The Fe isotopic composition of soils and waters from the Ravenna area (Northern Italy) and its significance for environmental studies

Stable Fe-isotope ratios have been measured by TIMS in samples of natural and cultivated soils as well as surface waters from a sector of the Ravenna area located next to the industrial zone that host particularly iron mills, and contoured by fields exploited by intensively agriculture. The soils, that can be classified as entisols in interdune lowlands and are composed of quartz, feldspars, calcite, muscovite, chlorite and dolomite, were analyzed following a three-step sequential chemical procedure, to obtain different extractable fractions and, thus, determine the potential sources of Fe in poorly-crystalline Fe oxides, crystalline Fe oxides and silicate-bound Fe (Wiederhold et al., 2007). Iron isotope ratios of bulk soil digests were compared with those of the different Fe fractions. In particular, the δ56Fe of the hydrochloric extracts of the soils, that can represent the metal fraction available to the plants and, thus, provide information on health of the vegetal ecosystem, range narrowly, increasing from slightly negative values in upper horizons to slightly positive values downward each profile. This trend suggests the Fe isotopes are little fractionated biologically during the pedogenesis. Preliminary analyses of waters from the irrigation network related to both natural and cultivated soils, display slightly negative δ56Fe. As a whole, the isotopic signatures of both soils and waters suggest the natural cycle of Fe in these materials is unaffected by significant anthropogenic contributions, despite the location of the study area just in front of major pollution sources. This conclusion is consistent with what has been acknowledged by a study on the distribution of potentially toxic metals and stable/radiogenic isotopes, carried out on the same materials from the Ravenna area. Lastly, the results corroborate previous information on fractionation mechanisms of Fe isotopes and help in the development of these isotopes as tracers for biogeochemical Fe cycling in nature