Miocene reversal of bottom water flow along the Pacific Margin of the Antarctic Peninsula:...

(2006) 93–116www.elsevier.com/locate/margeo

Marine Geology 228

Miocene reversal of bottom water flow along the Pacific Margin ofthe Antarctic Peninsula: Stratigraphic evidence from a contourite

sedimentary tail

F.J. Hernández-Molina a,⁎, R.D. Larter b, M. Rebesco c, A. Maldonado d

a Dpto. de Geociencias Marinas, Universidad de Vigo, 36200 Vigo, Spainb British Antarctic Survey (BAS), High Cross, Madingley Road, Cambridge CB3 0ET, UK

c Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/C, 34010 Sgonico, Trieste, Italyd Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC/University of Granada, Campus Fuentenueva, s/n 18002 Granada, Spain

Received 4 March 2005; received in revised form 15 December 2005; accepted 22 December 2005

Abstract

A Fossil Mounded Sedimentary Body (MB) has been identified in the sedimentary record on the central continental rise west ofAdelaide Island, on the Antarctic Peninsula Pacific margin. The growth patterns of the MB are defined through a detailed regionalstratigraphic analysis using multichannel seismic reflection profiles. The MB has an elongated NE trend. It overlaps and continuesto the NE of an extensive cluster of seamounts, and it developed between two non-depositional troughs. Nine seismic units havebeen identified: Unit 9 (the pre-MB stage), Unit 8 (MB growth stage), Units 7 and 6 (MB maintenance stage), Units 5 and 4(transitional stage), and Units 3, 2 and 1 (inactive stage). We interpret the MB as a patch drift plastered against the NE, lee side ofan obstacle, as a long Contourite Sedimentary Tail (CST), within a deep current that flowed northeastward. This segment of the riseis, however, affected at present by a SW-flowing branch of the Lower Circumpolar Deep Water (LCDW) from the Weddell Sea.The depositional patterns of the MB growth and maintenance stages, which are attributed an early Miocene age on the basis ofregional correlation of MCS profiles with DSDP Site 325 and ODP Leg 178 drill sites, provide the first evidence that bottomcurrents on the central continental rise flowed towards the NE at that time, probably as part of the Lower Circumpolar Deep Water(LCDW) of the Antarctic Circumpolar Current (ACC). We suggest that significant palaeocirculation and palaeoceanographicchanges occurred in this area, and probably more widely, during the middle Miocene or at the Miocene/Pliocene boundary.Although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of theAntarctic Peninsula's Pacific margin, they do indicate that the bottom current regime controlling the development of contouritedeposits may have changed over time and also that more than one water mass has probably affected their distribution.© 2006 Elsevier B.V. All rights reserved.

Keywords: Antarctic Peninsula Pacific margin; contourite deposits; seamounts; palaeoceanography; seismic stratigraphy; Miocene; Pliocene;Quaternary

⁎ Corresponding author.E-mail address: [email protected] (F.J. Hernández-Molina).

0025-3227/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2005.12.010

1. Introduction

Bottom contour currents in modern ocean basinsoften generate large sedimentary bodies (contouritedeposits or drifts), comparable in size to turbidite fans

mailto:[email protected]

http://dx.doi.org/10.1016/j.margeo.2005.12.010

Fig. 1. (A) Regional setting of the study area. The sea surface position of the Antarctic Circumpolar Current (ACC) fronts and present circulationpattern of bottom water masses is shown: (a) Circumpolar Deep Water (CDW) and (b) Lower Circumpolar Deep Water (LCDW) branch from theWeddell Sea. The Southern Boundary of the ACC (SB) dips northwestward, and LCDW from theWeddell Sea flows southwestward beneath it (SM =Seamount; HFZ = Hero Fracture Zone; bathymetric map from Rebesco et al., 1998). Grey-filled circles represent Ocean Drilling Program drill sites.(B) Sketch with the positions of seismic reflection profiles used, and the MB position on the continental rise of the margin. The main morphologicalprovinces are also marked.

94 F.J. Hernández-Molina et al. / Marine Geology 228 (2006) 93–116

95F.J. Hernández-Molina et al. / Marine Geology 228 (2006) 93–116

(Stow et al., 1986, 2002; Rebesco, 2005). Several driftclassification systems have recently been proposed,based mainly on morphologic, sedimentologic andseismic characteristics (McCave and Tucholke, 1986;Faugères and Stow, 1993; Faugères et al., 1999;Rebesco and Stow, 2001; Stow et al., 2002; Rebesco,2005). All drifts are related to the regional oceano-graphic conditions and the physiographic domainswhere they developed. Thus, it is possible to decode,from their morphologic, stratigraphic and sedimentarycharacteristics, the pathway and approximate flowvelocity of the water mass that was responsible fortheir development. This is particularly relevant whenburied contourite drifts and erosional discontinuities arefound in the sedimentary record of a basin, because it isthen possible to reconstruct it palaeoceanographicconditions.

We have recently completed a morphologic andstratigraphic interpretation of the region between 68°and 74°W and 65°–67°30′S (Fig. 1A), using multi-channel seismic profiles and multibeam echo sounderdata collected by several projects and research groups.In this area, on the central continental rise off AdelaideIsland, we found a large Fossil Mounded SedimentaryBody (MB) (Hernández-Molina et al., 2004). In thispaper we give a more complete description of thedevelopment of the MB, present new multibeam echosounder data that reveals additional seamounts withinthe MB area, and discuss the palaeoceanographicimplications of these observations.

2. Geological and oceanographic setting

There was continuous subduction at the Pacificmargin of the Antarctic Peninsula at least from earlyCretaceous time until the early Tertiary (Storey et al.,1996). During the Tertiary period, subduction stoppedalong most of the margin, as ridge-crest segments of theAntarctic–Phoenix spreading centre migrated into thetrench (Barker, 1982; Larter and Barker, 1991a). Ridge-crest segments arrived first at the southwestern part ofthe margin during the Palaeocene or Eocene, thenprogressively later towards the northeast. The study areais located between the Tula and Biscoe Fracture Zones,and is divided into two parts by the Adelaide FractureZone (Fig. 1A). These fracture zones formed at offsets inthe Antarctic–Phoenix ridge as it migrated towards aformer trench at the Antarctic Peninsula margin. Theridge segment between Tula and Adelaide FractureZones reached the trench at 19±0.8 Ma, and thesegment between Adelaide and Biscoe Fracture Zonesreached the trench at 16.7±0.7 Ma (Larter et al., 1997).

As the ridge migrated towards the trench, ocean floorformed on its NW flank was part of the Antarctic Plate,so features on this flank of the ridge never movedrelative to the Antarctic Peninsula after they wereformed (Larter and Barker, 1991a).

