The Mg Isotope Signature of Marine Mg-Evaporites

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Journal Article

The Mg isotope signature of marine Mg-evaporites

Author(s): Shalev, Netta; Lazar, Boaz; Halicz, Ludwik; Gavrieli, Ittai

Publication Date: 2021-05-15

Permanent Link: https://doi.org/10.3929/ethz-b-000473596

Originally published in: Geochimica et Cosmochimica Acta 301, http://doi.org/10.1016/j.gca.2021.02.032

Rights / License: Creative Commons Attribution 4.0 International

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Geochimica et Cosmochimica Acta 301 (2021) 30–47 www.elsevier.com/locate/gca

The Mg isotope signature of marine Mg-evaporites

Netta Shalev a,b,c,⇑, Boaz Lazar a, Ludwik Halicz b,d, Ittai Gavrieli b

a Institute of Earth Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, 91904 Jerusalem, Israel b Geological Survey of Israel, 32 Y. Leibowitz St., 9692100 Jerusalem, Israel c Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zu¨rich, Clausiusstrasse 25, 8092 Zu¨rich, Switzerland d Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Zwirki_ i Wigury 101, 02-089 Warsaw, Poland

Received 18 March 2020; accepted in revised form 19 February 2021; Available online 27 February 2021

Abstract

Marine Mg-evaporites are a small oceanic sink of magnesium, precipitating only from extremely evaporated brines. The 26 isotopic composition of Mg in seawater, d Mgseawater, has recently been shown to be an effective tool for reconstructing the Mg budget of the modern and past oceans. However, estimations of the Mg isotope fractionation between the Mg-evaporites 26 and their precipitating solution are required for full quantification of the isotope effect of the evaporitic sink on d Mgseawater, 26 as well as for utilizing ancient evaporitic sequences as an archive for past d Mgseawater. Here, we estimate the Mg isotope fractionation between Mg-evaporites and modern marine-derived brine along the course of seawater evaporation, up to degree evaporation of >200. The sequence of Mg-salts included epsomite (MgSO47H2O), kainite (KMgClSO43H2O), carnallite (KMgCl36H2O), kieserite (MgSO4H2O) and bischofite (MgCl26H2O). The following isotope fractionation values, either negative or positive, were calculated from the isotope difference between the salt and its precipitating brine, and from the evolution of d26Mg in the brine throughout the evaporation: Dcarnallite-brine = +1.1‰, Depsomite-brine = +0.59‰, Dbischofite-brine = +0.33‰, Dkieserite-brine = 0.2‰ and Dkainite- brine = 1.3‰. Magnesium isotopic compositions determined on minerals from different ages in the geological record corroborate well these results. Due to precipitation of multi-mineral assemblages having isotope fractionation values of opposing signs, the d26Mg value of the brine changes only slightly (<0.5‰) throughout the evaporation path, despite the considerable Mg removal (>50%). The isotope fractionations are shown to correlate with the number of water molecules coordinated to the Mg2+ and with Mg-O bond length in the mineral lattice. Given these isotope fractionations, it is calculated that a volume of 0.4 106–0.8 106 Km3 of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater d26Mg by only 0.1‰. This huge volume is by far larger than the volume of these minerals known to date in the global geological record. Therefore, it is concluded that the impact of 26 Mg-evaporites formation on d Mgseawater has been insignificant since the Proterozoic. The results of this study suggest that the Mg isotopic composition of Mg-evaporites preserved in the geological record of evaporitic basins may be used to: 1) quantify geochemical processes that fractionate Mg-isotopes within these basins, such as dolomitization; and 2) complete the secular variations curve of the marine d26Mg record using basins with well-established evaporitic sequences. Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/). Keywords: Magnesium isotopes; Magnesium salts; Marine evaporites; Seawater evaporation; Isotope fractionation; d26Mg; Chemical evolution of seawater

Abbreviation: DE, Degree of evaporation ⇑ Corresponding author at: Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zu¨rich, Clausiusstrasse 25, 8092 Zu¨rich, Switzerland. E-mail address: [email protected] (N. Shalev). https://doi.org/10.1016/j.gca.2021.02.032 0016-7037/Ó 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 31

