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The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d’Arguin (Mauritania)
Continental Shelf Research 19 (1999) 665 — 691
The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d’Arguin (Mauritania) R.F. Nolting*, A. Ramkema, J.M. Everaarts Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands Received 19 September 1997; received in revised form 20 March 1998; accepted 22 September 1998
Abstract Trace and major elements were measured in sediment cores collected at the shelf and continental slope of the Banc d’Arguin (Mauritania). Most of the sediments have a calcium carbonate content '50% and as a consequence have a low aluminium ((1.5%) and iron ((1.0%) content. The concentrations of the latter two elements increase down slope. Based on the major elemental composition, the sediments seem to be well mixed and concentration gradients are only observed on a spatial distribution. Concentrations of Cu, Cd, Zn, Ni and Pb are low and approximately equal to ‘‘natural background’’ values, indicating negligible anthropogenic influences. An exception is Cd for which very high values were found in the northern most part of the research area, close to Nouadhibou. A possible explanation for this high Cd content can be the existence of phosphate rich sediments with a high affinity for Cd. Small increases in the available trace metal concentrations (exchangeable fraction, 0.1 N HCl treated) observed near the sediment surface could be attributed to early diagenetic processes, and not to pollution effects. The inputs from the Saharan dust plume with low trace metal contents, the transport of small particles with higher trace metal content down slope together with the escape of redissolved trace metals from the sediment, and the absence of a supply of metal rich material to the Banc d’Arguin, has created a metal poor environment. There is clear evidence of trace metal redistributions, associated with Mn recycling. Mean enrichment factors, calculated relative to the crustal abundance, show that Zn, Pb and Cd are equal to unity and Cu and Ni below unity. These observations suggest that the Cu and Ni contents in sediments of Banc d’Arguin are depleted relative to the accepted values in the earth crust. The very low trace metal concentrations detected in these sediments can be taken as ‘‘baseline’’ values for unpolluted sediments in the global ocean coastal zone. 1999 Elsevier Science Ltd. All rights reserved.
*Corresponding author. Fax: 0031 222 319674. E-mail address: [email protected]. (R.F. Nolting) 0278—4343/99/$ — See front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 98 ) 0 01 0 9— 5
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1. Introduction Marine sediments are the ultimate sink for particulate material supplied by rivers and the atmosphere to the oceans (Yeats and Bewers, 1983). In urbanised regions this material is very often influenced by domestic, industrial and mining activities leading to increased trace metal concentrations (Bruland et al., 1974; Duinker and Nolting, 1976, 1977; Martin and Whitfield, 1983; Trefry et al., 1985; Martin et al., 1989). A large proportion of the metal enriched particles is deposited and buried on the shelf and continental slope. When these sediments remain more or less undisturbed, and sedimentation rates are constant, they can provide us with an historical pollution record (Bruland et al., 1974; Goldberg et al., 1977; Bertine, 1980; Eisma et al., 1989; Macdonald et al., 1991; Nolting and Helder, 1991). Biological, physical, chemical and diagenetic processes can change the sediment properties, but generally these influences are detectable (Zuo et al., 1991; Nolting and Helder, 1991; Nolting and Van Hoogstraten, 1993) and can be corrected for. Marine sediments are therefore well suited to study anthropogenic influences on the marine environment. Most studies concerning historical pollution records in marine sediments, have been performed in coastal areas under direct influence of rivers, either unpolluted or polluted (Goldberg et al., 1977; Lichtfuss and Bru¨mmer, 1981; Veron et al., 1987) and are restricted to temperature regions. In the context of the global study of continental shelves it is important to collect more data worldwide, and especially from less studied tropical regions. Few such studies have been performed on the continental slope off Africa. The objective of the present study was to gather more information about trace metal contents, as well as the processes that regulate their concentrations in coastal sediments, that are not influenced by riverine input or industrial activities. As a consequence, it can provide us with the urgently needed data from regions for which man can expect to be pristine, and local influences on the coastal region to be minor. For this reason it was an excellent opportunity to analyse sediment cores collected near the coast of Mauritania, West Africa. This area is a very remote one, situated in the tropics, almost undisturbed by human impact and only local fishermen make their living there. No big rivers or industries have an impact on the coastal environment where seagrass beds occupy the larger part of the tidal flats (Vermaat et al., 1993). As a consequence of an intense upwelling phenomena resulting in high productivity and the isolated situation, Banc d’Arguin is a favourite wintering place for birds (Leopold, 1993; Wolff et al., 1993b). In addition, the atmospheric input of pollutants is probably of minor importance here due to the prevailing winds, mostly of northerly direction and passing over the Saharan desert (Chester et al., 1979; Sidane ould Dedah, 1993). Chester et al. (1979) also showed that soil-sized atmospheric particulates, transported by the Atlantic northeast trades, dominate the supply of land-derived material to sediments in the equatorial North Atlantic. In the present study, concentrations of Cu, Cd, Zn, Ni and Pb were measured in sediment cores collected along two transects radiating into the Atlantic Ocean, across the continental slope perpendicular to the Banc d’Arguin (Mauritania) to investigate their concentration, distribution pattern and the processes that regulate them. A study
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of copper, zinc, lead and cadmium in surface sediment in relation to epibenthic invertebrates and the grain size fraction in this surface layer has already been published by Everaarts et al. (1993). Results of the whole expedition, including hydrology, upwelling system and ecological studies can be found in Wolff et al. (1993a).
2. Sampling and analysis Eight sediment cores were collected with a cylindrical box corer (i.d."30 cm) during a cruise with the Research Vessel ‘‘Tyro’’ from 7 until 29 May 1988. The box corer was designed to obtain undisturbed sediments and was closed during recovery to prevent contamination. Sampling locations are presented in Fig. 1, while their
Fig. 1. Map of Banc d’Arguin with station numbers and sampling positions.
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 1 Station number, its geographical position, water depth, organic carbon and nitrogen content and % grain size fraction (63 l in the surface layer of the sediments Station
Position
Southern transect 2b 19°20 1 N/17°08 9 W 2a 19°24 1 N/17°05 3 W 2 19°33 1 N/17°00 6 W 3 19°36 5 N/16°52 5 W Northern transect 5a 19°54 3 N/17°40 0 W 5 19°57 3 N/17°40 9 W 7 20°02 2 N/17°25 9 W 8 20°04 4 N/17°21 4 W
Water depth (m)
%C
%N
% grain size(63 l
1374 896 121 27
3.25 1.54 1.80 0.74
0.41 0.20 0.10 0.13
69 25 14 72
1080 112 54 31
2.19 1.18 1.59 0.93
0.28 0.13 0.16 0.12
39 8.9 32 27
geographical position and some hydrographical parameters are given in Table 1. Sub-samples were obtained by inserting perspex liners, diameter 6 cm, into the sediment. They were closed with rubber stoppers and stored deep frozen (!23°C) until further processing in the laboratory. The data obtained from the two transects are presented as a northern (St. 5a, 5, 7 and 8) and southern transect (St. 2b, 2a, 2 and 3). After thawing, the cores were cut in slices of 0.5 cm thickness at the surface increasing to 4 cm at the bottom of the core (see Appendix). Approximately 5 g of each slice was dried at 60°C and ground and homogenized with a teflon mortar and pestle. From this homogenized sediment, 100 mg was transferred into a 50 ml polypropylene measuring flask which was filled with 0.1 N HCl to release the exchangeable fraction (Duinker and Nolting, 1976). After 18 h the solution was filtered through a precleaned 0.45 lm cellulose nitrate filter and the filtrate stored for analyses. The residue and filter were transferred to teflon bombs, 5 ml HF and 1 ml Aqua Regia (HCl : HNO , 3 : 1) were added, and the sediments were digested at 110°C for 2 h. Sub sequently, the content of the bombs was poured into a 50 ml polypropylene measuring flask which contained 30 ml saturated boric acid, and filled up with Milli Q water to 50 ml (Rantala and Loring, 1977). This method was regularly checked at our laboratory, see below, and a loss of the total content of approximately 6% was found by summing up the leachable and residual fraction compared with direct total digestion (Van Hoogstraten and Nolting, 1991; Happee and Nolting, 1994). To avoid contamination all chemicals used were of Suprapur quality and all materials were intensively cleaned with acid and rinsed with ultra clean water before use. Cu, Cd and Pb were determined with graphite furnace atomic absorption spectroscopy (Perkin Elmer 5100, Zeeman GFAAS) and stabilized temperature platform furnace (STPF). For the determination of Ni a regular tube with wall atomisation was
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Table 2 Results of fivefold analyses of a representative sample. Presented are mean concentrations and standard deviation from the leachable and residual fractions. Total, represents the summation of the individual leachable and residual results of each sub-sample Element
Leachable (n"5)
Residual (n"5)
Total (n"5)
Cd lg g\ Cu lg g\ Ni lg g\ Pb lg g\ Zn lg g\ Mn lg g\ Fe% Mg% Al% Ca% Si%
0.27$0.02 0.82$0.12 n.d. 1.76$0.3 4.45$0.31 17.61$1.03 0.06$0.01 0.47$0.01 0.05$0.0 16.97$0.26
0.06$0.02 1.55$0.34 n.d. 5.79$0.4 26.2$2.2 59.24$0.97 0.95$0.05 0.38$0.02 2.83$0.03
0.33$0.03 2.35$0.31 n.d. 7.55$0.67 30.65$2.19 76.85$1.18 0.99$0.05 0.85$0.02 2.88$0.03 16.97$0.26 20.22$0.57
20.22$0.57
Table 3 Measured and certified values ($1 S.D.) of trace metals and major elements in the BCR samples 141 and 142. Values between brackets are not certified. N.D. means not determined
Element Cu lg g Cd lg g Ni lg g Pb lg g Zn lg g Mn lg g Fe O % Al O % CaO% SiO %
BCR 141 Determined 32$0.3 0.39$0.08 30.2$1.9 29.8#0.7 80.1$1.2 506$17 3.5$0.4 10.52$0.0 15.35$0.11 N.D.
Certified 32.6$1.4 0.36$0.10 (31$3) 29.4$2.6 81.3$3.7 (547$32) (3.74) (10.56) (42.58)
BCR 142 Determined 27.6$0.5 0.30$0.06 27.4$0.4 40.7$0.8 88.8$2.0 536$25 2.80$0.01 N.D. (17.98)
Certified 27.5$0.6 0.25$0.09 29.2$2.5 37.8$1.9 92.4$4.4 (569$26) (2.80) (9.48) 3.91$0.28 (4.94) 66.9$0.4 (68.22)
used. Zn was determined by flame AAS with a high-performance nebulizer (Perkin Elmer 2380). Major elements Fe, Mn, Al, Ca, Mg and Si were also determined by flame AAS. Concentrations were calculated using the standard addition method. To determine the reproducibility of the method, one representative sample was analysed five times for the exchangeable as well as the residual fraction (Table 2). To determine the accuracy of the method reference sediments, supplied by the EC Bureau of References (BCR141 and BCR142) were included during the entire analytical procedure (Colinet et al., 1983a,b). For all elements the measured values were in good agreement with the certified values (Table 3).
