Field Analysis:Redoximorphic
From SASSA
SASSA Home Page ⇒ Field Analysis Home Page ⇒ Why Record Soil Properies ⇒ Why Record Soil Colour ⇒ Redoximorphic Soil Colours
[edit] Redoximorphic colour patterns
As sediments accumulate buried deposits are isolated from the air and may be subject to waterlogging as the ground water level is rises in line with the new ground surface. The result can be the development of anaerobic burial conditions which overprints the stratigraphic section with characteristic redoximorphic colour patterns.
Iron in its oxidised form (ferric, Fe3+) usually imparts a reddish brown colour to the soil, however in anaerobic conditions iron accepts negatively charged electrons and is reduced (ferrous, Fe2+). Ferrous iron is more soluble than oxidised ferric iron and hence, iron is leached from the profile. In permanently waterlogged soils this results in a uniform grey or bluish/grey colour.
However, where the water table fluctuates, ferrous iron dissolved in the groundwater can be reoxidised and is deposited when it meets oxygenated areas of the soil, for example in the upper oxygenated zone of the water table, in the centre of peds, or along root channels. Iron can also be deposited where there is a higher pH, for example around limestone inclusions, and at other chemical and physical boundaries such as along cuts and at boundaries between depositional events. The result is reddish mottles, nodules and iron pans within an otherwise grey matrix.
Buried ‘turf lines’ (Ah horizons of buried soils) are often cemented by iron panning, which in the case of Bronze Age barrows in Jutland can enclose the entire core of the monument. In some instances these iron pans may persist even when the barrow itself has been ploughed out. Iron pans are also often found at the boundaries between different stratigraphic units, however it should be noted that they rarely follow the boundary faithfully, and their position can migrate slowly over time so they should not be relied on when drawing sections.
