A MICROMORPHOLOGICAL STUDY OF PEDOGENIC PROCESSES IN AN EVOLUTIONARY SOIL SEQUENCE FORMED ON LATE QUATERNARY RHYOLITIC TEPHRA DEPOSITS, NORTH-ISLAND, NEW-ZEALAND

The influence of time as a soil forming factor was studied on an evolu tionary sequence of five soils (1850 radiocarbo years BP-ca. 120,000 B P) developed in rhyolitic tephra deposits in New Zealand. New micromor phological observations were combined with existing macromorphological , chemical, textural, and clay mineralogical analyses to assess the ex tent 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 fragmen ts and fresh, easily weatherable minerals, as well as a low clay conte nt with an isotropic character. Comminution and neoformation are the m ain pedological processes. The older soils (ca. 120,000 BP) are charac terised by a high degree of alteration of primary minerals, little or no pumice fragments, clay illuviation, gleying, and a high clay conten t (>80%) consisting of a range of crystalline clay minerals including halloysite, gibbsite, kaolinite, and vermiculite, plus crystalline iro n oxides. Evidence of polygenesis can be observed in the micromorpholo gy of the older soils: (1) the range of secondary minerals is the resu lt of the integration and succession of soil-forming processes through time; (2) the accumulatory character of the tephra deposits has resul ted in stronger weathering of the lower horizons than upper horizons b ecause 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. Copyright (C) 1996 INQUA/Elsevier Science Ltd