The continental shelf and slope on the Pacific marginof the Antarctic Peninsula have a thick Miocene toQuaternary sedimentary cover with progradational andaggradational stacking patterns. Since Late Miocenetimes, prograding sequences formed as a consequenceof sediment transport and deposition beneath ice sheetswhich were grounded to the continental shelf edgeduring glacial maxima (Larter and Barker, 1989; Bartand Anderson, 1995; Larter et al., 1997; Barker andCamerlenghi, 2002). The continental rise is underlain bya thick succession of clastic sediments which have beenthe subject of many studies, using data from Deep SeaDrilling Project (DSDP) Site 325 (Leg 35), multichannelseismic reflection profiles, and more recently, from Leg178 of the Ocean Drilling Program (ODP) (Fig. 1A). Onthe upper continental rise there are several largeasymmetric sedimentary mounds (drifts) generallyhaving steep SW flanks, gently dipping, smoother NEflanks, and long narrow crests, separated from the baseof the continental slope and from each other by turbiditycurrent channels (Rebesco et al., 1996, 1997, in press;Pudsey and Camerlenghi, 1998; Pudsey, 2000; Rebescoet al., 2002; Lucchi et al., 2002). Through seismicstratigraphic analysis, Rebesco et al. (1996, 1997, 2002)identified six depositional units on the continental riseand summarised the history of sedimentation in threemain stages (Table 1): (1) a pre-drift stage (units M6 andM5; 36–15 Ma) characterised mainly by subparallelreflectors representing a dominantly turbiditic sequence;(2) a drift-growth stage (units M4 and M3; 15–5 Ma),showing substantial variations in thickness as aconsequence of increased bottom current activity andrapid glacial sediment supply from the margin; and (3) adrift-maintenance stage (units M2 and M1; 5–5.3 Ma tothe present), characterised by preservation of driftmorphology, with a fairly uniform drape of youngersediments on the NE gentle side and more condensedsuccession over the SW steeper side. The upper threeunits were thought to be composed mainly of glaciallyderived sediments of Late Miocene to Pleistocene age(Rebesco et al., 1997).

Among the deep-water masses in the SouthernOcean, two components can be distinguished (Orsi etal., 1995, 1999; Naveira-Garabato et al., 2002a,b, 2003).

1) The voluminous Circumpolar DeepWater (CDW),which is internally sub-divided into Upper CircumpolarDeep Water and Lower Circumpolar Deep Water


(UCDWand LCDW) and flows continuously to the east(Fig. 2B) within the Antarctic Circumpolar Current(ACC). (2) The deepest water mass around the Antarcticcontinent, although it lacks a circumpolar distribution(Fig. 2A), is the Antarctic Bottom Water (AABW)which is generated by a combination of brine rejectionon the continental shelf and by super-cooling of watermasses under floating ice shelves. The main source ofAABW is the Weddell Sea, which contributes approx-imately 60% of the total AABW production.

AABW in the Weddell Sea is composed of theWeddell Sea Bottom Water (WSBW) and the WeddellSea Deep Water (WSDW) (Fig. 2B). Above the WSDWflows the Warm Deep Water (WDW), which is a branchof the LCDW (Naveira-Garabato et al., 2002a,b).

A relatively cold and fresh branch of the WSDWspills over the South Scotia Ridge, spreads westwardthrough Drake Passage, and fills the bottom of the SouthShetland Trench (Fig 2B; Sievers and Nowlin, 1984).After passing the Hero Fracture Zone it loses its identitythrough mixing with the overlying LCDW (Naveira-Garabato et al., 2002a, 2003). A branch of LCDWderived from water that has circulated in the WeddellGyre, also flows along the slope on the southern flank ofthe South Shetland Trench above the WSDW. Thiswater mass continues southwestward along the Antarc-tic Peninsula Pacific margin (Nowlin and Zenk, 1988;Orsi et al., 1999; Naveira-Garabato et al., 2002a), whereis active at around 3500 m depth and follows contoursaround large drifts (Figs. 1A and 2) on the central andupper rise (Rebesco et al., 2002; Giorgetti et al., 2003).This LCDW branch was first inferred on the basis ofphysical properties of the water mass, short-term currentmeasurements and migration patterns of sedimentarywaves (see summary in Tucholke and Houtz, 1976;

Fig. 2. Summary of regional oceanographic framework and water mass dyn(AABW) with different density values (from Orsi et al., 1999). (B) SchemaBellingshausen Sea and Scotia Sea overlaid on regional bathymetry (IsobathCurrent (ACC) fronts (summary using data from the following authors: HolWhitworth and Nowlin, 1987; Foldvik and Gammelrsød, 1988; Nowlin and Z1995, 1999; Fahrbach et al., 1995, 1998; Barber and Crane, 1995; Hoffmann eal., 1999; Gordon et al., 2001; Meredith et al., 2001; Naveira Garabato et al.SAF = Subantarctic Front; PF = Polar Front; SACCF = Southern ACC Fronreference points, in alphabetical order: AI = Adelaide Island; AlI = AlexandeBP = Bruce Passage; BS = Bransfield Strait; BeS = Bellingshausen Sea; CTFalkland Escarpment; FP = Falkland Passage; FPL = Falkland Plateau; FRHero Fracture Zone; JB = Jane Basin; MEB = Maurice Ewing Bank; NGP =North Scotia Ridge; OP = Orkney Passage; OR = Orcadas Ridge; PB = PowPI = Peter Island; PhP = Philip Passage; SG = South Georgia; SFZ = ShackPacific Basin; SRP = Shag Rocks Passage; SSFZ = South Sandwich Fracture Z= South Scotia Ridge; SST = South Sandwich Ridge; WAP = Weddell AbyAntarctic Circumpolar Current; CDW = Circumpolar Deep Water; SPDWWater; UCDW = Upper Circumpolar Deep Water; AABW = Antarctic BottWater; WSBW = Weddell Sea Bottom Water; WSDW = Weddell Sea Deep

Tucholke, 1977). More recently its existence has beenverified by direct current-meter measurements, whichindicated mean current speeds of ∼6 cm/s andmaximum speeds of b20 cm/s, with potential tempera-tures of 0.11–0.13 °C, between water depths of 3475and 3338 m (Camerlenghi et al., 1997; Giorgetti et al.,2003).

3. Methodology

The present study is based on the analysis ofmultichannel seismic reflection (MCS) profiles collect-ed by the British Antarctic Survey (BAS) and the IstitutoNazionale di Oceanografía e di Geofisica Sperimentale(OGS). However, this work is part of a more extensiveregional work (Hernández-Molina et al., in preparation)which also utilises seismic reflection data from theBrazilian Antarctic Program, Rice University of Texas,and Spain (Project ANT99-0817), as well as multibeamecho sounding data from various sources. Results ofODP Leg 178 (Barker et al., 1999) and DSDP Site 325(Hollister et al., 1976) were also incorporated into thestudy. Correlation of seismic units and discontinuitieswith these sites was essential in order to analyse thetiming of the processes responsible for the developmentof depositional units and to establish the chronology ofgrowth patterns on the continental rise. The age of theoceanic crust, based on interpretation of marinemagnetic anomalies (Larter and Barker, 1991a), wasused to constrain the age of the oldest sediments on therise. Magnetic anomaly ages were based on themagnetic reversal timescale of Cande and Kent (1995).

