Earth and Atmospheric Sciences, Department of


First Advisor

Ross Secord

Date of this Version



Nguy, W.H. 2017. Middle Miocene Paleoenvironmental Reconstruction of the Central Great Plains from Stable Carbon Isotopes in Large Mammals. M.S. Thesis, Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Nebraska, USA.


A THESIS Presented to the Faculty of the Graduate College at the University of Nebraska in Partial Fulfillment of Requirements for the Degree of Master of Science, Major: Earth and Atmospheric Sciences, Under the Supervision of Professor Ross Secord. Lincoln, Nebraska, 2017.

Copyright © 2017 Willow H. Nguy


Middle Miocene (18-12 Mya) mammalian faunas of the North American Great Plains contained a much higher diversity of apparent browsers than any modern biome. This has been attributed to greater primary productivity, which may have supported greater browser diversity that commonly corresponds with densely vegetated habitats. However, several lines of proxy evidence suggest that open woodlands or savannas dominated middle Miocene biomes; neither of which support many browsers today. Stable carbon isotopes in mammalian herbivore tooth enamel were used to reconstruct vegetation structure of middle Miocene biomes.

Stable carbon isotopes in C3 dominated environments reflect vegetation density and herbivores in those environments record dietary values of vegetation in their tissues with predictable offsets. Tooth enamel was sampled from presumed browsers, mixed-feeders, and grazers, based on hypsodonty and microwear studies, from four late Barstovian (14.8-12.5 Mya) localities in Nebraska. Paleoenvironmental interpretations were made using a predictive model based on δ13C values in C3 vegetation in modern biomes. The model adjusts for differences in atmospheric δ13C between the Barstovian and present, diet-to-enamel enrichment, and latitudinal and altitudinal differences in δ13C plant values.

Mean δ13C faunal values plot in the upper range of values expected for C3 vegetation, suggesting open habitats. Means for several taxa plot in the range for water-

stressed C3 environments; a range that overlaps partially with the range for C4 vegetation. One individual has a high enough value to unequivocally indicate C4 consumption (δ13C=-6.0). The taxa that plot in the water-stressed range could potentially indicate up to 5% total C4 consumption, but this is lower than estimates made from paleosol carbonates that suggest 20% mean C4 biomass at this time. The narrow range in carbon isotope values suggests that browsers, mixed feeders, and grazers all consumed vegetation in mostly open areas. These results are consistent with other proxy data suggestive of savanna-woodland biomes during the middle Miocene, despite high browser diversity, which does not appear to have a modern analogue.

Advisor: Ross Secord