210Pb and210Po during the destruction of stratification in the Dead Sea

390

Earth and Planetary Science Letters, 71 (1984) 390-404 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[3]

21°pb and 21°po during

the destruction of stratification in the Dead Sea

M. Stiller and A. Kaufman Isotope Department, The Weizmann Institute of Science, Rehovot (Israel)

Received October 18, 1982 Revised version received September 20, 1983

The progressive weakening and final disappearance (in 1979) of the long-term meromictic structure of the Dead Sea are clearly reflected in the depth profiles of 21°Pb and 21°Po. In 1977/78, prior to overturn, dissolved 21°pb (35-50 dpm kg 1) predominated over particulate 21°Pb (1-2 dpm kg -1) in the oxic upper waters, whereas the reverse was true in the anoxic deep waters (16-20 dpm kg -1 particulate vs. 2-5 dpm kg -1 dissolved). The exact extent of the disequilibrium between 21o Pb and 226Ra is hard to evaluate in the upper oxic layers, because the progressive deepenings resulted in mixing with deep waters. By contrast, one can estimate the residence time of dissolved 21°pb in the unperturbed anoxic deepest layers, because these remained isolated, at about 3 years. Following the overturn of 1979, dissolved 21°pb exceeded particulate 21°Pb at all depths. The 21°po profiles of the stratified lake resembled in shape those of its grandparent 21°Pb, but with distinct characteristics of their own in the oxic upper waters where particulate 21°Po (8-12 dpm kg -1) was greatly in excess over particulate 21°pb, while dissolved 21°po (25-40 dpm kg 1) was slightly deficient. Immediately following the overturn, dissolved and particulate 21°Po were similar (about 15 dpm kg-l), at all depths. The destruction of the lake's meromictic structure was accompanied by a reduction of its 2a°pb inventory, while that of 21°po was almost unaffected. Thus, at overturn a transient state was created with the inventory of 21°po exceeding that of 2a°pb.

1. Introduction

The Dead Sea, situated at the lowest part of the Jordan Rift Valley is a terminal hypersaline lake. The "classical" Dead Sea, as described by Neev and Emery [1], exemplified a meromictic lake, permanently stratified by virtue of its salinity gradients; in 1959/60 the densities of the upper (mixolimnion) and lower (monimolimnion) water masses differed considerably: 1.205 and 1.233 g cm -3, respectively. A transition zone of about 40 m separated these distinct water masses. The level of the Dead Sea started to decline in the mid 1950's [2] as a result of both the diversion of Jordan River water and a diminished rainfall intensity. This decline caused the shrinkage and final detachment of the shallow southern basin in 1976 (Fig. 1) and an increase in the density of the 0012-821X/84/$03.00

© 1984 Elsevier Science Publishers B.V.

mixolimnion accompanied by gradual deepenings of the pycnocline layers. Finally a cruise conducted in February 1979 revealed that an overturn had already taken place [3]. The upper (UWM) and lower (LWM) water masses were characterized by slight differences in their major ion compositions [1]. In addition, the anoxic conditions which prevailed in the deep waters supported the presence of hydrogen sulfide [1,4] and of dissolved divalent iron [5]. Thus, in this paper the LWM will sometimes be referred to as the anoxic layer and the U W M as the oxic layer. The Dead Sea brines contain relatively high 226Ra activities: in February 1978 about 115 and 98 dpm kg-1 in the upper and lower water masses respectively [6]. In this paper we describe the distributions of dissolved and particulate 21°pb

391 35720

35"50' I

Aug 16,1978

~, 1979

Mar 29, t977 Feb 5,1979

12,13,1978 ,21, 1978

of these conditions has its characteristic steady state distribution. Although none of the measured profiles can be taken as to represent a true steady state condition, yet we shall treat the data in two groups: (1) Those of our first cruise (March 1977) will be used as an approximation of the steady state distribution during the stratified condition. (2) Those of all other cruises will be used to monitor the changes caused by the approach to lake overturn.

1978

Dec 20,1977

2. Sampling and analytical methods

Mar 31,197 7 1978

Mar 31,1977

978 978 978

Fig. 1. Map of the Dead Sea [14] with location of sampling sites. (t t : = 22 26 years) and of 2]°Po (t] 2 138 4 days) ~ / • the/ long-hved products of the 2 2 6 Ra decay" chain. The distributions of these isotopes have already been studied in a permanently anoxic basin, the Cariaco Trench [7]. A similar study in the Dead Sea is of particular interest for several reasons: (1) the atmospheric 2t°pb flux (0.15 dpm cm -2 yr t; [8]) is negligible compared to the in-situ production of 21°pb from 226Ra (about 76 dpm cm -2 y r - t ) ; (2) the unique opportunity existed to follow the changing distributions of these isotopes during the last stages of meromixis and at overturn, namely during the transition period from a partially anoxic to an entirely oxic water column; and (3) the distributions of 21°pb and 2~°po in the Dead Sea depend only on physical and chemical conditions since the primary productivity is very limited (0.2-1.0 mg C m -3 d a y - t ; [9]). The 21°pb and 21°po profiles were obtained while the Dead Sea was in the process of changing its condition from stratified to unstratified; each =

Water samples for this investigation were collected during seven cruises (Fig. 1) with 3.5-liter PVC Niskin bottles and stored in polyethylene bottles. The samples were always filtered as soon as possible after the cruises, through 0.45 /~m Millipore membranes. March 1977 and January 1978 cruises. Total 21°po and dissolved 21°po were treated similarly: 2°8po spike (in 3N HCL) and stable lead carrier (20 mg Pb) were added to weighed 200 ml samples and the polonium isotopes were coprecipitated with PbCrO 4. After separation of the precipitate by centrifugation and dissolution in hot 1N HC1, the Po isotopes were separated from the acid solution by self deposition (plating) on stainless steel disks at 90°C (4 hours) in the presence of 100 mg of ascorbic acid and determined by alpha-spectrometry. Plating of polonium isotopes was then repeated on the residual solution to accomplish their complete removal, thus leaving behind 21°pb free of 21°po; this solution was saved for subsequent 21°pb analysis. Corrections for 2]°Pb at sampling date were rarely needed, as the time interval between sampling and plating of 2l°Po was only several days. A few preliminary tests were conducted, wherein the efficiencies for polonium coprecipitation were compared between samples to which only lead carrier was added and samples to which iron chloride carrier (20 mg Fe) was used in addition to the lead carrier. Because similar efficiencies were obtained, only lead carrier was used in further runs.

