Beyond Lifetime Averages: Tracing Life Histories through Isotopic Analysis of Different Calcified Tissues from Archaeological Human Skeletons

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Isotopic measurements on human skeletons

The ratio of 13C to 12C, 15N/14N, 87Sr/86Sr and many other isotopes varies in a patterned fashion in the geosphere and biosphere. The composition of body tissues derives from foods eaten in life, so that isotopic analyses of archaeological human bones offer clues to aspects of diet and lifestyle. There has been a good deal of work on stable carbon and nitrogen isotope ratios in human skeletons (recent reviews include Schwarcz & Schoeninger (1991) and Katzenberg (1992)). Strontium isotope ratio measurements have been less widely applied. Isotope analyses are generally of post-cranial bone, especially ribs, since destructive analysis of ribs involves relatively little loss of morphological information. The most commonly used material for carbon and nitrogen analysis is bone collagen, the structural protein that makes up approximately 20% of fresh bone.

In a study of the isotopic ratios of different skeletal elements in animals raised on controlled diets, the 13C/12C and 15N/14N compositions of femora and humeri were identical (DeNiro & Schoeninger 1983). Schoeninger (1989b) compared carbon and nitrogen isotope ratios in vertebrae and femora of recent human skeletons buried in permafrost, and found no differences within skeletons. Available data also reflect negligible variations in isotope ratios due to sex (DeNiro & Schoeninger 1983; Lovell et al. 1986; Tieszen et al. 1989) and age (Hobson & Schwarcz 1986; Lovell et al. 1986; Tieszen et al. 1989).

The chemical constituents of teeth and bones are similar, but their respective histories of formation are very different. Tooth enamel forms in childhood, and does not undergo subsequent remodelling. Enamel is composed principally of hydroxyapatite, with little protein (LeGeros 1983). Dentine, a major component of teeth, forms mostly during childhood: primary dentine is laid down at the time of initial formation of the tooth. Later in life, a small amount of secondary dentine is added, mostly around the edges of the pulp cavity. Tertiary dentine may also be deposited in response to injury or decay (Hillson 1986). Dentine is rich in collagen, and isotope measurements of dentine collagen reflect diet, principally the diet consumed in childhood.

The first adult teeth to begin forming are first molars, which start to mineralize at birth, and complete their growth between 11 and 12 years of age. Third molars start to form in children of about seven, and the roots continue to grow in individuals in their early 20s (Hillson 1986). Different teeth therefore have isotopic compositions which reflect diet at different times of an individual's early life. Bone tissue, like teeth, forms early in life, but unlike teeth it 'turns over', i.e. it is resorbed and replaced, throughout life. The rate of turnover in healthy individuals is not precisely known. It varies at different times of life, slowing down in later years, and is also affected by nutritional and health status. Bone collagen in adults includes material accumulated over more than 10 years (Libby et al. 1964), so that its isotopic composition reflects a long-term dietary average. The vast majority of published stable isotope values for archaeological human skeletons represent such integrations. Dense post-cranial bone, such as that found in the shafts of the long-bones, remodels relatively slowly, and contains tissue deposited over many years. Spongy or cancellous bone, with its thin bony structures fed by abundant blood vessels, is likely to turn over faster than compact bone. Skeletal elements with a high proportion of cancellous bone, such as ribs, probably contain more material from the later part of an individual's life than do the shafts of long bones (Stenhouse & Baxter 1976). Thus different skeletal elements preserve records of diet and residence patterns at different stages of an individual's life.

Natural distributions of isotopes

Stable carbon isotope ratios (13C/12C) have been used as an index of the relative importance of [C. …