RARE EARTH AND TRACE ELEMENT DIFFUSION PERIODS FOR TERRESTRIAL AND MARINE FOSSIL BONES: IMPLICATIONS FOR BIOMOLECULE PRESERVATION IN DEEP-TIME

Diagenetically introduced trace (TE) and rare earth elements (REE) in modern and fossil bones are used in numerous studies, such as, oceanic redox conditions, fossil province, mitigating fossil poaching, and forensics. During fossilization the organic and bioapatite (dahllite) fractions of skeletal remains are diagenetically altered, leading to increases in mineral size, and incorporation of REE/TE into apatite crystallites. However, little is known about the time scale of elemental uptake and the driving mechanisms for diffusive processes. Here we analyzed marine and terrestrial fossil bones to assess the time period for diffusion (fossilization) and to better understand diffusion processes in fossilized material.

REE/TE concentrations were measured by LA-ICP-MS in three Miocene cetacean and dugongid (marine) bones collected in situ on the eastern seaboard, as well as five Eocene brontothere bones collected in/near Toadstool Park, NE.

REE/TE concentrations are high at bone surfaces, decrease inward, are generally smooth for most elements, and are consistent with diffusion/adsorption models. Regions of higher concentrations are associated with Haversian systems, suggesting multiple pathways for diffusion within bone. Diffusion periods were calculated using Fick’s second law, assuming diffusion from two bone surfaces, and using recently measured bone diffusion/adsorption values.

Diffusion periods differ between bones from the same bonebed accumulation, as well as between marine and terrestrial specimens. Diffusion periods for brontotheres range from ~20-1000 ka, with the exception of a highly silicified brontothere bone having a diffusion period of ~3 ka. In marine specimens, diffusion periods only ranged from ~20-40 ka. Shorter periods of uptake in marine specimens is directly related to the wetness value (w), however even when w=1 in terrestrial specimens, diffusion periods were generally shorter in marine bones.

Previous studies have suggested that soft tissue may be preserved in fossilized material if the period of fossilization (or diffusion) is shorter than decay. Diffusion periods for terrestrial specimens appear too long for biomolecule preservation, however shorter periods in marine specimens support the prospect for soft tissue preservation in deep-time.