Iron geochemical zonation in a tidally inundated acid sulfate soil wetland

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• 作者：Scott G. Johnston^a ; ^{scott.johnston@scu.edu.au} ; Annabelle F. Keene^a ; Richard T. Bush^a ; Edward D. Burton^a ; Leigh A. Sullivan^a ; Lloyd Isaacson^a ; Angus E. McElnea^b ; Col R. Ahern^b ; C. Douglas Smith^b ; Bernard Powell^b
• 关键词：Iron ; Tidal marsh ; Schwertmannite ; Jarosite ; Tidal forcing ; Sea-level rise
• 刊名：Chemical Geology
• 出版年：2011
• 出版时间：24 January 2011
• 年：2011
• 卷：280
• 期：3-4
• 页码：257-270
• 全文大小：1341 K

文摘

Tidal inundation is a new technique for remediating coastal acid sulfate soils (CASS). Here, we examine the effects of this technique on the geochemical zonation and cycling of Fe across a tidally inundated CASS toposequence, by investigating toposequence hydrology, in situ porewater geochemistry, solid-phase Fe fractions and Fe mineralogy. Interactions between topography and tides exerted a fundamental hydrological control on the geochemical zonation, redistribution and subsequent mineralogical transformations of Fe within the landscape. Reductive dissolution of Fe(III) minerals, including jarosite (KFe₃(SO₄)₂(OH)₆), resulted in elevated concentrations of porewater Fe²⁺ (> 30 mmol L⁻¹) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe²⁺-enriched porewater along the intertidal slope. Subsequent oxidation of Fe²⁺ led to substantial accumulation of reactive Fe(III) fractions (up to 8000 μmol g⁻¹) in redox-interfacial, tidal zone sediments. These Fe(III)-precipitates were poorly crystalline and displayed a distinct mineralisation sequence related to tidal zonation. Schwertmannite (Fe₈O₈(OH)₆SO₄) was the dominant Fe mineral phase in the upper-intertidal zone at mainly low pH (3–4). This was followed by increasing lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) at circumneutral pH within lower-intertidal and subtidal zones. Relationships were evident between Fe fractions and topography. There was increasing precipitation of Fe-sulfide minerals and non-sulfidic solid-phase Fe(II) in the lower intertidal and subtidal zones. Precipitation of Fe-sulfide minerals was spatially co-incident with decreases in porewater Fe²⁺. A conceptual model is presented to explain the observed landscape-scale patterns of Fe mineralisation and hydro-geochemical zonation. This study provides valuable insights into the hydro-geochemical processes caused by saline tidal inundation of low lying CASS landscapes, regardless of whether inundation is an intentional strategy or due to sea-level rise.