Mg-Rich Authigenic Carbonates in Coastal Facies of the Vtoroe Zasechnoe Lake (Southwest Siberia): First Assessment and Possible Mechanisms of Formation

The formation of Mg-rich carbonates in continental lakes throughout the world is highly relevant to irreversible CO2 sequestration and the reconstruction of paleo-sedimentary environments. Here, preliminary results on Mg-rich carbonate formation at the coastal zone of Lake Vtoroe Zasechnoe, representing the Setovskiye group of water bodies located in the forest-steppe zone of Southwest Western Siberia, are reported. The Setovskiye lakes are Cl−–Na+–(SO42−) type, alkaline, and medium or highly saline. The results of microscopic and mineralogical studies of microbialites from shallow coastal waters of Lake Vtoroe Zasechnoe demonstrated that Mg in the studied lake was precipitated in the form of hydrous Mg carbonates, which occur as radially divergent crystals that form clusters in a dumbbell or star shape. It is possible that hydrous Mg carbonate forms due to the mineralization of exopolymeric substances (EPS) around bacterial cells within the algal mats. Therefore, the Vtoroe Zasechnoe Lake represents a rare case of Mg-carbonates formation under contemporary lacustrine conditions. Further research on this, as well as other lakes of Setovskiye group, is needed for a better understanding of the possible role of biomineralization and abiotic mechanisms, such as winter freezing and solute concentration, in the formation of authigenic Mg carbonate in modern aquatic environments.


Introduction
The physicochemical parameters controlling the formation of Mg-carbonates, and especially, magnesite (MgCO 3 ), make up one of the most controversial scientific topics in biogeochemistry, mineralogy, and the geology of sedimentary rocks [1]. Similar to the formation of dolomite (CaMg(CO 3 ) 2 ) [2][3][4][5][6], the abiotic precipitation of magnesite at the Earth-surface pressures, as well as the temperatures, is kinetically limited due to the strongly pronounced hydrophilic properties of the Mg 2+ ion [7][8][9]. Instead, hydrated metastable phases, such as nesquehonite (MgCO 3 ·3H 2 O), dypingite The climate of the territory is moderately continental. The average annual temperature is 1.9 °C and the average annual precipitation is 381 mm [57]. The rainfall mainly feeds the water bodies in warm seasons and the snowmelt provides water in spring; however, the ground waters input may also contribute to the water balance in some cases. The landscapes of the territory surrounding the Setovskiye lakes are represented by the alternation of plowed fields and birch groves, with a predominance of Chernozems on watersheds and Planosols in depressions, which is typical for the Northern forest-steppe ecotone [58]. The agriculture represents the main source of anthropogenic impact on the environment within the area.

Field Study and Sampling
Water samplings of six lakes from Setovskiye group were carried out from 28 to 30 September 2018 ( Figure 1). This sampling period corresponds to a summer baseflow of the region, which is representative of the major period of authigenic carbonate mineral formation. Based on the results of preliminary observations, the coastal facies of the Lake Vtoroe Zasechnoye, where algae mats and microbialites were found during field studies, were selected for detailed sampling and analyses. The algal mats appeared as dense layers of dried cemented and partly mineralized algae formed in the  Table 1.
The climate of the territory is moderately continental. The average annual temperature is 1.9 • C and the average annual precipitation is 381 mm [57]. The rainfall mainly feeds the water bodies in warm seasons and the snowmelt provides water in spring; however, the ground waters input may also contribute to the water balance in some cases. The landscapes of the territory surrounding the Setovskiye lakes are represented by the alternation of plowed fields and birch groves, with a predominance of Chernozems on watersheds and Planosols in depressions, which is typical for the Northern forest-steppe ecotone [58]. The agriculture represents the main source of anthropogenic impact on the environment within the area.

