南京下蜀土的地球化学特征及其物源指示意义

GEOCHEMICAL CHARACTERISTICS OF THE XIASHU LOESS-PALAEOSOL SEQUENCE IN NANJING AND THEIR IMPLICATIONS FOR PROVENANCE

风尘堆积的物源研究对于揭示物源区的环境演化状况、重建古风场强度和古大气环流格局等都具有重要的意义。目前长江下游地区下蜀土的物质来源问题仍然存在争议。为了解决这一争议问题,本文以南京泰山新村下蜀土剖面为研究对象,开展了系统的地球化学研究。通过对南京下蜀土的常量元素、微量元素、Nd同位素进行测试,并与黄土高原同期黄土进行对比分析,探讨其对长江下游下蜀土的物源指示意义。南京下蜀土与北方黄土具有相似的常、微量元素UCC标准化曲线和稀土元素球粒陨石标准化曲线,但是它的Zr、Hf含量较高,Tl、Pb含量较低,且具有较高的SiO2/Al2O3、TiO2/Al2O3、Nb/Ta和GdN/YbN,较低的SiO2/TiO2、Zr/Hf、Y/Ho、Lu/Hf和εNd(0)值,说明南京下蜀土的物质来源明显有别于黄土高原黄土。南京下蜀土与长江中游下蜀土在地球化学特征方面有较多的相似性,指示其物质来源与中游下蜀土的物质来源相似,主要来源于长江中下游地区的松散沉积物。

The Xiashu loess is a typical Quaternary eolian deposit widespreading in Southern China and provides long terrestrial records of paleoenvironmental change in the subtropical region.Determining its provenance is of great significance for the reconstruction of paleo-atmospheric circulation pattern in this area.Previous studies were mostly conducted in the areas near the middle reaches of the Yangtze River,the provenance for the Xiashu loess in the lower reaches of the Yangtze River remains controversial.Here,we try to determine this provenance based on geochemical analysis,which is one of the important methods to judge the sources of eolian deposits.A Xiashu loess section(Taishanxincun:32°09'40.8"N,118°42'57.2"E)in Nanjing was analysed on their major-trace elements and Nd isotopic compositions to explore the provenance.This section is about 15.6 m in thickness,and contains three loess layers and three paleosol layers.Four loess samples were collected from L1,L2 and L3,and seven paleosol samples were collected from S1,S2 and S3 at the interval of 1.5 m.The results were compared with the data from the Lingtai section(34°59'N,107°45'E)formed at the same age in the Chinese Loess Plateau.We observed both similarities and obvious differences in the loess from these two places.The Nanjing Xiashu loess has similar UCC-normalized major-trace element abundances and chondrite-normalized rare earth element abundances as the Lingtai samples.However,the Nanjing Xiashu loess has relatively high abundances of Zr,Hf and ratios of SiO2/Al2O3,TiO2/Al2O3,Nb/Ta,GdN/YbN and low abundances of Tl,Pb and ratios of SiO2/TiO2,Zr/Hf,Y/Ho,Lu/Hf,εNd(0),which demonstrates a different provenance from that of the loess in the Chinese Loess Plateau.The Nanjing Xiashu loess has many similar geochemical characteristics as the Xiashu loess in the middle reaches of the Yangtze River.They both have high abundances of Zr,Hf and ratios of SiO2/Al2O3,TiO2/Al2O3,Nb/Ta and low abundances of Tl,Pb and ratios of Y/Ho,Lu/Hf,εNd(0),indicating a close similarity between their provenances.The loose sediments located nearby may be the primary dust source for the Xiashu loess in the lower reaches of the Yangtze River.