neogene history of the deep western boundary current at

Neogene history of the Deep Western Boundary Current atRekohu sediment drift, Southwest Paci¢c (ODP Site 1124)

Leah H. Joseph �, David K. Rea, Ben A. van der PluijmDepartment of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA

Received 25 January 2002; received in revised form 18 July 2003; accepted 26 January 2004

Abstract

ODP Site 1124, located 600 km east of the North Island of New Zealand, records post-middle Oligocenevariations in the Pacific Deep Western Boundary Current (DWBC) and New Zealand’s climatic and tectonicevolution. Sediment parameters, such as terrigenous grain size, flux, magnetic fabric, and non-depositional episodes,are used to interpret DWBC intensity and Antarctic climate. Interpretations of DWBC velocities indicate that theAntarctic Circumpolar Current reached modern intensities at V23 Ma, as the tectonic seaways expanded, completingthe thermal isolation of Antarctica. Periods of more intense bottom water formation are suggested by the presence ofhiatuses formed under the DWBC at 22.5^17.6, 16.5^15, and 14^11 Ma. The oldest interval of high current intensityoccurs within a climatically warm period during which the intensity of thermohaline circulation around Antarcticaincreased as a result of recent opening of circum-Antarctic gateways. The younger hiatuses represent glacial periodson Antarctica and major fluctuations in the East Antarctic Ice Sheet, whereas intervals around the hiatuses representtimes of relative warmth, but with continued current activity. The period between 11 to 9 Ma is characterized byconditions surrounding a high velocity DWBC around the time of the formation and stabilization of the WestAntarctic Ice Sheet. The increased terrigenous input may result from either changing Antarctic conditions or moredirect sediment transport from New Zealand. The Pacific DWBC did not exert a major influence on sedimentation atSite 1124 from 9 Ma to the present; the late Miocene to Pleistocene sequence is more influenced by the climatic andtectonic history of New Zealand. Despite the apparent potential for increased sediment supply to this site fromchanges in sediment channeling, increasing rates of mountain uplift, and volcanic activity, terrigenous fluxes remainlow and constant throughout this younger period.A 2004 Elsevier B.V. All rights reserved.

Keywords: paleoclimate; Southwest Paci¢c Deep Western Boundary Current; Antarctic Circumpolar Current; ocean gateways;ODP Site 1124; grain size and magnetic fabric analysis

1. Introduction

The Southern Ocean, and its surface and deepcirculation, are critical components of the convey-or system of heat transport of the world’s oceans.The Southern Ocean allows for interconnectionbetween the world’s oceans and the mixing of

0025-3227 / 04 / $ ^ see front matter A 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0025-3227(04)00023-4

* Corresponding author. Present address: EnvironmentalStudies Program, Department of Geoscience, Hobart andWilliam Smith Colleges, Geneva, NY 14456, USA.Tel. : +1-(315)-781-3954; Fax: +1-(315)-781-3860.

E-mail addresses: [email protected] (L.H. Joseph),[email protected] (D.K. Rea), [email protected](B.A. van der Pluijm).

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Marine Geology 205 (2004) 185^206

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water masses within the Antarctic CircumpolarCurrent (ACC). In addition, cold waters thatform, sink o¡ the Antarctic margin and mix inthis zone, become the bottom waters of the world,upwelling to the surface elsewhere. This transportsystem of water and properties of the individualwater masses link the polar regions to the globalclimate system as an integral part in the regula-tion of Earth’s climatic regime (Broecker, 1997;Orsi et al., 1999).

This study investigates the Southwest Paci¢cDeep Western Boundary Current (DWBC) o¡the eastern coast of New Zealand through thestudy of sedimentary characteristics obtained fromthe late Oligocene through Pleistocene sedimentof the Rekohu sediment drift (ODP Site 1124).Variation in the velocity of this DWBC mayhave implications for the development and £uc-tuation of the ACC to modern intensities, as theintensity of DWBC £ow is likely modulated bythe strength of ACC circulation, as well as cli-

matic conditions, ice sheet coverage and stabilityand the presence of sea ice (i.e. Keller and Barron,1983; Carter et al., 1996). Characteristics such asgrain alignment and grain size allow interpreta-tions of the bottom current intensities that canre£ect global climate conditions.

2. Background

2.1. Antarctic Bottom Water and the SouthwestPaci¢c DBWC

Waters upwell and/or di¡use globally from thedeep sea as part of oceanic thermohaline circula-tion in several locations and are often broad anddi¡use £ows. However, the sinking of watermasses, in balance with the upwellings, occurs inonly a few locations around the world, includingthe North Atlantic and the Southern Ocean,around Antarctica (Whitworth et al., 1999). A

Fig. 1. Present sediment transport systems around the New Zealand area (from Carter et al., 1999). Contours in meters.

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L.H. Joseph et al. /Marine Geology 205 (2004) 185^206186

conservative estimate of production of AntarcticBottom Water (AABW) is 8U106 m3/s, but valuesrange up to 13U106 m3/s (Orsi et al., 1995, 1999).

The Southwest Paci¢c is a gateway throughwhich approximately 40% of the deep waters en-ter the world’s oceans from Antarctica, making itsDWBC the largest single source of deep oceanwater (i.e. Carter et al., 1996; Carter and Wilkin,1999; Orsi et al., 1999). Presently, the ACC, andthus the DWBC as the deepest portion of theACC, travels through the passage between Ant-arctica and Australia, within the Balleny FractureZone in the southeast Indian Ridge, before turn-ing northeast at Macquarie Ridge (V60‡S) intothe Paci¢c Basin (Fig. 1; Carter et al., 1996; Car-ter and McCave, 1997; Orsi et al., 1999). At themouth of the Bounty Trough (V50‡S), the ACCand the DWBC uncouple, the ACC continuing inits eastward circuit and the DWBC travelingnorth. Bounded by the topography surroundingNew Zealand, the 1000-km-wide DWBC £owsalong the 3500-m contour on the CampbellPlateau, around Hikurangi Plateau, to followthe Kermadec Trench northward (Carter and

McCave, 1994; Carter et al., 1996, 1999). Esti-mates of strong and variable £ow from currentmeters indicate transport of the DWBC east ofNew Zealand and the Kermadec Ridge to be ap-proximately 16U106 m3/s ( Q 11.9U106 m3/s),with Circumpolar Deep Water (CDW) comprising15.8U106 m3/s ( Q 5.1U106 m3/s). Paci¢c DeepWater (PDW) comprises the remainder (Warren,1973; Whitworth et al., 1999).

