Marine Geology 255 (2008) 55–63

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Marine Geology

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The present and past bottom-current flow regime around the sediment drifts on thecontinental rise west of the Antarctic Peninsula

C.-D. Hillenbrand a,⁎, A. Camerlenghi b,1, E.A. Cowan c, F.J. Hernández-Molina d, R.G. Lucchi b,1,M. Rebesco b, G. Uenzelmann-Neben e

a British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdomb Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italyc Department of Geology, Appalachian State University, PO Box 32067, Boone NC 28608, USAd Dpto. de Geociencias Marinas, Universidad de Vigo, 36200 Vigo, Spaine Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12 01 61, D-27515 Bremerhaven, Germany

⁎ Corresponding author. Tel.: +44 1223 221340; fax: +E-mail address: hilc@bas.ac.uk (C.-D. Hillenbrand).

1 Present address: GRC Geociències Marines, DepartamBarcelona, Spain.

0025-3227/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.margeo.2008.07.004

a b s t r a c t

a r t i c l e i n f o

Article history:

Throughout the last decade Received 6 August 2007Received in revised form 17 June 2008Accepted 3 July 2008

Keywords:Antarctic Peninsulabottom currentCenozoicsediment driftLeg 178Antarctic Circumpolar Current

large sediment drifts located on the upper continental rise west of the AntarcticPeninsulawere the target of oceanographicmeasurements, bathymetricmapping, seismic investigations, shallowsediment coring, and deep-sea drilling byOceanDrilling Program (ODP) Leg 178. These studies concluded that formostof the lateNeogene andQuaternarya generally SW-warddirectedbottomcurrent affected the depositiononthe drifts. In particular during glacial periods, the deposition was additionally influenced by NW-ward directedtransport of terrigenous detritus supplied by turbidity currents from the Antarctic Peninsula continental slope.In a recent study, however, the palaeomagnetic signal of the drift sediments recovered at two ODP Leg 178 sites(1095 and 1101) was interpreted to provide spatial and directional information on the physical record of the NE-ward flowing Antarctic Circumpolar Current (ACC). Here we investigate the link between the clockwise flowingACC and the generally SW-wards flowing near-bottom contour current. We show that at the ODP Leg 178 siteson the western Antarctic Peninsula continental margin the ACC only affects the ocean circulation aboveca. 1000 m water depth. Therefore, the signal of ACC flow might only be archived in the ice-rafted debris (IRD)content of the drift sediments. However, the IRD from the Leg 178 sites accounts for only a small proportion ofthe drift sediments and is dominantly of local origin. Past variability of bottom-water flow around the sedimentdrifts was reconstructed on the basis of seismic studies and clay mineralogical and grain-size analyses. Thesereconstructions provide useful information about both downstreamdirection and velocity changes of the bottomcurrent and point to its SW-ward flow along the upper rise during at least the last 9.4 Ma.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562. Oceanography west of the Antarctic Peninsula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.1. The modern ocean circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572.2. The bottom-water mass and its origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.1. The sedimentary record of Neogene and Quaternary bottom-water flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.2. Bottom-current direction and the magnetic fabric of the drift sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.3. Were the sediment drifts bathed by the ACC during the past? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.4. The ACC component of the drift sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

44 1223 221646.

ent d'Estratigrafia, P. i Geociències Marines, Universitat de Barcelona, Facultat de Geologia C/ Martí i Franquès, s/n E-08028

l rights reserved.

56 C.-D. Hillenbrand et al. / Marine Geology 255 (2008) 55–63

1. Introduction

Twelve large sediment mounds interpreted as sediment drifts arelocated on the upper continental rise west of the Antarctic Peninsula(Fig. 1). The drifts are separated by deep-sea channels, which wereeroded by turbidity currents originating from the steep continentalslope. Sediment drift development started at about 15 Ma (e.g.Rebesco et al., 1997, 2002). The seven largest drifts were the target ofdetailed bathymetric studies (e.g. Rebesco et al., 1998, 2002;Dowdeswell et al., 2004; Amblas et al., 2006; Rebesco et al., 2007),acoustic subbottom profiling (e.g. Pudsey et al., 2002), reflectionseismic studies (e.g. McGinnis and Hayes, 1995; Rebesco et al., 1996,1997; Canals et al., 1998; Diviacco et al., 2006; Hernández-Molina etal., 2006a,b), shallow sediment coring (e.g. Camerlenghi et al., 1997a;Pudsey and Camerlenghi, 1998; Pudsey, 2000; Lucchi et al., 2002) anddeep-sea drilling by ODP Leg 178 (Barker et al., 1999, 2002).

