Rare earth elements in humic acids and soils of the archaeological site Stepnoe 7 (Southern Urals)

The growth of rare elements used in modern industrial production requires research activation with a view to their monitoring and regulation. The comparison of trace elements content in modern soils with ancient analogues withdrawn from active functioning, can help to assess the accumulation of elements in soils of the date hereof. Modern background soil, as well as paleosols and cultural layers of the archaeological site Stepnoe 7 of Bronze epoch, located in the Southern Urals in Chelyabinsk region are the objects of the study. Profile of modern soil background in morphology corresponds to leached chernozems, has a slightly alkaline environment and the magnetic susceptibility of organic-horizon 2.3–3.5×10–6 CGSE per/g of sediment. The humus horizon contains on average 3.2% of organic carbon, about 50% of it is humic acids, which exceeds the number of fulvic acids in more than 2 times and causes humate type of humus. Humic acids of the A horizons upper thickness of the soil profile contains 50.5 ± 3.4% C, 4.2 ± 0.3% H, 41.5 ± 2.87% O and 3.3 ± 0.3% N from mass. Buried soils and cultural layers have similar characteristics: the humus content is about 1%, it is characterized by humate composition, and the elemental composition of humic acids corresponds to steppe conditions of their functioning. Humic acids were allocated from 0.1n NaOH extracts after preliminary decalcification by their sedimentation 2n HCl, with the subsequent dissolution in alkali and resedimentation. The total content of rare earth elements in humic acids and soils was determined by multi-element neutron activation analysis in the laboratory of Tomsk Polytechnic University. A comparison of the content of rare earth elements in modern soils, paleosols and cultural layers with Clark in the lithosphere allows us to consider them non-contaminated lanthanides. Waning row of average element content is the same for all objects under study and looks as follows: Ce> Nd> La> Sm> Yb> Eu> Tb> Lu. However, for almost all of the elements the average content in the modern background soil is much higher than in the buried soils and cultural layers, although significant differences using the Student's t-test for the objects being compared are not identified for any metal. In order to establish the contribution of HA in the binding of rare earth element content of soils, the contents of elements in the preparations were recalculated to their number in the soil, per humic acid. The content of total organic carbon in the soil, the share of humic acids in the humus composition and carbon content in HA preparations were taken into account. The total percentage of all studied rare-earth metals in humic acids in relation to their content is 4/03% in the modern soil, 1.56% in the cultural layers and 1.28% in the buriedpaleosols. Thus, it clearly shows a tendency of increasing the content of rare earth elements in modern southern forest-steppe soil of Zauralye in the last four thousand years. The share of trace elements associated with humic acids of modern soils in the total pool of rare earth elements of more than 3 times higher than in paleosols and cultural layers.

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