2.2. The New Zealand microcontinent, continentalmargin, and tectonic development

The New Zealand microcontinent began mov-ing away from Antarctica between V80 and 60Ma, incurring the formation of the Tasman Seaand part of the Paci¢c Ocean (Fig. 2; Kamp,1986; Carter and Carter, 1987; Carter andMcCave, 1997). During the early Oligocene,New Zealand was almost completely submergedand terrigenous sediment sources were buried or£ooded (Rait et al., 1991; Carter et al., 1996,1999).

The North and South Islands of New Zealand

Fig. 2. Paleocirculation around Antarctica at 50, 30, and 20 Ma (from Carter et al., 1999; Lawver et al., 1992).

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L.H. Joseph et al. /Marine Geology 205 (2004) 185^206 187

are cut by the Australian^Paci¢c plate boundary,accounting for the di¡erences in ongoing geologicprocesses and topographies between the islands(Walcott, 1978; Carter et al., 1999). The AlpineFault did not form on the western South Islanduntil late Eocene; by early Miocene (V24 Ma)large amounts of sediment were shed from therising mountains along the Alpine Fault. Duringthe late Miocene (V6.5 Ma) there was an en-hanced episode of uplift along the Alpine Fault(Carter and Norris, 1976; Carter et al., 1999 andrefs therein). Currently, the Southern Alps arerecognized as one of the fastest rising mountainbelts in the world. New Zealand is thus a prom-inent sediment source to the oceans, providing 9%of the annual suspended £uvial input to theSouthwest Paci¢c, V2% to the oceans overall(Carter and McCave, 1997 and references there-in). East of the North Island, the Hikurangi Sub-duction Complex (HSC; or Trough) is locatedwhere the Paci¢c plate is subducting under theNorth Island, resulting in the Central VolcanicZone (Carter and Norris, 1976; Walcott, 1978;Carter et al., 1999).

2.3. Evolution of the ACC/DWBC systems

The opening of gateways to circumpolar ocean-ic circulation and deep water formation are in£u-enced by plate tectonics and the associated move-ment of the continents away from Antarctica(Fig. 2). By V43 Ma, the Australian block hadmoved far enough away from Antarctica to allowminor deep circulation through the Tasman Sea,although Tasmania and the South Tasman Risestill blocked deep water circulation. Around 34^30 Ma, the deep water connection between theIndian and Paci¢c oceans opened for the ¢rsttime (Kennett et al., 1972; Kennett, 1977, 1980;Carter et al., 1996; Exon et al., 2001). While theAntarctic Peninsula moved eastward with respectto South America, the Drake Passage remainedclosed until about early Oligocene time (Kennett,1977; Kennett and Stott, 1990; Lawver et al.,1992; Lazarus and Caulet, 1993; Veevers, 2000).Unrestricted latitudinal £ow of the ACC, similarto today, was fully established by the early Neo-gene. The delay between the opening of the Drake

Passage and the establishment of modern oceaniccirculation may be due to the time required fordeepening of the ocean basin between the conti-nents, associated plateaus (e.g. Kerguelen), andAntarctica. Estimates of more speci¢c time peri-ods vary widely (Kennett, 1977; Kemp, 1978; Za-chos et al., 2001).

Despite New Zealand’s continuous tectonism,the o¡shore region of the eastern New Zealandplateau has been relatively una¡ected by any ma-jor tectonic event since rifting in the late Creta-ceous and has acted primarily as a trailing-edgepassive margin (Carter et al., 1999). Flowing atbathyal to abyssal depths along the margin ofNew Zealand, the Southwest Paci¢c DWBC hasin£uenced sediment erosion and deposition east ofthe New Zealand microcontinent (Fig. 1, Carter etal., 1996). Today this is evidenced by the presenceof scoured bedrock, manganese nodule ¢elds, andsediment drifts around the eastern continentalboundary of New Zealand. The ¢rst evidence ofwidespread current activity and for signi¢cantsediment drift formation around the New Zealandmargin occurred in the late Eocene/early Oligo-cene. The Marshall Paraconformity (V33^27Ma), a regional unconformity noted by previousdrilling legs as well as onshore studies, indicatesthat circulation was well established and vigorousby mid^late Oligocene (e.g. Carter and Landis,1972; Kennett et al., 1972; Carter et al., 1996;Fulthorpe et al., 1996). The Southern Alps ofNew Zealand started to rise and shed sedimenteastward through the Bounty Trough to theSouth Island continental margin during the Mio-cene, while below 2000 m water depth sedimentdrifts began to form (Carter et al., 1996).

One of the goals of this study is to use theproperties of the sediment recovered from sedi-ment drifts to investigate the establishment and£uctuations in the ACC and DWBC as they de-veloped through ensuing major tectonic and cli-matic changes and reached modern intensities(Fig. 1; Carter et al., 1996, 1999). Potential sedi-ment sources include New Zealand, Antarctica,and reworked southerly sediment drifts (Carterand McCave, 1994; Carter et al., 1996, 1999).This study focuses on the last 27 my of sedimentcollected at ODP Site 1124, the Rekohu sediment

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drift near the mouth of the Hikurangi Channel(Fig. 1).

3. Study location

3.1. Rekohu Drift

The Rekohu Drift, a 250-km-long ridge-likefeature of mainly Miocene and older sediment,is located approximately 600 km east of the NorthIsland (Carter et al., 1996). The drift is built upona volcanic basement ridge and currently acts todivert the DWBC and serves as an e¡ective bar-rier/levee to the Hikurangi Channel (Fig. 1, Carterand McCave, 1994). Rekohu Drift lies betweenthe Bounty Trough to the south and KermadecTrench to the north and thus within the ‘inte-grated sediment source-transport-sink area’ ofthe eastern New Zealand Oceanic SedimentarySystem (ENZOSS, e.g. Carter et al., 1996).

