Recent sediment accumulation in the Norwegian Channel, North Sea

Recent sediment accumulation in the Norwegian Channel, North Sea

HENKDEHAAS,ELLEN OKKELS& TJEERDC.E.VAN WEERING

deHaas,H., Okkels,E.&van Weering,T.C.E.1996:Recentsedimentaccumulat ionin theNor wegi an Channe l,North Sea.Nor.geol.unders.Bull.430,57-65.

IntheNorwegian Channel35 boxcoreswere takenfor sedi mento logicalanalysis,and'lOPO(J.-and'37CSy-spect ros- copyto dete rminesedimentation rates.Grainsizeswere dete rmi nedand X-ray photog raphsof thecores were made.Inaddit ion, penetrat ing echo sounding linescovering a length of more than 5500 km were st udied.

Sedimentsente r theNorwegian Channel fromthesout hand thewestas suspendedload.The dry bulk densit ies of thesegenerallyfine-grainedsediments (siltyclays- silts)range from 0.35to1.79qxcrn',Mostof thecores showa surfacemixedlayerof lessthan2cm. Thesedimentat ion ratesmeasured in theNorweg ianChannel rangefrom30 to280 mmx100 yr',Highestsedi mentationratesarefoun din thenorth ernpart of the researcharea.Thetotalrecent drybulksediment accum ulationin theNorwegianChannel iscalculated tobe28x1O·tons'yr' .Penetr ating echoso- under datarevealthatsedimentation occursinthe deeper partsandinsmallprotectedbasinsalong theflanksof theNorwegian Channe l. Thecontrast betweenthe present-dayrelat ivelyhighsedimentat ionrat esandthe thin Holocenesedimentaryunit isexplained byachangeinthe deposit ionalsystemsomet ime during theHolocene.

deHaas,H.,Okkels,E.& vanWeering,T.CE.,NetherlandsInstitu tefor 5eaResearch, P.O.Bax59,7790AB DenBurg,The Nether lands.

Introduction

The NorwegianChannelis one of the largestand deepest recent sedimentary basinsof the North Sea and forms a major sink for fine-grained material in the North Sea (Eisma 1981,van Weering 1981,Eisma & Kalf 1987, van Weeringet al.1987,1993).Sedimentsenter theNorth Sea (Fig.1)from the AtlanticOcean in the north,through the English Channel, from the BalticSea and fromriver input.

Furthermore,sea floor erosion, coastal erosion,primary product ion and atmospheric input contrib ut eto thesedi-

Fig.1.TheNorthSea and thestudyarea(outlined).

60

50

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o

10

10 20

20 60

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ment load of the North Sea.The sedimentsin the North Sea are transported by an anticlockwise residual circulati- on (Otto et al. 1990). Recent sed iment accumulation occurs mainly along the eastern margin of the North Sea (t he Wadden Sea,the German Bight,the Skagerrak,the Kattegat and the Norwegian Channel) (McCave 1973, Eisma 1981,Eisma&Kalf 1987).

In recent years,application of210Pb geochronologyhas been usedto calculatesedi ment at ion and/oraccumulati- on rate sofrecent sedimentsin the depositional sinks of the North Sea (van Weering et al. 1987,1993,Zuo et al.

1989,Jorgensen et al. 1990, Denneqard et al. 1992a).

Othermet hodshave also been used,such as pollen studi- es (Zagwij n & Veenstra 1966, van Weering 1982, Henningm oen&Heeq 1985,Long et al. 1988),mass bud- get calcul ation sofsuspended sediments(Eisma1981),14C (Long etal. 1988,Moodley&vanWeering 1993),dinofla- gellat es(Long et al. 1986), stable isotopes (Erlenkeuser 1985) andforam inifera (van Weering 1982, van Weering&

Ovale1983,Ovale&vanWeering 1985).

In this paper we report on sedimentation and sedi- ment accumulation rates in the Norwegian Channel based on data coll ect ed during a cruise with the R.V.

'Aurelia' in1988 andtw o cruiseswith the R.V.'Pelagia' in 1991and 1993 (ENAM93)to the Norwegian Channel,and ondatafrom the literatu re.Thisst udy has aimedat deter- mining the total amountof sediment being deposited in the Norweg ianChannel, and to explain the majorsed i- mentary processes.

