Mineralogic and climatic interpretations of the Luochuan loess section (China) based on diffuse reflectance spectrophotometry

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 sp ectrophotometer from the near ultraviolet, through the visible, and into th e 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 derivat ive 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 mine rals 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 fac tor exhibits an inverse correlation with magnetic susceptibility; the hemat ite factor exhibits a weak positive correlation with susceptibility. Paleoc limatic 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 c onditions that are drier than today a "dry summer monsoon," whereas later P leistocene paleosols are characterized by a "wet summer monsoon." (C) 2001 University of Washington.