Biogeochemical indicators of environmental changes from 50 Ka to 10 Ka in a humid region of the Brazilian Amazon

Abstract

We present a geochemical record of a 113.6-cm sediment core (LPT V) from Lagoa da Pata, which is located in the forested upper Rio Negro basin. The record reveals significant changes in the environmental history of Amazonia during the late Quaternary. The results of biogeochemical analyses revealed three hydrological and climatic regimes from 50,000 to 10,000calyr BP. The first phase, between 50,000 and 26,300calyr BP, was characterized by a relatively wet climate as suggested by relatively high total organic carbon (TOC) and chlorophyll derivate concentrations, indicating high productivity linked to a high lake level. A decrease of the TOC and chlorophyll derivate accumulation rates between 43,100calyr BP and 26,300calyr BP marks a decrease in the productivity linked to a reduced lake level, indicating a decrease in moisture at the end of this phase. The second phase, between 26,300 and 15,300calyr BP, was characterized by a decrease in productivity, reaching a minimum at 21,950calyr BP, as indicated by a minima in sedimentary chlorophyll and TOC accumulation rates. Values of δ13C increased by 5‰ in relation to the preceding phase, indicating an influence of the C4 organic matter. High iron concentrations and accumulation rates, related to intense erosion of the lateritic crust in the watershed, were observed. All of the observations indicate a dry phase during this period. A third phase, from approximately 15,300 to 10,000calyr BP, was characterized by an increase in lacustrine productivity, as shown by an increased in TOC and chlorophyll derivate concentrations and accumulation rates. These increases likely correspond to a lake level rise due to a wetter climate.