Cenozoic Expansion of Grasslands and Climatic Cooling

Mountains, ocean currents, forests, and swamps have played an important role in regulating global climate for hundreds of millions of years, but the truly novel event of the Cenozoic was the evolution and expansion of grasslands, with their uniquely coevolved grasses and grazers. Neogene expansion of the climatic and geographic range of grasslands at the expense of woodlands is now revealed by recent studies of paleosols, fossils, and their stable isotopic compositions. Grasslands and their soils can be considered sinks for atmospheric CO2, CH4, and water vapor, and their Cenozoic evolution a contribution to long-term global climatic cooling. Grassland soils are richer in organic matter than are woodland and desert soils of comparable climates, and when eroded, their crumb clods form sediment unusually rich in organic matter. Grasslands also promote export of bicarbonate and nutrient cations to lakes and to the oceans where they stimulate productivity and C burial; this increased productivity and C burial occur because grasslands preferentially exploit fertile young soils in the first flush of weathering and their soils have a crumb structure with much higher internal surface area for weathering than soils of woodlands and deserts. Grasslands also promote regional climatic drying by virtue of their higher albedo and lower transpiration than woodlands of comparable climatic regions. Labile pools of C in grassland soils and their accelerated weathering rates early in soil development may also account for increased climatic instability over the past 40 m.yr. Unidirectional, stepwise, long-term climatic cooling, drying, and climatic instability may have been driven not by tectonic forcing but by the coevolution of grasses and grazers.