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Organic carbon and total nitrogen were determined with a Carlo-Erba NA 1500 CHNS analyser according to Verardo et al. (1990). The precision of the carbon and nitrogen determinations were better than 99.9% and the accuracy within 0.01%.
3. Results and discussion From four cores (2a, 3, 5a, 8) all slices were analysed and from the others some selected ones at different depths. The concentrations of the trace and major elements in the sediment cores are given in the appendix. Firstly, the surface concentrations of the major elements, calcium carbonate and minor elements with their spatial distribution are described, and next the vertical distribution in the sediment cores. 3.1. Spatial distribution The organic carbon content is highest at stations 2b and 5a (2.19—3.25%) (Table 1). The C/N ratio at all stations is around 8, excluding St. 2, (18) which indicates that most organic material in this area is predominantly of marine origin (Macdonald et al., 1991). This is not surprising taking into account that this part of the African coast is a major upwelling region, with high phytoplankton production and a supply of organic matter (dead debris) to the continental shelf (Wolff et al., 1993b; Loktionov, 1993). It has to be stressed that between 20 and 24°N, in an average year, 70—80% of the time northerly winds favour upwelling phenomena (Schemainda and Nehring, 1975). A short description of the sediment cores follows. The colour of most cores is light grey, at station 3 and 8 light grey pale and at station 2 light grey brown. No distinct colour differences with depth could be observed in all cores, only some small darker coloured bands were present at the upper surface layer of the cores collected at the deepest stations. Some shell remains were present in the sediments from shallow depth. Grain size distributions of the surface sediments were published by Everaarts et al. (1993). In Table 1 the fraction smaller than 63 l, clay and silt, is given and this fraction is at most stations higher than 25%. The grain size fraction '500 l is in most cores less than 5%, and only at station 2 a larger fraction of 23% is measured. It is interesting to note that this last fraction is more than 45% in sediments collected at very shallow water (5—18 m) close to the beach, demonstrating the escape of small particles from the coastal zone. Concentrations of Fe, Al, Si, Mg (as their oxides) and Ca as carbonate, in the surface sediments (upper cm) are presented in Fig. 2. The calcium carbonate concentration, calculated from the respective Ca content of each sample, increases in sediments along both transects from deeper to shallower water depth. This increase is more pronounced along the northern transect. These results are in contrast with the oxides of the other elements which show in general a decrease towards the shallow coastal area, where the lowest elemental concentrations for Fe (1.00% and Al(1.5% are found at the northern transect. SiO , (quartz, biogenic silica) along the northern transect only, decreases in concentration towards the coast. No clear trend for SiO is
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Fig. 2. Concentration of the major elements expressed as their oxides and calcium as calcium carbonate in the surface sediment of the different stations. (upper Fig. southern transect, lower Fig. northern transect).
observed along the southern transect. The sum of the oxides of Fe, Al, Si, Mg and Ca as calcium carbonate is lower in the sediments at the offshore stations 2b and 5a (80—90%) than at shallower depth (90—95%). On the other hand, the highest concentrations of organic carbon are found at these offshore stations. The increased
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Fig. 3. Concentrations of Cu, Cd, Pb, Ni, Mn and Zn (lg g\) in the surface sediment of the different stations, calculated on a calcium carbonate free basis. (left Fig. southern transect, right Fig. northern transect).
concentrations of aluminium- and iron-oxides at greater water depths, suggest a larger amount of clay minerals. This is supported by the Si/Al (weight/weight) ratio, which is 3.8—5.2 at the deepest stations and 6.6—11.8, maybe due to SiO variations, at the shallower part of the Banc d’Arguin. Deep sea clay has a Si/Al ratio of 2.9, but 5 or higher in siliceous sediments (Martin and Whitfield, 1983). Concentrations of trace metals in the bulk samples can be found in the Appendix, whereas those calculated on a calcium carbonate free basis are given in Fig. 3. Although the concentrations for most metals are low and comparable with other unpolluted sediments (Chester, 1990; Everaarts et al., 1993: Everaarts and Nieuwenhuize, 1995; Nolting and van Hoogstraten, 1993; Nolting et al., 1996) it is possible to
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observe some trends. Cu, Zn, Ni and to a minor extent Pb concentrations decrease from deeper waters towards shallower depths, particularly in the southern transect. There is, however, one exception: Cd concentrations tend to increase towards shallower waters, most pronounced in the northern transect where at station 8 a twentyfold increase in concentration occurs as compared to that at the deepest stations. The source for the enriched Cd concentrations in the northern transect is not known. Storage of iron ore, at Cap Blanc north of Banc d’Arguin could be a source (Sidane ould Dedah, 1993) but the existence of phosphate rich sediments with a high affinity for Cd is another possibility. Although other sources cannot be excluded, we have too little information from the area to draw fixed conclusions. It should be noted that almost 100% of the Cd content is in the exchangeable fraction, which means that the metal is adsorbed to sediment particles and in a mobile form. This trend is also evident after normalising relative to Al. The range of the element/Al;10\ ratios in all samples are 1.4—4 for Cu, 1.8 to 2.2 for Pb, 3.0—6.8 for Ni, 9.0 to 16.7 for Zn and 0.04 to 1.08 for Cd. These ratios are, with exception of Cd, comparable to or even lower than those published for continental soils and crust (Chester, 1990). We conclude that all elements have approximately a natural background concentration, with exception of Cd which seems to be enriched in surface sediments in the northern part of the Banc d’Arguin. 3.2. Depth distribution Depth profiles of Ca and Si for St. 2a, 3, 5a and 8 are given in Fig. 4. Concentrations for Fe, Al and Mg and results for the other stations can be found in the Appendix. No substantial differences could be observed in the vertical distribution pattern for the major elements. Fe/Al ratios are constant with depth for each site separately, and range from 0.36 for station 3 to 0.59 for station 7, with a mean value of 0.47. Based on this distribution, sediments of the Banc d’Arguin seem to be well mixed, and the sediment supply homogeneous. Differences can only be found in the horizontal distribution, as described previously. Depth profiles of Mn, Cu, Zn, Ni, and Pb for St. 2a, 3 (southern transect) and 5a and 8 (northern transect) are shown in Fig. 5. Although some of these profiles show an increase in concentration towards the surface layer, this is not necessarily an indication for anthropogenic input. It is more likely that early diagenetic processes are responsible for this phenomenon (e.g. Ridgway and Price, 1987; Macdonald et al., 1991). The sediments of Banc d’Arguin are very poor in Mn (40—80 lg g\), compared with other marine and coastal areas (Sundby et al., 1981; Sundby and Silverberg, 1985; Helder, 1989; Chester, 1990; Macdonald et al., 1991; Nolting et al., 1996; Nolting and Van Hoogstraten, 1993). The Mn concentration is lower than the values reported by Happee and Nolting (1994) in sediments of the upwelling area near Oman (170 lg g\). Mn increases in the exchangeable fraction in the upper surface layer, probably due to the reoxidation of upwardly diffused Mn> which is more pronounced at station 2a than at station 5a. Compared to areas with strong redox-cycling this increase is, however, very small (Nolting et al., 1996). No obvious signs for Mn recycling are seen in the calcareous sediments at stations 3 and 8. This may be due to a
Fig. 4. Depth profiles of Ca and Si (%) in sediments of St. 2a, 3 (southern transect) and 5a and 8 (northern transect).
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Fig. 5. Depth profiles of residual and total content of Cu, Pb, Ni, Mn and Zn (lg g\) in sediments of St. 2a, 3 (southern transect), 5a and 8 (northern transect).
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Fig. 5. (Continued.)
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Fig. 5. (Continued.)
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Fig. 5. (Continued.)
low supply of organic matter and, consequently, a lack of reducing conditions at depth in the sediment, or possibly under strong reducing conditions all reactive Mn has escaped to deeper water. Despite these differences between the stations, there is a consistency between the Mn geochemistry at each site, indicated by the down-core ratio between residual and total Mn, which is about equal (&0.70) at all stations. Depth profiles of Cu and Zn show more or less the same pattern as Mn. The enhanced Cu and Zn concentrations found in the surface sediment of some stations, reflected also in a small increase in the exchangeable fraction, suggest a correlation with Mn. Plotting Cu and Zn against Mn (Fig. 6), shows such a correlation at St. 2a for both Cu and Zn, and at St. 5a for Zn. This may indicate diagenetic redistribution of Cu and Zn controlled by the redox cycle of Mn, rather than a connection with anthropogenic input of these metals. We come to the conclusion, that in these unpolluted sediments, depth distributions of trace metals are mainly controlled by diagenetic processes, with a strong coupling to Mn (and possibly Fe). It should be emphasized that this coupling between trace metals and Mn redistribution, was visible due to the absence of nearby river inputs where sediment supply with elevated trace metal contents can highly influence levels of trace metals in bottom sediments, and hence mask these processes. Also the presence of the Saharan dust plume with low trace element concentrations (Chester et al., 1979; Kremling and Streu, 1993), preventing the deposition of metal enriched material to the shelf is another factor which strengthen these observations. On the basis of the mineralogical, chemical and biological composition of atmospheric particles, Chester et al. (1979) concluded that the transport of dust loadings to the equatorial North Atlantic dominates the land-derived material. Notably, the elemental aluminium ratios in soil-sized atmospheric particles collected in the Atlantic northeast trades (Chester et al., 1979), reflect the ratios found at our deepest stations. This lack of supply of metal rich material to the shelf and the presence of a high load of organic material, derived from planktonic species in these upwelling areas (Wolff et al., 1993b), can be responsible for the low
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Fig. 6. Relation between total Mn and Cu, Pb, Ni and Zn (lg g\) in the cores of St. 2a, 3 and 5a.
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Fig. 6. (Continued.)