Field Study and Sampling
Water samplings of six lakes from Setovskiye group were carried out from 28 to 30 September 2018 ( Figure 1). This sampling period corresponds to a summer baseflow of the region, which is representative of the major period of authigenic carbonate mineral formation. Based on the results of preliminary observations, the coastal facies of the Lake Vtoroe Zasechnoye, where algae mats and microbialites were found during field studies, were selected for detailed sampling and analyses. The algal mats appeared as dense layers of dried cemented and partly mineralized algae formed in the shoreline, while small greenish and creamy decaying microbialites cemented by carbonate material appeared within the shallow water ( Figure 2). The dried samples were used for mineralogical studies via scanning electron microscopy (SEM) and for the preparation of transparent thin sections for optical microscopic observations. Minerals 2019, 9, x FOR PEER REVIEW 4 of 19 shoreline, while small greenish and creamy decaying microbialites cemented by carbonate material appeared within the shallow water ( Figure 2). The dried samples were used for mineralogical studies via scanning electron microscopy (SEM) and for the preparation of transparent thin sections for optical microscopic observations. Water samples were taken from six main lakes representing the Setovskiye group. The pH, specific electric conductivity, and temperature were measured in the field using a Multi 3420 portable multiparameter meter (WTW, Xylem Analytics, Germany). Samples of lake waters were filtered on-site through disposable MILLEX Filter units (0.45 µm pore size, 33 mm in diameter) using a sterile plastic syringe and vinyl gloves. The first 20-50 mL of filtrate was discarded and the subsequent filtrate was collected into pre-washed 250 mL polypropylene bottles. Filtered samples of lake waters were taken in two bottles, one was acidified with bidistilled nitric acid to a concentration of 2% for the analysis of cations and trace elements, and the second was not acidified and used for analyses of dissolved organic (DOC) and inorganic (DIC) carbon and anions following standard techniques used in the Géosciences Environment Toulouse Laboratory [59]. Before analyses, filtered samples were stored in a refrigerator at 4-6 °C.

Analytical Methods
Hydrochemical studies of water samples from six lakes representing the Setovskiye group included measurements of DOC, DIC, cations, Сl − , and SO4 2− . The major anion concentrations (Cl − and SO4 2− ) were analyzed by ion chromatography (Dionex 2000i, Thermo Fisher Scientific, Waltham, Water samples were taken from six main lakes representing the Setovskiye group. The pH, specific electric conductivity, and temperature were measured in the field using a Multi 3420 portable multiparameter meter (WTW, Xylem Analytics, Germany). Samples of lake waters were filtered on-site through disposable MILLEX Filter units (0.45 µm pore size, 33 mm in diameter) using a sterile plastic syringe and vinyl gloves. The first 20-50 mL of filtrate was discarded and the subsequent filtrate was collected into pre-washed 250 mL polypropylene bottles. Filtered samples of lake waters were taken in two bottles, one was acidified with bidistilled nitric acid to a concentration of 2% for the analysis of cations and trace elements, and the second was not acidified and used for analyses of dissolved organic (DOC) and inorganic (DIC) carbon and anions following standard techniques used in the Géosciences Environment Toulouse Laboratory [59]. Before analyses, filtered samples were stored in a refrigerator at 4-6 • C.

