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Quaternary Science Reviews 49 (2012) 64e81

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Quaternary Science Reviews

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Recent glacially influenced sedimentary processes on the East Greenlandcontinental slope and deep Greenland Basin

Marga García a,*, Julian A. Dowdeswell a, Gemma Ercilla b, Martin Jakobsson c

a Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UKb Instituto de Ciencias del Mar, CSIC, Grupo de Márgenes Continentales, Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, SpaincDepartment of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 10 January 2012Received in revised form19 June 2012Accepted 27 June 2012Available online xxx

Keywords:Greenland BasinGlacially influenced sedimentary processesDebris flowsTurbidite systems

* Corresponding author. Instituto Andaluz dCSIC-Universidad de Granada, Avda. de las PalmerasSpain. Tel.: þ34 958 230000x190139; fax: þ34 958 5

E-mail addresses: marguita.garcia@gmail.com, m.g

0277-3791/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.quascirev.2012.06.016

a b s t r a c t

This paper presents the morpho-sedimentary characterization and interpretations of the assemblage oflandforms of the East Greenland continental slope and Greenland Basin, based on swath bathymetry andsub-bottom TOPAS profiles. The interpretation of landforms reveals the glacial influence on recentsedimentary processes shaping the seafloor, including mass-wasting and turbidite flows. The timing oflandform development points to a predominantly glacial origin of the sediment supplied to thecontinental margin, supporting the scenario of a Greenland Ice Sheet extending across the continentalshelf, or even to the shelf-edge, during the Last Glacial Maximum (LGM). Major sedimentary processesalong the central section of the eastern Greenland Continental Slope, the Norske margin, suggesta relatively high glacial sediment input during the LGM that, probably triggered by tectonic activity, ledto the development of scarps and channels on the slope and debris flows on the continental rise. Themore southerly Kejser Franz Josef margin has small-scale mass-wasting deposits and an extensiveturbidite system that developed in relation to both channelised and unconfined turbidity flows whichtransferred sediments into the deep Greenland Basin.

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1. Introduction

The large-scale morphology and landforms of high-latitudecontinental margins and basins are in most cases products of themajor recent sedimentary processes that have shaped the present-day seafloor. They have therefore been used widely for recon-structing sedimentary environments during the last glacial cycle.Reconstructions of the eastern Greenland Ice Sheet have beenbased on studies of the continental margin off Scoresby Sund andKejser Franz Joseph Fjord (68�N to 74�N) (e.g. Dowdeswell et al.,1994a,b; Evans et al., 2002; Ó Cofaigh et al., 2002; Wilken andMienert, 2006); two different scenarios have been proposed. Inthe first, glacier fluctuations were relatively limited during the lastglacial cycle with the ice sheet inferred to have reached only theouter fjords or the inner shelf during full-glaciation (Funder, 1989;Funder and Hansen, 1996; Elverhøi et al., 1998; Funder et al., 1998,2011). A second scenario suggested that the outer shelf, or even theshelf-edge, was the maximum extent for the ice sheet during the

e Ciencias de la Tierra,n�4, 18100 Armilla, Granada,52620.arcia@csic.es (M. García).

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last glaciation (Evans et al., 2002; Ó Cofaigh et al., 2002, 2004;Hakånsson et al., 2007). Recent studies further north, between76�N and 81.5�N, also support an ice sheet grounding as far as theshelf edge during the Last Glacial Maximum (Evans et al., 2009;Winkelmann et al., 2010). The understanding of past variations inthe dynamics of the Greenland Ice Sheet, and especially thebehaviour of fast-flowing ice streams, can provide importantconstraints on the likely dynamics of modern Greenland icestreams. These ice streams are, in turn, an important control on therate of global sea-level rise today (Gregory et al., 2004; Rignot andKanagaratnam, 2006).

High-latitude continental shelves, unless reworked by theploughing action of iceberg keels (e.g. Woodworth-Lynas et al.,1985; Dowdeswell et al., 2002; Dowdeswell and Bamber, 2007),often preserve assemblages of glacially produced landforms thatallow the reconstruction of the form and flow of Late Quaternaryice sheets (e.g. Canals et al., 2000; Ottesen et al., 2005; Evans et al.,2006; Mosola and Anderson, 2006). By contrast, continental slopesand deep basins in the polar seas are complex sedimentary settingsrecording the interplay of glacial, glacimarine, marine and mass-wasting processes (e.g. Laberg and Vorren, 1995; King et al., 1996;Hesse et al., 1999; Dowdeswell et al., 2004; Li et al., 2011). Thispaper presents a detailed description and interpretation of the

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landforms that characterize a sector of the East Greenland conti-nental slope and the Greenland Basin.We analyze landforms on thecontinental slope between 74�N and 76�N, and provide a newinterpretation of landforms in the deep Greenland Basin (Fig. 1).The geological significance and timing of the recent sedimentaryprocesses are interpreted in terms of their relation with the lastglacialeinterglacial cycle, in order to assess previously proposedhypotheses from adjacent margins regarding the extent of the EastGreenland Ice Sheet during the Last Glacial Maximum.

2. Methods and datasets

This work is based on mapping carried out during cruise JRC-51of RRS James Clark Ross (Fig. 1A). Swath bathymetry and associatedbackscatter data were obtained with a hull-mounted Kongsberg-Simrad EM120 multibeam system, operating with a nominalfrequency of 12 kHz and 191 beams, with a 1��1� beam configu-ration. This provided a resolution of approximately 50 m in 3000 mwater depth (the maximum depth in the study area being 3700 m).Navigation data were obtained from DGPS and the data processing

Fig. 1. A) Location of the study area on the regional IBCAO bathymetric map (Jakobsson et al.and cores obtained during the JRC-51 cruise that have been interpreted in this work. Blue dotTrough; S.K. I.: Store Koldeway Island; Hochst: Hochstetterbugten; Dove B.: Dove Bugt; KFJContinental shelf; CS: Continental slope; CR: Continental Rise; B: Basin. Profiles are locatedreferred to the web version of this article.)

and production of maps were performed with Caris and Fle-dermaus software.

Bathymetric and backscatter mosaics for the Greenland Basinare illustrated in Fig. 2. Fig. 2A shows the topography of the seafloorfrom the upper slope to the deep basin. The data cover depthsranging from 147 to 3730 m. Minimum depths occur at the VesterisSeamount, and maximum values are in the deep basin in theeasternmost part of the study area.

