Antarctica; Ice Sheet
• Online Access: https://doi.org/10.5194/cp-17-269-2021; https://cp.copernicus.org/articles/17/269/2021/

The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr . The change is marked by a global shift in deep-sea δ 18 O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO 2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO 2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO 2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO 2 forcing involving a large decrease in CO 2 of ca. 40 % ( ∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO 2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO 2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO 2 decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO 2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO 2 .