The distribution of 10Be and 9Be in the South Atlantic

Pergamon PII:

Deep-Sea Research I, Vol. 43. No. 7. pp. 987-1009, 1996 Copyright :b 1996 Elxvier Science Ltd Printed in Great Britain. All rights reserved S09674637(96)00049-0 0967-0637/96 $15.00+0.00

The distribution of “Be and 9Be in the South Atlantic C. I. MEASURES,* T. L. KU,? S. LUO,t J. R. SOUTHON,$ X. XI_Jt§ and M. KUSAKABEtq (Received 7 June 1995; in revisedform

31 January 1996; accepted 5 May 1996)

Abstract-Vertical and surface-water distributions of “Be and 9Be in the South Atlantic Ocean were studied. The major input of 9Be to the surface waters of the region is from the partial dissolution of eolian dusts, with the extent of the dissolution being about seven times that of aluminum from the dust particles. The gradients in surface-water “Be concentrations appear to reflect the magnitude of the local precipitation. The imprinting of the surface water signals onto the deep water masses appears rapid, particularly in regions of enhanced productivity. Bottom waters of Antarctic origin have characteristic isotope signatures that can be traced along their advective route into the Guinea Basin of the eastern Atlantic. Elevated 9Be concentrations in the Angola Basin are indicative of diagenetic input in this region of restricted circulation. The corresponding anomalous “Be in the bottom waters indicates historically lower surface-water “BepBe ratios in the region, perhaps as a result of the further southward penetration of the Inter-Tropical Convergence Zone at those times. The budgets of “Be and 9Be in the South Atlantic were estimated. The results show that crossequator transport of “Be and 9Be from the North Atlantic is 0.3 f lOs3atoms/year and 5.7 + 1O6mol/ year, respectively, accounting for less than 5% of “Be and about 10% of 9Be entering the North Atlantic. There is a net export across 50”s to the Antarctic/Indian/Pacific Oceans of 0.9 f lO23atoms/ year for “Be and 7.5 + lo6 mol/year for 9Be. Using “Be as a tracer, we evaluate accumulation rates of lithogenic minerals to be 1.7 f lOI g/year in the North Atlantic and 4.8 + 10” g/year in the South Atlantic. While the North Atlantic rate agrees with the observed eolian dust input, the estimated lithogenic flux in the South Atlantic is about twice the eolian input, suggesting that the riverine input of 9Be to the open ocean may become non-negligible in areas of low eolian dust flux. Copyright 0 1996 Elsevier Science Ltd

INTRODUCTION Down-core variation of cosmogenic “Be (half-life = 1.5 Myears) can be used to date marine sediments up to 15 Myears of age. As with any radioactive isotope the “Be concentration in a sediment reflects historic variations of its sedimentary accumulation as well as radioactive decay. It has been suggested (Measures and Edmond, 1982; Bourles et al., 1989; HenkenMellies et al., 1990) that use of the stable isotope 9Be to normalize the observed “Be concentrations might be an effective way to eliminate the effect of variations caused by non*University of Hawaii at Manoa, School of Ocean Earth Science and Technology, Department of Oceanography, 1000 Pope Road, Honolulu, HI 96822, U.S.A. t Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, U.S.A. $ Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. 4 Present address: Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA 91125, U.S.A. 7 Present address: Japan Marine Science and Technology Center, 2-l 5 Natsushima-cho, Yokosuka 237, Japan. 987

988

C. I. Measures et al.

etc.). The application of such a radiodecay processes (accumulation, diagenesis, normalizing scheme requires a knowledge of the degree to which the sedimentary isotope ratio reflects that of the overlying water column, as well as a detailed understanding of the processes that influence the oceanic distributions of the two isotopes. Understanding the oceanic behavior of “Be and 9Be is particularly important since the two isotopes have quite different sources and transport routes to the oceans. The results presented here are part of a continuing study whose goal is to determine the isotopic distribution of Be in the major water masses of the ocean and the factors that control it. The study has the added relevance to the paleoceanographic utilization of “Be/‘Be in view of the ratio’s systematic variations along the thermohaline advective flow lines (Ku et al., 1990; Wang et al., 1995). “Be is produced in the atmosphere by the interaction of cosmic rays with the nuclei of 0 and N. Although the cosmic ray flux has a latitude dependency, the processes that control the transfer of material from the stratosphere to the troposphere and its subsequent removal to the ocean result in a “Be input to the surface ocean that is dominated by precipitation, which washes the particle-reactive isotope out of the atmosphere. It has been estimated that only - 1% of the continentally deposited “Be is delivered to the oceans (Brown et al., 1992); the remainder is retained on soils as a result of its high distribution coefficient. The “Be that does enter river systems is largely removed in the estuarine/coastal zone, as is 9Be. The stable 9Be has an average concentration of - 2.8 ppm in crustal rocks (Taylor, 1964). Initial measurements of 9Be in rivers (Measures and Edmond, 1983) suggested that continental run-off would be an important transport route for this element to reach the ocean. Recent results (Kusakabe et al., 1991; Brown et al., 1992) have indicated that estuarine and coastal scavenging processes attenuate the river borne signal considerably, thus minimizing the importance of riverine input to the ocean. Hydrothermal vent fluids are enriched in 9Be by - lOOO-fold relative to ambient seawater. Scavenging associated with the oxidation of Fe and Mn in the non-buoyant hydrothermal plume as it disperses from the vent site appears to remove most of the hydrothermal Be close to its vent source (Bourles et al., 1994) reducing the impact of hydrothermal 9Be on the global cycle. Box model calculations (Kusakabe et al., 1991) and observation of enriched 9Be in the Mediterranean outflow water (Brown et al., 1992) suggest that, in a manner analogous to that of Al, the partial dissolution of continentally derived eolian dust in the surface oceans might be an important route by which 9Be is supplied to the oceans. More detailed work (Brown et al., 1992) has shown not only a considerable input of 9Be into the Mediterranean through eolian deposition but also an apparent removal of “Be as a result of isotopic exchange as the eolian particulate material settles through the water column. It appears then that the major delivery of both isotopes to the oceans occurs by atmospheric processes. The nature of these two processes, however, is quite different. In the case of “Be the pattern is dominated by washout of atmospherically produced material. In contrast, the 9Be input signal is dominated by the proximity of arid continental regions that contribute eolian material and the temporally variable processes that transport it. Estimates of dust input to the ocean have been obtained from various combinations of measured and extrapolated atmospheric dust loads, estimated deposition velocities and precipitation rates (Prospero, 1981; Duce et al., 1991). The picture that emerges from these estimates is that the major sites of dust deposition are restricted to the northwestern parts of the Indian and Pacific Oceans but occupy much of the central part of the Atlantic. In contrast global precipitation shows a similar, equatorial centered pattern in all three oceans. Given the delivery routes for the Be isotopes outlined above, it is to be expected that the