Methods

A total of 35 boxcoreswere collected in the Norwegian Channel with the RV.'Aurelia' (1988)and with the R.V.'

58 HenkdeHaas,El/en Okk els&Tjeerdc.E.vanWeerin g GU-BULL430. 1996

Fig.2.Locationsand sedimentatio nratesforthestations wheresedimenta- tionrateshave beendeterm ined.

Pelagia' (1991 and 1993).Immediately after retrieval of the cores the overlying bottom water was siphoned off and subsamples were made byinsertingwet PVC line rs (0:9 cm)into the boxcores. The liners were closed with plastic caps and sealed with tape. After sampling, the liners were stored upright at 4°C. Sedimentation rates were determined using a- andy-spect romet ry.21°Pbwas measured via its a-part icles emitting granddaughter 21OPO, which is assumed to be in equilibrium with 21°Pb.

Thesediment at ion rateswere calculated according to the (IC(Constant InitialConcentration)method.The method used is described in more detail by van Weering et al.

(1987). 137CS activity was measured using Aptec and Canberra germaniumy-ray detectors on 1 cm-thicksedi- ment slices with 35-65 g dry weight, depending on the porosity of the sediment. 21°Pb and 137CS analyses were performed onthe selected cores at1cm depth intervals.

The counting time for thea-andy-measurements was24 hours and 1 to 2 weeks,respectively.

X-radiographs of the split cores were made using a Hewlett-Packard43805N X-ray system Faxitron.The radi-

ographswere used to check the cores for bioturbation and other possible disturba nces. Sedimentation rates were determ ined onlyfrom cores not disturbed by bio- turbation or by sampling and/or storage. Twenty-five cores were considered suitable for sedimentation rate determination (Fig. 2). Grain-size analyses were perfor- med at the University of Utrecht usinga MalvernPart icle Sizermodel 26000 capableofmeasuring grain sizeswit- hin a range of 0.5-188mm. Orybulkdensities were deter- mined by taking subsamples of 5 cm' ofwet sedim ent whichwere driedat 60-700(for72 hour s andaft erwards weighed on a Met tl er P1200N balance. Sediment ation rates and dry bulk densities were corrected forcompact i- on during subsampling and storage of the samples. This was done by measuringthe compactio nof thesedime nts in the liners and assuming a linear compaction over the wholesedi ment column .Accum ulation rateswere calcu- lated using sedimentationratesand dry bulkdensit ies.

3.5kHz penetrating echosounderlinesrecordedin 1973, 1974,1975,1976 and 1993,and data from the literature were used to determinewhereinthe Norweg ian Channel recent sedimentation and erosion/non -deposition occurs.

Geolog ical settin g, hyd rog raphy and sedime nt transport

The presentmorphology of the NorwegianChannelis the product of glacial erosion followed by sedime ntation during the Pleistoceneand Holocen e(van Weering 1975, Otto et al. 1990, Pedersen et al. 1991, Holtedahl 1993, Pederstad et al. 1993). At -59°30'N the Norwegian Channel shallows to 280 m at a saddle point. Further sout h, theNorwegi an Channel deepensto morethan 700 m in the Skagerrak(Ott o et al. 1990).

An overviewof the dominant circulati on patternin the northernNort hSea isgiven by Ottoet al.(1990). Atlant ic water entersthe North Seamainlyalongthewesternslo- pe of the Norwegi an Channel and through the Orkney- Shetland infl ow.This water mass enters the Skagerrak along the sout hern slope as the Atlanti c Shelf Edge Current. Water coming from the southern North Sea enters the Skagerrakalongthe southern slope and partly flows into the Kattegat.Theremain dermixeswit hwater coming from the Baltic Sea,andtheAtlantic Shelf Edge Current.This watermass leavesthe Skagerrak as the less saline Norwegian Coastal Current, which flows to the nort h throu g h the Norwegian Channe l and finally into the NorwegianSea.Seasonal variationsin thispatte rn are the result of differences inwater temperatu reand wind st ress(Otto et al. 1990).The fract ion of the sediments which does notsettleintheSkagerrakis transportedout of the Skagerrak into the Norwegian Channel by the NorwegianCoastalCurrent as suspen ded load.Some of thismaterial settlesin the Norw egian Channel and the remainderis transported into the Norwegian Sea(Eisma 1981,vanWeering 1982).

