Late Holocene rainforest disturbance in French Guiana.

Palm swamp forest sediments collected in French Guiana provide new data about late Holocene rainforest. Two cores were collected in 'Les Nouragues' ecological station (4 degrees 05'N, 52 degrees 40'W). The lithology shows two different types of sediment, organic peat in the upper part and oxidized clay with low organic content and lacking pollen in the lower part, both separated by a gravel horizon. Radiocarbon dates show that this gravel horizon could have been deposited between 4500 and 3000yrBP. Pollen analysis carried out on the organic sediments record rainforest disturbances between ca 1520-1380 and 1060-860cal yrBP suggested by the presence of the pioneer tree species Cecropia together with other shade intolerant genera. Cecropia is recorded for a period that lasts between 660 and 320 years. This abnormal duration for presence of a pioneer species in rainforest is explained by brief and repeated changes in the composition of the canopy asssociated to perturbations of the palm swamp.