Salt precipitation and dissolution in the late Quaternary Dead Sea: Evidence from chemical and δ37Cl composition of pore fluids and halites
Introduction
Salt deposits are often found in geological records from marine settings, such as sediments pertaining to the Messinian Salinity Crisis (MSC), and sedimentary sections from closed terminal hypersaline lakes, such as the Great Salt Lake. Of these, halite is a common evaporite mineral that may accumulate to thickness of several km's (e.g. the MSC salt deposits; Hsü et al., 1973). Reconstruction of the paleo-environmental conditions and chemical composition of the halite deposits and its precipitating solutions can be done by evaluation of the chemical and isotope composition (e.g. 37Cl) of the crystals and their associated fluid inclusions, along with accompanying pore fluids (e.g. Eggenkamp et al., 1995, García-Veigas et al., 2009, Kiro et al., 2017, Levy et al., 2017).
Situated at the lowest accessible point on earth at ∼432 m below mean sea level (year 2017), the Dead Sea is currently one of the most concentrated natural water bodies with a salinity of ∼28%. Its negative water balance over the past decades has resulted in continuous lake level decline and halite precipitation (Steinhorn, 1983, Gavrieli et al., 1989, Gavrieli, 1997, Lensky et al., 2005, Sirota et al., 2017). The unique Ca-chloride composition of the brine comprising the Dead Sea solution began its evolution during the Miocene with the penetration of evaporated seawater into the tectonic depressions of the Kinnarot and Dead Sea basins. Further evaporation in the “Sedom Lagoon” resulted in the accumulation of a thick salt deposit that today is exposed at the Mount Sedom salt diapir to the south west of the Dead Sea (Zak, 1967; Fig. 1). Interaction of the solution with the surrounding Cretaceous limestone rocks further modified its composition, depleting it of bicarbonate and sulfate. The resulting Ca-chloride brine is characterized by Na/Cl < 1 and Ca/(HCO3 + SO4) > 1, composition that dictates the modes of primary minerals formation in the lakes that has since occupied the Dead Sea depression, i.e., mostly formation of primary aragonite or gypsum (Starinsky, 1974, Stein, 2014). Halite deposits can also be found in the sedimentary outcrops and drilled cores of the Dead Sea and precursor terminal water bodies of the region that evolved throughout the Pleistocene.
Over the late Quaternary the compositions of the hypersaline lakes filling the Dead Sea Basin varied significantly reflected the hydrological-climate conditions in the lake's large watershed (Stein, 2014). The alternating lake deposits comprising of authigenic evaporite deposits interbedded in the lacustrine sequences with clastic sediments provide high resolution sedimentary records that were extensively used for regional paleo-climate reconstruction (Stein, 2001, Stein, 2014; Stein et al., 2010, Torfstein et al., 2015, Palchan et al., 2017). While most of these studies were done at the exposed marginal terraces of the modern Dead Sea, information from the depocenter of the lake was gathered only following the drilling of cores at the deep floor of the Dead Sea in 2010–2011, the Dead Sea Deep Drilling Project (DSDDP) performed under the umbrella of the International Continental Drilling Project (ICDP) (Stein et al., 2011, Neugebauer et al., 2014). The drilling recovered a 456 m long sediment core (Core 5017-1-A) from 300 m below lake level (m bll) and provided for the first time a high-resolution sedimentary record of the deep lake spanning ca. 220 ka (Neugebauer et al., 2014, Neugebauer et al., 2016; Torfstein et al., 2015) (Fig. 1a and 1b).
The pore fluids extracted from the detrital sediments were used as proxies for compositional changes of the deep lake over time. The O and Cl− of the pore fluids were used to reconstruct the salinity in the deep lake of the last glacial Dead Sea (Lazar et al., 2014), which became diluted as a result of mixing with less saline waters. Moreover, the conservative Br− and Mg2+ concentrations varied in the lake as a response to the hydrological conditions in the lake's watershed, concentrating when the lake shrinked and diluting when the lake expanded (Levy et al., 2017).
The Na/Cl ratio also showed variability in the pore fluids. The variations in the conservative Br− and Mg2+ and non-conservative Na/Cl reflected changes in the lake volume and chemistry as a result of long-term regional climate changes, with a characteristic cyclic glacial–interglacial pattern as observed in global climate records such as atmospheric CO2 from Antarctic ice cores (Fig. 2; Lüthi et al., 2008, Levy et al., 2017). During interglacial periods, when lake volume decreased and conservative ion concentrations increased, the Na/Cl ratio in the pore fluids and halite fluid inclusions decreased (Levy et al., 2017, Kiro et al., 2017). Given the brine's Na/Cl < 1, a decrease in the ratio implies that halite precipitation took place as Na+ and Cl− would be removed at a ratio of 1:1. Conversely, during periods of lake expansion and decreasing conservative ion concentrations, such as the last glacial period, the Na/Cl ratios in the pore-fluids increased, suggesting that both Na+ and Cl− were added to the lake brine as a result of halite dissolution.
