FROM BONE TO STONE: THE INFLUENCE OF DEPOSITIONAL ENVIRONMENTS ON THE FOSSILIZATION OF VERTEBRATE BONE FROM THE PALEOGENE WHITE RIVER GROUP, BADLANDS NATIONAL PARK, SOUTH DAKOTA

Rare earth elements (REE) are incorporated into bioapatite during fossilization by diffusion from groundwater and surrounding sediments, producing REE signatures in fossils representing geochemical (pH, redox, sediment and solution compositions) conditions. Geochemical variations during fossilization may be time averaged, affecting the resulting signature; therefore, the evaluation of diffusion periods is necessary to determine the influence of time-averaging on REE concentrations in fossil bone and their fidelity for use as paleoenvironmental indicators. Diffusion periods were estimated using one-dimensional equations of Fick’s Second Law assuming saturated conditions and constant interfacial concentrations of La, Ce, and Nd. REE were measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of bones from fluvial and eolian-dominated environments. An average diffusion period of 80ka ± 25ka was obtained from two fossils from fluvial deposits and 430ka ± 250ka from a fossil in an eolian deposit. Cathodoluminescence (CL) of authigenic calcium carbonate spar in bones from fluvial deposits is entirely activated, suggesting saturated and reducing conditions. Within eolian deposits, CL of first stage micritic calcite is suppressed relative to second stage sparite, suggesting oxidizing vadose conditions. Luminescent banding is concordant with crystal facets, indicating fluctuating redox conditions due to repetitive wet-dry cycles. Superposed on the banded sparite is a zone of entirely activated sparite indicating a change from fluctuating redox to saturated, reducing conditions. Saturated conditions inferred from the reducing CL signal within bones from fluvial settings are probably the result of a shallower water table in these environments, whereas the bones from the eolian deposits were likely subject to repetitive wet-dry conditions before submersion in ground water that manifests as the central activated zone in CL. The saturated burial conditions of the fluvial samples and the largely unsaturated burial conditions of the eolian samples agree with their respective shorter and longer periods of REE diffusion, suggesting that saturated, reducing environments fossilize bones considerably faster than unsaturated, partially oxidizing environments.