6

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North Sea

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NGU-BULL430.1996 AivoLepl and&RodneyL.Stevens 59

ENAM18 Percentage

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ENAM 10 Perce ntage 25 50 751

ENAM 8 Perce ntag e

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Fig.3.Resultsofthegrain -sizeana lysesof the ENAMcores.Posit ionofthe graphsrela- tive to thegeographica l loca tion of the cores.

Results

Sediments

cores showed worm tubes at the sediment surface. The brownish colours of most of the top sediments of the cores indicate a well oxygenated environment.

Description Grain -sizeanalysisand dry bulk density

The sediments in the Norwegian Channel consistmainly of brownish,greenish and greyish silty clays. Some cores show a few cm thicksandy intervals(e.g. Sk91-4),whil e othersare sandy throughout thecore(e.g. Sk91-6).Core Sk91-2, taken high upon the western slope of the Norweg ianChanne l,consistsof medium to coarse sand.

Sometimes gravel is found (core NG88-6). Shells, shell fragments and echinoid spines are common.

Macroscopically the cores are homogeneous and lack almost completely prim ary sedi mentary structures. X- radi ogr aphs of the coresalso showveryfew primarysedi- mentarystruct ures.Coarse-grained intervals,quartz and claypebbles,anddarkand lightintervalscausedbydiffe- rencesin mineralogy can berecognis ed on thex-radi o- graphs.Macroscopicburrowscanberecogni sed inmost of the coresand on the X-ray images.The burrows have diameters rang ing from0.5mmto morethan10 mm.In many casesonlyafew narrow (0 =0.5 mm) or wider (0

±10 mm) burrows are present, leaving thefew primary sedimentary structures and the light and dark intervals intact,i.e.,the sediments are more orless undisturbed. In other cores the primary structures and colour banding are clearly disturbed, indi cat ing strong bioturbation (cores ENAM93-8, Sk91-13 and Sk91-14). Several box

The grain sizes have only been determined on the ENAM93 samples(Fig. 3). Most of these cores consist of clayey silts,the percentage of clay being usually less than 25%.In most ofthe cores the amount of very fine to fine sandis 5%or less. Cores ENAM93-10, 13 and 16 are relati- vely coarse,andthe amount of sandy materialsometimes exceeds 40% (t op of ENAM93-10). The finest-grained sedimentsare found in the central part of the Norwegian Channel. The grain-size distribution of the surface sedi- mentsis in agreement with the data provided by Qvale &

vanWeering(1985).

Drybul k densiti esvary fro m0.35 to 1.79 qxcrrr depen- dingonthe depth in the coreand the grain-sizedistribu - tion.

2 10Pb and 137Cs measurements and

sedimentation rates

The locations of theindividual stations and the sedimen- tation rates measured are given in Table 1. Cores ENAM93-8, ENAM93-15, NG88-6, NG88-7, NG88-10, NG88-15,Sk91-13and Sk91-14were not used for inter- pretatio ns,because they are strongly bioturbated. Core

60 Aivo Lepland&RodneyL.Stevens GU-BULL <:30,1996

ENAM9 ENAM 13

2,oPbexcess activity(Bq'kg") 210Pb excessactivity (Bq ' kg⁰¹) ENAM 9 ENAM 13

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Fig.4.Selected results ofthelIOPbmeasureme nts.Grey intervalindicates thickness of the surface mixed layer(SM L).Seetext for moredetails.

Sk91-2 was only 5cm long and thereforetoo short to be of any use.Core Sk91-4 show ed acompa ct ion of 53%

during sampling and storage,and is therefo re possibly too disturbed to allow a reliable deter mination of the sedimentati on rate.Fig.4 shows some of the results of

the 2lOPb measurement s. In general, the log arit hmi c

excess210Pb activity plotted versus core dept h shows a straightline belowthe surfacemixedlayer(SM L).Usually theSMLisvery thin.In mostcores thethicknessdoes not exceed2 cm(Fig.4).In some cores,howeve r, (ENAM93-8 and 15)the SML reachesa thicknessof15 cm or more.

