2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 1
Presentation Time: 8:00 AM-4:45 PM

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

FOX, David L., Department of Geology and Geophsyics, Univ of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455, dlfox@umn.edu

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.