PUBLICATIONS. Geochemistry, Geophysics, Geosystems

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1 PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE Key Points: Microscale distributions of soluble sulfate minerals in MSC evaporites were described A depositional environment is proposed with marked brines density stratification Evaporative concentration likely promoted a serious impact on the biota in the Caltanissetta basin Supporting Information: Supporting Information S1 Correspondence to: T. Yoshimura, yoshimura@aori.u-tokyo.ac.jp Citation: Yoshimura, T., et al. (2016), An X-ray spectroscopic perspective on Messinian evaporite from Sicily: Sedimentary fabrics, element distributions, and chemical environments of S and Mg, Geochem. Geophys. Geosyst., 17, , doi:. Received 16 DEC 2015 Accepted 16 MAR 2016 Accepted article online 24 MAR 2016 Published online 21 APR 2016 An X-ray spectroscopic perspective on Messinian evaporite from Sicily: Sedimentary fabrics, element distributions, and chemical environments of S and Mg Toshihiro Yoshimura 1,2, Junichiro Kuroda 1, Stefano Lugli 3, Yusuke Tamenori 4, Nanako O. Ogawa 1, Francisco J. Jimenez-Espejo 1, Yuta Isaji 2, Marco Roveri 5, Vinicio Manzi 5, Hodaka Kawahata 2, and Naohiko Ohkouchi 1 1 Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan, 2 Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba, Japan, 3 Dipartimento di Scienze Chimiche e Geologiche, Universita degli Studi di Modena e Reggio Emilia, Modena, Italy, 4 Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo, Japan, 5 Dipartimento di Fisica e Scienze della Terra, Universita di Parma, Parma, Italy Abstract The Messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the Mediterranean basin at Ma. The interpretation of the facies and stratigraphic associations of the Messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale. In this study, the spatial distributions of Na, Mg, S, O, Si, and Al in a potassicmagnesian salt and a halite layers of Messinian evaporites from the Realmonte mine on Sicily were determined using synchrotron based micro-x-ray fluorescence. The dominant molecular host site of Mg and S obtained by X-ray absorption near edge structure (XANES) is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals, thus shedding light on evaporative concentration processes in the Caltanissetta Basin of Sicily. Mg and S K-edge XANES spectra revealed the presence of highly soluble Mg-bearing sulfates. The massive halite layer unit C, contains less soluble minerals, thus did not exceed the stage of halite crystallization. We infer that as evaporative concentration increased, the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values, creating brines that were oversaturated with Mg-sulfate. Density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines. We propose lateral advection of dense Mg-sulfate brines that certainly affected marine biota. VC American Geophysical Union. All Rights Reserved. 1. Introduction The Messinian salinity crisis (MSC) is widely regarded as one of the most dramatic episodes of oceanic change in Earth s history [Hs uetal., 1973; Roveri et al., 2014]. However, the interpretation of the facies association, age, and stratigraphic position of the Messinian halite deposits is still under debate because of the absence of modern analogues of comparable scale, and has been the object of active research using robust archives of depositional environment data [e.g., Hardie, 1996; Natalicchio et al., 2014]. The Realmonte salt deposit of central Sicily includes a m thick halite unit [Decima and Wezel, 1971, 1973, Figure 1]. The mineralogy of these evaporites is dependent on the chemical composition of the brine, and thus preserves information of the depositional environment. Some of the Caltanissetta basin salt deposits contain potash and magnesium-salt layers mainly represented by kainite [MgSO 4 KCl3H 2 O]; in a few of these deposits, carnallite [KMgCl 3 6H 2 O], bischofite [MgCl 2 6H 2 O], and sylvite [KCl], and more rarely kieserite [MgSO 4 H 2 O] and langbeinite [K 2 Mg 2 (SO 4 ) 3 ] are observed [Decima and Wezel, 1971, 1973; Lugli et al., 1999]. The presence of potassic and magnesian (K-Mg) salt minerals reflect extreme hypersaline environments in the Caltanissetta basin during the MSC and, possibly, a certain degree of diagenetic modification of the original evaporite suite. Comparing the in situ geochemical signatures and sediment fabrics of these deposits with modern salt works allows a further understanding of the environmental conditions during the sedimentation processes. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1383

2 Figure 1. (left) Map showing the location of sampling points in Sicily. (right) Stratigraphic column of the Realmonte salt deposits modified after Lugli et al. [1999]. The mineral assemblage, being linked to parent brine chemistry, plays a fundamental role in the reconstruction of the sedimentary history of evaporites. Sulfate minerals are important components in understanding the hydrochemistry of sedimentary basins; the chemical composition and solubility of sulfate salts are given by thermochemical models [Harvie and Weare, 1980; Spencer, 2000]. The Ca sulfate salts, gypsum and anhydrite, are the least soluble, but in contrast, K-Na-Mg sulfate and chloride, including anhydrous, hydrous, and mixed salts, are considerably more soluble; hundreds of grams of these salts can dissolve in a kilogram of water. Therefore, to precipitate K-Na-Mg sulfate salts, seawater salinity must be considerably high [McCaffrey et al., 1987]. During seawater evaporation, the first Mg-bearing mineral that precipitates is sulfate salt. It follows that the mineral assemblages of deposited salts reflect the past depositional condition. While a rich variety of salt-adapted microbes exist in hypersaline environments, it is emphasized that Mg is not only an essential element for life but also inhibitory to cellular systems at elevated concentrations, up to 2.3M MgCl 2 [Hallsworth et al., 2007]. From this point of view, elucidation of the chemical forms of Mg and S is a key indicator of extreme evaporation or precipitation events because they are major constituents of highly soluble minerals. The microscale element distributions in geological materials are a widely employed approach for identifying differences in sedimentary conditions [e.g., Kuroda et al., 2005; Shanahan et al., 2008]. The distribution and chemical coordination of major and minor elements are crucial for evaluating the sedimentary processes of salt deposits. The scanning X-ray fluorescence (XRF) that utilizes synchrotron radiation (SR) sources with X-ray focusing optics, called micro XRF (l-xrf), makes it possible to generate trace element distributions with a micrometer-scale spatial resolution [e.g., Sutton et al., 2002; Frisia et al., 2005; Tamenori et al., 2014]. X-rays with photon energies above 4.0 kev are called hard X-rays, and those with lower energy are called soft X-rays. For the latter X-ray source used in this study, the energetically accessible elements, such as C, N, O, Na, Mg, Al, Si, P, and S, are simultaneously excited by the incident photon. The use of energy-tunable SR highlights speciation analysis by X-ray absorption near edge structure (XANES) [Spiro et al., 1984; Greegor et al., 1997; Frisia et al., 2005; Finch and Allison, 2007]. A XANES spectrum is a molecular-scale spectroscopy technique that can be used to identify the dominant host site of an element of interest. The spectral features relate to multiple scattering of the emitted photoelectrons, and reflect the geometrical arrangement of neighboring atoms. Moreover, X-ray focusing optics allow in situ spot XANES analysis (l-xanes) that directly constrains the phases involved in compositionally complex materials like geologic materials. Recent development of high-sensitivity analysis with a stable X-ray beam supplied by a third generation synchrotron radiation source offers a unique advantage over the conventional analytical techniques; the l-xrf and l-xanes analysis allow nondestructive mineral identification and speciation-specific chemical mapping at a high spatial resolution, these analysis require no extensive sample preparation and chemical pretreatment. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1384

