7

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 181: 7-14. 1981.

THE MASS-BALANCE OF SUSPENDED MATTER AND ASSOCIATED POLLUTANTS IN THE NORTH SEA

D. E ism a Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1970 AB Den Burg, Texel, Netherlands

INTRODUCTION Deposition is estimated from the distribution of fine­ The material in suspension in the North Sea comes grained deposits in the North Sea and from present from a variety of sources: rivers, coastal erosion, sea- knowledge of the rate of sedimentation in these areas. floor erosion, primary and secondary production, the A full discussion is given in Eisma (in press b). Ideally, atmosphere, the North Atlantic Ocean, the Channel, supply, expressed in million tonnes (dry weight) per and the Baltic. Pollutants enter the North Sea mostly year, should be equal to the sum of estimated depo­ from the same sources but chiefly from rivers and the sition and estimated outflow, expressed in the same atmosphere, as well as from direct discharges. They unit. interact or mix with the material in suspension and with bottom sediments. Complex processes are in­ volved, including adsorption, flocculation, precipita­ THE MASS-BALANCE OF SUSPENDED MATTER IN THE tion, and aggregation. This results in high concentra­ N ORTH SEA tions of trace metals and organic pollutants in parti­ The concentrations of suspended matter in the culate matter, the concentrations in suspended matter Atlantic flowing into the North Sea around Shetland and bottom deposits usually being several orders of are of the order of 0-1 to 0-2 mg -1-1 with an average magnitude higher than those of material in solution. near 0-2 m g-1-1 (Eisma and Kalf, 1978). This is Estuaries are important sites of conversion of trace slightly higher than the concentrations in the North metals in solution into a particulate form: this was Atlantic (Jacobs and Ewing, 1961), so probably some found in most of the estuaries that were studied particulate material has been taken up from the shelf. (summaries in Liss, 1976; Duinker, 1980; Eisma, The concentrations of suspended matter in the North in press a). Sea flowing into the Norwegian Sea along the Nor­ The dispersal of large concentrations of pollutants wegian coast are of the same order and between in particulate form makes it important to study the Shetland and Norway there were no significant dif­ flux of suspended matter through estuaries and the ferences between the inflow and outflow concentra­ nearshore seas towards the open ocean. Also it is tions in May 1977 (Eisma and Kalf, 1978) and January important to know the final destination of the suspen­ 1980 (unpublished data, NIOZ), although in both ded material, i.e. the areas where the suspended mat­ cases the outflow concentrations were slightly higher. ter, with associated pollutants, is deposited. In this On the basis of these data about 3xl06 tonnes-yr_1 paper the mass-balance of suspended matter in the at most may leave the North Sea in excess of what North Sea is discussed in relation to pollutant dis­ comes in from the Atlantic. Actually more material persal. The approach for calculating the mass-balance from other sources may leave the North Sea but this is relatively simple : the supply from the Atlantic, the is compensated for by material coming in from the Channel, and the Baltic as well as the outflow of Atlantic and remaining behind. suspended material into the Norwegian Sea are esti­ The concentrations of suspended matter in the mated by multiplying the average water flow with Atlantic flowing into the North Sea through the the average concentration of suspended matter, using Channel and the Strait of Dover-Calais increase in chiefly winter values because then there is no ad­ the