The morphology (gentle NE flank and steep SW side) and internalseismic architecture of most of the drifts and the sedimentary se-quences, which were recovered by numerous short sediment coresand at Leg 178 Sites 1095, 1096 (both drilled at Drift 7) and 1101

Fig.1.Detailed bathymetric map of the continental marginwest of the Antarctic Peninsula (Reof the Antarctic Polar Front and the modern sea-ice limits (taken from Pudsey and Camerlen1995) are also shown. Insets indicate locations of Figs. 2–5 and location of fossil mounded

(drilled at Drift 4), revealed that the drifts were built-up andmaintained bymainly terrigenous, fine-grained particles. This detrituswas initially supplied by turbidity currents that travelled through thechannels towards the abyssal plain. A near-bottom contour currentcaptured the fine-grained particles and transported them in a generalSW-ward direction before depositing them on the drifts together withdetritus directly supplied by the turbidity currents and by verticalsettling. The sedimentary components delivered by vertical settlingcomprise microfossils (mainly diatoms and planktonic foraminifera),ice-rafted debris (IRD), eolian dust, and fine-grained glaciogenicparticles raining out from meltwater plumes (e.g. Rebesco et al., 1996,1997; Pudsey, 2000, 2002a,b; Lucchi et al., 2002; Rebesco et al., 2002;Hillenbrand and Ehrmann, 2002, 2005).

At Drift 7 the interaction of a generally SW-wards flowing bottomcurrent with NW-wards flowing turbidity currents is assumed to bethe main control of deposition throughout the last 9.4 Ma, with thedominant role alternating between down-slope and along-slopetransport (Barker et al., 1999, 2002; Pudsey, 2002a; Rebesco et al.,2002; Hillenbrand and Ehrmann, 2002, 2005; Uenzelmann-Neben,2006). Also the deposition of the drift sediments further to the NEwas

besco et al., 1998) with locations of ODP Sites 1095,1096, and 1101 (solid dots). Locationsghi, 1998) and location of the southern boundary of the ACC (SB; taken from Orsi et al.,sedimentary body (Hernández-Molina et al., 2004, 2006a).

Fig. 2. Potential temperature in °C (left) and salinity in ‰ (right) along a hydrographicsection at 72°W (location see Fig. 1; modified from Whitworth et al., 1998).

57C.-D. Hillenbrand et al. / Marine Geology 255 (2008) 55–63

apparently controlled by this interplay of along-slope (i.e. SW-wards)and down-slope sediment transport since the middle Miocene (e.g.Rebesco et al., 1997, 2002). New palaeomagnetic data from Leg 178Sites 1095 and 1101, however, were linked to the deposition of driftsediments by the NE-ward flowing ACC (Parés et al., 2007). Thisconclusion challenges the view that the bottom-current flow isdirected towards SW and that the water mass generating the bottomcurrent originates in theWeddell Sea. Here we discuss oceanographic,geophysical and sedimentological data in the context of a hypotheticalconnection between the ACC and the drift sediments.

2. Oceanography west of the Antarctic Peninsula

2.1. The modern ocean circulation

The shelf edge and continental slope west of the AntarcticPeninsula (southwest of ca. 63°W) marks the boundary between thewestwind-driven ACC, which is theWorld's largest ocean current withan average net baroclinic transport of 97–154 million m3/s throughDrake Passage (e.g. Whitworth and Peterson, 1985; Orsi et al., 1995;Rintoul et al., 2001; Cunningham et al., 2003), and a flow regime withcyclonic gyres on the shelf (Fig. 1; Orsi et al., 1995; Hofmann et al.,1996; Smith et al., 1999). Usually, the ACC encompasses all watermasses from the ocean surface down to the seafloor (e.g. Whitworth,1988), but directly west of the Antarctic Peninsula the ACC onlydominates the upper water column while the seabed is affected by aSW-wards flowing current (Fig.1; see also Figs.1a and 2 in Hernández-Molina et al., 2006a). An oceanographic front, the southern boundaryof the ACC (SB; Orsi et al., 1995), runs along the shelf break of thewestern Antarctic Peninsula (Fig. 1). Orsi et al. (1995) defined theposition of the SB as the southernmost pole-ward extent of UpperCircumpolar Deep Water (UCDW). Episodic intrusions of UCDW ontothe western Antarctic Peninsula shelf are well documented (e.g. Smithand Klinck, 2002) and highlight the influence of the ACC on the oceancirculation above ca. 1000 m water depth in the study area (cf. Smithet al., 1999) as well as the spatial variability of the ACC frontal system(e.g. Moore et al., 1999; Rintoul et al., 2001).