The fractionation of stable isotope of chlorine (expressed in the Cl notation) during halite precipitation allows the evaluation of halite precipitation and dissolution processes and their reconstruction from the geological record (Eggenkamp et al., 1995, Stiller et al., 1998, García-Veigas et al., 2009, Luo et al., 2012). Stiller et al. (1998) investigated the hydrological and limnological system of the modern Dead Sea from a 37Cl standpoint. Additionally, geochemical and isotope analyses, including Cl, of the evaporite sequence of the Mount Sedom salt diapir indicate that evaporation of seawater and mixture of seawater and continental water played a role in the formation of these salts (García-Veigas et al., 2009).
Here, we focus on the process of halite precipitation and dissolution in the hypersaline Dead Sea brine during the last interglacial and glacial time intervals (ca. 132–14 ka). We combine chemical and Cl isotope data from pore fluids and halite samples that were recovered from the DSDDP core and sampled from the exposures of the Mount Sedom salt diapir. Mass balance calculations along with geological observations allow the quantification of halite dissolution and its effect on the lake chemistry during the last glacial period. Finally we show that the Mount Sedom salt diapir was a major source of the halite that dissolved and supplied the Na+ and Cl− to the lake, and as such was able to counteract the effects of lake level rise and increasing volume on the solution and maintain high salinity.
Section snippets
The Mount Sedom salt diapir
Mount Sedom is located at the south west of the modern Dead Sea and is built of sequences of halite, anhydrite and some dolomite that comprise the late Miocene Sedom Formation. The Sedom Formation is overlaid by sequences of lacustrine sediments comprising the Pleistocene Amora, Samra and Lisan Formations (Zak, 1967, Stein, 2014). The age of the Sedom Formation was estimated by 10Be atmospheric dating of the salt units and 10Be/26Al burial dating to lie between 3–6 Ma (Belmaker et al., 2013,
Sampling
A general description of the DSDDP core at 5017-1-A is given by Neugebauer et al. (2014) and the chronology of the core is given by Torfstein et al. (2015) and Kitagawa et al. (2017). Detrital sediment samples, 15–30 ml in volume, and numbering 126 in total, were collected during the drilling campaign of 2010/11 at intervals of several centimeters to 14.5 m from core catcher sections, and during 2013 at similar intervals but from accompanying core sections. The samples were kept airtight to
Results
The abundant impermeable halite layers found in the core sedimentary record along with the high viscosity of the accompanying pore fluids significantly hampered subsurface chemical diffusion in the upper 320 m blf interval of the core (ca. 132 ka). This is evident by the retention of the long-term concentration trends of Mg2+ (Fig. 3a – black) in comparison to pre-diffusion modeling concentration estimates in Levy et al. (2017) (Fig. 3a – purple). Given the retention of the Mg2+ profile, the
Chemical and δ37Cl isotope variations during halite precipitation and dissolution: ‘rules of the game’
Mg2+ and Br− are conservative in the Dead Sea pore fluids (Levy et al., 2017; see supplementary Fig. 3) and the trends of Mg2+ concentrations observed in the pore fluid profile is a manifestation of water balance changes, or the net addition/removal of H2O (Levy et al., 2017; Fig. 3a). The long-term net water balance changes in the Dead Sea are inherently the result of precipitation and evaporation changes in the Dead Sea watershed and follow a glacial–interglacial pattern. Thus, the
Summary and conclusions
Halite precipitation and dissolution in the Late Pleistocene Dead Sea had occurred as a response to water balance changes which were inherently related to regional climate changes. During periods of lake level drops and brine concentration, characteristic of the last interglacial (ca. 132–117 ka) concentrations (e.g. Mg2+) increased until the brine reached saturation with respect to halite, and halite precipitation commenced leading to a decrease in the Na/Cl ratio. The Cl of Cl− in the
Acknowledgments
We wish to thank Chongguang Luo, Hans Eggenkamp, an anonymous reviewer, and the editor for their reviews that considerably improved the paper. We wish to sincerely thank I. Swaed from the Geological Survey of Israel (GSI), E. Eliani-Russak, M. Adler, I. Bar-Or, N. Avrahamov, and A. Russak from Ben-Gurion University of the Negev, and G. Antler from Aarhus University for help in core catcher sampling and analysis. D. Stiber, G. Sharabi, O. Berlin and other members of the GSI geochemical division
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2019, Quaternary Science ReviewsCitation Excerpt :Chloride concentrations (Fig. 3a and i) also decrease within this depth interval, however the magnitude of dilution of chloride is much less than that of magnesium. Levy et al. (2018) proposed that this relatively ’moderate’ decrease was caused by an addition of chloride from the dissolution of halite at the Sedom salt diapir which counteracted the extent of dilution from freshwater. Halite dissolution is also evident in the increases in the sodium (Na+) concentration and the Na/Cl ratio (Levy et al., 2018).