The behaviour of the Leeuwin Current offshore NW Australia during the last five...

Global and Planetary Change 75 (2011) 119–132

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Global and Planetary Change

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The behaviour of the Leeuwin Current offshore NW Australia during the last fiveglacial–interglacial cycles

Michelle I. Spooner a, Patrick De Deckker b,⁎, Timothy T. Barrows c, L. Keith Fifield d

a Formerly of Department of Earth and Marine Sciences, The Australian National University, Canberra ACT 0200, Australiab Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australiac Geography, College of Life and Environmental Sciences, The University of Exeter, Exeter, Devon, EX4 4RJ, UKd Department of Nuclear Physics, Research School of Physics and Engineering, The Australian National University, Canberra ACT 0200, Australia

⁎ Corresponding author. Fax: +61 2 6125 5544.E-mail address: [email protected] (P. D

0921-8181/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.gloplacha.2010.10.015

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 July 2010Accepted 29 October 2010Available online 13 November 2010

Keywords:thermocline depthglacialinterglacialplanktonic foraminiferasea-surface temperatureMIS 11

The Leeuwin Current is an anomalous eastern boundary current along the western Australian coast. Toinvestigate its behaviour through time, we studied core MD002361 obtained from below the present-daypathway of the Leeuwin Current offshore the NW tip of Western Australia. Planktonic foraminiferaassemblages, sea-surface temperature estimates reconstructed from those assemblages, together with theδ18O and δ13C signals of near-surface dwelling foraminifera (Globigerinoides ruber), were used to reconstructthe vertical structure of the water column for the past 500 ka. Our findings indicate that the Leeuwin Currentwas present along the western coastline of Australia even during glacial periods. During those times, therewas a greater influence of South Indian Subtropical Water (STW) and South Indian Central Water (SICW) dueto a 3–4° northward migration of the Indonesian Throughflow Water/South Indian Central Water frontalsystem. This resulted in an overall 6–9 °C decrease in SST, paralleled by a thickening and greater homogeneityof the mixed layer. The increased influence of STW and SICW also suggests that the West Australian Current,which presently sits below the Leeuwin Current, was strengthened during the glacial periods and contributedto a weakening of the Leeuwin Current. Conversely, the Leeuwin Current was ‘stronger’ during interglacialperiods due to a thicker component of Indonesian Throughflow Water sourced from the Indo Pacific WarmPool. This was particularly the case during marine isotope stage 5.5 (MIS) and the ‘super’ interglacial MIS 11.

e Deckker).

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1. Introduction

Compared to other locations in the southern hemisphere, theLeeuwin Current is anomalous eastern boundary current transportingwarm, low salinity water formed within the Indonesian Throughflowand the Central Indian Ocean. The influence of this current extendsoffshoreWestern Australia fromNorthwest Cape in the north down toCape Leeuwin at the south-western tip of Western Australia and itcan, at times, extend as far as the west coast of Tasmania. The LeeuwinCurrent is seasonal, being more predominant during the Australwinter and has a temporal variability due to variation in the along-shore pressure gradient and prevailing equatorward winds. It is alsostrongly influenced by ENSO (El Niño Southern Oscillation) beingstrengthened during La Niña years and weaker during El Niño years.

Past investigations of the Leeuwin Current have been inconclusive,especially in regard to its occurrence during glacial periods. There isevidence that the Leeuwin Current either operated differently or wasabsent during glacial periods due to a strengthening of the opposing

West Australian Current and/or reduction of the along-shore pressuregradient. The studies of Wells and Wells (1994) and Wells et al.(1994) used planktonic foraminifera for sea-surface temperaturereconstructions from numerous cores located along the pathway ofthe Leeuwin Current for the past 130,000 years (see Fig. 1) andsuggested that the Leeuwin Current did not flow during glacialperiods and upwelling occurred during MIS 6. However, Wells andWells (1994) did not estimate temperature changes within the mixedlayer. However, Martinez et al. (1999) also found a reduction oftropical planktonic foraminifera species (e.g., Globigerinoides sacculi-fer) that coincided with an increase in intermediate (e.g., Neoglobo-quadrina dutertrei) and deep-dwelling species (e.g., Globorotaliainflata), suggesting a reduced Leeuwin Current. De Deckker (1997),Martinez et al. (1999), De Deckker (2001) and De Deckker et al.(2002), all suggested that the reduced input of low salinity water fromthe Indonesian Throughflow (ITF) and different sea-levels would haveencouraged the ITF incorporation into the South Equatorial Current(SEC) or South Java Current (SJC) enhancing these systems and,thereby, reducing the Leeuwin Current (see Fig. 1). The dominance ofthe SEC over the Leeuwin Current was also found by Takahashi andOkada (2000) whose study of calcareous nannofossils similarlyindicated that the Leeuwin Current was reduced during the Last

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Sumatra

Throughf lo

w

MD00-2361

ODP 762

ODP 760

Fr10 / 95 -GC17Northwest Cape

RS95GC21Gascoyne coast

CapesCurrent

MD01 -2378

90o 100o 110o 120o 130o

PacificOcean

Australia

Borneo

IndianOcean

140o

10o

0o

-10o

-20o

-30o

-40o

-140o-130o120o110o100o90o-40o

-30o

-20o

LC

WA

C

EGC

ECC

SJC

SEC

LC LC

LC

-10o

10o

0o

Fig. 1. Map showing the major surface/sub-surface currents in the eastern Indian Ocean and the Indo Pacific Warm Pool region. Abbreviations for those currents are: EEC: EastEquatorial Current; EGC: Eastern Gyral Current; LC: Leeuwin Current; SJC: South Java Current. Of interest is the Leeuwin Current which flows southward along the shelf break ofWestern Australia, while the West Australian Current [WAC] is located offshore and flows northward underneath the Leeuwin Current. The Leeuwin Undercurrent which sits belowthe LC is not shown here to maintain clarity. The coastal counter current [Capes Current] is shown near the south-western tip ofWestern Australia. The location of sites mentioned inthe text as well of MD00-2361 is shown as well as other cores discussed in this study.

120 M.I. Spooner et al. / Global and Planetary Change 75 (2011) 119–132

Glacial Maximum (LGM) and suggested that it did not reach thesouthern core site (111°49.75′E, 24°44.67′S) which they studied.

In addition, it has been suggested that theWest Australian Current(see Fig. 1) strengthened during glacial periods, thus reducing theinfluence of the Leeuwin Current (CLIMAP, 1984; Prell and Hutson,1979; Prell et al., 1979, 1980; Prell, 1985; Wells and Wells, 1994,1994; Okada and Wells, 1997; Takahashi and Okada, 2000; Barrowsand Juggins, 2005). Barrows and Juggins (2005) updated previousLGM SST estimates provided by Prell et al. (1980), and Wells andWells (1994), and indicated that lower SST off Western Australia mayhave been due to cooler waters entering the area via a strengthenedWest Australian Current, but that the Leeuwin Current probablyreached as far as 32°S. This observation is supported by calcareousnannofossil studies of Okada and Wells (1997) and Takahashi andOkada (2000). Further, Gingele et al. (2001)'s analysis of claymineralsfrom cores in the Indonesian Throughflow region and on theNorthwest Shelf gave insight into the pathways of the Indonesian

Throughflow (ITF) and the Leeuwin Current during the Last GlacialMaximum (LGM). This latter study suggested that the volume of theITF decreased during the LGM because less kaolinite and chloritereached the Timor Passage from the Banda Sea. In addition, offshoreNorthwest Cape, the same authors suggested that a reduction inchloritemay also have indicated a decreased Leeuwin Current becauseof the apparent reduction in the flow of the ITF.