Channel to 1-2-2-7 m g-1-1 (average 1-9 m g -I'1 mixture of living plankton. Supply from rivers and in the eastern Channel at salinities above 35 °/00, other sources has been estimated from published data. Eisma and Kalf, 1979). This suspended material is 8 D. Eisma probably picked up from the Channel coasts and the occur elsewhere in the North Sea but here deposition sea floor. Such uptake has been observed on the Varne largely stopped about 8400 ago (north of Dogger - Bank where comparatively large amounts of particles bank) or the deposits consist to a great extent of of 9 to 35 pm are brought into suspension and moved reworked tidal flat deposits (Oyster Grounds). In northward. The inflow from the Baltic has a content these areas there is only some small deposition of of suspended matter of about 1 mg • 1-1. Supply from carbonate material and organic carbon. Sedimenta­ rivers, coastal erosion, the atmosphere, and primary tion rates in the offshore areas of the North Sea have production is also relatively well known from the been based on pollen data (Outer Silver Pit, Skager­ available data but data on sea-floor erosion are in­ rak), 14C dating, 210Pb dating, trace metal content sufficient. Some quantitative estimates have been (German Bight, Elbe Rinne), and flux calculations made for the erosion on the Flemish Banks (Gossé, (Skagerrak, Kattegat), whereas sedimentation in the 1977) and the Varne Bank (Eisma and Kalf, 1979) estuaries, the Waddensea, and the Wash can be esti­ but for the other areas with indications of bottom mated from published data (Eisma, in press b). The erosion (off East Anglia and off northern Denmark) total amount deposited yearly in the North Sea is there are no estimates at all. The supply of suspended 21 to 31-5x 10® tonnes-yr-1, to which should be add­ matter to the North Sea from all sources together is ed an average amount of 2x10® tonnes-yr-1 that is of the order of 34x 10® tonnes *yr-1 (Table 1) plus an dredged and dumped on land (Table 2). About 7 x 10® unknown amount supplied from the sea floor. Also tonnes-yr-1, or 20 to 30%, are deposited nearshore an (unknown) amount should be added for the supply (estuaries, tidal flats, inland dumping), the remainder through the coastal waters of the Strait of Dover- being deposited further offshore, of which 50 to 70% Calais where suspended matter in concentrations up in the Skagerrak-Norwegian Channel and the Katte­ to more than 10 mg • 1-1 may have been supplied from gat. the Channel but also from the Southern Bight (by Comparing Tables 1 and 2 indicates that the total tidal dispersion or by inflow during northerly winds). estimated supply comes near the sum of the estimated Deposition of suspended matter in the North Sea outflow into the Atlantic and the estimated deposition. is restricted to a few areas : the Outer Silver Pit area, Adding some supply through the coastal waters of the German Bight, the Skagerrak-Kattegat and part the Strait of Dover-Calais and some additional supply of the Norwegian Channel, the Waddensea, the Wash, from the sea floor (indicated with question marks in the estuaries, and some very small areas in the South­ Table 1) brings the estimated supply well within the ern Bight (Fig. 1). Fine-grained bottom deposits also range of the estimated outflow plus deposition. Although this result looks rather good, the figures used should only be regarded as broad estimates. More Table 1. The supply of suspended matter to the precise data, however, cannot be obtained at present, North Sea chiefly because of uncertainties of the supply from tonnes • yr-1 (dry weight) seafloor erosion and of sedimentation rates. A third uncertainty involves the transport of fine-grained Atlantic Ocean 10 x 10« material