226Ra,210Pb and210Po in the Red Sea

Earth and Planetary Science Letters, 58 (1982) 213-224 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

213

[6]

226Ra, 21°pb and 21°po in

the Red Sea

Y. Chung, R.C. Finkel and K. Kim Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093 (U.S.A.)

Received October 16, 1981 Revised version received January 5, 1982

Profiles of 226Ra and dissolved 21°pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including 226Ra and dissolved and particulate 21°pb and 2t°po. These profiles show several distinct features: (l) 226Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved 21°pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water Zl°Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4) 21°pb and 21°po activities are essentially identical to each other in both particulate and dissolved phases although 2t°po activities appear somewhat lower; (5) about 20% of the 21°pb and 21°po in the water column resides on particulate matter. Assuming the atmospheric 21°pb flux to be in the dissolved form and at the lower level of the normal range, i.e. 0.5 d p m / c m 2 yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.

1. Introduction

The Red Sea is an elongated rift valley connected with the Mediterranean by the Gulf of Suez and the Suez Canal at the northwestern end, and linked with the Indian Ocean by the Strait of Bab el Mandeb and the Gulf of Aden at the southeastern end (Fig. 1). The exchange of water masses between the Red Sea and the Mediterranean is negligible compared to that which takes place between the Red Sea and the Indian Ocean across the Strait of Bab el Mandeb. The shallow sill depth at Bab el Mandeb (about 100 m) effectively limits exchange of water to the upper 150- to 200-m thick layer. This restricted exchange enhances the intense evaporative effects occurring in this region and makes the hydrography of the Red Sea markedly different from that of the Gulf of Aden and the Arabian Sea. Using the observed hydrographic properties together with the wind stress field and estimates of evaporation rate, pre-

vious investigators have been able to deduce circulation patterns for the Red Sea and to estimate the exchange rate across the sill as well as the renewal time for Red Sea water (e.g. [1-4]). A comprehensive account of the general circulation is given by Siedler [4]. A more detailed review of the physical and chemical oceanography of the Red Sea, including early historical investigations, climatology and the Red Sea brines, is given by Morcos [5]. During the GEOSECS legs of the 1977-78 INDOMED expedition, about 50 stations covering the entire Indian Ocean and the Red Sea were occupied, and profiles of various geochemical tracers including radioactive and stable isotopes were measured. The four Red Sea stations, occupied in December 1977, were located along the central rift valley with one station (405) at the northwestern end, two (406, 407) at the middle section, and one (408) at the southeastern end (Fig. 1). Station 406, at the Atlantis II Deep, was occupied for the study of the Red Sea brines.

0012-821 X/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company

214 35*

30*

.Gulf

of

4S°E

40*

GEOSECS Red Sea Stations

Suez

30*N

Contours in meter5

•405

25*

25*

20 °

20 °

•

IS •

.,.

15 ° .

• .

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StraH of Bob el Mondeb

35 •

40 °

45*

Fig. I. GEOSECS stations where 226Ra and dissolved 21°Pb profiles have been measured.

226Ra profiles were measured at all the Red Sea stations including the brines sampled at 406 (brines data not included in this paper). Dissolved 2~°pb profiles were measured at stations 405, 407 and 408. 2]°pb and 2~°Po in both the particulate and dissolved phases were measured at station 407 only. We present in this paper the results of these nuclide measurements from the Red Sea stations and also from a station in the Gulf of Aden just outside the Red Sea (409, Fig. 1). These measurements allow us to study the effect which water exchange and flow between the Red Sea and the Gulf of Aden across the sill at the Strait of Bab el Mandeb have on the distribution of these nuclides in the two different oceanographic regions.

2. General circulation, exchange and hydrography Circulation of the Red Sea waters is thermohaline, seasonal and wind-driven. During the

winter season, the low-salinity surface water of the Gulf of Aden is driven by the SSE wind into the Red Sea through the Strait of Bab el Mandeb. This water becomes progressively more saline and denser as it flows northwestward due to extensive evaporation and progressive cooling. It sinks and mixes with the water underneath, and possibly becomes part of the deep water or flows southeastward as a return flow, supplying the warm saline water which flows over the sill into the Gulf of Aden. During the summer, the wind blows uniformly from the N N W direction over the entire Red Sea area. It is possible that the surface water may flow southeastward and across the Strait of Bab el Mandeb due to the wind stress. If so, the Gulf of Aden water probably flows into the Red Sea, as required by mass balance, at a level below the wind-driven surface outflow and above the overflow of the warm, saline water. The Red Sea waters are strongly stratified, with the main thermocline as a transition layer separating the surface mixed layer and the deep-water layer. Vertical exchange and mixing across the transition layer is very limited, especially in the southeastern half of the basin. Since the water at the sill depth level in the Red Sea is significantly denser than that in the G u l f of Aden, an overflow is expected. The Red Sea overflow is clearly indicated by a tongue of •warm saline water in the Gulf of Aden, which can also be observed in the Indian Ocean. The rate of overflow has been estimated at about 10.5 × 1012 rrd/yr [4]. If the overflow withdraws only from the upper 150m layer, the renewal time is about 6 years. Fig. 2 shows the potential temperature (0), salinity (S) and density (o e) profiles from GEOSECS stations 405, 407 and 408 in the Red Sea and 409 and 410 in the Gulf of Aden (Fig. 1) in two different scales. These profiles shown as mean curves are based on the discrete hydrographic data which are stored in the GEOSECS data bank at Scripps. The 8 profiles show the main thermocline to be at about 50-200 m depth with the smallest gradient and lowest temperature at station 405 in the northwestern end. The Red Sea overflow is clearly seen as a 8 maximum at stations 409 (bottom) and 410 (600 m). The salinity in the mixed

