Radium in the Dead Sea: A Possible Tracer for the Duration of Meromixis

Radium in the Dead Sea: A Possible Tracer for the Duration of MeromixisAuthor(s): M. Stiller and Y. C. ChungSource: Limnology and Oceanography, Vol. 29, No. 3 (May, 1984), pp. 574-586Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2836304 .

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Limnol. Oceanogr., 29(3), 1984, 574-586 ? 1984, by the American Society of Limnology and Oceanography, Inc.

Radium in the Dead Sea: A possible tracer for the duration of meromixis'

M. Stiller Isotope Department, The Weizmann Institute of Science, 76 100 Rehovot, Israel

Y. C. Chung Scripps Institution of Oceanography, University of California at San Diego, La Jolla 92093

Abstract

Three profiles of 226Ra in the meromictic Dead Sea measured during 1963-1978 indicate that the radium activities in the upper water mass were higher than in the lower water mass. All three profiles indicate a similar radium inventory. The Jordan inflow is not the primary source of radium to the Dead Sea. Mineral springs and submerged seepages are probably more important contrib- utors.

The age of the meromictic structure is estimated by a model which requires that the radium inventory of the lake be at a steady state, that sometime in the past the radium profile of the lake had been uniform (when either the lake was monomictic or an overturn had ended an earlier meromictic phase) and that the contemporaneous profiles of radium have been built up by inflows of radium solely into the upper water mass, while the monimolimnion is relict, isolated, and loses radium only by radioactive decay. Supporting evidence is presented suggesting that the above conditions may be fulfilled. Thus, it is possible to estimate that the Dead Sea was meromictic for about 300 years before the turnover in 1979.

From tentative balances of the Dead Sea dissolved salts it is shown that a previous overturn might have occurred at a lower lake level than in 1979.

The Dead Sea is a highly saline, terminal lake. It is physiographically divided by an east to west sill-the Lisan Straits-into a very shallow southern basin and a much deeper and larger northern basin (Fig. 1).

According to Neev and Emery (1967), the lake level was maintained at about -436 m MSL, for a long time; that is, about 35 m lower than at present and the lake volume would thus have been restricted within its northern basin. Neev and Emery found supporting evidence that the level began to rise somewhat more than 1000 B.P. and that the waters of the Dead Sea have occupied both the deep northern basin and the shallow basin since about 500 B.P. High levels of the Dead Sea, -392 m MSL, were attained during the first three decades of the present century; since then the lake level has been decreasing, reaching ca. -403 m MSL in 1979 (Klein 1965, 1981).

Occasional chemical and physical analyses since 1864, and the detailed studies of Neev and Emery (1967) during 1959-1960

l This research was partially supported by a grant from the United States-Israel Binational Science Foun- dation (BSF), Jerusalem, Israel.

revealed that the Dead Sea was meromictic. The continued lowering of the lake level, especially the drastic drop during the 1960s and 1970s, led to the isolation of the southern basin in 1976 and also caused a stepwise deepening of the mixolimnion (the upper water mass) in the northern basin (Stein- horn 1981; Steinhom et al. 1979). Finally, the stratified structure was completely eroded in 1979 (Steinhorn et al. 1979), when the deep, "fossil," lower water mass mixed with the "younger" upper water mass.

The radium activities in the Dead Sea waters are among the highest ever measured in any waters; they are two to three orders of magnitude higher than in the oceans. This suggests that some of the inflows to the Dead Sea must be rich in radium and that most of the radium brought in is not scavenged to the sediments but accumulates in the lake waters until it is removed by radioactive decay. Contrary to a typical radium profile which shows a general increase with depth, in the Dead Sea the radium activity in the upper layers is greater than in the deep layers. The surface water radium content of the Dead Sea is about 1,600 times higher than that of the open oceans; in the deep waters

574



Dead Sea meromixis and 226Ra 575

it is only 250 times that of the deep waters of the northeast Pacific, which have the highest oceanic radium activities (Chung and Craig 1980).

We attempt here to interpret the different radium activities of the Dead Sea water masses and to use them as a tracer for the duration of the last meromictic phase which ended in February 1979.

We are indebted to J. R. Gat and H. Craig for discussions and encouragement and to D. Imboden for comments on an earlier ver- sion of this manuscript. We thank K. 0. Emery and an anonymous reviewer whose comments helped us improve our presen- tation. Thanks are also due to S. Kazas and to N. Bauman for performing radium, radon, and polonium-210 measurements.

Radium in the Dead Sea Radium was first measured in Dead Sea

surface waters by Gilboa (1963) and Mazor (1962) (Table 1). A radium profile sampled in 1963, and measured by Gat and Gilboa (Fig. 2a), has been published by Lerman (1971). One of us (M.S.) also measured a radium profile from the Dead Sea in March 1977; data are included in Table 1 and plot- ted in Fig. 2b. The most detailed radium profile was measured at the central northern basin by Chung and Craig (in prep.) in Feb- ruary 1978 (Fig. 2c). The major feature common to all three profiles is that radium activities are greater in the upper layers than in the deep waters (Fig. 2, Table 2). The transition zone between the upper and deep waters as seen in the radium profiles de- scends progressively from 1963 to 1978, re- flecting the general trend of mixolimnion deepening which is also clearly indicated by other tracers (Steinhom et al. 1979).

