Acid Sulphate Soil and Framboidal Pyrite in Northern Sweden

Sammanfattning: A soil profile was taken close to Skellefteå in Västerbotten, northern Sweden. The profile has an upper oxidised zone, 30 cm thick, underlain by a transitions zone, in this paper called accumulation zone, and a reduced zone at 160 cm depth. pH was measured in the field and samples from all zones were analysed for major and trace elements by ICP-SFMS. A batch test was performed on samples from the three zones. Optical microscopy and SEM were used to study mineralogy, focusing on the occurrence of framboidal pyrite. The results were compared with those of profiles in the Luleåarea further north, studied earlier. The results clearly show that the sampled soil profile in the Skellefteå area is of similar types as the soils in the Luleå area. A distinct upper oxidation zone has been developed in all profiles with a strong depletion of S, due to oxidation of iron sulphides when these sediments have been exposed to atmospheric oxygen. This oxidation appears to be stronger in the Luleå profiles than in theSkellefteå profile, possibly because they have oxidised a longer time. All the Luleå profiles and theSkellefteå profiles are acid sulphate soils. There is a depletion of sulphide-associated trace elements such as Cd, Cu, Ni and Zn in the oxidation zone due to the oxidation of iron sulphides with subsequent low pH and leaching. This depletion is more pronounced in the Luleå profiles than in the Skellefteå profile. There is a tendency for secondary accumulation in the accumulation zone, in the Skellefteå profile close to the groundwater table in particular. The batch test in the Skellefteå profile allowed for the observation that the highest concentrations occurred at the groundwater level. Framboidal pyrite occurs in the Skellefteå profile as well as in the Luleå profiles. In the Skellefteåprofile there is a positive correlation between organic matter, shells and framboidal pyrite, illustrating that organic matter was important for the formation of framboidal pyrite. Although most of the pyrite has been oxidized and dissolved in the oxidized zone, there was still some framboidal pyrite left, possibly protected by shells and organic matter.