Bottom currents interpreted from iceberg ploughmarks revealed bymultibeam data at Tromsøflaket, Barents Sea
Valérie Bellec a,⁎, Margaret Wilson a, Reidulv Bøe a, Leif Rise a, Terje Thorsnes a,Lene Buhl-Mortensen b, Pål Buhl-Mortensen b
a Geological Survey of Norway, Leiv Eirikssons vei 3, 7491 Trondheim, Norwayb Institute of Marine Research, Bergen, Norway
Received 5 July 2007; received in revised form 26 November 2007; accepted 29 November 2007
Abstract
Multibeam data have been acquired on eastern Tromsøflaket, a bank in the southwestern Barents Sea comprising a plateau (b200 m depth)bounded by glacially eroded troughs. These new data reveal the morphology of Tromsøflaket in spectacular detail and show that much of the areahas been intensively scoured by iceberg ploughmarks. Multibeam bathymetry and backscatter data together with samples and video data have beenused to interpret the distribution of surficial seabed sediments. The general pattern of this distribution reflects the morphology. Coarse sedimentsoccur on ridges and shallow bank areas, while finer sediments occur in depressions, on the slopes of the bank and in the deeper areas. Oneexception to this pattern is observed in the Sørøydjupet trough (ca. 300m depth) where coarse sediments occur. Recent sedimentation is dominatedby mud and fine sand.
Detailed examination of the multibeam backscatter data however reveals a high variability of sediments, especially in and around icebergploughmarks. We observe coarser sediments on the levees either side of the ploughmarks than inside the furrows. Moreover we frequently noteasymmetries in the distribution of fine sediments across these ploughmark features. Drawing inspiration from previous studies of bottom currentswhich use the distribution of sediments across bedforms (e.g. sand waves) we have interpreted the direction of bottom currents on Tromsøflaket byexamining the local distribution of fine sediments across iceberg ploughmarks, which are the most prominent bedform in this area. Furtherintegration of our results with existing grain-size - current-speed models have allowed estimates of the bottom current speeds to be obtained. Suchinterpretations are particularly valuable in this area where availability of bottom current measurements sparse.
Until recently, the bathymetry and the sedimentary environ-ment of the Barents Sea have been poorly mapped except for afew areas of petroleum exploration, e.g. the Goliat area on eastern
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
Tromsøflaket (Svitzer, 2001). The MAREANO programme wasinitiated in 2005 to address this lack of knowledge. Its objectivesare to survey and perform basic studies of the seabed's physical,biological and chemical environment. The information will besystematically archived in a marine area database that will coverNorway's coastal and marine regions and especially the Lofoten–Southern Barents Sea area (Fig. 1A).
This paper focuses on an area of the continental shelf in thesouthwestern Barents Sea (eastern Tromsøflaket; Fig. 1B),where water depth ranges from less than 50m to more than500m. The Barents Sea is characterized by bank areas, e.g.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 1. (A) Location of the MAREANO area. Regional currents are indicated by arrows. NAC = Norwegian Atlantic Current, NCC = Norwegian Coastal Current.(B) Bathymetry details of the study area. Shaded relief map corresponds to the area showing in this paper.
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Tromsøflaket, and transverse glacial troughs. The shallow bankof Tromsøflaket lies at of 150–200m depth and is bounded bysteeper slopes and two glacial troughs Sørøydjupet andIngøydjupet (Figs. 1 and 2). In the south, Sørøydjupet is 280–300m deep. Ingøydjupet, in the northeast, is deeper than 400m.
The aim of the present study is to better understand themodernsedimentation processes from newly available multibeam andground-truthing data. In light of the limited bottom current datafrom the area, we have used the distribution of sediments acrossiceberg ploughmarks, present across most of the study area, toinfer the direction of bottom currents. Qualitative estimates for thespeeds of bottom currents are obtained from the distribution of theerosion and deposition areas of mud and sand.
2. Regional setting
2.1. Quaternary geology
During the Late Weischelian maximum, the Barents Sea icesheet advanced to the shelf edge west of Tromsøflaket. Thesouthern Barents Sea was (quickly) deglaciated after 15 00014Cyears BP, with two periods of calving, at 15 000 and 12 50014Cyears BP in the Bear Island Trough (Vorren and Kristoffersen,1986; Hald et al., 1990; Siegert and Dowdeswell, 2002; Ottesenet al., 2005; Andreassen et al., 2007).
