MASSIVE LATE PLEISTOCENE TO HOLOCENE SOIL DENUDATION IN CENTRAL TEXAS
Temporal variation in the Sr isotope composition of fossils provides a unique tool to reconstruct Late Pleistocene to Holocene soil erosion on the Edwards Plateau in central Texas. In contrast to the thin soils of the modern Edwards Plateau, fossils of burrowing rodents, relict thick red upland soils, and red clay cave fills are evidence of a much thicker soil mantle in the Late Pleistocene to Early Holocene. Previous studies on Halls Cave vertebrate fauna and sediments propose a hypothesis for a massive soil erosion event in the Early Holocene. We test this hypothesis by applying a new technique, made possible by isotopically distinct limestone bedrock and silicate soil Sr sources, that relies on the Sr isotope composition of fossil animals and vegetation as a proxy for soil thickness. In central Texas, the exchangeable Sr isotope composition of soil varies spatially and temporally with soil thickness. Thin soil has exchangeable Sr with a low 87Sr/86Sr, close to values for the underlying Cretaceous marine limestone bedrock, whereas thick soil has a more radiogenic 87Sr/86Sr, similar to the continentally-derived soil silicate component. Since fossil animals and vegetation likely preserve the exchangeable Sr isotope composition of soil on the paleolandscape, denudation of the Edwards Plateau soil mantle will be represented by decreasing 87Sr/86Sr through time of Halls Cave fossils.
The 87Sr/86Sr values of fossil hackberry (Celtis) seed aragonite and rodent tooth enamel (Microtus and Geomys) from Halls Cave show a nearly linear decrease between 18 and 8 ka, suggesting progressive erosion of central Texas soils. In the Early Holocene, the 87Sr/86Sr of hackberry seed aragonite approaches the 87Sr/86Sr of a modern hackberry growing at the mouth of Halls Cave, supporting the hypothesis of thinner soil cover since the Early Holocene. Using the correlation of soil leachate 87Sr/86Sr with soil thickness observed in modern central Texas soils, we are able to relate the 87Sr/86Sr of ancient soils, as recorded by fossils, with soil thickness and constrain soil erosion rates. These results are significant as they represent a new method able to quantify high-resolution changes in soil erosion rates and provide a basis to address the effects of human landuse on natural soil erosion rates and biogeochemical cycles.