Boron isotope geochemistry of a brine-carbonate system in the Qaidam Basin, western China

Sedimentary Geology 383 (2019) 293–302

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Boron isotope geochemistry of a brine-carbonate system in the Qaidam Basin, western China Xiangru Zhang a,b,c, Qingkuan Li a,b,c, Zhanjie Qin a,b,c, Qishun Fan a,b,⁎, Yongsheng Du a,b, Haicheng Wei a,b, Donglin Gao a,b, Fashou Shan a,b a b c

Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China Qinghai Provincial Key Laboratory of Geology and Environment of Salt Lakes, Xining 810008, China University of Chinese Academy of Sciences, Beijing 10049, China

a r t i c l e

i n f o

Article history: Received 7 February 2019 Received in revised form 21 February 2019 Accepted 22 February 2019 Available online 28 February 2019 Editor: B. Jones Keywords: Boron isotopic composition Influencing factors Brine-carbonate system Salinity Qaidam Basin

a b s t r a c t The boron (B) isotope-pH proxy for marine carbonates has been widely used to reconstruct past ocean pH values. Unlike seawater, the apparent dissociation constant of boric acid (KB) and the δ11B values of brines from the terrestrial salt lakes in western China vary. However, relevant studies concerning the B isotope geochemistry of brine-carbonate systems in terrestrial salt lakes are limited. In this study, thirty-nine clastic samples from a sediment core (ISL1A) from the Qarhan Salt Lake in the Qaidam Basin were collected. These carbonates were analyzed for δ11B, B, Mg, Sr and Ca. Using our new data as well as published pH values, total dissolved solids (TDS), element concentrations, and δ11B values of brines in western China, we reached the following conclusions. (1) During evaporation process, salinity increases, the δ11B values of brines in salt lakes increase, and the pKB (−logKB) and pH values simultaneously decrease. (2) The large heterogeneity in the δ11B values of the carbonates (−2.74‰ to +7.64‰) from core ISL1A is mainly due to the δ11B values of the brines, rather than pKB and pH. (3) B isotopic fractionation with a small αcarbonate-brine of 0.997 in a brine-carbonate system conforms to the Rayleigh fractionation model. (4) Our comparison of the δ11B values of these carbonates with other records from core ISL1A suggests that the δ11B values of the carbonates in the arid Qaidam Basin can be used as a new proxy for the salinity of paleo-lake water. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Several studies have concluded that variation in the boron (B) isotopic compositions (δ11B) of marine carbonates only depend on the pH values (Spivack et al., 1987; Barth, 1993; Sanyal et al., 1997; Laurent et al., 2006). One of the important premises for the δ11B-pH proxy is that the δ11B values (+39.5‰) and pKB (-logKB) (KB is the apparent dissociation constant of boric acid) values of modern seawater have remained stable over the last 20 Ma (Barth, 1993). Therefore, the δ11B of marine biocarbonates (coral and foraminifera) (Spivack et al., 1993; Gaillardet and Allegre, 1995; Sanyal et al., 1995) and shallow marine abiotic carbonates (ooids and cements) (Clarkson et al., 2015; Zhang et al., 2017) have been widely used to calculate the past ocean pH values, which are an effective means for estimating atmospheric CO2 concentrations and reconstructing global climate change (Liu et al., 1999a, 1999b; Pearson and Palmer, 2000; Foster, 2008; Anagnostou et al., 2016). ⁎ Corresponding author at: Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China. E-mail address: [email protected] (Q. Fan).

https://doi.org/10.1016/j.sedgeo.2019.02.011 0037-0738/© 2019 Elsevier B.V. All rights reserved.

However, studies of B isotope geochemistry of brine-carbonate systems in terrestrial lake systems are rare. Liu et al. (2014) reported that the coupled high δ11B and low δ18O values of carbonates from a Lop Nor sediment core suggest a lacustrine depositional environment consisting of Late Miocene episodic lakes in the arid Tarim Basin, western China. Wei et al. (2014) concluded that the δ11B values of an acid soluble carbonate from a sediment core (25.0 m in depth) from Dongtai Salt Lake in the central Qaidam Basin can effectively indicate the pH values of the paleolake water. These studies have preliminarily highlighted our understanding of the δ11B values of carbonates (δ11Bcarbonate) under terrestrial conditions. Unlike seawater, the pKB and δ11B values of the brines (δ11Bbrine) in the terrestrial lakes of western China vary with salinity (Qi et al., 1993; Xiao et al., 1999; Golan et al., 2015). The variation in these values complicates the perception and application of the δ11Bcarbonate values of terrestrial brine-carbonate systems. For example, within a similar range of pH values, Sun et al. (1993) and Lv et al. (2014) observed heterogeneities in the δ11B differentials of the lake waters and carbonates in Qinghai Lake (1‰-2‰) and Damxung Co Salt Lake (6‰-12‰) in Tibet, respectively. Therefore, pH is not the only factor affecting the δ11Bcarbonate values of terrestrial salt lakes. The relationships between δ11Bbrine, pH, pKB and salinity and the main factors influencing

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the δ11Bcarbonate values of terrestrial lake systems need to be studied further. In this study, we present the B, Mg, Sr and Ca concentrations as well as the δ11B values of carbonates from a sediment core (ISL1A) from the Qarhan Salt Lake (QSL) in western China (Fig. 1). By combining our new data with reported pH values, total dissolved solids (TDS), element concentrations and δ11B values of brines from salt lakes on the Tibetan Plateau (Zhang, 1987; Qi et al., 1993; Xiao et al., 1999; Du et al., 2019), we aim to do the following: (1) establish relationships between the pH, pKB, δ11B values, and the salinity of brines; (2) discuss the variations in the δ11Bcarbonate values and the factors influencing these variations; (3) create a fractionation model for the brine-carbonate system; and (4) interpret the geological implications of the δ11Bcarbonate values of terrestrial salt lakes. This study is important for understanding the geochemical behavior and developing potential applications of the δ11Bcarbonate values in the geologic record. 2. Description of the study area Salt lakes are common in western China, especially in the Tibetan Plateau region, where different water types (carbonate type, sodium/magnesium sulfate subtypes, chloride type) of salt lakes form horizontal E-W zones (Zheng and Liu, 2009). The Qaidam Basin