3 In order to indentify the sedimentary environments where K-Mg salt layers in the Caltanissetta basin were deposited during the MSC, we investigated the spatial distribution of the elements (Na, Mg, S, O, Si, and Al) together with S and Mg K-edge XANES at scales of tens of micrometers in Messinian salts from the Realmonte mine and modern halite and gypsum from salt works in Trapani salina, Sicily (Figure 1a). With regard to evaporite deposits, however, knowledge of chemical environment and element distributions at this spatial resolution has not been explored. The XANES spectra obtained from modern samples were used as spectral reference data. Here we present an X-ray spectroscopic insight into the depositional environment of (1) a K-Mg salt layer and (2) a massive halite, respectively, belonging to unit B and C of the Realmonte salt mine [Lugli et al., 1999] that will help to better understand the basin-scale evaporization processes during the MSC. 2. Geological Setting The chronostratigraphic framework of the MSC [Roveri et al., 2014] comprises three main stages: stage 1 ( Ma), stage 2 ( Ma), and stage 3 ( Ma). This study focused on the sedimentary facies of stage 2 that consists of resedimented gypsum and primary evaporites dominated by CaCO 3, NaCl, and K salts [Roveri et al., 2008]. This stage has been recognized as the MSC acme comprising the TG14 and TG12 glacials, and was triggered by pan-mediterranean tectonic and climatic factors [Meijer and Krijgsman, 2005]. The stratigraphic position of the Sicilian salt unit suggests that this massive halite unit was probably deposited in <50 kyrs during this stage [Roveri et al., 2008; Manzi et al., 2009]. The thickness of the salt unit in the Caltanissetta basin is variable, but its internal stratigraphy appears to be relatively homogeneous and is characterized by a main halite body including, in its lower intermediate portion Mg and K-rich salts. The Realmonte salt deposit reaches a maximum apparent thickness of 600 m and has been divided into four main lithological units from base to top: A, B, C, and D [Decima and Wezel, 1971, 1973; Lugli et al., 1999, Figure 1]. Unit A, at the bottom, is composed of plate cumulates settled out from a stratified water column. The overlying unit B comprises plate cumulates of halite in a shallowing upward sequence containing as many as 12 kainite layers near the top of the unit. Kainite layers embedded within unit B are composed of fine to coarse-grained rocks [Garcia-Veigas et al., 1995] that are cumulitic in origin and have been deformed by tectonics with slump-type structures [Lugli et al., 1999]. Unit C consists of coarse and very pure halite cumulates of skeletal hoppers with chevron overgrowths, precipitated from a nonstratified water body with intercalated anhydritic laminae. These salt deposits are capped by unit D, skeletal halite, and anhydrite. Only the uppermost part of unit B shows a progressive appearance of large halite rafts together with localized dissolution pits filled with mud, characteristics that suggest an upward shallowing of the basin leading to desiccation with the formation of large expansion-contraction salt polygons [Lugli et al., 1999]. The salt layers of unit B show no evidence of bottom overgrowth, current structures, or dissolution and/or truncation surfaces. These particular characteristics indicate that evaporite precipitation occurred in a stratified water body, a feature that suggests a relatively deep water setting, which is below the wave base [Lugli et al., 1999]. 3. Material and Methods We collected a set of samples in the Messinian salt from the Realmonte mine (Agrigento; Caltanissetta basin, Sicily) and in the Sosalt commercial salt work at Trapani (western Sicily, Figure 1). One sample was collected in the upper portion of unit B located at the Rosone section above the topmost kainite-bearing layer (Figures 1 and 2a) and another one in unit C, 8.6 m above its lower boundary (Figure 2f). The first consists of a single halite cycle bounded by thin mud laminae and containing minor kainite (supporting information Figure S1). The latter consists of coarse halite cumulates with white patchy grains (supporting information Figure S2). The samples collected at Trapani consist of chevron halite crusts and small gypsum domal structures (Figure 3). The Realmonte samples were sliced into 5 mm thick slabs with a saw. For l-xrf measurements, the section perpendicular to the bedding plane was used and the cut surfaces were polished and then cleaned in an ultrasonic bath with organic solvents. After the l-xrf measurements, we prepared thin sections to investigate the sedimentary structure. The salt materials were mounted on a glass slide and ground smooth. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1385

