Barium content of paleosols from Central Italy
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 SO42-, 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.
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