# Chlorine and bromine isotope evolution within a fully developed Upper Permian natural salt sequence

H. G. M. Eggenkamp, P. Louvat, J. Griffioen, P. Agrinier

► To cite this version:

H. G. M. Eggenkamp, P. Louvat, J. Griffioen, P. Agrinier. Chlorine and bromine isotope evolution within a fully developed Upper Permian natural salt sequence. Geochimica et Cosmochimica Acta, 2019, 245, pp.316-326. 10.1016/j.gca.2018.11.010. insu-03586669

HAL Id: insu-03586669

https://insu.hal.science/insu-03586669v1

Submitted on 29 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Chlorine and bromine isotope evolution within a fully developed
Upper Permian natural salt sequence

HGM Eggenkamp$^{a,*}$, P Louvat$^{b}$, J Griffioen$^{c,d}$, P Agrinier$^{a}$

Abstract

The behaviour of chlorine and bromine isotopes in evaporite deposits differs significantly. We studied the isotope variations of both elements in a fully developed natural salt sequence from Zechstein evaporite deposits (Wuchiapingian, Upper Permian) in the Northern Netherlands. We observed that the Cl isotope variations follow previously predicted characteristics, showing slightly positive δ³⁷Cl (relative to seawater) in halite (NaCl) dominated layers (up to +0.05 ‰), decreasing to moderately negative values in carnallite (KMgCl₃·6H₂O) and bischofite (MgCl₂·6H₂O) dominated layers (down to -0.55 ‰). Bromine isotope variations, the first ever measured in marine evaporite samples, show a different characteristic. δ⁸¹Br values

<footnote>$^{a}$Équipe de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR 7154, CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France</footnote>

<footnote>$^{b}$Équipe de Géochimie des Enveloppes Externes, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR 7154, CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France</footnote>

<footnote>$^{c}$TNO Geological Survey of the Netherlands, P.O. Box 80015, Utrecht, The Netherlands</footnote>

<footnote>$^{d}$Copernicus Institute of Sustainable Development, Utrecht University, P.O. Box 80115, Utrecht, The Netherlands</footnote>

<footnote>*Corresponding author, email hans@eggenkamp.info</footnote>
decrease quickly in layers dominated by halite (from +0.2 to -0.5 ‰) and
increase again in layers dominated by carnallite and bischofite (up to -
0.1±0.2‰). These observations suggest that the Br isotope fractionation
(10³lnα) during precipitation of halite can be as high as +1.3, 3.8 times as
large as Cl isotope fractionation. The increasing δ⁸¹Br values during
precipitation of magnesium salts may suggest a negative fractionation (10³lnα)
of -0.9, meaning that the Br isotope ratio in the precipitated salt is lower than
in the brine. We suggest that the difference in behaviour between Cl and Br
can be explained by the fact that Br, as a minor component in the brine, has to
compete with Cl to fit into the crystal lattice of Cl salts, where it does not fit
perfectly due to its slightly larger ionic radius than Cl, resulting in relatively
large isotope fractionation. Further research is however needed to confirm or
reject this suggestion.

1. Introduction

Evaporites and their fluid inclusions are used to reconstruct the evolution of the chemical composition of seawater over geologic times (Horita et al., 2002; Lowenstein et al., 2003; 2005), which is very important to understand the evolution of our planet and of its climate, a major concern today for the future. Because chloride and bromide are supposedly conservative in seawater, measuring chlorine and bromine isotope ratios in evaporite deposits could bring additional valuable information regarding the conditions in which the huge salt deposits formed, and thus to ancient environmental settings of the oceans. But before addressing the long-term evolution of Cl and Br isotopes in the ocean, it is important to understand what their signatures in evaporite deposits represent, whether the seawater signal can be preserved or at least deduced from the evaporite compositions, or if these signatures have a different meaning (e.g. can trace certain conditions under which seawater evaporation and salt precipitation took place).

The evolution of chlorine stable isotopes within single salt sequences has been
discussed regularly since our first contribution on this subject (Eggenkamp et
al., 1995, 2016, Eastoe et al., 1999, 2007, Xiao et al., 1997, 2000; Sun et al.,
1998; Tan et al., 2009, Luo et al 2012, 2014, 2015, Eggenkamp, 2015a, Eastoe, 2016). In these studies, the evolution of salt that precipitates during the evaporation of saturated brines is discussed based on the stable chlorine isotope fractionation of the individual salt components NaCl, KCl and MgCl₂, the experimental evaporation of a seawater sample, or the salt composition of evaporites in non-marine inland basins. The general trend is that the chlorine isotope ratio (³⁷Cl/³⁵Cl expressed as δ³⁷Cl relative to SMOC, modern seawater, in ‰ unit) decreases during precipitation of salt from the evaporite brine. The δ³⁷Cl value of the initial precipitate is about +0.35‰, decreasing to a minimum value of about -0.69‰ when bischofite (MgCl₂·6H₂O) starts to precipitate. This change reflects the equilibrium isotope fractionation between chloride in brine and chloride in precipitated halite (Eggenkamp et al., 2016). The fractionation is less well understood for potassium and magnesium chloride precipitation: Eastoe et al. (1999) observed that the chlorine isotope ratio increases, whereas Luo et al. (2012, 2014) observed on the contrary that it continues to decrease. The chlorine isotope fractionation factor (10³lnα, where α is the ratio of ³⁷Cl/³⁵Cl in the salt to ³⁷Cl/³⁵Cl in the brine) for this last evaporation stage can thus be either lower or higher than 1. It is expected that the chlorine isotope variation in this last evaporation stage is very limited, as bischofite precipitates with 6 molecules of water, and thus by the time the brine is saturated in bischofite the total amount of water present in the brine is only 9.5 molecules per mole MgCl₂ (Atbir et al., 2000; Clynne and Potter, 1979; Table 4 in Eggenkamp et al, 2016). Hence, only a small amount of water will need to evaporate before the brine completely solidifies.

On the contrary, the evolution of bromine stable isotopes in salt sequences has not been studied in much detail yet. One study we know of investigated the bromine isotope fractionation of individual bromide salts such as NaBr, KBr, MgBr₂ and CaBr₂ during precipitation from their saturated solutions (Eggenkamp et al., 2016). From this study, one can hypothesize that only little bromine isotope variation (less than 0.2‰) will be observed in salt sequences. It has to be realised however that bromide, due to its low concentration and similar ionic radius as chloride, does not form evaporite minerals of its own,
so that the precipitation processes it observes can be significantly differing
from those of chloride.

A second study (Hanlon et al., 2017) investigated for the very first time the bromine and chlorine isotope variation in salts, associated waters and volatiles from a series of salt lakes in Nebraska and Kansas (USA). They observed clearly different behaviour of bromine and chlorine isotopes in these lake systems. <sup>37</sup>Cl was consistently more enriched in the salt than in the fluid, while <sup>81</sup>Br was consistently more enriched in the fluid compared to the precipitated salt.

In the present study, we first aim to characterise the evolution of the chlorine and bromine stable isotope ratios within one single natural salt sequence, the Zechstein salt sequence from the Northeastern Netherlands. The chlorine isotope variations in a salt deposit that consists of the full salt development sequence from halite to bischofite (Braitsch, 1962) should confirm (or contradict) the results from earlier chlorine isotope studies in evaporites.

Second, we aim to determine whether the same principles hold for the bromine isotope variation as for the Cl isotope variation, or if the bromine isotope variation is determined by other principles and, if so, which ones.

