REGIONAL VARIABILITY OF STABLE ISOTOPE AND RARE EARTH ELEMENT SIGNATURES IN FOSSIL VERTEBRATES FROM THE EOCENE-OLIGOCENE WHITE RIVER GROUP

Fossil bones from various sites in the Eocene-Oligocene White River Group of NE, SD, and WY differ from one another in lanthanide element (REE) and trace element (TE) signatures, and in bone-carbonate-fraction δ¹⁸O_C and δ¹³C_c isotopic values. The REE/TE signature in a bone is imparted during fossilization and is a function of the geochemical conditions in the meteoric zone, including redox, pH, and the degree of sediment-water interaction. Stable isotopes of δ¹³C_c and δ¹⁸O_C in the carbonate fraction of fossil bone also record conditions during fossilization, reflecting water stress, precipitation values, temperature and evaporation in the near surface fossilization environment. The combination of REE/TE/isotope signatures provides a new archive of paleoclimatic information, which is independent of animal type and avoids problems of interpreted paleoecology and preservational bias, that can be used to augment existing datasets. REE/TE/isotope geochemical signatures are regionally variable and change up-section. Local lateral changes within time-equivalent strata are a function of facies variation and degree of associated paleosol development, whereas vertical variations are a function of changing paleoclimatic conditions from Eocene “hothouse” to Oligocene “Icehouse” conditions in North America. REE/TE signatures also vary with isotopic composition. δ¹³C_c values in bone carbonate from Late Eocene fossils average ca. 1‰ heavier and δ¹⁸O_C values ca. 3‰ lighter at Flagstaff Rim (WY) than Toadstool Geologic Park (NE). REE signatures in fossils from central WY are more heavy-REE enriched and those from western NE more middle-REE and light-REE-enriched. These differences are consistent with greater and increasing water stress in WY and different paleosol development at the two sites.