关键词: 内布拉斯加, 黄土, 化学风化, Nd-Sr 同位素

Abstract: 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 87Sr/86Sr and lower εNd, 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.

Key words: Nebraska, Loess, Chemical wenthering, Nd-Sr isotope