Weathering of paleosols in Holocene and late Pleistocene tephras in Central North Island, New Zealand

Abstract

Buried soils formed on rhyolitic tephras in the central North Island, New Zealand, that were erupted between 900 and 40 000 years ago, have been examined by chemical methods to evaluate the rate of soil formation during the time that they were exposed to weathering. Rates of soil formation appear generally consistent with paleoclimate interpretation from vegetation evidence, and from ice-cores taken at Byrd Station, Antarctica. In the tephras that contain an abundance of volcanic glass in their sand fractions, the percentage of aluminium extracted by Tamm's acid oxalate reagent appears to be a fairly useful index of weathering. In the southern part of the area studied, andesitic ash as a contaminant could lead to overestimates of weathering rates by this method. Buried soils formed on air-fall rhyolitic ash resemble modern yellow-brown pumice soils of the New Zealand genetic soil classification, and those formed on this kind of parent material mixed with andesitic ash resemble modern yellow-brown loams, but both are lower in organic matter content than the modern soils. During the latter part of the Otira (Wisconsin or Würm) Glaciation a proportion of hydrothermally altered material appears to have been erupted along with some of the pumiceous ejecta.