The distribution of “Be and 9Be in the South Atlantic

989

“19Be isotope systematics in the deep ocean should evolve from an eolian dominated 9Be signal emanating from the Atlantic that is progressively eroded by the incorporation of the more uniformly deposited “Be along its advective flow path into the deep Pacific. The deep waters of the Pacific, the oldest end-member, have been well characterized (Kusakabe et al., 1987); similarly the young water masses of the North Atlantic have also been studied (Ku et al., 1990; Xu, 1994). The results indicate that the 9Be content of the deep water in the North Atlantic is only 15% lower than its deep Pacific counterpart. In contrast deep water “Be concentrations more than double, rising from ca 800 atoms/g in the Atlantic to more than 2000 atoms/g in the Pacific. Observing the evolution of Be isotope systematics between these two end-members allows us to separate the one-dimensional recycling function from the more general advective concentration along deep water flow lines. The SAVE (South Atlantic Ventilation Experiment) Leg III sampling in the Angola and Brazil basins of the South Atlantic provided an opportunity to observe the modification of the Be isotope ratio along advective flow lines between the North and South Atlantic. It also allows us to compare the distributions of the two Be isotopes in the essentially dead-end circulation of the Angola Basin in the east, to that in the advectively dominated Brazil Basin in the west. In addition, the transit region is one that is subject to a significant input of Sahara-derived eolian material, most notably to the Guinea Basin. This makes the surface waters in the region a useful “record” of the relative importance of eolian input to the global balance of 9Be, while also allowing the observation of the fate of eolian material after it enters the surface ocean. Furthermore, beryllium geochemistry provides an opportunity for studying the effect of the recycling process on a regionally variable input signal. The oceanic residence time of Be isotopes varies from several hundred years in the Atlantic to about 1000 years in the Pacific (Peng et al., 1989; Ku et al., 1990), on the same order as the ocean mixing time. As 9Be and “Be have disti n c t ly different sources, the progressive homogenization of the two isotopes along the advective flow path is a measure of the rate at which the internal cycling process can erase the geographic variability of the source. METHODS Samples were collected during Leg III of SAVE from Rio de Janeiro, Brazil to Abidjan, Ivory Coast between 28 January and 7 March 1988. Seawater for 9Be determination was collected during the large-volume stations using our own pre-cleaned 5 1or other 10 1Niskin bottles which had been equipped with silicone “0” rings and epoxy coated springs (ODF, Scripps) and were mounted in place of the normal ancillary samplers on the sides of the hydrowire-mounted 250 1 stainless steel Gerard bottles. Although it had been our intention to undertake 9Be determinations at sea, the reagent used for the shipboard determinations was found to have deteriorated during its, approximate, 3 months of storage on the ship. Consequently 11 samples were collected and stored in acid-washed linear polyethylene bottles, acidified with 4ml of 3 x Vycor distilled 6 N HCl. Shore-based determinations of 9Be were performed using electron capture detection-gas chromatography of the l,l, 1-trifluoro-2,Cpentanedione derivative (Measures and Edmond, 1986). The method has a detection limit of ca 2 pM (2 x IO-‘*mol/ 1= 18 x 1O- ’* g/l) and a precision of ca 5%. The accuracy is conservatively estimated to be somewhat lower than this at ca 8%, in order to allow for the extra error involved in fitting the standard curve from day to day.

990

C. I. Measures ef al.

With the exception of the 9Be data for the 750 m sample from Station 141, all data that were collected and processed successfully from the SAVE cruise are presented here. The Station 141 sample is excluded as it is believed to be contaminated; the contamination appears to be in the sample bottle as replicate determinations of this same stored sample yielded similar, but incongruent, data. Samples for “Be were taken as 50 1sub-samples from the stainless steel Gerard bottles. In all cases the “Be was pre-concentrated from water that had already been acidified and stripped for 14C determination. The exceptions to this were the uppermost sample at each station and the surface samples between the profiles. These samples were obtained by utilizing the ship’s sea chest pumping system. All samples were pumped into 120 1 plastic containers lined with a thin film of polyethylene. Into these samples was added hydrochloric acid (to adjust pH to - l-2), stable 9Be carrier, and iron chloride (for those samples previously acidified for i4C stripping no additional acid was added). After at least 6 h, to allow spike equilibration, ferric hydroxide as a carrier of Be isotopes was precipitated from the solution by adding ammonia to the solution to bring the pH to - 8. The sample stood in the covered container for about 12-24 h, allowing the precipitate to settle to the bottom. The supernatant was removed by peristaltic pump and the remaining precipitate was returned to the shore laboratory in a volume of < 500 ml. Shore-based purification of the precipitates was by the procedures of Kusakabe et al. (1987). The purified Be hydroxides were converted to Be0 by heating to - 1000°C in a platinum crucible and their “Be/‘Be ratios were measured by accelerator mass spectrometry as described by Southon et al. (1983). The overall precision of the analyses was typically 5-10%. In addition to sampling vertical profiles at Stations 114, 141, and 158, six “Be and 26 9Be surface water samples were obtained along the cruise track (Fig. 1). Fifty-liter aliquots of surface water for “Be samples were obtained directly from the ship’s seawater pumping system. To avoid contamination of 9Be from this system, uncontaminated samples for 9Be determination were obtained by pole sampling from the side of the ship (Measures and Edmond, 1990). RESULTS Sampling stations are shown in Fig. 1. The station co-ordinates and the Be data in surface waters at each location are listed in Table 1. The potential temperature-salinity characteristics of the water-column and the Be data of Stations 114, 141 and 158 are presented in Table 2 and plotted in Figs 2-5. As the physical, advective processes play an important role in establishing the distributions of Be, in what follows we point out certain salient features in the Be distribution in relation to the hydrography at each station. Station

114

The vertical distribution of 9Be at Station 114 (Table 2; Fig. 3) reflects the water mass structure. The surface - 120 m, composed of warm, high-salinity water advected into the region by the southward flowing Brazil Current (Strama et al., 1990), has slightly enriched 9Be concentration of - 12 pM. From 120 m down to about 950 m, 9Be concentrations are low at 8-l 1 pM. This part of the water column consists of the South Atlantic Central Water (SACW) and the Antarctic Intermediate Water (AAIW) with its low salinity, low silica core at 850 m. A rapid rise in 9Be concentrations from 11.2 to 16.8 pM between 950 and 1075 m is

991

The distribution of “Be and 9Be in the South Atlantic 6o’W

50-w

40-w

3O’W

1o’W

2O’W

0’

IO’E

10-N

IO’N

0’

10’S

20’S

30’S

40’s

30’S

_I--_._...”

6O’W

.. ‘ I

50-w

“-

I

401w

30-w

-.........

7

I

20-w

I

10-w

0’

40-s

10’E

Fig. 1. Map showing the location of the sampling stations during Leg III of the South Atlantic Ventilation Experiment (SAVE) the 4000 m contour is also shown.

coincident with the boundary between AAIW and the Circumpolar Intermediate Water (CPIW) with its minimum oxygen and temperature core at 1173 m. Below 1075 m, 9Be concentrations increase rapidly to - 24 pM at 1950 m, the sample closest to the upper boundary of the North Atlantic Deep Water (NADW) marked by the Si minimum of the Labrador Sea Water (LSW), found at 1830 m. From the depth of the LSW to approximately 3500m (the depth where the potential temperature is ca 2°C; the two degree discontinuity), 9Be concentrations remain extremely uniform around 24pM, a value that was also seen for NADW at the Sargasso Sea WBEX Station 22 (Ku et al., 1990). Filling the bottom of the water column below the two degree discontinuity is the Antarctic Bottom Water (AABW). A single 9Be sample within this water mass shows an enhanced concentration of 27.2 pM. The “Be profile (Table 2; Fig. 4) also generally shows the effect of the various water masses. Deep-water “Be concentrations in the North Atlantic are about 800 atoms/g (Ku et al., 1990) and they are - 1500 atoms/g in the Antarctic (Kusakabe et al., 1982). These two “end-member” values fit the observations here: between 1950 and 3500 m the NADW has “Be concentrations of 800-1000 atoms/g, reflecting the North Atlantic source. At > 3500 m depths below the two degree discontinuity, the two “Be values of 1437 and 1588 atoms/g are undoubtedly indicative of their Antarctic origin. The “Be/‘Be atom ratio at this station (Fig. 5) shows a general downward decrease from > 110 x 10V9 in shallow depths through the CPIW (91 x 10-9) into the NADW (57-

992

C. I. Measures

Table 1.

et al.

Station locations and Be data in the South Atlantic

Station

Latitude

Longitude

114 116 119 120 121 122 123 125 127 131 133 136 139 141 144 146 147 149 150 151 152 153 154 156 158 159 161 163 165 167 170

24”40’ S 24”55’S 25”25’ S 25”40’ S 25”56’S 26”16’S 26”36’ S 27”12’S 27”48’ S 28”37’ S 29’00’ S 3o”oo’ s 27”02’ S 24”55’ S 20”16’S 17”06’S 15’29’ S 12”23’ S 1O”49’ s 09”14’S 08”16’S 07”13’S 06”14’S 04”07’ s Ol”59’S Ol”3o’S OO”30’s OO”3O’N Ol”30’ N 02”45’ N 04”52’ N

38”21’W 35”Oo’ w 30”06’ w 28”31’ W 27”09’ W 25’18’W 23’36’W 20”20’ w 17”04’ w 12”36’W lO”25’W 05”OT w Ol”23’W Ol”O0’ E Ol”O0’ E Ol”O0’ E 01”OO’E 01”OO’E 01’00’ E 01’03’ E OO”14’E OO”35’w Ol”19’W 02”44’ W 04”02’ W 04”02’ W 04”oo’ w 04”02’ w 04”oo’ w 03”59’W 04”Oo’ w

surface ocean

9Be (PM)

“Be (atoms/g)

“Be/‘Be (x 10-9)

13.8 15.6 14.2 10.3 12.7 13.0 11.5 9.3 11.0 10.6 10.6 11.2 9.5 10.0 11.3 14.8 18.4 33.2 24.7 27.9 43.6 31.4 24.1 34.7 27.6 35.1 36.7 42.5 40.5 36.0 38.2

930*43

112k5

1216f67

196+11

1020+38

182+7

585k21

98+5

465 + 37

42+3

558+37 597 + 29

38k3 36+2

686+33

36&2

671+33

4Ok2

Be/Al EF 7.4 13.4 14.3 19.4 36.3 31.7 35.9 27.4 61.1 48.2 70.7 56.0 73.1 62.5 161.4 123.3 87.6 52.7 41.9 32.4 22.2 14.0 21.3 14.2 20.9 8.1 9.3 8.1 7.6 7.6 8.5

EF = (Be:Al),,,,I,/(Be:Al),,,,, 75 x 10p9). The single AABW sample at 3830 m has a ratio of 97 x 10e9, much higher than that of the NADW and close to that of the bottom water of the Drake Passage (Kusakabe et af., 1982).

