MIDDLE AND LATE HOLOCENE CLIMATE CHANGES IN NORTHEASTERN BRAZIL INFERRED FROM SEDIMENTARY ORGANIC MATTER INDICTORS OF WATER LEVEL VARIATIONS IN LAGOA DO BOQUEIRÃO, RIO GRANDE DO NORTE STATE

Changes in the water level of Lagoa do Boqueirão, Rio Grande do Norte State (Northeastern Brazil) are recorded by different histories of sediment accumulation in cores from the margin and center of the lake. Organic geochemical studies, backed by radiocarbon dating, reveal a history of variable hydrologic and climatic conditions over the last 7100 Cal yr BP. Comparison of total organic carbon concentrations, organic C/N ratios, and organic carbon and nitrogen isotope compositions in sediment horizons in the lake-center core with those in the lake-margin core show a progression of changes in the proportions of delivery of organic matter from macrophytes and phytoplankton that is related to water level variations. Lake level increased from 7100 Cal yr BP to peak at around 4500 Cal yr BP. From 4500 to 3200 Cal yr BP, the lake level decreased to remain at a stable, low level until 1200 Cal yr BP, when it began to rise. By comparison with other paleoclimate records from northern South America, we postulate that the paleohydrologic changes evident in Lagoa do Boqueirão were controlled by south-north shifts of the intertropical convergence zone in response to sub-Milankovitch solar cycles. In particular, the lake level variations were probably linked to regional forcings related to middle and late Holocene Atlantic and Pacific variability and their teleconnections.