Evidence of prehistoric wind erosion of the Mackenzie Basin, South Island, New Zealand: an assessment based on 137Cs and Kawakawa-Oruanui tephra

Abstract

Many authors have reported significant soil erosion resulting from the grazing of sheep, rabbit plagues and invasion of the exotic Hawkweed (Hieracium sp.) in the sub humid alpine region of Mackenzie Basin, South Island, New Zealand. In the present study, we investigated the soil redistribution of four study plots with varying vegetation depletion over historic (54 years) and long (25 ka) time scales. Historic soil loss, quantified by bomb fallout 137Cs, under plots of depleted short tussock and herbfield vegetation was no more than the adjacent undisturbed reference plot of red tussock (Chionochloa rubra). This indicates the present landscape characterised by soil and vegetation degradation is not due to erosion since 1953. There is no evidence from the present study to suggest that establishment and rapid invasion of Hieracium sp. and major periodic rabbit plagues have accelerated soil erosion over the past 54 years. By contrast, low topsoil thickness under Hieracium sp. indicates that Hieracium sp. is colonising bare ground and may have, at least in the short-term, a stabilising effect. Long-term soil loss was quantified by the profile distribution of volcanic glass originating from Kawakawa-Oruanui tephra (KOT). The peak concentration, and hence the tephra’s 25.4 ka isochron, occurred at a depth of 70–85 cm at the reference plot. The degraded plots showed significant decreases in glass concentration and depth to peak concentration with progressively shallower soils and vegetation depletion. This equated to a minimum erosion rate averaged over the past ~25.4 k years of 0.020 mm year–1 in the most eroded plot. The extent of bare ground and topsoil thickness were poor indicators of soil erosion status. The tephra results show a potentially long history of soil erosion that has predisposed soil and vegetation degradation within the European era.