The research of biochemical thermo- and salt-sensitive markers - long-chain alkenones in sediments of lakes of the North Minusinsk valley is an important contribution to the understanding of climatic changes in this region. We found that the total concentration of alkenones in the top layers of bottom sediments of saline lakes increases significantly at a salinity of about 20 g l-1. Also, it was found that the average chain length of alkenones in the surface sediments rises with increasing salinity and thus can be used as an indicator of salinity, and the ratio of alkenones C37/C38 varies widely, suggesting that this index can be used as a paleo-indicator of salinity. The high correlation between ,  and  indices with salinity indicates their potential for paleosalinity reconstruction. The obtained functional dependences were additionally verified in the core of bottom sediments of Lake Utichie-3 and confirmed the possibility of using alkenones as a paleo-indicator of climatically determined salinity changes in endorheic lakes. The taxonomic composition of haptophyte algae and alkenone composition in the studied lakes were comparable to lakes in prairie regions of North America.

Studying the pigment composition of bottom sediments of the meromictic Lake Shira allows us to learn about the past of the reservoir. Analysis of carotenoids of frozen cores of the upper layers of bottom sediments, and comparison with field observations shows the relationship between the hydrological regime of the lake and the content of okenone, a carotenoid of purple sulfur bacteria.

Long-chain alkenones are produced exclusively by certain species of microalgae Isochrysidales of the order of haptophytes (Haptophyta). These are polyunsaturated methyl and ethyl ketones C35–C42 with 2–4 trans double bonds in the aliphatic chain. The length of the hydrocarbon chain and the degree of unsaturation of double bonds of a given class of lipids can vary depending on the environmental conditions of their producers. This class of substances is well preserved in the bottom sediments (BSs) of seas and lakes and therefore can serve as a paleoindicator of climate change. Currently, studies are being conducted to identify the functional dependences of the composition of long-chain alkenones on the temperature and salinity of water in continental water bodies. In arid regions, saline closed-basin lakes sensitively react by changing the volume of water and its salinity to variations in the “precipitation–evaporation” balance, and, consequently, the sediments of such lakes are sources of information about climate humidity variations in the past. In this work, we analyzed the vertical profile of long-chain alkenones in the upper part of the BSs of the Utichye-3 salt lake located in the steppe region of southern Siberia and compared it with changes in the surface level and salinity of the lake water recorded over a period of about 100 years. It has been shown that the content of long-chain alkenones in sediments can increase during periods of high salinity. Consequently, the composition of alkenones can be used to reconstruct changes in salinity from BSs of closed-basin lakes, and, therefore, to reconstruct dry periods in the history of the climate of the steppe zone of southern Siberia.

New AMS14C-dated records of changes in the natural environment of southern Siberia from varve sediments of Lake Shira (Minusinsk Basin) are presented. The results allow reconstructing the history of vegetation, biomes, climate, relative productivity of the lake system, changes in the lake catchment erosion, and water levels in the lake itself over the past 2980 cal. years with an average temporal resolution of 21 years. The new reconstructions indicate a wet regional climate between 2980 and 2650 cal yr BP and its gradual aridization later. At the same time, the climate of the basin itself was more arid, thus providing the development of steppe and meadow-steppe communities around the lake in the last 2980 years. Variations in the Artemisia/Chenopodiaceae pollen ratio, considered as an indicator of moisture available to plants, suggest a slight increase in the moisture level in the Minusa Basin from 2980 to 50 cal yr BP and its noticeable decrease in the last 50 years. Reconstructions show that changes in vegetation around Lake Shira in the Late Holocene were mainly caused by large-scale circulation processes that changed the regional moisture balance. Steppe vegetation turned out to be very sensitive to moisture changes on a decadal scale. No clear pollen indicators of anthropogenic impact on vegetation were found in the new pollen record.

