Chemical Geology, 29 (1980) 211--226 211 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

OBSERVATIONS OF THE DISTRIBUTION OF MANGANESE OVER THE EAST PACIFIC RISE

G.P. KLINKHAMMER

Graduate School of Oceanography, University of Rhode Island, Kingston, R102881 (U.S.A.)

(Received July 30, 1979; revised and accepted March 12, 1980)

ABSTRACT

Klinkhammer, G.P., 1980. Observations of the distribution of manganese over the East Pacific Rise. Chem. Geol., 29: 211--226.

When seawater circulates through hot basalt at active spreading centers, the manganese concentration of the seawater increases by ~ l0 s times. Eventually, this hydrothermal water is discharged back into ambient seawater through vents and fissures on the sea floor. Mn vs. silica relationships in water samples collected from active vent fields on the Galapagos Rift are linear hut have different slopes. These relationships demonstrate that the Mn in the hydrothermal end-member mixes conservatively with ambient seawater (at least below 10°C) but the physical and chemical processes responsible for the Mn concentra- tion of the hydrothermal fluid must be variable. The Mn concentrations measured for the Galapagos system predict that hydrothermal emanations are major sources of Mn to the oceans but only if these data are not anomalously high. This paper presents profiles of Mn in bot tom waters at four locations along the East Pacific Rise (EPR) which support the hypothesis that Mn emanations comparable to those from the Galapagos are occurring along the EPR. Bottom water at one station in the Guatemala Basin, some 1000 km from the EPR crest, also has anomalously high Mn concentrations suggesting that this hydro- thermal Mn signal can be transported far from the source.

INTRODUCTION

The major role played by hydrothermal processes in the cycling of elements in the oceans is becoming increasingly apparent (Edmond et al., 1979a, b). One of the important consequences of hydrothermal circulation is the emana- tion of manganese. Mn is a prominent element in seawater geochemistry and a sensitive tracer for the discharge of hydrothermal fluids. This study presents our first direct observations of the areal distribution of Mn emanations.

The locations of stations used in this study are shown in Fig. 1. Stations PIO and P 1 7 are on the East Pacific Rise (EPR) and Galapagos Spreading Center (GSC), respectively. The first evidences of hydrothermal activity in the GSC were low heat flow measurements and bottom water temperature anomalies of a few hundredths of a degree (Sclater and Klitgord, 1973;

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Klitgord and Mudie, 1974; Williams et al., 1974). Deep tow studies of the area (Lupton et al., 1977; Weiss et al., 1977) were carried out during Leg I of the Pleiades Expedit ion of the R/V "Melville" in 1976. Water samples were col- lected from plumes above vents during Pleiades I and II, including one sample with a temperature anomaly of 0.2°C. More recently, water samples with temperatures 10°C above ambient water were collected from the vents them- selves with the submersible "Alvin" (Corliss et al., 1979).

Klinkhammer et al. (1977) demonstrated that hydrothermal emanations from the Galapagos Rift greatly affect the distribution of Mn in the overlying water column. The Mn concentrations in hydrocast samples taken above the GSC are as high as 15 nmol kg -~ or 50 times "background" levels. Even higher concentrations were measured in the anomalously warm waters sampled with "Deep T o w " (Weiss et al., 1977} and the submersible "Alvin" (Corliss et al., 1979). Mn data from the Galapagos were used by Weiss (1977) and Corliss et al. (1979) to estimate the hydrothermal flux of this element. These indirect estimates, based on heat and 3He normalizations, suggest that hydrothermal phenomena make a major contr ibution to the Mn budget of the oceans. One of the assumptions implied by these calculations is that hydro- thermal Mn emanations of this magnitude are not unique to the GSC. To determine how ubiquitous such emanations are, samples were collected at the other locations shown in Fig. 1.

Stations T2 and T4 are part of a detailed study of hydrothermal activity at 21°N near the Gulf of California. Station T4 is located on the Tamayo Frac- ture Zone while T2 is on the main extrusion zone of the EPR. The area sur- rounding T2 was surveyed extensively with deeply towed vehicles (Normark, 1976; Crane and Normark, 1977) and anomalously high temperatures of 3 . 1 0 - 2 - - 5 • 10 -2 °C were found 50 m above the sea floor.