near the bottom which was left out of the Channel 10 x 10« (+ ?) discussion. In the southern North Sea, at least in the Baltic 0-5x10« shallower parts, this is not important, but the data Rivers 4-5x10« from Rohde (1973) show that it may be of great Rhine 4- Meuse l-70x 10« Ems 0-07x10« importance in areas such as the Skagerrak. This Weser 0-95x10« implies that actually much more material may be Elbe 0-86x10« picked up from the sea floor and deposited in these Thames + areas than is known at present. Humber 1-47x10« 4-50x10« Atmosphere 1-6x10« THE MASS-BALANCE OF ASSOCIATED POLLUTANTS Coastal erosion 0-7x10« East Anglia + Holderness Mass-balance models describing the dispersal of Seafloor erosion =: 5 X 10« (+ ?) pollutants in the sea are much more complex than Flemish Banks: up to 2-4x 10« those for suspended matter and will often involve an Vam e Bank: cs 1-2x10« atmospheric phase, uptake by organisms, conversion Off East Anglia :; ? Skagerrak off Denmark : ? from solution into a particulate phase, remobilization, Primary production ^ 1 x 10« and reprecipitation. Restricting ourselves to suspen­ ded matter and bottom deposits, their mass-balance Total: = 34 x 10« (+ ?) can provide better insight into a) the pathways The mass-balance of suspended matter and associated pollutants in the North Sea 9 followed by the associated pollutants, b) their final destination, and c) a historical record. For the North Sea the budget of suspended matter as demonstrated here can provide a basis: the flux of associated pol­ lutants can be calculated for the offshore areas where suspended matter is deposited and an estimate can be made of the amounts that remain behind in the coastal areas (estuaries, tidal flats, inland dumping) and of the amounts that will eventually reach the open ocean. When dealing with pollutants, however, there are several complications that, when considering suspended matter alone, either are not present, or can be neglected. 1) Many pollutants enter the sea from a very loca­ lized source (point-source) such as a river, a city, or a discharge point of industrial waste. Near such a source usually strong gradients in pollutant con­ centration develop in the bottom sediment (Förstner and Wittmann, 1979), affecting larger areas of the sea floor when the discharges persist. Within such an area, near the source, there tends to be a strong correlation between pollutant content and the amount of fine-grained material in the bottom sediment be­ cause of the larger surface area of the finer particles (e.g. de Groot, 1973, who found a strong correlation between heavy metal concentrations and the per­ centage < 16 pm on a carbonate-free basis). Bad cor­ relations, however, were found by Nicholson and Moore (1981) with the percentage < 63 iim (r being 0-20, 0-21, and 0-29 respectively for Zn, Gd, and Cu, Œ 3 SANDY MUD 0-44 for Pb and 0-71 for Hg) and even worse cor­ Figure 1. Distribution of mud and sandy mud in the North Sea relations (with the percentage < 62-5 urn) were found (from Eisma, in press b). 1, Fladen Grounds and Moray Firth; by Taylor (1974) in Tor Bay (r being 0-01—0-16 for 2, Devils Hole area and Firth of Forth; 3, Oyster Grounds; Pb, Zn, Mn, Cu, Cr, Cd, Ni, Co, and Hg, following 4, Flemish Banks; 5, nearshore Dutch coast; 6, offshore off East Anglia; 7, nearshore off Lowestoft-Yarmouth; 8, Rhine- r). r increasing values for Intermediate values for were Meuse-Scheldt estuary; 9, Thames estuary; 10, Ems estuary; 11, Elbe-Weser estuary; 12, Waddensea; 13, the Wash; 14, German Bight; 15, Elbe Rinne; 16, Outer Silver Pit area; Table 2. The outflow into the North Atlantic and 17, Skagerrak—Kattegat; 18, Norwegian Channel; 19, Nor­ deposition in the North Sea wegian and Swedish fjords.

