Limnol. Oceanogr., 34(7), 1989, 1214-1223 0 1989, by the American Society of Limnology and Oceanography, Inc.

The geochemical evolution of halite structures in hypersaline lakes: The Dead Sea, Israel

Jiwchar Ganor and Amitai Katz Department of Geology, The Hebrew University of Jerusalem, 9 1904 Israel

Abstract

Depressions in the floor of the southern basin of the Dead Sea contain dense (1.26-l .29 g cm-3) brines which formed by evaporation at the lake’s surface. Absorbance of solar radiation heats these brines by about 2.4”C d-l, as long as the density gradient of the lake prevents overturn. As a result, bottom halite is dissolved in the warmer brines at about 0.4 g NaCl liter--’ ‘C-I. Continuous heating of the brine of these holes and cooling of the overlying liquids at night causes frequent overturns of the brine column, followed by mixing and cooling of the NaCl-rich brine. The extra load of halite is precipitated near the holes in the form of halite cones and mushroomlike structures. Whereas the initial NaCl supply to the growing halite structures comes from the floor of the lake, the later growth stage is supported by NaCl supplied from the upper (evaporating) brine layer.

Crude mass and heat balances show that the proposed model may well explain the occurrence and distribution of halite islands in the Dead Sea. The model may be applicable to any shallow enough (a few meters) hypcrsaline lake that is saturated with respect to halite.

The Dead Sea is composed of a Ca-chlo- ridic brine, the evolution of which has been proposed by Starinsky (1974). The physi- ography, limnology, and sedimentological regime of the lake have been studied and described by Neev and Emery (1967) among many others. The Dead Sea is comprised of two parts: a northern basin, 300 m deep, which forms the bulk of the lake’s volume and area, and a southern, shallow (mostly 2-4 m deep) basin (Fig. 1).

Dwindling precipitation since about 1930 (Klein 1985) and diversion of water from the Dead Sea drainage system for agricul- tural needs resulted in the continuous low- ering of the lake level at a rate of -50 cm yr-l . Finally, toward the end of the 1’960s the two basins became separated. Since then, the brine level in the southern basin is main- tained artificially. Evaporation of the brine results in enrichment of most of its dis- solved salts with respect to their concentra- tions in the northern basin. Exceptions are Na+ and S042- which are precipitated from

-- Acknowledgments

We are indebted to D. Anati (Weizman Institute of Science), N. Paldor, and I. Gavrieli (Hebrew Univer- sity) for their remarks and discussions of various as- pects of this paper. The careful and thorough reviews by A, Lerman (Northwestern University) and an anon- ymous reviewer significantly contributed to the dis- cussion of our results. Technical assistance in the lab- oratory and in the field was provided by E. Kasher, S. Meiri, M. Spitzer, and C. Kantor.

the southern basin water as halite and gyp- sum or anhydrite.

Past hahte sedimentation in the southern basin is represented by regularly deposited, horizontal salt strata. In recent years, how- ever, crystallization of prominent discor- dant halite bodies, shaped as cones and mushrooms, has replaced or supplemented the older ‘“normal” deposition mode. Con- sequently, the southern basin is presently being filled with a multitude of odd-looking halite islets. Occasionally these structures join to form long halite bars, which may reach several kilometers long. Yet, protrud- ing halite bodies are not homogeneously distributed throughout the basin. Some areas contain them in great abundance and others are almost island-free (Fig. I).

The dramatic change in the sedimenta- tion regime in the southern basin is intrigu- ing and presents the following questions: What is the cause for the inhomogeneous sedimentation rate in the basin? What is the mechanism initiating and supporting dis- cordant crystallization of NaCl? What was the reason for the particular timing of the change in the manner of halite crystalliza- tion?

Several suggestions have been made to answer these questions. Artificially intro- duced objects, such as stones, poles, and wires were proposed as crystallization foci for halite. Charrach (1986) called for NaCl- rich seepage through the floor of the basin

1214

Dead Sea halite structures 1215

which, upon admixture with the Mg-Ca-Cl brine of the pond, would salt-out the excess dissolved halite. The long halite bars have been explained in a similar way, as a salt- out product along tectonic fissures in the bottom of the lake that were activated dur- ing a recent (1979) earthquake. The present study is intended to re-examine the afore- mentioned problems and suggestions, to test the different solutions in the field and in the laboratory, and to evaluate the origin of these halite bodies.