There is some evidence of pulses of nutrient-rich water ‘upwelling’along the western coastline of Australia during glacial periods (Wellset al., 1994; McCorkle et al., 1994; Rogers and De Deckker, 2007).However, a fully developed upwelling system comparable to theeastern boundary currents offshore Peru and Namibia has never beenidentified. During interglacial periods, tropical and subtropicalplanktonic faunas have been used to indicate an enhanced LeeuwinCurrent at higher latitudes. For example, there appears to be evidencefor a stronger-than-present Leeuwin Current resulting in warmer seasaround the western and southern coasts of Australia during the last

121M.I. Spooner et al. / Global and Planetary Change 75 (2011) 119–132

interglacial (MIS 5e; ~125 ka). Wells and Wells (1994) indicated thatSST during MIS 5e was similar to today's (±1.3 °C error range) insummer and up to 3 °C warmer in winter. While coral reefdevelopment of the Ningaloo Reef system indicated that the lastinterglacial reef growth towards the end of the sea-level high stand(120–115 ka) was more extensive than Holocene reefal growth, likelyresulted from a stronger Leeuwin Current (Collins et al., 2003). PerthBasin deposits of Substage 5e also record a southerly expansion ofwarm water corals and other faunas consistent with shelf tempera-tures warmer than present (Kendrick et al., 1991).

This study investigates the dynamics of the surface layers andthe behaviour of the Leeuwin Current over the last 550 ka. Tocapture the presence and behaviour of the Leeuwin Current, deep-seacoreMD00-2361 (labelled hereinMD2361)was obtained frombeneaththe present-day pathway of the Leeuwin Current at a latitudewhere theLeeuwinCurrent canbedistinguished from the IndonesianThroughflow(ITF). The core is also located today on the south-westernmargin of theIndo Pacific Warm Pool (IDWP; see De Deckker et al., 2002 for termusage), and was chosen to decipher any changes in its extent throughtime. This study is based around the relative abundance of planktonicforaminifera, stable isotopic data, and temperature estimates of theupper part of the water column using the modern analogue technique(Barrows and Juggins, 2005).

The advantage of our study over previous investigations is thatcore MD2361 was sampled at high resolution. The modern analoguetechnique provides estimates of SST through time and the thickness ofthe mixed layer, the latter has not been previously quantified. Inaddition, we paid particular attention to MIS11 which may provide anunderstanding into the role of the Leeuwin Current in a futureenhanced greenhouse climate because this interglacial has beenconsidered to have similar orbital parameters to, but be warmer than,the present (Loutre and Berger, 2003).

2. Modern oceanography

The Leeuwin Current, off the coast of Western Australia, is uniquewhen compared to eastern boundary current systems in the SouthernHemisphere such as along the coastlines of Chile–Peru (HumboldtCurrent) and southwest Africa (Benguela Current). Most easternboundary currents transport cold, nutrient-enriched (upwelled)waters, flow equatorward, and are large sinks of heat from theatmosphere (Tomczak and Godfrey, 1994). Instead, the LeeuwinCurrent transports warm (tropical), low salinity waters poleward overa distance of 5500 km, and becomes well-defined near NorthwestCape (Western Australia; see Fig. 1) where waters from the centralIndian Gyre join with those from the Throughflow. At times, in[Austral] winter, the Leeuwin Current can extend as far as the westcoast of Tasmania (Ridgway and Condie, 2004). As such, the LeeuwinCurrent is one of the very few extended and possibly the longest zonalcoastal boundaries found anywhere in the world (Ridgway andCondie, 2004).

The Leeuwin Current is characterised by SST of ~29 °C in the north to~21 °C in the south in February, and ~27 °C in the north to ~16 °C in thesouth in August. This temperature gradient steepens south of ~22°S atthe southern boundary of the Indo Pacific Warm Pool and the ITF-SICWfront. Themain core of the LeeuwinCurrent is typically 100–200 mdeepand several km wide; it flows parallel to the coast at speeds up to0.45 m s−1, thus transportingup to5 Sv [=Sverdrup] andusually tracksalong the continental shelf edge (Feng and Meyers, 2003). Below theLeeuwin Current, there is a deeper (300–400 m) equatorward flow ofthe LeeuwinUndercurrent that today sits above themuch broaderWestAustralian Current [WAC] (Thompson, 1984; Domingues et al., 2007),which transports STW, SICW, andAntarctic IntermediateWater (AAIW)towards the tropics along the upper continental slope (Thompson,1984; Sprintall et al., 2002; Fieux et al., 2005; see Fig. 2).

There is some disagreement as to the source of the LeeuwinCurrent (Godfrey and Ridgway 1985; Holloway and Nye, 1985;Church et al., 1989; Cresswell and Peterson, 1993; Nof et al., 2002;Sprintall et al. 2002; Wijffels et al., 2002; Ridgway and Condie 2004;Domingues et al. 2007). Due to temperature and density character-istics, the general assumption is that the Leeuwin Current is a branchof the Indonesian Throughflow (Godfrey and Ridgway, 1985; Nofet al., 2002). This is also based on the strengthening of southwestcurrent pulses between February and June in the Timor Sea (Cresswelland Peterson, 1993) and the Northwest Shelf (Holloway and Nye,1985; Church et al., 1989; Cresswell and Peterson, 1993). Recentstudies, such as those of Wijffels et al. (2002) and Sprintall et al.(2002), regard the Leeuwin Current as being indirectly affected by theITF and variations in its strength are complicated by the interactionswith the South Java Current and the South Equatorial Current. Inaddition to its tropical source, it is believed that the Leeuwin Currentalso transports relatively cooler and saltier subtropical waters as abranch of the Eastern Gyral Current (Godfrey and Ridgway, 1985;Ridgway and Condie 2004;Wijffels et al., 1996; see Fig. 1). Domingueset al. (2007) also suggest a remote tropical source of the LeeuwinCurrent in the equatorial Indian Ocean, via the South Java Current.

Initial studies also believed that the variability in the LeeuwinCurrentwasdominatedby thenorthwardwind stress during theAustralsummer (McCreary et al., 1986; Kundu andMcCreary, 1986; Smith et al.,1991). Other studies promote the meridional pressure gradient, set upby sea-level differences and the excessive heating of theWarmPool andthe large amounts of cooling in the poleward region, as themain forcingmechanism (Thompson, 1984, 1987; Godfrey and Ridgway, 1985;Weaver and Middleton, 1989; Batteen and Rutherford, 1990; Godfreyand Weaver, 1991; Smith, 1992; Godfrey, 1996; Morrow and Birol,1998; Feng and Meyers, 2003). Recently, Ridgway and Condie (2004)confirmed that the along-shore pressure gradient was the dominantforce driving the Leeuwin Current along the western coastline ofAustralia, but also acknowledged that the seasonality in the strength ofthe Leeuwin Current is due to variations in both along-shore pressuregradient and the coastal wind stress (southwesterly winds).

Generally, the flow of the Leeuwin Current is strong enough tooverwhelm the upwelling tendencies of the coastal wind stress;therefore, upwelling via Ekman transport does not occur offshoreWestern Australia (Reason et al. 1999). However, due to seasonaldifferences in equatorward wind stress along the west coast ofWestern Australia, the Leeuwin Current flow is weaker betweenOctober and March (Pearce and Pattiaratchi, 1999; Meyers, 1996;England and Huang, 2005). The general pattern of wind change inrelation of the strength of the Leeuwin Current is described in detail inGodfrey and Ridgway (1985). The Leeuwin Current restricts theeastern arm of the Indian Ocean gyre to offshore regions (the WestAustralian Current), generating large-scale downwelling as it travelsalong the continental shelf break (Pearce, 1991; Smith et al., 1991). Itis postulated that without the Leeuwin Current, the prevailingequatorwardwind, that is driving northwards the Capes and NingalooCurrents along the Western Australian coastline, would almostcertainly produce strong coastal upwelling due to offshore Ekmantransport and this would be associated with enhanced productivity(Suess and Thiede, 1983; Veeh et al., 2000).