tonnes-yr-1 (dry weight) found by Nelson (1979) for the relation with the percentage < 2 (im in the Thames estuary (r being Outflow to the North Atlantic 1 l-4x 106 + < 3 x 106______0-26-0-67 for Ce, Th, Cr, Hf, Cs, Sc, Fe, Co, and Eu). Low values for r, however, do not necessarily imply 14-4x 106 that the pollutants are predominantly accumulated Deposition in the North Sea Estuaries l-8x 10® in the coarser size fractions: Waddensea + Wash 3 xio6 a) the fact that distance from the source is an im­ German Bight 3-7-5x10» portant parameter for the dispersal of pollutants ? Elbe Rinne implies that when samples are collected over a com­ Outer Silver Pit 1-4 xlO6 Kattegat 8 x 10® paratively large area, samples containing the same Skagerrak 4-7 xlO6______concentration of fine particles will contain different 21-31-5x10« concentrations of pollutants; Dumped on land 2 x 106 b) the polluting element or substance may be strongly associated with a certain component in the Total 37-5-48 x 106 sediment, such as organic matter (as found by Wollast, 10 D. Eisma

1977, off the Belgian-Dutch coast for Zn, Cu, Co, and Mn), or may also be incorporated in detrital minerals (as found by Nelson, 1979, in the Thames estuary for Hf, Sc, Cr, and Eu). Such components of the sediment may or may not be strongly correlated with the concentration of fine particles. 2) Areas of net erosion may include local areas of net deposition, areas of temporary deposition, or areas where fine-grained material containing pollutants may be exchanged against material without pollutants or with a lower or higher pollutant concentration. This would result in a net deposition, or a net supply, of pollutants that is not balanced by an equivalent deposition or removal of suspended matter. It is not RMm(Wool)' clear whether this is important in Nature, but a typical area where this may happen is the Flemish Banks where areas of net erosion intermingle with areas of net deposition, with an overall net erosion (Gossé, 1977). 3) In areas of non-deposition, i.e. most of the North Sea floor, organic material derived from plankton and associated pollutants may be deposited. Reworking of what is left after decomposition will result in burial of at least part of the material supplied in this way. This type of process, whereby organic matter is gra­ dually accumulated in bottom sediments, is going on in e.g., the Fladen Grounds in the northern North Sea. Here no suspended matter, or only a very small amount, has been deposited for about 9000 years (Jansen et al., 1979; Johnson and Elkins, 1979) but 14C data indicate the deposition of more recent organic material and reworking of the top 10 to 15 cm (Erlenkeuser, 1978). Reworking in this area can be caused by bottom fauna, very large storm waves, or bottom trawling. 4) In areas where sand or gravel forms the sea bed, fine-grained material is trapped in the pore space between the grains. Although this fine-grained ma­ terial may be only a very small fraction of the total sediment (less than 0-5% by weight), it may contain a high content of pollutants compared with the sand grains or the gravel because of the large surface area B of the fine particles and the presence of fine-grained organic material. 5) Hydrous oxydes can form a coating on sand Figure«2. A, distribution of sands with iron contents > 0-4% in grains and pollutants may be incorporated by co­ the 250 to 315 |rm fraction: shaded area, area investigated; dark shade, high iron content (from Eisma, 1975). B, distribu­ precipitation. Off the mouth of the Rhine iron-rich tion of iron content in the separate size fractions of sands from sands are formed in this way (Eisma et al., 1966; the dark shaded area (x-x) and of marine sands from the light Eisma, 1975). The coatings are found mainly on the shaded area (o-o). The iron content was determined with coarser grains (> 200 |jum; Fig. 2). They are probably 20% HC1 after removal of the heavy minerals (s.g. > 2-77). Isolines and numbers in A indicate the general distribution also formed on the smaller grains but these are more of salinity (in °/00 S) based on data available up to 1965. mobile and easily mixed with grains from elsewhere with lower iron content. The higher iron contents are confined to an area directly influenced by the outflow The mass-balance of suspended matter and associated pollutants in the North Sea 11 of the Rhine, which indicates that outside the direct small quantity is trapped) pollutant concentrations vicinity of river mouths the formation of coatings by may be relatively high in the bottom deposits. The precipitation or adsorption from the sea water is of concentrations of Zn, Cu, and Pb in the suspended minor importance. matter, however, are of the order of one