4 Figure 2. (a) K-Mg salt layer of unit B in the Realmonte mine showing folding and deformation. Scale bar at the center represents 1 m. (b) Transitional interval from clastic gray salts to opaque halite layer was used for l-xrf measurements and microscopic observations (yellow arrow shown in Figure 2a). (c) A section lying perpendicular to the bedding plane was cut and polished. The l-xrf measurements were performed at three spots (named spot 1 3 ) within 2 mm diameter cupper rings. (d) Stereomicroscopy images of l-xrf measurement spots. (e) The Church section [Manzi et al., 2012] and (f) the boundary between units B and C. Unit C shows the cyclic deposition of shale/anhydrite/halite caused by alternation of wet and dry periods. (g) Stereomicroscopy image of l-xrf measurement spot of unit C sample (red square shown in Figure 2g). The l-xrf measurements were conducted at the b-branch of the soft X-ray photochemistry beamline (BL27SU) at the SPring-8 synchrotron radiation facility, Japan Synchrotron Radiation Research Institute, using a two-dimensional approach for the partial fluorescence yield (PFY) measurement with a silicon drift detector (SDD) in the soft X-ray region [Tamenori et al., 2011] (Figure 4). A double crystal Si (111) monochromator ensured an energy resolution of 0.35 ev. Ohashi et al. [2001] has described details of the beamline. All XRF data were obtained at each mapping point, and the elemental imaging data were extracted from the XRF data set. Three l-xrf measurement spots, S1, S2, and S3 (Figures 2c and 2d), have been carried out in the kainitebearing layer. The polished slab was fixed on an aluminum sample holder with 2 mm diameter copper rings that was then installed in a vacuum chamber and fixed on a motorized XYZ stage. The height and width of the X-ray beam were 16.3 and 13.7 lm, respectively. The l-xrf measurements were taken at a photon energy of ev with an acquisition time of 1 s at 24 mm intervals for both the horizontal and vertical axes. The reproducibility for signal intensities was typically less than 5% (2SE) over ten replicates. For Mg K-edge XANES (Mg-XANES) measurements, powdered Messinian evaporites and modern salt-pan deposits from Trapani salina, Sicily (Figure 1) were fixed with conductive double-sided carbon tape onto an aluminum sample holder. Mg-XANES measurements were carried out at the c-branch of BL27SU. The light YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1386

5 Figure 3. Mg K-edge XANES spectra of Realmonte mine salts, Trapani salina salts, and reference minerals. For XANES measurements, evaporite samples were powdered and fixed with conductive tape onto a linear and rotatable manipulator. The XANES spectra of minerals and reagents have been published previously [Yoshimura et al., 2013b]. The beam size at focus point was 500 lm diameter circle with a photon flux of Ph/s. source was also radiation from a Figure 8 undulator, and the photon beam was dispersed by a soft X-ray monochromator with varied-line-spacing plane gratings [Ohashi et al., 2001]. XANES spectra were measured by scanning the undulator gap as well as the monochromator scan to maintain maximum intensity of the incident soft X-rays, and by scanning the width of entrance and exit slits to maintain constant resolving power. The photon energy resolution during the measurements was set at 250 mev. The beam size at the focus point was a 500 mm diameter circle with a photon flux of photons s 21 [Tamenori et al., 2007]. The results were recorded as fluorescence yield spectra with an SDD [Tamenori et al., 2011]. The selected energy range for Mg-XANES measurements was ev with an energy step of 0.2 ev, and an acquisition time of 1 s. Furthermore, l-xanes measurements were performed using the PFY method with an SDD for analysis of S K-edge XANES (S-XANES) [Tamenori et al., 2014]. The sample and detector layout was the same as that employed for the l-xrf measurements. The selected energy range for S-XANES measurements was ev with an energy step of 0.2 ev, and an acquisition time of 1 s. For reference standards, samples were powdered to ensure that sample orientation did not influence the results. In situ X-ray absorption spectra are influenced by the crystallographic orientation of sample crystals, thus certain spectral features can be orientation-dependent particularly for layered structures [Perez-Huerta et al., 2008]. Moreover, as in the case for samples that are thick relative to the absorption length, self-absorption may reduce the amplitude of resonances. It should be noted that both of these effects, however, only influence the magnitude of certain features, and not their peak positions. They do hamper direct matching to standards, i.e., quantitative identification or the following linear combination may be impacted, but qualitative interpretation is still valid. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1387

6 Geochemistry, Geophysics, Geosystems Figure 4. Two-dimensional fluorescence spectrum for evaporite taken in the S K-edge region. Partial fluorescence yield spectrum extracted from an integration of fluorescence counts in the channel range. The detection of the Ca and K fluorescence signal is difficult in soft X-ray experiments, since Ca and K K-edge are outside of the available photon energy region, and fluorescence decay probability induced by their L-edge excitation is too low. In order to obtain intensity maps of all major seawater cations, the distributions of Ca and K, together with Cl, Fe, Na, Mg, S, Si, and Al were analyzed in the other side of a cut surface of a slab for the l-xrf measurements (Figure 2c) using field-emission electron probe microanalyzer (FE-EPMA, JEOL JXA-8500F) at Japan Agency for Marine-Earth Science and Technology. The FE-EPMA measurements were taken at an accelerating voltage of 15 kev, a specimen current of 50 na, and at 50 mm intervals for the horizontal and vertical axis, respectively. We determined concentrations and isotope ratios of Total Carbon (TC, d13ctc) and Nitrogen (TN, d15ntn) by an isotope ratio mass spectrometer (Thermo Finnigan Delta Plus XP) equipped with an elemental analyzer (Flash EA1112) and ConFloIII interfaces [Ogawa et al., 2010]. Isotopic data are reported as per mil (&) deviations, and analytical precisions were 0.42& and 0.32& for d13c and d15n (1sd, n 5 10), respectively. We prepared four subsamples from three different sedimentary facies of the sample from unit B (S1, S2, S3, and another subsample from S1 layer). In addition, the powdered mine samples were decarbonated by chemical treatment with HCl in order to analyze the carbon and nitrogen isotopic ratio of organic fractions (d13ctoc and d15nton) in the same sample set. 4. Results 4.1. Sedimentary Facies and Spatial Distributions of Major and Minor Elements Kainite-Rich Interval of the Realmonte Salt Mine (The Rosone Section, Unit B) On the cut surface of the sample from the Rosone section, we observed a gradual color change from grayish to whitish parts (Figures 2a 2c). Three prominent features in the sedimentary facies, described below YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1388

