WESTERN NORTH AMERICAN GRASSLAND EXPANSION AND SEASONAL DRYING IN THE CENOZOIC: EVIDENCE FROM AUTHIGENIC CLAY AND CARBONATE OXYGEN ISOTOPES

Ecosystem structure and water availability in western North America (WNA) is strongly influenced by spatial patterns of aridity and precipitation seasonality, with precipitation seasonality transitioning from winter-dominated in the west to summer-dominated in the east. The vegetation history of WNA has been well-studied, with paleobotanical data demonstrating a spread of C3 grasslands approximately 26-22 Ma. However, testing possible links to precipitation and its seasonality is notoriously difficult because there are few direct constraints on sub-annual biases in the formation of paleoclimate proxy materials. To address this gap, we use a compilation spanning 38 to 50 ºN and 125 to 100 ºW of existing (n=2,708) and new (n=117) authigenic clay and carbonate oxygen isotope measurements from 50 to 2 Ma, taking advantage of the different formation timing of these authigenic minerals. We identify a positive shift (~3‰) in the oxygen isotope composition (δ¹⁸O) of authigenic clay minerals, interpreted to reflect integrated annual precipitation, that occurs at ~23 Ma, coincident with the expansion of open-habitat grasslands. In contrast, this positive shift in clay δ¹⁸O is not expressed in soil carbonates, which are interpreted to reflect summer precipitation. These distinct trends in clay and carbonate δ¹⁸O values point to a change in the seasonality of precipitation that led to drier winters permitting grasslands to expand at the expense of forests by the earliest Miocene.