During the 1960s and 1970s seabed photographs and variousoceanographic measurements indicated that a weak bottom currentflows SW-wards along the Antarctic Peninsula continental margin(between about 1000 m and 4000 mwater depth) to at least ca. 83°W(Hollister and Heezen, 1967; Tucholke, 1977 and references therein).More recent current measurements northeast of 63°W documented aSW-ward flow of surface, deep and bottom waters directly above theslope and the upper continental rise (Nowlin and Zenk, 1988).Between 68°W and 71.5°W current-induced winnowing is evidentfromvery coarse-grained seabed sediments recovered between 800mand 1600 m water depth (Pirrung et al., 2002; Dowdeswell et al.,2004). Elevated flow speed could be associated with either the SB and/or a SW-ward slope current. The first scenario is suggested by the SB'smean position close to the upper slope in this area (Fig. 1). The secondexplanation is supported by short-term current measurements thatindicate SW-ward flow with a speed of up to ca. 12 cm/s in about1000 m water depth at ca. 71°W (see Fig. 5 in Tucholke, 1977).Winnowing by a SW-ward slope current associated with the“Antarctic Slope Front” (ASF), which is observed almost all aroundAntarctica (e.g. Jacobs, 1991; Heywood et al., 2004), is less likely,because the ASF is thought to be absent between Drake Passage and120°W (Fig. 2;Whitworth et al., 1998). At ca. 80°Wawestward currentwith a speed of 12 cm/s was measured in about 3000 m water depth(Tucholke,1977). Currentmeasurements at 8m above the seabed fromthree locations on Drift 7 revealed an anticlockwise flow of a contourcurrent between about 3300 m and 3600 mwater depth with a meanspeed around 6 cm/s (maximum: 19 cm/s) (Fig. 3; Camerlenghi et al.,1997b; Giorgetti et al., 2003). In summary, multiple evidence points toa SW-ward current flow between ca. 1000 m and 4000 mwater depth

along the western Antarctic Peninsula margin from 63°W to at least80°W. In contrast, the lower continental rise at about 64°S and 76°W isaffected by NE-ward ACC flow with a speed of ca. 3–4 cm/s (Tucholke,1977), which is close to the mean through-flow speed of 4.5 cm/smeasured at ca. 3500 m water depth in central Drake Passage(Sciremammano et al., 1980).

2.2. The bottom-water mass and its origin

Camerlenghi et al. (1997b) interpreted the bottom-current flowaround Drift 7 (Fig. 3) to be generated by Weddell Sea Deep Water(WSDW) that originates in the southern Weddell Sea, flows antic-lockwise around the northern tip of the Antarctic Peninsula andextends to the west as far as the South Shetland Trench (Sievers andNowlin, 1984; Nowlin and Zenk, 1988; Whitworth, 1988; Orsi et al.,1999; Naveira Garabato et al., 2002a). However, the water massbathing the continental rise between 72°W and 77°W has a potentialtemperature (θ)N0 °C (Fig. 2; Camerlenghi et al., 1997b; Whitworthet al., 1998), whereasWSDW is characterised by −0.8 °Cbθb0.0 °C (e.g.Naveira Garabato et al., 2002b). Therefore, the water mass is probablymodified WSDW (cf. Giorgetti et al., 2003).

Most oceanographic studies restrict the westernmost spread ofWSDW to the South Shetland Trench (Sievers and Nowlin, 1984; Orsiet al., 1999; Smith et al., 1999; Naveira Garabato et al., 2002a).However, this conclusion takes into account only the salinity, potentialtemperature and density ofWSDW sensu stricto, but does not considermodified WSDW or flow directions determined by current measure-ments. In accordance with Naveira Garabato et al. (2002a), Hernán-dez-Molina et al. (2006a) suggested that a branch of LowerCircumpolar Deep Water (LCDW), which originates in the WeddellSea and overlies the WSDW in Drake Passage, continues SW-wardsalong the Pacific margin of the Antarctic Peninsula. An LCDWorigin ofthe bottom-water mass implies its modification by mixing processes,because the salinity of unmodified LCDW typically exceeds 34.70‰(Orsi et al., 1995) and its potential density parameter σ4 is b46.04(Naveira Garabato et al., 2002a). In contrast, the bottom-water salinitymeasured around Drift 7 is b34.70‰ and the potential density σ4

exceeds 46.05 (Giorgetti et al., 2003; see also Figs. 2 and 6 in chapter 2of Barker et al., 1999). Even if the bottom-water mass is modifiedLCDW, it is derived from the Weddell Sea, and its general flow isdirected against the main ACC flow (Naveira Garabato et al., 2002a;Hernández-Molina et al., 2006a).

