Abstracts of Symposium 55 and some papers from other related Symposia.

Pedological Evidences of a Dramatic Environmental Change of the Easter Island. Study of a Reference Profile

Diko LINUS JACOB, Roger LANGOHR, Soil Science Unit, Univ. Gent, Krijgslaan 281/S8, B-9000, Gent, Belgium.

A reconnaissance survey performed in 1990 by R.L. on the Easter Island had shown that the soils developed on the three main volcanoes, the Terevaca, Rano Kau and Poike are deeply weathered. In numerous sites it was possible to observe a several meters thick saprolite on the unweathered lava flow substratum. However the homogeneous soil cover, mostly present on such a deeply weathered rock, is largely missing on the Easter Island. Whenever the reddish homogenous cover soil is present, a stone layer and other evidences of a hiatus in the soil-sedimentary evolution can be observed in the basis of this part of the soil profile. This seems to indicate that a strong erosion phase has truncated the soils.

On the SW footslope of the Poike volcano, several meters deep and more than hundred meter long present-day erosion gullies did permit to study in detail the soil-sedimentary complex. At the level of the best developed profile site, five main pedostratigraphic units could be distinguished (the depth indications are approximate): I 0-25 cm: surface horizon developed in colluvial deposits of anthropogenic origin, as evidenced by the presence of small artefacts and charcoal fragments; II 25-100 cm: brown clayey unit with weak pedality, few slightly weathered rounded stones, the upper part of this unit corresponds to a (truncated?) surface horizon; III 100-115 cm: stone layer composed of lithic fragments similar to those observed in unit II; IV 115-250 cm : very clayey soil, well developed pedality, with mangans and ferriargillans along pores and ped faces; V 250+ cm - transition to a saprolite developed on basic volcanic rock.

The chemical, physical and mineralogical data of this profile show that unit IV corresponds to the Bt horizon of a deeply weathered soil. In this footslope position the soil has been only partly truncated. However, on the higher parts of the slope the whole soil has been eroded, including part of the saprolite. The rounded lithic fragments in the stone layer (III), and in the clayey unit (II) of the reference profile correspond most probably to corestones which came down from the higher landscape positions. However the relatively weak degree of weathering shows that the strong erosion phase has occurred in a not too far period, probably within the Upper Pleistocene. Further studies should permit to come to a better understanding and a more precise dating of this very severe erosion period.

Research funded by two grants from the Belgian National Science Foundation.

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