A QUANTITATIVE FRAMEWORK FOR RECONSTRUCTING C4 PLANT FRACTIONS IN ANCIENT BIOMES AND MAMMAL DIETS USING STABLE CARBON ISOTOPES

Stable carbon isotopes are widely used for reconstructing past terrestrial vegetation. Most modern plants utilize the C₃ photosynthetic pathway, which strongly selects light carbon (¹²C) over heavy carbon (¹³C) isotopes, resulting in very low stable isotope ratios (δ¹³C values) of plant tissue. An alternative mode of photosynthesis—the C₄ pathway—exists primarily in grasses and sedges of arid to subhumid grasslands and savannas. These plants are particularly tolerant of low atmospheric CO₂, aridity, and recurrent grazing and fire. The C₄ pathway results in overall lower discrimination against ¹³C and thus higher δ¹³C values in plant tissue compared to C₃ plants. For decades, researchers have utilized the clear separation of δ¹³C values between C₃ and C₄ plants to track the emergence and proliferation of C₄ grasses on nearly all continents throughout the latter portion of the Neogene Period, though most approaches are qualitative and suffer from a paucity of data sets from the early Neogene. This talk will present a new, robust data compilation of modern plants, which includes δ¹³C values, local climate and geographic data, and plant taxonomy. A Monte Carlo mixing model is used to quantify fractions of C₄ biomass for measured δ¹³C values in fossilized mammalian tooth enamel and pedogenic archives (organic matter and carbonates) based on modern reference values in the data compilation. Several surprising results emerge from these analyses, including Early and Middle Miocene occurrences of C₄ plants in North America and eastern Africa, and starkly different threshold values for non-zero C₄ fractions in δ¹³C observations compared to earlier approaches. This new, data-driven framework rectifies discrepancies between molecular clock predictions of C₄ origins and proxy data, and the results offer strong evidence that local ecological feedbacks (e.g., climate and/or disturbance) exerted dominant control on early C₄ plant emergence and proliferation compared to global drivers such as atmospheric CO₂.