Textural differentiation in chronosequences from eastern australia, III. evidence from elemental chemistry

Abstract

Proportions of resistant heavy minerals, zircon, rutile and ilmenite, xenotime and monazite, in the fraction > 2.96 g cm t−3 were estimated by X-ray fluorescent spectroscopic measurement of the elements zirconium, titanium, yttrium and phosphorus, respectively. Ratios of these elements in various fractions in soils of four terrace sequences showed statistically insignificant changes with depth, thus ruling out sedimentary layering as an explanation for textural differences. Formation of clay by more intense chemical weathering of coarse minerals in the B horizon was discounted on the basis of constant ratios of Fe 2O 3:ZrO 2, Na 2O:ZrO 2 and CaO:ZrO 2 in the silt fraction. In the oldest members of several sequences there was evidence of chemical weathering but moderate to strong textural contrast was already apparent in younger profiles. Calculations of losses and gains on the basis of the tri-acid resistant fraction of whole soil samples provided further strong evidence for the predominant role played by clay translocation and the minor contribution to the clay fraction of clay formation in place except in the oldest soils. It was concluded that clay illuviation was the main mechanism responsible for textural differentiation with weathering in situ playing a minor, albeit with time, an increasingly important role.