Iron geochemical zonation in a tidally inundated acid sulfate soil wetland

Scott G. Johnston, Annabelle F. Keene, Richard T. Bush, Edward D. Burton, Leigh A. Sullivan, Lloyd Isaacson, Angus E. McElnea, Col R. Ahern, C. Douglas Smith, Bernard Powell

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Abstract

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+ (>30mmol 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 8000μmol 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.

Original languageEnglish
Pages (from-to)257-270
Number of pages14
JournalChemical Geology
Volume280
Issue number3-4
DOIs
Publication statusPublished - 2011
Externally publishedYes

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acid sulfate soil
Wetlands
zonation
Sulfates
Sulfide minerals
porewater
Iron
wetland
Soils
toposequence
iron
Topography
Acids
Minerals
mineral
intertidal environment
Geochemistry
Mineralogy
Hydrology
Sea level

Cite this

Johnston, S. G., Keene, A. F., Bush, R. T., Burton, E. D., Sullivan, L. A., Isaacson, L., ... Powell, B. (2011). Iron geochemical zonation in a tidally inundated acid sulfate soil wetland. Chemical Geology, 280(3-4), 257-270. https://doi.org/10.1016/j.chemgeo.2010.11.014
Johnston, Scott G. ; Keene, Annabelle F. ; Bush, Richard T. ; Burton, Edward D. ; Sullivan, Leigh A. ; Isaacson, Lloyd ; McElnea, Angus E. ; Ahern, Col R. ; Smith, C. Douglas ; Powell, Bernard. / Iron geochemical zonation in a tidally inundated acid sulfate soil wetland. In: Chemical Geology. 2011 ; Vol. 280, No. 3-4. pp. 257-270.
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Johnston, SG, Keene, AF, Bush, RT, Burton, ED, Sullivan, LA, Isaacson, L, McElnea, AE, Ahern, CR, Smith, CD & Powell, B 2011, 'Iron geochemical zonation in a tidally inundated acid sulfate soil wetland', Chemical Geology, vol. 280, no. 3-4, pp. 257-270. https://doi.org/10.1016/j.chemgeo.2010.11.014

Iron geochemical zonation in a tidally inundated acid sulfate soil wetland. / Johnston, Scott G.; Keene, Annabelle F.; Bush, Richard T.; Burton, Edward D.; Sullivan, Leigh A.; Isaacson, Lloyd; McElnea, Angus E.; Ahern, Col R.; Smith, C. Douglas; Powell, Bernard.

In: Chemical Geology, Vol. 280, No. 3-4, 2011, p. 257-270.

Research output: Contribution to journalArticle

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T1 - Iron geochemical zonation in a tidally inundated acid sulfate soil wetland

AU - Johnston, Scott G.

AU - Keene, Annabelle F.

AU - Bush, Richard T.

AU - Burton, Edward D.

AU - Sullivan, Leigh A.

AU - Isaacson, Lloyd

AU - McElnea, Angus E.

AU - Ahern, Col R.

AU - Smith, C. Douglas

AU - Powell, Bernard

PY - 2011

Y1 - 2011

N2 - 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+ (>30mmol 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 8000μmol 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.

AB - 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+ (>30mmol 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 8000μmol 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.

KW - Iron

KW - Jarosite

KW - Schwertmannite

KW - Sea-level rise

KW - Tidal forcing

KW - Tidal marsh

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JO - Chemical Geology (Isotopic Geoscience) Section

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