ArticlePDF Available

IODP Expedition 318: From Greenhouse to Icehouse at the Wilkes Land Antarctic Margin

Authors:

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 318, Wilkes Land Glacial History, drilled a transect of sites across the Wilkes Land margin of Antarctica to provide a long-term record of the sedimentary archives of Cenozoic Antarctic glaciation and its intimate relationships with global climatic and oceanographic change. The Wilkes Land drilling program was undertaken to constrain the age, nature, and paleoenvironment of the previously only seismically inferred glacial sequences. The expedition (January–March 2010) recovered ~2000 meters of high-quality middle Eocene–Holocene sediments from water depths between 400 m and 4000 m at four sites on the Wilkes Land rise (U1355, U1356, U1359, and U1361) and three sites on the Wilkes Land shelf (U1357, U1358, and U1360). These records span ~53 million years of Antarctic history, and the various seismic units (WL-S4–WL-S9) have been successfully dated. The cores reveal the history of the Wilkes Land Antarctic margin from an ice-free “greenhouse” Antarctica, to the first cooling, to the onset and erosional consequences of the first glaciation and the subsequent dynamics of the waxing and waning ice sheets, all the way to thick, unprecedented "tree ring style" records with seasonal resolution of the last deglaciation that began ~10,000 y ago. The cores also reveal details of the tectonic history of the Australo-Antarctic Gulf from 53 Ma, portraying the onset of the second phase of rifting between Australia and Antarctica, to ever-subsiding margins and deepening, to the present continental and ever-widening ocean/continent configuration. doi:10.2204/iodp.sd.12.02.2011
Scientific Drilling, No. 12, September 2011 15
IODP Expedition 318: From Greenhouse to Icehouse at the
Wilkes Land Antarctic Margin
by Carlota Escutia, Henk Brinkhuis, Adam Klaus, and
the IODP Expedition 318 Scientists
doi: 10.2204/iodp.sd.12.02.2011
6FLHQFH5HSRUWV
Abstract
Integrated Ocean Drilling Program (IODP) Expedition
318, Wilkes Land Glacial History, drilled a transect of sites
across the Wilkes Land margin of Antarctica to provide a
long-term record of the sedimentary archives of Cenozoic
Antarctic glaciation and its intimate relationships with global
climatic and oceanographic change. T he Wilkes Land drill-
ing program was undertaken to constrain the age, nature,
and paleoenvironment of the previously only seismically
inferred glacial sequences. The expedition ( January–March
2010) recovered ~2000 meters of high-quality middle
Eocene–Holocene sediments from water depths between 400
m and 40 00 m at four sites on the Wilkes Land rise (U1355,
U1356, U1359, and U1361) and three sites on the Wilkes
Land shelf ( U1357, U1358, and U1360).
These records span ~53 million years of Antarctic history,
and the various seismic units ( WL-S4–W L -S9) have been
successfully dated. The cores reveal the history of the
Wilkes Land Antarctic margin from an ice-freegreen-
house” Antarctica, to the first cooling, to the onset and ero-
sional consequences of the first glaciation and the subse-
quent dynamics of the waxing and waning ice sheets, all the
way to thick, unprecedented “tree ring st yle” records with
seasonal resolution of the last deglaciation that began
~10,000 y ago. The cores also reveal details of the tectonic
histor y of the Australo-Antarctic Gulf from 53 Ma, por-
traying the onset of the second phase of
rif ting between Australia and Antarctica,
to ever-subsiding margins and deepening,
to the present continental and ever-wide -
ning ocean/continent configuration.
Introduction
Polar ice is an important component of
the modern climate system, af fecting
among other things global sea level, ocean
circulation and heat transport, marine pro-
ductivit y, air-sea gas exchange, and plane-
tar y albedo. The modern ice caps are, geo-
logically speaking, a relatively young
phenomenon. Since mid-Permian times
(~270 Ma), parts of Antarctica became
reglaciated only ~34 m.y. ago, whereas full-
scale, permanent Northern Hemisphere
continental ice began only ~3 m.y. ago
(Zachos et al., 2008; Fig. 1). The record of
Antarctic glaciation, from the time of first
ice-sheet inception through the significant
periods of climate change during the
Cenozoic, is not only of scientific interest
but also is of great importance for society.
