Sedimentary Processes on the Wilkes Land Continental Rise

REVIEW ARTICLE

Sedimentary processes on the Wilkes Land continental rise reflectchanges in glacial dynamic and bottom water flow

Andrea Caburlotto Æ R. G. Lucchi ÆL. De Santis Æ P. Macrı Æ R. Tolotti

Received: 19 February 2008 / Accepted: 25 January 2009 / Published online: 18 February 2009

� Springer-Verlag 2009

Abstract Four sediment cores were analysed in order to

determine the sedimentary processes associated with the

channel-ridge depositional system that characterise the

George V Land continental margin on the Wilkes Land.

The sedimentary record indicates that the WEGA channel

was a dynamic turbiditic system up to M.I.S. 11. After this

time, the channel became a lower-energy environment with

sediments delivered to the channel through high-density

bottom waters that we identify to be the high salinity shelf

waters (HSSW) forming on the shelf area. The HSSW

entrains the fine-grained sediments of the shelf area and

deliver them to the continental rise. The biostratigraphy

and facies of the sediments within the WEGA channel

indicate that the HSSW down flow was active also during

last glacial. The change from a turbiditic system to a low-

energy bottom current system within the WEGA channel

likely reflects a different ice-flow pattern, with ice-sheet

reaching the continental shelf edge only within the ice

trough (ice stream).

Keywords High salinity shelf water � Turbidity currents �Glacio-marine depositional processes � Marine isotopic

stage 11 � Glacial dynamic changes

Introduction

Deep-sea ridge deposits around Antarctica are object of

growing interest in geosciences for their relation to the

behaviour of the ice cap and their ability to record palae-

oclimatic changes. In the Wilkes Land continental margin

the study of these morphological features can provide

useful information for understanding the evolution of the

Eastern Antarctic ice sheet (EAIS).

The morphology on the Wilkes Land margin is char-

acterised by several submarine canyons cutting the slope,

and by a ridge-channel depositional system on the conti-

nental rise. On the studied area (between 143�E and

145�E), the ridges named C, A and B from west to east,

have approximately north–south elongated axes, perpen-

dicular to the margin. They are asymmetrical, with a long

gentle eastern side and short steep western side, lying

between 2,500 and 3,600 m of water depth. The relief is up

to 1,000 m in the proximal area, decreasing to about 300 m

in the centre. Ridges C and A are separated by the Jussie

canyon, while Ridge C is delimited on the eastern side by

the Buffon canyon. Both canyons reach the shelf break,

while the upper reaches of the WEGA Channel, separating

Ridge A and B, start from the upper continental rise

(Fig. 1). Seismic investigations indicated that the deep-sea

channels are the product of erosion by down-slope gravity

flows (Donda et al. 2003 Caburlotto et al. 2006). According

to Escutia et al. (1997, 2000), De Santis et al. (2003) and

Donda et al. (2003) the sediment ridges originated from an

interplay of turbidity and bottom currents: fine-grained

A. Caburlotto (&) � L. De Santis

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale,

OGS, Borgo grotta Gigante 42/c, 34010 Sgonico, Italy

e-mail: [email protected]

R. G. Lucchi

Department D’Estatrigrafia, P. i Geociences Marines, Universitat

de Barcelona, C/Martı i Franques, s/n, 08028 Barcelona, Spain

P. Macrı

Istituto Nazionale di Geofisica e Vulcanologia, INGV,

Via di Vigna Murata 605, 00143 Rome, Italy

R. Tolotti

Dip. Te. Ris., Universita degli Studi di Genova,

Corso Europa 26, 16132 Genoa, Italy

123

Int J Earth Sci (Geol Rundsch) (2010) 99:909–926

DOI 10.1007/s00531-009-0422-8

Mertz Bank

Shelf break

George V Basin

Adelie

Bank

Ridge “C”

Ridge “A” Ridge

“B”

Juss

ieu

Cha

nnel

WE

GA

Cha

nnel

Buf

fon

Cha

nnel

64° S142° E 143° E 144° E 145° E 146° E 147° E

65° S

66° S

67° S142° E 143° E 144° E 145° E 146° E 147° E

64° S

65° S

66° S

67° S

N

KILOMETERS

0 10 20 30 40 50

PC

-18

PC

-19

PC

-20

PC

-26

Mertz

GlacierAdelieCoast

Fig.5

Polynya

MCDW

HSSW

HSSW

MC

DW

Upwelled MCDW

HSSW

Fig. 1 Bathymetric map of the Wilkes Land continental margin between 142� and 147� long. E with core sites. Contour every 100 m (modified

after Caburlotto et al. 2006)

910 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926

123

sediments derived from turbid flows are entrained by

westward flowing bottom currents and deposited on the

eastern gentle slope of the ridges. This interaction of sed-

imentary processes explains the opposite asymmetry of the

ridges with respect to the effect of the Coriolis force only.

High salinity bottom currents originate on the shelf area

and move down-slope through the canyon system that

represents the main conducts for sediments and high

salinity water transfer to deeper environments. At the

present time, the Wilkes Land margin is characterised by

the presence of a long-lasting polynya, located west of the

Mertz glacier, which is responsible of the production of

high salinity shelf waters (HSSW) current (Bindoff et al.

2000). The Wilkes Land continental shelf, and in particular

the Adelie Depression close to the Mertz Glacier, is con-

sidered one of the main Antarctic area for HSSW

production (Gordon and Tchernia 1972; Rintoul 1998).

Plumes of cold and dense water flow over the continental

slope and rise and are probably channelled into the canyons

(Rintoul 1998). Northeward, the HSSW mixes with the

modified circumpolar deep waters (MCDW), producing the

cold and saline Antarctic bottom water (AABW) that

transports down slope oxygen and nutrients towards deeper

environments.

Several metres of cores have been collected from this

margin during the Deep Sea Drilling Project (DSDP) sites

168 and 269 (Hayes et al. 1975) as well as the USNS

Eltanin cruise (Payne and Conolly 1972), the deep freeze

79 cruise (Domack 1982), and the USGS 1984 cruise

(Hampton et al. 1987). Some of those cores were studied

by Escutia et al. (2003) in order to improve the sediment

age model and to better understand the relationship

between diachronous and coeval sedimentary processes

occurring across the shelf-slope and rise depositional sys-

tems. The cores located on the continental rise contain

evidences of down-slope sedimentary processes focussed

along the channels (turbidities and other gravity flows).

These deposits are interbedded with hemipelagites and

laminated sediments and their occurrence have been asso-

ciated with the glacial and interglacial climatic stages

during the Pleistocene (Escutia et al. 2003; Hampton et al.

1987).

The depositional model proposed for the Wilkes Lands

is similar to those provided for other ridge-channel systems

present elsewhere around the Antarctic margin, such as the

western Antarctic Peninsula (McGinnis and Hayes 1995;

Rebesco et al. 1996, 1997; McGinnis et al. 1997; Barker

et al. 2002), the Prydz Bay (Kuvaas and Leitchenkov 1992;

O’Brien et al. 2004; Grutzner et al. 2003) and the Weddell

Sea (Michels et al. 2001, 2002). On the continental rise

west of the Antarctic Peninsula, the ridges were identified

as sediment drifts and were studied in terms of Plio-

Quaternary sediment record of the glacial/interglacial

fluctuations, indicating that a record of climatic change is

preserved in these deposits (Pudsey 2000a, b; Lucchi et al.

2002; Cowan 2002; Macrı et al. 2006; Lucchi and Rebesco

2007). Sediment facies analyses allowed to recognise

sedimentary processes and to define the facies associated

with glacial/interglacial transition (Lucchi et al. 2002).

