More stringent procedures (Patterson and Settle 1976;. Patterson et al. 1976 ... the bow of a rubber Zodiac raft away from the mother ship using a samphng ...
Copper, nickel and cadmium in ocean waters
stations using Teflon-coated 2.5 L GO-FLO samplers deployed on a non-metallic Kevlar hydroUne. Subsamples for trace metal analysis were withdrawn before the GO-FLO samplers were detached from the Kevlar. These and other precautions were taken to minimise contamination of the samples during handling. Surface samples were collected from the bow of a rubber Zodiac raft away from the mother ship using a samphng protocol developed in co-operation with Dr C.C. Patterson's research group at the California Institute of Technology. Samples for trace metal analysis were stored unfiltered in pre-cleaned polyethylene bottles after acidification with 2 ml quartz-distilled HCl per Utre of seawater in the shipboard clean laboratory. Dissolved oxygen was determined immediately after sample collection by Winkler titration (Grasshoff 1976). Reactive phosphate and silicate were also determined using the methods of Murphy and Riley (1962) and Mullin and Riley (1955) respectively. Salinities were determined using an inductivelycoupled salinometer calibrated with lAPSO standard seawater. Cu, Ni and Cd were determined by flameless atomic absorption spectrometry (FAAS) after a 50:1 preconcentration by coprecipitation with cobalt pyrrolidene dithiocarboamate (Boyle and Edmond 1976). Analyses were conducted either using an Instrumentation Laboratories 251 instrument with a model 655 furnace atomiser, or using a Pye Unicam PU9000 instrument with a model 9095 video furnace. All trace metal analyses were carried out in a class 100 clean laboratory.
Keith A. Hunter and Frank W.T. Ho Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
INTRODUCTION
Trace metals such as Cu, Ni and Cd have a highly variable and unpredictable behaviour in the ocean. They have been considered far more "interesting" to study than, for example, the major ions, dissolved oxygen or biological micronutrients such as P and N, whose behaviour is well understood. We now know that most, if not all, except the most recent work on trace elements is useless, because the results are simply a measure of the contamination artifacts introduced during the collection, handling and analysis of water samples. More stringent procedures (Patterson and Settle 1976; Patterson et al. 1976; Schaule and Patterson 1981) designed to eliminate these problems have been systematically applied to the study of a number of trace metals in the ocean. Rehable data showed the concentrations of trace metals -varied in a predictable way and could be systematically related to other conventionally measured properties (Boyle et al. 1977). For example, Cd correlates very closely with phosphate (or nitrate) in oceanic waters (Boyle et al. 1976; Bruland et al. 1978, 1979; Bruland 1980; Knauer and Martin 1981; Bruland and Franks 1983) and a similar correlation has been observed between Zn and silicate (Bruland et al. 1979; Bruland 1980; Bruland and Franks 1983). Ni is best correlated with a combination of phosphate and silicate (Sclater et al. 1976; Boyle et al. 1981; Bruland 1980; Bruland and Franks 1983). The distribution of Cu in oceanic waters is influenced by biological processes giving rise to a substantial depletion in surface waters and correlation with phosphate or nitrate (Boyle and Edmond 1975). However, the vertical Cu distribution is complicated by the effects of scavenging by sinking particles at intermediate and mid-depths (Boyle et al. 1977; Bruland 1980) and also diffusion from sediments in bottom waters (Moore 1978; Klinkhammer et al. 1982). Most of these studies have been carried out in the north Pacific Ocean, but in other oceanic basins the deep water circulation system would be expected to influence the cycles of biolimited elements (Broecker and Peng 1982). The distributions of Mn, Cu, Ni, Cd and Zn in the Sargasso Sea, western north Atlantic (Bruland and Franks 1983) show the effects of deep water circulation on the horizontal segregation of trace metals. For example, silicate levels in the western north Atlantic are about-10 times lower than in the north Pacific because the deep water current system pushes silicate, a product of biological remineralisation, into the Pacific Basin from the Atlantic via the Indian Ocean. Zinc is closely correlated with silicate in both oceans, and has a Pacific:Atlantic concentration ratio of about 5. For Cd, the Cd:phosphate ratios for both oceans agree within \5%, indicating that the same mechanisms may control Cd cycling elsewhere in the world ocean. For Ni, the north Atlantic deep water concentrations are about 20% higher than those found in north Pacific waters of the same nutrient composition. Boyle et al. (1981) have demonstrated that the multiple correlation of Ni with phosphate and silicate depends on geographical location in a complex way. The present study is the first conducted, to our knowedge, in the Southern Hemisphere using recent clean laboratory techniques.