However, in the deep waters the radium activities differ significantly from one set of measurements to another although these waters were quite isolated during the period 1963-1978. The isolation of the deep waters during this period is indicated by the lack of change in density. In 1959-1960, the mean density of the deep waters, below 100 m, was 1.2338 ? 0.0012 g cm-3 (Neev and Emery 1967) and in 1975-1978, it was 1.2334 ? 0.0001 (> 100 samples: Steinhorn 1981). These values are practically identical

190 2600

Jordan R iver

-130

120 X

NRTHERN 10 BAS Fe b 1978

Dec 1977

00 | , eW bI ?Apr 19632 Arnon

Mar 1977

90 o

-80

,LISAN STRAITS

-70

vOport~flSOUTHERN

60 /

0 5 10km J I 1 1 1

Fig. 1. Map of the Dead Sea (Steinhom 1981) with location of some sampling sites. Local coordinates shown.

and significantly larger than those for the upper layer. Although waters supplied by deep, submerged springs and by spillage of the more concentrated waters from the southern basin might have altered the isolation of the monimolimnion somewhat, they appear to have no significant effect. Earlier density measurements of the monimolimnion, in 1864 and in 1919, also provide evidence for its "fossil" nature and isolation: in 1864 the mean density of the deep waters (recalculated to 21.3?C, the temperature of the deep waters in 1975- 1978) was 1.2308 ? 0.011 g cm-3 (7 sam-



576 Stiller and Chung

Table 1. Radium measurements in the Dead Sea.

Depth 226Ra Sampling date (m) (dpm kg-') Coordinates* Remarks

25Nov 57 surface 113.8?11.1 1847/0667 Mazor 1962t 23 Mar 63 surface 121.1?4.4 1866/0674 Gilboa 1963t 23 Mar 63 surface 122.9?2.2 1850/0663

27 Mar 77 35 97.9?10 1950/1040 Measured by 29 Mar 77 77 92.4?1.5 1910/0935 Stillert 29 Mar 77 87 98.3?7.1 1918/0935 29 Mar 77 137 78.2?2.9 1918/0935

189 79.0?4.9 1927/0948 206 78.7?2.6 216 80.5?0.3 289 80.4?2.3

20 Dec 77 surface 84.2?1.3 1950/1050 220 71.7?0.8

21 Feb 78 250 80.3?2.1 1945/1100 (81.4)?

275 74.7?1.8 (72.7)?

300 76.0?3.0 (78.2)?

Mar 78 End brinell 134.6?4.3 (p24?C 1.322)

* Local coordinates indicated on Fig. 1. t Activities given in the reference as pACi-liter-' were transformed into dpm kg-'; density was about 1.21 g.cm-3. : Method of Lewis and Assaf (1977) was used. ? 222Rn (dpm*kg-'). 11 Evaporated Dead Sea water released from the evaporation ponds into the Dead Sea.

ples); in 1919 it was 1.2329 ? 0.0007 (6 samples). In contrast, surface water densities were 1.174 and 1.165 g cm-3 in 1864 and 1919 (Assaf and Nissenbaum 1977).

The assumption of an isolated monimolimnion in terms of radium exchanges will be valid if bottom sources and spillage are negligible, and if the radium activity intro-

duced into the deep waters by exchange with overlying waters is also negligible. It can be shown that if the rate of fluid exchange between the upper and deep waters was in the range 4 x 10-4 yr-', at the early stages of meromixis, to 4 x 10-5 yr-', toward the 1 960s, the rate of change in radium activity in the deep waters would depend only on

Table 2. Averaged radium activities and radium inventories in the Dead Sea.

Sampling date, Layer Avg of measured 226Ra "Normalized" "Normalized" 226Ra respective lake Thickness vol Density avg of 226Ra inventory of lake

level, and volume (m) (km3) (ge m-3) (dpm kg-') (dpm liter-') (dpm kg-') (dpm)

Apr 63, 0-40 28.81 1.21 120 145.2 130.7 -398.5 m, 40-80 22.99 1.225 114.3t 19.55x1015 149.3 km3 80-M.D.* 97.47 1.233 90t 111.0 98.0

Mar 77, 0-100 61.40 1.232 96.2 118.5 118.7 -401.54 m, 100-135 17.05 1.233 108.4t 19.49x1015 146.6 km3 135-M.D.* 68.15 1.233 79.4 97.9 98.0

Feb 78, 0-150 85.78 1.233? 115.0 141.8 115.0 -402.16 m, 150-175 11.27 1.233 106.5t 146.2 km3 175-M.D.* 49.11 1.233 97.7 120.5 98.0 19.58x1015

* Maximal depth: -726 m. t Radium activity at the deepest point was not included in the average. t Estimated by interpolation between activities of the upper and lower layers. ? Density difference between upper and deep layers in February 1978 was about 0.0001 g cm-3.