392 Total 21°pb and dissolved 21°pb were determined on the residual acid solutions which had been stored for four months or more to allow ingrowth of 21°po. A new portion of 2°8po spike was added and the polonium isotopes were again separated by self deposition. The chemical efficiency for lead, generally 75-90% was measured by atomic absorption spectrometry (AAS) and corrections for ingrowth of 21°po from the 226Ra coprecipitated with PbCrO 4 were applied. Because the chromate ion is an effective scavenger for 226Ra, the efficiency of its coprecipitation with PbCrO 4 was assumed to be the same as the chemical efficiency for lead. Particulate 21°po and particulate 21°pb: the membrane filters were dissolved in 8 M HNO3, evaporated almost to dryness, redissolved in a 3N HCI solution containing the 208Po spike and again taken twice to near dryness with 3N HCI. The following steps were the same as those described above for dissolved 21°p0 and 21°pb. Corrections for ingrowth of 21°p0 from 226Ra were unnecessary as there is no more than 1.5 × 10 3 dpm 226Ra in the suspended matter on any filter. February 1978 cruise. Initially the procedure described above was followed, but by performing the polonium plating on Cu disks (due to a temporary shortage of stainless steel disks), the solutions became contaminated with dissolved copper which greatly reduced the plating efficiency during the 2a°pb determination step. Fortunately, a second series of samples (filtered immediately after sampling, acidified, and containing lead carrier) had been shipped to the Scripps Institution of Oceanography. Some of these samples were analyzed by Y.C. Chung and some upon our request, were kindly sent back to us and analyzed as further described for August and November 1978 samples. August 1978 and November 1978 cruises. For dissolved 2a°pb: filtering, acidification to pH 2, and addition of 20 mg of lead carrier were performed immediately after the cruise. The brine samples were stored in polyethylene bottles with 2°8po spike being added after the storage period. Further steps were identical with the procedure for dissolved 2mpo in the March 1977 cruise. Dis-

solved 21°pb at sampling date in the LWM was then calculated by taking the initial dissolved 21°po/21°pb ratio to be the same with the average ratio in the LWM in February 1978; in the UWM this ratio was taken for all samples as that of 50 m in August 1978 (Table 2). In both cases corrections for ingrowth of 21°po from 226Ra present in the stored brine samples were also applied. For particulate 2mpb, the membrane filters were rinsed, dried and stored in plastic boxes. After a certain period, the filters were dissolved as described previously, and the plating of polonium performed directly on the acid solutions of the dissolved filters. Particulate 2mpb was calculated by assuming an initial particulate 2mpo/Zl°Pb ratio of 6 for the UWM and of 1.23 and 1.2 for the LWM in August 1978 and November 1978 respectively (see Table 3). Since lead carrier was not used, a chemical efficiency determination by AAS was not needed. February 1979 cruise. Two separate aliquots from the same brine sample were filtered immediately after the cruise. 2mpo and 2mpb were measured by plating 21°po at two different dates each from a separate aliquot: for Po within several days and for 21°pb within several months of sampling. The procedural steps were: for dissolved 21°po as described for the March 1977 cruise, for particulate 2mpo as for particulate 21°pb in the August 1978 cruise and for dissolved and particulate 2mpb as in the August 1978 cruise. The 21°po data at two different dates of plating (t I and t2) enabled us to solve two equations in which the unknowns were 2a°po and 21°pb at the date of sampling (the equations and their solution are available on request; ingrowth from 226Ra in the dissolved phase was accounted for). This method has two advantages: neither repeated platings for the stripping of 21°pb nor assessment of chemical efficiency for lead by AAS are needed. Procedural blanks were never more than 0.1 dpm kg- 2. The corrections for ingrowth from 226Ra in the dissolved phase were based on a March 1977 profile [10] and on a detailed February 1978 [6] 226Ra profile. These profiles showed (after intercalibration) that 226Ra is a conservative tracer

393

since its activities in the UWM reflected the mixing with deeper layers caused by the deepening of the pycnocline. Therefore, the 226Ra activities for all other cruises were deduced according to the location of the pycnocline. In samples from the UWM the correction for ingrowth of Zl°Po from 226Ra was about 2-5% of the activity at plating for 2~°Pb analysis. For LWM samples these corrections were larger, between about 6% and 16%.

