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Dispersal, Concentration and Deposition of Suspended Matter in the North Sea

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Dispersal, Concentration and Deposition of Suspended Matter in the North Sea Journal ofthe Geological Society, London, Vol. 144, 1987, pp. 161-178, 11 figs, 1 table. Printed in Northern Ireland Dispersal, concentration and deposition of suspended matter in the North Sea D. EISMA & J. KALF Netherlands Institute for Sea Research, P.O.Box 59, Den Burg, Texel, The Netherlands Abstract: Suspended matter comes into the North Sea from the Atlantic Ocean, from the Channel, rivers, seafloor erosion, coastal erosion, the atmosphere, and from primary production. The contribu- tion of the last source is temporarily large when phytoplankton growthis abundant, but is soon decomposed, mineralized or consumed so that over the year the net effect is small. Accumulation and deposition of suspended matter occurs predominantly in only a few areas (the Waddensea, the German Bight, the Skagerrak/Norwegian Channel). The remainder of the North Sea floor consists of Pleistocene and Early Holocene relict deposits and of reworked sands and gravels. The mechanisms resulting in accumulation and deposition are discussed. There are strong indications that the amount of suspended matter flowing out from the North Sea into the Norwegian Sea along the Norwegian coast is only little more, or the same, as the amount coming in from the North Atlantic, although its composition may be different. Suspended matter and mud deposits in the Water circulation in the North Sea North Sea The circulation in the North Sea has been studied since the In the North Searegional bulk suspended matter studies late 19th century based at first on salinity data, some current have been made by sampling, weighing and using optical measurements and the flow of drift bottles-which methods (with more recently, some limlted SEM observa- culminated in the charts presented by Bohnecke (1922)- tions) mostly in the Southern Bight (summarized in Eisma & and subsequently also on current measurements of longer Kalf 1979). For the German Bight, data were published by duration, data fromsurface and bottom drifters and Krey (1956), Banse (1956), Martens (1978), Prober (1981), numericalmodels involving density differences, tides and Duinker (1981), Albrecht (pers. comm.), by Stolk (in Eisma windstress (Maier-Reimer 1977; Hill & Dickson1978; et al. 19%, and pers. comm.) and recently by Irion et al. Nihoul 1980; Becker 1981; Davies 1982). A general picture (this volume); for the Skagerrak/Kattegat/Norwegian of the residual surface circulation in the North Sea, mainly Channel by Joseph (1950), Malmberg (1964), Smed (ICES drawn after Hill & Dickson (1978), is given in Fig. 2. The 1969), Hljerslev (1971,1981), Rodhe (1973),Svansson circulation is largely anticlockwise with ocean water from (1975), Eisma & Kalf (1978) and Eisma et al. (19846); and the North Atlantic flowinginbetween Shetland and for the rest of the North Sea by Joseph (1955), Krey (1961), Norway,between Scotland and Shetland and through the Hagmeier (1961), and H~jerslev(1977). Strait of Dover, and mixingin the North Seawith fresh From these data the distribution of suspended matter in waterfrom rivers and low salinitywater from the Baltic the surface water of the North Sea isapproximately as (references for the approximate amounts given in Eisma indicated in Fig. 1. The concentrations close to the bottom, 1981). Large gyres are usually present SW of the Dogger where measured, are similar to those at the surface, i.e. Bank, in the German Bight, off southern Norway and in the usuallyless than 10 mg l-', but are higher in the coastal Skagerrak, and there are indications of smaller ones in the waters, in particular in the German Bight SE of Helgoland, Southern Bight off the Belgiancoast and East Angha where concentrations up to 130 mg 1-' have been measured (Nihoul & Ronday 1975), and in the Skagerrak along the close to the bottom (H. Albrecht pers. comm.; A. Stolkpers. Danish coast (Jammerbucht; Tomczak 1968). comm.). Higher concentrations near the bottom were also Where tidal flow is weak, as is generally the case north found at the Fladen Grounds (the 'FLEX area, H~jerslev of 54N, wind stress is an important factor, forcing the water 1977) where in August 1976 the concentrations increased downwind. This hasa particularly strong effectin partly from less than 0.4mg1-' at 80-100m depth to more than enclosed areas like the German Bight and the Skagerrak, 1mgl-' close tothe bottom at c. 125-m depth (mm. where the water can be piledup, resulting in astrong flow in 6.8 mg l-l). the opposite direction when the wind changes(Riepma Malmberg (1964) and Rodhe (1973) found higher near- 1980; Aure & Saetre 1981) but can affect the entire North bottom concentrations in the shallower parts of the Sea: also where the tidal