3. Methods

Core MD2361 was taken at 113°28.63′E, 22°04.92′S, at a depth of1805 mbsl, during the TIP 2000 expedition of the Marion Dufresne in2000. This core reached 42 m in length, however, only the upper1360 cm of the core was analysed in this study. The core was sampledat 4 cm intervals between 0 and 300 cm (primarily to obtain a detailedage model for this interval) and every 5 cm from 300 down to1360 cm. Each sample (3 cm3 of sediment) was soaked in dilute (3%)hydrogen peroxide solution until the clays disaggregrated from the

Fig. 2. Graph showing the age-depth relationship for the studied part of core MD2631. The chronology is based on (1) tie points [shown as open squares] linked with the isotopicSPECMAP nomenclature andwhich are labelled in Fig. 3 and (2) the radiocarbon dates [as open circles] presented in Table 1. Sedimentation rates may be deduced from this curve andshow some variation over the sampled interval.


microfossils, then wet sieved using the 63 μm mesh size to collect theplanktonic foraminifera tests and oven dried at 60 °C prior tomicroscope examination.

Approximately 15 Globigerinoides ruber were selected from the250 μm-size fraction for isotopic analysis. A total of 365 samples wereanalysed by the Finnigan MAT 251Mass Spectrometer at the ResearchSchool of Earth Sciences at The Australian National University.Samples were run concurrently with the international standardsNBS-19 (7 samples per run) and NBS-18 (1 sample per run), with ananalytical error for δ18O±0.08‰ and for δ13C±0.05‰ (Joe Cali, pers.com.). Core MD2361 was taken at approximately the same location ascore FR10/95-GC17 which had been extensively studied [see van derKaars et al., 2006, and references therein], but only covers 80 ka. Thislatter core already showed substantial changes in the water columnthrough time (see De Deckker (2001) for details).

3.1. Age model

The chronology of core MD2361 was constructed using: (1)oxygen isotope stratigraphy, (2) 14C accelerator mass spectrometeranalyses and (3) biostratigraphic last appearance of pink G. ruber.Twelve radiocarbon dates were obtained from 10 mg of G. ruber testsbetween 0 and 203.5 cm. Target preparations followed standardtechniques at the Tandetron of Gif-sur-Yvette (Tisnérat-Laborde et al.,2001). These samples were then dated at the Department of NuclearPhysics, Research School of Physics and Engineering, The AustralianNational University. Radiocarbon dates were calibrated usingthe CALIB 5.0 program (Stuiver et al., 2005), and the Marine04radiocarbon age calibration curve (Hughen et al., 2004). For theoceanic reservoir correction, the estimate of the regional ΔR meanfor NW Australia and Java of 67±24 (Bowman, 1985; Southon et al.,2002) was obtained from the CALIB Marine Reservoir CorrectionDatabase. Beyond the 14C limits, the age model was based on the

chronology of Bassinot et al. (1994) whose study captures isotopicevents in the tropical Indian Ocean that were accurately tuned to theastronomical time scale.

3.2. Planktonic foraminifera analysis

A total of 66,646 planktonic foraminifera were picked andidentified from core MD2361 samples. This is equivalent to approx-imately 436 individuals per sample, at an average sampling interval of9 cm. The species nomenclature used in this study follows thetaxonomy of Saito et al. (1981). Counts of planktonic foraminiferaweremade on splits of the N150 μm fraction, removing juveniles. Eachsample was split by an Otto-micro splitter until ~ 400 species werepresent in the final split. A full species list and abundance counts areavailable in BLUENET, the Australian Marine Science Data Network(www.bluenet.org.au) and will be on the journal's web site.

Present-day biogeographical provinces of planktonic foraminiferawithin the eastern Indian Ocean are mostly dictated by temperature,salinity, mixed layer and nutrient parameters of the various currentsystems in the region. Our knowledge of the ecology and present-daydistribution of planktonic foraminifera in the Indian Ocean (Bé andTolderlund, 1971; Bé andHutson 1977;Martinez et al., 1998; Ding et al.,2006) is the basis of palaeoceanographic reconstructions to determinethe behaviour of the Leeuwin Current over 5 glacial/interglacial cycles.

Principal component analysis (PCA) was performed on theabundance counts of foraminifera to understand the variance ofecological groups within the data. The C2 program, version 1.4(Juggins, 2003) was used to perform the statistical analysis. Inaccordance with other studies (Martinez et al., 1998), species withabundance below 1% were excluded from the analysis and raw countswere entered into the analysis to reduce error. A scree test (Cattell,1966) was used to determine how many Eigenvalues to retain.

http://www.bluenet.org.au


3.3. Modern analogue technique

Sea-surface temperatures (SST) were estimated from the plank-tonic foraminifera assemblage data using the modern analoguetechnique (MAT), in conjunction with the AUSMAT-F4 database(Barrows and Juggins, 2005). Each SST estimate was calculated as themean of the 10 best analogues from the global database, using thesquare chord distance as the dissimilarity coefficient. The annual SST(Tmean) and the temperature of the warmest (Tmax) and coolestmonths (Tmin) were estimated. Temperature at 50 m, 100 m, and150 mwater depthwas estimated the sameway using the Levitus andBoyer Atlas (1994). Finally we also estimated the mixed layer depth(in mbsl) using modern analogues and the database of Kara et al.(2003). This approachwas used previously in Spooner et al. (2005) fora core offshore Timor.

The distance to the nearest analogue, the mean distance and thestandard deviation were also calculated to assess the quality of theanalogue. The most precise variable is mean annual temperature witha root mean squared error of prediction (RMSEP) of 0.84 °C. Tmax alsohas a low RMSEP of 0.87 °C, whereas, Tmin SST has the greatest errorwith a RMSEP of 0.98 °C (Barrows and Juggins, 2005). Throughout, thequality of SST estimates is measured by the square chord distance(dissimilarity coefficient), with squared-chord distances of b0.2indicating good analogues. For further details, refer to Barrows andJuggins (2005). We note that the previous work by Wells and Wells(1994) that aimed at reconstructing past SST offshore WesternAustralia used the Indian Ocean transfer function FI-2 of Hutson andPrell (1980) which has since been superseded by the modernanalogue technique (Barrows et al. 2000).

4. Results

4.1. Age model

The radiocarbon-dated samples, and their corresponding ages arelisted in Table 1 and the isotopic stages of MD2361 are shown in Fig. 2.The presence of pink G. ruberwas noted at 120,800 years BP (505 cm)in accordance with the last appearance datum [=LAD] in the IndianOcean (Thompson et al., 1979). The sedimentation rate has beenestimated based on the isotope chronology and fluctuates throughoutthe studied part of the core (Figs. 2–3).

4.2. Isotopic analysis of the foraminifera

In core MD2361, the mean δ18O range from the LGM to presentwas 1.50‰. Subtracting the global sea-level LGM response of 1.05‰(Duplessy et al., 2002) leaves a Δδ18O of 0.45‰ to be explained bytemperature and/or salinity effects. The lightest δ18O value occurred

Table 1List of the radiocarbon ages obtained from ~10 mg of species G. ruber from coreMD2361. TheAMS targets at Gif-sur-Yvette [code GifA], but final analysis of the targets was carried out at t5.0 and calibration dataset of Hughen et al. (2004).

