Biostratigraphic characterization and Quaternary microfossil palaeoecology in sediment drifts west of the Antarctic Peninsula – implications for cyclic glacial–interglacial deposition

Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 237^263 www.elsevier.com/locate/palaeo

Biostratigraphic characterization and Quaternary microfossil palaeoecology in sediment drifts west of the Antarctic Peninsula ^ implications for cyclic glacial^interglacial deposition Giuliana Villa a; , Davide Persico a , Maria Cristina Bonci b , Renata G. Lucchi c , Caterina Morigi d , Michele Rebesco d a

Dipartimento di Scienze della Terra, Universita' di Parma, Parco Area delle Scienze, 157A, 43100 Parma, Italy Dipartimento per lo studio del Territorio e delle sue Risorse, Universita' di Genova, Corso Europa, 26, 16132 Genova, Italy Dipartimento di Geo¢sica della Litosfera, Istituto Nazionale di Oceanogra¢a e di Geo¢sica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy d Dipartimento di Scienze del Mare, Universita' Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy b

c

Received 14 February 2002; accepted 3 March 2003

Abstract The SEDANO Project recovered 19 gravity cores on sediment drifts from the Pacific continental margin of the Antarctic Peninsula [Camerlenghi et al. (1997a) High-resolution terrigenous sedimentary record of the sediment drifts on the Antarctic Peninsula Pacific margin. In: Ricci, C.A. (Ed.), The Antarctic region. Museo Nazionale dell’Antartide, Siena, pp. 705^710]. Fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils, diatoms, planktonic and benthic foraminifera, framed in a depositional process reconstruction. Barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles, respectively. Analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles. The unit boundaries were drawn more accurately by means of the microfossil distribution. On the basis of micropalaeontological and sedimentological evidence, Interglacial Unit C is correlated to Oxygen Isotope Stage 5, thus dating the unit boundaries at 127 and 70 ka. Peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment. A calcareous nannofossil cold-taxa association is present in most cores examined, and its consistent distribution within Interglacial Unit C indicates key environmental relationships. The occurrence of calcareous nannofossils has been related to temperature tolerance, sea-ice cover reduction, nutrient availability, and factors limiting primary productivity. Our results confirm that coccolithophorids occurred at southern high latitudes, in the western marginal basins of the Antarctic Peninsula, during short periods of the Late Quaternary. A foraminiferal assemblage, made up of sinistral Neogloboquadrina pachyderma and few benthics, occurs only in interglacial units. Correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes. A 2003 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +39-0521-905370; Fax: +39-0521-905305. E-mail address: [email protected] (G. Villa).

0031-0182 / 03 / $ ^ see front matter A 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00403-6

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Keywords: calcareous nannofossil; diatom; foraminifera; sediment drifts; Antarctic Peninsula

1. Introduction The Antarctic Ice Sheet is a key component of the world’s climatic system; and basic understanding of global climatic change can be gained by studying the interaction between ice sheet growth, ocean circulation and palaeoproductivity. Phytoplankton is one of the major groups responsible for primary productivity; nevertheless, calcareous microfossils have often been overlooked in the Late Cenozoic sedimentary record south of the Antarctic Polar Front, mainly because the water conditions do not favor the presence and preservation of these organisms. Quaternary sediments south of the Antarctic Divergence (AD) at about 63^65‡S, have been considered barren of calcareous nannofossils and only recently their presence has been documented (Villa and Wise, 1998; Villa et al., 2001; Busetti et al., 2003; Winter and Wise, 2001). Therefore, study of calcareous nannofossils in high latitude Quaternary sediments of Periantarctic basins, and their £uctuations and relationship with glacial^interglacial cycles, is of basic importance to help construct a more complete picture of the palaeoclimatic conditions of this interval. In order to verify the presence of calcareous nannofossils within Antarctic sediments and to correlate their distribution with those of other microfossil groups, and glacial^interglacial cycles, we have undertaken a micropalaeontological study on the sediments recovered by the ‘SEdiment Drift of the ANtarctic O¡shore’ (SEDANO) Project funded by the Programma Nazionale di Ricerche in Antartide (PNRA) and the British Antarctic Survey. The SEDANO Project acquired 19 gravity cores from the sediment drifts of the Paci¢c continental margin of the Antarctic Peninsula (Camerlenghi et al., 1997a; Pudsey and Camerlenghi, 1998). Seventeen cores were recovered along both a dip and a strike transect across Drift 7, in front of Alexander Island, while only two cores were collected along the NE side of Drift 4, located NE of Drift 7 (Fig. 1). Water depth for

the top of Drift 7 is, on average, 2800 m, and on Drift 4, 2600 m. Drift 7 is one of several sediment drifts which have developed since Middle Miocene time from the interaction of weak bottom water £ows with sediment entrained from turbidity currents, icerafted debris (IRD), meltwater turbid plumes, and biogenic activity (Rebesco et al., 1996; Lucchi et al., 2002). The occurrence of present-day weak SW-£owing contour-following bottom currents is shown by current meter mooring measurements (Camerlenghi et al., 1997b). Cores show a color alternation of muddy sediments corresponding to an alternation of glacial and interglacial cycles (Camerlenghi et al., 1997c; Pudsey and Camerlenghi, 1998). Interglacial sediments appear brownish, intensely bioturbated and contain microfossils and abundant IRD. Glacial sediments are gray, ¢nely laminated, with local bioturbation, and contain mm-thick silty turbidites with sporadic IRD layers. Sedimentological and compositional investigations show distinct di¡erences between the ¢nergrained glacial and the relatively coarser-grained interglacial sediments. Glacial sediments are inferred to have accumulated under a SW-£owing bottom current fed by the suspended load of turbidity £ows and meltwater turbid plumes, whereas interglacial sediments were produced mainly by hemipelagic and IRD fallout. Up to seven units (labeled A^G) were recognized within the studied cores on the basis of their sedimentological, compositional, and magnetic susceptibility characteristics (Lucchi et al., 2002) (Fig. 2). Fifteen of the SEDANO cores were sampled for an integrated biostratigraphic^palaeoecological study based on calcareous nannofossils, diatoms, planktonic and benthic foraminifera, to provide a biostratigraphic constraint for core correlation, to document calcareous nannofossil assemblages within the Antarctic sediments and to discuss the palaeoecological implications. The distribution and correlation of the di¡erent

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart

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Fig. 1. Location map of the study area (modi¢ed from Lucchi et al., 2002).

