A Ground-Truthing Study of the Nitrogen Isotopic Composition of Enamel Bound Organic Matter in Modern Bison Teeth

Abstract

This project develops enamel bound organic matter nitrogen isotope analysis for terrestrial paleoclimate reconstruction. Stable isotope ratios of organic N can provide insight into environmental processes because the 15N/14N of fixed N responds to different ecological and climatological conditions. The nitrogen isotopic composition of bone and tooth collagen has been used in other studies to reconstruct the ecology and environment of the past, yet enamel has been unused for organic isotopic analysis due to its low organic content even though it has the potential to be particularly resistant to diagenesis. This project analyzes modern bison tooth enamel from sites in the western United States with the purpose of developing a better understanding for how this enamel bound organic matter δ15N measurement may reflect the overall environmental δ15N and determining which environmental processes are the main drivers of variations in isotopic composition. δ15N records the availability of water in an ecosystem which varies with precipitation and temperature. The fact that stable nitrogen records a metric that is directly relevant to ecosystems makes it possibly more informative than other isotopes. Results from this ground-truthing study show that fractionating loss mechanisms are predominant in dry ecosystems and result in higher δ15N, while wetter environments have lower δ15N as hydrological loss increases in relative importance. It also shows that environments with high denitrification rates have more variability in δ15N values.