1. INTRODUCTION Table 1 The chemical formulas of some Mg-salts minerals. Magnesium is the third most abundant cation in the Mineral Symbol Chemical composition ocean and, due to its long residence time (13 Myr; Bischofite Bi MgCl ∙6H O Berner and Berner, 1996) relative to the mixing time of 2 2 Bloedite Bl Na2Mg(SO4)2∙4H2O the oceans, it is well-mixed. The concentration and isotopic Carnallite Car KMgCl3∙6H2O composition of Mg in seawater are determined by the ocea- Epsomite Ep MgSO4∙7H2O nic Mg budget, which is controlled by Mg supply from riv- Hexahydrite Hx MgSO4∙6H2O ers, and Mg removal, mainly by precipitation of carbonate Kainite Kai KMgClSO4∙3H2O ∙ minerals and hydrothermal reactions with the volcanic Kieserite Ki MgSO4 H2O ∙ oceanic crust and, to a lesser extent, by reverse weathering Leonite Le K2Mg(SO4)2 4H20 and precipitation of Mg-evaporites (e.g., Elderfield and Polyhalite Poly K2Ca2Mg(SO4)4 2H2O Schultz, 1996; Holland, 2005; Arvidson et al., 2006). Thus, Sylvite Syl KCl understanding and quantifying the Mg budget of the modern and ancient oceans are important to our understanding of how fundamental Earth processes, such as weathering, the degree of enclosure of the basin, the precipitating volcanism and sedimentation, have changed globally brine’s temperature, whether or not continuous reaction throughout the geological past, and how these processes with the already precipitated salts is maintained, and if are linked to Earth’s carbon cycle and long-term climate additional reactions within the evaporitic basin take place change (e.g., Holland, 2005; Elderfield, 2010). The isotopic (e.g., Eugster et al., 1980; Harvie et al., 1980; McCaffrey 26 et al., 1987; Shalev et al., 2018b). For example, when the composition of dissolved Mg in seawater, d Mgseawater, has recently been shown to be a reliable proxy for the recon- precipitating salts are continuously separated from the structions of the Mg budget of the modern and past oceans brine (i.e., fractional precipitation) during the course of ° (e.g., Tipper et al., 2006; Pogge Von Strandmann et al., modern seawater evaporation at 25 C, the precipitating 2014; Higgins and Schrag, 2015; Li et al., 2015; minerals include epsomite, kainite, carnallite, kieserite and Gothmann et al., 2017; Shalev et al., 2019; Xia et al., bischofite (e.g., Eugster et al., 1980; Shalev et al., 2018b; 2020). However, a full understanding of the isotopic com- Table 1). But, when the evolving solution is allowed to react positions of the oceanic Mg inputs and outputs and reliable with previously precipitated salts over the course of the 26 evaporation, the precipitating minerals include polyhalite, record of d Mgseawater in the past are needed in order to reconstruct the oceanic Mg-isotope budget. The scarcity epsomite, hexahydrite, carnallite, kieserite and bischofite of Mg isotope data from Mg-evaporites has thus far pre- (e.g., Eugster et al., 1980; Table 1). vented estimations of their potential effect, as a Mg-sink, To enable both the isotope characterization of the evap- d26 oritic Mg sink and the use of ancient Mg-evaporites as on the Mgseawater value, as well as their use as an archive 26 26 archives of past d Mgseawater, it is required to determine for past d Mgseawater. Magnesium-evaporites precipitate from extremely evap- the Mg isotope fractionation between the Mg-evaporites orated seawater, conditions that are typically reached only and their precipitating solution. Li et al. (2011) experimen- ‰ in fully or nearly enclosed basins. Such evaporites are found tally determined that epsomite (Table 1) is ca. +0.6 in large volumes in giant evaporitic basins (e.g., the Per- ‘heavier’ than its precipitating artificial Mg-SO4 solution. mian Zechstein basins in northern Europe or the Messinian However, as detailed above, epsomite is only one mineral basins around the Mediterranean; e.g., Warren, 2010), out of the five Mg-minerals that precipitate along the inferring that the evaporitic output flux of Mg from the course of evaporation of modern seawater (fractional path; ocean was not constant through time, and was higher in e.g., Eugster et al., 1980; Shalev et al., 2018b). Using quan- periods during which these giant evaporitic basins existed tum chemical density functional theory, Feng et al. (2018) (e.g., Arvidson et al., 2006). Magnesium-potassium evapor- calculated the equilibrium isotope fractionation between ite minerals in the geological record are important archives langbeinite, K2Mg2(SO4)3, and its precipitating solution, ‰ ° d26 for ancient brines and can be used to estimate past seawater to be +0.4 at 25 C. Based on this value and the Mg ‰ compositions and climate (e.g., Holland et al., 1986; of three Permian langbeinite samples (-3.9 ) they sug- d26 Hardie, 1991; Horita et al., 2002; Warren, 2010). As chem- gested that the Mg value of the Permian parent brine 26 ‰ ical deposits, these evaporites are direct recorders of the was extremely Mg-depleted, ca. 4 . However, the chemistry of ancient marine-derived brines (e.g., Babel mechanism of Mg isotope fractionation during mineral pre- and Schreiber, 2014), whereby variations in ocean chem- cipitation is still enigmatic. For example, the theoretical cal- istry are reflected in changes in the depositional records culations of Mg isotope fractionation during precipitation and sequences of Mg-K salt deposits. These vary between of carbonate minerals (Rustad et al., 2010; Schauble, the chloride type, composed mainly of sylvite and carnallite 2011) do not fit each other nor experimental data (e.g., (Table 1), and the sulfate type, characterized by Pearce et al., 2012; Li et al., 2015) and many studies suggest MgSO - rich minerals (e.g., Zharkov, 1981; Hardie, 1991; that kinetic effects play an important role in determining 4 d26 Lowenstein et al., 2001; Babel and Schreiber, 2014). The the Mg value of the precipitating mineral (e.g., sequence of marine Mg-evaporite minerals that precipitate Immenhauser et al., 2010; Pearce et al., 2012; Mavromatis along the course of evaporation depends not only on the et al., 2013; Oelkers et al., 2018). Furthermore, some chemical composition of the parent seawater, but also on authors suggest that additional factors, such as aqueous 32 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 speciation, amorphous precursor phases and organic collected. The bucket with the remaining salt was then thor- ligands, may play a role in determining the Mg isotope frac- oughly washed with DI water, and the brine was returned tionation during precipitation of different minerals from to the bucket and allowed to continue to evaporate, thereby aqueous solutions (Schott et al., 2016; Mavromatis et al., simulating a ‘fractional’ evaporation path. Following sam- 2017a, 2017b). Due to the lack of clear mechanistic under- pling, each brine sample was centrifuged and an aliquot of standing, natural and/or experimental observations are the centrifuged brine was weighed and diluted in a known needed as feedback to the theoretical calculations. weight of double-distilled water (DDW). Three types of In this study, we determined the Mg isotope fractiona- salts were sampled (Table SI-2): (1) suspended salt, which tion (Dmineral-brine) during precipitation of five of the abun- was separated from the brine sample by the centrifugation dant marine Mg-evaporite minerals: epsomite, kainite, (experiment ATL); (2) salts that were accumulated on the carnallite, kieserite and bischofite (Table 1). For that, we bottom of the bucket (indicated hereafter by ‘‘bc” (experi- evaporated modern seawater to extremely high degrees of ments G and W); and (3) a single salt crystal, in cases that evaporation (>200; for details see Shalev et al., 2018b) the salt crystals were large enough. Each salt sample was and determined the Mg isotopic composition, d26Mg, of immediately weighed and dissolved in a known weight of salt – brine pairs. The results were also compared with DDW. All salt samples (suspended, accumulated or crys- the d26Mg values of several Mg-evaporite mineral samples tals) contained also varying quantities of adsorbed brine from the geological record. We then discuss the possible and are therefore termed hereafter as ‘‘wet-salts”. All sam- effect of crystal H2O and Mg-O bond length on the direc- ples were analyzed for Na, K, Ca, Mg, SO4, Sr, Cl, Br and tion and magnitude of isotope fractionation. Finally, using Li concentrations (see methods in the Supplemental Infor- these findings we: (1) estimate the effect of the evaporitic mation), and selected pairs of contemporaneous brine and Mg sink on the d26Mg value of the global ocean; and (2) salt were analyzed for their Mg isotopic composition draw conclusions about possible future use of Mg- (Tables 2 and 3). The present paper presents and discusses evaporites as an archive for d26Mg value of ancient brines the results of these latter analyses. See Shalev et al. (2018b) and of past seawater. for a detailed discussion of the methods and their verifica- tion, and of the chemical evolution of the brine and the 2. METHODS mineralogy of the precipitating salts. Previous empirical studies suggested that, while all other 2.1. Evaporation experiments elements (such as Mg, Br, K, Na, and Rb) co-precipitate or precipitate as separate minerals during Mg-evaporite pre- Three evaporation experiments, labeled ATL, G and W, cipitation and are partly removed from the brine, Li is in which seawater was evaporated to extremely high degree the most conservative element at such extreme evaporation of evaporation (DE) of >200 were conducted in the labora- (e.g., McCaffrey et al., 1987; Warren, 2010; Babel and tories of the Geological Survey of Israel (GSI). Degree of Schreiber, 2014; Zilberman et al., 2017). Thus, it is the most evaporation, DE, is defined as the ratio of the mass of suitable element for determining the degree of evaporation, H2O in a given mass of ‘‘mean” modern seawater (salinity DE. Here, DE was calculated using the molal concentration _ 1 1 of 35 grskg ) divided by the mass of H2O in the brine (mol∙kgH2O) ratios of Li, mLi: remaining from the original mass of seawater after evapora- ðmLiÞ tion. To simulate natural conditions, we used evaporated DE ¼ sample ð1Þ Li ðm Þ Mediterranean seawater collected from the evaporation Li seawater pans of the Israel Salt Company (ISC). This initial brine where the subscript seawater denotes ‘‘mean” modern sea- contained noticeable organic matter. Buckets of ca. 20 liters water (salinity of 35 grskg_ 1 seawater) and Li concentration 1 each, were placed in a fume hood under a heating lamp, to of 0.0268 mmol∙kgH2O; McCaffrey et al., 1987). Errors on simulate natural conditions of dry and warm weather. The DE were propagated for each sample and are given in temperature of the brine, as measured during salt sampling, Tables 2 and 3 and in the supplemental Tables SI-1 and was not constant, but remained mostly in the range of SI-2. The potential error associated with the assumption 30–40 °C in experiment ATL and 20–30 °C in experiments that no Li is incorporated in the solid phase was not G and W (see Table 2, for selected samples; Tables SI-1 included in the errors given in Tables 2 and 3. However, and SI-2, for the full data; and Shalev et al., 2018b). After as shown and discussed in Shalev et al. (2018b), the good 46 days, experiment W was stopped, the organic matter pre- agreement of the DELi-based chemical evolution of brine sent in the brine was oxidized by H2O2 and UV irradiation, and the DELi of the onset and end of precipitation of each and the evaporation was then allowed to continue (experi- Mg-mineral in our evaporation experiment with: 1) the sea- ment Oxidized-W). This oxidation of the organic matter water evaporation experiment reported by McCaffrey et al. accelerated the evaporation rate and increased the maxi- (1987); 2) the DE calculated using Mg concentrations for mum attainable DE, but it does not have any further effect the lower DE, before the onset of Mg-salts precipitation; on the chemistry of the brine (Shalev et al., 2018b). and 3) thermodynamic simulation of the experiment, in The evaporating brines and precipitating salts were sam- which the DE is based directly on mass ratio of H2Oin pled periodically (Table SI-1). First, a sample of ca. 30 ml the original seawater and in the remaining brine, indicate brine was collected. Then, the remaining brine was that this error is insignificant (Shalev et al., 2018b). separated from the salts and pumped into a different The adsorbed brine present in the ‘‘wet-salt” samples pre-cleaned container, and a sample of the salt was could not be physically separated from the salts without Table 2 Chemical composition of the studied brine and salt samples from the evaporation experiments (after Shalev et al., 2018b). Exp.: experiment, n.d.: not determined, S: suspended salt, bc: salt from bottom and crust, Crys: a single large salt crystal sampled with tweezers, Hal: halite, MgSO4: Mg-sulfate, Kai: kainite, Car: carnallite, Bis: Bischofite. Brine composition Salt Li-corrected compositionc Estimated salt mineralogy .Sae ta./Gohmc tCsohmc ca31(01 04 33 30–47 (2021) 301 Acta Cosmochimica et Geochimica / al. et Shalev N.

