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

The Eocene-Oligocene transition (similar to 34 Ma) was one of the most pronounced episodes of climate change of the Cenozoic. In order to investigate this episode of global climate cooling in North America, we analyzed the carbon and oxygen stable isotope composition of the carbonate component of 19 perissodactyl (horse and rhino) tooth enamel samples from the Eocene-Oligocene rocks of the Cypress Hills Formation (southwestern Saskatchewan, Canada); we then compared the results with previously published data from the US Great Plains (Nebraska, South Dakota, and Wyoming). Average (+/- 1 sigma) perissodactyl enamel delta C-13 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 delta C-13 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 (+/- 1 sigma) perissodactyl enamel delta O-18 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 delta O-18 of meteoric precipitation across the transition in Saskatchewan. Enamel delta O-18 variability is much larger in the Oligocene vs. Eocene, indicating a large increase in temperature seasonality. This increase in enamel delta O-18 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 delta O-18 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 pCO(2) because climate models show that this mechanism produces uniform cooling at mid-latitudes. Published by Elsevier B.V.