Clim. Past, 17, 269–315, 2021 https://doi.org/10.5194/cp-17-269-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. 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 Steinthorsdottir1,3, Agatha M. de Boer1, Michiel Baatsen4, Anna von der Heydt4,5, 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´ nska´ 13, Paul A. Wilson14, and Zhongshi Zhang15,16 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 Correspondence: David K. Hutchinson ([email protected]) Received: 3 May 2020 – Discussion started: 18 May 2020 Revised: 17 November 2020 – Accepted: 18 November 2020 – Published: 28 January 2021 Abstract. The Eocene–Oligocene transition (EOT) was a thesise proxy evidence of palaeogeography, temperature, ice climate shift from a largely ice-free greenhouse world to sheets, ocean circulation and CO2 change from the marine an icehouse climate, involving the first major glaciation of and terrestrial realms. Furthermore, we quantitatively com- Antarctica and global cooling occurring ∼ 34 million years pare proxy records of change to an ensemble of climate ago (Ma) and lasting ∼ 790 kyr. The change is marked by a model simulations of temperature change across the EOT. global shift in deep-sea δ18O representing a combination of The simulations compare three forcing mechanisms across deep-ocean cooling and growth in land ice volume. At the the EOT: CO2 decrease, palaeogeographic changes and ice same time, multiple independent proxies for ocean tempera- sheet growth. Our model ensemble results demonstrate the ture indicate sea surface cooling, and major changes in global need for a global cooling mechanism beyond the imposition fauna and flora record a shift toward more cold-climate- of an ice sheet or palaeogeographic changes. We find that adapted species. The two principal suggested explanations of CO2 forcing involving a large decrease in CO2 of ca. 40 % this transition are a decline in atmospheric CO2 and changes (∼ 325 ppm drop) provides the best fit to the available proxy to ocean gateways, while orbital forcing likely influenced the evidence, with ice sheet and palaeogeographic changes play- precise timing of the glaciation. Here we review and syn- ing a secondary role. While this large decrease is consistent Published by Copernicus Publications on behalf of the European Geosciences Union. 270 D. K. Hutchinson et al.: The Eocene–Oligocene transition with some CO2 proxy records (the extreme endmember of proaches and highlight areas for future work. We then com- decrease), the positive feedback mechanisms on ice growth bine and synthesise the observational and modelling aspects are so strong that a modest CO2 decrease beyond a critical of the literature in a model–data intercomparison of the avail- threshold for ice sheet initiation is well capable of triggering able models of the EOT. This approach allows us to assess rapid ice sheet growth. Thus, the amplitude of CO2 decrease the relative effectiveness of the three modelled mechanisms signalled by our data–model comparison should be consid- in explaining the EOT observations. ered an upper estimate and perhaps artificially large, not least The paper is structured as follows: Sect. 1.2 defines the because the current generation of climate models do not in- chronology of events around the EOT and clarifies the ter- clude dynamic ice sheets and in some cases may be under- minology of associated events, transitions and intervals, sensitive to CO2 forcing. The model ensemble also cannot thereby setting the framework for the rest of the review. Sec- exclude the possibility that palaeogeographic changes could tion 2 reviews our understanding of palaeogeographic change have triggered a reduction in CO2. across the EOT and discusses proxy evidence for changes in ocean circulation and ice sheets. Section 3 synthesises ma- rine proxy evidence for sea surface temperatures (SSTs) and deep-ocean temperature change. Section 4 synthesises terres- 1 Introduction trial proxy evidence for continental temperature change, with 1.1 Scope of review a focus on pollen-based reconstructions. Section 5 presents estimates of CO2 forcing across the EOT, from geochemical Since the last major review of the Eocene–Oligocene transi- and stomatal-based proxies. Section 6 qualitatively reviews tion (EOT; Coxall and Pearson, 2007) the fields of palaeo- previous modelling work, and Sect. 7 provides a new quan- ceanography and palaeoclimatology have advanced consid- titative intercomparison of previous modelling studies, with erably. New proxy techniques, drilling and field archives of a focus on model–data comparisons to elucidate the relative Cenozoic (66 Ma to present) climates, have expanded global importance of different forcings across the EOT. Section 8 coverage and added increasingly detailed views of past cli- provides a brief conclusion. mate patterns, forcings and feedbacks. From a broad perspec- tive, statistical interrogation of an astronomically dated, con- 1.2 Terminology of the Eocene–Oligocene transition tinuous composite of benthic foraminifera isotope records confirms that the EOT is the most prominent climate tran- Palaeontological evidence has long established Eocene (56 to sition of the whole Cenozoic and suggests that the polar ice 34 Ma) warmth in comparison to a long-term Cenozoic cool- sheets that ensued seem to play a critical role in determin- ing trend (Lyell and Deshayes, 1830, p. 99–100). As modern ing the predictability of Earth’s climatological response to stratigraphic records improved, a prominent step in that cool- astronomical forcing (Westerhold et al., 2020). New proxy ing towards the end of the Eocene began to be resolved. This records capture near- and far-field signals of the onset of became evident in early oxygen isotope records (δ18O) de- Antarctic glaciation. Meanwhile, efforts to simulate the onset rived from deep-sea benthic foraminifera, which show an iso- of the Cenozoic “icehouse”, using the latest and most sophis- tope shift towards higher δ18O values (Kennett and Shackle- ticated climate models, have also progressed. Here we review ton, 1976; Shackleton and Kennett, 1975), which was subse- both observations and the results of modelling experiments quently attributed to a combination of continental ice growth of the EOT. From the marine realm, we review records of sea and cooling (Lear et al., 2008). In the 1980s the search was surface temperature, as well as deep-sea time series of the on for a suitable global stratotype section and point (GSSP) temperature and land ice proxy δ18O and carbon cycle proxy to define the Eocene–Oligocene boundary (EOB). Much of δ13C. From the terrestrial realm we cover plant records and the evidence was brought together in an important synthe- biogeochemical proxies of temperature, CO2 and vegetation sis edited by Pomerol and Premoli Silva (1986). The GSSP change. We summarise the main evidence of temperature, was eventually fixed at the Massignano outcrop section in glaciation and carbon cycle perturbations and constraints on the Marche region of Italy in 1992 (Premoli Silva and Jenk- the terrestrial ice extent during the EOT, and review indica- ins, 1993) at the 19.0 m mark which corresponds to the ex- tors of ocean circulation change and deep-water formation, tinction of the planktonic foraminifer family Hantkeninidae including how these changes reconcile with palaeogeogra- (Coccione, 1988; Nocchi et al., 1986). By the conventions of phy, in particular, ocean gateway effects. stratigraphy, Massignano is the only place where the EOB is Finally, we synthesise existing model experiments that defined unambiguously; everywhere else the EOB must be test three major proposed mechanisms driving the EOT: correlated to it, whether by biostratigraphy, magnetostratig- (i) palaeogeography changes, (ii) greenhouse forcing and raphy, isotope stratigraphy or other methods. (iii)