On the Wilkes Land continental margin, acoustic data

collected during the WEGA cruise carried out in February–

March 2000 onboard the R/V Tangaroa show some

morphological and stratigraphic differences between the

channels on the continental rise.

The purpose of this paper is to determine the changes in

depositional processes that occurred in the WEGA channel

during mid-late Pleistocene. We re-examined and analysed

the sediment cores collected in the channel area during the

WEGA cruise (Busetti et al. 2003). In addition, we defined

an integrated age model based on detailed diatom bio-

stratigraphic analyses, radiocarbon dating, and the records

of relative geomagnetic paleointensity (Macrı et al. 2005).

Grain size analyses were conducted in order to investigate

the changes in bottom current activity and in depositional

processes that occurred along the Wilkes Land continental

margin during Quaternary. The results have been inte-

grated and compared to the preliminary sedimentological

investigation previously carried out by Busetti et al.

(2003).

Stratigraphic setting

Post-rift Cenozoic evolution on the Wilkes Land margin is

characterised by deposition of thick sedimentary sequences

on the continental shelf, slope and rise.

The main unconformities within the Cenozoic sequences

have been interpreted as to represent the onset and devel-

opment of glacially dominated conditions on the Wilkes

Land continental margin (Eittreim and Smith 1987;

Tanahashi et al. 1994; Eittreim et al. 1995; Escutia et al.

1997, 2000; De Santis et al. 2003; Donda et al. 2003). Up

to the Miocene, glacial sequences deposited in a temperate

glacial environment with significant melt-water production,

and dominant high-energy turbiditic processes that pro-

duced fan lobes and channel-levee complexes.

The overlying glacial sequences indicate that deposition

was reduced on the continental rise, sediment drape

smoothing and partly in-filling the underlying relief, in a

generally low-energy environment, with reduced turbidite

activity and probably contour current influence. The sedi-

ment drape and the attenuation of the ridge relief after the

Miocene marks a transition from wet-based glaciers to

present polar conditions with dry-based ice systems on the

continent and on the over-deepened continental shelf

(De Santis et al. 2003; Donda et al. 2003).

Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 911

123

The Quaternary sedimentary environment of the deep

margin is affected by turbiditic down-slope sediment transfer

with minor contribution of along-slope contour currents.

Since mid-late Pleistocene, the WEGA channel is charac-

terised by transport and settling of sediment through weaker

downslope flows with respect to the Jussie and Buffon

channels. Caburlotto et al. (2006) recognised two different

sequences within the sedimentary section below the seafloor,

named from the deepest to the shallowest: WL-S10 and WL-

S11. The unconformity separating the two stratigraphic units

(WL-U10) clearly marks a change in the depositional setting

reflecting a rather gradual decrease in the down-slope fluxes

energy in the WEGA channel, and causing infill of previous

incisions by landwards (Southwards) shift of the sedimen-

tary depocentre. However, the geometric characteristics of

the seismic strata within seismic sequence WL-S11 indicate

a low-energy but still dynamic environment.

Materials and methods

This study has been carried out on four piston cores collected

during the WEGA cruise on January 2000, in the proximity

of the WEGA channel (Fig. 1): cores PC-18, -19, and -20

were recovered on the distal rise along a E–W oriented

transect from the WEGA channel moving across the eastern

gentle slope of ridge ‘‘A’’, while core PC-26 is located

nearby the crest of ridge ‘‘A’’, in a more proximal area.

The cores were analysed through radiographs and by

visual description on the fresh sediment surface, textural

characteristics, biostratigraphy, and palaeo-environmental

magnetism (Table 1).

Over 150 sediment samples were collected for grain size

analyses systematically taken every 10 cm along the whole

core’s length. Sediments were initially dried at 50�C to

determine the total weight, and were subsequently left to

disaggregate for 24 h in a 0.05% hexametaphosphate

solution. The suspension was wet-sieved at 62.5 lm and

the percentage of [62.5 lm and silt-clay (mud) fractions

were determined by weight of the dry sand fraction over

the total weight. The fine fraction was analysed at the

University of Trieste using a Sedigraph 5,100 for grain

sizes from 4 to 11 phi (62.5–0.5 lm). Grain size statistical

parameters (mean grain size and sorting) have been cal-

culated on the mud fraction according to Folk and Ward

(1978). Moreover, the grain size values of the mud fraction

have been treated with the cluster analysis, using the ‘‘k-

means’’ method (Swan and Sandilands 1995).

Biostratigraphic and diatom association studies were

conducted on cores PC-18 and PC-26. Diatom biostrati-

graphic markers were singled out through quantitative and

qualitative analyses on a total of 33 samples collected every

20 cm down-core. Sample preparation was made according

to Barde (1981 modified), for which dry sediments were

placed in 100 ml beakers with hydrogen peroxide at 16

volumes for 1 day, until complete desegregation of the

samples. The samples were then washed with distilled

water, and subsequently treated with HCl (10 volumes). The

reaction was allowed to progress for 30–40 min without

heating. The residues were diluted in a fixed quantity of

distilled water in order to maintain the same dilution

(gramme of dry sediment/water quantity) for each sample.

The slides were mounted with Naphrax. The absolute dia-

tom’s valve number was defined using an immersion 1,000

9—LM Reichert Jung-Polyvar microscope. When possible,

almost 300 diatom valves were counted for each slide

according to Schrader and Gersonde (1978) methodology.

When diatom concentration was lower, almost 100 valves

were counted along defined transects on each slide. Abso-

lute valve concentration is calculated as number of valves

for dry sediment according to Boden (1991).

AMS 14C dating were measured on bulk organic carbon

on four samples located at the top and bottom of core PC18

(15 cm and 322 cm bsf), at the bottom of core PC-19

(414 cm bsf) and at top of core PC-26 (16 cm bsf).

A detailed magnetostratigraphy of the cores was for-

merly conducted on u-channel samples in the Istituto

Nazionale di Geofisica e Vulcanologia of Rome.

Results

Core description

The cores close to the WEGA channel (PC-18 and -19) are

characterised by an alternation of brownish- and greenish-

grey muddy sediments, with pervasive bioturbation and

local concentration of coarser-grained terrigenous detritus of

ice rafted debris (IRD) (Fig. 2a). At the base of core PC-19,

Table 1 Type of investigation applied to each core; analysis made in

previous works are referred to references

Core Grain

size

Diatom

analysis

14C

datation

Paleointensity

PC 18 X X X Macrı et al. (2005)

PC 19 X X Macrı et al. (2005)

PC 20 X Macrı et al. (2005)

PC 26 X X X Macrı et al. (2005)

Fig. 2 Core logs with down-core textural and statistical grain size

parameter distributions of cores PC-18 and -19 (a) and cores PC-20

and -26 (b). Radiographs of the main litho-facies are shown as well as

the down-core distribution of textural clusters and related grain size

frequency curves of the mud fraction (see ‘‘Results’’). Marine Isotopic

Stages (M.I.S.) are indicated next to the core logs

c


123

phi104 5 6 7 8 9

4

2

6

8%

Cluster grain size frequency

cluster 3cluster 4

cluster 2cluster 1

cluster 1cluster 2cluster 3cluster 4

1

2

tran

sitio

n?

M.I.S.

1

2

M.I.S.

Sharp/gradual contact

Discontinuous/wispylaminationPlanar lamination

Homogeneous/bioturbated mud

Isolated pebbles (diameter > 1 cm)

Sparse sand and gravel

SlumpFault

LEGEND

sandsiltclay

TEXTUREa

PC-18 PC-19

0

1

2

3

dept

h (m

)

0

1

2

3

4

dept

h (m

)

20 40 60 80 1.2 1.6 2.0 2.4 2.8Sorting

20 40 60 8020 40 60 80 8 9 10Mean size (phi) Sorting

2.81.2 1.6 2.0 2.4Texture

8 9 10Mean size (phi)Texture

PC-26PC-200

1

2

3

4

dept

h (m

)

0

1

2

3

4

dept

h (m

)2-3-

4

5

6

7

8

9

10

11

12

13

14

15

1617

2-3-

4

5

6

tran

sitio

n?

