Evaluation of zircon U-Pb geochronology as a tool to determine soil provenance in a limestone terrane, Middle TN, USA

Abstract

Soil can form by bedrock weathering or by deposition of alluvial or aeolian material. To test the utility of detrital zircon UPb geochronology for determining soil provenance, and to determine the provenance of soils in Middle Tennessee, samples of soil and underlying bedrock were collected from two sites, along with alluvium from a nearby river and loess from West Tennessee. Multi-dimensional scaling plots were used to compare complex detrital zircon LA-ICP-MS UPb sample age distributions. Concentration ratios of immobile elements and Nd and Hf isotope compositions were also used as provenance indicators for comparison. At one site zircon UPb ages and bulk sample trace element and isotopic compositions of soil were similar to the underlying limestone bedrock, indicating soil formation by bedrock weathering. In contrast, at a second site provenance tests indicate that soil from a late Pleistocene river terrace was sourced from alluvium rather than the underlying bedrock. Provenance inferences based on zircon UPb ages were generally consistent with evidence from Zr/Hf ratios and Hf isotope compositions, which are all dominated by zircon, and Nd isotope compositions, which are not, although the best matches for different provenance indicators did not always agree. Zircon geochronology should be used with other soil provenance indicators to reduce effects of bias introduced by sedimentary sorting and sample processing.