Mineralogical and geochemical characteristics of bottom sediments of saline lakes in Transbaikalia

. The results of the research on the water mineralization, the mineral composition, ash content, and organic matter content of bottom sediments of stagnant saline lakes in Southeastern Transbaikalia are presented. It has been established that various ratios of feldspar, quartz, hydroxyl compounds, and carbonate minerals are universally found in the sediments. Sediments on the bottom contain a small amount of organic matter, which classifies them as highly siliceous, consisting mainly of sand and clay minerals. Changes in the mineralogical and geochemical characteristics of bottom sediments at different depths indicate changes in the hydrological regime of the studied lakes and water mineralization in these lakes over time.


Introduction
Southeastern Transbaikalia is home to numerous brackish and saline lakes [1].Such lakes are formed due to the peculiarities of the relief (with many depressions and low-level slopes) in combination with the semi-arid climate of the region, which contributes to periodic fluctuations in their water regime.The water regime of non-flowing lakes depends on climatic and weather conditions [2].As a result of changes in water levels, the physicochemical characteristics, as well as the content of macro-and micro components and biodiversity of the lakes, change [3].All these factors significantly affect the process of formation of bottom sediments of mineral lakes.Therefore, a comprehensive study of the composition and characteristics of bottom sediments of non-flowing lakes in Transbaikalia will allow identifying the features of their formation and accumulation, which indirectly serves as a basis for forming more accurate representations of the evolution of climatic, geological, and geochemical processes occurring in the studied region.

Research region, objects and methods
The studied lakes (Kudzhertay, Ukshinda, Barun-Shivertuy, Shvartsevskoye, Shikhalin-Nur) are located in the southeast of the Transbaikal region in the steppe latitude zone.The climate in the studied area is sharply continental with insufficient amount of atmospheric precipitation, cold and prolonged winter, and hot and dry summer.Annual fluctuations in air temperature are significant.The amount of atmospheric precipitation is unevenly distributed throughout the year and is characterized by temporal variability (wet or dry phases).Observations have revealed the cyclic nature of climatic changes and determined cycles with a duration of 4 and 11 years, as well as 25-30 years.Currently, the studied area is in a wet phase of the precipitation cycle, assumed to have begun in 2012 [4].The studied lakes are characterized by a high water mineralization, ranging from 13.4 g/dm 3 for Ukshinda Lake to 278 g/dm 3 for Kudzhertay Lake (according to research conducted in 2022).The water mineralization of Barun-Shivertuy, Shvartsevskoye and Shikhalin-Nur lakes is 45.1 g/dm 3 , 21.3 g/dm 3 and 20.8 g/dm 3 , respectively.Bacterial growths are observed at the bottom of the lakes, forming dense bacterial mats.Microbial growths develop due to the high salinity of the water, which limits the growth of higher plants in the lakes, creating conditions for the predominance of prokaryotic organisms [2].
Water and bottom sediment samples were taken in June 2022.Bottom sediment samples were taken using undisturbed columns with a length of 30-50 cm.Polypropylene tubes with a length of 100 cm and an internal diameter of 4.5 cm were used as sampling devices.Each sample was divided into layers of 1, 3, 5 or 10 cm, depending on the nature of the sediment (color, density, presence of any inclusions).Preparation of bottom sediment samples was carried out according to measurement methods (drying, calcination, and grinding).Measurement of water mineralization and ash content in bottom sediment samples of the studied lakes was carried out according to gravimetric methods.To determine the mineral composition of the bottom sediment samples, they were ground in an agate mortar with alcohol and analyzed using powder X-ray diffraction on a DRON-3.0X-ray diffractometer.X-ray diffraction patterns were identified using a phase search program.The semiquantitative ratio of components was calculated using corundum numbers.The obtained phase ratios are approximate.The content of organic carbon in bottom sediments was determined by the spectrophotometric method according to the standard working methodology.