3. Results: the vertical distribution of 21°pb and 21°Po general description Initially, three kinds of measurements were performed: "total 21°pb" and "total 21°Po" activity in unfiltered Dead Sea brines, "dissolved 21°Pb and 21°po" in the filtrates and "particulate 21°Pb and 21°po" in the residues from filtrations through 0.45-/~m Millipore membranes. Table 1 lists the data of March and December 1977. The combined activities of dissolved and particulate 21°po are, with only one exception, in reasonable agreement with the "total" activities, though the same comparison is less satisfactory for 2~°Pb. While the 2 1 ° p o data depend only on the efficiency of the 21°Po separation measured with the 2°8po spike, the 21°pb data depend, in addition, on the chemi-

cal yield of the lead carrier, measured by AAS. The analytical precision of this latter step is sometimes poorer then is the counting precision of the 2~°po/2°Spo ratio. The "total" activities were not measured on samples from subsequent cruises (data listed in Table 2). Also, in order to save 2°aPo spike, 2t°Pb was measured only in some of the samples from January 1978 and 21°Po was measured only in very few samples from the August and November 1978 cruises. A complete 2~°pb-Z~°Po analysis was performed for the cruise which monitored the turned over Dead Sea in February 1979. The upper and lower water masses of the Dead Sea before the overturn are characterized by strikingly different 21°pb distributions. The dissolved 21°pb activities found in the upper layers, are about 35 50 dpm kg -1, while in the deep anoxic layer, about 2-7 dpm kg 1. In contrast, the particulate 21°pb activities are very low, about 1-2 dpm kg ~, in the upper oxic layers and much larger, about 16-18 dpm kg 1, in the deep layers (Tables 1 and 2 and Fig. 2). Thus, in the oxic UWM, most of the 2t°pb activity is found in dissolved form while in the anoxic LWM most of it is in particulate form (Table 3). In the Cariaco Trench [7] very low dissolved 21°pb concentrations were found at the base of the oxic layer and were

TABLE 1 21°pb and 21°po in the Dead Sea in 1977 Station

Depth

21°po (dpm kg 1)

(m)

diss. (D)

21°pb (dpm kg l)

21Opo/21opb

part, (P)

total ~ (T)

T/(D+P)

diss. (D)

part. (P)

total a (T)

T / ( D + P)

diss.

part.

2 9 - 31 March 1977 3 10 31,7 3 35 40.8 6 77 40.4 6 87 37.1 6 137 3.7 5 189 2.5 5 206 3.9 5 216 3,3 5 239 5.5 5 289 5.2

13.3 7.7 10.7 11.8 15.6 16.3 17.4 18.3 17,9 18.5

44,1 46,7 43.4 47.3 25.3 19.3 21.6 23.5 22.6 23.5

0.98 0,96 0.85 0.97 1.31 1.03 1.01 1.09 0,97 0,99

50.8 57.2 48.6 46,2 5.0 3,4 8.5 7,3 12.3 9.4

0.9 1,0 1.4 1.4 15.8 20.3 15.4 17.4 14.3 18,4

48,2 52.4 61.0 52.2 22.6 19.4 21.3 24.6 26.2 25.2

0,93 0.90 1.22 1.10 1.09 0.82 0.89 1,00 0.97 0.91

0.62 0.71 0.83 0.80 0.74 0.74 0.46 0,45 0,45 0,55

14.8 7.7 7.6 8.4 0.99 0.80 1.13 1.05 1.26 1.01

20 December 1977 1 surface 28.6 1 220 2.1

13.7 19.1

44,3 21.5

1.05 1.01

lost lost

<1 17.5

lost 18.0

a Separate analysis of unfiltered sample

394

TABLE 2 21°Pb and 2I°po in the Dead Sea in 1978 and 1979 Station

Depth (m)

6 - 8 January 1978 2 surface 6 surface 5 20 2 40 6 80 2 100 5 140 2 160 4 180 2 220 6 220 3 237 2 280 2 300 4 305 6 315

2

320

2 0 - 21 February 1978 2 0.5 2 25 2 50 2 74 2 99 1 120 1 140 1 160 1 180 1 200 1 220 1 240 1 260 1 280 1 300 1 320 16 August 1978 4 4 4 4 4 4 4 4 4 4 4 4 4 4

surface 25 50 75 100 125 150 175 200 225 250 275 300 310

Zl°Po (dpm kg l)

21°pb (dpm kg 1)

21°po/21°pb

diss.

part.

diss.

part.

diss,

part.

26.6 25.7 25.3 37.7 26.5 29.2 15.1 2.5 1.0 2.2 1.4 1.1 2.9 4.0 2.2 1.0

11.1 8.5 8.4 10.6 9.9 9.7 14.2 18.2 24.0 25.0 23.8 17.4 17.8 15.8 18.2 15.1

35.9

7.2

0.74

1.5

lost 33.1

2.5 1.6

0.80

4.2 6.2

5.9 5.4 4.7 2.2

12.7 14.0 16.4 13.4

0.37 0.26 0.23 1.3

1.97 1.70 1.06 1.33

5.8

12.3

0.17

1.23

5.6

13.8

23.7 24.8 26.5 26.8 27.8 23.3 28.5 7.7 3.1 1.7 2.4 1.4 1.6 1.2 3.0 2.7

11.0 11.5 11.3 10.9 9.8 12.8 12.1 20.6 21.2 18.9 16.9 20.8 18.0 21.8 21.1 20.7

33.6

21.3

31.6 35.7 34.6 35.8 35.0 37.9 29.6 7.6 lost

0.75 0.69 0.77 0.75 0.79 0.61 0.96 1.01

a ~

a a

5.5 4.5 4.4 5.1

16.8 15.3 16.3

0.44 0.31 0.36 0.24

1.01 1.35 1.10

5.5 a

15.6

0.49

1.33

18.5 34.2 35.2 37.8 34.5 29.9 25.8 9.4 4.8 2.4 2.8 1.8 2.1 2.7

1.3 1.6 1.7 lost 1.7 1.7 2.9 8.7 15.6 17.7 17.3 19.6 13.8 18.7

0.95

1.23

395 TABLE 2 (continued) Station

Depth

21°po (dpm kg 1)

21°Pb (dpm kg l)

210po/210pb

(m)

diss.

diss.

diss.

part.

part.

