NEOGENE HISTORY OF C4 BIOMASS IN THE GREAT PLAINS, U.S.A

Grasslands are highly productive ecosystems that have historically supported highly coevolved communities of mammalian herbivores. The Great Plains is the largest continuous grassland in North America, and modern biomass in the region south of ca. 43° N is dominated by C₄ grasses. As with other modern C₄ grasslands, the evolution of the Great Plains and its associated fauna is a phenomenon of the late Cenozoic. However, diverse paleontological data (e.g., plant macrofossils, phytoliths, mammalian faunal structure, isotopic evidence of paleodiets) do not suggest a consistent chronology for the origin of the C₄-dominated grasslands of the Great Plains. We have used the stable carbon isotope composition (δ¹³C) of pedogenic carbonate from early Miocene to early Pleistocene sections in the Great Plains to examine the Neogene history of C₄ grasses in the region.

The mean δ¹³C value of Miocene samples is relatively high and invariant (-6.8±0.8‰, n=230), and δ¹³C values of co-occurring carbonate and organic matter indicate a substantial proportion (12-34%) of C₄ biomass in the region during the Miocene. Similar mean δ¹³C values for different carbonate morphologies (e.g., nodules, mortar beds) indicate that the abundance of C₄ biomass during the Miocene varied locally only on time scales short relative to carbonate accumulation within a soil. The proportion of C₄ biomass increased during the early Pliocene, and densely sampled transects along the same calcareous horizons have moderately variable C₃:C₄ ratios, suggesting maintenance of a habitat mosaic as open C₄-grasslands were becoming established during the Pliocene. C₄ biomass in the Great Plains did not reach modern levels until ca. 2.5 Ma.

A weak correlation between δ¹³C and δ¹⁸O values in Great Plains palesols suggests that long-term climate change is not the main control on C₄ biomass in the Great Plains. Contrasts between the Great Plains record and coeval paleosol records from other continents suggest regional factors rather than a single global factor control the evolution of C₄ grasslands. For the Great Plains, we propose that a decrease in the length of the growing season through winter cooling and the resulting concentration of consumer pressure led to the replacement of mixed habitats with an open C₄ grassland during the Pliocene and early Pleistocene.