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trace metal contents observed in the sediments. Diagenesis of this organic matter, creating reducing conditions, mobilising Mn and adsorbed metals and a subsequent transported of these elements from the shelf to the continental margin (Johnson et al., 1992), results in a metal poor environment. Ni shows more or less the same pattern as Cu and Zn, low concentrations at the Banc increasing seaward. One exception is the Ni concentration at St. 3, where no exchangeable fraction could be measured. There is no explanation for the absence of this fraction. All determinations were done at the same time and in the same batch. At all other cores the fraction ranges between 17 and 30%. Ni seems also to covary with Mn, with best correlations for St. 2a and 3 (Fig. 6). Cd does not show strong variations with depth in all cores (appendix 1). The high content at St. 8 is discussed in the preceding section dealing with the horizontal distribution. The Cd concentrations are consistently high down core at St. 5, 7 and 8 with a value of around 1 lg g\. The exchangeable fraction contributes almost 100% at all stations, indicating that the Cd measured in the sediment cores is mobile, and regulated by adsorption—desorption processes. In contrast to Cu, Zn and Ni, Cd does not show any positive relation with Mn. This is not surprising taking into account its opposite behaviour in relation to Mn in the marine environment, which means that when Mn shows an increase in concentration to the surface of sediments due to redox cycling, the Cd concentrations show a decrease. This contrasting behaviour is described for sediments as well as that for the water phase. As for example, Gobeil et al. (1987) and Gendron et al. (1986) have shown that Cd behaves opposite to Mn in sediment cores collected in the Laurentian Trough, during early diagenesis. In the river Scheldt, where the upper estuary is anoxic and clearly separated from the oxic lower estuary, Cd and Mn show the same ‘‘reverse’’ behaviour in the water phase (Duinker et al., 1982). Cd is closely related to the biogenic cycle, as evidenced by its relation to phosphate in the water column (Boyle et al., 1976; Knauer and Martin, 1981; De Baar et al., 1994). Due to degradation of organic debris in the oxygenated surface layer, Cd is normally released from the sediment and escapes to the water phase (Gobeil et al., 1987). However, if this process is valid for oxygen rich surface sediments, it may explain the very high and constant Cd concentrations found in the sediments of Banc d’Arguin. If reducing conditions exist in these sediments of Banc d’Arguin, the high Cd and low Mn content are a result of this situation. An example of this last reasoning is the high Cd concentration observed in sediments of the Laurentian Trough, associated with a high organic load for sinking particles resulting in an anoxic environment (Gendron et al., 1986). However, in the situation presented here the organic load cannot explain the observations, since there are no large differences in the organic carbon content between the two transects (Table 1). This organic content ranges between 0.7 and 1.8% and has a C/N ratio of around 9. Thus, it is more likely to assume that there is an unknown additional input of Cd (dissolved or particulate), which is fixed and stored in the sediment, or that phosphate rich sediments high in Cd content are the explanation for this peculiar situation. Pb is enriched in the top few centimeters of core 5a and to a lesser extent of core 2a. This increase is mostly attributed to the exchangeable fraction, and shows a clear
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 4 Pb concentrations in the sediment cores expressed on a calcium carbonate free basis from all stations (S. surface, B. bottom of core). Ratio of Pb content divided by the background concentration of 6 lg g\ (Pb.n) Station
Pb
Pb/Pb.n
2b S B 2a S B 2 S B 3 S B 5a S B 5 S B 7 S B 8 S B
11.8 11.92 11.01 7.98 6.91 7.67 9.07 8.87 11.30 7.70 6.93 5.83 7.50 5.44 7.37 6.82
1.97 1.99 1.84 1.33 1.15 1.27 1.51 1.47 1.88 1.28 1.12 0.97 1.25 0.91 1.23 1.14
correlation with Mn in core 2a. Pb is mostly supplied to the open marine environment by atmospheric inputs (Settle and Patterson, 1982; Veron et al., 1987; Martin et al., 1989; Nolting and Helder, 1991), and was used as an excellent tracer to study historical pollution records (Goldberg et al., 1977; Hamelin et al., 1990; Nolting and Helder, 1991), assuming that sedimentation rates are constant and bioturbation negligible (Zuo et al., 1991). Down-slope transport of surface sediments with a higher lead content together with the downward flux of Pb bound to large biogenic particles (Lambert et al., 1991), can explain the increased Pb values found at the deeper part of the Banc d’Arguin, as well as for the other trace metals. Although anthropogenic inputs are supposed to be the major source for the increased Pb concentrations found in marine sediments, an increase due to diagenetic changes cannot be excluded. Gobeil and Silverberg (1989) have shown that a part of the enriched Pb content of sediments from the Laurentian Trough could be explained by mobilisation and fixation processes, redox controlled by Fe diagenesis. This implies that the Pb enrichments in surface sediments should be interpreted very cautiously, in order to avoid an overestimation of the anthropogenic impact, leading to incorrect budget calculations. Expressed on a calcium carbonate free basis, the total Pb concentration in the sub-surface layer of all cores is 5—8 lg g\, increasing to &11 lg g\ in the surface layers of cores 2a, 2b and 5a (Table 4). Taking into account a background value of 12 lg g\ Pb in continental crust (Taylor, 1964), the down-core values presented here
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are about a factor of 2 lower. Veron et al. (1987) calculated a background lead content of &15 lg g\ in the silicate fraction of sediment cores collected in the North-East Atlantic. They argued that this background level does not necessarily represent a ‘‘natural’’ level. A background value of 8—10 lg\ Pb, corrected for calcium carbonate, was also found in sediment cores collected in the Mediterranean Sea (Nolting and Helder, 1991) and on the continental slope of Oman (Happee and Nolting, 1994). This indicates that a value of around 8—10 lg g\, or even lower is a more natural and realistic value for the Pb content found in the unpolluted, carbonate free, sediments around the African and Arabian continents, than the values of 12—15 lg g\ reported so far for the northern hemisphere. 3.3. Element to aluminium ratios Normalising elements relative to Al is widely used to compensate for differences in grain size variations and carbonate content. This method is also a powerful tool for the comparison of trace metal contents in sediments in different areas around the world and can also be applied to determine enrichment factors (EF). EFs are mainly used for the assessment of aerosols in order to understand the difference in composition in terms of desert and anthropogenic elemental sources (Chester et al., 1992; Molinari et al., 1993). In the present study the EF values are applied to evaluate the dominant source of the sediments and as indicators for pollution effects. Table 5 shows the calculated EFs of the analysed elements with respect to those determined in the crustal abundance (Taylor, 1964), employing the equation EF"(E/Al) /(E/Al) , in which (E/Al) and (E/Al) are the concentrations of the respective element E and of Al in the sediment and in the crustal material. (Molinari et al., 1993; Kremling and Strue, 1993). Enrichment factors close to 1 point to a crustal origin, while those with a factor '10 are considered to have a non-crustal source. It is clear from the data (Table 5) that all elements have an EF close to unity, with exception of Cd. The low EFs ((1) found for Cu and Ni are remarkable, and indicate that these elements are depleted in the sediments of the area of study, compared to those in the crustal abundance. The higher EF found for Cd in the northern transect is an indication for local inputs or a special situation, as discussed before. Comparing the EFs presented here with those reported by Kremling and Streu (1993), for sediment trap material collected at 21° 55.8’N, 25° 14.2’ W, off the coast of Mauritania, and those reported for Saharan aerosols (Murphy, 1985), a reasonable similarity (Table 6) can be established. EFs found in sediments of the Banc d’Arguin are at the lower end of those found in Saharan aerosols. Although, there are limitations in the comparison of aerosol derived particles with marine sediments, the tight (excluding the high Cd values) similarity between the obtained enrichment factors strongly suggests the crustal origin with Saharian inputs (Chester et al., 1979). Enrichment factors determined for trace elements in aerosols collected in urbanised areas (Murphy, 1985; Chester et al., 1992; Molinari et al., 1993), are orders of magnitude higher than those in the present study area. Also, the low Al/Si ratio of
684
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 5 Enrichment factors (EF) with respect to crustal ratios (Taylor, 1964), based on mean concentrations determined in whole cores. See text Station
Cu
Zn
Cd
Pb
Ni
2b 2a 2 3 5a 5 7 8
0.59 0.45 0.30 0.21 0.39 0.28 0.23 0.27
1.61 1.35 1.76 1.06 1.53 1.59 1.80 1.88
1.79 1.45 3.88 2.71 2.79 29.58 22.08 41.67
1.25 1.38 1.48 1.25 1.25 1.32 1.32 1.38
0.74 0.60 0.54 0.30 0.66 0.55 0.58 0.44
Table 6 Mean enrichment factors (EF$1 S.D) with respect of crustal ratios of all samples, compared to data reported for sediment trap material and Saharan dominated crustal aerosols (n"8). Cd data are separated in ranges, excluding the high values (n"5) Element
This work
1020 m
4120 m
Saharan aerosols
Cu Zn Cd n"5 Pb Ni
0.34$0.27 (Low) 1.57$0.27 (Low) 13.2$15.73 (High) 2.70$0.99 1.33$0.08 (Low) 0.55#0.13 (Low)
1.1$0.2 3.1$1.3 10.5$6.8
1.4$0.2 1.8$0.5 2.2$1.0
0.8—2.2 1.7—8.1 3.2—17
3.4$0.4 0.94#0.36
3.1$0.2 0.72#0.23
3.5—23 0.7—2.5
Sediment trap data from Kremling and Streu, (1993) collected at 1020 and 4120 meter water depth off the coast of Mauritania. Data from Murphy (1990).
&0.15 measured here differs significantly from the ratio of &0.40 in desert-dominated samples reported by Molinari et al. (1993). This indicates that sediment of the Banc d’Arguin has a high quartz content, that a part of the silicate fraction is of biogenic origin and not terrestrial, or that clay fractions are deposited further away from the shelf. Depth profiles of element/Al ratios for the four cores are shown in Fig. 7. Overall, the ratios do not show large variations with depth. Only Pb in core 5a shows an increase to the surface. In the upper surface layer, the effect of early diagenesis is reflected in the increase of Mn, Cu, Zn and Ni in cores 2a and 5a. Actually, the element/Al ratio support the conclusions made earlier for the vertical section, but some additional information can be derived from the profiles. The highest ratios are present at the deepest stations, which points at additional inputs of trace metals not related to Al. A possibility is transport of remobilised trace metals from the shelf, adsorption to small particles and sedimenting down slope, or
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
685
Fig. 7. Depth profiles of Cu, Pb, Cd, Ni, Mn and Zn, normalized against Al (weight/weight) 10\ in sediment cores of St. 2a, 3 (southern transect) and 5a and 8 (northern transect).
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
scavenging of dissolved metals from the water phase by small particles and a subsequent burial to the sediment. Fluxes of particles down column with the same element/Al ratios as at our deepest stations are reported by Kremling and Streu (1993), for a sediment trap station 1000 km west of the African coast. Transport of trace metals through the coastal zone, following remobilisation and adsorption processes resulting in metal enriched particles, from the shelf to the continental margin and deep sea was described by Bewers and Yeats (1989). Our data confirm their conclusion that a significant portion of the metals supplied to the coastal zone find their ultimate fate in the sediments of the deep sea. 4. Conclusions Sediments collected at the Banc d’Arguin are dominated by a mixture of calcareous and siliceous (quartz, biogenic silica) material and are well mixed. The calcium carbonate content is more than 50% in sediments at shallow water depth decreasing in an off-shore direction. Trace metal concentrations measured in the study area are low and comparable to those found in ‘‘pristine’’ environments. Calculated on a calcium carbonate free basis all trace metals, with exception of Cd, show increasing concentrations with water depth. Early diagenetic processes are responsible for the enriched Mn and trace metal concentration observed in surface layers, clearly showing that in unpolluted sediments diagenetic processes regulate trace metal distributions. Sub-surface Pb concentrations belong to the lowest ever determined in marine sediments and can be considered as natural concentrations. Export of the metal enriched surface layer down slope, mobilisation of metals and a shortage of supply of Mn and metal rich material to the Banc d’Arguin, is an extra factor that keeps the trace metal content in the sediments low. Normalisation of the elements relative to Al is used to determine enrichment factors (EF). Compared to the crustal abundance the EFs are amongst unity for Zn, Pb, and Cd (except for the coastal area of the northern transect) and below unity for Cu and Ni. Summarising, it can be concluded that the trace metal concentrations determined in the sediment of Banc d’Arguin should be regarded, as a ‘‘baseline’’ for unpolluted sediments in the global ocean coastal zone. Acknowledgements Special thanks are due to Captain J. de Jong and his crew of the R.V. Tyro and P.A.W.J. de Wilde for his stimulating interest as Chief-scientist during the cruise. Jeroen de Jong, Loes Gerringa and Wim Van Raaphorst gave valuable comments to earlier versions of the manuscript. This research has been carried out as a part of the Mauritania I Expedition, co-organised and funded by the Netherlands Marine Research Foundation (SOZ). Three anonymous referees gave valuable comments from which the paper benefited. This is publication No. 3211 of the Netherlands Institute for Sea Research.
