长江三峡巫山黄土稀土元素特征及古环境

Features of rare earth elements and paleoenvironment of Wushan Loess in the Three Gorges,Yangtze River

【研究目的】巫山黄土是位于北亚热带河谷的一处代表性的粉尘堆积,具有重要的区域环境指示意义。【研究方法】本文通过系统取样,对巫山黄土的稀土元素组成特征和古气候意义进行了分析和讨论。【研究结果】结果表明:(1)巫山黄土各样品的稀土元素含量相差不大,并具有同步变化。同时轻稀土含量远远高于重稀土含量,表现为轻稀土富集,重稀土亏损;(2)巫山黄土具有从老到新∑REE逐渐减小的特点,这可能与黄土风化作用的强度不同有关;(3)剖面中不同深度样品的REE分布模式具有相似性,且特征值δCe、δEu、La/Yb、Gd/Yb和ΣREE也非常相近;(4)所有样品稀土元素配分曲线均具有负斜率,La-Eu曲线较陡,Eu-Lu曲线较平缓的特点,揭示了轻重稀土元素之间具有一定的分异作用;(5)巫山黄土δEu值分布在纵轴的0~1区间,具有明显的Eu负异常。而δCe却分布在1附近,没有表现出明显的Ce异常,表明沉积物遭受的风化作用并不强。【结论】巫山黄土并未经受较强的化学风化作用,堆积时该区的气候晚期比早期更为干冷,且在堆积过程中没有发生明显的物源改变。

This paper is result of environmental geological survey engineering.[Objective]The representative dust deposit,the Wushan Loess located in China's northern subtropical river valleys,is a significant indication of environment near the Three Gorges region.[Methods]In this study,we collected samples from the Wushan Loess and analyzed rare earth elementary composition to discuss paleoclimatic significance of Wushan loess.[Results]Our new data show the four following implications.(1)The REE contents in each sample changes synchronously with little difference.The contents of LREE is much higher than that of HREE in each sample,suggesting the enrichment in LREE and loss in HREE.(2)The increasing of ∑REE with age in Wushan Loess is probably related to the strength of weathering the in Wushan Loess.(3)For samples from various depth,REE distribution patterns,δCe,δEu,La/Yb,Gd/Yb and ΣREE do not change significantly with depth.(4)The REE distribution patterns of all samples display negative slope,steep La-Eu curves and flat Eu-Lu curves,which suggests that certain differentiation between LREE and HREE has occurred.(5)The δEu values of Wushan Loess distributed within the interval of 0-1 on the vertical axis show obvious negative Eu anomalies andstable δCe values around 1.This implies that sediment in the upper Yangtze River did not suffer strong weathering in the period of Wushan Loess.[Conclusions]Wushan Loess have not been through intense weathering effect,the climate of its region while accumulation was colder and drier in the later period than it was in the earlier period,and no obvious change has been found in the source of its materials during the accumulation process.