Trace metal (Cd, Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)

Marine Chemistry 79 (2002) 1 – 26 www.elsevier.com/locate/marchem

Trace metal (Cd, Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea) Marie-He´le`ne Cotte´-Krief 1, Alain J. Thomas*, Jean-Marie Martin2 Institut de Bioge´ochimie Marine, CNRS ER 110, Ecole Normale Supe´rieure, 1 Rue Maurice Arnoux, 92120 Montrouge, France Received 29 May 2001; received in revised form 23 January 2002; accepted 18 February 2002

Abstract This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and export to deep waters). The concentrations of dissolved Cd, Cu, Ni and Pb were measured with this aim in January 1994 and June 1995 at vertical stations across slope, including stations with upwelling, and in the surface waters along the Celtic Sea shelf. Additionally, deep sea (from sediment trap data) and atmospheric fluxes were estimated. The metal profiles over the slope off the Celtic Sea are quite similar to open ocean profiles already described in the northeast Atlantic, and the concentrations in surface waters are only slightly enriched compared to the nearby open ocean (1.2 – 1.3
for Cd and Ni). The external sources to the system appear to be of weak influence: the fluvial input is locally strong at the coast and then ‘‘diluted’’ along the large continental shelf; the atmospheric deposition is not significant at the annual scale in comparison to the metal content in the upper waters of the shelf edge (at least for Cd, Ni and Cu). In the upwelling zone, a significant increase in concentrations was observed in the summer surface mixed layer (
2 for nitrate and Cd and
1.5 for Ni) in comparison to the non-upwelling zone. In winter, concentrations of bioreactive metals increased significantly in the surface waters in comparison to the low summer levels (
5 for nitrate and Cd). Our results suggest that upwelling and winter mixing act as regenerated sources that lead to the resupply of the bioreactive elements above the permanent thermocline with a low export to deeper waters. The tracing of the Mediterranean intermediate waters (MIW) from Gibraltar to the studied area shows indeed that its elemental content at the Celtic shelf edge is mainly due to the conservative mixing of the three ‘‘end-member’’ component waters which are thought to make up the MIW. The remineralization of organic matter within this water mass during its transport to the north would contribute only 20% of the nutrients and Cd concentrations recorded at the Celtic Sea shelf edge. According to the correlation found with nutrients in the 10 – 200-m layer, dissolved Pb would also be subjected to biological uptake and regeneration within the seasonal thermocline. Particulate scavenging removal of Pb would take place below the permanent thermocline throughout the water column. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nutrient; Dissolved metals; Cd; Cu; Ni; Pb; European continental margin; Celtic Sea; NE Atlantic Ocean; In situ uptake/ regeneration; Upwelling; Atmospheric deposition; Fluvial inputs; Advection

*

Corresponding author. Present address: UMR Sisyphe, University of Paris VI, case 123, 75005 Paris, France. E-mail address: [email protected] (A.J. Thomas). 1 Present address: Department of Marine Sciences, University of North Carolina, Chapel Hill, NC 27599, USA. 2 Present address: Joint Research Center, Institute for Environment and Sustainability, I-21020 Ispra, Italy. 0304-4203/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 ( 0 2 ) 0 0 0 1 3 - 0

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1. Introduction During the last two decades, much progress has been made in determining the oceanic and coastal distributions of trace metals (TMs). Some of them, like Cd, Ni, Cu or Mn, are relatively easy to measure and have been extensively studied. Others, like Pb, Fe or Zn, remain difficult to measure, and variable, scattered distributions are difficult to interpret. Biological processes, inorganic processes like desorption – adsorption on particles, sediment release and advection of water masses are primarily invoked to explain the oceanic distributions of trace metals (Bruland, 1980; Bruland and Franks, 1983; Schaule and Patterson, 1981, 1983; Danielsson et al., 1985; de Baar et al., 1994; Yeats et al., 1995; Saager et al., 1997), leading to the well-known classification of metals depending on their biogeochemical reactivity (recycled, scavenged and intermediate elements; see, e.g., Burton and Statham, 1990). Although most of the dissolved TM concentrations in coastal and shelf waters globally do not appear to differ very much from average open ocean waters (Martin and Thomas, 1994), various studies have shown local TM enrichments in ocean margin surface waters (Bruland and Franks, 1983; Kremling, 1985; Kremling and Hydes, 1988; Kremling and Pohl, 1989; Muller et al., 1994; Le Gall et al., 1999; Cotte´-Krief et al., 2000). Various sources have been considered to explain these TM enrichments in the ocean margins, including river inputs, atmospheric deposition and sediment diagenesis. In the past decade, the idea has emerged that upwelling could also bring trace metals to coastal surface waters (van Geen et al., 1990), but evidence is weak and based on only a few studies (van Geen and Luoma, 1993; van Geen and Husby, 1996; el Sayed et al., 1994; Saager, 1994; Johnson et al., 1999). Further investigations are needed. Even in the absence of obvious concentration enrichments, mass balance approaches have often demonstrated that elemental budgets in the ocean margins require external inputs from the open sea (e.g., Tusseau-Vuillemin et al., 1998 and Noe¨l, 1996 in the Gulf of Lions; Walsh, 1991 and Martin and Thomas, 1994 for the global ocean margin). A major difficulty in establishing elemental budgets is that the continental shelf has an open boundary with the ocean. An understanding of biogeochemical processes

at the shelf edge is, therefore, required prior to the modeling of elemental exchanges between the margin and the ocean. Processes that affect nitrate and dissolved Cd, Cu, Ni and Pb cycling at the shelf edge of the Celtic Sea (NE Atlantic Ocean) are investigated here from water column profiles recorded across the slope in both summer and winter and during a summer upwelling event, and from surface distributions established along the shelf in summer. We intend in this paper to: (1) determine to what extent the surface waters overlying the slope zone, which potentially may be transported toward the continental shelf, are enriched in dissolved TM as compared to the open ocean, (2) discuss the processes that may affect the surface-water concentrations (in situ regeneration, winter mixing, upwelling and external sources); and (3) compare the magnitude of the TM export to the deeper water to the lateral transport of metals by advection (in the case of the Mediterranean waters). This study is part of the extensive European Ocean Margin Exchange (OMEX) project (MAST) that aimed to examine the land –ocean exchange processes (physical, biological, biogeochemical, benthic and atmospheric processes) along the western European shelf edge. In the framework of this project, Le Gall et al. (1999) have partially considered metal distributions in the investigated area. Additional and original aspects of the present work relative to this earlier study are that we infer the seasonal drawdown of some trace metals in the upper water column associated with biological production during summer as well as the vertical resupply of the studied elements due to winter convective mixing and summer upwelling events.

2. Hydrography of the study area The study area (Fig. 1) is part of the European continental margin and includes the wide continental shelf of the Celtic Sea (more than 300 km wide) and the continental slope down to the abyssal plain at 4500-m depth. The hydrography of the northeast Atlantic Ocean has been thoroughly described by McCartney (1992), Tsuchiya et al. (1992) and van Aken and Becker (1996). The hydrography across the Celtic Sea shelf edge region (47 – 50jN; 7– 13jW) is quite similar.

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Fig. 1. OMEX study area at the shelf edge of the Celtic Sea and sampling stations.

2.1. Water masses T –S plots (Fig. 2A) were used to identify water masses. The surface waters are characterized by summer stratification that includes the surface mixed layer (0 –30 m) and the seasonal thermocline (30 – 100 m) (Fig. 2B). In winter, the surface mixed layer extends down to the permanent thermocline at 300– 400m depth (see later). Below these surface waters are the waters associated with the permanent thermocline, the northeast Atlantic central waters (NEACW), that extend to 500 –700-m depth, marked by a salinity minimum in our TS profiles. The NEACW result from the advection of the subpolar mode water (SPMW) formed by

winter deep convection in the northern North Atlantic (McCartney, 1992; Tsuchiya et al., 1992). The large salinity maximum at 850 –1000-m depth corresponds to the Mediterranean intermediate waters (MIW), originating from the deep Mediterranean outflow water (MOW) at Gibraltar. The depth and the intensity of this salinity maximum show little latitudinal trend in our study area (Fig. 2A), but variations are of greater amplitude at a larger scale (Measures et al., 1995). Between approximately 1250 and 3000 m is the northeast Atlantic deep water (NEADW), which includes the MIW, the Labrador sea water (LSW) and the Iceland – Scotland overflow water (ISOW) (McCartney, 1992). The LSW is recognized by a small

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Fig. 2. (A) TS diagrams and water masses (June 1995) in the Celtic slope region. Depth of water masses in brackets expressed in hundreds of meters. (B) Vertical profiles at a deep station (OM12): salinity, temperature, chlorophyll a (Statham, personal communication), attenuance and suspended matter (Hall, personal communication), dissolved oxygen, nitrate (Raabe, personal communication) and dissolved Cd and Pb (this work).