Backscatter imagery was produced using FM Geocoder (Fonsecaand Calder, 2005) (Fig. 2B). Values of mean backscatter rangefrom �20 to �55 dB. Backscatter represents the strength of thereturning sound signal transmitted from the multibeam system,and is dependent on the grain size, insonification-orientation andsea-surface conditions during the survey (e.g. Keeton and Searle,1996; Fonseca and Mayer, 2007; Kagesten, 2008; De Falco et al.,2010), and also on seafloor morphology and near-surface stratig-raphy. In general, coarser sediments will give a stronger reflectedsignal, with surface shape and roughness important in controllingbackscatter for coarse sediments (Kagesten, 2008). In theGreenland Basin, the mean backscatter pattern allows the

, 2008), showing the location of the sub-bottom TOPAS profiles, swath bathymetry datated lines mark the trend of former glacial troughs. F.Z.: Fracture zone; Norske T.: NorskeF.: Kejser Franz Josef Fjord. B) Profiles along the East Greenland margin. Legend: Csh:in A. (For interpretation of the references to colour in this figure legend, the reader is

Fig. 2. A) Swath bathymetric map of the study area, showing the Norske margin in thenorthern area, off the Norske Trough; the KFJ margin, off the Kejser Franz Joseph Fjord(KFJ fjord); and the deep basin, where volcanic edifices are the most prominentseafloor features. B) Backscatter map of the Greenland Basin, reflecting the charac-teristics of the shallow sediment. Backscatter values are one of the criteria used todefine acoustic facies.

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recognition of sedimentary features and has been used as a keyparameter for the definition of acoustic facies.

The TOPAS PS 018 system used to obtain sub-bottom profilesoperates with a primary frequency of 15 kHz and a secondary

frequency in the range of 0.5e5 kHz. It is capable of providinga vertical resolution better than 1 m. Processing of the data wascarried out with TOPAS software. Side-scan sonar imagery wasobtained with the Towed Ocean Bottom Instrument (TOBI) system(Flewellen et al., 1993). TOBI included a 30 kHz deep-tow side-scansonar with swath bathymetry capability, an 8 kHz chirp profilersonar, gyrocompass and pitch and roll sensor.

Acoustic facies have been defined based on the acousticamplitude, lateral continuity, geometry and internal configurationof reflections on sub-bottom TOPAS profiles (Damuth,1980), and onmean backscatter values (Fig. 2B). The interpretation of acousticfacies is based on their seismic characteristics and distribution, andfollows the work of several authors (e.g. Damuth, 1980; Hong andChen, 2000; Zaragosi et al., 2000; Droz et al., 2001; Loncke et al.,2002; Kottke et al., 2003).

Published regional datasets have been used to complete ourinterpretations. These include the IBCAO Version 2.0 bathymetricchart for offshore East Greenland (Jakobsson et al., 2008), a GLORIA6-kHz side-scan sonar mosaic (Mienert et al., 1993; Dowdeswellet al., 1996; Wilken and Mienert, 2006) and sedimentologicalinformation from cores retrieved from the Greenland Basin margin(Ó Cofaigh et al., 2002, 2004; Wilken and Mienert, 2006).

3. Study area: geological background and glacial history

The initial breakup of the Norwegian-Greenland Sea started inEarly Eocene time, through the activity of the Mid-Atlantic Ridge.Geodynamic evolution since then has included stages ofcompression, extension, magmatism, uplift and subsidence(Johnson and Heezen, 1967; Price et al., 1997; Lundin and Doré,2002; Voss and Jokat, 2009). The onset of glaciations in thenorthern hemisphere has been documented from the occurrence ofice-rafted debris (IRD) in deep-sea sediments (Eldrett et al., 2007;Tripati et al., 2008; Jakobsson et al., 2010). The Greenland IceSheet (GIS) dates from Eocene/early Oligocene and its glacialhistory includes a succession of at least four cooling pulses duringthe late Miocene, and a major intensification of glaciation duringthe Plio-Pleistocene. Glacial sediments were deposited on the outershelf and continental slope of the Greenland Basin during the earlyPliocene and late Pliocene, and a younger depositional eventoccurred during the Quaternary, probably corresponding to the lastmajor advance of the GIS during the Saalian (Vanneste et al., 1995;Solheim et al., 1998; Hakånsson et al., 2008). Large-scale troughmouth fans have not developed on the East Greenland margin dueto a combination of factors, including sediment characteristics anddelivery rate, meltwater volume and the size of the ice-sheetdrainage basins (Ó Cofaigh et al., 2004).

During the Late Weichselian, full-glacial ice-sheet advance inEast Greenland was until recently thought to have been restrictedmainly to fjord and inner-shelf locations (Funder, 1989; Funder andHansen, 1996; Funder et al., 1998, 2011). New swath-bathymetricobservations, however, have shown the presence of subglaciallyproduced mega-scale glacial lineations and moraine ridges on theouter shelf, providing clear evidence of ice advance across the shelf(Evans et al., 2009; Dowdeswell et al., 2010; Winkelmann et al.,2010). These findings provide direct support for earlier indirectinferences of an extensive Late Weichselian ice sheet, reaching atleast the outer continental shelf (Fleming and Lambeck, 2004; ÓCofaigh et al., 2004; Wilken and Mienert, 2006; Hakånsson et al.,2007). Deglaciation of the East Greenland shelf commenced about17,000e18,000 years ago (Evans et al., 2002; Ó Cofaigh et al., inpress), and hemipelagic sedimentation has been predominant onthe margin subsequent to ice retreat (Ó Cofaigh et al., 2002, 2004;Wilken andMienert, 2006). The timing of the deglaciation has beenestablished by radiocarbon dating of sediment cores from the

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Greenland Basin, where the transition from glacially influencedturbidites and debris-flow deposits to the most recent hemipelagicunit has been dated at about 13,000 years B.P. (Ó Cofaigh et al.,2004).

4. Physiography

The study area is delimited by the Jan Mayen Fracture Zone tothe south-west and the East Greenland Ridge to the north-east. Itcovers the upper slope, from 1200mwater depth, to the deep basinat depths of more than 3700 m (Figs. 1 and 2). The physiographychanges along the margin, from a narrow, less than 200 km-widemargin with a steep and narrow continental slope off Kejser FranzJoseph Fjord in the southern part of the study area (hereafter KFJmargin), to a much wider margin, up to 450 km wide, with a well-developed continental rise in the northern part of the study area offthe Norske Trough (hereafter Norske margin).

