Acid sulfate (AS) soils occupy significant areas along the boreal coastal plains of Finland. These soils are developed from oxidation of sulfide bearing sediments, which were formed under reducing conditions during the Litorina Sea stage (8000-3000 B.P.). Due to post-glacial isostatic land uplift these sediments are today commonly found at elevations between 0-45 m above the current sea level. AS soils develop when the sulfidic sediments (potential acid sulfate (PAS) soils) are oxidised due to oxygen penetration, often caused by drainage for agricultural purposes. The oxidation of PAS soils lead to an initial production and release of sulfuric acid, which commonly lowers the soil pH below 4. Studies have shown that, apart from sulfuric acid, certain toxic metals are also released from the AS soil. The release of acidity and toxic metals affect the environment in a harmful way, and several mass kills of fish have been documented.

In general, pyrite (FeS₂) is considered to compose the main part of the sulfidic material in PAS soils, with minor amounts of iron monosulfides (e.g. FeS and Fe₃S₄). The metastable FeS is usually converted into the more stable FeS₂. In Finnish PAS soils, however, FeS (and possibly Fe₃S₄) is frequently occurring and is commonly the dominant sulfur species. The persistence of FeS in Finnish PAS soils can probably be explained by the reducing conditions and the low sulfate concentration which prevailed in the brackish Litorina Sea. While the total sulfide concentration in Finnish PAS soils is typically in the range of 0.5-2.0%, sulfate sulfur and organic sulfur are usually only present in minor amounts.

The intention of this study is to investigate: (1) the distribution and behaviour (e.g. oxidation) of sulfur species in PAS and AS soils; and (2) the influence of sulfur oxidation on the mineral weathering. The mechanism behind the transformations of sulfur species, chiefly iron sulfides, in the PAS soil and in the transition zone between PAS and AS soils is of particular interest.

Several profiles (including the PAS and AS soil) have been collected from both recently emerged soils (near the shore line) and from older soils, which emerged approximately 4000 years ago (located further inland). The following sulfur species were determined (using a method developed at Åbo Akademi University): (1) iron monosulfides (FeS); (2) pyrite (FeS₂); (3) elemental sulfur (S⁰); (4) sulfate sulfur (SO₄^2-); and (5) organic sulfur (OrgS).

The initial results show that FeS is dominant (c. 60% of the sulfide content) in young PAS soils while FeS₂ (c. 80-90%) seems to be more common in older PAS soils. Sulfate sulfur, OrgS and S⁰ exists only in minor amounts in the un-oxidised PAS soil. The main part of S⁰ in PAS soils is believed to be a product of FeS and FeS₂ oxidation, and the concentration is generally greatest in the transition zone. Sulfate sulfur is the end product of the oxidation of FeS and FeS₂ (and possibly OrgS) and the greatest concentrations are found in the AS soil. Organic sulfur, which is believed to originate from assimilatory sulfate reduction, is present only in minor concentrations and oxidises slightly towards the surface.

Based on the initial results we have reason to propose that a variety of mechanisms is responsible for the transformation of FeS and FeS₂ in PAS soils and in the transition zone. There are, for example, indications of FeS (and possibly FeS₂) oxidising beneath the transition zone, even though the conditions (according to pH) are still reducing. This can be seen as a decrease of FeS upwards and a change in the FeS:FeS₂ ratio. The oxidation of FeS (and FeS₂) produces S⁰, which in return reacts with existing FeS to form additional FeS₂. A portion of S⁰ is also oxidised to SO₄^2-. In the transition zone, where FeS is almost completely consumed, S⁰ (from FeS₂ oxidation) can accumulate, before further oxidation to SO₄^2-. Isotopic studies of the sulfur species will be done and contribute to a better understanding of which mechanisms are of importance. Furthermore, sulfur isotopes will also be used in order to interpret the origin, relation and formation of especially FeS and FeS₂ in the PAS soil.