Cadmium in the southwest Pacific Ocean two factors significantly affecting the CdPO4 relationship in the ocean

Marine Chemistry 54 ( 1996) 55-67

Cadmium in the southwest Pacific Ocean Two factors significantly affecting the Cd-PO, relationship inthe ocean Isao Kudo *, Haruyo Kokubun, Katsuhiko Matsunaga Department of Fisheries Oceanography and Marine Science, Faculty of Fisheries, Hokkaido University. Hakadate 041, Japan

Received 9 November 1995; accepted 20 December 1995

Abstract Vertical profiles of dissolved Cd were determined in the southwest Pacific Ocean, which includes the Tasman Sea, the Solomon Sea, and the Coral Sea. Cd shows a nutrient type distribution, but weakly correlates with phosphate compared to the North Pacific Ocean when all data are plotted. The Cd/PO, ratio in surface layer is < 0.1 n M/FM and the ratio in the intermediate and deep water shows an increase approaching the Pacific value of * 0.3 n M/k M. The Cd/PO, ratio in the regeneration process has been estimated using an AOU (apparent oxygen utilization) respiration model. This ratio is affected by that in surface layer, so deep water receives a regeneration flux with a low Cd/PO, ratio. The preformed Cd and phosphate as estimated by the AOU relation are low in the surface layer and high in the deep layers. The intermediate water is a mixture of surface water and Antarctic Intermediate Water (AAIW). The chamcteristic Cd-PO, relationship in this area would be a consequence of mixing with two water masses which have different original Cd/PO, ratios. The factors affecting the Cd-PO, relationship in the world ocean are physical mixing and biological cycling, based on the results of the southern Pacific Ocean.

1. Introduction

The revolutionary development of trace-metal analyses in the last decade has allowed us to understand the biogeochemical cycles of trace metals in the ocean. Some trace elements such as Cu, Cd, Ni and Zn appear to have a similar distribution pattern as algal micro nutrients (Bruland, 1980; Danielsson, 1980; Boyle et al., 1981; Danielsson et al., 1985; Abe and Matsunaga, 1988; Jickells and Burton, 1988;

* Corresponding author. 0304-4203/96/$15.00 Copyright 0 SSDI 0304-4203(95)00100-X

Hunter and Ho, 1991). This indicates that these trace metals are involved in a similar cycling pattern as nutrients: surface biological uptake, subsequent sinking, and removal and regeneration at depth. This evidence suggests that these trace metals could be used as an indicator of biological activity or as a tracer of ocean circulation. Among these trace metals, Cd is well known to have the best correlation with phosphate in seawater. Boyle (1988) applied this Cd-PO, relationship to reconstruct the oceanographic circulation pattern in the glacial period. Recently, worldwide measurements of Cd clarify that this correlation is not always consistent within the whole ocean (Nolting et al., 1991; Saager et al.,

1996 Elsevier Science B.V. All rights reserved All rights reserved.

56

I. Kudo et al./ Marine Chemistry 54 (1996) 55-67

1992; De Baar et al., 1994). The slopes, and especially the intercepts of this relation are fairly variable from ocean to ocean. This indicates that cycling and input of Cd and phosphate may be different among individual oceans. The main object of the KH92-4 cruise was to study geochemical processes that affect ocean fluxes in the southwest Pacific Ocean: south of the equator and west of 175”E longitude. This area includes the Solomon Sea, the Coral Sea, the South Fizi basin and the Tasman Sea, all of these are > 4000 m deep basins. Measurements of Cd in the South Pacific have not been conducted since Boyle et al. (1976). Recently however, Hunter and Ho (1991), and Frew and Hunter (1992) reported the distribution of Cd in the Tasman Sea and Puyseger Trench, respectively. The purpose of this paper is to report on Cd in this area with specific emphasis on its relationship to phosphate. We further aim to improve the understanding of factors that control the distribution and cycle of Cd in the ocean.

2. Sampling and analysis The samples were collected during the KH92-4 cruise of R/V “Hakuho Maru” between September 16 and October 26 in 1992. The sampling stations are shown in Fig. 1. General oceanographic observations were conducted by a Sea-bird CTDO system equipped with a rosette of twenty-four Niskin bottles (General Oceanics). The suspending wire is made of titanium. The samples for Cd analysis were collected vertically by precleaned lever-action type Niskin bottles attached to a rosette system (deep layer) and a nonmetallic pumping system (O-250 m). In order to determine the sampling depth for pumping, a depth meter was attached to the intake, and nutrient and salinity data were compared to those of the CTDO cast. The water samples for Cd were transferred to Nalgene bottles (LDPE) and covered two-fold by polyethylene bags. Beforehand, bottles were soaked alternatively by 6 N hydrochloric acid and 7 N nitric acid and rinsed thoroughly with distilled water and Q-distilled water. The samples were acidified for each liter with 1 ml of 12 N ultrapure hydrochloric acid (Ciba-Merck) and stored at room temperature

Fig. 1. Map of the sampling stations in this study. Equator (SA5). Solomon Sea (SA7), South Fizi basin (SA 12 and - 14) and Tasman Sea @.A16 and -19).

until analysis. The samples were not filtered, thus the results include the dissolved and the acid-leachable Cd concentration. The acid-leachable fraction is generally negligible for open ocean waters. Analysis of Cd was conducted by the Co-APDC coprecipitation method (Boyle and Edmond, 1977). A subsample of 500 ml was concentrated to 5 ml of 1 N HNO,, the concentration factor being 100. The measurement was conducted by a Hitachi 180-70 polarized flameless Zeeman atomic absorption spectrometer. Quantification was made using a standard addition method to correct for matrix interference. The standard solution was prepared by diluting a commercially available cadmium chloride standard solution (1000 ppm, Wako Chemicals). The typical reagent blank for Cd was 0.002 nM. The precision estimated by replicate analysis (n = 12) at 0.2 nM was 53.8%. The recovery was 90 f 2%, which was estimated by analyzing the standard spiked samples. All handling was done in a laminar flow class 100 clean room. Nutrients and dissolved oxygen were analyzed on board by an Auto analyzer and automatic titrator (Hirama ART-31, respectively. All data except cadmium were quoted from the cruise report of KH92-4.