Analytical Methods
Hydrochemical studies of water samples from six lakes representing the Setovskiye group included measurements of DOC, DIC, cations, Cl − , and SO 4 2− . The major anion concentrations (Cl − and SO 4 2− ) were analyzed by ion chromatography (Dionex 2000i, Thermo Fisher Scientific, Waltham, MA, USA) with an uncertainty of 2%. The dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were determined using a TOC-Vscn analyzer (Shimadzu, Kyoto, Japan) with an uncertainty of 3% and a detection limit of 0.1 mg/L. The major elements were measured by quadrupole Agilent 7500ce ICP-MS system (Agilent Technologies, Santa Clara, CA, USA) with an uncertainty of ±5%. Indium and rhenium were used as internal standards. The international geostandard SLRS-5 (Riverine Water Reference Material for Trace Metals, certified by the National Research Council of Canada) was used to check the validity and reproducibility of each analysis. The primary diagnostics of collected samples was carried out using a Leica EZ4 D stereomicroscope (Leica Microsystems, Wetzlar, Germany) with an integrated digital camera. The studies of 10 representative samples were performed in thin sections of dried microbialites and algae mats using an Eclipse LV100POL polarization microscope (Nikon, Tokyo, Japan) and an Axio Vert reflected light microscope (Carl Zeiss, Oberkochen, Germany). The SEM-EDS analysis was performed using a TM3000 scanning electron microscope (Hitachi, Tokyo, Japan) with a Quantax 70 EDS attachment (Bruker, Billerica, MA, USA) at X100-5000 magnification and a JSM-6390LV scanning electron microscope (Jeol, Tokyo, Japan) with an INCA Energy 450 X-Max80 EDS attachment (Oxford Instruments, Abingdon, UK). The SEM observation were made under high vacuum (HV-mode), mainly in the elemental composition mode (BSE, registration of back scattered electrons). While performing the EDS analysis, the voltage was 15 and 20 kV for the first and second devices, respectively.
Confocal laser-scanning microscopy (CLSM) is often used to identify signs related to the activity of living organisms within stromatolites and microbialites [25,60,61]. Selected thin sections of algae mats and microbialites were examined using a Zeiss LSM 780 NLO confocal laser-scanning microscope (Carl Zeiss, Oberkochen, Germany). Lasers with wavelengths of 405, 488, and 561 nm were used as a source of fluorescence excitation in a microscope. Images illustrating the fluorescence of the studied samples were collected using an emission filter that transmits light with a wavelength between 505 and 539 nm (visible, green).
The mineralogical composition of the collected solid samples (two for microbialites and two for algal mats) was examined by a powder X-ray diffraction (XRD) technique. Diffractograms were recorded by a PANalytical X'Pert PRO diffractometer (Malvern Panalytical, Malvern, UK) using Co-Kα-radiation (40 mA, 40 kV) at a 2θ range from 5 • to 85 • and a scan speed of 0.03 • s −1 . Malvern Panalytical's HighScore Plus software with the ICSD database was used for the qualitative characterization of the crystalline products, and the mineral phases were quantified by Rietveld refinement. Corundum standards were always used to constrain the calibration. Analytical uncertainty of the quantification was within 1 wt.%.
Thermodynamic modeling included the calculation of saturation indices of aqueous solution with respect to various carbonates using the Visual MINTEQ ver. 3.1 software package [62]. The parameters including pH, temperature, and the concentrations of individual ions obtained from the measurements of these values in field and laboratory conditions were used as input parameters.

Water Chemistry of Setovskiye Lakes
The physical and chemical parameters of the studied lakes (Table 1) indicate that all lakes belong to the group of small, closed-water reservoirs with depths varying from 1.1 to 2.3 m. The lake waters are alkaline (pH values from 9.0 to 9.7); moderately or strongly saline (TDS values varied from 28 to 84 g/L) and belong to the Cl − -Na + -(SO 4 2− ) type. The Mg/Ca molar ratio in studied lakes varied from 38 to 66, except for Lakes Krugloe (9.3) and Vtoroe Zasechnoe (100), where these values were significantly lower and higher, respectively, than most of the Setovskiye Lakes. The microbialites were found only in the coastal zone of the Vtoroe Zasechnoe Lake, exhibiting the highest Mg/Ca ratio. The DOC and DIC concentrations varied from 69 to 112 and 240 to 365 mg/L, respectively. These results demonstrated significant spatial variability in hydrochemical parameters (notably salinity and Mg/Ca molar ratio) within the same group of closely located lakes. Note that the reasons of very high Mg/Ca ratios in some water bodies, as well as the variability of this parameter for the whole group cannot be explained based on the results of preliminary studies.

XRD Analysis of Minerals from Algae Mats and Microbialites from Vtoroe Zasechnoe Lake
The diffractograms of the most representative samples of microbialites (a) and algal mats (b) are presented in Figure 3. The interpretation of the XRD analysis results was quite difficult because the bottom sediments of Vtoroe Zasechnoye Lake are comprised of sands cemented with carbonates containing a significant amount of terrigenous components, mainly quartz and feldspars. This prevented the accurate determination of minor phases. Hydromagnesite (up to 13% ± 1%) was found in all samples of microbialites formed in shallow water. The XRD patterns, according to Reference [63], were used for the interpretation of the appearance of this hydrous Mg carbonate. Dolomite in small quantities was also likely present in one microbialites sample. Note that samples representing algae mats contained loeweite (Na 4 Mg 2 (SO 4 )·4.5 H 2 O), a typical mineral of marine and continental evaporate sediments [64], along with more common halite. the reasons of very high Mg/Ca ratios in some water bodies, as well as the variability of this parameter for the whole group cannot be explained based on the results of preliminary studies.

