Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Carbon isotopes and the evolution of C₄-dominated grasslands

Fossiliferous strata in the Meade Basin (southwest Kansas) preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from the Clarendonian North American Land Mammal Age (NALMA; 12.0-9.0 Ma, early late Miocene) to the early Irvingtonian NALMA (ca. 2.5-ca. 1.0 Ma, early Pleistocene). Faunas from these sections document the evolution of the small mammal community of the modern grassland ecosystem of the region, and the stable isotope composition of paleosol carbonates provides a means by which the environmental context of the evolution of the modern ecosystem may be documented. We used the stable carbon isotope composition (δ¹³C relative to Vienna Peedee belemnite [VPDB]) of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of C₄ grass abundance in the Meade Basin. Paleosol carbonate δ¹³C values reflect the proportion of C₃ (trees, shrubs, cool- climate grasses) and C₄ (warm-climate grasses) plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats. Paleosol carbonate δ¹³C values record a three-phase increase in the abundance of C₄ biomass during the Neogene in the Meade Basin. Late Miocene sections have mean δ¹³C values of -7.6‰ ± 0.90‰ (Clarendonian) and -6.5‰ ± 0.31‰ (Hemphillian NALMA, 9.0-4.9 Ma), consistent with 17% and 26% C₄ biomass, respectively. Miocene δ¹³C values from Meade are statistically identical to published δ¹³C values for Miocene paleosol carbonates elsewhere in the southern Great Plains, supporting the widespread presence of ~20% C₄ biomass on average in the region throughout the Miocene. The abundance of C₄ biomass increased between the end of the Hemphillian section and the beginning of the early Blancan NALMA (5.0-3.0 Ma). Early and middle Blancan (3.0-2.5 Ma) carbonates have statistically identical δ¹³C values (-4.9‰ ± 0.90‰ and -5.0‰ ± 1.10‰, respectively), suggesting a stable ecosystem during the early Pliocene, although high δ¹³C variability in densely sampled intervals suggests a high degree of landscape-scale variation in C₄ abundance. The final phase, geochronologically controlled by two well- characterized ashes (Huckleberry Ridge, 2.10 Ma; Cerro Toledo B, 1.47-1.23 Ma) and magnetostratigraphy, is a trend to higher δ¹³C values from the late Blancan to early Irvingtonian (ca. 2.5-ca 1.0 Ma) from -4‰ at the base of the section to ~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%) around the level of the Cerro Toledo B ash, indicating that a modern-like grassland ecosystem first appeared in the region ca. 1.3 Ma, although δ¹³C values do not remain consistently high through the rest of the section.