RESEARCH NOTES AND APPLICATION REPORTS NITROGEN AND FLUORINE DATING OF MOUNDVILLE SKELETAL SAMPLES

A large group of human bones were excavated during the 1930’s from the Moundville archaeological site in Alabama. If a chronology could be assigned to them, the bones might provide a guide to the changes which took place as Moundville flourished, since pottery and other objects were often buried with the individuals. A recent pottery classification based on style suggests three major time periods at Moundville (Steponaitis, 1978): Moundville I A.D. 1100-1250 Moundville I1 A.D. 1250-1400 Moundville 111 A.D. 1400-1550 Since all of the bones were treated upon unearthing with the preservative Alvar, the trade name for polyvinyl acetate, absolute dating by carbon-14 is not possible. In this preliminary study nineteen bone samples from fifteen individuals were analyzed for nitrogen and fluorine, since the concentrations of these elements will provide relative dates for the bones. Nitrogen dating is based on the fact that buried bones and teeth lose nitrogen over time. Fresh human bone contains about 5% nitrogen (Ortner et al. 1972), mostly in the form of the protein collagen. As bone decomposes, collagen breaks down into its constituent amino acids whch then leach out of the bone. Thus the nitrogen concentration decreases with time. The rate of nitrogen loss is dependent on environmental factors such as temperature, soil pH, ground water content and collagen-degrading micro-organisms (Ortner et al. 1972, Garlick, 1969, Von Endt 1979). One of the processes involved in fossilization is the accumulation of the fluoride ions in bone (Hoskins et al. 1955). Fluoride ions replace hydroxide ions in the mineral hydroxyapatite Ca,o(P04)6(0H)z. This takes place as a result of the higher molecular stability of fluoroapatite over hydroxyapatite (Hagen 1973, Tse et al. 1973). Like the loss of nitrogen, the rate of fluorine accumulation is dependent on environmental factors, such as temperature, soil pH, ground water content and soil fluorine content (Hagen 1973). In order to assume that the bones were exposed to the same environmental factors, which is a necessity for both nitrogen and fluorine dating methods, samples must be from the same vicinity. Another assumption involved in both of these dating methods is the comparability of bone types. Since fluorine diffuses into the bone through the surface, a concentration gradient is created. Thus thinner bones with a higher surface area to volume ratio will have a higher average fluorine content. In the case of nitrogen, thin porous bones probably lose nitrogen faster than more solid bones, owing to the higher water permeability. Therefore if the dimensions