Elsevier

Earth and Planetary Science Letters

Volume 400, 15 August 2014, Pages 94-101
Earth and Planetary Science Letters

Long-term freshening of the Dead Sea brine revealed by porewater Cl and δO18 in ICDP Dead Sea deep-drill

https://doi.org/10.1016/j.epsl.2014.03.019Get rights and content

Highlights

  • First attempt to establish the limnological history of the deepest Dead Sea brine.

  • Cl and δO18 were measured in pore-fluids extracted from Dead Sea ICDP deep cores.

  • Gradual decrease in Cl reflects brine freshening at 31–17 ka BP.

  • Brine-freshening reflects continuous turbulent mixing across the hypolimnion.

  • Gypsum saturation led to massive gypsum deposition at the end of this period.

Abstract

The geological evolution of the unique Dead Sea Ca-chloride brine has been the focus of many research efforts for several decades. These studies relied on the information obtained from sedimentary exposures of the marginal terraces of the modern Dead Sea, mostly documenting the history of the surface lake brine during its high stands periods. The present study is the first attempt to establish the history of the deepest part of the lake by direct measurements of the chemical and isotopic composition of pore-fluids that were extracted from cores drilled during 2011 by ICDP in the deep basin of the Dead Sea at water depth of 300 m. The vertical profiles of chloride (Cl) and oxygen isotopes (δO18) in pore brines reveal a substantial decrease in the salinity of the hyper-saline lake during the last glacial and particularly during MIS2 (3117 kaBP). The Cl concentration of the deep brine in the lake decreased gradually, reaching a minimum of less than 2/3 of its present value while the δO18 increased to maximum of 7 (3‰ higher than today). The low Cl indicates significant dilution of the bottom water mass (hypolimnion) of Lake Lisan (the last glacial predecessor of the modern Dead Sea) during its highest stand period. Beforehand, during the interglacial and later during the post-glacial and the Holocene the Cl concentrations and δO18 values were similar to those of the modern Dead Sea. The slow dilution of the deep Ca-chloride brine was caused probably by continuous turbulent mixing of the hypolimnion with the less saline high δO18 epilimnetic brine, across the epilimnion/hypolimnion interface (EHI). While the increase in δO18 during the salinity decrease of Lake Lisan is a result of “normal” evaporation of the less saline epilimnetic brine, the post-glacial δO18 decrease (contemporaneous with salinity increase) is attributed to the “reversed” behavior of δO18 during evaporation of high salinity brine. During the long freshening period the hypolimnion was enriched with dissolved sulfate supplied by the freshwater and transported by the turbulent mixing across the EHI until reaching gypsum saturation that commenced massive gypsum deposition at the end of this period, when full overturn took place.

Introduction

The lakes that filled the tectonic depression of the Dead Sea basin during the late Quaternary comprise mixtures of Ca-chloride brines that evolved from the late Neogene Sedom Lagoon and freshwater from the lake watershed (e.g. Neev and Emery, 1967, Stein, 2001). The Ca-chloride brine within the lake evolved further by mixing with inflowing brine springs and by deposition or dissolution of evaporitic minerals (e.g. aragonite, gypsum and halite). This, in turns reflects the prevailing limnological–hydrological conditions in the Dead Sea basin and its watershed (Stein et al., 1997; Stein, 2001; Gavrieli and Stein, 2006, Katz and Starinsky, 2009).

So far, most of the information on the evolution of the lakes was derived from the mineralogical and geochemical properties of the sediments recovered from exposures of the marginal terraces or shallow drillings along the lake's shores (Fig. 1). The sediments studied in these sequences consist mainly of the primary (evaporitic) minerals, aragonite and gypsum and fine-grained detritus (Katz et al., 1977, Kolodny et al., 2005, Migowski et al., 2006; Stein et al., 1997, Stein et al., 2010; Torfstein et al., 2005, Torfstein et al., 2008; Haliva-Cohen et al., 2012). The drilling project that was conducted under the wings of ICDP (International Continental Scientific Drilling Program) recovered a detailed core from the deep part of the Dead Sea (ICDP Site A, Fig. 1, Stein et al., 2011) providing a unique opportunity to study the solution history of the deep parts of the lake (i.e. the hypolimnion) by direct analyses of the chemical and isotopic composition of porewater brines.