a salt b Exp. Brine Exp. Brine Density Na K Ca Mg SO4 Cl Li DE(Li) DE(Li) Salt Type Li(wet) f Mg err. Mg Na K SO4 Cl Hal MgSO4 Kai Car Bis sample days temp. err. sample ° _ 1 ∙ 1 l ∙ 1 ∙ 1 Cgsml mmol (kgH 2O) mol g(wet-salt) Mol% Mol% mmol g(wet-salt) Mol% per total salts ATL ATL-B-5 38 34 1.2823 2090 489 6 2440 783 5930 1.54 58 4 ATL-B-6 49 37 1.3135 689 761 n.d. 3770 786 7290 2.57 96 7 ATL-S-6 S 0.66 61 2 1.5 5.3 0.2 1.5 5.6 78 19 3 0 0 ATL-B-7 50 n.d. 1.3176 612 634 n.d. 3790 788 7460 2.58 96 7 ATL-S-7 S 0.78 60 2 1.7 3.7 0.6 1.8 4.1 71 19 10 0 0 ATL-B-8 63 38 1.3274 425 327 n.d. 4560 704 8270 3.28 123 9 ATL-S-8 S 1.24 44 3 1.4 0.8 1.2 1.6 2.4 36 9 55 0 0 ATL-B-9 84 38 1.3650 106 41 n.d. 5920 352 11,300 4.99 190 13 ATL-S-9 S 3.20 13 5 0.6 0.0 0.1 0.3 0.3 0 75 0 25 0 ATL-B-10 112 n.d. n.d. 105 38 n.d. 5980 362 11,200 7.12 270 19 ATL-S-10 S 3.08 39 3 1.7 0.0 0.0 0.1 3.1 0 6 0 0 94 ATL-S-10-etd S 2.44 46 3 1.7 0.03 0.0 0.2 3.2 0 6 0 0 94 G G-B-20 67 28 1.3268 742 850 1 4320 997 7950 2.30 86 6 G-S-20-bc bc 0.75 48 3 1.3 3.1 0.3 1.3 3.7 71 23 6 0 0 G-B-26 87 27 1.3460 177 94 1 5290 529 9810 4.08 150 11 G-S-26-crys Crys 0.80 75 1 3.2 0.3 0.2 2.9 0.8 0 94 0 6 0 G-B-38 153 n.d. 1.3646 81 20 1 5890 306 11,300 6.04 230 16 G-S-38-bc bc 2.33 49 3 2.1 0.0 0.0 0.6 3.2 0 25 0 0 75 W W-B-1 0 29 1.2803 2260 430 3 2440 852 6130 1.26 47 3 W-B-9 26 26 1.2887 2000 510 4 2900 965 6170 1.42 53 4 Oxidized-W W-B-25 90 22 1.3913 92 30 2 6430 552 11,300 13.8 520 36 a Density was measured using a Mettler Toledo Densito 30PX density meter after sample centrifugation, except for samples ATL-B-5. b Molar fraction of Mg contributed by the solids in the ‘‘wet-salt” (see Eq. (3)). c Millimoles of each ion in the solid salt per g of ‘‘wet-salt”. d Sample washed with ethanol before dissolution. 4N hlve l eciiae omciiaAt 0 22)30–47 (2021) 301 Acta Cosmochimica et Geochimica / al. et Shalev N. 34

Table 3 Evaporation experiments-Mg isotopic compositions. Exp.: experiment, n-number of standard-sample-standard brackets, MgSO4: Mg-sulfate, Kai: kainite, Car: carnallite, Bis: Bischoﬁte. a a a c b Exp. DE(Li) DE(Li) error Brine sample Brine Salt sample Wet-salt Li-corrected salt Mg-salts mineralogy Dsalts-brine Dsalts-brine error 26 25 26 25 26 b d Mg 1SD 95% conf. d Mg 1SD 95% conf. n d Mg 1SD 95% conf. d Mg 1SD 95% conf. n d Mg Error MgSO4 Kai Car Bisch

‰‰‰mol(Mg-mineral) /mol(Mg-total) ‰ ATL 58 4 ATL-B-5 0.83 0.04 0.07 0.42 0.02 0.04 4 ------96 7 ATL-B-6 0.90 0.01 0.02 0.46 0.01 0.01 4 ATL-S-6 0.77 0.04 0.07 0.38 0.03 0.04 4 0.69 0.11 0.87 0.13 0.00 0.00 0.22 0.12 96 7 ATL-B-7 0.85 0.04 0.06 0.42 0.02 0.03 4 ATL-S-7 0.86 0.06 0.07 0.46 0.05 0.05 6 0.87 0.13 0.65 0.35 0.00 0.00 0.03 0.14 123 9 ATL-B-8 0.60 0.10 0.15 0.32 0.05 0.07 4 ATL-S-8 1.06 0.03 0.08 0.54 0.01 0.04 3 1.65 0.24 0.14 0.86 0.00 0.00 1.05 0.29 190 13 ATL-B-9 0.82 0.03 0.03 0.41 0.01 0.01 8 ATL-S-9 0.68 0.03 0.06 0.34 0.03 0.09 3 0.28 0.54 0.75 0.00 0.25 0.00 1.10 0.54 270 19 ATL-B-10 1.04 0.03 0.03 0.52 0.01 0.01 7 ATL-S-10 0.96 0.01 0.02 0.48 0.01 0.01 4 0.84 0.10 0.06 0.00 0.00 0.94 0.20 0.10 270 19 ATL-B-10 1.04 0.03 0.03 0.52 0.01 0.01 7 ATL-S-10et 0.10 0.01 0.02 0.50 0.01 0.01 4 0.95 0.08 0.06 0.00 0.00 0.94 0.09 0.09 G 86 6 G-B-20 0.72 0.04 0.04 0.38 0.05 0.04 8 G-S-20bc 0.58 0.07 0.05 0.30 0.07 0.05 10 0.42 0.11 0.80 0.20 0.00 0.00 0.30 0.12 150 11 G-B-26 0.82 0.06 0.05 0.45 0.06 0.06 7 G-S-26-crys 0.84 0.08 0.06 0.46 0.06 0.05 9 0.84 0.10 0.94 0.00 0.06 0.00 0.02 0.11 230 16 G-B-38 0.81 0.09 0.05 0.41 0.05 0.03 13 G-S-38bc 0.91 0.05 0.04 0.49 0.05 0.05 7 1.03 0.13 0.25 0.00 0.00 0.75 0.22 0.14 W 47 3 W-B-1 0.86 0.06 0.05 0.42 0.03 0.03 8 – – – – – – – – – – – – – – – – 53 4 W-B-9 0.71 0.05 0.04 0.36 0.02 0.02 7 – – – – – – – – – – – – – – – – Oxidized-W 520 36 W-B-25 1.08 0.09 0.09 0.58 0.16 0.17 6 – – – – – – – – – – – – – – – – a From Shalev et al. (2018b). b Calculated by error propagation. c The isotope difference between the Li-corrected salt and its precipitating brine. N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 35 damaging and losing part of the salt crystals. Thus, the 2002) using BioRAD Econo-Pac Chromatography Col- analyses of the salts were conducted on the dissolved umns, filled with Bio-Rad AGÒ 50W-X12 100–200 mesh ‘‘wet-salts”. In order to correct for the contribution of the resin. Samples, containing ca. 200 mg Mg, were loaded onto brine to the bulk chemical composition of the ‘‘wet salt” the columns. Then, the matrix was first rinsed with 25 ml of and to its Mg isotopic composition, a Li correction was 1.3 M HCl, and then with 20 ml of 2.3 M HCl. The Mg applied, assuming that the Li in the ‘‘wet-salt” is derived fraction was then eluted with 24 ml of 2.3 M HCl. The total solely from the adsorbed brine (i.e., no Li is present in procedural blanks contained <0.5% of the Mg amount pro- the salt lattice). The mineral assemblage in each salt sample cessed through column chemistry. Magnesium recovery was was then determined from iterating the chemical composi- >99% and the matrix separation (the molar ratio [K + Ca tion of the solid salt, containing no adsorbed brine, to yield + Na]/[Mg]) was <0.05 (after Galy et al., 2001). These the best-fit assemblage with minimum excess of ions. A parameters were verified for each sample using ICP-AES thermodynamic simulation of the evaporation experiment (Optima 3300, Perkin–Elmer). was later run, which enabled to distinguish between mineral Magnesium isotopes analyses were conducted using a assemblages that have the same chemical composition. See Nu Instruments Plasma II MC-ICP-MS (Shalev et al., supplemental information for further details and equations, 2018a). Samples were introduced into the mass spectrome- Table SI2 for the resulted mineralogy, and Shalev et al. ter via a DSN-100 desolvation system. Measurements were (2018b) for discussion on the limitation and validation of conducted using three Faraday cup collectors to measure these methods. the ion beam intensities at m/z 24, 25 and 26 simultaneously. Each measurement run (a block) comprises 20 inte- 2.2. Natural salts sampled from evaporitic sections grations of 10 s data acquisition, which is a total 200 s of data acquisition per measurement. The zero reference To supplement the experimental results, several samples points were reset simultaneously for all measured masses of natural Mg minerals from evaporitic sections were ana- by deflecting the potential of the electrostatic analyzer lyzed for their Mg isotopic composition. Samples were before measuring each block. Correction for instrumental obtained from the Permian geological deposit in Klodawa, mass discrimination was done using the standards-sample Poland (courtesy of I. Ploch), from the Messinian geologi- bracketing technique and the resulted isotope ratios are cal record in Realmonte Mine, Sicily, Italy, and in the reported as per mil deviation from the international refer- Ionian Sea (courtesy of P. Censi). In addition, modern car- ence material DSM-3 in the delta notation: nallite and its precipitating brine from the industrial evap- 2 3 ð 26Mg= 24MgÞ oration pans of the Dead Sea Works were analyzed d26Mgð‰Þ¼4 hisample 15 1 ð 26 = 24 Þ þð 26 = 24 Þ (courtesy of A. Katz). The mineralogy of the ancient Mg- 2 Mg Mg DSM3ð1Þ Mg Mg DSM3ð2Þ salt samples was established by XRD in the GSI laborato- 1000 ries (Table 4). The mineralogy of the modern Dead Sea car- ð Þ nallite was obtained following the procedure described 2 above for ‘‘wet-salts”. where DSM3(1) and DSM3(2) are the standards measured before and after the sample, respectively. Each sample was 2.3. Determination of Mg isotopic compositions measured by several brackets of standard-sample-standard, which were used for statistical analysis, where n is the num- The chemical pre-treatments and Mg isotope measure- ber of brackets run for the specific sample (Table 3). In ments conducted in the geochemical laboratories of the order to estimate the long-term reproducibility, a pre- GSI are detailed in Shalev et al. (2018a). Chemical separa- treated sample of modern Dead Sea brine (DSW-1) was tion of Mg was done by liquid chromatography (Galy et al., repeatedly measured (d26Mg = 0.67 ± 0.11‰, 2SD,

