FLUORIDATION OF A FOSSIL HORSE TOOTH FROM THE "PRE-COLGATE ERA"

In principle, light-stable-isotope and trace-element analyses of fossil bone and tooth apatite can reveal the climate, water source, and type of vegetation experienced by the animal under study. Postmortem chemical alteration during or after fossilization, however, can invalidate such chemical interpretations. Elevated fluoride concentrations are a typical signature of chemical alteration of fossil bioapatite, suggesting that fluoridation may be an important part of over-all alteration. To understand the chemical-mineralogical reactions inherent in fluoridation, we did real-time, in-situ Raman spectroscopic monitoring of the fluoridation of a modern horse tooth and compared our experimental results with spectra obtained on a 10-kya fossil horse tooth. The latter showed extensive, but inhomogeneous, fluoridation-recrystallization in the dentin and scattered micrometer-scale altered patches in the enamel. The goal of our experiments was to understand the fluoridation process sufficiently to evaluate if any original compositional (incl. isotopic) signatures of the bioapatite might be preserved in the fossil. In our experiments, cross-sectioned wafers of a modern horse molar were soaked in phosphate-buffered solutions with NaF concentrations ranging from 0.44 mg NaF/ml solution to 2 M NaF for periods up to 7 days. A fiber-optically-coupled probe constantly delivered a 10-µm focused laser beam to a fixed point on the dentin, while a Raman spectrum was collected every 30 minutes. Experimental data suggest that fluoridation occurs in a bioapatite-water system (e.g., bone or tooth buried in soil) not by diffusive exchange of fluoride for hydroxyl ions within the apatite, but rather by new grain growth, presumably by incorporating dissolved components of original bioapatite nanocrystals. We believe that the impetus for this replacement process is the significantly lower solubility of fluorapatite compared to bioapatite. Thus, many unaltered bioapatite crystals remain even as high-fluoride crystals are deposited within the tooth. Better understanding of the specific mechanism of fluoridation-alteration will affect the choice of fossil material for chemical analysis and interpretation, as well as indicate which chemical systematics are most likely to reflect original life signatures.