10 13 November 1978 2 surface 2 25 4 50 4 75 4 100 4 125 2 150 3 175 4 200 2 225 4 250 2 275 2 305

16.8 2.0 16.4 1.7 17.0 24.1 2.1 28.1 2.8 15.1 19.7 2.2 21.2 3.2 17.1 2.5 15.7 8.1 11.4 12.6 10.1 t7.7 3.8 18.3 20.7 3.8 16.6 possible contamination with surface water; data rejected

5 - 7 Februat T 1979 2 0 1 25 1 50 3 100 3 150 3 200 2 250 3 300

7.9 13.1 14.7 12.3 16.4 15.9 16.2 15.0

24.8 15.4 13.7 15.5 17.8 19.6 17.2 17.5

23.6 9.4 10.4 22.9 19.2 21.6 12.6 13.8

5.2 5.2 7.0 4.6 5.4 5.2 5.4 5.0

part

8.1 6.9

1.20

0.33 1.40 1.41 0.54 0.85 0.74 1.29 1.09

4.77 2.96 1.96 3.37 3.30 3.77 3.19 3.50

a Measured by Y. Chung at Scripps Institution of Oceanography and kindly made available to us.

TABLE 3 Percent of 21°pb and of Zl°Po present in the dissolved phase; averages of data

a

Date of

Oxic layers

sampling

depth range (m)

diss./(diss. + part.) (%)

Anoxic layers depth range (m)

diss./(diss. + part.) (%)

10-87 0-80

97.2+ 1.0 89.3 ± 8.6

137-289 220-315 220-320 200-310 250 275

30.2_+ 10.8 22.2_+ 7.9 23.8_+ 2.2 14.0+ 5.1 18.3+ 1.8b

137-289 160-320 200-320

18.3_+ 3.9 13.3 ± 8.4 9.0+ 3.0

21opb Mar. 29-30, 1977 Jan. 6-8, 1978 Feb. 20-21, 1978 Aug. 16, 1978 Nov. 10-13, 1978 Feb. 5-7, 1979

0-150 0-150 0-300

93.9+2.1 89.3+1.7 74.0_+ 8.5

10-87 0-120 0-140 0 300

77.3_+5.9 74.5 + 2.4 69.4_+ 2.8 44.1 _+8.5

2mpo Mar. 29-31, 1977 Jan. 6-8, 1978 Feb. 20-21, 1978 Feb. 5-7, 1979

a This is a simple average of all the data points and is not weighted for the varying volumes of the depth intervals. The data of the transition zone were excluded. b This is the average of only 2 data points.

396 F~-liculote Activity Density ( gr/cm 3 ) ( clpm/Kg) 1.233 I0 20 111 1.231 l I I .....

Dissolved Activity (dpm/Kg) 20 40

. . . .

-.f~

I00

-"

O~b

z,o" \

200

,. ~_',, ,~

March 4-1977 +

I:,

300 0

d'/" ",

I

2,I

\

~"1-1 0 0 I-(3..

,,,200

6-8 January

500 0 210p

i

2oo

.~ ZJOpb

"&ic

20-2, ~,b~,,o~!lg~ ~

,If

loo

/L

Aooo

zoob/

Lc

;

I{

16 Aug,,st 1978

""-.,,Z a£~/

i 1

%. I

or "~

z

p-

I

I

I

"'.

,,,zooL ," ~' l.;

o

*

/

I

I

t L

!

F I

"~..

l ".. ,o-,3 No,,~,s78 --:

7".

%0

ia'°Pd \

mo~-

:

:-

20

: 40

'

I

t

X

OXlC

J~=~ I=* ~/

I{

i

-~

/

I]

I"

.

i

'

':

~

L ~

-"- I

+ ?" ~ .... : ,' \z'°~o + ; ~ +

k~.

li I f

~ll:~O

I lOOp ~

I

.

i

fi o~c

I

f I0

20

1,231 1,233

Fig. 2. The vertical distributions of 2mpb (. . . . . . ) and of ) in the Dead Sea during 1977-1979. The density profiles at the dates of sampling have been measured by 21°po (

attributed to scavenging by oxides that form there from the upward transport of iron and manganese. In the Dead Dea, the poor resolution at the oxic/anoxic boundary of the March 1977 profile (the one which most closely represents its long-term meromictic distribution) prevents detection of the above-mentioned feature. However, in the other Dead Sea profiles, dissolved 21°pb appears to be lower at the base of the oxic zone than in overlying layers. But, as pycnocline deepenings caused mixing with anoxic waters which are poor in dissolved 21°pb, this feature cannot be unambiguously attributed to scavenging by oxides. The vertical profiles of 21°po resemble those of 21°pb in shape, but possess distinct characteristics of their own. In the oxic upper waters, particulate 21°po (about 8 12 dpm kg - ] ) is greatly in excess over particulate 2]°pb, while dissolved 21°po (25-40 dpm kg 1) is slightly deficient; typical dissolved 2]°po/21°pb ratios are 0.74 0.95 and particulate 21°po/2]°pb ratios are 4-8. It is interesting to contrast the dissolved 2]°po/21°pb ratios of less than unity in the oxic layer of the Dead Sea with those in the Cariaco Trench [7] where dissolved 2]°po greatly exceeds dissolved 21°pb. One of the explanations given [7] is the seasonal recycling of the 2]°po carried down to the sediments at periods of high plankton productivity. Thus, the low dissolved 21°po/2]°pb ratios in the Dead Sea may perhaps be due to its extremely low productivity and recycling of organic matter. In the deep anoxic layer, dissolved 21°po (1.5-5.5 dpm kg - ] ) is more markedly deficient than in the oxic layer while particulate 21°po (18-20 dpm kg - ] ) is in slight excess over particulate 210Pb. Typical ratios of dissolved 210Po/210 Pb and of particulate 21°po/2]°pb in the anoxic layer are 0.2-0.5 and 1.05-1.25, respectively. In February 1979, after the turnover, the general description given above no longer holds, as there are no more systematic differences with depth. The entire water column is characterized by a large scatter in data, dissolved 21°po/21°pb ratios

Steinhorn [14]. The presence of dissolved divalent iron [5] was taken as an indicator of anoxicconditions and the oxic/anoxic boundary is shown accordingly.