Northeastern Netherlands is particularly suitable for a study on chlorine and bromine isotopes in a fully developed salt sequence due to the large amount of magnesium salt deposits present (Coeleweij et al., 1978). Our samples are taken from the same drill core that was studied by Eggenkamp et al. (1995) and that was described in detail earlier by Buyze and Lorenzen (1986). The core was resampled for the current research. The samples were selected based on their mineral composition and were sampled from layers that were dominated by halite (NaCl dominated samples), carnallite (KMgCl₃·6H₂O dominated samples) or bischofite (MgCl₂·6H₂O dominated samples).

Eggenkamp et al. (1995) observed significant variation of <sup>37</sup>Cl/<sup>35</sup>Cl in the natural, fully developed salt sequence of Zechstein deposits (Wuchiapingian, Upper Permian) in the Netherlands. However, the trend was merely described qualitatively as no direct chemical analyses were made on the salt samples measured for Cl isotopes. In the present study, we took samples from the same boring as the earlier study and the chemical composition was
characterised in detail as well. Thus, the compositional evolution of the sequence can be considered in parallel with the evolution of the chlorine and bromine isotope ratios.

## 2. Material

Samples were taken from core TCI-2 (Tripscompagnie-2, original name TR-2) that was drilled in 1982 through the Veendam structure near Groningen, Northern Netherlands to explore the Zechstein (Wuchiapingian, Upper Permian) salt deposits for magnesium mining (Coeleweij et al., 1978; Buyze and Lorenzen, 1986, Figure 1). This core contains carnallite and bischofite in addition to kainite (KMgClSO₄·3H₂O) and halite. The core, therefore, allows the study of the final stages of the evaporation process.

<figure><img src="image_2.png" /><figcaption>Figure 1: Location of the Veendam Salt Structure in the Netherlands (after Coeleweij et al., 1978).</figcaption></figure>

The Zechstein (Wuchiapingian, Upper Permian) is a European sedimentary rock unit that contains massive marine salt deposits (Warren, 2010). Core TCI-2 is from the upper part of the Zechstein III Formation (Nederlandse Aardolie Maatschappij and Rijks Geologische Dienst, 1980). It samples subformations Zechstein-III-1, -2, and -3. All three subformations are again divided in a lower part a, which contains mainly halite, and a carnallite- and bischofite-rich upper part b. These formations were described in detail by Coeleweij et al. (1978) using the different, but geologically similar core VDM-4 (Veendam-4). The salt composition of core TCI-2 is summarised in Figure 2. Below 1770 m (Zechstein-III-la), the core contains massive halite with some sylvite (KCl) and
langbeinite ($K_2Mg_2(SO_4)_3$) layers. These are not primary minerals, indicating secondary processes due to conversion at temperatures above 83 °C (Braitsch, 1962). From 1770-1693 meter, a layered sequence (Zechstein-III-1b) of carnallite, bischofite, halite, and some kieserite ($MgSO_4·H_2O$) is found. From 1693 to 1654 m, formation Zechstein-III-2a with halite and some carnallite is found. Between 1654 and 1640 m, carnallite, halite, and kieserite are present. Here the core is red, indicating the presence of iron oxides, with some banding (Zechstein-III-2b). From 1640 to 1631 m, it contains halite with some carnallite (Zechstein-III-3a). Above this, the core contains halite, carnallite, and kieserite (Zechstein-III-3b, Haug, 1982). The two potassium-magnesium cycles, Zechstein-III-1b and -2b are very different (Coeleweij et al., 1978). Zechstein-III-2b shows no distinct subcycles, whereas in Zechstein-III-1b nine subcycles are present, each containing several mini-cycles (Eggenkamp et al., 1995). For the current study samples were selected from subformation Zechstein-III-2a which is predominantly halite at coring depths from 1678 to 1689 m, and subformation Zechstein III-1b for predominantly carnallite samples at depths between 1700 and 1704 m and for predominantly bischofite rich samples at depths between 1715 to 1719 m (see figure 2).

<figure><img src="image_3.png" /></figure>

Figure 2: Left, stratigraphic column of drillhole TCI-2 indicating the average mineral composition of the salt minerals per interval. Other salt minerals indicate mainly sylvite and langbeinite. Right, measured $\delta^{37}\text{Cl}$ (circles) and $\delta^{81}\text{Br}$ (diamonds) values of the samples taken from the indicated depths. Both isotope systems are plotted in the same column so that it is easily visible that the isotope behaviour of Cl and Br depends on the mineralogical composition of the salt. Compared to samples analysed in the research by Eggenkamp et al. (1995) samples are not taken evenly over the length of the core but from sections dominated by halite, carnallite and bischofite in order to obtain characteristic isotope data for samples dominated by these minerals.
Detailed information on the boreholes can be obtained from the Dutch government website on oil, gas and geothermal energy exploration and production in the Netherlands http://www.nlog.nl/en/. The core is stored at TNO Geological Survey (Zeist, The Netherlands).

Samples were taken from three depth intervals that are each dominated by one of the salt minerals halite, carnallite or bischofite. The first set of six samples, dominated by sodium salts, was taken at coring depths between 1678 and 1689 m (that is, due to the deviation of the boring, between 1672 and 1689 m below Dutch reference level, NAP) and consists on average of 91% halite, 4% kieserite and 5% carnallite by weight, based on the original analysis of the core. The second set of eight samples, dominated by potassium salts, was taken at coring depths between 1701 and 1704 m (between 1695 and 1699 m below NAP) and has an average composition of 36% halite, 8% kieserite, and 55% carnallite by weight. The third set of eight salts, dominated by magnesium salts, was sampled at coring depths of 1716 and 1719 m (1710 to 1713 m below NAP) and consists on average of 36% halite, 6% kieserite, 16% carnallite and 42% bischofite by weight.

### 3. Methods

Samples were prepared for chemical and isotopic analyses by diluting the whole sample (without separating into individual minerals) in two steps. The first step included dissolving about 1 g of sample in 9 g of MilliQ water for isotopic analyses. For elemental analyses, the above samples were diluted an extra 1000 times (30 mg of the first solution in 30 g of MilliQ water), leading to samples containing about 100 mg/L dissolved solids that could be analysed by ICP.

All samples were analysed for major element (Cl⁻, Br⁻, SO₄²⁻, Na⁺, K⁺, Mg²⁺ and Ca²⁺) and for Cl and Br isotopic compositions (δ³⁷Cl and δ⁸¹Br) at the Institut de Physique du Globe de Paris. Chloride and bromide were determined by ICP-MS on an Agilent 7900 ICP-MS. The cations and sulphate (measured as S) were analysed by ICP-OES on a Thermo Scientific iCAP 6000 Series. These concentrations allowed us to back calculate the chemical
compositions of the salts, which are reported in table 1 (to be found at the end of the document) in g per kg of salt.

Chlorine isotope compositions were measured according to the method developed by Kaufmann (1984) and described in detail in Eggenkamp (1994) and Godon et al. (2004). In short, Cl⁻ in the samples was precipitated as AgCl by adding AgNO₃, which was reacted with CH₃I to form CH₃Cl. ³⁷Cl/³⁵Cl ratios were subsequently determined with dual-inlet mass spectrometry on a ThermoFisher Delta PlusXP mass spectrometer. The results are reported in δ notation (δ³⁷Cl) defined as:

$$ \delta^{37}\text{Cl} = [(^{37}\text{Cl}/^{35}\text{Cl})_{\text{sample}}/(^{37}\text{Cl}/^{35}\text{Cl})_{\text{SMOC}} - 1] \times 1000 $$

with ‰ relative to SMOC (Standard Mean Ocean Chloride, Kaufmann et al., 1984) and an average standard deviation (1σ) of 0.05‰.