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
Appendix Concentrations of Cu, Cd, Ni, Zn, Pb and Mn (lg g\) and Fe, Al, Mg, Ca, CaCO and Si (%) in the sediment cores of Banc d’Arguin. ¹ is total concentrations and ¸ is the percentage exchangeable fraction Station
Dept (cm)
Cu ¹
¸
Cd ¹
¸
Ni ¹
¸
Zn ¹
¸
2b
1.0 8.0
18 18
11 9
0.19 0.22
100 100
32 27
31 34
65 62
17 14
2a
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
12 11 11 9 9 9 9 9 8 8
16 14 12 11 10 12 11 11 12 12
0.10 0.08 0.07 0.15 0.10 0.10 0.10 0.11 2.35 0.11
100 88 100 47 100 100 100 100 3.8 100
20 19 17 17 17 18 16 17 16 16
27 26 28 25 25 26 26 25 25 24
45 42 38 39 38 39 24 37 34 35
2
1.0 2.0 4.0 6.0
3 5 3 3
0 0 0 0
0.14 0.21 0.15 0.15
100 38 100 100
8 8 9 8
23 19 19 15
3
0.5 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0 18.0
4 5 6 4 4 4 4 3 4 4
10 16 6 6 10 9 11 10 10 10
0.16 0.19 0.18 0.18 0.17 0.16 0.17 0.17 0.19 0.18
94 100 100 100 100 100 100 100 84 100
9 9 9 8 8 7 7 7 7 7
5a
0.25 0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
9 9 9 10 9 8 8 8 8 8 8
12 12 10 6 12 12 9 10 12 12 11
0.17 0.19 0.22 0.22 0.22 0.22 0.23 0.23 0.26 0.27 0.22
100 100 100 100 100 96 100 100 100 100 100
5
0.5 1.0 8.0
3 3 3
21 13 14
1.01 0.80 0.91
7
0.5 1.0 8.0
2 2 3
15 11 25
0.84 0.75 0.67
Pb ¹
¸
Mn ¹
¸
8.9 8.7
32 29
112 95
40 29
17 17 18 16 16 14 16 12 11 13
7.1 7.2 6.4 6.6 6.6 6.1 5.7 6.2 6.0 6.0
24 26 21 21 19 19 15 19 14 16
123 96 88 69 65 59 56 58 52 58
60 50 47 36 26 29 27 26 23 26
24 24 22 22
31 22 29 23
3.0 3.6 3.9 3.6
30 17 26 21
36 39 37 42
31 31 29 28
0 0 3 0 0 0 0 0 0 0
26 28 28 27 24 24 25 27 27 28
9 13 9 14 11 6 8 13 11 13
5.4 5.6 5.4 5.2 5.1 5.4 5.4 5.0 5.1 14.5
26 24 20 21 21 22 22 24 23 8
79 82 81 77 77 78 76 77 77 76
27 26 26 25 23 23 23 24 24 23
19 19 18 23 20 9 21 21 21 20 21
28 30 28 24 26 30 23 24 26 24 26
53 44 49 43 44 41 39 42 42 43 43
31 20 26 14 17 14 18 17 20 23 19
6.8 6.3 6.1 6.7 6.2 5.6 5.4 6.0 4.9 4.4 4.4
44 43 44 37 44 38 41 33 34 33 35
105 98 85 79 79 76 77 79 73 74 73
48 43 35 28 28 30 29 28 31 28 31
100 100 100
7 6 7
27 29 26
19 18 20
27 26 34
2.8 3.4 2.8
30 24 30
27 37 33
41 33 38
94 100 100
8 8 7
24 26 24
23 22 21
32 26 25
3.2 2.7 2.4
33 31 24
37 37 36
27 27 25
688 8
Station
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 1.0 2.0 3.0 4.0 6.0
2 2 2 2 2
Dept (cm)
Fe ¹
31 25 4 7 8
1.28 1.36 0.97 1.13 1.14
¸
100 99 100 97 98
5 4 4 5 5
24 20 20 17 17
20 19 19 21 18
35 29 29 34 32
2.6 2.6 2.6 2.6 2.4
35 25 25 32 34
30 33 33 33 29
28 29 29 33 33
Al ¹
¸
Mg ¹
¸
Ca ¹
CaCO ¹
Si ¹
2b
1.0 8.0
2.34 2.29
8 6
4.66 4.59
3 3
1.10 1.08
42 41
10.03 11.09
25.08 27.73
17.74 18.18
2a
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
1.77 1.65 1.52 1.48 1.46 1.54 1.37 1.47 1.38 1.41
10 10 10 10 8 8 8 8 7 6
3.34 3.19 3.24 3.02 3.04 2.93 2.98 2.99 2.89 2.93
3 3 3 3 3 3 3 3 3 2
0.70 0.69 0.60 0.62 0.63 0.64 0.59 0.62 0.6 0.60
37 39 43 39 41 44 46 40 40 40
11.96 11.19 11.28 10.38 9.76 12.90 10.90 10.42 11.00 9.75
29.90 27.98 28.20 25.58 24.40 32.25 27.25 26.05 27.50 24.38
24.24 23.78 21.86 24.80 26.53 22.76 23.44 25.71 26.09 27.18
2
1.0 2.0 4.0 6.0
0.85 0.81 0.78 0.79
17 14 13 13
1.56 1.53 1.62 1.69
5 4 3 3
0.74 0.81 0.76 0.77
68 69 68 68
22.83 22.10 21.65 21.23
57.08 55.00 54.13 53.30
15.42 14.21 15.52 16.23
3
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0 18.0
1.06 1.06 1.09 1.10 1.03 0.99 0.99 1.00 0.99 0.99 0.96
8 8 7 7 7 7 7 7 7 6 6
2.90 2.95 2.98 3.02 2.91 2.75 2.83 2.79 2.73 2.91 2.70
2 2 2 2 2 2 2 2 2 2 2
0.75 0.75 0.76 0.80 0.77 0.67 0.72 0.71 0.71 0.69 0.69
59 60 61 60 61 66 63 65 65 64 64
16.39 16.55 17.00 16.83 17.03 16.83 17.07 17.01 17.00 17.04 17.04
40.98 41.38 42.50 42.08 42.58 42.08 42.68 42.53 42.50 43.73 42.60
19.25 22.01 20.55 20.55 19.98 20.23 20.15 20.14 18.62 20.73 19.86
5a
0.25 0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
1.59 1.60 1.56 1.52 1.54 1.45 1.44 1.50 1.42 1.43 1.44
9 10 9 8 7 8 7 7 7 6 6
3.32 3.32 3.29 3.39 3.38 3.25 3.21 3.32 3.16 3.23 3.23
3 3 3 3 3 3 3 3 4 3 3
0.76 0.75 0.73 0.75 0.78 0.77 0.77 0.77 0.77 0.76 0.74
49 48 45 49 53 53 53 53 55 52 53
15.95 16.01 15.89 16.12 16.17 16.36 16.26 16.04 17.03 16.40 17.08
39.88 40.03 39.73 40.30 40.43 40.90 40.65 40.10 42.58 41.00 42.70
17.23 17.12 20.18 17.48 17.34 16.54 17.29 17.66 16.37 17.28 16.81
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 5
0.5 1.0 8.0
0.63 0.64 0.62
16 13 13
1.35 1.45 1.40
4 3 4
0.63 0.60 0.61
73 72 74
23.87 20.11 20.85
59.68 50.28 52.13
15.96 16.60 16.46
7
0.5 1.0 8.0
0.87 0.88 0.84
20 22 21
1.61 1.53 1.42
4 5 4
0.83 0.81 0.73
71 72 68
23.24 22.94 22.70
58.10 57.35 56.75
12.75 15.71 15.47
8
1.0 2.0 3.0 4.0 6.0
0.54 0.49 0.57 0.58 0.53
17 18 14 16 17
1.19 1.00 1.34 1.28 1.11
3 4 3 3 4
0.61 0.70 0.65 0.67 0.66
75 79 71 76 80
25.64 28.23 24.63 25.65 25.79
64.10 70.58 61.58 64.13 64.48
11.32 9.72 12.56 12.25 10.98
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Ridgway, I.M., Price, N.B., 1987. Geochemical associations and post-depositional mobility of heavy metals in coastal sediments: Loch Etive, Scotland. Marine Chemistry 21, 229—248. Schemainda, R., Nehring, D., 1975. The annual cycle of the space temporal dislocation of the North West African upwelling region. Third international symposium of upwelling ecosystems, 25—28 August 1975, Kiel, Fed. Rep. Germany, 7pp. Settle, D.M., Patterson, C.C., 1982. Magnitudes and sources of precipitation and dry deposition fluxes of industrial and natural leads to the North Pacific at Enewatak. Journal of Geophysical Research 87, 8857—8869. Sidane ould Dedah, 1993. Wind, surface water temperature, surface salinity and pollution in the area of the Banc d’Arguin, Mauritania. Hydrobiologia 258, 9—19. Sundby, B., Silverberg, N., 1985. Manganese fluxes in the bentic boundary layer. Limnology and Oceanography 30(2), 372—381. Sundby, B., Silverberg, N., Chesselet, R., 1981. Pathways of manganese in an open estuarine system. Geochimica et Cosmochimica Acta 45(3), 293—307. Taylor, S.R., 1964. Abundance of chemical elements in the continental crust: a new table. Geochimica et Cosmochimica Acta 28, 1273—1285. Trefry, J.H., Metz, S., Trochine, R.P., Nelsen, T.A., 1985. A decline in lead transport by the Mississippi River. Science 230, 439—441. Van Hoogstraten, R.J., Nolting, R.F., 1991. Trace and major elements in sediments and in porewaters from the North Western basin of the Mediterranean Sea. NIOZ Rapport 1991—10, pp. 1—72. Verardo, D.J., Froelich, P.N., McIntyre, A., 1990. Determination of organic carbon and nitrogen in marine sediments using the Carlo Erba NA—1500 Analyzer. Deep Sea Research 37, 157—165. Vermaat, J.E., Beijer, J.A.J., Gijlstra, R., Hootsmans, M.J.M., Philippart, C.J.M., van den Brink, N.W., van Vierssen, W., 1993. Leaf dynamics and standing stocks of intertidal Zostera noltii Hornem. And Cymodocea nodosa (Ucria) Ascherson on the Banc d’Arguin (Mauritania. Hydrobiolog, 258, 59—72. Veron, A., Lambert, C.E., Isley, A., Linet, P., Grousset, F., 1987. Evidence of recent lead pollution in deep North-East Atlantic sediments. Nature 326, 278—281. Wolff, W.J., van der Land, J., Nienhuis, P.H., de Wilde, P.A.W.J. (Eds.), 1993a. Ecological Studies in the Coastal Waters of Mauritanie. Kluwer Academic Publishers, Dordrecht. Wolff, W.J., van der Land, J., Nienhuis, P.H., de Wilde, P.A.W.J., 1993b. The functioning of the ecosystem of the Banc d’Arguin, Mauritania: a review. Hydrobiologia 258, 211—222. Yeats, P.A., Bewers, J.M., 1983. Potential anthropogenic influences on trace metals distribution in the North Atlantic. Canadian Journal of Fisheries and Aquatic Sciences 40, 124—131. Zuo, Z., Eisma, D., Berger, G.W., 1991. Determination of sediment accumulation and mixing rates in the Gulf of Lions, The Mediterranean Sea. Oceanologica Acta 14, 253—262.
