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Temporary and definitive fixation of atmospheric lead in deep-sea sediments of the Western Mediterranean Sea
Pergamon
0025-326X(94)E0060-L
Marine Pollution Bulletin, Vol. 28, No. 12, pp. 727-734, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 01)25-326X/94 $7.00+ 0.00
Temporary and Definitive Fixation of Atmospheric Lead in Deep-Sea Sediments of the Western Mediterranean Sea FRAN(~OIS FERNEX*-~ and CHRISTOPHE MIGON~:§ *Laboratoire de GOodynamique sous-marine, CNRS UA. 132 G6osciences de l'Environnement, La Darse, BP 48, F-06230 Villefranche-sur-Mer, France +Environnements GOochimiques, Facultd des Sciences, Parc Valrose, F-06108, Nice Cedex 2, France ~;Laboratoire de Physique et Chimie Marines, Universit~ de Paris 6, CNRS 1NSU, La Darse, BP 8, F-06230, Villefranche-sur-Mer, France § CMCS, Universit~ de Corse, Grossetti, BP 52, F-20250 Corte, France
Most lead brought to the Mediterranean Sea has an anthropogenic origin and is mainly transported through the atmosphere. Atmospheric Pb was continuously collected at Cap Ferrat in 1986 and 1987. From this study, the estimation of the anthropogenic Pb flux on the whole Western Mediterranean was, averaged on 1986 and 1987 data, 4080 t. Assuming that the atmospheric anthropogenic Pb input varied in this course of time similarly to the consumption of Pb added to gasolines in France, the mean annual flux could be calculated: 3.95 kg km -2 yr-1, that is an annual input of 3360 t yr-1. Reaching the sea, this metal seems to become rapidly bound to phytoplankton. Grazing by zooplankton leads to the production of faecal pellets which frequently contain rather high metal concentrations. The sinking rate of pellets of various zooplankton species is high; within a few days pellets may reach deep-sea sediments. After deposition, Pb is released from this organic-rich material during early diagenesis. In most cases, it, therefore, returns to the overlaying water body by ascending diffusion. But, in a deep-sea area of approximately 80 000 km 2 where Mn oxide precipitation occurs in surficiai sediments, Pb seems to remain stored by coprecipitation processes. By considering the lead stored in 'excess' in the surficial sediment of the deep-sea area, we estimate that a mean annual anthropogenic Pb amount ranging from 800 up to 1080 t was stored every year from 1950. On the same area, taking into account the Pb loss at the straits, the 'direct' atmospheric input to the sea bottom is, on average, 184 t yr-L The remaining part, that is ( 8 0 0 - 1 0 8 0 ) - - 1 8 4 = ( 6 1 6 - 8 9 6 ) t yr-l, corresponds to an additional 'indirect' Pb flux in water due to Pb released from sediments of the surrounding areas where it does not remain stored.
The toxicity of lead is now well stated (Vernberg et al., 1974; Flament, 1985; Subramanian & Connor, 1991) and even once pristine locations are now contaminated (Bourton & Patterson, 1987; Nriagu, 1991). At least from 1960 to 1987, most of the lead brought to the Mediterranean Sea had an anthropogenic origin, chiefly due to the use of leaded gasoline (Pacyna et al., 1984; Pattenden & Branson, 1987; Hopper et al., 1989). Anthropogenic Pb is mainly transported through the atmosphere (Martin et al., 1989; Guieu et al., 1991; Migon, 1993) and it is easily solubilized in sea water (B6thoux et al., 1990; Chester et al., 1990). In the water body, lead divides into three: one part remains soluble, another becomes assimilated by planktonic organisms; another, bound to particulate matter, settles in the sea bottom. Coastal zones frequently display relatively high Pb levels in sediments and the concentrations globally decrease towards the open sea areas. Nevertheless, the surficial sediments of some deep-sea areas in the Western Mediterranean Sea exhibit higher concentrations (Arnoux et al., 1983) than neighbouring zones. Considering that continental shelf sediments are affected by pollution, it is attempted in this paper to determine whether there is an anthropogenic contribution to the Pb amount in offshore surficial sediments. A budget for anthropogenic Pb fluxes in the Western Mediterranean is presented in order to tentatively establish a relationship between the atmospheric inputs and the fixation in certain deep-sea sediments.
Sampling
A sampling station was established at Cap Ferrat (43°41'10"N, 7°19'30"E), on the southeastern coast of 727
Marine Pollution Bulletin
France. The characteristics of the site have been described in previous papers (Migon & Caccia, 1990; Migon et al., 1991); the fluxes measured here are presumably representative of long-range Pb transfer in the area. The low impact of local anthropogenic sources has been previously reported on the basis of a comparison (Migon et al., 1991) of aerosol samples from Cap Ferrat with those collected in sheltered areas or in the open Western Mediterranean basin by other workers (Bergametti, 1987; Dulac et al., 1987). Samples were continuously collected during 1986 and 1987. Atmospheric aerosol was sampled on Sartorius SMl1106 cellulose acetate membrane (pore size 0.45 gm; diameter 47 mm) with a filter holder Sartorius SM16510, raised at the top of a 6 m high mast and connected to pumps (Reciprotor 40 W) and counters (Gallus). The flow rate of the pumps was approximately 1 m 3 h -l and filtration was carried out over 4-8 h. Pb blank levels on filters were very low and reproducible (12-37 ng per filter). Wet deposition was collected with KFA-Julich automatic rain collector which only opens when it rains. Heavy metals existing essentially in dissolved form in rainwater under usual pH conditions (Chester et al., 1990; Migon et al., 1991), wet samples were filtered on Sartorius SM 11106 membranes previously cleaned with Suprapur 1-2 N HC1. The mean blank level on one filter was 3.3 ng of Pb. The pH values ranged from 3.7 up to 6.5, with an average of 4.6 and a standard deviation of 0.5. After pH measurements, samples were acidified to pH 2 in order to avoid adsorption, and then stored in teflon bottles until analysis. For further details, see Migon & Caccia (1990, 1993). Twenty-five surficial sediment samples (3-4 cm thick) were collected in the Mediterranean deep-sea area during the Biomede cruise for a study on the areal distribution of metals (Arnoux et al., 1983). These sediments were very thin grained and essentially constituted clays. Sediment box-cores were collected in 1987 (CF: 110 m, 43°40.5'N, 7°19.3'E; RP: 83 m, 4°51 ', 43°17.4'; SR: 1480 m 5°12 ', 42°58'; SM: 1800 m, 42°55.5 ', 5°16'; A: 2780 m, 41°6 ', 7°; B: 2800 m, 40°45.7 ', 5°16'). The cores were vertically sliced (2 cm) on board and each sample pressed in order to extract the interstitial water. The cellulose acetate filters (Sartorius) had a 0.2 gm porosity. In the whole studied area, the surficial sediments are all oxidizing. On the contrary, deep sediments are more reduced. In order to avoid oxidation, the contact with air is reduced to the minimum with transferring the sediment into the press, which is closed with a rubber membrane; the nitrogen pressure is applied on this membrane, which enables the sediment to be squeezed. The collected water is then acidified for storage (pH 1).
Analytical
Aerosol metal concentrations were measured by graphite furnace atomic absorption spectrophotometry. 728
Sections of the membrane filters (3 mm in diameter) were punched out and directly introduced into the carbon rod atomizer. The spectrophotometer used was a Varian AA 1275 equipped with a CRA 90 atomizer and ultra carbon pyrolitic crucibles. The detection limit was 50 pg. Reproducibility was better than 5%. The water samples were analysed for Pb by differential pulse anodic stripping voltammetry on a hanging mercury drop electrode. The measurements were performed with an E G & G Princeton Applied Research 264A polarographic analyser in conjunction with a 303 static mercury drop electrode. Detection limits were 100 ng 1-~. Blank values were at most equal to these detection limits; reproducibility was better than 5%. All analyses were carried out under laminar airflow benches in a class 100 clean room. The results of atmospheric measurements were published in previous papers (Migon & Caccia, 1990, 1993; Migon et al., 1991). Intercalibration tests were performed for rainwater with a good agreement. For further analytical details, see Migon & Caccia (1990, 1993). Pb and Mn from sediment were analysed by atomic absorption spectrometry. Metals were extracted from 1 g of dried sediment by means of 50 ml of a 8 N acid solution (2/3 H N O 3 + l / 3 HC10~), heated to dryness. Fifty millilitres of a HNO 3 solution (pH 1 or 1.5) was used for recovery, and then centrifuged. A simplified and partial sequential extraction procedure was utilized to extract metals from sediments of cores A and B. Sequential leaching techniques allow the metal bound to various geochemical fractions (or main sedimentary compounds) to be differentiated (see Chester & Hughes, 1967; Filipek & Owen, 1978; FOrstner et al., 1986). 1. Two grams of wet sediment were leached for 4 h with 150 ml of 0.5 M sodium acetate solution adjusted to pH 5 with acetic acid: this leaching essentially allowed extraction of exchangeable cations and the carbonate fraction. 2. After having been centrifuged under a nitrogen atmosphere, the residue from 1 was leached under a nitrogen atmosphere for 12 h with a hydroxylamine solution (NH2OH HC1 at 0.05 M) adjusted to pH 2.5 with acetic acid: this leaching mainly extracted the easily reducible fraction, in particular Mn oxides and poorly crystallized Fe hydroxides. 3. The residue of 2 was leached for 2 h at pH 5.5 with hydrosulphite (dithionite; 4 g for 50 ml): this extracted easily to moderately reducible phase, in particular free Fe oxides. The ship used for sampling was an oil burner and contamination is thus possible. Nevertheless, the Pb concentrations measured in the overlaying waters in the box-cores were at least one order of magnitude lower than those measured in the interstitial waters of the surficial sediments. Since these waters are the most susceptible to contamination such contamination should be neglected when compared with Pb levels into the surficial sediments, lntercalibration tests have shown that the results agree within 15-18% with the reference values (Loring, 1986).
