USING RARE EARTH ELEMENTS AS A TOOL TO DETERMINE THE AGE AND SOURCE OF THE MCPHERSON COLLECTION

Rare earth elements (REE) can be used to determine depositional environments of vertebrate fossils found as float in creeks of the Tunica Hills region of Louisiana. Fossils deposited in chemically distinct environments should have distinctive REE signatures, so comparisons of REE composition can be used to determine the fossil’s source and estimate its age. Specimens from the McPherson collection have been assigned an age of Pleistocene based on the supposition that the vertebrate fossils were weathering out of the Pleistocene loess, the Pleistocene terraces, or the Citronelle Formation; the age of which has been questioned. The fossils analyzed from the Miocene Pascagoula Formation, Pleistocene Peoria Loess and intermediate beds included: horses, mastodons, ground sloths, a rhinoceros, a large felid, a giant armadillo, a grouse, and the first occurrence of Synthetoceras in Louisiana. REE analysis is used to test the following questions 1) can REEs be used to determine the original stratigraphic position of the McPherson collections fossils and 2) can REE analysis help in determining paleoenvironmental conditions of the Tunica Hills region during the late Miocene to late Pleistocene. Discriminant analysis correctly identifies 90% of the in situ Miocene specimens and identifies all specimens originally thought to be Pleistocene as Pleistocene. The only incorrect identification was one in situ Tunica Hills/Kerry site specimen. This suggests that the REE signature of the McPherson collection specimens show no definitive age differences in the creek float material. The results are consistent when dentine and dentine/tusk/bone samples are compared. While the REE signatures do not show differences, the presence of Nannippus and Synthetoceras suggests that there is likely time averaging within the McPherson collection. While this does not suggest a difference in REE signatures between the Plio-Pleistocene, it may suggest a change in source water during this period.