NEW DEVELOPMENTS IN RARE EARTH ELEMENT (REE) ANALYSIS FOR THE PURPOSES OF PALEOENVIRONMENTAL INTERPRETATIONS

REE signatures in fossil vertebrates have successfully been used as method for Fossil Provenience, a Paleobathymetric indicator for the Western Interior Seaway during the late Cretaceous as well as an alkalinity and salinity indicator in lacustrine environments. Studies have shown that (" REE) in modern bones and teeth are generally less than 20 ppm. " REE in fossil bones may, however, be greater than 1,000 to 10,000 ppm . Therefore, more than ca. 95% of REE in fossil bone may be incorporated diagenetically into bone post mortem. Because most REE are introduced post mortem, REE signatures in fossils do not reflect the diet, trophic level, or phylogenetic position of the organism. REE and other trace elements are apparently incorporated shortly after deposition during early diagenetic recrystallization of the bone apatite, recording the depositional or early diagenetic conditions. Because the recrystallized apatite is thermodynamically more stable than the original material the substitutions are preserved. Due to changing diagenetic conditions REE signatures in fossils from different stratigraphic units may vary, providing unique identifiable variations in their depositional or early diagenetic environments. Prevailing conditions control what is available for replacement within bone apatite during the fossilization process. Understanding the factors influencing these conditions can be used in the analysis of REE signatures as a proxy to infer original paleoenvironmental conditions. In recent studies in both marine and terrestrial environments, REE analysis of fossil materials revealed that REE signatures vary stratigraphically between members and are virtually identical within the same member over a wide geographic area. In addition, interpretations of REE ratios show that REE compositions between units appear to result from differential mixing of waters and can be related to variations in the water mass during the fossilization process.