| 26Al/10Be burial dating, developed over the past dozen years, is a new dating method. In situ cosmogenic nuclides, including26Al and10Be, are produced when secondary cosmic-ray particles penetrate the ground surface and react with nuclei within mineral grains. The26Al/10Be ratio of about6.75remains nearly constant and is independent of varying geomagnetic latitude and altitude. If quartz grains from the surface, after a long period of exposure, are washed into a cave or buried sufficiently deep to ignore postburial production, then the inherited25Al and10Be concentrations decrease over time. As26A1decays approximately twice as fast as10Be, the Al/10Be ratio will decrease exponentially from the initial value. So the ratio can be used for dating burial events in the range of0.3-5Ma. Owing to its independence from other dating techniques and wide existence of quartz minerals, the material for dating,26Al/10Be burial dating has been widely applied to chronological studies of Early and Middle Pleistocene hominin sites.Xujiayao-Houjiayao site, discovered in the1970s, is located in the north of Nihewan Basin in Hebei and Shanxi Provinces. Twenty pieces of hominin fossils assinged to archaic Homo sapiens and a great number of stone artefacts represented by stone balls and of mammalian fossils were unearthed during several sessions of excavation. The chronological position of the site has been the topic of debate for a long time. The results from Paleontological correlation, U-series dating of fossil bones, infrared Optical Stimulated Luminescence and so on indicate an age of late Pleistocene. However, in recent years paleomagnetic study suggests that the geomagnetic reversal,3m below the cultural layer, should represent the Brunhes-Matuyama interface, in stead of Blake Episode as once considered. According to assumed sedimentation rates, these authors proposed an age of early to middle Middle Pleistocene, more precisely500ka.If the Paleomagnetic result holds, Xujiayao-Houjiayao site may be in the range of burial dating. A quartz sand sample and a quartzite gravel were taken from the lower cultural layer at a depth of12.5m at Locus74093of the site. The weighted mean of their26Al/10Be burial dates (0.01±0.12Ma and0.12±0.23Ma, respectively),0.03±0.22Ma (±2σ), indicates that the cultural deposits should not be earlier than250ka. This is at variance with the paleomagnetic dating, and should be of importance in resolving the above debate.Chemical separation and purification of Al and Be is important for getting dates from quartz. The chemical recovery and purity of Al is one of the limiting factors to high-precision age results. Yet several problems exist in Al procedure used by the Purdue Lab, namely1. grain size of the ion exchange resin is relatively large, resulting in redundant steps and unfavorable separating effect.2. H2O2and HNO3are used repeatedly to decompose H2C2O4, leading to time-consuming operation and possible cross contamination because of H2O2 decomposition. Based on results of conditional experiments, this paper proposes the following suggestions for refining the procedure:1. changing the grain size of resin and the composition of leacheate to elute Al from anion exchange column;2. recovering Al through cation exchange after measuring distribution coefficience of Al in cation exchange resin under different conditions. The result is that:using38～75μm, instead of75～150μm anion exchange resin can reduce the volume of eluting solution to20ml and improve the elution; under the condition of0.05mol/L H2C2O4-0.5mol/L HCl, Al can be quantitatively absorbed onto cation exchange resin, so it is feasible to transfer Al to cation resin, followed by eluting Al from cation column using4mol/L HCl. The whole recovery of Al, with a purity of98%, can reach91%±5%. |
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