The bottom sediments of the lakes contain biochemical markers of fecal intake, which have been renewed interest in the last few years. This analysis is one of the newest trends in paleolimnology. Fecal markers include sterols recovered by the intestinal microflora – stanols, which are indicators of fecal intake into reservoirs. Relative to other mammals, humans produce the largest amount of coprostanol. Therefore, based on its presence in bottom sediments, the dynamics of human presence in early times can be reconstructed, as well as the fecal anthropogenic load on reservoirs. Using the gas chromatography method with mass spectrometric detection, for the first time we estimated the contents of coprostanol, epicoprostanol, 5α-cholestanol and cholesterol in the bottom sediments of Lakes Peyungda and Zapovednoye (Evenkia, Krasnoyarsk Krai), and also estimated the indices R1 and R2, reflecting the human contribution to fecal intake. The absence of an increase in the proportion of coprostanol in both lakes may indicate a slight anthropogenic impact. Both in the modern period and in earlier times (about 5000 years for Peyungda and 2500 years for Zapovednoye), there were probably no settlements near the lakes.

Abstract. The core-top calibration study of 22 lakes indicates that in salt lakes the total alkenones increase sharply at a salinity of about 20 g/L, while alkenones are absent in the sediments of freshwater lakes. For the first time, this study shows that the Uk40 and Uk40 unsaturation indices are positively correlated with salinity and thus can be used to reconstruct salinity. The C37/C38 ratio is negatively correlated with salinity, and therefore this parameter can be used as an indicator of salinity. Also, the %C37:4 indicator is not correlated with salinity. It was found that the average chain length of alkenones increases with salinity, but the correlation was weak. Analysis of the alkenone composition and 18S rRNA suggests that all lakes are inhabited by Group II haptophytes, except for one freshwater lake, where the Group I of LCA-producing haptophytes was found. The taxonomic composition of haptophyte algae and the alkenone composition in the lakes were comparable to those in the lakes of the Canadian Prairies, apparently due to the similarity of climatic factors and the ionic composition of lake water in the two regions.

Detailed studies of processes of sediment-record formation are necessary for accurate sediment-derived paleo environmental reconstructions. In order to understand the deposition of sediments in meromictic Lake Shira (southern Siberia, Russia), we conducted a multi-year seasonal sediment trap study of sediment fluxes of chemical elements, organic and inorganic carbon, total nitrogen and photosynthetic pigments for a period from 2012 to 2017. In 2015 and 2016, a rare event of transition from meromixis (i.e. long-term hypolimnetic anoxia) to holomixis was observed, which was accompanied by the temporary disappearance of hydrogen sulfide from the water column in spring and a decrease in hydrogen sulfide in other seasons compared to the meromictic state. In general, we did not reveal any noticeable differences between the meromictic and holomictic states in the composition of sediments, except for okenone. The content of okenone, a specific carotenoid of purple sulfur bacteria, decreased in traps during the holomictic state and did not considerably increase two years after the meromixis restoration. Using a freeze-corer, we observed that the content of okenone in the uppermost 3-mm layer of sediments also decreased markedly from 2013 to 2017, as well as in traps. The sedimentation flux of molybdenum reflected the seasonality of hydrogen sulfide content in the water column, and it was maximal in winter, in contrast to all other sediment components. Other components reflected the seasonality of organic production and terrigenous inputs, with the maximum in late summer and autumn. An increase in the sedimentation rate of terrigenous elements in the autumn of 2012 was supposedly caused by the anomalous amount of precipitation in August 2012. Since detailed inter-annual observations of sedimentation processes are not often found in limnology, we hope that our results will be useful for reconstructing regional climate and, further, the understanding of sediment dynamics of stratified temperate water bodies.

Abstract. Detailed studies of processes of sediment record formation are necessary for accurate sediment-derived paleo-environmental reconstructions. We conducted the multi-year seasonal sediment evolution of sediment fluxes of chemical elements, organic and inorganic carbon, total nitrogen, and photosynthetic pigments for a period from 2012 to 2017. In 2013, 2016, 2017, and 2018, we estimated the contents of photosynthetic pigments in the uppermost sediment layers frozen in situ with a freeze-corer. In 2015 and 2016, a rare event of transition from meromixis (i.e. long-term hypolimnetic anoxia) to holomixis was observed, which was accompanied by the temporary disappearance of hydrogen sulfide from the water column in spring and a decrease in hydrogen sulfide in other seasons compared to the meromictic state. We have demonstrated that okenone and Mo in the Lake Shira sediments reflect the presence of hydrogen sulfide in the water column. However, the okenone showed smoothened multi-year dynamics without a pronounced seasonal one. Therefore, the okenone can be a proxy of sulphidic conditions in photic zone and weakly depend on seasons whereas Mo can be used as an indicator of winter periods when analyzed in cores with annual resolution. Sedimentation fluxes of other substances showed typical seasonal dynamics with a minimum in winter and a maximum in late summer and autumn. All chemical elements in the sedimentation flow can be roughly divided into those associated with organic matter and terrigenous-chemogenous. The components of the second group showed a pronounced peak of sedimentation in autumn 2012 and summer 2017 presumably due to the increased amount of precipitation at that time. This demonstrates the relationship between the terrigenous components and the climate humidity for this lake. Besides, it reflects the irregularity of annual varves composition.