The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d’Arguin (Mauritania) R.F. Nolting*, A. Ramkema, J.M. Everaarts Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands Received 19 September 1997; received in revised form 20 March 1998; accepted 22 September 1998
Abstract Trace and major elements were measured in sediment cores collected at the shelf and continental slope of the Banc d’Arguin (Mauritania). Most of the sediments have a calcium carbonate content '50% and as a consequence have a low aluminium ((1.5%) and iron ((1.0%) content. The concentrations of the latter two elements increase down slope. Based on the major elemental composition, the sediments seem to be well mixed and concentration gradients are only observed on a spatial distribution. Concentrations of Cu, Cd, Zn, Ni and Pb are low and approximately equal to ‘‘natural background’’ values, indicating negligible anthropogenic influences. An exception is Cd for which very high values were found in the northern most part of the research area, close to Nouadhibou. A possible explanation for this high Cd content can be the existence of phosphate rich sediments with a high affinity for Cd. Small increases in the available trace metal concentrations (exchangeable fraction, 0.1 N HCl treated) observed near the sediment surface could be attributed to early diagenetic processes, and not to pollution effects. The inputs from the Saharan dust plume with low trace metal contents, the transport of small particles with higher trace metal content down slope together with the escape of redissolved trace metals from the sediment, and the absence of a supply of metal rich material to the Banc d’Arguin, has created a metal poor environment. There is clear evidence of trace metal redistributions, associated with Mn recycling. Mean enrichment factors, calculated relative to the crustal abundance, show that Zn, Pb and Cd are equal to unity and Cu and Ni below unity. These observations suggest that the Cu and Ni contents in sediments of Banc d’Arguin are depleted relative to the accepted values in the earth crust. The very low trace metal concentrations detected in these sediments can be taken as ‘‘baseline’’ values for unpolluted sediments in the global ocean coastal zone. 1999 Elsevier Science Ltd. All rights reserved.
*Corresponding author. Fax: 0031 222 319674. E-mail address: [email protected]. (R.F. Nolting) 0278—4343/99/$ — See front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 98 ) 0 01 0 9— 5
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1. Introduction Marine sediments are the ultimate sink for particulate material supplied by rivers and the atmosphere to the oceans (Yeats and Bewers, 1983). In urbanised regions this material is very often influenced by domestic, industrial and mining activities leading to increased trace metal concentrations (Bruland et al., 1974; Duinker and Nolting, 1976, 1977; Martin and Whitfield, 1983; Trefry et al., 1985; Martin et al., 1989). A large proportion of the metal enriched particles is deposited and buried on the shelf and continental slope. When these sediments remain more or less undisturbed, and sedimentation rates are constant, they can provide us with an historical pollution record (Bruland et al., 1974; Goldberg et al., 1977; Bertine, 1980; Eisma et al., 1989; Macdonald et al., 1991; Nolting and Helder, 1991). Biological, physical, chemical and diagenetic processes can change the sediment properties, but generally these influences are detectable (Zuo et al., 1991; Nolting and Helder, 1991; Nolting and Van Hoogstraten, 1993) and can be corrected for. Marine sediments are therefore well suited to study anthropogenic influences on the marine environment. Most studies concerning historical pollution records in marine sediments, have been performed in coastal areas under direct influence of rivers, either unpolluted or polluted (Goldberg et al., 1977; Lichtfuss and Bru¨mmer, 1981; Veron et al., 1987) and are restricted to temperature regions. In the context of the global study of continental shelves it is important to collect more data worldwide, and especially from less studied tropical regions. Few such studies have been performed on the continental slope off Africa. The objective of the present study was to gather more information about trace metal contents, as well as the processes that regulate their concentrations in coastal sediments, that are not influenced by riverine input or industrial activities. As a consequence, it can provide us with the urgently needed data from regions for which man can expect to be pristine, and local influences on the coastal region to be minor. For this reason it was an excellent opportunity to analyse sediment cores collected near the coast of Mauritania, West Africa. This area is a very remote one, situated in the tropics, almost undisturbed by human impact and only local fishermen make their living there. No big rivers or industries have an impact on the coastal environment where seagrass beds occupy the larger part of the tidal flats (Vermaat et al., 1993). As a consequence of an intense upwelling phenomena resulting in high productivity and the isolated situation, Banc d’Arguin is a favourite wintering place for birds (Leopold, 1993; Wolff et al., 1993b). In addition, the atmospheric input of pollutants is probably of minor importance here due to the prevailing winds, mostly of northerly direction and passing over the Saharan desert (Chester et al., 1979; Sidane ould Dedah, 1993). Chester et al. (1979) also showed that soil-sized atmospheric particulates, transported by the Atlantic northeast trades, dominate the supply of land-derived material to sediments in the equatorial North Atlantic. In the present study, concentrations of Cu, Cd, Zn, Ni and Pb were measured in sediment cores collected along two transects radiating into the Atlantic Ocean, across the continental slope perpendicular to the Banc d’Arguin (Mauritania) to investigate their concentration, distribution pattern and the processes that regulate them. A study
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of copper, zinc, lead and cadmium in surface sediment in relation to epibenthic invertebrates and the grain size fraction in this surface layer has already been published by Everaarts et al. (1993). Results of the whole expedition, including hydrology, upwelling system and ecological studies can be found in Wolff et al. (1993a).
2. Sampling and analysis Eight sediment cores were collected with a cylindrical box corer (i.d."30 cm) during a cruise with the Research Vessel ‘‘Tyro’’ from 7 until 29 May 1988. The box corer was designed to obtain undisturbed sediments and was closed during recovery to prevent contamination. Sampling locations are presented in Fig. 1, while their
Fig. 1. Map of Banc d’Arguin with station numbers and sampling positions.
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 1 Station number, its geographical position, water depth, organic carbon and nitrogen content and % grain size fraction (63 l in the surface layer of the sediments Station
Position
Southern transect 2b 19°20 1 N/17°08 9 W 2a 19°24 1 N/17°05 3 W 2 19°33 1 N/17°00 6 W 3 19°36 5 N/16°52 5 W Northern transect 5a 19°54 3 N/17°40 0 W 5 19°57 3 N/17°40 9 W 7 20°02 2 N/17°25 9 W 8 20°04 4 N/17°21 4 W
Water depth (m)
%C
%N
% grain size(63 l
1374 896 121 27
3.25 1.54 1.80 0.74
0.41 0.20 0.10 0.13
69 25 14 72
1080 112 54 31
2.19 1.18 1.59 0.93
0.28 0.13 0.16 0.12
39 8.9 32 27
geographical position and some hydrographical parameters are given in Table 1. Sub-samples were obtained by inserting perspex liners, diameter 6 cm, into the sediment. They were closed with rubber stoppers and stored deep frozen (!23°C) until further processing in the laboratory. The data obtained from the two transects are presented as a northern (St. 5a, 5, 7 and 8) and southern transect (St. 2b, 2a, 2 and 3). After thawing, the cores were cut in slices of 0.5 cm thickness at the surface increasing to 4 cm at the bottom of the core (see Appendix). Approximately 5 g of each slice was dried at 60°C and ground and homogenized with a teflon mortar and pestle. From this homogenized sediment, 100 mg was transferred into a 50 ml polypropylene measuring flask which was filled with 0.1 N HCl to release the exchangeable fraction (Duinker and Nolting, 1976). After 18 h the solution was filtered through a precleaned 0.45 lm cellulose nitrate filter and the filtrate stored for analyses. The residue and filter were transferred to teflon bombs, 5 ml HF and 1 ml Aqua Regia (HCl : HNO , 3 : 1) were added, and the sediments were digested at 110°C for 2 h. Sub sequently, the content of the bombs was poured into a 50 ml polypropylene measuring flask which contained 30 ml saturated boric acid, and filled up with Milli Q water to 50 ml (Rantala and Loring, 1977). This method was regularly checked at our laboratory, see below, and a loss of the total content of approximately 6% was found by summing up the leachable and residual fraction compared with direct total digestion (Van Hoogstraten and Nolting, 1991; Happee and Nolting, 1994). To avoid contamination all chemicals used were of Suprapur quality and all materials were intensively cleaned with acid and rinsed with ultra clean water before use. Cu, Cd and Pb were determined with graphite furnace atomic absorption spectroscopy (Perkin Elmer 5100, Zeeman GFAAS) and stabilized temperature platform furnace (STPF). For the determination of Ni a regular tube with wall atomisation was
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Table 2 Results of fivefold analyses of a representative sample. Presented are mean concentrations and standard deviation from the leachable and residual fractions. Total, represents the summation of the individual leachable and residual results of each sub-sample Element
Leachable (n"5)
Residual (n"5)
Total (n"5)
Cd lg g\ Cu lg g\ Ni lg g\ Pb lg g\ Zn lg g\ Mn lg g\ Fe% Mg% Al% Ca% Si%
0.27$0.02 0.82$0.12 n.d. 1.76$0.3 4.45$0.31 17.61$1.03 0.06$0.01 0.47$0.01 0.05$0.0 16.97$0.26
0.06$0.02 1.55$0.34 n.d. 5.79$0.4 26.2$2.2 59.24$0.97 0.95$0.05 0.38$0.02 2.83$0.03
0.33$0.03 2.35$0.31 n.d. 7.55$0.67 30.65$2.19 76.85$1.18 0.99$0.05 0.85$0.02 2.88$0.03 16.97$0.26 20.22$0.57
20.22$0.57
Table 3 Measured and certified values ($1 S.D.) of trace metals and major elements in the BCR samples 141 and 142. Values between brackets are not certified. N.D. means not determined
Element Cu lg g Cd lg g Ni lg g Pb lg g Zn lg g Mn lg g Fe O % Al O % CaO% SiO %
BCR 141 Determined 32$0.3 0.39$0.08 30.2$1.9 29.8#0.7 80.1$1.2 506$17 3.5$0.4 10.52$0.0 15.35$0.11 N.D.