The bromine isotope compositions were determined on a Thermo Scientific Neptune multicollector ICP-MS. Samples were prepared in one of two possible methods. From samples with a high Br/Cl ratio, bromine is extracted by ion exchange chromatography on anion exchange resin AG 1X4 using 0.2M NH₄NO₃ as eluting solution (Louvat et al., 2016). For precise measurements 10 to 20 µg of bromine (in 2 mL solution) are necessary, so that samples with low Br/Cl-ratios could not be treated this way. As the exchange columns are filled with 1 mL of anion exchange resin with a maximum capacity of 1 meq/mL, the Br⁻ content in solutions with low Br/Cl is less than that required for isotope analysis. Modification of the extraction protocol by using larger volumes of resin would also have increased the final elution volume, thus diluting the Br concentration for MC-ICP-MS measurements even more, and was therefore not explored. For these reasons, bromine from samples with low Br/Cl ratios was extracted by oxidative distillation following Eggenkamp and Louvat (2018, after Murphy et al., 1954) In these samples, bromide is oxidised to bromine gas by a 14% boiling nitric acid solution and distilled into a 0.5 N ammonia solution. In this alkaline solution, Br₂ is trapped and most probably reduced back to bromide. This solution can be measured the same way as the bromide solution obtained by the ion exchange method. The
samples were introduced to the MC-ICP-MS through a small (20 mL) quartz spray chamber with a self-aspiring PFA nebulizer at 50 µL/min uptake rate. The measurements were done in low-resolution mode after careful tuning of the instrument, in order to minimize Ar₂H⁺ interference and to maximize ⁷⁹Br and ⁸¹Br signals (Louvat et al., 2016). The Ar₂H⁺ interference was corrected by blank subtraction. Bromine isotope ratios were measured by sample-standard bracketing with the NIST SRM 977 reference material solution at the same concentration as that of the sample solution. Blank was measured before each sample or standard, and each sample was measured three times. Seawater (MOMARSAT, Indian Ocean, sampled in January 2010 on the MD 175 cruise, latitude -32.45°S, longitude +84.01°E) was passed through the same Br extraction procedures as the samples to check both extraction yield and accuracy of the isotope ratio measurements. The results are finally reported in δ notation (δ⁸¹Br) defined as:

$$ \delta^{81}\text{Br} = [(^{81}\text{Br}/^{79}\text{Br})_{\text{sample}}/(^{81}\text{Br}/^{81}\text{Br})_{\text{SMOB}} - 1] \times 1000 $$

with ‰ relative to SMOB (Standard Mean Ocean Bromide; Eggenkamp and Coleman, 2000) and an average standard deviation (1σ) that is generally better than 0.1‰. Unfortunately the precision of some samples is worse than this. Some samples had too little bromide to be analysed using the ion exchange chromatography method, and needed to be analysed using the oxidation method that was still under development in the course of this project.

## 4. Results

The chemical composition, and δ³⁷Cl and δ⁸¹Br values of the salt samples are reported in Table 1. Because the Br/Cl ratio is used as a measure for the salt evolution it is also given. The three sets of samples can be well recognised in this table. Group 1, the halite dominated samples, (NL09 to NL14) is clearly dominated by NaCl. The samples in Group 2 (NL15 to NL22) were supposedly dominated by carnallite, but the compositions vary considerably (Table 1): K⁺
is frequently high but Na⁺ and also SO₄²⁻ can be high as well. In these
presumably carnallite-dominated samples, it is, however, expected that both
Mg²⁺ and Na⁺ are also regularly a common cation. In Group 3, the samples
dominated by bischofite (NL23 to NL30), the magnesium concentration is
indeed mostly high. There are, however, two samples in this group (NL26 and
NL29) that contain low magnesium concentrations and are dominated by NaCl.
These samples are indicative of the large compositional variability in the
sampled core, and especially in the Zechstein-III-1b unit. The sample
variability shows that NaCl dominated layers are well defined. In potassium
and magnesium dominated layers however, alternation between Mg-rich
layers and K-rich layers is quite common, most probably because of
intermittent inflow of less evolved brines in the basin at irregular time
intervals (Coeleweij et al., 1978).

It is well known that the Br/Cl ratio is related to brine and salt evolution: the more evolved a brine, the higher the Br/Cl ratio in the precipitate (Braitsch, 1962). As proposed by Valyashko (1956), the bromide content in evaporites can be used as a genetic and prospecting tool. Indeed, the Br/Cl ratio is low in first stage halite samples (down to 0.00025 in this dataset), and much lower than the Br/Cl in seawater (0.0034) as Br incorporates much less in the salt crystals than Cl. The ratio increases significantly with increasing Mg concentrations and reaches its highest values (up to 0.018) in samples with the highest Mg concentrations. Consistent with this, δ³⁷Cl decreases from about +0.05‰ in pure halite samples with the lowest Br/Cl ratios, to about -0.55‰ in pure bischofite samples with the highest Br/Cl ratios. A clear relationship is observed between the Cl isotope composition of the salts and the log-value of their Br/Cl ratios (Figure 3).

<figure><img src="image_4.png" /><figcaption>Figure 3: Relationships between δ³⁷Cl, Br/Cl ratio and the mineralogical composition of the samples.</figcaption></figure>
The relationship between the log value of Br/Cl ratio and δ⁸¹Br is not as straightforward as for δ³⁷Cl. The samples with the lowest Br/Cl ratios have positive δ⁸¹Br values (+0.2‰) that decrease to negative values (down to -0.5‰) with increasing Br/Cl ratio as long as halite is the most common salt in the deposit. When carnallite and bischofite become the most common salts in the deposit, δ⁸¹Br values appear to increase again with increasing Br/Cl ratios up to a value of about 0‰, albeit with considerable scatter, and a rather low r² of only 0.41. The two NaCl rich samples (NL26 and NL29) deviate from this general trend. They have high δ⁸¹Br values (+0.20 and -0.04‰) while their Br/Cl ratios are intermediate between the Br/Cl ratios of the sodium dominated and of the potassium and magnesium dominated samples. However, these two samples do not belong to group 1 (i.e., the halite rich samples) but they are the two above-mentioned NaCl rich samples that lie interbedded between the MgCl₂ dominated samples of Group 3 (Figure 4).

<figure><img src="image_5.png" /><figcaption>Figure 4: Relationships between δ⁸¹Cl, Br/Cl ratio and the mineralogical composition of the samples.</figcaption></figure>

Figures 3 and 4 reflect the variability in the δ³⁷Cl and the δ⁸¹Br data as function of the Br/Cl ratio of the samples. The background colours in the figures show the mineral composition of the samples so that the variability in the isotope ratios can visually be related to the sample composition. Globally the range in δ⁸¹Br values is larger than that for δ³⁷Cl (0.8‰ vs. 0.6‰), and unlike Cl isotopes, it is not possible to draw a single simple trend function through the bromine isotope data. Figure 5 shows how the chlorine and bromine isotope variations are related to each other. In this figure the salt precipitation evolution runs from right (high δ³⁷Cl values) to left (low δ³⁷Cl values). The trends that are visible are comparable to the trends visible in Figure 4, showing (from right to left) a decreasing δ⁸¹Br at high δ³⁷Cl values, and an increasing δ⁸¹Br at low δ³⁷Cl values. Just like in figure 4 samples NL26
and NL29 also fall outside the trend. The same background colours as in figure 3 and 4 are used to show the mineral composition of the samples so that the variability of the isotope ratios can again be visually related to the sample composition.