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salinity minimum at 1800 –2000 m, and a small salinity maximum identifies the ISOW at f 2600 m. The bottom water is the lower deep water (LDW) and results from the northward advection of the Antarctic bottom water (AABW) (McCartney, 1992). 2.2. Circulation On the shelf, the surface waters flow generally from west to east toward the English Channel (Pingree et al., 1982; Pingree and Le Cann, 1989; Pingree, 1993), and fluxes through the channel into the North Sea represent a significant output of water from the shelf system. Along the shelf edge, in autumn and winter, a densitydriven surface current flows northward along the west-

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ern European shelf edge from Portugal to north of Scotland, whereas in spring and summer, it reverses and flows southward due to the north trade wind regime (Wooster et al., 1976; Pingree, 1993; Pingree, personal communication). Below this seasonally variable surface current, a poleward undercurrent, associated with the MIW, flows along the slope between 800and 1200-m depth (Kaˆse and Zenk, 1987; Tsuchiya et al., 1992; van Aken and Becker, 1996). 2.3. Upwelling Along the shelf break, a summer surface cooling with a 0.5– 1.5 jC temperature anomaly is observed each year from May to September (Fig. 3) (Dickson

Fig. 3. Picture of the sea surface temperature recorded during the summer cruise. This is a composite infrared AVHRR picture (June 13 – 20, 1995) kindly supplied by P. Miller and S. Groom from the NERC Remote Sensing Data Analysis Service, Plymouth Marine Laboratory.

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et al., 1980; Pingree et al., 1982; Miller, personal communication). This cooling is associated with high chlorophyll levels (Pingree et al., 1986) and an enhancement of new production (Groom et al., 1997), supporting summer fishing activities (Dickson et al., 1980; Sournia et al., 1990). This seasonal event is the result of deeper water upwelling. The origin of this phenomenon is still uncertain (Dickson et al., 1980; Pingree et al., 1982; Sournia et al., 1990; Pingree and New, 1995), and the cold waters that mix with the surface waters may originate from below the seasonal thermocline. This mixing is thought to be mostly due to the propagation of exceptionally large internal waves, generated by the interaction of strong barotropic tidal currents with the steep shelf break topography (Dickson et al., 1980; Pingree et al., 1986; Pingree and New, 1995). Additional wind mixing seems to be necessary to enhance surface cooling (Huthnance, personal communication; Pingree and New, 1995).

3. Methodology Sampling was carried out in the framework of the OMEX program, jointly with the Southampton Oceanography Centre (SOC, UK) for the trace metals work in January 20– 31, 1994 (Charles Darwin cruise CD84) and in June 7 –16, 1995 (cruise CD94). For convenience, these cruises will be referred to as ‘winter cruise’ and ‘summer cruise’. A late summer cruise (D216) coordinated by the SOC took place in August –September 1995, whose results were presented elsewhere (Le Gall et al., 1999). Complete vertical profiles (up to 36 depths) were established at stations along two shelf break transects off the Goban Spur zone and at the station ‘Belgica’ (La Chapelle Bank, Fig. 1). Stations of the OM5– OM8 transect and station Belgica were sampled during both CD cruises. A complementary transect, where only surface samples were collected, was made across the shelf from Goban Spur toward the Bristol Channel in June 1995. 3.1. Sampling and pretreatment Water samples were taken using 10-l Teflon-coated GoFlo bottles (winter cruise) or new Lever Action Niskin bottles (summer cruise) fixed on a sampling rosette. GoFlo and Niskin bottles were equipped with

Teflon taps and carefully cleaned before use with Decon detergent and acid washes. Hydrographic parameters were obtained using a CTD profiler with an oxygen sensor and a transmissometer. The sampling rosette and CTD were also carefully cleaned, zinc sacrificial electrodes were removed, and all exposed corrodible metal parts coated in parafilm prior to the sampling. The six shelf surface samples were collected from a modified 3.5-kHz fish, which was towed from the starboard side of the ship. A tube made of lowdensity polypropylene led from the fish to a pneumatically operated Teflon pump on deck. About 230 samples were collected. Filtration was done through acid-washed polycarbonate membranes (NucleporeR 0.4-Am pore size, diameter of 142 mm) set up on an acid-cleaned Teflon filter holder. Filtration was performed (1) directly in line with the pump for the surface samples at a typical rate of about 1 l/min or (2) under high-purity nitrogen pressure (circa 0.8 bar) in line with the GoFlo and Niskin bottles. Filtrates were acidified to pH 2 with suprapure HNO3 and stored in high-density polyethylene, acid-washed bottles (NalgeneR) inside a plastic bag. Special attention has been devoted to minimizing contamination; all handling of samples was done in a clean container under laminar flow benches using ultraclean techniques, MilliQ water and suprapure reagents. 3.2. Analysis Trace metals (Cd, Cu, Ni and Pb) in the filtered samples were measured at Montrouge by graphite furnace atomic absorption spectrometry (AAS; SpectrAA 800 Zeeman, Varian). Prior to the AAS, trace metals in samples were concentrated according to a procedure adapted from Danielsson et al. (1982), which consists of complexing of metallic ions at pH = 4.5 F 0.5 with ammonium pyrrolidine dithiocarbamate/diethylammonium diethyldithiocarbamate Table 1 Blanks and seawater reference material (NASS-4, Canada)

Blanks (n = 19) NASS-4 (n = 22) Certified values Mean recovery

Cd (pM) Cu (nM)

Ni (nM)

Pb (pM)

2F1 182 F 21 142 F 27 128%

0.30 F 0.14 3.75 F 0.7 3.88 F 0.15 96%

6F4 24.5 F 4.7 62.7 F 24 39%

0.13 F 0.18 3.58 F 0.3 3.59 F 0.17 100%

Table 2 Hydrographic data, nutrients, and dissolved Cd, Cu, Ni and Pb for June 1995 cruise (CD94) (A) At vertical stations along the slope transects Depth (m)

Temperature (jC)

Salinity (psu)

Sigma-t

O2 (uM)

NO3a (AM)

PO4a (AM)

Sia (AM)

Cdb (pM)

Ni (nM)

Cu (nM)

Pbb (pM)

OM5 (49j30N, 11j00W)

1 10 20 40 100 160 182 1 20 50 100 200 350 500 651 801 917 1124 1160 1 13 56 100 201 350 500 556 700 850 1000 1250 1600 1848 2202 2500 2995

13.45 13.40 13.37 12.81 11.29 11.1 11.06 13.39 13.38 12.36 11.02 10.66 10.42 10.25 9.74 9.54 8.96 7.76 7.45 14.03 14.03 12.28 11.27 10.75 10.43 10.05 9.86 9.20 8.86 8.02 5.80 4.08 3.49 3.38 3.10 2.78

35.53 35.53 35.53 35.54 35.53 35.53 35.52 35.50 35.50 35.50 35.51 35.49 35.47 35.46 35.47 35.50 35.54 35.46 35.42 35.55 35.55 35.55 35.54 35.50 35.46 35.43 35.42 35.41 35.50 35.45 35.17 34.97 34.91 34.96 34.96 34.94

26.713 26.72 26.73 26.85 27.14 27.17 27.18 26.70 26.71 26.91 27.17 27.22 27.26 27.28 27.37 27.44 27.56 27.69 27.70 26.61 26.61 26.97 27.15 27.21 27.25 27.29 27.32 27.42 27.55 27.64 27.73 27.77 27.78 27.84 27.87 27.88

267.6 270.4 273.1 264.9 259.5 261.1 262.2 293.5 293.3 285.5 276.3 275.2 275.2 271.6 230.9 219.1 212.2 223.3 226.6 288.2 288.2 265.7 262.1 267.3 264.3 251.6 240.3 217.7 210.8 217.9 247.6 280.1 294.2 277.7 269.7 258.1

NA 1.94 2.02 4.90 10.16 10.50 10.30 NA 2.56 2.85 7.71 10.33 11.44 11.25 16.14 17.21 18.07 18.79 18.69 NA 0.56 4.75 10.15 11.03 11.81 14.43 14.92 17.43 17.75 18.28 18.31 17.54 NA NA NA 18.51

NA 0.11 0.16 0.23 0.41 0.47 0.41 NA 0.07 0.08 0.33 0.42 0.46 0.46 0.80 0.85 0.90 1.01 0.92 NA 0.03 0.22 0.40 0.48 0.54 0.65 0.70 0.87 0.93 0.92 0.90 0.88 NA NA NA 0.95

NA 1.02 1.02 0.92 2.97 3.52 3.45 NA 0.64 0.62 1.49 3.65 4.56 4.95 6.60 7.50 8.68 10.47 10.43 NA 0.16 0.87 3.20 4.01 4.79 5.92 7.21 8.73 9.58 10.45 11.52 10.91 NA NA NA 18.93

31 31 30 81 142 142 145 41 56 61 128 155 176 190 230 232 275 293 284 21 15 49 147 167 183 221 226 268 229 258 288 292 300 313 336 367

