Slash-and-burn agriculture practiced by several thousand small-scale farmers in the Tapajos region of the Brazilian Amazon has contributed to accelerated deforestation over the past decades. The present study aims to quantify and qualify changes in the transfer of terrestrial organic matter (TOM) to aquatic environments following deforestation. Lignin biomarkers have been analyzed from sediment cores collected in three floodplain lakes and in suspended particulate matter sampled during both wet and dry seasons. These analyses are interpreted with regard to lignin biomarker signatures of surface and deeper horizons of common soils, and of dominant plant species from forested and deforested environments. Dating the sediment cores with 210Pb allows reconstructing the successive deforestation cycles since the onset of European colonization two centuries ago. Further, satellite images coupled to a GIS approach is used to correlate the evolution of sedimentary TOM and anthropogenic land-use from 1986 to 2009. Over this period, sedimentation rates have sharply increased, and the nature of the sedimentary TOM has been shifted from being linked to primeval forest soils to degraded soils following deforestation for subsistence cropping and/or pasture lands. The intensity of changes in the nature of sedimentary TOM appears inversely related to the connectivity of flood lakes to the river. In the least connected flood lake, weathering of pasture soils in the watershed dominates TOM inputs particularly during the dry season. Massive deforestation in the Amazon thus triggers major changes in the nature of TOM transferred and sedimented in aquatic systems.
