Bone uranium

The previously proposed uptake models were mathematical assumptions and had no physical or chemical basis. Millard and Hedges, on the other hand, considered the chemistry of bone-uranium interactions. With the D-A model, they proposed that U was diffusing into bone as uranyl complexes, and adsorbing to the large surface area presented by the bone mineral hydroxyapatite (Millard and Hedges 1996). Laboratory experiments showed a partition coefficient between uranyl and hydroxyapatite under oxic conditions of 10" -10, demonstrating U uptake in the U state without the need for reduction by protein decay products as proposed by Rae and Ivanovich (1986). 

More information is needed on the absorption of various forms of uranium in young animals. Also, studies are needed on whether maternally stored bone uranium is mobilized to blood during pregnancy and lactation and whether this can increase exposure to the fetus and neonate. Child health data needs relating to exposure are discussed in Section 5.8.1, Data Needs Exposures of Children. 

Uranium -series dating of bone. Uranium concentrations in living bone are typically 0.1 ppm (Aitken 1990), whereas concentrations in fossil bone may exceed 1000 ppm. Theoretically, therefore. Quaternary age bones could be dated directly by the U-series method. 

The isotope molybdenum-99 is produced in large quantity as the precursor to technetium-99y, a radionucleide used in numerous medical imaging procedures such as those of bone and the heart (see Medical imaging technology). The molybdenum-99 is either recovered from the fission of uranium or made from lighter Mo isotopes by neutron capture. Typically, a Mo-99 cow consists of MoO adsorbed on a lead-shielded alumina column. The TcO formed upon the decay of Mo-99 by P-decay, = 66 h, has less affinity for the column and is eluted or milked and either used directly or appropriately chemically derivatized for the particular diagnostic test (100). 

Studies show that the main sites of uranium deposition ate the renal cortex and the Hvet (8). Uranium is also stored in bones deposition in soft tissues is almost negligible. Utanium(VI) is deposited mostly in the kidneys and eliminated with the urine whereas, tetravalent uranium is preferentially deposited in the Hvet and eliminated in the feces. The elimination of uranium absorbed into the blood occurs via the kidneys in urine, and most, - 84%, of it is cleared within 4 to 24 hours (8). 

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. 

Ayliffe LK, Veeh HH (1988) Uranium-series dating of speleothems and bones from Victoria Cave, Naracoorte, South Australia. Chem Geol 72 11-234... 

Yuan SX, Chen TM, Gao SJ, Hu YQ (1991) Study on uranium series dating of fossil bones from Zhoukoudian sites Acta Anthropol. Sinica 10 189-193 (in Chinese)... 

Bones and teeth, however, are primary archaeological materials and are common to many archaeological sites. Bones bearing cut marks from stone tools are a clear proxy for human occupation of a site, and in the study of human evolution, hominid remains provide the primary archive material. Hence, many attempts have been made to directly date bones and teeth using the U-series method. Unlike calcite, however, bones and teeth are open systems. Living bone, for example, contains a few parts per billion (ppb) of Uranium, but archaeological bone may contain 1-100 parts per million (ppm) of Uranium, taken up from the burial environment. Implicit in the calculation of a date from °Th/U or Pa/ U is a model for this Uranium uptake, and the reliability of a U-series date is dependent on the validity of this uptake model. 

The most commonly employed model is the early uptake (EU) model, where U is deemed to have been taken up sufficiently shortly after burial for the bone to approximate to a closed system. Justification for the validity of early uptake seems to have stemmed from Szabo s (1979) suggestion, later elaborated on by Rae and Ivanovich (1986), that Uranium is fixed in the bone in the U oxidation state, facilitated by the reduction of U by decay products of the organic phase of bone, collagen. Since the bulk of collagen is lost rapidly from the bone (on the U-series time-scale at least), it is assumed Uranium will be taken up rapidly, and then uptake will cease. 

An alternative to the early uptake assumption, linear uptake, assumes that bones and teeth continue to take up Uranium at a constant rate (Ikeya 1982 Bischoff et al. 1995), giving a U-series date something over twice that calculated using the EU assumption. Although more common for teeth than for bone, both EU and LU dates are often quoted for the same sample, with the implication that the true age of the sample probably lies somewhere in between. 

The D-A model predicts the distribution of uranium and U-series isotopes across a bone section (Figs. 3 and 4). Under constant conditions Uranium is diffusing from the inner and outer surfaces of the bone, giving a u-shaped Uranium concentration profile that gradually flattens with time to a uniform uranium distribution when the bone reaches equilibrium with the uranium in the groundwater. Because the uranium is equilibrating with the outer portions of the bone section first, closed system U-series dates approach the true age of the bone towards the surfaces, but are underestimated towards the centre. Further details of the D-A model are given in the Appendix. 

Bischoff JL, Rosenbaner RJ, Moench AF, Ku T-L (1995) Uranium series age equations for uranium assimilation by fossil bone. Radiochim Acta 69 127-135... 

Griin R, Thome A (1997) Dating the Ngandong humans. Science 276 1575 Hille P (1979) An open system model for uranium series dating. Earth Planet Sci Lett 42 138-142 Ikeya M (1982) A model of linear uranium accumulation for ESR age of Heidelberg (Mauer) and Tautavel bones. Jap J App Phys (Lett) 21 690-692... 

Rae AM, Hedges REM, Ivanovich M (1989) Further studies for uranium-series dating of fossil bones. Appl Geochem 4 331-337... 

Rae AM, Ivanovich M (1986) Successful application of uranium series dating of fossil bone. Appl Geochem 1 419-426... 

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. 

Uranium Series Dating. Reliable dating of bone is possible when using the technique known as uranium series dating (see Textbox 16). The technique, which is also based on measuring relative amounts of uranium, makes possible dating 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). 

Millard, A. R. (1993), Diagenesis of Archaeological Bone The Case of Uranium Uptake, Oxford Univ. Press, Oxford, UK. 

Ruffed grouse, Bonasa umbellus near uranium tailings discharge, Canada, Elliot Lake, 1987-88, Ra Bone vs. gut contents Liver vs. muscle... 

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