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239

240 G. Villa et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 237^263

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart Fig. 2. Core correlation on the gentle (NE) and distal (NW) sides of Drift 7. Lithologic units labeled A^G are reported as related to climatic changes.

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R: minuta

R: minutula

R: productella

Reticulofenestra sp:

P: lacunosa

H: carteri

large Gephyrocapsa sp:

medium Gephyrocapsa sp:

small Gephyrocapsa sp:

mediumGephyrocapsa sp:

H: carteri

9 165 1

2 48

1

17

3 10

329

2

4

189

51

24

1

1 1

2

2

R: productella

1

R: minuta

2 5 3

R: haqii

2 4 2

P: lacunosa

21 266 6

E: huxleyi

05-1-6/7 05-1-11/12 05-1-26/27 05-1-45/46 05-2-66/67 05-2-76/77 05-2-130/131 05-2-146/147 05-3-189/190 05-3-192/193 05-4-334/335 05-4-338/339 05-4-390/391 05-5-437/438 05-5-447/448 05-5-451/452 05-5-502/503 05-5-530/531

12

1

CORE 5 sample/depth

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

small Gephyrocapsa sp:

E: huxleyi

C: telesmos

C: pelagicus

C: leptoporus

1

C: pelagicus

01-1-5/6 01-1-24/25 01-2-39/40 01-2-50/51 01-2-82/83 01-2-108/109 01-2-136/137 01-3-160/161 01-3-210/211 01-4-288/289 01-4-325/326 01-4-345/346 01-4-365/366 01-5-410/411 01-5-414/415 01-5-434/435 01-5-454/455 01-5-474/475 01-5-494/495 01-5-500/501

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 1 sample/depth (cm)

Total abundance=mm2

Table 1 Range chart of calcareous nannofossils counted in the SEDANO cores

21

3

10

23

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241

242

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SED SED SED SED SED SED SED SED SED SED SED SED SED SED

R: productella

3

Reticulofenestra sp:

08-1-4/5 08-1-9/10 08-1-15/16 08-2-27/28 08-2-53/54 08-2-69/70 08-2-95/96 08-2-118/119 08-2-121/122 08-2-125/126 08-3-156/157 08-3-221/222 08-3-228/229 08-3-253/254

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart

R: productella

1

R: minutula

1

8 1

13

5 13 6

T: tuberosa

R: minutula

R: minuta

R: haqii

Reticulofenestra sp:

P: ovata

medium Gephyrocapsa sp:

3

2 9 4

P: ovata

small Gephyrocapsa sp:

P: lacunosa

E: huxleyi

2

P: lacunosa

1 16 9

H: carteri

medium Gephyrocapsa sp:

2 17 4

H: carteri

small Gephyrocapsa sp:

8 87 67

large Gephyrocapsa sp:

E: huxleyi

14 5 3

large Gephyrocapsa sp:

C: pelagicus

2 3

C: pelagicus

3 1 3

C: crassipons

39 177 102

C: crassipons

CORE 8 sample/depth (cm)

C: leptoporus

07-1-2/3.5 07-1-20/21 07-1-30/30 07-1-51/52 07-1-70/71 07-1-90/91 07-2-100/101 07-2-116/117 07-2-154/155 07-2-209/210 07-3-223/224 07-3-249/250 07-3-279/280 07-3-309/310 07-4-343/344 07-4-376/377 07-4-389/390 07-4-424/425 07-4-440/441 07-5-474/475 07-5-504/505 07-5-510/511 07-5-524/525 07-5-544/545 07-5-59/560 07-5-577/578

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 7 sample/depth (cm)

Total abundance=mm2

Table 1 (Continued).

1

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SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

09-1-2/3 09-1-20/21 09-1-40/41 09-1-56/57 09-2-60/61 09-2-88/89 09-2-127/128 09-2-166/167 09-3-196/197 09-3-208/209 09-3-214/215 09-3-224/225 09-3-234/235 09-3-240/241 09-3-254/255 09-3-264/265 09-4-310/311 09-4-339/340 09-4-411/412

4

3 6 2

3 1 3 1

3 3 2

7 6 99 21

2 15 2 382 5

R: minutula 1

R: productella

1 1 1 1 2

1 3 2

2

1

2 57 41 433 119 2

3 66 24 646 68 1

3

1

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart

R: productella

Reticulofenestra sp:

R: minuta

1

1 7 2

R: minutula

P: lacunosa R: ampla

1

P: ovata

H: carteri

small Gephyrocapsa sp:

2

1

1 1 5

P: lacunosa

2 8 3

large Gephyrocapsa sp:

medium Gephyrocapsa sp:

3 21 18

H: carteri

small Gephyrocapsa sp:

17 94 211

E: huxleyi

4 1 13 8

E: huxleyi

8 159 77 1589 230 3

3

3 2 7

D: hesslandii

1

C: pelagicus

4

1

C: pelagicus

2 3 5

C: crassipons

32 139 253

C: crassipons

CORE 9 sample/depth (cm)

C: leptoporus

08-4-272/273 08-4-293/294 08-4-313/314 08-4-333/334 08-4-353/354 08-4-372/373 08-5-432/433 08-5-453/454 08-5-463/464 08-5-473/474 08-5-483/484 08-5-493/494

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 8 sample/depth (cm)

Total abundance=mm2

Table 1 (Continued).

