摘要
本文报道对重庆市巫山县庙宇镇龙骨坡地层剖面的 4个动物牙珐琅化石的电子自旋共振法 ( ESR)测年结果 ,该剖面的第 7— 8层曾出土被原发掘者定为属早更新世的 ,目前有争议的人属化石和石制品。本文认为早更新世牙珐琅样品的 ESR早期铀加入模式年龄一般是真实年龄的上限 ,即年轻于真实年龄。 ESR测年结合该地层剖面的古地磁研究 ,给出该剖面第 7- 8层很可能与古地磁的奥杜威极性亚期相当 ,距今约 1 .8百万年。
Huang et al (1995) reported the discovery of fossil hominid materials (a single incisor and a broken mandible with P 4—M 1) and stone artifacts in the levels 7—8 of the Longgupo profile,Wushan county.The publication has evoked much controversy.The main objections are:(1) the phylogenetic position of the mandibular dentition is not hominid,but hominoid and (2) the incisor belongs to late Homo sapiens and is intrusive into the old deposits. \;This paper has the concern only with the dating problem of Longgupo Profile.Based on geomagnetic study of the profile and a single ESR age of a deer\|tooth from the level 4,Huang assigned the Olduvai subchron to the magnetically normal levels 7—8.However the published ESR age,0 75 Ma(Early U\|uptake) and 1 02 Ma(Lineal U\|uptake),could not exclude the possibility that the geological time of levels 7\|8 fits Jaramillo subchron(0 90—0 97Ma).\;In this work four animal tooth\|enamel samples collected from levels 2\|5 were dated with ESR method.The measured parameters and the final ESR EU and LU ages are summarized in the table 1.External gamma dose\|rate was estimated from the average U,Th and K contents of the surrounding sediments which are 3 14 μg·g -1 ,4 3μg·g -1 and 0 5% respectively.K a=0 15,radon loss=(30±15)% and moisture=(10±5)% were assumed in age calculation.The last three samples in table 1 show concordant EU ages of 1 1—1 34Ma,but sample W\|93002 is an exception,continuous U\|uptake indicated by its low 230 Th/ 234 U ratio and higher U\|content may account for its lower age.\;The authors suggest that the ESR\|EU ages of 1 1—1 34Ma serves only the lower limits of the true ages,because:(1) for samples of Earlier Pleistocene thermo\|fading of paramagnetic centres in samples should be taken into consideration;(2)the real uranium uptake pattern is always bracketed by EU and LU uptake models,if no significant recent uranium leakage from samples detected (this is the case for the samples studied in this work).Therefore the geomagnetically normal subchron of levels 7—8 should be Olduvai Subchron (or no younger than).Although the taxonomic classification of the ‘hominid’ fossils is still a matter of dispute and the certification of the artifacts is going on ,the new ESR dating results may be helpful in understanding of this site and the evolution of the Longfupo fauna.
出处
《人类学学报》
CSCD
北大核心
2000年第1期17-20,共4页
Acta Anthropologica Sinica
基金
国家自然科学基金资助项目 !课题号 :4 97731 99