Stations M7 and M9 were also surveyed with "Deep Tow" (Lonsdale and

213

Spiess, 1979a) as part of the Manganese Nodule Program (MANOP). Station M7 actually represents several hydrocasts taken east, west and on the axis of the EPR. Station M9 is in a plain in the Guatemala Basin about 1000 km east of the rise crest.

This paper first describes the results from the GSC where the evidence for a hydrothermal source for the anomalously high Mn concentrations in bo t tom waters is unequivocal. A comparison of these results with distributions at the other locations shown in Fig. 1 supports the content ion of widespread hydro- thermal emanation of Mn.

EXPERIMENTAL

Station numbers, locations, depths and the Mn concentrations of all samples in this report are tabulated in the Appendix.

Complete details of the analytical method involved in measuring Mn are given elsewhere (Klinkhammer, 1980a). Briefly, the data presented here are concentrations of " to ta l dissolvable Mn" (Bender et al., 1977). That is, the sea- water samples were no t filtered but were acidified to pH 2. 250-ml aliquots of these samples were spiked with S4Mn, adjusted to pH 8--9, extracted with 8-hydroxyquinoline in chloroform and back-extracted into 3 M HNO3. Efficiencies were determined by counting the 835-keV S4Mn gamma activity. The Mn concentrations in these acid concentrates were measured by atomic absorption in a graphite furnace.

This method detects at least 90% of the total Mn in seawater. Duplicate analyses are listed in the Appendix and plot ted in the figures as connected data points at the same depth. The precision of this measurement determined from 59 duplicate analyses is +9% (1 o).

RESULTS

Manganese over the Galapagos R i f t

Manganese results from stations PIO and P17 are presented in Figs. 2 and 3. Since the hydrocast samples in Fig. 2 were collected from several locations (Appendix), the concentrations are plot ted vs. depth relative to the bot tom. Typically, the Mn concentrat ion of deep water is about 0.3 nmol kg -1 (Klinkhammer and Bender, 1980). Near-bottom concentrations in the hydro- cast samples taken over the Galapagos Rift (station P17) and the EPR (3°22.8'S, 102°21.8'W) are 10--50 times greater than this "background". The Galapagos profile increases into the bot tom, reaching levels between 10 and 15 nmol kg -~ . The average Mn concentrat ion in the bo t tom 200 m at PIO is 4 nmol kg -1 .

While the maximum Mn concentrations at P1 7 are at least 5 times greater than any bo t tom concentrat ion found away from a spreading center, the bot- tom values at PIO are more typical of values observed in nepheloid layers

214

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Fig. 2. Mn vs. depth relative to the b o t t o m for a site on the Galapagos Rift (e) (86°W) and a station on the EPR (o) (3°22.8'S, 102°21.8'W).

Fig. 3. Mn vs. silica in water samples col lected by "Alvin" from vent fields on the GSC (Corliss et al., 1979) . The four vent fields were designated as the Garden of Eden (~), the Oyster beds (=), the Dandel ions (o) and the Clambake (e). The data po int on the silica axis represents ambient condit ions .

away from spreading centers (Klinkhammer and Bender, 1980). The differ- ences between the profiles at PIO and P17 may be the result of circulation or the intensity of hydrothermal activity. Observations from Alvin of flourishing and extinct geothermal animal communities on the Galapagos Rift (Corliss et al., 1979) show conclusively that hydrothermal emanations are episodic and transient in nature.

Corliss et al. (1979) analyzed Galapagos hydrothermal samples and showed that the silica content of water samples collected from vents are directly and linearly related to temperature. In fact, silica is the best indicator of the ex- tent of dilution of the hydrothermal end-member with ambient seawater. The Mn concentrations of water samples collected in four vent fields (Garden of Eden, Oyster Beds, Dandelions and Clambake) are plotted vs. silica in Fig. 3. The data point on the silica axis represents ambient conditions. The highest temperature of these samples was about 12°C, or 10°C above ambient. Generally, the data for any particular vent field falls on a simple dilution line, but each system has an unique hydrothermal end-member. These different Mn- silica relationships must reflect physical and chemical variations in the interac- tions of the circulating seawater and basalt.

