Paper No. 221-10
Presentation Time: 11:15 AM
RAPID AND BRIEF TRACE ELEMENT UPTAKE BY BONE AT THE STANDING ROCK EDMONTOSAURUS BONEBED, HELL CREEK FORMATION, CORSON COUNTY, SD: AN EXCEPTION TO LONG-TERM RARE EARTH ELEMENT UPTAKE
Recent discovery of prolonged, late-diagenetic rare earth element (REE) uptake in many fossils calls into question the mechanics of biomineral trace element incorporation. To explore this topic, we used LA-ICPMS to characterize intra-bone REE patterns at the Standing Rock Hadrosaur Site (SRHS) in Corson County, South Dakota, where traditional taphonomic features support rapid burial and brief diagenesis. REE concentrations are remarkably low, with two bones even presenting lanthanum (La) concentrations <350 ppm at the outer cortex edge. Such low concentrations are consistent with low permeability of the encasing mudstone limiting REE uptake. All bones exhibit a steep decrease in light REE concentrations with increasing cortical depth which, in tandem with low concentrations of middle REE and elements with moderate and low diffusivities through the middle cortex, strongly support rapid and brief diffusion. Comparatively flatter profiles for uranium than REE, despite similar diffusivities, support a recent hypothesis that trace element uptake occurs along a propagating recrystallization front. For SRHS, this pattern suggests that pore fluid flow and replenishment were not active, allowing fluid chemistry to change through diagenesis. This process produced fractionation trends in (La/Yb)N vs. (La/Sm)N space, namely a consistent decrease in (La/Yb)N and increase in (La/Sm)N with increasing cortical depth. We consider low variance in our ratio plots to reflect a lack of reworking rather than late-diagenetic overprinting because of universally low REE concentrations, rarity of bone modifications, confinement of bones to a single horizon, and retention of endogenous stable isotopes in teeth. Ce/Ce* and La/La* anomalies are generally absent and display minimal fluctuations with cortical depth, though La-corrected Ce/Ce** values are often slightly positive. While the latter could be caused by tetrad effects, we suggest they are reflective of oxidizing diagenetic conditions as evidenced by high bone scandium concentrations and abundant goethite concretions within the bonebed horizon. Regardless, such minimal variation supports lack of complex hydrologic regimes through diagenesis. Cumulative trace element evidence characterizes SRHS as an exception to the common trend of protracted REE uptake.