Fig. 3. Mean bottom-current flow directions (black arrows) recorded at the threecurrent meters (ST-01, ST-02, and ST-03) that were moored 8 m above the seabed onDrift 7. Grey arrows indicate locations of deep-sea channels. ODP Leg 178 Sites 1095 and1096 are also shown (taken from Rebesco et al., 2002). Reproduced with the permissionof the Geological Society (London).

58 C.-D. Hillenbrand et al. / Marine Geology 255 (2008) 55–63

Alternatively, the bottom-water mass might be Southeast PacificDeepWater (SPDW), which represents the densest watermass (besidesWSDW) west of the Antarctic Peninsula (Sievers and Nowlin, 1984;Smith et al.,1999). TheACC-origin of SPDWcombinedwith the SW-wardbottom-current direction measured at Drift 7 would imply that thedensest portion of SPDW branches off from the ACC (probably at thetopographic sill formed by Drake Passage) and is clockwise retroflectedback into the Southeast Pacific basin along the upper continental rise ofthe Antarctic Peninsula. Only in that case, past flow changes of the ACCcould be inferred from the drift sediments. Potential temperature(0.2 °C≤θ≤0.6 °C) and salinity (Sb34.71‰) of SPDW resemble those ofmodifiedWSDW (e.g. Sievers and Nowlin,1984; Naveira Garabato et al.,2002a). However, WSDW is denser than SPDW (Sievers and Nowlin,1984), and the potential density parameterσ4 can be used to distinguishWSDW (46.04bσ4b46.16) from SPDW (45.98bσ4b46.04) (NaveiraGarabato et al., 2002a). According to Giorgetti et al. (2003), the potentialdensity of the bottom-water mass bathing Drift 7 is characterised by46.05bσ4b46.10 pointing to modified WSDW. Another significantdifference between modified WSDW and SPDW is the concentrationof dissolved silica that typically exceeds 130–135 µmol/L in SPDW(Sievers and Nowlin, 1984; Smith et al., 1999; Naveira Garabato et al.,2002a). The dissolved silica concentration in a bottom-water sampletaken fromHole 1096B near the crest of Drift 7 is only 120 µmol/L (Table25 in chapter 5 of Barker et al.,1999; J. Schuffert, pers. comm., 2000) andthus supports the conclusion of Giorgetti et al. (2003) that the bottomcurrent is generated by modified WSDW. The WSDW is probablymodifiedbymixingwith theoverlying LCDW,whichalsohas its origin inthe Weddell Sea (e.g. Hernández-Molina et al., 2006a).

3. Discussion

3.1. The sedimentary record of Neogene andQuaternary bottom-water flow

Swath bathymetry data and acoustic subbottom and seismic profilesshowed that the morphology and internal architecture of most of thedrifts is consistent with a SW-ward bottom-water current following thebathymetric contours (Fig. 4; Rebesco et al., 1996, 1997, 1998, 2002;

Pudsey et al., 2002; Amblas et al., 2006; Hernández-Molina et al., 2006a,b; Uenzelmann-Neben, 2006; Rebesco et al., 2007). Only McGinnis andHayes (1995) and McGinnis et al. (1997) interpreted seismic profiles toindicate a mainly turbiditic origin for the drifts. Conversely, most otherauthors concluded that the sediment drifts were mainly formed bycontour currents with a contribution from turbidity currents and withthe importanceof these transport processes changing through time. Thisconclusion is corroborated by studies on marine sediments collectedfrom the drifts by piston- and gravity coring and deep-sea drilling atSites 1095, 1096 and 1101. The recovered sediments comprise muddycontourites,fine-grained turbidites,muddyhemipelagites, and IRDwithonlyaminor contribution frompelagic components such asdiatoms andforaminifera, which were mainly deposited during interglacial periods(PudseyandCamerlenghi,1998;Barker et al.,1999; Pudsey, 2000, 2002a,b; Lucchi et al., 2002; Mörz and Wolf-Welling, 2002; Hepp et al., 2006;Lucchi and Rebesco, 2007).