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Fig. 2. Potential temperature (0), salinity (S) and density (oe) versus depth (two different scales) to show differences between the Red Sea stations (405, 407, 408) and Gulf of Aden stations (409 and 410), and variation among the Red Sea stations. The Red Sea overflow is characterized by high 0 and S at around 600 m depth.

layer increases systematically from the Gulf of Aden to the northwestern end of the Red Sea, where a much smaller vertical variation is observed (station 405). The density profiles reflect the combined effect of the O and S distributions. The density in the upper 100-m layer increases rapidly from the Gulf of Aden toward the northwest as a result of the salinity increase and temperature decrease. Within the Red Sea, station 405 has slightly higher density than 407 and 408 except for the depth interval between 300 and 700m where the density is virtually identical. This suggests that the subsurface water above and below this interval probably flows southeastward. Fig. 3 shows the O-S curves of the same stations with density (oe) contours. The Red Sea deep water has the highest salinity with identical O-S curves everywhere. The core of the warm saline overflow water is marked by the distinct local 0 and S maximum at 600 m depth at station 410. At

409, the O and S maximum is at the bottom. All the water columns are stable as indicated by the density contours. The lower salinity of the nearsurface water at 408 dearly shows an inflow o f the Gulf water. In Fig. 4 we show both the oxygen and silica profiles of the same stations. The silica profiles show a mirror image of the corresponding oxygen profiles. The oxygen minimum (and Si maximum) deepens northwestward as its value increases (and

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Fig. 4. Oxygen and silica versus depth curves drawn from the discrete data plots. Notice the general mirror symmetry between the oxygen and silica profiles of the Red Sea stations: the oxygen minimum is correlated with the silica maximum; higher oxygen concentration is associated with lower silica concentration and vice versa. The mid-depth silica maximum and oxygen minimum are correlated with the 226Ra maximum (Fig. 5).

216 the Si maximum decreases). The distinct oxygen minimum and silica maximum at station 409 outside the Red Sea are induced by the overflow water which has higher oxygen and lower silica content than the Gulf of Aden ambient water. In the Red Sea, the layer of oxygen minimum and silica maximum observed between 300 and 700 m depth coincides with the layer of identical density profiles (i.e. a layer of essentially no horizontal density variation, Fig. 2). This suggests that, unless the oxygen consumption rate is enhanced at this depth, this layer is probably more stagnant and so has a longer renewal time than other layers. The highest 226Raconcentration is also observed in this layer, as we shall show later in this paper.

3. Sample collection and analytical methods Samples for 226Ra measurement were aliquots of the 14C samples which were collected and transferred directly from the Gerard barrels to 200-liter ~4C-stripping drums, acidified and then stripped of CO2 with nitrogen. The 226Ra samples were collected in 20-liter containers (jerry jugs) and returned to the laboratory. 226Ra was determined by the standard GEOSECS method [6,7] which measures the radon regenerated by 226Ra in a closed system (i.e. a 20-liter glass bottle). To avoid the scavenging and contamination effects observed earlier in the Pacific Gerard samples [8], 21°pb samples were collected exclusively from the Niskin bottles. The Niskin samples were filtered through 0.45-/~m nuclepore filters immediately after collection. Particulate 2~°pb and 2~°po were determined from these filters. The filtered waters were stored in 20-liter plastic containers and acidified with HC1 to a pH of about 1.5-2. A 2°8po spike and stable Pb carrier in the form of Pb(NO 3)2 were added to each sample along with a Co(NO3) 2 solution. After allowing at least one day for equilibration, APDC was added to precipitate the Co-APDC chelate which scavenged Pb and Po. The greenish precipitate was filtered onto a 0.45-/~m millipore filter and returned to the shore-based laboratory for analysis

of 21°pb and 21°po in the dissolved phase. In the case of particulate samples, Pb carrier was added at the time the nuclepore filter was decomposed in NH4OH. The 21°po activities in both the particulate and dissolved phases and the particulate 21°pb were measured only at station 407 by alpha particle spectrometry using the techniques of Bacon et al.