<figure><img src="image_6.png" /><figcaption>Figure 5: Relationships between δ³⁷Cl, δ⁸¹Cl and the mineralogical composition of the samples. Please note that the first precipitated salt is the salt with the highest δ³⁷Cl value that is located on the right of the figure. Salt evolution evolved to the left in this figure.</figcaption></figure>

# 5. Discussion

## 5.1. Br/Cl ratios

The bromide content of salts that precipitate from modern seawater has long been of interest (Boeke, 1908; Kühn, 1955; Valyashko, 1956, Braitsch and Herrmann, 1963, 1964; Herrmann, 1972, 1980; Herrmann et al., 1973; McCaffrey et al., 1987). It is related to the bromide (and chloride) content of the brine and to the crystal structure of each salt mineral, each of which incorporates a different amount of bromide relative to chloride (Braitsch and Herrmann, 1963). Eggenkamp et al. (1995) used the mineral composition during the different stages of salt precipitation in combination with the Cl isotope fractionation of the individual salt minerals to calculate the Cl isotope fractionation factors in four stages of evaporite precipitation, respectively dominated by halite, kainite (the first precipitating KCl salt: KMgClSO₄·3H₂O),
carnallite and bischofite precipitation. The same set of data can be used to estimate the bromide concentration of the salts during the precipitation sequence, assuming that: 1/ the bromide distribution factor, defined as $b=Br_{wt\% in salt}/Br_{wt\% in solution}$ (Braitsch and Herrmann, 1963) decreases from 0.14 to 0.073 during the first stage, when only halite precipitates; 2/ the distribution factor is 0.22 for kainite and 0.073 for halite when kainite is the most prominent precipitating salt; 3/ it is 0.52 for carnallite and 0.073 for halite when carnallite is the prominent precipitating salt and finally 4/ it is 0.66 when bischofite is the prominent precipitating salt (Braitsch and Herrmann, 1963). These distribution factors are experimentally determined for slowly precipitating salt in equilibrium with brine. It is then possible to compare the salt compositions calculated by Braitsch (1962) with those measured in the present study.

The chemical composition of ocean water in the Late Permian Epoch, from which the Zechstein deposits originate, was comparable to that of modern ocean (Horita et al., 2002; Lowenstein et al., 2003; 2005), except that the sulphate concentration was about 25% lower than today. For simplicity, the comparison is made with the salt compositions as calculated by Braitsch (1962), starting from today's chemical composition of seawater. The bromide distribution factors lead to predicted Br/Cl ratios systematically increasing during the halite phase from 0.00008 to 0.00021, during the kainite phase from 0.00064 to 0.00096, during the carnallite phase from 0.0021 to 0.0026 and during precipitation of bischofite from 0.0048 to higher.

This large range in Br/Cl ratios is not observed in our samples. The lowest Br/Cl ratio measured in nearly pure halite samples is higher than predicted, 0.00025 instead of 0.00008. On the other end of the spectrum, predicted Br/Cl ratios of bischofite agree much better with the expected ratios. The trend is clear, the Br/Cl ratio increases with increasing salt evolution (Figures 3 and 4).

The observation that the measured Br/Cl ratios are larger than the predicted values can be explained in three different ways. 1/ If the precipitation of the salt was out of equilibrium due to fast precipitation of salt, the factor b is larger as more Br is incorporated in the salt phase (Herrmann, 1980). 2/ Fluid inclusions that are present in the salt, containing the original brine with high
Br/Cl ratios, may increase the Br/Cl ratio of the analysed salt. 3/ High Br/Cl could also reflect either residual brines that were trapped between the salt deposits and evaporated at a later stage, or possibly a mixing of residual brine from the Zechstein Sea with more dilute seawater (originating from the Paleotethys Ocean?), which caused an extension of the halite precipitation stage but with increased Br/Cl ratio. As the Br/Cl ratio of seawater is 0.0034 and that of the residual brines it is even higher, only small amounts are necessary to significantly increase the Br/Cl ratios of the precipitated salts, especially during precipitation of Br-poor halite. Even when these discrepancies between predicted and measured Br/Cl ratios are taken into account, the Br/Cl ratio still appears as a good tracer of salt evolution stage because the Cl isotope composition and also the salt composition of the samples are well correlated with the measured Br/Cl ratio.

## 5.2. Cl isotope evolution

As expected from previous studies, the Cl isotope ratio of the samples decreases with increasing salt evolution (Figure 3) and reaches a minimum $\delta^{37}$Cl of about -0.55‰ in the samples with the highest Br/Cl ratios, which consist of almost 100% bischofite. The $\delta^{37}$Cl values of the samples show a good logarithmic relationship with the Br/Cl ratios (r²=0.951; Figure 3). According to calculations based on the various experimental determinations of Cl isotope fractionation during precipitation of salt samples (Eggenkamp et al., 1995, 2016; Luo et al., 2012, 2014, 2015; Eggenkamp, 2015a), either the lowest $\delta^{37}$Cl values should be reached right at the beginning of bischofite precipitation (Eggenkamp et al., 2016), or $\delta^{37}$Cl should continue to decrease even when Mg chlorides precipitate (Luo et al., 2014). Thus the predicted $\delta^{37}$Cl at the onset of bischofite precipitation is either about -0.5‰ considering the isotopic fractionation factors determined by Eggenkamp et al. (2016), or about -1.0‰ considering those of Luo et al. (2014). In the present study, $\delta^{37}$Cl is about -0.5‰ in samples from the last evaporation stages, which confirms that the Cl isotope fractionation factor for halite is close to 1.00035, as
determined by Eggenkamp et al. (2016), and not 1.00051, as determined by Luo et al. (2014).

The Cl isotope fractionation factor of bischofite cannot be deduced from the present dataset, as hardly any significant Cl isotope variation is found for the bischofite dominated samples. This is, however, an indication that the Cl isotope fractionation factor for bischofite is most probably very close to 1 as determined by Eggenkamp et al. (2016). With a value of 0.99998±0.00002, they concluded that the bischofite Cl isotope fractionation factor is not significantly different from 1. Here, it may be realised that the mole ratio MgCl₂:H₂O is 1:9.5 when a brine is saturated in bischofite (Atbir et al., 2000; Clynne and Potter, 1979) and 6 of these water molecules will precipitate with MgCl₂ in bischofite. So the loss of only little water from such brine will result in the total solidification of the remaining brine into bischofite without any possibility for further Cl isotope fractionation and thus any further change in the Cl isotope composition.

### 5.3. Br isotope evolution

The bromine isotope composition follows a different path than the Cl isotope composition during its evolution from halite to bischofite. Unlike Cl, there seems to be no relationship with the predicted isotope fractionation based on the experimentally determined isotope fractionation during Br salt precipitation (Eggenkamp et al., 2016). The relationship between the Br isotope composition and the brine/evaporite evolution as represented by the Br/Cl ratio is shown in Figure 4. As in Figure 3, the proportions of the major salt phases for each sample are shown in the figure so that the isotope composition can be compared easily with the salt evolution in the samples.