The upper glacigenic sequence is dominated by muddyglacigenic diamicton or silty sandy clay with scattered gravel.
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
The glacigenic diamicton is commonly overconsolidated,with onlya very thin cover (b1 m) of sand/gravel in the bank areas, and acover (1–15 m) of clay/silt in the deepest troughs. This sedimentcover was mainly formed during the deglaciation (b14 00014CyearsBP), and only small volumes of sediment have been depositedafter the ice retreated from North of Norway ca. 10 00014C yearsBP (Vorren et al., 1989; Hald et al., 1990; Sættem, 1991).
2.2. Oceanic currents
Three major ocean currents influence the Barents Sea: TheArctic Current comes from the north-northeast and dominatesnorth of 74°N; the Norwegian Atlantic Current (NAC) travelsalong the continental slope towards the north and turnseastwards in to the Barents Sea; The Norwegian CoastalCurrent (NCC) follows the coast from the southwest and turnseastwards in the Barents Sea as the North Cape current (Mosby,1968; Ersdal, 2001; Ingvaldsen et al., 2004; Asplin et al., 2006).The direction of the NAC, following the 500m contour with amaximum speed of 1.17 m/s at this depth, is strongly influencedby the large-scale topography of the continental shelf (Gjevik,2000), although typical speeds are 0.2–0.4 m/s (Gjevik, 1996).The velocity of the NCC is variable and surface current speedsexceeding 1m/s are frequently observed (Ersdal, 2001).
Bjerke and Torsethaugen (1989) indicated that Tromsøflaketis dominated by the NCC, and that several small and largeeddies are more or less permanent. By contrast, the model
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 2. Shaded relief map of multibeam bathymetry (cell size of 10 m) showing morphological features of the study area.
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produced by Vikebø and Ådlandsvik (2005) shows that theNCC predominates south of Tromsøflaket, while the NAC isdominant within our study area. The tidal current in the studyarea has a clockwise rotation. The residual currents (sum of allthe currents) are predominantly tidal.
On the northwest slope of Tromsøflaket, within our studyarea, currents were recorded by a moored current meterpositioning at 225 m depth (Bjerke and Torsethaugen, 1989).Close to the surface, currents have an average speed of 0.1–0.3 m/s. Maximum surface speeds of 0.75–0.8 m/s have beenmeasured on the northern edge of Tromsøflaket, and 0.45–0.7 m/s speeds recorded on the northwest slope of Tromsøflaket.The residual current decreases near the bottom, and is 0.02–0.04 m/s at 200m depth (near the bottom) in the winter. Close to225 m depth, Tryggestad (1981) recorded current velocities ofup to 0.48 m/s with a mean velocity of ca 0.1m/s at about 1mabove the sea bottom. Extreme values recorded by Bjerke andTorsethaugen (1989) at 225 m depth (1m above the bottom) arelower. They show 0.29 m/s over 1 year but could be more than0.4 m/s on a 100 years scale (extrapolated value using a three-parameter Weibull distribution). The tidal current shifts from
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
SW–NE at 25 m depth to E–W at 225 m depth with speedsvarying between 0.05 and 0.15 m/s.
The residual current at Tromsøflaket is much weaker than inthe near shore coastal current further south. Tidal currentstrength is at a maximum close to Sørøya (up to 0.5 m/s) anddecreases towards the northwest (NORSOK N-003, 1999).
Current measurements and models provide quite compre-hensive information of the currents close to the surface.Information close to the bottom is more limited. A few mea-surements have been taken with moored current meters butthese readings are very localised. To interpret the sedimentdistribution, we need detailed bottom current information in thewhole study area.
3. Materials and methods
3.1. Multibeam mapping
The study area (Fig. 1B) was mapped using a KongsbergSimrad EM1002 (95kHz) multibeam echosounder during threecruises (2005–2006). The multibeam provides two datasets:
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
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bathymetry which reveals the morphology of the seabed; andbackscatter which provides an indication of the nature of the seafloor. Since the acoustic reflectance (backscatter) is influencedmainly by sediment texture, it gives information on the nature ofthe seafloor, especially on the hardness and roughness. Hard/rough bottoms (rocks, coarse or compacted sediments) showhigher intensity values than soft/flat bottom (mud, non-compacted sediments). The datasets were processed to produceraster grids with cell sizes of 5 m and 10m, allowing detailedstudy of seabed features.