[10]. Both dissolved and particulate samples were dissolved in mineral acids and Po autoplated onto silver disks at pH 2. For the particulate samples, plating salts were then decomposed with nitric and hydrochloric acids and Pb precipitated with H2SO4. The PbSO4 precipitate was dissolved in ammonium acetate and passed through a small anion exchange column to remove residual Po isotopes. The samples were then allowed to sit for several months for grow-in of 21°po. 2°8po spike was added in the middle of the grow-in period. The 2t°pb activity was calculated from the level of regrowth of 21°po and from the Pb chemical yield as determined by atomic absorption. For the dissolved samples, nitric and perchloric acids were added to oxidize the plating salts. The samples were heated to dryness and then dissolved in ammonium acetate for further purification for 21°pb determination by beta counting. The chemical procedure for the preparation of PbSO4 beta-counting samples from the filtered APDC precipitate has been given elsewhere [9]. The dissolved 21°pb samples were counted for their Zl°Bi activities about a month after purification using gas-flow anti-coincidence 2~r beta counters. After 21°pb activities were determined the observed initial Zl°Po activities were corrected for decay and 21°pb production back to the sample collection time. As the dissolved 21°Pb activities in the Red Sea are very close to the detection limit by beta counting the data so obtained have a large uncertainty. In order to check the beta-counting results we redissolved station 407 samples and replated for 2~°po grown in from 21°pb for alpha counting. The results are discussed in the following section.

217

4. Results

4.1. 226Raprofiles The 226Ra results are given in Tables 1 and 2 and shown in Fig. 5. Except for a deep sample at station 407 (sample 294, 1539m) which shows unusually high activity, all the stations have similar profiles with comparable activities. Station 405 has essentially constant 226Ra concentrations below 4 0 0 m or a gentle m a x i m u m between this depth and the bottom ( - 1 2 0 0 m ) . Station 407 shows a broad m a x i m u m between about 300 and 800 m depth. Below 8 0 0 m 226Ra concentrations are somewhat lower than those of station 405. The surface water 226Ra concentration increases from the Gulf water at 7.4 d p m / 1 0 0 kg to 12.6 d p m / 1 0 0 kg at station 405 (Table 1). Although the

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ACTIVITY (dpm/lO0 kg) Fig. 5. 226Raand dissolved 21°Pb profiles measured in the Red Sea (405-408) and in the Gulf of Aden (409). The measurement uncertainty (2o) is less than or equal to the indicated error bar. The bottom depth of each station is marked. A 226Ra value at station 407 and a 21°pb value at station 405 appear to be unusually high and questionable. Some 226Ra values at station 407 (9.8, 10.6, and 12.6 dpm/100 kg at 179. 285, and 594 m depths, respectively) are perhaps too low compared to values at other stations (405 and 408).

trend of increase is similar to the salinity increase, the relative increase of 226Ra is much greater than that of salinity. Thus, evaporation alone cannot account for the 226Ra increase. If the single observed high value is typical in the northwest Red Sea, it implies that there is a 226Ra input into the surface water from shallow-water sediments. At station 405, the salinity minimum at about 100 m depth (Fig. 2) is marked by a distinct 226Ra minimum. A similar 226Ra minimum is observed at station 408 at shallower depth although no salinity minimum is seen at this station. This 226Ra minim u m presumably traces the Gulf water that advects into the Red Sea. Another interesting feature is that the 226Ra profiles at stations 405 and 408 are identical below the depth of the minimum although they are located at the opposite ends of the Red Sea (Fig. 1). This suggests that some values at station 407 (179, 295, 594 m) are perhaps too low (Fig. 5). Below 1000m depth, 226Ra, as seen at station 407, is constant, suggesting a well-mixed layer. Station 409 in the Gulf of Aden shows a minimum induced by the overflow. The layer of 226Ra m a x i m u m in the Red Sea lies at the same depth as the layer of the oxygen minimum and silica maximum (Fig. 4). This 226Ra m a x i m u m occurs at around 500 m depth and probably reflects a longer residence time of the water, as indicated by the oxygen minimum and silica maximum, which allows 226Ra to accumulate in this layer by diffusion from sediments at this depth level followed by horizontal mixing. At station 409 the induced silica maximum appears to correlate with the induced 226Ra m a x i m u m at around 3 0 0 m depth. However, 226Ra increases rather than decreases toward the bottom resulting in a local minimum at 400 m (Fig. 5). Apparently the bottom flux of 226Ra in the Gulf of Aden is sufficient to compensate the dilution effect of the overflow water which has lower 226Ra content than its ambient water. If we compare the 226Ra profiles of the Red Sea with those of the northern Indian Ocean or of the Arabian Sea (unpublished GEOSECS data), we see that the Red Sea values are higher than the open ocean values above 700 m depth and lower below this depth. The higher 226Ra concentrations

