GEOSECS Pacific and Indian Ocean 32Si profiles

Earth and Planetary Science Letters, 85 (1987) 329-342

329

Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

[6]

GEOSECS Atlantic 32Si profiles B.L.K. S o m a y a j u l u 1, R. R e n g a r a j a n 1, D. Lal 1, R.F. Weiss 2 a n d H. Craig 2 Physical Research Laboratory, Ahmedabad 380 009 (India) 2 Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.) Received July 20, 1987 Measurements of five cosmogenic 325i vertical profiles in Atlantic waters ( 2 7 ° N to 6 0 ° S ) are presented. The a m o u n t s of dissolved SiO 2 extracted range from 2 to 54 g; the a m o u n t s of water from which SiO 2 was extracted range between 540 kg and 270,000 kg. In additon, SiO 2 recovered from four surface particulate composites (64 ° N to 61 ° S) were also analyzed for 32Si. 32Si measurements were made by milking and counting the daughter activity, 32p. The net 32p activities range from 0.7 to 6.8 cph; typical errors in measurements of the 32p activities are 20-30%. The 32Si concentrations vary from 0.6 d p m / 1 0 6 kg of water in the North Atlantic surface waters to 235 d p m / 1 0 6 kg at 400 m depth in the circumpolar waters. The vertical profiles of 32Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and 32Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of 32Si (32Si/SiO2 ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2 - 4 times, than the bottom water values. These data are consistent with generation of the highest specific activity 32Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic 32Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high 328i specific activities. The 32Si inventories in the water column in the latitude belt 27 o N - 2 7 ° S are in the range ( 1 - 1 . 4 ) × 10-2 d p m 32Si/cm2, which is consistent with the expected fallout of cosmogenic 32Si. However, the 32Si column inventories south of 40 ° S are higher by a factor of - 5-7, whereas the corresponding Si inventories increase by only a factor of 3. This excess 32Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of 32Si dissolved from sinking particulates.

1. Introduction C o s m o g e n i c 32Si with a half-life o f - 140 years [1] is expected to be a useful tracer for studying oceanic circulation [2-5] and was measured during the GEOSECS expeditions to the Atlantic, Pacific and Indian Oceans. During the Atlantic expedition profiles of dissolved 32Si were collected using the in-situ adsorption technique based on ferric hydroxide-coated synthetic fibres [6]. As shown earlier [3,5,7], for a proper understanding of the deep-sea circulation using radioisotopes, it is necessary to study their concentrations in surface particulates as well as in the dissolved phases present in sea water at depths. Large-volume (5-20 tons) surface- and deep-water filtrations were carried out for the first time during the GEOSECS 0012-821X/87/$03.50

© 1987 Elsevier Science Publishers B.V.

expeditions and chemical mineralogical and radioisotopic studies were performed on the filtered particulate matter [8,9]. We report here the results of 32Si measurements of the vertical profile samples and of the surface particulates collected from the Atlantic Ocean GEOSECS expeditions [10]. 2. Methodology Two types of samples were collected: (1) vertical profiles of dissolved S i O 2 by the in-situ adsorption technique, and (2) S i O 2 recovered from the surface particulates collected by the largevolume filtration technique. The 32Si radioactivity of the SiO2 samples was measured via the counting of its daughter 32p activity.

330

2.1. Collection, recovery and purification of SiO: in vertical profile samples Preparation of fibre matrix. The dispersion of ferric hydroxide on synthetic acrilan fibres (Monsanto Chemicals, U.S.A.) was done in a two-step process, a modified version of the threestep process developed by Krishnaswami et al. [6], in TAJ M A L A L (a mini-factory set up at the Scripps Institution of Oceanography). Batches of fluffed acrilan fibres, each weighing about 1.7 kg, were soaked in 25% FeC13 solution at 8 0 - 8 5 ° C for 3 - 4 minutes. The hot fibres were then passed through a pair of rubber wringers mounted on a PVC frame and immersed in 25% N H 4 O H for 1-2 hours. The ammonia-soaked fibres were passed through a second pair of wringers after which they were packed in polyethylene bags and stored in steel barrels. A total of 3000 kg of fibres were processed for use in the Atlantic and Pacific GEOSECS expeditions. After washing with distilled water the treated fibres retained about 15% by weight of Fe(OH)3; this is about the same as that for fibres made by the three-step procedure [6].

Shipboard operations. Aboard R / V " K n o r r " at selected stations in the Atlantic, - 10 kg of fibre were fluffed and stuffed into each bag sampler (each sampler is a 1 m dia. cylinder with 6 segmented pockets made of 6 m m thick nylon mesh). Fourteen bag samplers were filled in this manner for exposure at seven depths in one case. The exposure depths for the samplers were determined by an STD cast on station, such that there is at least one sample from each of the principal water masses of the Atlantic including the surface water. For surface water exposure, two bag samplers were hung on the side of the ship in the " o p e n " position (Fig. lb). The subsurface samplers were used with a hydrographic wire. At each chosen depth two samplers were secured 10 m apart in the "closed" position. After all samplers reached the assigned depths in the "closed" position, a messenger was sent to trip them "open". The fibres were soaked with seawater for about eight hours when a second messenger was sent to "close" all the bags. A schematic diagram of the sampler in "closed", " o p e n " and "closed" positions is shown in Fig. 1. The fibres in drums were shipped to the Physical Research Laboratory for chemical processing and 32Si measurement. Of the

1"

a) DOWN CLOSED

~

CLOSED

Fig. 1. The plastic bag sampler with the "double-trip" mechanism secured to the hydrographic wire. The line in the middle of the bag indicates the position of the fibre matrix. (a) Sampler in "closed" position before lowering, (b) sampler tripped into "open" position by the first messenger, allowing flushing of seawater through the matrix, and (c) sampler tripped into "closed" position by the second messenger. In this figure, the "double-trip" mechanism (actual height approx. 30 cm) has been shown greatly enlarged relative to the bag sampler (approx. diameter 1 m) to show the functioning of the tripping mechanism and the release of the messengers.

seven casts attempted during the Atlantic G E O SECS expedition, the first three from the North Atlantic were unsuccessful due to various problems associated with the double trip mechanism and partly due to rough seas. The four successful casts were all in the South Atlantic (4 o S to 60 ° S). It therefore became necessary to collect at least one profile from the North Atlantic. This was achieved aboard R / V "Melville" during the second leg of I N D O M E D expedition at station 401 (at the same location as GEOSECS station 31). The locations of the Atlantic 32Si profile stations are shown in Fig. 2.

Extraction and purification of silica. The fibres from each sampler were washed with distilled water to remove any suspended matter picked up during exposure to seawater. The fibres from each small bag (1.7 kg) were dried in a laundromat drier and incinerated in a furnace at 500 ° C for 8 - 1 0 hours.

