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The Neogene transition from C^sub 3^ to C^sub 4^ grasslands in North America: stable carbon isotope ratios of fossil phytoliths


C^sub 4^ grasses form the foundation of warm-climate grasslands and savannas and provide important food crops such as corn, but their Neogene rise to dominance is still not fully understood. Carbon isotope ratios of tooth enamel, soil carbonate, carbonate cements, and plant lipids indicate a late Miocene-Pliocene (8-2 Ma) transition from C^sub 3^ vegetation to dominantly C^sub 4^ grasses at many sites around the world. However, these isotopic proxies cannot identify whether the C^sub 4^ grasses replaced woody vegetation (trees and shrubs) or C^sub 3^ grasses. Here we propose a method for reconstructing the carbon isotope ratio of Neogene grasses using the carbon isotope ratio of organic matter trapped in plant silica bodies (phytoliths). Although a wide range of plants produce phytoliths, we hypothesize that in grass-dominated ecosystems the majority of phytoliths will be derived from grasses, and will yield a grass carbon isotope signature. Phytolith extracts can be contaminated by non-phytolith silica (e.g., volcanic ash). To test the feasibility of the method given these potential problems, we examined sample purity (phytolith versus non-phytolith silica), abundance of grass versus non-grass phytoliths, and carbon isotope ratios of phytolith extracts from late Miocene-Pliocene paleosols of the central Great Plains. Isotope results from the purest samples are compared with phytolith assemblage analysis of these same extracts. The dual record spans the interval of focus (ca. 12-2 Ma), allowing us, for the first time, to investigate how isotopic shifts correlate with floral change. We found that many samples contained high abundances of non-biogenic silica; therefore, only a small subset of "pure" samples (50% of phytoliths by volume) with good preservation were considered to provide reliable carbon isotope ratios. All phytolith assemblages contained high proportions (on average 85%) of grass phytoliths, supporting our hypothesis for grass-dominated communities. Th

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