218 TABLE 1 226Ra and dissolved 21°pb in the Red Sea and the Gulf of Aden (station 407 data are given in Table 2) Sample No. a

Depth (m)

Station 405 (27"14'N, 34°28'E; i 181 m) 101, 102 7, 27 286 29 287 84 105 148 288 149 107 247 290 248 110 398 291 398 114 547 292 548 117 695 293 696 120 843 294 846 123 992 295 994 Station 408 (14°42'N, 42 ° lifE; 597 m) 113 15 286 18 287 57 288 76 290 83 116 84 291 102 117 103 292 175 118 182 293 307 120 326 122 434 294 435 295 577 124 587 Station 409 (12"07'N, 43"55'E; 687 m) 113 16 286 18 287 53 116 108 288 147 290 196 291 246 120 333 292 337 293 412 122 416 294 493 124 501 295 580

226Ra (dpm/100 kg) b

Dissolved 21°pb (dpm/100 kg) c

2.9±0.6 d 12.6±0.5 9.1±0.5 2.6±0.6 11.5±0.4 2.3±0.3 11.8±0.4 3,1±0.4 13.0~0.5 2.3±0.5 13.8±0.6 4.9±0.4 13.6±0.7 2.8±0.3 14.4±0.9 2.7±0.6 13.0±0.4 2.0m0.4 e 8.7±0.6 8.1±0.5 8.8±0.6 8.9±0.6 1.3±0,3 10.4±0.5 1.8±0.3 11.1±0.7 1.9~0.3 12.8±0.9 1.4±0.5 0.9±0.5 12.8±0.9 13.6±0.8 1.9±0.4 3.8±0.5 ~ 7.4±0.5 7.9±0.7 4.0±0.6 9,4±0.7 9.7±0.6 11.0±0.5 4.3±0.4 10.8~0.7 10.4±0.3 2.8±0.3 12.4±0.8 2.7±0.4 13.0±0.5

219 TABLE 2 226Ra and dissolved and particulate 21°pb and 21°po at station 407 (19°57'N, 38°29'E; 1957 m) Sample No.

Depth (m)

226Ra(dpm/100 kg) a

21°pb (dpm/100 kg) dissolved a b

102 387 104 390 106 286 287 108 288 110 290 112 114 291 116 292 118 293 120 122 294 124 295

29 70 I00 179 278 295 ,t,44 462 594 630 793 795 865 892 994 1092 1142 1291 1338 1488 1539 1665 1787

21°po (dpm/100 kg) particulate

dissolved b

particulate b

fl=

1.25±0.08

0.97±0.10

8.6±0.6 2.1±0.2

2.2±0.5

0.41±0.03

0.6±0.3

0

2.1 ±0.2

2.2±0.4

0.31--0.02

1.4±0.3

0.14±0.02

2.3±0.3

1.6±0.6

0.19±0.02

0.4--0.4

1.5 ±0.2

1.4±0.3

0.32±0.02

1.5±0.3

1.9±0.2 2.3±0.2

2.3--0.3 2.1±0.5

0.34±0.02 0.36±0.03

1.3--0.3 0.5±0.3

1.5±0.2

1.6±0.5

0.29--0.02

1.3±0,3

1.4±0.3

1.8±0.4

0.65±0.03

0.7±0,5

1.6±0.2 1.8±0.2

1.8-+-0.3 1.4-4-0.6

0.~±0.03 0.35±0.02

1,2--0.3

1.5--0.2

1.8±0.5

0.67±0.05

0.6±0.3

9.8±0.5 10.6±0.9 13.5±0.5 12.9±0.4 13.5--+-0.5 0,11±0.02

12.3±0.7 11.0±0.5 0.56±0.~

10.8±0.6

13.7±0.4 0.39±0.05

10.8±0.4

Quoted uncertainty is the greater of 1o counting statistics or 1o standard deviation of the mean of replicate measurements. b Quoted uncertainty is the greater of lo counting statistics or 1o standard deviation of the mean of replicate counting in different detectors or counters. c Quoted uncertainty is the quadratic sum of 1o counting statistics, the blank uncertainty and, where applicable, the uncertainty in chemical yield.