Methods of study Fieldwork included mapping the halite

structures and sampling the brines and ha- lite bodies. In addition, long-range (1 week) temperature profiles were taken at short in- tervals (every 10 min) by a computerized data-logging station (Campbell Sci. Instr. model 2 1 x) at one site to determine the tem- poral temperature-depth relationships in the lake. Mapping was carried out with stan- dard geological methods. Brine sampling was done from a small boat. The brines were collected along vertical profiles by means of an all-plastic peristaltic pump. Tempera- tures were measured in situ with a Fenwal Electronics model UUM3JI thermistor, with an accuracy of 4 0.5”C and a precision of &O.O5”C. The density of each brine sample was measured on-board by means of a Paar model DMA35 resonating density meter [precision = t-1 (r, where c = (p - 1) x l,OOO] and is presented after correction to in situ temperature.

Na, K, and Li were analyzed by atomic absorption spcctrometry and Ca, Mg, Sr, B, and S042- by ICP spectrometry. A Perkin- Elmer AAWCP model 6000 modular sys- tem was used for both spectrometric pro- cedures. Cl was determined by ion chro- matography with standard HPLC equipment and conductivity detection. A Vydac 302IC4.6 column was applied for the analysis. The electrical charge balance of the brine analyses was kept within +2%.

The dependence of the solubility of halite on the temperature of the brine was mea- sured as follows: 200-ml brine samples were equilibrated with halite in a shaking ther- mostatic bath at 16°C. After equilibration, the brines were sampled and diluted

Island distribution

Dense

Fig 1. Distribution map of halite structures in southern Dead Sea (based on July 1984 photos).

the

(x 1,000) for analyses of Na and K and the bath temperature was increased by 4°C. The process was repeated every few days until a temperature of 40°C was reached.

Salinity-density-temperature relation- ships in the brines were examined in the laboratory. Five identical, 400-ml subsam- ples were prepared from a larger field sam- ple. These were then brought to different concentrations by evaporation or dilution with distilled water. The samples were equilibrated in a shaking thermostatic bath at 2 1.5”C. Precipitated halite was separated, the filtrates were analyzed for Cl-, and their density was measured with a 56-ml pyc- nometer at a precision of +0.2 6. The pro- cedure was repeated at 1°C intervals for the temperature range of 25”-35°C.

Results Salt structures-The floor of the southern

basin of the Dead Sea exhibits two main types of structure-halite bodies and depressions.

The salt bodies show two typical mor- phologies: cone-shaped or cylindrical islets and mushroomlike islets with a conical or a cylindrical foot and an elliptic or circular

1216 Ganor and Kalz

Summer /eve/ ee- -p

- - ~..- -.- __ - - - - ________-_- -

-- - _ BRlNEL __ ____. -_ _ E

m -__- = 1.24 _ 1.27 gcms3 -IIlL --__. _. A

- -- - -- -

-- -_ -.--

Large hole 9 -1.290 g cmo3

Fig. 2. Schematic cross section through various salt bodies (not to scale). Denser brine fills large and small depressions on the bottom.

cap. The tops are typically concave, their centers being lower than their margins (Fig. 2). All the halite mushrooms display con- centric ridges on their tops. Cap diameters vary between a few meters and ~20 m.

During winter, the brine covers all the salt structures to a depth of several tens of cen- timeters, but in summer the caps of the mushrooms and the tops of some of the halite cones are exposed.

The floor of the basin is very irregular and contains many depressions such as holes and channels. The depth of the holes varies

o----o DENSITY (gcni3)

0 I 275 I 280 1.285 1.290

Fig. 3. Depth-temperature-density relationship in the brine within and above a large hole. Note the steep density and temperature gradients near the edge of the hole.

between less than a centimeter and >3 m. For the larger structures, a depth of about 2 m below the surface of the floor is very common. The channels may reach several kilometers in length, 50-200 cm in width, and several meters in depth. They are most- ly linear, but a few loop backward and may even cut themselves (Fig. 1). The channels are always bounded by halite islets, whereas a less striking association exists between the holes and the halite bodies.