Results and discussions
The column of lake sediment from Lake Kudzhertai, which is 28 cm long, was divided into three layers with thicknesses ranging from 8 to 10 cm.The stratigraphic structure of the sediment in the examined horizon is homogeneous in consistency with clay soil ranging in color from gray to black.The sequence of the stratigraphy is as follows: 28-18 cm -black clay sediment with a foul odor, homogeneous and plastic with no plant remains; the contact with the overlying layer is not clearly defined; 18-8 cm -grayish-black clay sediment with a strong foul odor, homogeneous in color and plastic, with plant remains and a change in sediment color marking the contact with the overlying layer; 8-0 cm -dense gray clay with a strong foul odor, without plant remains, and an uneven color, containing areas of dark gray color.The results of the X-ray structural analysis of the mineral composition of the lake sediment showed that the predominant minerals were feldspar and quartz.The horizon of 8-28 cm also contained significant amounts of dolomite, calcite, plagioclase, kaolinite, and hydrosilicates.The layer of 18-28 cm also contained X-ray amorphous material, possibly organic.Halite traces were found throughout the examined horizon.The distribution of ash content in the sediment was uniform, ranging from 83.8 to 87.2% (87.2 for the layer of 0-8 cm, 86.2 for the layer of 8-18 cm, and 83.8 for the layer of 18-28 cm), and the content of organic carbon ranged from 4.4 to 8.9 g/kg (4.36 -for the layer of 0-8 cm, 5.9 -for the layer of 8-18 cm, and 8.87 -for the layer of 18-28 cm), allowing Lake Kudzhertai's sediment to be classified as high-ash deposits and, according to the classification of sediment based on organic matter content, as aleurolite sands and clay silt (less than 10% organic content) 5.
The core of the lake sediment from Lake Barun-Shiver-Shivertui is clayey with a strongly pronounced sulfuric odor, and has a heterogeneous color ranging from dark gray to black in different horizons.The column was divided into layers with thicknesses ranging from 2 to 5 cm.The results of the X-ray structural analysis of the mineral composition of the lake sediment showed that the main mineral in the lake's sediment was quartz, varying from 40 to 65%.The minimum values of quartz content were observed in the intervals of 10-13 and 20-25 cm, and the maximum in the layer of 15-20 cm.
Field spars are also present in significant amounts in the sediment of the lake.Their mass fraction varies from 12 to 45%: minimum values are observed in the layer 10-13 cm, a sharp increase in the mass fraction of minerals in the layer 20-25 cm (up to 35%), and maximum values are observed in the layers 30-35 cm and 39-43 cm.The mass fraction of clay minerals (hydrosil and kaolinite) reaches 15% in the roof horizon and gradually decreases to trace amounts at a depth of 25-26 cm, with clay minerals containing only 5% in the layer 39-43 cm.The development of carbonate minerals -calcite (9-19%, with an increasing content from 25 to 10 cm), dolomite (6%), and aragonite, only detected in the interval 10-13 cm (10%) -is observed in the interval of 10-25 cm.It should be noted that calcite is evenly distributed in deeper layers of lake sediments (horizon 25-43 cm), and its content on average is 5%.Pyrite (4%) was also detected in the interval of 10-13 cm, and amphibole (5%) in the layer 35-43 cm.The ash content values of the studied horizon of lake sediments on Barun-Shivertuy Island vary from 82.3 to 96.6%, which allows them to be classified as highly ash mineral deposits.The minimum mass fraction of ash is recorded in the interval of 10-13 cm, and the horizon of 0-15 cm in general stands out with lower values of sediment ash content.The ash content is relatively evenly distributed at depths of 20-43 cm and ranges from 93-97%.The distribution of organic matter content in the core is logically related to the ash content.The highest values were recorded in the interval of 0-15 cm, with the lowest in the layer 25-26 cm (Table 1).The total length of the selected column of bottom sediments on Schwarzivskoe Island was 37 cm.The color of the sediment varies from dark gray (almost black) to light gray with increasing depth.The sediments contain plant remains and have a putrid odor in the 0-15 cm interval and a sulfurous odor in the 15-37 cm interval.The results of X-ray diffraction analysis of the mineral composition of the bottom