IMPLICATIONS FROM DIFFUSION MODELING OF TRACE ELEMENT UPTAKE BY BONES FROM THE STANDING ROCK HADROSAUR SITE, HELL CREEK FORMATION, CORSON COUNTY, SD

Uptake of rare earth elements (REE) and other trace elements by diffusion and adsorption during diagenesis is ubiquitous in vertebrate fossils. Any potential uses of these elements as proxies for paleoenvironmental/diagenetic conditions require determination of the amount of time-averaging in elemental uptake. As a case study, duration of trace element uptake was estimated for nine Edmontosaurus bones from the Standing Rock Hadrosaur Site, a mass death assemblage in the Cretaceous Hell Creek Formation in Corson County, South Dakota, by applying diffusion modeling to LA-ICPMS-derived intra-bone REE and uranium (U) profiles. All bones examined exhibit remarkably low REE and U concentrations, even at the outer cortex edge, and steeply declining concentrations with increasing cortical depth. Concentration-depth profiles are consistent with Fick's second law of diffusion except within secondary osteons surrounding vascular canals, which exhibit minor concentration spikes. Diffusion period estimates for individual bones, calculated using recently measured diffusion/adsorption coefficients for bone and fossil profiles of La, Ce, Nd, Gd, and U, are similar for all five elements. Although all nine fossils derive from the same small area of a single stratigraphic horizon, their calculated diffusion periods vary by more than an order of magnitude, ranging from roughly 10–167kyr. Why such variation exists remains unclear, especially considering that the bones remain unpermineralized and diffusion was therefore not hindered by closure of pore space. In fact, our cumulative data are consistent with a single phase of early trace element uptake. Low permeability of the encasing mudstone may have limited uptake and minimized long-term uptake through late diagenesis. When using REE or isotopic signatures for reconstruction of the paleoenvironment or diagenetic regime(s), duration of uptake constrains the temporal resolution of any interpretations. Therefore, bones yielding the shortest diffusion periods should be prioritized as they presumably provide the greatest temporal resolution. In addition, brief and rapid trace element uptake would imply minimal interactions with groundwater, implying high preservation potential for endogenous soft tissues and biomolecules in bones exhibiting brief uptake.