Radium-226 (dpm/kg) (dpm/kg) (dpm/kg)

80 100 120 80 100 120 80 100 120 ~~~~~I _ I I . I I

0 I0

80 " | 2 80

160 IApr 1963 ' Mar 1977 Feb 1978

1-60 I 160 a. w

240

(

240

320 (a) (c 320 . I I I I I I I I I

Fig. 2. Profiles of 226Ra in the Dead Sea: a- 1963 profile, measured by Gat and Gilboa (after Lerman 1971); b-March 1977 profile measured by Stiller; c-February 1978 profile measured by Chung and Craig (in prep.). Dotted line represents the March 1977 "normalized" profile.

the in situ decay of radium. It remains now to be shown whether in fact the fluid exchange rate did not exceed the above-men- tioned limits during most of the meromictic period. Very close to the disappearance of the meromictic structure, i.e. during 1976- 1977 and during 1978, the density gradients across the pycnocline which separated the deep waters of the Dead Sea were about 4.4 x 10-7 and 1.5 x 10-7 g-cm-3-cm-1.

Steinhom (1981) estimated that during this period some mixolimnetic waters penetrat- ed into the deep waters, representing an exchange rate (or water renewal) of about 0.007-yr-1 during 1976-1977 and of about 0.02-yr-' during 1978. In 1864 and 1919 the meromictic structure was dictated by much larger density gradients, about 115 x 10-7 g-cm-3-cm-', and in the 1960s they still were about 55 x 10-7 g.cm-3-cm-1.

Although the historical data may be subject to a larger uncertainty, it is reasonable to assume that for the period represented by these large density gradients the fluid exchange had been smaller than that in 1976- 1977 by an order of magnitude at least. Thus the radium input from upper layers due to fluid exchange is negligible.

Based on the above considerations of density stratification and lack of radium input into the monimolimnion, we believe that the differences between the three sets of measurements of radium activities in the deep waters are due to calibrations of radium standards. Because the 1978 profile measurements were made at Scripps by the regenerated radon method (Chung and Craig 1980) and were calibrated against an NBS radium standard prepared for and adopted by the global GEOSECS program, we have




Table 3. Radium measurements in the Dead Sea system.

226Ra

Coordinates* (dpm liter-1) Reference

Lake Kinneret surface waters Mar 62 1.1 Gilboa 1963

Mineral springs Ein Gedi (well), Feb 63 1875/0967 18.0 EinNoit Mar 62 1839/0676 19.5

Mar 62 20.4 Ein Boqeq Mar 62 1866/0671 6.7 HammeZohar Mar 62 1847/1640 29.3 Hamme Mazor Dec 54 1860/0920 75.5 Mazor 1962

Jan 55 17.8 Massada borehole Dec 54 1840/0860 88.8 Ein Mezad Boqeq Dec 54 1840/0677 22.2 Ein Boqeq May 55 1866/0671 <5.Ot Hamme Zohar spr. 1 Aug 53 1848/0645 71

Dec 53 55.5 Nov 54 46.6 Dec 54 37.7

Hamme Zohar spr. 2 Nov 54 1848/0645 64.4 Dec 54 40.0 Jan 55 53.3

Hamme Zohar spr. 3 Jan 55 1848/0645 46.6 Hamme Zohar spr. 4 Oct 57 1848/0638 55.5 Ein David Dec 58 1870/0977 <5.Ot

* Local coordinates indicated on Fig. 1. t Values listed as 0 in Mazor's (1962) table.

normalized the radium profiles of 1963 and 1977 (Table 2) to the 1978 profile with respect to its deep water mean value. The mean and standard deviation of 18 measurements on six duplicate deep water samples of Feb- ruary 1978 are 97.7 and 2.7 dpm kg-1. If radioactive decay was the sole removal process, then in 1963 the activity of the deep waters must have been 98.3 dpm kg-' and, in 1977, 97.74. With an uncertainty of +2.8% for the February 1978 radium activity in the deep waters, the decay correc- tions are not meaningful. Hence, we have chosen an activity of 98 dpm kg-l to represent the radium activity in the deep waters and have used the ratios between 98 dpm kg-' and the actual measurements in the deep waters in 1963 and in 1977 to nor- malize the radium activities in the respective upper layers (Table 2). The lake level dropped by about 3.7 m between 1963 and 1978; the layer volumes given in Table 2 are estimated from the hypsometric curve of the Dead Sea (Hall and Neev unpubl.) and the respective thickness of each layer.

The radium inventories of the three normalized profiles (Table 2) are practically

identical [within 0.23%, (19.54 ? 0.046) x 1015 dpm]. This similarity suggests a steady state inventory of radium in the Dead Sea, although the timespan is too short to prove it over a long period. Since the decay factor for 15 years (1963-1978) is only 0.64%, we cannot tell from the data whether there has been a steady state flux of radium or no flux at all. If the 1978 inventory were much greater than the 1963 inventory, then the inventory could not be at steady state, because a large input would be required during this relatively short period. The fact that the difference between the April 1963 and Feb- ruary 1978 inventories is within 0.15% sup- ports the steady state assumption, although the uncertainty due to analytical errors is much larger.

The sources of 226Ra to the Dead Sea Although it is not accurately known, the

age of the Dead Sea is believed to be of the order of about 10,000 years (Bentor 1961; Neev and Emery 1967). Over this period and with a (approximate) steady influx of radium, a steady state inventory could have been attained long ago. Historical data (As-




Table 4. Inflows of 226Ra into the Dead Sea.