331 W IO0*

E 80°

60°

20Q

40°

0°

20°

60* N

60°N

40 °

4O*

2 O*

z0*

o*

o*

20*

20*

40*

40 °

~

600S

i

i

Io0" 'do*' ~o*L 4'0" zo* W

89118)

I ~II1 I o*

60* S

z0* E

Fig. 2. L o c a t i o n s a n d s t a t i o n n u m b e r s o f the A t l a n t i c 32Si profiles. N u m b e r s in p a r e n t h e s e s are the m e a s u r e d 3 2 S i / S i O 2 r a t i o s ( d p m / k g ) in t h e b o t t o m w a t e r a t e a c h s t a t i o n .

The ash, about 15-20% of the original weight, was dispersed in distilled water to dissolve away sea-salts and the residue was boiled in 10 liters of 6M HC1 to dissolve Fe, cooled, filtered and washed with distilled water. The residue was boiled with 1 liter of 4M N a O H to dissolve the silica. SiO 2 was precipitated from the clear centrifuged N a O H solution by acidification with HC1. Usually about 60-70% of the SiO 2 present in the sample was recovered during the first extraction. The residue left after the first N a O H treatment was boiled with 6M HC1, washed with distilled water, and boiled again with 4M N a O H (about 500 ml per sample) and the remaining SiO 2 was recovered. The combined SiO 2 was repurified through N a O H dissolution followed by precipitation with HC1. Pure and dry SiO 2 (washed free of NaC1) was preserved in clean plastic bottles for 32p growth. The weight of SiO 2 recovered from Atlantic sampies ranged from 2 to 55 g. The SiO 2 extracted corresponded to a mass of 500-270,000 kg of sea water (Tables 1, 2). Two batches of Fe(OH)3 dispersed fibre, each of 20 kg and not exposed to sea water, were also processed. These yielded less than 0.2 g SiO 2 which on an average represents 2% of the SiO2 recovered from Atlantic samples.

2.2. SiO2 recovery from particulate samples The large-volume in-situ filtration technique is described in detail by Krishnaswami et al. [8]. Several filter ashes, after HCI leaching for radioisotopes, had to be combined and processed for SiO 2 by boiling with 4M NaOH, followed by filtration and acidification with HC1 [11]. Four composite samples were made from the Atlantic surface filtrations. The recovered weights of SiO 2 samples ranged from 0.6 to 5.0 g (Table 3). 2.3. 32p milking After at least three months of storage to allow 32p to grow in near equilibrium with the parent 32Si activity present in the pure SIO2, the samples were dissolved by boiling with 4M N a O H solution (40 m l / g SiO 2). Any insoluble material (usually < 10%) was removed by centrifugation. One percent of the clear solution (usually 5 ml) was removed for stable phosphorous determination which was carried out by the standard method of addition, using the colorimetric technique [12]. The inherent P concentrations ranged from 0.5 to 38 mg in individual samples. In samples where the inherent phosphorous concentrations were less than 40 mg, Mg2P2OT, a stable phosphorous carrier, was added to make the total Mg2P20 7 content of the sample 40 mg. The 32p milking was carried out using standard radiochemical procedures [13,14]. 2.4. 32p beta assay Radiochemically pure Mg2P20 7 obtained from each sample was powdered and made into a sandwich between two 0.9 m g / c m 2 mylar films in the circular hole in a 3.8 × 2.0 x 0.05 cm plastic sheet. Normally, four samples were processed in a batch and counted in the four 4~r beta detectors [14,15], contained in one system. Samples were counted undisturbed for a period of 30-45 days (two to three half-lives of 32p); the gross counting rate of each sample was determined every day. The background rates and counting efficiencies of the detectors were measured before and after each set of countings. 3. Results

Plots of gross beta activity as a function of e - x t (X = disintegration constant of 32p = 0.0485 d - ] and t = time in days from the date of milking)

332 TABLE 1 Hydrographic and SiO2 concentration data for the Atlantic profile samples [10] Station (Leg # ) (mean location; water depth)

Sample code

Mean depth a (m)

S b (%o)

8b ( ° C)

SiO2. (g/103 kg)

(1)

(2)

(3)

(4)

(5)

(6)

401 (Leg 2 °) (26 o 58.5'N, 53 °44.6' W; 6267 m)

(C + D) (E+ F) (G + H) (I +J) (K + L) (M + N)

10 1232 2140 3050 3979 4484

36.901 35.125 34.985 34.940 34.890 34.870

27.40 5.65 3.30 2.55 1.93 1.75

0.06 0.96 1.41 1.98 2.64 2.89

48 (Leg 4) (04 ° 00 'S, 29 o 00'W; 5079 m)

M (A + B) C E (G + H) K (I + J)

1 713 1183 1779 2780 3678 4475

36.08 34.465 34.780 34.959 34.921 34.905 34.750

26.30 4.55 4.26 3.60 2.58 2.10 0.625

0.078 1.92 1.66 1.17 1.92 2.01 6.00

58 (Leg 5) (27 o 00'S, 37 ° 01.4'W; 4602 m)

(M+N) ( K + L) (I + J) (G + H) (E+ F) (C + D) (A + B)

1 779 1773 2691 3367 4066 4464

36.40 34.335 34.850 34.920 34.890 34.725 34.672

21.83 4.80 3.23 2.65 2.13 0.75 - 0.12

0.036 1.44 1.80 1.86 2.52 6.39 7.50

67 (Leg 6) (44° 58'S, 51 ° 03.5' W; 5850 m)

(M + N) (I +J) (C + A) (F + L) (B + E) K (D + G)

1 151 447 1196 2792 3609 5385

34.59 34.631 34.178 34.425 34.825 34.730 34.665

12.28 8.60 4.10 2.56 2.43 1.094 - 0.190

0.024 0.234 0.882 3.48 3.78 6.48 7.71

89 (Leg 7) (60 o 01.5'S, 0 o 01.5' E; 5367 m)

(M + N) (K + L) (I + J) (G + H) C A

1 404 1202 2005 4000 4805

34.103 34.680 34.672 34.660 34.650 34.647

0.965 0.520 0.080 - 0.296 - 0.738 - 0.875

3.33 7.44 7.68 7.68 7.15 6.72

a Mean depth at which the two samplers, 10 m apart, were exposed. b S and 0 are obtained by interpolation of the data [10] above and below the sample exposure depths. Concentrations of silicic acid are expressed as equivalent weights of SiO2, where 1 g SiO2/103 kg = 16.7 #mol/kg. c Station 401 was occupied during INDOMED expedition on 13 November 1977.

were made for each sample. Some of the plots are s h o w n i n Fig. 3. L e a s t - s q u a r e s fits t o t h e d a t a [16] y i e l d e d t h e n e t 32p a c t i v i t y o n t h e d a y o f 32p s e p a r a t i o n f r o m p a r e n t 32Si. I n m o s t cases, t h e r e s i d u a l a c t i v i t y w a s less t h a n 2 c p h a b o v e t h e background counting rate. F r o m t h e 32p c o u n t i n g r a t e a n d t h e c h e m i c a l e f f i c i e n c y , c o u n t i n g e f f i c i e n c y , self a d s o r p t i o n fac-

t o r f o r 32p b e t a s i n t h e M g 2 P 2 0 7 s o u r c e , t h e g r o w t h f a c t o r , t h e 32Si d p m / k g SiO 2 ratio was c a l c u l a t e d [14]. S i n c e s t a b l e Si c o n c e n t r a t i o n p r o files w e r e m e a s u r e d a t e v e r y s t a t i o n [10], it is p o s s i b l e to c a l c u l a t e t h e 32Si c o n c e n t r a t i o n i n w a t e r a t e a c h d e p t h s a m p l e d ( T a b l e 2). A m i n i m u m o f two samples were remilked for each profile and the calculated 32Si/SiO2 ratios from the two sep-