2 1 2

243

244

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3

4 1 8

123 9 12

5 1 5

1

1 1

1 1 4

1 1

R: productella

6 1 3

1 1

R: minuta

1

1

Reticulofenestra sp:

1 1

1

11

1

1 1

2 2 1 2

1

1 1

1 1 1

1

R: productella

R: minuta

1

Reticulofenestra sp:

2

P: lacunosa

H: carteri

1

medium Gephyrocapsa sp:

1

P: lacunosa

medium Gephyrocapsa sp: 1 1

83 11 3 8 4 6 1

H: carteri

small Gephyrocapsa sp: 6 4 1

1 7 3 2 1 3 1 3

small Gephyrocapsa sp:

149 15 32

E: huxleyi

1

1 1

3 8 6

E: huxleyi

1 1

2 5

C: pelagicus

1 4 1 1 3

1 106 28 9 12 15 8 7

1 9 49

5 1

C: pelagicus

14-1-15/16 14-1-52/53 14-1-85/86 14-2-165/166 14-2-209/210 14-3-229/230 14-3-256/257 14-4-357/358 14-4-422/423 14-5-475/476 14-5-486/487 14-5-539/540 14-6-637/638 14-6-658/659

3 3

C: crassipons

SED SED SED SED SED SED SED SED SED SED SED SED SED SED

5 30 82

C: crassipons

CORE 14 sample/depth (cm)

C: leptoporus

11-1-3/4 11-1-14/15 11-1-19/20 11-2-60/61 11-2-119/120 11-3-178/179 11-3-249/250 11-4-278/279 11-4-310/311 11-5-407/408 11-5-420/421 11-5-430/431 11-5-440/441 11-5-450/451 11-5-460/461 11-5-470/471 11-5-480/481 11-5-490/491

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 11 sample/depth (cm)

Total abundance=mm2

Table 1 (Continued).

3 1 1

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medium Gephyrocapsa sp:

H: carteri 1 1

1

4

R: productella

small Gephyrocapsa sp:

1 1

R: minuta

E: huxleyi

9 3

Reticulofenestra sp:

C: pelagicus

1 12 86

P: lacunosa

C: crassipons

12 7

1

2 1

1

R: productella

16-1-13/14 16-2-98/99 15-3-172/173 16-3-222/223 16-3-260/261 16-3-277/278 16-4-298/299 16-4-339/340 16-4-375/376 16-5-404/405 16-5-437/438 16-5-460/461 16-5-483/484 16-5-506/507 16-5-518/519 16-6-559/560 16-6-625/626

4 3

E: huxleyi

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

1 7 3

C: pelagicus

CORE 16 sample/depth (cm)

2 52 106 1

C: leptoporus

15-1-12/13 15-1-28/29 15-1-78/79 15-2-159/160 15-2-191/192 15-3-209/210 15-3-239/240 15-3-281/282 15-3-311/312 15-4-342/343 15-4-377/378 15-4-409/410 15-5-457/458 15-5-477/478 15-5-487/488 15-6-559/560 15-6-587/588 15-6-632/633

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 15 sample/depth (cm)

Total abundance=mm2

Table 1 (Continued).

1 11

1 1

1

8

1

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart

245

246

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15

1 1

D: hesslandii

E: huxleyi

small Gephyrocapsa sp:

medium Gephyrocapsa sp:

H: carteri

P: lacunosa

1

2 1 2 1

2 1 1 4 4 2 2 1 2 5 1 3

14 3 21 91 355 73 33 170 35 180 360 159 1

1 1 6 3 2 5 2 1 4

1

8 1 16 72 194 9 23 1 26 161 290 150

1 1 4 123 41 4 2 1 5 60 2

R: scissura

R: productella

1 6

R: pseudoumbilica

R: minuta

3

R: productella

R: haqii

1 6

R: minutula

Reticulofenestra sp:

1 25

R: minuta

mediumGephyrocapsa sp:

4 18

Reticulofenestra sp:

small Gephyrocapsa sp:

1 4

R: ampla

E: huxleyi

1 7

C: pelagicus

19-1-16/17 19-1-66/67 19-2-98/99 19-2-130/131 19-3-237/238 19-5-410/411 19-5-420/421 19-5-430/431 19-5-440/441 19-5-450/451 19-5-460/461 19-5-470/471 19-5-466/467 19-5-480/481 19-5-490/491 19-5-500/501 19-5-511/512 19-5-512/513 19-5-520/521 19-6-543/544 19-6-552/553 19-6-583/584 19-6-647/648

C: pelagicus

SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED SED

10 84

C: crassipons

CORE 19 sample/depth (cm)

C: leptoporus

18-1-2/3 18-2-49/50 18-2-69/70 18-2-83/84 18-2-148/149 18-3-244/245 18-3-277/278 18-4-288/289 18-4-335/336 18-4-370/371 18-4-396/397

C: leptoporus

SED SED SED SED SED SED SED SED SED SED SED

Total abundance=mm2

CORE 18 sample/depth (cm)

Total abundance=mm2

Table 1 (Continued).

