Weathering and Wetness: The Merging of Laterite Formation with Ferrolysis.

An albic horizon may commonly result from ferrolysis in an aquic soil moisture regime. Lateritic bauxite is sesquioxidic residue from strong weathering which must have free drainage to remove bases and silica. An albic epipedon above ferruginous, aluminous laterite would therefore seem anomalous - unless the bleaching by an intermittent, perched water table only develops through ferrolysis after the laterite has formed.  In this paper the upper, bleached ferruginous gravels of ancient, lateritic regolith on the Darling Range of Western Australia are interpreted against a backdrop of related examples from Guyana, South Africa and Texas. These soils are viewed as products of gentle relief, forest vegetation, and periodically intense rainfall which saturates the upper solum, promoting the bacterial reduction of Fe. Some of the Fe is transferred laterally in solution, while some diffuses to zones of lower Fe²⁺ concentration. The transported Fe²⁺ subsequently oxidises and precipitates as lamination and pore sealant in nodular gravel and the underlying hardpan. Gravel abundance might partly be attributed to frequent disruption of the hardened laterite by roots of falling trees. Sparser forest vegetation appears to occur on areas where induration and periodic wetness are most intense. These forest glades have, instead of leaf litter, a surface mulch of dark, pisolitic gravel. We found a decline in the proportion of magnetic nodules with increasing signs of wetness. This could be because soil maghemite has less structural Al than hematite. The densest nodules are vitreous and almost black, representing extreme contraction of glaebules through diffusive retreat of ferrous Fe to a more oxidised interior during wet phases. Nodules may also contain Fe oxide precipitated on their surfaces. Thus iron initially enriched as a residuum of prolonged weathering becomes further concentrated through redox mobilization, both within glaebules and in parts of the soil mantle that gain seepage water from higher ground.