7 have been recognized in the K-Mg salt layer; for each one l-xrf spot measurement S1, S2, and S3 were obtained (Figure 2). The S1 interval consists of sand-sized halite crystals and intergranular clastic matrix. The larger halite crystals appear as cubes or occasionally have a plate shape (Figure 2d). The depositional facies is cube cumulate with some chevrons, testifying the shallowing upward trend of the halite sequence [Lugli, 1999]. Thin section observations revealed that the halite crystals have a cloudy fluid-inclusion-rich core with inclusion-free translucent rims. Some of the halite crystals consist entirely of clear halite. The distribution of Na is largely homogeneous within the halite crystals, except for spot occurrences of Si and Al where values exceeded 2000 and 300 counts/s, respectively (Figure 5a). The matrix consists of silt-sized (tens of micrometers in size), well sorted grains of halite and silicate mud. The tiny halite crystals may contain cloudy fluid inclusions, and Na is distributed homogeneously throughout the halite grains (Figure 5a). The intergranular cement is characterized by strong Mg, S, and O enrichment and weaker Na intensities, with XANES evidence of sulfate precipitates (discussed in section 5.1). The Si and Al values are mainly homogeneous in the sulfate cement and substantially higher in a well-defined patch in the lowermost part of S1. Compared with the high S region, both the sand and silt-sized halite crystals are characterized by low Si and Al intensities of <200 and <50 cps, respectively (corresponding values for the sulfur-bearing region are 1000s of Si and 100s of Al cps; Figure 5a). XRD analysis indicates that the main mineral is halite with no presence of anhydrite, gypsum, kainite, and polyhalite. S2 this is a gray halite interval (Figures 2c and 2d). Within the halite crystals, the distribution of Na is largely homogeneous, except for spot cooccurrences of Mg, S, O, Si, and Al (Figure 5b). Sulfate minerals are also cemented along intercrystalline spaces, but halite grains are not present. There is a zone of relatively high Mg concentration (>700 counts, 0.6 mm horizontal/20.3 mm vertical scales) accompanied by higher S and O content (Figure 5b). For Na, by contrast, an inverse distribution is observed in sulfate cement: decreasing Na signal intensity with increasing Mg, S, and O. Again within the sulfate cement, Si and Al are distributed homogeneously (Figure 5b). S3 as confirmed by the thin section view, this interval consists essentially of massive halite. The cloudy core of the crystals typically exhibits a banded structure: fluid-inclusion-rich bands alternate with fluid-inclusionpoor bands. Some of the halite crystals have no cloudy core, and consist entirely of clear halite. There are no observable dissolution features. The corresponding l-xrf data reveal that spot occurrences of Mg, S, O, Si, and Al are less evident than in S1 and S2 intervals, except for a rounded sulfate grain (300 lm in size, upper right of Figure 5c). It was not possible to conclusively identify this sulfate mineral in the XRD data due to its rare occurrence. The strong Na signal variability shown in the upper left part of Figure 5c along the exterior edge of the halite crystal can plausibly be related to an analytical artifact due to surface unevenness. The element distributions in the cross section of unit B were also determined by FE-EPMA (Figure 6). Na and Cl were distributed homogeneously throughout most of the measured area. In the bottom left corner, corresponding to a l-xrf measurement spot-s1 (Figure 2c), there was a zone of increased Mg, Fe, Al, and Si overlapping with low Ca and S intensities. Compared to the bottom part, the overlying interval is characterized by higher Ca and S, and lower Mg, Fe, Al, and Si intensities (Figure 6). This area corresponds to the l-xrf spot S2. Moreover, there is no clear change in K intensities between S1 and S2 intervals (Figure 6). In the top part of the EPMA measurement area, Ca, Mg, K, and S intensities are generally low, and there is absence of Fe, Al, and Si signals (Figure 6). The trends of the latter elements are associated well with a corresponding gradual color change from grayish l-xrf measurement spot S1 to whitish S3 parts Halite Interval of Realmonte Salt Mine (Unit C) Differently from unit B, the salt cycles of unit C show a simple sedimentary feature, and consist of massive halite. The distribution of Na was largely homogeneous within the measurement area (Figure 7). Under macroscopic observation, the sample selected for l-xrf measurement had the well-defined opaque (whitish) patches (Figure 2g, supporting information Figure S2). The corresponding element intensity data reveal spot occurrences of elevated O, S, and Mg spanning 1 mm in the longest diameter (Figure 7). The insoluble residue was obtained at the dissolution of the sample with distilled water. These minerals are waterinsoluble, and it should be noted that there does not appear to be an insoluble residue in the sample from unit B except for clay. The mineralogy of the insoluble fraction was determined by XRD to be composed of polyhalite. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1389

8 Geochemistry, Geophysics, Geosystems Figure 5. l-xrf intensity maps of Na, Mg, O, S, Si, and Al for measurement spot-1 (S1), spot-2 (S2), and spot-3 (S3) in the cross section of K-Mg salt later in unit B, the Realmonte mine (Figures 1c and 1d). Color scales indicate element concentrations in signal count per second. Magnesium enrichment is associated with corresponding changes in S. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1390

9 Geochemistry, Geophysics, Geosystems Figure 6. Intensity maps of Ca, K, Cl, Fe, and along with Na, Mg, O, S, Si, and Al for the cross section of unit B in the Realmonte mine salt (Figures 1c and 1d) analyzed by an electron probe microanalyzer (EPMA). Color scales indicate element concentrations in signal counts. The S1 and S2 intervals are characterized by higher intensities of Al and Si corresponding to the presence of clay minerals. The Mg intensity is markedly high in S1 interval. Figure 7. l-xrf intensity maps of Na, Mg, O, S, Si, and Al in the cross section of unit C in the Realmonte mine salt deposits (Figures 1c, 1d, and 2g). Color scales indicate element concentrations in signal count per second. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1391

