Arch Lake Woman: Physical Anthropology and Geoarchaeology

Radiocarbon Dating of the Skeleton

Radiocarbon dating of the Skeleton ✦ G eochronology studies of the burial site included radiocarbon measurements and chemical analyses of the skeleton. The primary objective was to determine the geologic age of the skeleton by directly dating the human bone. Two samples were tested for potential dating: 4 g of bone removed from the midshaft of the right femur and a right mandibular first molar. The suitability of the femur or molar for dating was assessed through quantitative amino acid analyses, which determined the quantity and quality of protein preserved. Table 3 summarizes the amino acid compositions of the human bone and tooth dentin compared to a modern sample. The protein (collagen) content of the femur is approximately one-tenth that of a modern bone; the tooth has one-tenth the protein of the ancient human femur, i.e., about 1% of modern. Modern bone and dentin contain approximately 2500 nanomoles (nmol) of total amino acids (AA) as collagen per milligram of bone. The femur contained 326 nmol AA’s/mg or 13.0% of a modern bone. The tooth dentin contained 26 nmol AA’s/mg or 1.1% of modern. Although there have been significant losses of protein from the Arch Lake human remains, the chemical composition of the remaining protein is more suitable for dating than would be indicated from the initial “nmol AA/mg” values . The amino acid composition of the bone is “collagenous”: the amino acid ratios are similar to collagen except for two amino acids, proline and glutamic acid, with proportions that are slightly altered relative to modern collagen. The tooth dentin has a collagen-derived composition because there are moderate changes in the R/1000 values of five key amino acids: hydroxyproline, aspartic and glutamic acids, proline, and arginine. The tooth’s amino acid composition is derived from collagen but not the same as collagen. Although the total amount of protein is significantly reduced, the amino acid spectrum of this protein is relatively well preserved despite oxidative diagenesis during burial. Both the cortical bone and tooth dentin were suitable for accurate 14 C dating because their remnant protein resembled collagen. However, too few 23 Study of the Arch Lake Site 24 TAbLe 3. QuANTiTATive AMiNO ACid ANALySeS OF ArCh LAKe huMAN bONe ANd TOOTh SAMPLeS. AMiNO ACid MOderN bONe r/1000 Sr-5259 FeMur r/1000 Sr-5260 TOOTh deNTiN r/1000 hydroxyproline 93a 96 69 Aspartic Acid 50 55 93 Threonine 19 18 21 Serine 33 13 18 Glutamic Acid 79 75 105 Proline 115 145 96 Glycine 327 336 336 Alanine 113 115 125 valine 20 27 37 Methionine 11 0 0 isoleucine 14 11 17 Leucine 31 24 30 Tyrosine 6 0 3 Phenylalanine 14 12 14 histidine 8 0 3 hydroxylysine 8 3 8 Lysine 28 30 23 Arginine 31 40 4 Total nmols of amino acids per mg sample 2500 326 26 % of modern bone or tooth 100 13.0 1.1 Amino Acid Composition Collagen Collagenous Collagenderived a Amino acid proportions are expressed in residues per thousand (r/1000) and as nanomoles of each amino acid per milligram (nmol/mg) of modern bone or dentin. 25 grams of dentin were available, and the tooth would not have yielded enough purified amino acid carbon for AMS dating. Therefore only the cortical bone was processed further. The techniques for chemical purification and dating of bone and the multiple-fraction dating protocol are discussed in Stafford et al. (1991), Stafford (1998), and Stafford et al. (1999). Specimens such as the Arch Lake skeleton require more stringent proof of their geologic age because ancient human skeletons are extremely rare and because the collagen content was low enough that one 14 C date would not have been definitive evidence for geologic age. Dating several different chemical fractions that are progressively purer chemically enables the removal of foreign carbon contamination to be monitored. If a sample’s 14 C age increases significantly with each progressive purification step, the presence of significant amounts of markedly different exogenous compounds is indicated. If the 14 C age changes by only a few hundred years with progressive purification, or if several different chemical fractions yield the same age, less contamination is indicated, and there is increasing certainty that the final age on the chemically purest fraction is the accurate age. The cortical bone was removed by hand-sawing using a brass jeweler’s saw blade 0...