sediments obtained using the powder method showed that the main mineral in the lake sediments is quartz.Its content ranges from 35 to 68%.It should be noted that the quartz content in different layers of the investigated sediment horizon hardly changes and is 50%.The exception is only the interval from 3-10 cm, where the mass fraction of the mineral is 68%, as well as the layer from 33-37 cm with the lowest quartz content (35%).The mass fraction of feldspars ranges from 25 to 45%.The 3-10 cm interval is characterized by the lowest content of feldspars, and the 10-20 cm interval has the highest content.At a depth of 10-33 cm, there is a gradual decrease in the mass fraction of feldspars from the roof to the sole of the interval, but then, in the layer from 33-37 cm, the content of feldspars sharply increases (from 30 to 40%).The distribution of clay minerals in the investigated sediment horizon is non-uniform.Trace amounts are detected in the 0-20 cm interval, but the mass fraction of clay minerals in the layer from 20-37 cm is 10-15%.Calcite is present in each investigated layer of the bottom sediments of Schwarzivskoe Lake, from trace amounts up to 15% by weight, with the highest value registered in the deepest of the investigated layers at a depth of 33-37 cm.In general, the development of calcite is noted in the upper intervals of bottom sediments (0-10 cm), then its amount sharply decreases to traces in the layer from 10-25 cm, but as the depth increases, a swift rise in the value of the mass fraction of this mineral is observed.In two sections of the sediments (the 10-15 cm and 33-37 cm layers), trace amounts of amphibole were found, and the interval from 20-25 cm contains 5% plagioclase.The study results showed that the value of the ash mass fraction does not significantly change in the interval of sediment depth from 3-33 cm.A significant difference is observed at a depth of 33-37 cm, where the ash content decreases to 91%, and also in the uppermost layer of the investigated horizon, where this indicator is slightly lower than in the underlying layers.Organic matter is distributed relatively evenly throughout the investigated sediment horizon, but there is an increase in the mass fraction of organic matter in the layers from 0-3 cm and 33-37 cm (Table 2).A column of bottom sediments from Lake Shikhalin-Nuur, 44 cm in length, is represented by sandy deposits that transition deeper into clays that have a sulfurous odor and contain occasional inclusions of fragmental material.Stones of varying sizes are most common within the 5-21 cm interval, with no occurrence in the lower layers until the 35-40 cm layer.Within the 25-44 cm interval, there are practically no traces of plant remains, though the upper layers of sediment do contain them.The sediment's color is non-uniform, ranging from gray to dark gray, with a brownish tint at the contact point between the core and sampler in the 15-42 cm interval.X-ray diffraction analysis of the mineral composition of the bottom sediments revealed that the lake's deposits are primarily composed of quartz and sodium and potassium feldspars, but the proportion of clay minerals is significant.The combined mass of clay minerals varied from 5 to 12 percent, represented by mixed-layered illite-smectite (49-100%), illite (51%), and chlorite (13%) single manifestations.The development of carbonate minerals occurs at a depth of 25-44 cm, with calcite as a singular occurrence throughout the explored horizon (depth of occurrence 35-40 cm, mineral mass fraction 5%).The content of dolomite ranges from 5 to 12 percent, with the maximum value of the mineral mass fraction being determined in the 35-40 cm layer.The presence of halite (5%) was discovered within the 30-35 cm interval, and amphibole (5%) in the 40-44 cm interval.The results of ash content measurement indicated that the sediment's ash content constantly increased as the accumulation progressed within the explored interval (from 83 to 98%), while the content of organic matter in the lake's bottom sediments correspondingly decreased (from 17 to 2%).The lake's shifting sediment type (based on organic content) is clearly traceable, with weakly sapropelic aleurolithic and clayey silts being replaced by aleurolithic sands and clayey silts as sediment accumulated over 40 cm 5.The study's findings are presented in Table 3. Table 3. Ash content and organic carbon content in bottom sediments of Lake Shikhalin-Nuur.