Certified 32.6$1.4 0.36$0.10 (31$3) 29.4$2.6 81.3$3.7 (547$32) (3.74) (10.56) (42.58)
BCR 142 Determined 27.6$0.5 0.30$0.06 27.4$0.4 40.7$0.8 88.8$2.0 536$25 2.80$0.01 N.D. (17.98)
Certified 27.5$0.6 0.25$0.09 29.2$2.5 37.8$1.9 92.4$4.4 (569$26) (2.80) (9.48) 3.91$0.28 (4.94) 66.9$0.4 (68.22)
used. Zn was determined by flame AAS with a high-performance nebulizer (Perkin Elmer 2380). Major elements Fe, Mn, Al, Ca, Mg and Si were also determined by flame AAS. Concentrations were calculated using the standard addition method. To determine the reproducibility of the method, one representative sample was analysed five times for the exchangeable as well as the residual fraction (Table 2). To determine the accuracy of the method reference sediments, supplied by the EC Bureau of References (BCR141 and BCR142) were included during the entire analytical procedure (Colinet et al., 1983a,b). For all elements the measured values were in good agreement with the certified values (Table 3).
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Organic carbon and total nitrogen were determined with a Carlo-Erba NA 1500 CHNS analyser according to Verardo et al. (1990). The precision of the carbon and nitrogen determinations were better than 99.9% and the accuracy within 0.01%.
3. Results and discussion From four cores (2a, 3, 5a, 8) all slices were analysed and from the others some selected ones at different depths. The concentrations of the trace and major elements in the sediment cores are given in the appendix. Firstly, the surface concentrations of the major elements, calcium carbonate and minor elements with their spatial distribution are described, and next the vertical distribution in the sediment cores. 3.1. Spatial distribution The organic carbon content is highest at stations 2b and 5a (2.19—3.25%) (Table 1). The C/N ratio at all stations is around 8, excluding St. 2, (18) which indicates that most organic material in this area is predominantly of marine origin (Macdonald et al., 1991). This is not surprising taking into account that this part of the African coast is a major upwelling region, with high phytoplankton production and a supply of organic matter (dead debris) to the continental shelf (Wolff et al., 1993b; Loktionov, 1993). It has to be stressed that between 20 and 24°N, in an average year, 70—80% of the time northerly winds favour upwelling phenomena (Schemainda and Nehring, 1975). A short description of the sediment cores follows. The colour of most cores is light grey, at station 3 and 8 light grey pale and at station 2 light grey brown. No distinct colour differences with depth could be observed in all cores, only some small darker coloured bands were present at the upper surface layer of the cores collected at the deepest stations. Some shell remains were present in the sediments from shallow depth. Grain size distributions of the surface sediments were published by Everaarts et al. (1993). In Table 1 the fraction smaller than 63 l, clay and silt, is given and this fraction is at most stations higher than 25%. The grain size fraction '500 l is in most cores less than 5%, and only at station 2 a larger fraction of 23% is measured. It is interesting to note that this last fraction is more than 45% in sediments collected at very shallow water (5—18 m) close to the beach, demonstrating the escape of small particles from the coastal zone. Concentrations of Fe, Al, Si, Mg (as their oxides) and Ca as carbonate, in the surface sediments (upper cm) are presented in Fig. 2. The calcium carbonate concentration, calculated from the respective Ca content of each sample, increases in sediments along both transects from deeper to shallower water depth. This increase is more pronounced along the northern transect. These results are in contrast with the oxides of the other elements which show in general a decrease towards the shallow coastal area, where the lowest elemental concentrations for Fe (1.00% and Al(1.5% are found at the northern transect. SiO , (quartz, biogenic silica) along the northern transect only, decreases in concentration towards the coast. No clear trend for SiO is
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Fig. 2. Concentration of the major elements expressed as their oxides and calcium as calcium carbonate in the surface sediment of the different stations. (upper Fig. southern transect, lower Fig. northern transect).
observed along the southern transect. The sum of the oxides of Fe, Al, Si, Mg and Ca as calcium carbonate is lower in the sediments at the offshore stations 2b and 5a (80—90%) than at shallower depth (90—95%). On the other hand, the highest concentrations of organic carbon are found at these offshore stations. The increased
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Fig. 3. Concentrations of Cu, Cd, Pb, Ni, Mn and Zn (lg g\) in the surface sediment of the different stations, calculated on a calcium carbonate free basis. (left Fig. southern transect, right Fig. northern transect).
concentrations of aluminium- and iron-oxides at greater water depths, suggest a larger amount of clay minerals. This is supported by the Si/Al (weight/weight) ratio, which is 3.8—5.2 at the deepest stations and 6.6—11.8, maybe due to SiO variations, at the shallower part of the Banc d’Arguin. Deep sea clay has a Si/Al ratio of 2.9, but 5 or higher in siliceous sediments (Martin and Whitfield, 1983). Concentrations of trace metals in the bulk samples can be found in the Appendix, whereas those calculated on a calcium carbonate free basis are given in Fig. 3. Although the concentrations for most metals are low and comparable with other unpolluted sediments (Chester, 1990; Everaarts et al., 1993: Everaarts and Nieuwenhuize, 1995; Nolting and van Hoogstraten, 1993; Nolting et al., 1996) it is possible to
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observe some trends. Cu, Zn, Ni and to a minor extent Pb concentrations decrease from deeper waters towards shallower depths, particularly in the southern transect. There is, however, one exception: Cd concentrations tend to increase towards shallower waters, most pronounced in the northern transect where at station 8 a twentyfold increase in concentration occurs as compared to that at the deepest stations. The source for the enriched Cd concentrations in the northern transect is not known. Storage of iron ore, at Cap Blanc north of Banc d’Arguin could be a source (Sidane ould Dedah, 1993) but the existence of phosphate rich sediments with a high affinity for Cd is another possibility. Although other sources cannot be excluded, we have too little information from the area to draw fixed conclusions. It should be noted that almost 100% of the Cd content is in the exchangeable fraction, which means that the metal is adsorbed to sediment particles and in a mobile form. This trend is also evident after normalising relative to Al. The range of the element/Al;10\ ratios in all samples are 1.4—4 for Cu, 1.8 to 2.2 for Pb, 3.0—6.8 for Ni, 9.0 to 16.7 for Zn and 0.04 to 1.08 for Cd. These ratios are, with exception of Cd, comparable to or even lower than those published for continental soils and crust (Chester, 1990). We conclude that all elements have approximately a natural background concentration, with exception of Cd which seems to be enriched in surface sediments in the northern part of the Banc d’Arguin. 3.2. Depth distribution Depth profiles of Ca and Si for St. 2a, 3, 5a and 8 are given in Fig. 4. Concentrations for Fe, Al and Mg and results for the other stations can be found in the Appendix. No substantial differences could be observed in the vertical distribution pattern for the major elements. Fe/Al ratios are constant with depth for each site separately, and range from 0.36 for station 3 to 0.59 for station 7, with a mean value of 0.47. Based on this distribution, sediments of the Banc d’Arguin seem to be well mixed, and the sediment supply homogeneous. Differences can only be found in the horizontal distribution, as described previously. Depth profiles of Mn, Cu, Zn, Ni, and Pb for St. 2a, 3 (southern transect) and 5a and 8 (northern transect) are shown in Fig. 5. Although some of these profiles show an increase in concentration towards the surface layer, this is not necessarily an indication for anthropogenic input. It is more likely that early diagenetic processes are responsible for this phenomenon (e.g. Ridgway and Price, 1987; Macdonald et al., 1991). The sediments of Banc d’Arguin are very poor in Mn (40—80 lg g\), compared with other marine and coastal areas (Sundby et al., 1981; Sundby and Silverberg, 1985; Helder, 1989; Chester, 1990; Macdonald et al., 1991; Nolting et al., 1996; Nolting and Van Hoogstraten, 1993). The Mn concentration is lower than the values reported by Happee and Nolting (1994) in sediments of the upwelling area near Oman (170 lg g\). Mn increases in the exchangeable fraction in the upper surface layer, probably due to the reoxidation of upwardly diffused Mn> which is more pronounced at station 2a than at station 5a. Compared to areas with strong redox-cycling this increase is, however, very small (Nolting et al., 1996). No obvious signs for Mn recycling are seen in the calcareous sediments at stations 3 and 8. This may be due to a
Fig. 4. Depth profiles of Ca and Si (%) in sediments of St. 2a, 3 (southern transect) and 5a and 8 (northern transect).
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Fig. 5. Depth profiles of residual and total content of Cu, Pb, Ni, Mn and Zn (lg g\) in sediments of St. 2a, 3 (southern transect), 5a and 8 (northern transect).
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Fig. 5. (Continued.)
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Fig. 5. (Continued.)
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Fig. 5. (Continued.)
low supply of organic matter and, consequently, a lack of reducing conditions at depth in the sediment, or possibly under strong reducing conditions all reactive Mn has escaped to deeper water. Despite these differences between the stations, there is a consistency between the Mn geochemistry at each site, indicated by the down-core ratio between residual and total Mn, which is about equal (&0.70) at all stations. Depth profiles of Cu and Zn show more or less the same pattern as Mn. The enhanced Cu and Zn concentrations found in the surface sediment of some stations, reflected also in a small increase in the exchangeable fraction, suggest a correlation with Mn. Plotting Cu and Zn against Mn (Fig. 6), shows such a correlation at St. 2a for both Cu and Zn, and at St. 5a for Zn. This may indicate diagenetic redistribution of Cu and Zn controlled by the redox cycle of Mn, rather than a connection with anthropogenic input of these metals. We come to the conclusion, that in these unpolluted sediments, depth distributions of trace metals are mainly controlled by diagenetic processes, with a strong coupling to Mn (and possibly Fe). It should be emphasized that this coupling between trace metals and Mn redistribution, was visible due to the absence of nearby river inputs where sediment supply with elevated trace metal contents can highly influence levels of trace metals in bottom sediments, and hence mask these processes. Also the presence of the Saharan dust plume with low trace element concentrations (Chester et al., 1979; Kremling and Streu, 1993), preventing the deposition of metal enriched material to the shelf is another factor which strengthen these observations. On the basis of the mineralogical, chemical and biological composition of atmospheric particles, Chester et al. (1979) concluded that the transport of dust loadings to the equatorial North Atlantic dominates the land-derived material. Notably, the elemental aluminium ratios in soil-sized atmospheric particles collected in the Atlantic northeast trades (Chester et al., 1979), reflect the ratios found at our deepest stations. This lack of supply of metal rich material to the shelf and the presence of a high load of organic material, derived from planktonic species in these upwelling areas (Wolff et al., 1993b), can be responsible for the low
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Fig. 6. Relation between total Mn and Cu, Pb, Ni and Zn (lg g\) in the cores of St. 2a, 3 and 5a.
