THE PAST 14,000 YEARS: EVIDENCE FOR PERIODS OF WARMING AND ARIDITY FROM RIVER VALLEYS OF THE CENTRAL GREAT PLAINS

Shifts in the seasonal distribution of precipitation and the nature of precipitation events interact with vegetation to cause hydrologic change that has geomorphic consequences in drainage basins. Over the past 14,000 years, erosion and sedimentation cycles in the Central Great Plains were driven by interactions among climate-related changes in vegetation communities that altered ground cover, and changes in the magnitude, frequency, and seasonal distribution of precipitation. These bioclimatic changes altered erosion, runoff, and sediment delivery relationships in drainage networks.

Stable carbon isotope (δ¹³C) data from buried soils in stream valleys of the Central Plains indicate that during the Pleistocene-Holocene transition, plant communities were dominated by cool-season C₃ vegetation. At that time, erosion rates were low on the uplands, and slow aggradation on valley floors was accompanied by cumulic soil development. However, from about 10,200 to 5700 years ago, a significant change in the composition of vegetation communities occurred on upland sediment-source areas. Specifically, there was a large increase in the proportion of C₄ biomass, which is interpreted as reflecting a shift to warmer and drier conditions. This episode coincides with the Altithermal (Holocene Climatic Optimum), a period of strong zonal airflow during summers that restricted the northward penetration of moist tropical air masses into the midcontinent. Stratigraphic evidence indicates that bioclimatic change at that time destabilized the uplands across the Central Plains, resulting in a substantial increase in sediment supply to stream valleys.

The δ¹³C record from buried alluvial soils shows a reduction in the percent of C₄ biomass since about 4,500 years ago. Stratigraphic evidence indicates episodic deposition that likely reflects a geomorphic response to fluctuations in environmental conditions (i.e., between relatively moist and dry), probably due to fluctuations in the mean position of air mass boundaries.

The record of bioclimatic and geomorphic change in the Central Plains during the Altithermal has important implications for predicting landscape response to projected increases in drought severity in the coming decades.