THE NEOGENE-QUATERNARY HISTORY OF C4 GRASSLANDS AND SAVANNAS IN NORTH AMERICA: INSIGHTS FROM STABLE ISOTOPE PROXIES

Stable carbon isotopes are widely used for reconstructing floral communities throughout the Cenozoic Era. Most modern plants utilize the C₃ photosynthetic pathway, which strongly selects light carbon (¹²C) over heavy carbon (¹³C) isotopes, resulting in low stable isotope ratios (δ¹³C values) of plant tissue. In contrast, many warm-season grasses and sedges use the C₄ photosynthetic pathway, which results in overall less discrimination against ¹³C and relatively higher δ¹³C values. A variety of physiological adaptations in C₄ grasses allows them to better tolerate low atmospheric CO₂, aridity, and recurrent grazing and fire compared to most other plants. Stable isotope proxies can readily distinguish signals of C₃ versus C₄ plants in the geologic record, which has provided insight into changing environments and climates across the Neogene and Quaternary Periods. For decades, researchers have applied generally qualitative methods to study the emergence and proliferation of C₄ grasses. This talk will apply a new, data-driven approach for quantifying fractions of C₄ biomass in paleosols and mammalian tooth enamel to re-examine the history of open-canopy biomes in North America. Our methodology leverages a new, global data compilation of modern plants, including their δ¹³C values, local climate and geographic data, and plant taxonomy. Several surprising results emerge from these analyses, including Early and Middle Miocene occurrences of C₄ plants in North America that rectify discrepancies between prior analyses of paleosols and fossil teeth. The results suggest that C₄ plants have been present on landscapes in low abundance for roughly 20 million years, several million years prior to their widespread proliferation in the Late Miocene.