3.20 3.12 3.12 3.50 3.59 3.50 3.57 3.66 3.40 3.47 3.50 3.71 3.55 3.74 4.13 4.29 3.99 NA 4.26 2.48 3.03 3.22 3.77c 3.33 3.36 3.34 3.59 3.75 3.63 3.64 3.91 3.79 3.72 4.04 4.39 4.83

1.03 1.19 1.02 1.15 1.15 1.25 1.32 1.10 1.11 1.25 1.23 1.35 1.26 1.18 1.62 1.20 1.17 1.42 1.41 0.97 1.72 1.42 1.33 1.41 1.39 1.29 1.41 1.36 1.40 1.25 1.40 1.40 1.51 1.78 1.97 2.37

144 109 191 203 202 268c 331c NA 174 187 213 215 210 172 201 213 156 NA 194 NA 148 90 133 132 191 200 144 193 180 178 NA 155 68 64 77 84

OM6 (49j13N, 12j36W)

OM7 (49j00N, 13j12W)

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Station

(continued on next page) 7

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Table 2 (continued ) (A) At vertical stations along the slope transects Station

Depth (m)

OM7 (49j00N, 13j12W)

OM9 (49j05N, 11j05W)

OM12 (48j35N, 13j20W)

3580 3631 1 20 50 100 250 400 567 600 717 770 853 972 1250 1400 1600 1800 2000 2500 3000 3300 3650 4000 4491 10 40 50 84 216 20 50 100 200

Salinity (psu) 2.58 2.55 13.85 13.85 12.85 11.16 10.68 10.35 9.61 9.59 9.47 9.32 9.00 8.04 5.91 4.61 3.81 3.56 3.46 3.05 2.74 2.63 2.55 2.51 2.52 13.42 13.11 12.96 11.78 10.93 14.08 12.52 11.16 10.76

Sigma-t 34.92 34.92 35.56 35.56 35.51 35.52 35.49 35.45 35.36 35.39 35.48 35.50 35.53 35.45 35.19 35.01 34.92 34.92 34.93 34.95 34.94 34.93 34.91 34.88 34.90 35.53 35.53 35.53 35.54 35.52 35.59 35.50 35.52 35.50

O2 (uM)

NO3a (AM) 27.89 27.89 26.65 26.65 26.82 27.16 27.22 27.26 27.31 27.34 27.43 27.47 27.55 27.64 27.73 27.74 27.76 27.78 27.80 27.86 27.88 27.88 27.88 27.86 27.88 26.72 26.78 26.81 27.05 27.20 26.62 26.88 27.16 27.21

PO4a (AM) 252.4 251.5 289.9 290.0 284.2 274.1 270.7 265.8 248.2 239.1 218.6 214.2 211.1 217.6 246.0 267.8 287.0 290.5 284.8 268.4 259.7 255.7 252.5 250.4 249.1 296.5 288.1 286.3 277.7 266.2 281.9 272.5 263.1 262.4

21.20 21.98 NA 0.71 1.77 9.21 10.84 11.52 14.85 14.88 16.61 16.96 17.57 17.77 17.76 18.11 17.78 17.66 NA 19.49 20.93 21.46 22.06 22.20 22.12 2.39 3.18 4.42 7.96 11.25 0.68 3.68 10.06 10.52

Sia (AM) 1.17 1.10 NA 0.03 0.16 0.42 0.47 0.57 0.78 0.77 0.88 0.86 0.90 0.95 1.03 1.02 0.96 0.93 NA 1.12 1.20 1.20 1.29 1.23 1.16 0.10 0.15 0.20 0.29 0.44 0.09 0.23 0.49 0.50

Cdb (pM) 34.24 41.80 NA 0.48 0.55 2.83 4.19 4.77 7.28 7.40 9.15 9.60 10.39 11.05 12.06 12.82 13.21 13.62 NA 27.51 34.02 36.32 38.86 39.65 46.82 1.08 1.11 1.27 1.87 3.62 0.88 0.05 0.50 3.26

Ni (nM) 395 405 72c 55c 29 142 164 171 224 239 281 281 265 274 298 296 296 292 319 333 363 377 325 421 378 46 59 79 115 160 15 45 147 157

Cu (nM) 5.07 4.81 2.73 2.57 2.62 3.06 3.09 3.17 3.42 3.40 3.79 3.63 3.67 3.48 3.95 3.86 3.97 3.72 NA 4.36 4.85 4.94 NA NA 4.50 2.89 2.93 2.90 2.87 3.10 2.18 2.37 2.78 2.95

2.75 2.75 1.15 1.06 1.27 1.34 1.24 1.18 1.22 1.27 1.24 1.30 1.32 1.32 1.39 1.39 1.37 1.42 1.44 1.68 2.17 2.29 1.72 2.60 3.54 1.43 1.10 1.15 1.54 1.04 0.97 1.00 1.21 1.13

Pbb (pM) 119 67 165 101 119 161 203 177 175 188 195 209 215 170 156 161 158 111 NA 70 73 97 NA NA NA 153 156 133 193 204 145 166 180 176

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OM8 (48j50N, 13j40W)

Temperature (jC)

10.55 10.19 10.16 9.70 9.82 9.39 8.70 7.49 5.59 4.73 4.26 3.75 3.52 3.10 2.79 2.62 2.53 2.51 2.50 15.01 11.77 11.57 11.50 11.35 10.88 10.64 10.35 10.30 10.08 10.04 9.85 9.65 8.17 7.10 5.77 4.80 4.73

35.48 35.44 35.45 35.38 35.45 35.52 35.54 35.40 35.14 35.04 34.99 34.94 34.95 34.96 34.95 34.93 34.91 34.91 34.91 35.64 35.56 35.56 35.56 35.56 35.56 35.55 35.56 35.56 35.58 35.62 35.69 35.71 35.58 35.43 35.25 35.13 35.13

27.24 27.28 27.29 27.31 27.35 27.48 27.61 27.68 27.73 27.75 27.77 27.78 27.81 27.86 27.88 27.89 27.89 27.89 27.89 26.46 27.07 27.11 27.13 27.15 27.24 27.28 27.34 27.35 27.41 27.45 27.54 27.59 27.73 27.76 27.80 27.82 27.83

260.9 271.4 260.5 256.0 231.3 213.6 211.9 223.6 250.5 265.2 275.4 284.6 284.1 271.8 259.3 254.9 250.8 250.0 249.8 282.3 268.1 268.2 269.1 272.5 248.9 241.6 232.0 229.6 217.6 212.6 208.1 207.8 219.4 231.8 248.6 258.3 258.8

10.97 11.45 14.12 15.22 16.24 17.62 18.01 18.34 18.16 17.98 17.81 17.67 17.87 19.57 21.00 21.69 22.22 22.52 22.38 0.03 0.13 1.54 7.17 8.70 12.52 13.13 14.34 14.55 16.36 17.26 NA 17.76 18.58 18.90 19.25 19.58 NA

0.54 0.53 0.71 0.71 0.87 1.10 1.30 0.92 1.01 1.05 1.04 1.08 1.02 1.18 1.22 1.32 1.36 1.37 1.22 0.07 0.09 0.18 0.41 0.52 0.74 0.71 0.65 0.69 0.84 0.89 NA 1.05 0.80 0.90 0.93 1.02 NA

3.89 4.30 6.42 7.48 8.33 9.91 10.86 11.86 12.18 12.34 13.02 13.70 16.23 27.76 35.29 41.38 45.15 46.62 46.29 0.31 0.23 0.56 2.05 2.81 4.77 5.17 6.13 6.40 7.79 8.76 NA 9.88 11.69 12.93 14.90 18.57 NA

173 192 213 233 247 266 281 284 328c 294 290 290 287 411c 346 341 397 397 NA 30 39 51 131 124 167 167 185 227 236 253 271 249 263 288 295 289 308

3.20 3.10 3.23 3.33 3.40 3.55 3.62 3.78 3.73 3.69 3.91 3.71 3.78 4.19 4.52 4.47 4.98 4.99 5.23 2.54 2.45 2.84 3.01 3.00 3.21 3.09 3.28 3.61 3.51 3.61 NA 3.66 3.81 4.96c 4.56c 3.94 4.13

1.11 1.06 1.28 1.17 1.26 1.16 1.32 1.31 1.35 1.36 1.40 1.37 1.48 1.76 2.00 1.83 2.40 2.62 2.62 1.01 0.96 1.04 1.21 1.11 1.13 1.06 1.03 1.00 1.27 1.15 1.19 1.33 1.34 1.56 1.45 0.96 1.12

207 254c 193 184 194 207 161 174 173 154 144 146 66 60 49 70 96 36 46 143 140 295c NA 105 115 111 106 312c 100 94 138 53c 25c 116 82 59 57

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Belgica (47j25N, 07j16W)

300 500 543 600 657 801 952 1102 1300 1450 1600 1800 2000 2500 3000 3500 4001 4300 4347 24 82 96 121 200 427 500 591 620 720 824 953 1000 1200 1300 1600 1875 1917

(continued on next page)

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(B) In surface water along the Celtic shelf Sample

Latitude (jN)

Longitude (jW)

Temperature (jC)

Salinity

Cdb (pM)

Ni (nM)

Cu (nM)

Pbb(pM)

1 2 3 4 5 6

48j 49j 49j 50j 50j 51j

09j 08j 07j 06j 05j 04j

13.90 14.58 14.51 14.42 14.10 13.20

35.58 35.50 35.24 35.16 35.17 32.94

65 53 90 116 213 368

1.69 2.80 2.69 2.70 2.54 9.08

4.29 2.95 3.14 3.56 3.79 6.64

191 158 NA 401 431 NA

40.50 16.04 45.16 25.11 53.29 17.55

19.06 20.23 30.63 37.31 29.88 12.00

NA: data not available. a Mike Orren and Tom Treacy (University College Galway) except for the station Belgica: Thomas Raabe and Uwe Brockman (University of Hamburg). b Corrected values from NASS-4 recoveries (see Methodology). c Questionable (not in the concentration range of data at equivalent depth).

