Constraints on aragonite precipitation in the Dead Sea from geochemical measurements of flood plumes
Introduction
The Dead Sea (DS) located at the lowest area on the continental Earth (now at 433 m below mean sea level-mbsl) is a remnant lake that inherited its solutions from previous lakes that occupied during the past 3 Ma the tectonic depression of the Dead Sea Basin (Neev and Emery, 1967; Stein, 2001; Stein, 2014 and references therein). The lakes' solution comprises a mixture between a Ca-chloride brine and freshwaters from the lakes watershed (Stein et al., 1997). The sedimentary sequences that were deposited from the lakes consist mainly of evaporites (e.g., primary carbonates, gypsum, halite) and detritus particles. Intervals of the sedimentary sequences display a laminated configuration with couplets of alternating fine detrital material and primary aragonite, or triplet with fine detritus material, aragonite and gypsum (Katz et al., 1977; Migowski et al., 2006; Prasad et al., 2004). The (a) high accumulation rates (0.6–1 m kyr−1), (b) the possibility to achieve calendar chronology (e.g., Bookman et al., 2004; Haase-Schramm et al., 2004) and (c) the use of chemical and isotope compositions of the primary aragonite and the fine detritus as monitors of the hydro-climate conditions in the lakes watershed, turned the laminated sequences of the DS lakes into a valuable hydroclimate archive of the Levant region (Stein, 2014 and references therein). The lakes' Ca-chloride brine is poor in bicarbonate and sulfate ions that are required for the deposition of primary aragonite or gypsum. These ions are provided to the lake with the incoming freshwaters (Stein et al., 1997). Thus, primary aragonite is deposited from a “cocktail” solution that comprises a mixture between the lakes' Ca-chloride brine and freshwaters (Barkan et al., 2001; Stein et al., 1997). Based on radiocarbon data Stein et al. (2013) described a model of “turbulent mixing” on the interface between the upper less saline water-body (epilimnion) and the lower brine (hypolimnion) filling the lake. The epilimnion/hypolimnion turbulent mixing model was also applied to explain the variations in the concentrations of Br−, and Cl− in the pore waters extracted from the sediments drilled at the deep floor of the lake by the ICDP (Lazar et al., 2014). However, not all the factors controlling aragonite precipitation in the DS lakes are fully understood. Defining these factors is crucial for understanding the DS carbon cycle and the interpretation of the paleo-hydrological data stored in the lake's sediments. Previous studies dealt with the carbonate cycle of the modern DS by measuring carbon system parameters along depth water-profiles recovered from the lake (Barkan et al., 2001; Golan et al., 2017; Luz et al., 1997) and by conducting laboratory experiments (Golan et al., 2017). However, up to date, no direct observations were conducted on the chemical composition and the carbonate/borate (alkalinity) system within the DS flood plumes, which represent the mixing between freshwater runoff (that bring bicarbonate and sulfate anions into the lake) and lake's brine (which provides the calcium cation). Here, we report for the first time on geochemical measurements within flood plumes in the modern DS. This “natural mixing experiment” between freshwater runoff loaded with suspended solids and DS brine provides new insights regarding the carbon cycle in the lake and the hydrological and limnological factors that control aragonite precipitation.
Section snippets
The Dead Sea Ca-chloride brine
The Dead Sea is located in the tectonic depression of the Dead Sea Basin (DSB) (Garfunkel, 1997; Neev and Emery, 1967) at the lowest elevation on the continents (currently its surface level stand at 433 m below mean sea level-mbmsl). The lake comprises a terminal hypersaline water-body with a Ca-chloride brine composition, where the molar ratios of Na+/Cl−<1 and (Starinsky, 1974). The DS brine evolved from evaporated seawater, which intruded into the DSB during the late
Sampling and methods
Samples for the present study were collected during flood events that occurred in the years 2014–2015. Overall seven flood plumes were sampled, three in the outlet of Wadi Darga, three in the outlet of Wadi Arugot and one in the outlet of Wadi David (Fig. 1). In each flood plume 5–7 surface water samples were collected from different locations within the flood plume (Fig. 1). Sampling was conducted using a 5 m inflatable motorboat (Suppl. Fig. 2). Sampling started a few hours after the
Major elements
Results are listed in Table 1. As a first evaluation of the geochemical processes that occur within the flood plumes, major element concentrations were plotted against the measured Cl− concentrations (as a measure of the mixing degree between DS brine and floodwater) together with modern Dead Sea composition measured during 2013 in an open Dead Sea profile (Golan et al., 2016) that provided the “non-flood” conditions. Open DS brine represents better than near shore brine the “non-flood” water
Summary
This study focuses on the behavior of carbonate and borate system in flood plumes that spread over the modern Dead Sea. The major findings are:
- a)
Na+, Mg2+, K+ and Cl− behave conservatively in the plume meaning that their concentrations are controlled by the degree of mixing between DS brine and flood freshwaters.
- b)
Ca2+, Sr2+ and boron are preferentially adsorbed on the flood suspended solids.
- c)
Adsorption of boron on the flood suspended solids reduced the borate alkalinity and enabled the bicarbonate
Acknowledgments
The authors wish to thank the laboratory technicians of the Geological Survey of Israel (GSI) for their help in the sample preparation and analysis. Jake Ben Zaken and Cohav Levi from “Salty landscapes” are thanked for their help in the field work. Rotem Golan in thanked for numerous fruitful discussions. The study was supported by the Israel Science Foundation (ISF) grant 1093/10 to RB and by the Dead Sea Deep Drill Center of Excellence (COE) of the Israel Science Foundation (ISF) grant 1736/11
References (77)
- et al.
Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’thermometry
Geochim. Cosmochim. Acta
(2008) - et al.
Changes in the thermo-haline structure of the Dead Sea: 1979–1984
Earth Planet. Sci. Lett.
(1987) - et al.
Dynamics of the carbon dioxide system in the Dead Sea
Geochem. Cosmochim. Acta
(2001) Southward migration of rain tracks during the last glacial, revealed by salinity gradient in Lake Lisan (Dead Sea rift)
Quat. Sci. Rev.
(2004)10Be dating of Neogene halite
Geochem. Cosmochim. Acta
(2013)Beryllium isotopes as tracers of lake lisan (last glacial Dead Sea) hydrology and the Laschamp geomagnetic excursion
Earth Planet. Sci. Lett.
(2014)Varves of the Dead Sea sedimentary record
Quat. Sci. Rev.
(2019)The climatic and physiographic controls of the eastern Mediterranean over the late Pleistocene climates in the southern Levant and its neighboring deserts
Glob. Planet. Chang.
(2008)- et al.
The Sahara-East Mediterranean dust and climate connection revealed by strontium and uranium isotopes in a Jerusalem speleothem
Earth Planet. Sci. Lett.
(2004) - et al.
Controls on the pH of hyper-saline lakes–A lesson from the Dead Sea
Earth Planet. Sci. Lett.
(2016)