The silica/Mn slopes for these vents were used by Corliss et al. (1979) to

215

estimate the hydrothermal flux of Mn to the oceans. These data predict a flux between 74.109 and 190.109 mol/yr. This range includes the value of 180.109 tool/yr, calculated by Weiss (1977) from Mn--3He relationships in "Deep Tow" samples collected at the same location. Since the global sedi- mentation rate of Mn is about 52.109 mol/yr. (Bender et al., 1977), the hydrothermal flux is a major source of Mn to the oceans if the flux from the Galapagos vents is not anomalously high.

Manganese at 21 °N

The Mn concentrations in hydrocast samples collected from the EPR (20°55.2'N, 109°1.9'W) and the Tamayo Fracture Zone (22°56.0'N, 108°5.9'W) are plotted vs. depth in Fig. 4. Typically, Mn profiles in the Northern Hemisphere have a surface maximum of about 3 nmol kg -1 , a similar maximum in the oxygen minimum and concentrations of about 0.3 nmol kg -1 in deep waters (Klinkhammer and Bender, 1980). Sometimes, samples collected within a couple of hundred meters of the bottom have concentrations of up to 3 nmol k4~ -1 due to resuspension of fine-grained particulates. The Mn distribution at 21°N above 2 km fits this general pattern reasonably well ex- cept that the concentrations between 1 and 2 km are somewhat higher than 0.3 nmol k g - ' . This higher "background" probably reflects the effects of up- welling and the proximity of station T2 to the coast and the Gulf of California.

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Fig. 4. Mn vs. depth for hydrocas t samples c o l l e c t e d over the EPR (e) (20°55.2'N, 109°1.9T~) and the Tamayo Fracture Zone (o) (22°56.0°N, 108°5.9T~). The dashed line at 2670 m is the bot tom depth at the EPR station.

216

The most interesting features of the Mn distributions at 21'~N are the very high concentrations near the bottom. Concentrations at station T2 on the EPR crest range from 6 to 13 nmol kg -1 while levels in the Tamayo Fracture Zone are even higher. The range of concentrations in these near-bottom samples (6--17 nmol kg -1 ) is similar to the signal in the Galapagos Rift. This consistency supports the contention that hydrothermal emanation of Mn is ubiquitous and rather uniform along this section of the ridge system.

Manganese at MANOP stations

The concentrations of all Mn samples collected at MANOP station 7 on the EPR are plotted vs. depth in Fig. 5. The results from four bottom-water hydro- casts are plotted separately in the same figure. The distribution at M9 in the Guatemala Basin is shown in Fig. 6. The Mn profiles above 2 km at these MANOP stations are discussed in another paper (Ktinkhammer and Bender, 1980).

Below 2 kin, the Mn concentrations generally increase into the bot tom at M7 and M9. The highest concentrations sampled are ~9 nmol kg -1 and occur in casts 3, 5 and 6 collected at station M7. Cast 6 (8°48.7'N, 104°13 .3~) was taken directly over the spreading axis. Cast 3 (8°47.5'N, 104°1.5'W) and cast 5 (8°47.6'N, 104°0.7'W) were collected SE of cast 6 about 24 km from the axis. Somewhat lower Mn concentrations were measured in samples taken from cast 4 (8°47.9'N, 104°17.1'W) collected 4 km west of the spreading axis.

The four bot tom Mn profiles at station M7 have distinctly different shapes.

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217

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Data from cast 4 have only slight curvature into the bo t tom while the rise crest profile (cast 6) has a sharper Mn gradient with more pronounced curva- ture. At locations east of the rise crest (casts 3 and 5), the Mn distributions have maxima above the bot tom.

The differences in these four Mn profiles are consistent with bo t tom cur- rent measurements. Three current meters were deployed 90 m above the sea floor at station M7. One meter was posit ioned on the crest and two east of cast 5. All three current meters indicated flow to the S-SE at an average velocity of 9 cm s -~ (Lonsdale and Spiess, 1979b). A current of this direc- tion and magnitude should advect hydrothermal Mn off the crest to the two locations east of the axis (casts 3 and 5) while the Mn profile west of the axis (cast 4) should be dominated by in situ hydrothermal sources on the bot tom.

While bo t tom currents and the shapes of the Mn profiles are consistent with an eastward componen t to the hydrothermal Mn signal along this section of the EPR, uncertainties in the magnitude and direction of this advection pre- clude any quantitative estimate of the effect of this signal on horizontal distributions. However, the data f rom station M9 suggest that advection of hydrothermal Mn may be extensive.