RESULTS AND DISCUSSION Vertical distributions in the Tasman Basin
The salinity profile (Fig. 1) shows clearly the presence of Antarctic Intermediate Water (AAIW) centred on a sahnity minimum at \000 m depth. A subsurface oxygen maximum corresponding to AAIW is also evident. The saUnity and oxygen profiles (Fig. 1) also show evidence of horizontal advection at the 200 m level. The nutrient profiles (Fig. 1) can be explained largely by the involvement of phosphate and silicate in biogeochemical cycling. A full understanding of these two nutrients is hampered by the availability of only 2000 m of hydroHne. Phosphate levels of 2.3 mmol/kg in the deepest waters studied are only slightly below those observed in the deep north Pacific (Bruland 1980), whereas silicate levels at 1800 m in the Tasman Basin are only half those found in the north Pacific below 1500 m. These differences are related to the coupling of nutrient regeneration to the deep ocean circulation system discussed earlier. As expected, the south Pacific results fall between those of the north Pacific and north Atlantic. The Cd profile (Fig. 2) closely resembles that of phosphate, as expected. Surface water concentrations are as low as 0.010 nmol/kg which is 1.5% of the concentrations found below 1500 m. These surface water levels compare with values of 0.002 nmol/kg found in oligotrophic waters of the north Atlantic (Bruland and Franks 1983) and north Pacific oceans (Bruland 1980). The detection hmit for our procedure is 0.010 nmol/kg, so that actual Cd concentrations in these waters may be lower. The correlation between Cd and phosphate is further emphasised when the results from other stations on this cruise were included (Fig. 3). The overall Cd:phosphate relationship for these waters is very close to those observed in the north Atlantic and north Pacific Oceans. These results show that the involvement of Cd in photosynthetic growth of surface layer plankton is almost certainly a universal mechanistic feature of oceanic biology. The Ni vertical profile (Fig. 2) resembles those found elsewhere, but the overall concentrations are significantly lower than those observed in north Pacific or north Atlantic waters of comparable nutrient status. Surface water Ni concentrations in the Tasman Basin average 0.9 nmol/kg, about half that in north Pacific and north Atlantic surface
SAMPLING AND ANALYTICAL METHODS During the August-September 1983 joint US/NZ cruise in the north east Tasman Sea of R/V Tangaroa, held in cooperation with the SEAREX program, vertical profile seawater samples were collected at a series of deepwater
35
waters (Bruland 1980; Bruland and Franks 1983; Boyle et al. 1981). Ni increases systematically with depth to 2 nmol/kg at 1600 m, compared with 6 nmol/kg in the western north Atlantic and 10 nmol/kg in the equatorial north Pacific. Nickel can be correlated with both silicate and phosphate because of its involvement in biogeochemical cychng of both organic tissue and the tests of organisms. The coefficients of correlation with these nutrients do vary, however, with geographical location (Boyle et al. 1981). The present results show that the involvement of Ni in this cycling is about one third that observed in north Pacific waters. Surface water concentrations of Cu average 1.3 nmol/kg (Fig. 2), compared with 0.5 nmol/kg in the equatorial north Pacific and 1.1 nmol/kg in the western north Atlantic. In the north Atlantic, Cu levels increase to only 1.5 nmol/kg at 1500-2000 m, but in the north Pacific to around 3 nmol/kg. Bruland and Franks (1983) have suggested that this indicates some involvement of Cu in biogeochemical cycling. On this basis, the increase in Cu in the Tasman Basin down to 1500 m, to somewhere between that observed in the north Atlantic and north Pacific is what we would have expected. But the trend is unclear because of the lack of data below 1500 m and the degree of scatter, which may indicate some residual contamination problems. The exact nature of the coupling between Cu and nutrient cycles in intermediate and surface water is unknown and is probably more complex than that of Cd or Ni. However, the Tasman Basin data are not inconsistent with the trends found elsewhere.
sampling program. Financial assistance for this project was provided by the New Zealand Oceanographic Institute, the US-NZ Cooperative Science Program, the N.Z. University Grants Committee, and Otago University.