This paper describes the results of chloride (Cl) and oxygen isotopes (δO18) analyses of pore fluids that were extracted from the deep central lake core (Site A, Fig. 1) during the drilling operation. The objective of the study was to reconstruct the limnological and geochemical properties of the deepest part of the lake and establish the mechanism that controlled lake configuration and salinity during the studied time interval. The paper focuses on the time interval of past 70 ka BP during which the tectonic depression of the Dead Sea basin was occupied by the last glacial Lake Lisan and the Holocene Dead Sea (e.g. Haase-Schramm et al., 2004, Migowski et al., 2006, Stein et al., 2010).

Section snippets

Geological settings

During the Neogene–Quaternary times a series of water-bodies filled the tectonic depressions along the DS rift. The history of these water-bodies commenced with the ingression of the late Neogene Sedom lagoon and continued after the disconnection of the lagoon from the open sea with the development of lakes succession that included the hypersaline mid to late Pleistocene Lake Amora, the last interglacial Lake Samra, the glacial Lake Lisan and the Holocene Dead Sea and the freshwater lakes

Sampling

During the ICDP–Dead Sea drilling project sediments cores were recovered from the center of the lake (Site A, Fig. 1) at water depth of 300 m. The drilling reached 460 m below the lake floor and the recovered cores were preliminary described on board.

The drilled-cores were sectioned into 1.5 m slices and kept in a dark refrigerated repository. Preliminary description and analyses of the sediments in all sections were conducted during 2012 by all members of the Dead Sea Deep Core consortium in

General description of the studied sequence in the core

This paper focuses on the last glacial and post-glacial period, the upper 190 m out of the total 460 m drilled core (a simplified stratigraphic description provided in Fig. 2). The sediments composing this part of the core are mainly detrital silt-size calcite and quartz grains, primary aragonite gypsum and halites (Stein et al., 2011, Neubereger et al., 2012). The sediments comprise several sedimentary facies (e.g. Machlus et al., 2000): (1) Sequences of laminated of aragonite and

Limnological configuration of Lake Lisan

The studies in the Dead Sea area on the exposed onshore terraces show that during most of the last glacial period (7014 kaBP), when Lake Lisan reached high stand of 200±40 mbmsl (Bartov et al., 2003, Torfstein et al., 2013) the lake was stratified, depositing the aad facies, the annual and seasonal laminated sequences of primary aragonite and silty-detritus material (Katz et al., 1977, Stein et al., 1997). The primary aragonite was deposited from the epilimnion that was supplied with

Summary

The pore brine data presented here provides the first direct measurement of the composition of the deep brine (hypolimnion) of the Dead Sea lakes during the late Pleistocene and Holocene periods (70 ka). Specifically, pore brine profiles suggest that during the Lisan period, Cl concentration of the hypolimnion decreased down to at least 70% of the modern Dead Sea value and δO18 value was 7 (almost 3‰ higher than modern).

The freshening of the last glacial Lake Lisan was accompanied by δO18

Acknowledgments

This study results from the ICDP Dead Sea drilling project. We thank all who participated in the drilling operations, opening and descriptions of the drilled cores. The scientific study is supported by an Excellence Center grant of the Israel Science Foundation (ISF) grant 1736/11 to B.L. (The Dead Sea Deep Drilling Project: The Dead Sea as a Global Late Quaternary Paleo-Environmental, Tectonic, and Seismological Archive). We thank Joris Gieskes for his thorough review that helped in improving

References (34)

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