Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River

Abstract

Palynological studies of a peat-bog sequence, with a basal date of 10,990±80 yr B.P., provide a history of vegetational and climatic changes in the Icatu River Valley located inside a large system of stabilized sand dunes at the middle São Francisco River at 10°24′S, 43°13′W in northeastern Brazil. The present day vegetation of the valley is `vereda' a Mauritia vinifera palm swamp forest bordered by a narrow strip of semi-deciduous tropical forest. The adjacent dunes are stabilized by arboreal caatinga and cerrado vegetation in some areas. The palynological profile of the Saquinho locality within this valley was divided into five pollen zones: Zone SA1 (10,990–10,540 yr B.P.) contains pollen of taxa found in present Amazonian and Atlantic forests, as well as from montane taxa, thus suggesting very humid climatic conditions synchronous with more reduced temperatures. Zone SA2 (10,540–6790 yr B.P.) has pollen spectra from a vegetation type dominated by Mauritia, suggesting progressive warming and high humidity levels. The period between 8920 and 8910 yr B.P. has rapid sedimentation rate, few thermoluminescence (TL) dates, pronounced increase of Mauritia and Ilex pollen suggesting a very moist climate. From 8910 to 6790 yr B.P. there is a progressive decline in forest taxa and a gradual increase of caatinga and cerrado taxa on the landscape. Zone SA3 (6790–ca. 6230 yr B.P.) does not contain palynomorphs, possibly indicating semi-arid conditions. Zone SA4 (ca. 6230–ca. 4535 yr B.P.) marks the return of mosaic vegetation composed of gallery forest, cerrado and caatinga taxa indicating moister climatic conditions. A marked decline in moisture levels characterizes the period between 4240 yr B.P. and Present, as suggested by an increase in caatinga and cerrado species and a decline in gallery forest taxa. Zone SA5 (ca. 4535 yr B.P.–Present) establishes the present vegetational and climatic pattern of the Icatu River Valley.