TABLE 1 (continued) Sample No. = Cast No. × 100+ Bottle No. Bottle No. ~ 86 = Gerard; Bottle No. g 24 = Niskin. b Uncertainty quoted is either Io counting statistics or Io standard deviation of the mean of replicate measurements, whichever is greater. c Uncertainty quoted is either 1o counting statistics or 1o standard deviation of the mean of replicate counting in different detectors or counters, whichever is greater. d Total :l°Pb measured from an unfiltered water sample. e Sample consists of 40% unfiltered and 60% filtered water. a

220 in the Red Sea surface water are due to concentration by evaporation of the surface water advecting from the Gulf of Aden and possibly also due to additional input from shallow-water sediments. The higher Red Sea 226Ra values above 700m depth derive from large area of sediments lying above 700 m depth. The portion of the Red Sea underlain by sediments shallower than 700 m in water.depth is at least 70% of the total surface area.

PARTICULATE PHASE 0 10 20 30 0

~ 1o

4.3. Profiles of dissolved 21°po, particulate 21°po and particulate 2t°pb at station 407 Particulate and dissolved 21°pb and 21°po data from station 407 are listed in Table 2 and plotted in Fig. 6. The dissolved 21°pb data as measured by both alpha and beta counting are in good agreement although larger uncertainties are observed in

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4.2. Dissolved 21°pb profiles The dissolved 21°pb data are listed in Tables 1 and 2, and plotted in Fig. 5 together with the 226Ra data to show distinct 21°pb-226Ra disequilibrium. Since the 21°pb activity level in the Red Sea is much lower than in the open ocean, the measurement uncertainty is at least - 10% and may be as high as -50% in some cases. However, at station 407, the additional 21°pb data obtained by alpha counting agree very well with those obtained by beta counting (Table 2). The overall dissolved 210Pb concentration in the Red Sea appears to be quite constant at about 2 dpm/100 kg from the surface to the bottom except for a high value at station 405 (sample 117, 695 m) which may be erroneous. T h e uniformity of dissolved 21°pb activity in both the vertical and horizontal directions and its extremely low values are the most striking features observed, considering the 226Ra distribution, which shows much higher concentrations with a distinct maximum around 500 m depth in all profiles. In the Gulf of Aden the dissolved 21°pb activities above 350 m depth as seen at station 409 are significantly higher than those observed in the Red Sea (Fig. 5) although the associated 226Ra c o n c e n t r a t i o n s are somewhat lower than the Red Sea values.

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Fig. 6. 2J°pb and 2i°po profiles of both dissolved and particulate phases at station 407. The dissolved-phase profiles are scattered due to large uncertainty of 21°pb values. These samples were reprocessed and plated for alpha counting. The new results are in good agreement with the beta-counting results, 21°pb and 21°po activities in the particulate phase are the same: both profiles show a slight increase with depth except for the surface water.

beta counting as expected. The dissolved 216pb profile has a mean activity of 1.8 dpm/100 kg. It appears that slightly higher activities are observed above 1000 m than below this depth. The dissolved 2~°Po activities are more scattered and tend to be lower than the dissolved 21°pb activities. The significance of this difference is difficult to assess at present due to the large measurement uncertainties resulting from the low sample activities. Particulate 21°pb and 21°po are identical within analytical uncertainty, and, except for higher activity at the surface, appear to increase slightly with depth. With an average particulate activity of about

221 0.5 dpm/100 kg for both 21°pb and 210po and an average dissolved activity of 1.8 dpm/100 kg for 21°pb and 1.0 dpm/100 kg for 21°po, the particulate activities account respectively for more than 20% and 30% of the total. In the open ocean particulate activities are generally only about 510% of the total activities with a tendency to increase toward the bottom (e.g. [8,10-12]). The extremely low activities of 21°po and 21°pb in the dissolved phase relative to 226Ra activities and the relatively large portion of these nuclides dwelling in the particulate phase suggest that settling particulate matter is probably acting as an effective scavenger in the Pb and Po removal processes.

5. 226Ra-Zl°Pb-Zl°Po relationships The measured 226Ra profiles in the Red Sea are nearly identical, all showing a distinct maximum around 500 m depth (Fig. 5). The dissolved 21°pb profiles are also identical but show much lower concentrations. The concentration of dissolved 21°pb is thus uniformly low in the entire Red Sea when compared with open ocean values. The 21°pb excess over 226Ra resulting from the atmospheric flux into the surface water, which is often observed in the open-ocean surface waters at low latitudes [8,10,11] is entirely lacking. This implies that either the atmospheric 21°pb flux is extremely small or the removal rate of 21°pb is exceptionally rapid. The removal processes may include strong boundary uptake at the shallow sediment/water interface. We will come back to this later in the paper. The activity ratio of dissolved 21°pb to 226Ra decreases from about 0.20 near the surface to about 0.15 in the maximum 226Ra layer, and then increases slightly to a constant of about 0.16 in the deep water below 1000m depth. The small variations of the activity ratio are probably related to horizontal mixing and boundary scavenging processes. The minimum ratio observed at the 226Ra maximum may indicate a boundary layer effect leading to enhanced 226Ra input and enhanced 21°pb removal. Assuming no atmospheric 21°pb flux, the apparent residence time for Pb in the surface layer, mid-depth 226Ra maximum layer,

