Fig. 5: Late and mid-Miocene latitudinal thermal gradient shown by coccolith clumped isotopes, alkenones and model simulations.

While alkenone temperatures calculated using widely-used calibrations suggest a small (late Miocene: from 11.6 to 5.33 Ma) or negligible (mid-Miocene: from last sample at 16–11.6 Ma) latitudinal thermal gradient (^1,8,56, and this study), coccolith clumped isotopes (as winter-spring at depth of production-dark blue star- and approximated surficial mean annual temperatures -pale blue star- temperatures) and Miocene model simulations^4,6,48 suggest a more modest polar amplification and a smaller flattening of the latitudinal thermal gradient. Modeled mean annual average 0–70 m water column temperatures from Lee et al.⁶ shown as a horizontal pink line below mean annual sea surface temperatures for ODP site 982, from which it differs by 0.9 °C. 95% confidence intervals for clumped isotope measurements are between 4 and 4.5 °C and for alkenone temperatures (BAYSPLINE calibration)²¹ are between 2.8 and 5.9 °C. Refer to Supplementary Fig. 9 to see the representation of this figure, including errors (95% CI) for the data produced in this study.