4-9 Reassessing the Neogene History of Grasses in the Great Plains, U.S.A. Using Compilations of Carbon Isotope Data from Modern Plants, Paleosols, and Fossil Mammals

Session: Recent Advances in Soil and Paleosol Science

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Multiple lines of evidence indicate the grassland ecosystems in the Great Plains of North America evolved in two stages. Plant microfossils and fossil mammalian herbivore teeth indicate open, grass-dominated habitats appeared 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 support this model, with the first C₄ consumers evident at 6.6 Ma at two localities in the southern Great Plains (Coffee Ranch and Optima). However, pedogenic carbonate δ¹³C values are interpreted as indicating 10-30% C₄ grass biomass since at least 23 Ma, broadly coincident with evidence for open habitats. Different interpretations of the carbon isotope records hinge on assumed endmember δ¹³C values for C₃ and C₄ plants in the past and how measured δ¹³C values are interpreted as fraction C₄ biomass (fC₄). We present new interpretations of the Neogene and Quaternary carbon isotope records in the Great Plains with a focus on Coffee Ranch and Optima. We use a global compilation of >15,000 published modern plant δ¹³C values and a Monte Carlo method for estimating fC₄ from measured δ¹³C values that applies linear mixing and accounts for natural variation in plant endmembers and uncertainties in ages of localities, δ¹³C of atmospheric CO₂ in the past, and isotopic enrichments associated with pedogenic carbonate and bioapatite. For a measured δ¹³C value, the MC method generates 10,000 fC₄ values, allowing statistical assessment of the C₄ component for a measured δ¹³C value. Our reanalysis provides statistical support for the original interpretation of the Miocene carbonate record, brings the mammalian isotopic record into agreement with the paleosol record, and indicates some mammals in the region were consuming C₄ grasses by ca. 17 Ma. Estimated fC₄ values of the four horse species at Coffee Ranch and Optima are statistically indistinguishable and indicate 40-50% C₄ in their diets. However, these fC₄ values are statistically higher than those for paleosol carbonates, other large-bodied mammal herbivores, and rodents, which indicate ca. 30% C₄ biomass each and are statistically indistinct from each other. Late Miocene changes in horse diets were not indicative of local or regional increases in C₄ grass abundance.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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