Some of the 2,oPb plot s show ano malously high or low 2,oPb values deeperin the core(ENAM93-13,Fig .4).These are thoug ht to be the resultof local burrow ingbelowthe SML, which resulted in the transport and subsequent mixing ofolder or youngersediment.Thisexplanatio nis supported bythe X-radiographswhich show some maj or burrow s atthedept hs conce rned,and bygrain-size ana- lysesshowing a variance in grain sizeat these specific depths.

Fig. 5 show s 137Cs-activity profi les of some of the ENAM93cores.The 1963 nuclear bomb testing and the 1986 ChemobylP'Cs-a ct ivitypeaksin coreENAM93-9 fall

Fig.5.Resultsof"'Cs measurem entsof ENAM1993cores.Grey interval indi- catesthickn ess ofthe surfacemixed layer(SM L).Arrows indi ca te expected depth of1963and1986activitypeaks.

withinthe bioturbated upper layerof the sediment and are therefore not clearlyvisibleintheprofi le.The 1986 peaksin coresENAM93-13 and 14arewithi n thebiotu r- bated layer,but smallincreases in137Cs-activityarevisible.

Bot h thesecores show a maximum inactivit y result ing from the 1963nuclear bomb testing.The peak at the base of the bioturbated layer of core ENAM93-16might be the result of the burialof 1986 sedime nt sdueto bioturbat i- on.This causes a pre-1986 137Cs-activity peak which is located toodeepinthe core.Thehig hactivit y inthe 3-4 cm int ervaljust below the biot ur bated layer maybe the result of the increased Sellafield releaseswhichpeaked around 1975 to 1980(Kunzendorf et al.thisvol ume).The sedime nt ation rateswere determined from the2'OPband 137Cs measurement s. The sedimentation rates in the Norweg ian Channelrange from 30to 280 mmx lOOyr' . Highestsedimentation rateswerefound in the northern part ofthe NorwegianChannel(Fig. 2).

From the lit erature it is know n that on the Fladen Ground, on the plateau northwest of the Norweg ian Channeland theupper western slope oftheNorweg ian Channel,thereislittlesedimentatio n,and possib lyerosi- on(Johnson&Elkins 1979,Rise&Rokoengen 1984).On

NGU-BULL430,1996 Aivo Lepland&RodneyL.Stevens 61

Fig,6.Location map of the acoustica l reflectionprofilesusedin this stu dy.

Numbers7A to 8 indicotetheposition of theprofiles show nin Figs7A to 8, respecti vely.

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58

2 4 6 the eastern slopeof the Norwegian Channel,towardsthe mainland, crystalline bedrock extending from land is locally exposed, which indicates that non-depositional condit io ns prevail there (Holtedahl 1993, Rise &Roko- engen 1984), but isolate d, pon ded Holo cene is likely to be present.Onthewestern slope ofthechannelthe local effectof erosiononthe platform isnot iced in the presen- ce and amount of older, reworked foraminifera in Nor- wegian Channel sediments(Qvale & van Weering 1985).

Acoustical data

In the period 1973-1976, 3.5 kHz echo-sounding lines were recorded across the Norwegian Channel between 58°and 63°N.Lines located northof 60^0Nare discussed in van Weering (1983).In 1993,some additi ona llines were recorded using a Datasonics Chirp echosoun der.

Togetherthese linescover alength of more than 5500km (Fig. 6).

North of 60^0N van Weering (1983)identified four sedi- mentary units reflecting depositional events during and afterthe last glaciation.These units can be tracedin the area south of 60^0N.The upper unit (unit 1) represents Holocene fine-grained sediments which overlie Weichselianglacialmarinedeposits(unit 2) (van Weering 1983, Holtedahl 1993).The Holocene and glacial marine sediments are not equallythick through outthe area.The averagethickness ofthe combined unitsin the centra l partof the Norw egianChannelis abo ut 10-20 m.Datings of a corefrom the central part of theNorwe g ian Channel show that herethe base of the Weichselian glacial marine unit lies at about 23 m, and the base of the Holocene at 3.75m depth(core Troll 3.1,Lehman et al. 1991,Lehman

& Keigwin 1992).In the southern part of the Norwegian

A

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50m Late Weich s el ia nl .! ^{Holoce n e cover}

