中国东部主要黄土分布区的Sr-Nd同位素物源示踪研究

摘要

不同地质背景物质的Sr-Nd同位素组成往往差异巨大,同时Sr-Nd同位素组成在搬运和沉积过程中相对元素组分更难被改变,因此是物源示踪研究最常用和有效的手段之一.该研究测试了中国东部代表性黄土堆积的Sr-Nd同位素组成,包括山东半岛、黄河下游沿岸黄土和下蜀黄土,发现与黄土高原黄土的Sr-Nd同位素特征存在明显差异,指示不同的物质来源.同时发现,中国东部主要黄土分布区的来源物质可能主要来自其附近的干旱区具有近源性.山东半岛及其滨海黄土主要来自黄泛平原和冰期暴露陆架的黄河沉积物;黄河下游沿岸黄土可能直接来自黄河河漫滩;下蜀黄土的细颗粒组成物源还没有定论,需要进一步研究.中国东部局部区域黄土的形成可能与中更新世革命以来冰期气候条件下的荒漠化有关.%The provenance study of the major loess deposits in eastern China can help understand the evolution since the late Quaternary in this area.Nd and Sr isotopes are the most commonly used source tracers for atmospheric dusts.This work applies the Sr-Nd isotope to trace the source of classical loess deposits in eastern China, including the Shandong Peninsula and its adjacent coastal area,the loess deposits along the lower reaches of the Yellow River and the Xiashu loess deposits in the lower reaches of the Changjiang. We found that the Sr-Nd isotope composition was different between the loess in eastern China and the loess on the Chinese Loess Plateau, suggesting a different source region for the loess in eastern China. The Sr-Nd isotope data indicate that the loess deposits in eastern China mainly came from the near source. The loess deposits in the Shandong Peninsula and its adjacent coastal area show similar εNdvalues and 87Sr/86Sr ratios ranging from 0.7140 to 0.7155, which were blown from the Yellow River sediments exposed on the floodplain and the shelf during glacial times.The loess in the lower reaches of the Yellow River show similar Sr-Nd isotope values with the sediments in the floodplain of lower reaches of Yellow River, suggesting that the floodplain of lower reaches of Yellow River was the major source for the loess in the lower reaches of the Yellow River.The Xiashu loess deposits show 87Sr/86Sr ratios ranging from 0.7194 to 0.7209,which is lower than that of modern sediments in the lower reaches of the Changjiang,and εNd values ranging from-11.3 to-12.0. The source region of fine grains in Xiashu loess has not yet been determined and needs further study.The formation of loess deposits in eastern China may be related to the desertification during the glacial climate since the middle Pleistocene transition.