Comment on “Implications of diagenesis for the isotopic analysis of Upper Miocene large mammalian herbivore tooth enamel from Chad” by L. Jacques, N. Ogle, I. Moussa, R. Kalin, P. Vignaud, M. Brunet and H. Bocherens [Palaeogeography, Palaeoclimatology, Palaeoecology 266 (2008) 200–210]

摘要

Jacques et al. (2008) provide new trace element and stable isotope data derived from Upper Miocene mammalian enamel from Toros-Menalla, Chad. They show evidence for chemical and crystallographic changes in Chadian fossil enamel during fossilization, including precipitation of secondary Fe, Si, Mn and Al-bearing oxides, F and S contamination as well as a reduction in Ca, P, Cl, Na and Mg content relative to modern enamel. Some of these highly contaminated samples also gave anomalously elevated δ18O (and to a lesser extent δ13C) values, when measured on a standard dual-inlet (DI) MultiPrep system. In contrast more plausible isotope values were observed for the same samples analysed in a Continuous flow (CF) system. The authors conclude that CF systems, which integrate a chromatographic column and a thermal conductivity detector (TCD), provide an improved means of eliminating contaminating gases relative to DI systems, where CO2 gas is purified by cryogeny. Therefore, they propose to use this approach as a technique to “attain or at least greatly approach primary isotopic signals”. We found this method paper a valuable contribution to the recognition of diagenetic processes during fossilization of enamel apatite and their consequences on the validity of paleoenvironmental interpretations based on the geochemical composition of bioapatites. However, we believe that the conclusion about the superiority of the CF approach compared to DI method is premature, and based on incorrect information about one of the mass spectrometers. In the second part of the discussion, we distinguish between two different aspects of diagenesis, namely contamination, and alteration.