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Fig.7.3.5kHzpenetrating echosounderprofi lesfrom theNorwegianChannel.Dashedlinesinsubsurface aretim emarkers.A)Profile showing thepinching out of theHolocene sedimentsonthewesternslope of theNorwegianChannel.Waterdepth 320-390m,verticalexaggeration 20x.B)Profileshowing thin Holocenesedimentarycoveron atopographic highinthecentra lpartofthe Norwegian Channel.Waterdepth400-440m,verticalexaggera tion SOx.C) Profile showingasmallloeal basinon the easternslope of theNorwegianChannel.Water depth 340-420m,vert icalexaggera tion 20x.

62 AivoLepland&RodneyL.Stevens NGU-BULL430, 1996

s .. ^"

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50m .,1<>

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Fig.8.Penetratingechosounderprofi leshowingthetruncat ion ofinternal reflectorsatthe sea floor,for details seeblow upsAtoC.Arrows indicate the trun- cation ofinternal reflectorsbythe sea-floorreflector.Waterdep th240-260m,verticalexaggeration30x.

Accumu lat io n rates

Based ontheacousticaldat a,themaximum surface area of that part of the Norwegian Channel in whichrecent sedimentation occursiscalculated to be 33,900 km'(Fig.

9).Using threeintervalsof sedimentati on rates(0-100, 100-200 and >200 mmx100yr')and calculating average sedimentation ratesand averagedry bulk densitiesfor each of theseintervals,themaximumtotalyearlydrybulk accumulation ratein this partof theNorweg ianChanne l is calculatedtobe28x10⁶tons.

Discussio n

Channel these layers can be thicker, but the boundary bet w een the Holocene sediments and theWeichselian glacial marine sediments beneath (van Weering 1983, unit2)can be very unclear.Thethicknessofthe Holocene cover can therefore not always be determined accurately.

Units 1 and 2 pinch out tow ards the margins of the Norweg ianChanneland towardsthesou t hand the nort h (Fig. 7A). On topographic hig hs the Weichselian and Holocene sedimentarycoveris sometimes extremely thin (Fig.7B).Local depressions of the seabed act assedim en- tary basins in which increased sedimentation occurs.

Especially along the eastern flank of the Norwegian Channel the pronou nced topographyresult sin the pre- sence of small local basins (Fig.7C).In som e cases the internal reflectorsof earlier depo sited sedi ments appear to be truncatedat thesea floor(Fig.8).

4 6 2

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584 - - -- - - - - - - - 62

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Fig. 9. Geographicaldistribution ofrecent sedimentation areas in the NorwegianChannel.

Alt hough the X-rad iographsclearly show the effect of bioturbation, the sedimentation rates calculated from

NGU-BULL430,1996

Table1.List ofstatio nsand"'Pb sedimentation rates(n.d.=no tdet e rmi- ned).

Sed ime ntatio n

Sta tion Po sitio n rate

No. North East (m m/1 00 yr)

Sk9 1-2 61"20.45' 2°00.70' n.d.

Sk91-3 61"30.79' 2°33.42' 190

Sk9 1-4 61"39.20' 3°24.90' n.d.

Sk91-6 62007.00' 1°19.78' 190

Sk91-7 62"09.83' 2°18.83' 220

Sk91-8 62'12.28' 3°17.75' 120

Sk91-9 62'12.81' 3°38.05' 60

Sk9 1-10 60°43.50' 4°29.70' 30

Sk9 1-11 60°42.7 1' 4°10.28' 280

Sk91-12 60°4 3.49' 3°20.89' 60

Sk91-13 60°42.62' 3°10.24' n.d.

Sk91-14 60°43.30' 3°13.08' n.d .

Sk91-15 59°30.60' 4°46.50 ' 120

Sk91-16 59"29.99' 4°27.30' 90

NG88-6 58°10.47' 4°37.61' n.d.

NG88-7 58°12.43' 4°40.25' n.d.

NG88-8 58°15.73' 4°52.12' 100

NG88-9 58"24.18' 5°07.83' 110

NG88-10 58"28.61' 5°18.92' n.d.

