Article in Climate of the Past

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

David K. Hutchinson¹, Helen K. Coxall¹, Daniel J. Lunt², Margret Steinthorsdottir^1,3, Agatha M. de Boer¹,Michiel Baatsen⁴, Anna von der Heydt^4,5, Matthew Huber⁶, Alan T. Kennedy-Asser², Lutz Kunzmann⁷,Jean-Baptiste Ladant⁸, Caroline H. Lear⁹, Karolin Moraweck⁷, Paul N. Pearson⁹, Emanuela Piga⁹,Matthew J. Pound¹⁰, Ulrich Salzmann¹⁰, Howie D. Scher¹¹, Willem P. Sijp¹², Kasia K. ́Sliwi ́nska¹³, Paul A. Wilson¹⁴,and Zhongshi Zhang^15,16

¹Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
²School of Geographical Sciences, University of Bristol, Bristol, UK
³Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden
⁴Institute for Marine and Atmospheric Research, Department of Physics, Utrecht University, Utrecht, the Netherlands
⁵Centre for Complex Systems Studies, Utrecht University, Utrecht, the Netherlands
⁶Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, USA
⁷Senckenberg Natural History Collections, Dresden, Germany
⁸Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, USA
⁹School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK
¹⁰Department of Geography and Environmental Sciences, Northumbria University, Newcastle upon Tyne, UK
¹¹School of the Earth, Ocean and Environment, University of South Carolina, Columbia SC, USA
¹²Climate Change Research Centre, University of New South Wales, Sydney, Australia
¹³Department of Stratigraphy, Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
¹⁴University of Southampton, National Oceanography Centre, Southampton, UK
¹⁵Department of Atmospheric Science, China University of Geoscience, Wuhan, China
¹⁶NORCE Research and Bjerknes Centre for Climate Research, Bergen, Norway

https://cp.copernicus.org/articles/17/269/2021/

Abstract
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 δ¹⁸O 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 CO₂ 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 CO₂ 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: CO₂ 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 CO₂ forcing involving a large decrease in CO₂ 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 CO₂ proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO₂ decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO₂ 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 CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO₂.