Fig. 3. Pictures from video lines. Video lines (A t
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
The bathymetry data were processed using KongsbergSimrad Neptune (data correction and cleaning) by Forsvaretsforskningsinstitutt (FFI). The backscatter data were pro-cessed using Kongsberg Simrad Poseidon (correction andmosaicing of the data) (Kongsberg Simrad, 1999-2001a,b).Backscatter data were further processed using a directionalcosine filter in Geosoft (Geosoft, 2005). This technique iseffective for reducing linear artefacts (nadir noise) associatedwith backscatter data, when line spacing and direction arefairly uniform.
o J) and sample location are shown in Fig. 4.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
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3.2. Video lines and sampling
Samples and seabed video were acquired during a cruise withRV Håkon Mosby in May-June 2006, both to calibrate themultibeam backscatter and for biological, geological, andgeochemical analysis. A total of 70 video transects (each 1 kmlong), 20 Van Veen grabs, 10 multicores and 16 boxcores werecollected. The video surveys were performed using the Instituteof Marine Research's towed video platform. The visualobservation platform, called CAMPOD, is a metal-framedtripod equipped with low light CCD and high definition (HD)video cameras. The lights (2 x 400W HMI) are mounted on thesides close to the two front legs about 1m above the “feet”. TheHD camera hasmanual zoom and focus and is mounted on a pan-and-tilt device. The CAMPOD was used both “parked” on theseabed for detail studies and in a drifting mode (drifting with theship along a predefined survey line). The height above theseabed is maintained constant manually by awinch operator whohas visual contact with the monitor. Geopositioning of the videolines was obtained via a transponder, and is accurate to ca. 2% ofthe water depth. Sediment grain size fractions were classifiedaccording toWentworth (1922) and Folk andWard (1957) scale.
Fig. 4. Multibeam backscatter map (cell size of 5 m) showing high values on Tromsølocation of the pictures of Fig. 3.
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
4. Results
4.1. Ground-truthing of sea-floor sediments
Video-transects and samples showed that there is consider-able variability in sediment types across the study area (seesupplementary data, Figs. 3 and 4). Variation occurs on severalspatial scales. On a local scale (decimetres to metres) we seefrom video that patches of mud, sand and gravel occur side byside. Nevertheless, a general broad scale (N1 km) pattern can berecognised: On Tromsøflaket, coarse sediments occur onelevated areas and finer sediments occur in depressions, pitsand iceberg ploughmarks. On the slopes of Tromsøflaket, thesediments are better sorted and finer-grained than on the banks.Towards the base of the slopes, the sediment is generally muddybut some patches of sand, gravel and boulders can be found. Indepressions, sediments are generally fine-grained (mud to finesand) and only a little coarse sediment (coarse sand, gravel) isfound. In Sørøydjupet trough, the top layer shows the presenceof gravel and stones. In Ingøydjupet trough, the sedimentconsists of silt and clay becoming more clayey towards thenortheast.
flaket and in Sørøydjupet, low values in other deep areas. Letters (A to J) show
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 5. Interpreted sediment map based on backscatter and ground-truthing data.
Table 1Summary of the trend of the main sediment classes
Class Sediments Commondepth
Areas
1 Mud b230 m Outside the bank, depressions2 Sandy mud,
some gravel190 m260≪210 m220≪400 m
Bank: depressionsSlope: west slope of bank, depressionsOutside the bank
3 Sand 300≪180 m Slopes of bank and depressions4 Gravelly sand 280≪160 m Bank, Sørøydjupet5 Sandy/muddy
gravelN190 m250≪200 mb200 m
BankNW slope of bankSørøydjupet
6 Gravel, stone,boulder
N150 m230≪210 mb290 m
Top of the bankNW slope of bankSørøydjupet
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4.2. Backscatter
OnTromsøflaket, backscatter intensity shows that, in general,high to very high values occur in three shallow areas interpretedas moraines (Vorren and Kristoffersen, 1986). Lower valuesoccur in depressions, pits and iceberg ploughmarks. Thenorthwest slope of Tromsøflaket has very high values, as doesthe Sørøydjupet trough. This initially seems to contradict thesedimentation pattern interpreted from the areas at the samedepth, about 300m depth, which, outside the bank, havegenerally medium to low values. Detailed examination howevershows that backscatter values display a very high variabilitylinked to small seabed reliefs, for example iceberg ploughmarksor pits.
