QUANTIFYING PERIODS OF FOSSILIZATION IN TERRESTRIAL AND MARINE ENVIRONMENTS USING RARE EARTH ELEMENTS

Concentrations of rare earth (REE), U, Th, and other trace elements (TE) were measured using LA-ICP-MS along transects across five Late Eocene brontothere bones from the terrestrial White River Group and four Miocene-Pliocene marine mammals from the Atlantic Coastal Plain. REE/TE concentrations tend to be highest at the bone surface and decrease rapidly with depth. REE concentration gradients are generally steeper in marine fossil bones and shallower in terrestrial fossil bones, indicating longer periods of REE uptake in terrestrial fossils. Histology may affect REE incorporation. REE concentrations are sometimes elevated in trabecular bone and Haversian systems, which may act as secondary diffusion pathways. In terrestrial fossil bones, REE concentrations increase at the edge of osteons and decrease toward the center of the osteon canals. However, in marine fossil bones, REE concentrations increase toward the center of osteon canals. In terrestrial bones, REE are strongly fractionated with depth, producing signatures varying from light-REE enriched at the surface to middle-REE depleted at depth. However, depth fractionation of REE is much less pronounced in marine bones. These differences in REE fractionation are consistent with greater influence of multiple secondary REE/TE diffusion paths in these marine samples. Minimum periods of lanthanide diffusion in the bones were calculated from concentration gradients using the diffusion adsorption (DA) model. Calculated periods of diffusion (fossilization) in terrestrial environments are ca. 5-70 ka (based on a wetness factor of 0.3). Periods of diffusion (fossilization) for marine environments range from ca. 1 to 4 ka. If these values are representative, diffusion-fossilization periods may be shorter in marine/lacustrine/spring environments, possibly due to constant water saturation. The fossilization process may be strongly affected by paleoenvironmental and hydrologic conditions. If primary periods of stable isotope incorporation in bone are similar to those of trace element diffusion, paleoenvironmental interpretations based on these isotopes must consider potential paleohydrologic influences.