DISCUSSIONS ON THE RELIABILITY OF U-SERIES DATING OF FOSSIL BONES FROM KARSTIC CAVES
Shen Guanjun(College of Geographical Sciences,Nanjing Normal University,Nanjing210097)
U-series dating of fossil bones has been widely applied,but its reliability has long been a subject of controversy.With the precision attainable with classical alpha spectrometry(±1σ,3.0% for 227Th/230Th,2.5% for 230Th/234U,1.0% for 234U/238U and hence 4.0% for 231Pa/235U) and according to two frequently quoted open system models,linear U addition and 10% U recent addition,this paper presents two figures comparing the real ages and corresponding apparent 230Th/234U,231Pa/235U and 227Th/230Th ages of a sample.From these comparisons,conclusions can be made that while a statistically significant discrepancy between the dates of two U-series methods attests a relatively recent and important uranium migration,their consistency within error range cannot be cited as a sufficient condition to guarantee the sample's closed-system behavior.Compared with open-air sites,limestone caves are generally considered to provide better conditions for preserving fossil bones.In the past two decades,in the course of dating Chinese archaeological sites mainly with intercalated speleothem formations,from 17 cave localities we have accumulated 46 alpha spectrometric 230Th/234U dates on fossil samples,among which 33 have been analyzed in parallel with 227Th/230Th.The results demonstrate that a greater part(25 out of 33) of these fossil samples give ages of two U-series methods consistent within ±1.5σ.However,at five sites where the dates on bones can be checked by comparing with overlying flowstone layers,three give substantially younger dates on bones,which is at variance with the depositional order.Fossil bones with calcite coating were considered as preferable materials for dating.However,four out of six samples we have studied give substantially younger dates in comparison with their encasing calcite.Based on the widely accepted reliability of relatively pure and dense speleothem calcites,bones yield mostly underestimated U-series dates in cave settings.Our results show also aberrant U-Th isotopic ratios for quite a few fossil specimens,the abnormality being difficult to account for with simple models of posterior U addition or loss.Besides,as compared with associated speleothem calcite a notable feature of fossil samples is their generally high 234U/238U ratios,of which no decreasing tendency is discernible with increasing antiquity of sites.To account for all these phenomena,we believe that fossil bones as a whole do not conform to a closed system.Rather uranium may cycle into or out of fossil bones in response to changing environmental conditions,such as pH and Eh of the surrounding pore water,and no known condition can securely protect them from being affected.If the U migration event is relatively recent and sufficiently important,two U-series methods will give discordant results.Otherwise with relatively stable geochemical conditions,a temporary equilibrium may be established,leading to apparently “good” samples with concordant dates of two U-series methods.With only two independent equations from two U-series decay schemes,it is literally impossible to solve precisely the chronological signal controlled by a variety of random parameters.Thus,in spite of its continued usefulness particularly for sites without any other dating material,the U-series dating of fossil bones should be of limited reliability.
【CateGory Index】： P597.3
【CateGory Index】： P597.3