(90°00′–98°20′E, 35°55′–39°10′N) is located in the northern part of the Tibetan Plateau. It is a closed fault-depressed basin with an irregular diamond shape. Three high mountains surround the basin, Qilian Mountain to the northeast, Altun Mountain to the northwest, and Kunlun Mountain to the south (Fig. 1a). The basin is known for its abundant salt resources since it contains both thick salt beds and brines (Chen and Bowler, 1986; Zhang, 1987). Currently, more than twenty salt lakes exist within the basin (Fig. 1a). The Qarhan Salt Lake (QSL) (94°42′36″–96°14′35″E, 36°37′36″– 37°12′33″N) is located in the eastern part of the Qaidam Basin at an elevation of 2677 m above sea level. It is the largest playa in this basin, with an area of 5856 km2 (Fig. 1). It extends ~168 km from west to east and 20–40 km from north to south. The QSL is characterized by thick salt deposits and abundant intercrystalline brines, which are remarkable for their K+ and Mg2+ concentrations. The total KCl reserves (194 × 106 t) of the QSL make it the largest base for potash fertilizer production in China (Cao and Wu, 2004). Ten shallow perennial and ephemeral saline lakes exist on the margin of the playa, including the Seni, Dabiele, Xiaobiele, West Dabuxun, Dabuxun, Tuanjie, Xiezuo, South Huobuxun, and North Huobuxun lakes (Fig. 1b). From west to east, the QSL is divided into four sections (the Bieletan, Dabuxun, Qarhan and Huobuxun sections) and thickness of the salt deposits gradually decreases 55–65 m to 15–25 m (Zhang, 1987).

Fig. 1. Map showing the location of the drill core and the salt lakes in the Qaidam Basin; (a) Map showing the location of Qarhan Salt Lake and other salt lakes in the Qaidam Basin, northeastern Tibetan Plateau; (b) Map of the drilling core (ISL1A) and the salt lakes in Qarhan Salt Lake area.

X. Zhang et al. / Sedimentary Geology 383 (2019) 293–302

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3.3. Element concentration analysis

Three halite-dominated evaporitic units, which are separated by clastic-dominated sediment units, have been observed in the salt deposits of the QSL (Yu et al., 2013). The sedimentary and stratigraphic records of Qarhan playa show that the evaporite sequences are stable and complete (Zhang, 1987; Yu et al., 2009). There are two main water sources (river waters and springs) flowing into Qarhan playa (Zhang, 1987; Lowenstein et al., 1989; Yuan et al., 1995). The Golmud River, which originates from the eastern Kunlun Mountain, is the main inflow with an annual average runoff volume of 19.2 × 109 m3 (Yu et al., 2009). The river water is enriched in Na+, Mg2+, Cl−, and HCO3− (Lowenstein et al., 1989). Spring water is discharged along a linear fault zone on the northern margin of the Qarhan playa. The spring water is enriched in Ca 2+ and is depleted in SO 42− and HCO 3− (Lowenstein et al., 1989; Yuan et al., 1995; Lowenstein and Risacher, 2009). These two water sources have been remained active in the QSL for the past 50,000 years (Fan et al., 2018). The B concentrations of the river water and spring water are 0.27–0.72 mg/L (Tan et al., 2001) and ~50 mg/L (Du et al., 2019), respectively. The B concentrations of the QSL brines range from 77.6 mg/L to 146.1 mg/L (Zhang, 1987). B(OH)3 and B(OH) 4− are the two main forms in which B is present in the solution when the B concentrations are b250 mg/L (Carrano et al., 2009). Previous comparisons of the B concentrations and TDS of brines indicate that B enrichment in the QSL primarily results from strong evaporation and concentration of the lake water (Du et al., 2019). In addition, the Mg 2+ concentrations increase and DIC concentrations (DIC is the sum of CO32− and HCO3−) decrease with increasing evaporation (Fan et al., 2018).

The three-step ion-exchange method was used to separate and purify the boron in samples. First, a moderate amount of sample was dissolved in 0.5 mol/L of HCl. Then, the liquid supernatant, containing about 15 μg B, was passed through a 0.5 mL Dowex 50 W × 8 resin column to eliminate Mg, Al and Fe. The resulting liquid was added to appropriate amounts of NH3·H2O. After this, the B in the solution was extracted using a 0.15 mL Amberlite IRA 743 resin column. About 10 mL of 75 °C 0.1 mol/L HCl were used to elute the B absorbed by the resin, and then, the solution was evaporated down to a drop on a clean electric hot plate at 65 °C. Finally, the residue was diluted with low-B water and was passed through a 0.5 mL mixed resin column (Dowex 50 W × 8; ion exchange II, v/v 1:1) to further purify the sample. Then, appropriate amounts of mannitol and cesium carbonate were added to the solution to obtain B/Cs and B/mannitol ratios of 2:1 and 1:1, respectively. The resulting solution was evaporated in a clean drying oven at 45 °C until the B concentration was about 1 μg/μL.

3. Materials and analytical methods

Table 1 The δ11B values and B, Ca, Mg, Sr concentrations of carbonates in core ISL1A.