Long-chain alkenones (LCAs)—lipides produced by some microalgae of the Haptophyta group in seas and continental water bodies—are promising paleomarkers of climate changes because they are well-preserved in bottom sediments. The hydrocarbon chain and the degree of unsaturation of double bonds of this class of lipides can vary depending on habitat conditions of their producers. For the first time, we have discovered LCAs in the bottom sediments of two saline meromictic lakes Shira and Uchum located in arid steppes of southern Siberia (the North Minusinsk Depression) and estimated the distribution of their total content, length, and the degree of unsaturation. The highly abundant population of producers of LCAs—haptophyte algae of the genus Isochrysis (Haptophyta)—is revealed in the water column of Lake Uchum. It is shown that the species composition of the haptophyte algae and composition of the LCAs in the lakes under study are similar to those in saline stratified lakes of North America (Saskatchewan, Canada) with a similar climate and salt composition. The abundance of the С37:4 alkenone in the bottom sediments in the last 100 years reflected the variations in lake salinity caused by the documented changes in the water level. In this way, it is shown that the LCAs may be used as a paleomarker of climate-driven changes in the water level of the saline lakes in southern Siberia.

Long-term measurements show that the sedimentation dynamics of okenone (a carotenoid of purple sulfur bacteria) reflects the abundance of purple sulfur bacteria, the hydrogen sulfide content, and the stratification stability in the closed saline meromictic Shira Lake (Southern Siberia, Russia). The profile of fossil okenone in bottom sediments has been compared with the data on the water surface level in the lake over the last 100 years. The peaks of okenone correspond to lake transgressions, and the drops in the okenone content coincide with periods of stable or decreasing water level. Our observations show that fossil okenone may be used as a qualitative proxy of meromictic conditions and as a quantitative proxy of the water level dynamics in closed stratified lakes. Therefore, okenone can be used for qualitative reconstructions of the water balance and climate humidity in arid zones.

The seasonal dynamics of the vertical structure of small saline Lake Uchum, located in the steppe arid zone of the south of Siberia (Krasnoyarsk krai), has been studied in detail for the first time. This lake is a meromictic water body. We have revealed a heterogeneous vertical distribution of plankton organisms and a dense population of purple sulfuric bacteria in the redox zone. The taxonomic composition and seasonal dynamics of phyto- and zooplankton are described. Presumably, the meromixis of Lake Uchum is due to the inflow of fresh water to the surface of the saline water body during the rise of its level in the early 20th century, similarly to lakes Shira and Shunet located nearby. The processes of salt displacement into the solution during the formation of ice, as well as the precipitation of salts in the winter, also contribute to the maintenance of permanent stratification. The information on the current state of the lake can be useful for reconstructing the climate by bottom sediments, as well as for creating a model of water quality and investigating the therapeutic properties of lake mud.