XRD Analysis of Minerals from Algae Mats and Microbialites from Vtoroe Zasechnoe Lake
The diffractograms of the most representative samples of microbialites (a) and algal mats (b) are presented in Figure 3. The interpretation of the XRD analysis results was quite difficult because the bottom sediments of Vtoroe Zasechnoye Lake are comprised of sands cemented with carbonates containing a significant amount of terrigenous components, mainly quartz and feldspars. This prevented the accurate determination of minor phases. Hydromagnesite (up to 13% ± 1%) was found in all samples of microbialites formed in shallow water. The XRD patterns, according to Reference [63], were used for the interpretation of the appearance of this hydrous Mg carbonate. Dolomite in small quantities was also likely present in one microbialites sample. Note that samples representing algae mats contained loeweite (Na4Mg2(SO4)·4.5 H2O), a typical mineral of marine and continental evaporate sediments [64], along with more common halite.

Hydrogeochemical Modeling
Calculations of saturation indexes (SI) for all reservoirs of the Setovskiye group using the Visual MINTEQ software package showed ( Table 2) that all the lakes are supersaturated with respect to huntite (SI > 4.52), ordered dolomite (SI > 3.89), non-ordered dolomite (SI > 3.28), magnesite (SI > 1.74), calcite (SI > 1.05), and aragonite (SI > 0.89). At the same time, with respect to monohydrocalcite, supersaturation was only observed for the least mineralized reservoir-Lake Krugloe (SI = 0.16). All studied reservoirs were undersaturated with respect to nesquehonite (SI ≤ −0.75). It is important to mention that the highest water saturation with respect to Mg-carbonates (huntite, hydromagnesite, and magnesite) was observed for the lakes Vtoroe Zasechnoe, where signs of biomineralization were found in the coastal zone and Solenoe Lake where the occurrence of Mg-carbonates was also previously reported [33]. However, one should keep the possible strong temporal heterogeneity of the Setovskiye group of water bodies in mind, since the lakes of the forest-steppe zone of Trans-Urals are subjected to significant fluctuations in the water levels within the seasons, which can affect strong variations of SI values during the year.

Characterization of Algae Mats from Near-Shore Zone of Vtoroe Zasechnoe Lake
Algae mats were found within a small sandy beach in the Southeastern part of the Vtoroe Zasechnoe Lake. The thickness of these biomats varied from 5 to 10 mm (Figure 4a). The upper surface of the algae mats was characterized by a cream-beige to white color due to the precipitation of microcrystals of evaporate minerals, mainly halite. The lower wetter surface was greenish-brown with a pink tint (Figure 4b). After drying, the algae mats retained their integrity and remained fairly strong and flexible, due to the presence of poorly mineralized algae filaments.
In transparent thin sections, the heterogeneous layered texture of algae mats related to the orientation of the algae filaments was clearly visible (Figure 4c). The layers of algae mats were cemented by massive pelitomorphic carbonate material, mainly hydromagnesite (white in reflected light). Separated carbonate aggregates were much less common; however, sometimes they were present as small solid intergrowths with diameters up to 0.6 mm, consisting of a rather dense material. Small evaporite neoformations were predominantly comprised of halite. The latter occurred as radially diverging thin needles, covering the outer surface of the algae mats (Figure 4c).
When studying algae mats using a scanning electron microscope, it can be seen that filaments oriented in one plane form the structural frame of algae mats (Figure 4d). In the upper layer, most of the filaments are mineralized, while in the lower part of the algae mats, a gradual transition from non-mineralized algae filaments to massive carbonate was observed (Figures 4e and 5). These aggregates were probably developed from already decomposed or decaying organic matter.   The carbonate material that covers some of algae filaments in the mineralized parts of algae mats is represented by plate-like radially divergent crystals that formed clusters of a dumbbell or star shape (Figure 6a,b), which likely correspond to hydromagnesite, based on the morphology of crystals, their clusters, and EDS spectra. In some cases, it was clearly visible that the precipitation of carbonates occurred in areas where organic matter was degraded. Well-formed associations of crystals coexisted or developed in areas with bacterial colonies and their relics, diatom skeletons of varying degrees of preservation, and cyanobacteria and EPS films (Figure 6c,d). The evaporitic minerals on the surface of dried algae mats were represented by both very large needle-shaped microcrystals (up to 0.5 mm) revealed by the results of optical microscopy and (10-25 µm) elongated bar-shaped microcrystals of halite revealed during SEM-EDS analysis. The carbonate material that covers some of algae filaments in the mineralized parts of algae mats is represented by plate-like radially divergent crystals that formed clusters of a dumbbell or star shape (Figure 6a,b), which likely correspond to hydromagnesite, based on the morphology of crystals, their clusters, and EDS spectra. In some cases, it was clearly visible that the precipitation of carbonates occurred in areas where organic matter was degraded. Well-formed associations of crystals coexisted or developed in areas with bacterial colonies and their relics, diatom skeletons of varying degrees of preservation, and cyanobacteria and EPS films (Figure 6c,d). The evaporitic minerals on the surface of dried algae mats were represented by both very large needle-shaped microcrystals (up to 0.5 mm) revealed by the results of optical microscopy and (10-25 µm) elongated bar-shaped microcrystals of halite revealed during SEM-EDS analysis.