The East Greenland continental shelf widens from minimumvalues of 120 km at the KFJ margin to amaximumwidth of 300 km attheNorskemargin,where it connectswith the Boreas Abyssal Plain totheeast (Figs.1 and2). Thecontinental shelf is cut by four troughswitha general NWeSE trend and widths of tens of kilometres. Glacialtroughs in the study area are, from south to north, the basinwardcontinuation of KFJ Fjord, the main trough formed by Hoch-stetterbugten and Dove Bugt, a trough off Store Koldewey, and theNorskeTrough (Fig.1). The shelf reachesdepths of 400m, except at themouths of glacial troughswhere depths increase tomore than 500m.

The continental slope extends to 2500e3200 m in water depth.On the KFJ margin the continental slope is relatively narrow(20e50 km wide), with maximum gradients of 5� (Fig. 1B). On theNorske margin the slope is up to 70 km wide, with gradients of1.5e3.5� reachingmaximumvalues of 9� on severalmargin-parallelscarps (Fig. 1B). In this area, a 60 km wide continental rise reachesdepths of 3500 m, with gentle gradients typically less than 2�. Thebasin extends to maximum depths of 3700 m. Characteristicfeatures in the basin are volcanic edifices, such as the solitaryVesteris Seamount which is 2850 m high (Haase and Devey, 1994).Our new bathymetric data reveals the existence of numerous NE-SW-oriented elongated edifices with maximum heights of 500 mabove the surrounding seafloor (Fig. 2A).

5. Acoustic facies

5.1. Acoustic facies description

Four major acoustic facies have been differentiated in theGreenland Basin. A non-penetrative facies (I) includes threesubtypes, showing no penetration of the acoustic signal. Regular,wavy and irregular seafloor reflections characterize subtypes IA, IBand IC, respectively. A transparent acoustic facies (II) has twosubtypes. Subtype IIA is represented by lenticular or mound-shaped bodies, up to 30 ms thick, whereas subtype IIB is charac-terized by thin lens-shaped bodies, less than 10 ms thick. A strati-fied acoustic facies (III) represents parallel-stratified reflectors andhas four subtypes; thin packages of parallel reflectors overlyingtransparent deposits (IIIA), continuous horizontal reflectors (IIIB),continuous undulating reflectors (IIIC), and deformed reflectors(IIID). A chaotic acoustic facies (IV) shows a wavy surface andchaotic internal character with no well-defined reflectors. The fouracoustic facies and their characteristics are illustrated in Table 1.

5.2. Acoustic facies distribution

The distribution of acoustic facies in the Greenland Basin isshown in Fig. 3.

The continental slope and rise are characterized mainly by twofacies: a transparent facies containing thin lenticular bodies (IIB)and a non-penetrative, wavy seismic facies (IB). In addition, a non-penetrative, regular acoustic facies (IA) is identified locally onsteeper areas.

The floor of the Greenland Basin is generally represented bystratified acoustic facies (Fig. 3). An undulating facies (IIIC) occursmainly in the shallowest, south-western part of the study area,where a locally chaotic acoustic facies (IV) is also identified. Thedeep basin generally presents stratified acoustic facies with paralleland horizontal (IIIB) or deformed reflections (IIID). Lenticular ormound-shaped bodies of transparent acoustic facies (IIA) charac-terize part of the deepest basin, locally covered by thin, layereddeposits (acoustic facies IIIA), in the northern part of the study area.Finally, non-penetrative, irregular facies (IC), with some patches oftransparent acoustic facies (IIB) beyond, characterise volcanicedifices in the basin.

6. Submarine landforms

Submarine landforms in the Greenland Basin have been iden-tified after examination of data from swath bathymetry, back-scatter, sub-bottom profiles and TOBI imagery. The landforms canbe grouped into four major categories: 1) erosive mass-wastingfeatures; 2) depositional lobes; 3) turbidite channel systems; and4) bedrock structural features.

6.1. Erosive mass-wasting features

Erosive mass-wasting features are identified on the continentalslope mainly in the Norske margin area (Figs. 1 and 4). Severalmajor slide scars, 30 km long and 1 kmwide, give a step-like shapeto the continental slope (Fig. 4B). The scars are up to 100mhigh andhave steep gradients of up to 9�. In addition, smaller scars areidentified at a wide range of depths, from 1200 to 3370 m (Fig. 4).These scars, 5e15 m high and only few kilometres long, are asso-ciated with depositional lobes.

Divergent channel-like depressions occur at depths of1900e3400 m, where the gradient is 0.1e1.4� (Fig. 4A). Individualchannels are 5e30 m deep, 0.5e3 kmwide and can be traced for atleast 40 km downslope. They occur basinward of the scars anddisplay a divergent pattern, suggesting they may be linked to thescars in terms of operating processes. Channels are identified asscarps on the sub-bottom profiles (profile BeB0 in Fig. 4B). The areaof erosive features on the Norske margin is characterized byacoustic facies IIB (thin transparent lens-shaped bodies) and locallyby acoustic facies IA (regular non-penetrative seafloor reflection)(Fig. 3).

6.2. Depositional lobes

Depositional lobes include a number of horizontally and verti-cally stacked lobes on the continental rise (Fig. 5A), and two largeisolated lobes on the continental slope (Fig. 5D). The stackeddepositional lobes occur on the Norske margin continental rise, atdepths of 3300e3600 m, where the gradient averages 0.4� (Fig 5B).They are elongate, up to 25 km long and 2e5 km wide witha NWeSE orientation (Fig. 5B). Individual lobes are separated bynarrow depressions up to 15 m deep, and are characterized bya wavy, non-penetrative seafloor reflector of acoustic facies IB. Thewavy surface can be recognized on the backscatter imagery asarcuate, concave-upslope stripes of alternating high and lowbackscatter (Fig. 5B). The lobes form a ridge-and-valley topography,up to 20 m deep and 0.8e1.3 km in wavelength (Fig. 5C). Down-slope of the depositional lobes, transparent deposits on the lower

Table 1Acoustic facies. Description based on seismic character and backscatter, distribution on the East Greenland margin and deep basin, interpretation and example from the sub-bottom TOPAS profiles.