The Southwest Paci¢c DWBC that bathes Re-kohu Drift, already decoupled from the ACC, hasmade its way north along the New Zealand con-tinental margin passing two major sites of sedi-ment injection, the Solander and Bounty Troughs(Carter and Mitchell, 1987; Carter et al., 1996).North of Chatham Rise, the DWBC is less ener-getic until it reaches the Rapuhia Scarp, but car-ries a larger sediment load. In addition to anyterrigenous sediment that may have been carriedthis far north from the Antarctic Basin, these twosites are likely sources for Rekohu Drift sediment.Eroded drifts south of Rekohu Drift can act assigni¢cant sediment sources as well ; upper Neo-gene sediment of Site 1124 contains reworked Eo-cene taxa likely released from the erosion of driftslying further south (Schuur et al., 1998; Carter etal., 1999).

Large amounts of sediment are delivered fromKaikoura and Cook Strait canyons (less than 10km from the coastal mountains and only a fewhundred meters from the shore) to the continentalmargin via turbidity £ows that move along the1400-km-long Hikurangi Channel (Carter et al.,1996, 1999). This constant supply is brought tothe mouth of the Hikurangi Channel, where therelatively accelerated DWBC sweeps sediment in-

put into a ‘fan-drift’ ^ a fan deposit that has beenextended 300 km downcurrent within the DWBC(Carter and McCave, 1997). In all 1200 m fromthe source, Hikurangi Channel is redirectednortheast by the Rekohu Drift. Therefore, theRekohu Drift may have as its sediment sourceturbidity currents over£owing the e¡ective leveeof Hikurangi Channel. Rekohu Drift formationpredates the development of the Hikurangi Chan-nel System which did not develop until Plioceneor even Pleistocene times when a submarine slideblocked the path of sediment directly to the Ker-madec Trench (Carter and McCave, 1994; Lewiset al., 1998).

3.2. Site selection ODP Site 1124

ODP Site 1124 (39‡29.901PS, 176‡31.894PW,3967 m below sea level) cored into the RekohuDrift. Through the Oligocene, the core is com-prised primarily of clay-bearing nannofossil oozeswith tephra layers, which are more frequent andthicker (up to 90 cm) near the top of the core. Theoldest noted tephra is located at approximately207 m below sea £oor (mbsf) and is of mid^lateMiocene age (Carter et al., 1999, 2003). Samplesextend through the Oligocene, however a numberof hiatuses are present at approximately 28.4,28.0, 27.1, 22.5^17.5, 16.5^15, and 14^11.1 Ma(Table 1). There are a few other small hiatusesas well as the 58^37 Ma hiatus that covers muchof the Eocene and extends beyond our samplingzone (Carter et al., 1999).

Table 1Ages of hiatuses from ODP Leg 181, Site 1124 (based onCarter et al., 1999)

Depth Start age End age(mcd) (Ma) (Ma)

440.93 58 37s 431.18 6 37.0 33.6s 425.38 33.6 33.1s 422.80 6 28.4 s 28.0s 421.13 6 28.0 s 27.1288.23 22.5 17.5267.00 16.5 15.0V237.50 14.0 11.1

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4. Methods

In all 272 separate samples of the Rekohusediment drift (adjacent scoop samples and2.2U2.1U1.4 cm paleomagnetic cubes) were ob-tained from ODP Site 1124B, C, and D cores.Care was taken to avoid sampling discrete ashlayers, however, dispersed ash was pervasive inthe top portion of the core.

4.1. Terrigenous sediment characterization

4.1.1. Chemical extractionScoop samples were subject to a chemical ex-

traction process based on the procedures de-scribed by Rea and Janecek (1981) with modi¢ca-tions by Clemens and Prell (1990) and Hovan

Table 2ODP Leg 181, Site 1124 age datums used for linear extrapo-lation of sample ages

Depth Age(mcd) (Ma)

21.06 0.4225.18 0.6539.85 0.9943.25 1.0750.30 1.2052.70 1.2164.50 1.7767.65 1.9571.25 2.1471.60 2.1581.60 2.5889.15 3.0490.60 3.1191.55 3.2293.20 3.3397.30 3.58106.60 4.18108.95 4.29111.60 4.48113.65 4.62115.65 4.80116.90 4.89118.30 4.98123.15 5.23135.10 5.89139.95 6.13143.10 6.24148.45 6.57154.70 6.93156.30 7.09157.25 7.14158.25 7.17161.55 7.34162.10 7.38162.60 7.43164.30 7.56166.80 7.65171.63 8.07175.73 8.23176.03 8.26180.53 8.70185.63 9.03189.13 9.23189.58 9.31199.23 9.74234.93 10.95235.38 11.05236.98 11.10237.00 11.10238.00 14.00238.23 14.18

Table 2 (Continued).

Depth Age(mcd) (Ma)

252.78 14.80255.58 14.89267.00 15.00267.00 16.50269.43 16.73288.23 17.50288.23 22.50334.43 23.68338.08 23.80350.18 23.99355.73 24.12381.83 24.73382.58 24.78385.08 24.84388.73 25.18394.43 25.50398.58 25.65401.93 25.82413.08 26.55420.73 27.03422.88 27.97425.38 33.06431.18 33.55440.93 37.00440.93 58.00458.71 62.50472.68 64.75

The youngest two datums were determined through biostra-tigraphy while the remaining datums are magnetostrati-graphic. Depths are provided in meters composite depth(mcd; Carter et al., 1999).

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(1995). This process removes calcium carbonate,oxides and hydroxides, zeolites, and biogenic sili-ca in order to isolate the terrigenous mineral com-ponent of marine sediment; however, it should benoted that ash survives this procedure. The terrig-enous wt% is calculated by comparing the weightof each sample before and after chemical extrac-tion. The age model is based on linear extrapola-tion primarily using both paleomagnetic and bio-stratigraphic data from the ODP Leg 181 InitialResults Volume (Table 2; Carter et al., 1999,2003).

4.1.2. Terrigenous mass accumulation ratesTerrigenous mass accumulation rates (MAR) in

g/cm2/ky were calculated using the equation:

Terrigenous MAR ¼

LSRUDBDUterrigenous weight %

where LSR is the linear sedimentation rate (in cm/ky), DBD is the dry bulk density (in g/cm3) ob-tained from shipboard bulk density values, andthe terrigenous wt% is determined using the chem-ical extraction method provided above. As a re-sult, the history of terrigenous input is indepen-dent of dilution/concentration e¡ects incurred byvariations in input rates of other sediment com-ponents, a di⁄culty often encountered when usingonly percentage data.

