Geochemical transformations of sulfur in salt lakes (Transbaikalia)

. The water column in brackish and saline lakes hosts various forms of sulfur including sulfide (hydrosulfide), elemental, thiosulfate, and sulfate sulfur. The unequal distribution of these reduced sulfur species indicates the presence of two opposing processes - sulfate reduction and oxidation of newly formed hydrogen sulfide. The scale of these processes varies among lakes, resulting in differing proportions of reduced sulfur forms. The bacterial reduction of sulfate ions is confirmed by a significant separation of sulfur isotopes into sulfide and sulfate ions, with the lighter isotope accumulating in the former and heavier isotope in the latter. In most soda, chloride, brackish, and salt lakes, sulfate reduction is the principal process responsible for relatively low sulfate ion content. Additionally, the presence of an oxidizing environment and sulfides in host rocks provide additional sources for sulfates, leading to the formation of sulfate-type lakes. The formation of specific types and subtypes of brackish and salt lakes is determined by processes such as water evaporation, dissolution of aluminosilicates, sulfate reduction, and oxidation of sulfides. The dominance of these processes contributes to the geochemical diversity of lakes.


Introduction
It is believed, the water's chemical type changes as the water salinity increases (i.e., from soda to sulfate to chloride).This sequential change in chemical composition is explained by the deposition of salts when they saturate lake waters.First, the least soluble calcium and magnesium carbonates fall out, then calcium, sodium, and other sulfates.
Our studies of brackish and saline lakes in South-Eastern Transbaikalia show that sulfate lakes are nearly non-existent within the study area.The surpassing increase in chloride ion content compared to the sulfate ion in lake waters does not correspond to the ratio of the main anions in groundwater, which provide the bulk of the salt supply to the lakes.Its equivalent concentrations are on average lower than that of the sulfate ion, therefore, before the lake water becomes supersaturated with respect to sulfate minerals, resulting in their subsequent precipitation (i.e., gypsum, mirabilite), sulfate ions must accumulate in higher concentrations than chloride ion, and sulfate ion becomes the main anion due to the increase in water salinity.
Under natural conditions sulfur oxidation states and species are distributed, depending on the redox environment.First of all, this refers to modern bottom sediments in organically enriched water bodies.Many forms of reduced sulfur compounds can be found in these reservoirs (e.g., iron sulfides, elemental sulfur, thiosulfates), as well as residual sulfates.Their nature is due to two opposite effects: the first is the chemical and bacterial oxidation of sulfides and the second is sulfate reduction.
The main goal of the research was to clarify the reasons for the discrepancy between the classical scheme of transformation of salt lakes with an increase in salinity and to identify the main mechanisms that determine the behavior of sulfur, transformation and stability of its forms under certain hydrogeochemical conditions.

Materials and methods
The work is based on the results of hydrogeochemical sampling, conducted during the summer from 2013 to 2020.The multiannual (2004-2017) and inter-seasonal observations were carried out on Lake Doroninskoye.There were 97 lakes were sampled during field work.
Sulfate ion (SO 4 2-) was analysed by the turbidimetric method in the form of barium sulfate, with a detection limit of 0.1 mg/L.Sulfide (S 2-) and elemental (S 0 ) sulfur were precipitated from 100 mL of each water sample using zinc acetate on "blue ribbon" filters.Thiosulfates (S 2 O 3 2-) and sulfites (SO 3 2-) were isolated using silver nitrate from the remainder of the filtrate.The elemental and thiosulfate sulfur (S(S 2 O 3 2-), after successive conversion to the sulfide form, as well as sulfide (S 2-and hydrosulfide HS -) sulfur, were determined by the photometric method.Considering the complexity associated with dividing thiosulfates and sulfites, they were determined together.Our method allows us to determine the concentration of the total S 0 in the water, including dissolved sulfur, mechanical sulfur and colloidal sulfur in the compositions of polysulfide ions (SS n 2-).The detection limit of this method for reduced sulfur forms is 5 μg/L.
The analysis of δ 34 S(S 2-) and δ 34 S(SO 4 2-) included two stages.At the first stage, the following reagents were added to the water samples: zinc acetate to determine the isotopic composition of S 2-and barium chloride (the sample was acidified to pH of 3) for determination of isotopic sulfur (SO 4 2-).At the second stage, the obtained solutions were filtered, and the precipitate formed was analysed in S configuration using a Flash EA-1112 analyser (Thermo Scientific, Germany), according to the standard protocol for converting sulfate and sulfide to SO 2 .The 34 S/ 32 S isotope ratios were measured using a MAT-253 mass spectrometer (Thermo Scientific, Germany).The measurements were performed with respect to the laboratory standard SO 2 gas, calibrated according to international standards IAEA-S-1, IAEA-S-2, IAEA-S-3 and NBS-127.The measurement results are presented in the generally accepted form: δ 34 S = (Rsample /Rstandard -1) and are expressed in (‰), where Rsample and Rstandard are 34 S/ 32 S ratio in the sample and standard, respectively.The reproducibility of the δ 34 S results was 0.1‰ (1σ) for standards (n = 10) and samples.Results are reported as δ 34 S values relative to the VCDT (Vienna-Canon Diabolo Troilite) standard.The sulfate-sulfide fractionation was calculated by the formula: (δ 34 S(SO 4 ) + 1000) / (δ 34 S(H 2 S) + 1000) (1)