Volume 2 8 / N u m b e r 1 2 / D e c e m b e r 1994 10
Results and Discussion A t m o s p h e r i c fluxes
The total atmospheric deposition of lead is the sum of the dry and wet inputs. For the calculation of the dry atmospheric flux, the mean Pb concentration in the aerosol (34.2 ng m-3; Migon et al., 1991) was multiplied by the mean dry deposition velocity, itself calculated on the basis of a total and wet deposition sampling at Cap Ferrat (0.25 cm s-~; unpublished data). This value of dry deposition velocity falls between low theoretical values (Dulac et al., 1987) and high experimental values obtained by cascade impactor data (Remoudaki, 1990). Moreover, it is very similar to the estimation of van Aalst (1988). Then, this dry deposition value is multiplied by the approximate number of dry days in the year and the annual dry flux is obtained (2 kg km -2 yr-1). The annual wet deposition of lead is the sum of the wet inputs associated with each rain event (that is the Pb concentration in rainwater multiplied by the rainfall amount). This wet flux was 3 kg km -2 yr -1 for 19861987. Therefore, the total atmospheric flux of Pb, as measured in 1986-1987 at Cap Ferrat, was approximately 5 kg km -2 yr -I (that is 4250 t yr -1 for the whole Western Mediterranean Sea). It was previously calculated that 91% of lead in the atmospheric aerosol had an anthropogenic origin in the Northwestern Mediterranean (Migon & Caccia, 1990). Also 99% of lead in rainwater originates from anthropogenic sources (Migon & Caccia, 1993). The low contribution of natural Pb emissions should be partly due to the low impact of volcanoes. Indeed, Mount Etna plumes (that is, the closest significant volcanic sources) should not affect the considered Mediterranean area (Martin et aL, 1984). It can thus be assumed that the contribution of volcanic Pb is negligible in this region. From the 1986-1987 data, it was calculated that the dry inputs represented approximately 40% of total deposition (then wet inputs account for 60%); then one can calculate that the total atmospheric flux of anthropogenic lead should be approximately 4.8 kg km -2 yr -I in 1986-1987, and the natural Pb input 0.2 kg km -2 yr-L Thus, for the whole Western Mediterranean Sea (surface area 0.85× 106 km2), the anthropogenic Pb annual input in 1986 or 1987 can be calculated as 4080 t yr -1. The annual flux of anthropogenic Pb on the whole Western Mediterranean from 1950 to 1987 can be evaluated by referring to the consumption of lead introduced in gasolines in France (Nicolas et al., 1994): the annual flux is assumed to be proportional to the consumption of lead from gasolines in France every year (Fig. 1). For the 38 years, the mean anthropogenic Pb flux was 3.95 kg km -2 yr -~, that is to say, a mean annual input of approximately 3360 t yr-1. According to Copin-Montrgut et al. (1986) and Brthoux et al. (1990), the surface waters flowing out from the Western Mediterranean to the Eastern basin through the Sicily Strait have higher Pb concentrations
Pb
8
1"7" 2
,9°0
,,'8°
,;,0
,;80
,9°0
year
Fig. 1 Variations in the course of time of the anthropogeuic Pb flux over the Western Mediterranean Sea. We assume that anthropogenic Pb was chiefly transported through the atmosphere, and its input into the sea was directly proportional to the consumption of lead from gasolines in France. The maximum yearly Pb consumption in gasolines in France was 14 500 t in 1976.
than the deep waters entering the Western basin through this strait. The loss of Pb for the Western Mediterranean is approximately 1200 t yr-L To a much lesser extent, there is also some loss at Gibraltar. So the budget at the straits shows a Pb loss for the Western Mediterranean of about 1400 t yr-L A mean annual load of 3 3 6 0 - 1 4 0 0 = 1 9 6 0 t of anthropogenic Pb accumulated, therefore, in the Western Mediterranean from 1950 to 1987. It can be assumed that a part of the load remained in the water body; there, Pb is either dissolved or it is adsorbed onto mineral particles or included in phytoplankton and faecal pellets (Fowler, 1977; Romeo et al., 1988). Another part settles down to bottom sediments with particles, chiefly faecal pellets. Storage in sediments
Generally, Pb concentrations in the sediments from near-shore areas (the infralittoral zone) are high; in the vicinity of a main sewer or a river mouth, they may exceed 8() or 100 ~tg g-l, which are in the main due to pollution. For instance, near the Rh6ne river mouth, it was estimated that, in sediments with concentrations up to 100 or 110 ~.g g-l, nearly half of Pb had an anthropogenic origin (Badie et al., 1983). Near a river mouth, the high metal concentrations in sediments can be related to short transit and high sedimentation rates which oppose the upward release of the metallic ion. Moreover, high organic matter concentrations lead there to oxygen depletion and to low redox conditions in sediment levels only a few centimetres below the interface, so that sulphate reduction occurs and metals can be trapped as insoluble sulphides. At a distance of an input point (for instance the Grand Rh6ne river mouth) the Pb concentrations in sediments of the continental shelf are lower. The concentration decline at a distance of an input point (for example a river mouth) chiefly results from metal release, first, during particulate matter transport in the seawater and, secondly, shortly after deposition (Fernex et al., 1986); there, the oxic conditions in the sediments down to a 729
Marine Pollution Bulletin
greater depth ( > 15 or 20 cm), owing to a lower organic matter content, prevent sulphide precipitation; metals can, therefore, be partly dissolved after deposition and released from the interstitial waters up to the overlaying seawater. Although generally > 8 5 % of particulate matter discharged by rivers settle on the continental shelf (0-200 m), approximately 20-30% of the discharged lead is incorporated in the water body, and it can be assumed that most of this 'labile' lead originates from human activities (natural lead is presumably bound to mineral particles, whose solubility is low, while anthropogenic lead should be associated with more degradable and more soluble particles). But, as mentioned above, the amount of anrhtopogenic lead transported from coastal areas to the open sea is very low when compared with the anthropogenic Pb amount which was brought to the whole Western Mediterranean through the atmosphere in 1986-1987; the coastal contribution to the open seawaters very probably does not reach 10 or 12%. Surprisingly, Pb is found at somewhat higher concentrations in the sediments of a deep-sea zone (where the surficial sediments are oxic) than in surrounding areas (Fig. 2). The difference between the concentrations in the richer and poorer deep-sea surficial sediments reaches 6-8 ~tg g-~. Globally, sediment cores collected whether in near shore areas or further offshore display increasing Pb concentrations from deeper levels (20-50 cm) to the top (Fig. 3; E! Moumni, 1994). The surficial sediments of all areas of the Mediterranean can, therefore, be assumed to be significantly contaminated by lead. The natural background should be given by concentrations observed in deeper levels corresponding to uncontaminated sediments, deposited during pre-industrial times. The depth where the sediments represent the natural inputs depends on the sedimentation rate, which rarely exceeds 100 cm per 1000 yr, except from the continental shelf (Chamley, 1971). In the contin-
Mn gg/g
ental shelf sediments, except from some areas where it can be slightly higher, the natural Pb concentration generally ranges from 25 to 35 ~tg g-i and from 15 or 16-25 in the sediments of offshore areas, in particular on the slope (Added et al., 1981); but it is higher in the deep-sea 'richer' zone (30-35 ~tg g-J; Fig. 3A). The highest Pb concentations in the surficial sediments of the deep-sea areas were observed where Mn concentrations are highest (Fig. 2). The richer sediments extend over a deep-sea zone of approximately 80 000 km 2. During the sequential extraction procedure with three steps, much more Mn was extracted from the upper levels of the richer zone than from deeper levels ( > 14 cm; Fig. 4). It appears that the enrichment of this metal in surficial sediments is due to dissolution of Mn oxides in the deeper levels and ion migration to the upper levels where dissolved Mn precipitated. The relatively high Pb concentrations in the sediments of the richer zone (Fig. 4) are surely related to the behaviour of this metal when reaching the bottom sediments. As a matter of fact it can be assumed that anthropogenic Pb introduced in surficial sediments by the settled matter (mainly organic) is removed from this matter during early diagenesis processes occurring there, and the interstitial waters become enriched. But in most zones of the Mediterranean Sea, the Pb 2+ concentrations in the interstitial water of surficial sediments remains too low to allow Pb oxides to precipitate. Then dissolved Pb flows from the interstitial water up to the overlaying seawater (Fig. 5). However, in the richer zone, relatively high Mn concentrations suggest that Pb was trapped in the surficial sediments during Mn precipitation, although Pb is present in interstitial waters in undersaturated concentrations as for Pb oxides. But actually, a trace metal like Pb 2+ can coprecipitate with a major dissolved metal, like Mn 2+ during Mn oxide formation if this major ion is present in apparent oversaturation with
Pb gg/g ~i~i~i~i~)i~i~i:i~i~i~i~i~i~i~iii~i !)i~i~ii~i~i~i~i~ii:~i~:~:~:~:~i~'~: i~i~i~i~i~i~i~i~i~?ii~ii~ii~i~i~ii!~ ¸ i~!~!i~ii~i~i~i!~i!~!i~i!iii~ii!~ii~ii~iiii~ii!i~!iiii~iiiiii~ii~i~i~!~ii~i!i)~J~i;i~ii!iiiiiii!ii~iii~;~iiiii~i~iiiiiiiiii!~!~ ¸¸;
Fig. 2 Distribution of Mn and Pb in deep-sea sediments of the Western Mediterranean. After Arnoux et al. (1983), modified. The shape of the contour maps is similar for the two metals.
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Volume 28/Number 12/December 1994
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Fig. 3 Vertical profiles of Pb extracted by means of concentrated acids (65% HNO3+35% HCIO4). RP: core collected south of the Rh6ne Pro-delta (83 m); CF: southeast of Cap Ferrat (110 m); SR: slope south of the Rh6ne delta (1450 m); SM: slope south of Marseille (1780 m); (A) abyssal plain (2780 m); (B) abyssal plain (2800 m).
