REFINEMENT OF RARE EARTH ELEMENT (REE) AND TRACE ELEMENT (TE) SIGNATURE APPLICATIONS IN FOSSIL VERTEBRATES AND ASSOCIATED SEDIMENT FRACTIONS:FOSSIL LAKE, OREGON

Rare Earth Elements (REE) and Trace Elements (TE) impart geochemical signatures that are locked within bioapatite of fossil vertebrates during the fossilization process. These signatures provide evidence of the depositional environment and are unique to their lithostratigraphic units.

One of the more significant Pleistocene sites is Fossil Lake, Oregon. Our field studies reveal nine exposed thin, rhythmic, fining-upward depositional packages, most separated by disconformities. Bones and associated sediment fractions analyzed from eight of the defined packages, and in units within these packages, have distinctly different REE signatures and TE ratios. REE signatures in fossils from lower packages suggest uptake from neutral pH waters. In contrast, up section REE signatures become increasingly heavy REE–enriched, with varying positive Ce anomalies in the upper units. REE signatures in fossils from the upper units are very similar to waters from modern alkaline suggesting diagenetic uptake in increasingly alkaline and saline waters. These REE changes also suggest increasing aridity up-section, a contention reinforced by the habitat preferences of the terrestrial vertebrates preserved.

In our most recent research, a unique opportunity presented itself. An articulated horse was found in-situ in the Ferruginous Sandstone unit at Fossil Lake. The recovered specimen transected subtle lithological variations within this unit. Since REE signatures reflect depositional environments, this specimen offered a unique opportunity to investigate the possible variations in a signature correlating to subtle changes in lithology. This was accomplished by collecting and analyzing bone and sediment fractions at 1 cm increments from the base of the specimen.

In the intervals from 1-7 cm there are no apparent changes in the REE signature or TE ratios for the bone or the associated sediment fractions. However, between the 8-10 cm intervals there are noticeable changes in the REE signatures, which mirror the changes in lithology. By relating subtle changes in lithology to variations in REE signature in both bioapatite and associated sediment fractions, this study offers a refined resolution for application of REE signatures to paleoenvironmental interpretations, fossil provenience and stratigraphic correlation.