10 Figure 8. S K-edge XANES spectra of the Realmonte mine salts. Micro-XANES measurements were performed at the focused mode at each measurement point. Black crosses in the l-xrf intensity map of S denote l-xanes measurement points. The spectra for reference materials (blue lines) are from Yoshimura et al. [2013a] X-Ray Absorption Near Edge Structure The stacked Mg K-edge XANES spectra collected from Realmonte mine, Trapani salina, and reference materials are shown in Figure 3. The Mg-XANES spectra from the mine evaporites exhibit pronounced peaks appearing between 1311 and 1317 ev. The changes in the relative intensities of these peaks were apparent between white and gray layers (Figure 3). The spectral shape of the main peaks of the gray layer and kainite samples are similar; however, a shoulder of the lower energy side of the peak at 1312 ev is not present in the kainite spectrum. Moreover, the intensity of this shoulder relative to the main peak at 1312 ev is higher for the spectrum of the white layer. Resonance on the high-energy side of the highest peaks appears in the spectra of the gray layer at ev and mine kainite at ev (Figure 3). Peaks at ev are the most intense for both the modern salina halite samples, and there are no prominent features above near edge energy regions (<1320 ev, Figure 3). Unlike the spectra of all the other samples, the overall shape of the modern salina gypsum crust spectrum is relatively featureless but similar to sulfate minerals (Figure 3). This may simply reflect the low concentrations of Mg and/or the complex speciation of Mg in the gypsum crust. No peaks associated with Mg-carbonates were identified in either the Trapani salt pan or the Realmonte mine samples. To identify the chemical form of the sulfur in the mine evaporites, we obtained S-XANES spectra by a selective measurement of specific microstructures of l-xrf measurement points of unit B, S1, S2, and S3 in Figure 2 (Figure 8 and supporting information Table S1). Regardless of the differences in sedimentary facies YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1392

11 Table 1. Carbon and Nitrogen Isotope Ratios and Content Data of Evaporites From Unit B Sample Sample d 13 C-TC (&) d 13 C-TOC (&) d 15 N-TN (&) d 15 N-TON (&) TC (ppm) TN (ppm) C/N (wt Ratio) S S S Detrital layer and microstructures, the l-xanes spectra showed the highest peaks at ev, and most of the S-XANES spectra showed a broad resonance at 2498 ev in the post edge regions. The S3-1 possesses a unique peak at ev, which corresponds to peaks observed in the magnesium and strontium sulfate spectra (Figure 8). In the spectra of S1-1 and S2-1, the peaks appearing in the post edge region are more evident than those of other measurement points. Small peaks, at approximately 2486 ev, which appeared on the high-energy side of the highest peak, are present in most spectra of the mine salts (Figure 8). The spectra measured within halite crystals (S1-2, S2-3, and S3-3) produced poor signal-to-noise ratios because of low S contents, and signal intensities decreased in the order S1-2, S2-3, and S3-3 (251, 89, and 40 count/s, respectively; supporting information Table S1). It is worth mentioning that the overall shape of S1-2 is similar to three other spectra from the S1 interval (Figure 8). For Unit C salt sample, l-xanes spectra obtained within the halite crystal, and at two points on the welldefined patches of elevated O, S, and Mg, are shown in Figure 8. All l-xanes spectra show a major peak at ev. The XRD analysis indicates that this mineral is polyhalite, thus resonance feature on the high energy side of highest peaks appeared in the spectra Unit C-1 and Unit C-2 can be used to specify polyhalite (Figure 8). Two resonances at ev and ev are similar to those of S1-4 and S2-2 of unit B. The spectrum of the Unit C-3 shows a different feature. A shoulder at ev that appears on the highenergy side of the highest peak was observed, but a low signal-to-noise ratio in the XANES analyses of this measurement point obscures the detailed feature of the spectrum. By contrast, the S-XANES spectrum of unit C-1 and C-2 exhibited no shoulder on the high energy side of the peak at ev C and N Contents and Stable Isotope Ratios Total carbon (TC) contents for S1, S2, and S3 samples are 978, 113, and 48 ppm, respectively (Table 1), and are substantially higher in the grayish parts. Total nitrogen (TN) contents show a similar pattern. The TN contents of S1, S2, and S3 are 66, 16, and 5 ppm, respectively (Table 1). Both TC and TN contents are notably higher in S1, and the corresponding CN ratio was significantly positive (14.9 wt. ratio, Table 1). Another subsample from a grayish part, which is taken from a few millimeters below the S1 shows considerably higher C and N contents (3550 and 153 ppm) and C/N ratio (23.2). The isotopic composition reveals distinct differences between sedimentary fabrics. The d 13 C TC values of S1 and S2 are 211.5& and 221.6&, respectively (Table 1). The d 13 C TC value of S3 was not determined because of the very low TC content (48 ppm). The decarbonated samples of S1, S2, and S3 intervals are characterized by lower d 13 C TOC values (<220&) than those of total carbon. The grayish interval possesses the highest d 13 C TC and d 13 C TOC values of 24.4& and 214.1&, respectively. The d 15 N TN value of the grayish interval is between 7.3& and 5.0&, but those of S2 and S3 were not determined because of very low TN content (Table 1). In the decarbonated samples, d 15 N TON ranges from 0.1& to 11.3& (Table 1). 5. Discussion 5.1. Magnesium and Sulfur Chemical Environments The spectral character of the mine samples is characterized by the presence of two types of Mg-bearing minerals: chloride and sulfate. The sulfate minerals in the gray layer of the Realmonte mine are dominated by Mg-bearing sulfate, with smaller amounts of MgCl 2 (Figure 3). Mg sulfates are the dominant component of seawater bittern and are characteristic of marine evaporites [Hardie, 1990]. The spectra of both magnesium sulfate (MgSO 4 ) and magnesium sulfate heptahydrate (MgSO 4 7H 2 O) show two pronounced peaks at approximately 1311 and 1316 ev, with a shoulder at approximately 1308 ev (Figure 3). The heights of these peaks are lower for MgSO 4 7H 2 O. There are three additional peaks in the MgSO 4 spectrum, at 1319, 1321, YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1393

