RARE EARTH ELEMENTS IN FOSSIL VERTEBRATES AS A POTENTIAL ARCHIVE OF PALEOCLIMATIC FLUCTUATION DURING THE HOTHOUSE-ICEHOUSE TRANSITION OF THE PALEOGENE WHITE RIVER GROUP, NORTHWEST NEBRASKA

Fossilization of bone, involving recrystallization and incorporation of rare earth and trace elements (REE, TE) into the apatite, occurs in soil vadose or near surface groundwater environments over periods of several thousand years during weathering and early stage diagenesis. Depending on environmental conditions at the time of fossilization (Eh, pH), and varied inputs from reactive soil minerals and water, fossil bones will preserve different REE signatures. Signatures are unaffected by weathering or bone processing prior to fossilization. These signatures are distinct and can be used to study provenance, time averaging, and taphonomy of fossil assemblages. The White River Group is a succession of non-marine strata that records the Eocene-Oligocene Transition (EOT) in northwest Nebraska. Earlier workers have suggested a dramatic cooling across the transition at this location (ca. 8^oC), but a negligible decrease in aridity. In contrast, paleosols and sedimentary facies suggest increasing aridity, although the same changes can be explained by increasing sedimentation rates. Bones were collected (ca. 100) from a 70 m vertical succession of terrestrial strata that spans the Eocene-Oligocene Transition (EOT). REE ratios and ratio variances change across the EOT, possibly as a function of climatic change from Hothouse to Icehouse conditions and/or greater degrees of ecosystem variability. Time series analysis of the Nd_N/Yb_N and other ratios reveals significant periodicities (p < 0.01) at ca. 1010, 720, and 520ka, near some Milankovitch orbital frequencies. Some bones were previously analyzed for δ¹³C and δ¹⁸O. The Nd_N/Yb_N correlation with δ¹³C is significant (p < 0.01) but that with δ¹⁸O is not. Bone δ¹³C has been proposed as an aridity proxy. Thus, REE signatures in bones may also be a paleoclimate and paleoenvironmental proxy that can be used to decipher the controls and influences on the EOT in this region. Surprisingly, time series analysis of δ¹³C shows no significant periodicities. This apparent disparity may be the result of the interactions of soil and atmospheric CO₂ and the location of the bone within a soil during the fossilization process. Analysis of bones from expanded sections to the west may preserve shorter-term periodicities and provide insight into the REE/δ¹³C relationship.