THE ROLE OF TREES IN THE EVOLUTION OF C4 GRASSLAND ECOSYSTEMS IN NORTH AMERICA, A MOLECULAR PERSPECTIVE

The evolution of grasslands was one of the most profound ecological changes in the Cenozoic. Understanding the history of forest to grassland transitions and the development of C₄ grasslands in particular is critical for understanding the evolution of many large mammals and the drivers of late Cenozoic climate change. During the expansion of C₄ grasses in the late Miocene and Pliocene in the North American Great Plains, significant variability in carbonate nodule δ¹³C values indicate heterogeneity in plant community composition (C₃ versus C₄) even as C₄ grasses were increasing in abundance. The character of the C₃ portion of these landscapes is not well described in terms of whether it represents woody or herbaceous vegetation. Longterm increases in C₄ are confounded on shorter time scales by variability in δ¹³C data, making the rate of increase in C₄ vegetation unclear. To resolve whether C₄ grasslands replaced C₃ grasses or C₃ trees, we extracted terrestrial leaf waxes included in carbonate nodules to analyze the δ¹³C values and the abundance of C₂₅ to C₃₅ n-alkanes as indicators of open grassland, savanna, and closed forest for the past 12 Ma in the Mead Basin, Kansas. A high resolution portion of the record shows landscape scale variability in the abundance of trees relative to grasses, which is also reflected in the δ¹³C values of the n-alkanes. Our record suggests that δ¹³C variability in archives of C₄ grassland formation do not necessarily represent a transition from C₃ grasses to C₄ grasses, but that trees in an open savanna were on the landscape as C₄ grasses rose to dominance in the Great Plains.