PALEOENVIRONMENTAL RECONSTRUCTION OF THE BARSTOW FORMATION, SOUTHERN CALIFORNIA, THROUGH THE MIDDLE MIOCENE CLIMATIC OPTIMUM

The Barstow Formation of southern California (middle Miocene) is widely known for its mammalian faunas, which are the basis for the Barstovian North American Land Mammal Age. The Barstow Formation encompasses the middle Miocene Climatic Optimum (MMCO), a warming period that occurred 17-14 Mya. Although faunal diversity and turnover patterns during this period are well studied, there is still uncertainty around the links between faunal change and environmental variation in response to middle Miocene warming. Thus, the Barstow Formation provides an opportunity to study how environments changed during this warming interval. We used compound-specific biomarker isotopes and phytolith (amorphous plant silica) abundance data to examine how vegetation and environments changed during the MMCO. We collected 142 sediment samples for biomarker and phytolith analyses at roughly 20-m intervals from 16 stratigraphic sections measured through the entire thickness of the Barstow Formation. Carbon and hydrogen isotopes of normal alkanes show two intervals of enrichment occurring prior to and during the peak of MMCO warming. These data suggest a period of drying during the MMCO. Phytolith yields were low overall for the Barstow Formation, but samples preserving phytolith assemblages reveal that mixed habitats featuring abundant grasses and palms dominated the landscape toward the end of the MMCO. The combination of biomarker and phytolith analyses provides new constraints on the change in continental ecosystems through the MMCO and adds environmental context for the evolution of the Barstow fauna during this time.