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Three-stage sequential extraction BCR was applied to surface sediments from the west part of Guanabara Bay to assess the mobility of Zn, Cu, Pb, Ni, Cr, and Mn. Results were satisfactory for the analysis of certificate standard material (BCR 701), with recoveries between 71 (Cu) and 123% (Cr). Evaluation of organic matter composition classified the area as eutrophic (CHO:PRT > 1), with aged organic detritus at some stations. Zn exhibited by far the greatest bioavailability, with 43.49% of its concentrations associated with the exchangeable fraction. Cu and Cr showed stronger affinity for organic matter, with 51.18 and 48.73% of their concentrations, respectively, bounded to the oxidizable fraction. Pb presented higher concentrations in the reducible fraction (45.41%). The strongest lithogenic contribution was shown by Ni (31.91%) and Mn (35.44%). PCA clearly showed the determinant role of organic matter and fine sediments in the distribution of metals in the study area and also a common source for these elements, with the exception of Cu. Risk Assessment Code (RAC) established Zn as the most concerning element in the study area. The decreasing mobility order, based on the sum of the three extractable fractions of BCR, was Pb > Cu > Cr > Zn > Ni > Mn. The comparison of the results with sediments quality guidelines (SQG) proved fractionation to be mandatory in the evaluation of effective ecological risk concerning trace elements in sediments.

Sedimentological and radiocarbon investigations are part of an ongoing research on the Bay-head delta of northeast Guanabara Bay, Rio de Janeiro State. Sediment accumulation indicates that the Holocene infill of the bay-head delta started around 8.2 kyr BP and was not in pace with the eustatic sea-level rise. Sediment accumulation was faster during the transgressive phase (0.56 cm.yr -1 ). However, during the regressive phase, progradation driven by baselevel fall was predominant over vertical sediment accumulation (0.02 cm.yr -1 ). Based on coring, three sedimentary units were defined: fluvial sands (U1), estuarine deposits (U2) and fluvial mud (U3).

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