397

being 0.96_+0.42 and particulate 21°po/21°pb ratios 3.35 + 0.78, thus suggesting that the water masses had not yet completely achieved either horizontal or vertical homogeneity. Low isotopic activities in the surface waters (compared to those of the rest of the UWM) are common to all dissolved 21°pb and 21°po profiles (Fig. 2). As regards 2wpb, this may be due to some loss of radon, the actual source of 21°pb, from the surface waters to the atmosphere. Scavenging of dissolved n°Pb by bottom sediments, as suggested by Bacon et al. [11], from the shallowest part of the lake may also occur, since the ratios of bottom-area/volume are highest in the upper 25 m (and also below 300 m). Dissolved 21°po simply follows the distribution of its parent in the upper 25 m. The relatively low activities in the surface layers also imply that the upper 25 m, which coincide more or less with the seasonal pycnocline, do not mix very efficiently with the rest of the upper water mass. Because our survey was not detailed enough to show otherwise, all calculations and inventories presented will assume horizontal homogeneity. However, some differences from site to site do seem to exist (see for example the January 1978 profile).

an erosion had taken place during the winter of 1976/77 in the top of the fossil layer and that a new transition zone formed between 100 and 140 m.

Despite these changes, the March 1977 profile is the one which most closely resembles the former steady state situation and is the one we shall use as a model for estimating the magnitude of the parameters which characterized the former steady state. Keeping these limitations in mind we will now proceed to the estimate of removal rates and residence times of the 2a°pb-21°po system in the Dead Sea in March 1977. The oxic UWM and the anoxic LWM will be considered separately. The following equations describe the "steady state" in which the rates of production of each isotope in a particular phase are assumed to balance the losses from it. For the UWM: d d . ~ 2 A I u = ~kzAdu + R z u A 2 u ,

"r2dc= 1 / Rdu

(1)

d d P P . R2uAzc = ~2AP2u + R2uAz c,

T2Pu= 1/RPu

(2)

d d . )k3Md2u = )k3Adu + R 3 u X 3 u ,

%dU = 1/Rdu 4. Steady state residence times

(3)

d d =X3A3Pu+ R3uA3c, v e . X3A2Pc + R3uA3u

.rPu = 1/Re3u The very stable meromictic structure of the Dead Sea described by Neev and Emery [1] for the early sixties gradually degenerated until its complete disappearance in the winter of 1978/79. One may distinguish between two major stages in the destruction of the meromictic layering: (1) before 1976 the density of the UWM gradually increased causing mixing and homogenization between the previous UWM (about 0-40 m) and the transition layer (about 40-100 m); during this phase there was no significant deepening of the pycnocline located at the top of the deep fossil water mass, and (2) beginning with winter 1976/77 and ending in winter 1978/79 there were successive deepenings of the meromictic pycnocline itself [3]. By comparing the density profile of March 1977 with those of previous years, one can observe that

(4)

For the LWM: ~.2A1L = ~.2AdL q_ R 2dL A 2dL ,

.

T2dL= 1/RdL

(5)

R 2dL A 2dL q- R 2Pu A 2 Pu = ~.2A2PL + R 2PL A 2PL , .

"r2PL= 1/RPL

(6)

~k3ddL = X3AdL %. R3LA3L, d d .

ydL = 1/RdL

(7)

R 3dL A 3dL + ~.3A2PL q- R 3Pu A 3Pu = ~.3A3PL q_ R 3PL A 3PL ,.

q'3PL= 1 / R Pt.

(8)

where X denotes radioactive decay constant (year a), A activity (dpm cm-2), R removal constant (year 1), and ~- mean residence time (years).

398

The subscripts 1, 2, 3 denote 226Ra, 2 1 ° p b and 2 1 ° p o respectively, the subscripts U and L the upper water mass (UWM) and the lower water mass (LWM) respectively and the superscripts d and p, the dissolved and the particulate phases respectively. The removal from the dissolved phase and the removal of particles are assumed to follow firstorder kinetics. The reciprocals of the removal constants are the respective mean residence times. Thus r d, the mean residence time in the dissolved phase, is dictated by removal processes other than radioactive decay, and r P is the mean residence time of particles in the water column, which are removed by settling. In the above equations the following inputs have been disregarded since they are very small: (1) the atmospheric flux of 2t°pb (about 0.15 dpm cm 2 yr 1 compared to the in situ production from 226Ra which is 34.9 dpm cm -2 yr -a in the upper 100 m, 44.8 in the upper 140 m and 76 in the whole water column), (2) the in situ production

of 2t°pb from 226Ra activity (3 d p m g 1) in suspended matter (about 5 mg 1 t), (3) the in situ production of 21°pb from excess radon at the lake floor (since excess radon was not detected in the bottom waters of the Dead Sea [6] its concentration can be no more than 3% of the radon in equilibrium with radium), and (4) loss of radon to the atmosphere across the surface of the Dead Sea has also been disregarded, although this flux, which has never been measured, is probably not negligible. However, we speculate here that the excess radon in the inflows and in shallow subsurface springs may counterbalance the losses across the air-lake interface so that the resultant 222Rn is approximately at equilibrium with 226Ra. Residence time calculations were performed in two ways: in the first, the transition layer 100-140 m (which is a mixture of U W M with LWM) was included within the LWM and in the second within the UWM. This procedure enabled us to obtain more reliable residence times for the oxic layer in the first run of the calculations and more reliable

