The geochemistry of soils on a catena on basalt at Khon Buri, northeast Thailand

Abstract

A geochemistry study was carried out on a catenary sequence of soils formed on basalt under a tropical climate. Four soil profiles developed on crest, backslope, footslope and valley floor positions on an undulating basaltic lava corrosion plain were sampled and analyzed by optical microscopy, XRD, XRF, ICP-MS and SEM/EDS analytical techniques. Aluminum, Si, Fe and Ti are the major elements in all soils across the catena. The crest soil profile contains uniform quantities of Al, Si, Fe and Ti in each horizon and very low amounts of Ca, Mg and K. This reflects the dominance of kaolin, iron oxide minerals, anatase and quartz. Backslope and footslope soils also consist mostly of Al, Si, Fe and Ti but they contain higher amounts of Mn as a result of a redox accumulation process. Other than quartz, ilmenite is the only sand-sized mineral. In the valley floor soil redox reactions occurring in stagnant water do not support crystalline iron oxide minerals. The high iron content of this soil is mostly present in authigenic smectite. This soil has higher amounts of Mg, Ca and K than upslope soils reflecting its accumulating position in the landscape and the in situ crystallization of carbonates and smectite. Factor analysis of chemical data shows quite different associations of elements in whole soils, fine sand, silt and clay fractions. The element compositions of the whole soil and in all size fractions show that the crest soil is most uniform, the backslope soil is quite uniform, the footslope soil is quite diverse and resembles the backslope soil, and the valley floor soil is highly diverse and quite different from the upslope soils. Three main elemental behavior groups exist for the whole soil. The different concentrations of elements in the Al group (Al, Ti, Li, Cs, Zr, Mo), and the Ca group (Ca, Si, K, Mg), result in the soils on crest and valley floor being distinctly different. The Fe group (Fe, Mn, P, Ga, Cr, Pb, Rb, V, Cu, Zn, Ni, Co, K, As) causes the backslope and footslope soils to overlap. Differences in concentration of the Mn group (Mn, Mg, Pb, Co, Pb, K) and Fe group (Fe, Rb, Ni, P, U, Zn, Ga, V, Al, Cu, As, Ti, Cs, Li, Mo) result in the soils on the backslope and footslope being different from the other two soils. No soils on the catena have elemental compositions that overlap for silt or clay fractions. Element concentrations in each size fraction and each landscape position can be attributed to differences in parent rock and differential weathering.