Diagenetic stability of non-traditional stable isotope systems (Ca, Sr, Mg, Zn) in teeth – An in-vitro alteration experiment of biogenic apatite in isotopically enriched tracer solution

Stable isotope ratios and trace element concentrations of fossil bones and teeth are important geochemical
proxies for the reconstruction of diet and past environment in archaeology and palaeontology. However, since
diagenesis can significantly alter primary diet-related isotope signatures and elemental compositions, it is
important to understand and quantify alteration processes. Here, we present the results of in-vitro alteration
experiments of dental tissues from a modern African elephant molar reacted in aqueous solutions at 30 °C and
90 °C for 4 to 63 days. Dental cubes with ≈ 3 mm edge length, comprising both enamel and dentin, were placed
into 2 mL of acidic aqueous solution enriched in different isotopes (²⁵Mg, ⁴⁴Ca, ⁶⁷Zn, ⁸⁶Sr, initial pH 1). Element
and isotope distribution profiles across the reacted cubes were measured with LA-(MC-)ICP-MS and EMPA, while
potential effects on the bioapatite crystal structure were characterised by Raman spectroscopy. In all experiments
isotope ratios measured by LA-(MC-)ICP-MS revealed an alteration of the enamel in the outer ≈ 200–300 μm. In
contrast, dentin was fully altered (≈ 1.4 mm) after one week at 90 °C while the alteration did not exceed a depth
of 150–200 μm during the 30 °C experiments. Then, the tracer solution started also to penetrate through the
enamel-dentin junction into the innermost enamel, however, leaving the central part of the enamel unaltered,
even after three months. The Raman spectra suggest an initial demineralisation in the acidic environment while
organic matter (i.e. collagen) is still preserved. In the 90 °C experiment, Raman spectra of the v₁(PO₄) band of the
dentin shift over time towards synthetic hydroxylapatite patterns and the Ca (and Sr) concentrations in the
respective solutions decrease. This indicates precipitation of newly formed apatite. Isotope and element concentration
profiles across the dental tissues reveal different exchange mechanisms for different isotope systems.
Magnesium is leached from enamel and dentin, while Zn is incorporated into the apatite crystal structure.
However, the distribution of both elements is not affected in the innermost enamel where their concentrations do
not change over the whole duration of the experiments. We found no correlation of reaction depth in the cubes
and experimental duration, which might be caused by natural variability of the dental material already at the
beginning of the experiment. Our alteration experiments in a closed system at high temperatures ≤90 °C and low
initial pH demonstrate that at least the central part of mm-thick mammalian enamel apatite seems to be resistant
against alteration preserving its pristine bioapatite mineral structure as well as its in-vivo elemental and isotopic
composition. The experiments assess diagenetic alteration in a novel multi-proxy approach using in-situ analyses
in high spatial resolution. It is demonstrated that the isotopes of Ca, Sr, Zn and Mg in the dentin are prone for
diagenetic alteration, while enamel is more resistant against alteration and could be used for dietary and
physiological reconstructions in fossil teeth.