Some aspects of vanadium and chromium chemistry in the English Channel

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Abstract

Dissolved and particulate vanadium and chromium concentrations along a transect between Cherbourg and the Isle of Wight were investigated in the English Channel. Seawater samples were collected at two different depths (surface and bottom) at five sampling stations during five cruises carried out between September 1994 and September 1995. Calculated mass flows through this Channel section were 7600 T yr−1 for vanadium (about 66% was in the dissolved phase) and 1650 T yr−1 for chromium (about 50% was in the dissolved phase). Dissolved chromium concentrations do not vary significantly along the transect. Seasonal variations in chromium distribution linked to biotic parameters were noted during the September 1994 cruise, when a significant relationship between particulate chromium and algal organic carbon was found. In addition, dissolved Cr (III) and Cr (VI) were well correlated with both algal and terrestrial organic matter. In the May cruise, during phytoplanktonic decay, particulate chromium was correlated with the detritic and bacterial organic fractions. These observations suggest interactions between chromium and biotic material. During the winter period, no relationship between chromium and POC was found. Dissolved and particulate vanadium concentrations varied, respectively, between 15 and 28 nmol l−1 and 2–32 nmol l−1. Values of dissolved vanadium showed depletion in the Channel with respect to oceanic waters. This loss in dissolved vanadium was higher near the English coast, but was compensated for by the increase in the particulate vanadium. There was no clear relationship between dissolved vanadium and algal organic carbon and it is inferred that the vanadium transfer cannot be explained by trapping with biotic material. On the other hand, the dissolved vanadium depletion could be attributed to the presence of ferric oxyhydroxide phases in particles, which have strong adsorbing properties for a range of dissolved metal ions.

Introduction

Chromium is present in seawater in two oxidation states. Chromium (III) is weakly soluble, forms hydroxy-complexes, is easily bound to suspended particles, and is an essential element for animals and humans (Chiffoleau, 1994). Chromium (VI) is the most prevalent form, and is more soluble and very toxic at high concentrations. On the other hand, vanadium is present only in the stable oxidized, soluble form, as V(V) (Sadiq, 1988; Wherli and Stumm, 1989) and it is not considered to be a significant environmental contaminant.

During the last decade, the behaviour of some dissolved and particulate metals in the English Channel has been studied by several authors (Tappin, 1988; Tappin et al., 1992; Hydes and Kremling, 1993; Dauby et al., 1993, Dauby et al., 1994). These studies have mainly focused on the determination of metal concentrations together with their seasonal fluctuations. With the FLUXMANCHE I project within the framework of the European Union Marine Sciences and Technologies (MAST) programme launched in 1990–1993, special attention was given to estimating particulate and dissolved metal fluxes flowing into the North Sea through the Strait of Dover. Unfortunately, no data about vanadium and chromium were available, but in the FLUXMANCHE II project, undertaken in the eastern part of the English Channel, vanadium and chromium concentrations were determined, and are reported here.

Moreover, the biogeochemical cycles of these two metals are poorly documented in this highly dynamic shelf sea (Jeandel, 1987; Dauby et al., 1994), and it was considered important in this study to give some insights into some aspects of vanadium and chromium chemistry in this system. Dauby et al. (1994) have reported a chromium enrichment in the organic-rich particulate phase in the Channel. In this FLUXMANCHE II project, some aspects of organic matter were also investigated (Bodineau et al., 1999) with particulate fatty acids (PFAs) used as biomarkers to identify the origin and sources of particulate organic matter (POM). Thus, it was important to follow the behaviour of V and Cr with respect to these available organic data.

Section snippets

Sampling and analysis

Along a Cherbourg-Isle of Wight transect (Fig. 1), sampling was done at five stations during five cruises in the period from September 1994 to September 1995 (Table 1). Seawater samples were collected at two different depths (5 m below the water surface and 5 m from the sea floor) with an 8 l Go-flo bottle, specially adapted for the trace metal sampling work. Usual precautions developed for the open ocean work were used: all bottles for sample storage were cleaned with a mixture of acids (HNO3,

Results

All dissolved and particulate metal data can be found in the FLUXMANCHE II database that can be accessed at the British Oceanographic Data Centre website (http://www.nbi.ac.uk/bodc).

Mean concentrations. The mean dissolved chromium concentration was found to be 3.3 nmol l−1±2 nmol l−1. Unlike vanadium, these data are comparable with data for North West Atlantic waters (range 3.5–5.2 nmol l−1, Boussemart and Van den Berg, 1994) and off Concarneau, South Bretagne (2.7 nmol l−1, Jeandel and Minster, 1984).

Chromium

Spatial distribution. Distributions of average concentrations for dissolved Cr (III) and Cr (VI) and total particulate Cr over all time periods are presented in Table 3. For each cruise, the water column in the Channel appeared homogeneous as surface and bottom concentrations were similar.

For the particulate phase, the lowest concentrations were observed in the central Channel waters whereas the highest ones were found near the English coasts where SPM loads were highest. For the latter, the

Conclusions

Both dissolved and particulate fluxes for vanadium and chromium have been calculated in the English Channel, between Cherbourg and the Isle of Wight. Concerning the dissolved compartment, vanadium and chromium fluxes have been, respectively, estimated at 5010 and 827 T yr−1 (the Cr (VI) flux is about 90% of the total dissolved chromium flux). Particulate fluxes have been estimated at 2580 and 803 T yr−1 for vanadium and chromium, respectively.

The total dissolved chromium concentration did not vary

Acknowledgements

We gratefully acknowledge the valuable suggestions and comments by Peter Statham from Southampton Oceanographic Centre, in reviewing the manuscript. This paper has also benefited from helpful comments from two anonymous referees. We are also indebted to the crews of the R/V Côte d'Aquitaine, the Thallia and the Noroı̂t for their cooperation in field sampling. This research was supported by the EEC programme Marine Science and Technology: Mast2CT940089.

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