Brines -The Dead Sea waters from the northern basin (-235 a) which enter the southern basin (260-290 C, depending on season and depth) bring about stratification of the briue column. This is further en- hanced by evaporation from the surface lay- er. Occasionally, the stratification is partly or completely obliterated by waves. During winter, only small brine volumes flow into the southern basin, evaporation rate is rel- atively low, and wind storms are common. Hence, a homogeneous lake is a winter phe- nomenon, and a sharp stratification marks most of summer.

Throughout the year, most of the depres- sions contain brines that are denser than the main water body (Fig. 3 and Table 1). Den- sity gradients may reach 1.6 (T cm- l. The enclosed brine temperature is mostly higher than that of the overlying water (Fig. 3). The temperature difference may become as high

Dead Sea halite structures 1217

Table 1. Analyses of brines from the southern Dead Sea, November 1986. Concentrations of Na, K, Mg, Ca, and Cl in g kg-’ solution; concentrations of Sr, Li, B, and SOd2- in mg kg-’ solution.

Sample Depth

(cm)

Temp (“0

Density (g cm-3) Na K Mg Ca Sr Li B Cl sod*-

357 420 33.8 1.290 7.77 9.52 55.0 21.4 409 17.6 69 218 77 358 400 34.0 1.290 7.73 9.45 54.3 20.6 406 17.7 68 214 73 359 375 34.3 1.289 7.83 9.57 54.6 21.2 402 18.0 64 217 78 360 350 34.6 1.289 7.87 9.55 53.2 21.0 397 18.0 65 215 84 361 325 34.9 1.289 7.92 9.49 55.2 21.0 402 18.1 65 216 84 362 300 35.0 1.289 8.01 9.58 54.9 21.2 411 17.7 65 219 84 363 275 35.0 1.289 7.94 9.53 54.9 22.0 406 18.2 68 215 95 364 250 35.0 1.289 7.96 9.52 53.3 20.5 392 17.9 69 219 96 365 225 34.9 1.289 7.97 9.52 53.4 20.3 396 18.0 70 215 89 366 220 34.2 1.289 8.04 9.56 56.0 21.1 419 18.0 66 216 99 367 215 34.1 1.288 8.27 9.48 53.3 20.8 402 17.8 66 214 105 368 210 33.2 1.288 8.33 9.44 55.2 21.3 412 17.7 67 215 98 369 205 32.0 1.287 8.44 9.39 53.4 20.3 390 17.8 66 211 103 370 200 30.1 1.287 8.98 9.14 52.3 20.4 390 17.6 64 211 104 371 195 29.1 1.287 10.06 9.15 53.0 20.7 400 17.4 68 208 125 372 190 28.9 1.276 11.34 8.75 51.1 20.3 379 16.9 61 208 171 373 175 28.8 1.276 11.45 8.96 53.0 20.2 394 16.9 61 203 189 374 150 28.8 1.276 11.15 8.91 51.0 20.0 386 16.9 61 201 183 375 100 28.8 1.276 11.20 8.40 51.0 20.1 384 16.9 61 203 186 376 50 28.8 1.276 11.02 8.63 49.6 19.7 374 16.9 63 202 192 377 0 28.8 1.276 11.08 8.55 51.0 19.9 371 16.8 63 204 195

as 12°C. On rare occasions, the temperature within the holes is equal to or even lower than that of its surrounding brine.

Large (a few meters) depressions typically show a sharp temperature gradient when ap- proached from above, followed downward by a moderate temperature decline to the bottom of the hole. The temperature max- imum is usually some lo-20 cm below the edge of the hole (Fig. 3).

The temperature gradient in the depres- sions is time-dependent (Fig. 4). That this must be so follows from the fact that the only heat source available to the brine in the depressions is solar radiation, which prevails during the day; at night, and most significantly between midnight and sunrise, appreciable cooling of the surface layer takes place. Occasional heating of the water body above the depressions at night (Fig. 4) thus reflects convective heat transfer from be- low. On the other hand, abrupt decreases in temperature reflect plumes of cool water de- scending from higher levels in the brine col- umn.

The concentrations and ionic ratios given in Tables 1 and 2 and in Fig. 5 show that Ca, Mg, K, Cl, Sr, B, and Li in the depres- sions are conservatively enriched by some

10% and that Na+ and SOd2- are depleted relative to the overlying main brine.