4.1.3. Terrigenous grain sizeGrain size analysis was performed on the ex-

tracted terrigenous component using a MultisizerIIE Coulter Counter with a selected analysisrange between 2 and 100 Wm. Terrigenous mediangrain size is represented here using both micronsand x units (x50 ; x=3log2(grain diametermm)).

4.2. Magnetic fabric analysis

Anisotropy of magnetic susceptibility (AMS)analyses were conducted on paleomagnetic cubesamples using a KLY-2 KappaBridge. Results ofAMS analyses show the bulk orientation of allgrains in a sample, called the magnetic fabric,and are represented in 3-D space by an ellipsoid;in most cases the greatest induced magnetization

parallels the long axis (Nye, 1957; 1985; Tarlingand Hrouda, 1993 and references therein). Theparameter PP mathematically represents the de-gree of anisotropy of the resulting magnetizationellipsoid (greater PP values represent a more de-veloped anisotropy of the magnetization ellipsoid)and the parameter T represents the shape factorof the ellipsoid. When 06T9 1, then the ellipsoidis oblate while if 319T6 0, the ellipsoid is pro-late.

Previous studies (e.g. Rea and Hovan, 1995;Boven and Rea, 1998; Joseph et al., 1998) haveprovided robust methods by which the mode ofterrigenous sediment transport and deposition canbe determined by appropriate analyses of a singlesample using a combination of AMS and terrige-nous grain size analyses. Grains deposited bymoving water, such as with sediment drifts andturbidites, show a distinct magnetic fabric; thosedeposited by random settling, eolian, pelagic, orhemipelagic, show no grain alignment along thelong axis. The terrigenous grain size distributionsallow discrimination between drift vs. turbidite,and eolian vs. hemipelagic deposition. In general,coarser median grain sizes combined with higherPP values indicate stronger depositional energies,however, due to source e¡ects, it is sometimesnecessary to rely more heavily on one parameter.

5. Results

Terrigenous MARs vary between near zero toalmost 10 g/cm2/ky, but are most commonly be-tween V0.5^2.5 g/cm2/ky (Figs. 3 and 4). Terrig-enous median grain size varies between V7.5 and6.0x and appears to increase in variability upcorewhile maintaining a base value of approximately7.5x throughout the interval sampled.

The middle^late Oligocene (V27 Ma) throughearly Miocene period (V23 Ma) exhibits an in-crease in terrigenous MARs from 6 0.5 to V3 g/cm2/ky, in a trend somewhat similar to that of thebulk magnetic susceptibility and terrigenous wt%(Figs. 4 and 5). Median terrigenous grain sizesexhibit two peaks in coarseness at V26.3 and23.8 Ma. Over this interval, carbonate wt% de-clines from nearly 80% at 27 Ma to less than

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30% at 23 Ma, changing most signi¢cantly be-tween 24 and 23 Ma.

Most of the early^middle Miocene was charac-terized by periods of non-deposition or erosion(chemical or physical), represented in the formof hiatuses (Fig. 4). The interpretation of the22.5^17.6-Ma hiatus is complicated by the lackof recovery; however, if deposition was continu-ous during this time period, LSRs would be com-paratively very low at V0.4 cm/ky.

Two small glimpses into the middle Miocenesediment drift environment are present betweenthe hiatuses. In both intervals, the MARs remaingenerally high, resulting from high calculatedLSRs and large terrigenous wt%. Bulk susceptibil-ities have increased and range up to 600U1036 SIand magnetic fabrics (PP) remain strong. Carbon-ate content is generally low, with more variationin the younger interval. Median grain size is rela-tively ¢ne, averaging around 7.6x in the olderinterval and coarsening slightly upcore throughthe younger interval. The extremely high MARvalues at 15 Ma (as well as 1.2 Ma) result froma short interval of very high calculated LSR, andlikely are an artifact of the resolution of the time-scale datums (Fig. 4A,B).

The late Miocene (V11^5.5 Ma) has an initial2-my period where MARs, bulk magnetic suscep-tibility values and PP values exhibit a distinct riseto a maximum at 10 Ma, followed by a drop invalues comparable to, or lower than, those in themiddle Oligocene (Figs. 4 and 6). This pattern ismimicked by the terrigenous wt%, although in amuted form. Carbonate wt% values are initiallylarger than those in the mid-Miocene and showa decline in values from 11 to 9 Ma. Terrigenousgrain size clusters tightly at V7.5x until 10 Ma,then coarsen slightly, with much more variation,until around 7 Ma.

MARs, carbonate wt%, and PP values remainvery low and fairly constant for the remainder ofthe Miocene, while terrigenous wt% values remainhigh. However, the bulk magnetic susceptibility

shows a gradual increase upcore to a relativehigh around the time of the Miocene^Plioceneboundary (V5 Ma). The ¢rst tephra layer occursat around 11.16 Ma (207 mbsf; Carter et al.,1999, 2003).

MARs of the terrigenous component remainvery low (6 1 g/cm2/ky), as do PP values, duringthe Pliocene interval (V5.5^1.8 Ma). Bulk suscep-tibilities and terrigenous wt% values gradually de-crease, while the carbonate wt% values graduallyincrease during this interval. The median grainsize is ¢ner between V5 and 4 Ma, followed bya general increase in both grain size and grain sizevariability through the end of the Pliocene.

During the Pleistocene, MARs vary up to 2 g/cm2/ky and PP values initially rise slightly. Bulkmagnetic susceptibilities remain under 100U1036

SI, except at the coretop. The carbonate weightabundance decreases in the past 1 my, while in-dividual measurements exhibit a great deal of var-iability between 5 and 80%. Grain size is alsoquite variable during the Pleistocene, varying be-tween 7.6 and 6.3x.

