Archive for October, 2013

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The enigma of the Dead Sea salts: Scientific questions and practical implications

October 4, 2013

The Dead Sea (DS) is a terminal lake in an arid region of the world. Thus, it is quite expected for it to accumulate salts as solutes that enter the lake have no way of leaving except by mineral (evaporite) precipitation. Most terminal lakes accumulate salts through time because solutes are continuously being delivered and none are removed in natural conditions. As lake deposits in the area go back as many as 30 million years, enough time has passed for significant salt accumulation to have occurred. However, a more detailed look at the nature of the solutes accumulated in the DS waters shows that this is not the entire story, as these solutes do not represent the weathering products of the geological materials present in the drainage basin. The high Mg++, K+ and Br as well as the anomalously low SO= concentrations are difficult to explain, as are some of the isotopic signatures of other components. Moreover, the evaporation of seawater does not lead to the formation of such as mixture either.

Various attempts have been made to explain the anomalous chemical nature of the DS. Most common ones rely on pure exogenic processes, while others employ a mixture of exogenic and endogenic processes. In this presentation, I will explore the various sets of rationales put forward to support each argument, and will also touch on why this is significant for the chemical industries based on the DS waters and how the proposed Red-Dead Canal may change the composition of this water.

General water for the evolution of saline water

Water is a powerful solvent, and so the accumulation of solutes within it should come as no surprise. Generally, as long as inorganic solutes are in supply, they will accumulate until the water becomes supersaturated with respect to an appropriate mineral phase.

It is often viewed that the hydrological cycle starts with rainwater, but by far the largest reservoir of water on the surface of the earth is seawater. Thus, the mineral evaporite precipitate sequence of seawater is well known: calcite (or aragonite); gypsum (or anhydrite), halite, sylvite and carnallite. Consequently, the chemistry of the residual waters reflects both concentration due to progressive removal of water as well as the removal of the solutes that make up the evaporite minerals.

The chemical evolution of seawater through evaporation thus follows a well-known progression which conforms to field observations and theoretical thermodynamic equilibrium calculations. Due to the progressive removal of calcium (in calcite and gypsum) and sulphate (in anhydrite), the residual water becomes a magnesium chloride water which is poor in sulphate and rich in chloride.

Dead Sea water characteristics

The waters of the Dead Sea don’t reflect the composition of evaporated marine water. At the same time, they don’t reflect the composition of evaporated fresh water as would be expected by thermodynamic modelling of river and stream waters feeding into the lake. Deviation between the expected (based on the assumption of marine starting chemistry) and the actual chemistry if Dead Sea water can be summarized as follows:

1-      The Ca/Mg ratio in Dead Sea water is higher than would be expected from evaporating sea water.

2-      The high Ca/Cl ratio

3-      The high Br/Cl ratio

4-      The low Na/Cl ratio

5-      The low SO4/Cl ratio

In addition, other chemical anomalies include

6-      B/Li ratios

7-      B isotope ratios significantly different than seawater.

Thus, an explanation is required to understand how Dead Sea waters evolved to their present state.

Current models

The model advocated by Katz and Starinsky (2009) involved extensive marine water modification. This starts with massive dolomitization of limestones in the area. The effect of this is to remove magnesium from solution and replace it with calcium. The second stage involves the reduction of sulfate in an anaerobic environment. The removal of sulfate through gypsum deposition is not enough to explain the low sulfate and low sulfate to chloride ratio. Numerous episodes of open and closed lagoon episodes acted to concentrate chloride and remove sodium according to this scenario.

An alternative model involves the interaction of non-marine brines with surface water to achieve the current brine chemistry of the DS. Different hypotheses have been advocated for the origin of these brines. Hardie (1990) offers an endogenic model for calcium chloride potash-rich brines, and reviews a number of cases from other areas of the worth where rift basins tend to accumulate such deposits. On the other hand, Rosenthal et al. (2006) view the brines in the DS area as being mostly of exogenic origin and of being compatible with the sedimentary environments that have prevailed in the area.

In trying to reach a conclusion as to the source of the DS salts in view of the various hypotheses that have been presented, Occam’s razor is probably the most useful tool to reach the best result. Occam’s razor stipulates that the hypothesis with the fewest assumptions is probably the closest one to the truth.

The marine water evaporation model of Katz and Starinsky (2009) requires that we start with marine water a priori. There is no empirical basis for this assumption. Moreover, the various dolomitization and sulfate reduction events are difficult to prove. Dolomite is present in the limestones of the region, but there is no evidence that these are related to the evolution of the DS brines. Although gypsum is also present in the area, its thickness and volume is not sufficient to explain the massive removal from sea water that this model proposes. Other geochemical anomalies, such as chlorine isotopes and bromine contents in DS region brines (Yechieli et al., 1996). Do not conform with the marine water source for the salts. Katz and Starinsky ignore this data, as their geochemical acrobatics could not reconcile this problem with their model.

As for the source of the brines that feed into the DS, the question of their source is also problematic. As Katz and Starinsky did, Rosenthal et al. (2006) ignore inconvenient evidence to the contrary of their hypothesis that the brines are of evaporitic concentration of various surface water, some of which might be marine. While some of it may be so, it is distressing to see that this line of research tends to simply ignore evidence that the proponents don’t like.

The endogenic source of the brines is also not straightforward. The chlorine isotope content in the adjacent brines does not support this idea very well either, as it mostly reflects rainwater origin. However, 36Cl is also accumulated in limestone as well due to cosmogenic interactions, although the resulting 36Cl/Cl ratios when weathering or rock/water interaction occurs may differ according to the length of exposure. As previously mentioned, rock/water interaction with limestone is insufficient to explain the chemistry of DS water (Abu-Jaber, 1998). This leaves the question open to the various proponents of each hypothesis.

The Red Sea/Dead Sea canal

There has been much discussion on the implementation of this project as a hydroelectric and desalination project as well as a tool for reestablishing a stable level for the DS. Mixing of these waters will ultimately lead to modification of the DS water. Abu-Jaber (2004) conducted a modeling exercise which showed that rapid increases in chloride and sodium concentrations will stabilize after supersaturation with respect to halite occurs. Slight increases in magnesium contents are also expected.

Practical implications

The obvious practical implications of these issues involves the chemical industries of the Dead Sea. In particular, if there is a reservoir of solutes such as potassium or bromine which differs from the DS may be of great significance, as these waters may be easier to treat and extract than the DS water, which is rapidly declining. This decline will ultimately raise the cost of production as pumping costs rise. Moreover, if the DS water stops becoming feedwater for the DS industries, this may slow down the decline of the DS level, which is accelerated by the current extraction processes there.

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