Lab code GifA Lab code ANUA Depth (cm) 14C dates

MD61-40175 29604 0–1 1870±170MD61-40176 29605 17.5–18.5 2810±170MD61-40177 29606 37.5–38.5 3070±180MD61-40178 29607 45.5–46.5 3280±190MD61-40180 29609 65.5–66.5 3960±190MD61-40181 29610 81.5–82.5 4860±180MD61-40182 29611 99.5–100.5 6030±190MD61-40188 29617 135.5–136.5 7710±200MD61-40186 29615 151.5–152.5 9670±210MD61-40187 29616 175.5–176.5 12,730±230MD61-40185 29614 187.5–188.5 15,180±250MD61-40191 29619 203.5–204.5 20,330±240

during theMIS 5e (−2.50‰) with the penultimate glacial period (MIS6.2) recording −0.49‰, with a mean δ18O range of 1.66‰. Thegreatest isotopic differences recorded in core MD2361 are, however,at the MIS 12 (−0.04‰) to MIS 11 (−2.49‰) transition (see Fig. 3).Nevertheless, the mean δ18O range was only slightly larger (1.62‰)than the LGM to present Δ δ18O and the estimated ice-volumecontribution to the δ18O signal at the MIS 11 to MIS 12 transition is~1.5‰ (Chappell, 1998). The range in δ13C is from a high of 2.11‰during MIS 10 to a low of −0.05‰ during MIS 5.

4.3. Faunal analysis of foraminifera

The most abundant species throughout core MD2361 is thesubtropical species G. ruber. High abundances ≥30% are seen duringthe interglacial periods MIS 11, 9, 7, 5 with peak abundance (37.7%)during the Holocene. Other tropical and subtropical species follow thispattern (see Fig. 4). The highest abundance of the tropical speciesGs. sacculifer occurs during the MIS 5e (18.2% peak; ave. 12.7%), withthe next peak of 13.7% (ave. 8.8%) during MIS 9. Of interest is that MIS11 only recorded a relative abundance peak of 11.4% (ave. 9.6%). Thetropical, deep-water dweller Pulleniatina obliquiloculata registers alarge increase in abundance during the last interglacial (MIS 5e) andduring MIS 7, with peak values of 17.9% (ave. 12.6%) and 19% (ave.9.4%) respectively compared to 6% (ave. 3.28%) during the Holocene(see Fig. 4). The tropical ‘upwelling’ species Globorotalia menardiidisplays two periods of abundance during the Holocene (14.3%; ave.8.3%) and MIS 11 (11.9%; ave. 5.9%) respectively, and an abundance of~2% during other marine isotope stages (see Fig. 4).

The transitional, deep-dwelling species Gr. inflata has the mostdistinct abundance change within core MD2361 (see Fig. 4). Thisspecies is absent during the interglacial periods but reaches highrelative abundances (~20–30%) during glacial periods. The highestrelative abundance recorded for this species is during the MIS 2 andMIS 3 transition with a value of 32.6%.

The sub-polar to transitional and upwelling indicator Ga. bulloidesrecords its greatest abundance at MIS 11 and MIS 10 of 22% and 17.5%respectively and remains around 10% through all the other isotopestages (see Fig. 4). As an indicator of cold, nutrient-rich water, theincrease in abundance of Ga. bulloides at MIS 11 is noteworthy. Anothernutrient indicator, the subtropical deep-chlorophyll-maximum speciesN. dutertrei, seems to sustain its abundance of ~10%duringMIS 14 toMIS11, perhaps indicating nutrients around the thermocline but thenreduces in abundance during glacial periods.

4.4. Principal component analysis

Analysis via the scree test of the Eigenvalues (Cattell, 1966)indicates that four components should be retained to describe the

two laboratory codes result from the fact that samples were initially prepared at LSCE ashe ANU [ANUA]. Ages are calibrated within two standard deviations obtained with Calib

14C cal years 95.4% (2 σ) cal age range Calibration data

1364 982–1738 (Hughen et al., 2004)2466 2024–2854 (Hughen et al., 2004)2781 2327–3233 (Hughen et al., 2004)3037 2580–3513 (Hughen et al., 2004)3879 3405–4388 (Hughen et al., 2004)5072 4607–5530 (Hughen et al., 2004)6391 5944–6815 (Hughen et al., 2004)8109 7689–8508 (Hughen et al., 2004)

10,472 9956–10,705 (Hughen et al., 2004)14,283 13,703–15,025 (Hughen et al., 2004)17,807 16,953–18,614 (Hughen et al., 2004)23,749 22,878–24,376 (Hughen et al., 2004)

δ18O

Age years BP0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 500,000

0.00

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8.0

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11.23

11.24

12.2

12.0

13.1113.13

13.2

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11.2

δ13C

Fig. 3. The δ18O and δ13C isotopic record of the planktonic foraminiferG. ruber for selected horizons in coreMD2661 spanning the last 550,000 years BP. The numbering on the δ18O curverefers to the isotopic stages identified by SPECMAP and available in Martinson et al. (1987) and extended down to 550,000 years.


variation within the species of core MD2361 The Eigenvalues arelisted below: Axis 1: 0.55; axis 2: 0.11; axis 3: 0.09; axis 4: 0.07 andaxis 5: 0.04. However, only the first two components, describing 65%of the variance, will be detailed in this paper. Refer to Spooner (2005)for further details of the PCA analysis conducted on core MD2361.

The species component scores for Component 1 and Component 2(see Table 2) were plotted in bi-plots to reveal ecological groupings(see Fig. 5). Species with high positive scores in the first componentand highly negative scores in the second component define Group 1.As indicated by the vector length the principal species causing themain variance in Group 1 is Gr. inflata, with a minor association ofGlobigerina falconensis, Globigerinita glutinata and Neogloboquadrinapachyderma (dextral). The species of this group appear to betemperate or transitional zone species (Gr. inflata and N. pachyderma)which dominate during glacial periods (see Fig. 6). Species G. inflata isan intermediate-water dweller living below 100 m (Martinez et al.,1999), and further suggests that this group represents transitional,oligotrophic water at depth.

Group 2 is dominated by Gs. ruber, which is most prolific insubtropical waters. The secondary species in this group are alsotropical to subtropical species but have an association to nutrient-richwater near coastal regions (N. dutertrei, Globigerinella aequilateralis,Globigerinella. calida, G. menardii and Globorotalia ungulata). Thedominance of Gs. ruber over the rest of the group, as indicated by thevector lengths, suggests that this group represents subtropicaloligotrophic waters (see Fig. 5). An increase in relative abundanceof this group 2 is timed to the interglacial periods (see Fig. 6). Inaddition, there is an inverse relationship with the temperate, sub-surface species of Group 1 (see Fig. 6).

Group 3 is defined by species having a positive score in Component2 and a negative score in Component 1. The principal species consist ofP. obliquiloculata with secondary associations with Ga. bulloides,Globorotalia truncatulinoides (s) and Turborotalia quinqueloba. Lesscorrelative associations as represented by Gs. sacculifer which liesbetween Group 2 and Group 3, with secondary associations withGr. tumida, Gs. conglobatus and Globigerinoides tenellus (see Fig. 5).This group appears to contain species that have a preference fordeeper water in a wide range of temperatures and the inclusion ofGr. truncatulinoides and T. quinqueloba suggest a homogenised mixedlayer.

Group 4 is dominated by Globigerina rubescens (see Fig. 5). Thissmall species occurs in low abundances at scattered localities intropical and subtropical waters (Bé, 1977), and lives predominantly inthe upper 50 m of the water column (Bé, 1977). There is an

association with Gr. truncatulinoides (dextral) which is found in thetropical–subtropical areas (Kennett et al., 1979; Andrijanic, 1988), butit lives in waters predominantly below 100–200 m (Schiebel andHemleben, 2005). In the abundance graph, this group displays noconvincing patterns (see Fig. 6).

4.5. Sea-surface temperature reconstructions

The sea-surface temperature record of MD2361 covers five inter-glacial and glacial periods (see Fig. 7). The annual mean temperaturerange for the interglacial periods recorded in MD2361 is MIS13=21.0–28.2 °C, MIS 11=26.7–28.6 °C, MIS 9=26.6–28.6 °C, MIS 7=22.0–27.9 °C, MIS 5=21.5–29.1 °C and the Holocene=25.6–28.4 °C (seeFig. 7). Note that MIS13, MIS 7 and MIS 5 had greater seasonalitycompared to the other stages.