1

2 8

1

4 20 17 2

1 1 1 5 1

1

1

3 2

63

94

1 3

2 8

1

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart

1 1

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247

Fig. 3. Calcareous nannofossil total abundance distribution in the studied cores, glacial and interglacial units are reported as in Fig. 2.

groups of microfossils, and their signi¢cance, will provide a contribution to the ongoing debate on the links among glacial^interglacial cycles, paleoproductivity, CO2 variation, and global climate.

area of 6 mm2 (Table 1). Fig. 3 shows the distribution, in the fossiliferous cores, of the total abundance as number of coccoliths/mm2 and is plotted against the lithologic units. 2.2. Diatoms

2. Methods 2.1. Nannofossils Fifteen cores (SED 01, 05, 06, 07, 08, 09, 11, 13, 14, 15, 16, 17, 18, 19, 20) were examined. Samples were collected from both glacial and interglacial facies and smear slide preparations of raw sediment were used for light microscope examination. In all 220 smear slides were examined using U1200 magni¢cation. Quantitative analyses were performed counting all coccoliths, within a ¢xed

Seventy-nine samples were collected from Cores 01, 09, 11, and 15 and studied using U1000 magni¢cation. Poor preservation is exempli¢ed by a high degree of fragmentation and species dissolution. For this reason, only a semi-quantitative abundance per sample was determined using the following codes: X (fragments), RR (very rare, 1^ 2 specimens per slide), R (rare, several specimens in 5^20 ¢elds of view), F (frequent, 1 specimen in 2^5 ¢elds of view), C (common, 1 specimen per ¢eld of view), A (abundant, 2 specimens per ¢eld

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248 G. Villa et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 237^263

PALAEO 3126 21-8-03 Cyaan Magenta Geel Zwart Fig. 4. Magnetic susceptibility curve (from Pudsey and Camerlenghi, 1998), and distribution of kaolinite traces (green) plotted against lithologic logs and calcareous nannofossil abundance (red).

G. Villa et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 237^263

of view), AA (very abundant, s 2 specimens per ¢eld of view).

249

3. Results 3.1. Nannofossils

2.3. Foraminifera 217 samples were selected for foraminiferal analysis from 12 cores (01, 05, 07, 08, 11, 14, 15, 16, 17, 18, 19, and 20). The samples were dried at 40‡C, weighed and washed through 63- and 150-Wm sieves. The fraction larger than 150 Wm was studied for planktonic and benthic foraminiferal analysis. A semi-quantitative analysis of the foraminiferal content was performed, species abundance is indicated with the numbers: absent (0), very rare (1), rare (2), common (3), frequent (4), very abundant (5).

Approximately one quarter of the SEDANO samples contain nannofossils. Coccoliths are generally rare with the exception of about 25 samples in which the total abundance is high and preservation is good. On Drift 7, two cores are barren of nannofossils (Cores SED 13 and 17). In the area of Drift 4, Cores SED 19 and 20 are both located in deep waters (nearly 3500 m depth), but while Core SED 19 contains fossils, Core SED 20 is barren, containing 4 m of slumped gray sediments (Lucchi et al., 2002). Calcareous nannofossil taxa are, in particular,

Fig. 5. Diatom total abundance distribution in the studied cores, glacial and interglacial units are reported as in Fig. 2.

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Table 2 Range chart of diatoms analyzed in SEDANO cores 1, 9, 11, 15.

Emiliania huxleyi, Gephyrocapsa spp., and coldwater indicators such as Coccolithus pelagicus and ‘small Reticulofenestrids’; with Calcidiscus leptoporus and Helicosphaera carteri subordinately present (Table 1). In all the fossiliferous intervals

the presence of nannofossils, though in variable abundance, is restricted to Interglacial Unit C. This allowed a better constraint of unit boundaries and correlation between cores. The palaeoecological implications will be discussed later.

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251

Table 2 (Continued).

3.1.1. Nannofossil biostratigraphy The common presence of Emiliania huxleyi indicates a Late Pleistocene age for all the fossiliferous samples. Its First Appearance Datum

(FAD) is an important biostratigraphic marker, dated between 285 ka (Ahagon et al., 1993) and 230 ka (Hills and Thierstein, 1989), and generally established by 268 ka (Thierstein et al., 1977),

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in the upper part of Oxygen Isotope Stage (OIS) 8. The younger bioevent, represented by the beginning of the Emiliania huxleyi Acme Zone, was not identi¢ed in our samples because the abundance of this species does not reach bloom values. The blooming life-style (still happening in the modern ocean) that occurred between V80 and 50 ka, is diachronous towards the poles. Gard (1988) at the high latitude of Norwegian and Arctic Seas correlates the E. huxleyi FAD with OIS 8 and its acme beginning at about 60 ka, in OIS 3. By analogy, at the southern high latitude, by means of nannofossils, Unit C is approximately dated between 285 ka and 60 ka, most probably corresponding to OIS 5 (Pudsey and Camerlenghi, 1998). Correlation of microfossil occurrence with magnetic susceptibility data further constrains the age assignment. The nannofossil abundance peak always falls between Units 23 and 25 of the magnetic susceptibility curve, suggesting that favorable conditions for the existence of nannofossils occurred (Fig. 4). 3.2. Diatoms Diatom abundance is highly variable through Cores 1, 9, 11, and 15, with high abundance intervals alternating with scarce abundance or barren sediments (Fig. 5). This behaviour is cyclic and generally shows a good correspondence with the lithologic succession as described by Lucchi et al. (2002) ; peaks of abundance are detected within Interglacial Units (A, C, E, and G), while lowest abundance corresponds with Glacial Units B and D; Unit F in Core 15, that has been described as a glacial unit, unexpectedly yields a high diatom content. Other exceptions are the intermediate and basal levels of Unit B in Core 11 and the lower part of Unit D in Core 01, where diatoms are common, but very poorly preserved and almost unidenti¢able. Diatoms are generally poorly preserved and show signs of dissolution and fragmentation in all SEDANO cores, sometimes preventing taxonomic classi¢cation. For example, it was di⁄cult to discriminate between the genera Thalassionema