TABLE 4 Residence times, settling fluxes and settling velocities of 21°pb and 21°po in the Dead Sea in March 1977 Layer thickness (m) 0-100 (oxic) Mean depth, h (m) 76.53 Volume (km 3 ) 61.22 22~Ra (10 12 dpm) ~ 8980 Residence times (years) for ~l°Pb: r d 23.5 rp 0.72 for2mpo: r d 1.5 (1.49) rp 1.6 (1.57) Particulate settling fluxes (dpm cm 2 yr 1) for 2mPb 19.7 b for 2mpo 66.9 b Settling velocities v (m yr i) for 21°pb 106 for 21oPo 48

100-325 (anoxic) 106.17 84.94 10600 3.6 4.7 1.0 (0.99) 2.1 (2.05) 43.5 105.7 22.6 51

0 140 (oxic) 101.35 81.08 11712 18.3 1.7 1.5 (1.54) 2.4 (2.36) 27.6 b 64.0 b 60 42

140-325 (anoxic) 81.35 65.08 7860 2.8 4.4 0.63 1.6 (1.64) 39.2 104.1 18.5 50

a For 0-140 m and 140-325 m the inventories of 2mPb and 21°po are given in Table 4. For the layer 0 100 m the inventories (10 12 dpm) are: 3790, 114, 2722 and 695 for dissolved and particulate 2mPb, and dissolved and particulate 2mpo respectively. By subtracting these data from the totals (dissolved or particulate) given in Table 4, one obtains the inventories of the 100-325 m layer. b According to the lake bathymetry, part of this flux settles on the shallow areas of the lake bottom and part of it enters the deeper waters. Therefore, the fraction of the particulate flux (of 2mpb and of 2mpo) which actually penetrates below 100 m (and which is used in equations (4) and (8)) is only 0.65; the fraction of the flux which penetrates below 140 m is only 0.594. The whole lake particulate flux for 21°pb and 21°po is 50.4 and 129 dpm cm 2 yr 1 respectively.

399

residence times for the anoxic layer in the second run. Table 4 summarizes the results of our calculations. The residence time of dissolved 2a°pb in the UWM (18-24 years) resembles that of the deep ocean which was found to be 20-90 years by Bacon et al. [11] and about 50 years by Somayajulu and Craig [12], and is much longer than that of the mixed layer of the ocean (1.4-2.3 years, [11]). It may be that this difference is due to the absence of biological recycling in the Dead Sea. The much shorter residence time, 2.8-3.6 years, estimated for the anoxic LWM is very similar to that found in the deep anoxic basin of the Cariaco Trench by Bacon et al. [7], who suggest that this rapid removal is related to the formation of an insoluble sulphide phase. The residence time of particulate Zl°pb with respect to settling is of the order of 1 year in the oxic waters and 4.4-4.7 years in the deep waters. Consequently the mean settling velocities (e) of 21°pb loaded particles (estimated either by dividing the settling flux by the average particulate activity or equivalently by dividing the mean depth by the residence time Tp) are much higher in the oxic UWM, about 100 m yr-1 vs. about 20 m yr 1 in the anoxic LWM. The simplest explanation for this difference is that the particles carrying the particulate 2t°pb activity in the UWM, mainly allochthonous, are larger than the freshly formed iron sulphide particles which probably carry the 21°pb particulate activity in the LWM. But there are other possibilities which cannot be excluded: (1) Since the separation of particulate from dissolved 21°pb was performed by 0.45-/zm Millipore membranes, the particulate filtering efficiency would necessarily differ for any two water samples whose particles had different size distributions. Thus, if the mean particle size were lower in the UWM, this along would make the particulate 2t°pb activity appear lower and yield an artificially high settling velocity in the UWM relative to the LWM. (2) According to our model all of the dissolved Zl°Pb which is "missing" from the UWM is transformed into particulate 2t°pb. However, if we assume that some of the dissolved 2t°pb is scavenged directly from the dissolved form at the oxic sediment interfaces (as suggested [11] for the

deep sea bottom), then equation (1) should be rewritten with an additional removal term on its right hand side. This would make the ( R 2aA 2d) u term smaller which in turn would make the ( R PzAz)u p term in equation (2) smaller too. Thus settling velocities in the UWM which are derived by this treatment will become smaller too and approach those estimated for the LWM. Smaller settling velocities of Zt°pb in the UWM could also be obtained if the additional removal term proposed above were to express radon loss to the atmosphere. In the UWM, the residence time of dissolved 2 1 ° p o is much lower than that of its parent (precursor) suggesting lower stability for 2t°po in oxic waters than for 2t°pb. That the residence time of dissolved 2t°po in the LWM is only about twice as low as in the UWM implies that the lack of oxygen does not have as dramatic an effect as it does for 21°pb. The explanation for this may be that polonium belonging to group Via, has a smaller tendency to form insoluble sulfides in the slightly acid (pH 6) anoxic deep waters of the Dead Sea. The residence times of particulate 21°po in the two environments are comparable (about 2 years) and so are the settling velocities of 21°poloaded particles (about 50 m yr t). It should be mentioned that in the dense (1.23 g cm 3) and viscous Dead Sea (3.2 centipoise at 22°C; [13]) Stokes velocities of settling particles are about one-fifth what they would be in a lake of similar dimensions. * Finally, it should be noted (Table 4) that the settling flux of 2 1 ° p o from both water masses, exceeds that of 21°pb, thus emphasizing the lower solubility of 21°p0 a n d / o r its association with larger particles. The reader should be reminded again that the actual steady state parameters might have been somewhat different from those derived from the * The Stokes velocities in a lake are, of course, independent of its dimensions. Yet, we mention "of similar dimensions" here because where mean residence times are determined from equations such as above, a mean particle settling velocity may be computed by v = h/'r p, where h is the mean depth. Thus it turns out that for two lakes of similar dimensions but of different densities and viscosities, ~-P must be smaller in the less dense and viscous lake.