Table 4 The Mg-isotopic composition of Mg-salts from diﬀerent locations and ages. Sample Location Age Mineralogy d26Mg [‰]SD[‰] 95% conf. [‰]n Realmonte-1 Realmonte mine, Sicilia Messinian Kainite 1.96 0.20 0.30 3 Realmonte-2 Realmonte mine, Sicilia Messinian Kainite 2.14 0.07 0.07 6 Realmonte-4 Realmonte mine, Sicilia Messinian Kainite 2.09 0.05 0.07 5 Klodawa-2 Klodawa mine, Poland Permian Kieserite, Halite 1.12 0.22 0.30 4 Klodawa-3 Klodawa mine, Poland Permian Polyhalite 0.43 0.07 0.07 6 Klodawa-6 Klodawa mine, Poland Permian Carnallite +1.39 0.03 0.04 4 KRYOS 0–1 Ionian Sea Bischoﬁte, 0.36 0.07 0.10 3 Anhydrite, Quartz G-5074-brine Industrial evaporation Pond 3, Present brine 0.71 0.09 0.14 4 Dead Sea Works G-5074-salta Industrial evaporation Pond 3, Present Carnallite +0.21 0.07 0.07 7 Dead Sea Works a Results for G-5074-salt are Li-corrected based on the analyses of the brine. 72% of the total Mg in the wet-salt was calculated to be in the solid phase. 36 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47

26 n = 35, Shalev et al., 2018a). The result of DSW-1, as well Some of the d Mgsalt values have relatively large errors. as the results of the international standards: Cambridge-1 In these cases, the Mg in the ‘‘wet-salt” originating from (2.62 ± 0.14‰, 2SD, n = 36) and IAPSO seawater the solid Mg-salt is small compared to the Mg from the (0.84 ± 0.09‰, 2SD, n = 12) are identical, within errors, salt adsorbed brine (i.e., small f Mg ), and the propagated error to the values reported by other laboratories and in previous therefore increases (Tables 2 and 3 and Fig. 1). Yet, the iso- literature (Shalev et al., 2018a and references therein). topic data clearly show that some salt samples are ’heavier’ d26 The isotopic compositions of the salts, Mgsalt, pre- than their precipitating brine while others are ’lighter’ than sented hereafter, are the compositions after correction for the brine (Table 3 and Fig. 1), suggesting that Mg-salts pre- the contribution of the Mg in the adsorbed brine. Using cipitation from extremely evaporated seawater involves iso- the Li-correction method (described in the supplemental tope fractionations of opposing signs. salt; information, equations SI1 – SI5), f Mg the fraction of Mg contributed by the solid salt (i.e., Mg that is incorpo- 3.2. Magnesium isotopic composition of natural Mg-salts rated into the mineral lattice) to the ‘‘wet-salt”, was calculated (Section 2.1 above and Shalev et al., 2018b). This The natural Mg-salt samples were found to have a range salt of d26Mg values of ca. 3.5‰ (2.14‰ to +1.39‰; Table 4 fraction, f Mg , is defined as: and Fig. 2), much larger than the range of values of the Mg- M salt 26 salt ¼ Mg ð Þ salts from the evaporation experiments. The d Mg values f Mg wet 3 M Mg of the analyzed natural carnallite, bischofite and polyhalite ‰ salt are higher than that of modern seawater ( 0.83 , accord- where M Mg is the calculated mass of Mg in the solid salt (in- ing to Young and Galy, 2004; Foster et al., 2010; Ling corporated into the mineral lattice), obtained from supple- et al., 2011), while d26Mg values of kieserite and kainite wet mental equation SI5, and M Mg is the measured total mass of are lower (Fig. 2). These results support the experimental ” salt Mg in the ‘‘wet-salt . Propagated errors on f Mg are shown results, which suggested a bi-directional isotope fractiona- in Table 2. tion during precipitation of evaporitic Mg-salts. 26 Then, d Mgsalt was calculated from a mixing equation, salt d26 ” 4. DISCUSSION using f Mg and the measured Mg of the ‘‘wet-salt , 26 26 d Mgwet, and of the corresponding brine, d Mgbrine: jk 4.1. Magnesium isotope fractionation during precipitation of d26 salt d26 Mgwet 1 f Mgbrine Mg-evaporites d26 ¼ hiMg ð Þ Mgsalt 4 salt f Mg The Mg isotope fractionation during precipitation of D 26 different Mg-minerals, mineral-brine, was estimated from The d Mgbrine value that was used in Eq. (4) above is of 26 26 the isotope difference between d Mgsalt and d Mgbrine of the brine sampled together with the salt sample. Salt sam- concurrently sampled salt and brine, respectively ples are either suspended salt or salt that accumulated on 26 26 (Dsalt-brine = d Mgsalt d Mgbrine; Table 3). In experiment the bottom of the bucket between two successive sampling ATL, suspended salts that were collected with the brine (over 3–4 days). It was shown by Shalev et al. (2018b) that samples and separated by centrifugation were analyzed. the total amount of Mg removed from the brine during In experiments G and W, salts were allowed to accumulate these 3–4 days was always <7% and it is therefore assumed on the bottom of the evaporation buckets between each two d26 that the Mg value of the brine did not change signifi- successive sampling (over 3–4 days). The total amount of d26 cantly between successive samples. The Mgsalt values Mg removed from the brine through Mg-salt precipitation and their propagated errors are shown in Table 3. during these 3–4 days was always <7% (Shalev et al., 2018b) and it is therefore assumed that the precipitation 3. RESULTS of the Mg-salts did not significantly change the d26Mg value of the brine between two successive sampling. Thus, the iso- 3.1. Magnesium isotopic composition of brines and Mg-salts tope difference, Dsalt-brine, of salt-brine pairs is taken here to during the evaporation experiments represent the instantaneous isotope fractionation. However, since all the salt samples that were selected for Despite the significant removal of Mg from the brine this study contained mixtures of two Mg-minerals (Table 3), through the precipitation of Mg-K-minerals (up to 60%, determination of the isotope fractionation during the pre- 50% and 80% of the Mg in experiments ATL, G and cipitation of the individual minerals (D ) was cal- d26 mineral-brine W, respectively), the Mg values of the evaporating brines culated by simultaneously solving two linear mixing ‰ ‰ varied within a narrow range of <0.5 , between 1.08 equations of the form: and 0.60‰ (Table 3 and Fig. 1). Falling within this range, D ¼ D þð ÞD ð Þ as the brine evaporated, a complex evolution was observed, saltbrine x mineralð1Þbrine 1 x mineralð2Þbrine 5 with values higher, similar or lower than the initial seawater where D is the measured isotope difference between ‰ salt-brine ( 0.83 ; Young and Galy, 2004; Foster et al., 2010; Ling the salt sample and brine sample (Table 3), mineral(1) d26 et al., 2011). The range of the Mg values of the salts, and mineral(2) are the two Mg minerals in the mixture d26 ‰ Mgsalt, however, is larger, >1.9 , with salts composition and x is the fraction of mineral(1) in the salt sample (Mg ‰ ‰ being between 1.65 and +0.28 ; Table 3 and Fig. 1). mole fraction, presented in Table 3; after Shalev et al., N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 37

Fig. 1. The d26Mg values of brines and salts from the evaporation experiments. Brines are shown in blue and salts in red (diamonds- ATL, squares- G, triangles- W and oxidized-W). The degree of evaporation (DE) of the salts is equal to the DE of the conjugate brines. Error bars on the isotopic compositions of the brines are 95% conﬁdence limit and errors on the isotopic compositions of the solid salts and on DE are calculated by error propagation. Seawater d26Mg value (0.83‰) is shown by grey solid line and the 2SD uncertainty on this value (0.09‰; Ling et al., 2011) is shown by grey dashed lines. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Dmineral(2)-brine, simultaneously. For any combination of salts containing the same two minerals, Eq. (5) is a linear function of the form: Dsalt-brine = f(x). A linear fit of this function yields both, Dmineral(1)-brine and Dmineral(2)-brine by extrapolating f(x) to two extreme cases: 1) x = 0, yielding Dsalt-brine = Dmineral(2)-brine; and 2) x = 1, yielding Dsalt-brine = Dmineral(1)-brine. An example for such extrapola- tion is presented by a plot for 4 different proportions (x) of epsomite and kainite (Fig. 3a). The errors on Dmineral (1)-brine and Dmineral(2)-brine were estimated by calculating the 95% confidence interval of the linear regression line (Fig. 3 and Table 5). Such error estimates require more than two different salt samples containing the same minerals. In case of just two salt samples, the errors were estimated from Eq. (5) when applying the maximum propagated errors on Dsalt-brine and x (Fig. 3b,c and Table 5). It should be noted that this approach assumes that Dmineral-brine for each mineral is constant throughout the experiment. Otherwise, the mixing of two or more salt mixtures (Eq. (5)) will not be linear. Fig. 2. Magnesium isotopic composition of various natural Mg- The determination of D for the natural salts salts from different geological ages. Error bars are the 95% mineral-brine confidence limits. The d26Mg of modern seawater (SW; Young measured in this study (Tables 4 and 5) is not possible since and Galy, 2004; Foster et al., 2010; Ling et al., 2011) and of the the precipitating brines are long gone. Yet, in order to obtain 26 evaporated Dead-Sea (DS) brine, from which the sample DS- first order estimates of Dmineral-brine, the d Mg values of the carnallite was precipitated, are presented for comparison. natural salts were subtracted from the Mg isotopic composition of modern seawater (d26Mg = 0.83‰). It should be noted however, that d26Mg of seawater varied somewhat 2018b). The two unknowns, Dmineral(1)-brine and during the Cenozoic Era (Higgins and Schrag, 2015; Dmineral(2)-brine, are the isotope differences between mineral Gothmann et al., 2017) and, possibly, throughout the (1) and mineral(2) and the precipitating brine, respectively. Phanerozoic Eon (Li et al., 2015; Xia et al., 2020). Further- A minimum of two salt samples containing a mixture of more, reactions (e.g., dolomitization) and/or mixing with dif- the same two minerals in different proportions (different x ferent Mg sources within the evaporitic basin may have in Eq. (5)), are needed to solve for Dmineral(1)-brine and changed the composition of the evaporating seawater 38 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47

