The distribution of dissolved vanadium in Eastern Canadian coastal waters

Estuarine, Coastal and Shelf Science (1992) 34, 85--93

T h e D i s t r i b u t i o n o f D i s s o l v e d V a n a d i u m in Eastern Canadian Coastal Waters

P. A. Y e a t s Physical and Chemical Sciences, Department of Fisheries and Oceans, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, Canada B2 Y 4A2 Received 20June 1990 and in revisedform 14 May 1991

Keywords: vanadium; distribution; geochemistry; Gulf of St Lawrence; Scotian shelf; Canada Coast Dissolved vanadium measurements from the Gulf of St Lawrence and the Scotian shelf are reported. T h e Gulf of St Lawrence results reveal an average vanadium concentration of 24 nM in the deep water of the Gulf decreasing to 17.6nM in the St Lawrence River. The Saguenay River had an even lower vanadium concentration of 5.1 nM. Linear increases in concentration with salinity were seen for both the St Lawrence estuary and the Saguenay fjord. In the Gulf of St Lawrence and on the Scotian shelf, depletion of vanadium in the surface waters and increasing concentrations with depth were observed. The vanadium-salinity relationships for these coastal waters both show increasing vanadium concentrations with increasing salinity and correlations that are indicative of extensive removal of dissolved vanadium from surface waters. On two offshore stations, the average vanadium concentration was 31 nM with no marked variation in concentration with depth. In general, the distributions can be explained in terms of the strengths of the input functions, be they rivers, atmospheric precipitation or oceanic advection, and the removal by biogenic scavenging processes.

Introduction A n a v e r a g e d i s s o l v e d v a n a d i u m c o n c e n t r a t i o n o f 1.19 I~g 1-~ (23-4 n M ) was r e p o r t e d for ocean w a t e r s west o f t h e U n i t e d K i n g d o m b y M o r r i s in 1975. S i n c e t h a t t i m e few r e p o r t s o f v a n a d i u m c o n c e n t r a t i o n s in s e a w a t e r h a v e b e e n w r i t t e n a n d t h o s e t h a t have, h a v e t e n d e d to c o n f i r m this a v e r a g e c o n c e n t r a t i o n for o c e a n waters. T h i s is u n l i k e t h e s i t u a t i o n for m o s t trace m e t a l s w h e r e r e c e n t r e p o r t s have r e s u l t e d in a d o w n w a r d r e v i s i o n in t h e a c c e p t e d c o n c e n t r a t i o n s o v e r t h e last 10 to 15 years. A p p a r e n t l y , v a n a d i u m is less p r o n e to c o n t a m i n a t i o n d u r i n g s a m p l i n g or analysis t h a n are m o s t o f t h e o t h e r t r a c e metals. T h e c o n c e n t r a t i o n s o f v a n a d i u m are also s o m e w h a t h i g h e r . I n t h e last few years, t h e r e has b e e n r e n e w e d i n t e r e s t in v a n a d i u m d i s t r i b u t i o n s , b o t h in the d e e p sea (Collier, 1984; H u i z i n g a & K e s t e r , 1982; M a r t i n & K n a u e r , 1982; W e i s e l et al., 1984; Z h o u & M u r r a y , 1982; • J e a n d e l et al., 1987) a n d the n e a r - s h o r e ( P r a n g e & K r e m l i n g , 1985; van d e r S l o o t et al., 1985; S h i l l e r & Boyle, 1987). I n this p a p e r I will d e s c r i b e the d i s t r i b u t i o n o f v a n a d i u m in 0272-7714/92/o 10085 + 09 $03.00/0

© 1992 Academic Press Limited

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waters off the eastern coast of Canada, specifically the St Lawrence estuary, the Gulf of St Lawrence and the Scotian shelf.