About 2 g of fine-grained sample were dissolved in 0.5 mol/L of HCl and then, the sample was centrifuged for approximately 10 min to collect the liquid supernatant to analyze it for B, Sr, Mg and Ca. The element concentrations were determined at the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences. The Ca and Mg concentrations were measured using ethylene diamine tetraacetic acid (EDTA) titration with errors of ≤0.5%. The B and Sr concentrations were determined using an ICAP 6500 DUO ICP-OES with errors of ≤3%. 3.4. Boron isotopic analysis

Depth/m δ11B (‰) Error (2σ, ‰) B (μg/g) Ca (mg/g) Mg (mg/g) Sr (mg/g)

3.1. Samples collection A 102-m-long sediment core (ISL1A) (37°03′50″N, 94°43′41″E) was drilled in the Bieletan section of the west-central QSL (Fig. 1b). The upper part of core ISL1A (0–51.0 m) consists of halite, silty sand, and clay. The lower part of core ISL1A (51.0–102.0 m) consists of silty sand and clay with organic matter, and it does not contain evaporites (Fan et al., 2014a, 2014b). The δ11B values of 43 halite samples from the upper 44 m of core ISL1A have been reported by Fan et al. (2015). Thirty-nine clastic samples (silt, sand, and clay sediments) from the upper 41 m of core ISL1A were collected to analyze for element concentrations and boron isotopic compositions. All of the samples were soaked in deionized water for about 2 h, and then, they were wet sieved with a 38 μm sieve to eliminate the exogenous and biogenic carbonates. The X-ray diffractometer (XRD) results indicate that the fine-grained carbonate samples are mainly composed of calcite (Fan et al., 2014a). All of the samples were analyzed for B and δ11B, and 16 of these samples were also analyzed for Mg, Sr, and Ca.

3.2. Experimental materials The experimental materials in this study are as follows: low-B water (obtained from passing sub-boiling temperature water through special boron resin), hydrochloric acid (HCl), ethylene diamine tetraacetic acid (EDTA), ammonium hydroxide (NH3·H2O) (high purified grade), Dowex 50 W × 8 resin (a strongly acidic cationic resin, H-form, 200–400 meshes, USA), Amberlite IRA 743 resin (a B special effect resin, 120–200 meshes), ion exchange II (an anionic resin, HCO3-form, 60–100 meshes, Germany); mannitol (1.82%), Cs2CO3 (12.3 g/L), graphite suspension, and standard material (NIST SRM 951, 11B/10B = 4.05537 ± 0.00004).

1.28 3.33 7.87 9.23 10.10 11.00 12.16 12.91 16.78 17.78 18.03 19.14 21.78 22.13 22.67 24.13 24.72 25.07 25.72 26.17 26.82 27.99 29.11 29.60 30.02 31.09 31.87 32.49 33.56 34.23 34.88 37.39 37.95 38.10 38.83 39.23 39.93 40.32 41.17

2.22 2.40 9.94 7.64 6.68 3.31 7.73 6.09 0.79 4.29 1.59 −2.74 1.26 6.03 0.11 −4.25 −2.43 2.17 −3.32 −1.38 −0.46 0.62 −1.28 3.15 2.00 −2.61 2.14 1.30 3.40 1.81 2.74 −4.04 5.35 0.82 1.06 4.42 −0.35 2.38 6.07

0.20 0.15 0.02 0.11 0.05 0.13 0.18 0.17 0.04 0.14 0.08 0.13 0.02 0.17 0.06 0.07 0.06 0.19 0.12 0.06 0.13 0.06 0.30 0.01 0.07 0.27 0.10 0.15 0.20 0.09 0.18 0.09 0.02 0.10 0.05 0.03 0.28 0.35 0.12

106.5 137.1 203.4 265.7 174.1 59.2 163.7 133.7 64.7 25.0 62.4 24.6 10.9 19.5 41.7 17.7 47.4 10.5 28.4 136.0 14.2 59.1 15.3 21.3 22.0 16.0 73.5 53.7 46.4 137.3 39.9 49.0 177.4 62.1 97.9 92.9 34.4 260.1 180.9

68.8 43.3 50.5

42.4 50.6 48.9

0.64 0.37 0.57

45.9

53.2

0.44

39.6

56.3

0.23

60.6

27.0

0.24

57.0

28.8

0.29

83.3 62.3

26.5 21.0

0.21 0.21

75.0 72.8 62.7

21.9 42.4 20.1

0.52 0.33 0.19

61.3

31.1

0.80

56.9

57.9

0.81

72.9

37.0

0.11

100.1

27.2

0.15

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No obvious B isotopic fractionation was observed (Zhang et al., 2016) and the B blank in this study was b40 ng. As a water-soluble element, B exists in solution in two forms, i.e., trigonal B(OH)3 and tetrahedral B(OH)4−, and has two stable isotopes, 10B and 11B, which are enriched in B(OH)4− and B(OH)3, respectively (Palmer et al., 1987; Vengosh et al., 1992; Barth, 1993; Zeebe, 2005). The 11B/10B ratio was measured using a Triton thermal ionization mass spectrometer (TIMS) (Thermo Fisher Scientific Inc., U.S.A.) at the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences using the graphite double coating method (Xiao et al., 1988). Static doublereceiving by a special double Faraday cup system was used to obtain ion flow intensities of 308 (133Cs210B16O2+) and 309 (133Cs211B16O2+). The 11B/10B ratio was determined after oxygen isotope correction. The B isotopic composition is reported as δ11B according to the following formula: δ11 B ð‰Þ ¼

h


11

i 
 B=10 B sample = 11 B=10 B standard −1  1000

ð1Þ

Standard NIST SRM 951 was used, and its measured (11B/10B) value in this experiment was 4.05537 ± 0.00004 (2σ, n = 4). All of the analytical precisions in this study were ≤±0.35‰ (2σ).

