LINKING CLIMATE AND FAUNAL TURNOVER IN THE EARLY EOCENE: MAGNETOSTRATIGRAPHY OF THE WASATCHIAN/BRIDGERIAN NALMA BOUNDARY IN THE WIND RIVER BASIN, WYOMING, USA

The Wasatchian/Bridgerian (Wa/Br) North American Land Mammal Age (NALMA) boundary represents a significant early Eocene faunal turnover that may be linked to global climate change during the Early Eocene Climatic Optimum (EECO). The EECO (~52-50 Ma) is the peak of the greatest warming trend in the entire Cenozoic. Interior West mean annual temperature rose to 23°C while global deep ocean temperature warmed to 15°C (14.5°C and 12°C greater than 20th century mean estimates, respectively). Mean annual precipitation, up to 140 cm/y, contributed to escalated floral diversity and habitat complexity. These subtropical conditions made possible the expansion into high latitudes by tropical foraminifera, broad-leaved evergreens, crocodiles, and arboreal mammals. Mammalian diversification intensified within Artiodactyla, Perissodactyla, Rodentia, and Primates, despite the absence of major immigration, while the once successful order, Pantodonta, went extinct.

The timing of the Wa/Br NALMA boundary relative to the EECO is uncertain. Previous magnetostratigraphic work in the Green River Basin of Wyoming placed the boundary in either Chron C22r or C23r of the Geomagnetic Polarity Timescale, leaving an uncertainty of nearly 2 Myr. This precludes precise correlation with climate proxy records. Consequently, related hypotheses about the role extreme climate change had on mammalian evolution become difficult to test. In order to better constrain the age of the Wa/Br boundary, we developed a magnetostratigraphic framework within the adjacent Wind River Basin. The stratotype section for the Wa/Br boundary is located in the northeastern part of this Laramide basin within the primarily terrestrial mudstone and siltstone floodplain deposits of the Wind River Formation. We collected 220 in-situ, oriented rock samples from 50 sites across the boundary covering 425 m of overlapping but discrete stratigraphic sections. Samples were subjected to thermal or alternating field stepped demagnetization and measured with a cryogenic magnetometer. Our results confirm the timing of the Wa/Br NALMA boundary within Chron C23r and, by correlation, its occurrence amidst rapidly increasing global temperature during, but just before the crest of, the EECO.