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Fig, 4 Deep-sea sediments. Profiles of easily extractable manganese and lead. The extraction sequence was as follows: (1) leaching by means of the acetic acid solution (pH 5); (2) hydroxylamine leaching (pH 2.5); (3) hydrosulphite leaching. It is assumed that the higher Pb concentrations in the upper levels resulted from pollution and the totality of anthropogenic Pb could be extracted by means of these three leachings.
/ phytop~ankton
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Fig. 5 Schematic representation of lead behaviour in the Western Mediterranean Sea. Although lead is not only exported through the Strait of Sicily but also through the Strait of Gibraltar, one strait alone is represented here.
731
Marine Pollution Bulletin
respect to its oxides, for example, Mn203 (Renard et al., 1976; Renard, 1979; Fernex et al., 1992). In such a case we have to consider the following reactions: 2Mn 2+ + 3 H 3 0 --" Mn203 + 6 H + + 2e-;
(a)
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(b)
Average atmospheric input of Pb 3360 + 180 t y-1 ^
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A defined Pb amount can coprecipitate if: NMneO3AGMn203 + N P b O z A G P b O 2 < 0, where NMn203 and NPbO2 are the number of moles reacting simultaneously in reactions (a) and (b) whose free energy changes A G can be calculated if the concentrations, the redox potential and p H are known. In the surficial sediment interstitial waters, at 2700 m, Mn an Pb concentrations were, respectively, 45 and 1.9 ~tg 1-~. This corresponds to activities of 173 and 0.25 nmol 1-~. The redox potential was 0.43-0.45 V and the p H was 8. Such values favour the coprecipitation process leading to an 'excess' of Pb concentration of 8-10 ~tg g-l. The upper levels (0-6 cm) of core A (in the area where the Mn concentrations are high) contain more esaily extactable Pb than deeper levels (Fig. 4). It can be assumed that the Pb 'in excess' in the upper levels results from the pollution by lead since 1950. Older atmospheric pollution by lead can be considered as negligible. The sedimentation rate is low in this deepsea area: about 2 0 - 2 5 cm per 1000 yr (Chamley, 1971; Kouame et al., 1983); but considering the Mn profiles, the similarity between the two first upper levels and the regular decrease in the concentration suggest a probable effect of bioturbation. Indeed, without any biological activity, the migration-reprecipitation process should strongly enrich only one level. For the case of Pb, the upper sediments were mixed and anthropogenic lead was, therefore, distributed in the 6 upper centimetres. As shown by the Pb profile in the sediments of the richer zone, 4 0 - 5 0 gg Pb in excess per cm 2 are stored in the upper levels (0-6 cm). This amount, which is assumed to have been due to atmospheric pollution during 38 years, corresponds to a mean flux of 1-1.35 ~tg cm -2 yr -~= 10-13.5 kg km 2 yr -~. Therefore, on average, 8 0 0 - 1 0 8 0 t of Pb were stored yearly in the 80 000 km 2 richer zone.
Flux budget
The average 'direct' atmospheric flux of anthropogenic Pb to the sea bottom is, for the 850 000 km 2 of the whole Mediterranean, 3360 t Pb yr -t. Taking into account the Pb loss through the straits, the 'direct' flux to the bottom should be 184 t yr -1 (2.3 kg km -2 yr -]) over the 80 000 km 2 of the richer zone. Anthropogenic Pb stored in the deep-sea zone corresponds, therefore, to a mean 'real' flux to the bottom which appears to be greater than the direct atmospheric Pb input of the same area (taking into account the loss through the Strait of Gibraltar). The difference is: ( 8 0 0 - 1 0 8 0 ) - 184 = (616-896) t yr -~, 732
atmosphc~.¢
. . . . . . . . . "~--
Fig. 6 Schematicsectionillustrating budgets of lead in the open part of the Western Mediterranean Sea. The total atmospheric input is the sum of the anthropogenic (3360 t yr ~) and natural (180 t yr ]) contributions.The fluxto the bottom sedimentswas higher than the flux at the atmosphere-seawaterinterface. Lead must, therefore, have been removed from surficial sediments in large areas of the Westernbasin. for the 80 000 km2; or (10-13.5) - 2.3 = 7.7-11.2 kg km -2 yr -1. These 616-896 t yr -~ which were not due to the direct flux probably originated from areas where anthropogenic lead was not definitively stored (Figs 5 and 6), but was released from sediment to the overlaying water where it could again be absorbed by phytoplankton. Grazing by zooplankton, in particular salps, leads to a complementary input of Pb to the bottom by means of faecal pellets. Nival et al. (1985), Andersen & Nival (1988) and Morris et al. (1988) report downward sinking velocity of salp faecal pellets up to 1100 m d -1. Few experimental data are related to the lead flux to sediments. On the one hand, some authors evaluated the organic matter downward flux either by means of sediment traps (Buscail et al., 1987; Miquel et aL, 1990) or by modelling (Nival et al., 1985; Andersen & Nival, 1988). On the other hand, Romeo et al. (1988) measured the P b : P ratio in particulate matter from sediment traps: 0.29 ixg Pb: ~tg P. Using the Redfield ratio, we obtain: 0.29 ~tg Pb:41 ixgC = 7 ~tg Pb:mg C -~. Buscail et al. (1987) measured the organic matter flux at 600 m in the Golfe du Lion which in average was 100 mg C m -2 d -1. Andersen & Nival (1988) calculated, for the Ligurian Sea, a faecal pellet flux of 30 mg C m -2 d ~. According to Peinert etal. (1991), the average flux at 2000 m can be estimated at 5 (or 6) mg C m -2 d -~. Considering this value and the concentration calculated from the results of Romeo et al. (1988), we obtain a Pb flux to deep sea sediments of 12.8 kg km -2 yr -j. This value compares well with the evaluation of the storage rate of anthropogenic lead (10-13.5 kg km -2 yr-1).
Conclusion A great part of anthropogenic Pb brought to the sea becomes bound to particles or phytoplankton. This
Volume 28/Number 12/December 1994 s o l i d m a t e r i a l settles d o w n a n d is p a r t l y d e p o s i t e d o n bottom s e d i m e n t s w e r e it b e c o m e s incorporated p r i n c i p a l l y b y t h e effects o f b i o t u r b a t i o n . A l l surficial s e d i m e n t s in t h e W e s t e r n M e d i t e r r a n e a n should, t h e r e f o r e , b e c o n t a m i n a t e d b y h u m a n activities. B u t the totality of the settled Pb generally does not remain s t o r e d in surficial s e d i m e n t ; a l a r g e p a r t is d i s s o l v e d d u r i n g t h e e a r l y d i a g e n e s i s , a n d , as t h e c o n c e n t r a t i o n s in the interstitial w a t e r r e m a i n t o o l o w to a l l o w P b o x i d e p r e c i p i t a t i o n , m e t a l i o n s m i g r a t e u p to t h e overlaying seawater (except from near shore areas where the high sedimentation rate prevents the release o f d i s s o l v e d m e t a l f r o m interstitial water). T h e dissolved Pb originating from sediment can again be assimilated by phytoplankton or adsorbed onto particulate matter. H o w e v e r , in a d e e p - s e a a r e a o f a b o u t 8 0 0 0 0 k m 2, t h e l o c a l r e d o x c o n d i t i o n s a l l o w e d P b to c o p r e c i p i t a t e w i t h M n d u r i n g M n o x i d e f o r m a t i o n in surficial s e d i m e n t . It c o u l d b e e s t i m a t e d that, b e t w e e n 1 9 5 0 a n d 1987, on average 800-1080 t of anthropogenic Pb were s t o r e d in t h e d e e p - s e a a r e a e v e r y year. T h i s is m o r e than the direct atmospheric Pb input over the same area ( t a k i n g i n t o a c c o u n t t h e loss o f a p p r o x i m a t e l y 1 4 0 0 t yr -1 t h r o u g h t h e straits o f G i b r a l t a r a n d Sicily): 184 t y r -~. T h e d i f f e r e n c e ( 6 1 6 - 8 9 6 t yr -~) p r o b a b l y corresponds to m e t a l r e l e a s e d f r o m the surficial sediment of surrounding areas where the redox c o n d i t i o n s d o n o t a l l o w a n t h r o p o g e n i c P b to b e c o p r e c i p i t a t e d w i t h M n . In d e e p - s e a areas, ' d e f i n i t i v e ' s t o r a g e o f a n t h r o p o g e n i c P b o n l y o c c u r s in s e d i m e n t s w h e r e M n o x i d e s p r e c i p i t a t e c l o s e to t h e i n t e r f a c e . As the waters flowing out of the Eastern Mediterr a n e a n t h r o u g h t h e Sicily S t r a i t c o n t a i n less l e a d t h a n t h e w a t e r s e n t e r i n g this b a s i n , it c a n b e a s s u m e d t h a t a s i m i l a r d e p o l l u t i o n p r o c e s s o c c u r s there. The authors wish to thank B. Gentili for technical assistance.
