PROVENANCE OF MODERN SOILS OF MIDDLE TENNESSEE ASSESSED USING ACCESSORY MINERAL U-PB GEOCHRONOLOGY

Society depends on soil for food production, but little is known about pedogenesis in areas underlain by limestone. Weathering of very pure limestone should not produce soil, yet soils are commonly observed on limestone bedrock. Studies of soils atop limestone in the Caribbean have found that the soils are derived almost solely from aeolian input of African dust due to the lack of impurities in the bedrock. In contrast to high-purity island carbonates, limestone bedrock in Middle Tennessee has a significant silt component. Furthermore, African dust is not believed to be a significant soil input in the region. Instead, modern soils in Middle Tennessee form from parent material that is comprised of some combination of three potential endmembers: 1) loess, sourced from the Mississippi River Valley; 2) alluvial sediments; 3) insoluble residue formed from the dissolution of “dirty” limestone.

Here, we estimate the proportions of these endmembers in soils from Middle Tennessee, which is mostly underlain by limestone bedrock, using zircon and monazite U-Pb age spectra and concentration ratios of immobile elements (e.g. Zr, Ti, Nb, and Ta) from analyses of soil-bedrock pairs and the potential endmember parent materials. No previous studies have attempted to recover accessory minerals from limestone for geochronology. The results give us a better understanding of the roles of loess and alluvium deposition and chemical weathering of limestone in soil development. In addition, age spectra of zircon or monazite within the limestone could potentially reveal sources of sediments deposited during limestone formation in the early to middle Paleozoic. We believe the new methods in this study could open up whole new areas of research, specifically application of accessory mineral geochronology for determining provenance of soils and the clastic portions of limestones.