heterogeneous (~10‰). In addition, the δ11Bcarbonates values are lower than the δ11Bbrine (+2.88‰ to +13.08‰) of the QSL (Table 3; Xiao et al., 1999; Du et al., 2019), but the isotopic heterogeneity of brines and carbonates is comparable. In this core, the δ11Bcarbonates values are high from 41.2 m to 31.5 m (corresponding to the lower salt layer), low from 31.5 m to 21.0 m (corresponding to the middle clastic layer), and high from 21.0 m to 0 m (corresponding to the upper salt layer). 4.2. Element concentrations The Ca, Mg, B, and Sr concentrations of core ISL1A are also presented in Table 1. These concentrations are highly variable, especially the B concentrations, which extend over an order of magnitude. Comparison plots of B/Ca, Mg/Ca, Sr/Ca and δ11Bcarbonate are shown in Fig. 2. There are strong positive correlations among B/Ca-δ11Bcarbonate (R = 0.92), Mg/Ca-δ11Bcarbonate (R = 0.83), and Sr/Ca-δ11Bcarbonate (R = 0.59) (Fig. 2). The Mg/Ca and Sr/Ca ratios of carbonates in lakes are used to indicate changes in the salinity of the lake water (Williams, 1966; Haskell et al., 1996; Zhou, 2004). Thus, the geologic implication of the δ11Bcarbonate in this region can be discussed. 5. Discussion

4. Results 4.1. Boron isotopic composition of core ISL1A

5.1. The relationships between pH, pKB, δ11B, and the salinity of brines from salt lakes in the Qaidam Basin

The δ11Bcarbonate values for core ISL1A are presented in Table 1. The δ Bcarbonate values vary from −4.25‰ to +9.94‰ (average +1.97‰). Most of the values cluster within the range of −2.74‰ to +7.64‰, indicating that the carbonates in this core are extremely isotopically

Compared with those of seawater, the variations in the pH and pKB of brines along with salinity make interpreting the δ11B values of brine-carbonate systems of terrestrial salt lakes complicated. The geochemical and isotopic characteristics of brines from the Qaidam

11

Fig. 2. Comparison of B, Mg, Ca and Sr concentrations and the δ11B for carbonates from core ISL1A. (a) B/Ca vs. Mg/Ca; B/Ca vs. Sr/Ca; (b) B/Ca vs. δ11B; (c) Mg/Ca vs. δ11B; (d) Sr/Ca vs. δ11B.

X. Zhang et al. / Sedimentary Geology 383 (2019) 293–302

Basin have been reported (Qi et al., 1993; Liu et al., 1999a, 1999b; Xiao et al., 1999; Du et al., 2019); however, systematic research is needed to explore the relationship among these characteristics. Previous studies on the TDS (salinity) and pH values of salt lakes on the Tibetan Plateau concluded that pH decreases with increasing salinity (Zheng and Liu, 2009). This negative correlation (R = 0.8) is also observed for salt lakes in the Qaidam Basin (Fig. 3a; Zhang, 1987; Du et al., 2019), which is consistent with the results of evaporation experiments conducted on lake water from Qinghai Lake, Yiliping Playa, and the Dongtai and Qarhan salt lakes (Fig. 3b; Sun et al., 1995; Wen et al., 2011; Wu et al., 2012). Why does the pH decrease with increasing salinity? Based on theoretical and PHREEQC (Pitzer database) calculations, Golan et al. (2015) demonstrated that in saline or hypersaline water with a high proportion of divalent cations, such as occur in the Qaidam Basin, boric acid is the dominant buffer system that determines the pH value (Golan et al., 2015). Thus, pH is obviously related to pKB (−logKB) (KB is the apparent dissociation constant of boric acid). pKB is controlled by water temperature, salinity and the divalent cations compositions (Hansson, 1973; Dickson, 1990; Golan et al., 2015). In the hyper-arid Qaidam Basin, which experiences only small annual variations in water temperature (b5 °C) (Yu et al., 2009), the effect of temperature on pKB can be ignored because the variation in pKB is only 0.04 for every 5 °C (Dickson, 1990). Therefore, the salinity and

Fig. 3. Variations in the pH values of salt lake brines with increasing evaporation and concentration. (a) The relationship between pH values and TDS of brine in Qaidam Basin. Fit line produced by solid circles reflects the relationship between pH and salinity of brines in the entire Qaidam Basin (Zhang, 1987); Fit line produced by squares reflects the relationship between pH and salinity of brine in Qarhan Salt Lake (Du et al., 2019). (b) Evaporation experiments on pH values of brines in Yiliping playa (Wu et al., 2012), Dongtai Salt Lake (Wen et al., 2011), Qarhan Salt Lake and Qinghai Lake (Sun et al., 1995).