333 TABLE 2 32Si in Atlantic profiles Sample code

Water depth (m)

SiO 2 recovered (g)

Equivalent volume (10 3 kg)

Net 32p activity (cph)

32Si/SiO 2 (dpm/kg)

39Si ( d p m / 1 0 6 kg)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Station 401 (27°N) (C + D) (E + F) (G + H)

10 1232 2140

16.08 20.14 54.48

268.0 21.0 38.6

3.0+0.7 2.8+0.7 0) 2.95-0.6 (ii) 1.8+0.4

10.1+ 7.3 + 3.0 + 2.8+ (M) 2.9+

2.3 1.9 0.6 0.6 0.6

(I + J) (K + L) (M + N)

3050 3979 4484

32.40 17.45 8.97

16.4 6.6 3.1

6.85-1.6 3.2+0.6 (i) 4.0+0.8 (~)2.8+o.3

8.1512.4+ 15.3 + 21.5 + (M) 18.9+

1.9 2.3 3.1 2.5 1.9

1

5.10

65.4

0)2.o+1.3 (ii)2.o+o.3

13.0+ 8.4 15.2+ 2.2 (M) 15.1_+ 2.2

(A + B) C

713 1183

18.16 6.72

9.5 4.0

2.15-o.3 0)3.5+1.o (ii)2.3+o.3

10.15- 1.5 25.3 _+ 7.2 16.6 + 2.2 (M) 17.3+ 2.1

E (G + H) K

1779 2780 3678

4.81 15.28 10.60

4.1 8.0 5.3

0.85-0.5 1.5±o.6 (i)1.7+o.6 (ii)2.O±l.1

10.3+ 6.6 4.95- 1.9 5.4+ 1.9 10.1 5- 5.5 (M) 5.95- 1.8

(I + J)

4475

16.87

2.8

0)5.45-1.1 (ii)2.7+o.7

14.4 + 3.0 9.5 + 2.5 (M) 11.55- 1.9

0.61+ 0.14 7.00 5- 1.8 4.10+ 0.8

16.0 + 3.8 32.7 + 6.1 54.6 + 5.5

Station 48 (4 "S) M

1.2 + 0.2

19.4 + 2.9 28.7 + 3.5

12.1 + 7.8 9.4 5- 3.6 11.9 5- 3.6

69.0 +_ 11.0

Station 58 (27 °S) (M+N)

1

7.22

200.6

0) 1.65-0.6 (ii) 3.05-0.4

18.3 + 6.0 20.7 + 2.7 (M) 20.35- 2.5

(K + L) (I+J) (G + H) (E + F) (C + D)

779 1773 2691 3367 4066

11.07 12.98 18.69 6.71 10.16

7.7 7.2 10.0 2.7 1.6

1.85-0.8 2.6+0.3 1.4+0.3 1.3+~4 0) 2.0+0.6 (ii) 1.65-0.3

9.7+ 9.458.0512.0517.9 + 13.5+ (M) 14.45-

(A + B)

4464

12.19

1.6

0.9+0.2

(M) 5.7+ 1.3

1

2.02

84.2

0) 2.1+0.4 (ii) 1.65-0.4

47.2 + 9.0 49.3 + 12.3 (M) 47.9+ 7.3

151

6.26

26.8

(i) 2.3±0.4 (ii) 2.3+0.7

31.2 + 5.5 25.3+ 7.7 (M) 29.25- 4.5

4.4 1.0 1.7 3.7 5.1 2.6 2.3

0.73 ___ 0.09

14.0 16.9 14.9 30.2

± 5+ +

6.3 1.8 3.2 9.3

92.0 5-15.0

42.8 + 9.8

Station 67 (45 °S) (M + N)

(i + J )

1.1 5- 0.2

6.8 5- 1.0

334 TABLE 2 (continued) Sample code

Water depth (m)

SiO 2 recovered (g)

Equivalent volume (103 kg)

(1)

(2)

(3)

(4)

(C + A) (F+L) (B+E)

447 1196 2792

3.29 3.32 3.16

3.7 0.95 0.84

Net 32p activity (cph)

32Si/Si02 (dpm/kg)

32Si ( d p m / 1 0 6 kg)

(5)

(6)

(7)

0.7 _+0.2 1.4_+0.5 (i) 1.7_+0.4 (ii) 1.2 _+0.5

19.4+ 22.3 + 25.7 + 14.6 _

5.6 7.9 6.0 6.1

17.1 + 4.9 77.6 + 27.0

17.7 _+ 5.7 25.1 _+ 7.9 19.3 + 9.6

114.7 +37.0

76.3 + 16.0

(M) K (D+G)

3609 5385

5.52 4.84

0.85 0.63

2.2 _+0.7 (i) 1.9_+0.6 (ii) 2.O _+1.0

175.8 +47.0

(M)

Station 89 (60 °S) (M+N) 1

24.10

7.2

(i) 2.1 _+0.6 (ii) 3.O _+O.4

9.4_+ 2.8 9.9_+ 1.3

32.6 + 4.0

(M) (K + L) (I + J) (G+H) C

404 1202 2005 4000

6.47 9.12 7.89 3.87

0.87 1.20 1.03 0.54

2.4 _+0.6 3.1 _+0.7 3.5_+0.8 (i) 1.4_+0.4 (ii) 2.0 _+0.4

31.5 _+ 16.9 _+ 28.6 _+ 20.8 _+ 22.4_+

7.9 3.8 6.6 6.0 5.0

234.4 + 59.0 129.8 + 29.O 219.6 + 51.0

18.2_+ 6.1

122.3 +41.0

155.2 +27.0

(M) A

4805

4.12

0.61

1.8 _+0.6

Errors indicated are l o counting statistics of 32p activity. (i) and (ii) indicate two separate 32p milkings and (M) denotes weighted mean.

TABLE 3 Si and 32Si in Atlantic surface particulates Sample

Location (lat., long.)

