PRESERVATION OF C-13/C-12 ORIGINAL RATIO AND C-14 DATING OF THE MINERAL FRACTION OF HUMAN BONES FROM SAHARAN TOMBS, NIGER

The remains of 40 skeletons from the necropolises of Iwelen and Adrar Bous, situated in the massif of Air, southern Sahara (Niger) have been investigated for their mineral content, carbon isotope ratio and C-14 activity. The impermeability of the graves allowed for a good preservation of the bones, which are in fact devoid of secondary calcite. Furthermore, the necropolis of Iwelen gave the opportunity to study tombs which cover a time interval of about four thousand years, which is rather exceptional for sub-Saharan Africa. The good state of preservation of the material enabled the study of the geochemical evolution of bone carbonate hydroxylapatite through time, with a significant statistical resolution. All analytical data allow us to conclude that the initial C-13/C-12 isotopic ratio of the carbonate hydroxylapatite, albeit accompanied by a change in the crystallinity index, has been preserved. Given the described local conditions, the increase in the crystallinity index cannot be linked to exchanges with the environment but clearly reflects the decrease of CO3 radicals in the bone material. A correlation is suggested between the crystallinity index and the residual content of organic matter. The C-14 dates obtained for the carbonate hydroxylapatite are identical to those of the material of comparison (leather, charcoal and associated collagen samples). We can therefore conclude that the potential carbon isotopic exchanges--either by adsorption of by substitution--between the carbonate hydroxylapatite on the one hand and the total dissolved inorganic carbon (TDIC) on the other, are too insignificant to be detectable through analysis. These results can be explained by the combining of two favourable factors: the waterproof nature of the graves and the arid climate. The dating of carbonate hydroxylapatite in the restricted conditions already described opens new prospects for establishing a chronology of the principal funerary monuments of the Sahara.