Variations of the Amazonian rainforest environment: a sedimentological record covering 30,000 years

Two cores (CSS 2 and CSS 10) were collected from two lakes situated in the Carajas area (6 degrees 35'S, 49 degrees 30'W) that show different stages o f silting up. The results of geochemical and petrographic organic analyses, backed up by C-14 radiometric dating, lead to a reconstruction of the vari ations in the hydrological regime of these lakes over the last 30,000 years . Evidence is revealed for a period of drying up, characterized by a sedime ntary hiatus between 22,000 and 13,000 C-14 years BP, followed by a period of refilling between 13,000 and 11,000 C-14 years BP. (15,400-12,900 cal ye ars BP). The latter interval is primarily characterized by a mainly elastic sedimentary input rich in quartz, kaolinite and iron hydroxides, leading o n to the authigenic formation of siderite. This type of sedimentation impli es the erosion of the small catchment basin during periods of transition be tween arid and humid climatic regimes. From 8000 to 4000 C-14 years BP (890 0-4500 cal years BP), as well as from 2700 to 1500 C-14 years BP (2780-1360 cal years BP), special hydrological conditions prevailed. They are charact erized in the sedimentary record by the presence of micro-laminae containin g wood charcoal interlayered with sponge spicule material. This interlamina tion reflects the rapid alternation of dry and humid periods. A comparison with the results obtained by pollen analysis shows that sedimentological in dicators, once filtered for strictly local effects, can provide a full reco rd of the regional scale variations in the environment. (C) 2001 Elsevier S cience B.V. All rights reserved.