Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

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Burton, ED, Bush, RT & Sullivan, LA 2006, ‘Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils', Geochimica et Cosmochimica Acta, vol. 70, no. 22, pp. 5455 - 5468.


We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26–3.54), FeIII-rich (27–138 μM) surface water with low molar Cl:SO4 ratios (0.086–5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28–30% of aqueous surface water Fe being present as the FeIIISO4 species). Subsurface sediments contained abundant pore-water HCO3 (6–20 mM) and were reducing (Eh < −100 mV) with pH 6.0–6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water FeII concentrations were high (>2 mM) and were constrained by precipitation–dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water FeII, precipitation–dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S−II concentrations. Such low concentrations of S−II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g−1 as Fe) compared to disordered mackinawite (up to 590 μmol g−1 as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.

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