297

divalent cations compositions are the two dominant factors that determine the pH values by controlling pKB. Generally, ionic concentrations increase with the increasing salinity. Higher salinity leads to lower pKB, which in turn, leads to lower pH values. The relationship between pKB and pH can be expressed as follows (Golan et al., 2015): 10−pKB ¼ f 4−brine =f 3−brine  10−pH

ð2Þ

where f4-brine and f3-brine are the fraction of B(OH)4− (borate) and B(OH)3 (boric acid) in the brine, respectively. If the pKB and pH values are known, the fraction of B(OH)4− and B(OH)3 in the solution can be calculated. The pH values of the brines from the QSL range from 6.0 to 7.1 (Table 3; Xiao et al., 1999; Du et al., 2019); however, no pKB values have been reported. Golan et al. (2015) studied the relationship between pH, pKB and the ionic strength of a pure MgCl2 and CaCl2 solution derived from a PHREEQC simulation. The results demonstrate that there are similar negative linear relationships between pH, pKB, and ionic strength in a pure MgCl2 and CaCl2 solution (Fig. 4). To obtain the pKB of the brines from the QSL, we calculated the ionic strengths of brines from the QSL (Table 4), and then, we plotted pH vs. ionic strength (Fig. 4). We observed that the data falls on the pure MgCl2 solution line. We used these results, the fact that Mg2+ is the dominant divalent cation in the brines from the QSL (Table 4; Zhang, 1987), and the pKB vs. ionic strength line of a pure MgCl2 solution to obtain the pKB (6.15–6.95) of brines in this region (Fig. 4). The above discussion elucidates the relationships between the pH, pKB and salinity of brines from the salt lakes in western China. The δ11Bbrine values were also reported by Qi et al. (1993) and Xiao et al. (1999). We also observed that in addition to increasing salinity during the concentration process, the δ11Bbrine values of these brines increase from carbonate-type to sulfate-type to chloride-type brines (Table 2; Fig. 5a). A positive correlation (R = 0.72) between the δ11Bbrine values and TDS of the salt lakes in the Qaidam Basin are shown in Fig. 5b (Table 2). A significant increase in the δ11B of seawater (δ11Bseawater) has also been observed during the evaporation of seawater (Fig. 5a; Vengosh et al., 1992). These comparisons indicate that δ11Bbrine values increase with the increasing salinity during evaporation, which is due to the fact that the lighter 10B enters the solid phase preferentially and the heavier 11B becomes enriched in the liquid phase (brine) (Palmer et al., 1987; Xiao et al., 1992; Sun et al., 1993).

Fig. 4. Calculated pKB and pH versus ionic strength for synthetic MgCl2 and CaCl2 solutions, derived using the PHREEQCE software and its Pitzer database (Golan et al., 2015). The QSL samples (square) fall close to the pure MgCl2 solution line of pH vs. ionic strength. The pKB (6.95–6.15) of the QSL were obtained from the pure MgCl2 solution line (Golan et al., 2015).

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Table 2 The pH, δ11B values and salinity of brines in the Qaidam Basin. Name Salt Lakes in Qaidam Basin Chaka lake Xiligou lake Keke playa Chaikai lake Gasikule lake Yiliping playa Xitai lake Da Qaidam lake Xiao Qaidam lake Dongtai lake Mangai playa Jiahu lake Kunteyi playa Niulang lake Zhinv lake Dalangtan playa Kunteyi lake Xiezuo lake Senie lake Dabielehu lake Dabuxun lake Qarhan playa Qarhan playa Salt lakes in Tibet Shawo lake Bangecuo lake Awengcuo lake a b c

pHa

δ11B (‰)a

pHb

δ11B (‰)b

TDS (g/L)c

6.8 7.6 6.8 6.9 7.6 7.3 7.8 7.9 7.8 7.8

11.68 19.76 15.77 12.69 17.48 10.75 4.85 4.07 1.55 2.56 18.17 14.78

4.60 8.50 11.90 9.90 17.90 4.70 3.10 5.30 −0.50 2.56 17.60 15.20 8.50 24.50 23.70 27.80 29.60 6.20 2.90 4.40 5.20 5.32 4.30

322.5 213.3 326.4 324.1 333.3 327.2 336.3 274.4 339.1 331.5

5.3 6.2

7.3 8.2 7.2 7.7 7.1 7.0 7.6 7.8 8.1 7.8 7.5 4.9 7.1 5.3 4.4 7.1 6.7 6.5 7.1 7.0 7.0 6.5

8.9 8.7 9.2

469.1 328.9 555.1 555.1

358.5 332.2 362.9 319.6 321.5 358.0

4.20 3.44 −4.06

Water typec Magnesium sulfate subtype Sodium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Sodium sulfate subtype Chloride type Chloride type Chloride type Chloride type Chloride type Chloride type Chloride type Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype Magnesium sulfate subtype

Carbonate type Carbonate type Carbonate type

From Qi et al., 1993. From Xiao et al., 1999. From Zhang, 1987.

In summary, in salt lake brines, pH and pKB decrease while δ11Bbrine increases with increasing salinity. An obviously negative correlation (R = 0.73) between δ11Bbrine and pH is observed for the QSL (Fig. 6; Table 3; Xiao et al., 1999; Du et al., 2019). The decrease in pH causes δ11 Bboric and δ 11 B borate to decrease at constant pKB (Palmer et al., 1987; Barth, 1993). However, based on Eq. (2), the pKB and pH values of the brines simultaneously influence the fraction of B(OH)4− and B (OH)3, and in turn, the δ11Bboric and δ11Bborate. Therefore, the effect of pH on the δ11B boric and δ11 Bborate values of the brines in salt lakes should be discussed. In addition, δ 11 B boric and δ 11 B borate increase with increasing δ11 Bbrine (Fig. 7). Because δ 11 B carbonate depends on the δ11Bboric and δ11Bborate values of the solution, it is necessary to investigate the factors affecting the δ11Bcarbonate values of brinecarbonate systems. 5.2. The influencing factors of and variation in the δ11Bcarbonante values of the QSL Previous studies have demonstrated that pH controls the degree of B isotopic fractionation between marine carbonates (mainly foraminifera and corals) and seawater by influencing the δ11Bborate values of the solution because B(OH)4− is the main species incorporated into the carbonate (Hemming and Hanson, 1992; Hemming et al., 1995; Sanyal et al., 1996, 2000). Due to constant δ11Bseawater (+39.5‰) (Spivack et al., 1987; Barth, 1993; Laurent et al., 2006) and pKB value (8.6; Dickson, 1990), pH value is the only factor influencing δ11Bcarbonate and the relationship between them can be represented by Eq. (3):   pH ¼ pKB − log δ11 Bseawater −δ11 Bcarbonate i .h α3–4  δ11 Bcarbonate −δ11 Bsw þ 103 ðα3–4 −1Þ

ð3Þ

where α3–4 denote the fractionation factor between boric acid and borate. The δ11 B values of marine carbonates increase with the increasing seawater pH.