REFERENCES
Boyle E.A. and Edmond J.M. (1975). Copper in surface waters south of New Zealand. Nature, London, 253; 107-109. Boyle E.A. and Edmond J.M. (1976). Determination of copper, nickel and cadmium in sea water by APDC chelate coprecipitation and flameless atomic absorption spectrometry. Analytica Chimica Acta 91; 189-197. Boyle E.A., Sclater F. and Edmond J.M. (1976). On the marine geochemistry of cadmium. Nature, London, 263; 42-44. Boyle E.A., Sclater F. and Edmond J.M. (1977). The distribution of dissolved copper in the Pacific. Earth & planetary Science Letters 37; 38-54. Boyle E.A., Huested S.S. and Jones S.P. (1981). On the distribution of copper, nickel and cadmium in the surface waters of the north Atlantic and north Pacific Oceans. J. Geophysical Research 86; 8048-8066. Broecker W.S. and Peng T.H. (1982). Tracers in the sea. Publication of the Lamont-Doherty Geological Observatory, Columbia University. Bruland K.W. (1980). Oceanographic distributions of cadmium, zinc, nickel and copper in the north Pacific. Earth & planetary Science Letters 47, 176-198. Bruland K.W., Knauer G.A. and Martin J.H. (1978). Zinc in northeast Pacific waters. Nature, London 271; 741-744. Bruland K.W., Franks R.P., Knauer G.A. and Martin J.H. (1979). Sampling and analytical methods for the determination of copper, cadmium, zinc and nickel at the nanogram per liter level in seawater. Analytica Chimica Acta 105, 233-245. Bruland K.W. and Franks R.P. (1983). Mn, Ni, Cu, Zn and Cd in the western north Atlantic. NATO Conf. Ser., (Ser.)4 1983, 9(Trace Met. Seawater), 395-414. Grasshoff, K. (1976). Methods of seawater analysis. Verlag-chimie, Weinheim. Klinkhammer G., Heggie D.T. and Graham D.W. (1982). Metal diagenesis in oxic marine sediments. Earth and planetary Science Letters 61; 211-219. Knauer G.A. and Martin J.M. (1981). Phosphorus-cadmium cycling in north-east Pacific waters. J. Marine Research; 39; 65-76. Moore R.M. (1978). The distribution of dissolved Cu in the eastern Atlantic ocean. Earth & planetary Science Letters 41; 461-468. Mullin J.B. and Riley J.P. (1955). The colorimetric determination of silicate with special reference to sea and natural waters. Analytica Chimica Acta 12; 162-169. Murphy J. and Riley J.P. (1962). A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27: 31-36. Patterson C . C , Settle D. and Glover B. (1976). Analysis of Pb in polluted coastal seawater. Marine Chemistry 4; 305-419. Patterson C. and Settle D.M. (1976). The reduction of orders of magnitude errors in lead analysis of biological materials and natural waters by evaluating and controlling external sources of industrial lead contamination introduced during sample collection, handling and analysis. N.B.S. Special Publication (D.M. la Fleur, ed.) 422, 321-351. Schaule B.K. and Patterson C.C. (1981). Lead concentrations in the north-east Pacific: evidence for global anthropogenic perturbations. Earth & planetary Science Letters. Sclater F.F., Boyle E. and Edmond J.M. (1976). On the marine geochemistry of nickel. Earth & planetary Science Letters 31, 119-128.