Methods

Water samples from the Gulf of St Lawrence were collected on Bedford Institute cruise 79-024 (25 August to 1 September and 1 October to 10 October, 1979) using General Oceanics Go-Flo samplers and Niskin samplers. The Go-Flos were modified for trace metal collection as described by Bewers and Windom (1982) and the Niskins as described by Bewers et al. (1974). Samples collected in the Gulf of St Lawrence were filtered through 0.4 lam Nuclepore filters attached directly to the Go-Flos. Estuary samples were collected with the Niskin bottles and filtered in a separate filtration apparatus. Filtered water samples were stored acidified to pI--I < 2 with Baker Ultrax HCI. Further details of the collection procedures and a map showing the Gulf of St Lawrence station locations are published elsewhere (Campbell & Yeats, 1984). T h e Scotian shelf samples were collected on Bedford Institute cruise 85-017 using the same techniques as those used for the Gulf sampling. The cruise track is shown in Figure 1. T h e analytical method for vanadium follows the method developed for chromium by Cranston and Murray (1978). It is very similar to the method reported by Weisel et al. (1984) for the analysis of A1, Pb and V. T h e p H of 150 ml of sample was adjusted to 8.0 with Ultrex ammonium hydroxide. One ml of ferrous hydroxide (made by adding 1-0 ml of 10% ammonium hydroxide to 50 ml of 0-01 M ferrous ammonium sulphate) was added

Distribution of dissolved vanadium

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to the samples and the samples shaken for 1 h. T h e precipitate formed during this period was filtered through acid-washed 0.4 lam Nuclepore filters and Milli-Q water was used to wash all the precipitate onto the filter. T h e filters were removed from the filtration funnels with plastic forceps and placed in 10 ml polyethylene vials. T h e precipitate was dissolved by adding 2 ml of 3 M HC1 (Ultrex) and shaking until the red colour disappeared from the filter. T h e s e samples were stored in the refrigerator and diluted 1:1 with Milli-Q water prior to analysis. Standards (and blanks) were prepared by spiking vanadium atomic absorption standards into the filtrate retained from the original sample analysis and re-separating these standards using the same procedure used for the original samples. T h e concentrated samples and standards were analysed by furnace atomic absorption spectrophotometry using a Perkin Elmer Zeeman 5000 spectrophotometer and H G A 500 furnace. Pyrolytically coated tubes were used and the samples were dried for 35 s (10 s ramp) at 120 °C, charred for 25 s (10 s ramp) at 1500 °C and atomized for 8 s at 2700 °C. T h e precision (l~r) for this analytical procedure is ___4.3°,o for samples with ,--20 n M vanadium concentration. T h e average analytical blank for a typical series of samples was 0-33 + 0.16 nM. T h e detection limit based on these precisions would be ~ 2 nM. A more realistic precision estimate that included sampling as well as analytical imprecision, would be something less than 10°o. I f I assume the seven samples from the freshwater regime represent the same water, I can calculate a standard deviation for these samples of 1.1 n M (coefficient of variation of 6-4°~0). Standard deviations for replicate analyses of actual samples (n usually 4) from the two cruises varied between 0.5 and 2 riM.

Results and discussion T h e measured dissolved vanadium concentrations in the G u l f of St Lawrence varied between 3.9 and 26.7 nM. T h e lowest concentrations (3-9-10.0 n M ) were all found in the brackish waters of the Saguenay fjord, a deep, shallow silled inlet that is the main tributary to the St Lawrence estuary system. T h e concentration in the Saguenay fjord generally increased with increasing salinity. T h e vanadium-salinity relationship in the Saguenay appeared to be linear but no samples were collected with salinities between 12 and 27 psu. This vanadium-salinity relationship can be described by V (nM) = 0"43 Sal + 4.9; n = 35, r = 0'926. T h e concentration found in one sample from the Saguenay River was 5-1 nM. In the rest of the system the concentrations varied over the fairly narrow range of 16-1-26-7 nM. T h e vanadium-salinity relationship for the St Lawrence estuary and G u l f of St Lawrence is shown in Figure 2. T h i s plot shows a general increase in concentration with salinity, with two groups of points that diverge from this relationship marked off. T h e first of these, in the low-salinity region, contains the surface samples from the region of the estuary where the turbidity is greatest. T h e turbidity m a x i m u m is a more or less constant feature of the St Lawrence estuary, occurring at roughly the same location at all times. A suspended matter vs. salinity plot for this cruise is shown in Figure 3 of Campbell and Yeats (1984). Several metals show marked changes in concentration in this region. Dissolved chromium concentrations were observed to decrease by about 50%, on this transect (Campbell & Yeats, 1984). On other cruises, similar decreases in iron, and increases in manganese have been observed (Bewers & Yeats, 1979). Decreases for chromium and iron have been attributed to precipitation and/or flocculation processes in

