Carbon isotope data on soil organic matter (SOM) were collected along an ecosystem transect 90 m in length that includes a tropical forest on the plateau, a transitional forest-savanna and savanna in a depression. Total organic carbon data show a significant increase in carbon content from sites representing forest ecosystem to sites representing savanna ecosystem. It was hypothesized that carbon accumulation in the depression is controlled by flooding conditions that slow down carbon decomposition and in part by carbon transport from the upper part of the transect (the savanna and the transition forest-savanna areas) into the depressions by water during the rainy season. The origin of the carbon was confirmed by using soil 13 C analysis. The savanna sites located in the depression showed δ 13C values between –19.5‰ and –22.5‰ indicating a mixture of C3 and C4 plants. The vegetation cover in the depression is predominantly C3 grasses with d 13C values of about –27‰ and –26‰. In the site under savanna located at an elevation slightly higher, the d 13C value was more enriched (–16‰) showing the predominance of C4 plants (d 13C of –13.6‰). At the forest-savanna transition and in the forest ecosystem the d 13C values were characteristic of C3 plants (–25‰ and –28.1‰). 14C and 13C data indicate that the organic matter of mixed origin has been deposited for at least the last 7000 years in the savanna depressions. The 13C pattern observed in the soil organic matter profiles indicate a predominance of C3 plants in the early part of the Holocene. About 7000 to 4000 years ago, the data show the influence of C4 plants, indicating forest regression associated with a drier climate than at present. The more recent 13 C records suggest forest expansion, and the return to a climate similar to the present.

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A mottled horizon in a laterite cover (without any duricrust) was studied by microscopy and quantitative chemical microanalysis. Apart from the voids, light red spots consisting of Fe-rich particles (≈ 2 µm) are set in clayey plasma. Dark red spots consisted of concentrations of Fe-rich particles. These patterns are inherited. On the border of structural or biological voids, where Fe-depletion features are systematic, gray or yellow spots result from dissolution of the Fe-rich particles and impregnation of the plasma by iron, respectively. The present Fe-depletion is the dominant process that explains the mottled differentiation and the absence of lateritic duricrust. To cite this article: V. Rosolen et al., C. R. Geoscience 334 (2002) 187-195. © 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS microscopy / scanning electron microscopy / quantitative chemical microprobe / iron / plinthic Cambisol / Brazil Résumé-Genèse d'un horizon tacheté par déferruginisation dans une couverture à latérite du Bassin amazonien. Un horizon tacheté d'une couverture à latérite (sans cuirasse) d'Amazonie a été étudié par microscopie et microanalyse chimique quantitative. Éloignées des pores, les taches rouges sont constituées de particules ferrugineuses (≈ 2 µm), enchâssées et isolées au sein d'une matrice argilo-quarzeuse ou en amas. Ce faciès est hérité. En bordure des pores, les taches grises ou jaunes témoignent respectivement d'une dissolution des particules ferrugineuses et d'une migration du fer au sein d'une matrice semblable. L'évolution est polyphasée avec, antérieurement, une ségrégation ferrugineuse, puis, actuellement, une déferruginisation, qui provoque une différenciation tachetée. La déferruginisation explique l'absence de cuirassement. Pour citer cet article : V.

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