Iceberg ploughmarks exhibit low values in scoured troughs(1–2 m deep), high to very high values on adjacent levees (1mhigh) and variable values in areas between them. Pits andpockmarks exhibit low values, but pockmarks may show highervalues in their central parts. The dimensions of pits are similarto those of pockmarks occurring in the Ingøydjupet trough. Inthis area pockmarks occur in clusters in muddy sediments. Bycontrast, pits do not occur in clusters and can be found in eithermuddy, sandy or gravelly sediments. They are often at thetails of iceberg ploughmarks. Several authors (e.g. Bass and
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
Woodworth-Lynas, 1988; Eden and Eyles, 2001) have indicatedthat these kinds of pits are likely formed by turning of icebergs.
5. Sediment map
The backscatter from a 95kHz multibeam echosounder such asEM1002 gives an indication of the nature of the surficialsediments down to 20–30 cm depth below the sediment surface.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
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The signal transmitted by the echosounder is more or lessreflected according to the nature of the seabed. For a muddyseabed, the signal is strongly absorbed and the values of thebackscatter are low. For a rocky or a gravelly seabed, the signal isstrongly reflected and the values of the backscatter are high. Otherfactors contributing to the value of the backscatter include sorting,seabed roughness, sediment compaction and seafloor fauna (e.g.Nitsche et al., 2004; Ferrini and Flood, 2006, Jackson et al., 1986;Lurton, 2003; Gardner et al., 2003). Samples and video-stationson Tromsøflaket show a very patchy sediment distribution withsand, mud and gravel side by side with a variation over shorterdistances than the resolution of the multibeam data (5 or 10mgrids). The sediment classification presented here has beenprepared to represent the seabed sediment distribution at a scaleof 1:100 000.
Based on the backscatter intensity, we have interpreted sixsediment classes (Fig. 5): mud (lowest values), sandy mud with
Fig. 6. Detailed views of backscatter data. (A) Iceberg ploughmarks filled by sandy mreliefs in area of mud in Ingøydjupet; (C) High variability of sediments; D) Interpre
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
gravel, sand, gravelly sand, sandy or muddy gravel and gravel–stones–boulders (highest values).
The distribution of sediment classes shows a strong link withthe morphology (Table 1). The main component of Tromsø-flaket, on the plateau, is gravelly sand. In shallower water, thesediments become coarse and more gravelly. Depressionsbetween ridges are progressively filled by sand, silt and mud.
On the northwest slope, the sediment can be very coarse (gravel,boulders) but generally becomes more finer-grained downslope.On the southeast slope, the sediment is finer than on the northwestslope and shows sandy mud deposition on flatter areas. InSørøydjupet, the sediment is very coarse. In the north andnorthwest deeper areas, sandymud class dominates with pockets ofmud inside depressions. In Ingøydjupet, mud is the dominant class.
Looking at the seabed in detail, we see a high variability ofsediment types. This variability is mainly due to small varia-tions in morphology (Fig. 6). Shallower than 190m depth, fine
ud in area of coarse sediment on the top of the bank; (B) Sand on NE-slopes ofted sediment map showing the position of the detailed pictures.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
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sediments (mud, silt) are only found in small depressions andiceberg ploughmarks (Fig. 6A). Patches of sand exist to-wards Ingøydjupet on the northern slopes of positive reliefs(Fig. 6B). Patches of sandy gravel and gravelly sand occur below250m depth on positive reliefs and iceberg ploughmark levees(Fig. 6C).
6. Direction of the bottom currents interpreted frombackscatter in iceberg ploughmarks
During the last deglaciation of the shelf and the coastal areas,the keels of icebergs ploughed the seabed, forming narrow andelongated depressions (Lien, 1983, 1986; Andreassen et al.,2007). Eroded sediments accumulated on the side of depres-sions, forming lateral levees or rims. Backscatter data show thatthe sediments of the levees are generally coarser-grained thanthe surrounding sediments. This has also been described fromside-scan sonar records and seismic profiles by other authorse.g. Belderson et al. (1973), Lien (1983, 1986), Harris andO'Brien (1998), Kuijpers et al. (2007) who indicated coarse-grained sediment levees of about 1 m height and furrows about2 m deep, filled with soft sediment (sand and mud).
