A New Approach to Understanding the Timing, Rates, and Causes of Quaternary Soil Denudation

University Of Texas At Austin, Austin TX

Investigators

Abstract

0074085 Stern The history of soil erosion in central Texas and many other regions is poorly known, yet understanding this history is important for studies of environmental change and efforts in soil conservation. We propose to quantify the temporal evolution of soil thickness in the vicinity of Hall's Cave, which is located in the regionally-extensive Edwards Plateau in Kerr Co., TX. Soil evolution will be quantified through application of a novel technique the uses the strontium isotope composition (87Sr/86Sr of mammalian fossils preserved in the cave-filling sediment. A trenched sediment sequence at Hall's Cave contains a rich paleontologic record, and a high resolution chronology was constructed for the past 16,000 years of deposition in the cave. As soils were eroded from the Edwards Plateau beginning around 8,000 years ago, the relative contribution to the exchangeable strontium pool of Sr derived from the limestone bedrock (low 87Sr/86Sr values) should have increased with respect to the silicates (high 87Sr/86Sr values) in the soil. Therefore, the strontium isotope ratio of biota living on the landscape will change through time and reflect the 87Sr/86Sr value of the exchangeable Sr pool in the soil, and thus the soil thickness. WE will construct a history of soil thickness, which we will then compare to independent chemical and biotic proxies for changing temperature, moisture, and soil character. Specific proxies included: (1) the mineralogy and elemental chemistry of the cave sediments as a constraint on the soil erosion history; (2) the oxygen isotope ratio of fossils (backberries, snails and speleothems as a record of precipitation patterns in the past; (3) the carbon isotope ratio of tooth enamel and humus as a proxy for the proportion of C4 grass in the paleoecosystem; (4) the palynology of the strata; and (5) the paleoecology indicated by the rich vertebrate fauna. Temporal correlation of these environmental proxies with the soil erosion timing and rate, as indicated by the Sr isotope variations, will provide essential evidence to evaluate the causes of the profound soil erosion that denuded the central Texas landscape of its once thick soil.