Paleoenvironmental Changes in a Lower Rio Negro Amazon Floodplain Lake (Anavilhanas Archipelago) Over the Last 4500 Years

Rio Negro is the largest black water river in the world which transports large amounts of organic carbon through the Amazon River system. The vegetation and soil in this region are recognized as an important reservoir in the global carbon pool. While tropical forests play a vital role in this process, the contribution of continental aquatic systems to carbon accumulation and its relationship with paleoclimatic changes remains unclear. Here, a 405 cm long core from Lago do Boto was studied to reconstruct the paleoenvironmental changes during the last 4,550 cal yr BP. The transition from the middle to late Holocene, occurring approximately between 5,550 and 4,270 cal yr BP, is marked by distinct hydrological changes in the Boto Lake record. This transition phase is characterized by low hydrodynamic, indicating relatively calm water conditions. Between 4270 and 3865 cal yr BP this area is characterized by high sediment dynamics, with sedimentary and organic indicators showing high variability representing distinct hydrological regimes This transition phase represents a shift in the dynamics of the lake system, potentially driven by changes in precipitation patterns, water flow, and the consequently geomorphological changes. The organic matter exhibited high C/N and δ13C values, indicating vascular allochthonous material with C4 signatures. Carbon accumulation averaged 60 g m-2 yr-1 and peaked at 196 g m-2 yr-1 at 4,050 cal yr BP. The transition between the Middle Holocene dry phase to a more humid period was marked by a high frequency of extreme flooding events recorded by a sandy deposition. In addition, this period is also characterized by vascular allochthonous C4 from paludose vegetation or/and extensive grasslands typical of sandy soil indicated by microscopical, elemental, and isotopic composition of organic matter. High fluxes of charcoal particles also indicated the occurrence of fires consistent with the dry period at the end of the middle Holocene. A gradual decrease in sediment dynamics between 3865 and 3,200 cal yr BP was observed with a change in vegetation to a more arboreal source. Between 3,200 and 2,100 cal yr BP, the lacustrine environment experienced low hydrodynamics concerning precedent phase and high autochthonous primary productivity due to being a more isolated system, as noted by an increase in chlorophyll derivatives and diatoms indicating a humid climate period without extreme flooding events. The carbon accumulation reached 18 g m-2 yr-1 during this period. Between 2,100 and 760 cal yr BP, there was a decreasing trend of autochthonous productivity, with decreasing concentrations and fluxes with total organic carbon reaching 15 g m-2 yr-1, indicating a shift to a climatic condition of lacustrine level decrease considering the already established lentic environment. In the last 760 years, and mainly the last 200 years, there was an increasing trend of autochthonous productivity and carbon fluxes, reaching 38.1 g m-2 yr-1. The aquatic environment of Boto Lake over the last 4500 cal years reveals that the Anavilhanas floodplain underwent significant variability, shifting the lake carbon accumulation between aquatic and terrestrial biomass.