Recent sedimentation is dominated by mud except on the topof Tromsøflaket where fine sand is found. Coarse or morecompacted sediments are often observable on ridges and hills.When the height of the hills, ridges or levees diminishes, muddysediments cover them. The finest sediments are commonlyfound on one side of the elongated depression formed by theiceberg ploughmarks. This local sedimentation indicates controlby bottom currents. Such deposition is only possible in negative
Fig. 7. Backscatter image showing sediment distribution in iceberg ploughm
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
features on the top of the bank. In the large depressions onTromsøflaket, between the three large moraines, and in thedeeper troughs (N200 m depth), muddy sediments may totallycover ploughmarks. Here too we observe a preferentialdeposition of the finest sediment on one side of the depressions.
Tidal currents typically reach 5–15 cm/s and currents canreach 30 cm/s (Bjerke and Torsethaugen, 1989). Such currents areable to erode non-consolidatedmud, silt and fine sand (Hjulstrom,1935). These fine-grained sediments need very weak currents tobe eroded if they are not compacted. Since we observe a muddycloudwhen the CAMPODvideo platform touches the seafloorweknow that there are fine, uncompacted sediments in this area. Anysuch eroded fine-grained sediment can be transported anddeposited on areas of lower currents (lee areas).
It is well known that on bedforms like sandwaves, which areoften used to determine current direction, fine sediments depositon the lee side of ripples due to decreasing current strength.Erosion may occur on the opposite exposed side (Allen, 1968,1982; Reineck and Singh, 1980; Kuijpers et al., 1993;Cunningham et al., 2005). If the direction of the currentchanges, the fine-grained sediment which settled on the formerlee side will be eroded again. It will be transported anddeposited on the new lee side. We should therefore consider thatthe finest non-consolidated sedimentation (mainly mud andclay) is the most recently deposited sediment. Since depositionof fine sediments on one side of iceberg ploughmarks shouldfollow this pattern, this gives us a possibility to estimate bottomcurrent directions by analysing the local distribution ofsediments across such topographic features (Fig. 7). Wheninterpreted across several topographic features in a particular
arks and schematic model for interpretation of bottom current direction.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 8. (A) Generalized (black arrows) and detailed (white arrows) directions of bottom current from iceberg ploughmarks (the interpreted velocity is presentedFig. 10), (B) Local gyre on Tromsøflaket, (C) Detailed map showing local influence of slope.
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area we gain a regional impression of the circulation of bottomcurrents. This approach complements and improves uponcurrent meter data which give bottom currents several tens ofcentimetres or more over the seabed and only record oneposition or one track.
The current direction map shown Fig. 8 is interpreted fromexamination of the sediment distribution in the vicinity oficeberg ploughmarks. It shows a general trend in the direction ofbottom currents from northwest to southeast on the bank, withminor deviations around the shallowest reliefs. In Sørøydjupet,the interpreted current direction is towards the northeast. In thenortheast and in Ingøydjupet, currents are from the north. Thecurrents seem to roughly follow the slope of Ingøydjupet like acontour current. A possible gyre appears to be present inside alarge depression on Tromsøflaket (Fig. 8B). Down-slopesediment transport occurs in areas of muddy sedimentation(Fig. 8C).
The interpreted directions of the bottom currents fit well withthe general direction of the NAC and the NCC surface currentspredicted by models (Vikebø and Ådlandsvik, 2005). Theseauthors used a current model (ROMS) off northern Norway(Fig. 9) to calculate currents at 50m depth (i.e. not bottomcurrents). Despite the differences in the depths considered, a
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
good match has been found between this model and our bottomcurrents interpreted from the fill of iceberg ploughmarks. Thecurrents have almost the same direction (from the northwest) inthe northern and western parts of the study area as well as inSørøydjupet. This suggests that the general direction of NACand NCC does not change much with depth (below 50m; Fig. 8).The dominant bottom current from the interpretation comes fromthe northwest, turning more southwards when it reaches theshallower sea bottom at the north flank of Tromsøflaket. Thesedirections correspond to the directions of the NAC. InSørøydjupet, the northeasterly current direction is similar tothe direction of the NCC.
Nevertheless, some differences appear eastwards: in themodel, the contour current on Ingøydjupet is located morenorthwards and the surface current direction outside the gyre isless easterly than the bottom current directions what implies anEkman transport. Moreover, the local gyre on the top of thebank is not present in the model.
