Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO2, pco2)1. Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (~12–5 Myr ago), an interval with pco2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere2, 3 and warmer-than-modern temperatures on the continents4.