Altered states: Effects of diagenesis on fossil tooth chemistry

Investigation of modern and fossil teeth from northern and central Kenya, u sing the ion microprobe, electron microprobe, and transmission electron mic roscope, confirms that fossil tooth chemistry is controlled not only by the diagenetic precipitation of secondary minerals but also by the chemical al teration of the biogenic apatite. Increases in the concentrations of Fe, Mn , Si, Al, Ba, and possibly Cu in fossil vs. modern teeth reflect mixtures o f apatite and secondary minerals. These secondary minerals occur in concent rations ranging from similar to 0.3% in enamel to similar to 5% in dentine and include sub-mu m, interstitial Fe-bearing manganite [(Fe3+ Mn3+)O(OH)], and smectite. The pervasive distribution and fine grain size of the second ary minerals indicate that mixed analyses of primary and secondary material are unavoidable in in situ methods, even in ion microprobe spots only 10 m u m in diameter, and that bulk chemical analyses are severely biased. Incre ases in other elements, including the rare earth elements, U, F, and possib ly Sr apparently reflect additional alteration of apatite in both dentine a nd enamel. Extreme care will be required to separate secondary minerals fro m original biogenic apatite for paleobiological or paleoclimate studies, an d nonetheless bulk analyses of purified apatite may be suspect. Although th e PO4 component of teeth seems resistant to chemical alteration, the OH com ponent is extensively altered. This OH alteration implies that bulk analyse s of fossil tooth enamel for oxygen isotope composition may be systematical ly biased by +/- 1 parts per thousand, and seasonal records of oxygen isoto pe composition may be spuriously shifted, enhanced, or diminished. Copyrigh t (C) 1999 Elsevier Scieizce Ltd.