The squared-chord distances reveal the quality of SST estimates arelowest duringMIS 5 andMIS 7 (see Fig. 7). Samples with higher relativeabundances of tropical species Gs. sacculifer and P. obliquiloculata,compared to the Holocene, coincide with larger square chord distancesand higher estimated temperatures. The results from MIS 5 and MIS 7should be viewed with caution due to high square chord distancesvalues.

The SST estimates for MIS10.2 and the LGM are similar at 20.1±0.84 °C and 21.3±0.84 °C respectively (see Fig. 7), but we requirecaution as error bars show that values can be identical. Also, theTmean of MIS 12.2 and MIS 6.2 are equivalent to the LGMwith Tmeanof 20.8 °C±0.84 °C and 20.7 °C±0.84 °C respectively. The mostextreme glacial to interglacial transition was from the penultimateglacial period (MIS 6) to the last interglacial (MIS 5) with a meantemperature range of 8.7 °C. A similar range of 8.6 °C was also foundfrom the transition of MIS 10 to MIS 9. The large transition in theisotope record from MIS 12 to MIS 11 has a temperature range of7.8 °C, whereas the smallest transition of 6.2 °C is from the LGM to theHolocene (see Fig. 7).

The presence of the IPWP is indicated by a summer SST (Tmax)above 28 °C (Martinez et al., 1999). The IDWP extends over the site atthe interglacial periods but retracted northwards during the glacialand cooler periods with a general Tmax range of 22.0–25.0 °C.Compared to themodern Tmax of 28.9 °C, the warmest Tmax estimateof 29.9 °C occurs during MIS 11 and there is good confidence of thisestimate with a low square chord distance. The coolest Tmax estimateof 22.2 °C occurs during MIS 10. Likewise, MIS 10 records the coldestTmin (coldest month) of 18 °C. Seasonality (Tmax minus Tmin)indicates that the glacial periods have a more extreme annual

image of Fig.�3

δ18O

δ13C

% Gs. ruber

% Gs. sacculifer

% Gt. rubescens

% P. obliquiloculata

% Gr. menardii

% N. dutertrei

% Gn. glutinata

% Gr. inflata

% Ga. bulloides

Age years BP0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 500,000

0

-3.0-0.5

2.50

400

200

150

200

150

200

200

300

20

10

10

20

10

10

10

10

10

10

20

5

5

-2.0

-1.0

0.5

1.0

2 6 8 124 14

Fig. 4. Diagram showing the relative abundance of planktonic foraminifera in core MD2361 against age, and the δ18O and δ13C record of Gs. ruber. The species Gs. ruber, Gs. sacculifer,Gr. menardii and P. obliquiloculata are those which favour tropical–subtropical water conditions. Gn. glutinata and N. dutertrei are species which prefer cooler and nutrient-rich waterconditions. Gr. inflata prefers transitional water masses, between subtropical and polar water temperatures. Ga. bulloides is a sub-polar–transitional ‘upwelling’ species. Glacialperiods are highlighted in pale grey.


temperature range of ~4 °C compared to ~2 °C during mostinterglacial periods (see Fig. 7).

To interpret the mixed layer depth estimates, a reference line isplaced on the ‘present-day’ mixed layer estimate of 36 mbsl (seeFig. 7). The mixed layer is thinner during interglacial periodscompared to glacial periods. The depth of the mixed layer is between39 and 31 mbsl with the shallowest mixed layer depth recorded inMIS 11. During glacial periods, the mixed layer depth was between 63and 34 m below the sea-surface (see Fig. 7). The changing thickness ofthe mixed layer is also indicated in the estimated temperature at50 m, 100 m and 150 m water depths. Temperature at 50 m wasgreatly reduced from ~27–28 °C, during the interglacial periods, to~20–21 °C, during the glacial periods. Conversely, the temperature at

150 m is more stable, ranging between ~17–19 °C±0.84 °C duringinterglacial periods and ~15–16 °C during glacial periods. Theexception is during MIS 5 with much higher temperatures beingrecorded at 150 m (22–24 °C), but with a high square chord distanceof 0.3 (see Fig. 7).

5. Discussion

The relative abundance counts indicate an alternation betweentropical–subtropical, oligotrophic conditions during interglacialperiods and subtropical–transitional, oligotrophic conditions at thesea and sub-surface during glacial periods (see Figs. 4 and 6).Importantly, this may indicate that there has been a change in the

Table 2List of the species scores for the first four components described by principalcomponent analysis for core MD2361.

Species Component 1 Component 2 Component 3 Component4

Ga. bulloides −0.30 0.55 −1.97 −1.01Ga. falconensis 0.20 −0.06 −0.81 0.82T. quinqueloba −0.03 0.27 −0.01 0.17Gt. rubescens 0.13 1.46 −0.97 0.43Gt. tenellus −0.08 0.09 −0.22 0.15Gs. ruber −1.92 −3.61 0.02 1.80Gs. sacculifer −0.74 0.02 1.57 −0.47Gs. conglobatus −0.10 0.09 0.42 −0.01O. universa −0.11 −0.08 0.18 0.10Gl. aequilateralis −0.28 −0.69 0.25 −0.32Gl. calida −0.12 −0.18 −0.02 0.06Gn. glutinata 0.20 −0.05 −2.63 1.17Gr. inflata 4.21 −1.65 0.65 0.41Gr. hirsuta 0.01 0.00 −0.04 −0.05Gr. truncatulinoides (s) −0.06 0.48 0.23 0.34Gr. truncatulinoides (d) 0.14 0.27 −0.02 0.14Gr. crassaformis −0.01 0.46 0.11 0.41Gr. menardii −0.56 −0.74 −0.31 −1.95Gr. tumida −0.05 −0.03 −0.01 −0.05Gr. ungulata −0.27 −0.47 −0.32 −0.24N. pachyderma (d) 0.08 −0.03 0.14 −0.41N. dutertrei −0.08 −0.97 0.58 −3.09P. obliquiloculata −0.54 1.39 2.60 1.43


water masses influencing the core site which could explain the δ18Oand temperature ranges recorded in core MD2361. The interrela-tionship of these data will be explored further below.

5.1. Reduced Leeuwin Current during glacial periods

SST estimates from core MD2361 indicate that the boundary of theIPWP (28 °C isotherm) was not located north of the core site duringglacial periods. The northward movement of the IPWP at the LGM to18°S was initially suggested by Martinez et al. (1999) and supported

Fig. 5. Diagram showing a bi-plot of Components 1 and 2 that explain 65% of variance in coreMinor species that do not influence the main variance of the two groups are removed for c

by Barrows and Juggins (2005). This significant shrinking of the IPWPwould reduce the amount of low salinity, warm source water (ITF)feeding the Leeuwin Current. However, the persistence of tropicalsurface dwelling species such as Gs. sacculifer through the entire coresuggests that tropical water was continually present during the last~550 ka (see Fig. 4).This observation is critical to our interpretationthat the Leeuwin Current remained at times active during glacials. Therelative abundance of Gs. sacculifer declined to its lowest value of 4%during the LGM from an average abundance of 10–12.5% duringinterglacial periods. The relative abundance of subtropical to tropicalsurface dwelling species Gs. ruber was also reduced during glacialperiods but it still remained a dominant species (~17.5%).

Today, the upper section of warm, low salinity tropical water (ITF)of the Leeuwin Current becomes colder and the tropical character-istics are diminished as the current moves southward due to mixingwith subtropical, salty waters (STW) and heat loss via evaporation tothe atmosphere (Rochford, 1969;Wyrtki, 1971; Fieux et al., 2005), yetthe geostophic parameters maintain the characteristics of theLeeuwin Current even at times as far as the western coastline ofTasmania (Ridgeway and Condie, 2004). The greater reduction inrelative abundance of G. sacculifer compared to G. ruber couldrepresent a greater component of STW and the reduction of ITFproperties from the surface layer over the core site. Presently, the STWis capped under the “fresher” ITW water in the middle of thethermocline with a salinity maximum near the upper thermocline(18 °C isotherm; Fieux et al., 2005).

