Pedogenic processes and domain boundaries in a Vertisol climosequence: evidence from titanium and zirconium distribution and morphology

Abstract

The occurrences of titanium (Ti) and zirconium (Zr) within eight Vertisols formed in a climosequence on the Upper Beaumont Formation of the Texas Gulf Coastal Plain were investigated in order to determine processes responsible for Ti and Zr redistribution during pedogenesis. Discontinuities defined by significant shifts in particle size distribution and the content (in volume percent) of Zr are present at 160 to 260 cm depth in each pedon. The discontinuities are interpreted to be functional boundaries, i.e., physico-chemical expressions of pedogenic domains, between an upper soil domain dominated by open-system pedogenesis and a lower, more closed-system domain dominated by chemical weathering. The depth at which the functional boundary occurs is dependent on physical and hydrogeochemical influences, which are largely a function of available water. Soil materials above the discontinuities are slightly coarser textured and enriched in Zr, whereas below the sediments are finer textured and have lower and more constant Zr contents. The Zr is associated almost exclusively with zircon and Zr contents correlate positively to the weight percent sand+coarse silt, with negligible Zr present in the <20 μm size fraction. By comparison, Ti occurs preferentially in the <20 μm size fraction and there are no marked discontinuities in Ti contents with depth. Scanning electron microscopy (SEM) of individual zircon and rutile grains show predominantly physical damage to zircons, whereas rutile grains appear to have been affected predominantly by chemical weathering. Thus, different processes dictate Ti and Zr content and distribution in these Vertisols, although both elements are often considered immobile in weathering profiles. Profile volume loss/gain (i.e., soil strain, ε), a mass-balance calculation that assumes Zr or Ti immobility during pedogenesis, indicate ε Zr values nearly four times greater than ε Ti. Large values of ε Zr within the upper soil domains are due primarily to sand and coarse silt additions to the Vertisols and preclude use of Zr as a basis of mass-balance calculations in these soils, despite its relative chemical stability. By comparison, Ti is generally conserved within the clay-rich soil profiles and is therefore better suited for mass-balance calculations of volume change and mobile element translocation during pedogenesis.