?

M.I.S. M.I.S.

b

20 40 60 80 100 1.2 1.6 2.0 2.4 2.8Sorting

40 60 8020 1.2 2.4 2.81.6 2.0

Sorting8 9 10 8 9 10

Mean size (phi)Texture Mean size (phi)Texture


123

between 360 and 390 cm, there is an interval of finely lam-

inated, not bioturbated sediments containing scattered IRD.

Core PC-20, located close to the crest of ridge ‘‘A’’,

contain the lithofacies observed in cores PC-18 and -19,

with bioturbated sediments containing scattered IRD. This

lithoface is locally interbedded with laminated sediments

having different down-core characteristics: in the upper

2 m of core PC-20 the mm-thick laminations are wispy/

discontinuous and include ice rafted detritus (grains/peb-

bles, Fig. 2b), whereas in the lower part (below 240 cm),

the laminations are mm- to cm-thick, well defined (sharp

colour contrast at the base), laterally continuous, and do not

include IRD. Moreover, some of the laminated intervals

present irregular/sharp bases (Fig. 2b).

The sediments of core PC-26, close to the crest of ridge

‘‘A’’ in a more proximal area, contain alternating brownish-

and greenish-grey muddy sediments. Similar to the other

cores, PC-26 contains fine-grained bioturbated sediments

with sparse IRD. Faulted and slumped sediments are fre-

quent in both cores PC-20 and -26 occurring with sharp/

irregular bases over laminated sediments.

Grain size

The sediment cores are generally characterised by fine-

grained sediments with predominant silt and clay fractions

(mean grain size usually within the very-fine silt/clay

fractions). The fraction coarser than 62.5 microns was

recovered only within IRD intervals (Fig. 2a, b).

A statistical approach to grain size analysis on the mud

fraction allowed us to outline the textural characteristics of

each lithological facies.

Four clusters were distinguished and associated with

lithofacies as follows (Fig. 2a).

Cluster 1 is the finest grained having mode within the

very-fine silt (7.5–8 phi). These sediments have a good

sorting and are associated with the laminated sediments at

the base of core PC-19 and PC-26 as well as the discon-

tinuous/wispy laminated intervals of cores PC-20.

Cluster 2 groups sediments having mode in the fine silt (7–

7.5 phi) and are mainly associated with the well-laminated

sediments at the base of core PC-20, and secondary to IRD-

rich sediments at the base of core PC-19, or in core PC-26.

Cluster 3 has the mode in the medium silt (6 phi) and

corresponds to the bioturbated, IRD-rich sediments form-

ing core PC-18 and most of the upper part of core PC-19

(Fig. 2a). These sediments are badly sorted.

Cluster 4 is the coarsest grained having mode in the

coarse silt (5 phi). These sediments have the highest sand

content and can be generally associated with IRD intervals.

The upper part of core PC-26 (in the proximal area) is

almost entirely formed by sediments belonging to cluster 4

regardless the presence of IRD layers (Fig. 2b).

Radio-carbon dating and age model

The results from AMS 14C dating on the bulk organic

carbon are reported on Table 2. The measured values have

been corrected for the reservoir effect, assuming a cor-

rection factor of 1,300 years, which is the standard

correction applied to Antarctic marine sediments (Berkman

and Forman 1996).

The results from the AMS 14C dating were integrated

with the biostratigraphic data as well as the high-resolution

analysis of magnetic properties of the WEGA sedimentary

sequences performed by Macrı et al. (2005) that allowed

the compilation of a WEGA relative paleointensity (RPI)

stack and a ChRM inclination stack, both spanning the last

300 ky. For this, an original age model was established for

the studied cores by tuning the individual RPI curves with

the global reference RPI stack SINT-800 of Guyodo and

Valet (1999). The reconstructed average sediment accu-

mulation rates resulted much variable among the various

cores (Fig. 3): cores PC-18 and -19 have the highest sed-

imentation rates (c.a. 13 and 19 cm/ky, respectively)

recording sediments deposited during the last glacial-

interglacial cycle (M.I.S. 1 and M.I.S. 2,3,4) and thus

providing an expanded record of the last 30 ky. Core PC-20

near the crest of the ridge in the distal area, has the lowest

sedimentation rate (about 0.6 cm/ky) having a condensed

sequence with sediments at the base of the core corre-

sponding to the M.I.S. 17 (c.a. 700 ky). Core PC-26, in the

proximal part of the crest of ridge ‘‘A’’, has a mean sedi-

mentation rate of 1.2 cm/ky, and contains sediments

spanning down to M.I.S. 6 (c.a. 200 ky).

Biostratigraphy

The absolute valve concentration is usually considered as a

signal of paleoproductivity. The biostratigraphic investi-

gation of the sedimentary record outlined clear evidences of

silica dissolution as documented by the spare presence of

the very lightly silicified Chaetoceros vegetative cells as

well as the partial dissolution of sea-ice forms such as

the F. cylindrus that is usually present in the sea-ice

associations (Armand et al. 2005). The biostratigraphic

reconstruction has been proposed taking note of this

Table 2 AMS 14C dates measured on bulk organic carbon

Core Depth Years B.P. Error Years B.P. corrected

for reservoir

PC 18 15 8,120 60 6,820

PC 18 322 27,570 270 26,270

PC 19 414 24,570 190 23,270

PC 26 16 11,420 60 10,120


123

problem, with a taxonomical identification of diatom spe-

cies and an estimation of the status of preservation of the

fossil’s assemblage.

From the taxa counts we identified three associations

that can be related to open water, sea-ice environmental

conditions and sediment’s reworking (Tables. 3, 4).

The open water association is generally dominant in both

cores, except in the lowest part of core PC 26. It comprises

mainly Fragilariopsis kerguelensis, Thalassiosira lenti-

ginosa, and the Thalassiotrix antarctica/longissima. This

association is generally well preserved and represents

pelagic open ocean environmental conditions. Within this

PC-18

0

1

2

3

dept

h (m

)

PC-190

1

2

3

4

dept

h (m

)

PC-200

1

2

3

4

dept

h (m

)PC-26

0

1

2

3

4

dept

h (m

)

0 10 20 30

0 10 20 30

Sed. rate(cm/kyr)

Age (kyr)

0 10 20 30

0 10 20 30 40

Sed. rate(cm/kyr)

Age (kyr)0 400 800

0 1 2

Age (kyr)

Sed. rate(cm/kyr)

0 200 400

0 1 2 3 4 5

Age (kyr)

Sed. rate(cm/kyr)

100500 150 200 250 300

PC-20

20100 30 40 50

PC-19

VADMS

Rel

ativ

e Pa

leoi

nten

sity

PC-18

Age (kyr)

350

PC-26

SINT-800

1.2

6.0

4.0

2.0

0.0

6.0

4.0

2.0

0.0

0.8

0.4

0.0

1.2

0.8

0.4

0.01000 400200 500300 600 800700

Relative Paleointensity

Age (kyr)

Rel

ativ

e Pa

leoi

nten

sity

SINT-800

b

a

Fig. 3 a The relative

paleointensity (RPI) records of

the WEGA cores were

compared with the SINT-800

reference curve of Guyodo and

Valet (1999) in order to obtain a

reliable age model (after Macrı

et al. 2005). b The obtained

sedimentation rates are

compared with the

lithostratigraphic log of the

core. Light grey line age versus

sedimentation rate calculated

between the tie points, dark lineage versus depth


123

Table 3 PC 18—percentage on total association of observed diatoms

Depth (cm) 10 30 50 70 92 112 132 152 173.5 193.5 213.5 233.5 253.5 273.6 293.6 313.6 333.6

Actinocyclusactinochilus

0.67 0.48 0.43 1.14 0.91 2.27 1.65 0.88 3.38 1.44 0.65 3.95 2.48 0.92 1.68 0.67 1.85

Actinocyclusehrembergii

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Actinocyclus ingens 0.00 0.00 0.22 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.67 0.31

Asteromphalus sp. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Aster. heptactis 0.00 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.48 0.00 0.00 0.00 0.35 0.00 0.00 0.00 0.00

Aster. parvulus 0.00 0.00 0.00 0.00 0.00 0.00 0.41 0.88 0.00 0.00 0.65 0.66 0.71 0.00 0.00 0.00 0.31

Chaetoceros cell.

veg.