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the turbidity maximum and increases for manganese to release of manganese from sediments or suspended particles. Perhaps release of dissolved vanadium from sediments or particles could also be occurring in the turbidity maximum. Direct release of vandium from sediments seems unlikely since the vanadium concentrations decrease with depth in the turbidity maximum, as shown for three turbidity maxium stations in Figure 2. Vanadium may be being released from the riverine suspended matter, but the literature does not suggest any mechanisms for such release. Alternatively, the increased concentrations could simply reflect some temporal or spatial variability in the river concentration. T h e river concentration would only have to increase by 10-15 % to account for the high concentrations. T h e second delineated group includes all the samples from the surface layer in the G u l f of St Lawrence. For the deep ( > 200 m) stations from the Gulf, concentrations are lowest at the surface, increase to a maximum at approximately mid-depth, then decrease towards the bottom. T w o examples of this patterns are illustrated in Figure 2. These profiles look very much like the nutrient profiles for the Gulf(Coote & Yeats, 1979) perhaps suggesting that biological removal is also occurring for vanadium. T h e mid-depth maximum, however, could also result from inward advection of offshore water with high vanadium concentrations. Although it is not clear if vanadium is being removed by active biological uptake or by passive scavenging by biogenic particles, it is clear that some process is removing vanadium from the surface waters since the surface layer concentrations are considerably depleted compared to the mixing line. I f the surface samples from the turbidity maximum are excluded, the vanadium-salinity plot for the St Lawrence estuary shows a straightforward linear relationship with the concentrations increasing from 17-6_+ 1.1 n M in the river to 23.5 n M at 31 psu. T h e equation that describes this relationship is

Distribution of dissolved vanadium

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V (nM) = 0.186 Sal + 17.7; n = 58, r = 0.800. In the Gulf of St Lawrence, an extension of the estuarine mixing line nicely bisects the points describing the deeper, high-salinity samples. T h e average concentration in the deep water is 24.0 nM. T h e surface samples from the Gulf fall distinctly below this line. For the Scotian shelf, concentrations varied from 21 to 35 nM. As was the case in the open Gulf of St Lawrence, concentrations increase with increasing salinity but the scatter is much greater. T h e relationship can be described by the equation V (nM) = 1-59 Sal - 23.8; n = 89, r = 0.46. A similar relationship can be calculated for the open Gulf data. In this case V (nM) = 0"962 Sal - 8"66; n = 44, r = 0.810. In both cases there is a significantly negative zero salinity intercept. According to these relationships, complete removal of vanadium would be expected by 10 psu. Complete removal obviously does not occur, but this extrapolation does indicate the significance of the surface removal of vanadium from coastal surface water. Extensive depletion of vanadium was observed by Prange and Kremling (1985) for the Baltic Sea. There is considerable scatter about the Scotian shelf line, partly a reflection of the limitations of the analytical precision, but also partly a reflection of spatial variability that is not exactly related to salinity. T h e surface waters on the north-western corner of the shelf, where water enters the shelf from the Gulf of St Lawrence, have the lowest concentrations. This is consistent with the observed low surface water concentrations in the Gulf. Surface concentrations on the shelf generally increase along shelf in a south-westerly direction and also increase offshelf. These trends are illustrated in Figure 1. In the bottom waters of the shelf, the highest concer~trations are found on the central part of the shelf, particularly in the deeper basins. On the deeper stations over the various basins on the Scotian shelf, there is a general increase in concentration with depth. T h e increases with depth are not very great and on four of the 11 deep stations, no measurable gradients were observed. For the other seven, increases from top to bottom varied from 1 to 8 nM. Three of these are illustrated in Figure 3. These increases with depth are paralleled by increases in salinity and the nutrients. For the shallower stations over the various banks ( < 100 m deep stations), no consistent trends were observed, some increasing, some decreasing, but in all cases except one the changes from surface to bottom were < 5 nM. Vanadium concentrations were also measured at two stations off the edge of the shelf in the waters of the Gulf Stream. T w o to four measurements were made of the vanadium concentrations of each sample from these two stations. T h e m e a n s _ one standard deviation for each of the results are shown in Figure 4. As can be seen from Figure 4, there is very little change in concentration with depth. T h e average concentrations for station 10, 31"2 ± 1.2 nM, and station 11, 30-6 _ 1-6 nM, are indistinguishable. These results can be compared to the profiles reported for the Pacific Ocean where increases with depth were observed for the top 500 m of the water column (Collier, 1984). These gradients were attributed to biological scavenging of vanadium. Atmospheric inputs to surface waters adjacent to eastern N o r t h America should be greater than in the Pacific, thus at least partly masking the effects of biological removal processes. T h e concentrations at the two offshore stations are higher than the shelf concentrations but only slightly above the values that would be expected from the Scotian shelf vanadium

