THE EFFECTS OF PROVENIENCE AND TAPHONOMY ON RARE EARTH AND TRACE ELEMENT SIGNATURES IN VERTEBRATE FOSSILS FROM THE EOCENE-OLIGOCENE WHITE RIVER GROUP, TOADSTOOL GEOLOGIC PARK, CRAWFORD, NE

Rare earth (REE) and trace elements (TE) were analyzed from 125 vertebrate fossils collected in Toadstool Geologic Park (Nebraska National Forest) in northwest Nebraska. Park strata expose members of the Eocene-Oligocene White River Group, which contain vertebrate fossils of the Chadronian-Arikareean NALMA. Analyses were conducted as part of a program to manage and protect fossil resources and determine whether chemical composition of fossils can be used to recognize the provenience of fossils and mitigate fossil poaching. In situ bone fragments, or fragments with a determined short transport distance, were collected from several members of the Chadron and Brule Formations. REE and TE are incorporated into bone during the fossilization process and their concentrations and ratios are a function of the degree of bone recrystallization, water composition, distribution coefficients, pH, redox, and reaction with REE carrier phases during early, near-surface diagenesis. Questions to be answered include: 1). Do fossils from different geologic units possess distinctive REE signatures? 2). Can fossils with undocumented stratigraphic affinity be assigned to their original geologic units? 3).What is the degree of regional geochemical variability within a particular stratigraphic unit? 4.) Can REE chemical fingerprints be used to aid in fossil poaching cases in which the provenience of the fossil is essential to prosecution? Our results suggest that REE signatures are distinct between individual stratigraphic units and that prefossilization taphonomic processes (weathering, processing [gnawing or root etching]) and fossil type (mammal vs. tortoise) have no effect on the REE signature. REE and TE ratios show distinct trends throughout the section, possibly reflecting climatic and environmental changes associated with the Eocene-Oligocene Transition, although this hypothesis requires further testing. Individual bone beds within the same stratigraphic unit can be distinguished. Discriminant analysis using REE and TE ratios allowed fossils from different strata at Toadstool Park to be assigned to the proper member 80% of the time and to be distinguished from those of Badlands National Park (South Dakota) with 99% accuracy. This may aid detection of fossil poaching.