Fig. 3. The Dsalt-brine values versus the fraction of mineral(1), x in Eq. (5), in two-mineral mixtures collected during the evaporation experiments. (A) Mixtures of epsomite and kainite; (B) mixtures of kieserite and carnallite; and (C) mixtures of kieserite and bischoﬁte. Errors on Dsalt-brine were calculated by error propagation (presented in Table 3). Errors on x were propagated from the error on the Li-corrected chemical compositions of the salt (Shalev et al., 2018b). Grey-solid lines are the linear regression (equation and R2 are presented where possible). The isotope fractionations of the pure minerals, Dmineral(1)-brine and Dmineral(2)-brine, were calculated by extrapolating the lines for x = 0 and x = 1 as explained in the text (results are presented in Table 5). The dotted lines show either the 95% conﬁdence interval on the linear regression (in panel A) or the mixing lines (Eq. (5)) when applying the maximum errors on Dsalt-brine and x (in panels B and C).

(e.g., Geske et al., 2015a, 2015b; Shalev et al., 2021) and the from two sets of mineral mixtures in the evaporation exper- salts themselves may have been subjected to late recrystal- iments: kieserite-carnallite for which Dkieserite-brine = lization. Exception to this is the modern carnallite from the 0.37‰ (samples ATL-9 and G-S-26-crys) and kieserite- Dead Sea evaporation pans, for which we measured also bischofite for which Dkieserite-brine = 1.8‰ (samples ATL- 26 the d Mg of the precipitating brine which allowed direct 10 and G-S-38bc). These Dkieserite-brine values, however, determination of Dcarnallite-brine. are similar within errors (see Table 5 and Fig. 4). Similarly, Results of Dmineral-brine for each mineral are presented in the Dkieserite-SW of the natural Permian kieserite (0.29‰) Table 5 and Fig. 4. Despite large errors in some samples, falls within the errors of the Dkieserite-brine obtained from the Dmineral-brine values for each mineral cluster within a the kieserite-carnallite mixtures (0.37‰; Table 5 and rather narrow range, corroborating the isotope fractiona- Fig. 4). The isotope fractionation for bischofite, tions of opposing signs. It should be noted that these dis- Dbischofite-brine, is 0.33‰, based on the kieserite-bischofite tinctive clusters are observed despite the non-constant mixtures in the evaporation experiments. It is equal, within (semi)natural conditions of the experimental and natural errors, to the Dbischofite-SW = 0.47‰ calculated from the nat- salt precipitation (such as temperature, precipitation rate, ural Permian bischofite (Table 5 and Fig. 4). presence of organic matter, brine composition, etc.). Note In summary, we suggest the following isotope fractiona- that the values of Depsomite-brine, Dcarnallite-brine and tions for the investigated evaporite Mg-salts: Depsomite- Dbischofite-brine are positive and the values of Dkainite-brine brine = +0.59 ± 0.31‰, Dkainite-brine = 1.3 ± 0.43‰, and and Dkieserite-brine are negative. Dbischofite-brine = +0.33 ± 0.19‰. Due to large errors, The isotope fractionation of epsomite, calculated from Dcarnallite-brine and Dkieserite-brine are further discussed in the evaporation experiments, Depsomite-brine= +0.59‰ (see Section 4.2. errors in Table 5), is identical, within errors, to the equilib- 26 rium value of 0.58‰ determined experimentally by Li et al. 4.2. Evolution of d Mgbrine during brine evaporation and (2011) on epsomite precipitated from Mg-SO4 artificial Mg-salts precipitation solution (Fig. 4). The Dkainite-brine value obtained from the evaporation experiments, 1.30‰, is similar to the The Mg isotopic composition of the dissolved Mg, d26- Dkainite-SW of the three Messinian natural samples, assuming Mgbrine, during the course of experimental evaporation of they precipitated from a brine having Mg isotopic compo- seawater remained close-to-constant, within ca. ±0.25‰ sition of modern seawater. The Dcarnallite-SW=+2.22‰, of the value of the original seawater (Table 3 and Fig. 1). calculated from Permian carnallite versus modern seawater, This is despite the significant fraction of Mg that was is within the large error obtained for Dcarnallite-brine from the removed from the brine during the experiments, in the form evaporation experiments (Table 5 and Fig. 4). However, the of Mg-salts (up to 60%, 50% and 80% of the Mg was isotope fractionation determined on the Dead Sea carnal- removed in experiments ATL, G and oxidized-W, respec- lite, Dcarnallite-brine = +0.92‰, is significantly lower than tively) and the significant isotope fractionation that accom- both the calculated Permian and evaporation experiments panies the Mg-salts precipitation. We conclude that this 26 values. It should be noted that the brine composition of small variation in d Mgbrine stems from the precipitation the Dead Sea, which is a Ca-chloride brine, is very different of multi-mineral assemblages having opposite Mg-isotope from evaporated modern seawater and is characterized by fractionations. This observation stands in contrast to the 26 very low sulfate concentration. The isotope fractionation previously suggested evolution of the d Mgbrine value in 26 during kieserite precipitation, Dkieserite-brine, was determined closed system (>0.4‰ decrease in d Mgbrine at >50% Mg Table 5 Mineral-specific isotope difference. c c Mineral Source Calculation method Salt samples Mineral Error on Dmineral-brine or Uncertainty(+) Uncertainty(-) a a b used for calculation abundance in sample abundance Dmineral-SW [‰] Epsomite Evaporation experiments Mixing line ATL-6 0.87 0.001 0.59 0.31 0.31 ATL-7 0.65 0.005 ATL-8 0.14 0.09 .Sae ta./Gohmc tCsohmc ca31(01 04 39 30–47 (2021) 301 Acta Cosmochimica et Geochimica / al. et Shalev N. G-B-20 0.80 0.03 Epsomite Li et al. (2011) Experimental 0.58 0.16 0.16 Kainite Evaporation experiments Mixing line ATL-6 0.13 0.001 1.30 0.43 0.43 ATL-7 0.35 0.005 ATL-8 0.86 0.09 G-B-20 0.20 0.03 Kainite Natural sample, Messinian Relative to modern SW Realmonte-1 1.13 0.30 0.30 Kainite Natural sample, Messinian Relative to modern SW Realmonte-2 1.31 0.07 0.07 Kainite Natural sample, Messinian Relative to modern SW Realmonte-4 1.26 0.06 0.06 Carnallite Evaporation experiments Mixing line ATL-9 0.25 0.09 5.5 17 4.0 G-B-26-crystal 0.06 0.03 Carnallite Natural sample, Permian Relative to modern SW Klodawa-6 2.22 0.04 0.04 Carnallite Natural sample, Relative to its precipitating brine G-5074-salt 0.92 0.22 0.22 Modern Dead Sea salt pond Kieserite Evaporation experiments Mixing line ATL-9 0.75 0.09 0.37 0.41 1.8 G-B-26-crystal 0.94 0.03 Kieserite Evaporation experiments Mixing line ATL-S-10 0.06 0.003 1.8 1.1 1.3 G-S-38bc 0.25 0.01 Kieserite Natural sample, Permian Relative to modern SW Klodawa-2 0.29 0.30 0.30 Bischofite Evaporation experiments Mixing line ATL-S-10 0.94 0.003 0.33 0.19 0.18 G-S-38bc 0.75 0.01 Bischofite Natural sample Relative to modern SW KRYOS 0–1 0.47 0.10 0.10 a From Shalev et al. (2018b), based on the chemistry of the solid sample. Errors are propagated from the error on the Li-corrected chemical compositions of the salt (Shalev et al., 2018b). b The isotope difference between the ancient mineral sample and modern seawater. c For natural samples, the uncertainty is the analytical 95% confidence interval. For samples from the evaporation experiment, uncertainty is calculated as shown in Fig. 3 and is, therefore, asymmetric in some cases. 40 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 mineral-brine =0

