Fraction 2. Metals Bound to Carbonates. Dissolution of the sediment carbonate fraction is essentially complete as evidenced by the disappearance of the dolomite X-ray dif- fraction peak following treatment with the acetate buffer (pH 5.0). Further evidence is provided by the kinetics of the leaching step (Figure 1); increasing the leaching time from 5 to 24 h does not result in higher calcium concentrations in the leachate.