Habitat Patchiness during the Transition to the Modern, C4-Dominated Grassland Ecosystem in the Great Plains

The carbon isotope composition (δ¹³C) of pedogenic carbonate is a reliable means to reconstruct the relative abundance in past ecosystems of C3 (trees, shrubs, cool-growing season grasses) and C4 (predominantly warm-growing season grasses) plants. Biomass in modern prairies of the central Great Plains (Kansas) is dominated by C4 grasses (ca. 70%). Paleosol carbonates in the Meade Basin (SW Kansas) record the evolution of the modern, C4-dominated grassland in two stages: an initial increase from the late Miocene (ca. 20% C4 biomass) to the early Pliocene (ca. 40% C4), and a subsequent increase during the late Pliocene-early Pleistocene to modern levels. Early Pliocene paleosols across the Meade Basin (ca. 45 km²) exhibit high within section variability in δ¹³C values, suggesting high landscape scale variability in C4 abundance. Such heterogeneity is consistent with discrete patches of C3- and C4-dominated habitat that migrated laterally on time scales short relative to carbonate accumulation during the transition to the modern ecosystem. Dense lateral sampling of a <4 m thick sequence of paleosols in multiple outcrops along 1.5 km of Keefe Canyon provides a means to examine landscape variability in C4 abundance in detail. In each outcrop, short correlative sections were measured and sampled vertically every 5-20 m along the outcrop. Mean δ¹³C value of these samples is identical to other early Pliocene samples. Within outcrops, C4 abundance varied from 20-75% stratigraphically. In some outcrops, all sections exhibit the same temporal variations between C3- and C4-dominated states, but in others the sections do not vary coherently, reflecting complex spatial patterns of patch migration on short time scales. During the evolution of the Great Plains ecosystem, a complex mosaic of habitat patches persisted with local heterogeneity superimposed on the long term, two stage increase in C4 abundance from the late Miocene to the early Pleistocene.