Altered states: Effects of diagenesis on fossil tooth chemistry

Investigation of modern and fossil teeth from northern and central Kenya, using the ion microprobe, electron microprobe, and transmission electron microscope, confirms that fossil tooth chemistry is controlled not only by the diagenetic precipitation of secondary minerals but also by the chemical alteration of the biogenic apatite. Increases in the concentrations of Fe, Mn, Si, Al, Ba, and possibly Cu in fossil vs. modern teeth reflect mixtures of apatite and secondary minerals. These secondary minerals occur in concentrations ranging from ~0.3% in enamel to ~5% in dentine and include sub-μm, interstitial Fe-bearing manganite [(Fe³⁺, Mn³⁺)O(OH)], and smectite. The pervasive distribution and fine grain size of the secondary minerals indicate that mixed analyses of primary and secondary material are unavoidable in in situ methods, even in ion microprobe spots only 10 μm in diameter, and that bulk chemical analyses are severely biased. Increases in other elements, including the rare earth elements, U, F, and possibly Sr apparently reflect additional alteration of apatite in both dentine and enamel. Extreme care will be required to separate secondary minerals from original biogenic apatite for paleobiological or paleoclimate studies, and nonetheless bulk analyses of purified apatite may be suspect. Although the PO₄ component of teeth seems resistant to chemical alteration, the OH component is extensively altered. This OH alteration implies that bulk analyses of fossil tooth enamel for oxygen isotope composition may be systematically biased by ±1‰, and seasonal records of oxygen isotope composition may be spuriously shifted, enhanced, or diminished.