Sediment deposited around the time of theBruhnes^Matuyama boundary (0.78 Ma; Candeand Kent, 1995) was sampled more intensivelyto provide a high resolution sequence with whichto investigate the presence (or lack) of an orbital-ly-forced sediment signal. Fig. 7 shows the sedi-ment parameters plotted on an orbitally-tunedtimescale determined for Site 1123, tied to theSite 1124 record using proxy carbonate data(Hall et al., 2002). Between 0.8 and 0.5 Ma,MARs, initially at V2 g/cm2/ky, drop to 6 1 g/cm2/ky at V0.64 Ma and remain there for therest of the interval. Carbonate wt% values showa several-fold £uctuation, and overall show a gen-erally increasing trend from 0.74 to 0.5 Ma, mir-rored by the terrigenous wt%. Terrigenous mediangrain sizes are very variable throughout, whilebulk magnetic susceptibility shows almost no var-iation from its low values 6 100U1036 SI. PPvalues are also fairly low and constant throughout

Fig. 3. Results of sediment characterization from Site 1124. Core recovery (A) represented in black, non-recovery in white (Carteret al., 1999), trends in terrigenous MAR (B), linear sedimentation rate (C), terrigenous median grain size (D), carbonate wt%(E), terrigenous wt% (F), bulk magnetic susceptibility (G), and magnetic fabric (H) at Site 1124 for V460 mcd.

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the interval, showing an increase at approximately0.58 Ma.

6. Discussion

Overall, ¢ve distinct intervals are noted basedon the sediment parameters described above. Thebreaks between these intervals are at 24.5, 9.0,5.8, and 1.5 Ma. Depositional environmental de-terminations using magnetic fabric and terrige-nous grain size parameters were constructed foreach of these units separately and are provided inFig. 8A^E. Overall, the terrigenous median grainsize, which is generally ¢ne throughout, plots pri-marily in the low-energy environment ¢eld, whilethe magnetic fabrics vary between those charac-teristic of low- and middle-energy (pelagic inter-layer and sediment drift types, respectively) de-positional environments. In general, PP valuesincrease from relative lows after hiatuses and ex-hibit an increase just prior to hiatuses. Terrige-nous median grain size exhibits a distinct baselineat V7.5x throughout the entire record (Figs. 3and 4). This likely represents either a source e¡ector an e¡ect of the travel path and distance, ratherthan localized sluggish current transport as themagnetic fabric indicates signi¢cant current in£u-ence in the older portion of the record.

6.1. 27^24.5 Ma: strong and £uctuating DWBC

The interval between 27 and 24.5 Ma is char-acterized by low bulk magnetic susceptibilities,terrigenous wt% values, and MARs (Figs. 4 and5). However, terrigenous grain sizes are variableand PP values are relatively high. The magneticfabrics suggest a pronounced sediment drift-likeenvironmental energy while terrigenous deposi-tion remains relatively low.

This interval in the late Oligocene before anymajor New Zealand tectonic uplift, following twomid-Oligocene hiatuses, likely represents the slow-

ing down of the DWBC to non-scouring, deposi-tional velocities. The £ow, however, remainsstrong, as indicated by the magnetic fabric, al-though median grain sizes are ¢ne.

6.2. 24.5^9.0 Ma: continued strength of theDWBC

The latest Oligocene through the late Mioceneis a period of strong and variable current activity.This is evidenced not only by the three majorepisodes of non-deposition/scouring of sediment,but also by strong magnetic fabrics and large ter-rigenous inputs between hiatuses (Fig. 4).

In the latest Oligocene^earliest Miocene, terrig-enous input increases gradually while PP valuesdecrease slightly, although fabrics still remainstrongly oriented. This is indicative of either acontinued slowdown of the Paci¢c DWBC possi-bly with an increased supply of material, allowingfor drift deposition while still maintaining strong£ows.

The presence of hiatuses through increased cor-rosiveness or physical erosion (or non-depositionof sediment) is largely in£uenced by the path ofbottom water £ows and changes in the intensityof ocean circulation (Keller and Barron, 1983).The hiatuses noted in this study, with the win-dows of time recorded between them, imply astrong, but variable DWBC throughout theearly^middle Miocene, with episodes of erosion/non-deposition followed by periods when eitherthe DWBC decelerated, indicated by the relativelyweak magnetic fabric values immediately afterhiatuses, and/or sediment supply increased enoughto allow signi¢cant deposition.

Between the youngest hiatus and 9 Ma, a peakin MARs, PP, and bulk magnetic susceptibility at10 Ma is quite distinctive (Figs. 4 and 6). In thisinterval, the magnetic fabrics become more ori-ented in concert with the higher terrigenous input,implying a greater amount of deposition within afaster current. Here, following a slowdown of the

Fig. 4. Results of sediment characterization from Site 1124. Trends in terrigenous MAR (A), linear sedimentation rate (B), terrig-enous median grain size (C), carbonate wt% (D), terrigenous wt% (E), bulk magnetic susceptibility (F), and magnetic fabric (G)at Site 1124 for V30 My.

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DWBC from scouring intensities that created thehiatus, the comparison of maxima in sedimentproperties indicates that the current has becomestronger once again around 10 Ma (Fig. 6). Amajor increase in sediment transport to theDWBC upcurrent of Rekohu Drift may surpassthe carrying capacity of the water mass and de-position may result at continued high velocities.This terrigenous input peak may represent moreextensive erosion of sediment upcurrent from Re-kohu Drift. It is also possible that the increase interrigenous input at about 10 Ma re£ects in-creased injection of sediment from the mountainsof New Zealand to the deep-sea channel/fan sys-tems (Carter et al., 1996), possibly through theSolander or Bounty channels. However, an hiatuswas noted at Site 1122 (Bounty Fan) from V10to 4 Ma and turbidite deposition did not occur atthe Bounty Fan until approximately the earlyPleistocene, implying that at least this path hadnot yet reached the in£uence of the DWBC (Car-ter et al., 1999).

