1. School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK
2. Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
3. Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA

Correspondence to: Paul N. Pearson¹ Correspondence and requests for materials should be addressed to P.N.P. (Email: pearsonp@cardiff.ac.uk).

Abstract

Geological and geochemical evidence^{1, 2, 3} indicates that the Antarctic ice sheet formed during the Eocene–Oligocene transition⁴, 33.5–34.0 million years ago. Modelling studies^{5, 6} suggest that such ice-sheet formation might have been triggered when atmospheric carbon dioxide levels () fell below a critical threshold of 750 p.p.m.v., but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear. Here we use the boron isotope pH proxy^{7, 8} on exceptionally well-preserved carbonate microfossils from a recently discovered geological section in Tanzania^{9, 10} to estimate before, during and after the climate transition. Our data suggest that a reduction in occurred before the main phase of ice growth, followed by a sharp recovery to pre-transition values and then a more gradual decline. During maximum ice-sheet growth, was between 450 and 1,500 p.p.m.v., with a central estimate of 760 p.p.m.v. The ice cap survived the period of recovery, although possibly with some reduction in its volume, implying (as models predict¹¹) a nonlinear response to climate forcing during melting. Overall, our results confirm the central role of declining in the development of the Antarctic ice sheet (in broad agreement with carbon cycle modelling¹²) and help to constrain mechanisms and feedbacks associated with the Earth's biggest climate switch of the past 65 Myr.

1. School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK
2. Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
3. Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA

Correspondence to: Paul N. Pearson¹ Correspondence and requests for materials should be addressed to P.N.P. (Email: pearsonp@cardiff.ac.uk).

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