Pb Isotopes Study in Recent Sediments from Paraty Bay (Rio de Janeiro, Brazil): Antropogenic Versus Oceanic Signatures

Abstract

We carried out Pb isotopes studies in sediments fromParatyBaylocated in the SE area of theRio de JaneiroStateto discriminate the signature of metal contamination from domestic sewage, industries, shipyards and geogenic sources. The studied samples have been chosen in two sectors of the Bay, including the Paraty area andJuatingaBay. Sample preparation procedures included: 1) drying and granulometric separations (200 mesh); 2) weighting; 3) complete dissolution and 4) Pb isotope analysis obtained by MC-ICP-MS (ratios 208 Pb/ 204 Pb, 207 Pb/ 204 Pb, 206 Pb/ 204 Pb). Preliminary results when plotted in the 207 Pb/ 204 Pb versus 208 Pb/ 204 Pb and 207 Pb/ 206 Pb versus 206 Pb/ 204 Pb diagrams showed three groups of signatures. The first group (samples PY-01, PY-03 PY-106 and PY-127) presented 207 Pb/ 206 Pb values in the range from 1.191 to 1.209. The isotope values may be interpreted as contribution of a pollutant sources of Pb not yet identified. The second group (PY-11, PY-13, PY-21, PY-22, PY-46, PY-51, PY-109 and PY-111) showed 207 Pb/ 206 Pb values in the range from 1.188 to 1.222. The last group (samples PY-123 and PY-135) presented higher 207 Pb/ 206 Pb values from 1.321 to 1.322 and probably represented oceanic signature. The Pb isotope data of the second group may be interpreted as the result of oceanic water and pollutant sources mixture from the continent. According to the literature the anomalous Pb signature is similar to anthropogenic source. The Pb isotope results here reported are coherent with the reported data from the continent area. We may conclude that significant isotopic differences in Pb signatures are consequence of anthropogenic Pb isotope signature. In this way, Pb isotope is an important tool to trace the changing sources of pollutant Pb (and other metals) in ocean environment.