Upbuilding pedogenesis in multisequal tephra-derived soils in the Waikato Region

Abstract

Soil formation is classically regarded as a 'topdown' process whereby soil thickness, horizonation and vertical anisotropy increase steadily (not necessarily linearly) over time. However, where incremental additions to the land surface are common (e.g. loess, tephra, alluvium or colluvium), the process of soil upbuilding becomes important. The degree of soil expression is governed by the relative rates of pedogenesis and aggradation, which are unlikely to remain constant over time (Almond and Tonkin 1999). The interplay between these processes was examined by Johnson and Watson-Stegner (1987) and others. Soils on aggrading landscapes undergo 'developmental upbuilding' (progressive pedogenesis) if rates of addition allow assimilation of the deposits and profile deepening, or 'retardant upbuilding' (regressive pedogenesis) if rates are so rapid that horizonation is prevented. In upbuilding soils, each increment of soil below the A horizon has experienced processes that are characteristic of all horizons above it (Almond and Tonkin 1999). Many Quaternary landscapes in NZ comprise well-studied stratigraphic sequences of deposits, including widespread loess and tephra, in which there are repetitions of soil horizons and buried paleosols within the vertical profile (multisequal soils). Rates of aggradation and the relative influence of soil upbuilding processes on soil evolution in such sequences have been examined closely in only a few studies. Using tephrochronology, I examine soil upbuilding in multisequal tephra-derived soils on a Waikato transect.