The Mn concentrations at station M9 (Fig. 6) begin to increase into the bot- tom at ~2 .2 km, ~1 .4 km above the bot tom. There are at least five possible sources for this anomalous Mn: (1) resuspension of fine-grained material off the bot tom, (2) diffusion of Mn from sediments below station Mg, (3) advec- tion of hydrothermal Mn from the GSC, (4) reduct ion of Mn-oxides in hemi- pelagic sediments along the coast and advection to station M9, or (5) advec- tion of hydrothermal Mn from the EPR.

Most of these possible sources can be eliminated as strong candidates: (a) Resuspension. The fact that the Mn signal extends 1.4 km above the bot-

tom makes resuspension an unlikely possibility.

218

(b) Diffusion. The Mn concentration in pore waters squeezed from the top 10 cm of sediment at station M9 is nearly the same as the bot tom-water con- centration (Klinkhammer, 1980b). Therefore, the steady-state flux of Mn from sediments at this location could not support the anomaly we observe in deep waters.

(c) Advection from the GSC. Fig. 7 is a potential temperature (0)--salinity (S) plot of bo t tom waters at stations M7, M9 and P1 7. The bo t tom water at station M9 is distinctly different from bo t tom water at P17 (the solid line). This difference is consistent with general circulation patterns. Bot tom water (2.6 km depth) south of the GSC flows east and north through channels near South America and is effectively trapped in the Panama Basin SE of station M9 by the Cocos Ridge where the depth is less than 2 km (Laird, 1971; Lonsdale and Malfait, 1974).

(d) EPR or hemi-pelagic sediments. Neither of these candidates can be eliminated as the source of the

ancmalous Mn we observe in the deep waters at station M9, but three observa- tions support a hydrothermal origin: (1) waters at station Mg, having anomalously high Mn concentrations (the shaded points in Fig. 7), overlap the

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Fig. 7. Po ten t i a l t e m p e r a t u r e (0) vs. sa l in i ty (S) for waters f rom 1800 m to the b o t t o m a t s t a t ions M7 and M9. The solid line is a l inear bes t - f i t t h r o u g h da ta f r o m 1900 m to the bot - t o m at s t a t i on P17. T he shaded data points (o) r ep resen t wate r samples which had anoma lous ly h igh Mn c o n c e n t r a t i o n s ( > 1.0 n m o l kg - I ). The open circles (o) are wate r types wi th Mn c o n c e n t r a t i o n s < 1.0 n m o l kg -1 . The water mass ou t l i ned by the dotted line inc ludes the wa te r types having anomalous ly high Mn concentrat ions at M7.

219

O--S regime of bo t tom water at the EPR (station M7, Fig. 7, the dashed line); (2) currents measured over the rise crest had an easterly component ; and (3) the top of the Mn anomaly at station M9 occurs at about the same depth as the depth of the EPR crest.

If the Mn anomaly below 2.2 km at station M9 is hydrothermal in origin, this observation has important implications: (1) horizontal distributions of Mn could be used to deduce mean circulation patterns near active spreading centers; (2) Mn-oxides forming in the water column could scavenge other ele- ments spreading a metalliferous component far from hydrothermal sources; and (3) Mn profiles near active spreading centers could provide our most precise estimate of the hydrothermal flux of this element to the oceans.

CONCLUSIONS

(1) The Mn concentrations of b o t t o m waters over the GSC are 30--50 times "background" levels. The Mn concentrations in vent samples taken from the same location show conclusively that this anomalous Mn is hydrothermal.

(2) The Mn anomalies in bo t tom waters at 21°N and 8°N are similar to the signal over the GSC, suggesting that hydrothermal Mn sources of comparable magnitude exist at these three locations.

(3) Mn--silica relationships in water samples collected from vents on the Galapagos Rift are linear but variable between vent fields. This observation implies that hydrothermal Mn mixes conservatively with seawater in these vent fields but that chemical and physical processes controlling the Mn con- tent of the hydrothermal end-member are variable.

(4) Below 2 km, the Mn concentrations at one station in the Guatemala Basin are anomalously high. Hydrographic evidence suggests that this anomaly is a hydrothermal signal.

