Abstract
A paper published in 2013 concerning the elemental composition of 50 paleosols from Central Italy (Tuscany and Abruzzo) noted that Barium values were exceptionally high, a fact which could be not justified by the presence of neighbouring outcrops of igneous rocks. This current paper aims to explain the origin and significance of barium enrichment in these Pleistocene paleosols (Xerults, Xeralfs, Udalfs and Xerepts) formed on alluvial siliciclastic deposits mainly derived from sandstones and shales and, in a few cases, insoluble residues of Mesozoic carbonate rocks. Ba content was considered in relation to the molecular weathering ratio, soil pH and total Mg content. A qualitative inverse correlation was suggested between Mg and Ba for most soils. Maximum Ba values were found both in acid and alkaline soils, while minimum values were observed in neutral soils. This suggests the existence of different pedoclimatic environments. The acid soil solution supports the Ba release from minerals and soil components and its precipitation in the presence of SO<sub>4</sub><sup>2-</sup>, clays and Fe/Mn nodules, whereas other bases are removed. In contrast, the soils during seasonal low humidity periods or extended events of drought have a different pH: the soil solution then has a higher base concentration and therefore a greater likelihood of Ba precipitation bound to clays, metal oxides or even as insoluble sulphate (barite). Thus the soils with higher and lower pH are enriched in Ba both directly and indirectly in relation to ionic strength of the soil solution. The significant enrichment in Ba is ascribed to intense and very long pedogenetic processes during past climatic conditions of a wet- warm subtropical type, which acted on the Italian paleosols of Pleistocene age. The recurring shrink-swell pattern would have increased the rate of chemical weathering, the rate of barium release in the soil solution and its subsequent immobilisation in neoformed complexes as barite and others.
Highlights
In 2013 a paper concerning the natural uranium content of Fe/Mn nodules from 50 paleosols of Central Italy (Magaldi et al 2013) was published (Figure 1)
Principal component analysis (PCA) indicated that association between the trace elements was determined by the composition of different parent materials from which paleosols originated
Average Ba contents of 3 soil group are shown in Table 1a, together the Ba content of selected Fe/Mn nodules from 3 paleosols of Xeralfs + Xerults Group (Table 1b)
Summary
In 2013 a paper concerning the natural uranium content of Fe/Mn nodules from 50 paleosols of Central Italy (Magaldi et al 2013) was published (Figure 1). Uranium and in particular barium, could not be grouped with other elements. It was noted that Ba values were exceptionally high in all soils. These values were not justified by the presence of neighbouring outcrops of igneous rocks or any contribution of past or actual winds. Barium is an important element in the Earth’s crust, with an average content of 425 mg/kg, as a component of many igneous and clastic silicate rock types (Hanor 2000). The Ba content of soils in semiarid sub-tropical regions is about 400 to 1000 mg/kg, compared to maximum values of 2000-3000 mg/kg in humid tropical regions (Aubert and Pinta 1971). Ba2+ ions can eventually react with sulphate ions to form barite (BaSO4), and with bicarbonate to form witherite (BaCO3)
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