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vs. salinity relationship. These concentrations are, however, considerably higher than

most others reported for pelagic N o r t h Atlantic waters. Morris (1975) reported an average concentration of 23 n M for open ocean waters west of the U . K . and Weisel et al. (1984) found an average of 24 n M for surface waters of the Sargasso Sea. Likewise, the concentration determined in this study for the NASS-1 seawater reference material, which is a 1300-m deep Sargasso Sea sample, was 26 n M , in good agreement with the value for NASS-1 reported by Prange and Knochel (1985). Vanadium wasn't reported by the producers of the reference material. On the other hand, Huizinga and Kester (1982) reported in an abstract for an oral presentation higher concentrations (36-45 n M ) for north-west Atlantic waters but these results have not subsequently been published. Shiller and Boyle (1987) found ~ 31 n M concentration in a surface water sample from the G u l f of Mexico and Jeandel et al. (1987) found an average concentration of 35 n M at a station in the eastern part of the Sargasso Sea with 29 n M at the surface. It would appear from these comparisons that vanadium concentrations increase from the low concentrations on the Scotian shelf to m a x i m u m concentrations in the G u l f Stream, perhaps followed by decreasing concentrations into the Sargasso Sea. T h e s e

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Figure 4. Vertical profilesof dissolved vanadium for the two Gulf Stream stations (cruise 85-017). (a) Station 10, (b) station 11. trends are the same as the observed changes in surface dissolved aluminum concentrations for a transect between Nova Scotia and the Sargasso Sea (Moore & Millward, 1984). Surface A1 distributions are generally related to atmospheric inputs of AI (Hydes, 1979; Moore & Millward, 1984). Since the atmospheric transport of vanadium is also important, it is likely that the observed distribution of vanadium is also related to the strength of the atmospheric input fluxes. In general, concentrations increase from 5 and 18 n M in the two largest freshwater sources to the G u l f of St Lawrence to 18 to 25 n M in the Gulf, 20 to 30 n M on the Scotian shelf and 31 n M in the G u l f Stream. T h e s e levels and the trends from low concentrations in low-salinity coastal waters to high concentrations offshore agree with most other observations on vanadium. Shiller and Boyle (1987) reported an average river concentration (15 n M ) that is very similar to the concentrations found here for the St Lawrence and Saguenay Rivers. Prange and Kremling (1985) reported m u c h lower vanadium concentrations for the Baltic than for the adjacent N o r t h Atlantic. This is also consistent with m y observations of lower concentrations in the G u l f of St Lawrence than on the two offshore stations. Prange and Kremling attributed the depletion in the Baltic waters to scavenging of vanadium by terrigenous and/or biogenic particulate material. T h e G u l f and Shelf results presented here would suggest that these scavenging processes are more generally important than just in the Baltic. T h e form of the vanadium profiles from the G u l f of St Lawrence may be indicative of biogenic scavenging processes but whatever the process, it is apparently more effective at removing vanadium in the coastal waters than at the offshore stations. T h e observations that are reported here give some indication of the types of removal processes that are important for vanadium. Vanadium is not scavenged from the water of the St Lawrence estuary. This is unlike the picture for Cr and Fe, which are extensively