I. Kuio et al./Marine Chemistry 54 (1996) 55-67

3. Results

(IM of phosphate are present. Antarctic Intermediate Water (AAIW), which is characterized by low salinity (34.4-34.5) and high dissolved oxygen content, is observed between 600- 1000 m depth at all stations (Y. Nozaki, pers. commun.). This AAIW has OS-O.6 nM of Cd and 2-2.5 p.M of phosphate. The concentration of Cd and phosphate below AAIW is consistent with other reports of the South Pacific (Boyle et al., 1976; Frew and Hunter, 1992). Hunter and Ho (1991) reported the only distribution of Cd for the Tasman Sea albeit restricted to the eastern part. Vertical profiles of phosphate and Cd at our stations SA14 and -16 are combined with those at St.

3.1. Distribution of Cd and phosphate in the southwest Pacific Ocean

The distributions of phosphate and Cd in this study are shown in Fig. 2. Results of Cd analyses and associated hydrographic data are also listed in Appendix A. Cd shows a similar distribution pattern with that of phosphate as has previously been demonstrated by many authors. A general depletion of Cd and phosphate in surface water is found at all stations except SA16 where 0.05 nM of Cd and 0.3

0

0.5

1

1.5

2

2.5

57

0

3

c 0

0.2

0.4

0.6

0.6

1

Fig. 2. Vertical profiles of phosphate (a) and dissolved (162”40.6’E, 35”50.6’S) of Hunter and HO (1991).

0

0.2

0.4

0.6

0.6

1

Cd (b) at SA7, -14 and -16 along with

0 St.

0.5

1

1.5

2

25

3

,,,,,,,,,,,,,,,.,, 0.2

0.4

57 (170”19.9’E,

0.6

0.6

34’46.8’s)

1

and St. 64

I. Kudo et al. /Marine

58

56 and 64 by Hunter and Ho (1991), which are the nearest stations to our respective stations. Reasonable agreement was found between them for phosphate and Cd values, though only a few data are comparable below 1000 m depth due to a limited number of clean samplers for trace metals in this study and lack of data below 2000 m from Hunter and Ho (1991). Hunter and Ho (1991) also reported on Cd concentration near New Zealand, being O.l0.2 nM higher than expected from the general CdPO, relationship. They attributed this excess Cd to a terrestrial source. We did not observe a terrestrial influence because our stations were apparently far enough from land to eliminate any such influence.

Chemistry 54 (1996) 55-67

PO po.2 PO po

0

20

40 60 AOU (rkl)

80

100

Fig. 3. Relation between AOU and phosphate in the upper layer. The lines denote the ideal regeneration ratio to AOU for phosphate of 1:138 (kmol PO, /p,mol AOU). PO: means preformed phosphate concentration.

3.2. Phosphate regeneration with reference to AOU The vertical distribution of phosphate generally shows surface depletion and gradual increase with depth as a result of uptake by phytoplankton at the surface and settling removal to deeper layers. These biogenic particles are subject to decomposition with settling, and nutrients are regenerated to seawater. This decomposition and regeneration process consumes dissolved oxygen in seawater. This degree of consumption is expressed by the AOU (apparent oxygen utilization) content which is defined by: AoU =

(02)saturation.S,T

-

(‘2

)measured

where the saturation value is calculated from given S, T of the water sample. Redfield et al. (1963) established the stoichiometric equation for average phytoplankton composition and its respiratory decomposition as follows: (CH,O) ,&NHs) ,6HjPO4 + 1380, + 106C0, + 16HN0, + H3P0, + 122H,O

(1)

Fig. 3 shows the plots of AOU and phosphate for the upper layer (O-300 m). Phosphate increases with AOU and shows a good correlation at individual stations with a slope of 7.2 x 1O-3 (p,MpO,/pMAou), which satisfies Redfield’s equation (POJAOU = 1:138). Although the slope is almost the same, an AOU-zero intercept is different for each station and shows an increase towards the south. At the southernmost station SA16 this relation has an intercept of N 0.3 p_M while the slope is

parallel with others. This means that this water has excess phosphate independent of AOU content. This excess phosphate originated from phosphate in the surface water that was saturated with oxygen. Redfield et al. (1963) defined this excess phosphate as preformed phosphate (PO:). The preformed phosphate was calculated by the following equation: PO,0 = PO,m- AOU/138

