Teaching a new trick to old data: reassessing the Neogene history of C4 grasses in the Great Plains of North America

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• David Fox, University of Minnesota
• Tuesday 31 October 2023, 12:00-13:00
• Department of Earth Sciences, Tilley Lecture Theatre.

If you have a question about this talk, please contact Dr Rachael Rhodes.

Modern grasslands are responsible for a disproportionate fraction of terrestrial primary productivity relative to their areal extent, and the origin of these ecosystems and their associated mammalian faunas over the Cenozoic has been the focus of extensive research. Multiple lines of evidence indicate the origin of the modern grassland ecosystems in the Great Plains of North America proceeded in two stages. Plant microfossils (phytoliths) and fossil mammalian herbivore teeth indicate that open, grass-dominated habitats first appear in the Early Miocene and included grasses using the C3 photosynthetic pathway, with C4 grasses emerging later in the Miocene to Pliocene. Stable carbon isotope ratios (δ13C values) from tooth enamel of large-bodied mammalian herbivores largely support this model, with the first C4 consumers first evident by 6.6 Ma in Texas and then at ca. 6.25 Ma in Nebraska. However, pedogenic carbonate δ13C values are consistent with C4 grasses being present since at least 23 Ma, broadly coincident with the first phytolith evidence for open habitats. Key to the different interpretations of the carbon isotope records are the assumed endmember δ13C values for C3 and C4 plants in the past and whether and how measured δ13C values are interpreted as fraction C4 biomass (fC4). Here we reinterpret published carbon isotope records from the Neogene and Quaternary of Nebraska using a global compilation of more than 5,000 modern plant δ13C values and a Monte Carlo method for estimating fC4 from measured δ13C values. This approach applies linear mixing that accounts for natural variation in plant endmembers and uncertainties in the δ13C of atmospheric CO2 in the past and the fractionations associated with pedogenic carbonate and bioapatite. For a measured δ13C value, the MC method generates a distribution of 10,000 fC4 values. Estimation of fC4 from δ13C values of modern mammals from tropical forests and Paleogene fossil mammals for which C4 grasses were not available indicate that median fC4 values are good predictors of C4 in diet, but the 5th percentile of the distributions allows for statistical assessment of the C4 component. We focus on the record from Nebraska for its long duration and to minimize the role of latitudinal diachroneity in increased abundance of C4 grasses. Our reanalysis brings the paleosol and mammalian isotopic records into agreement, provides statistical support for the original interpretation of the Miocene carbonate record from the Great Plains, and indicates that mammals in the region were consuming C4 grasses during the Early Miocene, much earlier than heretofore recognized and possibly before anywhere else on Earth.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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• Department of Earth Sciences Seminars (downtown)
• Department of Earth Sciences seminars
• Department of Earth Sciences, Tilley Lecture Theatre
• ps635

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