SiO 2 recovered

Net 32p activity

32Si/SiO2

code

start

end

(g)

(cph)

(dpm/kg)

AP 1

64°N 22°W

50°N 43°W

0.62

(i) 0.8 + 0.3 (ii) 0.9 + 0.4

70.7 + 26.0 51.9_+23.0 (M) 59.8+ 17.0

AP 4

36 o N 54°W

02 ° S 05°W

0.70

1.7+0.4

49.8 + 12.0

AP 2

30 o S 39°W

55 o S 39°W

2.46

(i) 1.8_+0.3 (ii) 2.1 + 0.5

35.3 _ 5.9 33.0 + 7.9 (M) 34.5 _.+ 4.7

AP 3

50 ° S 66°W

61 ° S 12°E

5.00

(i) 2.8 -+ 0.9 (ii) 2.1 + O.3

17.2+ 5.5 20.6 + 2.9 (M) 19.9+_ 2.6

(i) and (ii) denote two separate 32p milkings and (M) denotes weighted mean. Errors quoted represent one sigma propagated counting statistics.

335 (a)

32Si ( d p m / l O 6 kg I0

I

I0

I00

'

o

5 3

I

'

I

IOO ,

\\ )

1//

"~ 2000

3 I

I

I

5000

I

4OOO

i

5OO0

4

6ooc

L9

40__! ~, ,

58

48

'~',oo-i

,~

BLANK-1

6

2

I

5

56 2

I

7

".= 8

~

I0

~00 '

io(3o

~

A

t

LO

SILICATE

t LO 0.8

I I 0.6 0.4

t / 0.2 0

(~) 1.0 0.8

0.60.4

0.2

0

I i

e--xt Fig. 3. Plots of d a i l y gross beta activity vs. e - x t for six s a m p l e s a n d two blanks. The d o t t e d lines i n d i c a t e the m e a n b a c k g r o u n d c o u n t i n g rates; this is n o t s h o w n in Blank-1 since the m e a n b a c k g r o u n d rate is close to the s a m p l e rate in this case.

arate milkings were found to be in good agreement. Sample location and Si and 32Si data for particulates are given in Table 3.

3 I

32Si (dpm/106

kg)

I0 ~0 I00 300

I0 50 ~00 300

I

1

I00

(FM/kg)

i

i

i

i

1000

"~ 2000

Q. taJ a

5000

I

4000

4. Discussion

5000

The dissolved 32Si concentrations in the Atlantic Ocean between 27 o N and 60 o S (Table 2, Fig. 4a, b) range from 0.6 dpm/106 kg in the North Atlantic surface waters (station 401) to 235 dpm/106 kg at 404 m in the circumpolar waters (station 89). The 32Si/SiOz ratios (Table 2) range from 3 d p m / k g in the intermediate waters at station 401 to 48 d p m / k g in the surface waters of the Argentine basin (station 67). In the top 1 km depth the gradients in 32Si concentration are about an order of magnitude smaller than in Si concentration, except for the circumpolar station where vertical mixing effects are pronounced for both Si and nSi. At greater depths, 32Si concentration profiles follow more closely the Si profiles (Fig. 4a, b). In the Atlantic surface waters, 3zsi concentrations increase from 0.6 dpm/106 kg at 2 7 ° N (station 401) to 32.6 dpm/106 kg in the cir-

6000

I 67 -

~

-

8.,..99

-

'

~

,~ 5'o ,~o~oo

,~ ~ 40

SILICATE (/~M/kg) Fig. 4. (a) D i s s o l v e d Si ( d a s h e d lines) a n d a b s o l u t e n S i (solid lines) c o n c e n t r a t i o n s as a f u n c t i o n of w a t e r d e p t h in the c e n t r a l A t l a n t i c stations (401, 48, 58). Si a n d 32Si d a t a are given i n [10] a n d T a b l e 2 respectively. (b) D i s s o l v e d Si ( d a s h e d lines) a n d a b s o l u t e 32Si in the S o u t h A t l a n t i c a n d C i r c u m p o l a r w a t e r s (stations 67 a n d 89).

cumpolar region (station 89). The increase, however, is not uniform between 27 ° N and 45 ° S; 32Si concentrations at these stations are 0.6 and 1.2 d p m / 1 0 6 kg respectively. The Si concentrations of these surface waters are also low, varying from 0.025 to 0.08 g SIO2/103 kg, with the lowest value occurring at station 67 (Table 1). In the circumpolar surface waters, the Si and 32Si conc e n t r a t i o n s increase to 3.3 g SiO2/103 kg and 33 d p m / 1 0 6 kg respectively.

336 The 32Si/SiOa ratio in surface waters increases from 10 d p m / k g at 2 7 ° N , to 48 d p m / k g at 45°S, and then decreases to 10 d p m / k g in the circumpolar region (station 89). The low 328i/SiO2 ratio in the circumpolar waters is obviously due to the higher concentration of Si. On the other hand, the higher 32Si/SiO2 ratio at 45°S (station 67) appears to be due to the expected higher fallout of 32Si at this latitude. It should have been possible to check on this by making observations in the northern hemisphere; however, the cast we attempted at 45 ° N was unsuccessful.

4.1. 32Si/Si02 ratios in surface particulates The 328i/SiO2 ratio in surface particulates is highest 59.8 d p m / k g in the composite (64-50 o N) latitude sample, decreasing steadily southward, the lowest value being 19.9 for the (50-61°S) sample. Unfortunately, we do not have any dissolved 32Si measurements between 6 4 ° N and 27 ° N to make a direct comparison between the particulate and dissolved 32Si/SiO2 ratios. The ratios for the two North Atlantic samples, 50-60 d p m / k g , (AP1 and AP4, Table 3) are not inconsistent with the values obtained for siliceous sponges collected from 23 o N to 7 4 ° N [17], which range from 27 to 80 d p m 32Si/kg SiO 2. The dissolved 32Si//SiO2 ratio in the North Atlantic at 2 7 ° N (station 401), 10 d p m / k g SiO 2 (Table 2), is lower than the particulate value (sam-

52Si/Si 0 2 o

IO

(dpm/kg] zO

pie AP4, Table 3) by about a factor of four. However, since the composite particulate samples cover a wide latitude range and the relative amounts of Si and 32Si from each latitude sample are not known, the difference may not be significant. In the South Atlantic, particulates collected from 30 o S to 55 o S (sample AP2, Table 3), have a 3 a s i / s i o 2 ratio of 34.5 + 4.7 d p m / k g , which is consistent with the value of 47.9_+ 7.3 d p m / k g obtained for surface waters at 45°S. The measured 32Si/SiO2 ratio of 19.9+ 1.8 d p m / k g in particulates collected from 50°S to 61°S is higher than that of the dissolved 32Si/SiO2 ratio (9.8 + 1.2 d p m / k g ) at 60 ° S. This discrepancy may be due to the composite particulate sample having larger contributions from regions closer to 50°S where silicon concentrations are higher.