In fact, the match is good at depths shallower than 200m butpoorer for deeper areas. Many of the observed differences couldbe attributable to differences in the depths as Vikebø andÅdlandsvik (2005) used 50m depth currents and we interpretednear-bottom-currents. Also we would expect variation in the
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 9. Comparison between currents modelled byVikebø and Ådlandsvik (2005) at 50m depth (A) and direction of bottom currents from the iceberg ploughmarks fill (B).
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circulation due to the fact that the NAC is a topographicallycontrolled flow (Gjevik, 2000).
7. Evaluation of bottom current speed
Since Hjulstrom (1935), many authors (e.g. Shields, 1936;Vanoni, 1977; Gao and Collins, 1992; Li and Amos, 2001;Yang, 2006) have studied the relationship between mean grain-size and current speed. A lot of transport equations existincluding many different kind of parameters (Yang, 2006). Weuse the diagram of Hjulström giving by Nichols (1999). This
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
kind of diagram gives clear indication of the relationshipbetween the mean grain-size and the flow speed needed toeroded or transport the particles.
Our sediment classes ranges from mud to boulders. Muddeposition indicates very low erosion. According to theHjulström diagram, the speed should be around or lower than5 cm/s. When the speed is higher than 5 cm/s, muddy particlesbegin being eroded. This corresponds to the sandy mud classeswith mud deposit inside the depression. With the increasing ofthe current, sandy mud deposit occurs less and less and only indepressions. Fine sand replaces sandy mud into the depression,
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 10. Interpreted current speed (grey scale) and direction (black arrows) of bottom currents.
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when the bottom current speed reaches about 20 cm/s in thesurrounding areas. Gravel is eroded when the bottom current ishigher than 50 cm/s. In this case, there is no deposition of finesediment (mud and to fine sand) even inside pits or icebergploughmarks.
The NCC reaches more than 40 cm/s in Sørøydjupet andoccasionally more than 100 cm/s (Poulain, 1995; Ersdal, 2001).Typical speeds of the NAC are 20–40 cm/s (Gjevik, 1996) witha residual current speed in the order of 2–4 cm/s close to thebottom and a maximum current speed of 30 cm/s at 1 m abovethe sea bottom (on Tromsøflaket at 225 m depth). At 100–150mdepth, extreme current speeds can reach 50 cm/s (Bjerke andTorsethaugen, 1989). Since the current is tidally influenced,temporary deposition of sandy mud on Tromsøflaket is likely tobe possible during periods of slack waters or neap tides, but alsoduring periods of very low wind speeds. This is followed byerosive phases when tidal currents or wind speed increase.
Fig. 10 summarises the current speed and direction obtainedfrom the interpretation of multibeam data. From our interpreta-tion, the (maximum) interpreted speeds of the near-bottomcurrent (b10 cm above the sea bottom) are about 50 cm/s on thebank, between 150 and 200 m depth, and 20 cm/s in areasdeeper than − 220m, except in Sørøydjupet, where the inter-preted speed of the bottom currents can be higher than 50 cm/s.
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
This shows a good match with the current velocities obtained byother methods.
8. Recent deposition and erosion
Modern sediment deposition is restricted to mud and sand.Modern deposits appear to be very thin, and reliefs of 1–2 mheight (e.g. levees) are rarely covered by fine-grained sediments,except in the deepest areas. Mud and sand deposition occursright across the study area, but deposits are mainly confined todepressions (a few metres to several kilometres wide) andiceberg ploughmarks, especially on the top of Tromsøflaket.
Fine-grained sediments are not deposited above ca. 150mdepth, indicating stronger currents in those areas (N20 cm/s;Figs. 10 and 11). Towards the deepest areas, more fine-grainedsediments are deposited when currents decrease, especially onsoutheast slopes where the current comes from the northwest.
On the north-northwest slopes of Tromsøflaket, sand isrestricted to the lower part of the slope. Moreover, very coarse-grained deposits are present. On the east-southeast slopes, sandand sandy mud occur over large areas (Fig. 5). These slopesprobably correspond to the “lee” slopes of the bank, confirmingthe presence of a current originating from the north–northwest(Figs. 8 and 11).
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
Fig. 11. Interpreted origin and impact of the bottom currents.