Sulfur in the rocks of the coastal lakes and bottom sediments
The chemical analysis of the bulk fraction of the lake coastal rocks showed that, on average, in intrusive rocks the sulfur content is an order of magnitude higher than those in sedimentary and metamorphic rocks.The former also contain large amounts of iron, silicon, potassium and magnesium.X-ray phase analysis of bottom sediments shows that the mineral composition of feldspars (mainly albite, orthoclase and microcline), carbonates (calcite and dolomite) includes an X-ray amorphous phase consisting of finely dispersed minerals.Natron and mirabilite are present in Lake Borzinskoye, a highly saline soda lake.Most of the bottom sediments are composed of black oily silts with a pronounced smell of hydrogen sulfide.Colloidal iron monosulfide -hydrotroilite (FeS n •H 2 O) imparts a black color to silts.
The thickness of silt is more than 0.5 m in the coastal part of many lakes.The average content of sulfide sulfur (i.e., hydrogen sulfide and oxygen-soluble sulfides) in lake silts is 0.26 mg/100 kg of sediment, with a maximum value of 15.5 mg/100 kg in Lake Doroninskoye.The value of the redox potential Eh decreased to -423 mV, averaging -250 mV in all lakes (Table 1).The presence of an anaerobic environment and relatively high levels of organic matter contribute to the active process of sulfate reduction.According to data, the rate of sulfate reduction averages 69.04 mg S/kg/day in lakes with salinity up to 30 g/L, whereas the number of sulfate-reducing bacteria is 10 4 -10 5 cells/mL.When the salinity increases, the rate of sulfate reduction decreases and it amounts to tenths of mg S/kg/day in highly mineralized lakes.For example, in soda hypersaline Lake Borzinskoye (320 g/L) the measured rate of sulfate reduction is 0.886 mg S/kg/day.The sulfate reduction process runs intensively during both summer and the freezing periods.The psychrophilic haloalkaliphilic community becomes active in this period.It participates in the decomposition of organic matter, even at negative water temperatures, which can reach from -2 to -8 ºС in brines.