and deep water below 1000 m is calculated to be 8, 5.6 and 6 years, respectively. These Pb residence times which are comparable to the fluid renewal time of the upper 200-m layer, are considerably shorter than those found in open-ocean deep water (e.g. [8-10]). In the Gulf of Aden, much higher dissolved 21°pb is associated with somewhat lower 226Ra in the shallow water, resulting in a much higher activity ratio ( ~ 0.4) and so a much longer residence time ( ~ 20 years). In the Cariaco trench, an anoxic environment, Bacon et al. [12] found an even smaller activity ratio (about 0.05), indicating a residence time of less than 2 years for Pb due to rapid particle scavenging. Po is more reactive than Pb in general in the near-surface layer of open-ocean waters as indicated by higher 21°po than 21°pb in the particulate phase and vice versa in the dissolved phase [10,13]. In the Red Sea dissolved activities are consistent with this pattern; however, particulate 21°po and 21°pb are equal within experimental uncertainty.

6. 226Ra inventory and flux We can estimate the net 226Ra loss resulting from the exchange of water masses across the Strait of Bab el Mandeb if the transport volume and the 226Ra activities of the inflow and outflow waters are known. The evaporation rate has been estimated at 0.88 × 1012 m3/yr and the transport of the outflow at 10.5 × 1012mS/yr [4]. If the total volume of the Red Sea water is constant (i.e. no significant sea level change), then the inflow transport is about 11.4× 1012 m3/yr. With a mean activity of 7.5 dpm/100 kg for the Gulf surface water and l 1 dpm/100 kg for the outflow Red Sea water, the net loss of 226Ra is about 3 × 1014 dpm/yr. At steady state, this loss and the decay loss have to be balanced by an input from the bottom sediments. The total 226Ra in the Red Sea as integrated from our data over the entire volume is about 27.5 × l015 dpm (in a total volume of about 225 × 1012 m3). The decay loss is about 1.2 × l013 dpm/yr, about 4% of the loss by overflow. The 226Ra flUX from sediments, required mainly to balance the loss by overflow, is about 3.1 × 1014 dpm/yr. Taking the effective sediment

222

surface area for 226Ra diffusion to be about 0.44 × 1012 m 2, we obtain a 226Ra flux of about 7 × l0 s d p m / m 2 yr or 2.2 × 10 -9 dpm/cm 2 s. This value (about 3 atoms/cm2s) is quite similar to that estimated for the northeast Pacific where there is a strong 226Ra source, and is about three times the global average 226Ra flux [14].

7. 2t°Pb flux and residence time

The sources of 21°Pb for the Red Sea include in-situ production by 226Ra decay, net gain resulting from water exchange across the Strait of Bab el Mandeb, and atmospheric input. The first two sources can be estimated readily from our present data. The atmospheric flux in the Red Sea area is not known. Turekian et al. [15] have estimated the atmospheric 2~°pb flux in the mid-latitude regions of the northern hemisphere, and have found that the flux varies from about 0.5 to 2 d p m / c m 2 yr dependent on the proximity of continents and air mass trajectories. Similar fluxes have also been obtained by others based on 21°pb mass balance calculations in the open-ocean mixed layer [10,11]. In order to assess the effect of the atmospheric 21°Pb flux on the Pb removal rate in the Red Sea, we have considered both the upper and the lower limits of this estimated flux. With the lower limit of the atmospheric 21°pb flux of 0.5 dpm/cm 2 yr and the surface area of 0.44 × 1012 m2, the total atmospheric flux in the Red Sea is about 2.2 × 1015 dpm/yr. The upper-limit flux of 2 dpm/cm 2 yr yields a total atmospheric flux of 8.8× 1015 dpm/yr. As the atmospheric 21°pb flux is strongly enhanced by rainfall, the lower limit is more likely for the Red Sea area due to its arid condition. The total 226Ra content of the Red Sea integrated from our data is about 27.5 × 1015 dpm. The 21°pb produced by the decay of this 226Ra (neglecting effects of the radon loss in the surface layer and of the radon excess in the bottom water) is about 0.86 × 1015 d p m / y r which is about 40% of the total lower-limit atmospheric flux or 10% of the total upper-limit flux. Since the dissolved 21°pb activity of the surface water in the Gulf of Aden is about 2 dpm/100 kg higher than that of the overflow Red Sea water

