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Thirty very deep aquic spodosols in swales of the Bassendean Dune System, Western Australia were investigated for their potential to acidify due to their contents of acid-forming sulphur minerals. Pyrite commonly occurs in B, Bh, C and sometimes A and E horizons of these soils. Two morphologies of pyrite are present: single crystals of pyrite are euhedral cubic or octahedral (≤ 1 μm) and framboidal pyrite aggregate grains range from 1 to 10 μm. Elemental sulphur in the form of anhedral particles 10–500 nm may occur in E, B and Bh horizons and is strongly associated with organic matter and jarosite sometimes occurred in the A horizon. Rare marcasite and mackinawite are other acid generating sulphide minerals and indicate the diverse redox and other chemical conditions that exist in these spodosols. Consequently drying and aeration of these soils due to drainage and possibly also to climate change acidifies both soil and groundwater. These spodosols are therefore acid sulphate soils. Acid buffering is mostly minor and is provided by organic matter and allophane. These observations of acid generating sulphides in spodosols may indicate that very acid nonaquic spodosols elsewhere were once saturated and contained pyrite before the profiles experienced permanent or episodic oxidising conditions. |
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