Sulfur in water
Hydrogen sulfide occurred as H 2 S in the waters, including hydrosulfide (HS -) and sulfide ions (S 2-).The stability of the mentioned species is determined by the pH value of the water (Borzenko and Zamana 2011).Hydrogen sulfide migrates mainly as the HS -species in soda lakes (more than 90%), and it is represented in relatively equal proportions of molecular Н 2 S and ionic HS -species in chloride and sulfate lakes.In both cases, sulfur has an oxidation state of -2, therefore we express it as S 2-.
The relatively great concentrations of S 2-in the water column are found in Lake Doroninskoye (370200 μg/L), Lake Khodatuy (6700 μg/L), Lake Kudzhertay (10300 μg/L), Lake Malyye Yakshi (9100 μg/L), Lake Shvartsevskoye (1900 μg/L), Lake Bolshaya Bulugunda (1250 μg/L) (Table 2).A high number of microorganisms involved in the sulfur cycle give a bright red colour to water in these lakes, where colonies of purple bacteria use hydrogen sulfide as an electron acceptor to oxidize organic matter.
Mostly, elevated levels of hydrogen sulfide are recorded in lakes with relatively low concentrations of dissolved oxygen or its complete absence.In such lakes, Eh values vary from negative values to the upper limit of their life (100 mV).
The hydrogen sulfide content varies from the detection limit (5 μg/L) to 370200 µg/L with a mean of 508.7 μg/L in soda lakes, maximum concentration in sulfates was found in Lake Barun-Shivertuy (57 μg/L) with an average value of 19.0 μg/L.In chloride waters, maximum sulfate concentration was found in Lake Bol'shaya Bulugunda (1250 μg/L) with an average of 469.3 μg/L.The content of hydrogen sulfide in lakes of subtype II averages 6.75 μg/L, its maximum content was measured in Lake Zhilino (41.3 μg/L) In addition to S 2-, elemental sulfur (S 0 ) and thiosulfate sulfur (S(S2O3 2-)) were the most stable species in lake waters.The range of variation of S 0 concentration in waters range from its complete absence to a maximum value of 3800 μg/L (Lake Malyye Yakshi), with an average value of 208.9 μg/L.According to both the maximum and average values, its content was higher in soda lakes (228.8 μg/L), In sulfate and chloride lakes they were 148.8 and 116 μg/L, respectively.S 0 was predominant (47.7%), the second was S 2-(31.3%), the proportion of S(S2O3 2-) was only 21% in the total reduced sulfur (⅀Sred).
The measured content of S 0 was often greater than its solubility limit (6 μg/L), which indicates its various species in water.Sulfur in lakes can be present in the dissolved form, which is a part of the polysulfide ions (SS n 2-), as well as in suspended and colloidal forms.The presence of S 0 in the form of the smallest suspended particles found on the surface of bacterial fouling and directly on the surface of the water was observed in Lakes Kudzhertay, Khodatuy, Malyye and Bol'shiye Yakshi, Doroninskoye.
Its presence in the form of polysulfides are formed under alkaline conditions according to the reaction: Is confirmed by the consistency of the distribution of S 0 and S 2-in the region of their elevated concentrations.
Another possible route for the formation of S 0 can be associated with the interaction between hydrogen sulfide and iron hydroxide, in which S 0 is formed at one of the stages, and iron sulfides may be the final product according to the reaction: The concentration of S(S 2 О 3 2-) varies from 5.00 to 630 μg/L in Lake Malyye and Bol'shiye Yakshi.The maximum values are found in soda lakes (119.7 μg/L), in sulfate and chloride lakes they are lower (20.9 and 52.0 μg/L, respectively).The randomness in the distribution of S 0 S 4+ with respect to S 2-and S 0 is explained by its dual nature, since S(S 2 O 3 2-) is an intermediate product of anaerobic reduction of sulfates to hydrogen sulfide or oxidation of the latter.
The lack of order in the distribution of reduced sulfur in the water column of lakes is the result of two oppositely directed processes of the reduction of sulfates and oxidation of hydrogen sulfide.However, there is a strictly defined pattern in the behavior of all reduced forms of sulfur in the lakes with depth -a decrease in oxygen and sulfate ions, the transition of the redox potential Eh to negative values and the increase in the concentration of hydrogen sulfide to the bottom.
The behavior of sulfur in the water column in the deepest soda lakes, Bain-Tsagan and Doroninskoye, was studied in more detail.According to the obtained data, S 2-is dominant in ΣS red in the oxygen-free bottom layers of these lakes, the S 2-content gradually decreases at the surface layers, while the contents of S 0 increase, the oxidation of S 0 leads to the accumulation of S 0 S 4+ in the transition region of the reductive from to the oxidative environment.
Long-term seasonal studies of Lake Doroninskoye have shown that sulfate reduction actively occurs not only in summer, but also during the freezing period against the background of a decrease and even complete disappearance of dissolved oxygen in the water column.The increase in the contents of S 2-and the simultaneous decrease in the contents of SO 4 2-are observed during this period.In this period the number of sulfatereducing bacteria is 100 cells/mL at a sulfate-reduction rate up to 29.8 mg S/dm 3 /day.The proportion of S 2-in ∑S red increases up to 50% at the end of the freezing period, the maximum concentration of S 2-was recorded in March 2013, which amounted to 370.2 mg/L.The Eh value decreased to a minimum of -423 mV; the content of SO 4 2-, on the contrary, decreased to 28 mg/L.
The increase in DOC of more oxidized compounds of S 0 (up to 560 μg/L) and SO 4 2-(up to 290 mg/L) is noted during the period of ice boom, on the contrary, the content of S 2-(<5 μg/L) decreases.In summer, when water is warm, bacterial mats are formed and by the autumn they decompose.Therefore, the next peak of S 2-content is recorded during this period.According to observations, the content of S 2-reached 4.2 mg/L in the surface layers.
Considering that most lakes have depths of a few meters or less, the ice cover (ice thickness up to 1.5 m) significantly reduces the volume of lake water and the number of hydrobionts sharply increases in a unit volume.The oxygen cost for respiration and the chemical oxidation of reduced chemical elements (e.g., sulfur, nitrogen, iron) increase, which leads to the complete disappearance of oxygen in the water column.These anaerobic conditions contribute to the development of the corresponding microbiota.Hot summers contribute to the rapid heating of small lakes and the development of bacterial mats, which are an additional powerful source of easily mineralized organic matter; its decomposition also leads to oxygen consumption, therefore, an increase in the content of S 2-is noted during this period.

