Sedimentary Processes on the Wilkes Land Continental Rise
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Transcript of 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
912 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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
Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 913
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
914 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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
Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 915
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
916 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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
Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 917
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
918 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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
Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 919
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.
920 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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)
Int J Earth Sci (Geol Rundsch) (2010) 99:909–926 921
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
922 Int J Earth Sci (Geol Rundsch) (2010) 99:909–926
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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