Station 141

This station is located on the eastern side of the South Atlantic and south of the Benguela/ Angola, front. The surface water at this station is less saline and cooler than the tropical water at Station 114. It is influenced by both the northward flowing Benguela current, advecting cooler waters of southern origin and the offshore transport of wind-driven coastal upwelling. The 10 pM 9Be and 585 atoms/g “Be concentrations in the surface water at this station are much lower than those of Station 114, but are consistent with the values seen in other surface samples in this region (Table 1). The distribution of “Be/‘Be for depths < 3500 m at this station shows a remarkable

The distribution of “Be and ‘Be in the South Atlantic Table 2.

Water-column Be data ‘%/%e

Depth(m)

Be(pM)

'"Be(atoms/g)

(at./at. x10-y

StationII4 0

13.8

930+43

ll2k5

105

IO.1

995+77

l&1*3

240

8.4

92Ok48

182+9

349

9.9

759543

l27+7

497

8.3

799*43

160+9

6Oil

9.7

789+43

135+7

700

9.7

829+67

142kll

850

10.2

819+38

133k6

948

11.2

849+34

126k5

1075

16.8

925+38

91+4

1950

23.8

1068&30

75k2

2200

23.9

82Ok34

57+2

2600

24.3

960+42

66+3

3000

24.1

877+33

6Ok2

3400

23.2

95Ok42

68+3

1437152

3680 3830

27.2

1588k48

97*3

Station141 10.0

585k21

98+5

72

9.9

815k28

137*5 166*8

0

190

7.3

729&34

291

9.8

608+2l

103+4

415

13.2

859+34

108t4

534

12.9

839k43

108k6

642

13.9

lOl5+48

I2li6

751'

25.8

93Oi67

60+4

II30

16.0

906+37

94*4

1649

18.0

1090+38

101+4

2144

23.0

1065k38

77+3

2642

24.1

1095+38

76k3

3144

30.0

ll8lf52

65A3

3643

34.2

119lk77

58t4

4152

34.8

950*2x

45+1

0

27.6

671+33

40+2

35

20.2

60

18.1

274+29

66&3

145

16.9

633k34

62k3

200

16.6

578k34

58k3

300

19.9

754*33

63k3

570

20.0

597k29

50*2

730

18.3

735+29

67k3

910

17.9

627+33

58+3

1580

23.5

823k37

58+3

2cQo

28.5

lOl9k46

59*3

2500

27.9

965+37

57k2

3000

26.3

999+33

63+2

3500

27.8

l239k46

74*3

4000

29.8

1219+46

68k3

Station158

4500

27.7

1224&42

73+3

4850

28.7

1210+42

70?2

993

994

C. I. Measures et al.

(a)

0))

30

+

0

0

+ 0 0

25

2

114 141 158

20

I

0 a

15

2 ij a

10

1

+

?J ‘S 5 ij a

0°" 0

d? St;

5

O-0

4

2

1

0

0 0

p

:

+

”

o 5

+

#+ 6

E $ 7 ._

6

I

+ 0 0

114 141 158

I

+ +* +

0 34

34.5

35

35.5 Salinity

Fig. 2.

36

36.5

31

34.2

34.4

34.6

34.1

34.9

Salinity

Potential temperature and salinity at SAVE Stations 114, 141 and 158.

similarity to that at Station 114, clearly reflecting identical water masses at the two locations. In the upper km the two profiles parallel each other, with the Station 141 ratio approximately 20 x 10e9 lower. In particular there is a pronounced shallow maximum at both stations occurring at a potential density of 26.3 (190-240 m). At 1130 m the Station 141 ratio becomes greater than that at Station 114 by approximately 10 x lop9 units. The values in both profiles parallel each other while the ratio drops from ca 94 x 10m9 at 1130 m to 65 x lop9 at 3100 m. Below 3500 m, the deep water ratios of the two stations diverge. Rather than a rise of “Be/‘Be as seen at Station 114, the ratio at Station 141 continues to decrease, reaching its lowest value, 45 x 10K9, in the deepest 4152 m sample. This low “Be/‘Be ratio is a result of high 9Be (- 35 PM) and low “Be (950 atoms/g) concentrations reflect the near absence of “Be-rich AABW at Station 141. The 9Be concentration of - 35 pM is among the highest observed in deep waters of the world oceans and may result from the long residence time of bottom water on the eastern side of the South Atlantic, allowing the accumulation of diagenetically released 9Be from sediments.

Station 158 The intense rainfall at this equatorial station results in a cap of low-salinity equatorial water from the surface to ca 40m covering the SACW. The surface 9Be concentration, 27.6 pM, is twice the value seen at Station 114; “Be by contrast at 671 atoms/g is similar to that seen at Station 141 (Table 2). Throughout the SACW the 9Be concentrations are some 5-10 pM higher than those seen in this water mass at the other two stations. The high surface

995

The distribution of “Be and 9Be in the South Atlantic

g

-3000

sa B

+

WBEX 22

d

SAVE 158

A

SAVE 141

--)_

SAVE 114

-5000

-6ooo 0

10

20

30

40

9BepM Fig. 3.

Vertical distribution of 9Be at the three South Atlantic stations and also the Western Boundary Experiment (WBEX) Station 22 in the North Atlantic at 34”N 63”W.

9Be level at Station 158 could be due to its proximity to the Sahara Desert, a major source of eolian dust. The closer proximity of Station 158 to the cross-equatorial branch of the southward flowing NADW results in the Si minimum of the LSW at ca 1700-l 800 m being more strongly pronounced than at Station 141. Similarly the salinity maximum of the NADW (34.973) is also well developed, rivaling that seen at Station 114. Beryllium-9 shows the familiar strong increase in the transition from the intermediate waters to the NADW with values rising from 18 pM at 900 m to 28 pM at 1900 m. From the top of the NADW to the base of the water column (4726m) 9Be concentrations are quite uniform, averaging 28 f 1.1 pM, some 3-5 pM higher than at Station 114. “Be concentrations in the deep waters rise from 627 atoms/g at 9 10 m to 1019 atoms/g at 2000 m, similar to the values seen at Station 114 at the top of the NADW. Within the deep waters the “Be data indicate two layers of uniform concentration: the first, from 1900 m to

996

C. I. Measures

et al.

-2ooo

-5000

400

600

800

1000

1200

1400

1600

lo Be atoms&m Fig. 4.

Vertical

distribution

of “Be at the same stations

as Fig. 3.

2900 m, averages ca 1000 atoms/g, and the second, from 3400 m to the bottom, averages ca

1220 atoms/g. Unlike Stations 114 and 141, which show much higher “Be/‘Be ratios in the intermediate waters of Antarctic origin, the “Be/‘Be profile at Station 158 is, for the most part, extremely uniform (Fig. 5). Apart from the low surface water ratio of 40 x 10p9, induced by the high surface water 9Be concentration, the “Be/‘Be profile in the upper 3000m is invariant at (61+ 4) x 10u9. This ratio does rise below 3000 m to (71+ 3) x 10p9, coincident with the deeper of the two uniform “Be layers described above. The profile bears a resemblance to that at Station WBEX 22 of the North Atlantic (Ku et al., 1990) reflecting the similarity of water mass structure between the two stations. Compared to Station WBEX 22, Station 158 shows slightly higher “Be/‘Be ratios. The difference can be attributed to the presence of Antarctic components at this near-equator station and to the “nutrient effect” of “Be input to deep waters (Peng et al., 1989; Ku et al., 1990).

