Color variations of the Quaternary Red Clay in southern China and its paleoclimatic implications

Abstract

The red clay and red weathering crust are widely distributed in southern China. To study the possible relationship between soil color and paleoclimatic environment, three color parameters, lightness (L⁎), redness (a⁎) and yellowness (b⁎), of twelve Quaternary Red Clay (QRC) profiles in southern China were measured using a colorimeter. Colors of the QRC profiles vary vertically: a⁎ and b⁎ generally increase downward and L⁎ decreases downwards. Colors of the QRC also show spatial variation: a⁎ and b⁎ generally increase towards the equator; whereas L⁎ increases away from it. Both a⁎ and b⁎ are positively significantly correlated with clay (<2μm) content, free Fe (Fed), Fed/Fet ratios and other soil weathering indices of the QRC (ρ<0.05), and the correlations between a⁎ and the weathering indices are much stronger than those between b⁎ and the indices. a⁎ mainly reflects hematite content in soils. Compared with magnetic susceptibility (χ), a⁎ of the QRC is a more promising paleoclimatic indicator, although it is sometimes disturbed by uneven parent materials and dissolution of hematite by water logging. The upward decrease of a⁎ of the QRC profiles reflects a cooling and drying paleoclimate since the end of the Last Interglacial, and the spatial variation of a⁎ coincides with more optimum hydrothermal conditions in lower-latitude areas of southern China.