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Sequential extractions were performed on small amounts of particulate and sediment samples (6 to10 mg) from the Mississippi River mixing zone. The leachates were analyzed for Sr concentration and 87Sr/86Sr isotope ratio. Mn and Fe contents were also measured as their oxyhydroxides are potential carrier phases for Sr. The largest fraction of Sr in the solid phase (particulates and sediments) was found to be present in the residual, refractory fraction (>70% of total). By comparison with the corresponding sediment, particulates appear to have higher concentrations of nonresidual, labile Sr (30% vs. 15%). Carbonate components seem to play an important role as carriers for labile Sr in particulates and sediments. Changes in the composition and content of the solid phase may significantly modify both the 87Sr/86Sr isotope ratio of the total labile fractions and that of the bulk components. However, such modifications, under normal conditions, exert little measurable influence on the Sr isotope composition of the dissolved phase. [Copyright &y& Elsevier] more...