Characterization of Initial Microbialites from the Shallow Coastal Zone of the Vtoroe Zasechnoe Lake
Microbilites are represented by thin (1-2 cm), intermittent beige-colored crusts that are unevenly distributed over the area and follow the contours of the coastline. They occurred in the shallow coastal zone of the lake, at 10-30 cm depth. After drying, the neoformations disintegrated into isometric and rather fragile pieces (Figure 7a,b). The microbialites had a laminated structure in which three separate layers can be distinguished: Upper, middle, and lower layers. The upper layer

Characterization of Initial Microbialites from the Shallow Coastal Zone of the Vtoroe Zasechnoe Lake
Microbilites are represented by thin (1-2 cm), intermittent beige-colored crusts that are unevenly distributed over the area and follow the contours of the coastline. They occurred in the shallow coastal zone of the lake, at 10-30 cm depth. After drying, the neoformations disintegrated into isometric and rather fragile pieces (Figure 7a,b). The microbialites had a laminated structure in which three separate layers can be distinguished: Upper, middle, and lower layers. The upper layer with a thickness of less than 1 mm had a lighter color and contained fewer terrigenous impurities. The middle layer (3-5 mm thick) had a greenish color and exhibited an uneven degree of mineralization of organic matter. The lower one (4-6 mm thick) is darker in the upper part with a pinkish tinge and was largely enriched with terrigenous material, mainly quartz grains. with a thickness of less than 1 mm had a lighter color and contained fewer terrigenous impurities. The middle layer (3-5 mm thick) had a greenish color and exhibited an uneven degree of mineralization of organic matter. The lower one (4-6 mm thick) is darker in the upper part with a pinkish tinge and was largely enriched with terrigenous material, mainly quartz grains. The study of microbialites in transparent thin sections using a polarizing microscope demonstrated that carbonate material is present in all the studied samples. Carbonates in the studied samples occur both as cement, and in the form of small separated aggregates. The cements consisted The study of microbialites in transparent thin sections using a polarizing microscope demonstrated that carbonate material is present in all the studied samples. Carbonates in the studied samples occur both as cement, and in the form of small separated aggregates. The cements consisted of small microcrystalline aggregates of carbonate minerals (up to 90%) and only in rare cases contained thin films of clay material. Two types of unevenly oriented lenticular interlayers, formed by different carbonates, can be distinguished (Figure 7c,d). One of the interlayers is enriched in dense pelitomorphic (finely crystalline) carbonate. A looser carbonate material formed aggregates consisting of slightly larger microcrystals and constituted the second type of interlayers. The terrigenous component of microbialites was mainly represented by poorly sorted quartz sand.
The SEM study of samples demonstrated that the bulk of the microbialites is a mixture of carbonate material and terrigenous particles. In this case, the terrigenous particles are enclosed in a mineral matrix, which is preserved even when individual grains fall out (Figure 8a,b). Thin mineralized films covered mineral grains and filled the space between them. Poorly mineralized residues of bacterial communities formed EPS films. In some cases, these films were covered with halite precipitations or were not mineralized at all. In most cases, the surfaces of such films were encrusted with micro-aggregates