0.13 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Chaetoceros spore 2.97 6.52 11.26 20.45 5.45 13.64 16.87 7.96 2.42 2.52 1.29 4.61 7.45 3.36 5.03 2.33 3.40

Choretroncriophilum

0.00 0.00 0.00 0.00 0.00 2.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Cocconeis sp. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.65 0.00 0.00 0.00 0.00 0.33 0.00

Coscinodiscusoculusiridis

0.00 0.00 0.00 0.00 0.00 0.00 0.41 0.00 0.00 0.00 0.00 1.32 0.00 0.92 0.00 0.33 0.00

Dactyliosolenantarcticus

7.83 1.93 3.25 4.55 0.00 1.14 3.29 4.42 1.45 2.52 3.87 1.32 3.55 0.00 1.96 0.67 2.78

Denticulopsishyalina

0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.36 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Dentyculopsis sp. 0.00 0.00 0.00 1.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Denticulopsis cf.

hustedtii0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.28 0.67 0.00

Denticulopsismaccollumii

0.00 0.00 0.00 0.00 0.00 1.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.28 0.33 0.00

Eucampia antarctica 0.81 0.72 0.22 0.00 3.64 17.05 2.88 3.54 6.76 5.04 9.68 6.58 3.55 7.03 7.26 3.67 5.25

Fragilariopsisrhombica

3.24 4.59 6.28 0.00 0.00 0.00 0.00 0.88 1.45 0.00 0.00 0.00 1.06 0.00 0.56 0.33 1.23

Fragilariopsiscilyndrus

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fragilariopsis curta 1.89 5.56 2.38 4.55 3.64 12.50 2.47 0.88 0.48 0.36 0.65 0.00 1.77 2.14 0.84 0.67 0.00

Fragilariopsiskerguelensis

67.21 56.28 58.87 45.45 56.36 20.45 44.44 48.67 62.80 62.23 56.77 57.24 57.80 58.10 47.49 62.00 58.64

Fragilariopsisobliquecostata

0.81 1.21 0.43 9.09 9.09 10.23 10.70 6.19 7.25 8.63 10.32 10.53 10.99 6.12 15.08 2.33 3.70

Fragilariopsisseparanda

0.81 0.48 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.08 0.00 0.00 0.00 0.00 0.28 0.00 0.00

Fragilariopsissublinearis

1.08 0.72 1.08 1.14 6.36 5.68 4.53 7.96 1.45 0.36 1.29 0.00 0.71 3.06 0.28 0.67 0.31

Gen. sp. incognito 0.54 2.66 1.73 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.66 0.00 0.00 0.00 0.00 0.00

Paralia sulcata 0.00 0.00 0.00 0.00 0.00 0.00 0.41 0.00 0.00 0.36 0.00 0.66 0.35 0.31 0.00 0.00 0.31

Porosira glacialis 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.54 0.00 0.36 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Porosirapseudodenticulata

0.00 0.00 0.00 0.00 0.00 2.27 0.00 0.00 0.48 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Proboscia alata 0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.88 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.33 0.00

Rhizosolenia barboi 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Rhizosoleniastilyformis

0.13 0.24 0.22 1.14 0.91 1.14 1.23 0.88 1.45 1.08 0.00 1.97 0.35 0.92 0.56 0.33 0.00

Rhiz. cf. costata 0.00 0.24 0.00 0.00 0.00 0.00 0.00 0.88 0.00 0.36 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Rouxia antarctica 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Rouxia cf. leventerae 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.32 0.00 0.00 0.00 0.00 0.00

Rouxia sp. 0.00 0.00 0.00 1.14 0.00 0.00 0.41 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00


123

association, F. kerguelensis is the dominant form. This is a

high productivity endemic species in the Southern Ocean,

and its recovery is useful for past ecosystems/hydrographic

reconstructions (Crosta et al. 2005; Cortese and Gersonde

2007).

The sea-ice taxa association is represented mainly by

Fragilariopsis curta together with F. rhombica, F. separanda,

F. obliquecostata, Chaetoceros spores and E. Antarctica var.

recta (Fryxell and Prasad 1990; Armand et al. 2005).

In sea-ice permanent cover conditions diatom valves are

rare (low productivity) and usually badly preserved with

evidence of silica dissolution.

Fragilariopsis curta and Chaetoceros spp. can be also

linked to seasonal diatom blooms or high productivity. In

these cases the absolute valve concentration in the sedi-

ments is higher, with lower silica dissolution. F. curta is

commonly found in surface waters along the Antarctic

coastal areas characterised by winter highly consolidated

ice conditions (9–11 months/year sea ice cover) and sum-

mer water temperature ranging from -1.3 to 2.5�C

(Armand et al. 2005). We considered Chaetoceros resting

spores within the sea ice taxa association as indicators of

sea ice coverage, according to Leventer et al. (1996) and

Crosta et al. (1997).

Fragilariopsis separanda is considered a sea-ice corre-

lated species although it is present at a wide range of

temperature. In the literature the maximum abundances of

this species has been observed where sea ice conditions last

4.5–9 months/year, with free ice summer conditions

(Armand et al. 2005). This taxa was mainly found in core

PC-26, having a relatively high percentage within the

species association.

Fragilariopsis obliquecostata is usually associated with

permanent sea-ice conditions with cool water production

(Bianchi and Gersonde 2004) while Eucampia antarctica is

considered ubiquitous in modern sediments.

The reworked taxa group comprises few Plio-Pleist-

ocenic markers that were found as Actinocyclus ingens and

Rouxia taxa such as Rouxia leventerae and Rouxia

constricta.

Because of the low recovery (generally lower than 2%)

and bad preservation of these species within the assem-

blage associations, these taxa have been interpreted as

reworked. The only exception is at the bottom of the core

PC-26 at 390 cm, where we found indications of very low

paleoproductivity with a relatively high presence of well

preserved R. leventerae, that we consider biostratigraphi-

cally in situ.