Aalst, R. M. van (1988). Input from the atmosphere. In Pollution of the North Sea: An Assessment (Salomons, W., Bayne, B. L., Duursma, E. K. & FiSrstner, U., eds), pp. 275-283. Springer, Berlin. Added, A,, Fernex, E & Mangin, J. P, (1981), Repartitiondes oligoelements m/:talliques darts les sediments marins devant l'embouchure du Grande Rhdne. In Ves Journdes Etud. Pollutions (U.N.E.P.; Cagliari, 1980), pp. 535-543. C.I.E.S.M., Monaco. Andersen, V. & Nival, P. (1988). A pelagic ecosystem model simulating production and sedimentation of biogenic particles: role of salps and copepods. Mar. E~ol. Progr. Ser. 44, 37-5/). Arnoux, A,, Chamley, H., Bellan-Santini, D., Tatosian, J. & Diana, C. (1983). Etude mineralogique et chimique des s~diments profonds de Mediterrande Occidentale. In Vles Journ&s Etu. Pollut. (U.N.E.P.; Cannes, 1982), pp. 385 395. C.I.E.S.M., Monaco. Badie, C., Added, A., Fernex, E, Rapin, E & Span, D. (1982). Determination de la part qui revient ~ la contamination dans l'apport en metaux par le Rhdne, In Vles JournOes Etud. t'ollut. (U.N.E.P.; Cannes, 1982), pp. 65-72. C.I.E.S,M., Monaco. Bergametti, G. (1987). Apports de mati&re par voie atmospherique 5, la Mediterrande occidentale. PhD Thesis, Universit6 Paris 7,296 pp. Bethoux, J. P., Courau, P., Nicolas, E. & Ruiz-Pino, D. (1990). Trace metal pollution in the Mediterranean Sea. Ocean. Acta 13,481-488. Boutron, C. E & Patterson, C. C. (1987). Relative levels of natural and anthropogenic Pb in recent Antarctic snow. J. geophys. Res. 92, 8454-8464. Buscail, R., Guidi, L, & Bovee, E de (1987). Flux de matibre organique, variation des parametres biochimiques et dynamique biologique dans le Canyon Lacaze-Duthiers, Golfe du Lion. CoIL intern. Ocdan. Perpignan, C1ESM (Monaco); p. 73. Chamley, H. (1971). Recherche sur la s6dimentation argileuse en
Mdditerran&. Sc. gOologiques, Serv. Carte geol. Alsace-Lorraine (Strasbourg), 35; 210 p. Chester, H. & Hughes, M. J. (1967). A chemical technique for the separation of ferromanganese minerals, carbonates minerals and adsorbed trace elements from pelagic sediments. ( h e m . Geol. 2, 249-262. Chester, R., Nimmo, M., Murphy, K. J. & Nicolase, E. (1990). Atmospheric trace metals transported to the Western Mediterranean: data from a station on Cap Ferrat. Water PolL Res. Report 20, EROS 2000 (Martin, J. M. & Barth, H. eds), pp. 597-612. Envir. Res. Progr., Commiss, of European Communities. Copin-Montegut, G., Courau, R & Nicolase, E. (1986). Distribution and transfer of trace elements in the Western Mediterranean. Mw: Chem. 18, 189-195. Dulac, E, Buat-Menard, R, Arnold, M. & Ezat, U. (1987). Atmospheric input of trace metals to the western Mediterranean Sea: 1. Factors controlling the variability of atmospheric concentrations. J. geophys. Res. 92, 8437-8453. el Moumni, B. (1994). Etude sedimentologique et geochimique des depdts au Quaternaire rdcent de la partie orientale de la Marge Mediterrandenne Marocaine (Met d?Mboran). Rev. Geologie Mediterr. (in press). Fernex, F., Fevrier, G., Benaim, J. & Arnoux, A. (1992). Copper, lead and zinc trapping in Mediterranean deep-sea sediments: probable coprecipitation with Mn and Fe. ('hem. GeoL 98,293-306. Fernex, F.~ Span, D., Flatau, G. & Renard, D. (1986). Behavior of some metals in surficial sediments of the Northwest Mediterranean Continental Shelf. In Sediments and Water hueractions (Sly, P. G., ed.), pp. 353-370. Springer, New York. Filipek, L. & Owen, R. (1978). Analysis of heavy metal distribution among different mineralogical states in sediments. Can. J. Spectrosc. 22, 31-34. Flament, P. (1985). Les metaux-traces associes aux aerosols atmospheriques: apports au milieu marin du littoral Nord-Pas-deCalais. PhD Thesis, Universit~ Sciences et Techniques Lille. Forstner, U., Calmano, W., Kersten, M. & Salomons, W. (1986). Mobility of heavy metals in dredged harbor sediments. In Sediment and Water Interactions (Sly, P. G., ed.), pp. 371 380. Springer, New York. Fowler, S. W. (1977). Trace elements in zooplankton particulate products. Nature 269, 51-53. Guieu, C., Martin, J. M., Thomas, A. J. & Elbaz-Poulichet, E (1991). Atmospheric versus river inputs of metals to the Gulf of Lions. Mar. Pollut. Bull. 22, 176-183. Hopper, J. F., Ross, H. B., Sturges, W. T. & Barrie, L. A. (1991). Regional source discrimination of atmospheric aerosol in Europe using the isotopic composition of lead. ~'llus 43B, 45-6(/. Kouame, A., Fernex, F. & Poutiers, J. (1983). Sedimentation dans le secteur du Ddme TI au Sud de Toulon. (Programme ECOMED), Petrole et "l~'chnique 300, 18-20. Ass. fran~. Technic. P&role, 18-20. Loring, D. H. (1986). ICES lntercalibration for trace metals in marine sediments. Bedford Institute of Oceanography, Dartmouth, 60 pp. Martin, D., Cheymol, D., lmbard, M. & Strauss, B. (1984). Climatology of forward trajectories of Mount Etna plume over a 18-year period. BulL Volcan. 47, 1115-1123. Martin, J. M., Elbaz-Poulichet, F., Guieu, C., Loye-Pilot, M. D. & Hart, G. (1989). River versus atmospheric input of material to the Mediterranean Sea: an overview. Mar. ('hem. 28, 159-182. Migon, C. (1993). Riverine and atmospheric inputs of heavy metals in the Ligurian Sea. Sci. 7otalEnviron. 138,289-299. Migon, C. & Caccia, J. L. (1990). Separation of anthropogenic and natural emissions of heavy metals in the Western Mediterranean atmosphere. Atmos. Environ. 24A, 399-405. Migon, C. & Caccia, J. L. (1993). Estimation of anthropogenic and natural heavy metals in the Northwestern Mediterranean rainwater and total atmospheric deposition. Chemosphere 27, 2389-2396. Migon, C., Morelli, J., Nicolas, E. & Copin-Montegut, G. (199l). Evaluation of total atmospheric deposition of Pb, Cd, Cu and Zn to the Ligurian Sea. Sci. Total Environ. 105, 135-148. Miquel, J. C., Fowler, S. W. & La Rosa, J. (1990). Vertical fluxes of particulate material in a frontal zone off Corsica. Rapp. Comm. int. Met Meditern 32,280. CIESM (Monaco). Morris, R. J., Bone, Q., Head, R., Braconnot, J. C, & Nival, R (1988). Role of salps in the flux of organic matter to the bottom of the Ligurian Sea. Mar, BioL 97, 237-241. Nicolas, E., Ruiz-Pino, D., Buat-Menard, P. & Bethoux, J. P. (1994). Abrupt decrease of lead concentration in the Mediterranean Sea: a response to antipollution policy. Geophys. Res. Lett. (submitted). Nival, P., Braconnot, J. C., Andersen, V., Oberdorff, T., Choe, S. M. & Laval, R (1985). Estimation de l'impact des Salpes sur le phytoplancton en Mer Ligure. Rapp. Comm. int. Mer Mediterr. 29, 283-286. CIESM (Monaco). Nriagu, J.-O. (1991). Human influence on the global cycling of trace 733
Marine Pollution Bulletin metals. In Heavy Metals in the Environment (Farmer, J. G., ed.), pp. 1-5. CEP Consultants, Edinburgh. Pacyna, J. M., Semb, A. & Hansen, J. E. (1984). Emission and longrange transport of trace elements in Europe. Tellus 36B, 163-178. Pattenden, N. J. & Branson, J. R. (1987). Relation between lead in air and in petrol in two urban areas of Britain. Atmos, Environ. 21, 2481-2483. Peinert, R. D., Fowler, S. W., La Rosa, J., Miquel, J. C. & Teyssie, J. L. (1991). Vertical flux and microplankton assemblages in the Gulf of Lions during spring 1990. Wat. Pollut. Res. Rept 28, EROS 2000 (Martin, J. M. & Barth, H., eds), pp. 413-424. Envir. Res. Progr., European Comm. Remoudaki, E. (1990). Etude des processus contr61ant la variabilit6 temporelle des flux atmosph6riques de polluants et de poussibres min6rales en M6diterran6e occidentale. PhD Thesis, Universit6 de Paris 7; 223 pp. Renard, D. (1979). Etude g6ochiimque de l'incorporation du
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Manganbse et des filements associ6s dans les nodules polym&aliques. Thesis Dr Et., Univ. Paris VII, Lab. Gdoch. Eaux; 253 pp. Renard, D., Michard, G. & Hoffert, M. (1976). Comportement gdochimique du Cuivre, du Nickel et du Cobalt '~ I'interface eausddiment. Mineral. Deposita 11, 380-393. Romeo, M., Gnassia-Barelli, M., Nicolas, E. & Carre, C. (1988). Importance du microplancton g61atineux darts le stockage et le transfert de m6taux traces en M6diterran6e nord-occidentale. Rapp. Comm. int. Mer M~diterr 31, 35. CIESM, Monaco. Subramanian, K. S. & Connor, J. W. (1991). Lead contamination of drinking water. J. Environ. Health 54, 29-32. Vernberg, W. B., Coursey, P. J. & O'Hara, J. (1974). Multiple environmental factors on physiology and behaviour of the fiddler crab Uca pugilator. In Pollution and Physiology of'Marine Organisms: Recent Advances (Vernberg, W. B. & Vernberg, F. J., eds). Academic Press, New York,
0025-326X(94)E0060-L
Marine Pollution Bulletin, Vol. 28, No. 12, pp. 727-734, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 01)25-326X/94 $7.00+ 0.00
Temporary and Definitive Fixation of Atmospheric Lead in Deep-Sea Sediments of the Western Mediterranean Sea FRAN(~OIS FERNEX*-~ and CHRISTOPHE MIGON~:§ *Laboratoire de GOodynamique sous-marine, CNRS UA. 132 G6osciences de l'Environnement, La Darse, BP 48, F-06230 Villefranche-sur-Mer, France +Environnements GOochimiques, Facultd des Sciences, Parc Valrose, F-06108, Nice Cedex 2, France ~;Laboratoire de Physique et Chimie Marines, Universit~ de Paris 6, CNRS 1NSU, La Darse, BP 8, F-06230, Villefranche-sur-Mer, France § CMCS, Universit~ de Corse, Grossetti, BP 52, F-20250 Corte, France
Most lead brought to the Mediterranean Sea has an anthropogenic origin and is mainly transported through the atmosphere. Atmospheric Pb was continuously collected at Cap Ferrat in 1986 and 1987. From this study, the estimation of the anthropogenic Pb flux on the whole Western Mediterranean was, averaged on 1986 and 1987 data, 4080 t. Assuming that the atmospheric anthropogenic Pb input varied in this course of time similarly to the consumption of Pb added to gasolines in France, the mean annual flux could be calculated: 3.95 kg km -2 yr-1, that is an annual input of 3360 t yr-1. Reaching the sea, this metal seems to become rapidly bound to phytoplankton. Grazing by zooplankton leads to the production of faecal pellets which frequently contain rather high metal concentrations. The sinking rate of pellets of various zooplankton species is high; within a few days pellets may reach deep-sea sediments. After deposition, Pb is released from this organic-rich material during early diagenesis. In most cases, it, therefore, returns to the overlaying water body by ascending diffusion. But, in a deep-sea area of approximately 80 000 km 2 where Mn oxide precipitation occurs in surficiai sediments, Pb seems to remain stored by coprecipitation processes. By considering the lead stored in 'excess' in the surficial sediment of the deep-sea area, we estimate that a mean annual anthropogenic Pb amount ranging from 800 up to 1080 t was stored every year from 1950. On the same area, taking into account the Pb loss at the straits, the 'direct' atmospheric input to the sea bottom is, on average, 184 t yr-L The remaining part, that is ( 8 0 0 - 1 0 8 0 ) - - 1 8 4 = ( 6 1 6 - 8 9 6 ) t yr-l, corresponds to an additional 'indirect' Pb flux in water due to Pb released from sediments of the surrounding areas where it does not remain stored.