Fig. 4. Magnesium isotope fractionations (Dmineral-brine) during precipitation of evaporitic Mg-minerals. Blue squares- isotope fractionation determined based on salt-mixtures from the evaporation experiments; Purple squares- isotope fractionation determined based on salt-mixtures and brine evolution in the evaporation experiments (see Section 4.2 below); Red circles- isotope difference between ancient natural salts and modern seawater; Green triangle- isotope fractionation between Dead Sea carnallite and its precipitating brine; Orange diamond- result from Li et al. (2011): isotope fractionation between epsomite and artificial Mg-sulfate solution. The grey horizontal line represents no isotope fractionation. See text and Table 5 for Dmineral-brine and error calculation procedures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) removal), which was based only on data from epsomite (Li 2018b; see also the supplemental information). The differ- et al., 2011). Thus, our results stress the importance of ence in the isotopic compositions between the precipitating accounting for all the different Mg-evaporite salts present Mg mineral assemblage and the brine at step(n + 1), Dtotal 26 26 in evaporitic sequences in future Mg-isotope studies. (n+1) (=d Mgsalts(n+1)- d Mgbrine(n+1)), was calculated from Furthermore, given the known mineral assemblages in the measured Mg mole fraction of each mineral phase in the 26 our evaporation experiment, this small change in the d - salt sampled in each step, xmineral (Table 6; from Shalev Mgbrine value along the course of the evaporation can be et al., 2018b), and from the corresponding isotope fraction- used to better constrain the Dmineral-brine values. Here, we ation of this mineral, Dmineral-brine: use this approach to obtain better estimates of the Mg isotope fractionation of the minerals carnallite and kieserite Dtotal ¼ xepsomite Depsomitebrine þ xkainite Dkainitebrine that showed rather large errors in the evaporation experi- þ x D þ x D ments (Fig. 4). For that purpose, we modeled the evolution carnallite carnallite brine kieserite kieserite brine 26 þ x D ð7Þ of d Mgbrine throughout evaporation experiments G and bischofite bischofite brine D Oxidized-W, given the isotope fractionations, mineral-brine, The values of D and D were ‰ ‰ carnallite-brine kieserite-brine for epsomite (0.59 ), kainite ( 1.3 ) and bischofite allowed to change between 0.7‰ and 5.5‰, and between ‰ (0.33 ), as calculated above (Section 4.1), and the detailed 1.8‰ and 0.0‰, respectively (Table 5 and Fig. 4). The brine chemical composition and precipitating mineral 26 resulting d Mgbrine values were compared to the three mea- assemblages, taken from Shalev et al. (2018b) and summa- sured d26Mg values in the carnallite and kieserite pre- d26 ‰ brine rized in Table 6. Initial Mg value is 0.83 (modern cipitation phases available from experiments G and d26 seawater). The expected change in Mgbrine between two Oxidized-W: G-B-26, G-B-38 and W-B-25 (Table 3 and successive samplings of mineral assemblages was calculated Fig. 5). The sum of the deviations between the modeled using a mass balance equation: and the measured values was minimized to achieve a unique d26 ¼ d26 D best-fit output using Microsoft Excel Solver. Then, the MgbrineðÞ nþ1 MgbrineðÞ n 1 f stepðÞ nþ1 totalðÞ nþ1 ranges of Dcarnallite-brine and Dkieserite-brine, for which the ð Þ 26 6 modeled d Mgbrine is within the errors of the measured where d26Mg and d26Mg are the d26Mg val- value for all three samples were calculated to obtain uncer- brine(n+1) brine(n) D ues of the brine in step n + 1 and step n, respectively, and tainty estimations on the best-fit values. The carnallite-brine f is the mole fraction of the remaining Mg in the value is constrained by this approach to range between step(n+1) ‰ ‰ D brine in step n + 1 relative to the previous step, n (Table 6). 0.7 and 1.9 and the best fit yields a carnallite-brine value ‰ D The values of f were calculated based on the mea- of 1.1 . The kieserite-brine value is constrained to range step(n+1) ‰ ‰ sured Mg concentrations in the brine in both steps n and between 0.0 and 1.0 and the best fit yields a Dkieserite-brine value of 0.2‰ (Figs. 4 and 5). n + 1 and the DE(Li) at each step (from Shalev et al., Table 6 26 Evolution model for d Mgbrine : data, inputs and output of the best fit. Data Input Output (best fit) a b c 26 Sample DE(Li) Err. DE(Li) fRemain Err. fRemain MgSO4 type Mg mole fraction of each mineral in the Mg- d Mgbrine salts mixture xMineral d f MgSO4 Kai Car Bisch Dtotal fRemain fstep (corrected)e Initial 50 1 Eps 1.00 0.83 g G-S-2-bc 50 4 94 7 Eps 0.75 0.25 0.00 0.00 0.12 0.98 0.98 0.832 G-S-11-bc 58 4 98 7 Eps 0.00 1.00 0.00 0.00 1.30 0.98 1.00 0.832

G-S-12-bc 61 4 98 7 Eps 0.50 0.50 0.00 0.00 0.36 0.98 41 1.00 0.832 30–47 (2021) 301 Acta Cosmochimica et Geochimica / al. et Shalev N. G-S-15-bc 65 5 98 7 Eps 0.81 0.19 0.00 0.00 0.24 0.98 1.00 0.832 G-S-16-bc 67 5 98 7 Eps 1.00 0.00 0.00 0.00 0.59 0.98 1.00 0.832 G-S-17-bc 67 5 100 7 Eps 1.00 0.00 0.00 0.00 0.59 0.98 1.00 0.832 G-S-18-bc 69 5 98 7 Eps 1.00 0.00 0.00 0.00 0.59 0.98 1.00 0.832 G-S-19-bc 74 5 86 6 Eps 1.00 0.00 0.00 0.00 0.59 0.92 0.94 0.868 G-S-20-bc 86 6 91 7 Eps 0.80 0.20 0.00 0.00 0.21 0.91 0.99 0.870 G-S-21-bc 90 6 88 7 Eps 0.30 0.70 0.00 0.00 0.72 0.88 0.97 0.845 G-S-22-bc 94 7 83 6 Eps 0.47 0.44 0.09 0.00 0.16 0.83 0.94 0.833 G-S-23-bc 100 7 82 6 Eps 0.42 0.29 0.29 0.00 0.28 0.82 0.99 0.834 G-S-24-bc 103 7 80 6 Eps 0.36 0.00 0.64 0.00 1.12 0.80 0.98 0.856 G-S-25-bc 134 9 70 5 Kies 0.00 0.58 0.42 0.00 0.16 0.70 0.88 0.820 G-S-26-bc 152 11 63 5 Kies 0.84 0.00 0.16 0.00 0.25 0.63 0.90 0.822 G-S-27-bc 165 12 61 5 Kies 0.42 0.00 0.51 0.07 0.51 0.61 0.97 0.838 G-S-29-bc 193 14 60 4 Kies 0.00 0.00 0.00 1.00 0.33 0.60 0.98 0.846 G-S-31-bc 202 14 58 4 Kies 0.23 0.00 0.00 0.77 0.12 0.58 0.97 0.852 G-S-32-bc 203 14 56 4 Kies 0.73 0.00 0.00 0.27 0.33 0.56 0.97 0.850 G-S-33-bc 204 14 53 4 Kies 0.24 0.00 0.00 0.76 0.11 0.53 0.95 0.860 G-S-34-bc 222 16 48 4 Kies 0.27 0.00 0.00 0.73 0.08 0.48 0.90 0.878 G-S-35-bc 232 16 47 3 Kies 0.29 0.00 0.00 0.71 0.07 0.47 0.97 0.883 G-S-36-bc 233 16 45 3 Kies 0.16 0.00 0.00 0.84 0.18 0.45 0.97 0.891 W-S-22-bc 298 21 44 3 Kies 0.02 0.00 0.02 0.95 0.33 0.44 0.97 0.902 W-S-23-bc 340 24 34 3 Kies 0.11 0.00 0.01 0.88 0.25 0.34 0.77 0.968 W-S-24-bc 481 34 25 2 Kies 0.01 0.00 0.00 0.99 0.32 0.25 0.74 1.053 W-S-25-bc 515 36 23 2 Kies 0.05 0.00 0.00 0.95 0.28 0.23 0.91 1.080 a Degree of evaporation, calculated on a Li-scale (Shalev et al., 2018b). b The fRemain value is the mole fraction of Mg remaining in solution at any time, relative to initial content of Mg (in moles) at the beginning of the experiment, before the onset of Mg evaporites precipitation. c The approach used for determination of MgSO4 type and the mineralogy of the precipitating salts is detailed in Shalev et al. (2018b). d The Dtotal values were calculated using Eq. (7). e Some corrections, within the analytical errors of the measured data, were made in the input fRemain values, in order to enable a smooth decrease in this parameter, which would otherwise produce errors in the calculations. f The fstep value is the mole fraction of Mg remaining in the brine at step n + 1, relative to the previous step, n. g 26 Initial d Mgbrine = 0.83‰ is modern seawater value. 42 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47

