Rare earth elements as an investigative tool into the source, age, and ecology of late Miocene to late Pleistocene fossils from the Tunica Hills, Louisiana
Rare earth elements were investigated for their potential to determine provenance and depositional environments of vertebrate fossils found as float in creeks of the Tunica Hills region of Louisiana. Elemental analyses are used to test the hypotheses that fossils span multiple time periods, from late Miocene to late Pleistocene, and that elemental profiles can distinguish terrestrial depositional settings from possible estuarine deposits. In addition, the animals present were then used to test the hypothesis that Tunica Hills had late Pleistocene environmental conditions much like the modern Great Lakes region. This study is important because it provides new insight on the vertebrate fossils of Louisiana to better the current understanding of the paleontology of the central Gulf Coast region. Calcium is replaced by similar-sized rare earth elements in bone and tooth dentine very quickly upon death (10,000-30,000 years), and the signature is not altered by additional diagenetic effects. The change in source of sediment supply from the Tennessee River, a postulated source during the Miocene, to the average Midwest continental signature, during the late Pleistocene, would have affected the overall rare earth element compositions available during fossilization. The loess material may not have had enough to complete the uptake of REE, which would have an influence on the profiles seen. The 38 fossils analyzed from the Pascagoula Formation, Peoria Loess and intermediate beds included horses, mastodons, ground sloths, a rhinoceros, a large felid, a giant armadillo, and a grouse. The first occurrence of Synthetoceras in Louisiana is represented by one analyzed fossil specimen. Cluster analysis separated in situ fossils from loess deposits from the Pascagoula Formation fossils, based on rare earth element profiles. The results on lithologically diverse and stratigraphically complex intermediate deposits, including reworked Citronelle Formation, are less clear, and shell material and nodules analyzed were not useful for stratigraphic control. In situ samples from the Pascagoula Formation support an estuarine depositional environment in the Tunica Hills during the late Miocene or early Pliocene. Rare earth element analysis shows promise, but more comparative data is needed to help unravel the history Gulf Coast region.