Table 3 continued

Depth (cm) 10 30 50 70 92 112 132 152 173.5 193.5 213.5 233.5 253.5 273.6 293.6 313.6 333.6

Stellarima microtrias 0.00 0.00 0.00 0.00 0.91 1.14 0.41 0.00 0.48 0.00 0.00 0.00 0.71 0.00 0.00 0.00 0.31

Stephanopyxis sp. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Stephanopyxisgrunowii

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Stephanopyxis turris 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiosiraantarctica

0.13 0.00 0.00 0.00 0.00 0.00 0.41 0.00 0.00 0.00 0.65 0.66 0.71 0.00 0.28 0.00 0.62

Thalassiosiraeccentrica

0.13 0.48 0.00 0.00 0.00 1.14 0.00 0.88 0.00 1.08 0.00 0.66 0.00 0.00 0.56 0.00 0.31

Thalassiosiragracilis

1.48 1.69 1.08 0.00 0.91 0.00 0.41 0.00 0.00 0.72 0.00 0.66 0.35 0.31 0.84 1.00 0.62

Thalassiosira cf.

inura0.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.48 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiosiralentiginosa

9.31 14.98 11.90 10.23 9.09 6.82 8.23 9.73 7.25 9.71 10.32 5.26 6.74 14.68 15.36 22.00 18.83

Thalassiosiraoestrupii

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiosiraoliverana

0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.97 0.72 0.65 0.00 0.00 0.31 0.84 0.33 0.31

Thalassiosira tumida 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.48 0.00 0.65 0.00 0.00 0.00 0.00 0.00 0.00

Trinacria excavata 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Trinacria pileolus 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Xantiopyxis 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.29 0.00 0.00 0.00 0.00 0.33 0.00

Centrales ind. 0.00 0.00 0.43 0.00 0.00 0.00 0.41 0.00 0.00 0.00 0.00 1.32 0.00 0.92 0.00 0.00 0.31


123

Table 4 PC 26—percentage on total association of observed diatom

Depth (cm) 2 12 21 65 85 100 130 165 191 198 215 260 320 345 370 390

Actinocyclus actinochilus 0.21 0.82 0.20 2.50 1.43 1.90 1.07 0.71 0.40 1.64 1.52 1.07 0.00 0.00 0.61 3.70

Actinocyclus ingens 0.42 0.00 1.57 1.88 0.00 0.00 0.54 0.18 0.00 0.00 0.00 0.00 0.00 0.00 0.61 0.00

Actinocyclus karstenii 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.07 0.00 0.00 0.00 0.00

Aster. parvulus 0.21 0.00 0.20 0.63 0.29 1.27 0.80 0.00 0.20 0.23 0.22 0.54 0.00 0.00 0.30 0.00

Chaetoceros cell. veg. 0.00 0.82 0.00 3.13 1.15 0.00 1.88 0.36 0.00 0.00 0.87 0.00 0.00 0.00 0.00 0.00

Chaetoceros spore 25.21 28.22 28.74 28.13 21.20 29.21 9.38 28.24 37.15 27.80 16.52 30.56 0.00 35.71 23.40 14.81

Choretron criophilum 0.21 0.00 0.00 0.00 0.29 0.00 0.27 0.18 0.40 0.23 0.00 0.27 0.00 0.00 0.00 0.00

Centrales ind. 1.06 0.27 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.22 0.00 0.00 0.00 0.00 0.00

Coscinodiscus oculusiridis 0.00 0.00 0.00 0.00 0.00 0.32 0.00 0.18 0.20 0.47 0.00 0.00 0.00 0.00 0.00 0.00

Dactyliosolen antarcticus 2.97 3.01 0.39 1.88 2.29 1.27 2.41 0.53 3.16 2.57 1.52 2.68 0.00 0.00 0.30 0.00

Denticulopsis cf. hustedtii 0.00 0.00 0.20 5.00 0.00 0.00 0.00 0.00 0.00 0.23 0.00 0.54 0.00 0.00 0.00 0.00

Denticulopsis maccollumii 0.00 0.00 0.00 0.63 0.00 0.00 0.27 0.00 0.00 0.00 0.00 0.00 30.00 7.14 0.00 5.56

Eucampia antarctica 1.27 0.27 0.39 11.88 5.16 1.59 9.12 2.13 5.73 6.78 0.87 4.56 0.00 0.00 1.82 1.85

Fragilariopsis cilyndrus 0.42 0.27 0.98 0.00 0.00 0.00 0.00 0.00 0.00 0.23 0.43 0.54 0.00 0.00 0.30 0.00

Fragilariopsis curta 6.36 9.86 11.81 4.38 9.74 7.62 7.51 5.68 3.56 6.78 8.26 3.22 0.00 0.00 20.67 1.85

Fragilariopsis kerguelensis 38.14 38.63 38.19 19.38 30.37 27.94 39.68 27.71 28.46 29.91 53.70 36.46 0.00 7.14 8.21 31.48

Fragilariopsis obliquecostata 1.48 0.55 0.39 5.00 9.46 11.11 9.12 12.26 2.96 4.67 0.87 1.88 0.00 0.00 2.13 1.85

Fragilariopsis separanda 8.47 9.59 8.46 2.50 7.45 0.95 3.75 12.61 6.32 7.94 3.26 3.49 0.00 0.00 31.61 0.00

Fragilariopsis sublinearis 0.42 0.55 0.20 0.63 3.72 3.17 1.07 0.53 0.40 0.93 0.65 0.27 0.00 0.00 4.56 3.70

Odontella spores 0.00 0.00 0.20 0.00 0.00 0.00 0.00 0.36 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Pennales ind. 0.00 0.82 1.38 0.63 0.00 0.00 0.00 0.89 0.00 0.23 0.00 0.00 0.00 0.00 0.00 0.00

Paralia sulcata 0.21 0.00 0.00 0.63 0.00 0.32 0.27 0.18 0.20 0.00 0.43 0.27 20.00 0.00 0.61 1.85

Porosira glacialis 0.64 1.37 0.00 0.00 0.00 0.95 0.00 0.36 0.00 0.00 0.22 0.27 0.00 0.00 0.00 0.00

Porosira pseudodenticulata 0.64 0.00 0.20 0.63 0.29 0.63 1.34 0.00 0.20 0.00 0.00 0.00 0.00 0.00 0.30 0.00

Proboscia alata 0.00 0.00 0.20 0.00 0.00 0.00 0.27 0.00 0.00 0.00 0.00 0.27 0.00 0.00 0.00 0.00

Rhizosolenia costata 0.00 0.00 0.00 0.00 0.29 0.00 0.00 0.00 0.00 0.00 0.00 0.27 0.00 7.14 0.00 0.00

Rhizosolenia stilyformis 0.64 0.82 0.20 1.25 0.29 0.95 0.27 0.36 0.20 0.23 0.00 0.80 30.00 14.29 0.30 3.70

Rhizosolenia hebetata 0.00 0.00 0.20 1.25 0.00 0.00 0.27 0.18 0.59 0.47 0.22 0.27 0.00 0.00 0.30 0.00

Rhizosolenia hebetata morf. bidens 0.00 0.00 0.00 0.00 0.00 0.00 0.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.30 0.00

Rouxia constricta 0.00 0.27 0.00 0.00 0.00 0.00 0.00 0.00 0.20 0.00 0.65 0.00 0.00 0.00 0.00 0.00

Rouxia leventerae 0.00 0.00 0.00 0.63 0.00 0.32 0.54 0.53 0.40 0.93 0.22 0.54 0.00 0.00 0.91 18.52

Rouxia cf. diploneides 0.00 0.00 0.00 0.63 0.00 0.00 0.54 0.00 0.20 0.00 0.43 0.00 0.00 0.00 0.00 0.00

Stephaopyxis turris 0.00 0.00 0.00 0.00 0.00 0.32 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiosira antarctica 0.85 0.00 0.79 0.63 3.44 5.40 0.00 0.89 0.79 2.10 0.00 0.80 0.00 7.14 0.00 0.00

Thalassiosira eccentrica 0.64 0.00 0.00 0.00 0.00 0.00 1.07 0.36 0.00 0.00 0.00 0.54 0.00 0.00 0.61 0.00

Thalassiosira gracilis 1.27 1.37 0.00 1.25 0.29 0.63 1.61 0.71 2.17 2.10 1.30 0.80 0.00 0.00 0.30 1.85