Acoustic facies Description Distribution Interpretation

I. Nonpenetrative

IA. Regular seafloor reflection with no subbottomreflections Backscatter: �30/�20 dB

Locally on thecontinental slope

Scars, steep areas

IB. Wavy non-penetrative seafloor reflectionBackscatter: �40/�20 dB

Norske margincontinental rise

Slide deposits withpressure ridges

IC. Irregular seafloor reflection with nosubbottom reflections Backscatter: irregular

Volcanic edifices Volcanic activity

II. Transparent

IIA. Transparent lenticular or mounded-shapedbodies, up to 30 ms TWTT thickBackscatter: �40/�35 dB

Deep basinDistal turbidite systemdepositional lobes

IIB. Thin (<10 ms TWTT) transparent lens-shapedbodies Backscatter: �40/�20 dB

Continental slopeand rise

Mass movement deposits

III. Stratified

IIIA. Thin (w10 ms TWTT) package of continuoushorizontal parallel reflections overlying transparentdeposits Backscatter: �35/�25 dB

Distal part of thedeep basin

Non-channelized turbiditedeposition covering distalturbidite deposits

IIIB. Continuous, horizontal parallel reflectionsBackscatter: �40/�30 dB

Deep basin Non-channelized turbiditedeposition

IIIC. Continuous, undulating parallel layeredreflections Backscatter: �40/�30 dB

Proximal parts of thedeep basin

Sediment waves onturbidites

IIID. Parallel layered deformed reflectionsBackscatter: �45/�30 dB

Deep basin, close tovolcanic edifices

Deep basin, close tovolcanic edifices

IV. Chaotic

IV. Chaotic facies, wavy surfaceBackscatter: �40/�35 dB

Foot of the KFJ margincontinental slope

Debris flows

Fig. 3. Mapofacoustic faciesdistributionon theEastGreenlandmarginandbasin, showingthemajor four types (non-penetrative, transparent, stratifiedandchaotic) andsubtypes (Table1).

Fig. 4. Erosive mass-wasting landforms of the Greenland Basin slope (location in Inset). A) Shaded-relief swath image of the Norske Trough margin. B) Sub-bottom profiles showingthe non-penetrative seafloor, thin transparent lenses and scars. See the location of profiles in A.

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Fig. 5. Depositional mass-wasting features in the Greenland Basin (locations Inset). Shaded relief swath image (A) and backscatter map (B) of the Norske Trough margin showingthe distribution of stacked lobes in the profile CeC0 . D) Shaded relief swath image of the KFJ margin and location of sub-bottom profile DeD0 , with isolated lobe.

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continental rise (acoustic facies IIB) connect with turbidite chan-nels (Fig. 5A).

In addition, two large isolated depositional lobes have beenidentified at the foot of the slope on the KFJ margin, at depths of1800e2100 and 2450e2550 m (Fig. 5C). Only the more westerlylobe can be mapped in its entirety. It is elongate downslope, and is22 km long and 8 km wide, with a maximum relief of 30 m. Itdisplays a characteristic highly irregular surface in the bathymetryand chaotic acoustic facies IV in sub-bottom profiles and is affectedby the excavation of channels on its northern side (Fig. 5C).

6.3. Turbidite systems

Twomajor turbidite systems have been identified on our swath-bathymetric dataset. The first is a channel at the foot of the conti-nental rise on the Norske margin, with a WSWeENE orientationand maximum width of 3.7 km (Figs. 2A, 5A). The data only covera 50 km-long part of this system, revealing a strongly asymmetricalprofile, with steep SE walls (25 m high, up to 4�) and smoother,almost diffuse NWwalls. The second is a complete turbidite systemthat extends for more than 500 km from the KFJ margin continentalslope to the deep basin (Figs. 2A, 6 and 7). Although this turbiditesystem has already been reported (Mienert et al., 1993;Dowdeswell et al., 2002; Ó Cofaigh et al., 2004; Wilken andMienert, 2006), the new data analyzed in this work allowa detailed characterization of the complete system. Four sub-systems are identified, forming a continuum from the upperslope to the distal part of the Greenland Basin; tributary channels,channels with sediment-wave fields, channel-levee complexes, anddistal channels and associated depositional lobes.

6.3.1. Tributary channelsA series of tributary channels is observed on the KFJ margin

continental slope, at 1350e2000 mwater depth, where the averagegradient is 1.2� (Figs. 5D, 6A, C). These channels have a generalWNWeESE to EeW trend, are spaced less than 1 km apart anddisplay a convergent pattern downslope. The channels are0.8e1.6 km wide and 5e25 m deep. Acoustic facies IC is typical ofthese tributary channels, which have an irregular, non-penetrativeseafloor reflector.

6.3.2. Channels and sediment-wave fieldsTributary channels continue basinward through sediment-wave

fields, which are present for about 100 km downslope at depths of2000e2600 m, where the regional gradient averages 0.35� (Figs. 6,7). Channels are relatively straight, WSWeENE to SWeNE-oriented, and up to 60 km long (Fig. 6A). They are 10e30 m deep,1e2 km wide and have V-shaped profiles. Most channels endabruptly in the proximity of a major trunk channel, whereas someothers seem to open directly into it. The WeE-trending trunkchannel is relatively straight, and wider (up to 3.5 km) and deeper(up to 35 m) than the smaller tributary channels.

Sediment-wave fields develop at depths ranging between about2000 and 2600 m (Fig. 6). Two families of sediment waves aredifferentiated. The first family corresponds to large waves thataffect the upper 100 m of the sedimentary record and are roughlyparallel to the regional gradient (Fig. 6E). They occupy an area of atleast 1170 km2. The waves are up to 25 m high and have a wave-length of 1e2.6 km and an upslope-migration trend (Fig. 6, profileBeB0). Individual sediment waves are laterally continuous for tensof kilometres, and some of them seem to be continuous across thechannels. Vertically, two sets of waves are identified on the TOPASprofiles (Fig. 6E). The deeper set (at least 20m thick) is composed ofrelatively symmetrical waves (average upslope flank length:-downslope flank length 1:1.1) with a higher migration than the

shallower set, which affects the upper 40m of the sediment columnand is composed of more asymmetrical waves (average upslopeflank length:downslope flank length 1:3.3).

A second family of sediment waves is composed ofmuch smallerfeatures that can be clearly identified on the TOBI side-scan sonarimages at the distal terminations of the small channels (Fig. 6,profile CeC0). These waves only affect the upper 10e20 m of thesedimentary record, are relatively irregular in shape and trend, onlytens of metres long and typically 5e10 m high; they sometimesdisplaying barchan-shapes. Individual sediment waves appear to beoriented oblique to the regional gradient, but they display a highvariability in orientation.

The predominant acoustic facies on the channels and sediment-wave fields is type IIIC (continuous, undulating parallel layeredreflections, Table 1), but acoustically transparent deposits can alsobe observed underlying most of the sediment-wave fields.