Clay mineral analyses on the drift sediments revealed that themineral smectite, which mainly originates from volcanic rocks aroundthe South Shetland Islands, is enriched during interglacial periods,whereas the mineral chlorite, which is predominantly derived frombasic volcanic and metamorphic rocks in the Antarctic Peninsulahinterland, is enriched during glacial intervals (Pudsey, 2000; Lucchiet al., 2002; Hillenbrand and Ehrmann, 2002, 2005). The clay mineralinvestigations document a SW-ward transport of fine-grainedterrigenous particles predominantly during interglacial periods anda greater supply of detritus from the Antarctic Peninsula mainland(predominantly by turbidity currents) during glacial periods back toabout 9.4 Ma (Hillenbrand and Ehrmann, 2002, 2005). However, thesestudies do not suggest a bottom-current reversal, i.e. direct influenceof the ACC on drift deposition. The interpretation of the clay mineralfluctuations was also supported by geochemical studies on Pliocenesediments from Site 1095 (Hepp et al., 2006).

In general, the grain-size variations within the mud (i.e. silt plusclay) fraction of sediments from Drift 7 are minor. Pudsey andCamerlenghi (1998), Lucchi et al. (2002), Mörz and Wolf-Welling(2002), Pudsey (2002a,b) and Lucchi and Rebesco (2007) analysedstatistic grain-size parameters (such as median, sorting and skew-ness), which show that a low-energy hemipelagic environment withweak bottom currents prevailed throughout the last 9.4 Ma and thebottom-current speed remained fairly constant. The mud fractiondeposited on the NW-flanks of Drifts 3, 4, 4a, 5, 6 and 7 during inter-glacial periods of the late Quaternary was a little coarser than duringglacial periods (Pudsey and Camerlenghi, 1998; Ó Cofaigh et al., 2001;Lucchi et al., 2002; Pudsey, 2002a). Ó Cofaigh et al. (2001) attributedthis trend to a higher bottom-current velocity, which might point to aSW-ward shift of the Antarctic Polar Front (APF) (cf. Pudsey, 2000;Pudsey et al., 2002). However, the concentration of silt-sized diatomsin the interglacial sediments is much higher than in the glacialdeposits (e.g. Pudsey and Camerlenghi, 1998; Pudsey, 2000, 2002a;Lucchi et al., 2002), which we consider as the more likely explanationfor the interglacial coarsening of the drift sediments.

The upper Miocene to Pliocene sedimentary sequence at Site 1095lacks a pronounced glacial–interglacial cyclicity in diatom abundance(Barker et al., 1999). Its mud fraction is slightly coarser in laminatedintervals assumed to be of glacial origin (Pudsey, 2002a). The coarsergrain size in the laminated intervals of the late Miocene and Plioceneis probably caused by a higher input of silty detritus via turbiditycurrents (cf. Pudsey, 2002a). Clay mineral assemblages indicate thatsome of the laminated intervals might be interglacial contourites (Bartet al., 2007). The coarser grain size of those intervals may result fromhigher diatom contents or a slightly higher bottom-current speed.

3.2. Bottom-current direction and themagnetic fabric of the drift sediments

In a recent paper, Parés et al. (2007) determined the azimuth ofbottom-water flow in drift sediments, which were recovered from

Fig. 4. Colour-shaded relief bathymetric map of Drift 7 with locations of a field of NE–SW striking sediment waves (yellow line), deep-sea channels (red hatched lines) and ODP Leg178 Sites 1095 and 1096 (vertical exaggeration ×7.5, illumination fromnorth; modified fromRebesco et al., 2007). Reproducedwith the permission of the Geological Society (London).

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Drift 4 (Site 1101) and Drift 7 (Site 1095). The used reoriented data ofthe anisotropy of magnetic susceptibility (AMS) were unsuitable forrevealing the downstream (or upstream) direction or the strength ofthe palaeo-bottom current, but their azimuth documented sedimenttransport at both sites either towards NW or SE, which Parés et al.(2007) related to ACC flow. In fact, the current direction reconstructedfrom the AMS data disagrees with both NE-ward directed ACC flowand the SW-ward flow of the bottom current inferred from the otherstudies. However, a NW-ward downstream direction at Site 1095could be explained by turbidity current transport, because the distalflank of Drift 7 was significantly affected by turbidite deposition(Barker et al., 1999; Pudsey, 2002a; Mörz and Wolf-Welling, 2002;Hepp et al., 2006).