Saline Lake Shira (Southern Siberia, Russia) was meromictic through the observation period 2002–2015. During the under-ice periods of 2015 and 2016, complete mixing of the water column was recorded for the first time, and hydrogen sulphide temporarily disappeared from the water column of the lake; i.e. in those years the lake turned to holomixis. In the summer of 2015, a sharp increase in chlorophyll a, organic carbon, zooplankton, and phytoflagellates was observed in the lake, which was probably due to the release of nutrients from the monimolimnion. Purple sulfur bacteria completely disappeared from the lake after the first mixing in 2015, and did not reappear despite the restoration of meromixis in 2017. Thus, it was demonstrated that purple sulfur bacteria are sensitive to the weakening of the stratification of Lake Shira. Based on the data of the seasonal monitoring of temperature and salinity profiles over the period 2002–2017, it was presumed that the main cause of deep mixing in 2015 was the weakening of the salinity gradient due to strong wind impact and early ice retreat in the spring of 2014. In addition, it was shown that in previous years a significant contribution to the maintenance of meromixis was made by an additional influx of fresh water, which caused a rise in the lake level in the period 2002–2007. Thus, we identified a relationship between the stratification regime of the lake and the change in its level, which provides valuable information both for the forecast of water quality and for reconstruction of the Holocene climate humidity in this region of Southern Siberia from the sediment cores of Lake Shira.

According to the results of seasonal monitoring, in 2007–2013 purple sulfur bacteria morphologically similar to Thiocapsa sp. Shira_1 (AJ633676 in EMBL/GenBank) predominated in the anoxygenic phototrophic community of the water column of the meromictic Lake Shira (Khakassia, Siberia). No pronounced seasonal periodicity in the total cell number in the water column was revealed during the period of observation. In some years cell number during the period when the lake was covered with ice was reliably higher than in summer. The absence of seasonal periodicity was probably due to the low amplitude of seasonal variations in temperature and illumination in the redox zone, resulting from its relatively deep location (12–16 m). The year-to-year dynamics was characterized by a reliable decrease of the total cell number in 2009–2010 and maxima in 2007 and 2011–2012. Canonical correlation analysis revealed that water temperature in the redox zone was the best predictor of the PSB abundance in Lake Shira. Water temperature, in turn, depended on the depth of mixing of the water column. Intense mixing in 2009–2011 was probably responsible for decreased PSB abundance in the lake. On the other hand, the absence of deep winter mixing, resulting in stable conditions in the chemocline, favored the preservation of relatively high PSB biomass. Prediction of circulation depth, which depends mainly on the weather conditions and dynamics of the water level, is required for prediction of PSB abundance in Lake Shira. These results may be useful for paleolimnological reconstructions of the history of the lake based on the remnants of purple sulfur bacteria in bottom sediments.

Phototrophic sulfur bacteria form dense accumulations in the chemocline zones of stratified lakes where light reaches the sulfide-containing layers of water. Many works are dedicated to the ecophysiology of these microorganisms in meromictic lakes. However, the role of these microorganisms in the trophic network of these ecosystems, the ways of biomass utilization, and the contribution to the turnover of biogenic elements have so far been insufficiently understood. This work deals with the analysis of many years’ seasonal dynamics of the biomass of purple sulfur bacteria and the physicochemical conditions of their environment in Lake Shunet (Siberia, Khakassia, Russia), unraveling the causes of their anomalous development in the chemocline of this lake, as well as the comparative analysis of such type of ecosystems. Lake Shunet is characterized by markedly pronounced stratification and the high density of purple sulfur bacteria (PSB) in the chemocline, which is comparable to that of Lake Mahoney (Canada) where the number of PSB is the greatest among those known in the world. It was shown that, in the period 2002–2009, the total amount of bacterio-chlorophyll a in the water column of Lake Shunet increased and did not correlate with the seasonal variations in temperature and illumination in the chemocline. It was established that PSB cells in the purple layer experienced the effect of self-shading. The sedimentation rate of purple sulfur bacteria in Lake Shunet was low due to the pronounced density gradient in the chemocline zone. Thus, the high number of PSB in the chemocline was due to the combination of strong illumination, a high sulfide concentration, and a high water density gradient, which was responsible for stable stratification and contributed to the accumulation of the cells in a narrow layer. The data obtained could be useful for the paleoreconstruction of climatically deter-mined changes in the level of the lake and its periods of meromixis by the presence of carotenoids and bacte-riochlorophylls in the bottom sediments.

The concentrations of carotenoids buried in the bottom sediments of Lake Shira (Siberia, Khakassiya) have analyzed for the period of the last 2300 years. The bottom sediments were found to contain carotenoids, which are molecular markers of the corresponding groups of Phototrophic organisms. The bottom sediments of Lake Shira were shown to be a promising object for climate reconstructions of the Late Holocene in southern Siberia.