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

Edward D. Burton, Richard T. Bush, Leigh A. Sullivan

Research output: Contribution to journalArticle

103 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)5455-5468
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number22
DOIs
Publication statusPublished - 2006
Externally publishedYes

Fingerprint Dive into the research topics of 'Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils'. Together they form a unique fingerprint.

  • Cite this