(2) where PO? denotes the measured phosphate concentration and all units are in pM. This preformed phosphate is nearly zero at SA5, -7, -12 and -14 and increases southward. The AOU in the upper layer shows temporary negative values (i.e. 0, supersaturation) due to gas exchange with the atmosphere and photosynthetic activity. Our stations are located almost in the tropical and subtropical areas, so the water column is stratified without showing extreme 0, supersaturation. Preformed phosphate values are constant in the upper 300 m and the increase of phosphate agrees well with the AOU increase. Thus, we think the application of the AOU concept to the upper layer is valid in our case. Fig. 4 is a phosphate-AOU plot for the deep layer (> 2000 m). Phosphate shows a good correlation with AOU and the slope satisfies with the PO,/O, ratio (1: 138) at each station. Preformed phosphate falls in the range of 1.1-1.5 FM, which agrees with the PO: value found in the South Pacific by Broecker and Peng (1982). Broecker et al. (1985) also provided evidence in support of a different ratio of oxygen consumption to phosphate production of 175 in deep

I. Kudo et al./Marine

Chemistry 54 (1996) 55-67

3.5

3.0

3.0

2.5

59

. . . I . I . 1 I I , I I . . , I . . .I SA12 0 meas. : oo”-: 0 preform. . 0

2.5 2.0 1.5 1.0

I

0.5

A

SA14

+

SA16

100

50

150

200

in the deep layer.

North Atlantic waters. As a value of 175 would be slightly high for our PO,-AOU plot, we use the classical value of 138 in the South Pacific Ocean. Fig. 5 shows the plots of AOU and PO, for all samples at SA12 and -16. Although the lower- and higher-AOU regions, which represent the upper and deep layer, respectively, show a slope almost equal to the Redfield ratio, intermediate water shows a steeper slope than other layers. Fig. 6 shows the change of measured and preformed phosphate with ur at SA12. Preformed phosphate shows nearly zero at uT < 26.1, which corresponds to O-300 m depth. The increase in the measured phosphate to = 0.5 PM is due to the regeneration of phosphate in this layer. At err between 26.1 and 27.4, both measured and preformed phosphate increase with uT, and fi-

r”“‘““‘““25l PO*%.

E 3

Pop1

2.5

PO $0.5

Jz 2 2.0

P040=0

.o

E 1.5 f

1.0

0

50

loo

150

200

j8

1.5 1.0

L

0.5

:

250

300

AOU (CIhl) Fig. 5. Relation between AOU and phosphate for all depth at SA12 and - 16. The solid lines denote the same in Fig. 3.

0

0

0

b 0 80

O

0

q

N-un

:

q

0

’ ’ 1o’ ’ en’ B

24

Fig. 4. Relation between AOU and phosphate The solid lines denote the same in Fig. 3.

3.0

;

0.0

250

AOU (IrM)

3.5

2.0

f

0.0 0

5

25

Fig. 6. Relation between phosphate at SA 12.

26 Sigma-T

27

uT and the measured

20

and preformed

nally a constant PO: value is attained at 1.4 Q4. The layer showing this constant PO: is corresponding to AAIW and AABW (Antarctic bottom water). This gradual increase of PO:, therefore, would be produced by the apparent conservative mixing of two water masses where the end-member of southern origin is deemed to have a high preformed phosphate. In this layer, the increase of measured phosphate comes from in situ regeneration of phosphate from biogenic particles as well as the gradual increase of preformed phosphate. As the plots for other stations exhibit the same pattern (not shown), we divide the water column into three layers for individual stations by the relationship between preformed phosphate and ur (i.e. upper, intermediate and deep layer). The intermediate layer is defined as a layer affected by the mixing of two water masses. 3.3. Cadmium-phosphate plots Fig. 7 shows the relationships of Cd and phosphate in this study. Plots are shown separately for the upper and deep layers defined from the preformed phosphate values. The plots for the upper layer are further grouped into the tropical and subtropical stations (SA5, -7, -12 and -14 in Fig. 7a), and the subantarctic station @A16 in Fig. 7b), judged from the surface phosphate concentration. In the subantarctic station, a sizable amount of phosphate is present in the surface. Results by correlation analysis show a good linearity between cadmium and phos-

a

I. Kudo et al./ Marine Chemistry 54 (1996) 55-67

0

0 0 A

phate in the individual plots for the upper layer. The regression lines are as follows: Tropical and subtropical stations:

(a)

SA5 SA7 SA12 SA14

1

Cd = 0.001 + (0.074 x 10-3) XPO,

xPO,

0.6

0.0

Phosphate @M)

0.2 1

+

SA16

)

t

B 5

1

+ + 0.1

3

++

oL* 0

”

I’. 0.2

x

W

/

s

”

’

0.4

I

’ c n

0.8

3

4

Phosphate (JIM) 1.5 L”““

1.0

0.0 0

1

2 Phosphate (JIM)

Fig. 7. Relation between phosphate and cadmium in the southwest Pacific Ocean. a. Tropical and subtropical station for the upper layer. b. Subantarctic station for the upper layer c. All stations for the deep layer. Solid line in (a) and (b) represents the repression line, and that in (c) represents the regression line for deep waters in the world ocean with the band width of 95% confidence level after De Baar et al. (1994) (see text).

