Depression wetland formation by redox-driven iron and silica cycling

Abstract

This paper investigates the origin and geomorphic evolution of Shadow Vlei, a depression wetland located approximately 30 km northwest of Gqeberha, South Africa. The wetland is on sandstone on the African Erosion Surface. The depression wetland has a rim of high-lying ground surrounding a central depression. Fluctuations between wet and dry periods create both highly reducing conditions in soils during wet phases in the depression, and highly oxidising conditions in the depression margin. Under reducing conditions, iron(III) oxides that are insoluble, are reduced to soluble iron(II), which is transported to and trapped in the margins of the depression under oxidising conditions when the water table declines and iron(II) is oxidised to iron(III). There is also a moderate flux of silica from the surface in the centre of the depression to the margins, which is suggested to be associated with maximum alkalinity associated with iron(III) reduction, and decreased alkalinity in the margins associated with iron(II) oxidation. However, the main flux of silica is from the surface in the centre of the depression to depth of greater than 1.5 m, again associated with variations in pH linked to iron-cycling. These reactions have thus caused a net volume loss in the centre of the depression, causing sagging, and a net volume gain at the margins of the depression, causing swelling. Wetland formation associated with alteration of landform morphology as a consequence of alternating redox conditions represents a novel mechanism that has not yet been described.