The following observations regarding the brines have been made in the laboratory. The solubility of halite in the brine increases with temperature by some 0.4 g liter-l ‘C-l (Fig. 6). The density-temperature relation-

HOURS ho, la,00 2O;O 22pO OOPO O’:“/

I 371.7

I I I I 1.8 19 20 21

TIME 1 day)

Fig. 4. Time-depth-temperature relationship in the brine above a large hole at night. At a depth of 170 cm, gradual cooling takes place. Sinking of cool brine from the surface is indicated by steep thermal steps at depths of 86 and 140 cm about 1900 and 2 100 hours. At the same time the upper 36-cm-depth layer is warmed by upwelling warm water (Fig. 4 is an enlarged detail of Fig. 9).

1218 Ganor and Katz

‘s P

d

t p"

400 d b

I-_--f - --- ..-- -L 4 6W/Mg)eq. x100 -

Fig. 5. Conservative and nonconservative behav- ior of some dissolved components in the brine (see text).

Table 2. Equivalent ratios of selected solutes in the southern Dead Sea. -- ---- --

Depth K:Mg L.i : Ca Na : Cl SO,*- : Ca Sample (cm) Mg:Ca (x100) (~1,000) (x100) (~1,000)

357 420 4.24 5.38 2.37 5.50 1.50 358 400 4.34 5.42 2.48 5.56 1.49 359 375 4.25 5.46 2.46 5.56 1.55 360 350 4.18 5.58 2.47 5.64 1.68 361 325 4.33 5.35 2.49 5.65 1.66 362 300 4.28 5.42 2.42 5.64 1.66 363 275 4.12 5.40 2.39 5.68 1.81 364 250 4.29 5.56 2.53 5.61 1.95 365 225 4.34 5.54 2.56 5.72 1.84 366 220 4.37 5.31 2.46 5.74 1.96 367 215 4.23 5.53 2.47 5.96 2.12 368 210 4.27 5.32 2.40 5.96 1.92 369 205 4.33 5.47 2.53 6.16 2.12 370 200 4.23 5.43 2.49 6.57 2.14 37 1 195 4.22 5.38 2.42 7.45 2.52 372 190 4.15 5.32 2.41 8.40 3.53 373 175 4.3 1 5.26 2.41 8.71 3.89 374 150 4.22 5.43 2.44 8.54 3.83 375 100 4.18 5.12 2.43 8.52 3.86 376 50 4.15 5.41 2.47 8.42 4.07 377 0 4.23 5.21 2.43 8.36 4.08

TEMPERATURE 1°C)

Fig. 6. The dependence of halite solubility on tem- peraturc in southern Dead Sea brine (laboratory data).

ship in the brine is characterized by a -0.45 g “C- l slope (Fig. 7). The density of the brine increases with increased chlorinity by 0.87 Q (g Cl)- ’ liter- 1 (Fig. 8).

1.31

1.30

m ‘E v

,“1.29

z v,

!z 1.28

1.27

1.26;

-~“-o~o~o 293 --o---Lo

-x-Ax -x--x,x

-x-x -X-::;

Ei 30 35 TEMPERATURE (“CI

Fig. 7. The relationship between density and tem- perature at chlorinities between 250 and 293 g Cl li- ter- I.

Dead Sea halite structures 1219

1.31 -

l.30-

z v,

zi 0 1.28 -

250 260 270 280 290

Cl (g litef’l Fig. 8. The relationship between density and chlo-

rinity at 25”, 30”, and 35°C.

Discussion The origin of the brine in the depres-

sions-All the brines sampled in the south- ern basin, regardless of their (variable) total salinity or sampling site, have similar ionic ratios (except the ones including Na+ and SOd2-). Therefore, a common origin may be assigned to these solutions. The salinity and density differences between the brines are a consequence of their evaporative con- centration at the surface of the lake. The depletion in Na+ and SOd2- in brines from depressions is brought about by NaCl and CaSO, mineral crystallization. The higher temperature in the depressions is caused by absorption of solar radiation, as with solar ponds, and is stabilized by the density gra- dient. The fact that the main temperature variations in the depressions closely follow fluctuations in solar radiation (Fig. 9) rules out thermal seepage into these sites as an explanation for the observed heat-up mech- anism of the brine.

