In Situ Cosmogenic Nuclide Burial Dating: Recent Progressesand Prospectsin Chronological Studiesof Early Hominin Sitesin China
详细信息
- 责任者：Shen Guanjun(School of Geography Science ; Nanjing Normal University ; Nanjing 210097)
- 刊名：Quaternary Sciences
- 出版年：2012
- 卷期：32(3)
- 页码：p.382-387
- issnNo：ISSN1001-7410
- isbnNo：CN11-2708/P
摘要
This paper introduces the basic principles of cosmogenic 26Al/ 10Be burial dating.The two nuclides are produced in situ in quartz at a known atomic ratio of 6.75 :1 by secondary cosmic ray particles.If quartz minerals,after a long period of exposure near the ground surface,are suddenly washed into a cave or deeply buried,then production of cosmogenic nuclides drastically slows and the inherited radionuclide concentrations decay exponentially over time.Because 26Al decays approximately twice as fast as 10Be,the 26Al/ 10Be ratio also decreases exponentially over time with an apparent half-life of 1.48Ma.This offers a means to date burial events in the range of 0.3~5.0Ma.
There are two types of uncertainties in burial dating.The first is statistcal counting error of AMS,typically with a ±1σ error of ca.3 % for 10Be and ca.5 % for 26Al,leading thus to an error of ca.120ka for age results.The second is a combination of uncertainties in physical constants including the half-lives of 10Be and 26Al,and the ratio of their production rates,which may lead to a systematic error of 60~100ka on the accuracy of burial dating.
Apart from these errors,far more important uncertainties may possibly be brought about by samples’unconformity to the prerequisites of burial dating,notably the one of "simple steady-state erosion model".This model requires that quartz minerals experienced only one exposure-burial cycle in the past ca.10 million years.The results accumulated show that the 26Al/ 10Be ratio in most soil-mantled landscapes falls near the value of steady-state erosion.However,samples with complicated burial histories do exist.The recently introduced method of isochron burial dating,21 Ne-26Al- 10Be three nuclides burial dating and inter-comparison with other well established dating methods may finally reveal whether the samples are suitable to dating.
The 26Al and 10Be concentrations of three quartzite artifacts and one quartz sand sample from Layers 8~9 of Zhoukoudian Locality 1 can be fitted into an isochron.This isochron passes the origin of the coordinate plane and the age derived from its slope is consistent within errors with the weighted mean of the four individual burial ages,providing supporting evidence for the samples’suitability to burial dating.
Cosmogenic nuclide burial dating is a radioisotopic method with its well-founded basis in physics,and it is independent of other dating methods.Its timescale fills a "blank period" in Quaternary geochronology in China.This method should be widely applicable because it requires only quartz,a mineral that is often available at hominin sites.Its potential for further development is well demonstrated by the important methodological progresses in the past few years in precisely determining the half-life of 10Be,in markedly improving the precision of AMS 26Al measurement,and in introducing isochron method and the third cosmogenic nuclide,21 Ne,into burial dating.It is expected that in the coming years the application of this dating technique will contribute substantially to addressing the issues concerning the first hominin appearance and the mode of early human evolution in China.
There are two types of uncertainties in burial dating.The first is statistcal counting error of AMS,typically with a ±1σ error of ca.3 % for 10Be and ca.5 % for 26Al,leading thus to an error of ca.120ka for age results.The second is a combination of uncertainties in physical constants including the half-lives of 10Be and 26Al,and the ratio of their production rates,which may lead to a systematic error of 60~100ka on the accuracy of burial dating.
Apart from these errors,far more important uncertainties may possibly be brought about by samples’unconformity to the prerequisites of burial dating,notably the one of "simple steady-state erosion model".This model requires that quartz minerals experienced only one exposure-burial cycle in the past ca.10 million years.The results accumulated show that the 26Al/ 10Be ratio in most soil-mantled landscapes falls near the value of steady-state erosion.However,samples with complicated burial histories do exist.The recently introduced method of isochron burial dating,21 Ne-26Al- 10Be three nuclides burial dating and inter-comparison with other well established dating methods may finally reveal whether the samples are suitable to dating.
The 26Al and 10Be concentrations of three quartzite artifacts and one quartz sand sample from Layers 8~9 of Zhoukoudian Locality 1 can be fitted into an isochron.This isochron passes the origin of the coordinate plane and the age derived from its slope is consistent within errors with the weighted mean of the four individual burial ages,providing supporting evidence for the samples’suitability to burial dating.
Cosmogenic nuclide burial dating is a radioisotopic method with its well-founded basis in physics,and it is independent of other dating methods.Its timescale fills a "blank period" in Quaternary geochronology in China.This method should be widely applicable because it requires only quartz,a mineral that is often available at hominin sites.Its potential for further development is well demonstrated by the important methodological progresses in the past few years in precisely determining the half-life of 10Be,in markedly improving the precision of AMS 26Al measurement,and in introducing isochron method and the third cosmogenic nuclide,21 Ne,into burial dating.It is expected that in the coming years the application of this dating technique will contribute substantially to addressing the issues concerning the first hominin appearance and the mode of early human evolution in China.
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