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

Table 2 (continued)

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(APDC/DDDC, 1%) into an organic phase (1,1,2trichloro-1,2,2-trifluoroethane) followed by back extraction into suprapure HNO3. Blanks (MilliQ water), seawater reference material (NASS-4, National Research Council of Canada) and a ‘‘homemade’’ standard solution were run following the protocol described above to assess the quality of the extraction and analysis. All the above sample treatments were conducted using ultraclean techniques in order to minimize sample contamination. Blank values were in a reasonable range (Table 1), representing less than 1% of the average concentration in the marine samples (summer cruise) for Cd and less than 10% for Cu, Ni and Pb. The concentrations obtained on 22 aliquots of the NASS-4 (Table 1) were within the range of the certified values for Cu and Ni; for Cd, the recoveries were systematically higher by a factor of 1.3; for Pb, the recoveries were systematically lower by a factor of 0.4. The same recoveries were found for the ‘‘homemade’’ standard solution. The reproducibility of the analysis for these latter two metals was satisfactory being 11% and 19% RSD, respectively. Additional analyses were performed in order to validate the data for Cd and Pb (Cotte´, 1997): eight aliquots of the seawater reference material and 24 seawater samples (station OM8) were reanalyzed (after extraction) by ICP/MS (Hewlett Packard 4500) at the Institute for Environment and Sustainability (Ispra, Italy). For Cd, the recovery obtained by the ICP/MS analysis was 99 F 7%, indicating that the f 20% overestimation shown by the AAS analysis was inherent to the AAS analysis (probably due to some sort of interference or unaccounted blank). Reanalysis of the 24 seawater samples by ICP/MS led to the same conclusion: AAS values were higher than those measured by ICP/MS, but were identical if corrected by the equivalent NASS-4 recovery. Opposite results were found for Pb: seawater and NASS-4 samples showed higher concentrations when measured by ICP/MS, but a recovery of 100% was never reached (80 – 85%). Accurate measurements of Pb in the open ocean remain difficult, as is well illustrated by the certified NASS-4 value that has a coefficient of variation of F 38%. As all samples could not be reanalyzed by ICP/MS, the concentrations of Cd and Pb obtained by GFAAS were divided by the NASS-4 average recovery (1.28 and 0.39, respectively).

11

Winter (CD84) and summer (CD94) data are reported in Cotte´ (1997) and in the OMEX database that can be obtained from the British Oceanographic Data Centre (http://www.bodc.ac.uk). The complete summer data set is given in Table 2 since these data are mainly discussed here. Nutrients were measured by Tom Treacy and Michael Orren at the University of Galway and Thomas Raabe and Uwe Brockmann at the University of Hamburg.

4. Results 4.1. Region of the shelf break and the continental slope Results presented in this section are those of the vertical stations located across the Celtic slope (transects OM5 –OM8, OM9– OM12 and station Belgica, Fig. 1) during the summer cruise, which corresponds to oligotrophic conditions. Changes in the surface waters during the winter cruise are discussed later. 4.1.1. Concentrations and vertical profiles 4.1.1.1. Oxygen and nutrients. In the summer mixed layer, dissolved oxygen decreases by 5– 10% from the surface to 100-m depth, while surface or subsurface chlorophyll maxima reach 0.3 –1.0 Ag l  1 (see, e.g., the station OM12, Fig. 2B). The overall oxygen profile is characterized by a small maximum near 500 m and a distinct minimum near 1000 m. Below, the NEADW is well oxygenated. According to several studies (Tsuchiya et al., 1992; Measures et al., 1995; Cotte´, 1997), the oxygen minimum is not linked to the presence of the MIW despite its appearance in our profiles. It is partly a relict from the extreme oxygen minimum of the productive northwest African upwelling waters, and its depth does not vary with latitude along the East Atlantic Basin, whereas the depth of the salinity maximum does. Nitrate (Fig. 2B) and phosphate (not shown) show the usual uptake-regeneration profiles, with discontinuities associated with the hydrographic structure. Below the nutrient-depleted surface waters, nitrate increases in the NEACW. A sharp gradient occurs near 500 m into the oxygen minimum layer. The depth of the oxygen minimum (900 – 1000 m) corresponds to a

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

12

nitrate maximum, and the core of the LSW (1600 – 2000 m) to a slight nitrate minimum. Concentrations are the highest in bottom waters—up to 23 AM for nitrate, 1.4 AM for phosphate and 46 AM for silicate (Table 2)—reflecting the presence of silicate-rich Antarctic bottom waters (Yeats et al., 1995; Saager et al., 1997). These profiles are consistent with the rapid drop of the chlorophyll a content below 100-m depth (Fig. 2B) and the gradual decrease of the light attenuation (and the total suspended matter content). 4.1.1.2. Trace metals. Cd: Summer Cd concentrations range from 65 F 43 pM in the 0 –100 m surface waters to 310 F 26 and 382 F 31 pM in deep (NEADW) and bottom (LBW) waters, respectively (Table 3). Our concentrations in the slope area are higher (by 10 – 30%) than the more oceanic stations of Saager et al. (1997), and this is especially so for the deeper waters (Table 3). Our values are also higher than those published by Le Gall et al. (1999) in the same area, but are in good agreement to the nearby stations of Danielsson et al. (1985, stations 88 and 89). The Cd

profiles perfectly mimic the nitrate and phosphate (not shown) distributions over the whole water column, with, in particular, a surface depletion down to 15 pM and a rapid concentration increase in the seasonal thermocline and below in the permanent thermocline (Figs. 2B and 4). This confirms the nutrient-like Cd distributions already described in the study area (Danielsson et al., 1985; Cossa et al., 1992; Le Gall et al., 1999) and, more generally, in the East Atlantic Ocean (De Barr et al., 1994; Landing et al., 1995; Yeats et al., 1995; Saager et al., 1997). Ni: Ni concentrations range from 3.0 F 0.4 nM in the surface waters to 4.9 F 0.3 nM in the LDW waters (Table 3). Concentrations in surface waters appear to be lower at the off slope stations than at the shelf stations (Fig. 4), and concentrations are even lower in the NE Atlantic open ocean (2.4 F 0.8 nM, Table 3). Concentrations are otherwise in good agreement with Celtic or Atlantic published values (Le Gall et al., 1999; Saager et al., 1997). Vertical distributions show a nutrient-like behavior, but the amplitude of the concentration variations over the water column is much smaller, and the resemblance with nitrate is less obvious than for Cd

Table 3 Average concentration ( F r) of nutrients and metals in the various water masses of the Celtic slope area in summer (winter values for the surface layer only). Comparison with concentrations in equivalent water masses of the NE Atlantic Ocean (from previous work)

Depth range n Temperature (jC) Salinity NO3 (AM) PO4 (AM) Si (AM) Cd (pM) Ni (nM) Cu (nM) Pb (pM)

Area

Source of data

Surface (summer)

Surface (winter)

NEACW

MIW

NEADW

LDW

Celtic area

this study

Celtic area

this study

0 – 100 27 12.78 F 1.10

0 – 100 14 11.21 F 0.21

101 – 500 18 10.70 F 0.39

501 – 1250 30 8.99 F 1.19

1251 – 3000 22 4.01 F 1.11

3001 – 4500 10 2.55 F 0.05

Celtic area Celtic area Celtic area Celtic area Celtic area NE Atlantic Celtic area NE Atlantic Celtic area NE Atlantic Celtic area NE Atlantic

this this this this this (1) this (1) this (1) this (2) (3)

35.54 F 0.03 4.1 F 3.4 0.20 F 0.13 1.1 F 0.9 65 F 43 58 F 50 3.0 F 0.4 2.4 F 0.8 1.18 F 0.19 1.04 F 0.31 162 F 43 145 F 8 203 F 11