Depth of sedimentary deposits, cm
Ash content., % The mass fraction of organic matter, % The core sample of the lake sediments from Uksinda, measuring 42 cm in length, was divided into 8 layers with a thickness ranging from 3-8 cm.The stratigraphic sequence from bottom to top is as follows: 42-34 cm -fine-grained light grey sand with plant residues and no characteristic odor, contact with the overlying layer is clearly defined by soil characteristics (plasticity and consistency); 34-26 cm -light grey clay with plant residues and no characteristic odor, a thin layer (less than 1 mm) of yellow-brown color, likely oxidized iron, is observed on the external wall of the core (in contact with the sampler), contact with the overlying layer is clearly defined by soil characteristics (plasticity and consistency); 26-18 cm -dusty grey-brown sand with a slight odor of decay and plant residues, a non-uniformly distributed pelitic fraction of sediment is observed at a depth of 23-26 cm; 18-15 cm -medium-grained grey-brown sand with a slight odor of decay and plant residues, contact with the overlying layer is clear and different in color and composition; 15-11 cm -fine-grained grey sand with a slight odor of decay and plant residues, contact with the overlying layer is clearly defined by a change in color; 11-7 cmfine-grained dark grey sand with a decay odor and plant residues, contact with the overlying layer is clearly defined by a change in color (the upper layer has a darker shade and a distinct sulfuric odor); 7-5 cm -fine-grained sand (with a small amount of finer fraction) black-grey in color with a sulfuric odor and plant residues, contact with the overlying layer is clearly defined by a change in color (the upper layer has a lighter shade); 5-0 cm -finegrained sand (with a small amount of finer fraction) dark grey in color with no characteristic odor and plant residues.The analysis of the stratigraphic structure of the lake sediments from Uksinda shows that the first 18 cm from the top mainly consists of fine to medium-grained sand.The layer between 18-26 cm consists of sand and aleurite in almost equal proportions, with a slightly larger fraction of fine-grained sand present.The boundary of this sediment layer with the underlying layer is not clearly defined due to the unevenly distributed clay inclusion.A homogeneous layer of clay is present at a depth of 26-34 cm, followed by sand in the deeper layer.X-ray diffraction analysis of the mineral composition of the lake sediments from Uksinda shows that the deposit mainly consists of feldspar and quartz.The content of feldspar varies from 35-50%, with the highest concentration in the upper layer of the studied horizon (50%), and relatively evenly distributed in the lower layers.The content of quartz reaches a maximum of 60% in the layer of 5-7 cm, while in other layers, it varies slightly and remains around 50%.Hydroslud, calcite, dolomite, and clay minerals are present in smaller quantities.The fine fraction of the upper layer contains hydroslud, kaolinite, calcite, dolomite, and halite, as well as a possibility of the presence of a mixed layer mineral of hydroslud-smectite.The lower layer (at a depth of 7-15 cm) contains clay minerals and hydroslud, with a mass fraction of 5%, and traces of carbonate minerals (calcite and dolomite) are also present.Overall, the mineral composition of the lake sediments from Uksinda is homogenous throughout the studied horizon.The bottom sediments of Lake Ukshinda were also examined for the content of ash and organic matter.The results are presented in table 4. The ash content is evenly distributed and ranges from 88.7-98.7%,which allows the classification of the lake sediments from Uksinda as mineralized sediments mainly composed of clays and sands.A relatively significant decrease in the ash content is observed only in the interval of 18-34 cm.The amount of organic matter in lake sediments is extremely low (about 1-2% in the investigated horizon as a whole).An exception is made for layers of 18-26 cm (the mass fraction of organic matter is 5.7%) and 26-34 cm (the content of organic matter reaches 11%).The results of spectrophotometric determination of the content of organic carbon in each of the intervals of the investigated horizon showed that its concentration does not exceed the limit of determination of the technique (the limit is 1.7 g/t) over the entire thickness of the deposits subjected to the study, except for the layer of 26-34, where the content of organic carbon is 2.06 g/t.

Conclusion
Thus, it can be concluded that the bottom sediments of the non-flowing mineral lakes of Transbaikalia differ in their mineralogical and geochemical characteristics.The mineral composition of the bottom sediments of the studied lakes is similar, dominated by fragmented material such as quartz and feldspars, which can be explained by the characteristics of the landscape of the studied area.However, an essential fact for this study is the presence of authigenic minerals in the bottom sediments, the mass content of which does not exceed 10-15%.Their formation is caused by hydrogeochemical interactions and depends on the chemical and physicochemical properties of the lake water and surrounding rocks.It should be noted that the mineral composition of the bottom sediments of one lake is also heterogeneous and varies depending on the depth of the sediment.This indicates the instability of hydrological and hydrogeochemical regimes of the lake and indicates changes in the chemical composition of its water at different times.The high value of the sediment solubility, as well as the low content of organic matter, indicates that the development of living organisms under conditions of high water mineralization is difficult.

Table 1 .
Ash content and organic carbon content in bottom sediments of Lake Barun Shiveretuy.

Table 2 .
Ash content and organic carbon content in bottom sediments of Lake Schwarzivskoe.

Table 4 .
Ash content and organic carbon content in the bottom sediments of Lake Uksinda.