(Tables 1 and 2, Fig. 5), there is a net gain of 21°pb in the Red Sea due to the advective transport and exchange of waters across the Strait. Assuming negligible diffusive transport, the net advective transport of 21°Pb into the Red Sea is estimated at about 0.25 × 1015 dpm/yr. Thus the total annual input rate of 21°pb into the Red Sea from these sources is about 3.31 × 1015 dpm, of which about 66% is due to the atmospheric flux. The portion of the atmospheric flux increases to about 90% when the upper limit is assumed. 21°pb in the Red Sea is removed by radioactive decay, by particulate scavenging [ 18,19] and perhaps by boundary scavenging [10]. Since the 21°pb inventory is quite low, the removal rate by decay is also quite small, i.e. 0.14× 1015 d p m / y r in the dissolved phase and 0.04X 1015 d p m / y r in the particulate phase. Assuming that the atmospheric 21°pb flux is at its lower limit (0.5 dpm/cm 2 yr) and is in the dissolved form, the rate of 2t°pb transfer from the dissolved phase to the particulate phase or directly to the sediments can be obtained by subtracting the decay rate (in dissolved phase, 0.14× 1015 dpm/yr) from the total input rate (3.31 × 1015 dpm/yr). This transfer rate is 3.17 × 1015 dpm/yr. Since the 21°Pb inventory in the dissolved phase is about 4.6 × 1015 dpm, the residence time for Pb in the dissolved phase is about 1.5 years. If the atmospheric 21°Pb flux is all in the particulate form, then the transfer rate from the dissolved phase to the particulate phase is only 0.97 X 1015 d p m / y r (the difference between the total transfer rate and the total atmospheric flux), corresponding to a residence time of about 5 years for the dissolved Pb. This is essentially what we have obtained from the observed 21°pb/226Ra activity ratio. The residence time for Pb in the particulate phase can be estimated assuming that particulate scavenging is the only removal process other than radioactive decay. Since the particulate 2~°pb concentration is about 0.5 dpm/100 kg of water filtered, the particulate 21°pb inventory is about 1.15 × 1015 dpm which has a decay rate of 0.04 X 1015 dpm/yr. Thus the net 21°pb removal rate by particulate scavenging is 3.13 × 1015 dpm/yr. The residence time for Pb in the particulate phase is

223 thus only 0.4 years. These short residence times (1.5 years for the dissolved Pb and 0.4 years for the particulate Pb) result from the relative importance of the assumed atmospheric flux which is unfortunately not well known. With the upper-limit atmospheric 21°Pb flux of 2 d p m / c m 2 yr, the residence times would be reduced to about one fourth of the above estimates. If there is no atmospheric 21°pb flux into the Red Sea, the residence time for Pb would be significantly longer. Our estimates based on the 21°pb-EErRa disequilibrium in the water column yield a mean residence time of about 6 years. If the mean particulate matter concentration and the mean deposition rate in the Red Sea are known, one can examine the consistency of 21°pb flux into sediments and also can evaluate indirectly the level of the atmospheric 21°Pb flux by measuring 21°pb concentrations in the top sediments. The surface-water particulate concentration in the Red Sea ranges from about 40 to 60/~g/kg based on large-volume underway filtrations carded out during the GEOSECS expedition [16]. Deep-water particulate concentrations are most likely lower than the above values. Taking 50 /~g/kg as a representative concentration and a mean particulate residence time of 0.4 years in a 500-m water column, we obtain a particulate flux or a deposition rate of about 6.4 mg/cm 2 yr. This flux is at most about 20% of that estimated from 14C dating on sedimentary cores [17]. The specific 2~°Pb activity of the particulate matter is 0.1 d p m / m g if the particulate concentration is taken as 50 #g/kg. Thus the 21°pb flux into the sediments is 0.64 d p m / c m 2 yr, of which 0.5 d p m / c m 2 yr, is the presumed lower-limit atmospheric flux. Measurement of the 21°pb concentration in the interfacial sediments may allow one to estimate the atmospheric 21°Pb flux in the Red Sea area indirectly.

8. Conclusions Our data on 226Ra, 2|°pb and 21°po in the Red Sea show that these isotopes exhibit a different behavior in this partially enclosed basin than has

been observed in open-ocean regions. 21°pb and 2~°po occur at significantly lower concentrations than at open-ocean sites. 226Raactivity is relatively high in Red Sea surface waters due to the effects of evaporative concentration and transport from shelf sediments. The extensive exchange of water between the high-EE6Ra Red Sea water and the low-E26Ra Gulf of Aden water results in a net 226Ra loss from the Red Sea by circulation. The 226Ra flux from bottom sediments required to balance this advective loss and the much smaller decay loss is about three times the global average sedimentary flux and about equal to that estimated for the northeast Pacific. The surface-water 21°pb excess which is commonly observed in lowlatitude open-ocean regions is entirely lacking. About 20% of the 21°pb and 2J°Po in the water column reside on particulate matter in comparison to generally less than 10% in open-ocean regions. Estimates of the 21°pb residence time depend strongly on the magnitude and form (bound or soluble) of the atmospheric flux assumed. Dissolved Pb has a residence time between 1.5 and 5 years for most likely values of the atmospheric flux. The residence time of particulate Pb is less than 0.4 years if all the Pb is removed only by sinking particles.