35.55 F 0.03 8.0 F 0.5 0.66 F 0.38 3.3 F 2.8 104 F 18

35.50 F 0.04 11.1 F 1.6 0.52 F 0.10 4.1 F 0.9 166 F 23 118 F 64 3.3 F 0.2 2.6 F 0.6 1.21 F 0.11 1.39 F 0.48 189 F 58 130 F 12 166 F 6

35.48 F 0.12 16.9 F 1.4 0.88 F 0.14 9.1 F 1.8 255 F 28 222 F 28 3.7 F 0.3 3.5 F 0.8 1.28 F 0.12 1.38 F 0.34 163 F 49 92 136 F 16

35.01 F 0.13 18.7 F 1.1 1.03 F 0.10 18.1 F 7.9 310 F 26 246 F 34 4.1 F 0.4 3.9 F 0.9 1.56 F 0.33 1.73 F 0.4 101 F 43 37.5 F 5 97 F 52

34.91 F 0.01 22.0 F 0.4 1.24 F 0.09 41.7 F 4.5 382 F 31 319 F 29 4.9 F 0.3 4.9 F 0.6 2.51 F 0.51 2.77 F 0.37 76 F 30 24 32 F 11

Ocean Ocean Ocean Ocean

study study study study study study study study

3.5 F 0.7 1.52 F 0.39 163 F 52

n: Number of samples used to calculate the averages. (1) Saager et al. (1997). Selected data: JGOFS-1: station 36/37 (32j56N, 20jW) and station 21/22 (46j59N, 20jW); JGOFS-3: station 20 (32j59N, 20j50W); and JGOFS-4: station 1 (58j30N, 20j30W), station 3 (47j40N, 20j50W) and station 12 (40j36, 20jW). (2) Bru¨gmann et al. (1985). Selected data: station 85 (59jN, 20jW), station 87 (48j30 N, 20jW) and station 88 (48j30, 15jW). (3) Lambert et al. (1991). Selected data: Fluxatlante station 5 (33jN, 20jW) and Fluxatlante station 6 (40j08N, 21j08W).

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26 Fig. 4. Vertical profiles of metals in summer (CD94) at stations along the Celtic slope (shelf and upper slope, lower slope and abyssal plain). Note that the minimum of the (X) axis scale for Ni is not at 0. The selected scale allows a better comparison with profiles of nitrate and Cd.

13

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M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

(note: a few questionable high values were measured in samples from the Belgica station). Cu: Cu concentrations vary from 1.2 F 0.2 nM in the surface waters to 2.5 F 0.5 nM in the deepest waters. Concentrations increase weakly over the upper 2000-m depth interval, and then significantly with depths greater than 2000 m (Fig. 4). Minor remineralization in surface waters and additional benthic input have been invoked by Saager et al. (1997) to explain this trend. Similar concentrations and profiles have been observed in our study area and nearby (Le Gall et al., 1999; Danielsson et al., 1985). Pb: Pb concentrations range from 40 to 250 pM. Our data (Table 3) seem to indicate no significant decrease compared to concentrations recorded 10 –15 years before in the NE Atlantic (Bru¨gmann et al., 1985; Lambert et al., 1991), contrary to the reduction

observed in NW Atlantic waters from leaded gasoline regulation (Wu and Boyle, 1997). This observation should be considered cautiously based on the uncertainties in the absolute concentrations of our Pb data (see Methodology). The profiles reveal reproducible features that are identified from the summer stations (Figs. 2B and 4) as follows. (1) Peaks are observed near the surface (0– 15 m, stations OM5, OM6, OM7 and OM8). These surface maxima are in accordance with the 0 – 20-m measurements reported around the British Isles by Bru¨gmann et al. (1985). Similar features, attributed to aeolian inputs, were observed in the central North Pacific by Schaule and Patterson (1981), in the western North Atlantic (Sargasso Sea) by Schaule and Patterson (1983) and Boyle et al. (1986) and in the eastern North Atlantic (35 –48jN) by Lambert et al. (1991).

Fig. 5. Comparison between the summer (black points) and the winter (white points) profiles at stations without upwelling (OM7 and 8) in the upper 400 m.

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(2) Just below, the Pb concentrations are lower and then increase by a factor of about 1.5 – 2 toward the permanent thermocline. This summer trend is observed in many previous studies (Schaule and Patterson, 1981, 1983; Bru¨gmann et al., 1985; Lambert et al., 1991) and will be discussed later. (3) Below the thermocline, the profiles show an irregular decrease with apparent relationships with the hydrographic structure. At 1800 – 2000-m depth, there is a sharp concentration decrease to a low and relatively uniform deepwater level. The same tendency was observed in the studies mentioned above and is thought to reflect scavenging removal.

15

As shown in Fig. 4, the summer vertical distributions of dissolved metals in the slope region are, on the whole, characteristic of the open ocean environment. The main difference with the NE Atlantic open ocean concerns the concentrations in the surface waters: they are slightly enriched at the shelf edge in Cd and Ni (enrichment factor, i.e., ratio Celtic concentration/NE Atlantic concentration (Table 3) of 1.2 and 1.3, respectively), but not significantly in Cu and Pb (enrichment factor of 1.1 and 0.9, respectively). 4.1.2. Seasonal variability in the surface waters Below the permanent thermocline, the hydrography and the elemental vertical profiles were indistinguish-

Fig. 6. Surface (0 – 20 m) concentrations along transects from the Celtic Sea shelf edge to the coast including the data from Muller et al. (1994) and Harper (1991).

16

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

able during the summer and winter cruises (Cotte´, 1997), in contrast to the upper water layers (0 –400 m), which were affected by seasonal and local variations. In summer, the upper waters were well stratified with high vertical concentration gradients of nutrients and trace metals (Cd, Ni and possibly Pb), while these species and temperature were completely homogeneous over the upper 300– 400-m depth in winter (Fig. 5). This homogeneity is a consequence of the winter vertical mixing relative to the general deep convection of the North Atlantic surface waters. In summer, the surface waters (0 –100 m) are impoverished in nutrient and metals compared to the winter as a result of stratification and biological uptake: average concentrations between winter and summer decreased by 36% for nitrate, 41% for Cd, 14% for Ni and 22% for Cu (Table 3). Relatively similar removal rates between winter and summer were found by Kremling and Pohl (1989) in the surface waters of the NE Atlantic (56 F 16% for Cd, 4% for Ni and 12% for Cu). In contrast, the average surface concentration of Pb remained unchanged with season. 4.2. Continental shelf waters We now consider the results obtained in the summer transect from the shelf break to the Bristol Channel (Fig. 1), for which only surface samples were taken. Our data for this shelf transect (Fig. 6) are consistent with measurements in these shelf waters by Kremling and Pohl (1989). There is also a good agreement with measurements reported along another transect extending further into the English Channel by Muller et al. (1994) (Fig. 6). The only marked difference is that our data in the Bristol Channel, confirmed by samples analyzed by Harper (1991) and Achterberg et al. (1999), are much higher. There is a welldefined concentration increase from the Celtic shelf edge toward the less saline waters of the English Channel and the same trend is observed and amplified near the marine end-member of the Severn estuary in the Bristol Channel. Dissolved trace metal concentrations at the entrance of the Bristol Channel at 32.9 salinity reached 0.37 nM Cd, 6.6 nM Cu, 9.1 nM Ni and > 0.4 nM Pb (Table 2B), which are three to six times higher than the average slope surface values. Data of Harper (1991) and Achterberg et al. (1999)

show that concentrations continue to increase into the estuarine zone to approximately 2.2 nM Cd, >30 nM Cu, 15 nM Ni and 0.6 nM Pb at 29 salinity. At a larger scale, metal enrichments in coastal waters close to river mouths around England have been observed by Kremling and Hydes (1988) and Achterberg et al. (1999).