4.2. 32Si/ Si02 vertical profiles The vertical distributions of 32Si/SiO2 ratios in the Atlantic are plotted in Figs. 5, 6 and 7. The single North Atlantic profile is shown in Fig. 5, the three South Atlantic profiles in Fig. 6, and that of the circumpolar region in Fig. 7. At 27 o N (station 401) the 32Si/SiO2 ratio (Fig. 5) varies from a minimum value of 2.9 at 2140 m, to a m a x i m u m of 18.9 d p m / k g at 4484 m (AABW). The decrease from 10 at surface to 3 d p m / k g at 2140 m and the subsequent increase to 20 d p m / k g are nearly linear. In the Equatorial Atlantic (Fig. 6) at 4 ° S (station 48), the pattern of 32Si/SiO2 depth variation is similar to that at 27 o N, but the respective changes are smaller.

o

52Si / S i 0 2

I

o,

i~

o~

"E .at

(dpm/kg)

zo ~-

2

i

i

-g2~ t.fl o

i

5 o_

4 5F

5

Fig. 5. 328i//8iO2 ratio vertical profile at North Atlantic station 401 (Table 2).

4 61

I

67 "--~

,

--

° ['~7~

,

~

,

Fig. 6. 32Si/SiO2 ratio vertical profile in the South Atlantic (Table 2).

337 "~i~Si/Si 0 z (dDm/kcJ) I0

ZO

~,0

40

2

4

--

I-----

7.32Si/SiO2 ratio vertical profile at Circumpolar station 89 (Table 2). Fig.

I n the South A t l a n t i c (Figs. 6 a n d 7), the gradients in 32Si/SiO2 ratios decrease further as one goes to the south. A t 45 ° S (station 67) a n d 60 ° S (station 89) the ratios b e c o m e essentially depth-ind e p e n d e n t (except for the surface samples).

4.3. 32Si and Si inventories T h e available data allow calculation of integrated c o l u m n inventories of 32Si at several lati-

tudes in the A t l a n t i c Ocean. T h e data are however, primarily limited to South Atlantic; in the N o r t h A t l a n t i c only one station, at 27 ° N, is available. T h e results for c o l u m n - i n t e g r a t e d inventories are given i n T a b l e 4 for 32Si, stable Si, a n d 32Si/SiO2 ratios. I n this table we have also listed the 3 a s i / s i o 2 ratios for the N o r t h A t l a n t i c deep waters ( N A D W ) a n d the A n t a r c t i c B o t t o m Waters (AABW). T h e Si data are from Bainbridge [10]. Other data i n T a b l e 4 are from the present work. A t 5 7 ° N the m e a s u r e d 32Si/SiO2 ratio in siliceous sponges is 60 d p m / k g . I n view of the good vertical mixing, we assume this a l s o to be the 32Si/SiO2 ratio in the water c o l u m n at 57 ° N ; a n d we therefore a d o p t 60 d p m / k g as the water colu m n - a v e r a g e d ratio. M u l t i p l y i n g this with the Si i n v e n t o r y at the location (Table 4), we find that the 32Si i n v e n t o r y is 1.31 × 10 -2 d p m / c m 2 which is the same (within the errors of the 32Si measurements) as the inventories at 27 ° N, 4 ° S a n d 27 ° S (Table 4, Fig. 8). T h e integrated c o l u m n 32Si i n v e n t o r y essentially r e m a i n s c o n s t a n t at (1.3 +_ 0.1) x 10 -2 d p m / c m 2 b e t w e e n 57 ° N a n d 27 ° S. It then rises steeply to a value of 8.48 × 10 -2 d p m / c m 2 at 60 ° S. The Si i n v e n t o r y does n o t follow the same p a t t e r n exactly. T h e p r i m a r y differences are in the

TABLE 4 32Si and Si inventories and 32Si/SiO2 ratios Station No. 5 27 401 40 48 58 67 89 North Atlantic (4 station average) South Atlantic (4 station average) Atlantic (8 station average)

Latitude

32Si

57°N 42°N 27 o N 4°N 4° S 27 o S 45 o S 60 ° S

32Si/SiO2 (dpm/kg)

Inventory (dpm/cm2)

Si (g SiO2/cm2)

water column average

1.31×10 -2 1.20×10 -2b 1.42 × 10 -2 1.25 x l 0 -2b 1.12 x 10- 2 1.22 x 10- 2 5.52 × 10-2 8.48 x 10- 2

0.22 0.47 1.11 0.68 1.26 1.18 2.74 3.69

60 a 25.5 12.8 18.4 8.9 10.3 20.1 22.9

1.30 X 10-a

0.68

19.1

4.18 × 10-2

2.36

17.7

2.82 X 10-2

1.53

18.4

NADW: North Atlantic Deep Water; AABW: Antarctic Bottom Water. a From [17], see text for discussion, b Obtained by interpolation.

NADW

AABW

8.1±1.9

18.9±1.9

5.9±1.8 12.0±3.7 19.0±5.0 20.0±5.1

11.5±1.9 10.1±1.9 22.8±6.1 20.0±5.1

338 gradients south of 27°S; the gradients in the concentrations of Si inventory, south of 27 o S, are much smaller than those observed for 32Si. The above results and the available data for 32Si/SiO2 ratios in siliceous sponges from North Atlantic surface waters [17] are plotted in Fig. 8. We observe a steep increase in the column-averaged 32Si/SiO2 ratios both north and south of 40 ° N and 40 °S respectively, the percentage increase being larger in the northern waters. By comparison with the 32Si column inventories, we can infer the cause(s) of this increase. The 32Si column inventories are seen to be uniform between 5 7 ° N and 2 7 ° N but values south of this (at 45 °S and 60 o S) are higher by factors of 5 to 7. The annual deposition of 32Si has been measured at six stations at 1 0 - 3 2 ° N [13,19]. Earlier measurements [13] at these stations may have had some contributions from nuclear weapon-produced 32Si, but the data subsequent to 1966 [19] are free from this uncertainty. The zonal fallout, F, for 1 0 - 3 2 ° N for 1967-1970 [20] ranges between (1.5-3.6) × 10 -5 dpm 32Si/cm-2 yr with

o

@

AABW

5

i

-o

c o l u m n overago

Wator

NADW

32

24

o 8

L

L

I

I

L

~

I

o

8

4

o

~6 4

2

x 2

I []

o

0

I° 80*

o

°l 40 °

•

I

v

I 0°

N ~LATITUDE--'-,,,-

•

o I

1 40*

0 8o o

S

Fig. 8. The measured 32Si/SiO2 ratios in NADW and AABW, and the mean water column ratios, are plotted vs. latitude in the upper figure. The column inventories of Si and 32Si at the profile locations are shown in the lower figure (ordinates for both Si and 32Si are marked). The boxed points are based on 32Si/SiO2 ratios measured in siliceous sponges (see text).