and Thalassiothrix (here designated as Thalassionema^Thalassiothrix group), as well as for Thalassiosira at a speci¢c level. The assemblage composition (Table 2) in levels with high abundance is similar in all cores : Fragilariopsis kerguelensis, Thalassionema^Thalassiothrix group, Thalassiosira lentiginosa and Thalassiosira spp. are dominant ; Actinocyclus actinochilus, Eucampia antarctica var. recta, Thalassiosira gracilis, Fragilariopsis ritscheri and F. sublinearis are less common ; reworked taxa from Miocene and Plio^Pleistocene, including: Denticulopsis spp., Trinacria excavata, Stephanopyxis spp., and Actinocyclus ingens are always rare. Only the lower part of Unit D in Core 01 is different: Fragilariopsis kerguelensis, Thalassionema^ Thalassiothrix group, Thalassiosira lentiginosa, and Thalassiosira spp. are rare, while Eucampia antarctica var. recta, Fragilariopsis obliquecostata, and Chaetoceros spp. (specially resting spores) are dominant. 3.2.1. Diatom biostratigraphy Age diagnostic diatom species are recorded primarily in the interglacial units; all cores were assigned to the Thalassiosira lentiginosa Zone (0.0^ 0.6 Ma; Harwood and Maruyama, 1992) based on the presence of Thalassiosira lentiginosa and T. antarctica, and the absence of Actinocyclus ingens and T. elliptipora. The base of the T. lentiginosa Zone is de¢ned by the Last Appearance Datum (LAD) of Actinocyclus ingens, a wellknown datum from deep-sea sediments in the Southern Ocean, previously calibrated at 0.62 Ma, and recalibrated by Bohaty et al. (1998) to 0.66 Ma. T. antarctica has a range 6 0.67 Ma and the last occurrence of T. elliptipora is dated at 0.70 Ma (both ages recalibrated by Bohaty et al., 1998). The Hemidiscus karstenii acme interval (from 0.42 to 0.19 Ma according to Gersonde and Barce¤na, 1998) is generally useful in regions north of the Polar Front Zone and only in some areas south of it (see Kerguelen Plateau). This species was detected only in Cores 11 and 15, in low abundance. A fragmented specimen present in Core 15 (Unit E) cannot be considered a useful

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marker. In Core 11 the presence of very rare specimens in three levels of Unit C is not reliable, therefore the assignment to the Thalassiosira lentiginosa C Subzone (Gersonde and Barce¤na, 1998), above the LAD of Hemidiscus karstenii at 0.19 Ma, is assumed. Cores 01, 11 and 15 also contain very rare fragments of Rouxia spp. (probably cf. R. antarctica). The last occurrence of Rouxia spp. has been dated at V0.35 Ma (Abbott, 1974); V0.37 Ma (Akiba, 1982); and V0.15 Ma (Pichon, 1985, in Bohaty et al., 1998). In our cores we cannot rely on these taxa, as they are present only in very rare and poorly preserved fragments, and may be re-

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worked. The specimens or fragments of older diatoms, interpreted as reworked, are present in very low abundances. 3.3. Foraminifera Biogenic carbonate rich intervals alternate with barren sediments. Foraminifera are present in the upper part of cores corresponding to Unit A (OIS 1), and more conspicuously in Interglacial Unit C (Figs. 6 and 7). In Unit C left-coiling Neogloboquadrina pachyderma dominates the planktonic foraminiferal assemblage, left-coiling Neogloboquadrina incompta

Fig. 6. Planktonic foraminifera total abundance distribution in the studied cores, glacial and interglacial units are reported as in Fig. 2.

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and right coiling Neogloboquadrina pachyderma occur in low abundances. Globorotalia scitula and Globigerina bulloides occur rarely only in Cores 01 and 11. In Cores 05 and 08, planktonic foraminifera are present also in the upper centimeters corresponding to OIS 1. Abundance in these upper levels is lower than in Unit C and only two species (left-coiling N. pachyderma and left-coiling N. incompta) are present in the assemblage. The benthic foraminiferal fauna of the cores is characterized by few species of calcareous and agglutinated taxa. The dominant calcareous spe-

cies are Epistominella exigua, Nuttallides umbonifer, Cibicidoides wuellerstor¢, and Oridorsalis umbonatus, whereas the dominant agglutinated genera are Cribrostomoides, Cyclammina and Trochammina. Benthic foraminifera are present only in the interglacial units. In Unit A, benthic foraminifera are present in Cores SED 01, 05, 07, 08, 11, 15, 16, and 18 and the benthic assemblages are mainly composed of agglutinated species. Calcareous benthic taxa are always present in Unit C, except for Core SED 17. Agglutinated taxa are present also at the base of Unit C, in Cores 01, 05, 07, 08, 14 and 17 (Fig. 7). In two

Fig. 7. Benthic foraminifera total abundance distribution in the studied cores, glacial and interglacial units are reported as in Fig. 2.

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cores (SED 16 and 18) agglutinated benthic foraminifera also occur in the older interglacial unit (Fig. 7).

