EOCENE-OLIGOCENE LATITUDINAL CLIMATE GRADIENTS IN NORTH AMERICA INFERRED FROM STABLE ISOTOPE RATIOS IN PERISSODACTYL TOOTH ENAMEL

The Eocene-Oligocene transition (~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 (horses and rhinos) 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 (±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 (MAP) in Saskatchewan across the transition. Average δ¹³C values in Saskatchewan 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 rainwater δ¹⁸O 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 rainwater δ¹⁸O in North America, which suggests no change in mean annual temperature gradients across the transition. This result supports the hypothesis that explains the climate change of the transition with a drop in atmospheric pCO₂ because climate models show that this mechanism produces uniform cooling at mid-latitudes.