It appears that the first bromide salts precipitated from the Zechstein Sea have positive (≈ +0.2‰) δ⁸¹Br values. Then δ⁸¹Br decreases quickly in halite-dominated samples to ≈ -0.5‰. As with δ³⁷Cl, there appears to be a fairly good relationship between δ⁸¹Br and the log-value of the Br/Cl ratio in the most halite-dominated samples (Br/Cl ≤ 0.0012), with r² = 0.86. The slope of the regression line is significantly steeper than for δ³⁷Cl, suggesting a much
larger isotopic fractionation for Br than for Cl during halite precipitation. As a first approximation, it is presumed here that the slope on the log(Br/Cl-ratio) vs. δ³⁷Cl or δ⁸¹Br has a linear relationship with the isotope fractionation 1000lnα. On the Cl plot (Figure 3), the slope is -0.130, equivalent to a fractionation factor (1000lnα) of 0.35 as determined by Eggenkamp et al. (2016). For Br, the slope at low Br/Cl values is -0.498 (Figure 4). This value is 3.8 times greater in magnitude than for Cl, and suggests an isotopic fractionation factor (1000lnα) of 1.3 (α = 1.0013) for Br during halite precipitation, following the formula (-0.498/-0.130)*0.35=1.3, where -0.498 is the slope of the Br isotope values, -0.130 the slope of the Cl isotope values and 0.35 1000lnα for Cl. It has to be realised that this is only a first approximation based on a single set of observations on natural samples, which needs to be confirmed by more observations, including experimental. However, the suggestion of such a large isotope fractionation might explain the large variation of Br isotope values in certain environments.

On the contrary, in the carnallite and bischofite dominated part of the sequence (Br/Cl ratio > 0.005), the δ⁸¹Br values appear to increase again, although with considerable scatter, from about -0.5‰ to about -0.1±0.2‰ for the most evolved samples. The r² of 0.41 suggests only a weak relationship between δ⁸¹Br and the log-value of the Br/Cl ratio (see Figure 4). The positive slope of this regression (+0.342) suggests an isotope fractionation of -0.9 (α = 0.9991). This value is determined by dividing the log(Br/Cl) vs δ⁸¹Br slope of the high Br/Cl Br isotope trend by the general slope log(Br/Cl) vs δ³⁷Cl, and multiplying with the Cl isotope fractionation factor. This gives a value of 1000lnα(+0.342/-0.130)*0.35=-0.9 where +0.342 is the slope of the Br isotope values, -0.130 the slope of the Cl isotope values and 0.35 is 1000lnα for Cl (during halite precipitation). This value suggests that the bromide that precipitates in carnallite and bischofite is isotopically lighter than bromide in the brine. It also is possible that, in the very evolved brines from which these salts precipitate volatile halide gases have escaped in a similar fashion as suggested by Hanlon et al. (2017). Hanlon et al. (2017) find that halide volatiles that escape from terrestrial lakes during evaporation are enriched in ⁷⁹Br, indicating that the remaining brine will increase in ⁸¹Br, a trend that is
also suggested in our samples. Given the only weak indications for the proper
fractionation processthe suggested Br isotope fractionation during carnallite
and bischofite precipitation clearly needs further confirmation in future
studies.

Two samples (NL26 and NL29) with intermediate Br/Cl ratios have δ⁸¹Br values that strongly deviate from the observed trends (+0.2 and -0.04‰, for Br/Cl of 0.0022 and 0.0028, respectively). Their δ⁸¹Br values indicate that these samples are among the first salts precipitating from a saturated solution, while their intermediate Br/Cl ratios rather suggest a brine that is more evolved than the rest of the halite samples. Unlike the other halite rich samples, these two samples are from the bischofite dominated group of samples and their halite content that is over 80%, is much higher than their neighbouring samples. This suggests, as previously discussed, that less evolved or seawater-admixed brines may have been present within the bischofite dominated salt layers at certain periods, and that some of the salts (of a less evolved type) precipitated from them.

Considering the very small Br isotope fractionation measured during
precipitation of bromide salts from saturated bromide solutions (α<sub>salt-brine</sub> =
0.99993±0.00005 for NaBr, 1.00000±0.00001 for KBr and 1.00006±0.00006
for MgBr<sub>2</sub>; Eggenkamp et al., 2016), no variations beyond ±0.1‰ from the
bromine isotope composition of the oceans are expected for any salt sequence
if the same process as for chlorine isotopes applies. The total variation of
about 0.8‰ for the δ<sup>81</sup>Br of the presently studied natural Zechstein salt
sequence thus indicates that a different explanation needs to be sought.

Realising this, the process of uptake of bromide in chloride salt minerals can
indeed not be compared to the uptake of chloride in these minerals. While the
brine is saturated in chloride when chloride salts precipitate, the brine is not
saturated with bromide and the minerals formed are not bromide but chloride
salts. The incorporation of bromide as impurity in Cl salt minerals is a
different process that is probably considerably more complex due to
interactions between chloride and bromide ions (and possibly other ions) both
in the solution and at the crystal lattice. Although these interactions also play
a role for chloride, they are negligible, while they play a major role for
bromide due to the low concentration of bromide ions as compared to chloride ions in the brine and even more so in the chloride salt. In brines that predominately precipitate halite, where the amount of bromide relative to chloride incorporated in the crystals is about one tenth of that present in the brine (Braitsch and Herrmann, 1963), the uptake of bromide in Cl salt minerals appears to result in a considerable positive bromine isotope fractionation (1000lnα) of 1.3‰. For potassium and magnesium rich brines, that precipitate carnallite and bischofite, the bromide content relative to chloride in the crystals is over half that present in the brine (Braitsch, 1962). The uptake of bromide in carnallite and bischofite now appears to result in an also considerable, but negative bromine isotope fractionation (1000lnα) of -0.9‰.

Although these crystallisation effects might be reasonable explanations for the isotopic fractionation effects during the incorporation of bromide in chloride salts, additional laboratory experiments and computer simulations are needed for confirmation and to understand the exact processes. Isotope analysis of other fully developed salt sequences would also provide insights of whether the effects observed here for the Dutch Zechstein salt deposits are universally present.

The possibility of large isotope fractionation due to uptake of bromide in salts during precipitation is still speculative, but after Hanlon et al. (2017), this is the second study that provides evidence that precipitation of salt from brine results in large bromine isotope fractionation. It is well possible that different processes than those proposed above have taken place. Eggenkamp (2015b) discussed several of these potential effects that could cause unexpectedly large bromine isotope variation, as observed in most bromine isotope studies. Most fractionation processes that are fairly well understood and that do not involve redox reactions, show much smaller fractionation factors for Br than for Cl. For example, fractionation factors for kinetic process are about two times smaller for Br than for Cl (e.g. Eggenkamp and Coleman, 2009), as Br is about twice as heavy as Cl. Fractionation factors during salt precipitation are more erratic, but can be up to 5 times smaller for Br than for Cl (Eggenkamp et al., 2016). In the case of redox reactions, the fractionation factors for Br are
also much smaller than for Cl, as was already theoretically determined in
detail at a very early date by Urey and Greiff (1935) and Urey (1947) who
found that the reaction between HX and X₂ (X = Cl or Br) would lead to a
fractionation between the two phases of 3‰ for Cl, but only 0.4‰ for Br.
Further oxidation of X₂ to XO₃⁻ would result in considerable fractionation of up
to 46‰ for Cl and (only!) 8‰ for Br. Although isotope fractionation factors
are much smaller for Br than for Cl, Eggenkamp (2015b) argued that, because
Br is much easier to oxidise than Cl, it still can lead to a considerably larger
total isotope variation.