The distribution of “Be and 9Be in the South Atlantic

--+-

SAVE 158

+

SAVE 141

997

1 lo” ‘~9 Be atom/atom Fig. 5.

Vertical profile of the “19Be ratio at the same stations as Fig. 3.

Surface waters

The data (Table 1) show distinct variations in “Be. Values of 930 atoms/g at Station 114 in the Brazil Current rise to 12 16 atoms/g at Station 120 in the center of the Brazil Basin and then decrease along the track to a minimum of 465 atoms/g in the central Angola Basin at Station 147. Values increase to the north reaching ca 670-690 atoms/g in the northern Angola/southern Guinea basins. The more detailed 9Be data set shows a rather different trend. The elevated values in the Brazil Current (13%15.6pM) decrease to 9.3-11.5 pM across the center of the gyre (Stations 123-139). Moving north through the Angola Basin and into the Guinea Basin 9Be concentrations start to increase dramatically, with the highest concentration, 42.5 pM, being found just north of the equator. Between Stations 114 and 141 the variation in “Be/‘Be ratios is driven by the variation in the “Be, the latter varying by > 100% while the 9Be values only vary some 50%. The ratio increases from 112 x 10v9 in the Brazil Current to 182-196 x 10e9 in the center of the gyre.

998

C. I. Measures

et al

The value decreases to 98 x 10m9 in the southern Angola Basin at Station 141. From Station 147, north of the Angola/Benguela front, the ratio is driven almost entirely by the variation in 9Be which increases by a factor of four over the Station 141 value; this results in a dramatic decrease in the ratio from 98 x lop9 to -40 x 10e9 from 15”s to 2”s.

DISCUSSION The three stations with vertical profiles allow the processes that are responsible for the distribution of Be in the oceans to be separated to some degree and examined in relative isolation. We shall do this by using station WBEX 22 in the Sargasso Sea, North Atlantic, reported by Ku ef al. (1990) as representative of the water that is advected southwards into the SAVE region. This will be compared with the three SAVE profiles and we will then attempt to separate the one-dimensional (vertical) from the three-dimensional (advective plus vertical) processes.

Geochemical

cycling of “Be and 9Be in the South Atlantic

The deep waters at Station 114 in the Brazil Basin of the western South Atlantic represent the southward advection of the NADW in the Western Boundary Undercurrent (WBUC) and should therefore provide a means of examining the net one-dimensional input to this water mass in the relatively short time that it takes this water mass to travel from the North Atlantic to the South Atlantic (Weiss et al., 1985). Station 158 in the Guinea Basin of the eastern South Atlantic represents NADW that has also been advected south to the equator in the WBUC, where this water joined the cross-equatorial current system taking it across the Mid-Atlantic Ridge into the eastern basin. Station 141 in the southern Angola Basin off the eastern Atlantic has its deep circulation restricted by the Mid-Atlantic Ridge and the Walvis Ridge. The slower circulation in this basin allows us to examine the sedimentary diagenetic input to the deep waters in direct comparison to the waters that are feeding it from the north. Since we wish to examine the effect that one-dimensional processes play in enriching the deep waters with the various isotopes that are scavenged from the surface waters, we will begin with a discussion of the surface water distributions. Surface waters. The distribution of 9Be (Fig. 6) in the surface waters is remarkably similar to that reported for aluminum (Measures and Edmond, 1990). In the case of Al it was argued that the observed distribution was the result of the partial dissolution in surface waters of particulate eolian material of Saharan origin that had been drawn into the Inter Tropical Convergence Zone (ITCZ) and then rained out into surface waters of the Gulf of Guinea from where it was subsequently distributed by the prevailing currents. The similarity of the 9Be distributions to those of Al suggests that the same process is responsible for its distribution. The averaged observed surface water concentrations of Al and Be in the Guinea Basin (Stations 159-168) are 48.7 nM and 38.2 pM, respectively. If these concentrations were derived entirely from partial dissolution of eolian material, and if this material contains 8% Al and 2.8 ppm Be by weight (Taylor, 1964), then the apparent solubility for Be is about 7.5 times higher than for Al in seawater. This solubility difference is of the same order (to within a factor of two) as that deduced previously. The percentages of Be and Al in seawater dissolved from aerosol dust have been determined by leaching experiments and by mass

The distribution of “Be and 9Be in the South Atlantic

I

I

Surface I

waters I

I

I

999

50

?? ??

Al

0

9Be

0 40

30

g SD 2

20

10

,O Station number Fig. 6.

Surface water distributions of Al and Be in the South Atlantic.

balance calculations. The range of values given for the percentage of Be solubilized from eolian particulates in seawater is 1040% (Kusakabe et al., 1991; Brown et al., 1992; Xu, 1994). The corresponding range for Al is 1.5-10% (Maring and Duce, 1987; Prosper0 et al., 1987; Brown et al., 1992). Support for the Saharan origin of the particulate material contributing the dissolved 9Be can also be derived from the ratio of Be to Al in the surface waters. In the same manner that Brown et al. (1992) calculated an enrichment factor (EFaefined as the (Be:Al),,,,& (Be:Al),,,,J for water column data from the Mediterranean we can also calculate the EF for the surface waters presented here. The results (Table 1) show that the EFs for the Guinea Basin Stations 158-168 (our presumed site of eolian input) are very similar to the value found in the outflow of the Saharan-dominated waters of the Mediterranean (EF -7) (Brown et al., 1992). The variation of the calculated EF in the surface waters of the South Atlantic can be used as an indicator of the relative behavior of Al and Be in the surface waters. Although neither has been measured directly, there is good reason to believe that the residence times of Al and Be in surface waters are significantly different from each other. Using observed concentrations of Al in the oligotrophic Pacific Ocean, Orians and Bruland (1986) estimated that the residence time of Al was 3-4 years in surface waters of this region. Peng et al. (1989) in their lo-box numerical model of oceanic Be distributions found that by using a residence time of 50 years for Be in surface water their box model was able to produce Be distributions that mimicked observed oceanic distributions quite well. With a differential residence time of perhaps a factor of 15, the ratio of Be/Al, and hence the EF in a parcel of surface water, will increase with time, i.e. the ratio of Be/Al in a parcel of water advected from an eolian point source has the property of time and could potentially be used to indicate the importance of eolian input of “new” Be relative to the recycling of the “old” Be

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in the surface waters. This effect can be seen quite clearly in the surface water data in Table 1. The evolution in EF from -7 at Station 114 in the Brazil current to - 161 at Station 144 follows the anticyclonic circulation of the surface waters in the South Atlantic. Station 144, where the maximum EF value is found is on the southern side of the Angola/Benguela front, the point at which the northward flowing Benguela Current turns westward (Gordon and Bosley, 1991) as it encounters the southward flowing Angolan Current (Shannon et al., 1987). North of the front EF values rapidly decline as low EF surface waters from the Gulf of Guinea are brought south by the Angolan Current. The EF values in the Guinea Basin are - 8, similar to those seen in the Brazil Current at Station 114. The similarity of these ratios might suggest that the elevated values of Al and Be at the western end of the cruise track originate in the Gulf of Guinea and are recirculated within the gyre. This lack of change in the EF contrasts strongly with the rather rapid increase in the EF ratio seen across the Brazil Basin from Stations 114 to 127. This increase may be associated with the addition of “old” Be carried by the Antarctic current, which flows north-eastward and converges with the Brazil Current. An alternative explanation is that the Station 114 value is the result of a more localized dust input from South America into this region. Clearly more detailed sampling in the Brazil Current, to the south in the Antarctic and to the north in the equatorial regions is required to differentiate between these possibilities. For “Be the surface water distribution is considerably different: a maximal value of 1216 atoms/g is found at Station 120 in the western part of the basin, while a minimum of 465 atoms/g is found at Station 147 (15S, 0O”E). This distribution pattern can be interpreted in terms of regional precipitation variations. The annual precipitation in the South Atlantic in a region between - 5”s and 28”s along 0”E is less than 25 cm, with a minimum centered around 15’S (Hdflich, 19X4),where we also found a “Be minimum in the surface water. Along the track from Stations 114 to 141, the annual precipitation decreases from > 100 cm in the west to - 25 cm in the east. This east-west gradient in precipitation appears to be responsible for the higher surface-water “Be concentration in the western basin of the South Atlantic. The surface water “Be/‘Be ratios reflect the disparate inputs of the two isotopes. The “Be/‘Be ratios vary from 36 x 10V9 to 196 x 10e9. The lower values, similar to those seen in the North Atlantic, are found in the northeastern part of the South Atlantic; they are associated not only with the lower precipitation there, but also with the large eolian dust input. In contrast, the higher “Be/‘Be ratios are observed in the eastern basin where the “Be input by precipitation is higher and the eolian input of 9Be is lower. Deep waters. The similarity of the 9Be concentration in the NADW between 2 and 3.5 km in the North Atlantic WBEX Station 22 (24.7f 1.3 PM) to that seen in this water mass at Station 114 (23.9 f 0.5 PM) implies that there has been little or no net change in the deep water 9Be content during its transit from the North Atlantic. The “Be data average of 902&-63 atoms/g in the NADW at Station 114, is barely significantly above the 830 f 30 atoms/g seen at WBEX 22. Consequently the “Be/‘Be shows a small increase from the WBEX value of (55 &4) x low9 to (63 Ifr5) x 10e9 at Station 114. The more notable increase in “Be compared to 9Be is to be expected given their relative increases in concentrations along the advective flow line from the Atlantic to the Pacific (15% for 9Be and > 200% for “Be). In the NADW at Station 158, 9Be values are 27.1 IfI 1.1 pM, some 3 pM above the WBEX

