Precise timing and rate of massive late Quaternary soil denudation

Abstract

Strontium isotopes are a unique tool to study soil-erosion dynamics. Changes in Sr isotope ratios ( 87 Sr/ 86 Sr) provide a record of late Quaternary landscape denudation of the Edwards Plateau of central Texas, United States. The use of Sr isotopes as a tracer for soil erosion is based on the observation that, in central Texas, the 87 Sr/ 86 Sr ratio of soil correlates with soil thickness. Plants and animals express the 87 Sr/ 86 Sr ratio of exchange- able Sr in the soil. Therefore, we use changes in Sr isotope ratios through a well-dated stratigraphic sequence of fossil plants and animals in Hall's Cave, Kerr County, Texas, as a proxy for temporal changes in soil thickness. By using this record we are able to characterize late Quaternary climate-driven soil-erosion dynamics on the Edwards Pla- teau. We find that continuous erosion removed at least 180 cm of soil at a constant min- imum rate of 11 cm/k.y.; this continuous phase of erosion ended ca. 5 ka. The Sr isotope record of soil erosion is consistent with late Quaternary environmental change in central Texas that has been independently modeled by using local and regional climate records. However, the rate of this climate-driven soil-erosion event was an order of magnitude slower than recent soil erosion caused by human land use. These results link erosion to century- to millennial-scale climate change and are cautionary evidence that even greater landscape degradation may result from coincident climatic variability and anthropogenic influences.