Apparent Discrepancy in Contamination History of a Sub-Tropical Estuary Evaluated Through super(210)Pb Profile and Chronsotratigraphical Markers

Zn and Cd concentrations, stable lead isotopes, and super(210)Pb-derived chronology in a sediment core sampled at Sepetiba Bay were determined. Sediment sampling was carried out during 1995 in the Coroa Grande mangrove, located in the vicinity of Ilha da Madeira plant. Samples were collected at 0.1 m in water depth during low tide with a 1 m PVC tube in the tidal flat in front of the mangrove. Sediments were dried at 40-50 degree C to minimize loss of volatile elements and then crushed in an agate mortar. Polonium activities were calculated taking into account decay corrections for super(210)Po and super(208)Po. Sediment accumulation rates evaluated by Zn and Cd profiles used as time-markers were higher than those calculated from super(210)Pb-based chronology models.