Annual input* F, Ra content Ra, Ra flux (m3.yr-1) (dpm m-3) (dpmyra)

Jordan River, at Allenby Bridget 1.5x 109 1-2x 103 1.5-3x 1012

Inflows south of Allenby Bridge and floods 0.5 x 109 1-2 x 103: 0.5-1 x 1012 Measured springs around the Dead Sea 3 x 107 40 x 103 1.2 x 1012

3.2-5.2x 1012

Estimated z F,Ra, (from steady state model) 8.4x 1012

Unknown sources: ? 5.2-3.2x 1012

Rivulets on eastern shore 7.8 x 107

Unmeasured springs on both shores ? ? Submerged springs ? ?

*Neumann 1958; Dalin 1982. t About 13 km north of the Jordan inflow into the Dead Sea. t Assumed to be the same as in the Jordan River. ? Calculated by subtracting 3.2-5.2 x 1012 dpm yr-' from 2; FRa, = 8.4 x 1012 dpm-yr-'.

saf and Nissenbaum 1977) indicate that the Dead Sea has been meromictic for >100 years. Radium profiles in the 1960s and 1970s with activities greater in the mixolimnion than in the monimolimnion indicate that radium did enter into the upper layers of the lake during this meromictic stage. The assumption of a steady state inventory requires a minimal radium flux necessary to account for the situation de- scribed above. If the inventories estimated earlier, (19.54 ? 0.046) x 1015 dpm, represent the steady state radium inventory of the Dead Sea, then in order to maintain it, an average annual input of about 8.4 x 1012 dpm-yr-1 is required.

Radium activities measured in mineral springs and in Lake Kinneret are given in Table 3 and an estimate of the inflows of radium to the Dead Sea is summarized in Table 4. Lake Kinneret (Sea of Galilee) has a relatively high radium content in com- parison to other freshwater lakes, due to the inflows of hot spring waters. The Jordan River then conveys the mixed water into the Dead Sea. The radium content of the Jordan River is quite comparable to Gil- boa's (1963) Lake Kinneret value. In addition to the Jordan River input, the high radium content of the Dead Sea is also con- tributed by its own hot springs. There is historical evidence that hot springs were ac- tive at both Lake Kinneret and the Dead Sea during Roman times about 2000 B.P.

Aragonite laminae (Stiller et al. in prep.) separated from a 5.5-m-long core of the Dead Sea (representing about 7,000 years of records) have similar oxygen- 18 content. Thus the hydrologic balance of the Dead Sea has been about the same during this whole period. Also, in view of the climatic history of the region it is not unreasonable to believe that the same sources have been providing a more or less steady influx of radium into the Dead Sea for several mil- lennia.

The discharge of the Jordan River at Al- lenby Bridge is about 1.5 x 109 m3 yr-1. Since 1964, this inflow has gradually decreased to about 5 x 108 m3 yr-I due to damming of the outflow from Lake Kin- neret (at Degania) and other irrigation proj- ects on both shores of the Jordan River. However, the radium flux through the Jor- dan has probably remained about the same as before the damming, because radium- rich salty springs such as Tiberias hot springs, discharging previously into Lake Kinneret were diverted in 1967 and have since then spilled into the Jordan, several kilometers south of Degania. We assume therefore that the radium activity measured in Lake Kinneret before diversion of the salty springs (1 dpm liter-1: Gilboa 1963) closely represents the activity in the Jordan River. An upper limit of 2 dpm liter-' would allow other sources of radium flowing into the Jordan River to be included, and the




corresponding flux of about 3 x 1012 dpm. yr- I would also allow for discharges > 1.5 x 109 m33 yr-', since the radium concentration is most likely below the upper limit. Our recent measurements of Jordan River radium near the Dead Sea confirm this as- sessment.

The total measured flux of springs around the Dead Sea is 3 x I07 m3 yr-'. The radium activities in the springs are quite different from one another (Table 3: between 7 and 89 dpm liter-l with an average 39.6 ? 26). Also, the radium activity of the same spring varies with time (Table 3: 38-71 dpm liter-l at Hamme Zohar spring No. 1). With these large spatial and temporal vari- ations it is difficult to estimate accurately the total radium flux from these springs. In Table 4 we assume that 40 dpmdliter-1 is a reasonable average radium activity for the springs measured. Their radium flux is indeed comparable to that of the Jordan River even if the estimate is off by a factor of 2.

In addition, there are several unknown sources of radium (see Table 4). For the rivulets (e.g. Arnon and Zered) of the eastern shore and for springs on the eastern shore, some of which are hot (e.g. Zarka Mayin) and possibly radioactive, there are no radium measurements. If the rivulets on the eastern shore and the unmeasured springs on both shores contribute a radium flux comparable to that of the measured springs, then about 2-4 x 1012 dpm yr-I are still unaccounted for and must be attributed to the radium flux of submerged springs. From hydrological considerations it seems reasonable that these submerged springs and seepages are located under shallow waters. When the lake level dropped to below -403 m in 1979, several tiny seepages, most of them with a strong H2S odor, emerged along the shores, which had been submerged when the lake was higher. Neither the input of the submerged spring waters nor their radium activities are known. If their mean radium activity is of the same order of magnitude as that of the measured springs, then an inflow of about 0.5-1 x 108 m3 yr-I can be expected, in terms of lake level rise, roughly about 5-10 cm- yr-1 or a small fraction of the evaporation rate of about 1.8 m yr-I (Neumann 1958). The water influx of the

submerged springs may well be overesti- mated if their mean radium activity is > 40 dpm liter-' or if the rivulets and hot springs on the eastern shore contribute more radium than we estimate.

