A COMPARISON OF THE EARLY DIAGENETIC ENVIRONMENT IN INTERTIDAL SANDS AND MUDS OF THE MANUKAU HARBOR, NEW-ZEALAND

The early diagenetic environment of intertidal sandy sediments (sands) and muddy sediments (muds) is described and compared from two cores t aken from an unpolluted part of the Manukau Harbour, New Zealand. Extr action techniques characterized the form of the trace elements (Fe, Mn , S, C, Pb, Zn, Cu) at different depths in the sediment. Dissolved for ms of Fe, Mn, and S were measured in interstitial water. Nonresidual m etal concentrations, humic acid, FeS, and FeS2 are an order of magnitu de higher in the muds than in the sands because of dilution by unreact ive sand particles. Muds contain a larger proportion of metals in the mobile fractions; exchangeable (Mn), carbonate (Mn, Fe, Zn), and easil y-reducible oxide (Fe, Mn, Zn, Pb). This is due to greater surface are a (for Mn adsorption); the favorable conditions for MnCO3, FeCO3, and FeS precipitation; and higher concentrations of easily reducible iron oxide and humic acid. Therefore, compared to the sands, muds are more important as reservoirs for toxic metals, both in terms of quantity an d availability. At either site there was very little difference betwee n the forms of Zn, Pb or Cu identified by sequential extraction as sed iments changed from oxic to anoxic conditions. One reason for this is that the amounts and proportions of some of the important components t hat bind metals, viz., amorphous iron hydrous oxides, humic acids, and FeS2, do not change much. Other components that do change with redox conditions, for example, manganese phases and FeS, are only minor comp onents of the sediment. Redox conditions, then, have relatively little effect on trace-metal partitioning