magnitude 6) The historical record in bottom deposits can be higher than those in the offshore sandy sediments disturbed by the burrowing activity of bottom fauna, (Table 3; there are no data on DDT or PCBs in the by reworking through waves and currents, and by suspended matter). The suspended material is in a mobilization of pollutants within the sediment. Sedi- continuous flux northward and shoreward, whereas mentological data in combination with isotope data in the sandy areas of non-deposition the contact can provide information on the physical and biological between the bottom-sediment particles and the sea reworking of bottom deposits. The effects of mobili­ water, and consequently the accumulation of pollu­ zation and upward or downward transport of pollu­ tants from the water into the sediment is a process tants following the gradients in the interstitial water extended over many years. In spite of this, pollutant and in the sediment are less easily detected. The con­ concentrations in the bottom sands are low compared centrations of pollutants in solution in the interstitial with those in suspended matter, at least for Zn, Cu, water are, where measured, very low as compared and Pb : on a yearly basis, therefore, the incorporation with those in the sediment. Aston and Stanners (1979) of these pollutants in the sandy-bottom sediments compared the 134Cs/137Cs record in an estuarine sedi­ must be very small compared with the amounts in­ ment core with diffusion models : their data suggested corporated in the fine-grained suspended material. that the original record had not been significantly This indicates that the areas in the North Sea where altered during the past 6 years. However, the pos­ suspended material is being deposited, are also the sibility of some disturbance of the original record over principal areas where these pollutants in particulate a long period of time cannot be excluded. Clear form are accumulated. records of increasing trace metal contamination over time have been found in the North Sea in the German Bight (Förstner and Reineck, 1974; Dominik et al., CONCLUSIONS 1978), and for Pb off the Belgian coast (Wollast, The mass-balance of suspended matter in the North 1977), and they show similar patterns as found else­ Sea indicates that an amount of the order of 40 million where (the Baltic, California coastal basins, Osaka tonnes (dry weight) of suspended matter (37-5 - Bay, Ned Bedford harbour (Erlenkeuser et al., 1974; 48xl06 tonnes) is yearly moving through the North Bruland et al., 1974, Matsumoto and Yokota, 1978; Sea. About 14-5xlO6 tonnes flow out into the Nor­ Förstner et al., 1978). wegian Sea, about 2xl06 tonnes are yearly dumped In order to assess the importance of suspended on land, and the remainder (21 - 31-5xl06 tonnes) material for the dispersal of pollutants it is essential is deposited within the North Sea. About 5x10® tonnes, to know the effects of small-particle trapping, organic- or 16 to 25%, is deposited nearshore in estuaries and matter deposition, formation of coatings, and other tidal-flat areas; the remainder is deposited offshore forms of association with bottom-sediment particles (50-70% in the Skagerrak-Norwegian Channel and in offshore North Sea sediments outside the areas the Kattegat). Data on the concentrations of pollu­ where suspended matter is being deposited. This has tants in suspended matter and bottom deposits indicate not been investigated but some idea of their combined that offshore Zn, Cu, and Pb in particulate form are effect can be obtained from the pollution surveys in predominantly accumulated in the areas where sus­ bottom sediment and suspended matter made by pended matter is deposited. Only small amounts are Hoenig (1978) and Henriet (1978) in the eastern yearly incorporated in the offshore sandy deposits Southern Bight off the Belgian-Dutch coast. Their results for the bottom sediments, summarized in Table 3. Content of Zn, Cu, and Pb (in ppm) in Figures 3 and 4, indicate that the concentrations of suspended matter and in offshore bottom sands off Zn and Cu are highest nearshore where also the con­ the Belgian-Dutch coast (from Hoenig, 1978). centration of fine particles (< 62 um) is highest. For Average content in parentheses Pb the higher concentrations are found further off­ shore in sandy sediments with very low concentrations Zn Cu Pb of particles < 62 p,m. Organic pollutants (DDT, PCBs) occur offshore as well as nearshore in com­ In suspension 171-838 38-508 88-571 paratively high concentrations with a tendency to be (415) (204) (239) higher nearshore. This shows that also in areas where Bottom sands 8-225 < 5-20 < 0-1-36 no suspended matter is deposited (or only a very (31) (« 7 ) (10) 12 D. Eisma