a mean value of (2.5 + 0.8) × 10 5 dpm 32Si/cm2 yr. The corresponding expected column inventory of 32Si in the ocean, assuming complete retention in the water column after fallout (i.e.~no migration or removal to sediments) is F/?t d p m / c m 2 column. This gives us a value of (5.05 + 1.6) × 10 - 3 d p m / c m 2 for the column inventory in the 0 - 3 0 o latitude belt, using 140 years for the half-life of 32Si. The value expected for the 30-90 o belt would be expected to be higher due to contributions from the stratosphere. Following Lal and Peters [20], the expected relative mean fallout of a longlived isotope in the 0-30 ° and 30-90 ° belts would be 1 : 1 . 4 and the maximum fallout would be at 40 o (N or S). Therefore the absolute mean fallout in the latitude belt (30-90 o) is expected to be (7.1 _+ 2.3) × 10 - 3 a t o m s / c m 2 rain based on production rate estimates [20]. The corresponding steady state column inventory would be (7.1 + 2.3) × 10 - 3d p m / c m 2. The observed mean column inventory of 32Si at 2 7 ° N to 27°S ( 8 . 9 + 0 . 3 ) X10 -3 d p m / c m 2 is somewhat higher than the expected value of (5 _+ 1.6) × 10 -3 d p m / c m 2. This discrepancy may be partly due to the present uncertainty in the production rate. (A value of 200 years for the half-life would give a better agreement.) However, the column inventories south of 40 o S are too large to be due t o the expected fallout from the stratosphere or to southward transport of silicon. Theoretical fallout models would predict at most a 1.4 times higher fallout in the 30-90 ° belt as discussed above; the observed values are 5 - 7 times larger. In the light of the foregoing discussion, we conclude that: (1) The observed column inventories between 5 7 ° N and 27°S are consistent with expectations based on cosmic ray production of 32Si. (2) The high 32Si/SiO2 ratios are 4 0 ° N and 5 7 ° N reflect to the substantially lower silicon concentrations in these waters. (3) The high 32Si/SiO2 ratios, and also the higher 32Si column inventories, at 45 °S and 60 °S are far too large to be due to direct fallout from the atmosphere. The excess cannot be attributed to nuclear weapon contributions, since this contribution amounted at best only to a few years of cosmic ray production in the northern hemisphere [13,19]. Furthermore, our observations are in the

339 Southern Ocean where the nuclear weapon fallout is an order of magnitude smaller. We can also rule out the source of excess 32Si being the melting of accumulated Antarctic snow in recent years. Recent dissolution of surface siliceous ooze samples can add to the 32Si column inventory but this would not explain the high column-averaged 32Si/SiO2 ratios at 45 ° S and 60 ° S. The only plausible source of excess 32Si inventory in the Southern Ocean is the southward advective transport of dissolved Si and 32Si. This hypothesis is in fact consistent with simple budget calculations of water, Si and 32Si. If the total southward deep-water transport to the Southern Ocean is on the order of 40 Sv [21], and the volume of the Southern Ocean south of the Polar Front not counting surface waters is 130 × 10 6 k m 3 [22], then the water flushing time for the Southern Ocean is about 100 years. The replacement time for dissolved Si in the Southern Ocean is on the order of 130 years, depending on the assigned water flow rates and Si concentrations for the southward deep-water transport from the Atlantic, Indian and Pacific Oceans. It is interesting to note that a 10 Sv southward flow of N A D W with the 32Si concentration we measure in this water mass at 45°S (see below) carries a 32Si flux into the Southern Ocean which is roughly twice the amount injected directly to the sea surface by tropospheric fallout south of the Polar Front. Thus, within the uncertainties of these calculations, it appears that the approximately constant 32Si/SiO2 ratios, but the much higher 32Si column inventory in the Southern Ocean, is primarily sustained by the southward deep-water advection of dissolved Si and 32Si. This implies that (1) a substantial loss of 32Si occurs from the water column north of the Polar Front and (2) the Southern Ocean Si and 32Si inventories are very sensitive to changes in the water circulation, and may have varied considerably in the past. More detailed 32Si budgets cannot be presented at this time since 32Si data are not yet available for the Indian and Pacific Oceans. 4.4.

32Si in

different water masses

Considering the fact that we have only 5 profiles, we consider only the principal water masses: N A D W and AABW. The 32Si/5iO2 ratios for N A D W and AABW are listed separately in Table

4. The 32Si specific activity in N A D W decreases from 27 ° N to 4 ° S, and increases steadily thereafter. The increase is about a factor of 3. In the case of AABW the specific activity remains unchanged from 60 °S to 45 ° S. This value is 20 d p m 32Si/kg SiOz, which decreases to 10 d p m 32Si/kg SiO 2 at 27°S. The value at 4 ° S is about the same but that at 27 ° N is about 2 times higher. The above features in 32Si observed for N A D W and AABW differ markedly from those for 14C [23]. In fact, based on 32Si data alone, one could not characterize these as distinct water masses because the 32Si specific activities go through a minimum at 4°S. Based on 14C data, one would have expected considerable steady decrease in specific activities due to ageing of water. The decrease (for both 14C and 32Si) would, however, be expected to be partially cancelled by in-situ dissolution of surface particulates but the effect would be much greater for 32Si [3]. One would even expect an increase in 32Si/Si ratio with distance for a fast horizontal transport, if in-situ dissolution is appreciable. Based on 14C data, Stuiver [23] noted an ageing by about 160 years for the N A D W between 4 2 ° N and 32°S, and 80 years ageing between circumpolar and equatorial AABW waters. For 32Si, we see ageing only between 2 7 ° N and 4 ° S for N A D W (about 50 years) and between 45 °S and 27°S for AABW (about 100 years), as shown in Fig. 8. This apparent ageing effect is nullified as the waters move southward or northward respectively for N A D W and AABW, primarily due to in-situ dissolution of sinking surface particulates. The surface particulates have a much higher specific activity (20-50 d p m 32Si/kg SiO2) compared to that in the N A D W or AABW waters ( - 10-20 d p m 32Si/kg SiO2). 5. Comparison of 328i with 39Ar A set of 14 measurements in the deep waters of N o r t h Atlantic (near the equator to 45 ° N) collected during the T T O cruises have been reported by Schlitzer et al. [24] for cosmogenic 39Ar (half-life, 269 years). The principal advantage of 39Ar as a tracer for studying ocean circulation arises from the fact that it is a conservative tracer. However, as in the case of 32Si, the measurements reported so far have large uncertainties compared

340 to the range of measurements. Of the 14 measurements reported for the Atlantic [24], with the exception of one value (39Ar = 39% modern) all others ranged from 45 + 5% to 57 + 5% modern. In the case of 32Si, one observes distinctly different 32Si/SiO2 ratios for the deep Atlantic waters, compared to surface waters. This feature is not seen for 39Ar, and is understandable considering that in the case of 32Si, appreciable amounts of high 32Si specific activity recent particulate silica are dissolved in the waters at the water-sediment interface. When higher precision 39Ar measurements become available, it would be useful to combine 32Si and 39Ar data to estimate several useful physical and chemical parameters, e.g. horizontal-vertical advection/mixing, and the 32Si source function at the water-sediment interface. 6. Conclusions