NG88-11 58"36.68' 5°24.71' 120

NG88-12 59'11.92' 4°51.39' 40

NG88-13 59°13.8 1' 4°26.94' 60

NG88-14 59°15.47' 4°08.85' 40

NG88-15 59°12.70' 3°42.88' n.d.

NG88-16 59°13.07' 3°30.46' 90

NG88-17 59°59.45' 3°17.80' 140

NG88-19 60001.72' 3°55.14' 160

ENAM 8 59"30.10' 3°41.25' n.d.

ENAM 9 59"30.03' 4°05.29' 40

ENAM10 60"20.60' 4°39.32' 60

ENAM 11 60°19.98' 3°22.06' n.d.

ENAM 13 61°19.94' 3°52.17' 70

ENAM 14 61°19.77' 3°35.58' 100

ENAM 15 61°19.25' 3°15.87' n.d.

ENAM 16 61°19.96' 2°34.64' 110

ENAM 18 62003.69' 2°55.58' 190

downcore21°Pb profiles are considered to reflect the real sedimentation rates. This assumption is based on the work of Nittrouer et al.(1984),who concluded that sedi- mentation rates on the Washingtonshelf can be calcula - ted from the 21°Pb profile below the surface mixedlayer.

Furthermore, the maxima in the 137Cs-activit y profiles coincide well with the expected depths of the 1963 nuclear bomb test and 1986 Chernobyl accident peaks, thus supporting the sedimentation rates determined by the21°Pb method.

The thin Holocene sedimentary cover in the central part of the northern Norwegian Channel,in the orderof 4 m orless,and the even thinner layer near the flanksof the Norweg ianChannel donot correspo nd with theextrapo - lated recent sedimentati on ratesmeasured by the 21°Pb method,whichare in the orde rof100-200mmxl00yr'.

Therelativ ely thin Hol ocenesedimentary cover in relati- on to the highrecentsedi mentationratesinthe nort hern part of the areacannot be exp lainedbyastr on ger com- paction of the sediments.The contrast between the pre- sent-day sedimentation rates and the thin Holocene sedi- mentary cover can,however,be explained by a changein sedimentaryregime sometime during the Holocene. This

AivoLepland&RodneyL.Stevens 63

explanationis supported by the grain-sizedistr ibution in two piston cores taken about 30 miles NNW (station ENAM93-15, Table 1) and 45 miles SSW (ENAM93-11, Table 1) of core Troll 3.1 described by Lehman et al.

(1991)and Lehman & Keigwin(1992).In these two cores a coarsening upwards starts at -1 m and -2 m core depth, respectively.In both ENAM cores the grain size increases from clayey silt to coarse silt-very fine sandy silt,Echo- sounding lineslocated very near the three cores mentio- ned, do suggest that on the locati ons where the two ENAM93 cores and core Troll 3.1were taken,similarsedi- mentary processes took place during the Late Weichselian and Holocene.The only difference is foundin the total thickness of the sedimentary unit.In the south (near ENAM93-11),the unit is about 1.5times as thick as on the locationof core Troll 3.1. In the north,near station ENAM93-15, the thickness is just slightly less than near core Troll 3.1. The mechanism behind this change in depositional regime and possible related change in hydrological conditions is not yet known.Further study of the piston cores will most probably give the answer to thisquestion.

Van Weering (1983) concluded that in the northern part of the Norwegian Channel the deposition of the Holocene deposits (unit 1)ceased around 8000 BP.The results of the210Pb measurements presented here clearly show that this is not the case,as was already suggested by Nagy & Ofstad (1980).The absence of present-day sedimentation along the eastern flank of the Norwegian Channel,as concluded on the basisof the echo-sounding data (Fig. 8), is supported by thework ofNagy&Ofstad (1980) who studied foraminifera along two transects across the Norwegian Channel.

Several authors have reported erosion and redistributi- on of sediments on the shallow sea floor of the North Sea.

Erosion of the Flemish Banks(sout hern North Sea) was reported by Eisma&Kalf (1987).The tidal current near the Dogger Bank is not strong enough to transport sea-floor sedi ments.Sea-floor erosion and sediment transport in thisarea, however,do occur during storms (von Haugwitz

&Wong 1988),Sea-floor photographs of the Norwegian

sector of the North Sea indicate local present-daysea-flo - or erosion (Rise & Rokoengen 1984, NIOZ unpublished data). Foraminiferal studies also point to local erosion and reworking of sediments(Qvale & van Weering 1985).