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removed from these waters (Campbell & Yeats, 1984), and indicates that for the St Lawrence estuary at this time, scavenging of vanadium by terrigenous particles or resuspended bottom sediments was not important. T h e particulate organic content of the river water on this cruise was 0.3 m g I - 1(Tan & Strain, 1983), which corresponds to 5% of the total suspended matter concentration. Flocculation and preferential settling of this organic matter in the estuary has been observed (Kranck, 1979), but none of these organic processes appear to affect the vanadium distribution. I n the open G u l f of St Lawrence, depletion of vanadium from the surface waters is clearly seen. T h e shapes of the vanadium profiles indicate a correlation with phosphate or nitrate that would suggest an active biological incorporation, but less direct interactions could also produce this covariance. T h e Pacific Ocean profiles (Collier, 1984) also show surface depletion but no strong correlation with phosphate is evident from these data, nor did I see a phosphate-related depletion of vanadium on the two stations off the edge of the Scotian shelf. T h e G u l f of St Lawrence and Scotian shelf results and those from the Pacific Ocean taken together would suggest removal of vanadium by biogenic particles but not an input, uptake and regeneration cycle that simply mimics the behaviour of phosphate or nitrate. T h e residence time for vanadium based on river inputs is quite long ( ~ l0 s years; Martin & Whitfield, 1983; Shiller & Boyle, 1987) as would be expected for an element that forms oxyanions in seawater. Inclusion of atmospheric inputs would reduce this residence time somewhat. A review of atmospheric inputs of trace metals (Chester & M u r p h y , 1990) indicates that the atmospheric input of vanadium, accounting for the limited solubility of aerosol vanadium, would be approximately 20% of the fluvial flux. T h e behaviour of vanadium in coastal waters is, however, quite different from that of other trace metals, such as molybdenum, that also form oxyanions. These elements are expected to behave nearly conservatively increasing from low concentrations in freshwaters to high concentrations in seawater. C h r o m i u m is an exception because of the importance of the C r ( I I I ) oxidation state. Vanadium shows extensive depletion in coastal waters indicative of an important removal process that occurs in these regions. Offshore, the profiles are unlike those of metals such as A1 or Pb whose vertical distributions are dominated by atmospheric inputs and scavenging processes, but resemble m u c h more closely the distributions of the quasi-conservative metals such as molybdenum. Similar scavenging processes or active biological uptake could cause the surface depletion observed in the Pacific Ocean while the subsurface m a x i m u m could result from advective transports associated with ventilation of the thermocline as demonstrated by Boyle et al. (1986) for lead, or from regeneration. T h e general absence of surface depletion in the offshore Atlantic profiles would reflect a different balance between the rates of surface inputs and removal by scavenging, biological uptake and advection than that seen in the Pacific.

Acknowledgements T h i s work was supported in part with funding from the Panel on Energy Research and Development. T h e vanadium analyses were conducted by Ms C. Anstey and Ocean C h e m Ltd, D a r t m o u t h N.S.

References Bewers, J. M. & Windom, H. L. 1982 Comparison of sampling devices for trace metal determinations in seawater. Marine Chemistry 11, 71-86.

