AMS 14C dating of bioapatite: advantage and disadvantage
Dr Alexander Cherkinsky1, Dr. Gurazada V. Ravi Prasad1, Mrs Hong Sheng1
1University of Georgia, Athens, United States
Since the early days of radiocarbon dating, the analysis of bone material has been a problem due to frequent discord between the dates of the bone material and associated charcoal, and/or between different fractions isolated from a single bone. Recent publications show that such problems still exist and suggest that they are caused by both the poor preservation of collagen and the difficulties in removing contaminants from the bone extracts used for dating.
Most methods of bone preparation for 14C dating used today are designed to extract and purify (with varying degrees of success) a fraction of the organic residue. In general, the goal of these methods is to isolate collagen or some individual compounds such as protein or amino acids of collagen. However, for poorly preserved bones, the problem becomes acute, as they often do not even have collagen for AMS dating. The mineral fraction does not usually undergo microbiological decomposition, hydrolysis, denaturation, and dissolution over millennium scales, so that only exceptional conditions do. Quantities sufficient for analysis survive into the Pleistocene, the exception is permafrost condition where collage could be well preserved for the tens and even hundred thousand years. However, it may be exposed to isotopic exchange with environmental carbonates. The problem thus becomes one of separating the diagenetic carbonates without destroying the bioapatite.
We have used the technique of treating with the weak acid with periodical evacuation and re-pressurization of bone sample to remove the secondary carbonates. Then pretreated samples were treated with 100% phosphoric acid to recover of CO2 from bioapatite. carbon dioxide was graphitized using iron catalyst and radiocarbon concentration was measure using 0.5MV NEC tandem pelletron AMS system. We also compare the mineral and organic fractions for bones and teeth of different ages and differing degrees of preservation.
The obtained results have shown that preservation of the original mineral carbon depends on the conditions of the environment in which bone and tooth samples were buried. The samples, which have been buried in wet environments with ground water saturated by carbonates, usually have been enriched with 14C from isotopic exchange with environmental carbonates. Depletion of 14C of bone bioapatite deposited in the caves and in the limestone is also possible. However the results for the samples from arid, semi-arid and non-carbonate environments are in a good agreement with the dates on other materials.
We demonstrate that that proper pretreatment of bone and tooth samples permits the separation of diagenetic carbonates from bioapatite, as long as the carbon in these samples has not degraded completely. Both bone or tooth tissue can be used for paleodietary studies and radiocarbon dating with taking in account of the environmental condition of the burials.
I am working at the Center for applied isotope Studies, University of Georgia for 15 years, before I was a director of Radiocarbon Laboratory at Geochron Laboratories Inc in Cambridge, Massachusetts for 13 years and even before that I was head of Radiocarbon laboratory in Institute of Geography, Russian Academy of Science, Moscow.