THE ENVIRONMENTAL CONTEXT OF CHANGING MAMMAL DIVERSITY THROUGH THE MIDDLE MIOCENE CLIMATIC OPTIMUM IN THE BARSTOW FORMATION, MOJAVE DESERT, CALIFORNIA (Invited Presentation)

The Middle Miocene Climatic Optimum (MMCO; 17–14 Ma) coincided with high mammal diversity and significant tectonic and volcanic activity in western North America. The Barstow Formation in southern California is one of the few continental formations in North America to span the MMCO and subsequent cooling of the Middle Miocene Climatic Transition (MMCT; beginning ~14 Ma). The Barstow Formation preserves rich mammalian assemblages that form the basis of the Barstovian North American Mammal Age (16–12.5 Ma) and are representative of high mammal diversity in western North America during the middle Miocene.

Multiproxy environmental records are key to interpreting how ecosystems changed through time in relation to climatic and tectonic influences. We combined isotopic analyses of carbon and hydrogen from sedimentary n-alkanes and bulk soil organic matter with analyses of facies, biosilica (phytoliths and diatoms) assemblages, and microscopic charcoal to reconstruct vegetation composition, habitat structure, hydrology, and fire activity through the formation. The depositional environments in the basin changed from alluvial fans and playa lakes early in basin history to floodplains and wetlands over time. The δ¹³C and δD of n-alkanes derived from terrestrial plants indicate wet conditions in fluvial environments during the peak of the MMCO compared to earlier and later environments. Temporal and spatial fluctuations in carbon and hydrogen isotopes reflect changes in moisture, driven partly by facies and vegetation composition in the Barstow Basin, as well as regional tectonic activity and climatic variability. During the MMCO, phytolith assemblages are dominated by forest indicators in riparian habitats, and mammal richness in the formation increased over time. After 14 Ma, grass morphotypes constitute major components of phytolith assemblages, and microscopic charcoal abundance significantly increased, indicating a shift to drier, more open-canopy habitats and greater fire frequency after the MMCO. Mammal richness remained high in the late-forming environments of the Barstow Basin. The establishment of dry savannas in southern California coincides with the beginning of the MMCT, cooling sea-surface temperatures in the Pacific Ocean, and the shift to seasonal precipitation regimes.