6.3.3. Channel-levee systemIn the more proximal part of the Greenland Basin, a channel-

levee system is present for more than 150 km at depths rangingfrom 2600 to 3200 m and gradients of 0.1e0.25� (Fig. 7). Themorphology of the main channel changes abruptly at 2660 mwaterdepth, becoming deeper (50e60 m deep) and asymmetrical,showing a 20e30 m higher leveed southern wall (Fig. 7A). Theprofile is generally smooth, with marked rims and steep flanks ofup to 5� and a general trend parallel to the regional gradient. Asecond leveed channel merges with the main channel from itsnorthern flank at w2820 m water depth and sinuosity increasesbasinwards in the vicinity of several volcanic edifices (Fig. 7A). TOBIside-scan sonar images show terraces, up to 40 m high, instabilitiesaffecting the rims of the channel, braided channel floors and over-excavations within the thalweg (Fig. 7B). The channel widthincreases from w1 km to 2.8 km in the distal, sinuous part of thechannel. A small sediment-wave field covers an area ofw40 km2 atthe southern side of the channel, atw2700mwater depth (Fig. 7A).Waves are 5e15 m high and wavelength is 0.7e1.6 km. Continuousand deformed parallel stratified acoustic facies (IIIB and IIID,respectively) predominate in this area, whereas the thalweg of thechannel shows high-amplitude reflections or infilling by trans-parent deposits. Transparent deposits occur buried at the channelsflanks, and also fill the thalweg in distal areas (Fig. 7D).

6.3.4. Distal channel and depositional lobesThe distal part of the turbidite system occurs in the deep basin,

at water depths >3200 m, where the regional gradient is very low,typically <0.15� (Fig. 8). Narrow (1e1.6 km wide) and relativelyshallow (5e30 m deep) channels are identified on this flat area,displaying SWeNE toWeE trends and symmetrical, V- or U-shapedprofiles. Channels usually present relatively low values of back-scatter (�45 to �40 dB) (Fig. 8B). The channels lose theirmorphological identity at about 3580 m water depth. The flat,deepest areas where channels disappear show higher backscattervalues of �25 to �20 dB (Fig. 8B). The predominant acoustic faciesat the termination of channels are transparent lens- or mound-shaped bodies (IIA), thin packages of continuous parallel reflec-tions overlying transparent deposits (IIIA) in the northernmost partof the deep basin, and continuous parallel reflections (IIIB) in therelatively higher areas surrounding volcanic edifices (Fig. 8).

6.4. Structural features

Landforms on the seafloor of the Greenland Basin also includetwo types of structural features: structural scarps and volcanicedifices (Figs. 1, 8A). Structural scarps occur in the deep basin, andproduce a step-like morphology, becoming steeper in a north-

Fig. 6. Tributary channels and channels and sediment-wave field subsystems of the major turbidite system in the Greenland Basin (located in inset). A) Shaded relief swath imageshowing the location of the sub-bottom TOPAS profiles and TOBI imagery. C) Profile AeA0 showing the tributary channels. D) Sub-bottom profiles showing fields of large (BeB0) andsmall (CeC0) sediment waves. E) Sub-bottom profile DeD0 showing the two sets of waves.

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Fig. 7. Turbidity-current channel-levee system, distal channels and depositional lobes in the Greenland Basin. A) Shaded relief swath image of the channel-levee system, withcross-profiles showing the levee on the SE side of the channel, and the locations of sub-bottom profile AeA0 and TOBI images. B) and D) TOBI images of the channel, displayinginstability features and terraces on the walls. D) Sub-bottom profile crossing the main channel showing the horizontal parallel reflections and acoustically transparent deposits.

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easterly direction. Individual scarps are up to 30 m high and can betraced for tens of kilometres on the basin floor. Volcanic edifices arewidespread along the deep basin at depths >2800 m (Figs. 1, 2A).The largest example is Vesteris Seamount (2850 m high, 35 km indiameter), but a series of elongated NE to SW-oriented volcanicridges is also identified in the distal part of the basin (Figs. 2A, 8A).

Some of the edifices crop out on the present-day seafloor, formingcrests up to 700m high, 5 kmwide and 25 km long, whereas othersare buried by subsequent sedimentation and form smooth,100e200 m high mounds. Outcropping volcanic edifices displayirregular, non-penetrative acoustic facies IC (Table 1). Irregularpatches of relatively high reflectivity (�35 to �25 dB) are also

Fig. 8. Distal depositional lobes in the deep Greenland Basin (located in inset). A) Shaded relief swath image and B) backscatter map of the deep basin. Profiles BeB0 , CeC0 and DeD0

show the distal depositional lobes as transparent deposits, locally covered by a thin layered deposit.

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identified on the basinward side of some volcanic edifices in thedeep basin (Fig. 7C), suggesting relatively recent activity. We do notdiscuss these bedrock features further here.

7. Discussion: recent sedimentary processes

The landforms identified on the seafloor of the East Greenlandcontinental slope, rise and deep basin are shown in Fig. 9. The mapalso includes channels identified beyond our study area by previousworkers (Mienert et al., 1993; Wilken and Mienert, 2006). Thefigure shows the complex distribution of landforms.

The Norske margin is dominated by large slide scars anddownslope-trending divergent channels on the continental slope,and stacked lobes on the continental rise (Fig. 9). A turbidite channel

flows at the base of the continental rise. By contrast, the KFJ marginpresents a markedly different set of landforms. Here, a few isolatedlobes coexist with the upper reaches of a well-developed turbiditesystem consisting of tributary channels on the slope that merge,within a sediment-wave field, into a major trunk channel (Fig. 9).The basin beyond is dominatedby themain turbidite channel and itslevees. In the deepest part of the basin there are more individualchannels that transport sediment to the most distal, north-easternareas of the basin. Structural features (scarps and volcanicedifices) in the deep basin appear to influence the location of themain channel and the areas of depositional from themost distal partof the turbidite system (Fig. 9). The analysis and interpretation ofsubmarine landforms enables discussion of the recent sedimentaryprocesses operating on the Greenland Basin margin.

Fig. 9. Map of the Greenland Basin margin showing the distribution of the landforms identified in this work. The IBCAO regional map and published GLORIA data have been used asbackground bathymetry.

M. García et al. / Quaternary Science Reviews 49 (2012) 64e81 75

7.1. Glacigenic mass-wasting processes on the continental slope

Slide systems are composed of scars, channel-like depressionsand lobes, and dominate recent deposition on the Norske margin(Fig. 9). Large lobes, with a maximum run-out distance of 130 kmfrom the most proximal slide scars, are present; they are them-selves affected by smaller-scale slide processes, as suggested by thepresence of smaller slide scars. The ridge-and-depression shapedsurface of the lobes is interpreted as a series of pressure ridges(Fig. 4). These morphological characteristics suggest that lobes arecohesive masses composed of failed material (Hampton et al., 1996;McAdoo et al., 2000). The acoustically transparent depositsimmediately basinward of the depositional features that evolve tostratified deposits towards the turbidite channel (Fig. 5C) areinterpreted as debris flow deposits resulting from disintegrativeslides, composed of the fluidized part of the failed material(McAdoo et al., 2000). Part of the sediment transported by thesedebris flows may feed into the turbidite channel at the foot of thecontinental rise (Fig. 5A). The two isolated depositional lobesidentified on the KJF margin are also interpreted as debris flows ordisintegrative slides, as suggested by their acoustic facies andmorphology (Fig. 5D).

