MN UPTAKE IN FOSSIL TEETH: SIMULATING FOSSIL DIAGENESIS IN CROCODYLIAN AND MAMMALIAN TEETH

Modern teeth and bones lack significant concentrations of Mn. Like Fe, however, Mn is a common diagenetic addition to fossil teeth and bones, often as a trace element in the tissues or in secondary minerals that form in the porosity elements or on the surface. Teeth and bones sometimes show Mn signals in their ESR spectra.

To simulate diagenetic Mn uptake in fresh teeth during fossilization, ten modern crocodylian and six modern bear teeth were suspended in a 500 ppm MnCl₂ solution buffered at pH = 4.8-5.0 for 2-10 weeks. During the experiment, the solution was replaced every two weeks to reduce algal growth. SEM EDX analyses were performed to examine uptake rates and diffusion gradients. Within four weeks, both bear and crocodylian teeth uptook Mn and developed Mn-rich crusts, some with well formed acicular crystals, precipitated in scattered locations on the exposed surfaces.

For all the teeth analyzed, Mn/[Mn+Ca+P] ratios in the enamel or dentine did not exceed 10 atom%. In the secondary crusts, however, Mn/[Mn+Ca+P] ratios exceeded 50 atom%. The Mn uptake rate depended on the tooth's surface:volume ratio and the tissue type. After eight weeks immersion, Ca concentrations and Ca/P ratios increased exponentially with depth from the exposed surface. In both enamel and dentine, Mn concentrations decreased exponentially with depth inside the tooth. Mn/Ca ratios were inversely correlated with distance from the surface exposed to the Mn solutions. Significant correlations in the elemental ratios hint that Mn may be ionically substituting for Ca in the hydroxyapatite.