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 preserv ation of the bones, which are in fact devoid of secondary calcite. Fur thermore, the necropolis of Iwelen gave the opportunity to study tombs which cover a time interval of about four thousand years, which is ra ther exceptional for sub-Saharan Africa. The good state of preservatio n of the material enabled the study of the geochemical evolution of bo ne carbonate hydroxylapatite through time, with a significant statisti cal resolution. All analytical data allow us to conclude that the init ial 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 crystallin ity index cannot be linked to exchanges with the environment but clear ly reflects the decrease of CO3 radicals in the bone material. A corre lation is suggested between the crystallinity index and the residual c ontent of organic matter. The C-14 dates obtained for the carbonate hy droxylapatite are identical to those of the material of comparison (le ather, charcoal and associated collagen samples). We can therefore con clude that the potential carbon isotopic exchanges--either by adsorpti on of by substitution--between the carbonate hydroxylapatite on the on e hand and the total dissolved inorganic carbon (TDIC) on the other, a re too insignificant to be detectable through analysis. These results can be explained by the combining of two favourable factors: the water proof nature of the graves and the arid climate. The dating of carbona te hydroxylapatite in the restricted conditions already described open s new prospects for establishing a chronology of the principal funerar y monuments of the Sahara.