The distribution of “Be and 9Be in the South Atlantic

1001

Station 22 or western basin (Station 114) values. The “Be values (982 f 24 atoms/g for 23 km depth range) are also higher than those at Station 114 (902 &-66 atoms/g). In contrast the “Be/‘Be ratio is marginally lower at (60-L-4) x 10m9 than the Station 114 value (63 f 5 x 10-9). It is necessary to explain why the enrichment of “Be and 9Be in deep waters has the order: Station 158 > Station 114 > WBEX 22. Since the source of the enrichment must be from the one-dimensional scavenging of Be in surface waters and its subsequent remobilization in the deep waters, the “Be and 9Be concentrations would increase with aging of the deep waters. As described above, the NADW at Station 114 arrives from the North Atlantic as a plume hugging the eastern boundary of South America. The transit time from the North Atlantic to this station is short, probably not much longer than the -23 year ventilation age calculated for upper NADW at the equator from Freon distributions (Weiss et al., 1985). The deep water at Station 158 is fed by the cross-equatorial branch of the NADW that splits off from the western basin flow and crosses into the eastern basin through the Mid-Atlantic Ridge (MAR) at the Romanche Fracture Zone, situated at - 1”N. The enrichment of “Be and 9Be in the deep waters at Stations 158 and 141 relative to those at Station 114 could indicate that the deep waters in the eastern South Atlantic are older than those in the western basin. On the other hand, it could be argued that there is less 9Be and “Be in the surface waters of the western equatorial Atlantic and hence less surface water scavenging and subsequent remobilization of them into the deep waters that flow down the eastern boundary to Station 114. We have no surface water data in this region to either confirm or deny this hypothesis. However, the general basin-wide enrichment for Al seen in the North Atlantic would tend to suggest that north of the ITCZ there is no large east/west gradient in surface water concentrations away from the Saharan source. In addition the gyre recirculation of the Guinea Basin surface water signal proposed above would tend to mitigate any sharp input gradients in the equatorial regions. A simpler argument that can be made for the differences seen at these stations is that the one-dimensional scavenging-regeneration process is acting more rapidly or over a longer period of time on the NADW that passes into the eastern basin. The - 10 uM lower oxygen concentrations seen in this water mass at Station 158 compared to Station 114 are consistent with either of these explanations. In the absence of any data to determine the average flow rate of the NADW to each of these stations, it is not possible to rule out the longer residence time explanation. There is, however, a mechanism that could increase the rate of surface water scavenging above the flow path of the NADW that flows towards Station 158. Equatorial upwelling in the Atlantic is predominantly found in the eastern part of the basin where the southeast trade winds cross the equator. The upwelling leads to enhanced production which in turn results in an increase in the amount of scavenging and transfer of surface-derived signals to the deep water. This is a similar phenomenon to that reported by Anderson et al. (1990) for the enhanced boundary scavenging of “Be. In their case this enhanced transport mechanism was manifested by enhanced deposition of “Be in sediments at coastal margins. Anderson et al. (1990) speculated that in an ocean with uniform deepwater Be this enhanced boundary scavenging would lead to a depletion of water-column Be concentrations in coastal regions. For an element like Be, that undergoes multiple scavenging and release cycles, enhanced scavenging serves to increase the rate at which enriched surface water signals are transferred to the deep ocean, and thus enhanced scavenging can result in higher steady-state concentrations in the deep waters underlying such areas.

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The changes in the deep water “Be and 9Be seen at Station 141 are also the result of superimposing the one-dimensional cycling of the surface water inputs onto the underlying advection scheme. The circulation of the eastern South Atlantic has been reviewed recently by Warren and Speer (1991). In the Angola Basin, they report a general southeast flow of water originating from the western South Atlantic across the basin at 2 km depth. At greater depths (between 2.4 and 4 km) the MAR becomes a western boundary to the basin and the water at this depth originating from the southwest corner of the Walvis Ridge becomes a northward-flowing western boundary current. The observed Be isotope concentrations in the 2-3 km depth range at Station 141 are consistent with this circulation scheme. During the anticlockwise gyre circulation ofwaters from Station 114 to Station 141 we would expect a surface water scavenging/deep water regeneration process to modify the isotope signal of the deep waters towards the values of the overlying surface waters. The evolution of the deep waters between Station 114 and Station 141 reflects this pattern. Between 2 and 3 km at Station 141 the average 9Be concentrations are 23.5 pM, almost identical to the 24.1 pM observed at Station 114. For “Be the concentrations rise from 902 + 66 atoms/g at Station 114 to 1080 k 21 atoms/g at Station 141 and the “Be/‘Be at Station 141 is 76-J x 10V9 compared to 63 ) 5 x lop9 at Station 114. These changes are consistent with the surface water values of this part of the gyre which have the lowest values of 9Be, but the highest “Be concentrations (930-l 2 16 atoms/g) and the highest “Be/‘Be ratios (112-196) x lop9 (Table 1). Bottom water. Antarctic Bottom Water is found occupying the water column beneath the NADW complex at Station 114. The single data point for 9Be in this water mass shows an elevated value of 27.2 pM. This value is consistent with values determined for AABW in the deep Cape Basin of ca 27 pM (C. I. Measures, unpublished data, 1991) and the Circumpolar Current ca 26 pM (Kusakabe et al., 1987). The “Be values almost double from the NADW to the underlying AABW with the highest value reaching 1588 atoms/g near the bottom at Station 114. This bottom value is consistent with the ca 1500 atoms/g found in the Circumpolar Water in the Drake Passage (Kusakabe et al., 1987). The “Be/‘Be ratio for this water mass 97 x low9 is also close to that seen in the deep waters of the Circumpolar Water (Kusakabe et al., 1987). The bottom waters at Station 158 consist of an admixture of AABW and lower NADW that flow through the Romanche Fracture Zone to ventilate the bottom waters of the eastern South Atlantic. The composition of this inflowing water has been estimated by Broecker et al. (1980) to be an approximately even mix of southern component water (i.e. AABW) and northern component water (i.e. lower NADW). This assessment has been confirmed by using samples from within the Romanche Fracture Zone. The “Be data at Station 158 in the Guinea Basin indicate a uniform deep water layer below 3 km with considerably elevated values. The origin of this enrichment would appear to be the admixture of the lower NADW and AABW as it quantitatively matches the composition scheme outlined above. Applying the Broecker et al. (1980) water mass composition to the observed loBe contents of the two water masses at Station 114 (50% of of 1269 atoms/g in the 1588 atoms/g + 50% of 950 atoms/g) yields a “Be concentration mixture, remarkably close to, and within the analytical error (40-50 atoms/g) of, the observed average of 1223 atoms/g. The relatively small difference in 9Be concentrations of 4 pM between the lower NADW and AABW would yield an expected 2 pM increase in the bottom waters at Station 158.