Sediments removed from the sea floor of the southern North Sea are transported northwards by the residual current (Eisma & Kalf 1987). Erlenkeuser & Pederstad (1984)found fine-grained sedi ment from a southern ori- gin (southern North Sea and the Baltic Sea) in the Skagerrak.Hass(1993) concluded thatthe sedimentation pattern in the Skagerrak isinfl uenced by short-term cli- matic changes.Heshow ed thatthe average sedimentati- on rates in a core in the Skagerrak clearly increased duringthe stormy'Little Ice Age' (1550-1890 AD.)com- pared to the four centuries before the LittleIce Age and the present-day situation. An increased frequency of windsinfluenced the bottom currents and therefore ero-

64 AivoLepland&RodneyL.Stevens GU-BULL 430.1996

sion and transport of sedime nts.The sedime nt s in the Norwegian Channel are transported andsupplied by the Jutland Current, the Baltic Current , and also by the Norwegian riversand fjord s(Bj 6rklund et al.1985);thus sedimentserodedfrom theNorthSea sea floor and sedi- mentstransported out ofthe Balti c Sea may be deposite d in the Norwegian Channel. In the Norwegian Channel, wherecurrent velociti esarelowe rand waveacti vity less pronounced,the sedimentssettle outofsuspension and in the deeperpartsof the NorwegianChannelthe hydro- logical conditions favour permanent deposition of the sedi men ts.This isclearly seen in Fig. 7A, which shows that depo sit ion occurs only in the lower part s of the flanks.Another examp le isgiven inFig.7B, which shows that on local topographichighs inthe deep central part of the st udy area the sedimentary coveris much thinner than in thesurround ingdeeper parts.Localdepressio ns higher up the slope s provide enough shelter for sedi- ment s to settle permanent ly,as isclearly demonstrated in Fig . 7C Thetruncationof reflec torswit hin olde r depo- sits show n in Fig.8indicat esthat previously deposited sediment s areeroded.

Conclusions

Dow ncore210Pb activitymeasurem ent s allow the det ermi- nationof recent sediment ati onand mass accum ulat ion ratesintheNorwegian Channel. Therecent sedi ment ati- on ratesin theNorw egian Channelrangefrom 30to280 mmx100 yr' . Highest sedimentat ion ratesare found in the nort hern part of theNorw egi anChannel. Sed im ent s ente r the Norweg ianChannelfromthe souththroug h the Skagerrakand from the westfrom theNort h Sea plateau, most probably as suspend ed load. In the Norwegia n Channel, lowe r hydrodynamic activity allow s the sedi- ment sto settle out of suspensio n.Recent deposit io n of sediment s occursin thedeeper central part and in small protecte d basins higher up onthe eastern flank of the channel. In the centralnorthern Norweg ian Channelthe thicknessof the Holocene sedim entary cover isgenerally -4 m, and become sthinne r towar dsthe flanks of the basin. In the sou t hern part the Holocene deposits are thicker. The contra st between the thi n Holocene cover andthe relatively high present-daysedimentatio n rat esis explained by a chang ein the hydrologicaland depositio- nal regimein the Norwegian Chann elsom etime during the Holocene.Inthe Norw egianChannel 28x10·tons of drybulksedimentis depositedannually.

Acknowledgements

Wethankthecaptain andcrew ofthe RV.'Pelagia'fortheirassista nce and for a pleasant stay onboardduring the cruises. Jack Schilling , CerilloWillems,Ton Kuypers,TroelsLaierandYvo Kok arethanked for theirassistanceduring sampling.WimBoerassistedwith the'''Csmea- surements. Grain-size analysis was carried out by Marjan Reit h (Departmentof Earth Scien ces,Universityof Ut recht).Thecritical revie- wing by Helmar Kunzendorf and Edward King improved the manus-

cript.Birthe Bakcheckedthe Englishtext .Thisresearchis supported by theNetherlandsOrganisationfortheAdva ncem ent ofScience(NWO) undergrant 753-718-229 to Tj.e.E.v.w.This isNIOZ contribution no.

3093.

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