Several factors may be involved in the location and timing ofslides on the Greenland Basin slope (Locat, 2001; Locat and Lee,2002). The most important is the likelihood of rapid sedimentdelivery to the shelf edge at the mouths of cross-shelf troughs,linked to the presence of full-glacial fast-flowing ice streams(e.g. Laberg and Vorren, 1995; Dowdeswell et al., 1996, 1997;Dowdeswell and Siegert, 1999; Ó Cofaigh et al., 2003, 2004;Evans et al., 2009). The increased loading due to the rapidaccumulation of glacial sediment would favour the occurrence ofslides. In addition, tectonic effects may also be important trig-gers for mass-wasting, including post-glacial seismicity linked toisostatic readjustments during and following deglaciation andearthquakes (Hampton et al., 1996; Piper et al., 1999; McAdooet al., 2000; Lee, 2009; Locat et al., 2009). The most importantsource of seismicity on the region is related to the Greenlandfracture zone on the East Greenland Ridge (Fig. 1A), whichaffects particularly the Norske margin (Bungum and Husebye,1977; Gregersen, 1986, 2006; Poulsen and Simonsen, 2006;Voss et al., 2007). Finally, the location of slides is probablyalso influenced by relatively weak stratigraphic layers, suggestedby the lateral continuity of slide scars across the continentalslope (Fig. 4A).

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7.2. Turbidite systems

Turbidite systems predominate on the distal continental slopeand deep basin (Figs. 6, 7, 8A, 9). The major turbidite system in theGreenland Basin was active during full-glacial and deglacialconditions, with activity largely ceasing after about 13,000 years BPaccording to dating of sediments from the levees beyond thechannel (Ó Cofaigh et al., 2004; Wilken and Mienert, 2006). Theimplication of the timing of this activity is that it was linked to therelatively rapid full-glacial and deglacial delivery of sediments andmeltwater when the Greenland Ice Sheet extended across thecontinental shelf (Evans et al., 2009; Winkelmann et al., 2010).

Physiography is important as it determines the potential fortransformation of glacially derived sediment on the upper slopeinto debris flows and turbidity currents (Migeon et al., 2011).Although slope angle has been demonstrated to have small effecton the initiation of slides (Hühnerbach et al., 2004; Migeon et al.,2011), it determines the post-failure behaviour of the displacedslide masses (Migeon et al., 2011). This may explain why mass-wasting deposits conserve their cohesion and form stacked debrislobes on the continental rise of the Norske margin, where thegradients are relatively low (Fig. 9). In contrast, on the KFJ margin,higher slope gradients may be linked to translation into turbiditycurrents. The presence of structural features also appears to have aneffect on the distribution of turbidite systems. Structural scarpsaffect both the trends and location of turbidite channels in the deepbasin, whereas volcanic edifices have a control on the location ofdistal turbidite deposits which are restricted to the eastern side ofthe deeper edifices (Figs. 4 and 8).

The detailed morphological study carried out in this work alsoenables some discussion of the evolution of the turbidite system.The tributary channels on themid-slopewere probably produced byintermittent mass-wasting at shallower depths which translatedinto turbidity currents. The abrupt morphological change at 2660mwater depth (Figs. 6A, 7A), where the channel becomes deeper andasymmetrical with the development of prominent levees, can beexplained by either of two processes. The first hypothesis involvesa change in flow characteristics due to variations in regional slope.Such a change in the regional gradient is observed, marking thetermination of the sediment-wave field (0.36�e0.05�) about 15 kmproximal to the point where the trunk channel over-deepens(Fig. 6A). This physiographical effect could have producedahydraulic jump, increasing turbidityflow thickness anddecreasingvelocity; this would favour overspill from the channel and deposi-tion of levees (Garcia and Parker,1989;Mulder andAlexander, 2001;Ercilla et al., 2002; García et al., 2006). While levee deposition maybe explained by this effect, it does not explain the sudden increase inchannel depth that is probably linked to an increase in flowvelocity,rather than to a decrease. A second hypothesis therefore involvesa change from a predominantly confined turbidity flow, responsiblefor the development of a classical turbidite system (includingchannels, levees and distal lobes), to predominantly unconfinedturbidity flow that would favour the development of the observedsediment-wave fields and the partial burial and deactivation of theproximal part of the turbidite system. The hypotheses are notmutually exclusive, and a combination of the two, operating atdifferent times, could explain the morphology of the system.

Depositional lobes in the deep basin can alternatively be inter-preted as debris-flow deposits, as suggested by their thickness,erosive character at the base and the occurrence of channelseroding their upper limit (sub-bottom profile BeB0 in Fig. 8).Debris-flow deposits would be composed of glacigenic sedimentsupplied to the continental slope during full-glacial conditions andearly stages of deglaciation, when the ice-sheet reached distalpositions on the continental shelf.

7.3. Sediment waves

Sediment-wave fields in the Greenland Basin (Fig. 6) areattributed to the activity of turbidite processes, as suggested by thepredominance of laminated sand and mud turbidites in sedimentcores recovered from the proximal turbidite channel area (ÓCofaigh et al., 2002, 2004), and their occurrence in an area domi-nated by debris flows (Fig. 9) (Belderson et al., 1984; Faugeres et al.,1993). Morphological characteristics such as the upslope migrationof sediment waves and the orientation of crestlines parallel to theregional slope also suggest a link to turbidites rather than contourcurrents (Figs. 6 and 9; Normark et al., 1980; Wynn et al., 2000a,b;Wynn and Stow, 2002). The observation that some wave crests canbe traced across the channels (Fig. 6A) also points to an originrelated to unconfined currents rather than related to spillover fromthe turbidite channels.