5. Discussion Two major results emerge from the previous section: from a regional perspective, there are clear concentration gradients for Cd, Ni, Cu and Pb in the surface waters from the slope region toward the continent (by a factor of three to six times), but weak gradients from the NE Atlantic open ocean waters to the slope (enrichment factor at the slope of 0.9– 1.3). The vertical water column distribution of these trace elements over the continental slope and the shelf edge does not differ significantly from the neighboring open ocean. From a seasonal perspective, the upper waters (0 –400 m) near the shelf edge exhibit a strong seasonal variability (surface summer drawdown of 36% for nitrate, 41% for Cd, 14% for Ni and 22% for Cu), but seasonal changes were not investigated over the shelf itself. In this section, we examine (1) the processes affecting trace metals above the permanent thermocline (plankton uptake, upwelling/winter mixing, atmospheric and fluvial input) and (2) the processes leading to the export and transport of trace metals to/within the deep waters. 5.1. Processes affecting trace metals above the permanent thermocline near the shelf edge 5.1.1. In situ uptake The dissolved Cd profiles confirm the well-known nutrient-like behavior of this element. In contrast, the rather invariant Cu profiles provide no evidence of the involvement of this metal in the biological uptake/ regeneration cycle. The surface summer depletion of Ni is much lower than for Cd (Fig. 4) and indicates a smaller uptake by plankton. The case of our Pb profiles deserves special attention. In summer, the dissolved Pb concentrations decrease with depth in the 10 – 200-m layer (surface

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

maxima excluded), which indicates a transfer of dissolved Pb to the particulate phase, either by biological uptake or scavenging. Moreover, this tendency is accompanied by a significant correlation between Pb and phosphate (R2>0.89, Fig. 7) or Pb and nitrate (R2>0.87, not shown), although this is based on only a few samples at each station (between 10 and 200 m). Similar summer Pb decreases have been previously observed (Schaule and Patterson, 1981, 1983; Bru¨gmann et al., 1985; Lambert et al., 1991), but a lack of nutrient data or too small a number of samples in the layer of interest prohibited the comparison with nutrient concentrations. However, a relationship between Pb and phosphate can be observed in some of the data reported in summer by Boyle et al. (1986) near Bermuda (station S) between the mixed layer and the main thermocline (high surface values excluded, Fig. 7). Our summer data suggest a removal of dissolved Pb to organic particulate matter (by either active uptake or passive sorption) above the seasonal thermocline, and a partial release of Pb to solution in deeper layers (by organic matter remineralization) above the permanent thermocline. The absence of significant concentration gradients in winter profiles (50 – 400 m, Fig. 5) supports this interpretation. Once released in solution, dissolved Pb is removed from the water column by adsorption onto settling inorganic particles, leading to the low, uniform concentration levels observed in the deep waters. This explanation is in agreement with scavenging models of metals (see, e.g., Moran and Moore, 1992) where scavenging is described as a two-

17

step process: a reversible association between metals and biogenic particles in surface waters followed by an irreversible adsorption and removal of released metals onto settling particles at depth. 5.1.2. Modifications due to upwelling In the upwelling zone, an enrichment in the surface waters was expected for the nutrients and the nutrientlike trace metals, namely Cd and Ni, as a result of deepwater outcropping. The in situ measurements confirm the presence of the upwelling detected by satellite imagery (Fig. 3): the stations where upwelling occurred (OM5, 6 and 9) are readily identified by their lower temperature (Fig. 8A) over the shelf break (Fig. 1). Fig. 8B shows that nutrient and metal profiles are only slightly affected by this process: for nitrate and Cd, there is an enrichment by a factor of about 2– 3 in the cold surface layer above the seasonal thermocline and perhaps some impoverishment between 100 and 200 m, which is poorly characterized by the limited number of samples. For Ni, as compared to the stations without upwelling where concentrations increase with depth, the profiles at the upwelling stations are also enriched (by about 50%, but much less than for nitrate and Cd), with some homogenization of the waters above the permanent thermocline. When the surface maxima are not considered, the Pb profiles show the same characteristics as the Ni profiles, with a nearly two-fold increase with depth at the stations without upwelling and some near-surface enrichment where upwelling occurs; at the shallow OM5 station, there is

Fig. 7. Relationships in summer between Pb and phosphate in the surface layer; this work and Boyle et al. (1986, Sargasso Sea).

18

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

Fig. 8. Impact of upwelling: (A) TS diagram of the surface samples (0 – 20 m) along the Celtic slope (transects OM5 – 8, OM9 – 12 and underway data) identifying the stations with upwelling. (B) Comparison in summer between stations (0 – 400 m) with (OM5, 6, 9, white points) and without upwelling (OM7, 8, 12, black points).

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

a regular increase up to 330 pM at 180-m depth that was not observed elsewhere and remains questionable (Fig. 8B). Thus, for these elements, the upwelling zone is characterized by some degree of enrichment of the surface waters and a certain smoothing of the seasonal stratification. The limited number of samples between 100- and 200-m depth does not allow us to verify the likely occurrence of lower concentrations in this depth range, which would indicate a redistribution of these elements by upwelling in the 0 –400-m water layer, with no obvious need to invoke a deeper origin. Since our measurements correspond to an algal bloom situation, some enrichment of these elements may have been masked by subsequent biological removal. In contrast, there is no discernible indication of Cu enrichment in the upwelling waters. 5.1.3. Modifications due to winter convection Modifications due to winter convection are illustrated in Fig. 5, with summer and winter profiles at stations where upwelling was not observed (stations OM7 and OM8). In summer, the thin surface mixed layer is separated from the deeper waters by the seasonal thermocline where temperature decreases by about 3 jC down to 100-m depth. In winter, this thermocline is eroded by vertical mixing. The nitrate and Cd profiles are more homogenous in winter, in particular, in the upper 300 m, than in summer (Fig. 5); thus, there is a large enrichment of the surface waters (it is likely that the 50-m data may be extrapolated to the surface) relative to summer. For nitrate and Cd, the winter resupply is obviously much more important than the upwelling effect. For Ni, a limited enrichment in the 0– 50-m layer (relative to summer) seems to occur, and its magnitude is equivalent to the enrichment by upwelling. For Pb, a comparable, limited surface enrichment is expected since we previously suggested the nutrient-like behavior of this element. This trend, which seems to be visible from our profiles (Fig. 5, surface maxima excluded), needs to be confirmed by more detailed sampling in the depth range of interest. For Cu, however, it is unlikely that winter convective mixing produces any significant change of the surface concentrations. Our results would thus suggest a simple redistribution in winter of the most bioreactive elements within the water masses above the permanent thermocline.

19

This is validated by the nitrate – salinity diagram (Fig. 9) obtained for the 0– 350-m waters from summer and winter properties. The winter nitrate – salinity properties are gathered into the triangle formed by the summer end-member waters properties (surface, 100 and 350 m, Fig. 9) showing that a simple conservative mixing of water masses present in summer above the permanent thermocline may produce the observed winter water mass. Application of this test of conservative mixing to Cd and Ni gives the same result (not shown). 5.1.4. Comparison of inventories The preceding discussion indicates that the seasonal effects seem essentially to result in a redistribution of the nutrients and some of the trace metals above the permanent thermocline without large exchanges with the deeper waters. We tried to test this interpretation by considering elemental inventories. Inventories were calculated by summing estimates of depth-integrated concentrations over depth intervals between 0- and 400-m depth (i.e., S[mean concentration Dz
dz], where the depth intervals, Dz, used are 0– 5, 5 –30, 30– 75, 75– 150, 150– 300 and 300– 400 m). Inventory estimates based only on the dissolved phase are reported in Table 4. The amount of trace metals bound to the particulate phase would not modify significantly these estimates. Indeed, a rough approximation of inventories due to metal particulate fraction in the 0 – 400-m layer in summer gives 5 Amol m  2 Cd and Cu and 15 Amol m  2 Ni, values that are negligible relative to estimates based on dissolved phase (Table

Fig. 9. Nitrate – salinity property plot for the upper 400 m (station OM7) showing that the winter properties are the result of the conservative mixing of the summer water masses.

20

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

Table 4 Metals inventories in the upper layer (0 – 400 m) in winter and in summer, and comparison with atmospheric fluxes Inventory (0 – 400 m) [lmol m  2] All Celtic stations Summer (June 1995), n = 39 Winter (January 1994), n = 30

Cd

Ni

Cu

Pb

NO3

50.4 39.8

1286 1015

493 518

74.6 53.1

3795
10 + 3 3287
10 + 3

3.6 – 6.6 3.4

5.4 – 7.7 14.5 – 16.4 3.9

3.6 – 5.8 4.2 – 4.9

1.0 – 1.6%

5.1 – 10.9%

Total dissolved Atm flux [lmol m  2 year  1] Brittany coasta (1) 0.54 0.125 – 0.180 NE Atlantica (2) North Atlantic (3) 0.179

Annual atmospheric deposition as percentage of the inventory = Flux/inventory 0.2 – 1.4% 0.3 – 0.7%

Estimated in Cotte´ (1997); printed in bold are range used in the calculation flux/inventory a. Using (1) coastal deposition (total soluble) from Cabon and Le Bihan (1996) and Cabon (personal communication) measured between 1992 and 1995 at the station F1 (Brittany, France) and (2) NE Atlantic deposition (rain + soluble aerosols) from Lim and Jickels (1990), Church et al. (1990) and Duce et al. (1991). (3) Chester and Murphy (1990), as reported in Kuss and Kremling (1999).