The geological origin of radium in the Dead Sea is not likely to be from porewaters originally trapped in the Lisan Formation, although we cannot exclude their contribution to the mineral composition of the springs. (Lake Lisan was, until about 16,000 B.P., the larger but less saline Pleistocene precursor of the Dead Sea.) Evidently radium is not a suitable clock for this timespan even if the system had been a closed one. The origin of radium from salt domes and igneous and sedimentary sources be- neath the deep part of the Dead Sea is also unlikely because the highest radium con- centrations are found in the upper waters.

Dead Sea porewaters which migrate land- wards and are then flushed back by meteoric waters could be a source of radium for some of its shoreline springs. For instance in the case of the Hamme Zohar springs, the ratio of radium in springs to radium in the Dead Sea is about the same as the chloride ratio and may indicate a relatively fast recirculation. On the other hand, Ein Gedi (also called Hamme Yesha) contains about 50% Dead Sea water (Mazor 1969), but the radium content is only 18 dpm liter-l, suggesting a slower recirculation. However, as the Dead Sea itself seems to be the source of radium to the above springs, the question of the geological origin remains unan- swered. The source of radium to the Jordan River is apparently related not to the Dead Sea but to the Tiberias and other hot springs of Lake Kinneret which are very rich in radium. Waters more saline than the Dead Sea and very rich in radium (1,500-2,200 dpm . liter-l) have been found in the Sedom area in the vicinity of the southern basin of the Dead Sea (Mazor 1962). The source of radium in these brines is obviously not related to the present Dead Sea; it could be that brines such as those of the Sedom area (or those of Tiberias hot springs type) after mixing with connate waters emerge as shoreline or submerged springs. Another potential source of radium might be the leaching by meteoric waters of uranium-rich




Table 5. Estimated annual coprecipitation of radium with gypsum and with aragonite in the Dead Sea.

226Ra in Depth in Measured 2'0po* fresh layert Estimated annual sediment depositiont Coprecipitated Ra

Laminae (cm) (dpm g-1) (g.yr-1) (dpm yr-')

Gypsum 130 0.10 0.17 2x10"l 0.034x 1012 Aragonite 312 1.06 4.03 2xO1l0 0.806x1012

* Analytical method: acid dissolution in presence of 208po tracer, coprecipitation with iron hydroxide, acid dissolution and self-plating of polonium isotopes; detection by alpha spectrometry.

t Radium-226 was assumed to be at radioactive equilibnum with 210po. Its activity in a freshly deposited laminae was estimated by correcting for radioactive decay in the sediments, at a sedimentation rate of about 1 mm yr-'.

t Neev and Emery 1967.

(100 ppm U) phosphate rocks occurring in the southern and western part of the Dead Sea watershed.

Our relatively weak radium budget, which is due to the fact that some radium sources are poorly known or unaccounted for, calls for further investigations on the hydrological balance of the Dead Sea (all inflows and evaporation), its submerged springs (loca- tions, water, and radium fluxes), and radium measurements of all its surrounding springs.

So far, we have considered that radium is being introduced into the lake only in dissolved form and that none of it is lost to the sediments. But, two questions should be clarified: how much radium coprecipitates with authigenic aragonite and gypsum, and what is the fate of particulate radium after it is transported into the lake by suspended particles.

To answer the first question we have ana- lyzed the polonium-2 10 activities of gypsum and of aragonite laminae separated from a 5.5-m-long sediment core taken at 20-m water depth (at 31?25'N, 35?23'E: Stiller et al. in press). Below 1r-m depth of sediment the 210Po activities are believed to be at radioactive equilibrium with 226Ra. Very little radium is expected to coprecipitate with gypsum, about 0.03 x 1012 dpm yr-', whereas more radium may accompany the deposition of aragonite, about 0.8 x 1012 dpm yr-1 (Table 5). The latter could be of some importance for the radium balance in the lake. It means that the annual radium input must be about 10% larger than 8.4 x 1012 dpm *yr-1 in order to maintain the present inventory of 1.95 x 1016 dpm at steady state.

The second question, the fate of radium

entering the lake in suspended form, depends on whether it all remains on particles and reaches the sediments or is partially leached or dissolved into the water. Sedi- ments from the Jordan River bed and those at shallow depths from the Jordan delta have 226Ra activities of 3.5-4.0 dpm g-1. The top layers of the Dead Sea sediments from both shallow and deep areas of the lake are close to the above values, in the range of 2.5-5 dpm g-' (Stiller in prep.). Thus, these data suggest that there is no significant leaching of particulate radium, nor scavenging of dissolved radium by settling particles. The data are consistent with the contention that there is no appreciable amount of radium coprecipitated with authigenic minerals, as indicated by our radium measurements on gypsum and aragonite in cores. Since there is no observable radium gradient near the Dead Sea bottom the diffusive radium flux from bottom sediments is most likely negligible compared to the other radium sources of the Dead Sea. This condition is actually required in order to determine the duration of the meromictic phase.