MCS data collected by BAS on RRS Discoverycruise 172 during the 1987/1988 austral summer wereacquired with an 800-m long, 32-channel streamer and a

amics. (A) Distribution of the flow pattern of Antarctic Bottom Watertic circulation patterns of the deep water masses in the Weddell Sea,s of 1500, 4000 and 6000 m). Also shown are Antarctic Circumpolarlister and Elder, 1969; Tucholke, 1977; Whitworth et al., 1982, 1994;enk, 1988; Domack et al., 1992; Locarnini et al., 1993; Orsi et al., 1993,t al., 1996; Camerlenghi et al., 1997; Pudsey and Howe, 1998; Arhan et, 2002a,b, 2003; Hillenbrand et al., 2003). Legends of the ACC fronts:t; SB = Southern Boundary of the ACC. Legend of the physiographicr Island; BAP = Bellingshausen Abyssal Plain; BB = Burdwood Bank;= Chile Trench; DP = Discovery Passage; FI = Falkland Island; FE == Falkland Ridge; GB = Georgia Basin; GP = Georgia Passage; HFZ =Northeast Georgia Passage; NGR = Northeast Georgia Ridge; NSR =ell Basin; PCM = Pacific Continental Margin of Antarctic Peninsula;leton Fracture Zone; SOI = South Orkney Island; SPB = Southeasternone; SShI = South Shetland Island; SSI = South Sandwich Island; SSRssal Plain; YT = Yaghan Basin. Legend of the water masses: ACC == Southeast Pacific Deep Water; LCDW = Lower Circumpolar Deepom Water; WSC = Weddell Scotia Confluence; WDW = Warm DeepWater.


4-airgun source with a total volume of 15.8 l. Theanalogue signals from pairs of channels in the streamerwere summed before being digitized, so the recordeddata effectively represent 16 hydrophone groups, each50 m long. MCS data collected by OGS on R/V OGS-Explora during the austral summers of 1989/1990 and1991/1992 were acquired with a 3000-m long, 120-channel hydrophone streamer towed at a depth of 8 m.During the 1989/1990 season, the source consisted of 36airguns with a total volume of 45.2 l., whereas in the1991/1992 season the source consisted of 40 airgunswith a total volume of 72 1. On all three cruises, shotswere fired every 50 m, giving 8-fold coverage on theBAS data, and 30-fold coverage on the OGS data. Thedata sampling interval on all three cruises was 4 ms. Thedata were processed with a standard processingsequence, but the data from line BAS878-19 shown inFigs 3 and 4 have been f–k migrated.

For the purposes of the present paper, we havefocused on interpretation of lines BAS878-19 andBAS878-20 from BAS (hereinafter referred to as linesBAS19 and BAS20), and OGS lines IT89-048 andIT92-110 (hereinafter referred to as lines IT48 andIT110) (Fig. 1B). The 3D definition of the MB is basedon these seismic lines. These are the only presentlyexisting lines available to define the body. Althoughthere is no tie line along the length of the body,reflections on all lines can be traced to DSDP Site 325(via a tie to line IT89049, in the case of line IT48), andso the stratigraphic interpretations are tied throughcorrelation to this site. In addition, we present newmultibeam echo sounding data over the central conti-nental rise collected using the Kongsberg SimradEM120 system (12 kHz, 1°×1° beams) on RRSJames Clark Ross during cruise JR104 (January–February 2004). These data were processed using theSimrad ‘Neptune’ software package.

4. The fossil mounded sedimentary body (MB):morphosedimentary features and seismicstratigraphic analysis

We have identified a MB on the central continentalrise offshore from Adelaide Island between 65°S and65°30′S and 71°45′W–72°45′W (Fig. 1B). The MB(Seismic Units 8 to 6 in Figs. 3 and 4) is buried 300 to150 m below the sea floor, which is at a water depth of3600 m, and at which it has no morphologicalexpression. It has a mounded, elongated shape, over-lapping and continuing to the NE of an extensive clusterof seamounts. Buried seamounts are identified on lineIT110 and line BAS20, suggesting a cluster with a

general E–W trend (Figs. 1B and 3). In addition, therecently collected multibeam echo sounding data revealtwo seamounts that rise above the surrounding sea floorwithin the northern and central part of the MB, betweenlines BAS19 and IT48 (Fig. 5).

The seismic profiles show that during its develop-ment the MB was bounded by two marginal troughs.Towards the northeast the MB becomes less pronouncedand has a more subdued external shape. The MB is atleast 48 km long and up to 25 km wide (Fig. 1B) whereit overlaps the seamount cluster (line BAS19), but it isnarrower to the northeast (≤20 km on line IT48). Thethickness is variable, both across and along the body.Below we describe the development of the MB as it isobserved mainly on two MCS profiles: line BAS19 inthe area where the MB overlaps the seamount cluster,and line IT48 to the NE of the seamounts. In theoverlapping area the MB is around 850 ms (twt) thick onaverage, but it decreases in thickness NE of theseamounts to around 400 ms (twt) on average (Fig. 3).

4.1. Oceanic crustal basement and seamounts

The reflection interpreted as the top of the oceanicbasement is located at around 6 s (twt) on average. Afeature of the top of the basement on line BAS19 is ashallow depression below the MB and correspondingclosely to its extent (Figs. 3B and 4). The depression isasymmetrical, with a deeper area in the SE, where thetop of the basement reaches a maximum two-way time(twt) of 6240 ms (Figs. 3B and 4).

We infer the general distribution of the cluster ofseamounts based on lines BAS20 and IT110, togetherwith the new multibeam echo sounding data (Figs. 1B,3A and 5), but in detail the three-dimensionalmorphology of the cluster is not well defined. Online IT110, close to the MB, two buried seamounts canbe seen (Fig. 3B). The peak of the largest one is 900ms (twt) above the average level of the oceanicbasement, and this seamount is at least 12 km wide atits base. A smaller seamount is located furthernorthwestward along the same seismic line. Its peakis 200 ms (twt) above the nearby oceanic basement,and it is about 5 km wide at its base. It is likely thatthis profile does not cross their true summits, andrepresents a partial view of larger seamount structures.If we consider the total area occupied by these twoburied seamounts at their base, it is at least 20 kmwide, a dimension comparable with the width of theMB (up to 25 km).

The seamounts observed in the multibeam datathat emerge above the sea floor are located within the


northern and central part of the MB (Fig. 5). The peaksof these seamounts are at around 2800 m water depth,800 m above the average depth of the surrounding seafloor. The total area occupied by these seamounts on thesea floor is about 5 km wide and 8.9 km long, but theyundoubtedly cover a greater area at the top of oceanicbasement (Fig. 5).

4.2. Seismic units

Nine seismic units (SU), characterizing five evolu-tionary stages of the MB stratigraphic evolution, havebeen identified in the sedimentary succession aboveoceanic basement (Figs. 3, 4 and Table 1).

4.2.1. Seismic unit 9 (the pre-MB stage)SU9 lies directly above the basement, and is

identified within the area of the seamount cluster onlines IT110 and BAS19 (Fig. 3A and B). Within thedepression in the top of the basement below the MB online BAS19, this unit is well developed (Fig. 3B), andhere it reaches a thickness of up to 150 ms (twt).Reflectors within this unit have high amplitudes, goodlateral continuity, an aggradational configuration, andonlap onto the basement. This unit cannot be resolvedunambiguously on line IT48, which is outside theseamount area (Fig. 3C).

