USING FOSSIL BONE GEOCHEMISTRY AND AGE REPORTS TO RECONSTRUCT DIAGENESIS AT THE PIG DIG SITE, BADLANDS NATIONAL PARK, SD

The Conata Picnic Ground Site of Badlands National Park (Pig Dig) was a prolific Orellan assemblage excavated between 1993 and 2008. While a geochemical analysis has been conducted on rocks from the site, the geochemistry of the fossils has not been previously described. To decipher fossil geochemistry, tooth wear and X-ray fluorescence (XRF) analyses were conducted on specimens from the Pig Dig.

Measures of individual animal ages (age reports) using comparative tooth wear analysis were used to separate the large (Archaeotherium, Subhyracodon) and small (Leptomeryx, Mesohippus) specimens from the site into sub-full-grown, full-grown, and old-aged mass bins. Using pre-existing mass estimates for the taxa, the total masses of organic matter (i.e., soft tissues) for each taxon were calculated. Masses were then used to infer the release of carbon, nitrogen, and iron (Fe), resulting from the decomposition of that previously living soft tissue mass. These nutrient release analyses revealed that Archaeotherium contributed the most nutrients to the locality upon decomposition, and Leptomeryx contributed the least.

XRF analyses were also completed to quantify fossil and host rock geochemistry from the site, and revealed significant enrichment of magnesium (Mg), aluminum (Al), silicon (Si), and Fe in the fossil bone and teeth compared to modern bone and teeth. However, no significant differences were found between the large and small taxa using major element chemistry. XRF analyses also revealed that the rock samples were enriched in Fe, while the fossil samples were enriched in calcium and phosphorus, and that the bone-bearing layer of the Pig Dig was significantly depleted in Si, Fe, Mg, and Al when compared to the surrounding layers.

Because the XRF results did not support geochemical differences between the large and small taxa shown by the nutrient release estimates, other factors may have influenced the uptake of elements during fossilization. These could include a high concentration of decomposing individuals which made the mass differences meaningless, or an upper limit of elemental uptake during fossilization that was reached by both the large and small taxa. Further work is encouraged to understand the impacts of decomposition on locality geochemistry and elemental uptake during fossilization.