Can the Br isotope variation in the Dutch Zechstein samples be explained by
isotope fractionation due to redox reactions? Bromide is relatively easy to
oxidise to Br₂ under ultraviolet light (e.g. Halman and Porat, 1988), but only
under acid conditions. Most probably, such conditions do not exist during
evaporative precipitation of salts from seawater. Bromide may also be oxidised
by biological reactions where bromide is converted to bromomethane (as
described e.g. by Blei et al., 2012) and such reactions do not need to take
place in acid solutions. In both processes gaseous products are formed that
would escape from the aqueous environment. Hanlon et al. (2017) suggested
that this type of processes might happen in terrestrial evaporating salt lake
environments which have very high amounts of organic matter. They observed
very negative δ⁸¹Br values in gases that escaped from the brine with values
down to -1‰ vs. SMOB. The composition of the gas samples was not
determined, but it was suggested it consisted of oxidised Br species. As the
gaseous products are of a higher oxidation state than the original bromide
they are expected to be isotopically heavier, as was calculated by Urey and
Greiff (1935), and the δ⁸¹Br of the residual bromide would be lower.
Considering the very low δ⁸¹Br values in the gases measured by Hanlon et al.
(2017) it may be possible that the process is different from a strict redox
reaction and fractionation might be complicated by complex biological
reactions. It is not strictly impossible that this effect might also have taken
place in our salt sequence, either in the brine, before the precipitation of salt,
or in the precipitated salt due to the activity of for example halobacteria (e.g.
Shubert et al., 2009). We believe however that in our salt sequence the
proposed process is highly unlikely to have taken place. Indeed, to obtain the
significantly large isotope variation as observed in our salt sequence, a very
large percentage of the bromide should have been oxidised and released to
the atmosphere. As a result, the bromide content in the salt should be
considerably reduced as a result. On the contrary, the bromide content in our
samples appears to be even higher than expected from plain evaporation of
seawater (as given by e.g. Braitsch, 1962).

Diffusion can also lead to significant bromine isotope variations (Eggenkamp
and Coleman, 2009). Large isotope variations are, however, only expected
when large concentration differences between end-members are encountered,
which is not the case here. Moreover, very negative δ<sup>81</sup>Br values are only
reached in the samples with low concentrations during diffusion, while
positive δ<sup>81</sup>Br values are difficult to obtain by diffusion only (Eggenkamp and
Coleman, 2009). For the present dataset, it is unlikely that diffusion could
have played a role.

Despite our lack for explanation, the present study of Cl and Br isotopes in a
fully developed salt sequence shows large bromine isotopes variations in salt
deposits. They are still enigmatic in the sense of present day knowledge of
bromine stable isotope behaviour, which is still in its infancy. Bromine
deserves more research on both experimental and natural samples to explore
the source of its isotope variations and promote its use as a geochemical
tracer. Of the processes discussed above, the physico-chemical process that is
responsible for the uptake of bromide in the chloride salts might be the most
relevant.

6. Conclusions

We measured the chlorine and bromine isotope variations in a single salt
sequence from Dutch Zechstein deposits. The evaluated salt deposit contains
the full salt sequence from early stage pure halite deposits to last stage pure
bischofite deposits. The results show that chlorine and bromine isotopes
undergo very different processes while they precipitate from a saturated brine
and get transformed into salt deposits. The salt evolution can be tracked using
the Br/Cl ratio, which increases regularly in the evolving salt deposits from
low values in pure halite to high values in pure bischofite deposits. The
isotope analyses indicate that δ³⁷Cl decreases with increasing Br/Cl ratio,
from about +0.05‰ in halite samples, to about -0.55‰ in bischofite samples.
This is in line with previous research and confirms the earlier established Cl
isotope fractionation factor αsalt-brine of 1.00035. Bromine isotopes show a
different behaviour, with decreasing δ⁸¹Br values (from +0.3 to -0.5‰) during
the precipitation of halite and increasing δ⁸¹Br values (from -0.5 to 0.0‰)
during carnallite/bischofite precipitation. These values, based on the
relationship between the isotope composition and the log(Br/Cl ratio), suggest
a Br isotope fractionation factor αsalt-brine of 1.0013 during halite precipitation
and a isotope fractionation factor αsalt-brine of 0.9991 during
carnallite/bischofite precipitation. We suggest that the processes that are
responsible for the distribution of bromide between a chloride saturated
seawater-derived brine and chloride salts are responsible for two different
effects. 1/ They may cause the larger than expected Br isotopic fractionation
factors compared to the values expected for the precipitation of pure bromide
salts from pure bromide brines. 2/ They may also cause the large differences
in Br isotope fractionation factors during bromide incorporation in different
Cl salts. We argue that other processes such as photo- or bio-oxidation and
diffusion that may in theory fractionate bromine isotopes in salt deposits
cannot account for the large δ⁸¹Br variations observed here. Further
experimental and theoretical studies on the bromine isotope fractionation
accompanying incorporation of bromide in chloride salts are necessary to
fundamentally understand Br isotope ratios in natural salt sequences.

Acknowledgements

This research is part of the project "BRISOACTIONS" that has received
funding from the European Union's Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agreement No 702001.
We are grateful to Gilian Schout (Utrecht University), Alex Klomp and Jan-
Willem Weegink (both TNO Geological Survey) with their help to select the
samples. We thank Pierre Burckel (IPGP) for the analyses of Cl and Br by ICP-MS and Laure Cordier (IPGP) for the analyses by ICP-OES. Parts of this work were supported by IPGP multidisciplinary program PARI, and by Paris-IdF region SESAME Grant no. 12015908. We would like to thank the Associate Editor, Brian W. Stewart, Randy Stotler and one anonymous reviewer for their constructive and helpful comments that improved the quality of the paper significantly. This is IPGP contribution No. 3974.

References

Atbir A., Marrouche A., Atil H., Boukbit L., El Hadek M. and Cohen-Adad R. (2000) Calcul des diagrammes de phases des systemes binaires H₂O-MClₙ (M = Mg²⁺, Fe²⁺, Fe³⁺). *J. Therm. Anal. Calorim.* **61**, 849-860.

Blei E., Heal M.R. and Heal, K.V. (2012) Long-term CH₃Br and CH₃Cl flux measurements in temperate salt marshes. *Biogeosciences* **7**, 3657-3668.

Boeke H.E. (1908) Über das Krystallisationsschema der Chloride, Bromide, Jodide von Natrium, Kalium und Magnesium, sowie über das Vorkommen des Broms und das Fehlen von Jod in den Kalisalzlagerstatten. *Z. Kristallogr.* **45**, 346-391.

Braitsch O. (1962) Entstehung und Stoffbestand der Salzlagerstätten. Springer Verlag.

Braitsch O. and Hermann A.G. (1963) Zur Geochemie des Broms in salinären Sedimenten. I. Experimentelle Bestimmung der Br-Verteilung in verschiedenen natürlichen Salzsystemen. *Geochem. Cosmochim. Acta* **27**, 361-391.

Braitsch O. and Hermann A.G. (1964) Zur Geochemie des Broms in salinären Sedimenten. II. Die Bildungstemperaturen primärer Sylvin- und Carnallitgesteine. *Geochem. Cosmochim. Acta* **28**, 1081-1109.

Buyze D. and Lorenzen H. (1986) Solution mining of multi-component magnesium-bearing salts: a realization in the Netherlands. *CIM Bull.* **79**, 52-60.
Clynne M.A. Potter II R.W. (1979) Solubility of some alkali and alkaline earth chlorides in water at moderate temperatures. *J. Chem. Eng. Datas* **24**, 338-340.

Coeleweij P. A. J., Haug G. M. W. and van Kuijk H. (1978) Magnesium-salt exploration in the Northeastern Netherlands. *Geol. Mijnb.* **57**, 487-502.

Eastoe C.J., Long A. and Knauth L.P. (1999) Stable chlorine isotopes in the Palo Duro Basin, Texas: evidence for preservation of Permian evaporite brines. *Geochim. Cosmochim. Acta* **63**, 1375-1382.

Eggenkamp H.G.M. (1994) *δ*37Cl; *The Geochemistry of Chlorine Isotopes* (Ph.D. Thesis) Utrecht University (150 pp.).

