TERRESTRIAL CLIMATE CHANGE ACROSS THE EOCENE-OLIGOCENE TRANSITION BASED ON STABLE ISOTOPE RATIOS IN TEETH FROM THE CYPRESS HILLS (SASKATCHEWAN, CANADA)

The transition from the Eocene to the Oligocene epochs, at ~ 34 Ma, is characterized by a change in climate from a greenhouse Earth to an icehouse Earth, which still persists today. Until recently, most of the studies on this transition have focused on marine climate changes, with significantly less research done on the terrestrial record. In this study, we analyzed the carbon and oxygen isotope composition of the carbonate component of 94 enamel samples, which were subsampled from 17 molars and premolars of fossil horses and rhinos. The samples were collected from 7 sites in the Cypress Hills Formation (Saskatchewan, Canada) which spans almost 30 Myr, from ~44 Ma (Eocene, Uinta NALMA) to ~16 Ma (Miocene, Hemingfordian NALMA).

The δ¹³C of mammalian teeth reflects the composition of the diet and, in a pure C3 ecosystem, can serve as a proxy for aridity and vegetation openness. In addition, the δ¹⁸O reflects rainwater composition with a minor humidity effect; the average δ¹⁸O value may serve as a proxy for changes in mean annual temperature (MAT) whereas the δ¹⁸O variability may serve as a proxy for temperature seasonality (i.e., MART, mean annual range of temperature).

Enamel δ¹³C values are consistent with expected pure C3 diets. Average (±1 SD) enamel δ¹³C (vs. PDB) in the Eocene (-8.7±0.3‰; n=40) and Oligocene (-8.9±0.7‰; n=54) are indistinguishable, suggesting no major change in aridity and vegetation openness across the transition. With respect to δ¹⁸O, average values (vs. SMOW) in the Eocene (20.3‰; n=40) and Oligocene (20.5‰; n=54) are also indistinguishable, suggesting no change in rainwater composition across the transition. Because seawater δ¹⁸O increased by ~1‰ from the Eocene to the Oligocene due to the ice volume effect, these data may indicate either a small decrease in MAT across the transition or a larger decrease in MAT along with a shift in atmospheric circulation. In addition, the δ¹⁸O standard deviation significantly increases from the Eocene (2‰; n=40) to the Oligocene (3.5‰; n=54), probably suggesting higher Oligocene vs. Eocene MART. These results are consistent with those of similar studies previously conducted in Nebraska, South Dakota, and Wyoming (Zanazzi et al., 2007; Zanazzi and Kohn, 2008; Zanazzi et al., 2009).