Stable sulfur isotope dynamics in an acid sulfate soil landscape following seawater inundation

In 2002 a tidally driven seawater exchange remediation strategy was successfully implemented on a severely acidified tropical coastal landscape dominated by acid sulfate soils (ASS) in northern Australia. This study examined changes in the stable sulfur isotope signatures in a range of sulfide and sulfate (SO₄) fractions at three sites with different levels of exposure to the tidally driven seawater exchange remediation. δ³⁴S in the acid soluble SO₄ fraction (e.g. jarosite) was less depleted in ³⁴S than the corresponding sulfide, indicating a degree of fractionation during sulfide oxidation and jarosite precipitation. The δ³⁴S of jarositic-SO₄ was similar at all three sites indicating the appreciable stability of jarositic-SO₄ even after extended exposure to seawater. δ³⁴S of the water soluble, exchangeable and schwertmannitic-SO₄ reflect conditions post remediation and indicate the relative contributions from two potential SO₄ sources – a lighter SO₄ derived from the oxidation of pyrite, and a heavier SO₄ derived from the seawater. The δ³⁴S of the contemporary surficial sulfide accumulations also reflect a SO₄ contribution from seawater used for remediation and were isotopically different from the relict sulfides found at depth at all sites. δ³⁴S of water soluble sulfate allowed the progress of the remediation to be traced down the soil profile. This study demonstrates the utility of stable sulfur isotope signatures in various sulfide and SO₄ fractions to trace the sulfur geochemical pathways occurring in soils, in this case as a result of the introduction of tidally driven sea water.