Contemporary iron stone formation due to acid sulfate soil processes

One of the defining factors affecting soil formation is parent material: it may therefore be expected that sulfidic parent materials leave acid sulfate soil ‘inprints' within the soil profiles developing on such parent materials. Indeed, acid sulfate soil processes have been thought to be responsible for relict soil features (e.g. Fanning, pers. comm.; Rabenhorst et al 2004). This presentation describes the occurrence of a contemporary iron stone layer (i.e. an indurated layer cemented by iron minerals) within the channel of the Talbragar River in central New South Wales, in Eastern Australia. A long term study has been conducted at this site since 2001 examining sulfidisation of soils and sediments as a result of dryland salinisation. During a recent prolonged drought at this study site an opportunity to examine the effects of oxidation of the accumulated sulfides in the sediments was presented. One such manifestation of oxidation within the sulfidic sediments was the development of an ironstone layer up to 15 cm thick. The contemporaneous nature of these formations was demonstrated by the occasional inclusion of living plant roots within the ironstone layer. Results are presented that demonstrate that the iron minerals cementing the sediments were released from the iron sulfide minerals underlying the ironstone formation. The very low watertables resulting from the drought conditions allowed oxidation of the sulfides and resulted in enrichment of both sulfates and iron into the surrounding matrix. In the case of iron, the accumulation of these oxidation products was in the soil layer immediately overlying the sulfidic sediments and exposed to the atmosphere. The released iron deposited as iron minerals cementing the ironstone layer. It is likely that this sulfidic oxidation process, observed contemporaneously at the Talbragar River study site, may provide an explanation for the formation of ironstone layers in other geomorphic and geological strata.