4.2.2. Seismic unit 8 (MB growth stage)This unit characterises the formation and growth

stage of the MB, with marginal troughs present on bothsides of it on line BAS19, which crosses the MBbetween the buried seamounts on line IT110 and theemergent seamounts observed in the multibeam data. Inthe MB, SU8 is about 400 ms (twt) thick, on average,within the seamount area (line BAS19, Fig. 3B) andapproximately 150–200 ms (twt) thick to the NE of theseamounts (line IT48). The unit has a slightlyasymmetric cross-section on line BAS19 being a littlethinner on the southeastern side of the MB (350 ms,twt) than on its northwestern side (400–450 ms, twt).The unit has a mounded external shape and anaggradational stacking pattern in the central part ofthe mound. Within this unit, seismic facies in the MBchange from a high acoustic response with reflectorshaving high lateral continuity at the base to slightlyweaker acoustic response with less continuous reflec-tors at the top. In general, a more reflective acousticresponse is presented on the southeast side of thedeposits. The marginal trough on the SE side of theMB is slightly more developed than the one to the NW(Figs. 3B and 4).

4.2.3. Seismic Units 7 and 6 (MB maintenance stage)Both SU7 and SU6 have similar mounded morphol-

ogy to SU8, with marginal troughs on both sides of theMB. Once again, the marginal trough on the SE side isslightly more developed than the one to the NW, and itsseismic facies are characterised by a high acousticresponse, which laterally changes to a weaker acousticresponse (Fig. 4C). This particular facies is observed onboth sides of the MB; however, it is more developed onthe SE side than on the NW side. A small furrow can bedetected in the central part of the MB, particularly in theupper discontinuities of these two units (Figs 3 and 4B).

SU7 has an average thickness of 100 ms (twt) withinthe seamount area, but gradually thickens from NW toSE over the MB (line BAS19, Fig. 3B). The unit isapproximately 50–60 ms (twt) thick to the NE of theseamounts (line IT48). Seismic facies of this unit arecharacterised by a weak acoustic response with anaggradational configuration.

SU6 has an asymmetric cross-section on line BAS19(Fig. 3B) with a lesser thickness on the southeastern side(350 ms, twt) and a greater thickness on the northwest-ern side (400–450 ms twt). The thickness of this unitdecreases to approximately 100–150 ms (twt) to the NEof the seamounts (line IT48). It has a draping shape withan aggradational stacking pattern and a weak-to-transparent acoustic response.

4.2.4. SU5 and SU4 (transitional stage)SU5 is observed regionally over the continental rise,

and at Site 325 has an average thickness of ∼100 ms(twt). However, in the MB area, this unit is not presenton line BAS19 because it is truncated by thediscontinuity at the base of SU4 (Fig. 4A). The unit ispresent over part of the MB to the NE of the seamounts(line IT48, Fig. 3), with a thickness between 50 and 150ms (twt). Data in that area are inadequate to determinethe morphology and seismic facies of this unit.However, SU5 has an aggradational stacking patternwith a weak-to-high acoustic response in other areasaway from the MB.

SU4 represents an important change in the deposi-tional style on the margin. Interpretation of seismicreflection data over the wider region shows that its lowerboundary represents the most prominent unconformityin the entire study area. Associated with this surface, thedevelopment of the marginal troughs is enhanced. Atthis stage, the trough to the NW of the MB was clearlylarger than the one to its SE (Figs. 3B and 4). Thethickness of the unit decreases from ∼300 ms (twt) onthe SE margin to ∼200 ms (twt) on the NW margin ofthe MB (line BAS19, Fig. 3B) within the seamount area,

Fig. 3. Multichannel seismic profiles across the MB: (A) Seamounts area (IT110); (B) proximal area of the Sedimentary Tail (BAS19, f–k migrateddata) and (C) distal area of the Sedimentary Tail (IT48). See position of the lines in Fig. 1B.


Fig. 4. (A) and (B) F–k migrated multichannel seismic profile (BAS19) with a detailed stratigraphic interpretation of the MB and adjacent areas and its correlation with DSDP Site 325. Note the mounded morphology and the related marginal troughs on both sides, and the small drift deposits that developed either side ofthe MB in Seismic Unit 4. Also, note a small troughs in the upper discontinuities of Seismic Units 7 and 6 in the middle of the MB (explanation on the text). Unit 5 is in shaded dark grey to emphasize how it is truncated by the basal discontinuity of Unit 4. See position of the line in Fig. 1B. (C) Detail of the sei smic faciesof the marginal trough on the SE side of the MB. High-amplitude reflections are interpreted as representing coarse lags in the marginal troughs. Location of data panel shown in A.

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F.J. Hernández-Molina et al. / Marine Geology 228 (2006) 93–116 105

Fig. 5. Swath bathymetry data showing seamounts that rise 800 m above the surrounding sea floor located within the northern and central part othe MB, between the positions of the seismic lines BAS19 and IT48. Multibeam data are displayed with shaded-relief illumination from thenorth.

t

l

,

f

and from ∼250 ms (twt) on the SE margin to ∼200 ms(twt) on the NWmargin to the NE of the seamounts (lineIT48). Seismic Unit 4 exhibits high-to-very highacoustic response with reflections with high amplitudeand good lateral continuity. This unit has an aggrada-tional stacking pattern over the MB, but farther NW, amounded and elongated sedimentary deposit progradingtowards the SE is observed on lines BAS19, IT110 andIT48, separated from the MB by the marginal trough(Fig. 4A).

4.2.5. Seismic Units 3, 2 and 1 (inactive stage)The lower boundary of SU3 represents an important

unconformity with marginal troughs on both sides of theMB developed on top of SU4, although the trough onthe NWmargin is largest. Within the seamount area, thisunit is absent on top of the MB within the proximal areas(line BAS19), being restricted to its flanks with anaggradational, onlapping stacking pattern (Figs. 3Band 4). On the NW flank of the MB, this unit reaches amaximum thickness of 260 ms (twt), and has amaximum thickness of 300 ms (twt) on the SE margin(Fig. 4). To the NE of the seamounts, this unit is presentover the MB with an aggradational stacking pattern andan average thickness of 100–150 ms (twt). SU3exhibits high acoustic response and contains reflectionswith high lateral continuity. A mounded elongated drift

deposit is present to the SE of the MB, i.e. on itsopposite side with respect to the sedimentary mounddeveloped within SU4 (Fig. 4). This mound, which isseparated from the MB by a marginal trough, progradesnorthwestward and evolves laterally southeastward to asheeted drift with an aggradational stacking pattern(Fig. 4), and closely spaced small troughs can beobserved in the transition area between the two kinds ofdeposit. In contrast, no depositional mounds are presenwithin SU3 on the NW margin of the MB, where theseismic succession shows a more regular aggradationastacking pattern (Fig. 4).

Both SU2 and SU1 contain the uppermost deposits ofthe sedimentary record. The upper boundary of SU1 isthe present seafloor and they are bounded by anunconformity at the base of SU2. The average combinedthickness of SU1 and SU2 is approximately 150–200 ms(twt) within the seamount area (line BAS19), andapproximately 150 ms (twt) to the NE of the seamounts(line IT48) (Fig. 3B and C). Detailed description of theseunits is outside of the scope of the present paper.