4.3. The effect of crystal H2O and Mg-O bond length on the gested that the difference in isotope fractionation between direction and magnitude of isotope fractionation epsomite and carbonate minerals is due to the fact that all six H2O molecules of the aquo-ion are retained in the The results presented above show that the magnesium epsomite (MgSO4∙7H2O) structure whereas no H2O mole- 2+ isotope fractionation values (expressed as Dmineral-brine) cules are present in the carbonate lattice. Thus, Mg associated with precipitation of evaporitic Mg-salts are aquo-ion must be fully dehydrated (resulting in negative either negative or positive, depending on the specific min- isotope fractionation value as explained above) before 2 eral. Negative isotope fractionation value was previously bonding with CO3 to form the carbonate mineral (Li reported for calcium-carbonates, dolomites, magnesite, et al., 2011). brucite and hydromagnesite (e.g., Galy et al., 2002; Recently, Hindshaw et al. (2020) suggested that, for syn- Higgins and Schrag, 2010; Rustad et al., 2010; Schauble, thesised Mg-phyllosilicate minerals and brucite, the direc- 2011; Mavromatis et al., 2012; Pearce et al., 2012; tion and magnitude of the isotope fractionation is Mavromatis et al., 2013; Oelkers et al., 2018). Magnesium determined by the difference in Mg-O bond-length between ions are strongly hydrated in aqueous solution: each the Mg in solution (2.066–2.08 A˚ ; Hindshaw et al., 2020 2+ Mg is bonded to six H2O molecules to form an octahe- and references therein) and in the mineral lattice, so that 2+ dral aquo-ion, [Mg(H2O)6] (e.g., Richens, 1997), and the isotope fractionation is negatively correlated with the hence, some authors explained the negative isotope frac- bond-length. This explanation is in accordance with some tionation value during carbonate precipitation by preferen- experimental data (e.g., Wimpenny et al., 2010; Li et al., 24 2+ tial dehydration of the ‘light’ [ Mg(H2O)6] aquo-ions 2014), but contradicts many of the field observations (e.g., (e.g., Immenhauser et al., 2010; Rustad et al., 2010; Opfergelt et al., 2012; Dunlea et al., 2017; Schuessler Schauble, 2011; Mavromatis et al., 2012; Mavromatis et al., 2018; Huang et al., 2018; Shalev et al., 2019; Voigt et al., 2013). However, additional factors, such as aqueous et al., 2020; Santiago-Ramos et al., 2020). Thus, additional speciation, amorphous precursor phases and organic research is needed to settle this controversy. As shown ligands, may also play a role in determining the Mg isotope below, our results may support both the dehydration frac- fractionation during precipitation of different minerals tionation mechanism (e.g., Li et al., 2011) and the Mg-O from aqueous solutions (Shirokova et al., 2013; Schott bond-length dependency (e.g., Hindshaw et al., 2020). Note et al., 2016; Mavromatis et al., 2017a; Mavromatis et al., that these two mechanisms may be related and/or have a 2017b). combined effect on the total isotope fractionation. Positive Mg-isotope fractionation value was previously The Mg-minerals that precipitated during the present reported for epsomite (Li et al., 2011). These authors sug- research contained 2 to 6 H2O molecules bonded to the Mg2+ ion in their lattice (references in Table 7). If indeed the isotope fractionation of Mg, Dmineral-solution, during the formation of each of the Mg-minerals depends on the number of H2O molecules that are removed from the 6 coordination H2O molecules of the Mg-aquo-ion, then Dmineral-solution should be negatively correlated to the difference between 6 and number of H2O molecules bound to 2+ ] Mg ion in its lattice. Indeed, Dmineral-solution correlates well 2 (R = 0.83) with the number of H2O molecules that are

Mg [ needed to be removed from the Mg-aquo-ion in order to 26 form each of the minerals (Table 7 and Fig. 6A). The more H2O molecules are removed the more negative is Dmineral-solution, in accordance with the explanation of pref- 24 2+ erential dehydration of the ‘light’ [ Mg(H2O)6] aquo- ions (negative isotope fractionation value, see paragraph above). On the other hand, the fact that some Dmineral-solution values are positive suggests that besides the preferential dehy- 24 2+ 26 dration of the ‘light’ [ Mg(H O) ] aquo-ions, the Fig. 5. Modeled d Mgbrine throughout the seawater evaporation 2 6 2+ 2+ experiments. Model calculations assume: Depsomite-brine = +0.59‰ incorporation of Mg or [Mg(H2O)6] into the mineral Dkainite-brine = 1.3‰; Dbischofite-brine = +0.33‰. The best fit (black lattice during the precipitation is accompanied by a mecha- 26 line) is obtained by using Dcarnallite-brine = +1.1‰ and nism that favors the heavier Mg isotope, Mg, at least for D ‰ kieserite-brine = 0.2 . The dashed black lines are obtained from these minerals. This is demonstrated by the high and posi- the same parameters as the best fit line, while applying the tive Dmineral-solution of carnallite (Fig. 6A), which has six maximum allowed isotope fractionations on kieserite H O molecules bound to the lattice Mg2+ (Table 7) and D ‰ D ‰ 2 ( kieserite-brine = 1.80 ) and carnallite ( carnallite-brine = +5.5 ). hence requires no dehydration prior to the incorporation Measured values are also shown for reference (blue marks) with of Mg-aquo-ion into the mineral lattice. The fact that error bars of 2SD. The three brine samples that were used for the determination of the best-fit model are marked by red circles. (For bischofite and epsomite, which also have 6 Mg-bound interpretation of the references to colour in this figure legend, the H2O molecules (Table 7), have lower (but still positive) iso- reader is referred to the web version of this article.) tope fractionation values may be attributed to differences in N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 43

other crystal properties, such as the Mg-O bond-length (Table 7 and Fig. 6B). Except for the mineral dypingite (Table 7), all minerals with longer Mg-O bond than the

Hawthorne Mg-aquo-ion have negative Dmineral-solution values, while all minerals with shorter bond have positive Dmineral-solution values, in accordance with the data from . Included are only Hindshaw et al. (2020; Fig. 6B).

; Kieserite – D Li et al. (2011) We conclude that: 1) The sign of mineral-solution (negative or positive) is not determined by the anion in the pre-

mineral-solution cipitating Mg-mineral (carbonate, sulfate, or chloride), for D example, the isotope fractionation value of epsomite evolution) Mavromatis et al. (2012) evolution) Mavromatis et al. (2012) References Li et al. (2015);(2010) Higgins and Schrag (MgSO4∙7H2O) is positive, while that of kieserite (MgSO4∙- H2O) is negative; 2) The Dmineral-solution is negatively corre-

lated with both the number of H2O molecules that are

Robinson et al. (1972) dehydrated from the Mg-aquo-ions to form a particular Mg- mineral and with the Mg-O bond-length in the form- ing mineral, as suggested in earlier studies (e.g.,

. Immenhauser et al., 2010; Li et al., 2011; Mavromatis ; Kainite – et al., 2013; Hindshaw et al., 2020; Fig. 6A and B); and 3) The Dmineral-solution is positive when mineral formation 2+ requires no pre-dehydration of the [Mg(H2O)6] aquo- ion and/or when Mg-O bond length is shorter in the mineral lattice than in the solution. Schott et al. (2016) Uncertainty + Uncertainty 0.11 0.11 4.4. Geological implications Schlemper et al. (1985)

4.4.1. The eﬀect of deposition of evaporitic Mg-salt on the d26

c secular variations in oceanic Mg ; Dolomite –

] The isotope fractionation effect of rather large amount 0.2 0.2 0.8 This study (experiments + brine 1.30 0.43 0.43 This study 1.75 0.63 0.07 0.07 1.31 0.11 0.11 ; Carnallite – mineral-brine ‰ D [ of Mg that go into Mg-evaporites could potentially have a significant effect on the d26Mg value of the precipitating reservoir. Applying a Rayleigh distillation model to the isotope fractionation data shows that precipitating of 7% of the brine’s dissolved Mg inventory is required to yield Schott et al. (2016) 0.1‰ change in d26Mg of the associated brine, which is in the mineral formula was used. roughly the analytical error on d26Mg. This is true if Mg 2+ a

] precipitates as a mono-Mg-mineral phase with the largest Agron and Busing (1985) ˚ Mg-O bond length [A 2.069

O/Mg isotope fractionation (i.e., kainite or carnallite). However, 2 ; Dypingite – the present study demonstrates that, in the more common 2+ case of precipitating multi-mineral assemblages at such higher degree of evaporation, the isotope fractionation ; Bischofite – effect may be even lower. This is due to the isotope fractionations of opposing signs of the different salts that precipitate throughout the evaporation of marine derived brines. O bonded to Mg

2 According to the above calculation, the eﬀect of Mg- in various Mg evaporite minerals, their average Mg-O bond length and their associated isotope fractionations, b 1 0 2.115 a salts formation on the d26Mg of seawater is negligible, 2+ because a removal of 7% of the dissolved Mg reservoir in the ocean as kainite or carnallite (that results in d26Mg 5 ∙ 2 O 2.5 2.077 change of 0.1‰), requires a precipitation of 2 6 6 3