+ (0.382 X 10-3) (r=0.88,

n=7)

(4) The equation from the tropical and subtropical stations indicates a lower slope of 0.074 (nM/pM), which is roughly one-fourth of the global ratio with almost zero intercept. Contrarily, the equation from the subantarctic station has a slightly higher slope of 0.382 (nM/p,M) than the global ratio with negative intercept ( - 0.094 n M). All data from depths deeper than 1000 m were plotted in Fig. 7c. Plots for the deep layer show scatter to some extent and tend to deviate to lower values, but this cluster falls within the recent overall oceanic relationship for deep waters given by De Baar et al. (1994): Cd = -0.259

”

0.6

(3)

n=30)

Subarctic station:

Cd = -0.094

0.4

(r=0.91,

+ (0.413 x 10-3) x PO,

(5) Although Cd at deep depth shows the “normal” ratio to phosphate, that means a similar ratio as for the world ocean, Cd at a shallower depth exhibits deficiency to some extent. Nutrients are usually depleted in the surface layer of tropical and subtropical regions. Phytoplankton growth in these areas is generally restricted by the rate of supply of nutrients from the aphotic zone. As nutrients and Cd are supplied to the euphotic zone by diffusion or advection from the aphotic layer into the upper layer, the ratio of Cd to phosphate in the supply flux would be identical to the slope in the Cd-PO, relationship in the upper layer (Fig. 7a). If we can assume that a steady state is attained in this region, the Cd/PO, ratio in the removal flux would be the same as that in the supply flux. 3.4. Cd regeneration

with reference

to AOU

The Cd-PO, relationship in the tropical and subtropical southern oceans exhibits low Cd/PO, ratios in the upper layer. This low-Cd/PO, water may influence the ratio in phytoplankton that grows in

I. Kudo et al./ Marine Chemistry 54 (1996) 55-67

these regions. Consequently, biogenic particles that fall to the deep layer would have a lower Cd/PO, ratio compared to that in the North Pacific. If Cd is incorporated into phytoplankton cells, this regeneration model would be also applicable to Cd. Morel and Hudson (1985) suggested that Redfield’s model could be extended to Cd and gave a molar ratio of 0.001 to PO,. Fig. 8 shows the relationship for Cd with AOU in the upper layer. Cd shows a good correlation with AOU at each station and has a slope of 7.2 X 10eJ (nMc,/~MM,,,). A combination of the slopes in the P-AOU and Cd-AOU relationships satisfies with the slope found in the Cd-PO, relationship (Fig. 7a) of Cd/PO, = 0.07 (n M/PM). This relationship at SA16 is a little different from other stations, showing a higher slope and intercept. Cd in the deep layer shows the same correlation with AOU (Fig. 9>, although some scatter and outlying data make the tendency not so apparent compared to phosphate. As some observed Cd data within 10 m above sediment are higher than the trend, probably being influenced by sediment or overlying water, we excluded these data from the plots. The general trend is the same for the upper layer with the exception of the intercept concentration. The preformed Cd in the deep water is extrapolated to be 0.45-0.55 nM. Preformed phosphate and Cd mean the initial concentration in surface water that has been saturated with oxygen before they cascaded to

0.3 Cd ‘=0.05 (a=O.3)

0.2

0.8

S 0.6 L 3

0.4 0.2 0 0

50

100 150 AOU (@.I)

Fig. 9. Relation between AOU and dissolved The solid lines denote the same in Fig. 8.

200

250

Cd in the deep layer.

deep layer. These properties would behave conservatively after seawater left surface, so they can be used as a chemical tracer. Broecker and Peng (1982) reported that deep water formed in the northern Atlantic has a considerably lower PO: content than that originating in the Antarctic Ocean. This higher preformed phosphate would be produced when the Antarctic circumpolar water mixes with water originated from surface water of the Weddell Sea during winter months. This process could supply oxygen to old circumpolar water. High surface Cd concentrations in the Antarctic were reported by Nolting et al. (1991) and Nolting and De Baar (1994), ranging from 0.4 to 0.76 nA4. This water does not always produce all parts of AABW, so preformed Cd in AABW would be lowered to some extent as a result of mixture with other source waters. 3.5. Vertical change of Cd/ PO,

s 5 3

61

Cd ‘=0.05 (aeO.1)

0.1

Cd ‘=O (a-0.1) 0 0

20

40

60

80

loo

AOU @M) Fig. 8. Relation between AOU and layer. The solid lines denote the ideal AOU (a: 135, nmol Cd/Fmol AOU), to phosphate (nmol Cd/pmol PO,). mium concentration.

dissolved Cd in the upper regeneration ratio of Cd to where a is the ratio of Cd Cd’ means preformed cad-

Fig. 10 shows the vertical change of Cd/PO, in the water column. The Cd/PO, ratios below 2000 m show an almost constant ratio of 0.26 + 0.02, but decrease gradually toward the upper layer showing around 0.1 or less. This trend has been reported in all other oceans (Saager et al., 1992; De Baar et al., 1994). As already shown in Fig. 7a, the upper layer shows some deficiency of Cd as expected by the general trend of the Cd-PO, relationship. This lowCd/PO, water would produce biogenic particles that have a low Cd/PO, ratio. This has been confirmed

I. Kudo et al. /Marine Chemistry 54 (1996) X5-67

62

4. Discussion

Cd/PO ,(nM@M)

0

0.1

0.2

0.3

0.4

0.5

:....,..“,....,...‘,..,.I

Fig. 10. Vertical distribution of the ratio Cd/PO, (nmol/pmol) in the southwest Pacific Ocean.

by the AOU relations that show the low regenerated Cd/PO, ratio in the upper layer. The Cd/PO, ratio in the deep layer is N 0.26, which falls between the ratios of the Antarctic and northeast Pacific Oceans (De Baar et al., 1994). The Cd/PO, ratio is plotted against ur in Fig. 11. The Cd/PO, ratio is correlated with oT as is the case for PO:. Intermediate water has Cd/PO, ratios of 0.1-0.3 and a or of 26.0-27.4. The upper layer has low Cd/PO, ratios, while the deep layer has high Cd/PO, ratios. Gradual change of PO: would be a result of apparent conservative mixing with two water masses as is shown by the relationship between PO: and ur (Fig. 6). A change in the Cd-PO, relationship through the water column would be attributable to a mixing with water masses which have different Cd/PO, ratios.

