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《QUATERNARY SCIENCES》 1995-02
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PROGRESS AND PROBLEMS IN QUATERNARY DATING

Chen Tiemei (Department of Archaeology, Peking Unibersity)  
It has been about 50 years for different radiometrical and derivative dating methods to be applied in succession to QuaternarY research. A basic chronological framework has been established for Quaternary. However, further study needs highly accurate and precise time-scales. To understand the mechanism of global climate change, slight deviations from synchronism such as differential climate changes between oceanic and continental areas or between the two hemispheres, must be detailedly detected. To shed light on the more than two thousand year-lasting debate on the chronology of the first three dynasties in Chinese history, a precision of better than 50 years in radiocarbon dating is needed. Therefore high reliability and high precision are a centre issue in Quaternary dating. The last decade has seen some important improvements common to van ons dating techniques.1. Tiny sample quantity. The accelerator-mass-spectrometry 14C (AMS14C)technique only demands one milligram of carbon, while conventional 14C dating techniques require several grams of carbon. The mass-spectrometry uranium series(MU)method and the laser heating single grain 39Ar-40Ar method demand very tiny sample quantity, too. These methods simplify procedures of sampling and the mechanical or chemical separation of samples to allow some objects which can not be dated by conventional methods to be dated.2.High efficiency. An AMS14C lab can usually date 2 000-3 000 samples per year, while a conventional one is able only to deal with 200-300 samples per year.The MU method shortens the measurement time of one sample from decades of hours took by conventional methods to 2-3 hours. At the Geochronological Centre of Berkeley, for example, the 39Ar-40Ar system can load 289 samples each time and takes only about ten days to finish their dating. Automatization and computer-control raise the efficiency of the thermoluminescence (TL) dating system. The high efficiency much reduces random errors in dating. Generally, the more the dating data of a geological object, the higher the reliability of dating.China's Quaternary dating has made significant progress 5 too. The AMS14C system at Peking University works routinely with a precision of about 1% and a background of 0.006MC. The Academy of Atomic Energy successfully measured the growth rate of manganese nodules with AMS19Be; the academy finished the setting of its 36Cl beam line. 39Ar-40Ar laser microprobes are working in several institutes. Use of such microprobes has made it clear that the abnormal high ages obtained previously by the K-Ar method for Quaternary lava aer due to the presence of trapped grains in lava. A new Finnigen MAT 262 massuspectrometer will be introduced into University of Science and Technology in 1995. The institute of Hydrology, Ministry of Geology and Mineral Resources and the Institute of Geology,State Bureau of Seismology are setting up their optic stimulated luminescence (OSL)system.Much attention has been paid to highly precise dating of loess, paleosol and lake sediments of Quaternary as they contain the precious records of palaeoclimate changes. The variations of 10Be concentration and magnetic susceptibility in Quaternary loess-paleosol profiles have shown to correlate with the variation of oceanic δ18O. The correlation can be used as a derivative time-scale for those profiles. However the derivative time-scale is based on a hypothesis that global climate changes have a exact synchronism, but the hypothesis itself remains still to be proved. Usually,the upper parts of those profiles are dated with the 14C method, but the accuracy and precision of dating are not perfect. Radiocarbon dating of microfossils such as leaves, seeds, animal remains, pollen and phytolith may improve the situation. The thermoluminescence(TL) method extends the dating range beyond the radiocarbon dating limit, but the cross-check between TL and 14C dates for the young samples should be done to set up all adequate TL experimental procedure.At present, unfortunately, ages of sediments earlier than Middle Ple
【Fund】: 国家自然科学基金
【CateGory Index】: P534.6
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