A C K N O W L E D G M E N T S

I wish to thank Ray Weiss (Scripps) for his help in obtaining many of these samples. John Edmond (M.I.T.) and Michael Bender stimulated much of the thought behind this paper and Susan typed the manuscript. This work was sup- por ted by the United States N.S.F:-I .D.O.E. with grant number OCE 77-05184 (MANOP).

APPENDIX

In this Appendix are listed the l oca t ions and manganese c o n c e n tr a t i o n s of waters col- lected at the s ta t ions s h o w n in Fig. 1. The vent samples were c o l l e c t e d w i t h " A l v i n " (Corliss et al., 1979). T w o Mn c o n c e n t r a t i o n s l i s ted for the same depth are duplicate analyses o f the same sample.

220

(A) Silica and Mn concent ra t ions of vent samples

Vent field Dive

Garden of Eden 728

Sample Bottle Silica sampler (umol k g - ' )

A 3 G837 699 A 7 G552 732 B6 G321 635 A 4 G331 657

Mn (nmol kg ~)

9.8 11.1

9.1 9.1

Oyster Beds 723

726

A1 G417 414 5.6

B8 G542 399 5.8 A 3 G549 365 5.1 B4 G535 501 8.6 A 6 G528 566 10.6

Dandelions 716

723

A 2 G890 338 6.7 A 3 G888 384 8.2 A 4 G885 375 6.6

A 6 G432 274 2.4 A 5 G426 231 0.9 A 3 G423 335 6.6 A 3 G422 6.8

Clambake 715

71 7

719

722

727

B1 G860 344 11.5 B2 G864 327 10.4 B3 G868 264 6.9 A 6 G8 78 440 17.5 A5 G883 476 20.0 B4 G870 486 20.0 A 7 G882 532 25.5

B I G902 578 23.7 B3 G909 478 17.7

A1 G929 522 21.8 A1 G930 21.8

A 4 G411 600 27.3

B2 G829 602 27.3 B2 G839 25.5

(B) Mn concent ra t ions

Area Stat ion Cast (expedi t ion)

Galapagos 56 (Pleiades-II)

of samples near EPR and GSC

Locat ion . . . . . . . . . . . . . depth lat. (°'N) long. (°'W) (m)

0 3 5 . 5 6 86 8.95

B o t t o m Water Mn column (nmol kg -1) height (m)

2,682 1,597 2.13 2,605 11.3 2,669 11.9

APPENDIX (continued)

Area (expedition)

Galapagos (Pleiades-I)

EPR (Pleiades-I)

EPR

221

Station Cast Location Bottom Water Mn depth column (nmol kg -1)

lat. (°'N) long. (°T¢) (m) height (m)

57

63

66

23

87

17

10

2

0 35.79 86 8.65 2,750 2,445 4.68 2,645 8.77--9.17 2,744 14.6 2,749 16.4

047.83 86 8.99 2,500 2,494 9.79 2,497 11.9 2,499 10.9

0 47.63 86 10.15 2,498 1,656 2.60 2,478 9.17

0 35.65 86 4.65 2,708 1,886 2.97 2,385 5.53 2,705 13.3

0 35.79 86 5.59 2,710 2,704 13.7 2,709 13.7

0 48.6 86 9.3 2,496 1,324 1.42--1.53 1,477 0.84 1,630 O.96 1,782 1.60 1,909 2.00 2,034 1.72 2,134 3.33--3.24 2,233 4.50 2,38O 6.63 2,429 7.95--6.74

3 22.8 S 102 21.8 2,910 1,550 0.46 1,800 0.58--0.66 2,000 0.31 2,200 0.42 2,500 1.24 2,600 1.75--2.33 2,720 3.93 2,810 4.17 2,900 3.59--3.44

20 55.2 109 1.9 0 3.22 57 1.97

263 2.80 468 2.60--3.11 672 2.04 875 1.98

1,078 1.71 1,280 1.37 1,483 1.38

222

APPENDIX (continued)

Area Station Cast (expedition)

Location Bottom Water Mn ~- depth column (nmolkg -~)

lat. (°'N) long. (°'W) (m) height (m)

1,686 1.44 1,860 1.13 1,992 1.33 2,094 1.58 2,197 1.80 2,300 9.43 2,404 12.5 2,456 8.70 2,509 6.92--6.26 2,561 6.70 2,619 6.19