5. Chronostratigraphic constraints

The age of the seismic units can be assessed bycorrelation with DSDP Site 325 (Hollister et al., 1976)through which lines BAS19 and IT110 both pass

F.J. Hernández-Molina et al. / Marine Geology 228 (2006) 93–116106

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SsMCmth4minMcsS

pproximately 60 km NWof the MB (Figs. 1B and 4A).eflections on line 48 can also be traced to Site 325, viatersecting line IT89049 (Fig. 1B). In addition, aegional stratigraphic study of the continental marginff Adelaide Island has been carried out (Hernández-olina et al., in preparation), taking into considerationhronostratigraphic constraints from other drill sites ine region (ODP sites 1095–1097) (Barker et al., 1999),ombined regional seismic datasets, and the stratigraphicterpretations proposed by previous authors (Larter andarker, 1991b; Larter and Cunningham, 1993; McGin-is and Hayes, 1995; Rebesco et al., 1996, 1997;cGinnis et al., 1997; Barker et al., 1999; Rebesco et al.,002; Barker and Camerlenghi, 2002). Table 1 sum-arizes the age constraints on the seismic unitsonstituting the MB.SU9 (pre-MB stage) thins seaward and is too thin to

e resolved on line BAS19 where it passes through Site25 (Fig. 4). At the drill site, the age of the oceanicasement is late Oligocene (anomaly 6Cn.3n: Larter andarker, 1991a), 24 Ma on the magnetic reversal timecale of Cande and Kent (1995). The MB lies parallel toe trend of the magnetic anomalies, between anomalyAAn (21.8 Ma) and the young edge of anomaly 6Bn22.6 Ma). Therefore, we can be certain that theediments in SU9 above the igneous basement areounger than 22.6 Ma. Excluding variations in thicknesscross the MB, SU8 gradually increases in thicknesswards the margin, which suggests that most of it waseposited after the Antarctic−Phoenix ridge segment inis spreading corridor reached the margin, i.e. no earlieran 17.4 Ma. Consequently, most of SU8 must haveeen deposited during the latter part of the Earlyiocene. Therefore, the thin sediments in SU9 representery slow accumulation over an interval of severalillion years after the volcanic basement was formed,uring which time sediment supply from the margin waslocked by the intervening spreading ridge.Biostratigraphic constraints on sedimentation rates at

ite 325 indicate that it contains a hiatus or condensedequence spanning most of the Middle Miocene (b6 m/a between 15 and 8 Ma; Hollister et al., 1976).orrelation along line BAS19 to Site 325 indicates thatost of SU6 in the region of the MB must be older thane Mid-Miocene condensed sequence at Site 325 (Fig.), i.e. N15 Ma. Therefore, SU8, SU7 and SU6 (MBaintenance stage) were deposited rapidly during anterval of b2.4 Ma in the latter part of the earlyiocene (Fig. 4, Table 1). The lower boundary of SU5orrelates with a reflection located 460 ms (twt) belowea floor (bsf) on line BAS19 where it passes throughite 325 (Fig. 4; Larter and Barker, 1991b). There is

little doubt that this reflection is from the same boundaryas the reflection picked at 475 ms (twt) bsf on theoriginal single-channel site survey data for Site 325(Hollister et al., 1976). The 15 ms difference in thepicked two-way times represents only about half of thedominant wavelength of the data on line BAS19, andprobably results from a combination of differences inseismic wave shape between the two datasets andinterpreters picking different phases of the waveform.This reflection was originally interpreted as corres-ponding to a level below the Middle Miocene hiatus orcondensed sequence (Tucholke and Houtz, 1976:Hollister et al., 1976), even though this implied a ratherhigh average interval velocity below the sea floor of2.25 km/s. However, it is associated with an unconfor-mity that can be traced all across the continental rise online BAS19, and on this basis Larter and Barker (1991b)re-interpreted it as corresponding to the MiddleMiocene hiatus or condensed interval at Site 325. Thehiatus or condensed interval has been attributed to areduction in terrigenous supply as a result of marginuplift following interaction of the ridge-crest with themargin (Larter and Barker, 1991b). Therefore, SU5 isMiddle–Late Miocene in age (Table 1). However,beneath the MB this unit is either represented by avery condensed sequence or it is missing, with S4directly overlying S6.

We correlate the prominent erosional discontinuity atthe base of SU4 (transitional stage) to a seismicreflection identified at 0.370 s (twt) below the seafloor(bsf) at Site 325 (Fig. 4A). We consider the upperdiscontinuity of SU4 to correlate to Horizon R, definedpreviously by Tucholke and Houtz (1976), which wehave traced to 0.225 s (twt) at Site 325. This correlationimplies that SU4 has an Early Pliocene age (Table 1).The youngest units are SU3, SU2 and SU1 (inactivestage); SU3 developed from the Early Pliocene to theLate Pliocene, and SU2 and SU1 from the Late Plioceneto the Recent (Table 1).

Seismic profile intersections allow us (Table 1) tocorrelate our SU with the units defined by previousauthors on the continental rise (Larter and Barker,1991b; Rebesco et al., 1997, 2002), and with the resultsof the Ocean Drilling Program Leg 178 (Barker et al.,1999; Volpi et al., 2001; Barker and Camerlenghi,2002). SU9, SU8, SU7 and SU6 correlate with the pre-drift stage of the upper rise drifts of Rebesco et al.(1997, 2002). This implies that growth and maintenancestages of MB developed before the upper rise drifts. SU5 correlates with M3 sequence of these authors, coevalwith the drift-growth stage of the upper rise drifts. Weinterpret the lower boundary of S4 as correlating with


,

,,,,

,

t

;

t

,

,

the base of the prograding shelf sequences, previouslyreferred to as the “base of glacial margin sequences”(Larter et al., 1997). SU4 represents most of the MBtransitional stage and the period when the mostsignificant change in the depositional style of themargin occurred, after its transformation from an activeto a passive margin in the Early Miocene (Hernández-Molina et al., 2003, in press), and it correlates with thebase of the M2 sequence of Rebesco et al. (1997, 2002).This change in the depositional style at the end of theMiocene–Early Pliocene has been associated with thedevelopment of the “glacial margin sequences” on theshelf by Larter et al. (1997), which they interpreted asmarking the onset of frequent advances of grounded iceto the shelf break. At this time there was a change togreater influence of down-slope sedimentary processeswhich interacted with the along-slope sedimentaryprocesses within a palaeoceanographic setting similarto the present day (Hernández-Molina et al., 2003, inpress). Our correlation implies that the MB transitionalstage is synchronous with the onset of development ofglacial margin sequences on the shelf. SU3, SU2 andSU1 correlate with the upper part of the M2 and M1Sequences of the sediment drifts of the upper rise(Rebesco et al., 1997, 2002), representing the drift-maintenance stage of the upper rise drifts.