0 I 2 3 4 5

TIME (days)

Fig. 9. The temperature-time relationship in the brine. Note the superposition of secondary thermal events on the major daily heating cycle which corre- sponds to solar radiation (S.R.).

The depressions -The depressions in the southern basin floor present two questions: What process formed them in the first place? How are the depressions preserved, includ- ing their strikingly fresh and steep walls, in view of the very high halite sedimentation rate in this region (- 16 cm yr-l, Ganor 1986)?

Charrach (1986) postulated a tectoni‘c or- igin for the channels in the floor of the basin. This is unlikely because some channels are loop-shaped (Fig. 1). The general pattern and orientation of the channels suggest that they may be related to construction opera- tions; this idea is also supported by the rec- ollection of people who worked in this re- gion during the 1960s. Other nonlinear holes and depressions may simply represent normal bottom irregularities-common in the bottom of any lake.

The preservation of the negative bottom structures can be attributed to the increased solubility of halite at higher temperatures, as observed in our laboratory experiments. The solar heating of the enclosed brine may explain not only dissolution of bottom salt, but may also indicate that infalling salt crys- tals could have dissolved before reaching the sediment interface.

Vertical transport in the water column- The proposed explanation for preserving the

1220 Ganor and Katz

28

27-- ’ I I I 8 9 10 II 12

TIME (hours)

Fig, 10. Time-depth-temperature relationship in the brine above a large hole during the day. Abrupt heating of the upper brine column at 5 1- and 8 1 -cm depth is caused by convection from below of warmer brine (see text).

depressions requires a process that includes filling the depressions with dense brine, heating by solar effect, and resulting in halite dissolution and outward disposal. At the same time, the depressions must be refilled with new, cool and dense brine. That such conditions are indeed met is demonstrated by time-temperature profiles (Figs. 4 and 10). Figure 4 shows distinct cooling events around 1900 and 2 100 hours at depths of 86 and 140 cm. These toolings may be ex- plained by the fall of a cool and dense brine parcel from the surface, which arrived at the respective thermistors at the times in- dicated. During the same period, the over- lying brine at 36 cm warmed. Because the overall process took place at night, it is ob- vious that heating of the upper brine layer could have been caused only by hotter brine displaced from below, rather than by solar radiation. Figure 10 displays similar brine cooling at 168- and 232-cm depth during the day, accompanied by warming of the overlying brine at 8 l- and 5 1 -cm depths. The sudden change in the heating rate is very likely brought about by upward con- vection of hotter brine from below.

Laboratory experiments (p. 1215 and

Figs. 6-8) and field evidence (Table 1 and Fig. 3) provide the necessary data to define the stability conditions for the brine body.

A small change in the brine density can be defined by

da = (d&s) ds -t (da/dT) dT = 0.87 ds - 0.4 dT (1)

and, for an isopycnal profile (da = 0),

dT 0.87 2.2”C z-=0.4= g Cl liter-’ * (2)

Equation 2 means that the increase in density of the brine caused by a concentra- tion increase of 1 g Cl liter-l can be com- pensated by an increase in temperature of 2.2”C. However, Fig. 6 shows that the halite solubility increases with temperature by 0.4 g Cl liter- l “C-l. If we assume fast equilib- rium between the brine and halite, this val- ue can be substituted into Eq. 1 and we obtain

da = -0.05 dT. (3)

It is instructive to examine the field evi- dence in light of the last relationship. The density of the brine column increases by about 12 c units between 190- and 2 1 O-cm depth with a parallel temperature increase of 4°C. Hence, it would take an (absurd!) increase of about 240°C to destabilize this column by heating alone. Rather, it is likely that vertical brine transport is mainly in- duced by surface evaporation, which is re- sponsible for the formation of the invading cooler and denser brines described earlier (Fig. 4). These brines sink into the holes, displacing upward the older brine that was heated by solar radiation inside the depres- sion.

It remains to be seen if double-diffusive instability, winds, or both can cause the ob- served overturn of brine in the southern Dead Sea. Turner (1973) has shown that double-diffusive instability may occur in a gravitationally stable water column, whose deeper part is warm and salty, as a result of self-enhanced oscillations. The respective conditions, expressed by Rayleigh numbers, have been worked out (Turner 1973, equa- tion 8.1-6) and are given below.