flow is stronger, as in the Southern Skagerrak along the N Danish coast, indicating that bottom Bight and Straits of Dover, wind stress cancompletely sediment is being stirred up. This is also suggested by the reverse the normal direction of flow for a number of days data of H~jerslev(1971, 1981). During most of the year the (Carruthers 1926; Lee 1970; Riepma 1980). Fine particles in upper 10-20m of the water column are more turbid than suspension start to settle out when the current velocity the deeper water because of plankton growth. The dispersal, becomesless than 10-20 cms-'. Where the maximum concentration and deposition of suspended matter in the tidalvelocity is lower, as in the Skagerrak andin the North Sea is the subject of this paper and will be discussed southern part of the Norwegian channel (Fig. 2), wind stress chiefly in relation to the water circulation and wave activity. and density differences can induce currents that are much 161 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/161/4888858/gsjgs.144.1.0161.pdf by guest on 02 October 2021 162 D. EISMA & J. KALF 40 20 0" 2" 4" 6" 8" 10" Channel down to the Skagerrak (Dooley 1974). When in the central North Sea during the winter the water cools down, the cold water moves along the bottom into the Skagerrak, whereit mixeswith the water alreadypresent there and forms the deep Skagerrak water (Ljeen 8t Svansson 1972). SI0 Sources and dispersal of suspended sediment The principal sources of suspended matter (non-living) in 60" the North Sea are (McCave 1973; Eisma 1981): the North Atlantic Ocean, from where c. 10 X 10" t a-' (or 59O c. 30% of the total yearly suspended load of the North Sea of 35-40 X 10" t a-'), enters with the inflowing water in very 58' low concentrations; the Channel (c. 10 X 10" t a-'), the Baltic (c. 0.5 X 10" t a-'), 57 rivers (c. 4.8 X 10" t a-'), seafloor erosion (c. 6-7.5 X 10" t a-',probably more, 56" including uptake of material in the Northern North Sea), coastal erosion (c. 0.7 X 10" t a-'), the atmosphere (c. 1.6 X 10" t a-'), 55" primary production (c. 1X 10" t a-'). 54" 53O 52" 51" 50' 490 Fig. 1. Distribution of suspended matter in the North (fromSea Eisma 1981). Heavy lines and numbers represent suspended matter concentrations inmg 1-'. The numbers between brackets were obtained during a few cruises made in 1977-1979. The others are based on this data with additions from the literature. Arrows general direction of flow; broken arrows, subsurface flow. stronger: in the upper c. lOOmof the Skagerrak current velocities of more than 20cm S-' have been measured regularly and can be up to 90 cm S-' (as was found off the Danish coast: ICES 1969; Dahl 1978). Residual bottom currents in the shallow Southern North Sea are verysimilar to those at the surface, except nearshore where the residual currents tend to be directed more landward near bottom and more offshore at the surface. This is caused by small density differences between the surface water and the bottom water (Dietrich 1955; Kao 1981). Also in the deeper Central and Northern North Sea and in the Norwegian Channel and the Skagerrak there is ,2,5- -rnox. lido1 current subsurface flow in a direction different from the flow at the speed at meon surface. In the Central and Northern North Sea this flow is springtide (cm S-' irregular and depends onthe regional variations inwind rivers 12901 stress and seawater temperature, but further E a more permanent subsurface flowof North Atlantic water is Fig. 2. General residual circulation in the North Sea (after Hill& present. This water enters the North Sea between Norway Dickson 1978). Transition zone from Pingree& Griffiths (1978); and Shetland and becomes a subsurface current near maximum tidal current speed at mean spring tide from North Sea Shetland, following the western side of the Norwegian Atlas (MAFF, 1981), inflow and outflow data cited in Eisrna(1981). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/161/4888858/gsjgs.144.1.0161.pdf by guest on 02 October 2021 SUSPENDEDMATTER SEANORTHIN THE 163 The residualcirculation transports material insuspension Mud deposition from the Northern North Sea southward, picks up material Most of the suspended matter deposited in the North Sea from the Southern Bight and moves this along the eastern (50-70%)is deposited in the Skagerrak-Kattegat- side of the Southern North Sea from the Englishand Norwegian Channel, with the remainder in the German Belgian-Dutch-German-Danishcoasts to the Skagerrak Bight, on the tidal flats in the Waddensea and the Wash and from there along the coast of Norwaytowards the (primarilyin the largeembayments: the Dollard, the Norwegian Sea (Fig. 1). From the Southern Bight to the N Leybucht, the Jade), and in rivermouths (Eisma 1981a,b). there isalso a marked decrease in the suspended matter Off the Belgian coast, on the FlemishBanks, both concentration.
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