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.