6. Genesis of the MB and palaeoceanographicimplications

6.1. Effect of seamounts on an impinging flow

Knowledge of how seamounts and seamount chainsinteract with ocean circulation is important from ageological point of view. Akin to island mass effects(Heywood et al., 1990), seamounts generate seamounteffects, with important effects on oceanographic pro-cesses (Roden, 1987), marine biota (Rogers, 1994),sedimentation and erosion rates (Davies and Laughton,1972; Roberts et al., 1974), and hence palaeoceano-graphic interpretations (Roden, 1987). The streamlinedistortion around obstacles is relevant to sedimentdistribution around seamounts (Fig. 6).

The hydrodynamic processes resulting from interac-tion between ocean currents and a seamount arecomplex, but two end members may be considered inwhich either advection processes or vorticity interactiondominate (Fig. 6A and B):

1. If vorticity interaction processes are dominant, theimpinging flow generates a pair of oppositely rotatingeddies (double vortex generation) on the flanks of theseamount, with flow intensification of the left-hand side

of the obstacle when looking downstream (in thenorthern hemisphere). Both eddies interact with eachother (in time and space) and with the mean flow in acomplicated manner to produce many transient andtrapped features (Roden, 1987, 1991; Zhang and Boyer1991). Seamounts also condition the generation ofinternal waves downcurrent (Roden, 1987, 1991).

2. If the mean velocity of the flow is lower, advectionprocesses may dominate. In this case, flow local to theseamount is clockwise around it (in the northernhemisphere), and an anticyclonic eddy-column-likeupwelling cone (Taylor column) appears near its top(Taylor, 1917; Gould et al., 1981; Eriksen, 1991; Roden1991; Ou, 1991; Zhang and Boyer, 1991; Bograd et al.1997), although when the flow rate oscillates over timeeddies are periodically shed from the seamount (Roden1991). Water mass stratification, in general, inhibitsvertical motion and the vertical extent of topographi-cally generated disturbances. Eddies atop seamountsthen become bottom trapped, and if the stratification isvery strong, a surface cyclonic eddy may appear abovethe deep anticyclonic eddy (Roden, 1991).

Where a bottom current encounters major solidobstacles, such as seamounts, its flow is constricted andaccelerated, and thereby its competence is increasedwhich may result in erosion in front of the obstacles(Fig. 6C).

Many seamounts have marginal troughs (scours ormoats), which develop preferentially one side of theseamount relative to the flow direction depending onwhich hemisphere the seamount is in (Fig. 6A andC), with accelerated flow to the left of the seamoun(looking downstream) in the northern hemisphere anddecelerated flow to the right (Roberts et al., 1974Gould et al., 1981), resulting in both locally fasterand slower accumulation than in areas distant fromobstacles.

Downstream from an obstacle, the two separate flowsdecelerate and develop small turbulent eddies, whichtend to return back towards the obstacle. The positiverelief of obstacles also causes upwelling and down-welling eddies, internal waves, and transverse turbulenrolls of the shear layer in the flow over them (Fig. 6Aand B), and many of these processes could occur in adeep water mass (Roden, 1987). The net result of theseprocesses can be the generation of a shadow zonedownstream of large obstacles or groups of obstacleswhere turbulence intensity slowly diminishes and thecurrents are slower relative to the marginal troughs, asproposed by Allen (1982, 1994) and Zhang and Boyer(1991). In this shadow zone behind an obstacle adepositional tail may develop (Davies and Laughton


1e

6

Mwsssas

hcgrinMsCUmndaMhsfsinoinas(b(

cbMrp(Mncitp

972; Masson et al., 2003), with a mounded andlongated external morphology (Fig. 6A and C).

.2. MB depositional model

The morphology and seismic characteristics of theB enable us to infer the depositional processes thatere responsible for the development of these depo-its. We interpret the MB as a mounded, elongatededimentary body produced by the redistribution ofediments by bottom currents that were divertedround and disturbed by the extensive cluster ofeamounts.We interpret the marginal troughs flanking the MB as

aving been produced by separate, turbulent, fasterores on both sides of the seamounts (Fig. 6A),enerating a pair of oppositely rotating eddies, aseported previously around seamounts (Roden, 1987),fluencing deposits as described by Allen (1982, 1994).arginal troughs are expected on the right side of

eamounts in the southern hemisphere (McCave andarter, 1997), so that during the deposition of Seismicnits 8, 7 and 6, the greater development of thearginal trough on the SE margin of the MB suggestsortheastward-directed main flow on this margin. Theevelopment of a down-flow shadow zone, as discussedbove explains the northeastward continuation of theB to line IT48. Gaps between seamounts might alsoave accelerated flow, resulting in the scouring ofediments and their redeposition downstream as thelow decelerated: this might explain some details of thetratigraphic patterns we observe, such as small troughsthe central part of the MB, in the upper discontinuitiesf SU8, SU7 and SU6 (Figs. 3 and 4). Therefore, weterpret the MB, as a patch drift, plastered between andgainst the NE (lee) side of the extensive cluster ofeamounts, as a large Contourite Sedimentary TailCST) deposited in the shadow zone of the seamountsy the action of a deep current flowing northeastwardFig. 6A).The MB depositional model was developed by

onsidering the likely effect of the extensive cluster ofuried seamounts on an impinging northeastward flow.ore recently, new multibeam echo sounding data have

evealed additional, larger seamounts within the centralart of the MB, northeast of where it is most developedon line BAS19). We consider the interpretation that theB developed in a shadow zone is still correct, but it isow clear that this zone was partly within the widerluster or seamounts rather than entirely downstream of. The position of the newly discovered seamountsrobably explains why the two main current cores that

formed the marginal troughs remained so widelyseparated for such a long distance downstream of theburied seamounts.

6.3. Stratigraphic considerations and palaeoceano-graphic implications

SU9 is resolved only in the depressions of thebasement close to the seamounts (Figs. 3 and 4). Thisunit is thin and sheeted, filling the irregularities of thebasement and without a mounded morphology. We referto this unit, developed during the Early Miocene, as thepre-MB stage, and interpret it as being composed mainlyof distal turbidites that accumulated slowly before thesegment of the Antarctic–Phoenix Ridge between theAdelaide and Biscoe fracture zones reached the margin.

SU8 (MB growth stage), and SU7 and SU6 (MBmaintenance stage) were deposited during the latter partof the Early Miocene. The observation that the marginaltrough on the SE side of the MB was slightly moredeveloped than the one on its NW side during thedeposition of these units suggests that there wasinteraction of a northeast-flowing contour current withthe extensive cluster of seamounts on the centralcontinental rise at this time. The position and orientationof the MB in relation to the seamount cluster is alsoconsistent with flow in that direction having concen-trated deposition in shadow zones between anddownstream of seamounts. Correlations with Site 325indicate that SU8, SU7, and SU6 are older than the mainphase of development of sediment drifts on the nearbyupper continental rise (M4 and M3 Units), the driftgrowth stage of Rebesco et al. (1997, 2002). The growthof the MB during deposition of SU 8 suggests avigorous bottom current regime on the central conti-nental rise in the latter part of the Early Miocene.Previously, the earliest indications of strong bottomcurrent activity on the continental rise west of theAntarctic Peninsula have been observed at a stratigraph-ic level equivalent to SU5, corresponding to the start ofthe Middle Miocene. The characteristics of SU7 andSU6 suggest a similar pattern of bottom current flowwas also active during their deposition. However, incontrast to SU8, these units show less of an increase inthickness over the MB and few internal variations inseismic facies, suggesting more sluggish currents. Theseismic facies of these units suggest fine- to very fine-grained sediments (silts and clays) similar to those coredin the nearby Middle Miocene–Recent sediment drifts(Barker et al., 1999; Pudsey, 2000; Lucchi et al., 2002).We envisage that the fine-grained components ofsediments supplied by gravity flows from the adjacent