Thalassiosira fasciculata 0.00 0.00 0.00 0.63 0.00 0.00 0.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiosira lentiginosa 5.30 1.37 3.94 1.88 1.15 3.81 5.36 2.66 5.34 2.57 5.65 5.63 10.00 0.00 1.82 3.70

Thalassiosira oestrupii 0.21 0.00 0.39 0.00 0.00 0.00 0.00 0.53 0.00 0.00 0.22 0.00 0.00 0.00 0.00 0.00

Thalassiosira oliverana 0.42 0.00 0.59 0.00 0.57 0.00 0.54 0.18 0.20 0.47 0.00 0.80 0.00 0.00 0.00 0.00

Thalassiosira tumida 0.00 0.27 0.00 0.00 0.00 0.00 0.27 0.00 0.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Tricothoxon Fragments 0.00 0.00 0.00 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Thalassiotrix long/ant. group

fragments

1.27 0.82 0.00 0.63 0.29 0.32 0.27 0.53 0.20 0.00 1.30 0.27 0.00 7.14 0.00 0.00

Trinacria excavata 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.14 0.00 0.00

Trinacria pileolus 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.14 0.00 0.00

Xantiopyxis 0.00 0.00 0.00 0.63 0.57 0.00 0.00 0.00 0.00 0.47 0.00 0.00 10.00 0.00 0.00 5.56

Centrales ind. 1.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.34 0.00 0.00 0.00 0.00


123

Core PC-18 contains the record of last climatic cycle

(Fig. 4). In the lower part of the core, from the base of core

to 132 cm, the absolute valve concentration is generally

low. In this interval the sea ice forms (F. curta, F. rhombica,

F. obliquecostata and E. antarctica) increase progressively

up to 132 cm having an opposite trend with open ocean

species. We associated this interval to the last glacial phase.

Above this interval, at 132 cm, 112 cm, and 70.5 cm,

the low absolute valve concentration is associated to sea-

sonal sea ice and/or low temperature water specimens such

as Chaetoceros spores, E. antarctica var. recta with

F. obliquecostata. This interval was associated with the last

glacial–interglacial climatic transition phase.

In the upper part of the core (above 50 cm) an increase

in paleoproductivity characterised by open water speci-

mens like F. kerguelensis, (dominant) and T. lentiginosa

with occasionally Chaetoceros spores, is interpreted as the

present interglacial productivity phase.

Core PC-26 contains the record of the past three climatic

cycles (Fig. 4). The bottom of the core (390 cm) is char-

acterised by low paleoproductivity (rare diatoms) although

there is a relative high presence of well preserved

R. leventerae. We referred the base of core PC-26 to the

glacial M.I�S. 6 in the T. lentiginosa/F.kerguelensis Zone

subzone a (Zielinski et al. 2002).

At 370 cm the sea-ice taxa are dominant and repre-

sented mainly by F. separanda (dominant with over 30%

within the association) and F. curta. The occurrence of

F. separanda together with the high percentage of F. curta

and the low percentage of F. kerguelensis suggests a rel-

ative high productivity phase that we related to the retreat

of the seasonal sea ice edge during the transition between

glacial interglacial stages.

The slump above the depth of 370 cm marks the tran-

sition between T. lentiginosa/F. kerguelensis concurrent

range zone subzone a and T. lentiginosa/F. kerguelensis

Concurrent Range Zone subzone b. The boundary between

subzones a and b is defined by the LOD of R. leventerae at

the end of M.I.S. 6 (0.13 My).

Chaetoceros spores (dominant) together with F. kerg-

uelensis, F. obliquecostata and F. separanda are very

abundant at 165 cm, suggesting a high-productivity epi-

sode. Variability within water temperature and sea-ice

extension could explain this high productivity phase that

we correlated to interglacial M.I.S. 5.

From 165 cm to 65 cm the sediments contain F. Kerg-

uelensis and Chaetoceros spores as dominant species, with

a relative high percentage of sea-ice species. The occur-

rence of F. obliquecostata within this taxa association

together with evidences of low paleoproductivity (poor

20 60 100

PC 18

Legend

% Sea ice species

% Open ocean species

% Reworked

4*10

1,2*

10

0

1

2

3

dept

h (m

)

Tha

lass

iosi

ra le

ntig

inos

a / F

. ker

guel

ensi

s

5

6

4*10

1,2*

105

6

Tha

lass

iosi

ra le

ntig

inos

a / F

. ker

guel

ensi

sb

0

1

2

3

4

dept

h (m

)

PC 26

20 60 100

Abs

olut

e va

lve

conc

entr

atio

n(n

° of

val

ves/

dry

sedi

men

t)

Abs

olut

e va

lve

conc

entr

atio

n(n

° of

val

ves/

dry

sedi

men

t)

a

Diatom

zone

s

Diatom

zone

sFig. 4 Distribution of open

water, sea-ice and reworked

diatom species along cores PC-

18 and -26. See Tables 3 and 4

for taxa percentages


123

specimen recovery) lead us to relate this interval to M.I.S.

4, 3 and 2.

In the upper section above 65 cm to the top of the core,

the paucity of data does not allow a detailed biostrati-

graphic reconstruction. However, the high productivity and

the constant high presence of open-water specimens (e.g.

F. kerguelensis) recorded in the upper 20 cm of the core

allowed us to associate this interval to the last glacial-

interglacial climatic transition phase.

Discussion

Sediment facies and related depositional processes

We distinguished two main depositional settings: (1) at the

crest of ridge ‘‘A’’ (cores PC-20, -26); and (2) the WEGA

channel and the western gentle side of ridge ‘‘A’’ (cores

PC-18, -19).

At the crest of ridge ‘‘A’’ both cores PC-20 (distal) and

PC-26 (proximal) contain condensed sequences spanning

down to M.I.S. 17 and 6, respectively (700 and 200 ky

approximately). Evidences of sediment instability in the

form of slumped intervals having sharp/irregular bases are

frequent, especially within the older units. In the proximal

area, core PC-26 is mainly formed by coarse-grained

(cluster 4) bioturbated sediments with abundant IRD. These

sediments are characterised by a generally high productivity

with diatom assemblage formed by open-water taxa with a

constant supply of sea-ice taxa indicating a strong influence

of the sea-ice extension over the sedimentation.

On core PC-20 (distal), the bioturbated, IRD-rich sedi-

ments are generally finer-grained (prevailing cluster 3) than

in core PC-26. In addition, the diatom’s assemblage in PC-

26 indicates the absence of sea-ice taxa throughout inter-

glacials, which became dominant during glacial intervals.

We related the bioturbated, IRD-rich sediments to hemi-

pelagic sedimentation associated with fall out rain of

terrigenous sediments driven from the land through water

plumes and/or icebergs. This interpretation is consistent

with the poor sorting and the oceanward decrease of the

bulk texture of the sediments forming this lithofacies. The

absence of lamination within these sediments rule out

the presence of sheer stress at the sea bottom associated

with tractive currents (bottom and/or turbidity currents).

Core PC-20 contains also laminated sediment. In the

upper 2.4 m of the core (present time to M.I.S. 11) the

discontinuously/wispy laminated intervals are very fine

grained (cluster 1), bioturbated, and contain scattered

detritus of IRD. Similar facies were described from the

Pacific margins of the Antarctic Peninsula by Lucchi et al.

(2002) and Lucchi and Rebesco (2007) and were associated

with bottom currents. This interpretation is coherent with

both texture and structure of these intervals that indi-

cate low-energy tractive currents. Contrary to what was

described by Lucchi and Rebesco (2007), however, in this

study the laminations are more pronounced with little

bioturbation, suggesting a more energetic environment.