0.5

s $

0.3

0 i-

0.2 0.1 0

24

25

26

27

26

Sigma-t Fig. Il. Relation between crT and Cd/PO, ratio for all stations.

It is well accepted that the distribution of Cd in the ocean is well correlated with phosphate. It has been reported recently that this correlation, however, varies from basin to basin (Nolting et al., 1991; Saager et al., 1992; De Baar et al., 1994; Nolting and De Baar, 1994). This may imply that several mechanisms control the cycles of phosphorus and cadmium in the ocean. One of them is physical entrainment as suggested by Frew and Hunter (1992). This is also well supported in this study by the PO: and Cd/PO, vs. ur relationships. The Cd-depleted surface water is entrained with AAIW from the Southern Ocean and this mixture would produce a Cd-PO, relationship that has a high slope (OS-O.6 nM/pM) and a negative intercept as reported for the Southern Ocean (Nolting et al., 1991; Nolting and De Baar, 1994). This phenomenon does not seem to occur in the northern Atlantic Ocean, another place of deep water formation. Although a similar entrainment would occur in the case of deep water formation, the data in the Arctic Ocean (Danielsson and Westerlund, 1983) show a similar Cd/PO, ratio throughout the water column. The source water is the only one there, so a vertical change of the Cd/PO, ratio cannot be expected in the northern Atlantic. Until now, the study about trace-metal distribution referring its relationships to nutrients has been plotted between only two properties throughout the water column. As is shown, the water column generally consists of several water masses. If two properties have the same biogeochemical cycles and inventories among these water masses, the relationship between these properties would show a uniform ratio throughout the water column. Cadmium has a similarity in its distribution with phosphate, so the relationship between them shows an almost unity ratio for the greater part of the ocean, except in the southern hemisphere. To understand the biogeochemical cycles of trace metals in the ocean, correlation analysis should be done for the same water mass. The other important factor that affects the cycle of Cd is biological activities. Considering the biogeochemical cycles of Cd in the ocean, one usually refers to phosphate because of its similar residence time. However, the change of the Cd/PO, ratio in the biogenic particles that form in the euphotic layer

I. Kudo et al. / Marine Chemistry 54 (1996) 55-67

and sink towards the deeper layers is not taken into account. This has been documented in this study by the correlation with AOU. As the Cd/PO, ratio in the surface water and the slope in the Cd-AOU relationship is low in the subtropical southwest Pacific Ocean, phytoplankton seems to grow having low Cd/PO, ratio in this area. By culture experiment, Abe and Matsunaga (1988) demonstrated that the Cd/PO, ratio in phytoplankton is affected by this ratio in ambient seawater. This implies that the ratio in the surface water affects the ratio in phytoplankton and in the regeneration flux. This has been confirmed by the fact that the slope of the Cd-AOU relationship is higher at the southernmost station (SA16), where the Cd/PO, ratio in the surrounding water is also higher (0.17) than other stations. It is further examined why AABW has a high Cd/PO, ratio though it receives regeneration flux of a low Cd/PO, ratio. We have already divided the observed concentrations of phosphate and cadmium into two portions: preformed and regenerated. The preformed fraction consists of 70-85% and 50% of Cd and phosphate, respectively, in the Pacific deep water (PDW). The contribution of the regenerated portion is small in PDW, while the upper layer is virtually dominated by regenerated Cd and PO,. Thus, the regenerated Cd/PO, ratio would easily affect to the in situ ratio in the upper layer, but PDW would respond slowly to the regeneration flux. Tsunogai and Noriki (1991) have summarized particulate fluxes of carbonate carbon and organic carbon observed -in various oceans. Particle fluxes are one or two orders of magnitude larger in the biologically productive polar, subpolar, hemipelagic and coastal seas than in the greater part of the pelagic ocean area. This evidence implicates that organic fluxes carrying P and Cd from the surface to the deep ocean are much smaller in our study site than in polar and subpolar areas. Thus, the increase of regenerated P and Cd is small and the change of Cd/P ratio is not so prominent compared to other ocean areas. Cadmium is highly depleted in surface water relative to phosphate in the subtropical South Pacific Ocean. These phenomena of Cd depletion at the surface are not particular to the southern hemisphere. Boyle et al. (1981) and Bruland (1980) reported surface depletion of Cd relative to phosphate, which leads to a decreasing Cd/PO, ratio toward the sur-

