摘要
热电离质谱法可用比经典的α能谱法少一个数量级的样品,实现高一个数量级的测量精度。本文报道了用这一新技术测定北京猿人遗址1-2层钙板样BZC-3的铀系年代。据四次平行分析的加权平均值,该样应为41±1万年,代表了北京猿人五号头骨的最小年龄。四次分析(样品未混匀)的结果在误差范围内一致、α能谱二种铀系法结果一致及加速器质谱法未能检出-(14)C,表明样品有较好的封闭性。本文结果不支持亚洲直立人演化较慢和中国有二种人类(晚期直立人与早期智人)并存的假说。
The Peking Man site at Zhoukoudian in northern China presents one of the world's most important paleolithic site. A reliable chronology for the site is vital to our understanding of the physical evolution and cultural development of the human species.Earlier efforts have provided the following time frame for the strata at the site: the bottom (13th) layer-ca. 700 ka, the lowest human fossil-containing (10th) layer-ca. 500 ka and the uppermost (1-3) layers -ca. 230 ka. However, on close examinations of the dating methods used in arriving at these age estimates, one finds that certain assumptions introduced, hence the reliability of the results, have remained problematical. Furthermore, sample BZC-3, taken from an intercalated travertine in Layers 1-2, has been reported recently to have a 230Th/ 234U age of 421-54+110ka, an age much older than the previous age assignment (Shen and Jin, 1993).That relatively pure, compact and well-crystallized speleothem samples such as BZC-3 can be precisely dated and has been well documented. The large error limits quoted above result from the precision achievable by the a spectrometric measurements on samples of >350ka of age. The newly developed thermal ionization mass spectrometric (TIMS) analyses of U and Th isotopes have promised to greatly reduce these error limits. With this promise in mind, we haveinitiated a program of TIMS dating of Zhoukoudian speleothems in hopes of refining the chronology of the Peking Man.We report here the preliminary results on the TIMS dating of sample BZC-3. The dating was done using the facilities at Centre de Recherche en Geochimie Isotopique et en Geochronologie (GEOTOP), University of Q.uebec at Montreal, Canada. Ultrasonically cleaned sample grains (~ 0.5 cm in size) of translucent calcites, free of visible porosity and detrital contaminants, were carefully hand-picked for TIMS analyses, following procedures adapted from Edwards et al (1986/ 87). The 233U-236U-229Th spike used was calibrated against a Precambrian uraninite standard (HU-1).The age derived from the weighted mean of four measurements is 414 ± 13 ka. The integrity, of BZC-3 as a closed-system is indicated by (1) the consistency within analytical error of 234U/ 238U and 230Th/ 234U in four subsamples (not homogenized aliquots); (2) the concordant 230Th/ 234U and 227Th/ 230Th dates measured by a spectrometry; and (3)the undetectable amount of 14C in the sample as measured by accelerator mass spectrometry.While supporting the 421 ka age reported earlier, the present more precise age of 414 ± 13 ka for BZC-3 points to a much greater age for the Zhoukoudian upper strata than previously thought. The No.5 skull of Peking Man, a representative of the youngest Homo erect us in China, was excavated just below these strata and has been assigned an age of 230 ka. It is mainly based on this age assignment that hypotheses have been formulated concerning a possible overlapping existence in China of two species of Homo (erectus and sapians) and a possible slower evolution rate of Asian Homo erectus compared with their African counterparts. None of these hypotheses is supported by the present dating results that the youngest member of the Peking Man family should be at least 400 ka old.The techniques of TIMS as applied to 230Th/ 234U dating have begun making significant impact on refining the Pleistocene chronology. As well, their capabilities have recently extended to the measurement of 231Pa. This paper represents the first attempt at TIMS dating of speleothem from an early paleolithic site. By careful search for suitable calcite samples from Zhoukoudian stratigraphic sequence and by stringent cross-checks between 230Th/ 234U and 23lPa/ 235U signals to be acquired by TIMS on such samples, we anticipate that temporal records of Zhoukoudian and other important early paleolithic cave sites can be deciphered with greater clarity than ever before.
出处
《人类学学报》
CSCD
北大核心
1996年第3期212-217,共6页
Acta Anthropologica Sinica
基金
国家自然科学基金资助项目
资助号49363008
关键词
北京猿人遗址
遗址
年代
热电离质谱
铀系法
Peking Man site, Thermal ionization mass spectrometry (TIMS), Speleothem, U-series disequilibrium dating methods