Bottom currents possibly remobilised fine-grained sedi-

ments conveyed into the system by other processes such us

sediment-laden water plumes, benthic sediments re-sus-

pension, etc.

In the older units of core PC-20 (below M.I.S. 11), the

laminated intervals are coarser-grained (cluster 2), not

bioturbated with well-defined laminations. The absence of

IRD within lamination indicates a rapid sedimentary pro-

cess that we associate with low-density turbidity currents.

This interpretation is coherent also with the presence of

sharp/irregular bases of some laminated intervals that can

be related to a higher-energy depositional process with

respect to contour currents. Evidences of turbidity-current

processes along the WEGA channel have been revealed

also by sub-bottom profiles. According to Caburlotto et al.

(2006), the unconformity WL-U10, which was traced from

the WEGA channel up to site PC-20 at the crest of ridge

‘‘A’’, signs a change within the sedimentary system passing

from a turbiditic system with sediment erosion within the

channel thalweg, to a lower-energy depositional processes

that filled the channel. We correlated unconformity

WL-U10 within core PC-20 (Fig. 5) marking the change

from prevailing turbidity currents to low-energy-deposi-

tional bottom currents, recorded by the very-fine grained

(cluster 1), laminated facies with scattered IRD located

above the unconformity.

On the WEGA channel and western gentle side of ridge

‘‘A’’, cores PC-18 and -19 contain bioturbated medium-

grained sediments (cluster 3) containing scattered IRD,

with seasonal sea-ice bioproductivity. The high-sedimen-

tation rate (13 cm/ky) calculated in core PC-18 during

M.I.S. 1 and 2 (last glacial/interglacial cycle) is consistent

with a sedimentary depocenter located along the WEGA

channel. The lack of erosional surfaces and/or laminations

indicate a low-energy depositional process that can be

associated with either sediment settling from a nepheloid

layer and/or very low-energy bottom currents. In the latter

case, weak sheer stress at the sea bottom may have had

generated faint laminations that were subsequently masked

by intense bioturbation.

The deeper part of core PC-19, correlated to M.I.S. 2

(glacial), contains discontinuous/wispy laminated sedi-

ments with scattered IRD having textural (cluster 1) and

structural characteristics similar to the laminated sediments

recovered in the upper part of core PC-20. We associated

the presence of lamination to a minor biological activity

during the glacial that did not mask the primary sedimen-

tary structures.


123

The WEGA channel Quaternary history

The Quaternary depositional history of the WEGA channel

largely differs from the surrounding Jussieu and Buffon

channels where high-energy turbiditic processes are

believed to dominate the deposition since Cenozoic time

(Payne and Conolly 1972; Escutia et al. 2003, 2005). The

Jussieu and Buffon channel’s heads cut into the shelf edge

and thus were directly fed at the glacier trough’s mouth with

a large amount of terrigenous debris that moved across the

shelf trough ice-streams. The V-shaped cross profiles of the

Jussieu and Buffon channels indicate high-energy erosive

turbidity currents. Sandy turbidites were sampled very close

to the Jussieu channel thalweg (Busetti et al. 2003). On the

contrary, the WEGA channel, which originates seaward of

the Mertz bank, has the upper reaches in the middle slope

and is separated from the shelf edge by a pronounced

bathymetric bulge representing a morphological barrier for

Sea floor

WL-S11

WL-S10

WEGA channel

WL-U10WL-U9

PC 20

PC 19PC 18

4.0

4.1

4.2

4.3

4.4

4.5

4.6

TWT

(s)

WEST

Jussieu Channel

Ridge “A”

2 km

EAST

LINE WEGA 2601

1.5 1.6 1.7

(gm/cc)

1440 1460 1480

(m/s)density P- wave

WL-S11

WL-S10

PC 194.3 m5.8 ms

PC 183.5 m4.7 ms

PC 204.7 m6.3 ms Sea floor

unconformity WL-U10

m b

sf

5

4

3

2

1

00

8

6

4

2

TW

T (

ms)

M.I.

S. 1

M.I.

S. 2

M.I.

S. 1

M.I.

S. 2

tran

sitio

n?

a

b

5

6

7

8

9

10

11

12

13

14

15

1617

2-3-4

-1 0 1 2

Specmapstacked O-18

-1 0 1 2

Specmapstacked O-18

-1 0 1 2

Specmapstacked O-18

Fig. 5 a Line drawing of acoustic profile W 2601 crossing the

WEGA channel and the ridge ‘‘A’’ (see Fig. 1 for seismic line

location). The stratigraphic units WL-S11 and WL-S10, the uncon-

formity W L-U10 and the location of the cores PC-18, -19 and -20 are

indicated (modified after Caburlotto et al. 2006). b Core correlation

along the acoustic profile W 2601. The combination of the integrated

age model with the down-core logs of the bulk density and P wave

velocity (after Busetti et al. 2003) allowed identifying the unconfor-

mity WL-U10 within core PC-20 (see text for discussion). For each

core the age has been plotted versus the SPECMAP time scale

developed by Imbrie et al. (1984), based on normalised planktonic

records (normalised O18 vs time)


123

sediment delivery to the slope. Notwithstanding, the

stratigraphic sequence recovered in core PC-20 indicates

that the WEGA channel experienced a more articulated

sedimentary history than the Jussieu and Buffon channels.

According to our integrated age model, core PC-20

recovered a condensed sedimentary sequence spanning

down to M.I.S. 17 and thus contains the unconformity WL-

U10 described by Caburlotto et al. (2006) on the echo-

sounding record (Fig. 5). Assuming a P-wave velocity

average of 1,480 m/s, we identify in core PC-20 the WL-

U10 unconformity at a depth of 2.4 m bsf corresponding to

a negative peak on both P-wave and bulk density in the

down-core profiles measured by Busetti et al. (2003).

Although the seismic profile suggests a hiatus or a change

in sedimentation rates at the unconformity, no (clear)

indications for a time gap were discovered in the lithology

observed on core PC-20.

Low-density, cm-thick, turbidites were recovered in the

deeper part of core PC-20 corresponding to stratigraphic

unit WL-S10. These turbidites consist of fine-grained

laminated sediments having sharp or irregular bases.

Within unit WL-S10, the echo-sounding profiles indicate

the occurrence of debris flows along the channel axis and/

or at the base of its side walls suggesting a higher-energy

sedimentary environment than present time (Caburlotto

et al. 2006). The wide U-shaped cross section of the paleo-

WEGA channel as well as the development of continuous

strata inside its levees, however, suggest less powerful

down-slope gravity flows than those observed into the

Jussieu and Buffon channels, with depositional processes

dominating over erosion.

We claim that the low-density turbidites observed in

core PC-20 derived from underwater sediment plumes

originated by melting of the grounded-ice at the shelf brake

and/or minor local sediment instability along the WEGA

channel. Pervasive weak laminations within the fine-

grained sediments suggest the co-presence of slow bottom

currents that reworked the fine fraction delivered into the

system through down-slope gravity processes. The exis-

tence in this area of westward flowing bottom currents is

also supported by the geophysical data. Caburlotto et al.

(2006) indicated on the echo-sounding record the presence

of two types of wavy bedforms having different orientation

with respect to the crest of ridge A: the first type is parallel

to the crest of ridge A and was associated with turbidity

current over-banking sediments; the second type appears as

upslope migrating sediment waves obliquely oriented with

respect to the crest of the ridge and was related to westward

flowing bottom currents. The asymmetry of the WEGA

channel levees during WL-S10 (the western one being

more pronounces) was associated with westward deviation

of down-slope gravity flows by effect of both the Coriolis

force and west-flowing bottom currents. De Santis et al.

