A comparison of the early diagenetic environment in intertidal sands and muds of the Manukau Harbour,New Zealand |
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| Authors: | R B Williamson T M Hume J Mol-Krijnen |
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| Institution: | (1) NIWA Ecosystems, National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand |
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| Abstract: | The early diagenetic environment of intertidal sandy sediments (sands) and muddy sediments (muds) is described and compared from two cores taken from an unpolluted part of the Manukau Harbour, New Zealand. Extraction techniques characterized the form of the trace elements (Fe, Mn, S, C, Pb, Zn, Cu) at different depths in the sediment. Dissolved forms of Fe, Mn, and S were measured in interstitial water. Nonresidual metal concentrations, humic acid, FeS, and FeS2 are an order of magnitude higher in the muds than in the sands because of dilution by unreactive sand particles. Muds contain a larger proportion of metals in the mobile fractions; exchangeable (Mn), carbonate (Mn, Fe, Zn), and easily-reducible oxide (Fe, Mn, Zn, Pb). This is due to greater surface area (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 and availability. At either site there was very little difference between the forms of Zn, Pb or Cu identified by sequential extraction as sediments changed from oxic to anoxic conditions. One reason for this is that the amounts and proportions of some of the important components that 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 components of the sediment. Redox conditions, then, have relatively little effect on trace-metal partitioning in the sediment matrix of these unpolluted sediments. |
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| Keywords: | Sequential extraction Diagenesis Estuary Sediment Heavy metals |
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