63

face in the tropical and subtropical North Pacific Ocean. This decrease of Cd/PO, ratio in the upper layer is a general phenomenon in the world ocean (De Baar et al., 1994). In the subarctic North Pacific (I. Kudo, unpublished data) and the Antarctic (Nolting and De Baar, 1994) however, such depletion is not observed due to enhanced supply from the deep layers. What has forced the depletion in the surface layer? At first, riverine input must be considered. Edmond et al. (198 11, Boyle et al. (19821, and Sharp et al. (1982) reported Cd and phosphate concentrations in estuaries. They show high Cd/PO, ratios of 0.3-0.7, that approximately resemble or are higher than the ratio in the average ocean. This rivet-me input may only influence coastal areas. The horizontal gradient for phosphate and cadmium has not been reported in the open ocean. Generally, elements involved in the biochemical cycles of the ocean are distributed to the surface waters through vertical mixing and atmospheric transport. The atmospheric input for Cd does not appear to affect significantly its distribution (Bruland, 1980). Boyle et al. (198 1) argued that this depletion is the result of a change of the discrimination factor which is dependent upon the ambient phosphate concentration. Bruland (1980) explained these phenomena as a preferential removal of cadmium and a faster regeneration of phosphate than cadmium in the upper thermocline. Diatoms are dominant phytoplankton in a highly productive area and appear to be a main carrier of Cd and P to the deep layer. The Cd/P ratio in diatoms is similar to that of seawater (Abe and Matsunaga, 1988; I. Kudo, in prep.>. In general, oligotrophic ocean areas are not dominated by diatoms, but by CaCO,-forming plankton. The Cd/P ratio in the CaCO,-forming phytoplankton is one-tenth of that in seawater (Frew and 1992). Boyle (1988) reported that Hunter, foraminifera and other calcareous organisms are responsible for < 2% of vertical Cd transport. This means that CaCO,-bearing particles are less important in the vertical transport of Cd, but there is a possibility that cadmium incorporated into the calcareous phase could suffer some separation from phosphate during regeneration cycles. Although the respiratory decomposition of organic particles takes place mainly in the upper 1000 m, the dissolution of

I. Kudo et al./Marine

64

CaCO, occurs below the lysocline (generally below 3000 m>. CaCO,-forming plankton dominates the export production in an oligotrophic ocean, so this separation would produce a depletion of Cd in the surface water in these areas. If the preferential removal of Cd against phosphate occurs in the euphotic zone, it would come from the difference of carrier phase (i.e. organic tissue and the hard parts). In summary, a correlation analysis between cadmium and phosphate should be done within the same water mass to understand the biogeochemical cycles in the ocean. The negative P-zero intercept and higher slope that is observed in the southern hemisphere would be produced as a consequence of mixing with two water masses which have different original Cd/PO, ratios. The Cd/PO, ratio in seawater also change, reflecting the ratio in biogenic particles and regeneration flux. The distribution of Cd and its relationship to phosphate in the ocean is controlled by both oceanographic structure and biogeochemical cycles. Unfortunately, we do not have enough data sets to extend the AOU regeneration model to other ocean areas, but if this model is applied properly, cadmium-phosphate cycles would

Chemistry 54 (1996) 55-67

be well understood about the whole ocean. Based on the understanding of these characteristic behaviors of Cd, this trace metal could be useful as an indicator for biological activity or a tracer for ocean circulation.

Acknowledgements We are grateful to the captain, officers and crew of R/V “Hakuho-Maru”, and Dr. Y. Nozaki (Chief Scientist) and other scientists on board for their help and cooperation during the KH92-4 cruise, conducted by the Ocean Research Institute, University of Tokyo. The authors express special thanks to Mr. T. Stewart for reading an earlier version of the manuscript. The constructive suggestions given by anonymous reviewers and Dr. F. Millero, Editor-inChief, led to improvements in the manuscript. This study was partially supported by a grant from the Ministry of Education, Science and Culture of Japan (Ocean Fluxes - Their Role in the Geosphere and the Biosphere).

Appendix A. Seawater analysis results and associated hydrographic data in this study

=P& (m)

Sal. (p.s.u.)

SA5 (000.54’S, 9 30 50 74 97 299 498 991 1486 1980 2473 2963 3451 3940

Temp. (“0

AOU

UT

bmol/l)

149’=57.34’E):

34.200 34.499 34.787 34.986 35.132 34.837 34.594 34.561 34.609 34.633 34.657 34.673 34.675 34.684

28.94 28.36 27.23 25.30 23.88 12.37 8.06 4.48 2.82 2.29 1.81 1.62 1.57 1.53

21.44 21.79 22.26 23.10 23.47 26.42 26.96 27.36 27.58 27.65 27.71 27.74 27.75 27.75

198.2 201.3 197.7 160.2 151.3 137.0 109.8 99.5 115.6 122.2 134.8 145.1 150.4 155.8

0.4 - 1.3 5.8 49.5 63.8 131.2 185.7 221.9 219.2 216.9 208.4 200.0 195.0 189.7

0

0.02 0.02 0.26 0.41 1.38 2.24 2.90 2.85 2.87 2.77 2.72 2.67 2.65

0 n.d. 0 0 0 0.41 0.86 1.26 1.23 1.26 1.23 1.24 1.22 1.24

0.010 0.002 0.057 0.034 0.050 0.218 0.678 n.d. n.d. 0.793 0.661 n.d. 0.656 n.d.

1. Kudo et al./Marine

5149 (B - IO)

34.687

SA7 (14”15.38’S, IO 30 50 74 99 149 200 250 298 497 597 1001 1488 1980 3001 3941 4639

1.61

Chemistry 54 (1996) 55-67

65

27.75

161.6

183.9

2.58

1.22

0.878

22.76 22.79 22.80 23.29 24.24 142.8 25.49 25.90 26.26 26.97 27.10 27.40 27.56 27.66 27.73 27.76 27.75

205.8 206.2 206.2 192.8 153.5 79.0 147.7 150.0 147.3 177.6 175.4 155.8 143.3 146.8 185.2 187.9 187.9

-3.1 - 1.3 -0.8 12.9 59.3 0.54 84.3 93.7 111.1 118.3 130.8 169.1 190.6 191.9 157.5 155.4 155.3