4. Discussion 4.1. Nannofossil palaeoecology Our results con¢rm that coccolithophorids occurred at southern high latitudes, in western marginal basins of the Antarctic Peninsula, during short periods in the Late Quaternary. In contrast, modern coccolithophorids are restricted to latitudes north of the AD (Findlay, 1998; Pudsey, 2000). Previous work only reports poorly preserved nannofossils in low abundance in Holocene core sediments from the Polar Front area (north of 60‡S) and for about 100 km to the south (Pudsey and Howe, 1998). The only living calcareous nannoplankton found in the Weddell Sea (Winter et al., 1999), are typical of a subtropical association and di¡erent from the cool-water taxa characteristic of the cores studied. Nannoplankton, represented by poorly calci¢ed holococcoliths and tiny coldwater coccolithophorids, was found in water samples from the Weddell Sea, south of the Antarctic convergence (Thomsen et al., 1988). These forms are adapted to extreme environmental stress, and we emphasize that the modern assemblages are completely di¡erent from those encountered in the SEDANO cores, which are composed mainly of heterococcoliths. Rich assemblages are described from latitudes between 60 and 65‡S, in ODP Legs 113, 119 and 120 (Wei and Wise, 1990; Wei and Thierstein, 1991; Wei and Wise, 1992). In contrast, a few reports document the presence of rare nannofossils in Quaternary sediments of the southern high latitudes (Villa and Wise, 1998; Pudsey, 2000; Villa et al., 2001; Winter and Wise, 2001; Busetti et al., 2003, and new data are under acquisition from Leg 188 (Pospichal, 2003; Fontanesi and Villa, 2002). The signi¢cance of nannofossil occurrence in the extreme southern areas, like that of the SEDANO cores, with the latitudinal range of 66‡ and 68‡, in restricted intervals of the Qua-

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ternary, is interpreted as a signal of surface water conditions which allowed the coccolithophorids to survive ; their presence is here thought to represent a useful proxy for sea surface temperature (SST), sea-ice coverage, and nutrient availability and hence, palaeoproductivity. Modern sea surface conditions at southern high latitudes, like those of the studied cores, do not permit coccolithophorids to thrive, and their distributions re£ect temperature, light, and nutrient control. Villa et al. (2002) discussed the factors that govern the presence of calcareous nannoplankton and put forth several conclusions. Firstly, a minimum temperature of approximately 2‡C, in surface^subsurface waters, is required for coccolithophores to survive. Today, a maximum temperature variable of 1.0^1.6‡C is reported for this latitude (Olbers et al., 1992; Clark and Leakey, 1996). Also, coccolithophores can only survive in areas with less than 9 months sea-ice cover; this duration is the longest present sea-ice period in the southwestern part of the study area (Gloersen et al., 1992). Sedimentological data con¢rm a retreat of the ice sheet cover during interglacials with an extent similar to the present day (Lucchi et al., 2002, and references therein). Lastly, in modern waters, nutrients (e.g. nitrate and phosphate) increase poleward, and a decrease in coccolithophorids abundance was detected by Findlay (1998) in the Australian sector of the Southern Ocean. Calcareous nannoplankton seem to prefer nutrient depleted waters such as those strati¢ed of subtropical gyres (Brand, 1994). High levels of nutrients are regarded as a¡ecting negatively the coccolithophorid productivity ; thus even in conditions of relative nutrient starvation, like those recorded during interglacials, coccolithophores probably did not diminish. 4.2. Diatom palaeoecology Although diatom assemblages lack light-silici¢ed taxa, most diatom taxa present in the SEDANO cores are extant. We therefore attempt an interpretation of palaeoenvironmental conditions based on the known distribution of modern taxa. The uppermost part of Unit A contains the highest abundance of diatoms, except for Core

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11, as already pointed out by Pudsey (2000), in Sediment Drifts 1^6, of the Western Continental Rise (Antarctic Peninsula). The assemblage is very similar to the ‘oceanic subantarctic complex’ (Kozlova, 1964) and indicates high productivity, open ocean conditions with little sea-ice in£uence. Fragilariopsis kerguelensis and Thalassiosira lentiginosa are the primary constituents of this assemblage and are characteristic of Antarctic open ocean waters with an optimum temperature between 0‡ and 10‡C. High abundance of these Southern Ocean endemic taxa indicates sea-ice free waters (Burckle et al., 1987; Zielinski and Gersonde, 1997). The consistent presence of the Thalassionema^Thalassiothrix group is further linked to high primary productivity. The genus Thalassiosira, which in Antarctic waters is associated with high productivity areas in open ocean, also indicates open marine conditions (Leventer and Dunbar, 1988; Leventer, 1992). Accordingly, taxa typical of the neritic environment associated with sea-ice are present in low abundance (i.e. Actinocyclus actinochilus, Eucampia antarctica var. recta, Fragilariopsis curta, F. obliquecostata, F. sublinearis, Stellarima microtrias, Thalassiosira antarctica). Assemblages with similar characteristics, indicative of interglacial conditions, are present in recent Antarctic open ocean biogenic sediments, with minimal marine sea-ice in£uence. High diatom abundance is also recorded in Units C, E and G, with the exception of Core 9 (low abundance in Unit C). The latter may be explained by the more marginal position of the core, where water was probably more corrosive to silica. The highest peak of nannofossils was recorded in this core in Unit C, and this should exclude the hypothesis of a longer cover of sea-ice during the year as postulated by Pudsey (2000). The decrease of diatoms in favor of nannofossils may be due to dissolution of silica or oligotrophic conditions. Core 15 Unit F, postulated as glacial according to sedimentological data (Lucchi et al., 2002), has a high diatom content, similar to the interglacial units. A milder glacial interval could be a possible explanation, combined with the distal position on the drift, that could have experienced sea-ice free waters during summer periods. This ‘glacial’ is