6.3. 9.0^5.8 Ma: lack of current in£uence and lowterrigenous input

After 9 Ma a decline in sediment dry bulk den-sity occurs: this is the boundary between litholog-ic units 1C and 1B where nannofossil ooze andsilty clay change downsection to nannofossil chalkand mudstone (Carter et al., 1999). Determina-tions of MARs, however, take this factor intoconsideration. Although the terrigenous wt% ishigh (showing an inverse variation with carbonatewt%), the terrigenous MARs remain low andsteady between 9 and 5.8 Ma, despite the poten-tial enhancement in the MARs from ash inputand other increases in sediment supply likely tohave been associated with intensi¢ed collisionalong the New Zealand plate boundary (Carteret al., 1999 and references therein; Carter et al.,2003). This implies that the additional sediment

generated by these processes did not have a path-way to Rekohu Drift ; the Hikurangi Channel wasnot yet directed eastward and the Bounty FanChannel did not yet reach the DWBC’s in£uence.Terrigenous grain sizes remain ¢ne and the in-creased variability in median grain sizes may re-sult from the introduction of ash as part of theterrigenous component. McCave and Carter(1997) noted a relatively coarse grain size in themodern sediment of Rekohu Drift that likely re-sulted, at least partially, from the presence of theTaupo ash.

Some complications exist for the remainingportion of the record. An initial macroscopictephra layer occurs around the start of the lateMiocene, representing the onset of volcanism onthe North Island of New Zealand. Once volca-nism begins, the ash input to the sediment recordbecomes signi¢cant with ash layers (s 1 cm) mak-ing up 6% of the late Miocene and younger sedi-ment column. These ash layers provide a wonder-ful opportunity for dating as well as correlation ofNew Zealand shore-based studies of volcanism(Carter et al., 1999, 2003). However, because ashcannot be removed by the extraction process usedin this study, it causes complications in determin-ing both depositional environment and terrige-nous input. Peaks in bulk magnetic susceptibilitiesdo not appear to correspond with visible ashlayers. Although care was taken not to sampletephra layers, glass was still visible in smear-slidesof many post-extraction samples and is likelypresent in the sediment column as dispersed ashor microscopic tephra layers.

PP values, the magnetic fabric, are very lowvalues between 9 and 5.8 Ma, indicating the lackof current in£uence (or a very small one) at thistime. It is possible that this drop in PP values at9 Ma represents only the lithologic change frommore to less consolidated sediment; however, thisis a signi¢cant drop to consistently low values.Previous studies using unconsolidated sediment

Fig. 5. Expanded version of the period between 27.5 and 22.5 Ma. Terrigenous MAR (A), terrigenous median grain size (B), car-bonate wt% (C), terrigenous wt% (D), bulk magnetic susceptibility (E), and magnetic fabric (F). The black line represents the dis-tinct data points, the gray line is smoothed. The smoothed curves have the geometric weight applied to the current point andQ10% of the data range.

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from drifts and turbidites (Joseph et al., 1998)show higher PP values in similar sediment, indicat-ing the presence of currents.

While bulk magnetic susceptibility and ex-tracted terrigenous wt% trends mimic each other,the terrigenous wt% trend is muted in comparisonto the bulk magnetic susceptibility. This may alsobe a result of the addition of the ash componentto the system and the various e¡ects the introduc-tion of ash would impose on each of these anal-yses.

6.4. 5.8^1.5 Ma: ash and the Hikurangi Channelin£uence

Ash is a signi¢cant component of the Pliocenesection at Site 1124 as well. In all 41 visible tephralayers were noted with this age interval, averagingapproximately one layer every 1.4 m (Carter etal., 1999, 2003).

The formation of the Hikurangi Channel mayhave occurred during the Pliocene or early Pleis-tocene (Lewis et al., 1998; Carter et al., 1999).The formation of this channel altered the sedi-ment distribution pathways of the Kaikoura Can-yon (located at the foot of the coastal mountains)and Northern Island areas from the HikurangiTrough (or subduction complex), cutting acrossthe Hikurangi Plateau to the continental margin.Currently, Rekohu Drift acts as an e¡ective bar-rier to the Hikurangi Channel turbidity £ows andoverbank £ow from the channel onto the drift haslikely occurred. Despite this potential additionalinput to the terrigenous MAR of this sedimentdrift, the terrigenous £uxes remain low throughthe late Pliocene, and the magnetic fabric is con-sistently weak. As carbonate wt% increases, ter-rigenous wt% and bulk magnetic susceptibilityshow the corresponding decline expected in atwo-component system. The increasing variationin terrigenous grain size likely represents the com-bination of ash and hemipelagic muds. This time

period is typi¢ed by low terrigenous input to thesite, implying that interpretations of a later (earlyPleistocene) formation of the channel (Lewis etal., 1998) is favored. Bottom £ow energy, as mea-sured by the magnetic fabric, is quite low.

6.5. 6 1.5 Ma: tephra

The youngest 1.5 my of the record at Site 1124exhibits some increased variation in sediment pa-rameters. MARs increase slightly, as do terrige-nous wt% values, yet bulk magnetic susceptibilityvalues generally decrease (Fig. 4). Magnetic fabricdata suggest that depositional energies vary be-tween low and moderate velocities (Fig. 8).

Gradually increasing in depositional episodes,ash is even more of a complicating factor thanduring the proceeding time period (5.8^1.5 Ma),averaging one macroscopic tephra layer every1.2 m and including some thicker layers (meanthickness 13.6 cm, maximum thickness 92 cm;Carter et al., 1999, 2003). The presence of ash inthis section of the Pleistocene sediment may beresponsible for the di¡erences in the trends ofbulk magnetic susceptibility and extracted terrig-enous wt%, two independent measurements of ter-rigenous content. Signi¢cant ash contributioncould also a¡ect magnetic fabrics.

The interval around the Bruhnes^Matuyamaboundary (0.3^0.8 Ma), a generally cold episoderepresenting the initiation of high amplitudechanges in ice volume with primarily 100-ky fre-quencies, experienced an enhanced ACC and bot-tom water £ow according to previous studies (i.e.Bandy et al., 1971; Kuijpers, 1989). The time pe-riod was sampled at a higher resolution with theintention to investigate Milankovitch forcing ofNew Zealand climate. While this site is likely re-cording the climatic history of New Zealand, be-fore this determination can be made the ashshould be removed from the samples, new MAR’scalculated, and terrigenous median grain sizes re-

Fig. 6. Expanded version of the period younger than 12 Ma. Terrigenous MAR (A), terrigenous median grain size (B), carbonatewt% (C), terrigenous wt% (D), bulk magnetic susceptibility (E), and magnetic fabric (F). The black line represents the distinctdata points, the gray line is smoothed. The smoothed curves have the geometric weight applied to the current point and Q10%of the data range.