(2003), Donda et al. (2003) and Escutia et al. (2003) sug-

gested that along the Wilkes Land continental margin the

interplay between down-slope and along-slope currents

was active also during pre-Quaternary time.

Unconformity WL-U10 (between M.I.S. 12 and M.I.S.

11, Fig. 5) marks an important change within the deposi-

tional history of the WEGA channel: after this time there

are no evidences of turbidity flow related deposition, while

strong erosive currents still affect the Jussieu and Buffon

channels (Busetti et al. 2003; Caburlotto et al. 2006). After

M.I.S. 11, the WEGA channel became a depositional

morphologic depression affected by very low-energy down-

slope processes. The decreased energy of the sedimentary

environment within the WEGA channel likely reflects a

change of either the ice-flow pattern on the continental shelf

or its extension over the shelf so that the head of the WEGA

channel was deprived from direct glacial debris input.

Lambert et al. (2007), Jouzel et al. (2007) and the

EPICA community members (2004) reported that both

deep-sea and ice-core records indicate that the transition

between M.I.S. 12 and M.I.S. 11 (the Mid-Brunhes Event,

about 420 ky BP) signs an important change within the

characteristics of glacial/interglacial cyclicity with warmer

and longer interglacials, and colder but shorter glacial

stages in the younger sequences corresponding to a rela-

tively lower ice-volume and higher greenhouse gas

concentration in the atmosphere. Presti et al. (2005)

observed that during last glacial maximum (LGM) the

deepest part of the Mertz trough on the Wilkes Land

continental shelf was not occupied by grounded ice sheet.

On the contrary, there was a floating ice shelf likely

grounded on the shallowest areas of the shelf confirming

that at least during LGM the extension of the grounded ice

sheet was smaller than during previous glacial maxima. We

think that after M.I.S. 11, the minor ice-volume during

glacials would have been confined within the ice-shelf

glacial troughs so that only ice-streams reached the shelf

edges delivering sediments to the upper slope (e.g. the

Jussieu and Buffon channels). The volume of the ice-shelf

was not large enough to climb morphological barriers such

as the Mertz bank and thus did not pass over the bathy-

metric bulge facing the WEGA channel that was deprived

of direct sediment input by the glacier.

The high-sedimentation rates measured on cores PC-18

and -19 within the WEGA channel, as well as the sediment

texture and structure observed in the upper part of core PC-

20 and -26 suggest that after MIS 11 the channel was not an

inactive feature but rather a low-energy environment. The

seismic record indicates that above unconformity WL-U10

the decreasing down-slope current activity resulted with

infill of the previous incisions with landward shift of the

sedimentary depocentre (Caburlotto et al. 2006). Sediment

deposition occurred mainly within the channel axis and its


123

western levee. We discussed above that the bioturbated and

not laminated sediments observed in core PC-18 and -19

can be associated with either deposition by sediment set-

tling from a nepheloid layer and/or through very low-

density and energy down-slope flows.

We related this low-energy down-slope flow to the

HSSW forming on the Mertz shelf by cooling of the

modified circum-polar deep water (MCDW) in the area of

the Mertz Polynya (Gordon and Tchernia 1972; Rintoul

1998). This hypersaline, cold and dense water, fills the

shelf area and periodically spills-off the shelf edge flowing

down-slope along existing slope canyons (Bindoff et al.

2000). In analogy to other areas of dense water production

(Ivanov et al. 2003; Shapiro et al. 2003; Bergamasco et al.

2003), the HSSW behaves as a gravity current driven by

salinity contrast that moves along the WEGA channel

entraining fine-suspended particles that are carried towards

deeper environments.

The relatively well-sorted Holocene silts observed

above the glacial diamicton recovered in the cores from the

Mertz trough (Harris et al. 2001; Presti et al. 2003) were

interpreted as due to winnowing of fine-grained sediment

by bottom density-currents associated with the HSSW

formation (Dunbar et al. 1985; Domack 1988; Baines and

Condie 1998; Harris and O’Brien 1998; Presti et al. 2003).

The fine-grained sediments are transported down-slope

along the WEGA channel to feed a thick nepheloid layer,

having the highest density in the lowermost few 100 m of

the water column (Eittreim et al. 1971). The presence of

both seasonal-sea-ice and open-water diatom assemblages

within the present interglacial deposits sustain the

hypothesis of the continental shelf origin of the fine-

grained sediment recovered in the rise area.

Contrary to what was observed in the glacial sediments

recovered in other parts of Antarctica that are barren, not

bioturbated (Lucchi and Rebesco 2007 and references

therein), in cores PC-18 and -19 last glacial sediments are

bioturbated and the diatom assemblage indicate a reduced

palaeo-productivity containing both seasonal-sea-ice and

open-water species, typical of shelf areas. These observa-

tions suggest that down-slope flowing dense waters

endured during last glacial.

We can suggest two possible mechanisms for dense

water production during glacials: (1) the permanence of the

polynya, possibly of reduced size and/or northward located,

which would have allowed the formation of the HSSW also

during glacial time (Lucchi and Rebesco 2007), or (2) the

cooling of the MCDW beneath the floating ice sheet,

similar to what happens today below the Ross Ice Shelf

(Bergamasco et al. 2003; Rivaro et al. 2003).

In both cases the formation of colder and denser water

masses that sink below the ice sheet and periodically spill

over the shelf edge would provide deeper environments

with nutrients and oxygen to the benthic fauna (low pro-

ductivity during glacials) and with sediments to fill the

bottom nepheloid layer (high sedimentation rates within

cores PC-18 and -19).

Conclusions

The study of sedimentary facies confirms that the deposi-

tional system on the George V Land continental margin is

dominated by down-slope gravity process in the form of

turbidity currents and/or high salinity cold bottom waters.

The crest of ridge ‘‘A’’ is highly instable. In the proxi-

mal area of the crest bioturbated, IRD-rich sediments

derive mainly from settling of sediment-laden water

plumes and ice-rafted debris.

In the distal area of ridge ‘‘A’’, a condensed sequence

recovered sediments down to M.I.S. 17 containing the

unconformity WL-U10 previously identified in the eco-

sounding record. The older sedimentary sequence is dom-

inated by turbidity currents with interplay of along-slope

bottom contour currents (pervasive weak laminations),

while above the unconformity wispy-discontinuous lami-

nations were associated with bottom down-slope currents.

After M.I.S. 11, no turbiditic flows occurred along the

WEGA channel, while strong erosive current still affect the

Jussieu and Buffon channels. The decreasing energy of the

sedimentary environment within the WEGA channel likely

reflects a change of the ice-flow pattern on the continental

shelf possibly associated with a minor ice-shelf volume

occurring during late Quaternary glacial stages.

High salinity shelf waters forming on the shelf and

periodically over-spilling the shelf edge, deliver sediments

to the deeper environment feeding the bottom nepheloid

layer within the WEGA channel. High-sedimentation rates

within the channel derive from sediment settling from the

nepheloid layer and/or direct input by the HSSW. This

process was active also during last glacial allowing the

survival of the benthic fauna.

Acknowledgments We acknowledge Captain and crew of the R.V.

Tangaroa for their skilful support during the WEGA 2000 cruise. We

thank G. Kuhn, M. Weber and R. Stein for detailed review that greatly

improved the manuscript. This work was funded by the Programma

Nazionale delle Ricerche in Antartide (PNRA) under the WEGA

Project. The first author benefits from an OGS doctoral fellowship in

Polar Science at the University of Siena (Italy). The grain size

analysis has been made at the ‘‘Laboratorio Antartide’’ at the

Department of Geological, environmental and Marine Sciences of the

University of Trieste (Italy).

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Sedimentary Processes on the Wilkes Land Continental Rise