The toxicity of lead is now well stated (Vernberg et al., 1974; Flament, 1985; Subramanian & Connor, 1991) and even once pristine locations are now contaminated (Bourton & Patterson, 1987; Nriagu, 1991). At least from 1960 to 1987, most of the lead brought to the Mediterranean Sea had an anthropogenic origin, chiefly due to the use of leaded gasoline (Pacyna et al., 1984; Pattenden & Branson, 1987; Hopper et al., 1989). Anthropogenic Pb is mainly transported through the atmosphere (Martin et al., 1989; Guieu et al., 1991; Migon, 1993) and it is easily solubilized in sea water (B6thoux et al., 1990; Chester et al., 1990). In the water body, lead divides into three: one part remains soluble, another becomes assimilated by planktonic organisms; another, bound to particulate matter, settles in the sea bottom. Coastal zones frequently display relatively high Pb levels in sediments and the concentrations globally decrease towards the open sea areas. Nevertheless, the surficial sediments of some deep-sea areas in the Western Mediterranean Sea exhibit higher concentrations (Arnoux et al., 1983) than neighbouring zones. Considering that continental shelf sediments are affected by pollution, it is attempted in this paper to determine whether there is an anthropogenic contribution to the Pb amount in offshore surficial sediments. A budget for anthropogenic Pb fluxes in the Western Mediterranean is presented in order to tentatively establish a relationship between the atmospheric inputs and the fixation in certain deep-sea sediments.
Sampling
A sampling station was established at Cap Ferrat (43°41'10"N, 7°19'30"E), on the southeastern coast of 727
Marine Pollution Bulletin
France. The characteristics of the site have been described in previous papers (Migon & Caccia, 1990; Migon et al., 1991); the fluxes measured here are presumably representative of long-range Pb transfer in the area. The low impact of local anthropogenic sources has been previously reported on the basis of a comparison (Migon et al., 1991) of aerosol samples from Cap Ferrat with those collected in sheltered areas or in the open Western Mediterranean basin by other workers (Bergametti, 1987; Dulac et al., 1987). Samples were continuously collected during 1986 and 1987. Atmospheric aerosol was sampled on Sartorius SMl1106 cellulose acetate membrane (pore size 0.45 gm; diameter 47 mm) with a filter holder Sartorius SM16510, raised at the top of a 6 m high mast and connected to pumps (Reciprotor 40 W) and counters (Gallus). The flow rate of the pumps was approximately 1 m 3 h -l and filtration was carried out over 4-8 h. Pb blank levels on filters were very low and reproducible (12-37 ng per filter). Wet deposition was collected with KFA-Julich automatic rain collector which only opens when it rains. Heavy metals existing essentially in dissolved form in rainwater under usual pH conditions (Chester et al., 1990; Migon et al., 1991), wet samples were filtered on Sartorius SM 11106 membranes previously cleaned with Suprapur 1-2 N HC1. The mean blank level on one filter was 3.3 ng of Pb. The pH values ranged from 3.7 up to 6.5, with an average of 4.6 and a standard deviation of 0.5. After pH measurements, samples were acidified to pH 2 in order to avoid adsorption, and then stored in teflon bottles until analysis. For further details, see Migon & Caccia (1990, 1993). Twenty-five surficial sediment samples (3-4 cm thick) were collected in the Mediterranean deep-sea area during the Biomede cruise for a study on the areal distribution of metals (Arnoux et al., 1983). These sediments were very thin grained and essentially constituted clays. Sediment box-cores were collected in 1987 (CF: 110 m, 43°40.5'N, 7°19.3'E; RP: 83 m, 4°51 ', 43°17.4'; SR: 1480 m 5°12 ', 42°58'; SM: 1800 m, 42°55.5 ', 5°16'; A: 2780 m, 41°6 ', 7°; B: 2800 m, 40°45.7 ', 5°16'). The cores were vertically sliced (2 cm) on board and each sample pressed in order to extract the interstitial water. The cellulose acetate filters (Sartorius) had a 0.2 gm porosity. In the whole studied area, the surficial sediments are all oxidizing. On the contrary, deep sediments are more reduced. In order to avoid oxidation, the contact with air is reduced to the minimum with transferring the sediment into the press, which is closed with a rubber membrane; the nitrogen pressure is applied on this membrane, which enables the sediment to be squeezed. The collected water is then acidified for storage (pH 1).
Analytical
Aerosol metal concentrations were measured by graphite furnace atomic absorption spectrophotometry. 728
Sections of the membrane filters (3 mm in diameter) were punched out and directly introduced into the carbon rod atomizer. The spectrophotometer used was a Varian AA 1275 equipped with a CRA 90 atomizer and ultra carbon pyrolitic crucibles. The detection limit was 50 pg. Reproducibility was better than 5%. The water samples were analysed for Pb by differential pulse anodic stripping voltammetry on a hanging mercury drop electrode. The measurements were performed with an E G & G Princeton Applied Research 264A polarographic analyser in conjunction with a 303 static mercury drop electrode. Detection limits were 100 ng 1-~. Blank values were at most equal to these detection limits; reproducibility was better than 5%. All analyses were carried out under laminar airflow benches in a class 100 clean room. The results of atmospheric measurements were published in previous papers (Migon & Caccia, 1990, 1993; Migon et al., 1991). Intercalibration tests were performed for rainwater with a good agreement. For further analytical details, see Migon & Caccia (1990, 1993). Pb and Mn from sediment were analysed by atomic absorption spectrometry. Metals were extracted from 1 g of dried sediment by means of 50 ml of a 8 N acid solution (2/3 H N O 3 + l / 3 HC10~), heated to dryness. Fifty millilitres of a HNO 3 solution (pH 1 or 1.5) was used for recovery, and then centrifuged. A simplified and partial sequential extraction procedure was utilized to extract metals from sediments of cores A and B. Sequential leaching techniques allow the metal bound to various geochemical fractions (or main sedimentary compounds) to be differentiated (see Chester & Hughes, 1967; Filipek & Owen, 1978; FOrstner et al., 1986). 1. Two grams of wet sediment were leached for 4 h with 150 ml of 0.5 M sodium acetate solution adjusted to pH 5 with acetic acid: this leaching essentially allowed extraction of exchangeable cations and the carbonate fraction. 2. After having been centrifuged under a nitrogen atmosphere, the residue from 1 was leached under a nitrogen atmosphere for 12 h with a hydroxylamine solution (NH2OH HC1 at 0.05 M) adjusted to pH 2.5 with acetic acid: this leaching mainly extracted the easily reducible fraction, in particular Mn oxides and poorly crystallized Fe hydroxides. 3. The residue of 2 was leached for 2 h at pH 5.5 with hydrosulphite (dithionite; 4 g for 50 ml): this extracted easily to moderately reducible phase, in particular free Fe oxides. The ship used for sampling was an oil burner and contamination is thus possible. Nevertheless, the Pb concentrations measured in the overlaying waters in the box-cores were at least one order of magnitude lower than those measured in the interstitial waters of the surficial sediments. Since these waters are the most susceptible to contamination such contamination should be neglected when compared with Pb levels into the surficial sediments, lntercalibration tests have shown that the results agree within 15-18% with the reference values (Loring, 1986).