In addition, the glacial relative abundance of Gs. ruber, Gs. sacculiferand Gs. bulloides in core MD2361 are of a similar magnitude to therelative abundance of these species measured between 24 and 30°S(south-eastern Indian Ocean) in the core top study of Martinez et al.(1998), an area presently influenced by a greater component of theSTW compared to the ITW. The estimate of the mixed layer depth inMD2361 indicates that it became thicker during glacial periods.Estimates of water temperatures at the 50 m water depth by the MATreveal a significant reduction of water temperature at 50 m compared

MD2361 and which also reveals four ecological groups of foraminifera within the data.larity around the origin of the plot.

δ18O δ13C % Group 1 % Group 2 % Large Group 3 % Group 4

Age

yea

rs B

P

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

450,000

500,000

0.0 0.5 1.0 1.5 2.0 2.5 -0.5 0.5 1.0 1.5 2.0 2.5 0 10 20 30 40 50 60 10 20 30 40 50 60 70 0 10 20 30 40 50 0 4 8 12 16

Fig. 6. Diagram showing the respective percentages of the four ecological groups revealed by analysing Component 1 versus Component 2 with PCA analysis in core MD61. Thegroups are compared to the δ18O and δ13C record of Gs. ruber through time.


to the estimated temperature at 150 m (see Fig. 7) and suggestinggreater influence of STW within the mixed layer.

In core MD2361, the relative abundance of tropical and subtropicalspecies was reduced during glacial periods (see Fig. 4). However, therelative abundance change displayed by species living at the sea-surfacewas minor compared to sub-surface species (see Fig. 4). This is a key tounderstanding the flowof the Leeuwin Current during glacial periods. Itis shown that transitional species Gr. inflata was the most influentialspecies of Component 1 which caused themost variability (54%) withinthe component groups (see Table 2).

Throughout the record of core MD2361, Gr. inflata appears duringglacial periods with peak abundance leading the δ18O maxima. Thetwo curves seem to coincide well except for the earliest part of therecord. Conversely, during interglacial periods, Gr. inflata records verylow abundances of b1% (see Fig. 4). The occurrence of Gr. inflata hasbeen previously linked to South Indian Central Waters (SICW) byMartinez et al. (1999) in the south-eastern Indian Ocean. The increasein the abundance of Gr. inflata may indicate that there is an increasedinfluence of SICW over the core site during glacial periods. Previously,Duplessy et al. (1989) suggested enhanced ventilation of intermedi-ate-water masses during the LGM. An aspect of the modern-dayseasonal variability and weakening of the Leeuwin Current is thestrengthening of the WAC during the Austral summer. The WACcarries STW and SICW and therefore, the data from core MD2361suggest that the WAC was more dominant during glacial periods. Thisis in agreement with previous findings of Prell et al. (1980), Wells andWells (1994), Barrows et al. (1996), Okada andWells (1997), Barrowsand Juggins (2005) and Holbourn et al. (2005).

Nevertheless, we note that McGowran et al. (1997) cited the workof Wells and Wells (1994) and Okada and Wells (1997) and statedthat the flow of the Leeuwin Current was decreased or even absentduring glacial periods. This was also repeated by James et al. (1999)who stated that the Leeuwin Current did not flow into the GreatAustralian Bight, and may not have reached Rottnest Island offshorePerth. We believe the latter statement to be correct only for thesouthern portion the Western Australian coast. Further evidence forsuggesting that the Leeuwin Current was present during glacialsoffshore Northwest Cape is shown by the counts of upwelling speciessuch as Gs. bulloides, Gr. menardii and Gn. glutinatawhich did not fig. 7increase in relative abundance during glacial periods (see Fig. 4). Dueto the lack of evidence for sustained upwelling and the persistence ofwarm water species, it is possible that a thin veneer of warm waterstill existed to prevent the movement of cold nutrient-rich waters,characteristic of the WAC, from reaching the sea surface. This wouldbe analogous to what is presently seen further south of the coreposition from Perth to Cape Leeuwin where SSTs of ~20 °C are

observed and prevent prolonged steady state processes of upwelling(Gersbach et al., 1999). Benthic foraminifera analysed by Murgese(2003) and Murgese and De Deckker (2007) in core Fr10/95 GC17(very close to the location of core MD2361) showed high percentagesof C. wuellerstorfi between 31,000 and 18,000 years BP, spanning theLGM, therefore indicating oligotrophic conditions and increasedventilation by lateral advection of active bottom currents.

In regard to upwelling, another consideration is that the STW andSICW, being oligotrophic water masses, may not carry enoughnutrients to support a phytoplankton bloom. Modern-day ‘upwelling’seen along the Gascoyne coast is not significant because nutrientlevels are low as a result of being sourced from the base of theLeeuwin Current thermocline in the Capes and Ningaloo Currents(Gersbach et al., 1999; Hanson et al., 2005).

In addition, the relative abundance of deep-chlorophyll-maximum(DCM) species N. dutertrei also indicates that waters around thethermocline were disrupted during glacial periods, especially fromMIS 10 until today. Presently, near the location of core MD2361, awell-defined DCM is located near the nutricline at around 70 m belowsea level (Hanson et al., 2005). The thicker mixed layer suggested inglacial periods by the MAT would result in the thermocline/nutriclinemoving deeper in the water column, thus reducing light availabilityand thereby reducing the productivity of the DCM layer. Each glacialperiod except forMIS 12 andMIS 4 resulted in a reduced abundance ofN. dutertrei (see Fig. 4). This is supported by Martinez et al. (1999)who observed a conspicuous reduction of N. dutertrei and increase inGr. inflata during the LGM. The thickness of the mixed layer may be aresult of ‘caballing’ (downwelling) along the ITW-SICW front assuggested by Martinez et al. (1999), due to similar characteristics ofthe glacial ITW (colder and denser) and the SICW.

The relative abundance counts of MD2361 do not indicateupwelling during glacial periods. However, when comparing the SSTdata of core MD2361 with the study of Barrows and Juggins (2005),there appears to be a conspicuous cold cell (~3 °C cooler) around thelocation of core MD2361 and core Fr10/95-17 (analysed by Martinezet al. 1999) when compared to the estimates from surrounding coresduring the LGM. Similar Tmean estimates inMD2361 support that thiscold cell developed during all of the recorded glacial periods in thiscore. During the glacial periods, the SST (Tmean) was ~6–8 °C coolercompared to interglacial periods in core MD2361.

The establishment of large areas of cold surface water has beendocumented previously off the coast of Western Australia bynumerous studies (Prell and Hutson (1979), Prell et al. (1979,1980), CLIMAP members (1984), Prell (1985), Wells and Wells(1994), and Martinez et al. (1999)). In addition, Wells and Wells(1994) found anomalous cooling of−6 to−7 °C for cores during both

image of Fig.�6

δ18O

δ13C

Tmean

Tmax

Tmin

Seasonality(°C)

Mixed layerdepth(mbsl)

Temperatureat 150m

(°C)

Square chorddistance

Age (BP)

0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 500,0000.0

-2.5-0.5

2.516

3216

3216

320

6-70

-3016

16

2812

240.0

0.3

1.5

0.5

-1.5

24

24

24

4

2

-50

20

24

28

20

24

16

20

0.2

0.1

(°C)

(°C)

(°C)

Temperatureat 100m

(°C)

Temperatureat 50m

(°C)

2 4 6 8 10 12 14



glacial and interglacial extremes along the Gascoyne Coast but only 3–4 °C during MIS 6. This led to the argument that the Leeuwin Currentwas absent because of the large change in SST. However, reanalysis ofSST's of their cores by Martinez et al. (1999) and Barrows and Juggins(2005) combinedwith the results of MD2361, indicate that this area isisolated, only−3 °C cooler, and it appears that warmwater surroundsthe cold cell at the sea-surface (see Fig 8). The continued presence,although reduced, of tropical to subtropical foraminifera in MD2361and Fr10/95-17 (Martinez et al. 1999) during the LGM also suggestseasonal influences of warmwater. If the Leeuwin Current was absentduring glacial periods, one would expect a significant SST change allalong the coast. However, Barrows and Juggins (2005) showed thatthe isotherms were depressed southward near the coast during theLGM, therefore arguing that the Leeuwin Current was still active.

