Holocene Paleoenvironmental Reconstruction in the Eastern Amazonian Basin: Santa Ninha, Maracá and Comprido Lake

Abstract:

A synthesis of the impacts of the Amazon River hydrological changes on the sedimentation process of organic matter (OM) in three different floodplain lakes (Santa Ninha, Maracá, and Comprido lakes) is presented in this study.Today the Santa Ninha and Maracá lakes are directly and permanently connected with the main channel of the Amazon River, in contrast to Comprido Lake, which is indirectly and periodically influenced by the Amazon River due to its greater distance from the main channel. All three lake sediment records showed a reduced river inflow due to dry climatic conditions during the early and middle Holocene followed by an increased fluvial input during the wetter late Holocene. In Santa Ninha and Maraca Lakes, the reduced river inflow periodwas recorded by sediments with a lowabundance of smectite (~20 wt.%), a clay mineral mainly transported by the fluvial system, high total organic carbon (TOC) contents (~8.2 wt.%). During the late Holocene, a higher smectite abundance (~43 wt.%) and a lower TOC content (~1.4 wt.%) pointed to greater dilution by riverine lithogenic matter. In Comprido Lake, a sedimentation gap occurred during the early and middle Holocene. The wetter late Holocene, since 3000 cal years BP, was characterized by high TOC values (~9 wt.%), but the smectite content was low (~14%). This combination suggests that the sedimentary input to Comprido Lake from the local catchment area became dominant during the wet-climate late Holocene due to the large distance of the lake from the Amazon River main channel. Consequently, our study shows that the sedimentation processes of OM in Amazonian floodplain lakes were strongly influenced by variations in the hydrodynamic regime of the Amazon River during the Holocene. However, the impacts of the variations on the three floodplain lakes were different depending on the distance of each lake from the main channel of the Amazon River.