4-10 Holocene Formation of Pedogenic Hematite in Cool Temperate Forest Soils

Session: Recent Advances in Soil and Paleosol Science

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Abstract:

Soil color is one of the most important and reliable indicators of pedo-environmental conditions. Reddening (or rubification) in soils and paleosols is commonly linked to the accumulation and/or transformation of free Fe oxides, with redder (5YR–10R hue) colors indicating higher hematite content. Hematite formation is generally favored under conditions of intense weathering and/or low water activity, hence its prevalence in tropical and dryland soils. As a result, the potential for pedogenic hematite formation in relatively young soils, particularly in cool temperate climates, has been largely overlooked, and by some researchers, discounted altogether. Here, we report on and explain contemporary formation of pedogenic hematite in cool temperate forest soils, drawing on data from several fine-loamy Glossudalfs (Retisols) with reddish (5YR) subsoils in northern Michigan (USA) and Estonia. In these soils, establishment of perched water in the subsoil during seasonal wet periods triggers the dissolution of free Fe oxides under low Eh (reducing) conditions, and the concomitant release of Fe ions into solution. The presence of highly reactive dissolved organic carbon compounds, produced and translocated from surface leaf litter, helps to facilitate dissolution reactions and to temporarily inhibit Fe oxide recrystallization. Once soluble, local gradients in matric potential favor migration of Fe to drier zones, where high Eh (oxidizing) conditions combined with neutral to alkaline pH values allow rapid hydroxylation and polymerization of Fe ions into the intermediate oxyhydroxide phase ferrihydrite. These metastable Fe oxide forms precipitate initially as finely disseminated concentrations on the surface of voids. During intervening dry periods, subsoil desiccation causes the dehydration and transformation of freshly deposited ferrihydrite to hematite. Repeated wetting-drying cycles drive the gradual segregation of Fe and development of diagnostic eluvial-illuvial features and horizons that typify these soils. This model (1) reports on the broader range of environments that may favor the conditions – strong seasonality in soil wetness, Fe oxide instability, and transient stabilization of ferrihydrite – necessary for hematite formation, and (2) emphasizes caution when using iron oxide mineralogy in paleosols to interpret past environmental conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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