FE³⁺ UPTAKE IN FOSSIL TEETH: SIMULATING FOSSIL DIAGENESIS IN CROCODYLIAN AND MAMMALIAN TEETH

Crocodilians, including crocodiles, alligators, and gavials, are poikilothermic quadrupeds that first evolved in the Triassic. Modern crocodilians' ubiquitous distribution through tropical and subtropical marine and freshwater systems, combined with their evolutionary stability make them useful for performing dating or geochemical analyses in these systems. Under ESR, some crocodylian fossils show strong Fe³⁺ interference signals, which obscure the hydroxyapatite dating signal. Rarely do mammalian teeth show the same Fe³⁺ interference signal, and normally only in teeth from freshwater fluvial systems. Signal subtraction to remove the Fe³⁺ interference peak allows the teeth with Fe³⁺ interference to be dated by ESR.

To simulate diagenetic Fe³⁺ uptake in fresh teeth during fossilization, ten modern crocodylian and three modern bear teeth were suspended in a 500 ppm Fe(NO₃)₃ solution for 2-10 weeks. During the experiment, the solution was replaced every two weeks to reduce algal growth. Within two weeks, all the teeth uptook Fe³⁺, which caused strong Fe³⁺ interference signals in their ESR spectra. SEM EDX analyses were performed to examine uptake rates and diffusion gradients.

The Fe uptake rate and magnitude depended on the tooth's surface:volume ratio and the tissue type. Although both bear and crocodylian teeth showed a slow growth in Fe³⁺ peak heights with time, larger surface-volume ratios probably caused more extensive uptake in the smaller teeth. In both enamel and dentine, Fe concentrations decreased exponentially with depth inside the tooth. Fe/Ca ratios were inversely correlated with distance from the surface exposed to the Fe solutions. Ca concentrations and Ca/P ratios increased exponentially with depth away from the exposed surface. Significant correlations in the elemental ratios hint that Fe may be ionically substituting for Ca, possibly at the Ca-2 site in the hydroxyapatite.