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This paper presents new multi-proxy records of the Holocene environmental and climatic changes in the Western Caucasus revealed from a continuous sediment sequence from mountainous Lake Khuko (Caucasus State Natural Biospheric Reserve, 1744 m a.s.l.). Palaeoecological analyses of a sediment core for grain size, magnetic susceptibility, loss on ignition, and pollen allowed us to determine five principal climatic phases with several subphases since 10.5 ka BP. The age model is based on seven accelerator mass spectrometry 14C dates, supplemented by 210Pb data for the uppermost part of the sediment core. Warm periods (10.5–6.7, 6.7–5.5, 3.5–2.4, 0.8–0.5 ka BP) were characterized by high biological productivity in the lake as indicated by high organic matter content and expansion of forests, typical of modern low and middle mountain zones, as indicated by the increase in abundance of Quercus, Ulmus, Corylus, and Tilia in the pollen assemblages. Cold periods (5.5–3.5, 2.4–0.8, and 0.5 ka BP–present) are marked by a consistent decrease in organic matter content in lake deposits and possibly higher intensity of the catchment erosion. The changes in pollen assemblages (for instance peaks of Abies, Picea, and Pinus) suggested a potential elevational decline in the boundaries of vegetation belts and expansion of high-altitude woodlands. Abrupt changes in the lake ecosystem were identified between 4.2 and 3.5 ka cal BP marked by a short-term variation in sediment regime shown by variation in organic matter content, magnetic susceptibility values, and sediment grain size. This was probably caused by climatic fluctuations in the Western Caucasus region as a result of complex shifts in the ocean-atmosphere system during the 4.2 ka event. Overall, the first Holocene multi-proxy continuous lake sediment record provides new insights into the climate history in the Western Caucasus. [ABSTRACT FROM AUTHOR]

Over the past 30 years, there has been a catastrophic reduction of the glacierized area in the mountains of the Greater Caucasus, with the values reaching 0.69% per year. Physical mechanisms accountable for such intense melting are investigated in this study. The main trends in the temperature‐moisture regime of the Caucasus and adjacent areas for the period 1982–2015 were recovered based on instrumental data and the ERA‐Interim data reanalysis. It is demonstrated that there is statistically significant warming for the summer season in the region as a whole. No statistically significant changes were detected in the precipitation regime, despite the increase in the integral moisture content of the atmosphere and in the potential convective energy. The integral moisture content growth is compensated by the increase in the moisture divergence. This happens due to the intensification of large‐scale descending atmospheric motions. As a result, the seasonal and annual precipitation amounts do not change significantly. Such effects are possibly the consequence of the global process of "widening of the tropics." This process is most clearly manifested in the expansion of the Hadley cell and the northward shift of its descending branch. This process can lead to an increased frequency of anticyclones over the southern regions of Europe during the warm part of the year. This, in turn, leads to a negative cloud cover trend as well as an increase in the closely related radiation balance. Apparently, this process is the cause of the intensive reduction of the glaciation area in the North Caucasus. [ABSTRACT FROM AUTHOR]

The specific activities of natural (210Pb, 226Ra, and 232Th) and artificial (137Cs, 239,240Pu, and 241Am) radionuclides in the sediments of two North Caucasus lakes were determined. The two lakes, Lake Khuko and Lake Donguz-Orun, differ in their sedimentation conditions. Based on the use of unsupported 210Pb ex and both Chernobyl-derived and bomb-derived 137Cs as chronological markers, it was established that the sedimentation rates in Lake Khuko over the past 55–60 y did not exceed 0.017 cm y−1. Sedimentation rates in Lake Donguz-Orun were found to be more than an order of magnitude higher. In the latter case, the sedimentation rates for the period from 1986 to the present were over 1.5 times higher than they were for the period 1963–1986. The differences in sedimentation rates were due to differences in the rates of denudation of their respective catchment areas. The specific activities of artificial radionuclides (137Cs, 2600 Bq kg−1; 239,240Pu, 162 Bq kg−1; and 241Am, 36 Bq kg−1) and their ratios in the sediments of Lake Khuko show that their deposition was mainly due to global stratospheric fallout of technogenic radionuclides associated with nuclear bomb testing during 1954–1963—rather than fallout from the Chernobyl accident. Several factors, including the mode of precipitation, features of the surface runoff, and location of Lake Khuko, were responsible for the accumulation of artificial radionuclides. Image 1 • Both 137Cs and 210Pb ex were used for evaluation of sedimentation rates in the two Caucasus lakes. • Sedimentation rate in Donguz-Orun lake located in proglacial zone increase in 1,5 times since 1986. • Global warming is the main reason of increasing denudation rates in periglacial zone. • High concentrations of technogenic radionuclides were determined in sediments of Lake Khuko. • The possible contribution of different sources of Khuko lake sediment contamination is determined. Main finding: It was determined that global warming is the primary reason the denudation rates in the periglacial zone are increasing. The possible sources of the radioactive contamination of the Khuko lake sediment were also determined. [ABSTRACT FROM AUTHOR]

The purposes of this study are to determine the content and origin of anthropogenic fallout radionuclides (FRN) in soils of Mount Khuko, located in the western sector of the Caucasus Mountains and to assess the possibility to use them for evaluation of sediment redistribution for the alpine grasslands,. The field study was carried out in August 2019 near the top of Mount Khuko, located in the western part of the main Caucasus Mountain Ridge. Integral and incremental soil samples were collected from the different morphological units of the studied area. The content of 137Cs and 241Am in soil samples was evaluated using laboratory gamma-spectrometry. A part of samples was selected for Pu isotopes extraction and then alpha-spectrometric analysis. It was established that the 137Cs contamination of soils in the studied area has at least two sources of origin. The first source is the 137Cs bomb-derived fallout after the bomb tests in 1950–60th, which is widespread across the globe. The second source is 137Cs Chernobyl-derived fallout High random variability (Cv = 25–42%) was found within reference sites, located at the undisturbed areas on the local flat interfluves due to high variability of soil characteristics (grain size, density, organic matter content etc.). However minimum spatial variability (range 12,2–14,3 kBq/m2) was identified for the mean value of 137Cs inventories for all 5 reference sites located in the different parts of the studied area. It is difficult to separate individual peaks of the bomb-derived and Chernobyl-derived 137Cs falloutin sediment sinks with low sedimentation rates. Application 239,240Pu as an additional chronological marker allows to identify the origin of above mention peaks in the soils of alpine grasslands and of dry lake bottom. • Radioactive contamination with anthropogenic FRN of soils in the Western Caucasus has a heterogenous origin. • Variable deposition and lateral migration of 137Cs complicates interpretation of the vertical distribution. • 239,240Pu is implemented as an additional chronomarker for evaluation of sedimentation rates. [ABSTRACT FROM AUTHOR]