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

Abstract

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 (δ 13 C relative to Vienna Peedee belemnite [VPDB]) of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of C 4 grass abundance in the Meade Basin. Paleosol carbonate δ 13 C values reflect the proportion of C 3 (trees, shrubs, cool-climate grasses) and C 4 (warm-climate grasses) plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats. Paleosol carbonate δ 13 C values record a three-phase increase in the abundance of C 4 biomass during the Neogene in the Meade Basin. Late Miocene sections have mean δ 13 C values of −7.6‰ ± 0.90‰ (Clarendonian) and −6.5‰ ± 0.31‰ (Hemphillian NALMA, 9.0–4.9 Ma), consistent with 17% and 26% C 4 biomass, respectively. Miocene δ 13 C values from Meade are statistically identical to published δ 13 C values for Miocene paleosol carbonates elsewhere in the southern Great Plains, supporting the widespread presence of ∼20% C 4 biomass on average in the region throughout the Miocene. The abundance of C 4 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 δ 13 C values (−4.9‰ ± 0.90‰ and −5.0‰ ± 1.10‰, respectively), suggesting a stable ecosystem during the early Pliocene, although high δ 13 C variability in densely sampled intervals suggests a high degree of landscape-scale variation in C 4 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 δ 13 C 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% C 4 biomass. The abundance of C 4 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 δ 13 C values do not remain consistently high through the rest of the section.