However, unlike seawater, the δ11Bcarbonate values of salt lakes are influenced by the variation of pH, pKB and δ11B values of brines, and the precipitated carbonate minerals (calcite and aragonite) (Xiao et al., 2008; Mavromatis et al., 2015; Noireaux et al., 2015), which complicates the evaluation of the main factors controlling δ11Bcarbonate. To decipher the relationships between these factors, some basic Eqs. (4)–(7) are used. The δ11Bbrine can be expressed by Eq. (4) according to mass balance of the fractions and isotopic compositions of the boric acid (f3-brine, δ11Bboric) and borate (f4-brine, δ11Bborate) in the brine: δ11 Bbrine ¼ f 3−brine δ11 Bboric þ f 4−brine δ11 Bborate

ð4Þ

Due to that the fact that both aqueous boric acid and borate are incorporated into calcite without isotopic fractionation (Xiao et al., 2008; Mavromatis et al., 2015; Noireaux et al., 2015) and the fraction of boric acid in calcite is linearly correlated with the fraction of boric acid in solution (f3-brine) (Noireaux et al., 2015), δ11Bcarbonate (calcite is the dominant carbonate mineral in the QSL; Fan et al., 2014a) can be expressed by Eq. (5): δ11 Bcalcite ¼ 0:35f 3−brine δ11 Bboric þ ð1–0:35f 3−brine Þδ11 Bborate

ð5Þ

Using the coefficient α3–4 = 1.026 (Klochko et al., 2006; Nir et al., 2015), Eqs. (4) and (5) can be simplified to Eqs. (6) and (7), respectively. δ11 Bbrine ¼ 0:026f 3−brine δ11 Bborate þ δ11 Bborate þ 26 f 3−brine

ð6Þ

δ11 Bcalcite ¼ 0:35δ11 Bbrine þ 0:65δ11 Bborate

ð7Þ

Relying on the above equations, the possible factors (pH, pKB, δ 11 B brine) influencing δ 11 B carbonate will be discussed using a case study of the QSL. The pH and pKB values directly affect δ11Bcalcite by determining the f3-brine and f4-brine values of the brines (Eq. (2)). Based on Eq. (2), for

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Fig. 6. Plot of pH versus δ11B for brines from salt lakes in the Qaidam Basin. Fit line produced by red circles represents brines from salt lakes in the entire Qaidam Basin (Table 2; Qi et al., 1993; Xiao et al., 1999); Fit line produced by blue squares represents brines from Qarhan Salt Lake (Table 3; Xiao et al., 1999; Du et al., 2019) and fit line produced by purple triangles represents brines from Da Qaidam Salt Lake (Xiao et al., 1992).

main factor influencing the variation in δ11Bcalcite in the study area and the high δ11Bcalcite might indicate a high δ11Bbrine. 5.3. Fractionation model for the brine-carbonate system Wei et al. (2014) reported that the δ11Bcarbonate values of the acid soluble phase from a sediment core collected from the Dongtai Salt Lake in the central Qaidam Basin vary exponentially with the molar ratio of B/Ca. They concluded that this equilibrium isotopic

Fig. 5. Variations in the δ11B values of salt lake brines and seawater with increasing evaporation and concentration. (a) Variation in the δ11B values of salt lake brines with different water types (square, rhombus, triangle) (Table 2; Zhang, 1987; Qi et al., 1993; Xiao et al., 1999), the δ11B values of evaporated seawater (circles) vs. degree of evaporation (calculated from bromide concentrations) (Vengosh et al., 1992); (b) The relationship between the δ11B and TDS of salt lakes in the Qaidam Basin (Table 2; Zhang, 1987; Xiao et al., 1999).

the brines from the QSL, f3-brine increase from 41.5% to 47.1% with a decrease in pH of 7.1 to 6.0 (Table 3) and a decrease in pKB of 6.95 to 6.15 (Fig. 4). If δ11Bbrine is constant, δ11Bborate will decrease by ~1.5‰ (Eq. (6)) and δ11Bcalcite will decrease by ~1.0‰ (Eq. (7)). If only the influence of pKB and pH are considered, the variation (1‰) in the calculated δ11Bcalcite value is significantly smaller than that (10.38‰) of the calcite samples (−2.74‰ to +7.64‰) from core ISL1A from the QSL. This inconsistency implies that pH and pKB have only a limited influence on δ11Bcalcite values of the QSL. The δ11Bbrine value should be another influencing factor. Based on Eq. (7), δ11Bbrine and δ11Bborate both influence the δ11Bcalcite value. In Fig. 7, δ11Bbrine and δ11Bborate exhibit similar trends at any given pH value. Considering the fact that δ11Bbrine increases from +2.88‰ to +13.08‰ in the QSL, δ11Bcalcite will also increase by ~10.2‰ (Eq. (7)), which is approximately equivalent to the measured variation in δ11Bcalcite of core ISL1A (10.38‰). Therefore, δ11Bbrine might be the

Table 3 The pH and δ11B values in QSL. Name a

Senie lake Dabielehu lakea Xiezuo lakea Intercrystalline brine in Qarhan sectiona Intercrystalline brine in Bieletan sectiona Intercrystalline brine in Dabuxun sectiona Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab Intercrystalline brine in Qarhan playab a b