short and has the characteristics of a transitional facies (Lucchi et al., 2002). Diatoms generally are scarce in Units B and D, identi¢ed as glacial facies. Local and temporary polynyas may have contributed to increased diatom deposition of Unit B in Core 11 and in the lower part of Unit D in Core 01. For the latter, the assemblage composition seems to con¢rm this hypothesis. Open ocean taxa are rare, while Chaetoceros spp. (primarily resting spores), Fragilariopsis obliquecostata and Eucampia antarctica var. recta are consistently present. Fragilariopsis obliquecostata, a cold-temperature taxon, is considered indicative of neritic environments strongly in£uenced by sea-ice (Gersonde, 1986) or even associated with summer sea-ice (Gersonde and Zielinski, 2000). The diatom taxa Fragilariopsis sublinearis, Odontella weiss£ogii, Porosira glacialis and P. pseudodenticulata are also associated with sea-ice (Burckle, 1984; Burckle et al., 1990; Fryxell and Prasad, 1990; Cunningham and Leventer, 1998; Harwood et al., 2000). Eucampia antarctica var. recta is frequently associated with sea-ice presence (Burckle, 1984; Burckle et al., 1990; Fryxell and Prasad, 1990), or open-water ice-edge conditions (Harwood et al., 2000; Cunningham and Leventer, 1998). On the other hand, Zielinski and Gersonde (1997) maintain that the link of this species with sea-ice is not de¢nite and that its increase could be due to reworking and concentration by bottom currents. They also note high abundance of this species principally during the glacial intervals. It is noteworthy that in Core 01 this taxon is consistently present in conditions clearly related to sea-ice proximity. The high abundance of Chaetoceros spp. resting spores may be related to increased productivity (Zielinski and Gersonde, 1997) particularly in ice-edge areas. Chaetoceros spp. are abundant in Southern Ocean neritic environments in£uenced by ice or near to ice-shelves. From these considerations, the presence of abundant Chaetoceros resting spores validates the supposition of polynyas as postulated by others (Zwally et al., 1985; Pudsey, 2000). Temperature preferences of modern diatom

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species (Zielinski and Gersonde, 1997; Whitehead et al., 2001) can provide SST estimates for interglacial intervals. The occurrence of Thalassionema spp. and Dactyliosolen antarcticus indicates a summer SST s 0‡C. In support of this interpretation Fragilariopsis kerguelensis and Thalassiosira lentiginosa reach the maximum abundance between 0‡C and 10‡C. Fragilariopsis curta, F. sublinearis, Thalassiosira oliverana and Eucampia antarctica var. recta (taxa unable to form resting spores in order to evade the upper temperature tolerance limits) (Whitehead et al., 2001) occur in waters colder than 1, 2, 5 and 5‡C, respectively. Therefore, we can document a SST between 1‡C (if Fragilariopsis sublinearis is present) and 5‡C. Poor diatom preservation in most of SEDANO samples, even in association with high biogenic silica levels, may be related to dissolution engendered by the Weddell Sea Bottom Water which is corrosive to silica (Gordon, 1966; Nowlin and Zenk, 1988). High fragmentation of valves can be associated partly to bioturbation, as also noted by Pudsey (2000) within glacial units. 4.3. Foraminiferal palaeoecology The living planktonic foraminiferal assemblage, south of 60‡S latitude, is composed almost entirely of left-coiling Neogloboquadrina pachyderma (Be¤, 1969; Donner and Wefer, 1994; Kemle-von Mu«cke and Hemleben, 1999). Based on the distributional pattern of living planktonic foraminifera, left-coiling N. pachyderma is a deep-dwelling species, herbivorous and symbiont lacking. It is found in not-strati¢ed, very nutrient rich waters, where temperature is usually lower than 8‡C (Reynolds and Thunell, 1986; Donner and Wefer, 1994). In Unit C, the presence of other planktonic species and morphotypes, other than left-coiling Neogloboquadrina pachyderma, may re£ect the in£uence of a more temperate current. For example, right-coiling N. pachyderma is related to higher seasonal SST (Be¤ and Tolderlund, 1971) and its strong preference for the deep chlorophyll maximum re£ects a gradient in nutrients (less fertile super¢cial waters) (Ravelo et al., 1990). The benthic microfauna is mainly composed of

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epifaunal or shallow infaunal species (Gooday, 1988; Lutze and Thiel, 1989, Corliss and Emerson, 1990). The microhabitat can be indicative of trophic requirements of the taxa. In this case the dominant benthic microfauna is typically of oligotrophic environment. Epistominella exigua is reported from a large number of bathyal and abyssal sites usually oligotrophic and with high oxygen content, but where seasonally large amount of phytodetritus settle on the sea£oor (Gooday, 1988; Smart et al., 1994; Schmiedl et al., 1997). According to several authors (Bremer and Lohmann, 1982; Mackensen et al., 1993, 1995; Schmiedl et al., 1997), Nuttallides umbonifer inhabits the abyssal part of the oceans at the highest latitude. This taxon prefers the carbonate corrosive environment between the calcite lysocline and calcite compensation depth (Mackensen et al., 1993; Schmiedl et al., 1997). Schmiedl et al. (1997) found N. umbonifer together with a high abundance of agglutinated forms, that usually dominates where the water carbonate content is low (Kaminski, 1985; Mackensen et al., 1995). The benthic microfauna of Unit C indicates a highly oxygenated, predominantly oligotrophic bottom environment, but with seasonal organic matter £ux to the sea£oor. During Unit A, probably highly corrosive water and an extremely oligotrophic environment determined the low benthic foraminiferal density. 4.4. Palaeoproductivity and its palaeoceanographic implications In the SEDANO cores, from the distribution and the features of the microfossil assemblages, we can make some remarks on palaeoproductivity. Quaternary palaeoproductivity in polar regions is a debated topic. There is evidence that high productivity in the Antarctic region, south of the Polar Front, occurred during the interglacial periods (Charles et al., 1991; Mortlock et al., 1991; Bonn et al., 1998), dominated by warmer circumpolar deep water and short periods of seaice cover. Studies on sediment traps in the Weddell Sea demonstrate that seasonal sea-ice changes

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control productivity (Fischer et al., 1988; Dunbar, 1984). This line of reasoning con£icts with the general assumption that CO2 reduction during glacial intervals was connected to enhanced primary productivity, as, for example, postulated in the ‘iron

hypothesis’ (Martin, 1990). Similar conclusions were suggested by Lyle et al. (1988), who studied equatorial Paci¢c cores, and demonstrated that high deposition of Corg , equivalent of palaeoproductivity, occurred at 18 ka and during OIS 6, both periods representing glacial intervals. Ac-

Fig. 8. Model for the environmental conditions triggering palaeoproductivity during glacial and interglacial intervals, in the continental margin of the Antarctic Peninsula (modi¢ed from Bonn et al., 1994; Pudsey, 2000).

