Вестник МГТУ. 2020, Т. 23, № 1.

drilling to a depth of 15 m, including water depth. Over the three years, 278 sediment samples were collected from various depths. Fig. 1. Map of the study area. Inset: MR - Murmansk Region, NOR - Norway, SWE - Sweden, FIN - Finland Рис. 1. Карта района исследования. Врезка: MR - Мурманская область, NOR - Норвегия, SWE - Швеция, FIN - Финляндия All studied sediments are sapropel - the most common type of lake sediments in the Republic of Karelia (Синькевич и др., 1995). The proportion of organic matter in sapropel ranges between 15 and 90 %, however in the majority of the studied lakes, the sediments contained 40-50 % organic matter and can be classified as organic or organosilicate sediments. Only the sediments in Lake Lamba located in the city of Petrozavodsk (Слуковский и др., 2017) belongs to the organo-ferrous type, which is also often found in the region. After collection, all samples were placed into plastic containers, which were labeled and packed in a cooler bag. Then the samples were delivered to the laboratory and placed in a refrigerator, in which they were stored until dry at a temperature of about 4 °C according to the methodological guidelines. For further study, the sediment samples were dried to an air-dry condition at room temperature and then to an absolutely dry condition in an oven at a temperature of approximately 105-110 °C. Laboratory tests were carried out at the Analysis Center of the Institute of Geology of the Karelian Research Centre of the Russian Academy of Science, in Petrozavodsk, Karelia. Sediment samples were decomposed with acid in an open system. For analysis, sub-samples 0.1 g each were used. The samples were placed in 50 ml Teflon glasses, 0.1 ml of a solution containing 8 ppb 161 Dy was added (to monitor chemical yield during the decomposition of samples) and then several drops of deionized water. Then, 0.5 ml of HClO4 (Perchloric acid fuming 70 % Supratur, Merck), 3 ml of HF (High Purity, TU 6-09-3401-88), 0.5 ml of HNO3 (High Purity GOST 11125-841) were added and evaporated until intense white vapor was observed. HF, HNO3, HCl underwent additional purification in a PTFE/PFA SubboilingEco IR distiller. The glasses were cooled, their walls were washed with water, and the solution was again evaporated to wet salts. Then, 2 ml of HCl (High Purity, GOST 14261-772) and 0.2 ml of a 0.1 M solution of Н3ВО3 (AR grade) were added and evaporated to a volume of 0.5-0.7 ml. The resulting solutions were transferred into polyethylene bottles and diluted with deionized water to 20 ml. For analysis, basic solutions diluted by a factor of 20 were used. To prepare blank samples, the above procedure was repeated in Teflon glasses without samples. Together with the analyzed samples, blank samples and one standard (control) sample (sediments from Lake Baikal BIL-1 - GSO 7126-94) were decomposed. The content of chemical elements (Li, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, W, Tl, Pb, Bi, Th, U) in the sediment samples was measured by the mass spectral method using an XSeries 2 ICP-MS instrument by Thermo Ficher 1GOST 11125-84. Super pure nitric acid. Specifications. Moscow, 2006. 2 GOST 14261-77. Hydrochloric acid super pure. Specifications. Moscow, 1988.