Alternatively, the magnetic fabric data could indicate the localrather than the general flow direction of the bottom current. Thecurrent meter measurements from the central part of Drift 7demonstrate that the modern bottom-water flow closely followsthe bathymetric contours (Fig. 3; Camerlenghi et al., 1997b; Rebescoet al., 2002; Giorgetti et al., 2003). Thus, its local downstreamdirection switches between W on the NE flank of the drift, SW at itsoceanward side, and SSE on its SW flank. Sites 1095 and 1101 arelocated at the NW-side of Drift 7 and Drift 4, respectively (Figs. 1, 4

and 5). Depending on the precise location of the sites in respect tothe drift-crest axis, we can expect local downstream flow towardsW,SW or SSE. The orientation of bathymetric contours at Site 1095suggests local current flow towards NW (Fig. 3). Moreover, high-resolution multibeam data from Drift 7 (Fig. 4) reveal that Site 1095is located at the southwestern edge of a field of NE–SW strikingsediment waves (Rebesco et al., 2007). Although the long axis of mudwaves in the deep sea and the prevailing bottom-current directionmay form angles of up to 40° (e.g. Flood, 1994), also the orientation ofthe sediment waves in the vicinity of Site 1095 points to localcontour-current flow towards NW. Bathymetric contours at Site 1101are orientated in a general NE–SW direction (Fig. 5). However, high-resolution multibeam data reveal local undulation of the contours atthe NW-side of Drift 4 (e.g. small ridge in Fig. 5). Therefore, the localdownstream direction of the bottom current at Site 1101 may beeither NW or SE.

We conclude that the magnetic fabric data of sediments indicatelocal bottom-water flow towards NW at Site 1095 and towards NWorSE at Site 1101. These directions are consistent with the general SW-ward setting of the contour current against the main flow direction ofthe ACC. The AMS data at both sites could only be linked to ACC flow, ifthey would clearly document a NE-ward setting current. This would

Fig. 5. High-resolution bathymetric map of Drifts 1, 2, 3, 3a and 4 with ODP Leg 178 Site 1101 (modified from Rebesco et al., 2002). The arrow at the NW-side of Drift 4 indicates asmall, local ridge near Site 1101. Reproduced with the permission of the Geological Society (London).

60 C.-D. Hillenbrand et al. / Marine Geology 255 (2008) 55–63

indeed point to a bottom-current reversal in respect to the present-day flow regime.

3.3. Were the sediment drifts bathed by the ACC during the past?

Pudsey (2000) and Pudsey et al. (2002) describe enrichment of(sand-sized) calcareous foraminifera in sediments deposited atshallow locations of the drifts during the late phase of Marine IsotopeStage (MIS) 5. The authors interpret these layers to indicate win-nowing by either the SW-ward flowing bottom current or the ACC.They prefer the first hypothesis, because the outflow of bottom-watermasses produced in the Weddell Sea intensified during lateQuaternary interglacial periods (e.g. Pudsey, 2002c; Diekmann et al.,2003). Pudsey et al. (2002) also attributed the thinning of all driftstowards NWand the relative thinness of Drifts 1, 2, and 3 to condenseddeposition and/or erosion induced by the ACC. The distal NW-flanks ofthe drifts are furthest away from the Antarctic Peninsula, the mainsource for the terrigenous detritus. Therefore, we can expect a thin-ning of the sediment drifts towards NW. However, we do not excludean impact of ACC flow on Drifts 1, 2 and 3, which lie far to the NE ofSites 1095, 1096 and 1101 (Fig. 1).

A hint for an influence of the ACC on deposition on the uppercontinental rise west of the Antarctic Peninsula comes from seismicdata collected froman area betweenDrifts 4 and 5 below3500mwaterdepth (Fig. 1; Hernández-Molina et al., 2004, 2006a). There, a fossilmounded sedimentary body (patch drift), which is located directly tothe NE of a small group of seamounts, was interpreted to indicate a NE-ward bottom current related to the ACC that affected the sedimenta-tion on the rise from the early Miocene until the middle or even lateMiocene (Hernández-Molina et al., 2006a). However, it is unclear, if thesuggested NE-ward bottom current influenced (i) all sediment drifts,(ii) the central parts of the drifts, or (iii) sedimentation on the driftsduring the time period spanned by the sedimentary sequences re-

covered from theODP Leg 178 sites. Adepositionalmodel for theDrift 7area, which is based on the geometry of seismic reflectors, indicatesthat the SW-setting bottom current weakened from 15 Ma to 5.3 Ma,but that a NE-ward setting current can be ruled out for the time after9.4 Ma (cf. Rebesco et al., 1997, 2002; Uenzelmann-Neben, 2006). Thisconclusion is confirmed by the clay mineral data from Sites 1095 and1096 (Hillenbrand and Ehrmann, 2002, 2005).

