Journal article
Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils
Geochimica et Cosmochimica Acta, Vol.70(22), pp.5455-5468
2006
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Abstract
<p>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), Fe<sup>III</sup>-rich (27–138 μM) surface water with low molar Cl:SO<sub>4</sub> 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 Fe<sup>III</sup>SO<sub>4</sub> species). Subsurface sediments contained abundant pore-water HCO<sub>3</sub> (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 Fe<sup>II</sup> concentrations were high (>2 mM) and were constrained by precipitation–dissolution of siderite. The near-neutral, reducing conditions also promoted SO<sub>4</sub>-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 Fe<sup>II</sup>, 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<sup>−1</sup> as Fe) compared to disordered mackinawite (up to 590 μmol g<sup>−1</sup> 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.</p>
Details
- Title
- Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils
- Creators
- Edward D Burton - Southern Cross UniversityRichard T Bush - Southern Cross UniversityLeigh A Sullivan - Southern Cross University
- Publication Details
- Geochimica et Cosmochimica Acta, Vol.70(22), pp.5455-5468
- Identifiers
- 1340; 991012821327902368
- Academic Unit
- School of Environment, Science and Engineering; Southern Cross GeoScience; Science; Faculty of Science and Engineering
- Resource Type
- Journal article