Fig. 6. (A) Sketch with the main hydrodynamic features related to an incoming flow with a seamount (summary using data from the followingauthors: Taylor, 1917; Davies and Laughton, 1972; Roberts et al., 1974; Owens and Hogg, 1980; Gould et al., 1981; Roden, 1987, 1991; Saunders1988; Eriksen, 1991; Ou, 1991; Zhang and Boyer, 1991; Rogers, 1994; Bograd et al., 1997; McCave and Carter, 1997; Wright, 2001; Masson et al.2003). (B) Sketches of streamlines for vortex shedding regime in oscillatory free stream impinging on two obstacles such as seamounts (from Zhangand Boyer, 1991): 1.—Relatively smooth streamline field with wave-like undulations in the near wake, 2.—quite three-dimensional motion in thenear wake with relatively stagnant region extending far downstream; and 3.—in the absence of a strong advective current, interaction of CorioliForce with obstacles produces a chain of closed vortices. (C) Formation of marginal troughs (moats) and sedimentary tails when bottom currentencounter major obstacles, such as a seamount (from Davies and Laughton, 1972).

,,

ss

F.J.Hernández-M

olinaet

al./Marine

Geology

228(2006)

93–116110

Table 1Chronostratigraphic framework for the seismic units of the MB

The units and discontinuities have been laterally correlated with the results from DSDP Site 325. A correlation with the regional units of other authors and ODP sites is also presented.


,,

t

,

,tt

,

t

,

continental margin were entrained and transportednortheastward in a nepheloid layer by a persistentambient contour current. Presently, however, LCDWfrom the Weddell Sea flows over this area in theopposite direction, and consequently we suggest that amajor palaeoceanographic change has occurred in thearea at some time since the Early Miocene.

SU5 is not present in the MB area because it istruncated by the basal discontinuity of SU4. Conse-quently, the palaeoceanographic change could havetaken place during deposition of SU5. However, on thecontinental rise west of the Antarctic Peninsula, the baseof SU4 (transitional stage) may hold the key tounderstanding the palaeoceanographic change that weinfer, because it represents an important change in thedepositional style on the continental rise. The prominenterosion surface at the base of SU4 is indicative of moreenergetic bottom current flow in comparison with theearlier SU5, and the more developed marginal trough tothe NWof the MB at this time suggests it was formed bya strong core of a current flowing southwestwards, i.e. inthe opposite direction to the current inferred to havebeen associated with the growth and maintenance stages(SU8–SU6) of the MB. In this new oceanographicscenario the seamounts located within the northern andcentral part of the MB would have affected the currentflowing southwestwards in the opposite sense to the waythe seamount cluster to the SW affected the northeast-ward flowing current during the deposition of SU8, SU7and SU6. Hence a new aggrading, mounded unitdeveloped over the central part of the MB, to the SWor the large seamounts observed on the multibeam echosounding data. Based on the chronology of the seismicunits, the change in bottom current regime took placeduring the interval represented by the SU5 or SU5/SU4boundary over the MB, but presently available ageconstraints leave a large degree of uncertainty regardingthe precise time at which the change in bottom currentflow occurred. The change could have taken place at anytime between the beginning of the Middle Miocene andthe end of the Miocene (i.e. 15–5 Ma). The prominenterosional surface at the base of SU4, the high-to-veryhigh acoustic response, and the high lateral continuity ofreflections within SU4 indicate a more energetic deepwater regime, compared with the previous units SU7–SU5.

This segment of the Pacific margin continental rise ispresently affected by a LCDW branch flowing south-westward from the Weddell Sea. Farther north lies theaxis of the east-flowing ACC (Nowlin and Zenk, 1988;Camerlenghi et al., 1997: Giorgetti et al., 2003; NaveiraGarabato et al., 2003). Stronger bottom currents

following similar paths to the modern ones have beeninterpreted to have generated the major sediment driftsalong the upper continental rise during the Middle andLate Miocene (Rebesco et al., 1996, 1997; Barker et al.1999; Rebesco et al., 2002; Barker and Camerlenghi2002). The development of the MB during the earlyMiocene indicates the predominance of northeastwardcontour current flow on the central continental rise athat time. During the Early and Middle Miocene, therewas certainly a deep water pathway through DrakePassage for Circumpolar Deep Water (CDW) (Barkerand Burrell, 1977), but, there may not have been a routefor WSDW and LCDW from the Weddell Sea to reachthe continental rise west of the Antarctic Peninsulathrough the southern Scotia Sea. The first incursion ofWSDW into the central Scotia Sea has been interpretedas having occurred during the Middle Miocene, whenplate movements had produced sufficient gaps in theSouth Scotia Ridge to allow deep water inflow from thenorthern Weddell Sea (Maldonado et al., 2003; Bohoyo2004; Maldonado et al., 2005). At the same time LCDWfrom the Weddell Sea probably started to flow into theScotia Sea and move towards the southwest. Thereforea vigorous northeastward flowing contour currenrelated to one of the frontal zones of the ACC mighhave operated closer to the Margin in the Early Mioceneand generated the MB on the central rise (Fig. 7A).

On the basis of the chronostratigraphic constraints onthe data presented here, oceanographic change couldhave begun in the Middle Miocene, at the same time asthe drift-growth stage defined by Rebesco et al. (19972002). However, we suggest that the prominent erosivesurface at the SU5/SU4 boundary could represent theonset of a stage of more intense bottom water circulationand a major reorganization of deep water flows towardsa regional oceanographic pattern more similar to thepresent one (Billups, 2002). As a result, LCDW from theWeddell Sea may have extended over the central risealong the Antarctica Peninsula, displacing CDW andforcing it to move to a more distal position (Fig. 7B).

SU3, SU2 and SU1 buried the MB (inactive stage)and, consequently, indicate that a new bottom currenregime was established. Therefore, bottom flows similarin pattern and strength to those found presently haveprobably been established during the Pliocene andQuaternary. SU3, SU2 and SU1 record two main stagesof the glacially dominated margin during which down-slope processes were dominant over along-slopeprocesses (Hernández-Molina et al., 2003, in press)favouring MB buried on the rise.