The distribution of “Be and 9Be in the South Atlantic

1003

However, as discussed above the deep waters of this station are already enriched by - 3 pM with surface-water scavenged 9Be and therefore the weaker advective signal has been obscured. The bottom waters at Station 141 are composed of Guinea Basin water that overflows the - 4300 m sill at the Guinea Rise (Warren and Speer, 1991). This water flows southward as a boundary current turning into the interior where it fills the bottom of the Angola Basin. Since there are no deep outlets for this water from the basin, continuity requires that there is upwelling of the deep water. At Station 141 there is a considerable enrichment of 9Be below 3000 m. While we would expect to see an increase over the bottom water values at Station 158 the values at Station 141 reach -35 pM at the bottom-some 6 pM above Station 158, and are the highest reported for open ocean stations to date. They are however less than some of the bottom water values reported for the nearshore stations in San Nicolas Basin (33-48 PM) (Kusakabe et al., 1982). The enrichment in both of these cases may well be a result of the limited circulation in these basins, resulting in a significant diagenetic flux from the sediments imprinting the lower water column. The “Be data below 3000m (with the exception of the deepest sample) are very close (1186 atoms/g) to the values seen at Station 158 (1223 atoms/g), and are consistent with the southward propagation of Guinea Basin bottom water. At first sight the depleted “Be value in the deepest sample from Station 141 would appear to simply be the result of the leakage of the 250-l Gerard water sampler and contamination with water from higher up in the water column. However, the concordance of the salinity from the Gerard sampler with that from the attached Niskin bottle excludes this possibility. The “Be/‘Be ratio for this sample is entirely consistent with the samples in the water column above it, falling on the extension of a smooth trend with the “Be/‘Be ratio falling from -77 x 10e9 at 2100m to 45 x lop9 at 4152 m. If these anomalously low values for “Be and “Be/‘Be are not artifacts then they must arise either from advection into the basin or from diagenetic processes in the sediments. An advective source can be dismissed for two reasons. Firstly, the basin at this depth is closed to circulation from all directions except the north and the isotopic characteristics of the water in the Guinea Basin do not match those observed here. Secondly, even if passageways existed in the MAR to ventilate the basin at this depth, there is no water mass in any of the other adjacent basins that matches the properties (high ‘Be-low “Be) observed. If advection is limited to water originating from the Guinea Basin then only an end-member containing no admixed AABW but still highly enriched 9Be would satisfy the observed distributions. At this stage we know of no evidence to support a relatively recent cessation of the contribution of AABW to the Romanche Fracture Zone through flow. A diagenetic explanation seems most likely since it is known that the water at the bottom of the Guinea Basin has a relatively old radiocarbon age of 130 + 40 years (Broecker et al., 1980). In addition the elevated Si values seen in the bottom waters have been ascribed to diagenetic input (Chan et al., 1977). For diagenesis to explain the observed distribution of Be isotopes requires remobilization of a sediment component with an anomalously low “Be signal. This can only be achieved by one of two mechanisms. Either the material is old enough that radioactive decay has attenuated the “Be concentrations, or the original material had low “Be concentrations. Since the remobilization processes that involve the recycling of Be into pore waters are likely to involve the Fe/Mn redox cycles and occur near the sediment-water interface, it is unlikely that decay can play a significant role for an

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et al.

isotope with a 1.5 Myear half-life. The only plausible explanation that this leaves is that the original sedimented material had an anomalously low “Be/‘Be isotope ratio. The results above indicate that the deep water signal is imprinted by the surface water ratio and so must be the sediments. The surface water “Be/‘Be ratio for Station 141 is 98 x 10e9 more than twice that of the deepest sample. Indeed nowhere in the water column is the ratio as low as seen in the bottom sample. The isotope ratio of the material that is being remobilized from the sediments does not reflect the contemporary surface water ratio. The surface water “Be/‘Be ratio however is considerably lower (42 x 10p9) at Station 147 approximately 1000 km away, north of the Angola-Benguela front. Since the sedimentary remobilization process is integrating over a considerable period of time the anomalous ratio could be explained by either a significant seasonal variation in the “Be/‘Be of the surface water in this region, a significant cross-front transport of particulate material as it settles through the water column, or that the processes bringing the elevated 9Be to surface waters extended much further south historically. The more southward penetration of the Saharan 9Be could be accomplished by either moving the AngolaBenguela front further south or having a larger southern oscillation of the ITCZ during the austral summer. Deposition

and interhemispheric

transport

of “Be and 9Be in the Atlantic

Ocean

The Atlantic Ocean’s conveyor circulation carries the NADW from the northern to southern hemispheres at a rate of 20 x lo6 m3/s (Broecker and Peng, 1992). This southward flow in the deep ocean is compensated by the northward flow in the upper ocean above - 1000 m (Hall and Bryden, 1982) and the exchange constitutes a major process responsible for the interhemispheric transport of carbon in the oceans (Brewer et al., 1989; Broecker and Peng, 1992). To evaluate the interhemispheric transport of “Be and 9Be, we list in Table 3 the average concentrations of “Be and 9Be and their ratios in the South and North Atlantic Oceans based on the data of the present study for the South Atlantic and those in the North Atlantic by Ku et al. (1990) and Xu (1994). It is seen that the “Be/‘Be ratio increases along the conveyor circulation, from 47 x 10e9 in the surface North Atlantic to 72 x 10e9 in the deep South Atlantic. This increase is due to different source functions of “Be and 9Be: dominant fluvial and eolian input of 9Be in the North Atlantic as opposed to the more-orless uniform pluvial input of “Be over the world oceans (Peng et al., 1989; Ku et al., 1990). The high “Be/‘Be ratio of 91 x 10Y9 in the upper South Atlantic is mainly due to the low 9Be concentration. Continuing particle scavenging from surface waters and their regeneration in deeper parts of the water column (nutrient-like effect) cause the concentrations of both Be isotopes to increase along the advective flow lines. But the extent of the increase is different between the two isotopes, leading an increase of the “Be/‘Be ratio from the surface North Atlantic toward the surface North Pacific (Ku et al., 1990). From the Table 3 data and the aforementioned transport rate of 20 x lo6 m3/s, we estimate net southward transports of 0.3 x 1O23atoms/year and 5.7 x lO”mol/year for “Be and 9Be, respectively (Table 4). Since the atmospheric input of “Be over the North Atlantic is of the order of 7.0 x 1O23atoms/year-taking an average atmospheric “Be production rate of 1.2 x lo6 atoms/cm2/year (Monaghan et al., 1985/86) over an area of 58 x lo6 km2we note that < 5% of “Be is transported to the southern hemisphere by the conveyer circulation. This implies that most of the “Be supplied from the atmosphere is removed to sediments of the North Atlantic basin before being advected into the South Atlantic. The

The distribution of “Be and ‘Be in the South Atlantic Table 3.

1005

Average concentrations of “Be and ‘Be in the upper and deep oceans of the Atlantic Ocean*

“Be (atoms/g)

‘Be (PM)

‘“Be/gBe (IO-‘)

North Atlantic Upper ocean Deep ocean

700 + 40 820+80

25+3 23&2

47+6 59*7

South Atlantic Upper ocean Deep ocean

770 f 80 1080+ 10

1454 25&-2

91k28 72k6

Oceans

*Upper ocean refers to the GlOOO m depth and deep ocean to that of > 1000 m.

sediment removal rate in the North Atlantic is about 6.7 x 1O23atoms/year, neglecting the continental input of “Be as it is effectively removed in estuaries and coastal areas (Kusakabe et al., 1990; Brown et al., 1992). Sodium hydroxide leaching experiments show that the “Be/‘Be ratio in the authigenic phase(s) of sediments is in equilibrium with that of deep waters (Wang et al., 1996). If so, the sedimentation rate of authigenic 9Be in the North Atlantic can be estimated from the sedimentation rate of “Be and the deep water “Be/‘Be ratio as: (6.7 x 1023) + (59 x 10e9) + (6.02 x 1023)= 1.9 x lO’mol/year. About 40% of 9Be in deep-sea sediments has been found to be authigenic (Bourles et al., 1989; Wang et al., 1996) giving a total 9Be accumulation rate of 4.8 x lO’mol/year. The sum of this accumulation rate and the southward transport of 5.7 x lo6 mol/year is balanced by a total input of 5.4 x 10’mol/year in the North Atlantic. Therefore about 10% of the total input of 9Be is transported to the South Atlantic by the conveyor circulation. If we assume that the source of 9Be in the ocean is chiefly from eolian minerals with a Be concentration similar to that of average crustal rocks (2.8 ppm), then the total input of aerosol minerals to the North Atlantic would be 1.7 x lOi g/year, an estimate comparable to the value of 2.2 x lOi g/year given by Duce et al. (1991). It should be noted that the negligible interhemispheric transport of “Be in the Atlantic as deduced from our mass balance calculations in no way conflicts with the relatively long oceanic residence time of Be (i.e. similar to or slightly shorter than the ventilation time of the deep ocean; Ku et al., 1990). In the North Atlantic, the “Be scavenging (deposition) rate (atoms/cm3/year) is equal to its deep-water concentration divided by its residence time. Expressed in the same unit as the “Be scavenging rate, the net cross-equator transport of “Be is equal to the concentration difference between the deep North Atlantic and surface South Atlantic divided by the water residence time in the deep

Table 4.