0.08 0.04 0.05 0.08 0.35 0 0.63 0.82 1.16 1.79 2.09 2.67 2.88 2.89 2.66 2.51 2.58

n.d. n.d. n.d. 0 0 0.039 0 0.13 0.33 0.9 1 1.12 1.41 1.46 1.47 1.42 1.42 1.42

0.004 0.005 0.007 0.006 0.032 0.039 0.079 0.044 0.079 n.d. 0.295 0.533 n.d. n.d. 0.720 n.d. 0.820

25.18 25.25 25.25 25.36 25.44 25.64 25.77 25.93 26.45 26.71 26.95 27.19 27.39 27.65 27.72 27.73 27.73 27.72

n.d. 227.6 229.9 233.5 233.9 207.5 196.4 195.0 190.1 193.7 209.8 194.1 173.2 155.8 158.4 158.4 159.3 159.8

n.d. 1.8 0.9 -1.3 0.0 30.4 46.0 50.0 74.6 87.1 88.0 122.3 153.5 183.0 183.9 183.9 183.9 182.6

0.04

n.d.

0.06 0.05 0.00 0.13 0.26 0.34 0.82 1.20 1.69 2.25 2.55 2.75 2.71 2.70 2.73 2.7 I

0.05 n.d. 0 0.01 0.01 0.01 0.26 0.56 1.03 1.34 1.41 I .39

1.38 1.35

0.011 0.004 0.004 0.001 0.007 0.002 0.005 0.028 0.07 1 n.d. 0.418 n.d. 0.528 0.575 n.d. 0.663 n-d. 0.708

25.72 25.76 25.77

234.3 233.9 246.8 220.9 212.0 212.0 191.9 188.3 187.9

4.9 5.8 7.1 20.1 31.3 50.4 92.4 110.3 110.3

0.14 0.12 0.09 0.19 0.30 0.63 1.29 1.70 1.72

0.10 0.08 0.04 0.04 0.06 0.26 0.61 0.88 0.90

0.011 0.013 0.006 0.002 0.013 cd. n.d. 0.456 0.47 1

154°20.06’E):

34.961 34.975 35.000 35.472 35.841 35.839 21.84 35.636 35.363 35.048 34.541 34.468 34.514 34.597 34.642 34.697 34.716 34.717

26.79 26.76 26.69 26.32 24.12 25.06 19.45 17.01 14.12 7.97 6.53 4.03 2.92 2.32 1.95 1.83 1.88

(B - 9) SA12 (27”15.37’5, 10 21 44 75 97 148 198 249 398 495 689 991 1238 1976 2959 3450 3936 4519

35.619 35.701 35.698 35.690 35.699 35.634 35.531 35.744 35.075 34.757 34.505 34.393 34.474 34.626 34.674 34.679 34.680 34.681

SA14 (32”30.23’S, 10 32 49 74 99 299 495 694 694

35.616 35.615 35.614 35.570 35.519 35.222 34.792 34.606 34.606

175”25.04’E): 20.20 20.05 19.72 19.36 19.00 18.16 17.26 16.56 12.87 10.24 7.68 5.17 3.84 2.34 1.89

1.85 1.87 1.93

1.35 1.33

170”32.51’E): 17.86 17.87 17.84 17.45 16.94 13.35 9.68 7.59 7.59

25.80 25.92 26.49 26.88 27.03 27.03

I. Kudo et al./~ar~~e

66

993 1484 1978 2959 3204 3378 (B - 7)

34.525 34.591 34.642 34.679 34.682 34.683

5.19 3.02 2.37 1.93

1.94 1.94

Chemistry 54 (1~6~ 55-67

27.25 27.53 27.65 27.71 27.71 27.7 1

177.2 161.6 158.0 156.2 185.2 184.8

138.8 171.4 180.8 185.7 185.3 184.8

2.05 2.37 2.44 2.48 2.50 2.48

1.02 1.10 1.10 1.10 1.12 1.14

n.d. n.d. n.d. n.d. 0.682 0.839

26.52 24.44 26.39 26.52 26.56 26.60 26.63 26.69 26.77 26.87 27.02 27.16

264.2 263.3 257.5 250.8 254.4 240.6 241.5 216.9 225.0 214.2 192.4 187.0 173.2 172.7 194.6 202.2 204.9

-7.1 -6.7 0.9 11.1 8.0 24.1 24.6 57.1 54.9 76.2 109.4 125.9 150.0 165.6 150.0 147.3 146.0

0.31 0.32 0.32 0.40 0.41 0.50 0.56 0.88 0.98 1.37 1.76 2.05 2.31 2.38 2.24 2.28 2.29

n.d. n.d. 0.31 0.31 0.35 0.37 0.37 0.46 0.57 0.80 0.95 1.11 1.19 1.15 1.13 1.18 1.20

0.057 0.050 0.032 0.037 0.039 0.119 0.121 n.d. 0.320 n.d. n.d. 0.432 0.625 n.d. n.d. 0.644 0.647

n.d. n.d. n.d. n.d. n.d. n.d. n.d.

0.002 0.020 0.007 0.004 0.007 0.006 0.007

SA16 (40”2.16’S, 155’8.36’E): 11 30 50 7.5 99 149 200 298 397 595 792 990 1235 1975 2955 3933 4175

35.404 35.392 35.374 35.319 35.308 35.269 35.228 34.994 34.839 34.620 34.5 11 34.453 34.480 34.658 34.73 34.713 34.712

14.32 14.29 14.00 13.32 13.26 12 91 12.68 11.30 10.34 8.65 7.15 5.60 4.26 2.40 1.64 1.11

1.09

27.34 27.66 27.78 27.81 27.81

SA19 (27”14.O’S, 155’25.O’E): 10 30 50 75 100 150

35.556 35.557 35.565 35.584 35.588 35.549

nd. 23.15 23.14 22.5 1 21.50 21.50

n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d.