From Xiao et al., 1999. From Du et al., 2019.

pH

δ11B(‰)

Error (2σ, ‰)

7.1 7.0 6.5 7.0 6.5 6.7 6.5 6.1 6.5 6.0 6.2 6.3 6.3 6.0 6.3 6.2 6.3 6.5 6.5 6.5 6.3 6.0

2.90 4.40 6.20 5.32 4.30 5.20 11.00 9.90 9.67 10.04 9.32 10.81 13.08 9.87 10.43 10.10 9.60 9.56 11.23 10.22 11.39 10.66

0.14 0.17 0.10 0.10 0.18 0.13 0.22 0.12 0.05 0.10 0.22 0.11 0.13 0.05 0.17 0.02

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Fig. 7. δ11Bborate vs. δ11Bbrine at pH values of 7.1 and 6.0 with pKB values of 8.6 (Seawater, salinity 35 g/L; Dickson, 1990) and 6.29 (hypersaline water, salinity 348 g/L; Golan et al., 2015), respectively; α3–4 is 1.026 (Nir et al., 2015).

fractionation process conforms to the Rayleigh fractionation model. Rayleigh fractionation is expressed by the following equation: δ11 Bcarbonate ¼ 1000 ðαbrine−carbonate −1Þ lnF þ δ11 Bbrine

ð8Þ

where F represents the fraction of boron in the carbonate and is simplified as the molar ratio of B/Ca. Based on Eq. (8), a small fractionation factor between the carbonate and brine (α carbonate-brine ) (0.994) was obtained for the Dongtai Salt Lake (Wei et al., 2014). In the QSL, δ11Bcarbonate is mainly controlled by δ11Bbrine, so they vary simultaneously with each other. In addition, ln (B/Ca) vs. δ11Bcarbonate exhibits a good linear correlation (R2 = 0.76) (Fig. 8a). This comparison suggests that the fractionation model of the brine-carbonate system in the QSL also follows the Rayleigh fractionation model. For Rayleigh fractionation (Eq. (8)), the 3.013 gradient of the fit line yields a αcarbonate-brine of 0.997. Based on experiments of carbonate precipitation from artificial seawater (Fig. 9; Sanyal et al., 1996, 2000; Xiao et al., 2008; Noireaux et al., 2015), the small αcarbonate-brine of the QSL (0.997, this study) and the Dongtai Salt Lake (0.994, Wei et al., 2014) correspond to high pH values (≥9.6). However, the reported pH values of brines from the QSL and the Dongtai Salt Lake range from 7.8 to 6.0 (Tables 2, 3). This discrepancy can be interpreted by considering the difference between brines and seawater. The equation for the fractionation factor between carbonate and brine (αcarbonate-brine) can be expressed by Eq. (9). By substituting Eqs. (2), (6) and (7) into Eq. (9), αcarbonate-brine can be expressed as a function of f3-brine (Eq. (10)).     αcalcite−brine ¼ δ11 Bcalcite þ 1000 = δ11 Bbrine þ 1000 αcalcite−brine ¼ ð0:0091f 3−brine þ 1Þ=ð0:026f 3−brine þ 1Þ

ð9Þ ð10Þ

Based on Eq. (10), the smaller the f3-brine is, the smaller αcarbonate-brine (close to 1) will be. According to Eq. (2), a small f3-brine can be obtained for low pH hypersaline waters, where the pKB of the brine is smaller

Fig. 8. Rayleigh fractionation model for B isotopes in a brine-carbonate system. (a) linear fit on a plot of δ11Bcarbonate vs. lnF; (b) parallel lines (blue dashed and solid) are the ideal Rayleigh fractionation model; nonparallel lines (blue dashed and red solid) are the fractionation model for carbonate and brine from the QSL. The increase in the pKB and pH values produces a 1‰ decrease in the δ11Bcarbonate of the QSL (The calculation process is explained in detail in Section 5.2).

(Golan et al., 2015) than those of seawater (8.6; Dickson, 1990) or other dilute solutions (Hansson, 1973). These results imply that a small αcarbonate-brine will occur in low pH conditions. For example, based on the pKB data for the Dead Sea brine (6.29) reported by Golan et al. (2015), its αcarbonate-brine is 0.997 at pH = 7 (calculated using Eqs. (2) and (10)). Therefore, compared with seawater, the small αcarbonate-brine of salt lakes does not directly correlate with high pH values. In addition, αcalcite-brine does not change significantly with pH in hypersaline waters. This is due to the fact that pKB and pH values vary simultaneously with salinity and their values are always close to each other (Fig. 4), so pH has little effect on the variation in f3-brine (Eq. (2)) and then, the αcalcite-brine (Eq. (10)). Using αcarbonate-brine, the calculated δ11B range of paleo-lake water in the QSL is +0.26‰ to +10.64‰, which are in the range of modern brines (+2.88‰ to +13.08‰) (Du et al., 2019) and almost overlaps with those of the halite (−0.35‰ to +5.84‰) in core ISL1A (Fan et al., 2015). This explanation verifies that the calculated αcalcite-brine value is reasonable and the fractionation process in the brine-carbonate system in the Qaidam Basin follows the Rayleigh fractionation model. Further study on the fractionation model of terrestrial brine-carbonate systems is still needed, e.g., the influence of pKB and pH on fractionation (brine-carbonate) (Fig. 8b). 5.4. Geologic implication of the δ11Bcarbonate values of the QSL The discussion in Sections 5.1 and 5.2 indicates that δ11Bbrine increases with increasing salinity during concentration and δ11Bcarbonate increases with increasing δ11Bbrine in salt lakes. These conclusions indicate that δ11 Bcarbonate positively correlates with salinity in brine-carbonate systems, which is demonstrated by the B/Ca, Sr/Ca and Mg/Ca ratios of the carbonate in core ISL1A (Fig. 2) (the latter