An alternative hypothesis for sediment waves would be theirorigin in relation to gravitational deformation processes. Thishypothesis is not favoured by the morphological characteristics,such as the upslope migration, the asymmetric reflectivity and thevariations in sediment thickness across the waves. Finally, anocean-current influence on the origin of sediment waves cannot beclearly identified in the seafloor morphology of the GreenlandBasin. The East Greenland Current flows along the margin fromFram Strait to Denmark Strait as a strongly directional, south-westward flow running parallel to the depth contours (Aagaard andCoachman, 1968; Swift and Aargaard, 1981; Woodgate et al., 1999;Rudels et al., 2005). Although this current is likely to undergointensification due to topographical constraints at the Jan MayenFracture Zone (Fig. 1A), its effect on the origin of the sedimentwaves is unclear.

If we assume a turbidity-current origin for the sediment waves,we can use their morphology to make inferences and calculationsabout the characteristics of the flows that generated them. Inaddition, spatial and temporal changes in morphological parame-ters such as size, symmetry and migration rate provide informationabout both spatial and temporal variations in turbidity-currentflow regime.

The characteristics of a turbidity current can be inferred usingnumerical models (Bowen et al., 1984; Wynn et al., 2000a,b; Ercillaet al., 2002). The internal Froude number (Fi), flow thickness andflow velocity have been estimated (Fig. 10, Table 2; see modeldetails in Appendix 1). These calculations have been applied tosectors of the sediment-wave fields that have distinctive slope-gradient values (Fig. 10). Transect 1 was traced along an inter-channel area between tributaries on the slope where the smallsediment waves have been identified. Transect 2 parallels the trunkchannel on the main levee in the southernmost part of the system(Fig. 10; Table 2).

In general, modelled values of Fi decrease with reduction inslope gradient, indicating less turbulent flow (Bowen et al., 1984). Inthe Greenland Basin sediment-wave fields, Fi is in the range0.96e1.975, in general agreement with the estimated values of Fifor turbidity currents that have generated coarse-grained sedimentwaves (Fi ranging 1e2; Piper and Savoye, 1993; Mulder et al., 1997;Wynn et al., 2000a). No sediment waves occur in areas where theflow is sub-critical (i.e., significantly Fi < 1). Predicted flow thick-nesses across the entire wave field are from 27 to 335 m, witha maximum at the centre of the sediment-wave fields. Values forTransect 1, on the small sediment-wave field, are significantly lower(25e75 m) than for Transect 2 on the main levee, where they rangefrom 93 to 335m.Model-predicted velocities range between 11 and72 cm s�1, decreasing downslope. Maximum values may representthe velocity at the head of the turbidity current, where sedimentconcentration is higher, while minimum values would be found at

Fig. 10. Shaded relief swath image of the area of sediment-wave fields (located in inset), showing the position of the long-profile transects. Profiles show the sectors into whichtransects have been divided based on their gradient, for the analysis of the characteristics of turbidity-current flow (see Table 2).

M. García et al. / Quaternary Science Reviews 49 (2012) 64e81 77

the tail of the current. Calculated values are similar to those of otherturbidity-current produced wave fields (Normark et al., 1980;Bowen et al., 1984; Piper and Savoye, 1993; Wynn et al., 2000b;Ercilla et al., 2002).

Spatial variations can also be inferred from the analysis of thetwo distinct families of sediment waves on the Greenland Basinseafloor (Fig. 6D). Their presence suggests either the coexistenceof two different mechanisms of wave formations, or a differentdegree of maturity between the two types of sediment waves. Thesecond option is favoured: first, there are no significant physio-graphical variations to justify the existence of two coeval flowswith different characteristics; secondly, the smaller sedimentwaves develop above aw10 m thick acoustically transparent bodyinterpreted as a small slide deposited at depths of 20e25 m belowthe seafloor. Sediment waves developed on top of this depositwould therefore be more recent in origin, and may not havereached the same degree of maturity. Temporal variations in theflow characteristics are suggested by the existence of at least twosets of sediment waves showing distinctive symmetry andmigration rates (Fig. 6D).

7.4. Timing

The study of submarine landforms on the East Greenlandmargin and basin reveals a clear predominance of debris-flow and

Table 2Characteristics of the turbidite flow inferred from physiographic and morphologic paranumerical models used for calculations.

Sector Gradient (�) H (m) WL (m

Transect 1 1 1 No waves e

2 0.7 5e15 500e73 0.635 5e10 10004 0.457 5e15 500e9

Transect 2 1 0.524 15e30 1300e2 0.227 5e10 1300e3 0.06 No waves e

4 0.219 5e15 1000e5 0.16 No waves e

turbidity-current processes on the continental slope, continentalrise and deep basin. The timing of deposition of these landforms isinterpreted in relation to glacial processes, in particular to therelatively high volumes of sediment supplied by the ice sheet to thecontinental margin at the LGM (e.g. Dowdeswell and Siegert, 1999).This implies that most of the observed submarine landforms wereproduced during full-glacial conditions, once the ice-sheetgrounding zone reached its most advanced position, and duringthe early stages of deglaciation, when sediment supply to the slopewas still high. This is in agreement with an ice sheet grounded atthe outer continental shelf or even at the shelf-edge during the LateWeichselian glaciation of Northeast Greenland (Ó Cofaigh et al.,2002, 2004; Wilken and Mienert, 2006; Evans et al., 2009;Winkelmann et al., 2010). The development of turbidity currentsis linked to both physiography and sediment supply, making thisprocess likely to be active during both full-glacial and deglaciationconditions. A temporal change in turbidite processes (from chan-nelised to unconfined flows) could be related to a progressivedecrease in sediment supply as deglaciation proceeds, deactivatingturbidite channels but maintaining sediment transport in the formof unconfined flows and the associated development of sediment-waves fields. Submarine mass-wasting would be favoured by bothrapid full-glacial sediment delivery and the triggering action ofisostatic movements (Hampton et al., 1996; Piper et al., 1999;McAdoo et al., 2000; Lee, 2009; Locat et al., 2009).

meters of the margin and the sediment waves. See Appendix 1 for details on the

) Fi (av) Flow thickness (m) Flow velocity (cm/s)

2.05 e e

00 1.71 27e38 11e35/13e421.635 59 16e50

00 1.38 41e75 11e35/15e48

2000 1.485 93e144 18e58/23e722000 1.975 217e335 18e57/23e72

0.5 e e

1500 0.96 173e259 16e50/19e620.82 e -

Fig. 11. A) Synthesis of the major sedimentary processes affecting the East Greenland margin and deep basin. B) Sketch of the differential controls on recent sedimentation on theNorske margin and the KFJ margin.