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analyzed. Hall et al.’s (2001, 2002) examination ofsoutherly New Zealand ODP Sites 1123 and 1124generally indicates coarser grains, increased car-bonate dissolution, and a higher terrigenous inputto these sites during glacial periods, althoughsome major pulses noted at Site 1123 were missingor muted at Site 1124. Currently, some of oursediment parameters from Site 1124 show a pos-sible correlation to glacial^interglacial cycles inFig. 7 when plotted with N

18O of benthic forami-nifera from Site 1123 over 1.2 Ma (Hall et al.,2001), but as mentioned above, more detailedwork needs to be completed.

6.6. Implications

Changes in the velocity of the Southwest Paci¢cDWBC will re£ect changes in the intensity of bot-tom water production around Antarctica, partic-ularly as the Southwest Paci¢c DWBC is the larg-est single source of bottom water to the world’soceans (i.e. Carter et al., 1996; Stickley et al.,2001). Bottom water production, in turn, is linkedto circulation around Antarctica, climatic condi-tions, ice sheet coverage and stability and thepresence of sea ice (i.e. Keller and Barron, 1983;Carter et al., 1996). Studies of various sedimentparameters during more recent periods link glacialintervals around New Zealand to increased bot-tom water production from Antarctica (i.e. Hallet al., 2001, 2002; Stickley et al., 2001).

The record representing the period between 27and 9 Ma shows the presence of a strong DWBCat the depth of Site 1124. Whereas the hiatusesmake it di⁄cult for exact interpretations, theMARs are generally consistently high, 1.5^2 g/cm2/ky (between the hiatuses) starting at approx-imately 23 Ma. In a similar fashion, other sedi-ment parameters are consistent throughout thisinterval. Thus, while the proto-ACC and deepwaters have been forming since the early Oligo-

cene, the full establishment of this system to mod-ern circulation patterns likely occurred at V23Ma as deepening of the gateway between SouthAmerica and the Antarctica Peninsula (the DrakePassage) continued. This generally correspondswith others estimates of ACC establishment, com-pleting the thermal isolation of Antarctica andallowing for signi¢cant bottom water formation(Kennett, 1977; Lawver et al., 1992; Zachos etal., 2001).

Evidence exists from the Antarctic continentand o¡shore Antarctica of signi¢cant variationin ice-volume before and after the middle Mio-cene (e.g. Webb and Harwood, 1991 and referen-ces therein; Carter and McCave, 1994; Flowerand Kennett, 1994, 1995; Zachos et al., 2001)and these £uctuations are re£ected in the recordof DWBC velocities provided by this study. LateOligocene warming, as indicated by oxygen iso-topes, begins V28 Ma and reaches its initial max-imum just after 27 Ma (Wright and Miller, 1993).This warming may have caused the DWBC toslow down enough to allow deposition. Climatebegins to degrade V25 Ma and the ¢rst majorMiocene glacial period (Miocene isotope-1 [Mi-1]) occurs around 23 Ma, giving rise to the ¢rsterosional/non-depositional period of the Miocene.The peak in MAR’s at 24 Ma, associated withslightly slower currents based on PP values is in-terpreted as the increase in terrigenous materialcoming from the rising mountains along the Al-pine Fault boundary (Carter et al., 1999) althoughits exact pathway from New Zealand is uncertain.

The hiatus between V22.5 and 17.5 Ma, result-ing from erosion, carbonate dissolution or lowsedimentation rates, is interpreted as a time ofincreased bottom water formation and thermoha-line circulation. A widespread hiatus was noted byKeller and Barron between V21 and 17 Ma,(NH1, accounting for timescale di¡erences).While this is a relatively warm episode climati-

Fig. 7. Expanded version of the Bruhnes^Matuyama boundary plotted on an orbitally-tuned timescale determined for Site 1123,tied to the Site 1124 record using proxy carbonate data (Hall et al., 2002). Terrigenous MAR (A), terrigenous median grain size(B), carbonate wt% (C), terrigenous wt% (D), bulk magnetic susceptibility (E), and magnetic fabric (F). Sampled at higher resolu-tion. Glacial periods determined from N

18O values from Site 677, representing glacial periods 10^22.

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cally, Kennett (1995) suggests that this unusualconnection between warm climates and increasedintensity of Antarctic circulation results from thecomplete establishment of the ACC and the PolarFront Zone just prior to this time period. Thee¡ect of this is to isolate the Antarctic systemfrom signi¢cant warming in the north.

The period from 17 to 14.5 Ma is generallywarm, although oxygen isotope high Mi-2 occursnear the midpoint of this interval, an interval ofincreased deep water production occurred at V15Ma (Wright and Miller, 1993), and changing cir-culation patterns were noted by Keller and Bar-ron (1983) between V16.5 and 15 Ma (NH2).The glacial period probably stimulated the inten-sity of the DWBC and caused non-deposition orerosion between 16.5 and 15 Ma. The warmerperiods are likely associated with records of highMARs and strong magnetic fabric, indicating asediment supply that exceeded the carrying ca-pacity of the current, yet the current was fastenough to leave its signature in the form of mag-netic fabric. A recent study by Hall et al. (2003)on nearby Site 1123 indicates DWBC intensi¢ca-tion during 15.5^12.5 Ma due to increased pro-duction of Southern Component Water.

The ¢nal Miocene hiatus between V14 and 11Ma at Site 1124 is associated with major expan-sion of the East Antarctic Ice Sheet; a shift inN18O of nearly 1.0x to more positive values oc-curs during this interval (Flower and Kennett,1993; Wright and Miller, 1993; Kennett, 1995).A widespread hiatus (NH3) was noted at approx-imately 12.5^11.6 Ma (Keller and Barron, 1983).The expansion of the East Antarctic Ice Sheetwould be expected to strengthen both atmosphericand thermohaline circulation, the latter throughenhanced bottom water formation and ACC cou-pling with the DWBC.

