Implications of diagenesis for the isotopic analysis of Upper Miocene large mammalian herbivore tooth enamel from Chad

摘要

High resolution chemical and textural investigation of Upper Miocene mammalian tooth enamel from Chad using electron microprobe and scanning electron microscope techniques provide direct evidence of extensive diagenetic alteration when compared with some modern and contemporaneous fossil specimens. Chemical and textural changes in Chadian fossil enamel during fossilization include large amounts of secondary Fe, Si, Mn, and Al-bearing oxides - both as coarse precipitation in cracks and permeating the apatite - high fluorine contamination (i.e., apatite alteration), Ca, P, Cl, Na and Mg depletion, and sulphur contamination. Such diagenetic contamination significantly affects the oxygen and carbon isotope analysis of carbonate in apatite, altering the delta O-18 and delta C-13 biogenic signals towards more positive values. Isotopic alteration is however greater for oxygen than for carbon. Indeed, isotopic analysis of the heavily oxide and S-contaminated Chadian fossil tooth enamel samples on a classical Dual Inlet-MultiPrep system versus three Continuous Flow-IRMS mass spectrometers show great differences in both the delta O-18 and delta C-13 values. While the MultiPrep-Dual Inlet approach provides carbon and oxygen isotopic results that are indubitably altered and generally inconsistent with any biological values, isotopic analysis on Continuous Flow systems - which automatically filter out contaminating gases - allow to attain or at least greatly approach primary isotopic signals. Some of the Chadian fossil enamel samples analysed in this work come from the very important Upper Miocene TM266 fossil locality that yielded Sahelanthropus tchadensis a.k.a. "Toumai", the oldest member of the hominid clade known so far. At this site, as in others, a few or no other environmental proxies than isotopic analysis of biomaterials are available for palaeoenvironmental and palaeoecological reconstructions. Therefore, to obtain reliable environmental information from the time of this ancient Hominid, recovering uncontaminated biogenic isotopic signals is crucial and from these results it is now technically possible.