3.4. The ACC component of the drift sediments

Lucchi et al. (2002) observed minor contents of the clay mineralkaolinite in sediments deposited at the gentle sides of Drifts 4 and 7(oceanwards of Sites 1096 and 1101) during late Quaternary interglacialperiods, including the interglacial–glacial transitions. The authorsattributed the interglacial presence of kaolinite to either dust supplyfrom South America, which was favoured by Villa et al. (2003), or IRD-supply from the Amundsen Sea that is the closest Antarctic source forclay enriched in kaolinite (Hillenbrand et al., 2003). Today, the ACCdrives the surface waters above the drifts (Fig. 1). Icebergs that werecalved fromglaciers draining into theAmundsen Sea can drift eastwardswithin the ACC to the continental rise west of the Antarctic Peninsula.Thus, IRD-supply of clay enriched in kaolinite to the sediment driftsduring interglacial periods as suggested by Lucchi et al. (2002) would beconsistent with the modern ocean circulation.

In contrast,wedonot favour significant eolian supply of kaolinite tothe sediment drifts. During the present interglacial period dust blownto the Antarctic Peninsula is mainly derived from South Americansources (e.g. McConnell et al., 2007) that are dominated by smectiteand/or illite (e.g. Zárate, 2003; Gaiero et al., 2004). Holoceneaccumulation rates of terrigenous detritus (MARterr) on the continentalrise west of the Antarctic Peninsula range from at least 5 g/m2 yr(calculated fromdata in Pudsey, 2000; Lucchi et al., 2002) to 65 g/m2 yr(Hillenbrand et al., 2003) (cf. Ó Cofaigh et al., 2001). These values agree

61C.-D. Hillenbrand et al. / Marine Geology 255 (2008) 55–63

with a MARterr of 9.5 g/m2 yr in a sediment trap moored at 14 m abovethe seafloor on Drift 7 (Harland and Pudsey, 1999). However, themodern dust accumulation rate in an ice core from the northernAntarctic Peninsula is only 17 mg/m2 yr (McConnell et al., 2007), morethan two orders of magnitude lower. During glacial periods, dustsupply to Antarctica increased, but so did the importance of thekaolinite-poor South American sources (e.g. Delmonte et al., 2004;Gaiero et al., 2004).

Throughout the late Quaternary, IRD is usually a minor componentof the drift sediments (e.g. Pudsey and Camerlenghi, 1998; Pudsey,2000, Ó Cofaigh et al., 2001; Lucchi et al., 2002). If we assume that(i) the sand fraction (N63 µm) in surface sediments is mainly terrig-enous, (ii) its content is a proxy for IRD, and (iii) IRD is equallydistributed in the sand, silt and clay fractions, the resulting bulk IRDaccumulation rate (MARIRD) for the late Holocene would range fromca. 0.6 g/m2 yr (calculated from data in Pudsey and Camerlenghi,1998;Pudsey, 2000; Lucchi et al., 2002) to 6.5 g/m2 yr (Hillenbrand et al.,2003) and would account for just about 10–15% of the MARterr. Thisestimation demonstrates that MARIRD on the continental rise duringthe present interglacial period plays a subordinate role, but never-theless may influence the clay mineralogical composition of the driftsediments. A northward shift of the SB during glacial intervals wouldexplain the absence of kaolinite in the corresponding drift sediments(Lucchi et al., 2002).

Theoretically, the IRD component of the drift sediments mightreflect changes in the ACC flow on longer time scales. However,MARIRD and its spatial and temporal variability was additionallycontrolled by a number of regional factors, such as ice-sheet dynamics,sea-ice extent, surface water temperature and bottom-current win-nowing (e.g. Ó Cofaigh et al., 2001; Cowan, 2002; Cowan et al., 2008).Throughout at least the last 7 Ma the IRD concentration reached itsmaximum elsewhere on the continental rise during the transitionfrom glacial to interglacial periods and during interglacials (e.g.Pudsey and Camerlenghi, 1998; Pudsey, 2000; Ó Cofaigh et al., 2001;Pudsey, 2002a,b; Cowan, 2002; Lucchi et al., 2002; Hepp et al., 2006).In general, more IRD was deposited during the late Pliocene and earlyPleistocene than during the late Miocene (Site 1095) and lateQuaternary (Sites 1095, 1096, 1101) (e.g. Pudsey, 2002a,b; Cowanet al., 2008). The lithologies of dropstones (N2 mm) recovered fromthese sites can all be matched with the local Antarctic Peninsulageology (Hassler and Cowan, 2002; Pudsey, 2002a; Cowan et al.,2008). The absence of far-travelled clasts is not surprising consideringthe proximity of the drill sites to the shelf edge, which marks themaximum extent of grounded ice-sheet advance.

