IRON OXIDE CRYSTALLINITY IN FLUVIAL TERRACES, RENO RIVER VALLEY, ITALY

The ratio of amorphous to total secondary iron oxides of soils developing in Quaternary sediments has been shown to decrease systematically as a function of soil age for chronosequences (McFadden and Hendricks, 1985). In the valley of the Reno River (Fiume Reno) south of Bologna, Italy, a soil chronosequence is being developed for a suite of fluvial terraces (Qt1 – Qt9) ranging in age from Middle Pleistocene to Late Holocene. In an effort to help quantify pedogenic alteration as a function of surface age, Fe oxide crystallinity was examined for 10 soils on 6 terrace units.

For each soil profile, horizon(s) with the greatest degree of soil development as indicated by structure, color and texture (typically a B horizon) were chosen for Fe extraction. Extraction solutions of oxalate (McKeague and Day, 1966) for amorphous Fe oxides and dithionite (Mehra and Jackson, 1960) for total amorphous and crystalline Fe oxides were used. The extracts were analyzed by atomic absorption spectrometry. The oxalate-dithionite ratio is an indicator of iron oxide crystallinity. As the ratio decreases, amorphous Fe oxides are depleted relative to crystalline Fe oxides.

Preliminary results indicate that the oxalate-dithionite ratio is highest for Qt9 (latest Holocene) and lowest for Qt1, a terrace of unknown age. For terraces of intermediate age and position, however, the oxalate-dithionite ratio does not always decrease with increasing terrace age as hypothesized. When more than one profile is examined among terraces of similar age, the amount of dithionite-extractable Fe varies between profiles by 0.0 to 2.8 weight percent Fe₂O₃, averaging 1.5 wt% difference. Extractable iron varies less, however, within a single profile for the horizons sampled. In profiles for which more than one horizon was sampled, the amount of dithionite-extractable Fe varies between horizons by 0.0 to 0.9 weight percent Fe₂O₃, averaging 0.3 wt% difference. Field observations suggest that some soil profiles incorporate varying amounts of colluvial cover. Possible explanations for variable extractable iron content between profiles are 1) differences in initial mineralogy of sediment between sampling locations, 2) effects of Fe cycling for soils that are more influenced by the water table, and 3) contribution of reworked secondary Fe oxides by hillslope processes.