Buried bones

Most bone is, at present, dated by radiocarbon dating the carbon-containing components of the bone (see Textbox 52). Relatively large samples are 

Uranium Series Dating. Reliable dating of bone is possible when using fhe fechnique known as uranium series dating (see Textbox 16). The technique, which is also based on measuring relative amounts of uranium, makes possible dahng very old bones, beyond the range that can be dated with radiocarbon, that is, over 40,000 years and up to 500,000 years old (Schwarcz 1997 Ivanovich and Harmon 1992). 

Fiuorine Dating. Probably the oldest scientific dating technique is fluorine dating, which, although seldom used today, is discussed here because of its historic interest. Fluorine dating of bone centers on an irreversible process whereby the inorganic component of buried bone is slowly and gradually, transformed into a more stable compmmd (see Textbox 67) (Carnot 1893 Middleton 1845). 

Sample Maximum age (years) Fiuorine content (%) Fluorine phosphate ratio 

Once fluoride ions react with bone, they are not easily dissolved out or exchanged by other elements. If bone is buried for long periods of time, the relative amount of fluorine in the bone gradually increases as a function of time the fluoridation process continues until the maximum amount of fluorine (necessary to convert all the hydroxyapatite to fluorapatite) is reached. The total concentration of fluor in carbonated fluorapatite can reach levels as high as above 3%. There is ample room, therefore, for an increase in the relative amount of fluorine in buried bone. Determining the relative amount of fluorine in buried bone may thus serve as a tool for dating bone. 

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The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

Millard, A.R. and Hedges, R.E.M. 1995 The role of the environment in uranium uptake by buried bone. Journal of Archaeological Science 22 239-250. 

Unquestionably, there are limitations to such a procedure, and because calculations for two specimens failed, they lead to an even higher deviation from the control signal. These limits have to be defined by future work. The model experiment is only an approach to collagen diagenesis in buried bone. In nature, more than one bacterial species feeds from bone protein, and a... 

Once uranium is incorporated into buried bone, shell, coral, or speleothems, the isotope uranium-235 decays, initially into the short-lived isotope (thorium-231) and then into long-lived protoactinium-231. Uranium-238, on the other hand, decays first into two successive short-lived isotopes (thorium-234 and protoactinium-234) and only then into a long-lived isotope, uranium-234 (see Fig. 12). The decay of uranium-235 to long-lived protoactinium-231 is used to date events up to 150,000 years in age that of uranium-234 (derived from uranium-238) to thorium-230 is of use for dating events within the time range 1000-500,000 years. 

FIGURE 79 Bone carving. A seventh-century b.c.e. decorative carving on bone that was inlayed in wood, Tel Malhata, Israel. Bone has been crafted into practical and decorative objects since the dawn of time. Bone carvings hold great detail, and the surface polish that can be achieved is high. Many bone-made objects have survived, partly because it was widely used, but also because buried bone is generally well preserved in many types of soil. 

Fluoride ions may be relatively abundant in groundwater at one location and practically absent in that at another site hence the rate of fluoridation of the bone (the rate of increase in the relative amount of fluorine in the bone) varies from site to site. For instance, bones buried for a short time at a site in which the groundwater is rich in fluoride may acquire much more fluorine than bones buried for a very long time at a place where there is little fluoride in the groundwater. Therefore, fluorine analysis does not provide a tool for estimating the absolute age of buried bone, but only for dating bones at the same site, comparative to each other. The relative amount of fluoride in buried bone at a particular site thus provides a clue as to the length of time the bone has been buried. 

Summarizing, it can be said that the accumulation, depletion, or alteration of some compounds or elements in buried bone take place at definable functions of time. Determining the relative amounts of such compounds or elements in bone provides, therefore, information about their age. The information thus obtained should, however, be interpreted with caution and any inherent errors taken into consideration. 

Figure 1. Ca and P concentrations in Egyptian bones. Key o, samples from mummies A, buried bones O and buried samples corrected for dilution by the organic component and soil contamination.

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