4). The above values are based on concentration of the suspended particulate material (f 320 ug/l in the 0 – 100-m layer and f 50 ug/l in the 100 – 400-m depth range, Fig. 2B) and estimated trace metal content of the particulate fraction of 100 nmol g  1 Cd and Cu and 300 nmol g  1 Ni in the 0 –400-m depth range. Values of 93 F 11 nmol g  1 Cd, 88 F 12 nmol g  1 Cu and 240 F 68 nmol g  1 Ni have been measured in particulate matter collected from a depth of 7 m in July at the NE Atlantic open ocean station L2 (f 48jN, f 20jW) by Kuss and Kremling (1999). The differences between the summer and winter inventories (Table 4) do not exceed 20 –30% for Cd, Ni and Pb and 5% for Cu. The inventories are always lower in winter, except for Cu. We doubt that the winter deficiencies are really significant since the reproducibility of the metals analyses is of equivalent range (F 11% for Cd, F 19% for Ni and Pb and F 8% for Cu, Table 1). Thus, these inventory estimates suggest that the yearly nitrate and metal stocks above the permanent thermocline are rather constant, within, on average, F 15%. The mass balance above the permanent thermocline will reflect inputs by upwelling, atmospheric deposition and rivers and export associated with particulate organic matter; it will also be affected by lateral advective transport and exchanges at the sediment –water interface. Our results suggest that, at the annual scale, a nearly steady state balance is achieved

between these exchanges. This suggests that (1) the elements are mostly redistributed inside the upper waters, as predicted by the test of conservative mixing (Fig. 9) or/and (2) the TM external inputs are similar to the quantity exported to deep water by settling particles. We do not have enough data to establish a complete mass balance near the shelf edge, but the relative importance of the external sources may be evaluated as follows. 5.1.4.1. River inputs. The overall landward enrichment is essentially due to mixing of marine and fresh waters, as described in more detail by Muller et al. (1994), and may also be related to an increased atmospheric deposition. Muller et al. (1994) showed evidence of the intrusion of marine waters onto the large Celtic shelf and despite seasonal variation in the ocean surface waters, the intrusion acts as a diluent of the land-based sources, which induce high metal concentrations in the vicinity of the coast only (Fig. 6). The dilution effect is observed in the whole English Channel (Laslett, 1995). This is in agreement with current data that indicate a surface eastward flow over the shelf (Pingree and Le Cann, 1989; Pingree, 1993). It is thus unlikely that the surface inventories west of Goban Spur will be influenced by river inputs. 5.1.4.2. Atmospheric inputs. Some inputs from the atmosphere are expected in the studied area because of

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

(i) the likely influence of the prevalent North American westerly regime, which may account for 58% of Pb deposited to the European basin (Veron et al., 1994) and (ii) the proximity of the European continent, with metals transported by easterlies. Various estimates of the atmospheric metal deposition are presented in Table 4. They include the total soluble deposition at a coastal station in Brittany, France (Cabon and Le Bihan, 1996; Cabon, personal communication) and the fluxes of dissolved elements by the rain and the dissolution from aerosol particles over the northeast Atlantic Ocean estimated from Pb flux (Duce et al., 1991) and metal/Pb ratios in samples (Lim and Jickells, 1990; Church et al., 1990). Compared to fluxes to the North Atlantic estimated by Chester and Murphy (1990, Table 4, as reported in Kuss and Kremling, 1999), fluxes at the Brittany coast are somewhat higher, while the estimated fluxes to the northeast Atlantic are in good agreement for Cd and Ni, but may be overestimated for Cu. Despite the large uncertainties that characterize these estimates, the importance of the atmospheric deposition of these metals at a yearly scale can be appreciated by comparison with the inventories evaluated previously. The annual deposition of Cd, Ni and Cu by the atmosphere represents less than 1.5% of the estimated metal content in the 0 –400-m waters and 5– 10% in the case of Pb (Table 4). If only the 0 –100-m layer is considered, the latter range increases to about 25%, i.e., a renewal time (time to replace the Pb stock in the 0 – 100-m layer) of about 4 years. This time is of the same order of magnitude as the estimate of 1.1 F 0.1 years evaluated in the 0 –100-m mixed layer of the study area by Lambert et al. (1991).

21

These estimates suggest that the inventories of Cd, Ni and Cu near the Celtic sea shelf edge should not be significantly affected by the atmospheric deposition on a yearly time scale. In contrast, the calculation for Pb implies a more significant atmospheric input. Consistent with this, Pb is the only element for which surface maxima were observed. 5.2. Export and transport to/within the deeper waters 5.2.1. Extrapolations from sediment trap data The above discussion of the shelf edge zone of the Celtic Sea is based on data which are limited in both time and space, and does not allow us to infer the trace metal export by settling organic debris through and below the permanent thermocline. Deployment of sediment traps at 600-m depth (near the top of the MIW) in the OMEX area on the continental slope allowed Antia et al. (1999) to estimate mass and POC exports of 14 – 19 g dry weight (DW) m  2 year  1 and 2.2 g C m  2 year  1, respectively. At 1500-m depth, fluxes are higher (Table 5) with significant lateral transport of materials from the slope (Antia et al., 1999). Unfortunately, there is no record of metal fluxes for the OMEX traps. Some trap metals data have been measured during a 14-month period by Kuss and Kremling (1999) further to the west of our study area (f 47j50N, 19j50W; site L2). At this site, mass and POC fluxes were no more than two times lower than at the OMEX sites at approximately equivalent depths (Table 5). This is in agreement with our expectations since the greater productivity near the shelf edge and the slope effect are likely to produce a larger export of particles to the deep waters. Therefore, we expect the

Table 5 Trap data used in estimating metal exports to deep waters in the OMEX area

OMEX areaa

NE Atlanticb

a b

600 m 1440 m 3220 m 1000 m (trap 42 + 49) 2000 m (trap 50 + 58) 3500 m (trap 43 + 46)

DW (g m  2 year  1)

POC (g m  2 year  1)

14 – 19 28 – 43 43 7 – 26 14 – 24 14 – 34

2.2 2.3 – 3.7 2.3 0.8 – 1.9 1.3 – 1.8 0.6 – 1.1

Cd (10  6 mol m  2 year  1)

Cu (10  6 mol m  2 year  1)

Ni (10  6 mol m  2 year  1)

0.44 *

4.4 *

5.5 *

0.11 **

5.3 **

4.2 **

Antia et al., 1999. Sites OMEX 2 and 3. Kuss and Kremling, 1999. Site L2 (f 47j50N, 19j50W) (*: rough estimates from their Fig. 2; **: from their Table 2).

22

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

metal fluxes in the OMEX zone to be about twice as high as those of Kuss and Kremling (Table 5), all other factors being assumed equal. This leads to the following rough estimates for metal fluxes at 600– 1000-m depth in the OMEX area: 0.8 Amol Cd m  2 year  1, 10 Amol Ni m  2 year  1 and 8 Amol Cu m  2 year  1. Fluxes at 3500-m depth would be lower for Cd (0.2) and Ni (8) and slightly higher for Cu (10). Pb fluxes were not measured by Kuss and Kremling (1999). Kuss and Kremling (1999) observed at the L2 site an amazing similarity between sediment trap fluxes in the deep ocean (2200 and 3500 m) and estimated atmospheric fluxes for Cd, Cu and Ni, supporting the idea that the atmospheric input approximately balances the vertical flux of these elements to the deep ocean. We believe that this may also be true in the OMEX area since trap fluxes at 3500 m are not much different from the estimated atmospheric deposition (Tables 4 and 5). The trace metal fluxes estimated at 600– 1000-m depth represent annually only about 2% of the metal inventories in the 0 – 400-m layer (and thus less than 2% if we consider the inventories for a larger depth of water column). This result reinforces our suggestion that export of metals below the permanent thermocline is small relative to recycling of metals due to seasonal process above the permanent thermocline. 5.2.2. Regeneration and transport in the Mediterranean intermediate waters We have shown that the vertical profiles of Cd and Ni reveal obvious relationships with oxygen and nitrate profiles and with water column stratification (Figs. 2 and 4); for example, the maximum Cd concentration observed at the depth of the MIW (i.e., at the maximum of salinity) coincides with the maximum nitrate concentration and the oxygen minimum. Below the thermocline, changes in metal concentrations are consistent with the occurrence of a different water mass. This is also true for Cu and Pb at f 1800-m depth, for instance. This indicates some control of concentrations by both hydrography (water masses) and biology (regenerative inputs). We attempt below to separate the respective contributions of these processes for the case of the Mediterranean waters, which can be traced easily from the Gibraltar Strait to the Celtic Sea. The degree of mixing of the Mediterranean outflow water (MOW) with the

adjacent water masses (NEACW and NEADW) varies significantly with latitude. Indeed, the contribution of the MOW in the mixture decreases mostly as the result of dilution with the NEACW, from about 17% at a station located in front of the Gibraltar Strait (station 4, 34jN – 13jW, Landing et al., 1995; Measures et al., 1995) to 7% at the Celtic Sea station OM8 (Table 6). These percentages have been calculated from the salinity and potential temperature properties of the three proposed end-members waters making up the MIW using the barycentre method (Cotte´, 1997; Open University Course Team, 1989). A similar MOW contribution has been found by Measures et al. (1995) at the station located in front of Gibraltar (station 4 mentioned above). A station located in the Alboran Sea to the east of the Gibraltar Strait (Station D7, EROS 2000, Yoon, 1994; Yoon et al., 1999) was chosen to represent the MOW end-member. Table 6 Evaluation of conservative mixing in the evolution of Mediterranean Intermediate Water; contributions of different water masses and the mixing ratio CCONS/CMIW Contribution in the mixing

