新疆伊犁河谷沙漠沉积的稀土元素特征及其环境意义

RARE EARTH ELEMENT CHARACTERISTICS OF DESERT SEDIMENTS IN ILI VALLEY AND THEIR ENVIRONMENTAL IMPLICATION

以伊犁河谷西部塔克尔莫乎尔沙漠内的可克达拉（TKP）剖面作为研究对象开展了稀土元素特征分析，结合方解石变化特征，探讨沙漠沉积稀土元素与环境变化间的关系。研究结果表明TKP剖面的稀土元素总量（不含Y）平均为130．90μg／g，变化范围为32．88～238．58μg／g，整个剖面分配模式相似，缓右倾斜型，轻稀土适度富集，δCe，δEu和LREE／HREE比值相近，Eu中度负异常，未见明显Ce异常，指示剖面物质来源的一致性和总体干旱、半干旱的气候环境。包括Ce在内的各稀土元素、∑REE以及LREE／HREE在垂向变化上高度相关，并与方解石含量存在较好的相关关系，表明其与气候变化密切相关，在干旱区的沙漠沉积中稀土元素对古气候变化具有良好的指示作用，并据此重建了晚全新世以来伊犁河谷的气候演变过程。

For the similar chemical properties and little change during weathering, transportation and diagenesis, Rare Earth Elements（REE）are widely used as trace elements in provenance studies of various sediments. At the same time,REE characteristics also can record the evolution of environment in detail, therefore,they are widely used as an important indicator in studies of paleoclimate. Relationships between REE characteristics and environmental change on eolian deposits have intensively been investigated in the Chinese Loess Plateau and the middle and lower reaches of the Yangtze River, however, their relationships between REE and paleoenvironment recorded by eolian sediments in arid Central Asia are still poorly understood. Ili Valley of Xinjiang dominated by the westerlies is an ideal place for understanding the climate change of Central Asia. The Kekdala section （TKP, 43°58＇13＂N, 80°32＇39＂E, elevation 605m）is located in the hinterland of Takelmukul desert （43° 50＇00＂ -44° 15＇40＂N, 80°5＇00＂-80°54＇30＂E） in the western of Ili Valley. The section is about 360cm thick and the whole section can be divided into 10 couples of alternative sand and paleosols. OSL dating showed that it developed since 3.7ka. This paper reports REE results of TKP section and discusses the relationship between REE of desert sediments and environmental change in combination with calcite. The results show that the EREE values vary from 32. 88μg/g to 238.58 μg/g with an average value of 130. 90μg/g, slightly higher than those of modern desert and lower than those of Chinese loess in average（ 171 μg/g）. The REE distribution patterns in different units of the section are similar in shape, which are moderately rich in light rare earth element（LREE） and negative slopes. The ratios of LREE/HREE are close to each other, as well as 8Ce and 8Eu, with unclear Ce anomaly and moderately negative Eu anomaly. All these features indicate a consistence of profile material sources and arid/semi-arid environment. REE contents （including Ce）have good relationship with each other, so do EREE, LREE/HREE and calcite. These demonstrate that REE characteristics of desert sediments are closely related with climatic change and can be used as effective paleoclimatie proxies in this area. Therefore,the paleoelimate evolution of Ili valley in Xinjiang since Late Holocene was reconstructed. From 3.7ka to 2.1 ka the climate was characterized by dry with relatively warm and humid during 2.5 -2.3ka and 3.4 -2.9ka. It was warm and moist from 2. lka to 0.5ka with small fluctuations. Since then the Ili valley has experienced weak chemical weathering and poor pedogenesis environment with aridity and less precipitation.