TRACE-ELEMENT COMPOSITION IN SEDIMENTS OF THE AMAZONIAN LAKE CRISTALINO

Lake Cristalino is a small lake adjacent to the Negro River near Manaus (03 degrees 08'S, 60 degrees 06'W) and not far from the Amazonas River, in the central Amazon basin. The lake is fed seasonally by waters of the Negro River, a blackwater river with low levels of nutrients and suspended solids (7 g m(-3)). However, some investigations have established that Lake Cristalino has a high sedimentation rate (0.4-0.5 cm year(-1)) similar to those in the alluvial floodplain lakes of the Amazonas River (suspended solids 200-300 g m(-3)). Sediment cores were taken during the low-water period and the trace-element composition and the natural radioactivity in the lake were examined. The results show a core (31 cm length) relatively uniform in concentrations of trace elements (Br, Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Sc, Sm, Ta, Tb, Th, Yb and Zn), and the presence of Cs-137 in the first half. Concentrations of trace elements in Lake Cristalino sediments were not correlated with concentrations in the sediments of its parent river, the Negro River, or with concentrations in soils of the local area. However, significant correlation was found between the sediments of the lake and those of the Amazonas River. On the basis of these results, and water-level data at Manaus port, it is concluded that the lake occasionally receives variable amounts of sediment from the Amazonas River.