Elsevier

Quaternary Science Reviews

Volume 221, 1 October 2019, 105876
Quaternary Science Reviews

Constraints on aragonite precipitation in the Dead Sea from geochemical measurements of flood plumes

https://doi.org/10.1016/j.quascirev.2019.105876Get rights and content

Highlights

  • The chemical composition of flood plumes where the mixing between runoff and Dead Sea brine occurs was measured.

  • Dissolution of calcite dust blown from the Sahara is the source of bicarbonate which is required for aragonite precipitation.

  • Apart from high dust fluxes, aragonite precipitation requires a water balance that enables a stratified lake configuration to develop.

  • When the epilimnion borate alkalinity is low enough, bicarbonate entering via runoff can react with the brines’ Ca2+ and precipitate aragonite.

Abstract

The laminated sequences of the Holocene Dead Sea (DS) and its late Pleistocene precursor Lake Lisan comprise primary aragonite and fine detritus that record the hydro-climate conditions of the late Quaternary Levant. Several studies suggested that the primary aragonite precipitated due to mixing between runoff that brought bicarbonate to the lake and the lake's Ca-chloride brine. However, the factors controlling the aragonite precipitation were not robustly established. Here, we addressed this issue by measuring the chemical composition (pH, Na+, K+, Ca2+, Mg2+, Sr2+, Cl, Br, B, alkalinity) of flood plumes where the mixing occurs. The results indicate that: (a) Na+, Mg2+, K+ and Cl are conservative during the floodwater-brine mixing whereas Ca2+ and Sr2+ adsorb on flood's suspended sediments; (b) Boron (an important alkalinity species in the DS) adsorption on flood's suspended load enabled the bicarbonate that entered the lake via runoff to react with the Ca2+ thus facilitating aragonite precipitation (c) Dissolution of calcite dust blown from the Sahara during winter storm is the source of bicarbonate which is required for aragonite precipitation. These observations explain the occurrence of aragonite laminae both during the wet last glacial period and during the dry last 3000yr. Although the water input during these two periods was completely different, they both were characterized by high dust fluxes and a stratified lake configuration in which the boron concentrations in the epilimnion were low enough to enable the bicarbonate that entered the lake via runoff to react with the lake brine Ca2+ and precipitate aragonite.

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 Ca2+SO42+HCO3>1 (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

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