12 and 1330 ev. The spectrum of another hydrated Mg-Al sulfate (pickeringite; MgAl 2 (SO 4 ) 4 22H 2 O) is similar to those of the other two sulfate compounds; however, the peak position is at a slightly higher energy. Moreover, the spectral peaks for the hydrated sulfates exhibited a smoother spectrum than anhydrous MgSO 4 (Figure 3). The differences in the sulfate spectra might reflect changes in the geometric and chemical environment of magnesium related to the presence or absence of crystal water, which has been demonstrated for other elements [Yoshimura et al., 2013a]. The two pronounced peaks below 1316 ev appear to be distinguishing features of sulfate compounds. Kainite [MgSO 4 KCl3H 2 O] is a monoclinic K-Mg sulfate with a crystal structure characterized by Mg and K octahedra and sulfur tetrahedra to form sheets that are linked together by Mg octahedra [Robinson et al., 1972]. The spectrum of kainite was similar to that of the gray layer of the mine sample; however, the peak position of the first peak at ev is at a slightly lower energy. Note that the spectrum for the gray layer of the mine halite exhibited another broad peak at ev, which does not correspond to any reference material spectra. As discussed below, the possible dominant Mg-bearing phase in the Realmonte salts are Na-Mg sulfates, which can be attributed to this peak. The spectral features we obtained are at the lower energy side of the sulfate peaks. The peak appeared at around 1311 ev, which agrees well with the spectra of the Trapani halite samples (Figure 3), and is also in agreement with those of MgCl 2 [Nakanishi et al., 2010]. If Mg dissolves in fluid inclusions then the XANES spectra should resemble to the spectra of Mg 21 -aquocomplex, which exhibits a smoother spectrum. The XANES spectrum suggests that magnesium is not predominantly in a dissolved phase of fluid inclusion, but rather as a solid phase. The salinity in the man-made evaporation ponds and crystallizer pools of solar saltern systems do not exceed those of NaCl-saturated brines, and further concentration would be required for Mg salts and bischofite precipitation. Therefore, the local environment of Mg in both mine and salina halite may indicate a coprecipitation of minor amount of Mg with halite. The Mg concentrations of halite are used as a normalizer to determine minor element (K, Ca, and Br) incorporations [Herut et al., 1998], because Mg incorporation into the halite lattice is expected to be negligible [McCaffrey et al., 1987]. Nevertheless, our data suggest that magnesium in the halite crystals could also occur as cation substitutions at suitable sites in the crystal lattice or as precipitation of a trace amount of MgCl 2. The overall shape of the spectrum for the gypsum crust collected in Trapani salina is similar to those of the other sulfate compounds; however, the peaks were difficult to identify from our spectrum (Figure 3), which may simply reflect low concentrations of Mg. The variations in S K-edge l-xanes of mine salts and reference materials suggest that the sulfur is predominantly in sulfate compounds (Figure 8). For the inorganic sulfate standards, there were characteristic resonances associated with the presence of sodium, magnesium, potassium, calcium, and strontium sulfate in the S K-edge spectra. No peaks associated with CaSO 4 and K 2 SO 4 were identified in the S-XANES spectra of mine and salina halite in XRD, indicating that the sulfur in the samples is not predominantly from Ca and K sulfate. The l-xrf signal intensity of S showed a positive correlation with Mg (Figure 9), indicating that S speciation and distribution are dominated by sulfate salts associated with Mg. As evaporation proceeds, soluble Mg and K-bearing sulfates start to precipitate when seawater is highly concentrated more than approximately 70 and 90 times, respectively [McCaffrey et al., 1987], thus this fact hampers the use of these soluble minerals as an indicator of an advanced stage of evaporation. Moreover, some amount of Na is also distributed in intergranular space associated with S (Figure 5a), and the overall spectral character of S-XANES may have also been affected by the presence of Na-bearing sulfates. Various hydrated sulfates are potentially precipitated over a wide range of physicochemical conditions in the evaporitic basin [Spencer, 2000]. The common Na and Mg-bearing sulfates in evaporites consist of thenardite [Na 2 SO 4 ], mirabilite [Na 2 SO 4 10H 2 O], bl odite [Na 2 Mg(SO 4 ) 2 4H 2 O], konyaite [Na 2 Mg(SO 4 ) 2 5H 2 O], epsomite [MgSO 4 7H 2 O], and hexahydrite [MgSO 4 6H 2 O]. The experimentally obtained l-xanes spectra of mine salts were more pronounced for photon energy above 2484 ev and varied between measurement spots (Figure 6); this hampers the use of XANES as an indicator of the dominant S-bearing phase in the candidate sulfates. Assuming that Na 2 SO 4 and MgSO 4 are the dominant sulfates, the mixing ratio can be provided by a simulation of the sample spectrum by the linear combination of those of Na 2 SO 4 and MgSO 4 with a molar step of 1% (supporting information Figure S3) since it has not yet been shown spectra of a suite of Na and Mg-bearing sulfate minerals. YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1394

