MINIMAL CHANGE IN VEGETATION STRUCTURE ACROSS THE EOCENE-OLIGOCENE BOUNDARY IN NEBRASKA AS INDICATED BY STABLE ISOTOPES IN MAMMALS

Global climate change occurred across the Eocene-Oligocene (E-O) boundary at the onset of Antarctic glaciation. This change is manifested in the North American mid-continent by paleosols that suggest a change from humid, mosaic open forest in the latest Eocene (Chadronian North American land-mammal age, NALMA) to semi-arid, wooded scrubland in the earliest Oligocene (Orellan NALMA). In this study we use stable carbon values from the carbonate component of ungulate tooth enamel (δ¹³C_E) to infer the types of habitats inhabited by mammals in northwestern Nebraska across the E-O boundary as a corollary to the changes seen in paleosols. δ¹³C_E values in mammalian herbivores are a direct reflection of the vegetation they consume with predictable enrichment, and thus can be used to estimate the δ¹³C values of ancient vegetation. This requires a correction for change in the δ¹³C value of atmospheric CO₂ from E-O time until present, and an estimate of diet-enamel enrichment based on modern mammals. We use δ¹³C values in modern vegetation from a variety of biomes as a basis for habitat interpretation across the E-O boundary.

Sampled specimens were collected from the Chadron and Brule formations in northwestern Nebraska. 12 taxa were examined in this study, six of which (Merycoidodon, Agriochoerus, Archaeotherium, Mesohippus, Subhyracodon, and Hyracodon) occur in both the Chadronian and Orellan NALMAs. These taxa have δ¹³C_E values (~-9‰, VPDB) that plot in the range predicted for xeric wooded scrubland. Mean values do not change from the Chadronian to the Orellan, suggesting that a wooded scrubland was well established in the Chadronian landscape and persisted into the Orellan. The Chadronian brontothere Megacerops has δ¹³C_E values (-10.9 ±0.70‰, VPDB), suggesting a slightly more closed, or more mesic microhabitat, plotting in the range predicted for savanna woodlands and dry forests. The extinction of brontotheres at the end of the Chadronian along with the absence of tree-sized root traces in the Orellan suggests that woodland microhabitats were either lost or became more restricted during the earliest Oligocene. However, overall, our isotopic evidence suggests that early Oligocene climate change had only a minor impact on the vegetation structure in Nebraska.