Forms and origin of alumina in the A2 horizon of the tropical podzols of Tahiti (French Polynesia)

Abstract Above 1100 m, on the moderate slopes of basaltic planezes in Tahiti, the rather cold climate and the abundant rainfall (3200 mm) allow the organic matter to accumulate. The soils are podzols developed on a substratum desilicated and rich in aluminium. They are characterized by a thin humo-ferruginous spodic horizon and a thin gibbsitic albic horizon (A2). Forms and origin of alumina in this horizon are studied here. Microphormic observations of soil thin-sections, analysis through the scanning electron microscope allow to distinguish three forms of alumina: 1 - inherited gibbsitic microgeodes; they are formed in the natural microsites of the weathering horizon whose alveolar structure is preserved. They have been subjected, without any visible modification, to the pedogenic evolution, before podzolisation appears. 2 - orange-coloured “plaquettes” likely to reach 1 mm including alumina, titanium, and iron. They are often juxtaposed to organic accumulations. These “plaquettes” are most often perforated and altered by numerous mycelial hyphae. Their presence raises the problem of their origin (probably “phlobophenes”) that remains to be defined and of their evolution affected by the fungial lysis. 3 - secondary alumina in the form of a thin edge round the patterned or non patterned organic matter. Its observation through the U.V. fluorescence can account for the stability of the organic matter in the A2 horizon rich in aluminium. Its suggests an evolution in the podzolisation process.