Tamayo 4 1 22 56.0 Fracture Zone

108 5.9 2,735 9.01 2,835 9.36 2,940 17.4 3,043 15.8 3,147 13.7 3,251 13.7 3,351 12.7--12.1

EPR 7 Bucket 8 45.9 (INDOMED-I) 2

104 10.3 2,571 0 2.64 10 1.75 21 2.91 30 2.42

153 1.02 364 1.77 469 2.33--2.86 522 2.15 575 1.17 627 1.11

1,043 0.49

EPR east of axis 7 3 8 47.5 104 1.5 2,028 1.09 2,130 0.84 2,338 2.13 2,441 1.98 2,545 8.01 2,646 8.70 2.748 6.57---6.77 2,852 9.14 2,903 8.10 2,954 8.05 2,985 10.3--10.7 3,028 6.52--7.02 3,039 6.97 3,049 7.70

APPENDIX (continued)

Area (expedition)

Station Cast

223

Location Bottom Water Mn depth column (nmol kg -1)

lat. (°'N) long. (°~V) (m) height (m)

EPR west of axis 7 4 8 47.9

EPR east of axis 7 5 8 47.6

EPR ridge ax/s 7 6 8 48.7

104 17.1 2,985 1,482 0.49 1,691 0.66 1,864 0.64 2,066 2.44 2,221 2.35 2,324 3.06 2,427 3.91 2,478 3.75 2,530 4.39 2,581 4.44 2,632 3.77 2,784 4.86 2,884 7.61 2,985 7.15

104 0.7 3,059 1,050 0.66 1,155 0.71 1,260 0.58 1,365 1.06 1,470 0.55 1,574 0.49 1,679 0.56 1,784 0.82 1,887 0.58 1,992 O.69 2,247 1.07 2,349 4.92 2,451 7.32 2,553 9.28 2,655 9.01 2,757 9.32

104 13.3 2,541 1,972 1.06 2,075 O.93 2,177 1.07 2.281 2.64 2,384 6.26 2,405 7.55 2,414 8.O5 2,421 8.17 2,429 8.50 2,437 8.45 2,446 6.57 2,454 7.08 2,463 9.21 2,471 8.77 2,488 9.14

224

A P P E N D I X ( con t inued)

Area Sta t ion Cast ( exped i t i on )

Loca t i on

lat. (°'N)

Guatemala Basin 9

7 7 8 48.3

9 13

9 14

9 15

Bucket 11

6 33.3

6 34.5

6 33.7

6 33.9

B o t t o m Water d e p t h

long. (~~¢~) (m)

103 57.7 2 ,986

Mn c o l u m n ( n m o l k g ' ) height, (m)

35 2.97 42 2.55 49 2 .26- -2 .24 56 2.44 63 2.39 71 2.73 78 2.55 85 2.44 94 2.57

102 1.66 109 1.71 t 1 8 1.47

92 48.0 0 3.40 16 2.O8

150 O.82 249 0.97 347 1.11 446 1.88 546 0 .97 645 1.06 696 1.06 894 0.73

92 50.7 3 ,570

92 53.5 3 ,557

92 48.1 3 ,553

1 090 1 289 I 488 1 685 1 885 2 082 2 280 2 477

2 ,658 2 ,957 3 ,306 3 ,408 3 ,510 3 ,535 3 ,55]

427 457 5O3 534 565 58O

1 .00- -1 .18 0.49 0.51 0.55 0 .58 0.47 0.98 1.18

1.42 2 .95- -3 .33 3.22 3.26 3.53 4 .31- -4 .06 3.57

1.77 3 .42- -3 .97 1.02 0.86 0.80 0.93

APPENDIX (continued)

225

Area Station (expedition)

Cast Location Bottom Water Mn depth column (nmol kg -I)

lat. (°'N) long. (°'W) (m) height (m)

9 17 6 33.3 92 47.4 3,557

636 1.00 • 641 1.20--1.27 645 0.91

3,232 3.51 3,307 3.35 3,457 3.4O 3,477 3.33 3,497 3.82 3,504 3.48 3,511 3.30 3,517 4.13 3,522 3.37 3,527 3.53 3,532 4.33 3,539 3.57 3,542 4.55 3,547 5.01

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