Volume 2 8 / N u m b e r 1 2 / D e c e m b e r 1994 10
Results and Discussion A t m o s p h e r i c fluxes
The total atmospheric deposition of lead is the sum of the dry and wet inputs. For the calculation of the dry atmospheric flux, the mean Pb concentration in the aerosol (34.2 ng m-3; Migon et al., 1991) was multiplied by the mean dry deposition velocity, itself calculated on the basis of a total and wet deposition sampling at Cap Ferrat (0.25 cm s-~; unpublished data). This value of dry deposition velocity falls between low theoretical values (Dulac et al., 1987) and high experimental values obtained by cascade impactor data (Remoudaki, 1990). Moreover, it is very similar to the estimation of van Aalst (1988). Then, this dry deposition value is multiplied by the approximate number of dry days in the year and the annual dry flux is obtained (2 kg km -2 yr-1). The annual wet deposition of lead is the sum of the wet inputs associated with each rain event (that is the Pb concentration in rainwater multiplied by the rainfall amount). This wet flux was 3 kg km -2 yr -1 for 19861987. Therefore, the total atmospheric flux of Pb, as measured in 1986-1987 at Cap Ferrat, was approximately 5 kg km -2 yr -I (that is 4250 t yr -1 for the whole Western Mediterranean Sea). It was previously calculated that 91% of lead in the atmospheric aerosol had an anthropogenic origin in the Northwestern Mediterranean (Migon & Caccia, 1990). Also 99% of lead in rainwater originates from anthropogenic sources (Migon & Caccia, 1993). The low contribution of natural Pb emissions should be partly due to the low impact of volcanoes. Indeed, Mount Etna plumes (that is, the closest significant volcanic sources) should not affect the considered Mediterranean area (Martin et aL, 1984). It can thus be assumed that the contribution of volcanic Pb is negligible in this region. From the 1986-1987 data, it was calculated that the dry inputs represented approximately 40% of total deposition (then wet inputs account for 60%); then one can calculate that the total atmospheric flux of anthropogenic lead should be approximately 4.8 kg km -2 yr -I in 1986-1987, and the natural Pb input 0.2 kg km -2 yr-L Thus, for the whole Western Mediterranean Sea (surface area 0.85× 106 km2), the anthropogenic Pb annual input in 1986 or 1987 can be calculated as 4080 t yr -1. The annual flux of anthropogenic Pb on the whole Western Mediterranean from 1950 to 1987 can be evaluated by referring to the consumption of lead introduced in gasolines in France (Nicolas et al., 1994): the annual flux is assumed to be proportional to the consumption of lead from gasolines in France every year (Fig. 1). For the 38 years, the mean anthropogenic Pb flux was 3.95 kg km -2 yr -~, that is to say, a mean annual input of approximately 3360 t yr-1. According to Copin-Montrgut et al. (1986) and Brthoux et al. (1990), the surface waters flowing out from the Western Mediterranean to the Eastern basin through the Sicily Strait have higher Pb concentrations
Pb
8
1"7" 2
,9°0
,,'8°
,;,0
,;80
,9°0
year
Fig. 1 Variations in the course of time of the anthropogeuic Pb flux over the Western Mediterranean Sea. We assume that anthropogenic Pb was chiefly transported through the atmosphere, and its input into the sea was directly proportional to the consumption of lead from gasolines in France. The maximum yearly Pb consumption in gasolines in France was 14 500 t in 1976.
than the deep waters entering the Western basin through this strait. The loss of Pb for the Western Mediterranean is approximately 1200 t yr-L To a much lesser extent, there is also some loss at Gibraltar. So the budget at the straits shows a Pb loss for the Western Mediterranean of about 1400 t yr-L A mean annual load of 3 3 6 0 - 1 4 0 0 = 1 9 6 0 t of anthropogenic Pb accumulated, therefore, in the Western Mediterranean from 1950 to 1987. It can be assumed that a part of the load remained in the water body; there, Pb is either dissolved or it is adsorbed onto mineral particles or included in phytoplankton and faecal pellets (Fowler, 1977; Romeo et al., 1988). Another part settles down to bottom sediments with particles, chiefly faecal pellets. Storage in sediments
Generally, Pb concentrations in the sediments from near-shore areas (the infralittoral zone) are high; in the vicinity of a main sewer or a river mouth, they may exceed 8() or 100 ~tg g-l, which are in the main due to pollution. For instance, near the Rh6ne river mouth, it was estimated that, in sediments with concentrations up to 100 or 110 ~.g g-l, nearly half of Pb had an anthropogenic origin (Badie et al., 1983). Near a river mouth, the high metal concentrations in sediments can be related to short transit and high sedimentation rates which oppose the upward release of the metallic ion. Moreover, high organic matter concentrations lead there to oxygen depletion and to low redox conditions in sediment levels only a few centimetres below the interface, so that sulphate reduction occurs and metals can be trapped as insoluble sulphides. At a distance of an input point (for instance the Grand Rh6ne river mouth) the Pb concentrations in sediments of the continental shelf are lower. The concentration decline at a distance of an input point (for example a river mouth) chiefly results from metal release, first, during particulate matter transport in the seawater and, secondly, shortly after deposition (Fernex et al., 1986); there, the oxic conditions in the sediments down to a 729
Marine Pollution Bulletin
greater depth ( > 15 or 20 cm), owing to a lower organic matter content, prevent sulphide precipitation; metals can, therefore, be partly dissolved after deposition and released from the interstitial waters up to the overlaying seawater. Although generally > 8 5 % of particulate matter discharged by rivers settle on the continental shelf (0-200 m), approximately 20-30% of the discharged lead is incorporated in the water body, and it can be assumed that most of this 'labile' lead originates from human activities (natural lead is presumably bound to mineral particles, whose solubility is low, while anthropogenic lead should be associated with more degradable and more soluble particles). But, as mentioned above, the amount of anrhtopogenic lead transported from coastal areas to the open sea is very low when compared with the anthropogenic Pb amount which was brought to the whole Western Mediterranean through the atmosphere in 1986-1987; the coastal contribution to the open seawaters very probably does not reach 10 or 12%. Surprisingly, Pb is found at somewhat higher concentrations in the sediments of a deep-sea zone (where the surficial sediments are oxic) than in surrounding areas (Fig. 2). The difference between the concentrations in the richer and poorer deep-sea surficial sediments reaches 6-8 ~tg g-~. Globally, sediment cores collected whether in near shore areas or further offshore display increasing Pb concentrations from deeper levels (20-50 cm) to the top (Fig. 3; E! Moumni, 1994). The surficial sediments of all areas of the Mediterranean can, therefore, be assumed to be significantly contaminated by lead. The natural background should be given by concentrations observed in deeper levels corresponding to uncontaminated sediments, deposited during pre-industrial times. The depth where the sediments represent the natural inputs depends on the sedimentation rate, which rarely exceeds 100 cm per 1000 yr, except from the continental shelf (Chamley, 1971). In the contin-
Mn gg/g
ental shelf sediments, except from some areas where it can be slightly higher, the natural Pb concentration generally ranges from 25 to 35 ~tg g-i and from 15 or 16-25 in the sediments of offshore areas, in particular on the slope (Added et al., 1981); but it is higher in the deep-sea 'richer' zone (30-35 ~tg g-J; Fig. 3A). The highest Pb concentations in the surficial sediments of the deep-sea areas were observed where Mn concentrations are highest (Fig. 2). The richer sediments extend over a deep-sea zone of approximately 80 000 km 2. During the sequential extraction procedure with three steps, much more Mn was extracted from the upper levels of the richer zone than from deeper levels ( > 14 cm; Fig. 4). It appears that the enrichment of this metal in surficial sediments is due to dissolution of Mn oxides in the deeper levels and ion migration to the upper levels where dissolved Mn precipitated. The relatively high Pb concentrations in the sediments of the richer zone (Fig. 4) are surely related to the behaviour of this metal when reaching the bottom sediments. As a matter of fact it can be assumed that anthropogenic Pb introduced in surficial sediments by the settled matter (mainly organic) is removed from this matter during early diagenesis processes occurring there, and the interstitial waters become enriched. But in most zones of the Mediterranean Sea, the Pb 2+ concentrations in the interstitial water of surficial sediments remains too low to allow Pb oxides to precipitate. Then dissolved Pb flows from the interstitial water up to the overlaying seawater (Fig. 5). However, in the richer zone, relatively high Mn concentrations suggest that Pb was trapped in the surficial sediments during Mn precipitation, although Pb is present in interstitial waters in undersaturated concentrations as for Pb oxides. But actually, a trace metal like Pb 2+ can coprecipitate with a major dissolved metal, like Mn 2+ during Mn oxide formation if this major ion is present in apparent oversaturation with
Pb gg/g ~i~i~i~i~)i~i~i:i~i~i~i~i~i~i~iii~i !)i~i~ii~i~i~i~i~ii:~i~:~:~:~:~i~'~: i~i~i~i~i~i~i~i~i~?ii~ii~ii~i~i~ii!~ ¸ i~!~!i~ii~i~i~i!~i!~!i~i!iii~ii!~ii~ii~iiii~ii!i~!iiii~iiiiii~ii~i~i~!~ii~i!i)~J~i;i~ii!iiiiiii!ii~iii~;~iiiii~i~iiiiiiiiii!~!~ ¸¸;
Fig. 2 Distribution of Mn and Pb in deep-sea sediments of the Western Mediterranean. After Arnoux et al. (1983), modified. The shape of the contour maps is similar for the two metals.
730
Volume 28/Number 12/December 1994
g-1
pg Pb 20
40
60
80
100
1
I
I.~
II
I
0
~
10
GF
2O
i
I
20
RP
I
30
I
' SR
I
20
I
I
10
I
E tj
4O
0
I I
4050 20
40
I
I
0
20
Ii
40
I
I
1
oE 10
0 0
I
0
I
I II
lo
A
20
O~
I B
~o
2O
20 30
4O I
I Jl
I
L
I I I I
, SM
Fig. 3 Vertical profiles of Pb extracted by means of concentrated acids (65% HNO3+35% HCIO4). RP: core collected south of the Rh6ne Pro-delta (83 m); CF: southeast of Cap Ferrat (110 m); SR: slope south of the Rh6ne delta (1450 m); SM: slope south of Marseille (1780 m); (A) abyssal plain (2780 m); (B) abyssal plain (2800 m).
Natural
Pollution
Pb
;: ::.:]!t:]
0-2 [:.::.'1
2-4 [.;t
4-81:i::::~ 8-8
.::,::::::~]
f:.::::-I
=
8-10 [:::::.]
t3
1~12 "F-.:,:-t
m m
;:::-:.]