13 Figure 9. Comparison of relationships between fluorescence signals (in counts/s) between l-xrf measurement spot-1 (gray circle), 2 (yellow square), and 3 (navy triangle, Figure 2c). According to the linear combination fits for mine spectra, Mg molar fractions are 34% 60% except for relatively scattered spectra obtained within halite crystals (S1-2, S2-3, and S3-3 spots, Figure 8, Table 1). Although the sulfate mineralogy of evaporites is fairly complex due to the similar solubility of soluble Na, Mg, Ca, K-bearing salts and the complex solution chemistry of brine reacting with earlier-formed precipitates [e.g., Spencer, 2000], some general points can be inferred. The sulfate mineral that forms from the simple evaporation of recent seawater is gypsum, which may be later turned into anhydrite, glauberite, polyhalite, and finally Mg sulfate precipitates. Within the massive halite (S3) interval in unit B, sulfur intensity values were mostly low throughout the measurement area, but substantially high in a well-defined ellipsoidal patch (upper right of Figures 5c and 8). The sulfur l-xanes spectrum obtained from this measurement point (S3-1 in Figure 8) only exhibited a peak at ev suggesting the occurrence of a unique mineral phase. Evidence of localized occurrences of elevated element concentrations imply a risk of contamination, such as precipitation of evaporite minerals from fluid inclusions during sample preparation. In such cases, these artificial minerals should precipitate on the surface of pits of fluid inclusions, also causing changes in elastic scattering of X-ray due to surface unevenness, but no change of elastic scattering intensity was found along this ellisoidal mineral, thus the observed patches cannot be attributed to the artificial contamination at the spatial resolution used in this study, i.e., tens of micrometer scale. Data on the petrographic study of the Realmonte mine have shown the occurrence of diagenetic polyhalite in the units B and C, and polyhalite displays the following fabrics: (1) ellipsoidal-shaped radial spherulites; (2) cross-shaped aggregates of parallel fibers; and (3) rectagular and pseudohexgonal-shaped radial aggregates [Garcia-Veigas et al. 1995]. The FE-EPMA analysis indicates that the S1 interval in the unit B is YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1395

14 characterized by markedly high Mg concentrations (Figure 6). Moreover, relatively high K concentrations are visible in the grayish intervals and higher Ca concentrations are found in overlying S2 interval. The S-XANES spectra of the S1-4 and S2-2 are similar to those of nodular polyhalite [K 2 MgCa 2 (SO 4 ) 4 2H 2 O] of the S l-xanes measurement points Unit C-1 and C-2 (Figure 8), supporting the presence of polyhalite in the grayish layers of unit B. Polyhalite precipitates at a middle stage of evaporation, and it would coexist with massive deposition of halite [Harvie and Weare, 1980]. Therefore, the occurrence of polyhalite in units B and C is consistent with the depositional environments at halite saturation Depositional Processes The Rosone section sample displays the following facies (Figures 2d and 5) from bottom to top: (1) muddy gray halite with Na-Mg sulfate and tiny silt-sized halite crystals, (2) slightly muddy light-gray halite with smaller amounts of Na-Mg sulfate cement, and (3) massive milky-white halite. The key to understanding evaporite deposition lies in the recognition of sedimentary history through a sequence of stages. Evidence of a close relationship between the element distributions and the sedimentary facies is apparent from a comparison of l-xrf and microscopic images. As the section measured in this study is only 5 cm in length, and deposited over a short time period, it can be assumed that the level of the main water body did not fluctuate to a significant extent as a result of evaporation or precipitation events during deposition. Therefore, the coexistence of minerals of different solubility, halite, and Na and Mg-bearing sulfates, indicates that the depositional facies corresponds to particular hydrological and paleoceanographic conditions. The higher Si and Al signals are associated with clay mineral fractions and shows mainly homogeneous Si and Al distributions within the Na-Mg sulfate (Figure 5). The Si and Al signals were found to originate from the clay fraction. In the present case, the Si and Al distributions suggest that the crystal growth of Na-Mg sulfates occurred in the hypersaline brine with suspended clay; therefore, the Na-Mg sulfates were emplaced as syndepositional precipitates. The S1 interval also contains large milky crystals, up to a few millimeters in size, and silt-size crystals of halite (Figure 2). Halite crystallization starts at the brine surface as small plates and hopper crystals, followed by bottom overgrowth resulting in the development of chevrons and cornets [e.g., Kendall and Harwood, 1996; Schreiber and Tabakh, 2000, and references therein]. The plate shapes indicate that the halites are subaqueous cumulates resulting from precipitation at the brine-air interface. The absence of any mud matrix within these halites suggests that crystal growth occurred in a water body without suspended mud. Therefore, the sedimentary features are interpreted as evidence of episodic brine inflows with halite crystals and siliciclastic materials. The fine-grained halite could also be formed because of the mixing of waters with different salinities [Herut et al., 1998]. The mixing of marginal brine with the main water body might also result in rapid precipitation of tiny halite crystals at an interface between two distinct brine types. Carbonate isotopic information can help us to constrain previous interpretations. Marine carbonate reservoirs have a d 13 C value close to 0& and d 13 C values of organic matter range from 220& to 230& [Hoefs, 2008]. Desiccation of a basin would result in concentric distribution of evaporite minerals. Carbonate is the first salt to precipitate from brine, and it may be distributed on the outer rims of an evaporative basin [Hs u et al., 1973]. The input of carbonate fragments in the surrounding strata is a potential source of carbon in the S1 interval. In that case, the TC derived from the carbonate would have higher isotope ratios than that derived from organic matter. According to the measured d 13 C TOC values of decarbonated samples range from 214.1& to 226.1&, the observed higher d 13 C TC value of the grayish interval, including S1 (24.4 and 211.5&) relative to S2 (221.6&), indicates that the higher C contents and C/N ratios of this interval are due to the presence of a trace amount of carbonate minerals (only around thousand ppm; Table 1). The origin of this carbonate, e.g., coeval chemical precipitate of the halite or a product of sulfate bacterial reduction, is not clear. These particular characteristics allow us to reconstruct the hydrological condition during salt deposition of the section measured in this study because the cooccurrence of Mg sulfate and halite cumulates is of key factor to interpreting the depositional environment (Figure 10). The presence of Mg sulfates suggests that salt precipitation began in a stratified water body that experienced a significant evaporative concentration. As evaporative concentration increased, the density of the brine at the shallow margin of the basin increased with increasing salinity. The higher influence of reflux of seawater offshore set up a lateral facies change in the precipitating minerals because of the horizontal salinity gradient in the basin (Figure 10). The YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1396