0.07 0.05 0.03 0.05 0.05 0.09

200

35.553

21.26

n.d.

n.d.

n.d.

0.11

PG: denotes preformed phosphate (see text). Data except Cd were quoted from the cruise report of KH92-4 (some representative data were listed from this cruise report). n.d. = no data; (B - x): x m above bottom.

References Abe, K. and Matsunaga, K., 1988. Mechanism controlling Cd and PO, concentrations in Funka Bay, Japan. Mar. Chem., 23: 145-152. Boyle, E.A., 1988. Cadmium: Chemical tracer of deep water pal~e~ography. P~e~e~ography, 3: 471-489. Boyle. E.A. and Edmond, J.M., 1977. Determination of copper, nickel, and cadmium in sea water by APDC chelate coprecipitation and flameless atomic absorption spectrometry. Anal. Chim. Acta, 91: 189-197. Boyle, E.A., Sclater, F.R. and Edmond, J.M., 1976. Gn the marine geochemistry of cadmium. Nature (London), 263: 42-44. Boyle, E.A., Huested, S.S. and Jones, S.P., 1981. Gn me distribution of copper, nickel, and cadmium in the surface waters of the North Atlantic and North Pacific Ocean. J. Geophys. Res., 86: 8048-8066.

Boyle, E.A., Huested, S.S. and Grant, B., 1982. The chemical mass balance of the Amazone Plume, II. Copper, nickel, and cadmium. Deep-Sea Res., 1IA: 1355-1364. Broecker, W.S. and Peng, T.-H., 1982. Tracers in the Sea. Eldigio, New York, NY, 690 pp. Broecker, W.S., Takahashi, T. and Takahashi, T., 1985. Sources and tlow patterns of deep-ocean waters as deduced from potential temperature, salinity and initial phosphate concentration. J. Geophys. Res., 90: 692-S-6939. Bruland, K.W., 1980. Oceanographic distribution of cadmium, zinc, nickel, and copper in the north Pacific. Earth Planet. Sci. Lea. 47: 176-198. Danielsson. L.-G., 1980. Cadmium, cobalt, copper, iron, lead, nickel and zinc in the Indian Gcean. Mar. C&em., 8: 199215. Danielsson, L.-G. and Westerlund, S., 1983. Trace metals in the Arctic Ocean. In: C.S. Wong et al. (Editors), Trace Metals in Seawater. Plenum, New York, NY, pp. 85-95.

I. Kudo et al./Marine Chemistry 54 (1996) 55-67 Danielsson, L.-G., Magnusson, B. and Westerhmd, S., 198.5. Cadmium, copper, iron, nickel and zinc in the north-east Atlantic Ocean. Mar. Chem., 17: 23-41. De Baar, H.J.W., Saager, P.M., Nohing, R.F. and van der Meer, J., 1994. Cadmium versus phosphate in the world ocean. Mar. Chem., 46: 261-281. Edmond, J.M., Boyle, E.A., Grant, B. and Stalhud, R.F., 1981. The chemical mass balance in the Amazon plume, 1. The nutrients. Deep-Sea Res., 28A: 1339- 1374. Frew, R.D. and Hunter, K.A., 1992. Influence of southern ocean waters on the cadmium-phosphate properties of the global ocean. Nature (London), 360: 144-146. Hunter, K.A. and Ho, F.W.T., 1991. Phosphorus-cadmium cycling in the northeast Tasman Sea, 35-40”s. Mar. Chem., 33: 279-298. Jickells, T.D. and Burton, J.D., 1988. Cobalt, copper, manganese and nickel in the Sargasso Sea. Mar. Chem., 23: 133-144. Morel, F.M.M. and Hudson, R.J.M., 1985. The geobiological cycle of trace elements in the aquatic system: Redfield revisited. In: W. Stumm (Editor), Chemical Processes in Lakes. Wiley-lnterscience, New York, NY, pp. 251-281.

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Nolting, R.F. and De Baar, H.J.W., 1994. Behaviour of nickel, copper, zinc and cadmium in the upper 300 m of a transect in the Southern Ocean (57”-62% 49”W). Mar. Chem., 45: 225242. Nolting, R.F., De Baar, H.J.W., Van Bennekom, A.J. and Masson, A., 1991. Cadmium, copper and iron in the Scotia Sea, Weddell Sea and Weddell/Scotia confluence (Antarctica). Mar. Chem., 35: 219-243. Redfield, AC., Ketchum, B.H. and Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: M.N. Hill (Editor), The Sea, Vol. 2, Wiley-Interscience, New York, NY, pp. 26-77. Saager, P.M., De Baar, H.J.W. and Howland, RJ., 1992. Cd, Zn, Ni and Cu in the Indian Ocean. Deep-Sea Res., 39: 9-35. Sharp, J.H., Culberson, C.H. and Church, T.M., 1982. The chemistry of the Delaware estuary. General considerations. Limnol. Gceanogr., 27: 1015-1028. Tsunogai, S. and Noriki, S., 1991. Particulate fluxes of carbonate and organic carbon in the ocean. Is the marine biological activity working as a sink of the atmospheric carbon? Tellus, 43B: 256-266.