Eocene-Oligocene latitudinal climate gradients in North America inferred from stable isotope ratios in perissodactyl tooth enamel

Average (± 1σ) perissodactyl enamel δ13C values (vs. V-PDB) in the Eocene (− 8.8 ± 0.3‰) and Oligocene (− 9.0 ± 0.3‰) are indistinguishable, suggesting no major change in mean annual precipitation in Saskatchewan across the transition. The δ13C values in Saskatchewan indicate the presence of arid ecosystems and are slightly higher than those in the US Great Plains, suggesting drier conditions at higher latitudes. With respect to oxygen isotopes, average (± 1σ) perissodactyl enamel δ18O values (vs. V-SMOW) in the Eocene (19.8 ± 2.0‰) and Oligocene (20.1 ± 3.6‰) are also indistinguishable, suggesting no change in the δ18O of meteoric precipitation across the transition in Saskatchewan. Enamel δ18O variability is much larger in the Oligocene vs. Eocene, indicating a large increase in temperature seasonality. This increase in enamel δ18O variability is much larger than that recorded in the US Great Plains, suggesting that higher latitudes are more sensitive to major episodes of climate change with respect to temperature seasonality. Finally, our data indicate no major change in the Oligocene vs. Eocene latitudinal gradient in local water δ18O in North America, which suggests no change in mean annual temperature gradients across the transition. This result supports the hypothesis that ascribes the climate change of the transition to a drop in atmospheric pCO2 because climate models show that this mechanism produces uniform cooling at mid-latitudes.