of dolomite. of small microcrystalline aggregates of carbonate minerals (up to 90%) and only in rare cases contained thin films of clay material. Two types of unevenly oriented lenticular interlayers, formed by different carbonates, can be distinguished (Figure 7c,d). One of the interlayers is enriched in dense pelitomorphic (finely crystalline) carbonate. A looser carbonate material formed aggregates consisting of slightly larger microcrystals and constituted the second type of interlayers. The terrigenous component of microbialites was mainly represented by poorly sorted quartz sand.
The SEM study of samples demonstrated that the bulk of the microbialites is a mixture of carbonate material and terrigenous particles. In this case, the terrigenous particles are enclosed in a mineral matrix, which is preserved even when individual grains fall out (Figure 8a,b). Thin mineralized films covered mineral grains and filled the space between them. Poorly mineralized residues of bacterial communities formed EPS films. In some cases, these films were covered with halite precipitations or were not mineralized at all. In most cases, the surfaces of such films were encrusted with micro-aggregates of dolomite. Two unevenly alternating types of layers, which were clearly visible in thin sections, were also distinguished via the SEM-EDS technique (Figure 7e). These two layers had different carbonate associations and different density. Looser interlayers containing approximately equal proportions of Mg-calcite and hydromagnesite (Figure 7f) represented the first one. Mg-calcite was presumably Two unevenly alternating types of layers, which were clearly visible in thin sections, were also distinguished via the SEM-EDS technique (Figure 7e). These two layers had different carbonate associations and different density. Looser interlayers containing approximately equal proportions of Mg-calcite and hydromagnesite (Figure 7f) represented the first one. Mg-calcite was presumably formed solely by the mineralization of EPS films and precipitates in the form of spitted sheaf-like microcrystals (Figure 8a,c). Sometimes it formed loose spherical aggregates consisting of crystals and non-mineralized EPS films (Figure 8d-f). The biofilms were not completely replaced by carbonate material and remained a biopolymeric matrix during the formation of Mg-calcite. As a result, Mg-calcite crystals almost never coalesced and formed a very loose, weakly connected layer inside organo-mineral aggregates. The second layer within the microbialites was dominated by hydromagnesite, which often formed radiating clusters of fine-layered microcrystals. In such cases, carbonate material replaced organic matter almost completely (Figures 7g and 8c). Associations of hydromagnesite microcrystals in such interlayers exhibited a characteristic structure that inherited the initial cellular-alveolar structure of decaying EPS [11].
The dolomite typically occurred in layers with a predominance of hydromagnesite and a high degree of degradation of organic matter. This mineral was found in the form of isometric microcrystals encrusted in mineralized EPS films. Microcrystals had an angular, poorly delineated shape, while some crystals had a sub-rhombohedral shape. Dolomite micro-aggregates often appeared within the massive deposits of hydromagnesite (Figure 8f-h). It is noteworthy that the formation of dolomite microcrystals was only characteristic of voids mineralized by hydromagnesite. The formation of magnesite rhombohedral microcrystals was also observed in the mass mineralized by hydromagnesite (Figure 8i). However, unlike dolomite, this mineral was much less common.