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Fig. 6. (Continued.)
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trace metal contents observed in the sediments. Diagenesis of this organic matter, creating reducing conditions, mobilising Mn and adsorbed metals and a subsequent transported of these elements from the shelf to the continental margin (Johnson et al., 1992), results in a metal poor environment. Ni shows more or less the same pattern as Cu and Zn, low concentrations at the Banc increasing seaward. One exception is the Ni concentration at St. 3, where no exchangeable fraction could be measured. There is no explanation for the absence of this fraction. All determinations were done at the same time and in the same batch. At all other cores the fraction ranges between 17 and 30%. Ni seems also to covary with Mn, with best correlations for St. 2a and 3 (Fig. 6). Cd does not show strong variations with depth in all cores (appendix 1). The high content at St. 8 is discussed in the preceding section dealing with the horizontal distribution. The Cd concentrations are consistently high down core at St. 5, 7 and 8 with a value of around 1 lg g\. The exchangeable fraction contributes almost 100% at all stations, indicating that the Cd measured in the sediment cores is mobile, and regulated by adsorption—desorption processes. In contrast to Cu, Zn and Ni, Cd does not show any positive relation with Mn. This is not surprising taking into account its opposite behaviour in relation to Mn in the marine environment, which means that when Mn shows an increase in concentration to the surface of sediments due to redox cycling, the Cd concentrations show a decrease. This contrasting behaviour is described for sediments as well as that for the water phase. As for example, Gobeil et al. (1987) and Gendron et al. (1986) have shown that Cd behaves opposite to Mn in sediment cores collected in the Laurentian Trough, during early diagenesis. In the river Scheldt, where the upper estuary is anoxic and clearly separated from the oxic lower estuary, Cd and Mn show the same ‘‘reverse’’ behaviour in the water phase (Duinker et al., 1982). Cd is closely related to the biogenic cycle, as evidenced by its relation to phosphate in the water column (Boyle et al., 1976; Knauer and Martin, 1981; De Baar et al., 1994). Due to degradation of organic debris in the oxygenated surface layer, Cd is normally released from the sediment and escapes to the water phase (Gobeil et al., 1987). However, if this process is valid for oxygen rich surface sediments, it may explain the very high and constant Cd concentrations found in the sediments of Banc d’Arguin. If reducing conditions exist in these sediments of Banc d’Arguin, the high Cd and low Mn content are a result of this situation. An example of this last reasoning is the high Cd concentration observed in sediments of the Laurentian Trough, associated with a high organic load for sinking particles resulting in an anoxic environment (Gendron et al., 1986). However, in the situation presented here the organic load cannot explain the observations, since there are no large differences in the organic carbon content between the two transects (Table 1). This organic content ranges between 0.7 and 1.8% and has a C/N ratio of around 9. Thus, it is more likely to assume that there is an unknown additional input of Cd (dissolved or particulate), which is fixed and stored in the sediment, or that phosphate rich sediments high in Cd content are the explanation for this peculiar situation. Pb is enriched in the top few centimeters of core 5a and to a lesser extent of core 2a. This increase is mostly attributed to the exchangeable fraction, and shows a clear
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 4 Pb concentrations in the sediment cores expressed on a calcium carbonate free basis from all stations (S. surface, B. bottom of core). Ratio of Pb content divided by the background concentration of 6 lg g\ (Pb.n) Station
Pb
Pb/Pb.n
2b S B 2a S B 2 S B 3 S B 5a S B 5 S B 7 S B 8 S B
11.8 11.92 11.01 7.98 6.91 7.67 9.07 8.87 11.30 7.70 6.93 5.83 7.50 5.44 7.37 6.82
1.97 1.99 1.84 1.33 1.15 1.27 1.51 1.47 1.88 1.28 1.12 0.97 1.25 0.91 1.23 1.14
correlation with Mn in core 2a. Pb is mostly supplied to the open marine environment by atmospheric inputs (Settle and Patterson, 1982; Veron et al., 1987; Martin et al., 1989; Nolting and Helder, 1991), and was used as an excellent tracer to study historical pollution records (Goldberg et al., 1977; Hamelin et al., 1990; Nolting and Helder, 1991), assuming that sedimentation rates are constant and bioturbation negligible (Zuo et al., 1991). Down-slope transport of surface sediments with a higher lead content together with the downward flux of Pb bound to large biogenic particles (Lambert et al., 1991), can explain the increased Pb values found at the deeper part of the Banc d’Arguin, as well as for the other trace metals. Although anthropogenic inputs are supposed to be the major source for the increased Pb concentrations found in marine sediments, an increase due to diagenetic changes cannot be excluded. Gobeil and Silverberg (1989) have shown that a part of the enriched Pb content of sediments from the Laurentian Trough could be explained by mobilisation and fixation processes, redox controlled by Fe diagenesis. This implies that the Pb enrichments in surface sediments should be interpreted very cautiously, in order to avoid an overestimation of the anthropogenic impact, leading to incorrect budget calculations. Expressed on a calcium carbonate free basis, the total Pb concentration in the sub-surface layer of all cores is 5—8 lg g\, increasing to &11 lg g\ in the surface layers of cores 2a, 2b and 5a (Table 4). Taking into account a background value of 12 lg g\ Pb in continental crust (Taylor, 1964), the down-core values presented here
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
683
are about a factor of 2 lower. Veron et al. (1987) calculated a background lead content of &15 lg g\ in the silicate fraction of sediment cores collected in the North-East Atlantic. They argued that this background level does not necessarily represent a ‘‘natural’’ level. A background value of 8—10 lg\ Pb, corrected for calcium carbonate, was also found in sediment cores collected in the Mediterranean Sea (Nolting and Helder, 1991) and on the continental slope of Oman (Happee and Nolting, 1994). This indicates that a value of around 8—10 lg g\, or even lower is a more natural and realistic value for the Pb content found in the unpolluted, carbonate free, sediments around the African and Arabian continents, than the values of 12—15 lg g\ reported so far for the northern hemisphere. 3.3. Element to aluminium ratios Normalising elements relative to Al is widely used to compensate for differences in grain size variations and carbonate content. This method is also a powerful tool for the comparison of trace metal contents in sediments in different areas around the world and can also be applied to determine enrichment factors (EF). EFs are mainly used for the assessment of aerosols in order to understand the difference in composition in terms of desert and anthropogenic elemental sources (Chester et al., 1992; Molinari et al., 1993). In the present study the EF values are applied to evaluate the dominant source of the sediments and as indicators for pollution effects. Table 5 shows the calculated EFs of the analysed elements with respect to those determined in the crustal abundance (Taylor, 1964), employing the equation EF"(E/Al) /(E/Al) , in which (E/Al) and (E/Al) are the concentrations of the respective element E and of Al in the sediment and in the crustal material. (Molinari et al., 1993; Kremling and Strue, 1993). Enrichment factors close to 1 point to a crustal origin, while those with a factor '10 are considered to have a non-crustal source. It is clear from the data (Table 5) that all elements have an EF close to unity, with exception of Cd. The low EFs ((1) found for Cu and Ni are remarkable, and indicate that these elements are depleted in the sediments of the area of study, compared to those in the crustal abundance. The higher EF found for Cd in the northern transect is an indication for local inputs or a special situation, as discussed before. Comparing the EFs presented here with those reported by Kremling and Streu (1993), for sediment trap material collected at 21° 55.8’N, 25° 14.2’ W, off the coast of Mauritania, and those reported for Saharan aerosols (Murphy, 1985), a reasonable similarity (Table 6) can be established. EFs found in sediments of the Banc d’Arguin are at the lower end of those found in Saharan aerosols. Although, there are limitations in the comparison of aerosol derived particles with marine sediments, the tight (excluding the high Cd values) similarity between the obtained enrichment factors strongly suggests the crustal origin with Saharian inputs (Chester et al., 1979). Enrichment factors determined for trace elements in aerosols collected in urbanised areas (Murphy, 1985; Chester et al., 1992; Molinari et al., 1993), are orders of magnitude higher than those in the present study area. Also, the low Al/Si ratio of
684
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 Table 5 Enrichment factors (EF) with respect to crustal ratios (Taylor, 1964), based on mean concentrations determined in whole cores. See text Station
Cu
Zn
Cd
Pb
Ni
2b 2a 2 3 5a 5 7 8
0.59 0.45 0.30 0.21 0.39 0.28 0.23 0.27
1.61 1.35 1.76 1.06 1.53 1.59 1.80 1.88
1.79 1.45 3.88 2.71 2.79 29.58 22.08 41.67
1.25 1.38 1.48 1.25 1.25 1.32 1.32 1.38
0.74 0.60 0.54 0.30 0.66 0.55 0.58 0.44
Table 6 Mean enrichment factors (EF$1 S.D) with respect of crustal ratios of all samples, compared to data reported for sediment trap material and Saharan dominated crustal aerosols (n"8). Cd data are separated in ranges, excluding the high values (n"5) Element
This work
1020 m
4120 m
Saharan aerosols
Cu Zn Cd n"5 Pb Ni
0.34$0.27 (Low) 1.57$0.27 (Low) 13.2$15.73 (High) 2.70$0.99 1.33$0.08 (Low) 0.55#0.13 (Low)
1.1$0.2 3.1$1.3 10.5$6.8
1.4$0.2 1.8$0.5 2.2$1.0
0.8—2.2 1.7—8.1 3.2—17
3.4$0.4 0.94#0.36
3.1$0.2 0.72#0.23
3.5—23 0.7—2.5
Sediment trap data from Kremling and Streu, (1993) collected at 1020 and 4120 meter water depth off the coast of Mauritania. Data from Murphy (1990).
&0.15 measured here differs significantly from the ratio of &0.40 in desert-dominated samples reported by Molinari et al. (1993). This indicates that sediment of the Banc d’Arguin has a high quartz content, that a part of the silicate fraction is of biogenic origin and not terrestrial, or that clay fractions are deposited further away from the shelf. Depth profiles of element/Al ratios for the four cores are shown in Fig. 7. Overall, the ratios do not show large variations with depth. Only Pb in core 5a shows an increase to the surface. In the upper surface layer, the effect of early diagenesis is reflected in the increase of Mn, Cu, Zn and Ni in cores 2a and 5a. Actually, the element/Al ratio support the conclusions made earlier for the vertical section, but some additional information can be derived from the profiles. The highest ratios are present at the deepest stations, which points at additional inputs of trace metals not related to Al. A possibility is transport of remobilised trace metals from the shelf, adsorption to small particles and sedimenting down slope, or
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
685
Fig. 7. Depth profiles of Cu, Pb, Cd, Ni, Mn and Zn, normalized against Al (weight/weight) 10\ in sediment cores of St. 2a, 3 (southern transect) and 5a and 8 (northern transect).