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Bewers, J. M. & Yeats, P. A. 1979 T h e behavior of trace metals in estuaries of the St. Lawrence basin. Le Naturaliste Canadien 106, 149-161. Bewers, J. M., Hall, W. W. & Macauley, I. D. 1974 A modified Niskin bottle for trace metal sample collection. Bedford Institute of Oceanography Report BI-R-74-2. Boyle, E. A., Chapnick, S. D. & Shen, G. T. 1986 Temporal variability of lead in the western North Atlantic. Journal of Geophysical Research 91, 8573--8593. Campbell, J. A. & Yeats, P. A. 1984 Dissolved chromium in the St. Lawrence estuary. Estuarine, Coastal and Shelf Science 19, 513--522. Chester, R. & Murphy, K. J. T. 1990 Metals in the marine atmosphere. In Heavy Metals in the Marine Environment (Furness, R. W. & Rainbow, P. S., eds). CRC Press, Boca Raton, Florida, pp. 27-49. Collier, R. W. 1984 Particulate and dissolved vanadium in the North Pacific Ocean. Nature 309, 441-444. Coote, A. R. & Yeats, P. A. 1979 Distribution of nutrients in the Gulf of St. Lawrence. Journal of the Fisheries Research Board of Canada 36, 122-131. Cranston, R. E. & Murray, J. W. 1978 T h e determination of chromium species in natural waters. Analytica Chimica Acta 99~ 275-282. Huizinga, D. L. & Kester, D. R. 1982 T h e distribution of vanadium in the Northwestern Atlantic. EOS 63, 990. Jeandel, C., Caisso, M. & Minster, J. F. 1987 Vanadium behavior in the global ocean and in the Mediterranean Sea. Maline Chemistry 21, 51-74. Kranck, K. 1979 Dynamics and distribution of suspended particulate matter in the St. Lawrence estuary. Le Naturaliste Canadien 106, 163--173. Martin, J. H. & Knauer, G. A. 1982 A comparison of particulate reactivities ofAg, Cd, Co, Fe, Mn, Mo, Ni, V and Zn observed during V E R T E X II. EOS 63, 960. Martin, J.-M. & Whitfield, M. 1983 The significance of the river input of chemical elements to the ocean. In Trace Metals in Sea Water (Wong, C. S., Boyle, E., Bruland, K. W., Burton, J. D. & Goldberg, E. D., eds), Plenum Press, New York, pp. 265-296. Moore, R. M. & Millward, G. E. 1984 Dissolved-particulate interactions of aluminium in ocean waters. Geochimica et Cosmochimica Acta, 48~ 235-241. Morris, A. W. 1975 Dissolved molybdenum and vanadium in the Northeast Atlantic Ocean. Deep Sea Research 22, 49-54. Prange, A. & Knochel, A. 1985 Multi-element determination of dissolved heavy metal traces in sea water by total-reflection x-ray fluorescence spectrometry. Analytica Chimica Acta, 172, 79-100. Prange, A. & Kremling, K. 1985 Distribution of dissolved molybdenum, uranium and vanadium in Baltic Sea waters. Marine Chemistry" 16, 259-274. Shiller, A. M. & Boyle, E. A. 1987 Dissolved vanadium in rivers and estuaries. Earth and Planetary Science Letters 86, 214-224. Tan, F. C. & Strain, P. M. 1983 Sources, sinks and distribution of organic carbon in the St. Lawrence estuary, Canada. Geochimica et Cosmochimica Acta 47, 125--132. Van der Sloot, H. A., Hoede, D., Wijkstra, J., Duinker, J. C. & Nolting, R. F. 1985 Anionic species of V, As, Se, Mo, Sb, T e and W in the Scheldt and Rhine estuaries and the Southern Bight (North Sea). Estuarine, Coastal and Shelf Science 21,633-651. Weisel, C. P., Duce, R. A. & Fasching, J. L. 1984 Determination of aluminum, lead and vanadium in North Atlantic seawater after coprecipitation with ferric hydroxide. Analytical Chemistry 56, 1050-1052. Zhou, J.-Y. & Murray, J. W. 1982 T h e distribution of vanadium, chromium and manganese in the Northeast Pacific. EOS 63~ 989.