Вестник МГТУ. 2020, Т. 23, № 1.

a difference in the organic matter content in the sediments from the lakes of Karelia and regions located further north, since Pb, being a chalcophile element, and Cr, Co, and Ni, being siderophile elements, accumulate to a significantly greater extent in mineral deposits, which are more common in the Murmansk Region and Scandinavia. In addition, these regions may have generally increased geochemical background levels of these elements due to a large number of Pb, Cr, Co, and Ni ore occurrences and deposits. It was also found that the background concentrations of Zn, Cr, V in the lake sediments from the south of Karelia are close to the respective values in the sediments from small lakes in southern Norway ( Rognerud et al., 2000). On the other hand, the Norwegian sediments have a slightly higher enrichment factor of Co, Pb, and Mn, compared to the Karelian lakes, which we attribute to the multiple ore occurrences of the respective elements in southern Norway and a slightly lower enrichment factor of Cu, Ni, and Cd, compared to the Karelian lakes. The latter may also be associated with the geochemical features of each of the regions. For example, elevated Cu concentrations in the surface waters in the southern part of Karelia and increased background levels of Cd in the rivers of Karelia were previously noted ( Белкина и др., 2012). Fig. 4. Average background levels of heavy metals in the sediments of the lakes in the south of Karelia, Murmansk Region, and Scandinavian countries Рис. 4. Средние фоновые уровни содержания тяжелых металлов в донных отложениях озер юга Карелии, Мурманской области и стран Скандинавии The averaged concentrations of rare-earth elements (REE) in the sediments of lakes in the south of Karelia have a similar distribution pattern with the trend for the upper continental crust (Fig. 5), whose composition is usually adopted to analyze the behavior of lithophilic elements (Интерпретация..., 2001; Sun et al., 1989; Wedepohl, 1995). In both cases, enrichment in light REEs and a negative Eu anomaly are typical. However, at the same time, the total REE content in the upper continental crust is much higher than in the sediments of Karelian lakes, which is due to the fact that in sediments with a high organic matter content, which also applies to the studied lake sediments, the total REE content tends to be low. The accumulation of these elements is usually controlled by the terrigenous fraction of the sediment, therefore the total REE content is usually higher in the mineral sediment, where the organic fraction does not exceed 10-15 % (Страховенко, 2011). In addition to REE, whose levels are low - compared to the average continental crust - in the background layers of the sediments of small lakes in the south of Karelia, the levels of other chemical elements, except Mo, are also low. Mo ores and ore occurrences are common in Karelia ( Минерально-сырьевая..., 2006), which probably contributed to its accumulation in pre-industrial times. On the other hand, the upper sediment layers of the studied lakes are significantly enriched in elements such as Pb, Cd, Tl, Zn, Cu, W, V compared with the average continental crust, which is explained by the anthropogenic factor (Слуковский, 2018). A comparative analysis of the concentrations of various chemical elements in the lake sediments collected for this study and in the river sediments collected in Karelia’s reference areas (Геохимическое., 2004) showed a fairly close level of accumulation within the same region (Fig. 6). A minor difference is noted only for Cu, Ba, and Zn, which are better represented in lake sediment with a high content of organic matter, and for Li, Cr, and Sr, which accumulate more intensively in stream and river sediments due to the abundance of the terrigenous fraction, which predominantly contains lithophile elements. But in general, the data collected for lake sediments