Isotopic composition of sulfur
Variations in the sulfur isotopic composition of dissolved sulfates in the lakes under consideration fit into the 34 S content observed in the waters of continental reservoirs.The average value of δ 34 S(SO 4 2-) has equaled 10 ‰, although the range of values varies from -8.4 ‰ (Lake Kharanor) to 27.4 ‰ (Lake Khodatuy).A low value (-2.74 ‰) was found in Lake Hara-Torum.In the first case, no signs of sulfate reduction were detected, and in the second, the S 2-content was near its detection limit (6 μg/L).
According to the average values of δ 34 S, the lightest sulfur composition can be found in sulfate lakes (7.7‰) and a few soda lakes of subtype II (4.3 ‰) with a variation range from -0.9 ‰ (the soda lake Grishkino) up to 9.3 ‰ (the sulfate lake Barun-Shivertuy).The weighting in the latter can be explained by the presence of sulfate reduction both in the bottom sediments and in the water column of the lake.In the water column, the concentration of S 2-reaches 57 μg/L.In this case sulfate reduction is of subordinate importance, and the lightened isotopic composition of sulfate sulfur (Lakes Kharanor, Khara-Torum, Ganga-Nur, Kharaganash and others) is a consequence of the oxidation of isotopic-light sulfides of water-bearing rocks.The sulfur isotopic composition of ultrabasic and basic rocks averages 1.2 ‰, sulfur of effusive rocks is slightly heavier (mean 1.9 ‰).It is clear that lakes located within these geological formations inherit light sulfur.
According to the geology of the region, sulfate and soda lakes of subtype II are located within intrusive formations bearing sulfide mineralization, hence the oxidation of sulfides (usually pyrite) leads to an increase in the concentration of SO 4 2-in groundwaters, and then in lakes.For example, in the catchment of Lake Kharaganash (soda lake, subtype II) in spring water (Ca-HCO 3 -SO 4 ) the SO 4 2-content (20.3 mg/L) was several times greater than the Cl -content (2.0 mg/L), which definitely indicates the presence of an additional sulfur source, which were sulfides of water-bearing rocks (i.e., pyrite, chalcopyrite, sphalerite).
The opposite situation developed in Lake Chodatuy (soda, subtype I), this lake is characterized by relatively heavy isotope sulfur SO 4 2-and the relatively high concentration of S 2-(6700 μg/L in waters), as well as a low sulfate coefficient value, К SO4/Cl = 0.025.The value of the ratio δ 34 S(SO 4 2-) exceeds not only the values for sulfur SO 4 2-of ocean water (20.1 ± 0.8 ‰) and modern evaporites (24.3 ‰), but the well-known upper limit of the range for acidic rocks (26.7 ‰), it is second only to sulfates of salt domes (up to 62 ‰ in individual samples; all values are given according to Grinenko and Grinenko (1974).The same order of magnitude values of δ 34 S was determined in subtypes I and III of soda lakes (Lakes Tsagan-Nur, Kudzhertay, Malyye Yakshi, Gashkoy, Zun-Torey) with high concentrations of hydrogen sulfide and low K SO4/Cl < 0.5.
In general, a greater presence of S 2-and heavy sulfur isotopes in SO 4 2-leads to lower value of the K SO4/Cl ratio.The influence of sulfate reduction on the enrichment of SO 4 2-in heavy isotopes can be traced according to data obtained from Lake Borzinskoye and a well located in its catchment.The sulfur isotopic ratio of dissolved sulfates from the well was -2.3 ‰, and the δ 34 S(SO 4 2-) value of atmospheric precipitation averaged 6.7 ‰.Thus, dissolved SO 4 2-will not have a δ 34 S(SO 4 2-) reading (measured value of lake water is 12 ‰) in any proportions of mixing atmospheric precipitation with groundwater.According to the obtained data, the content of S 2-was 480 μg/L in Lake Borzinskoye, and δ 34 S 2-was -28 ‰, and the fractionation coefficient was 1.040 ‰.This separation is possible only with bacterial reduction of sulfates.
Significant fractionation of sulfur isotopes was observed in most lakes with a relatively high content of reduced sulfur.The isotopic composition of S 2-sulfur ranged from 13.0 ‰ (Lake Doroninskoye) to -30.8 ‰ (Lake Malyye Yakshi).Moreover, the maximum fractionation (1.042) with a relatively high content of hydrogen sulfide (1900 μg/L) and a low sulfate coefficient (K SO4/C l = 0.37) was noted in Lake Shvartsevskoye (soda, subtype III).A relatively high value of δ 34 S(S 2-) in Lake Doroninskoye was most likely the result of the repeated reduction process of SO 4 2-, as a result S 2-was gradually enriched in the 34 S heavy isotope.On the other hand, the weighting of S 2-can be explained by the high rate of sulfate reduction, which was noted in this reservoir.
The influence of bacterial reduction of sulfates on the separation of sulfur isotopes was especially observed in seasonal and depth changes not only in the contents of reduced sulfur, but also in the isotopic ratios between S 2-and SO 4 2-in Lake Doroninskoye.The range of variation of the isotopic densities of SO 4 2-did not exceed of 18 -27.2‰ with an average value of 20.3 ‰, and S 2-ranged from 13 to -7.4 ‰ with an average of 3.54 ‰.The maximum heavy SO 4 2-sulfur was determined during the freezing period (the fractionation coefficient average was 1.0356 ‰).With a decrease in the SO 4 2-content and an increase of S 2-in depth, the first enriches in 34 S and the second becomes isotopically lighter.During navigation season, when oxygen content increases and hydrogen sulfide content decreases, SO 4 2-sulfur becomes isotopically lighter.On the contrary, SO 4 2-becomes isotopically heavier, when water temperatures are high, bacterial fouling develops and S 2-content increases in the surface layers.
This means that the oxidation processes of reduced sulfur do not contribute significantly to the separation of sulfur isotopes in its geochemical transformations.Hence, the relatively isotopically light sulfur SO 4 2-is a consequence of the oxidation process of isotopically light S 2-.

