美国比格内尔山黄土的地球化学特征及其古气候意义

Geochemical Characteristics of the Loess in Nebraska, U.S.,and Their Paleoclimatic Implications

黄土是美国内布拉斯加州地表分布范围最广的沉积物之一,其中末次冰期形成的皮奥瑞亚黄土(Peoria loess,PL)质量累积速率(Mass Accumulation Rate,MAR)在比格内尔山(Bignell Hill,BH)达到世界最高。探究此地区黄土快速沉积的原因一直是北美中部大平原黄土研究的热点。本文通过对内布拉斯加州中部BH样品的Nd、Sr同位素、主微量和稀土元素的分析,发现内布拉斯加黄土的化学成分相对均一,与上部大陆地壳(UCC)和中国洛川黄土相比,Fe、Mg值较低,K、Na含量相对较高。BH黄土的化学风化指数(CIA)值平均为65.7,小于洛川黄土,显示出较弱的化学风化强度。Sr-Nd同位素结果显示,除了普遍认为的怀特组沉积为黄土的主要源区外,还存在另一个^(87)Sr/^(86)Sr比值更高,ε_(Nd)更负的源区,推测这个源区为北美劳伦冰盖冰川沉积,其对黄土的贡献达30%~45%。BH黄土剖面的CIA、Na/K和碳酸盐含量变化表明,在PL形成时期(约25~18 ka),化学风化较弱,气候干冷,有助于风尘产生与堆积。

Loess is one of the most extensive surficial geologic deposits in Nebraska, U.S.. The deposition rate of the Peoria Loess was extremely fast during the last glacial period, especially in Bignell Hill, the Mass Accumulation Rate is highest in the word. It is still controversial to interpret the cause of the mass dust in the central Great Plain. In this paper, the compositions of Nd-Sr isotopes, major,trace elements and rare earth elements of the loess deposit were investigated on the Bignell Hill section in Nebraska. The results suggest that chemical compositions of the Nebraska loess are relatively homogeneous. Compared with the upper continental crust and the Luochuan loess in China, the contents of Fe and Mg are relatively low, and the contents of K and Na are relatively high in the Nebraska loess. Nd-Sr isotopic compositions indicate there is another source with higher ^(87)Sr/^(86)Sr and lower ε_(Nd), except the primary provenance of White River Group. The second source is likely to be glacial deposits which were derived from the area of Laurentide Ice Sheet, its contribution even as high as 30%~45%. The chemical index of alteration of Nebraska loess is about 65.7, lower than Luochuan loess in China, showing a weak chemical weathering intensity. The variation of CIA, Na/K and carbonate content in Bignell Hill section reveal a dry and cold climatic condition and weak weathering during Peiria Loess deposit(25~18 ka). This climatic condition was beneficial to the production and deposition of dust thus generated extremely high mass accumulation rate during that time.