A micromorphological study of pedogenic processes in an evolutionary soil sequence formed on Late Quaternary rhyolitic tephra deposits, North Island, New Zealand

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The influence of time as a soil forming factor was studied on an evolutionary sequence of five soils (1850 radiocarbo years BP-ca. 120,000 BP) developed in rhyolitic tephra deposits in New Zealand. New micromorphological observations were combined with existing macromorphological, chemical, textural, and clay mineralogical analyses to assess the extent of polygenesis in the soils. The younger soils (1850 BP to ca. 22,600 BP) are characterised by initial weathering and little secondary mineral formation. This status is expressed by abundant pumice fragments and fresh, easily weatherable minerals, as well as a low clay content with an isotropic character. Comminution and neoformation are the main pedological processes. The older soils (ca. 120,000 BP) are characterised by a high degree of alteration of primary minerals, little or no pumice fragments, clay illuviation, gleying, and a high clay content (>80%) consisting of a range of crystalline clay minerals including halloysite, gibbsite, kaolinite, and vermiculite, plus crystalline iron oxides. Evidence of polygenesis can be observed in the micromorphology of the older soils: (1) the range of secondary minerals is the result of the integration and succession of soil-forming processes through time; (2) the accumulatory character of the tephra deposits has resulted in stronger weathering of the lower horizons than upper horizons because of continuous pedogenesis. Such polygenesis in the older soils supports the view that the older the soil, the more polygenetic it is likely to be.