Mn
12-14 14.16
" ::; ;:: ;:. ::::."1
~
~g/g
16-18 0
r
]
r
/
200
400
600
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8 oo
ACETIC ACID
[
' 1200
Pb '
IIl
,
#g/g
' 1400
2;
[ HYDROXYLAMINE
~
i
25
DITHIONITE
Fig, 4 Deep-sea sediments. Profiles of easily extractable manganese and lead. The extraction sequence was as follows: (1) leaching by means of the acetic acid solution (pH 5); (2) hydroxylamine leaching (pH 2.5); (3) hydrosulphite leaching. It is assumed that the higher Pb concentrations in the upper levels resulted from pollution and the totality of anthropogenic Pb could be extracted by means of these three leachings.
/ phytop~ankton
P b ....
'
"
-
-
P
atmospheric
~
zooplankton /
,,
,
,
/
!
input /
/z
./phytoplank'ton
zooplankton
,,,,
f
/
/
/ /
bZ
/
Sea level
°
"-: :.:.: [" :" : i:. .': :,~;.
fecal ,pellels I
....... ~ ~ ' ~ , "': 517::':---::'-'.
~ L ....
Pb02
.
,
I
coprecipitated
~' f e c a l
,
with
Mn
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~
~
,
,
~(
Mn-,-"
- / ..:
oxides
Fig. 5 Schematic representation of lead behaviour in the Western Mediterranean Sea. Although lead is not only exported through the Strait of Sicily but also through the Strait of Gibraltar, one strait alone is represented here.
731
Marine Pollution Bulletin
respect to its oxides, for example, Mn203 (Renard et al., 1976; Renard, 1979; Fernex et al., 1992). In such a case we have to consider the following reactions: 2Mn 2+ + 3 H 3 0 --" Mn203 + 6 H + + 2e-;
(a)
Pb 2+ + 2H20--" PbO2 + 4 H + + 2e-.
(b)
Average atmospheric input of Pb 3360 + 180 t y-1 ^
/
/
I ~ / ~ /
/
Anthropogenic.
~ /, ~Pbnux
/
,,~ , * ,
A defined Pb amount can coprecipitate if: NMneO3AGMn203 + N P b O z A G P b O 2 < 0, where NMn203 and NPbO2 are the number of moles reacting simultaneously in reactions (a) and (b) whose free energy changes A G can be calculated if the concentrations, the redox potential and p H are known. In the surficial sediment interstitial waters, at 2700 m, Mn an Pb concentrations were, respectively, 45 and 1.9 ~tg 1-~. This corresponds to activities of 173 and 0.25 nmol 1-~. The redox potential was 0.43-0.45 V and the p H was 8. Such values favour the coprecipitation process leading to an 'excess' of Pb concentration of 8-10 ~tg g-l. The upper levels (0-6 cm) of core A (in the area where the Mn concentrations are high) contain more esaily extactable Pb than deeper levels (Fig. 4). It can be assumed that the Pb 'in excess' in the upper levels results from the pollution by lead since 1950. Older atmospheric pollution by lead can be considered as negligible. The sedimentation rate is low in this deepsea area: about 2 0 - 2 5 cm per 1000 yr (Chamley, 1971; Kouame et al., 1983); but considering the Mn profiles, the similarity between the two first upper levels and the regular decrease in the concentration suggest a probable effect of bioturbation. Indeed, without any biological activity, the migration-reprecipitation process should strongly enrich only one level. For the case of Pb, the upper sediments were mixed and anthropogenic lead was, therefore, distributed in the 6 upper centimetres. As shown by the Pb profile in the sediments of the richer zone, 4 0 - 5 0 gg Pb in excess per cm 2 are stored in the upper levels (0-6 cm). This amount, which is assumed to have been due to atmospheric pollution during 38 years, corresponds to a mean flux of 1-1.35 ~tg cm -2 yr -~= 10-13.5 kg km 2 yr -~. Therefore, on average, 8 0 0 - 1 0 8 0 t of Pb were stored yearly in the 80 000 km 2 richer zone.
Flux budget
The average 'direct' atmospheric flux of anthropogenic Pb to the sea bottom is, for the 850 000 km 2 of the whole Mediterranean, 3360 t Pb yr -t. Taking into account the Pb loss through the straits, the 'direct' flux to the bottom should be 184 t yr -1 (2.3 kg km -2 yr -]) over the 80 000 km 2 of the richer zone. Anthropogenic Pb stored in the deep-sea zone corresponds, therefore, to a mean 'real' flux to the bottom which appears to be greater than the direct atmospheric Pb input of the same area (taking into account the loss through the Strait of Gibraltar). The difference is: ( 8 0 0 - 1 0 8 0 ) - 184 = (616-896) t yr -~, 732
atmosphc~.¢
. . . . . . . . . "~--
Fig. 6 Schematicsectionillustrating budgets of lead in the open part of the Western Mediterranean Sea. The total atmospheric input is the sum of the anthropogenic (3360 t yr ~) and natural (180 t yr ]) contributions.The fluxto the bottom sedimentswas higher than the flux at the atmosphere-seawaterinterface. Lead must, therefore, have been removed from surficial sediments in large areas of the Westernbasin. for the 80 000 km2; or (10-13.5) - 2.3 = 7.7-11.2 kg km -2 yr -1. These 616-896 t yr -~ which were not due to the direct flux probably originated from areas where anthropogenic lead was not definitively stored (Figs 5 and 6), but was released from sediment to the overlaying water where it could again be absorbed by phytoplankton. Grazing by zooplankton, in particular salps, leads to a complementary input of Pb to the bottom by means of faecal pellets. Nival et al. (1985), Andersen & Nival (1988) and Morris et al. (1988) report downward sinking velocity of salp faecal pellets up to 1100 m d -1. Few experimental data are related to the lead flux to sediments. On the one hand, some authors evaluated the organic matter downward flux either by means of sediment traps (Buscail et al., 1987; Miquel et aL, 1990) or by modelling (Nival et al., 1985; Andersen & Nival, 1988). On the other hand, Romeo et al. (1988) measured the P b : P ratio in particulate matter from sediment traps: 0.29 ixg Pb: ~tg P. Using the Redfield ratio, we obtain: 0.29 ~tg Pb:41 ixgC = 7 ~tg Pb:mg C -~. Buscail et al. (1987) measured the organic matter flux at 600 m in the Golfe du Lion which in average was 100 mg C m -2 d -1. Andersen & Nival (1988) calculated, for the Ligurian Sea, a faecal pellet flux of 30 mg C m -2 d ~. According to Peinert etal. (1991), the average flux at 2000 m can be estimated at 5 (or 6) mg C m -2 d -~. Considering this value and the concentration calculated from the results of Romeo et al. (1988), we obtain a Pb flux to deep sea sediments of 12.8 kg km -2 yr -j. This value compares well with the evaluation of the storage rate of anthropogenic lead (10-13.5 kg km -2 yr-1).
Conclusion A great part of anthropogenic Pb brought to the sea becomes bound to particles or phytoplankton. This
Volume 28/Number 12/December 1994 s o l i d m a t e r i a l settles d o w n a n d is p a r t l y d e p o s i t e d o n bottom s e d i m e n t s w e r e it b e c o m e s incorporated p r i n c i p a l l y b y t h e effects o f b i o t u r b a t i o n . A l l surficial s e d i m e n t s in t h e W e s t e r n M e d i t e r r a n e a n should, t h e r e f o r e , b e c o n t a m i n a t e d b y h u m a n activities. B u t the totality of the settled Pb generally does not remain s t o r e d in surficial s e d i m e n t ; a l a r g e p a r t is d i s s o l v e d d u r i n g t h e e a r l y d i a g e n e s i s , a n d , as t h e c o n c e n t r a t i o n s in the interstitial w a t e r r e m a i n t o o l o w to a l l o w P b o x i d e p r e c i p i t a t i o n , m e t a l i o n s m i g r a t e u p to t h e overlaying seawater (except from near shore areas where the high sedimentation rate prevents the release o f d i s s o l v e d m e t a l f r o m interstitial water). T h e dissolved Pb originating from sediment can again be assimilated by phytoplankton or adsorbed onto particulate matter. H o w e v e r , in a d e e p - s e a a r e a o f a b o u t 8 0 0 0 0 k m 2, t h e l o c a l r e d o x c o n d i t i o n s a l l o w e d P b to c o p r e c i p i t a t e w i t h M n d u r i n g M n o x i d e f o r m a t i o n in surficial s e d i m e n t . It c o u l d b e e s t i m a t e d that, b e t w e e n 1 9 5 0 a n d 1987, on average 800-1080 t of anthropogenic Pb were s t o r e d in t h e d e e p - s e a a r e a e v e r y year. T h i s is m o r e than the direct atmospheric Pb input over the same area ( t a k i n g i n t o a c c o u n t t h e loss o f a p p r o x i m a t e l y 1 4 0 0 t yr -1 t h r o u g h t h e straits o f G i b r a l t a r a n d Sicily): 184 t y r -~. T h e d i f f e r e n c e ( 6 1 6 - 8 9 6 t yr -~) p r o b a b l y corresponds to m e t a l r e l e a s e d f r o m the surficial sediment of surrounding areas where the redox c o n d i t i o n s d o n o t a l l o w a n t h r o p o g e n i c P b to b e c o p r e c i p i t a t e d w i t h M n . In d e e p - s e a areas, ' d e f i n i t i v e ' s t o r a g e o f a n t h r o p o g e n i c P b o n l y o c c u r s in s e d i m e n t s w h e r e M n o x i d e s p r e c i p i t a t e c l o s e to t h e i n t e r f a c e . As the waters flowing out of the Eastern Mediterr a n e a n t h r o u g h t h e Sicily S t r a i t c o n t a i n less l e a d t h a n t h e w a t e r s e n t e r i n g this b a s i n , it c a n b e a s s u m e d t h a t a s i m i l a r d e p o l l u t i o n p r o c e s s o c c u r s there. The authors wish to thank B. Gentili for technical assistance.