North Sea sediment North Sea sediments Zn (ppm) Cu (ppm) • <20 • 20-50 • 5-10 • 50-100 • 10-15 • 100-200 • 15-20 • >200 : • 20-30 • >30 -52'

•V

North Sea sediments 5B°30'- Pb (ppm) i I 0-1% ■ <10 I— I 1-2% • 10-15 • • « 2-4% • 15-20 B S 4-8% • 20-25 B 8-16% • 25-30 . . . • >30 • • -52 C > ‘

S/°30'-

2‘30‘

• >32% <62>un CZ5 > 2% < 62 /im

-52

■S/°J0'

Figure 3. Distribution of A, Zn; B, Cu; and C, Pb in bottom deposits off Belgium and southern Holland (from Hoenig, 1978); D, weight loss at 550°C (from Billen, 1978); and E, content of fine-grained material < 62 (Jm (redrawn from Gullentops et al., 1977). The mass-balance of suspended matter and associated pollutants in the North Sea 13

/• 3 0 ' 2° 2 *3 0 ' 3° 3 *3 0 ' 4° 4 ’ 3 0 ' 5° 1*3 0 ' 2° 2 *3 0 ' 3° 3 *3 0 ' 4° 4 *3 0 ' 5°

ppb ppb

■ > 3.75 H >10

53°- ■ 2 .2 5 - 3.75 5 3 - ■ 5-10

■ < 2.25 DDT PCB average average concentration concentration 5 2 *3 0 '- - 5 2 "3 0 '

- 52‘ 52°- - 52‘

51*3 0 '- - 5 /‘ 3 0 ‘ 51*3 0 '- - 51*3 0 '

51 - -51 51- -51 '

1*3 0 " 2 *3 0 ' 3 *3 0 ' 4 *3 0 ' 1*3 0 ' 4 * 3 0 '

Figure 4. Distribution of A, DDT; and B, PCBs in bottom deposits off Belgium and southern Holland (from Henriet,1978).

(which are areas of non-deposition). The mass-balance Zonderhuis, J. 1966. Iron and trace elements in Dutch coastal of suspended matter therefore offers a base for estimat­ sands. Neth. J. Sea Res., 3 (1): 68-94. Eisma, D., and Kalf, J. 1978. Suspended matter between Norway ing fluxes of these pollutants (and probably others) and Shetland and in the Sognefjord. Internal Reports NIOZ towards the deposition areas as well as outflow into 1978-13. the Atlantic Ocean. From the historical record this Eisma, D., and K alf,J. 1979. Distribution and particle size of can be extended into the past. suspended matter in the Southern Bight of the North Sea and the eastern Channel. Neth. J. Sea Res., 13 (2): 298-324. Erlenkeuser, H. 1978. The use of radiocarbon in estuarine re­ search. In Biogeochemistry of estuarine sediments, pp. 140-153. Ed. by E. D. Goldberg. UNESCO, Paris, 293 pp. REFERENCES Erlenkeuser, H., Suess, E., and Willkomm, H . 1974. Industriali­ zation effects, heavy metal and carbon isotope concentrations Aston, S. R., and Stanners, D. A. 1979. The determination of in recent Baltic sea sediments. Geochim. cosmochim. Acta, 38 : estuarine sedimentation rates by 184Cs/m Cs and other artificial 823-842. radionuclide profiles. Estuar. & coast, mar. Sei., 9 (5) : 529-541. Förstner, U., Müller, G., and Stoffers, P. 1978. Heavy metal Billen, G. 1978. A budget of nitrogen recycling in North Sea contamination in estuarine and coastal sediments: sources, sediments off the Belgian coast. Estuar. & coast, mar. Sei., chemical association and diagenetic effects. In Biogeochemistry 7 (2): 127-146. of estuarine sediments, pp. 49-69. Ed. by. E. D. Goldberg. Bruland, K. W., Bertine, K. K., Koide, M., and Goldberg, E. D. UNESCO, Paris, 293 pp. 1974. History of metal pollution in southern California coastal Förstner, U., and Reineck, H.-E. 1974. Die Anreicherung von zone. Environ. Sei. Technol., 8: 425-432. Spurenelementen in den rezenten Sedimenten eines Profilkerns Dominik, J., Förstner, U., Mangini, A., and Reineck, H.-E. 1978. aus der Deutschen Bucht. Senckenb. mar., 6 (2): 175-184. Pb-210 and Cs-137 chronology of heavy metal pollution in a Förstner, U., and Wittmann, G. T. W. 1979. Metal pollution in sediment core from the German Bight (North Sea). Senckenb. the aquatic environment. Springer-Verlag, Berlin-New York, mar., 10 (4/6) 213-228. 486 pp. Duinker, J. C. 1980. Suspended matter in estuaries: adsorption Gossé, J . G. 1977. A preliminary investigation into the possibility and desorption processes. In Chemistry and biochemistry of of erosion in the area of the Flemish Banks. Rijkswaterstaat, estuaries. Ed. by E. Olausson and I. Cato. Wiley and Sons, Dir. Waterh. en Waterbew. Fys. Afd. Nota FA 7702, 20 pp. Chichester-New York-Toronto, 452 pp. Groot, A. J., de, 1973. Occurrence and behaviour of heavy Eisma, D. 1975. Dissolved iron in the Rhine estuary and the metals in river deltas, with special reference to the Rhine and adjacent North Sea. Neth. J. Res. Sea, 9 (2) : 222-230. Ems rivers. In North Sea science, pp. 308-325. Ed. by E. D. Eisma, D. Suspended matter as a carrier for pollutants in es­ Goldberg. M. I. T. Press, Cambridge, Mass., 500 pp. tuaries and the sea. In Studies in marine environmental pollu­ Gullentops, E., Moens, M., Ringele, A., and Sengier, R. 1977. tion. Ed. by R. A. Geyer. (In press a). Geologische kenmerken van de suspensies en de Sedimenten. Eisma, D. Supply and deposition of suspended matter in the Projet Mer, Rapport final, 4 (Sédimentologie) : 1—45. North Sea. In I.A.S. Special Volume North Sea Symposium. Henriet, J. 1978. Recherche des pesticides dans l’eau de mer et (In press b). dans les sédiments marins. Projet Mer, Rapport final, 6 Eisma, D., Das, H. A., Hoede, D., Raaphorst, J. G., van, and (Inventaire des polluants), 163-185. 14 D. Eisma