State-of-the-art climate models (DeConto
and Pollard, 2003a, 2003b; Huber et al.,
2004; DeConto et al., 2007; Pollard and
DeConto, 200 9) combined with paleocli-
matic proxy data (Pagani et al., 2005) sug-
gest that the main triggering mechanism
for inception and development of the
6FLHQFH5HSRUWV
Figure 1. Updated Cenozoic pCO2 and stacked deep-sea benthic foraminifer oxygen
isotope curve for 0 Ma to 65 Ma. NB: Updated from Zachos et al. (2008) and converted
to the “Gradstein timescale” (Gradstein et al., 20 04). Mi-1 = Miocene isotope Event 1,
Oi-1 = Oligocene isotope Event 1, ETM2 = Eocene Thermal Maximum 2, PETM =
Paleocene/Eocene Thermal Maximum, ETM1 = Eocene Thermal Maximum 1.
-1
0
1
2
3
4
5
010 20 30 40 50 60
Miocene OligocenePlio.
Plt.
12°
Ice-free temperature (°C)
Full-scale and permanent
Partial or ephemeral
PETM (ETM1)
Atmospheric CO 2 (ppmv)
4000
0
1000
2000
3000
5000
Age (Ma)
CO2 proxies
Boron
Alkenones
010 20 30 40 50 60
Anthropogenic peak
early Eocene
minimum
PaleoceneEocene
Early Eocene
Climatic Optimum
Nahcolite
Trona
Cret.
ETM2
Antarctic ice sheets
N. Hemisphere ice sheets
?
N. Hemisphere sea ice
?
?
Episodic perennial
Seasonal
?
Mid-Miocene Climatic
Optimum
Mid-Eocene Climatic
Optimum
δ18 O (‰)
Oi-1
Mi-1
16 Scientif ic Drilling, No. 12, September 2011
6FLHQFH5HSRUWV
estimates have not been experienced since before the ice
sheets in Antarctica formed.
Since their inception, the Antarctic ice sheets appear to
have been very dynamic, waxing and waning in response to
global climate change over intermediate and even short
(orbital) timescales ( Wise et al., 1991; Zachos et al., 1997;
Pollard and DeConto, 20 09). However, not much is known
about the nat ure, cause, timing, and rate of processes invol-
ved. Of the two main ice sheets, the West Antarctic Ice Sheet
(WAIS) is ma inly mari ne based a nd is considered less stable
(Florindo and Siegert, 2009). The East Antarctic Ice Sheet
(EAIS), which overlies continental terrains that are largely
above sea level, is considered st able and is believed to
respond only slowly to changes in climate (Florindo and
Antarctic ice sheet was the decreasing levels of CO2 (a nd
other greenhouse gases) concentrations in the atmosphere
(DeConto and Pollard, 2003a, 2003b; Figs 1, 2). The opening
of critical Southern Ocean gateways played only a second-
ary role (Kennett, 1977; DeConto and Pollard, 2003a; Huber
et al., 2004). With current rising atmospheric greenhouse
gases resulting in rapidly increasing global temperatures
(Intergovernmental Panel on Climate Change [IPCC], 2007;
ww w.ipcc.ch/), studies of polar climates are prominent on
the research agenda. Understanding Antarctic ice- sheet
dynamics and stability is of special relevance because, based
on IPCC (2007) forecasts, atmospheric CO2 doubling and a
1.8°C–4.2°C temperature rise is expected by the end of this
century. The lower values of these estimates have not been
experienced on our planet since 10–15 Ma, and the higher
Ice-sheet thickness (m)
4000
3500
3000
2500
2000
1500
1000
500
0
Δ Sea level (m)
0204060
Δ δ18 O
0.0 0.2 0.4 0.6
Ice volume (106 km3)
01020
Time (10 6 modely)
2
4
6
8
10 2.0
2.2
2.4
2.6
3.0
3.2
3.4
3.6
2.8
CO2 (PAL)
60°S
70°
8
PB
WL
LG
0E
0E
0E
0E
Figure 3. Combined IODP Expedition 318 transit and drill sites.
