The evolution of the continental and coastal environments during the last climatic cycle in Brazil (120 Ky. B.P. to Present)

Abstract

Studies done in Brazil, under the CNPq (Brazil) and ORSTOM (France) agreement, allowed us to envisage the general trend of climatic changes that occurred during the last 60,000 years. At first, examination of successive accumulation and erosion phases, recorded within Central Brazilian valley deposits, provided evidence of importam changes in slope vegetation cover and in the hydraulic regime of water courses, and hence characteristics of precipitation and climate regimes. Studies of Tamandua river valley deposits in the town of Sao Simao (State of Sao Paulo) showed that between 32 and 21 ky. B.P. the climate was humid, whereas between 17 and 11 ky. B.P. it was dry with very scarce and strong rains. From 10 ky to 8.5 ky. B.P. the climate was humid too. Finally, after 7.5 ky. B.P. the deposits revealed the existence of several dry episodes within a globally humid climate. Palynological and sedimentological analyses of lacustrine deposits sampled by vibrocorers in the Eastern Amazon (Carajas, State of Para) and in Central Brazil (Salitre, State of Minas Gerais) permitted us to outline the evolutionary history of the vegetation cover within these regions during the last 60,000 years for Carajas and 30,000 years for Salitre and thus to have a preliminary idea about their past climates. Then, it was possible to show that at Carajas regression of the evergreen rainforest occurred four times, at about 60, 40, between 23-11 and 7-4 ky. B.P. Moreover, apparently the last rainforest regression episode was quite different from the previous three episodes. At Salitre, the most characteristic is the indication of the existence of Araucaria forest between 12 and 8.5 ky. B.P. This Araucaria forest location is clearly much further north than at present, which suggests that during that time the climate was wetter and colder. Finally, studies developed along the central Brazilian coast allowed us to show evidence of wind-/driven change in littoral dynamics during the last 5 ky. A detailed examination of Doce river (State of Espirito Santo) mouth beach-ridge geometry has shown inversions in littoral drift direction as a consequence of change in wave and then in wind regimes, reflecting characteristics of atmospheric circulation during the last 5 ky.