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Mineralogic and Climatic Interpretations of the Luochuan Loess Section (China) Based on Diffuse Reflectance Spectrophotometry

Published online by Cambridge University Press:  20 January 2017

Junfeng Ji ,
William Balsam  and
Jun Chen
Junfeng Ji
Affiliation:
Institute of Surface Geochemistry, State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing, 210093, China
William Balsam
Affiliation:
Department of Geology, University of Texas at Arlington, Arlington, Texas, 76019
Jun Chen
Affiliation:
Institute of Surface Geochemistry, State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing, 210093, China
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Abstract

We examined the top 135 m, that is, the entire Pleistocene, of the classic Luochuan section on the Chinese Loess Plateau with a diffuse reflectance spectrophotometer from the near ultraviolet, through the visible, and into the near infrared. From the reflectance data we calculated sample brightness which, with some caveats, is a reasonable proxy for magnetic susceptibility. Mineralogic changes were identified by factor analyzing the first derivative of the percent reflectance data and examining samples with high factor scores. Two factors which explain about 96% of the cumulative variance are distinguished by the relative proportion of hematite and goethite, the minerals that are responsible for the color changes in the loess sequence. Both hematite and goethite are present in both loess and paleosol but goethite dominates in loess whereas hematite dominates in paleosol. The goethite factor exhibits an inverse correlation with magnetic susceptibility; the hematite factor exhibits a weak positive correlation with susceptibility. Paleoclimatic interpretations are drawn from comparison of susceptibility to the concentration of spectrally identified hematite. Based on this comparison, paleosols in the early Pleistocene Wucheng Formation are characterized by conditions that are drier than today, a “dry summer monsoon”, whereas later Pleistocene paleosols are characterized by a “wet summer monsoon”.

Keywords

loessLuochuanreflectance spectrophotometryhematitegoethitepaleosolmagnetic susceptibility
Type
Research Article
Information
Quaternary Research , Volume 56 , Issue 1 , July 2001 , pp. 23 - 30
Copyright
University of Washington

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