RECONSTRUCTION OF THE DIETS AND HABITATS OF LATE MIOCENE HERBIVORES FROM THE DOVE SPRING FORMATION, MOJAVE DESERT, SOUTHERN CALIFORNIA

In the late Miocene, there was a worldwide, rapid increase in C4 ecosystems (RICE). Recent studies from the Great Plains and the Barstow Formation suggest that there may have been a significant amount of C4 grasslands prior to the RICE event. This study examines stable carbon isotopic ratios of herbivore tooth enamel from the Dove Spring Formation (DSF) of southern California, which is geologically younger than the Barstow Formation. Taxa sampled in this study include the Camelidae, Antilocapridae, Gomphotheriidae, Equidae, Merycoidodontidae, Castoridae, and Rhinocerotidae families. The enamel δ¹³C values of all taxa sampled are -10.1±1.1‰ (n=89), ranging from -13.3 to -6.7‰. Past studies have used -8‰ as the cutoff for a pure C3 diet, which would be indicative of a percentage of C4 grasses in diets of some of the DSF herbivores and support the idea of an early regional spread of C4 grasslands. All specimens whose carbon isotopic signature was higher than -8‰ were from the Equidae and Antilocapridae families, similar to the results of the study of the Barstow Formation ungulates. However, a cutoff value of -8‰ for a pure C3 diet is reliant on the modern atmospheric δ¹³C value. Proxy data indicate that the δ¹³C value of atmospheric CO₂ has varied significantly in the geologic past. The estimated Miocene δ¹³C value of atmospheric CO₂ was ~2-3‰ higher than the modern value, making the pure C3 diet cutoff value higher. With a higher end member δ¹³C value for a pure C3 diet, much of the reported evidence of an early spread of C4 grasses in areas such as the Great Plains and the Barstow Formation may be a false signal from water-stressed C3 plants and the amount of C4 biomass in the geologic past may have been greatly over-estimated. After accounting for the changes in the δ¹³C of atmospheric CO₂, tooth enamel δ¹³C values from the DSF indicate that late Miocene herbivores in the area had primarily C3 diets and lived in an environment dominated by C3 plants.