铀系不平衡测年法在我国秦岭南麓黄土钙结核中的应用

秦岭南麓汉江流域上游地区分布着厚达二、三十米的黄土堆积,与秦岭以北地区黄土相比差异明显。汉江流域黄土中的钙结核分布十分广泛,钙结核内壁附着纯净致密的碳酸盐晶体,而秦岭以北黄土钙结核中却少有碳酸盐晶体析出。铀系不平衡测年是一门发展成熟、测年精度高、测年范围相对较广的第四纪重要测年方法,本文尝试利用铀系测年法对汉江流域上游柳陂黄土剖面钙结核样品内的碳酸盐晶体进行了铀系测年分析。柳陂剖面厚约17 m,可辨认出多层古土壤与黄土地层。自柳陂剖面中下部获取的4件钙结核样品测年结果分别为距今115.57±0.66 ka、130.15±0.65 ka、303.97±14.47 ka和356.40±11.60 ka,随着地层深度增加,^230Th年代增长,且与磁化率揭示的土壤地层序列基本一致。所以初步推断柳陂黄土剖面的地层序列为S1、L2、S2-I、S2-II、L3和S3。研究表明,秦岭南麓钙结核的^230Th年龄可为黄土地层的独立定年提供重要参考。根据碳酸盐钙结核形成机制,认为钙结核中碳酸盐晶体的^230Th年代能够指示相应黄土地层的最小年龄。

APPLICATION OF THE U-SERIES DATING METHOD IN THE CARBONATE NODULES OF LOESS IN THE SOUTHERN QINLING MOUNTAINS,CHINA

Loess distributed along the Hanjiang River Valley in the Southern Qinling Mountains comprises extensive carbonate nodules.These carbonate nodules enclose pure and dense carbonate crystals,which are different from that in the Chinese Loess Plateau.U-series,which can be applied to carbonate materials,is an important dating method used in the Quaternary.In this study,we used the U-series method to date the carbonate nodules in the Liubei loess section.The Liubei section(32°50'00.80"N,110°45'02.64"E;180 m a.s.l.)is located in Liubei Town,Shiyan City,Hubei Province.The depth of the Liubei section is 17 m,which could be divided into three loess and four paleosol layers.We dated the carbonate crystal using six carbonate nodule samples.However,only four samples could be dated because the other two samples failed in the chemical procedure.The four samples were dated 115.57±0.66 ka,130.15±0.65 ka,303.97±14.47 ka and 356.40±11.60 ka and increased with the section depth.Based on the ^230Th dating results and the magnetic susceptibility correlation with the Luochuan loess section,we deduce that the Liubei section is composed of S1,L2,S2-I,S2-II,L3 and S3.According to this study,we suggest that the carbonate nodules could provide an absolute age support for the loess in the southern Qinling Mountain Range.Although the carbonate nodule mechanism is complex,and the ^230Th age of carbonate crystals could indicate the minimum age of the corresponding loess deposition.