400

March 1977 profile. However, we believe that the above calculations, though not completely exact, provide an insight into the 21°pb-2]°po system in the stratified Dead Sea.

can be seen that the percent of dissolved 2mpb in both oxic and anoxic layers gradually decreases, suggesting enhanced adsorption on particles. 21°po follows the same trend (Table 3). A second way of following the gradual changes occurring in the 21°pb-Z]°Po system is to compare the inventories of the 1977-1979 cruises (Table 5 and Fig. 3). The inventories were calculated by multiplying the average activity of each layer (oxic and anoxic) by its respective volume of water. The activities of the transition layers were included with the inventories of the oxic layers. It should be kept in mind that during destratification simple mixing of the oxic layers with deeper layers, in the absence of any chemical interaction, would result in a gradual increase of the oxic inventories at the expense of the anoxic ones. Summing up the destratification period, by February 1979 only 60% of the total 21°pb inven-

5. The response of 21°pb and 21°Po to the gradual destruction of meromixis The vertical profiles of 2]°pb and 2]°Po during 1977-1979 changed gradually as the destratification of the Dead Sea proceeded (Fig. 2). The changing profiles demonstrate not only the mixing between layers with different 2]°pb (or 21°Po) activities, but also the occurrence of chemical interactions. One way to quantify the changes in the 2]°pb-21°po system accompanying the destruction of meromixis is to compare dissolved vs. total 21°Pb (or 21°po) during 1977-1979 (Table 3). It

Anoxic layer

200 I00

Part. 210po ~

l~J~oxic layer

o i ~ 21o~

0 0

500

- 500 L Oiss2fOpo

400 300

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400

Oxic layer

300

200 I00

O0

,.xlX ~.

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300

t 2lOpb

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1978

1979

1977

1978

1979

Fig. 3. The inventories(dpm cm -2) of 21°pb (at left) and of 21°Po(at right) in the whole lake (bottom), in the oxic layer (middle) and in the anoxic layer (top) during 1977-1979.

401

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402 tory of March 1977 was present in the lake. As regards dissolved 21°pb, it seems apparent (Table 5 and Fig. 3) that the destratification process was accompanied by chemical interactions. Both oxic and anoxic dissolved 21°pb inventories decrease gradually from 1977 towards the end of 1978; the dissolved anoxic 21°Pb inventory decreases much faster than would be expected by its diminishing volume, though any increased removal by sulphide phases can be ruled out. (The slight increase in anoxic dissolved 21°Pb in November 1978 is not relevant since stratification was already very poor and some intrusion of oxic water, rich in dissolved 2~°Pb, was possible as suggested also by tritium measurements [14].) Finally, after the turnover, the dissolved 21°pb inventory of the whole lake was only 61.5% of what it had been at the beginning of our survey in March 1977. It seems therefore that 21°pb had been efficiently removed from the water column during this period in two ways: by adsorption on relatively rapidly settling particles, and by a direct loss of dissolved 21°pb without the intermediation of suspended particles. The first of these involves both iron hydroxide particles and resuspended bottom particles. Concomitant with the meromictic pycnocline deepenings, iron hydroxide, which has been formed by the oxidation of dissolved Fe(II) from previously anoxic zones, may adsorb 21°pb and be at least partially responsible for the decrease of dissolved 21°pb in the oxic zone and even more so in the anoxic zone. (It seems unlikely that dissolved 21°pb could be scavenged from the water column by manganese oxides as suspended manganese was at the detection limit [15].) The removal of dissolved 21°pb may also be enhanced by the resuspension of bottom sediments underlying the previous (before it deepened) pycnocline zone and the subsequent adsorption of dissolved 21°Pb to the resuspended particles. The second mechanism mentioned above is also related to the destratification process: bottom surfaces which had previously been both, anoxic and very rich in dissolved Fe(II), [5] become exposed to oxic conditions; such freshly oxidized surfaces may have efficiently scavenged dissolved 21°pb directly from solution. As for particulate :l°Pb, from March 1977 to November 1978, two trends are observed: the in-

ventories of particulate oxic 21°pb do increase, but less than expected by mixing with anoxic layers rich in particulate 21°Pb; and the particulate anoxic 21°pb inventories diminish somewhat more than expected by the diminishing volume of the anoxic water mass. Both of these observations suggest that removal of particulate 21°pb from the lake has occurred. This removal, in both the UWM and the LWM, may have occurred by means of a relatively rapid sinking phase, which was not as abundant in the lake waters before 1977, like iron hydroxide particles. The inventories of dissolved oxic 21°po and of dissolved anoxic 21°po generally follow the same decreasing trends towards the turnover as do the inventories of dissolved 21°pb, indicating the loss of dissolved 21°po from the water column. However, these losses being more moderate than those of 2~°Pb, we find in February 1979 73.9% of the dissolved 21°Po inventory of March 1977 (vs. only 61.5% for 2mPb). The trends of the particulate oxic and particulate anoxic 2~°Po inventories are not very distinct. The large increase in particulate 21°po in February 1979, is a remarkable feature which becomes even more outstanding when one compares it with the impoverished particulate 21°Pb inventory. It is because of the relatively large particulate 21°po inventory of February 1979, that one faces the peculiar situation in which the whole lake (dissolved+ particulate) inventory of 21°po became larger than that of its parent 21°pb: the ratio between the 21°po and 21°pb inventories in February 1979 had reached 1.4; in March 1977 it was 0.9 and in early 1978 it might have reached unity. It is suggested here that unsupported 21°po found in the lake in February 1979 will decay to the point where the 21°Po : 21°pb inventories of the mixed lake will approach the ratio of about 0.94 which was measured in the oxic layer in March 1977. As to the process responsible for the increase in the inventory of particulate 21°po in February 1979, we propose that during late 1978 the deepening of the pycnocline zone and related mixing had triggered resuspension of bottom sediments which in turn supplied particulate 21°Po to the lake waters (evidence is presented below). Unfortunately, the few measurements of particulate 21°po in Novem-

