Apparent discrepancy in contamination history of a sub-tropical estuary evaluated through 21Pb profile and chronostratigraphical markers

Zn and Cd concentrations, stable lead isotopes and 21Pb-derived chronology were determined in a sediment core sampled at Sepetiba Bay (South-eastern Brazil). During the last decades, the bay's watershed has been modified by the increase of industrial activities and human interventions. In particular, Zn and Cd ore treatment plants were built near the coast in 1960 and 1970, respectively, and water has been diverted from the adjacent Paraiba do Sul River watershed since 1950. The core collected at shallow depth near the industrial area exhibits four successive events: (i) at 50 cm depth, a change in the 206Pb/207Pb ratio from about 1.162 to more than 1.18 might be the result of the Sao Francisco Channel opening and water diversion from Paraiba do Sul river; (ii) at 40 cm depth, Zn concentration starts to increase (up to 0.8 mg g-1) (iii) above 30 cm depth, relatively high Cd concentrations (up to 1.6 μg g-1) are observed and (iv) at 16 cm depth, change in unsupported 21Pb slope is probably related to a waste dam built to prevent strong metal contamination in the bay. Sediment accumulation rates evaluated by Zn and Cd profiles used as time-markers are higher than those calculated from 21Pb-based chronology models. Using the constant initial concentration (CIC) model both events are supposed to date back to about 1884 and 1902, respectively, while using the constant rate of supply (CRS) model it shifts to about 1925 and 1935. Such discrepancies are probably assigned to the fact that these models do not take into account site-specific local sedimentation dynamics. In the study area, particles deposition seems to be controlled by enrichment with unsupported 21Pb transported by runoff from a mangrove flat bank. Chronology derived from a model that assumes an exponential increase of the initial 21Pb activity fits well with the estimated rates obtained from historical events