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The deeper areas exhibit three sedimentation patterns(Fig. 11). In the western areas, deeper than 220 m, the sedimentdistribution shows high variability (Fig. 6C), strongly linked tothe presence of iceberg ploughmarks. At positive reliefs of sometens of centimetres height, there is erosion or non-deposition.Where small depressions several tens-of-centimetres in heightoccur, fine-grained particles are deposited. The velocity of thecurrents probably varies around the erosion/deposition bound-ary for silt and mud (around 10 cm/s; Hjulstrom, 1935).
Although below 300 m water depth, Sørøydjupet is coveredby gravel, stones and boulders with only minor sand. Thisindicates strong currents (N20 cm/s) which prevent deposition offine-grained particles as shown in previous studies (Poulain,1995; Ersdal, 2001). Surface current velocities higher than 1 m/sare common. Halfway between Sørøydjupet and the top ofTromsøflaket, the mean grain-size decreases, indicating a re-duction of current speed.
Previous studies (Svitzer, 2001) have indicated that betweenTromsøflaket and Ingøydjupet, below 310m, the seabed consistsof glacial clay covered by a thin veneer of very soft to soft claywith occasional lithic clasts and shell fragments. This veneer isless than 1 m but may locally reach a thickness of few metres.The veneer thickens towards the northeast and becomes morethan 10 m thick in Ingøydjupet. This is in accordance with our
Please cite this article as: Bellec, V., et al., Bottom currents interpreted from icebergGeol. (2008), doi:10.1016/j.margeo.2007.11.009
sediment map (Fig. 5) which shows that this area is covered bymud with some sand and gravel, sometimes boulders. Homo-geneous mud can also be found in a few areas between the top ofTromsøflaket and Ingøydjupet (Fig. 6).
The sediment map also shows the presence of two sandyareas on the northeast side of positive reliefs close toIngøydjupet. This could be due to currents from the north–northeast, which erode clay and silt, leaving coarser silt andsand. Currents follow the slopes of Ingøydjupet, eroding thefinest sediment (probably the soft clay) and leave a strip ofcoarser muddy areas surrounded by more homogeneous mudup-slope and down-slope (Fig. 6B).
9. Conclusions
Backscatter data used together with bathymetry data frommultibeam surveys, and ground-truthing with video andsampling, have enabled detailed mapping of the sedimentdistribution on eastern Tromsøflaket. The direction of bottomcurrents has been interpreted from the distribution of recentsediments in iceberg ploughmarks. Fine sediments generallydeposit on the lee side of the furrow. An indication of thestrength of the bottom currents was obtained from thedistribution of muds and sands.
ploughmarks revealed by multibeam data at Tromsøflaket, Barents Sea, Mar.
13V. Bellec et al. / Marine Geology xx (2008) xxx–xxx
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According to our model, the Norwegian Atlantic Currentcomes from the northwest across the bank but also from the northacross Ingøydjupet. Its speed near the seafloor ranges from verylow (less than 5 cm/s) in Ingøydjupet to high (more than 50 cm/s)on the shallower parts of Tromsøflaket. The Norwegian CoastalCurrent comes from the southwest in Sørøydjupet with speedshigher than 50 cm/s. The resulting current directions are rathersimilar to the direction of the currents at 50 m depth obtained inother studies. The few differences that have been observed areattributed to differences in water-depth.
We observe a strong relationship between morphology andsediment distribution. Modern sediments are represented by athin cover of mud to fine sand. Coarse sediments onTromsøflaket and in Sørøydjupet represent likely morainescovered by lag deposit. Gravelly sand and lags are widelydistributed on Tromsøflaket and in deeper areas of bathymetrywhere the NAC is sufficiently strong to prevent deposition orerode the finer fractions.
Sand and sandy mud have settled in depressions onTromsøflaket, and also on its southeastern slope. Sandy mudand mud dominate in deeper areas, indicating a decrease ofcurrent strength with depth. The strength of the current variesaround the erosion / deposition transport boundary of the mud/silt fraction. Many small, positive reliefs (1–2 m high) showcoarser sediment without deposition of fine sediment.
Detailed mapping of sediment distributions across icebergploughmarks provide strong indicators for modern bottom currentdirections. Such techniques could be readily applied to similarareas of iceberg ploughmarks, or topographic features withmultiple orientations, where amode ofmodern transport is present.
Acknowledgements
This work has been done in the framework of theMAREANO program, and provides part of the scientificdocumentation of the marine area database, providing decisionsupport for the holistic, ecosystem based management of theLofoten–Barents Sea.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.margeo.2007.11.009.
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