We have presented data for 5 profiles of dissolved 32Si concentrations in Atlantic waters between 27 ° N and 60 o S, and for composite surface particulates between 6 4 ° N and 61°S. The concentrations of dissolved 32Si are observed to vary by three orders of magnitude. The 32Si specific activities (32Si/SiO2) in water, however show less than order of magnitude variation. This is quite according to expectation; the biological processes controlling the concentration of silicon also regulate the dissolved concentrations of 32Si. Cosmogenic 32Si is added in surface waters by precipitation where it equilibrates with dissolved silicon and is removed quickly from surface water by siliceous organisms [2,3]. Thus the 32Si/SiO~ ratios are highest in surface waters, where 325i is added from the atmosphere and stable Si concentrations are low. Since Si concentrations normally increase downward, 32Si/Si ratios decrease progressively with depth due to mixing and decay. An important means of downward transport of surface dissolved silicon is the sinking of biological particulates. Dissolved Si data can be explained in terms of a model where only part of the particulates dissolve enroute, but a greater part reaches the ocean floor undissolved. Of the particulate flux to the sediments, an appreciable part then dissolves before being buried by sedimentation. Our measurements of 32Si/SiO2 ratios in water

clearly show the 32Si source is in surface waters, and in deep and bottom waters. All the depth profiles of 32Si/SiO2 show a minimum at intermediate depths, 2 - 4 km. The profile for station 67 shows a pronounced maximum in the surface waters, but not at the water-sediment interface; the latter is obviously due to the effect of high vertical mixing in these waters. We have discussed the column inventories of 32Si at the five profile stations. The data are adequate to obtain fairly accurate values of the standing crop of 32Si, which can be compared with the expected fallout pattern of 32Si. The standing crop in the latitude belt 4 0 ° N to 27°S is in excellent agreement with expectations, but large excesses are found at latitudes south of 40 o S, by a factor of 5-7. We propose that the excess column inventories south of 40°S are due to southward transport of dissolved silicon by N A D W . The complicated 32Si source function, injections at surface, water-sediment interface and an appreciable southern transport by water masses, lead to a complex distribution of 32Si in the principal water masses. Consequently this distribution differs markedly from that of 14C, whose injection occurs primarily at the surface by gas exchange. "Ageing" of 32Si is observed only over short distances. The absence in these water masses of uniform gradients in 32Si/SiO2 ratios along the entire range of latitude represented by our samples arises from the combined effects of (a) appreciable in-situ dissolution of surface particulates, and (b) vertical mixing. The salient features of the above data may be summarized as follows: (1) Highest 32Si/SiO2 ratios are observed in the surface waters where dissolved silicon concentrations are low and cosmogenic 32Si is injected. There seems to be a maximum in the ratio in the surface waters at 45 o S; this can be understood as due to vertical upwards flux of 32Si from intermediate waters having high 32Si/SiO2 ratios. (2) The integrated 32Si column inventories in the Atlantic ocean in the latitude interval 27 o N to 27 o S are - 10 -2 d p m 32Si/cm2. The inventories at 45 °S and 60 °S are higher by a factor of about 5. This can be understood as due to influx of dissolved 32Si via horizontal advection of water masses into the Southern Ocean. The replacement time of 32Si and Si in the Southern Ocean is on the

341 o r d e r of 100 years, a n d one expects large secular v a r i a t i o n in 'the i n v e n t o r i e s d u e to changes in the o c e a n circulation. (3) T h e N A D W a n d A A B W w a t e r masses d o n o t show progressive ageing as g a u g e d b y the m e a s u r e d 32Si/SiO2 ratios. A f t e r an initial ageing, the ratios increase with d i s t a n c e traversed. This o b s e r v a t i o n is c o n s i s t e n t with e x p e c t a t i o n since high 32Si/SiO2 surface p a r t i c u l a t e s dissolve in the w a t e r mass, enroute. These results clearly establish the usefulness of 32Si as a tracer for s t u d y i n g the d y n a m i c s of p a r t i c u l a t e a n d dissolved silicon in the oceans, a n d for u n d e r s t a n d i n g the b u d g e t of Si in the S o u t h e r n Ocean. T h e c o n t r a s t i n g b e h a v i o u r o f 32Si with that of 14C, as e v i d e n c e d f r o m the observations of 14C/12C ratios in N A D W a n d A A B W is p r i m a r i l y a result of the fact t h a t the vertical recycling of Si a n d 32Si, d u e to d i s s o l u t i o n of m i x i n g particulates, is m u c h m o r e i m p o r t a n t for the i s o t o p e 32Si. T h e p r e s e n t results suggest that the p r e c i s i o n of the m e a s u r e m e n t s should b e imp r o v e d ( + 10% w o u l d b e desirable). This w o u l d allow d e t e r m i n a t i o n of in-situ dissolved silicon fluxes in i n d i v i d u a l w a t e r masses, as well as studies of m i x i n g in the Pacific a n d I n d i a n O c e a n s using a d v e c t i o n / d i f f u s i o n models. M o r e extensive 32Si m e a s u r e m e n t s in the w a t e r masses feeding the S o u t h e r n Ocean, a n d m o r e d e t a i l e d m e a s u r e m e n t s in the i n t e r m e d i a t e waters, are also especially desirable.

Acknowledgements This p a p e r is d e d i c a t e d to the late F r e d e r i c k D i x o n of SIO, w h o was r e s p o n s i b l e for c a r r y i n g o u t all the casts d u r i n g the G E O S E C S trial cruises a n d for establishing the s h i p b o a r d p r o c e d u r e s used t h r o u g h o u t the p r o g r a m ; we owe him a great d e b t o f g r a t i t u d e for his enthusiastic p a r t i c i p a t i o n in this p r o g r a m a n d his m a n y c o n t r i b u t i o n s to the research. W e are grateful to the m e m b e r s o f the G E O S E C S O p e r a t i o n s G r o u p for fluffing a n d stuffing the t r e a t e d fibres, a n d for the fibre cast work. W e t h a n k Shale N i s k i n for the design a n d c o n s t r u c t i o n of the b a g s a m p l e r s a n d for strong s u p p o r t a n d e n c o u r a g e m e n t t h r o u g h o u t the p r o gram. T h e extensive efforts of Everett H e r n a n d e z a n d H e l m u t K u e k e r in the c o n s t r u c t i o n a n d m a i n t e n a n c e of the fibre f a c t o r y " T A J M A L A L "

are greatly a p p r e c i a t e d . W e are also t h a n k f u l to V.J. Jeevaraj, M . M . Satin, P.K. T a l e k a r a n d J.P. B h a v s a r for help with the Si e x t r a c t i o n a n d to N . R . M a n c h a n d r a , A.R.S. P a n d i a n a n d G . D . P a n chal for the electronics setup at P R L . W e are p a r t i c u l a r l y t h a n k f u l to Prof. S. K r i s h n a s w a m i for his help d u r i n g the initial stages a n d for several useful discussions. Chief Scientists w h o p l a n n e d a n d e x e c u t e d the 32Si casts o n the A t l a n t i c G E O S E C S legs were: H. Craig, Leg 4; W. Broecker, Leg 5; D. S p e n c e r a n d K. Park, Leg 6; a n d H. Craig, Leg 7. T h e s a m p l i n g at station 401 ( S I O e x p e d i t i o n I N D O M E D ) was carried o u t b y W . Price, Chief G E O S E C S T e c h n i cian, who c o n t r i b u t e d to m a n y aspects of the 32Si w o r k at sea. T h e design, i n s t r u m e n t a t i o n a n d testing of the N i s k i n b a g samplers, fibre processing a n d ship t i m e were f u n d e d b y the U.S. N a t i o n a l Science Foundation, IDOE, through NSF grant IDO7104197A03 to the I s o t o p e L a b o r a t o r y , SIO.