Acknowledgements Valerie Craig provided valuable help in collecting and processing many of the 21°pb samples at sea. C. MacIsaac helped with the 21°pb and 21°po analyses. H. Craig and two anonymous reviewers made valuable comments on the manuscript. This work was supported by a grant (OCE 77-24526) to the Isotope Laboratory and the Mt. Soledad Laboratory of the Scripps Institution of Oceanography from the International Decade of Ocean Exploration of the National Science Foundation, as part of the Indian Ocean GEOSECS program. Additional support to the Isotope Laboratory was provided by another grant (OCE 79-27284) from the Marine Chemistry Section, Ocean Sciences Division of the NSF.

224

References 1 E.F. Thompson, The general hydrography of the Red Sea, in: The John Murray Expedition 1933-34 Scientific Reports, Vol. 2. Meteorological, Chemical and Physical Investigations (1939) 83. 2 E.F. Thompson, The exchange of water between the Red Sea and the Gulf of Aden over the "sill", in: The John Murray Expedition 1933-34 Scientific Reports, Vol. 2. Meteorological, Chemical and Physical Investigations (1939) 105. 3 A.C. Newmann and D.A. McGill, Circulation of the Red Sea in early summer, Deep-Sea Res. 8 (1961) 223. 4 G. Siedler, General circulation of water masses in the Red Sea, in: Hot Brines and Recent Heavy Metal Deposits in the Red Sea, E.T. Degens and D.A. Ross, eds. (SpringerVerlag, NewYork, N.Y., 1969) 131. 5 S.A. Morcos, Physical and chemical oceanography of the Red Sea, Oceanogr. Mar. Biol. Annu. Rev. 8 (1970) 73. 6 Y. Chung, H. Craig, T.-L. Ku, J. Goddard and W.S. Broecker, Radium-226 measurements from three GEOSECS intercalibration stations, Earth Planet. Sei. Lett. 23 (1974) 116. 7 Y. Chung and H. Craig, 226Ra in the Pacific Ocean, Earth planet. Sei. Lett. 49 (1980) 267. 8 Y. Chung, K. Kim, M.D. Applequist and H. Craig, Lead-210 in the Pacific: particulate and dissolved concentrations, Earth Planet. Sei. Lett. (in press). 9 Y. Chung, 2t°pb and 226Ra distributions in the Circumpolar waters, Earth Planet. So. Lett. 55 (1981) 205. 10 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 (1976) 277. 11 Y. Nozaki, J. Thomson and K.K. Turekian, The distribution of 2t°Pb and 21°po in the surface water of the Pacific Ocean, Earth Planet. Sei. Left. 32 (1976) 304. 12 M.P. Bacon, P.G. Brewer, D.W. Spencer, J.W. Murray and J. Goddard, Lead-210, polonium-210, manganese and iron in the Cariaco Trench, Deep-Sea Res. 27A (1980) 119. 13 M.P. Bacon, D.W. Spencer and P.G. Brewer, Lead-210 and polonium-210 as marine geochemical tracers: Review and discussion of results from the Labrador Sea, in: Natural Radiation Environment III, Vol. 1, T.F. Gesell and W.F. Lowder, eds., U.S. Dep. Energy Rep. CONF-780422 (1978) 473. 14 Y. Chung, Radium-barium-silica correlations and a twodimensional radium modal for the world ocean, Earth planet. Sci. Lett. 49 (1980) 309. 15 K.K. Turekian, Y. Nozaki and L.K. Benninger, Geochemistry of atmospheric radon and radon products, Annu. Rev. Earth Planet. Sci. 5 (1977) 227. 16 S. Krishnaswami, M.M. Satin and B.L.K. Somayajulu, Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean, Earth Planet. Sei. Lett. 54 (1981) 81. 17 M.A. Geyh and A. Hohndorf, The contribution of complementary 14C and T h / U analysis to the stratigraphy of the Red Sea sediments, Geol. Jahrb. Reihe D, 17 (1976) 79. 18 H. Craig, S. Krishnaswami and B.L.K. Somayajulu, 2i°pb226Ra: radioactive disequilibrium in the deep sea, Earth Planet. Sci. Lett. 17 (1973) 295. 19 B.L.K. Somayajulu and H. Craig, Particulate and soluble 2~°pb activities in the deep sea, Earth Planet. Sei. Lett. 32 (1976) 268.