Interhemispheric

transport of “Be and ‘Be in the Atlantic Ocean

Transport

“Be (1O23atoms/year)

‘Be ( lo6 mol/year)

Northwards Southwards Net transport

-4.9 5.2 0.3

-8.8 14.5 5.7

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North Atlantic. From these two definitions, it follows that if the two residence times are comparable, the ratio of interhemispheric transport “Be to local scavenging for “Be should approach zero as the “Be concentration ratio of the deep North Atlantic to surface South Atlantic approaches unity. The observed concentration ratio of - 0.95 (Table 3) therefore, points to a negligible cross-equator transport of “Be, even though “Be in the deep North Atlantic has a residence time of 2-3 hundred years.

Budgets of “Be and 9Be in the South Atlantic

Ocean

The geochemical budgets of the two Be isotopes may be computed using the radiocarboncalibrated ocean mixing model of Broecker and Peng (1987). The model depicts that the South Atlantic exports deep waters (as the NADW) to the Antarctic at a rate 20 x lo6 m3/s, while receiving 4 x lo6 m3/s each from the Antarctic as the AABW and the AAIW + CPIW and 12 x lo6 m3/s from the Pacific+ Indian intermediate waters (PIIW). The “Be concentrations in these water masses are estimated as follows: 1080 atoms/g in the South Atlantic part of NADW (Table 3) 830 atoms/g in both AAIW + CPIW (Station 114) and 1450 atoms/g in the AABW (Kusakabe et al., 1982, 1987). Using these estimates and assuming the “Be concentration in PIIW to be similar to that of the upper 1000 mat Station 141 (800 atoms/g), we calculate that water exchange results in a net export of 0.9 x 1O23atoms/year of “Be from the South Atlantic to the Antarctic/Pacific/Indian Oceans. Taking into account the 0.3 x 1O23atoms/year advected into the South Atlantic from the North Atlantic (Table 4) we see that there is a net loss of 0.6 x lo*’ atoms/year of “Be from the South Atlantic due to advective transport. We calculate the “Be flux to the surface waters of the South Atlantic to average about 0.9 x lo6 atoms/cm*/year (i.e. three quarters of the global mean value of 1.2 x 1O6 atoms/cm*/year), based on the fact that rainfall between 0” and 45”s is about one quarter less than the mean precipitation of the South Atlantic + Antarctic (Hiiflich, 1984). Taking 50”s as the southern boundary gives an area of 32 x lo6 km* for the South Atlantic, and thus an atmospheric “Be input of 2.9 x 1O23atoms/year. Therefore, the total deposition of “Be in the sediments is 2.3 x 1O23atoms/year, equal to the surface water inputs minus the advective loss. Similar budget estimations can be made for 9Be. The 9Be concentrations are about 8 pM for AAIW and 12 pM for CPIW (C. I. Measures, unpublished data), averaging 10 pM for these two water masses. This value also applies to PIIW, based on the 9Be profile from Cape Basin at 30”s (C. I. Measures, unpublished data). The 9Be concentration in AABW is about 25 pM (Ku et al., 1990) similar to that in the deep South Atlantic (Table 3). With the above 9Be concentration values, the export of 9Be from the South Atlantic through its southern boundaries with the Antarctic and Indian Oceans is estimated to be 7.5 x 106mol/year. Since the North Atlantic supplies 5.7 x lo6 mol/year via advection, ocean circulation results in a net export of 1.8 x lo6 mol/year from the South Atlantic. The accumulation of authigenic 9Be in South Atlantic sediments can be calculated from the “Be sedimentation rate of 2.3 x 1O23atoms/year and the deep-water “Be/‘Be of 72 x lop9 (Table 3) to be 5.3 x lo6 mol/year. Assuming that 40% of the sedimentary 9Be is authigenic (Wang et al., 1996) the total sedimentation rate of 9Be is 13 x 106mol/year. To balance the loss of 9Be by advection and sedimentary deposition requires an influx of 9Be of 14.8 x lo6 mol/year, or 4.8 x lOI g/year of lithogenic particles from the continents (assuming 2.8 ppm 9Be in continental rocks). This estimate is significantly lower than that for the North Atlantic discussed above, but higher than the eolian dust flux observed by

The distribution

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Duce et al. (1991) by about a factor of 2. If our model calculation and the dust flux estimate are both correct, then the difference implies that in areas with low influx of eolian dusts such as the South Atlantic, a sizable fraction of detrital sediments (and the 9Be contained therein) could originate from riverine input and be delivered to the deep sea via transport of fine clay particles. CONCLUSIONS This work has shown that the input of Saharan material has a significant effect on 9Be concentrations in the equatorial regions of the Atlantic Ocean, whereas the “Be in the surface waters is associated with local precipitation. The surface water signals are rapidly transferred to the deep water, particularly in regions where enhanced primary productivity exist. Diagenetic processes are most important in regions where deep water circulation is slow. In the Angola Basin where this is most apparent the isotopic ratio of the remobilized material suggests that there may have been significant differences in the surface water ratios in the past. Water-column variations of “Be and 9Be reflect the difference in source and watermass in the South Atlantic. The similarity in distributions of “Be and 9Be (at < 3500 m) between Stations 114 and WBEX 22 reflects the relative rapidity of the western boundary current in the Atlantic Ocean. Enhanced “Be and 9Be concentrations in deep waters of the eastern basin indicate longer deep-water residence times there. Higher “Be/‘Be ratios in the intermediate waters at Stations 114 and 141 indicate that these waters originate mainly from AAIW and CPIW. It is estimated from the “Be and 9Be data that bottom waters at Stations 114 and 158 contain N 100% and N 50% of AABW, respectively, whereas little AABW is present at Station 141-consistent with the circulation pattern proposed by Warren and Speer (1991). Geochemical model calculations show that the interhemispheric transport of dissolved loBe and 9Be from the North to the South Atlantic is 0.3 x 1O23atoms/year and 5.7 x lo6 mol/year, respectively, accounting for ~5% of “Be and w 10% of 9Be supplied to the ocean surface from the atmosphere. The net export of Be isotopes from the South Atlantic to the Antarctic/Indian/Pacific Oceans is about 0.9 k 1O23atoms/year (loBe) and 7.5+ lO”mol/year (9Be). Based on the Be isotope budgets, we estimated the accumulation rate of lithogenic minerals to be 1.7 + lOI g/year in the North Atlantic and 4.8f 1013g/year in the South Atlantic. While the North Atlantic rate agrees with the observed eolian dust input (Duce et al., 1991) that for the South Atlantic is higher than the eolian input by about a factor of 2. This implies that particulate 9Be of riverine source may contribute to the pelagic sedimentary flux of ‘Be in areas of low eolian dust flux such as the South Atlantic. The observed negligible inter-ocean transport of 9Be suggests that the variation of deep-water “Be/‘Be ratio could chiefly reflect the variation of “Be concentration. This would facilitate the use of authigenic “Be/‘Be in sediments as time and property tracers. Acknoll,lengernents-We would like to thank Bill Smethie (project co-ordinator), Bill Jenkins. and Don Olson (Chief Scientists, Leg III) for finding the space on a very crowded ship which allowed us to participate in Leg III of SAVE. We are also indebted to the same people and the other SAVE PI’s for allowing us to make use of the hydrographic data, the excellent quality of which is a testament to the professional skill of the members of the PACODF team. We wish to thank D. Kaminski for frequent, and too many others to name for occasional, help with the surface sampling. We would like to thank the officers and the crew of the R.V. Knorr whose skills made the

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cruise so successful. For the AMS measurement of “Be, we are indebted to D. Erle Nelson and John S. Vogel for their generous support and assistance. We would like to thank the two anonymous reviewers for their comments. This work was supported by NSF grants OCE-8717227 (to C.I.M.) and OCE-9000450 (to T.L.K.), and by the Canadian NESRC (to DEN) and McMaster University Accelerator Laboratory. Contribution no. 4138 of the School of Ocean Earth Science and Technology of the University of Hawaii.