Discussion
The Vtoroe Zasechnoe Lake is an extremely interesting natural environment to study the processes of the modern formation of Mg-carbonates. Unlike most forest-steppe drainless reservoirs of Southwestern Western Siberia with high alkalinity and high Mg/Ca values, the magnesium in Vtoroe Zasechnoe Lake is precipitated mainly in the form of hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O), and not Mg-calcite or dolomite. In the upper layer of the bottom sediments and in the coastal zone of the Vtoroe Zasechnoe Lake, there are favorable conditions for the deposition of hydromagnesite due to the high concentrations of Mg 2+ , HCO 3 − , and CO 3 2− ions, which cause the supersaturation of waters with respect to this mineral. However, this condition is necessary but not sufficient for the formation of Mg-carbonates; such surface water supersaturations are characteristic of a number of other water bodies in the forest-steppe zone of the Trans-Urals region, where formation of Mg-carbonates does not occur [33]. Figure 9 illustrates the relationships between dissolved inorganic carbon (DIC) concentration and Mg/Ca molar ratio for Setovskiye Lakes and other lacustrine environments of the world, where authigenic Mg-rich carbonates were also reported. The biplot shows that the DIC values in Trans-Urals forest-steppe reservoirs are significantly lower than in lakes and hydromagnesite-magnesite playas of the Cariboo Plateau, British Columbia, Canada [35,65], and similar or even a bit higher than for water bodies of the Turkish Lakes Region [66,67] and the alkaline lake Eras from Spain [68]. At the same time, the Mg/Ca molar ratios of Vtoroe Zasechnoe was rather high, especially in comparison to the Turkish Lakes and the Slime Lake of British Columbia. In total, the Mg/Ca ratios for all water bodies of the Setovskiye group were comparable or even higher than for most of the studied environments, except for some lakes and playas of the Cariboo Plateau. Accordingly, we suggest that the formation of hydromagnesite in the Vtoroe Zasechnoe Lake is related to the activity of living organisms, unique to this lake of the region. Similar to other cases of alkaline lakes, where the precipitation of Mg-carbonates prevails over Mg-Ca and Ca-carbonates [12], in the Vtoroe Zasechnoe Lake, the most intensive mineralization is confined to microbialites and algae mats in the coastal zone. This feature was also noted in previous studies of sediment cores from the Setovskiye lakes [33], where the hydromagnesite prevailed in the upper layer of bottom sediments, and sharply decreased its content with depth, due to the ceasing of phototrophic biological activity. It is therefore possible that algae, which form a dense mat of red and green color near the coastline in the summer period, are responsible for the precipitation of Mg-carbonates in Vtoroe Zasechnoe Lake. During this time of the year, intensive photosynthesis leads to the increase in the concentration of СO3 2− ions and pH, creating highly supersaturated solutions, as it is known in other environments of living algal mats [25,[69][70][71][72].
Another important observation is that the zones of hydromagnesite, and probably Mg-calcite and other carbonates precipitation, are confined to the degrading organic matter, such as EPS films, algae and cyanobacteria filaments, and cells of bacterial colonies. The associations of hydromagnesite microcrystals likely inherit the characteristic cellular alveolar structure of degrading EPS films [73], as is known for calcite-forming cyanobacteria [25,74]. It is possible that diatoms can play an important role in the precipitation of carbonates due to their potentially significant contribution to the formation of EPS [61,75,76]. Further, the specific features of the initial geometry of diatom frustules, having a high surface area and cation-binding sites [77], can contribute to the formation of nucleation zones via their service as templates for authigenic mineral nucleation. The skew-shaped form of Mg-calcite crystals in some cases may be related to the processes of biomineralization [12,78,79], although such mineral forms can also occur as result of abiotic precipitation [80]. For isolated microcrystals of Mg-Ca and Mg-carbonates that can correspond to dolomite and magnesite, it seems that carbonate nanoparticles, produced by bacteria in order to avoid cellular burial, act as the nucleation centers [81,82]. This mechanism was also evidenced for cyanobacteria, which possess a self-protection mechanism against calcification [83].
To sum up, the sequence of carbonate mineralization in coastal facies of the Vtorore Zasechnoe Lake can be tentatively described as follows. At the first stage, the formation of Mg-calcite occurs under the conditions of a low degree of decomposition of organic matter. At the second stage, radiating hydromagnesite microcrystals from cluster aggregates along the cellular-alveolar matrix precipitates under conditions of massive degradation of EPS. At the final stage, when organic matter becomes deficient, Mg-Ca and Mg-carbonates microcrystals in the form of nano-globules precipitate in the vicinity of bacteria and at the cell surfaces. Recently, a similar sequence was described for the Accordingly, we suggest that the formation of hydromagnesite in the Vtoroe Zasechnoe Lake is related to the activity of living organisms, unique to this lake of the region. Similar to