The age of meromictic phase traced by 226Ra

As shown in Fig. 2 the depth profiles of radium in the Dead Sea are not uniform, but tend to become so as the mixolimnion becomes thicker. The fact that the activities in the mixolimnion were greater than in the monimolimnion suggests that radium was introduced mainly into the upper layers of the Dead Sea in amounts larger than needed to counterbalance the decay within the mixolimnion, but, as a steady state radium inventory is assumed, these amounts are equal to the decay within the entire lake. Thus,




R 'Rooverturnvrtur

4 \ Ra IRaoverturn/ Voverturn

V3

Fig. 3. Schematic representation of 226Ra profiles in the Dead Sea during meromixis ( ) and at overturn (---).

the depth profiles of radium become useful in determining the age of the meromictic phase, with the mixolimnion acting as a "young" lake which accumulates radium and the monimolimnion as an isolated body of water for radium decay. The radium profile can be a measure of the duration of the meromictic phase if it persists for more than a few tens and for less than several thou- sands of years. At overturn this information will be wiped out by complete vertical mixing.

If a steady state inventory of radium in the Dead Sea was achieved long ago, the unit volume radium activity must have been the same at each overturn, provided that overturn occurred within a similar lake volume. By the February 1979 overturn, the lake level had dropped to about -403 m and the lake volume had decreased to about 145.4 km3; this yields a unit volume radium activity of about 134 dpm liter-1. As a first approximation it is assumed that the unit volume radium activity at the previous overturn was the same as at the overturn of February 1979. After that previous overturn, the Dead Sea might have been monomictic for a certain period during which it maintained a homogeneous, steady state radium profile. Only when permanent stratification started to develop did the shape of the 226Ra profile gradually change.

A radium profile taken anytime during the meromictic phase can be used to calculate the (meromictic) age, t, by either of the following two equations if the three conditions discussed previously are fulfilled: that the radium inventory of the lake is at

steady state, that at the onset of the meromictic structure the radium profile was uniform, and that radium enters only into the mixolimnion while the monimolimnion is isolated.

RaIVI + Ra2V2 rt

- z (F,Ra,) f exp(-Xt) dt

+ Rao,exp(-Xt)(VO, - V3) (1)

and

Ra3= Rao,exp(-Xt) (2)

where RaI, Ra2, Ra3 are the 226Ra activities in the upper, intermediate, and deep layers (dpm m-3) and V1, V2, V3 are the respective volumes (m3). The other terms are defined as follows: Vov is the lake volume at overturn taken as 145.36 km3; Rao, is the unit volume radium activity at overturn, Raov = RaL/ Vov (dpm rM-3) where RaL is the inventory of radium in the whole lake; X is the decay constant of 226Ra (=0.0004279 yr-'); t is the age of the meromictic phase (years), i.e. the timespan between the onset of the meromictic structure and the date of sampling of the radium profile; z (F,Ra,) is the sum of radium inflows, i.e. F, (m3 yr-') times the respective Ra, radium activities (dpm m-3). This quantity is assumed to be constant in time; for a radium inventory at steady state z (F,Ra,) = XRaL; also

rt J'exp(-Xt) dt -1 /X[exp(-Xt) -1].

Equation 1 is the radium inventory of the mixolimnion, the upper water mass including the transition layer (Fig. 3). As discussed earlier, we assume that all radium inflows are confined solely to the mixolimnion. The total amount of radium in the upper layer depends on t, the period which has elapsed since it lost contact with the monimolimnion, and on all inflows of radium accumulated within this water mass, i.e. the total radium fluxes.

Equation 2 describes the radium activity in the monimolimnion which decays from its overturn value, according to the interval since this water mass was isolated from the overlying waters. Based on evidence discussed above we may assume that during




Table 6. The estimated age of the meromictic phase.

Ra3 Ra.,, Age, t Data source* (dpm liter-l) (yr)

Apr 63 111.0 123.5 249 Mar 77 97.9 108.7 244 Feb 78 120.5 134.5 257

* Averages of the measured activities of radium (Table 2) have been used for calculations of t.

the whole meromictic phase the monimolimnion was perfectly isolated and that there was no transport of radium into it. The monimolimnion might have been eroded from its top, but such water layers are included in the intermediate water mass, i.e. in Eq. 1. As the presence of 230Th could not be detected in the Dead Sea brines in any appreciable amount (Somayajulu and Craig in prep.), we assumed that the in situ pro- duction of 226Ra from decay of 230Th is negligible. Algebraically, Eq. 1 converges into the much simpler Eq. 2. The only reason for showing Eq. 1 is that it clearly describes our concept about the radium accumulation within the upper layers during the meromictic phase.