CONCLUSIONS
Copper and Ni data for other stations on this cruise (not presented) confirm the trends discussed. Vertical profiles of Cd show a phosphate-type distribution in these north east Tasman Sea waters with average concentrations in waters below 1000 m that are within 15% of those observed at similar depths in the deep north Pacific. This similarity underlines the dominant role of internal biogeochemical cycling in the distribution of this element. Nickel shows a mixed nutrient-type distribution, indicating involvement in a shallow water regeneration cycle, like that of phosphate, and a deeper regeneration process, like that of silicate. However, the concentrations of Ni in surface and intermediate waters are significantly lower (Vi-Vi) than in north Pacific and north Atlantic waters of the same nutrient status. This difference may be a result of large differences in the rate of supply of this element to different oceanic regions. The smaller concentration gradient of Ni across the thermocline in the north east Tasman Sea than in the north Pacific greatly reduces the influence of vertical mixing on the surface water distribution. The cycle of Cu is less affected by biogeochemical processes than is that of Cd or Ni because of scavenging effects, so that vertical mixing does not have a major influence on the supply to surface waters. As a result, the gradient in Cu concentration in the north east Tasman as one approaches the N.Z. continental shelf is not great. ACKNOWLEDGEMENTS
The authors are grateful to the SEAREX team (W. Fitzgerald, H. Maring, A. Pszenny, J. Kim, N. Tindale and J. Hunt) for their assistance during the cruise, and to the master and crew of 'Tangaroa' for their co-operation with the unconventional requirements of our
36
O x y g e n( ml k g ' ' ' )
FIG,
1
3.0
Station
4.0
5.0
6 4 : Hy d r o g r a p h i c
6.0
0
parameters
40
(
35*50'S
8 0 S i U c a t ^tjjM. k g
162*36H
)
C o p p e r { nM.kg 0
200
0
1
2
)
Nickel 0
0
-
1
o
0
600
0
0
o
o
o
0
cP
0
o
1
0 o o
1
dissolved
1
trace
metal
-
0
o
-
Total
CP
0
0
1,6 0 0
-
0
0
•
0
o o
1.400
2
0
0
1,2 0 0
1,800
o o
1,0 0 0
0.8
o 0
o O
Q14
)
0
0 o
0
Cadmi um ( nM.kg
0
0
o
800
0
o
0
400
2
0 o
-1
( nM.kg'"^ )
0 t
1
profiles
station
64
1
Phosphate FIG.3
Cadmfum
concentration
( pM.kg ' )
against
phosphat?
stations
6 4 o ; 54
o CO
o
o
3.
-4-
O
CXI
r4
O CM'
O
o
40
'en
c
-
1.5
-
LJ
0
0.5
1.0 Phosphai-e
F I G. 5
Nickel
1.5 (
c on c e n t r a t - i on
-1 pM.kg ' againsf
2.0
2.5
phosphate
station
) 64
APPENDIX 1: NORTH EASTERN TASMAN RESULTS ( MICRONUTRIENTS ) North e a s t e r n tasman v e r t i c a l p r o f i l e
Depth
Phosphate
Silicate
( 35 50'S 162 36'E )
Dissolved
Salinity
Oxygen (m)
(>imol/kg)
(>inol/kg)
(ml/kg)
(%.)
0
0.16
1.6
5.89
35.588
25
0.30
2.1
5.91
35.586
49
0.38
2.8
5.b9
35.580
75
0.46
1.8
5.52
35.567
100
0.50
2.0
5.40
35.523
149
0.53
2.5
5.37
35.423
249
0.53
2.2
35.349
280
5. 17
349
1.05
5.4
4.63
35.022
448
0.93
4.8
4.99
34.955
498
1.35
7.7
4.56
34.744
573
1.43
8.6
4.50
34.677
689
1.51
12.6
4.47
34.623
747
1.56
14.1
4.44
34.598
788
1.67
15.5
4.45
34.558
886
1.71
19.2
4,45
34.514
985
1.89
21.4
4.38
34.480
996
1.93
26.7
4.35
34.480
1083
1.98
33.8
4.26
34.448
1245
2.01
50.0
1395
2.18
61.1
3.32
34.541
1594
2.34
73.9
3.68
34.577
1793
2.30
78.4
3.32
34.622
.
34.492
42
APPENDIX 2: NORTH EASTERN TASMAN RESULTS ( TRACE I-IETALS ) North e a s t e r n Tasman v e r t i c a l p r o f i l e ( 35°50'S 162°36'E )
Depth
Copper
Nickel
Cadmium
(nmol/kg)
(nmol/kg)
(nmol/kg)
0 25 49 75 100 149 249 349 448 498 573 689 747 788 886 985 996
1.34
0.90
0.010
1.44
1.06
0.064
1.38
0.86
0.106
1.38
1.06
0.106
1.22
0.85
0.095
1.34
0.86
0.075
1.49
1.15
0.095
1.13
1.26
0.223
1.07
0.95
0.244
1.76
1.06
0.275
1.50
1.26
0.292
1.27
1.87
0.402
1.33
1.87
0.424
0.91
1.26
0.385
1.07
1.60
0.456
2.01
1.84
0.402
2.12
1.80
0.352
1083
2.01
2.06
0.660
1245
2.33
1.73
0.511
1395
1.86
2.04
0.670
1.96
0.700
2.25
0.710
(m)
1594 1793
1.29
43
