SPECIES-ENVIRONMENT RELATIONSHIPS DURING THE MID-MIOCENE CLIMATIC OPTIMUM IN THE JOHN DAY BASIN, CENTRAL OREGON

During the mid-Miocene climatic optimum (MMCO), approximately 15-16 Ma, global temperatures rose 3-4 °C, providing a potential analog for the magnitude (but not the rate) of current climate change. The MMCO climatic warming has been documented in deep sea cores as well as terrestrial deposits, including the paleosols of the Mascall Formation in the John Day Basin of central Oregon (Bestland et al., 2008). Here the fossil occurrences in the Mascall Formation (Barstovian) are analyzed to determine if there is a concurrent change in the mammalian fauna. The base of the Mascall Formation is dated at 16. 4 Ma and the formation spans the MMCO. It contains sequences of paleosols, each representing at least ~100,000 years of soil development, and tuff deposits that provide refined age constraints. This exceptional paleontological, temporal and paleoenvironmental data provides an ideal setting for testing hypotheses about species-environment interactions in the face of pronounced climate change.

Fossil localities were relocated and placed in a detailed stratigraphic framework to study faunal change through time. To link fossil organisms to their environment, at each fossil locality, paleosol deposits were classified and samples taken for stable isotope and XRF analyses. The faunal composition of the Mascall Formation, including changes within the formation, is compared to earlier faunas of the area that were deposited in cooler and drier climates (upper John Day Formation). Comparison of the John Day faunas to coeval ones elsewhere in North America were used to determine whether the climate and faunal changes in the Mascall Formation are due to regional or local phenomena. This study provides a basis for understanding the long-term relationships between organisms and their environment in the fossil record as climate changes. Future work will build on this to examine species distributions through the Mascall Formation using ecological niche modeling to determine whether or not species tracked their preferred habitat as the climate changed. These results will be informative for studies examining the future distribution of species in the face of the current global climate change.