Abrupt Late Eocene Climate Change in the North American Mid-Continent

We analyzed the stable isotope composition of 220 fossil bones and 18 teeth collected at Flagstaff Rim, central Wyoming, and discovered an abrupt shift in climate at ~35.4 Ma (timescale of Gradstein et al., 2004). The eight teeth older than 35.4 Ma belong to Subhyracodon (an ancestral rhino), whereas the ten teeth younger than 35.3 Ma belong to Leptomeryx (a small, deer-like artiodactyl).

In pure C3 ecosystems, enamel d¹³C reflects habitat preference (i.e., degree of vegetation openness) and/or plant water-stress. Similarly, bone d¹³C tracks trends in vegetation and aridity. Enamel d¹⁸O mainly depends on rainwater composition with a minor humidity effect. Both soil water composition and soil temperature determine instead bone CO₃ d¹⁸O. Hence, combined enamel and bone CO₃ d¹⁸O values may be used to distinguish changes in water composition from changes in temperature.

Bone d¹³C shows no change between 35.7 and 35.0 Ma, with an average (±1 SE) value of -6.97±0.05‰ (V-PDB). In contrast, average enamel d¹³C increases from -9.08±0.06‰ to -8.54±0.10‰ at ~35.4 Ma; this could reflect an increase in aridity or simply differences in dietary and habitat preferences between Subhyracodon and Leptomeryx. Values for CO₃ d¹⁸O abruptly increase by 3.9‰ for enamel (from 21.04±0.12‰ to 24.95±0.41‰, V-SMOW) and 1.7‰ for bone (from 20.47±0.10‰ to 22.19±0.11‰), at ~35.4 Ma, coincident with a lithologic change from claystones and conglomeratic sandstones to tuffaceous siltstones. These data possibly indicate an increase in aridity and/or mean annual temperature, and might reflect the climatic effect of the late Eocene extraterrestrial impacts or the opening of the Drake Passage and consequent strengthening of the Antarctic Circumpolar Current.