References 1 The half life of 32Si is not well known at present. The earliest value based on the estimated production rate of 32Si was 710 years (M. Lindner, New nuclides produced in chlorine spallation, Phys. Rev. 91, 642, 1953). A geochemical estimate of 300 years was obtained based on intercomparison of 21°pb and 32Si activities in a Gulf of Mexico sediment core (D.J. DeMaster, The half-fife of 32Si determined from a varved Gulf of California sediment core, Earth Planet. Sci. Lett. 48, 209, 1980). Subsequently, two independent half-fife estimates have been made. One of these is due to two groups (D. Eimore, N. Anantaraman, H.W. Fulbright, H.E. Gove, H.S. Hans, K. Nishiizumi, M.T. Murrell and M. Honda, Half-fife of 32Si from tandem accelerator mass spectrometry, Phys. Rev. Lett. 45, 589, 1980; M. Kutschera, W. Henning, M. Paul, R.K. Smither, E.J. Stephenson, Y.L. Yentna, D. E. Alburger and G. Harbottle, Measurement of the half-fife of 32Si via accelerator mass spectrometry, Phys. Rev. Lett. 45, 592, 1980) who used an accelerator mass spectrometer to determine the number of 32Si atoms; they obtained consistent values from independent experiments, the mean value being 105+18 years. The second estimate is based on the measured decrease in the counting rate of 32Si source over a period of 4 years and gives a value of 172 + 4 years (D.E. Alburger, G. Harbottle and E.F. Norton, Half-life of 32Si, Earth Planet. Sci. Lett. 78, 168, 1986). For this paper we have adopted a value of 140 + 20 years for the 32Si half-fife (r = 1/X = 202 years). 2 B.L.K. Somayajulu, D. Lal and H. Craig, 32Si profiles in the South Pacific, Earth Planet. Sci. Lett. 18, 181, 1973. 3 D. Lal, Cosmic ray produced radionuclides in the sea, J. Oceanogr. Soc. Jpn. 20, 600, 1962.

342 4 D. Lal, Characteristics of large scale oceanic circulation as derived from the distribution of radioactive elements, Proc. 2nd. Int. Oceanogr. Congr., Morning Rev. Lectures 30, 1968. 5 H. Craig, Abyssal carbon and radiocarbon in the Pacific, J. Geophys. Res. 74, 5491, 1969. 6 S. Krishnaswami, D. Lal, B.L.K. Somayajulu, F.S. Dixon, S.A. Stonecipher and H. Craig, Silicon, radium, thorium and lead in seawater: in situ extraction by synthetic fibre, Earth Planet. Sci. Lett. 16, 84, 1972. 7 H. Craig, A scavenging model for trace elements in the deep see, Earth Planet. Sci. Lett. 23, 149, 1974. 8 S. Krishnaswami, D. Lal and B.L.K. Somayajulu, Investigations of gram quantities of Atlantic and Pacific surface particulates. Earth Planet. Sci. Lett. 32, 403, 1976. 9 S. Krishnaswami and M.M. Satin, Atlantic surface particulates: composition, settling rates and dissolution in the deep sea, Earth Planet. Sci. Lett. 32, 430, 1976; S. Krishnaswami, M.M. Sarin and B.L.K. Somayajulu, Chemical and radiochemical investigations of surface and deep particulates of the Indian Ocean, Earth Planet. Sci. Lett. 54, 81, 1981. 10 A.E. Bainbridge, Hydrographic data 1972-1973, GEOSECS Atlantic Expedition, 121 pp., National Science Foundation, Washington, D.C., 1981. 11 S. Krishnaswami and M.M. Sarin, Procedures for the simultaneous determination of Th, U, Ra isotopes, 21°Pb, 55Fe, 32Si and 14C in marine suspended phases, Anal. Chim. Acta 83, 143, 1976. 12 J. Murphy and J. Riley, A modified simple solution method for the determination of phosphate in natural waters, Anal. Chim. Acta 27, 1962: J.D.H. Strickland and T.R. Parsons, A practical handbook of seawater analysis, Fish. Res. Board Can. Bull. 167, 621 pp., 1968. 13 D.P. Kharkar, V.N. Nijampurkar and D. Lal, The global fallout of 32Si produced by cosmic rays, Geochim. Cosmochim. Acta 30, 621, 1966.

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14 B.L.K. Somayajulu, Study of marine processes using naturally occurring radionuclides, 100 pp., Ph.D. Thesis, Bombay University, 1969. 15 V.N. Nijampurkar and B.L.K. Somayajulu, An improved method of 32Si measurement in ground waters, Proc. Ind. Acad. Sci. 80A, 289, 1974. 16 C.S.R. Murthy, A programming system on multiple regression analysis, PRL Tech. Note TN-83-33, 34 pp., 1983. 17 D. Lal, V.N. Nijampurkar, B.L.K. Somayajulu, M. Koide and E.D. Goldberg, 32Si specific activities in coastal waters of the world oceans, Limnol. Oceanogr. 21, 285, 1976; D. Lal and B.L.K. Somayajulu, Possible applications of cosmogenic 32Si for studying mixing in coastal waters, in: Isotope Marine Chemistry, E.D. Goldberg, K. Saruhashi and Y. Horibe, eds., p. 145, Uchida Rokakuho, Tokyo, 1980. 18 H.B. Clausen, Dating polar ice by 32Si, J. Glaciol. 12, 411, 1973. 19 D. Lal, V.N. Nijampurkar, G. Rajagopalan and B.L.K. Somayajulu, Annual fallout of 32Si, 21°pb, 22Na, 355 and 7Be in rains in India, Proc. Ind. Acad. Sci. 88A, 29, 1979. 20 D. Lal and B. Peters, Cosmic ray produced radioactivity on the earth, Handbuch der Physik 46, 551, 1967. 21 A.L. Gordon, General ocean circulation, in: Numerical Models of Ocean Circulation, p. 39, U.S. National Academy of Sciences, Washington, D.C., 1975; M. Stuiver, P.D. Quay and H.G. 0stlund, Abyssal water carbon-14 distribution and the age of the world oceans, Science 219, 849, 1983. 22 E.C. Carmack, Water characteristics of the Southern Ocean south of the Polar Front, in: A Voyage of Discovery, M. Angel, ed., p. 15, Pergamon Press, 1977. 23 M. Stuiver, The 14C distribution in west Atlantic abyssal waters, Earth Planet. Sci. Lett. 32, 322, 1976, 24 R. Schlitzer, W. Roether, U. Weidmann, P. Kalt and H.H~ Loosli, A meridional 14C and 39Ar section in Northeast Atlantic deep water, J. Geophys. Res. 90, 6945, 1985.