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
scavenging of dissolved metals from the water phase by small particles and a subsequent burial to the sediment. Fluxes of particles down column with the same element/Al ratios as at our deepest stations are reported by Kremling and Streu (1993), for a sediment trap station 1000 km west of the African coast. Transport of trace metals through the coastal zone, following remobilisation and adsorption processes resulting in metal enriched particles, from the shelf to the continental margin and deep sea was described by Bewers and Yeats (1989). Our data confirm their conclusion that a significant portion of the metals supplied to the coastal zone find their ultimate fate in the sediments of the deep sea. 4. Conclusions Sediments collected at the Banc d’Arguin are dominated by a mixture of calcareous and siliceous (quartz, biogenic silica) material and are well mixed. The calcium carbonate content is more than 50% in sediments at shallow water depth decreasing in an off-shore direction. Trace metal concentrations measured in the study area are low and comparable to those found in ‘‘pristine’’ environments. Calculated on a calcium carbonate free basis all trace metals, with exception of Cd, show increasing concentrations with water depth. Early diagenetic processes are responsible for the enriched Mn and trace metal concentration observed in surface layers, clearly showing that in unpolluted sediments diagenetic processes regulate trace metal distributions. Sub-surface Pb concentrations belong to the lowest ever determined in marine sediments and can be considered as natural concentrations. Export of the metal enriched surface layer down slope, mobilisation of metals and a shortage of supply of Mn and metal rich material to the Banc d’Arguin, is an extra factor that keeps the trace metal content in the sediments low. Normalisation of the elements relative to Al is used to determine enrichment factors (EF). Compared to the crustal abundance the EFs are amongst unity for Zn, Pb, and Cd (except for the coastal area of the northern transect) and below unity for Cu and Ni. Summarising, it can be concluded that the trace metal concentrations determined in the sediment of Banc d’Arguin should be regarded, as a ‘‘baseline’’ for unpolluted sediments in the global ocean coastal zone. Acknowledgements Special thanks are due to Captain J. de Jong and his crew of the R.V. Tyro and P.A.W.J. de Wilde for his stimulating interest as Chief-scientist during the cruise. Jeroen de Jong, Loes Gerringa and Wim Van Raaphorst gave valuable comments to earlier versions of the manuscript. This research has been carried out as a part of the Mauritania I Expedition, co-organised and funded by the Netherlands Marine Research Foundation (SOZ). Three anonymous referees gave valuable comments from which the paper benefited. This is publication No. 3211 of the Netherlands Institute for Sea Research.
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691
Appendix Concentrations of Cu, Cd, Ni, Zn, Pb and Mn (lg g\) and Fe, Al, Mg, Ca, CaCO and Si (%) in the sediment cores of Banc d’Arguin. ¹ is total concentrations and ¸ is the percentage exchangeable fraction Station
Dept (cm)
Cu ¹
¸
Cd ¹
¸
Ni ¹
¸
Zn ¹
¸
2b
1.0 8.0
18 18
11 9
0.19 0.22
100 100
32 27
31 34
65 62
17 14
2a
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
12 11 11 9 9 9 9 9 8 8
16 14 12 11 10 12 11 11 12 12
0.10 0.08 0.07 0.15 0.10 0.10 0.10 0.11 2.35 0.11
100 88 100 47 100 100 100 100 3.8 100
20 19 17 17 17 18 16 17 16 16
27 26 28 25 25 26 26 25 25 24
45 42 38 39 38 39 24 37 34 35
2
1.0 2.0 4.0 6.0
3 5 3 3
0 0 0 0
0.14 0.21 0.15 0.15
100 38 100 100
8 8 9 8
23 19 19 15
3
0.5 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0 18.0
4 5 6 4 4 4 4 3 4 4
10 16 6 6 10 9 11 10 10 10
0.16 0.19 0.18 0.18 0.17 0.16 0.17 0.17 0.19 0.18
94 100 100 100 100 100 100 100 84 100
9 9 9 8 8 7 7 7 7 7
5a
0.25 0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
9 9 9 10 9 8 8 8 8 8 8
12 12 10 6 12 12 9 10 12 12 11
0.17 0.19 0.22 0.22 0.22 0.22 0.23 0.23 0.26 0.27 0.22
100 100 100 100 100 96 100 100 100 100 100
5
0.5 1.0 8.0
3 3 3
21 13 14
1.01 0.80 0.91
7
0.5 1.0 8.0
2 2 3
15 11 25
0.84 0.75 0.67
Pb ¹
¸
Mn ¹
¸
8.9 8.7
32 29
112 95
40 29
17 17 18 16 16 14 16 12 11 13
7.1 7.2 6.4 6.6 6.6 6.1 5.7 6.2 6.0 6.0
24 26 21 21 19 19 15 19 14 16
123 96 88 69 65 59 56 58 52 58
60 50 47 36 26 29 27 26 23 26
24 24 22 22
31 22 29 23
3.0 3.6 3.9 3.6
30 17 26 21
36 39 37 42
31 31 29 28
0 0 3 0 0 0 0 0 0 0
26 28 28 27 24 24 25 27 27 28
9 13 9 14 11 6 8 13 11 13
5.4 5.6 5.4 5.2 5.1 5.4 5.4 5.0 5.1 14.5
26 24 20 21 21 22 22 24 23 8
79 82 81 77 77 78 76 77 77 76
27 26 26 25 23 23 23 24 24 23
19 19 18 23 20 9 21 21 21 20 21
28 30 28 24 26 30 23 24 26 24 26
53 44 49 43 44 41 39 42 42 43 43
31 20 26 14 17 14 18 17 20 23 19
6.8 6.3 6.1 6.7 6.2 5.6 5.4 6.0 4.9 4.4 4.4
44 43 44 37 44 38 41 33 34 33 35
105 98 85 79 79 76 77 79 73 74 73
48 43 35 28 28 30 29 28 31 28 31
100 100 100
7 6 7
27 29 26
19 18 20
27 26 34
2.8 3.4 2.8
30 24 30
27 37 33
41 33 38
94 100 100
8 8 7
24 26 24
23 22 21
32 26 25
3.2 2.7 2.4
33 31 24
37 37 36
27 27 25
688 8
Station
R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 1.0 2.0 3.0 4.0 6.0
2 2 2 2 2
Dept (cm)
Fe ¹
31 25 4 7 8
1.28 1.36 0.97 1.13 1.14
¸
100 99 100 97 98
5 4 4 5 5
24 20 20 17 17
20 19 19 21 18
35 29 29 34 32
2.6 2.6 2.6 2.6 2.4
35 25 25 32 34
30 33 33 33 29
28 29 29 33 33
Al ¹
¸
Mg ¹
¸
Ca ¹
CaCO ¹
Si ¹
2b
1.0 8.0
2.34 2.29
8 6
4.66 4.59
3 3
1.10 1.08
42 41
10.03 11.09
25.08 27.73
17.74 18.18
2a
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
1.77 1.65 1.52 1.48 1.46 1.54 1.37 1.47 1.38 1.41
10 10 10 10 8 8 8 8 7 6
3.34 3.19 3.24 3.02 3.04 2.93 2.98 2.99 2.89 2.93
3 3 3 3 3 3 3 3 3 2
0.70 0.69 0.60 0.62 0.63 0.64 0.59 0.62 0.6 0.60
37 39 43 39 41 44 46 40 40 40
11.96 11.19 11.28 10.38 9.76 12.90 10.90 10.42 11.00 9.75
29.90 27.98 28.20 25.58 24.40 32.25 27.25 26.05 27.50 24.38
24.24 23.78 21.86 24.80 26.53 22.76 23.44 25.71 26.09 27.18
2
1.0 2.0 4.0 6.0
0.85 0.81 0.78 0.79
17 14 13 13
1.56 1.53 1.62 1.69
5 4 3 3
0.74 0.81 0.76 0.77
68 69 68 68
22.83 22.10 21.65 21.23
57.08 55.00 54.13 53.30
15.42 14.21 15.52 16.23
3
0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0 18.0
1.06 1.06 1.09 1.10 1.03 0.99 0.99 1.00 0.99 0.99 0.96
8 8 7 7 7 7 7 7 7 6 6
2.90 2.95 2.98 3.02 2.91 2.75 2.83 2.79 2.73 2.91 2.70
2 2 2 2 2 2 2 2 2 2 2
0.75 0.75 0.76 0.80 0.77 0.67 0.72 0.71 0.71 0.69 0.69
59 60 61 60 61 66 63 65 65 64 64
16.39 16.55 17.00 16.83 17.03 16.83 17.07 17.01 17.00 17.04 17.04
40.98 41.38 42.50 42.08 42.58 42.08 42.68 42.53 42.50 43.73 42.60
19.25 22.01 20.55 20.55 19.98 20.23 20.15 20.14 18.62 20.73 19.86
5a
0.25 0.5 1.0 1.5 2.0 3.0 4.0 6.0 8.0 10.0 14.0
1.59 1.60 1.56 1.52 1.54 1.45 1.44 1.50 1.42 1.43 1.44
9 10 9 8 7 8 7 7 7 6 6
3.32 3.32 3.29 3.39 3.38 3.25 3.21 3.32 3.16 3.23 3.23
3 3 3 3 3 3 3 3 4 3 3
0.76 0.75 0.73 0.75 0.78 0.77 0.77 0.77 0.77 0.76 0.74
49 48 45 49 53 53 53 53 55 52 53
15.95 16.01 15.89 16.12 16.17 16.36 16.26 16.04 17.03 16.40 17.08
39.88 40.03 39.73 40.30 40.43 40.90 40.65 40.10 42.58 41.00 42.70
17.23 17.12 20.18 17.48 17.34 16.54 17.29 17.66 16.37 17.28 16.81
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R.F. Nolting et al./Continental Shelf Research 19 (1999) 665—691 5
0.5 1.0 8.0
0.63 0.64 0.62
16 13 13
1.35 1.45 1.40
4 3 4
0.63 0.60 0.61
73 72 74
23.87 20.11 20.85
59.68 50.28 52.13
15.96 16.60 16.46
7
0.5 1.0 8.0
0.87 0.88 0.84
20 22 21
1.61 1.53 1.42
4 5 4
0.83 0.81 0.73
71 72 68
23.24 22.94 22.70
58.10 57.35 56.75
12.75 15.71 15.47
8
1.0 2.0 3.0 4.0 6.0
0.54 0.49 0.57 0.58 0.53
17 18 14 16 17
1.19 1.00 1.34 1.28 1.11
3 4 3 3 4
0.61 0.70 0.65 0.67 0.66
75 79 71 76 80
25.64 28.23 24.63 25.65 25.79
64.10 70.58 61.58 64.13 64.48
11.32 9.72 12.56 12.25 10.98
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