REFERENCES Anderson R. F., Y. Lao, W. S. Broecker, S. E. Trumbore, H. J. Hofman and W. Wolfli (1990) Boundary scavenging in the Pacific Ocean: a comparison of “Be and *“Pa. Earth and Planetary Science Letters, 96, 287-304. Bourles D., G. M. Raisbeck and F. Yiou (1989) loBe and 9Be in marine sediments and their potential for dating. Geochimica et Cosmochimica Acta, 53, 443452. Bourles D. L., E. T. Brown, C. R. German, C. I. Measures, J. M. Edmond, G. M. Raisbeck and F. Yiou (1994) Examination of hydrothermal influences on oceanic beryllium using fluids, plume particles and sediments from the TAG hydrothermal field. Earth and Planetary Science Letters, 122, 143-157. Brewer P. G., C. Goyet and D. Dyrssen (1989) Carbon dioxide transport by ocean currents at 25”N latitude in the Atlantic Ocean. Science, 246, 477479. Broecker W. S. and P.-H. Peng (1987) The role of CaCOs compensation in the glacial to interglacial atmospheric CO1 change. Global Biogeochemical Cycles, 1, 15-39. Broecker W. S. and T.-H. Peng (1992) Interhemispheric transport of carbon dioxide by ocean circulation. Nature, 356, 587-589. Broecker W. S., T. Takahashi and M. Stuiver (1980) Hydrography of the central Atlantic-II. Waters beneath the two-degree discontinuity. Deep-Sea Research, 27, 397419. Brown E. T., C. I. Measures, J. M. Edmond, D. L. Bourles, G. M. Raisbeck and F. Yiou (1992) Continental inputs of beryllium to the oceans. Earth and Planetary Science Letters, 114, 101-l 11. Chan L. H., D. Drummond, J. M. Edmond and B. Grant (1977) On the barium data from the Atlantic GEOSECS Expedition. Deep-Sea Research, 24, 613-649. Duce R. A., P. S. Liss, J. T. Merrill, E. L. Atlas, P. Baut-Menard, B. B. Hicks, J. M. Miller, J. M. Prospero, A. R. Arimoto, T. M. Church, W. Ellis, J. N. Galloway, L. Hansen, T. D. Jickells, A. H. Knap, K. H. Reinhardt, B. Schneider, A. Soudine, J. J. Tokos, S. Tsunogai, R. Wollast and M. Zhou (1991) The atmospheric input of trace species to the world ocean. Global Biogeochemical Cycles, 5, 193-259. Gordon A. L. and K. T. Bosley (1991) Cyclonic gyre in the tropical South Atlantic. Deep-Sea Research, 38(suppl.), S323-S343. Hall M. M. and H. L. Bryden (1982) Direct estimates and mechanisms of ocean heat transport. Deep-Sea Research, 29, 339-359. Henken-Mellies W. U., J. Beer, F. Heller, K. J. Hsii, C. Shen, G. Bonani, H. J. Hofmann, M. Suter and W. Wiilfli (1990) “Be and 9Be in South Atlantic DSDP Site 519: Relation to geomagnetic reversals and sediment Earth and Planetary Science Letters, 98, 267-276. composition. Hoflich 0. (1984) Climate of the South Atlantic Ocean. In: Climates of the oceans, world survey of climatology, H. Van Loon, editor, Vol. 15, Elsevier, Amsterdam, 191 pp. Ku T. L., M. Kusakabe, C. I. Measures, J. R. Southon, G. Cusimano, J. S. Vogel, D. E. Nelson and S. Nakaya (1990) Beryllium isotope distribution in the western North Atlantic: a comparison to the Pacific. Deep-Sea Research, 37, 795-808. Kusakabe M., T. L. Ku, J. S. Vogel, J. R. Southon, D. E. Nelson and G. Richards (1982) “Be profiles in seawater. Nature, 299, 712-714. Kusakabe M., T. L. Ku, J. R. Southon, J. S. Vogel, D. E. Nelson, C. I. Measures and Y. Nozaki (1987) Distribution of “Be and 9Be in the Pacific Ocean. Earth and Plunetary Science Letters, 82, 231-240. Kusakabe M., T. L. Ku, J. R. Southon and C. I. Measures (1990) Beryllium isotopes in the ocean. Geochemistry Journal, 24, 263-272. Kusakabe M., T. L. Ku, J. R. Southon, S. Liu, J. S. Vogel, D. E. Nelson, S. Nakaya and G. L. Cusimano (1991) Be isotopes in rivers/estuaries and their oceanic budgets. Earth and Planetary Science Letters, 102, 265-276. Maring H. B. and R. A. Duce (1987) The impact of atmospheric aerosols on trace metal chemistry in open ocean surface seawater. 1. Aluminum. Earth and Planetary Science Letters, 84, 381-392.

The distribution

of “Be and ‘Be in the South Atlantic

1009

Measures C. I. and J. M. Edmond (1982) Beryllium in the water column of the central North Pacific. Nature, 297, 51-53. Measures C. I. and J. M. Edmond (1983) The geochemical cycle of ‘Be: a reconnaissance. Earth and Planetary Science Letters, 66, 101-l 10. Measures C. I. and J. M. Edmond (1986) Determination of beryllium in natural waters in real time using electron capture detection gas chromatography. Analytical Chemistry, 58, 2065-2069. Measures C. I. and J. M. Edmond (1990) Aluminum in the South Atlantic: steady state distribution of a short residence time element. Journal of Geophysical Research, 95, 5331-5340. Monaghan M. C., S. Krishnaswami and K. K. Turekian (1985/86) The global-average production rate of “Be. Earth and Planetary Science Letters, 76, 279-281. Orians K. J. and K. W. Bruland (1986) The biogeochemistry of aluminum in the Pacific Ocean. Earth and Planetary Science Letters, 78, 397410. Peng T. -H., T. L. Ku, J. Southon, C. Measures and W. S. Broecker (1989) Factors controlling the distribution of “Be and ‘Be in the ocean. In: From mantle to meteorites, a garland of perspectives (festschrtft for Devendra Lal), K. Gopalan, V. K. Gaur, B. L. K. Somayajulu and J. D. Macdougall, editors, Indian Academy of Sciences, Bangalore, pp. 201-204. Prosper0 J. M. (1981) Eolian transport to the world ocean. In: The sea, Emiliani, editor, Vol. 7, Wiley and Sons, New York, pp. 801-874. Prosper0 J. M., R. T. Ness and M. Uematsu (1987) Deposition rate of particulate and dissolved aluminum derived from Saharan dust in precipitation at Miami, Florida. Journal of Geophysical Research, 92, 14,714 14,723. Shannon L. V., J. J. Agenbag and M. E. L. Buys (1987) Large and mesoscale features of the Angola-Benguela front. African Journal of Marine Science, 5, 1 l-34. Southon J. R., J. S. Vogel, I. Rowikow, D. E. Nelson, R. G. Korteling, T. L. Ku, M. Kusakabe and C. A. Huh (1983) The measurements of “Be concentrations with a tandem accelerator. Nuclear Instrumentation Methods, 205, 251-251. Strama L. A., Y. Ikeda and R. G. Peterson (1990) Geostrophic transport in the Brazil Current region north of 20”s. Deep-Sea Research, 37, 1875-1886. Taylor S. R. (1964) Abundance of chemical elements in the continental crust: A new table. Geochimica et Cosmochimica Acta, 28, 1273-1285. Wang L., T. L. Ku, S. Luo and J. R. Southon (1995) ‘“Be/gBe as paleoceanographic proxy: Weakening of Atlantic Western Boundary Undercurrent during the LGM, EOS 76, p. F688. Wang L., T. L. Ku, S. Luo, J. R. Southon and M. Kusakabe (1996) s6Al-“Be systematics in deep-sea sediments. Geochimica et Cosmochimica Acta, 60, 109-I 19. Warren B. A. and K. G. Speer (1991) Deep circulation in the eastern South Atlantic Ocean. Deep-Sea Research, 38, S281-S322. Weiss R. F., J. L. Bullister and M. J. Warner (1985) Atmospheric chlorofluoromethanes in the deep equatorial Atlantic. Nature, 314, 608-610. Studies of Beryllium Isotopes in Marine and Continental Natural Water Systems, Xu X. (1994) Geochemical Ph.D. Thesis, University of Southern California, 315 pp.