Table 4 Chemical compositions of intercrystalline brines in QSL. Data from Zhang, 1987. The sum of CO3 and HCO3 is given as DIC; ionic strength is calculated by the Equation: I = 1/2 ∑ M ∗ Q2 (M represents the molar concentration of ions, Q represents ionic charge). Section Bieletan Dabuxun Qarhan Huobuxun

+ Na+ (mol/L) K+ (mol/L) Mg2+ (mol/L) Ca2+ (mol/L) Cl− (mol/L) SO2− 4 (mol/L) Li (mmol/L) B (mmol/L) DIC (mmol/L) Ionic strength (mol/L) pH

1.01 1.82 3.10 3.62

0.59 0.49 0.31 0.08

2.67 1.94 1.19 0.88

0.01 0.05 0.03 0.09

6.76 6.19 5.68 5.59

0.07 0.02 0.07 0.02

17.86 3.75 2.25 1.48

13.52 9.23 7.18 7.78

3.39 3.59 5.60 3.60

9.76 8.33 7.16 7.97

6.5 6.7 7.0 6.7

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δ11Bcarbonate can be used as a new proxy for the salinity of paleo-lake water in salt lakes. 6. Conclusions

Fig. 9. The B isotope fractionation factor between carbonate and artificial seawater (αcalcite-brine). Results were obtained from laboratory experiments with controlled pH values (Sanyal et al., 1996, 2000; Xiao et al., 2008; Noireaux et al., 2015).

two ratios are an indicator of salinity; Williams, 1966; Haskell et al., 1996; Zhou, 2004). By comparing our δ11Bcarbonate values with previously reported records of core ISL1A, we found δ11Bcarbonate varies (Fig. 10b) synchronously with the various element contents and ratios (Fig. 10c, f and g) and with other isotopic proxies (Fig. 10d and e) in the same core, suggesting that they represent similar geologic processes. The stratigraphic units of core ISL1A (Fig. 10a), the fact that the high δ11Bcarbonate values correspond to high δ11Bhalite (Fig. 10d; Fan et al., 2015; Du et al., 2019) and δ 18 O carbonate values (Fig. 10e; Fan et al., 2014a), the Mg/Ca and Sr/Ca ratios of the carbonates (Fig. 10f and g), and the coarse grain size of the clastic sediments (Fig. 10h; An, 2012) in the lower and upper salt layers all suggest a high salinity paleolake in a dry climate with low effective moisture. In contrast, the low δ11Bcarbonate, δ11Bhalite, and δ18Ocarbonate values, the low Mg/Ca and Sr/Ca ratios, and the fine grain size of the clastic sediments in the middle clastic layer of the core, suggest a relatively wet climate with high effective moisture. These comparisons demonstrate that

Arid western China contains numerous salt lakes, which are enriched in valuable resource elements (such as K, Li and B). Relevant studies of the B isotope geochemistry of brine-carbonate systems in terrestrial lakes are rare. In this study, we collected clastic sediments from sediment core ISL1A from the Qarhan Salt Lake in the eastern Qaidam Basin and analyzed the B, Sr, Mg, Ca contents and B isotopic compositions of the carbonates. The relationships between the pH, pKB, δ11Bbrine, and salinity of brine from the terrestrial salt lakes in western China were discussed. Comparisons of these values suggest that boric acid is the dominant buffer system in the brines from the Qaidam Basin and salinity determines the apparent dissociation constant of the boric acid (KB), and in turn, the pH value. In addition, δ11Bbrine values increase while pKB (−logKB) and pH decrease during the concentration of brines from salt lakes in western China. Based on this interpretation and the element ratios and δ11Bcarbonate values of the QSL samples, we reached the following conclusions. (1) The heterogeneity of the δ11Bcarbonate values is mainly controlled by the δ11Bbrine values, which correspond to the salinity of the brine, rather than pKB and pH. (2) The boron isotopic fractionation between carbonate and brine, which has a small fractionation factor (0.997), follows the Rayleigh fractionation model. (3) The δ11Bcarbonate values of core ISL1A from the QSL coincided with other variables (δ11Bhalite, δ18Ocarbonate, Mg/Ca, and Sr/Ca ratios, and mean grain size), suggesting that δ 11B carbonate can be used to determine the salinity of paleo-lake water. Acknowledgments We thank Dr. Yunqi Ma and Dr. Zhangkuang Peng for their help and guidance in the laboratory. We are grateful to editor and reviewers for their meaningful suggestions. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41872093, 41502096), Foundation of Qinghai Science & Technology Department (2017-ZJ-931Q) and One-Thousand Innovative Talent Project of Qinghai Province (Grant to QS Fan).

Fig. 10. Comparison of the δ11Bcarbonate record of core ISL1A and other proxy records. a: lithology of core ISL1A (Fan et al., 2014a). b, c: δ11Bcarbonate values and B concentrations of carbonates (this study). d: δ11Bhalite values (Fan et al., 2015). e: δ18Ocarbonate values (Fan et al., 2014a). f, g, h: Mg/Ca (mol/mol), Sr/Ca (mol/mol) of carbonate and mean grain size (μm) (An, 2012). The blue and yellow bars represent the periods of high and low δ11Bcarbonate values in core ISL1A, respectively.

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A dissertation submitted to Guangzhou institute of Geochemistry, Chinese Academy of Sciences, Guangzhou (in Chinese).