Summer

NEACW MOW NEADW

0.75 0.06 0.19

^ Ratio CCONS/ C NOW CMIW

Summer

Oxygen Nitrate Phosphate Silicate Cd Ni Cu

1.17 0.85 0.89 0.86 0.89 1.01 1.04

185 AM (1) 8 AM (2) 0.4 AM (3) 9 AM (4) 85 pM (5) 4.5 nM (6) 1.8 nM (7)

Winter

OM7 OM8 OM12 Belgica All stations 0.78 0.07 0.15

0.70 0.08 0.22

0.71 0.09 0.21

0.64 0.05 0.32 Winter

OM7 OM8 OM12 Belgica All stations 1.18 0.84 0.87 0.80 0.85 0.98 0.97

1.21 0.84 0.82 0.83 0.97 0.96

1.16 0.79 0.71 0.84 0.74 0.93 0.79a

1.15 0.78 0.91 0.83 1.00 1.02

Selected values representing the MOW end-member: salinity. 38.43, temperature: 12.9 jC. (1) Boyle et al., 1985 (184 F 11 AM). (2) Woodward, 1994. (3) Saager et al., 1997. (4) Boyle et al., 1985 (9.2 F 1.1 AM); Chou and Wollast, 1992 (9 AM). (5) Yoon, 1994 (96 F 21pM); Boyle et al., 1985 (71 F 13 pM). (6) Yoon, 1994 (4.7 F 0.4); Boyle et al., 1985 (4.2 F 0.3 nM). (7) Yoon, 1994 (1.60 F 0.14 nM); Boyle et al., 1985 (2.0 F 0.2 nM). a This Cu profile is more scattered than the other ones.

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

Here, we examine whether the dissolved metals behave conservatively during the transport and the mixing of the Mediterranean outflow water as it travels north. With this aim, the metal concentrations expected in the MIW in the case of a conservative mixing (CCONS) of MOW, NEACW and NEADW have been calculated by: CCONS ¼ ðCMOW
% mixingMOW Þ þ ðCNEACW
% mixingNEACW Þ þ ðCNEADW
% mixingNEADW Þ where % mixingMOW, % mixingNEACW and % mixingNEADW are calculated as described above (Table 6); CNEACW and CNEADW are the metal concentrations measured for each station at the specific depth of the corresponding water mass; and CMOW is the MOW end-member concentration (see Table 6 for the selected values). The calculation has been performed for four individual deep stations (OM7, 8, 12 and Belgica) for the summer cruise and at a ‘‘mean’’ Celtic Sea station (average of all stations) for the winter cruise. We selected nitrate, phosphate, silicate, oxygen, Cd and Ni to examine the mixing of elements associated with the biological pump, and Cu as a counterexample. Pb is not suitable for this test since its behavior below the permanent thermocline is thought to be governed by vertical scavenging processes. Results are expressed by the ratio CCONS/CMIW (Table 6), i.e., the predicted, ‘‘conservative mixing’’ values expressed in percentage of the measured ones. In other words, a ratio CCONS/CMIW different from 1 reflects the modification introduced in the Mediterranean water during its transport from Gibraltar to the Celtic Sea area. Around 80% of the concentrations of nitrate, phosphate, silicate and Cd measured in the MIW are due to the conservative mixing of the Mediterranean water with the Atlantic waters (Table 6); the 20% in excess must be attributed to another source. The ratios CCONS/CMIW close to 1 for Cu and Ni indicate a near conservative behavior during the transport and mixing of the Mediterranean water. At the same time, the oxygen content of the MIW shows a consistent deficit of about 20% compared to the conservative mixing value. This suggests that the Mediterranean water has been subjected to significant oxygen consumption during its transport. Our approach, however,

23

may be an oversimplification of the true mixing process since preformed oxygen-poor waters partly inherited from the northwest African upwelling system (Tsuchiya et al., 1992; Saager et al., 1997) may contribute to the mixing. The ratio of the ‘‘excess’’ concentrations (CMIW  CCONS) calculated at the four deep stations averages DN/DP/DCd = 15.9/1/0.00026. This ratio is roughly consistent with the ‘‘extended Redfield’s ratio’’ defined by Bruland et al. (1991) as N/P/Cd =16/1/0.0004. This indicates that the ‘‘excess’’ of nutrients and Cd in MIW is likely the product of remineralized organic matter exported from surface waters in the study region and/or during the transport of the Mediterranean water to the Celtic shelf edge area. We thus conclude that nutrients and Cd concentrations in the MIW near the shelf edge of the Celtic Sea are controlled primarily by conservative mixing of component waters (despite uncertainties on the true mixing processes), but there are also discernible in situ regeneration processes. Cu and Ni are clearly less involved in the biological cycling. This confirms the dominance of the advection in controlling trace metals levels, as observed at a large scale in the deep waters of the Atlantic Ocean by Yeats et al. (1995) and Saager et al. (1997).

6. Conclusions External, continental sources (i.e., fluvial and atmospheric) of trace metals do weakly influence the dissolved levels of Cd, Cu, Ni and Pb near the shelf edge of the Celtic Sea. (1) Most of the profiles over the slope are quite similar to profiles in the open northeast Atlantic, and the concentrations at the surface are only slightly enriched compared to the nearby open ocean (enrichment factor of 1.2 – 1.3 for Cd and Ni). (2) The fluvial sources of these metals are locally strong at the coast (enrichments of a factor of 3 –6 compared to slope area), but are ‘‘diluted’’ along the large continental shelf by the marine water masses. (3) The atmospheric fluxes of Cd, Cu and Ni at a yearly time scale do not affect significantly the trace metal content in the 0– 400-m waters: they represent annually less than 1.5% of the inventories. In comparison, the calculation for Pb (5– 10%) would imply a more significant atmospheric input, and correspondingly,

24

M.-H. Cotte´-Krief et al. / Marine Chemistry 79 (2002) 1–26

Pb was the only element of those studied for which surface maxima were observed. Regenerated sources (i.e., upwelling and winter mixing) do more significantly influence the distributions of nutrients, Cd, Ni and possibly, Pb in the OMEX area: they return the bioreactive metals to the surface waters, while these metals are removed from the surface by biological uptake in summer and remineralized below. (1) In summer, the upwelling zone, which is geographically limited to the surface waters of the shelf break, is characterized by some degree of enrichment in nitrate and Cd (concentrations
2) and Ni (
1.5) compared to the summer surface oligotrophic level. The enrichment of these metals may have been masked by subsequent biological uptake. (2) After the winter mixing, the surface waters of the entire slope area are also enriched in nitrate and Cd (
5) and Ni (
1.5) compared to the summer surface oligotrophic concentrations. Our results suggest that upwelling and winter mixing lead to the redistribution of nutrients and nutrient-like metals above the permanent thermocline (i.e., in the 0 –400-m water layer) without large export of metals to deeper waters as estimated from a comparison of elemental inventories with vertical fluxes roughly evaluated from sediment trap data. As a matter of fact, the elemental contents in the Mediterranean intermediate water (MIW) are almost totally (Cu and Ni) or mainly (80% for nutrients and Cd) consistent with the conservative mixing of the three ‘‘end-member’’ component waters, which are thought to make up MIW. The ‘‘excess’’ of 20% for the latter elements is likely acquired from remineralization process occurring in the above waters during the transport of the Mediterranean outflow water from Gibraltar to the Celtic Sea shelf edge. The complex vertical profiles of Pb within the upper water column are thought to reflect the combined effects of atmospheric inputs at the surface, biological uptake and regeneration within the seasonal thermocline (according to the correlation with nutrients in the 10 – 200-m layer) and particulate scavenging removal throughout the water column.

Acknowledgements Discussions with Roland Wollast (Universite´ Libre de Bruxelles) and Ce´cile Guieu (Laboratoire de

Physique et Chimie Marines, Villefranche sur Mer) have been considerably appreciated during this work. The original manuscript has been really improved thanks to the constructive comments from Marco Grotti and two anonymous reviewers. Peter Statham (Southampton Oceanographic Centre) and Larry Benninger (University of North Carolina at Chapel Hill) have also been of great help in reviewing the script. All of them are gratefully acknowledged. We would like to thank the OMEX community for sharing the data and particularly, Tom Treacy and Michael Orren (University of Galway, nutrient data), Thomas Raabe and Uwe Brockmann (University of Hamburg, nutrient data), Peter Miller and Steve Groom (NERC Plymouth Marine Laboratory, satellite imagery), Peter Statham (Southampton Oceanographic Centre, hydrological parameters and chlorophyll a) and Ian Hall and Nick McCave (University of Cambridge, SPM data). We thank the officers and crew of the RRS Charles Darwin for their assistance at sea, as well as the Research Vessel Services team for its technical support. This research was sponsored by an OMEX EU contract (# MAS2-CT93-0069) and a 3-year doctoral grant from the French Department of Research and Education. Associate editor: Dr. Edward Boyle.

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