PALEOENVIRONMENTAL CHANGES AND MERCURY DEPOSITION IN A REMOTE AREA OF AMAZONIA

Geochemical analysis of four cores collected in Lagoa da Pata (Morro dos Seis Lagos, Sao Gabriel da Cachoeira, Amazonas State) showed important changes in the environmental history of this area during the last 45,000 years. Three distinct sections were clearly identified in the cores. They consist of upper and lower organic-rich layers, separated by an inorganic layer which represents a short period of rapid accumulation around 18000 ¹⁴C yr BP. The first phase between, at least 45500 and 26000 B.P., it was observed a decrease in lacustrine productivity until the last glacial maximum, probably associated to a decrease in water level. In the LPT V core values of d¹³C (around -32‰) are associated to high C/N ratio related to woody material contribution. The second phase at around 18000 ¹⁴C yr BP, appears to have been a sudden input of clastic material. This was represented by a sandy facies, which exhibits lower carbon and water contents and high bulk density. This event may have been due to the occurrence of sudden and torrential rains, typical of dry climate, at about 18 ka BP. At the third phase, between at least 18000 ¹⁴C yr B.P. until the present occurred an increase in lacustrine productivity attested by an increment in the chlrophyll derivates and the carbon flux mainly after 4000 ¹⁴C yr BP attested in the LPT III core wich have the highest resolution in this phase in comparision with the other sudied cores. It probably corresponds to a lake level rise increasing the production of the system. The C/N values dropped indicating an increase in algae organic matter contribution. The d¹⁵N decrease in this phase. The mercury accumulation rate after the last glacial maximum is found to be larger than the period betwenn 45500 to 25000 ¹⁴C yr BP with higher fluxes between 9000 and 4000 ¹⁴C yr BP in LPT III, IV and LPTVI core. In the four cores analysed the mercury flux is greater after the last glacial maximum and mainly in the Holocene. These large accumulations rate of mercury is probably associated with an increase in oceanic degaussing or a high frequency of forest fires during the Holocene.