403 ber 1978 do not allow us to make an inventory but we speculate that if our resuspension hypothesis is true, this inventory might have been even larger than that of February 1979. The resuspended sediments must have had a relatively high 21°po/21°pb ratio since the effect is readily observed in the particulate 21°po but not in the 21°pb inventory. In fact, particles freshly settled at the lake floor in regions which directly underlie the upper water mass are indeed expected to have high 210Po/21o Pb ratios since they are derived from the oxic layer in which this ratio is about 6. The resuspension hypothesis is supported by the following evidence: (1) A large increase in the suspended iron inventory of the lake was observed in November 1978 by Nishri and Stiller [5]; they suggested that the enhanced destratification involved the resuspension at the water-sediment boundary zone, of sediments which were rich in freshly formed (and recently settled) iron hydroxide. (2) The particulate 21°pb inventory of November 1978 being the same as in August 1978 constitutes an exception to the generally decreasing trend of the particulate 21°pb inventories, suggesting that part of the particulate 21°pb in the November 1978 inventory was derived from resuspended sediments. According to Nishri and Stiller [5] part of the resuspended sediments had already resettled by February 1979 implying that the particulate 21°Po inventory, had it been measured, was even larger in November 1978 than in February 1979. (3) Particulate 21°po activities (unfortunately only 2 data points exist) in the upper water mass in November 1978 are the largest ever measured in the oxic waters of the Dead Sea.

(2) The meromictic steady state distribution of 21°Pb was disrupted by several processes related to the destruction of the stratification. In addition to inherent changes in the 2a°pb profiles caused by mixing of the oxic UWM with underlying anoxic layers, dissolved 21°pb was scavenged from the water column by freshly formed iron hydroxides and by freshly oxidized bottom interfaces. The effect of the latter two processes overrode the diminished scavenging by sulfide phases. (3) The gradual increase in particulate 21°Po suggests that concomitant with the turnover, resuspension of bottom sediments from intermediate depths has occurred. (4) Due to the high density and viscosity of the Dead Sea the "steady state" residence times with respect to settling of particulate 21°Pb and of particulate 21°po are about 5 times larger (and the settling velocities about 5 times smaller) than in a freshwater lake.

6. Conclusions

References

(1) The gradual approach to the overturn of the Dead Sea was clearly reflected by the vertical profiles of dissolved 21°pb and of particulate 21°pb which had sharp activity gradients at the gradually changing depth of the interface between the oxic and anoxic water masses and thus implemented the pycnocline deepenings.

Acknowledgements We are indebted to Dr. Y. Chung for letting us use his unpublished dissolved 2t°pb data of the February 1978 cruise, and for many very helpful critical comments on the manuscript, to K. Kim who performed some of the analytical work for the above mentioned measurements and to Prof. H. Craig for his interest in carrying out this study. Thanks are due to Dr. M. Bacon for his careful review and to Prof. J.R. Gat for critically reading the manuscript. The skillful technical help of Mrs. N. Bauman is gratefully acknowledged. This work was partially supported by grant No. 936 of the U.S.-Israel Binational Science Foundation.

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404

4

5 6 7

8

9 10

and W. Weiss, The Dead Sea: deepening of the mixolimnion signifies the overture to overturn of the water column, Science 206, 55-57, 1979. A. Nissenbaum, The geochemistry of the Jordan River-Dead Sea system, Ph.D. Thesis, University of California, Los Angeles, Calif., 1969. A. Nishri and M. Stiller, Iron in the Dead Sea, Earth Planet. Sci. Lett. 71,405-414, 1984 (this issue). Y. Chung and H. Craig, Radium in the Dead Sea, Earth Planet. Sci. Lett. (in press). M.P. Bacon, P.G. Brewer, D.W. Spencer, J.M. Murray and J. Goddard, Lead-210, polonium-210, manganese and iron in the Cariaco Trench, Deep-Sea Res. 27A, 119-135, 1980. A. Pruzhansky, R. Levin and F. Yaron, 21°Pb content of rainfall in the Israel Coastal Plain, NCRN-(TN)-001, Nuclear Res. Center, Negev, 1977. A. Oren, Bacteriorhodopsin-mediated CO 2 photoassimilation in the Dead Sea, Limnol. Oceanogr. (in press). M. Stiller and Y.C. Chung, Radium-226 in the Dead Sea: a

11

12

13

14

15

possible tracer for the duration of meromixis (submitted to Limnol. Oceanogr.). M.P. Bacon, D.W. Spencer and P.G. Brewer, 21°pb/226Ra and 21°po/21°pb disequilibria in seawater and suspended particulate matter, Earth Planet. Sci. Lett. 32, 277-296, 1976. B.L.K. Somayajulu and H. Craig, Particulate and soluble 21°pb in the Deep Sea, Earth Planet. Sci. Lett. 32, 268-276, 1976. A. Nishri, Geochemical behaviour of manganese and iron in the Dead Sea, P h . D . Thesis, Weizmann Institute of Science, Rehovot, 1982. I. Steinhorn, Physical and hydrographic study of the Dead Sea during the destruction of its long term meromictic stratification, Ph.D. Thesis, Weizmann Institute of Science, Rehovot, 1981. A. Nishri, The geochemistry of manganese in the Dead Sea, Earth Planet. Sci. Lett. 7l. 415--426, 1984 (this issue).