The Great Plains is the largest continuous grassland in North America, and biomass south of ca. 43° N is dominated by C₄ (warm growing season) grasses.  Although the evolution of the Great Plains ecosystem is a late Cenozoic phenomenon, diverse paleontological data (plant macrofossils, phytoliths, mammalian faunal structure and paleodiets) do not suggest a consistent chronology for the origin of C₄-dominated grasslands in the region.  I use stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions of 230 paleosol (fossil soil) carbonates from Miocene sections (23-6.5 Ma) across the Great Plains and 452 paleosol carbonates from the Meade Basin, southwest Kansas that range in age from late Miocene to early Pleistocene (12-1 Ma) to examine the history of C₄ grasses and climate in the region.  
Results/Conclusions

Mean δ¹³C of Great Plains Miocene samples is relatively high and invariant (‑6.8±0.8‰ VPDB, n = 230) and indicates 12-34% C₄ biomass in the region throughout the Miocene.  The Meade Basin record indicates a multi-phase increase in the abundance of C₄ biomass.  Miocene δ¹³C values from Meade are statistically similar to Miocene paleosol carbonates elsewhere in the Great Plains.  The abundance of C₄ biomass increased between the end of the Miocene and the beginning of the early Pliocene (5-4 Ma).  Early and mid (4-3 Ma) Pliocene carbonates have statistically identical δ¹³C values (‑4.9±0.90‰, ‑5.0±1.10‰, respectively) that suggest a stable ecosystem with about 40% C₄ biomass for several million years.  High δ¹³C variability in densely sampled intervals during the early to mid Pliocene indicate a high degree of landscape scale patchiness in C₄ abundance.  Finally, during the late Pliocene to early Pleistocene (>2.1-ca 1 Ma), δ¹³C values increase steadily from ‑4‰ to ca. 1‰ at the top, corresponding to an increase from almost 50% to 65% C₄ biomass.  The abundance of C₄ biomass first reaches modern levels for the region (78±10.9‰) at ca. 1.3 Ma, although δ¹³C values do not remain consistently high through the rest of the section.  Mean carbonate δ¹⁸O values decrease from the late Miocene (25.3±0.72 VSMOW) to the early and mid Pliocene (21.8±0.87 and 22.1±0.69‰, respectively), and exhibit a trend in the late Pliocene-early Pleistocene from ca. 25 to ca. 21‰.  The trends in δ¹⁸O values suggest that C₄ biomass increased coincident with some combination of a decreasing temperature, increasing proportion of winter precipitation recharge of soil water, and/or increasing in soil moisture.