Abstract

Neogene paleosols of the Great Plains and central Oregon provide abundant evidence of grasslands of the past. The characteristic surface horizon (mollic epipedon) of grassland soils (Mollisols) can be recognized in paleosols from granular ped structure, abundant fine root traces, dark color, and common carbonate or other easily weatherable minerals. The fossil record of such soils indicates a three-stage evolution of grasslands. Eocene to Oligocene rangelands are represented by paleosols with near-mollic soil structure and fine root traces similar to soils of modern desert grasslands with scattered bunch grasses and shrubs. Paleosols with mollic epipedon and shallow (less than 40 cm down into the profile) calcic horizons are evidence for the appearance of sod-forming short grasslands during the early to middle Miocene. Mollic paleosols with deeper (some 1 m or more) calcic horizons represent tall grasslands and have not been found older than late Miocene. Early stages in the evolution of grassland soils correspond to climatic coolings near the end of the Eocene and middle Miocene. The best understood climatic cooling and drying during the late Miocene (Messinian or 5-7 Ma) is synchronous with the expansion of tall C 4 grasslands.

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