The cold cell development at the location of coreMD2361may be asignal of the ITF-SICW frontal system below the sea-surface. Theestimates of SST and the thickness of the mixed layer is suggestivethat the ITF-SICW frontal system (that presently meets at the sea-surface at 24°S) had moved 3–4° northward during the LGM.However, it is also possible that the reduction in SST is over estimateddue to the influx of sub-surface species, which was noted by Martinezet al. (1999), and this cold band may be more of an indication of whatis happeningwith the thermoclinewaters (sub-surfacewaters) ratherthan at the sea surface.

Analysis of benthic foraminifera by Wells et al. (1994) did not finda strong signal for productivity during the LGM. However, theseauthors suggested that during the penultimate glaciation (MIS 6)upwelling was established within the areas of low SST due to“depleted δ13C” [sic]. In core MD2361, there is no evidence ofupwelling during MIS 6 and instead the faunal assemblages show asimilar pattern as the LGM with the dominance of Gr. inflata. Theabsence of upwelling during MIS 6 has also been suggested by Okadaand Wells (1997) because small placoliths (calcareous nannofossils)did not increase in abundance during MIS 6. In addition, these authorsalso found that the reduced abundance of the nannofossil Florisphaeraprofunda indicated lower temperatures in the lower photic zonerather than increased productivity in the upper photic zone duringMIS 2 and 6. Takahashi and Okada (2000) suggested the LeeuwinCurrent was weakened at the site of core Fr 10/95-GC17 but did notreach core site Fr10-95-GC20 located further south (24°44′S) duringthe LGM. However, these assumptions were based on the anglebetween the bridge and long axis of several types of Gephyrocapsa(nannoplankton) to infer SST which cannot be converted to anumerically estimate (Takahashi and Okada, 2000).

A study on organic carbon of cores from the Exmouth Plateau andthe Perth Basin by Veeh et al. (2000) showed little support forenhanced productivity off Western Australia during the LGM. This issignificant because a previous study by McCorkle et al. (1994) on thesame core material used by Veeh et al. (2000) suggested that therewas a significant increase in productivity during the LGM. McCorkleet al. (1994) used mass accumulation rates of benthic foraminifera toindicate increased productivity during the LGM but Veeh et al. (2000)suggested the mass accumulation rate [=MAR] may be elevated dueto recycled organic matter during sea-level lowstands of the LGM (seeMurgese and De Deckker, 2007).

5.2. Enhanced Leeuwin Current during interglacial periods

The expansion of tropical water (ITW) over the MD2361 core sitewould provide a favourable habitat for tropical, oligotrophic planktonic

Fig. 7. Plots of the sea-surface temperature record based on the faunal assemblages of MD2calculated using the modern analogue technique and the AUSMAT-F4 database (Barrowswarmest (Tmax) and coolest months (Tmin), as well as seasonality and the depth of the mixand 150 mwater depth are also displayed. For further details, see text. The square chord distof b0.2 indicate good analogues.

foraminifera. An increase in the abundance of these species couldtherefore infer the presence/strength of the Leeuwin Current duringthese periods. Comparison between the interglacial periods recorded incoreMD2361 such as theHolocene and Isotope Stages 7, 9 and 11 showsthat the relative abundances of Gs. ruber and Gs. sacculifer wereequivalent to modern-day abundances (see Fig. 4). However, anincrease in relative abundance of tropical species G. sacculifer duringMIS 5e suggests that the Leeuwin Current may have been warmer [andtherefore probably stronger] compared to the present-day. In addition,the relative abundance of another tropical species, P. obliquiloculata,indicates warmer sub-surface conditions and possible enhancement ofthe Leeuwin Current during MIS 5 and 7 (see Fig. 4).

PCA analysis of Component 1 versus Component 2 combinedtropical surface and sub-surface species into Group 3. The peak inrelative abundance of Group 3 indicates that the Leeuwin Current waspossibly deeper during MIS 5, MIS 7 and MIS 11 compared to thepresent (see Fig. 6). In comparison, the Holocene period is dominatedby the subtropical Group 2 and MIS 9 was equally dominated bysubtropical and tropical groups (see Fig. 6). The increased influence ofITW in the mixed layer and the reduction of STW and SICW bothsuggest that the WA Current had weakened and, therefore, it can beinferred that the Leeuwin Current was stronger as a result.

The increased influence of ITW during interglacial periods is alsosuggested by the SST estimates which indicate most interglacialperiods were equivalent or warmer than the mid-Holocene Tmean of27.5 °C. During MIS 5e, a peak Tmean of 29.1 °C is recorded, while MIS11 records a peak Tmean of 28.6 °C (see Fig. 7). Seasonality is mostreduced during MIS 5 with only 1 °C difference between Tmax andTmin (see Fig 7), indicating warmer surface and sub-surface watersespecially when assessing the Tmin (~September).

The results from core MD2361 show that the tropical componentof the Leeuwin Current was deeper with a greater influx of tropicalITW into the flow. A greater influx of tropical waters suggests astronger, along-shore pressure gradient and, therefore, supportsprevious findings of an enhanced Leeuwin Current during MIS 5.Wells and Wells (1994) found a Tmin SST increase of 3 °C during MIS5e. In addition, the increase in the development of coral fringing reefsystems along the coast of Western Australia (Collins et al., 2003) alsosuggests an enhanced Leeuwin Current during MIS 5e.

5.3. Upwelling events

Upwelling during interglacial periods initially appears counterin-tuitive, i.e. if the Leeuwin Current was removed, then an upwellingsystem equivalent to that seen to offshore Peru or Namibia wouldexist. Upwelling pulses are seen today along the shallow shelfalregions along the western coastline driven by the cool equatorwardcoastal Ningaloo and Capes Currents during the Austral summer(Pearce and Pattiaratchi, 1999; Gersbach et al., 1999; Hanson et al.,2005). Hanson et al. (2005) describe the processes of upwelling to be acombination of a) the depth of the Leeuwin Current's nutrient-depleted mixed layer, b) the strength and duration of upwellingfavourable winds, and c) the geographic location. Presently, coreMD2361 is located under the pathway of the oligotrophic LeeuwinCurrent but, according to Hanson et al. (2005), this is near theboundary with the more nutrient-rich shelfal waters driven by theNingaloo Current.

There is evidence for upwelling in core MD2361; however, thisoccurs during interglacial periods. A major upwelling event wasindicated in the later stages of MIS 11 and the Holocene with a large

361 versus the δ18O and δ13C record of G. ruber through time. Various parameters wereand Juggins, 2005). Estimates of the annual SST (Tmean) and the temperature of theed layer (Dann) are shown. In addition to SST parameters, temperature at 50 m, 100 m,ance (dissimilarity coefficients) are indicated at the top of the diagram. Note that values


increase (12%) in the relative abundance of Ga. bulloides (see Fig. 4).Conditions of upwelling are also indicated by the tropical upwellingspecies Gr. menardii, towards the transition to MIS 10. Although anupwelling signal is not present in MIS 5 and 7 in core MD2361,nannofossil evidence along the western coastline indicates mild‘upwelling’ for MIS 7 and 5 in core RS96G21 (Okada andWells, 1997),but this core is further away from the Australian coast line.Nevertheless, some caution is required in inferring ‘upwelling’ usingthese abundances. For example, Ga. bulloides reaches high abundancesin the southeast Indian Ocean (up to 60%) where there is no upwellingat all. Although N. dutertrei is a better upwelling indicator, it isnevertheless lowest during interglacials.