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cordingly, Sarnthein et al. (1988) showed that the increase in CO2 that started from Termination 1 (15 ka) could be explained by a decrease in palaeoproductivity at low^mid latitudes. At the same time they report a reversed trend from the high latitude oceans covered by sea ice during the Last Glacial Maximum, where they detected an increase in new production during Termination 1. Several other studies (Bonn et al., 1994; Dymond et al., 1992; Frank et al., 1995; Nurnberg et al., 1997; Bonn et al., 1998; Pudsey, 2000) utilized opal and biogenic barium as palaeoproductivity proxies in the Southern Ocean and showed that high peaks of barium correspond to interglacials. Frank et al. (1995) in gravity cores form the Weddell Sea demonstrated that biogenic barium was in£uenced by increased palaeoproductivity at the beginning of the interglacials. Nurnberg et al. (1997) demonstrated the applicability of barium as a productivity proxy in the Southern Ocean. They compared the barium content in cores of the southern South Atlantic, in an area extending from the subantarctic zone to the winter sea-ice zone. The southernmost of their cores were characterized by a high barium vertical rain rate both in the Holocene and in OIS 5.5; but only the latter event is recorded northward as far as the Polar Front boundary, demonstrating that, during the warmer interglacial OIS 5, an area wider than today experienced a higher export of barium. From these data, the authors concluded that productivity is primarily controlled by sea ice £uctuations. In summary, according to many authors, productivity increased during interglacials, within the Antarctic zone, mainly because phytoplankton thrives in conditions of intensi¢ed light. Our study supports this interpretation: sediments of Unit C clearly indicate enhanced productivity during an interglacial, on the basis of the presence of calcareous nannofossils and planktonic foraminifera ; in addition to diatom assemblages indicative of high productivity in interglacial units. From microfossil distributions, higher productivity and warmer SST than those present today are inferred for the time equivalent to OIS 5. Hoddell (1993) showed that, during OIS 7, 9 and 11 (interglacials), the warm in£ux of North Atlantic Deep Water

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(NADW) at ODP Hole 704A (South Atlantic) was stronger than during glacials. Calcareous microfossils may have taken advantage of the NADW heat transfer, which, like today, a¡ects the sea-ice coverage (Charles and Fairbanks, 1992; Howard and Prell, 1994). In contrast, at very high latitudes, reduced productivity is ascribed during glacial intervals, due to lower SST, and permanent sea-ice cover, which limits light availability in the photic zone (Fig. 8). Clay mineral assemblages provide further support of the microfossil interpretation (Lucchi et al., 2002). Two di¡erent assemblages are present in the SEDANO cores : glacials are dominated by chlorite, and interglacials contain a relatively higher percentage of smectite. Traces of kaolinite are present only within interglacials or in interglacial-transitional (deglaciation) sediments ; none of the glacial or glacial-transitional (glaciation) sediment samples contain kaolinite. The presence of traces of kaolinite correlates well with the occurrence of nannofossils (Fig. 4). Lucchi et al. (2002) argued that there are two possible mechanisms that can explain the selective presence of kaolinite in the interglacial sediments of this margin. The ¢rst hypothesis envisages an eolian transport of dust from Patagonia (South America). Alternatively, the kaolinite is supplied by iceberg advection from southwestern areas (Amundsen Sea). Our micropalaeontological results support the ¢rst hypothesis that implies a widely extended area free from long term sea-ice cover.

5. Conclusions The most striking result from the analysis of the SEDANO cores is the presence of Quaternary calcareous nannofossils south of 65‡S. These occurrences have not been fully documented in previous reports and are only recently described in more detail in Villa et al. (2002). Together with planktonic foraminifera, calcareous nannofossils are present only in Interglacial Unit C, the age of which, on the basis of micropalaeontological and sedimentological evidence, is correlated to OIS 5, thus dating the unit boundaries at 127 and 70 ka.

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The SST for interglacial OIS 5 on the basis of microfossil preferences, is assumed to range between 1^2‡C and 5‡C. We present evidence of higher productivity during interglacial cycles at southern high latitudes, in a region of seasonal sea-ice expansion. From the calcareous nannofossil occurrence, we infer higher productivity for OIS 5, enhanced by SST warmer than at present. Calcareous nannoplankton may have taken advantage of increased NAWD heat delivery to the Southern Ocean, which in turn reduced the sea-ice coverage (Hoddell, 1993) and consequently allowed sunlight, critical to phytoplankton growth, to reach the photic zone. The presence of traces of kaolinite is diagnostic of interglacial conditions and is interpreted as an indicator of reduced sea-ice cover. In contrast, reduced productivity is inferred during glacial intervals, at southern high latitudes, due to lower SST, and permanent sea-ice cover. Evidence from the diatom assemblage indicates that during glacial intervals (e.g. in Core 11 Unit B and in Core 01 Unit D) increased diatom abundance may be tied to the presence of local and temporary polynyas. Further study on glacial^interglacial cycles, including from the older record, may provide some insight into microfossil distribution in the Southern Ocean and its potential response to global change.

Acknowledgements We wish to thank A. Camerlenghi, S. Bohaty, P. Quilty for critical reviews and helpful suggestions. F. Florindo edited and improved the last version. This study was supported by PNRA Grant 99-01 to G.V.

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