other cases of alkaline lakes, where the precipitation of Mg-carbonates prevails over Mg-Ca and Ca-carbonates [12], in the Vtoroe Zasechnoe Lake, the most intensive mineralization is confined to microbialites and algae mats in the coastal zone. This feature was also noted in previous studies of sediment cores from the Setovskiye lakes [33], where the hydromagnesite prevailed in the upper layer of bottom sediments, and sharply decreased its content with depth, due to the ceasing of phototrophic biological activity. It is therefore possible that algae, which form a dense mat of red and green color near the coastline in the summer period, are responsible for the precipitation of Mg-carbonates in Vtoroe Zasechnoe Lake. During this time of the year, intensive photosynthesis leads to the increase in the concentration of CO 3 2− ions and pH, creating highly supersaturated solutions, as it is known in other environments of living algal mats [25,[69][70][71][72].
Another important observation is that the zones of hydromagnesite, and probably Mg-calcite and other carbonates precipitation, are confined to the degrading organic matter, such as EPS films, algae and cyanobacteria filaments, and cells of bacterial colonies. The associations of hydromagnesite microcrystals likely inherit the characteristic cellular alveolar structure of degrading EPS films [73], as is known for calcite-forming cyanobacteria [25,74]. It is possible that diatoms can play an important role in the precipitation of carbonates due to their potentially significant contribution to the formation of EPS [61,75,76]. Further, the specific features of the initial geometry of diatom frustules, having a high surface area and cation-binding sites [77], can contribute to the formation of nucleation zones via their service as templates for authigenic mineral nucleation. The skew-shaped form of Mg-calcite crystals in some cases may be related to the processes of biomineralization [12,78,79], although such mineral forms can also occur as result of abiotic precipitation [80]. For isolated microcrystals of Mg-Ca and Mg-carbonates that can correspond to dolomite and magnesite, it seems that carbonate nanoparticles, produced by bacteria in order to avoid cellular burial, act as the nucleation centers [81,82]. This mechanism was also evidenced for cyanobacteria, which possess a self-protection mechanism against calcification [83].
To sum up, the sequence of carbonate mineralization in coastal facies of the Vtorore Zasechnoe Lake can be tentatively described as follows. At the first stage, the formation of Mg-calcite occurs under the conditions of a low degree of decomposition of organic matter. At the second stage, radiating hydromagnesite microcrystals from cluster aggregates along the cellular-alveolar matrix precipitates under conditions of massive degradation of EPS. At the final stage, when organic matter becomes deficient, Mg-Ca and Mg-carbonates microcrystals in the form of nano-globules precipitate in the vicinity of bacteria and at the cell surfaces. Recently, a similar sequence was described for the bio-mineralization of Mg-rich calcites following the degradation of microbialites in Lake Las Eras in Spain [12].
The obtained results represent the first step towards understanding the role of biomineralization processes in the formation of Mg-carbonates in forest-steppe lakes in the Southwest of Western Siberia. Further investigations should include more accurate identification of minor phases, the use of high-resolution transmission electron microscopy, and characterization of seasonal dynamics of mineralization in the Setovskiye lakes, especially the possible role of winter freezing and solute concentration in this process. These should be coupled with the assessment of genetic and metabolic diversity of specific groups of microorganisms contributing to carbonate mineral formation.

Conclusions
The Vtoroe Zasechnoe Lake of the forest-steppe Southwestern Western Siberia is an interesting and rare example of authigenic formation of modern hydrous Mg-carbonates. Here, we demonstrated the formation of hydromagnesite and minor Mg-Ca and Mg-carbonates, which are most likely dolomite and magnesite, in microbialites and algae mats of the coastal facies of Lake Second Zasechnoe. This lake is characterized by the highest Mg/Ca ratio among all the reservoirs of the studied group of water bodies. It is possible that this chemical environment allows for the substantial increase of solution pH during algal photosynthesis, thus creating favorable conditions for authigenic carbonate mineral formation. The association of Mg-carbonates to EPS films and bacterial cells, as well as the characteristic morphology of crystals, suggest the leading role of biomineralization processes in their formation. The obtained results represent an interesting example of the preferential precipitation of Mg-carbonates in the semiarid conditions of the Northern forest-steppe.
It is important to note that, unlike other environments for which similar processes were reported, the Setovskiye lakes are located in the conditions with rather low average annual temperatures and these shallow water bodies fully or partially freeze during winter. The role of water freezing and thawing cycles leading to solute concentration and possible oscillations of saturation degree on carbonate minerals formation in these lakes should be a subject of further studies.