The periods of increased abundance of ‘upwelling’ species occurwhen the mixed layer reduces in thickness and seemingly permits theupwelling of more nutrient-rich waters to the sea-surface (see Fig. 7).This is significant because upwelling waters of the Ningaloo Currentare sourced from the base of the thermocline where DCM layersdevelop (Hanson et al., 2005).

The reason upwelling may not be occurring during otherinterglacial periods may be due to wind direction. Wyrwoll andMiller (2001) and van der Kaars and De Deckker (2002) suggestedthat the NW Monsoon was intensified at approximately 14,000 yearsBP and this may provide more ideal wind conditions for upwellingnear the core location. In core MD2361, ‘upwelling’ species peakedbetween 15,000 and 5000 years BP. Due to similar orbital parametersduring MIS 11 and at present (Berger and Loutre, 2002; Loutre andBerger, 2003), a greater influence of the NWMonsoon or similar windforces may also have occurred during MIS 11.

Another factor that could be contributing to the upwelling signal isthe increase of fluvial discharge as a consequence of increasedprecipitation. From 14,000 to 5000 years BP, the region wascharacterised by substantial summer rains (Veeh et al., 2000; Gingeleet al. 2001; van der Kaars and De Deckker, 2002), which is paralleledwith the upwelling signal in core MD2361. The timing of theupwelling events during MIS 11 and the Holocene is associated withthe deposition of red clays within core MD2361. These clays areterrigenous in origin [but are not deposited at sea by aeolianprocesses; see Gingele et al., (2001) and Gingele and De Deckker(2004)] and occur for every interglacial in this studied part of the core.However, the upwelling species did not increase during otherinterglacial periods with similar sedimentation of red clays such asin MIS 5.

5.4. MIS 11

Core MD2361 captures MIS 11 which is believed to be an analoguefor a future greenhouse climate (Loutre and Berger, 2003). In coreMD2361, the mean SST at the warmest peak (MIS 11.3) was 28.6 °Ccompared to the present-day estimate of 27.5 °C, which is just beyondthe error range 0.83 °C at 1σ. The Tmax estimates 28.9 °C and 29.9 °Cand Tmin of 25.7 °C and 27.4 °C were recorded for the Holocene andMIS 11 respectively (see Fig. 7). The SST estimates together with thePCA analysis of core MD2361 indicate a thicker component of tropicalwater within the Leeuwin Current. Therefore, the Leeuwin Currentcould positively reinforce the characteristics of a future greenhouseclimate by delivering a greater amount of warm, low salinity water tohigher latitudes. Nevertheless, we note the extremely long andsomewhat stable temperature conditions that pertained during thatinterglacial are very different from those that existed over the last-glacial–interglacial. Previously, Lea et al. (2003) determined that MIS11 in ODP Hole 806B on the Ontong Java Plateau, on the other side ofthe Warm Pool, had recorded the warmest interglacial period of thelast 450,000 years. This is in line with core MD2361. Lea et al. (2003)further noted that MIS was unusually long, viz. 20,000 years. It may bethat those prolonged ‘stable and very warm’ conditions permittedformation of carbonate reef platforms, such as occurred along the east

coast of Australia where the Great Barrier Reef became establishedafter the Brunhes–Matuyama boundary, within a very broad time‘envelope’ of 600±280 ka (Alexander et al., 2001).

It has been difficult to compare the findings between our core andthose of Holbourn et al. (2005) carried out on the long core MD01-2378 obtained from the Timor Sea. Holbourn et al. (2005) concen-trated their investigation substantially on the benthic foraminiferarecord to determine past productivity conditions. This we wereunable to do with our data because we used different proxies, but wenote, for example, that the δ13C record of the planktonic foraminiferG. ruber in core MD01-2378 is very different from that of MD00-2361(done on the same species) prior to the establishment of MIS 9.3.The productivity signal in the Timor Sea is strongly influenced bymonsoonal winds in the area (Holbourn et al. 2005), in contrast to therecord offshore Northwest Cape which would have been controlled,mostly by the Throughflow, and its arm the Leeuwin Current and theWest Australian Current underneath it.

6. Conclusion

The results of this study provide evidence that the LeeuwinCurrent was still active in the core area of MD2361 during the last fiveglacial periods. Some studies have argued that the Leeuwin Currentwas absent during glacial periods because the tropical–subtropicalcomponent of the current was reduced. Our study suggests that thephysical characteristics of the Leeuwin Current were altered but thiswas relative to general decreases in global temperatures and SSTdifference between the Leeuwin Current and surrounding waters. Inaddition, it is believed the forcing mechanism of the current was stillin place, permitting its flow to higher latitudes or directly into thecentral Indian Ocean. The data indicate a reduction in temperatureand an increase in salinity at the MD2361 core site, suggesting lessITW was being delivered to the Leeuwin Current from the ITF. It issuggested that the characteristics of thewater above theMD2361 coresite are similar to what is found presently at approximately 25–32°Sdue to the relative abundances of the planktonic foraminiferacompared to the core top study (‘present-day’ abundances) ofMartinez et al. (1998). In general, the faunal assemblages changedfrom tropical–subtropical to subtropical–transitional assemblages atMD2361 site during glacial periods. Indeed, with stronger trade windsin the glacial world, steric height differences between the Pacific andIndian Oceans are likely to have been even greater than today, drivingan albeit cooler, Leeuwin Current.

The findings from this study also add to the regional understand-ing of the dynamics of the Leeuwin Current during interglacialperiods, with the Leeuwin Current appearing to be stronger duringMIS 5, MIS 7, MIS 11 and periodically during MIS 9, compared to theHolocene. The Leeuwin Current is inferred to be stronger and/ordeeper due to increases in SST and the response of sub-surface speciesto warmer temperatures suggesting a deeper mixed layer. Thewarming of the sub-surface layers was aided by the southwardmovement of the ITW/SICW front.

A stronger and deeper flow of the Leeuwin Current has significantclimatic implications, especially for an enhanced greenhouse climate.An analogousMIS 11 had a SST approximately 2 °C warmer during thesummer and winter compared to present. This is significant becausethe Leeuwin Current provides a conduit from the warmest oceano-graphic feature on Earth, the Warm Pool, to the Southern Ocean andmay potentially aid in the reduction of Antarctic polar ice.

Acknowledgements

The TIP2000 cruise on board the RVMarion Dufresnewas led by DrMichele Fieux who provided much information while at sea on theoceanography of the Indian Ocean and on the Indo Pacific Through-flow to the senior author. Dr François Guichard was also instrumental


in obtaining coreMD2361 and provided logistical support.We are alsograteful to Mr Yvon Balut for his effort in obtaining the core with hislegendary CALYPSO corer and the French Polar Institute [IPEV] forfunding the cruise. We are also very grateful to Joe Cali for the isotopicanalyses, Dr Martine Paterne who helped the senior author preparethe AMS targets at Gif-sur-Yvette, and Dr Elisabeth Michel whodiscussed various aspects of the isotopic stratigraphy with the seniorauthor. PDD acknowledges the Australian Research Council for aDiscovery grant that partly funded this project. Dr Davide Murgesehelped logging and sampling the core at the ‘carothèque’ at Gif-sur-Yvette, France.

We are grateful for the comments offered by Stephen Gallagherand an anonymous reviewer that helped improve the originalmanuscript.

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