At the beginning of the late Miocene, the peri-

od from 11 to 9 Ma exhibits some of the mostpositive N

18O values of the entire Miocene andcorresponds with a signi¢cant sea-level drop.The West Antarctic Ice Sheet was growing andestablishing itself, although highly unstable duringthe late Miocene (e.g. Haq et al., 1987; Kennett,1995). This growth would be expected to have hada signi¢cant impact on bottom water formation.The sediment record provided here indicates notonly an increase in MARs at this time, but also anenhancement of the strength of magnetic fabric.At the peak of terrigenous input, the currents arethe strongest, as opposed to the trend at 25^23Ma when increases in measures of magnetic fabricare associated with decreases in terrigenous £ux.The increase in terrigenous input at this site mayhave resulted from signi¢cant Antarctic erosionby the WAIS, which contributed signi¢cant sedi-ment amounts to the Weddell and Bellinghausenabyssal plains and likely to the Ross Sea area aswell (Kennett, 1995). Analysis of sediment fromKerguelen Plateau (Site 744) also indicates a sig-ni¢cant increase in MARs at V10 Ma (Joseph,2001). However, although diatom tracers fromAntarctica are present in New Zealand sedimentdrifts (Stickley et al., 2001), this path may be toolong for large amounts of the denser terrigenousgrains to travel. Another possible source includesthe increase in sediment supply from New Zea-land noted at V10 Ma (e.g. Carter et al., 1996);provenance studies would help resolve the truesource, or sources, of this increase in terrigenous£ux. A warming following this cold episode lastedto V7 Ma (Kennett, 1995).

The distinct lack of current in£uence at Site1124 from 9 Ma to the present does not indicatethat the DWBC is e¡ectively non-existent, espe-cially as its modern presence has been con¢rmedthrough physical oceanography (i.e. McCave andCarter, 1997). Site 1121, which is located directly

Fig. 8. Environmental velocity determinations using magnetic fabric (left) and grain size (right) parameters (see Joseph et al.,1998). (A^E) represent di¡erent time slices (oldest to youngest), determined from changes in the trends of the data in Fig. 4. Thesmall shapes are the results from ODP Site 1124 samples, while the larger shaded areas behind them indicate previously de¢nedenergy ¢elds (Joseph et al., 1998). The pelagic/hemipelagic ¢eld indicates a low energy depositional environment, the sedimentdrift ¢eld indicates a moderate energy, and the turbidite ¢eld indicates a high energy environment.

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in the path of the ACC before it decouples fromthe DWBC, exhibits an hiatus over 50 my be-tween V56 to 1.5 Ma, representing signi¢cantcurrent strengths at times within this period, andnecessarily in the youngest portion. Sedimentfrom Site 745 (Joseph et al., 2002), indicates var-iable and unstable ice sheets present on Antarcticauntil around 4 Ma, with the ice sheet becomingwell established near sea level since then. Thus,one would expect £uctuations in the DWBC asice sheet £uctuations near the coastline a¡ectdeep water formation. Other records indicate a£ow-speed maximum in the same general area asSite 1124 at V8.4 Ma, likely due to the formationof the West Antarctic Ice Sheet (Carter andMcCave, 1994). Therefore, the lack of current in-£uence at Site 1124 is a localized e¡ect, possiblydue to a small shift of the core of the current fromthe location of Site 1124. Thus the interval be-tween 9 and 1.5 Ma at Site 1124 more likely rep-resents variations in New Zealand history andevolution, rather than that of the DWBC. Fluc-tuations in sediment parameters may correspondto glacial/interglacial episodes however, reanalysisof the data after the removal of ash may aid theseinterpretations as would determination of terrige-nous provenance.

7. Conclusions

Through the use of sediment characteristics, in-cluding terrigenous £ux calculations, terrigenousgrain size analysis, magnetic fabrics, and the tim-ing of erosional/non-depositional events, the his-tory of the Southwest Paci¢c DWBC betweenV27 and 9 Ma is presented from Site 1124. Thesesediment parameters allow interpretations ofDWBC velocities that indicate the establishmentof the ACC to modern pathways and intensities atapproximately 23 Ma, as the tectonic seaways ex-panded, completing the thermal isolation of Ant-arctica. The sediment parameters used in thisstudy appear to be very sensitive to £uctuationsof the Southwest Paci¢c DWBC between 23 and9 Ma, and are in concordance with major Antarc-tic episodes of glacial development and circulationchanges. The hiatus between 22.5 and 17.6 Ma

represents an increase in thermohaline circulationaround Antarctica due to the recent opening ofcircum-Antarctic gateways, while the hiatuses at16.5^15 and 14^11 Ma represent glacial periodson Antarctica and major £uctuations in the EastAntarctic Ice Sheet separated by periods of rela-tive warmth. The time period from 11 to 9 Ma ischaracterized by conditions surrounding the for-mation and stabilization of the West Antarctic IceSheet.

The sediment record at Site 1124 from 9 Maand younger contains a history of New Zealandclimate and tectonics. The lack of indications ofcurrent activity during this time period is likely alocal e¡ect caused by the relative movement of theDWBC away from the 1124 location. Depositio-nally, it is not a sediment drift environment dur-ing this time.

The delivery of terrigenous sediment from NewZealand to the continental margin increases atapproximately 10 Ma, and volcanic activitystarted during the late Miocene. The HikurangiChannel shifted to its path across the HikurangiPlateau to the continental margin during the Plio-cene or Pleistocene. Despite all these potentialsediment inputs, terrigenous MARs remain fairlylow and constant through the Pleistocene.

Acknowledgements

We thank the Leg 181 scienti¢c party, sta¡, andcrew of the JOIDES Resolution, without whichthis paper would not have been possible; the in-formation gained is certainly a group e¡ort. EunPark was of much assistance in the laboratory.We appreciate the helpful comments and thought-ful reviews provided by Lionel Carter, Ben Flow-er, Ted Moore, Josep Pares, and Don Zak. IanHall graciously provided the orbitally-tuned time-scale for the youngest part of our record. Post-cruise funding was provided by JOI^USSAC. Thisresearch used samples and data provided by theOcean Drilling Program (ODP). The ODP issponsored by the U.S. National Science Founda-tion (NSF) and participating countries undermanagement of Joint Oceanographic Institutions(JOI), Inc.

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neogene history of the deep western boundary current at

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