Conclusion
The water column of brackish and saline lakes in South-East Transbaikalia contain hydrogen sulfide and its derivatives S 0 S 4+ and S 0. The presence of S 0 along with S 2-indicates the simultaneously occurring multidirectional oxidation processes of hydrogen sulfide and sulfate reduction.However, the presence of S 2-is clear evidence that the processes of hydrogen sulfide formation surpass the process of sulfur oxidation to a sulfate species of sulfur that is the most stable in water.The relationships between hydrogen sulfide and products of its incomplete oxidation are determined by many factors such as the content of dissolved oxygen, the corresponding microflora, and the kinetic parameters of each process stages in these natural environments.Microbiological processes are associated with low concentrations of sulfate ions in lake waters; it is confirmed by sulfur isotopic data.
Additional sources of sulfur are required for the formation of sulfate and soda lakes with a relatively high content of sulfate ions.Sulfides of the host rocks act as a source of sulfur, their oxidation leads to the accumulation of sulfate ions in the lakes.

Table 1 .
The content of hydrogen sulfide, Eh in the bottom sediments of lakes.

Table 2 .
The content of hydrogen sulfide and its derivatives, δ 34 S values of dissolved sulfate and sulfide ions, Eh and in water of the distinguished types and subtypes of lakes.
n-quantity of samples, -no data due to the absence of hydrogen sulfide or its low contents.