Sulfur biogeochemical cycling and novel Fe–S mineralization pathways in a tidally re-flooded wetland

• Published in: Geochimica et cosmochimica acta 2011-06, Vol.75 (12), p.3434-3451
• Main Authors: Burton, Edward D., Bush, Richard T., Johnston, Scott G., Sullivan, Leigh A., Keene, Annabelle F.
• Format: Article
• Language: English
• Subjects: acidification; acidity; biogeochemical cycles; dissolved organic carbon; drainage; groundwater; iron; jarosite; mineralization; oxidation; pyrites; sediments; soil water; sulfur; wetlands
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.03.020

Sulfur biogeochemical cycling and associated Fe–S mineralization processes exert a major influence over acidity dynamics, electron flow and contaminant mobility in wetlands, benthic sediments and groundwater systems. While S biogeochemical cycling has been studied intensively in many environmental settings, relatively little direct information exists on S cycling in formerly drained wetlands that have been remediated via tidal re-flooding. This study focuses on a tidal wetland that was drained in the 1970s (causing severe soil and water acidification), and subsequently remediated by controlled re-flooding in 2002. We examine SO 4 2 - reduction rates and Fe–S mineralization at the tidal fringe, 7 years after the commencement of re-flooding. The initial drainage of the wetland examined here caused in-situ pyrite (FeS 2) oxidation, resulting in the drained soil layers being highly acidic and rich in SO 4 2 - -bearing Fe(III) minerals, including jarosite (KFe 3(SO 4) 2(OH) 6). Tidal re-flooding has neutralized much of the previous acidity, with the pore-water pH now mostly spanning pH 5–7. The fastest rates of in-situ SO 4 2 - reduction (up to ∼300 nmol cm −3 day −1) occur within the inter-tidal zone in the near-surface soil layers (to ∼60 cm below ground surface). The SO 4 2 - reduction rates correlate with pore-water dissolved organic C concentrations, thereby suggesting that electron donor supply was the predominant rate determining factor. Elemental S was a major short-term product of SO 4 2 - reduction, comprising up to 69% of reduced inorganic S in the near-surface soil layers. This enrichment in elemental S can be partly attributed to interactions between biogenic H 2S and jarosite – a process that also contributed to enrichment in pore-water Fe 2+ (up to 55 mM) and SO 4 2 - (up to 50 mM). The iron sulfide thiospinel, greigite (Fe 3S 4), was abundant in near-surface soil layers within the inter- to sub-tidal zone where tidal water level fluctuations created oscillatory redox conditions. There was evidence for relatively rapid pyrite re-formation within the re-flooded soil layers. However, the results indicate that pyrite re-formation has occurred mainly in the lower formerly drained soil layers, whereas the accumulation of elemental S and greigite has been confined towards the soil surface. The discovery that pyrite formation was spatially decoupled from that of elemental S and greigite challenges the concept that greigite is an essential precursor requ