Chemical trends in a perhumid soil catena on the Turrialba volcano (Costa Rica)

The variation in chemical composition of soil samples (XRFS data) from a soil catena of 14 soil profiles on the northern slope and foot slope of the andesitic Turrialba volcano (3300 m) has been analysed to test the two hypotheses, which underlie our interpretation that this catena is a weathering sequence. The first hypothesis is that all parent material is chemically homogeneous and the second hypothesis is that development of chemical variation is the net result of progression of weathering and of diversity of weathering, which both depend on altitude via gradients in temperature and precipitation. The aim of this purely chemical approach is to find out if trace elements are significant “tracers” for different weathering paths leading to different amorphous or crystalline products, for example imogolite, allophane, halloysite, gibbsite, and ferrihydrite. The results show that volcanic ash is sufficiently homogeneous to detect the well-known dominance of a general weathering trend with relative depletion of mobile elements (Mg, K, Si, etc.) and relative enrichment of less-mobile elements (Ti, Fe, Al, Ce, Cr, V, Zr, etc.). The least-mobile elements were Ti, V, and Zr. From their contents, a Weathering Index for all samples has been derived that describes 95% of their total variance (hypothesis 1). In addition to this general trend, Ti, V, and Zr showed systematic deviations that could be defined as a chemical differentiation factor. In combination with the general weathering trend, this explained 99% of the total variation of Ti, V, and Zr (hypothesis 2). The contents of other (major, minor, and trace) elements used can only partially be described by the Ti–V–Zr-derived Weathering Index and Chemical Differentiation. The level of explanation decreases roughly with increasing mobility of the chemical element in the system. Triangular plots of element ratios suggest that contents of mobile elements are influenced by too many weathering processes. Trace elements did not show a more specific Chemical Differentiation than major and minor oxides. The leaching pattern of Mg indicates the presence of three different processes: (1) short-term preferential leaching of basic cations at high altitude, (2) subsequent allophane formation between 660 and 1220 m altitude, and (3) long-term loss of basic oxides (and not of silica) upon ageing of fluvio-volcanic sediments in the foot plain below 240 m. Fe shows anomalous depletion (redistribution) due to oxidation and reduction, and Al shows extra depletion in two of the allophanic profiles.