Dead Sea halite structures 1221

a

a = -(wanAT a

(ap/as)as S (4)

where $@a and %?s are the temperature and salinity Rayleigh numbers of the brine, and s, T, and p designate the salinity, tempera- ture, and density of the brine.

Substituting our Dead Sea data (see above) into Eq. 4, we obtain aa = 0.04 @. The stable oscillation field lies below a line whose slope is defined by (Turner 1973, equation 8.1.9 and figure 8.2)

Slope = (Y/K + K,/K)/(Y/K + 1). (5)

In Eq. 5 V, K,, and K designate the viscosity, the salt diffusion coefficient, and the ther- mal diffusion coefficient. Because K is about? two orders of magnitude larger than K,, the slope from Eq. 5 will be smaller than 0.04 only if V/K is dO.03. Such a demand is un- reasonable for the Dead Sea brine. Hence, the Rayleigh numbers for the Dead Sea de- fine a point which lies within the stable os- cillation field, and therefore double-diffu- sive instability is ruled out as the cause of the observed convection. This conclusion is in accord with the study of Weinberger (1964) on the behavior of solar ponds.

The observed exchange of brine within the water column cannot be attributed to mixing by winds, because it represents daily phenomena that also occur in the absence of winds. This does not rule out wind as an important mixing agent of southern basin waters. Several severe storms, mainly dur- ing winter, which homogenized the brine column, have been observed during the present study. The frequency of such events is much lower and the behavior of the brines is decisively different than that described above.

The origin of the halite islets-The pro- posal that halite is salted-out near depres- sions because NaCl-saturated brines seep through the floor of the lake at these sites and mix with the southern Dead Sea water (Charrach 1986) is unlikely in view of the following evidence. None of the brines (over 500) we analyzed displayed any sign of con- tamination by imported brines. Rather, the chemical composition of all these samples could readily be explained by conservative

evaporation of northern basin water within the southern basin.

The degree of saturation of various brine mixtures with respect to halite has been tested by computer simulation. The com- position of the southern basin brine was used as one end member, and various brines sampled in drillholes and seepages along the west coast of the southern Dead Sea rep- resented the other end. The thermodynamic simulation program of Gavrieli et al. (1989) for Dead Sea brines was applied. The results of this attempt prove that most of the sim- ulated mixtures are slightly undersaturated with respect to the mineral. Moreover, hy- drological considerations limit the avail- ability of the appropriate brines to volumes far below the mass balance requirement of the halite structures.

We propose, instead, that the formation of the halite islands is a two-stage process. In stage 1, which extends from the begin- ning of growth until the islet nears the sur- face, the expansion of the salt body is es- sentially vertical. During this part of the process, the dissolved salt load is trans- ported to the growing structure from the bottom of the lake. Stage 2 is distinguished by the concentric addition of halite to the growing body at and near the water surface and consequent lateral growth of its cap. Here, the source of the NaCl added to the island is located in the surface brine layer.

A detailed description of the proposed evolution follows. Stage 1: Brine at the sur- face evaporates, sinks through less dense layers and fills available bottom depres- sions. Density stratification allows solar heating of the deeper brine by a few degrees. Heating of the depression results in under- saturation of its brine with respect to halite, followed by local dissolution of this min- eral. Dissolution may consume wall mate- rial as well as suspended halite crystals de- rived from the upper layer. Brine in the holes, which is heated during the day by solar effect, is continuously replenished by denser, cooler brine formed outside the holes by surface evaporation. The density differ- ential, supported by the heating, causes the expulsion of brine from the holes and its dispersion around them. Expelled brine mixes with overlying waters and is thereby

1222 Ganor and Katz

cooled. Consequent oversaturation with re- spect to halite is achieved and the extra load is precipitated near the depression.

Stage 2: During stage 1, the growing halite structure feeds mainly on NaCl dissolved from the bottom. Hence, this process must become attenuated as the transport distance increases. This is the onset of stage 2, when the overall halite saturation profile (the sat- uration-depth gradient over a hypothetical smooth lake bottom) starts to exert its in- fluence on the halite body. The surficial na- ture of evaporation dictates that the super- saturation increases with decreasing brine depth. Therefore, it is reasonable that the growth rate of halite islands accelerates upon their approach to the surface of the lake, which is clearly reflected by the lateral (mushroomlike) expansion of the halite is- land caps (Katz et al. 1988).