Eggenkamp H.G.M. (2015a) Comment on "Stable isotope fractionation of chlorine during the precipitation of single chloride minerals "by Luo, C.-g., Xiao, Y.-k., Wen, H.-j., Ma, H.-z., Ma, Y.-q., Zhang, Y.-l., Zhang, Y.-x. And He, M.-y. [Applied Geochemistry 47 (2014) 141-149]. *Appl. Geochem.* **54**, 111-116.

Eggenkamp H.G.M. (2015b) Why are variations in bromine isotope compositions in the Earth's history larger than chlorine isotope compositions? *Annales UMCS, Sectio AAA* **70**, 183-194.

Eggenkamp H.G.M. and Coleman M.L. (2000) Rediscovery of classical methods and their application to the measurement of stable bromine isotopes in natural samples. *Chem. Geol.* **167**, 393-402.

Eggenkamp H.G.M. and Coleman M.L. (2009) The effect of aqueous diffusion on the fractionation of chlorine and bromine stable isotopes. *Geochim. Cosmochim. Acta* **73**, 3539-3548.

Eggenkamp H.G.M. and Louvat P. (2018) A simple distillation technique to extract bromine for MC-ICP-MS isotope analyses. *Rapid Comm. Mass Spectrom.* **32**, 612-618.

Eggenkamp H.G.M., Kreulen R. and Koster van Groos A.F. (1995) Chlorine stable isotope fractionation in evaporites. *Geochim. Cosmochim. Acta* **59**, 5169-5175.

Eggenkamp H.G.M., Bonifacie M., Ader M. and Agrinier P. (2016) Experimental determination of stable chlorine and bromine isotope
fractionation during precipitation of salt from a saturated solution. *Chem. Geol.* **433**, 46-56.

Godon A., Jendrzejewski N., Eggenkamp H.G.M., Banks D.A., Ader M., Coleman M.L. and Pineau F. (2004) A cross calibration of chlorine isotopic measurements and suitability of seawater as the international reference material. *Chem. Geol.* **207**, 1-12.

Halman M. and Porat Z. (1988) Photooxidation of bromide to bromine in Dead Sea water. *Solar Energy* **41**, 417-421.

Hanlon C., Stotler R., Frape S. and Gwynne R. (2017) Comparison of 681Br and 637Cl composition of volatiles, salt precipitates, and associated water in terrestrial evaporite saline lake systems, *Isot. Env. Health Studies* **53**, 446-465.

Haug G. (1982) *Report on the results of drillhole TR-2*. Internal Shell Report.

Hermann A.G. (1972) Bromine distribution coefficients for halite precipitated from modern sea water under natural conditions. *Contr. Mineral. Petrol.* **37**, 249-252.

Hermann A.G. (1980) Bromide distribution between halite and NaCl-saturated seawater. *Chem. Geol.* **28**, 171-177.

Hermann A.G., Knake D., Schneider J. and Peters H. (1973) Geochemistry of modern seawater and brines from salt pans: main components and bromine distribution. *Contrib. Mineral. Petrol.* **40**, 1-24.

Horita J., Zimmermann H. and Holland H.D. (2002) The chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites. *Geochim. Cosmochim Acta* **66**, 3733-3756.

Kaufmann R.S. (1984) *Chlorine in groundwater; stable isotope distribution*. PhD thesis University of Arizona, Tucson (132 pp.)

Kaufmann R., Long A., Bentley H. and Davis S. (1984) Natural chlorine isotope variations. *Nature* **309**, 338-340.

Kühn R. (1955) Über den Bromgehalt von Salzgesteinen, insbesondere die quantitative Ableitung des Bromgehaltes nichtprimärer Hartsalze onder Sylvinite aus Carnallit. *Kali u. Steinsalz.* **1**, **H. 9**, 3-16.

Louvat P., Bonifacie M., Giunta T., Michel A. and Coleman M. (2016) Determination of bromine stable isotope ratios from saline solutions by "wet
plasma" MC-ICPMS including a comparison between high- and high-resolution modes, and three introduction systems. *Anal. Chem.* **88**, 3891-3898.

Lowenstein TK, Hardie LA, Timofeeff MN, and Demicco RV (2003) Secular variation in seawater chemistry and the origin of calcium chloride basinal brines. *Geology* **31**, 857-860.

Lowenstein TK, Timofeeff MN, Kovalevych VM, and Horita J (2005) The major-ion composition of Permian seawater. *Geochim. Cosmochim. Acta* **69**, 1701-1719.

Luo C.G., Xiao Y.K., Ma H.Z., Ma Y.Q. Zhang Y.L. and He M.Y. (2012) Stable isotope fractionation of chlorine during evaporation of brine from a saline lake. *Chin. Sci. Bull.* **57**, 1833-1843.

Luo C.G., Xiao Y.K., Wen H.J., Ma H.H., Ma Y.Q., Zhang Y.L., Zhang Y.X. and He M.Y. (2014) Stable isotope fractionation of chloride during the precipitation of single chloride minerals. *Appl. Geochem.* **47**, 141-149.

Luo C.G., Xiao Y.K., Wen H.J., Ma H.H., Ma Y.Q., Zhang Y.L. and He M.Y. (2015) Reply to the comment on the paper "Stable isotope fractionation of chlorine during the precipitation of single chloride minerals". *Appl. Geochem.* **54**, 117-118.

McCaffrey M.A., Lazar B. and Holland H.D. (1987) The evaporation path of seawater and the coprecipitation of Br⁻ and K⁺ with halite. *J. Sediment. Petrol.* **57**, 928-937.

Murphy T.J., Clabaugh W.S. and Gilchrist R. (1954) Separation of iodide, bromide and chloride from one another and their subsequent determination. *J. Res. Nat. Bur. Stand.* **53**, 13-18.

Nederlandse Aardolie Maatschappij B. V. and Rijks Geologische Dienst (1980) Stratigraphic nomenclature of the Netherlands. *Verh. Kon. Ned. Geol. Mijnb. Gen.* **32**.

Schubert B.A., Lowenstein T.K., Timofeeff M.N. and Parker M.A. (2009) How do prokaryotes survive in fluid inclusions in halite for 30,000 years? *Geology* **37**, 1059-1062.

Sun D.P., Shuai K.Y., Gao J.H., Liu W.G., Zhou Y.M. and Ma Y.H. (1998) A preliminary investigation on stable isotopic geochemistry of chlorine for the
sylvinite deposits of chloride-type. *Geoscience* **12**, 80-85 (in Chinese with English abstract).

Tan H.B., Ma H.Z., Zhang X.Y., Xu J.X. and Xiao Y.K. (2009) Fractionation of chlorine isotope in salt mineral sequences and application: research on sedimentary stage of ancient salt rock deposit in Tarim Basin and western Qaidam Basin. *Acta Petrol. Sin.* **25**, 955-962 (in Chinese with English abstract).

Urey H. C., 1947. The thermodynamic properties of isotopic substances. *J. Chem. Soc.* 562-581.

Urey H.C. and Greiff L.J. (1935) Isotope exchange equilibria. *J. Am. Chem. Soc.* **57**, 321-327.

Valyashko M.G. (1956) Geochemsitry of bromine in the processes of salt deposition and the use of the bromine content as a genetic and prospecting criterion. *Geochemistry (Geokhimiya)* 570-589.

Warren J.K. (2010) Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits. *Earth Sc. Rev.* **98**, 217-268.

Xiao Y.K., Liu W.G. and Zhou Y.M. (1997) Isotopic compositions of chlorine in brine and saline minerals. *Chin. Sci. Bull.* **42**, 406-409.