We interpret the occurrence of the MB in thesedimentary succession of the central continental rise


ig. 7. Sketches with (A) the inferred past circulation pattern of bottom water masses for the MB growth and maintenance stages and (B) presenirculation pattern of bottom water masses. (a) Circumpolar Deep Water (CDW) and (b) LCDW branch from the Weddell Sea.

andHcWdc

1

2

Fc

s evidence that this area was affected by CDW flowingortheastward as a contour current within the ACCuring the latter part of the Early Miocene (Fig. 7A).owever, since at least the Late Miocene–Early Plio-ene the area has been affected by LCDW from theeddell Sea flowing southwestward (Fig. 7B), asescribed above. These palaeoceanographic changesould be explained by three possible hypotheses:

. Migration and coexistence hypothesis. During mostof the Miocene, the influence of WSDW along theSouth Scotia Ridge, and LCDW from the WeddellSea along the Pacific margin of the AntarcticPeninsula, was less extensive than it has been duringthe Late Miocene, Pliocene and Quaternary, and as aresult it was restricted to the upper rise. Consequent-ly, CDW within the ACC circulated closer to themargin and its northeastward flow generated the MBon the central rise. Later, a more intense bottom watercirculation and reorganization of bottom currentsduring the Late Miocene or Early Pliocene couldhave been associated with increased LCDW outflowfrom the Weddell Sea. As a result, LCDW from theWeddell Sea may have extended over the central rise,and CDW was displaced to a more distal position.Such a palaeoceanographic change over the centralrise could have produced the ‘inactive’ stage of theMB. According to this hypothesis, the CDW fromthe ACC on the central rise and the LCDW fromthe Weddell Sea on the upper rise coexisted duringthe Middle to Late Miocene.

. All change hypothesis. The MB was developed bynortheastward-flowing CDW, which was the only

t

water mass flowing over the entire margin during theEarly Miocene. Later, an oceanographic regimesimilar to the modern one, but with more energeticbottom currents, was established as a result ofincreased outflow of LCDW from the Weddell Seaand reorganization of bottom currents betweenMiddle Miocene and Early Pliocene times. The MBwas fossilised during the Pliocene and Pleistocene asweaker southwestward-flowing LCDW from theWeddell Sea allowed down-slope sedimentary pro-cesses to dominate. According to this hypothesis,there was a widespread change in thermohalinecirculation, either near the beginning of the MiddleMiocene (start of the drift-growth stage on the upperrise described by Rebesco et al. (1997, 2002) orduring Late Miocene/earliest Pliocene times (cf.Billups, 2002).

3. Diversion of ACC fronts hypothesis. A study ofsatellite-derived sea surface temperature data revealsthat the mean path of the Polar Front is stronglysteered by the topographic features of the SouthernOcean (Moore et al., 1999). At present the meanpath of the Polar Front in Drake Passage lies just tothe north of a prominent ridge along the southernpart of the Shackleton Fracture Zone (SFZ).Livermore et al. (2004) have recently suggestedthat this ridge was only uplifted since spreadingstopped on the West Scotia Ridge in Drake Passageduring the Late Miocene. In this case, the path of thePolar Front, and probably also the SACCF and SB,may have been farther south in Drake Passage andon the Antarctic Peninsula continental rise beforethe latest Miocene. Uplift of the ridge might have


,

t

,

,

caused northward migration of the Polar Front,allowing the other fronts to migrate northward aswell, and this could explain some of the changesobserved around the Late Miocene to Miocene–Pliocene boundary. According to this hypothesis,there would not necessarily have been any change inLCDW outflow from the Weddell Sea in theMiddle–Late Miocene. When the Polar Front andother ACC fronts were displaced northwards, as aresult of the uplift of the ridge at the ShackletonFracture Zone, the WSDW and LCDW from theWeddell Sea would have been able to spread acrossthe rise. In this hypothesis the two flows coexist butthe ‘driving’ mechanism was the northward dis-placement of the ACC fronts.

7. Conclusions

The analysis and interpretation of three parallel MCSprofiles and multibeam echo sounding data collectedover the continental rise on the Pacific margin of theAntarctic Peninsula can be summarised with thefollowing conclusions:

a) A Fossil Mounded Sedimentary Body (MB) has beenidentified in the Early Miocene sedimentary recordover the oceanic crust on the central continental riseoffshore from Adelaide Island.

b) The MB has an elongated NE trend, overlapping andcontinuing to the NE of an extensive cluster ofseamounts, and it developed bounded by twomarginal troughs. It is over 48 km long and rangesfrom nearly 25 km wide where it overlaps theseamount cluster to about 20 km wide NE of theseamounts. Its thickness is variable, across and alongthe body. In the area where it overlaps the seamountcluster it measures ∼850 ms (twt), but thicknessdecreases towards the NE (∼400 ms twt).

c) Nine seismic units have been identified in thesedimentary record: Unit 9 (pre-MB stage); Unit8 (MB growth stage); Units 7 and 6 (MB mainte-nance stage); Units 5 and 4 (transitional stage); andUnits 3, 2 and 1 (inactive stage).

d) We interpret the MB as a patch drift plasteredbetween and against the NE (lee) side of theextensive cluster of seamounts, as a large ContouriteSedimentary Tail. It was deposited in a “shadowzone” between and behind the seamounts that wasgenerated by the action of a deep current flowingnortheastward.

e) The central and upper continental rise are presentlyaffected by a southwestward-flowing branch of

LCDW from the Weddell Sea. The occurrence ofthe MB in the Miocene sedimentary record providesevidence that during the Early Miocene, the contourcurrent flow on the central rise was towards the NEprobably involving CDW within the ACC. Thereforea palaeoceanographic change occurred in the area asome time between the start of the Middle Mioceneand the end of the Miocene.

f) Our stratigraphic interpretations are consistent withprevious work in the region, which mainly focussedon sediment drifts along the upper continental rise(Rebesco et al., 1996, 1997, 2002; Barker et al.1999; Barker and Camerlenghi, 2002). However, ourobservations do indicate that Miocene contouritedeposits in the region were not all produced by thesame water masses.

g) Our results are an example of how contouritedeposits, moats, and marginal troughs present inthe deep marine environment can provide evidencefor the reconstruction of palaeoceanographicchanges.

Acknowledgements

This research has been partially supported bySpain's Inter-ministerial Science and TechnologyCommittee (CYCIT), through Project ANT99-0817We thank the officers, crew, technical support staffand scientists who sailed on RRS Discovery cruise172 (1988) and R/V OGS-Explora cruises in 1989and 1992. On the OGS-Explora cruises, seismic datawere collected as part of Italy's PNRA (ProgrammaNazionale di Ricerche in Antartide). PNRA sup-ported Michele Rebesco's contributions through theSEDANO (Sediment Drifts of the Antarctic Offshoreproject).

This work has been carried out during research stagesat BAS funded by the Secretaría de Estado de Educacióny Universidades (PR2002-0271 and PR2003-0421), andwithin the scientific Spanish–British agreement on thestudy of the continental margin offshore from AdelaideIsland.

Dr. Alberto C. Naveira Garabato (National Oceano-graphic Centre, Southampton, NOCS) is thanked for hisfull and constructive reviews of the OceanographicSetting chapter and the Fig. 2 of this paper. We aregrateful to Dr. Carol Pudsey for processing the multi-beam echo sounding data in Fig. 6 and commenting onan earlier version of themanuscript. Finally, we thank thepositive comments and suggestions of David J.W. PiperPaul C. Knutz and one anonymous reviewer who helpedus to improve the present paper.


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Miocene reversal of bottom water flow along the Pacific Margin of the Antarctic Peninsula:...

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