Also, once they reach the surface, the ha- lite bodies start to behave as sediment traps for floating halite crystals by attenuating the flow of brine in their vicinity. Although am- ple masses of floating halite crystals are vis- ible during summer, we believe that in situ crystallization of this mineral, rather than its trapping, is the predominant process during stage 2.

Semiquantitative model calculations --It is of interest to test our model in view of the available data and to check if it can account at least semiquantitatively for the observed structural relations between the halite islands and their precursors, the bot- tom depressions. The average annual solar radiation reaching the region of the Dead Sea (lo) comes to 250 J (m2 s-‘)-I; at a brine depth of 2 m it can be calculated (applying an empirical equation developed for solar ponds in this area by D. Anati (pers. comm.) to be

Z, = Z,[O.69 exp(-0.62) + 0.31 exp(-4.6z)] (6)

where Z, is the solar radiation at a brine depth of z (m).

At the basin floor depth of 2 m, then,

t 2m = 250 x 0.21 = 50 J (m2 s-l)-‘.(7)

The heat capacity of the Dead Sea water is 3.6 x lo6 J (m3 OC-l)-l (D. Anati pers. comm.). Assuming a similar value for the heat capacity of the southern basin brine

and neglecting heat losses to the bottom, we estimate the heating rate of a brine column in a 50-cm-deep hole to be

dT 50 1 -dt = 3.6 x lo6 xo.5

-278 x 10-60Cs-1 . (8)

or 2.4”C d-l. Our laboratory experiments indicate an

increase in the solubility of halite in the brine with temperature of about 0.4 g liter- * “C- l, which, c ombined with the aforemen- tioned heating rate, yields a daily increase in halite content of 2.4 x 0.4 = 0.96 g NaCl liter-l.

A “typical” hole, 0.5 m deep and 1 m2 in area, has a volume of 500 liters. Its halite dissolution potential is 0.96 x 500 = 480 g d-l or 175 kg yr- l. The halite-floored southern basin has been isolated from the main lake for about 17 yr. Hence some 175 x 17= 2,975 kg of halite could have been processed through this hole. Dividing this mass by the bulk density of the halite islands (1,900 kg m-3), we find that the volume of the islet core which may build nearby is - 1.5 m3, which is very common in the lake. This estimate is obviously a maximum be- cause a 100% yield has been assumed for the overall process. However, our model does not require a 1 : 1 islet-hole relation- ship nor is such the rule in the field. In fact, virtually all the areas in which halite bodies occur are perforated by a large number of small irregular depressions, all of which may act as solar dissolution holes to support the fewer larger holes in NaCl dissolution and halite island construction.

Conclusions The preservation of bottom depressions

on the halite-covered floor of the southern basin of the Dead Sea under an annual sedi- mentation rate of some 15-20 cm of halite is closely related to the accumulation of ha- lite islands around and in the vicinity of these structures. The relation stems from the following process. Initially, dense brine, which is forming via evaporation of water from the surface, sinks into pits and holes on the bottom. Consequently, a solar pond elect sets in, and the brines in these holes are heated. As a result, the brines become

Dead Sea halite structures 1223

undersaturated with respect to halite and dissolve this mineral from the bottom and walls of the depressions that contain them. The warm, NaCl-enriched brine is then ex- pelled from the holes by newer, cooler, fresh brine. Upon emergence and dispersion around the holes, the brine cools, and pre- cipitates its extra load of halite in the form of embryonic halite islets. These, in turn, grow slowly upward until they approach the oversaturated layer in the upper brine col- umn. It is within this zone that rapid lateral growth of the islet tops begins, resulting in their typical “mushroom” shape.

Calculations involving the salinity and temperature gradients, as well as the vis- cosities of the Dead Sea brines, rule out dou- ble-diffusive instability as an important fac- tor in the observed intracolumn mixing of the brines. Neither the frequency of wind storms nor their mixing effects are compat- ible with the mixing behavior measured in the present study.

References CHARRACH, J. 1986. Genesis of halite islands in the

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Submitted: 2.5 March 1988 Accepted: 31 May 1989 Revised: 1 August 1989 i