Hoenig, M. 1978. Polluants inorganiques dans les sédiments et Matsumoto, E., and Yokota, S. 1978. Accumulation rate and les suspensions de la Mer du Nord et de l’Escaut. Projet Mer, heavy metal pollution in Osaka Bay sediments. Jour. Oceanogr. (Inventaire des polluants): 193-209. Soc. Japan, 34: 108-115. Jacobs, M. B., and Ewing, M. 1961. Suspended particulate Nelson, L. A. 1979. Minor elements in the sediments of the matter: concentration in the major oceans. Science, 163: Thames estuary. Estuar. & coast, mar. Sei., 9 (5): 623-629. 380-383. Nicholson, R . A., and Moore, P .J. 1981. The distribution of Jansen, J. H. F., Weering, T. C. E., van, and Eisma, D. 1979. heavy metals in the superficial sediments of the North Sea. Late quaternary sedimentation in the North Sea. In The This volume, pp. 35-49. quaternary history of the North Sea, pp. 175-187. Ed. by Rohde, J. 1973. Sediment transport and accumulation at the E. Oele, R. T. E. Schüttenhelm and A. J. Wiggers. Acta Uni- Skagerrak-Kattegat border. Report no. 8, Oceanografiska versitatis Upsaliensis, Uppsala, 248 pp, Institutionen, Göteborgs Universitetet, Göteborg. Johnson, T. C., and Elkins, S. R. 1979. Holocene deposits from Taylor, D. 1974. Natural distribution of trace metals in sedi­ the northern North Sea : evidence for dynamic control of their ments from a coastal environment, Tor Bay, England. Estuar. & mineral and chemical composition. Geologie Mijnb., 58 (3) : coast, mar. Sei., 2 (4): 417—424. 353-366. Wollast, R. 1977. Propriétés physico-chimiques des sédiments et Liss, P. S. 1976. Conservative and non-conservative behaviour des suspensions de la Mer du Nord. Projet Mer, Rapport final, of dissolved constituents during estuarine mixing. In Estuarine 4 (Sédimentologie) : 123-143. chemistry, pp. 93-130. Ed. by J. D. Burton and P. S. Liss. Academic Press, London, New York, 229 pp.