15 Figure 10. Schematic depositional environment for the measured sample. deep basin is considered to be density stratified, with heavy brines sinking to the bottom, and overlain by more dilute brines that are oversaturated in terms of halite. As the bottom waters were replaced by the dense brines, sinking from the shallow marginal, silt-size halite and siliciclastic mud with trace amounts of carbonates were transported with these brines, away from coastal areas. The Mg sulfate precipitated directly as intergranular cement along with the siliciclastic mud. The pronounced vertical density stratification of the water column was an important component of the deep-water model of evaporite sedimentation. Cyclic evaporite-shale deposition was caused by alteration of dry and wet periods that triggered a significant change in the hydrological budget. This would have produced stratification of the water column and anoxia at the basin floor during wet seasons [Manzi et al., 2012]. It is worth emphasizing that a similar geographic setting was envisioned for the deposition of K-Mg salts of unit B, and the present study dealt with an advanced stage of evaporation in the same basin. The S2 is composed of large cloudy halite crystals together with intergranular pores filled with sulfate and smaller amount of siliciclastic mud (Figure 10). The presence of siliciclastic mud may suggest that the bottom water still contained suspended matter during the deposition of the S1 and S2 intervals, but was gradually getting clearer by the time of deposition of S3 (Figure 10). Nevertheless, the deposition of S2 and S3 is also explained by multiple events. Another explanation for the mud in intergranular position is dissolution pits especially in the chevron facies [Lugli et al., 1999]. In thin section, a commonly observed microstructure in the S2 and S3 intervals is composite halite grains, locally rich in fluid inclusions but also containing irregular patches of clear halite free of fluid inclusions. The cloudy halites have been interpreted as primary crystals [Lowenstein and Hardie, 1985], whereas the clear halite has been proposed as early pore space filling or YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1397

16 diagenetic recrystallization or replacement [e.g., Lowenstein and Hardie, 1985; Schleder and Urai, 2005, and references therein]. The clear halite may represent syndepositional cement formed during early diagenesis, rather than recrystallization, because diagenetic recrystallization and replacement processes usually completely eliminate cloudy halite textures. Given the depositional environment described above, there is a need for an external trigger for this depositional system. The triggers of density stratification are speculative; however, we suggest gravitational instability as one possibility. The soluble salt layers, up to 10 m thick, are intercalated in the halite body and the K-Mg salt layers, and are only found below the exposure surface. The repeated occurrence of K and Mg salt deposition during this period would be an early stage of a significant reduction in the connections between the Atlantic Ocean and the basin, followed by the complete exposure of the halite basin by rapid salt accumulation [Lugli et al., 1999; Manzi et al., 2012] Implications Seawater evaporation caused the precipitation of an ordered sequence of minerals with increasing solubility. The most common marine evaporite minerals are calcite and/or aragonite, gypsum, and halite. Magnesium salt precipitation starts when seawater is concentrated over 70 times [McCaffrey et al., 1987], and evaporites enriched in magnesium sulfates and complex salts are derived from a relatively simple, evaporative concentration of seawater [Hardie, 1990]. The biological productivity in the water column is strongly limited under hypersaline environments, and very low TOC values are observed (Table 1). A small number of halophilic Archaea can grow at high MgCl 2 concentrations. Although Mg is an essential element for life, high Mg concentration is inhibitory to cellular systems at elevated concentrations, up to 2.3M MgCl 2 [Hallsworth et al., 2007]. As magnesium salt precipitation starts at high concentration, Mg 21 concentration would be 4M [McCaffrey et al., 1987], which exceeds limits of microbial activity. Despite the presence of kosmotropic solutes, which would compensate for the net chaotropicity of the brine, it is suggested that the salt concentration in the brine exceeded that required for the occurrence of microbiota. In the Caltanissetta basin, the repeated occurrences of K-Mg salt layers are only observed slightly below the exposure surface [Lugli et al., 1999, 2006, 2008]. The alternation of halite and soluble salt layers further suggests the recurrence of density stratification events during the advanced stage of the evaporitic basin. At this stage, it is possible that repeated inflows of marginal brine and the stratification had a serious impact on the benthic biota of the basin. Similar soluble salts also occur in Racalmuto, Corvillo, and Pasquasia mines [Lugli et al., 2006, 2008], located a few tens of kilometers away from the Realmonte mine. These samples must be tested to see whether the extreme evaporative concentration and density stratification have been widespread in the Caltanissetta basin: it can be regarded as the events preceding the development of the spectacular vertical fissures cut through the Sicilian salt. 6. Conclusions We investigated element distributions of Na, Mg, S, O, Si, and Al in Messinian evaporite deposits from two different lithological units of the Realmonte mine on Sicily. The potassic-magnesian salt layers of the halite deposits display the following facies from bottom to top: muddy gray halite, slightly muddy light-gray halite, and massive milky-white halite. By discerning the dominant Mg and S-bearing phase in the geological materials, XANES analysis can contribute to the refinement of our understanding of geochemical environments and to the reconstruction of past hydrological and geological events. Our data allow us to propose a simplified depositional environment for the Caltanissetta Basin that indicates the presence of temporal water stratification involving enriched Mg brines, coetaneous, and persistent halite rain and diagenetical processes. The main conclusions obtained in this study are as follows: 1. The variations in Mg and S K-edge XANES of Sicily salts and reference materials suggested that multiple types of Mg-bearing sulfate minerals are distributed in the Realmonte mine salts. The l-xrf data reveal that occurrences of a rounded Mg-bearing sulfate grain in a massive halite layer, and such ellipsoidal sulfur-hotspots plausibly indicates the occurrence of polyhalite. 2. The microscopic observation and l-xrf element mapping of gray layer in unit B sample show that this layer consists of fine-grained halite crystals associated with clay, and intergranular pores filled with Mgbearing sulfate cement. Compared to the ellipsoidal patch of sulfate minerals in massive halite layer, the YOSHIMURA ET AL. A X-RAY SPECTROSCOPY OF MSC EVAPORITES 1398