Xiao Y.K., Liu W.G., Zhou Y.M., Wang Y.H. and Shirodkar P.V. (2000) Variations in isotopic compositions of chlorine in evaporation controlled salt lake brines of Qaidam Basin, China. Chin. *J. Oceanol. Limnol.* **18**, 169-177.
<table><caption>Table 1: Results of the chemical and isotopic analyses of the samples from core TCI-2. δ³⁷Cl errors are defined as the 1 standard deviation of multiple measurements (mostly 2, sometimes 3). δ⁸¹Br errors are defined as the 2 standard deviation of 5 δ⁸¹Br measurements based on measurements of the sample and of a bracketing solution (Std-Smpl-Std-Smpl-Std-Smpl-Std). The bracketing standard solutions are prepared to the same Br and Cl concentrations as the analysed samples, in order to avoid linearity bias and matrix effects.</caption><thead><tr><th>Sample</th><th>Depth</th><th>Na<sup>+</sup></th><th>K<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>Ca<sup>2+</sup></th><th>Cl<sup>-</sup></th><th>Br<sup>-</sup></th><th>SO<sub>4</sub><sup>2-</sup></th><th>δ<sup>37</sup>Cl</th><th>δ<sup>81</sup>Br</th><th>Br/Cl</th></tr><tr><td></td><td>(m below surface)</td><td colspan="8">g/kg salt</td><td>%<sub>o</sub> vs. SMOC</td><td>%<sub>o</sub> vs. SMOB</td><td>weight units</td></tr></thead><tbody><tr><td>NL09</td><td>1678.0</td><td>391.00</td><td>1.36</td><td>2.47</td><td>0.80</td><td>601.44</td><td>0.21</td><td>10.50</td><td>-0.12±0.07</td><td>+0.27±0.07</td><td>0.00035</td></tr><tr><td>NL10</td><td>1679.1</td><td>393.47</td><td>1.17</td><td>0.73</td><td>0.05</td><td>603.19</td><td>0.32</td><td>0.42</td><td>-0.06±0.03</td><td>-0.10±0.05</td><td>0.00054</td></tr><tr><td>NL11</td><td>1681.5</td><td>394.04</td><td>2.52</td><td>4.12</td><td>1.52</td><td>608.77</td><td>0.22</td><td>12.53</td><td>+0.04±0.04</td><td>+0.26±0.05</td><td>0.00036</td></tr><tr><td>NL12</td><td>1682.5</td><td>397.63</td><td>5.14</td><td>4.83</td><td>0.40</td><td>617.82</td><td>0.24</td><td>8.28</td><td>+0.05±0.02</td><td>+0.20±0.08</td><td>0.00038</td></tr><tr><td>NL13</td><td>1688.2</td><td>360.97</td><td>0.22</td><td>14.34</td><td>0.45</td><td>561.36</td><td>0.14</td><td>54.13</td><td>-0.02±0.01</td><td>+0.16±0.05</td><td>0.00025</td></tr><tr><td>NL14</td><td>1689.2</td><td>324.47</td><td>0.22</td><td>28.85</td><td>1.21</td><td>517.30</td><td>0.20</td><td>106.64</td><td>-0.04±0.02</td><td>+0.11±0.11</td><td>0.00039</td></tr><tr><td>NL15</td><td>1700.7</td><td>158.93</td><td>83.94</td><td>55.03</td><td>3.10</td><td>479.03</td><td>2.88</td><td>7.38</td><td>-0.35±0.10</td><td>-0.50±0.11</td><td>0.00602</td></tr><tr><td>NL16</td><td>1701.5</td><td>354.40</td><td>4.01</td><td>5.12</td><td>16.60</td><td>561.01</td><td>0.47</td><td>44.22</td><td>-0.14±0.01</td><td>-0.36±0.05</td><td>0.00084</td></tr><tr><td>NL17</td><td>1701.6</td><td>96.57</td><td>98.13</td><td>74.23</td><td>4.65</td><td>433.12</td><td>3.47</td><td>40.20</td><td>-0.48±0.07</td><td>-0.46±0.19</td><td>0.00800</td></tr><tr><td>NL18</td><td>1702.6</td><td>56.48</td><td>118.27</td><td>82.49</td><td>2.85</td><td>428.51</td><td>4.20</td><td>11.50</td><td>-0.42±0.01</td><td>-0.41±0.13</td><td>0.00980</td></tr><tr><td>NL19</td><td>1702.7</td><td>11.71</td><td>68.64</td><td>122.41</td><td>3.30</td><td>228.38</td><td>2.63</td><td>291.19</td><td>-0.46±0.03</td><td>-0.26±0.24</td><td>0.01151</td></tr><tr><td>NL20</td><td>1703.6</td><td>333.88</td><td>13.24</td><td>12.24</td><td>2.24</td><td>551.32</td><td>0.66</td><td>17.95</td><td>-0.26±0.01</td><td>-0.42±0.05</td><td>0.00119</td></tr><tr><td>NL21</td><td>1703.7</td><td>54.51</td><td>116.43</td><td>82.68</td><td>2.07</td><td>426.92</td><td>4.17</td><td>5.67</td><td>-0.51±0.01</td><td>-0.09±0.07</td><td>0.00977</td></tr><tr><td>NL22</td><td>1704.5</td><td>6.53</td><td>7.94</td><td>113.65</td><td>0.39</td><td>346.22</td><td>4.82</td><td>1.49</td><td>-0.51±0.04</td><td>+0.03±0.05</td><td>0.01391</td></tr><tr><td>NL23</td><td>1715.9</td><td>1.61</td><td>136.28</td><td>96.91</td><td>0.00</td><td>402.50</td><td>5.67</td><td>2.44</td><td>-0.53±0.01</td><td>-0.20±0.06</td><td>0.01409</td></tr><tr><td>NL24</td><td>1716.8</td><td>23.37</td><td>99.76</td><td>94.94</td><td>0.09</td><td>332.03</td><td>4.09</td><td>101.70</td><td>-0.49±0.03</td><td>-0.23±0.21</td><td>0.01232</td></tr><tr><td>NL25</td><td>1716.9</td><td>27.32</td><td>128.52</td><td>92.64</td><td>0.01</td><td>421.41</td><td>5.47</td><td>3.18</td><td>-0.50±0.01</td><td>-0.32±0.09</td><td>0.01298</td></tr><tr><td>NL26</td><td>1717.7</td><td>334.23</td><td>23.21</td><td>18.20</td><td>1.67</td><td>581.81</td><td>1.27</td><td>4.58</td><td>-0.19±0.02</td><td>+0.20±0.15</td><td>0.00218</td></tr><tr><td>NL27</td><td>1718.6</td><td>9.16</td><td>0.66</td><td>125.26</td><td>0.00</td><td>359.55</td><td>6.22</td><td>25.36</td><td>-0.50±0.01</td><td>-0.13±0.17</td><td>0.01730</td></tr><tr><td>NL28</td><td>1718.7</td><td>7.73</td><td>2.09</td><td>122.18</td><td>0.00</td><td>361.12</td><td>6.24</td><td>7.53</td><td>-0.53±0.04</td><td>+0.04±0.04</td><td>0.01728</td></tr><tr><td>NL29</td><td>1718.8</td><td>313.74</td><td>29.21</td><td>21.29</td><td>0.24</td><td>567.08</td><td>1.56</td><td>2.99</td><td>-0.26±0.04</td><td>-0.04±0.32</td><td>0.00275</td></tr><tr><td>NL30</td><td>1719.7</td><td>1.18</td><td>0.00</td><td>129.19</td><td>0.00</td><td>349.83</td><td>6.33</td><td>36.93</td><td>-0.51±0.04</td><td>-0.33±0.08</td><td>0.01810</td></tr></tbody></table>