M. García et al. / Quaternary Science Reviews 49 (2012) 64e8178

Since about 13,000 years BP, hemipelagic deposition has beenthe dominant sedimentary process in most of the basin. After thistime, mass-movement and turbidity-current activity appears tohave largely ceased, and an open-marine ice-distal environmentbecame established in the Greenland Basin (Hebbeln et al., 1998;Evans et al., 2002; Ó Cofaigh et al., 2004; Wilken and Mienert,2006). Although the resolution of the sub-bottom profiles doesnot allow the recognition of a hemipelagic draping layer, thesesediments compose the top of most of the sediment cores recov-ered from the margin and basin, except on the Norske margincontinental rise and distal basin. Here, corer penetration was pre-vented by relatively coarse surficial sediment composed of sandy,foraminifera-rich mud. This suggests that some downslope transferof coarser-grained debris may have been active during the Holo-cene in this part of the deep basin.

8. Summary and conclusions

This paper presents the first study of seafloor landforms on theEast Greenland continental slope between 74�N and 76�N andprovides further morpho-sedimentary interpretations on land-forms from the Greenland Basin (Mienert et al., 1993; Ó Cofaighet al., 2002, 2004; Wilken and Mienert, 2006), based on the anal-ysis of swath bathymetry datasets and sub-bottom TOPAS profiles(Fig. 11A). This work has enabled the interpretation of sedimentarylandforms in terms of recent sedimentary processes and theirtiming with relation to glacialeinterglacial cyclicity. The EastGreenland continental margin shows two areas where distinctsedimentary processes predominate; the major processes andlandforms are summarised in Fig. 11B.

The more northerly Norske Trough margin is characterized bymass-wasting processes, with scarps and channel-like depressionson the slope and debris-flow lobes deposited on the entire conti-nental rise (Fig. 11B). This suggests a relatively high sedimentsupply to the margin during glacial conditions, with the

interpretation of a Greenland Ice Sheet extending across thecontinental shelf during the Last Glacial Maximum, perhaps as anice stream in Norske Trough. Mass-wasting would also have beenfavoured by tectonic effects, including both post-glacial seismicityrelated to isostatic adjustments during deglaciation and regionalseismicity.

By contrast, the more southerly Kejser Franz Josef marginpresents small-scale mass-wasting deposits in the form of isolateddebris-flow lobes that most likely developed coevally with anextensive turbidite system composed of tributary valleys, majorchannels and sediment-wave fields, channel-levee systems anddistal channels and depositional lobes (Fig. 11B). Turbidite-channelsystems cover practically the entire Greenland Basin (Mienert et al.,1993; Ó Cofaigh et al., 2004; Wilken and Mienert, 2006). Thesesystems are the result of two types of flows. Channelised turbidityflows are responsible for the transport of sediment from theproximal continental slope to the deep basin through well-developed leveed-channels. Sediment involved in turbidite trans-port is interpreted as being supplied to the continental slope byglacial processes, based on the proposed chronology of the system,which became largely inactive about 13,000 years BP (Ó Cofaighet al., 2004; Wilken and Mienert, 2006). The channelisation ofsediment into turbidite channels, rather than the development ofmajor mass wasting (as in the case of the Norske margin) may berelated to a relatively lower sediment supply and/or reducedtectonism. Unconfined flows ranging 11e72 cm s�1 produce theextensive sediment-wave fields on the levees. Sediment waveshave developed coevally with small slides, showing differentdegrees of maturity, and their morphological characteristics alsopoint to temporal changes in the characteristics of the flows.

This paper demonstrates the glacial influence on recent deep-sea sedimentation in the Greenland Basin. Although continentalshelves have been the focus of most the studies aiming to recon-struction of Late Quaternary glacial environments, through identi-fication of landforms related to ice-sheet grounding on the seafloor,

M. García et al. / Quaternary Science Reviews 49 (2012) 64e81 79

the study of more distal continental slope-basin systems whichhave a glacial influence can also provide valuable information aboutthe extent of past ice sheets.

Acknowledgements

We thank Neil Kenyon and Colm Ó Cofaigh for their help withdata acquisition on Cruise JR51 of the RRS James Clark Ross. Thecruise was funded through UK NERC Grant GST/02/2198 to JulianDowdeswell. A Marie Curie Intra-European Fellowship (7thFramework Programme, European Commission) has supported theresearch of Marga Garcia in the Scott Polar Research Institute,University of Cambridge. Writing of the paper was completed whileJulian Dowdeswell was Visiting Cambridge Fellow at GatewayAntarctica, University of Canterbury, New Zealand, funded throughthe Erskine Bequest. M. Jakobsson received support from theSwedish Research Council (VR) and the Bert Bolin Centre for ClimateResearch at StockholmUniversity throughagrant fromFORMAS.Weare grateful to the editor and reviewers for their suggestions thathave contributed to improve the quality and clarity of this work.

Appendix 1. Calculations of turbidity-current characteristics

First, the internal Froude number is calculated. This value, incombination with wavelength of sediment waves, is used to esti-mate flow thickness; then, flow velocity is calculated based on theinternal Froude number and sediment concentration.

Internal Froude number (Fi)

Fi for a turbidity current is calculated using (Bowen et al., 1984):

Fi2 ¼ sin b=Cf þ E; Fi ¼ internal Froude number

where b is slope angle, Cf is drag coefficient and E is entrainmentcoefficient.

Suggested values for the drag coefficient (Cf) in channelisedturbidity currents are in the range of 3e5 � 10�3 (e.g. Bowen et al.,1984). For unconfined flows, a lower figuremay bemore applicable.Therefore, a range of Cf values is used, varying from a minimumpossible of 5 � 10�4 for smooth, artificial beds, to a maximum of5 � 10�3 for unchannelised flows crossing a gravel bed. Theentrainment coefficient (E) for most turbidity currents variesbetween 5 � 10�4 and 6 � 10�3 (Bowen et al., 1984). Generally, Fidecreases with a reduction in slope gradient, and is lower in lessturbulent flows (Bowen et al., 1984).

Flow thickness (h)

The relationship between sediment-wave length (L), flowthickness and internal Froude number (Fi), is expressed as (modi-fied from Normark et al., 1980):

h ¼ L=2pFi2

Flow velocity (u)

u is calculated based on internal Froude number and sedimentconcentration using the following equation (Piper and Savoye,1993):

u2 ¼ DpCghFi2

where Dp is grain density � density of fluid within TC/seawaterdensity, C is volume concentration, g is gravitational acceleration

and h is flow thickness. C is a dimensionless number representingsediment concentration. It is relatively poorly constrained inmodels and can vary depending on the type of flow. For unconfinedturbidity currents formed of fine-grained sediments, values of 10�5

and 10�4 may be suitable (e.g. Bowen et al., 1984; Piper and Savoye,1993).

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