Nearshore sedimentation as a record of landuse change and erosion: Jurujuba Sound, Niterَi, SE Brazil

The Southeast of Brazil has been subject to successive phases of land disturbance since the sixteenth century that has ultimately resulted in widespread urbanisation. This study details mass movements and fluvial erosion triggered by construction and slash and burn cultivation, on steep slopes in a suburb of Niterَi city, following rapid development since it was joined to Rio de Janeiro by a bridge in 1974. The transport of sediment as a result of human impact is traced to near shore deposits within a nearby enclosed bay that has acted as a sediment sink since its formation by Holocene sea level rise. Cores from this bay reveal an initial influx of sediment coinciding with the arrival of European colonists, but the most rapid sedimentation is related to the recent urbanisation of the catchment. This is commonly associated with intense erosion of deeply weathered regolith and soil triggered by mass movements, quarrying and the excavation of level sites for construction. In the coastal region the coarse fraction of the sediment is mainly composed of construction debris, angular quartz, feldspars and rock fragments. Geochemical analyses of a sedimentary core collected in the centre of the bay showed a recent enrichment of metals (Pb, Ni, Cu, Cr, Zn and Mn) over approximately the last forty years. This is also matched by an increase over time in the foraminifera species Ammonia tepida and a decline in the species Buliminella elegantissima. A. tepida is commonly found in restricted and highly polluted environments, whereas, B. elegantissima is more sensitive to environmental deterioration. Pollen analysis shows a gradual decrease in forest since the European settlement and an increase in field vegetation that has accelerated in recent years, together with the introduction of exotic species. In order to understand human impact on near shore sediments, complementary techniques such as geochemical, micropaleontological and sedimentological are shown to be very important tools in this field of research.