The normalization which has been performed (Table 2) indicates the consistency of the three radium profiles in terms of inventory. Estimates of the period between the onset of the meromictic phase and the date of sampling, i.e. the age t, are given in Table 6, in which the actual data (see average of measured 226Ra in Table 2) have been used. The February 1978 data yield an age of 257 years. Since the 1963 and 1978 profiles are 15 years apart, their age estimates differ only by 7 years (249 + 15 -

25 7). The age derived from the March 1977 data differs slightly from the other two but is reasonable.

The estimated age of about 260 years at the 1978 sampling means that the meromictic structure of the Dead Sea before the 1979 overturn became more or less stabi- lized around A.D. 1720. Information on the conditions of the lake before this meromictic phase would have been wiped out by a previous overturn if the lake had been meromictic. Of course, one cannot rule out the possibility that the Dead Sea was monomictic before A.D. 1720.

The age estimate of the meromictic phase appears to be supported by sedimentological evidence: halite crystals were found (Stiller et al. in press) in a sediment core taken from the deepest part of the northern basin (at 31?35'N, 35?27'E). The core has been dated at its bottom by 14C (Stiller and Kaufman in prep.) and the inferred age at the depth of the halite crystals is about 265 years. If these crystals have not grown diage- netically or have not been redeposited from elsewhere, the following sequence of events can be deduced from their presence: about 265 years ago there was an episode of su- persaturation with respect to halite, probably triggered by a low stand of the Dead Sea (in other words conditions favorable to precipitation in a monomictic lake). After that, the lake level began to rise, the surface water became more diluted, and a meromictic structure was established.

The sources of error in the age estimate of the meromictic phase are the method- ological error in measuring radium, the uncertainty of the steady state inventory, and the lake volume at the former overturn. [See derived variables in Eq. 2, which can be re- written as t = (1/X)ln(RaL/ V0,Ra3).] The radium measurements of February 1978 indicate an error of 2.8% (Chung and Craig in prep.); the steady state inventory might be in error by 0.35% (see below) and the lake volume at the previous overturn (see below) by about 0.9%. By assigning V0,v as an independent variable not correlated with Ra3 and RaL, we have calculated the prop- agated age error to be ? 61 years, or ? 23.5%. Apparently the error of the radium measurements is the largest single source for the uncertainty of age estimate in the model. The magnification of age error is due to the fact that the age is a logarithmic function of Ra3/RaOv.

Processes related to the manufacture of potash from the Dead Sea, namely the precipitation of halite and carnallite from Dead Sea brines and the subsequent release into the water of the supernatant concentrated brines (end brines) might affect the radium inventory of the lake. Each kilogram of end brine pumped back into the Dead Sea at the end of the evaporation process represents about 1.85 kg of Dead Sea waters which




Table 7. Estimates of the Dead Sea volume at A.D. 1720 and the dependence on them of the calculated meromictic age.

Dead Sea, Feb 79, total salts (Steinhom 1981) 276.4 g kg-l x 1,233.48 kg-m-3 x 145.4 km3 4.957 x 1016

Salts deposited in the evaporation ponds of the Dead Sea Works Ltd. until Feb 79 from about 2 x 109 m3 of brine g0.025 x 1016

4.982 x 1016

Estimated net* Jordan contribution during z260 yr (-- 1.5 x 1012 g-yr-') 0.039 x 1016

Estimated contribution by mineral springs ;Jordan 0.039 x 1016

Estimated total salts in the Dead Sea at ZA.D. 1720 z S = 4.904 x 1016 g

Conditions of a mixed Dead Sea at -A.D. 1720 Mero- mictic

T S p V.4 Lake level? age (OC) (g kg-1) (g-cm-3) (km3) (m) (yr)

Like the deep waters 1975-1978 21.3 276.0 1.23339 144.1 ~ -404.8 282 Less salty than above 21.3 275.4 1.23309t 144.4 -404.3 277 More salty and warmer than above 22.0 276.7 1.23366t 143.7 -405.3 286 * After precipitation of aragonite and gypsum. t Calculated from relationship to temperature and salt concentration (Steinhorn 1980).

Vv =- X 10-12. S p

? Lake levels calculated from lake volumes given in column 5. Sediment volume, about 0.3 km3, accumulated within 300 years has been disregarded.

were pumped into the evaporation ponds about 1 year earlier. Thus, if radium does not coprecipitate with minerals in the evaporation ponds, the radium activity in the end brines in 1978 should have been about 220 dpm kg-1; however, the activity measured in a sample of end brine collected in March 1978 (Table 1) was only 165 dpm kg-1 (after normalization to the Scripps radium standards). During the last 20 years, about 2 x 109 m3 of Dead Sea waters have been pumped into the evaporation ponds, and this means that about 7 x 1013 dpm of 226Ra has been lost at the floor of the evaporation ponds or, in other words, about 0.35% of the February 1978 inventory (which we believe is the most accurate one) is missing. If this estimate is correct, then the revised inventory, including the above amount, would yield a meromictic age estimate (269 years) which is larger than that given in Table 6 by about 10 years.

The level of the Dead Sea at the onset of the meromictic phase

The volume ofthe completely mixed Dead Sea just before meromixis became established is an important parameter in the age determination of the meromictic phase. We have tried to compute the volume and the

corresponding lake level of the mixed Dead Sea before the onset of the meromictic structure from its total dissolved salts inventory at the beginning of the 18th century and from the salinity of the "fossil" monimolimnion (Table 7).