O 6 2.045∙ 1.1∙ 0.8 0.4 This study (experiments + brine

O 20.4 2.076 10 –0.8 10 Km of kainite or carnallite. This volume 3H 2 2 O 6 2.065 0.59 0.31 0.31 This study; ∙ 2 ) O 6 2.060 0.33 0.19 0.18 This study 2 (OH) O 2 2.077 4 3 2 4 2 total ) Giester et al. (2000), Schott et al. (2016) 6H of Mg-salts is about half of the volume of giant evap- 3 ∙ 3H 7H H ∙ 3 ∙ ∙ 6H 3 ∙ 4 4 oritic basins, such as the Messinian Mediterranean basin, 2 (CO O) 5 the Jurassic basin in the Gulf of Mexico and the Permian 2 Fortes et al. (2006), Schott et al. (2016) ∙ 6 ∙ 6 3 (H Zechstein basin (1.0 10 –2.0 10 km ). As giant evaporitic basins contain mainly halite and calcium-sulfates with

O molecules bonded to the Mg just minute amounts of Mg-K-salts unites consisting of 2 ; Nesquehonite –

C. multi-mineral assemblages (e.g., Warren, 2010; Babel and ° Schreiber, 2014), the formation of these salts had virtually no measurable eﬀect on the Mg-isotopic composition of From: Epsomite – The structure of DypingiteAt in 25 unknown and thus the number of H c a b seawater. However, better constraints on the original vol- Table 7 Bischoﬁte MgCl Number of H Nesquehonite MgCO Kieserite MgSO et al. (1987) Dypingite Mg Kainite KMgClSO Carnallite MgKCl Dolomite CaMg(CO Epsomite MgSO minerals that precipitated in low-temperature, sedimentaryMineral environments, and in which Mg is a major ion. Formula H 44 N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47

Fig. 6. Magnesium isotope fractionation (Dmineral-solution) associated with precipitation of Mg minerals. Mineral symbols are specified in Table 1. (A) The isotope fractionation versus the number of H2O molecules that are dehydrated from the 6 coordination Mg-aquo-ion to form the mineral lattice. Data from current research (red circles) and literature (blue triangles) are taken from Table 7 (minerals details and references are specified there). (B) The isotope fractionation versus Mg-O bond length. For comparison, the average (2.072 A˚ ; grey solid line) and range (2.066–2.08 A˚ ; grey rectangle) of the Mg-O bond length in Mg-aquo-ion is shown (Hindshaw et al., 2020; and references therein). Synthesised phyllosilicate minerals and brucite are also shown (green asterisks; Wimpenny et al., 2010; Li et al., 2014; Wimpenny et al., 2014; Ryu et al., 2016; Hindshaw et al., 2020; and references therein). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

umes and mineralogy of ancient evaporites (that may have Mg-salts facies depends primarily on the precipitation since been dissolved and, therefore, need to be inferred sequence of the Mg-minerals. In our experiment, due to from the preserved record) are needed to exclude an episo- the specific mineral assemblages precipitated, 26 dic evaporitic event in the past that significantly changed d Mgbrine remains almost constant. However, because of the d26Mg value of the ocean. the large isotope fractionations associated with the precipitation of some of these minerals, any deviation from this 26 26 4.4.2. Magnesium-evaporites as an archive for d Mgbrine and specific sequence may significantly modify the d Mgbrine 26 d Mgseawater value (similarly to the change in Ca isotopic composition Magnesium-evaporites are chemical deposits, in which due to precipitation of gypsum; Harouaka et al., 2014). Mg is a major element, and it is thus suggested that they Dolomite formation occurs in many evaporitic environ- can be used to reconstruct the Mg isotopic composition ments (e.g., Babel and Schreiber, 2014). Since dolomite for- 26 of their precipitating brine, d Mgbrine, provided the respec- mation has a negative Ddolomite-solution value, it may increase 26 tive Dmineral-brine values are well established. It should be the d Mgbrine value (e.g., Higgins and Schrag, 2010; Li noted that, further research of potential effects on the iso- et al., 2015; Bla¨ttler et al., 2015; Shalev et al., 2021). We tope fractionation, such as kinetic or temperature effects, postulate that the d26Mg of Mg-evaporite sequences precip- may help to better constrain these Dmineral-brine values. Also, itated within a basin in which dolomite formation has further research is required to evaluate the potential effect occurred prior to the Mg-evaporites precipitation will be of post-depositional alterations, such as recrystallization, higher than d26Mg of sequences precipitated in contempo- on the Mg isotopic composition of these evaporites and, raneous basin with no dolomite formation. Hence, we sug- for natural sequences, local mineralogical, petrographic, gest that d26Mg values of ancient Mg-evaporites may be and sedimentological study should supplement the Mg iso- used to identify and quantify such processes that may have topes measurements to enable reconstruction of the condi- occurred within the evaporitic basin prior to their tions under which these evaporites formed. In addition, it is precipitation. suggested that the high Mg concentration in these evapor- For the above reasons, it is suggested that future recon- 26 ites, together with the large isotope fractionations, may structions of d Mgseawater of ancient seawater from overprint the d26Mg signal derived from other low-Mg Mg-evaporites require careful measures. Such measures archives, such as Ca-carbonates or fluid inclusions in halite, may include adopting sample selection criteria together if these Mg-evaporites are present in the same system, for with detailed investigation of the entire evaporitic sequence, example, as mineral inclusions (e.g., Xia et al., 2020). similarly to the methodologies used in studies dealing with The d26Mg value of the marine-derived precipitating the geological history of seawater from the composition of brine may also be modified by reactions occurring in fluid inclusions in halite (e.g., Lazar and Holland, 1988; evaporitic environments. Such reactions may include prior Horita et al., 2002; Brennan et al., 2013). The facts that precipitation of different Mg-evaporites or dolomite forma- the three Messinian kainite samples have the same d26Mg 26 tion. As shown here, the evolution of d Mgbrine during the value (within error, Table 4 and Fig. 2; ca. 2.1‰), and N. Shalev et al. / Geochimica et Cosmochimica Acta 301 (2021) 30–47 45 that, applying the Dkainite-brine from our evaporation exper- mental brine by UV irradiation. We thank Aliza Ravitzky and 26 iment (1.3 ± 0.43‰), the Messinian d Mgseawater is the staff of the Israel Salt Company for helping with brine collec- obtained (close to the modern value, 0.83‰; e.g., tion from the Atlit salt pans. Natural salt samples were provided Gothmann et al., 2017), suggest that, in some cases, by courtesy of Izabela Ploch (samples from Klodawa, Poland), d26Mg of Mg-evaporites may serve as proxy for past Paolo Censi (samples from Sicily and the Ionian Sea), and Amitai Katz (samples from the Dead Sea industrial pans). This research d26Mg . seawater was funded by the DFG-trilateral project TRION (Ei272/30-1; BL and LH); The Israeli Ministry of Science, Technology and 5. SUMMARY Space (Eshkol scholarship, NS); The Israeli Ministry of National Infrastructures, Energy and Water resources (IG); and Dalia and This study focuses on evaluating the Mg isotope frac- Dan Maydan Fellowship (NS). The part of the research conducted tionation between the five Mg-salts that precipitate during at ETH-Zurich was funded by an ETH postdoctoral fellowship the course of modern seawater evaporation and the (FEL-14 16-1) and a Swiss NSF Ambizione grant marine-derived brine. These Mg-salts include: epsomite (PZ00P2_185988) to NS. Comments by Philip Pogge von Strand- mann and two anonymous reviewers greatly improved the manu- (MgSO4∙7H2O), kainite (KMgClSO4∙3H2O), carnallite (KMgCl ∙6H O), kieserite (MgSO ∙H O) and bischofite script. Special thanks are due to Matthew S. Fantle, GCA 3 2 4 2 Associate Editor, for his handling of the paper and his fruitful (MgCl ∙6H O). The precipitation of these minerals is 2 2 review and comments. accompanied by Mg isotope fractionations, Dmineral-brine, that are either negative or positive, depending on the mineral. The evaporation path of seawater is characterized by APPENDIX A. SUPPLEMENTARY MATERIAL precipitation of multi-mineral Mg-salt assemblages that have opposite signs of D . Hence, despite the large mineral-brine Supplementary data to this article can be found online at (>50%) removal of Mg from the evaporating brine that pre- https://doi.org/10.1016/j.gca.2021.02.032. cipitated these Mg-minerals, the d26Mg value of the brine changed only slightly (within a range of 0.5‰). REFERENCES The following isotope fractionations between Mg-evaporites and their precipitating brine were D ‰ D Agron P. A. and Busing W. R. (1985) Magnesium dichloride determined: epsomite-brine = +0.59 ± 0.31 , kainite-brine = hexahydrate, MgCl2.6H2O, by neutron diffraction. Acta Crys- ‰ D ‰ 1.3 ± 0.43 , bischofite-brine = +0.33 ± 0.19 . The best tallogr. C 41, 8–10. fit for Dcarnallite-brine is +1.1‰ (ranging between +0.7‰ Arvidson R. S., Mackenzie F. T. and Guidry M. (2006) MAGic: a and +1.9‰) and best fit for Dkieserite-brine is 0.2‰ (ranging phanerozoic model for the geochemical cycling. Am. J. Sci. 306, between 1.0‰ and 0.0‰). 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