LATE EOCENE TRENDS IN CLIMATE AND ECOSYSTEM STRUCTURE IN SOUTHWESTERN MONTANA BASED ON CARBON AND OXYGEN ISOTOPE RATIOS IN TOOTH ENAMEL

To investigate late Eocene trends in climate and ecosystem structure in the mid-continent, we analyzed the carbon and oxygen isotope composition of the carbonate component of 57 tooth enamel fragments from various fossil localities in southwestern Montana. Samples belong to generally unknown ungulate taxa and cover a time interval from 38 to 33.8 Ma.

Tooth enamel is thought to be resistant to diagenetic alteration and to maintain its original biological isotope signal. Therefore, in pure C₃ ecosystems, enamel δ¹³C reflects habitat preference (i.e., degree of vegetation openness) and/or plant water-stress. Enamel δ¹⁸O mainly depends on rainwater composition with a minor humidity effect. In mid-latitudes, rainwater δ¹⁸O is directly correlated with mean annual temperature. Therefore, enamel δ¹³C can be used to investigate changes in aridity and ecosystem structure, and enamel δ¹⁸O to reconstruct changes in mean annual temperature.

Enamel δ¹³C values are consistent with expected pure C3 diets. Average (± 1SE) δ¹³C values (vs. V-PDB) in the late Duchesnean (-8.04±0.19‰), medial Chadronian (-8.74±0.19‰), and late Chadronian (-8.22±0.16‰) NALMA’s are similar. In contrast, the early Chadronian NALMA shows a significantly lower average enamel δ¹³C (-10.22±0.34‰). These data might indicate higher mean annual precipitation (MAP) supporting a more closed environment between 37 and 35.7 Ma. With respect to δ¹⁸O, average (± 1SE) values (vs. V-SMOW) are 19.85±1.09‰ (late Duchesnean), 22.04±0.34‰ (early Chadronian), 20.52±0.47‰ (medial Chadronian), and 17.83±0.13‰ (late Chadronian). Low δ¹³C-high δ¹⁸O values in the early Chadronian suggest warmer and wetter conditions, with progressive cooling and drying, possibly coupled with shifts to atmospheric circulation, towards the Eocene-Oligocene climate boundary at 33.8 Ma.