The MARIRD has been calculated for Site 1101 using the terrigenousfraction from 250 µm to 2 mm (Cowan, 2002; Cowan et al., 2008). IRDflux can be estimated for Sites 1095 and 1096 using the bulk sandfraction N63 µm (Wolf-Welling et al., 2002) as an IRD proxy. Use of thebulk sand fraction is complicated by the occurrence of sandyturbidites (in particular at Site 1095) and sand-sized foraminifera(Barker et al., 1999; Pudsey, 2002a,b; Wolf-Welling et al., 2002). AtSites 1095, 1096 and 1101 the fraction N63 µm exceeds 15% in only afew discrete layers (Pudsey, 2002a,b; Wolf-Welling et al., 2002),indicating that the deposition of IRD was small compared to thatof terrigenous mud supplied by contour and turbidity currents.Apparently, the deposition of IRD, which may have been linked topast variability of the ACC, played only a subordinate role for thesedimentation at the drifts during most of the time. Therefore, fine-grained IRD is unlikely to have affected the bottom-current signalarchived in the fraction b63 µm of the drift sediments (see alsoMcCave and Hall, 2006).

4. Conclusions

• The modern bottom current on the upper continental rise west ofthe Antarctic Peninsula is SW-setting, thereby following the bathy-

metric contours. The current is probably formed bymodifiedWSDWor modified LCDW derived from the Weddell Sea, whereas its originin an SPDW branch of the ACC is less likely.

• Clay mineral assemblages and grain-size data from the drift sedi-ments indicate that throughout the last ca. 9.4 Ma their depositionwas controlled by the SW-ward flowing contour current and NW-ward flowing turbidity currents. The depositional setting on thedrifts was characterised by a weak, non-erosive bottom current formost of the time.

• Seismic studies of Drift 7 give no evidence for a bottom-currentreversal during the last ca. 9.4 Ma (but do not contradict a possibleNE-ward bottom-current flow below 3500 m water depth betweenDrifts 4 and 5).

• The only ACC-derived component in the drift sediments may be thesmall fraction of IRD that was transported within the NE-wardflowing surface waters of the ACC. However, IRD contributed onlyvery little to the deposition of the drift sediments during the last9.4 Ma.

• Current directions inferred from the anisotropy of magnetic suscep-tibility (AMS) of the drift sediments disagree with both the generalSW-ward flow of the bottom current and themain ACC flow towardsNE. The AMS data from Sites 1095 and 1101 probably indicate localbottom-current flow that is unrelated to ACC flow. Considering thatthe bottom-current flow closely follows the bathymetric contours, alocal NW-ward or SE-ward downstream direction at both siteswould not contradict its general SW-ward flow.

• Past variations of the bottom-current speed may provide importantinformation about the dynamics of floating and grounded icemassesin the southern Weddell Sea, where the bottom-water mass has itsorigin, rather than about the variability of the ACC (cf. Pudsey, 2000;Hillenbrand and Ehrmann, 2002).

Acknowledgements

This research used data provided by the Ocean Drilling Program(ODP). The ODP is sponsored by the U.S. National Science Foundation(NSF) and participating countries under management of JointOceanographic Institutions (JOI), Inc. The work was supported bythe British Antarctic Survey (BAS) GRADES-QWAD project, the ItalianProgramma Nazionale di Ricerche in Antartide (PNRA), Spain'sComisión Interministerial de Ciencia y Tecnología (CYCIT) (ProjectsREN2001-2143/ANT & CGL2004-05646) and the Secretaría de Estadode Educación y Universidades (PR2006-0275). The authors acknowl-edge the captains, officers, crew, support staff and scientists, whoparticipated in the various cruises, and thank R. Larter (BAS), A.Naveira Garabato (NOC Southampton), D. Piper and two anonymousreviewers for their helpful comments and suggestions.

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