Aalst, R. M. van (1988). Input from the atmosphere. In Pollution of the North Sea: An Assessment (Salomons, W., Bayne, B. L., Duursma, E. K. & FiSrstner, U., eds), pp. 275-283. Springer, Berlin. Added, A,, Fernex, E & Mangin, J. P, (1981), Repartitiondes oligoelements m/:talliques darts les sediments marins devant l'embouchure du Grande Rhdne. In Ves Journdes Etud. Pollutions (U.N.E.P.; Cagliari, 1980), pp. 535-543. C.I.E.S.M., Monaco. Andersen, V. & Nival, P. (1988). A pelagic ecosystem model simulating production and sedimentation of biogenic particles: role of salps and copepods. Mar. E~ol. Progr. Ser. 44, 37-5/). Arnoux, A,, Chamley, H., Bellan-Santini, D., Tatosian, J. & Diana, C. (1983). Etude mineralogique et chimique des s~diments profonds de Mediterrande Occidentale. In Vles Journ&s Etu. Pollut. (U.N.E.P.; Cannes, 1982), pp. 385 395. C.I.E.S.M., Monaco. Badie, C., Added, A., Fernex, E, Rapin, E & Span, D. (1982). Determination de la part qui revient ~ la contamination dans l'apport en metaux par le Rhdne, In Vles JournOes Etud. t'ollut. (U.N.E.P.; Cannes, 1982), pp. 65-72. C.I.E.S,M., Monaco. Bergametti, G. (1987). Apports de mati&re par voie atmospherique 5, la Mediterrande occidentale. PhD Thesis, Universit6 Paris 7,296 pp. Bethoux, J. P., Courau, P., Nicolas, E. & Ruiz-Pino, D. (1990). Trace metal pollution in the Mediterranean Sea. Ocean. Acta 13,481-488. Boutron, C. E & Patterson, C. C. (1987). Relative levels of natural and anthropogenic Pb in recent Antarctic snow. J. geophys. Res. 92, 8454-8464. Buscail, R., Guidi, L, & Bovee, E de (1987). Flux de matibre organique, variation des parametres biochimiques et dynamique biologique dans le Canyon Lacaze-Duthiers, Golfe du Lion. CoIL intern. Ocdan. Perpignan, C1ESM (Monaco); p. 73. Chamley, H. (1971). Recherche sur la s6dimentation argileuse en
Mdditerran&. Sc. gOologiques, Serv. Carte geol. Alsace-Lorraine (Strasbourg), 35; 210 p. Chester, H. & Hughes, M. J. (1967). A chemical technique for the separation of ferromanganese minerals, carbonates minerals and adsorbed trace elements from pelagic sediments. ( h e m . Geol. 2, 249-262. Chester, R., Nimmo, M., Murphy, K. J. & Nicolase, E. (1990). Atmospheric trace metals transported to the Western Mediterranean: data from a station on Cap Ferrat. Water PolL Res. Report 20, EROS 2000 (Martin, J. M. & Barth, H. eds), pp. 597-612. Envir. Res. Progr., Commiss, of European Communities. Copin-Montegut, G., Courau, R & Nicolase, E. (1986). Distribution and transfer of trace elements in the Western Mediterranean. Mw: Chem. 18, 189-195. Dulac, E, Buat-Menard, R, Arnold, M. & Ezat, U. (1987). Atmospheric input of trace metals to the western Mediterranean Sea: 1. Factors controlling the variability of atmospheric concentrations. J. geophys. Res. 92, 8437-8453. el Moumni, B. (1994). Etude sedimentologique et geochimique des depdts au Quaternaire rdcent de la partie orientale de la Marge Mediterrandenne Marocaine (Met d?Mboran). Rev. Geologie Mediterr. (in press). Fernex, F., Fevrier, G., Benaim, J. & Arnoux, A. (1992). Copper, lead and zinc trapping in Mediterranean deep-sea sediments: probable coprecipitation with Mn and Fe. ('hem. GeoL 98,293-306. Fernex, F.~ Span, D., Flatau, G. & Renard, D. (1986). Behavior of some metals in surficial sediments of the Northwest Mediterranean Continental Shelf. In Sediments and Water hueractions (Sly, P. G., ed.), pp. 353-370. Springer, New York. Filipek, L. & Owen, R. (1978). Analysis of heavy metal distribution among different mineralogical states in sediments. Can. J. Spectrosc. 22, 31-34. Flament, P. (1985). Les metaux-traces associes aux aerosols atmospheriques: apports au milieu marin du littoral Nord-Pas-deCalais. PhD Thesis, Universit~ Sciences et Techniques Lille. Forstner, U., Calmano, W., Kersten, M. & Salomons, W. (1986). Mobility of heavy metals in dredged harbor sediments. In Sediment and Water Interactions (Sly, P. G., ed.), pp. 371 380. Springer, New York. Fowler, S. W. (1977). Trace elements in zooplankton particulate products. Nature 269, 51-53. Guieu, C., Martin, J. M., Thomas, A. J. & Elbaz-Poulichet, E (1991). Atmospheric versus river inputs of metals to the Gulf of Lions. Mar. Pollut. Bull. 22, 176-183. Hopper, J. F., Ross, H. B., Sturges, W. T. & Barrie, L. A. (1991). Regional source discrimination of atmospheric aerosol in Europe using the isotopic composition of lead. ~'llus 43B, 45-6(/. Kouame, A., Fernex, F. & Poutiers, J. (1983). Sedimentation dans le secteur du Ddme TI au Sud de Toulon. (Programme ECOMED), Petrole et "l~'chnique 300, 18-20. Ass. fran~. Technic. P&role, 18-20. Loring, D. H. (1986). ICES lntercalibration for trace metals in marine sediments. Bedford Institute of Oceanography, Dartmouth, 60 pp. Martin, D., Cheymol, D., lmbard, M. & Strauss, B. (1984). Climatology of forward trajectories of Mount Etna plume over a 18-year period. BulL Volcan. 47, 1115-1123. Martin, J. M., Elbaz-Poulichet, F., Guieu, C., Loye-Pilot, M. D. & Hart, G. (1989). River versus atmospheric input of material to the Mediterranean Sea: an overview. Mar. ('hem. 28, 159-182. Migon, C. (1993). Riverine and atmospheric inputs of heavy metals in the Ligurian Sea. Sci. 7otalEnviron. 138,289-299. Migon, C. & Caccia, J. L. (1990). Separation of anthropogenic and natural emissions of heavy metals in the Western Mediterranean atmosphere. Atmos. Environ. 24A, 399-405. Migon, C. & Caccia, J. L. (1993). Estimation of anthropogenic and natural heavy metals in the Northwestern Mediterranean rainwater and total atmospheric deposition. Chemosphere 27, 2389-2396. Migon, C., Morelli, J., Nicolas, E. & Copin-Montegut, G. (199l). Evaluation of total atmospheric deposition of Pb, Cd, Cu and Zn to the Ligurian Sea. Sci. Total Environ. 105, 135-148. Miquel, J. C., Fowler, S. W. & La Rosa, J. (1990). Vertical fluxes of particulate material in a frontal zone off Corsica. Rapp. Comm. int. Met Meditern 32,280. CIESM (Monaco). Morris, R. J., Bone, Q., Head, R., Braconnot, J. C, & Nival, R (1988). Role of salps in the flux of organic matter to the bottom of the Ligurian Sea. Mar, BioL 97, 237-241. Nicolas, E., Ruiz-Pino, D., Buat-Menard, P. & Bethoux, J. P. (1994). Abrupt decrease of lead concentration in the Mediterranean Sea: a response to antipollution policy. Geophys. Res. Lett. (submitted). Nival, P., Braconnot, J. C., Andersen, V., Oberdorff, T., Choe, S. M. & Laval, R (1985). Estimation de l'impact des Salpes sur le phytoplancton en Mer Ligure. Rapp. Comm. int. Mer Mediterr. 29, 283-286. CIESM (Monaco). Nriagu, J.-O. (1991). Human influence on the global cycling of trace 733
Marine Pollution Bulletin metals. In Heavy Metals in the Environment (Farmer, J. G., ed.), pp. 1-5. CEP Consultants, Edinburgh. Pacyna, J. M., Semb, A. & Hansen, J. E. (1984). Emission and longrange transport of trace elements in Europe. Tellus 36B, 163-178. Pattenden, N. J. & Branson, J. R. (1987). Relation between lead in air and in petrol in two urban areas of Britain. Atmos, Environ. 21, 2481-2483. Peinert, R. D., Fowler, S. W., La Rosa, J., Miquel, J. C. & Teyssie, J. L. (1991). Vertical flux and microplankton assemblages in the Gulf of Lions during spring 1990. Wat. Pollut. Res. Rept 28, EROS 2000 (Martin, J. M. & Barth, H., eds), pp. 413-424. Envir. Res. Progr., European Comm. Remoudaki, E. (1990). Etude des processus contr61ant la variabilit6 temporelle des flux atmosph6riques de polluants et de poussibres min6rales en M6diterran6e occidentale. PhD Thesis, Universit6 de Paris 7; 223 pp. Renard, D. (1979). Etude g6ochiimque de l'incorporation du
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Manganbse et des filements associ6s dans les nodules polym&aliques. Thesis Dr Et., Univ. Paris VII, Lab. Gdoch. Eaux; 253 pp. Renard, D., Michard, G. & Hoffert, M. (1976). Comportement gdochimique du Cuivre, du Nickel et du Cobalt '~ I'interface eausddiment. Mineral. Deposita 11, 380-393. Romeo, M., Gnassia-Barelli, M., Nicolas, E. & Carre, C. (1988). Importance du microplancton g61atineux darts le stockage et le transfert de m6taux traces en M6diterran6e nord-occidentale. Rapp. Comm. int. Mer M~diterr 31, 35. CIESM, Monaco. Subramanian, K. S. & Connor, J. W. (1991). Lead contamination of drinking water. J. Environ. Health 54, 29-32. Vernberg, W. B., Coursey, P. J. & O'Hara, J. (1974). Multiple environmental factors on physiology and behaviour of the fiddler crab Uca pugilator. In Pollution and Physiology of'Marine Organisms: Recent Advances (Vernberg, W. B. & Vernberg, F. J., eds). Academic Press, New York,