In February 1979 the inventory of dissolved salts was 4.96 x 1016 g (Steinhom 1981). Salts deposited on the floor of the evaporation ponds during the last three decades should be added to this (see Table 7). At the beginning of the 18th century the total inventory of dissolved salts must have been slightly lower than in 1979, because dissolved salts brought in by inflows have accumulated in the Dead Sea during 1720- 1979. We assume, tentatively, that the salt flux from mineral springs more or less equals that of the Jordan. According to Bentor (1961), the saline springs have supplied more salts to the Dead Sea than the Jordan River. It follows that the salts inventory at A.D. 1720, 4.90 x 1016 g (Table 7), was smaller than that of 1979 by about 8 x 1014 g.

If we assume that the properties of the monimolimnion during 1975-1978 (Stein- horn 1981) represent the conditions of the mixed Dead Sea at the onset of meromixis, then we can calculate the lake volume at that time (V0v in Table 7) by dividing the




total amount of salt present in the lake at about A.D. 1720, viz. 4.90 x 1016 g, by the unit volume salt content. Slightly different conditions, namely a slightly lower or larger salinity (lower than ? 0.3%, lower part of Table 7) have also been considered. In all three cases the calculated volume of the lake appears to have been somewhat smaller and the respective lake level lower than at the 1979 overturn. Supporting evidence for lake levels being lower than -403 m at the beginning of the 18th century can be found in the reconstructed Dead Sea levels of Klein (1981), which were at about -404.5 m MSL at around A.D. 1720-1740 and 1760. If the amount of salts deposited in the evaporation ponds and in the southern basin as it dried up balances the amount brought in by inflows during the last 260 years (which is unlikely), then the previous overturn might have taken place in a similar volume (and at about the same lake level) as in February 1979. But, if the lake volume was indeed somewhat <145.4 km3 (in Table 6, it was assumed to be 145.4 km3), then the unit volume radium activity Raov would have been somewhat larger than that used in Ta- ble 6, and accordingly the age estimate (by Eq. 2) becomes larger than 260 years by about 20 years (Table 7).

Our age estimate of the meromictic phase of about 260-280 years disagrees with that of Assaf and Nissenbaum (1977) who, by relying on the evidence of a relatively low lake level, -402 mat A.D. 1810-1815 (Klein 1965), and on a surface density measurement at A.D. 1819 which was 1.24 g-cm-3, believed that around 1810-1815 the lake was homogeneous with densities of about 1.235. But another surface density measurement from the same period performed in 1817 by Gay Lussac (cited by Nissen- baum 1970) indicates a surface density of only 1.2283 at 17?C. Also, the total salt inventory in 1815, which must have been somewhat smaller than in 1979 (roughly about 4.94 x 106 g) cannot be accommo- dated in a volume of about 146.2 km3 (level -402 m) with a homogeneous density of 1.235 g.cm-3 unless the temperature was as low as 12?C. Or, in other words, a somewhat smaller amount of salts than in 1979 cannot be dissolved in a larger volume (146.2 km3

at A.D. 1815 vs. 145.4 km3 in February 1979) and still have, roughly, the same density. What becomes evident from the above ar- gument is that the level of the lake must have been close to or lower than -403 m MSL if the previous overturn occurred at a temperature close to 20?C.

A meromictic age of only about 170 years, as suggested by Assaf and Nissenbaum (1977), implies that the first assumption of our model, i.e. a steady state inventory of radium, is not valid. This can be shown by combining Eq. 1 and 2:

RaIV, + Ra2V2 + Ra3V3

- z (FiRa,/X)(I - exp[-Xt]) + Ra3Vov. (3)

The right-hand side of Eq. 3 represents the radium inventory RaL; in 1978 with t = 170 years, we get z FRa, = 12.3 x 1012 dpm yr-1. This radium flux is about 50% larger than that required by the steady state model. It also implies a smaller radium inventory in 1810 than in 1978. Thus not only do the data for density and lake levels disprove the 1810 overturn (as shown above), but also it would be very difficult to adjust the radium budget of the Dead Sea (Table 4) for such a large radium influx.

It should be noted that if only the first two conditions of the model, i.e. a steady state inventory and an initial uniform profile, are fulfilled, but not the third one (no radium fluxes into the deep waters), then an additional term for radium fluxes of submerged springs into the monimolimnion must be added to Eq. 2. For instance, if from the total radium flux of 8.4 x 1012 dpm yr-1, 2 x 1012 dpm yr-I have entered the monimolimnion (below 80 m), then the meromictic age estimate would become about 400 years. However, our age estimate of only about 280 years, derived by completely excluding radium fluxes into the deep waters, seems to be supported, as men- tioned before, both by sedimentological evidence (the halite crystals) and by the low lake level, -404.5 m, which prevailed during the first half of the eighteenth century and which was favorable for a turnover to occur.




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Submitted: 4 June 1982 Accepted: 2 November 1983



Radium in the Dead Sea: A Possible Tracer for the Duration of Meromixis

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Transcript of Radium in the Dead Sea: A Possible Tracer for the Duration of Meromixis