Sulfur dynamics in boreal acid sulfate soils rich in metastable iron sulfide—The role of artificial drainage

The sulfur dynamics of boreal brackish-water sediments rich in metastable iron sulfide (average elemental composition of FeS1.1), which upon oxidation have a huge impact on stream-water chemistry, were studied. Two cores, extending from the upper oxidized layer (acid sulfate soil) into the underlying iron sulfide-bearing sediment (potential acid sulfate soil) were collected at a site close to the sea level and at another site about 40 m above the latter in a region of current isostatic land uplift (Finland). The data clearly show, in contrast to what is often argued, that these notorious acidic soils are formed not as a result of the natural uplift but because of extensive ditching of farmlands. Above the depth of artificial drainage, S and Ni are abundantly lost, while beneath that level pyrite is abundant and metastable iron sulfide is increasing with depth, and the Ni concentration is relatively stable. In the narrow zone between the reduced and oxidized layers, the processes are dynamic and result in preservation of elemental S at one of the locations. The sulfur isotopic composition for metastable iron sulfide and pyrite in the investigated acid sulfate soils were found to be distinctly bimodal and roughly corresponding to a similar distribution in stream-water sulfate earlier reported from the same region. This indicates that pyrite is the main source of sulfate and acidity in some affected drains, while in others it is metastable iron sulfide.