Figure 2. Simulated initiation of East
Antarctic glaciation in the earliest
Oligocene using a coupled general
circulation model (GCM) ice-sheet
model (from DeConto and Pollard,
2003a). These results are principally
forced by gradual lowering of
atmospheric levels of CO2 in the
simulated atmosphere. Note that the
glaciation takes place in a “two -step”
fashion reminiscent of the two-step
į18O increase recorded in benthic
foraminiferal carbonates across the
Eocene– Oligocene transition (Coxall
et al., 2005). The first step results in
glaciation in the Antarctic continental
interior, discharging mainly through
the Lamber t Graben (LG) to
Prydz Bay (PB). The second step
results in the initial connection and
subsequent rapid expansion of the
ice sheet, reaching sea level in the
Wilkes Land (WL) at a later stage.
PAL = Preindustrial Atmospheric
Level.
Scientific Drilling, No. 12, September 2011 17
-1012345
0
10
20
30
40
50
60
MioceneOligocene Plio.
Plt.
δ18O (‰)
U1359
8° 12°
Ice-free temperature (°C)
Full-scale and permanent
Partial or ephemeral
PETM (ETM1)
Age (Ma)
Paleocene Eocene
Early Eocene
Climatic Optimum
Cret.
ETM2
Antarctic ice sheets
N. Hemisphere ice sheets
?
N. Hemisphere sea ice
?
?
Episodic perennial
?
Mid-Miocene Climatic
Optimum
U1355
U1357
U1360
U1358
U1356
U1361
Shelf sites Rise sites
Seasonal
WL-U8?
IcehouseGreenhouse
WL-U5
WL-U4
WL-U3
51.06 Ma
51.90 Ma
Clay/Silt
Sand
Diatom ooze
Nannofossil ooze
Sandy mud
Limestone
Lithology
Rise
Site
U1356
Rise
Site
U1359
Rise
Site
U1361
Shelf
Site
U1358
Shelf
Site
U1360
Clast-rich
sandy diamict
Clast-poor
sandy diamict
Timescale
25
30
35
40
45
50
55
Age (Ma)
0
5
10
15
20
Eocene Oligocene
Paleo-
cene Miocene
Plio-
cene
cene
Holocene
C18
C17
C16
C15
C13
C12
C11
C10
C9
C8
C7A
C7
C6C
C6B
C6AA
C6A
C26
C25
C24
C23
C22
C21
C20
C19
C5E
C5D
C5C
C5B
C5A
D
C5A
C
C5AB
C5AA
C5A
C5
C4A
C4
C3B
C3A
C3
C2A
C2
C1 C1n
C3n
C2An
C3An
C4n/C4An
C5n
C5r.2n
C5An
C5An
C3An
C2An
C3n
C1n
C2n
C5n
1000
975
950
925
900
875
850
825
800
775
750
725
700
675
650
625
600
575
550
525
500
475
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0
Depth (mbsf)
C24n
C12n
C9n
C8n
C7An
C5Dn
C5Cn
C5AAn
C5ABn
C5ACn
C5ADn
C23n.2n
C11n.1n
C10n
C5Bn.2n
600
575
550
525
500
475
450
425
400
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0
375
350
325
300
275
250
225
200
175
150
125
100
75
50
25
0
25
0
Insucient paleomagnetic data
C5AB
C5AA
C5A
C5
C4A
C4
C3B
C3A
C3
C2A
C2
C1
0
5
10
Miocene Plio-
cene Pleisto-
cene
Holo-
cene
1
2
3
7
8
9
6
4
13
11
12
h
h
Pleist.
Plio.
C12r
C23n.2n
50
25
0
Pleist.
C12r
14.3 m
h
h
U5
U3
Timescale
A
B
Figure 4. [A] Stacked deep-sea
benthic foraminifer oxygen isotope
curve for 0–65 Ma (updated from
Zachos et al., 2008) converted into
“Gradstein timescale” (Gradstein
et al., 2004) and combined with
chronostratigraphy of the sites drilled
during Expedition 318. ETM2 =
Eocene Thermal Maximum 2, PETM =
Paleocene/Eocene Thermal Maximum,
ETM1 = Eocene Thermal Maximum 1.
[B] Chronostratigraphic framework for
sites drilled during Expedition 318.
Timescales are those of Gradstein et
al. (2004). [11x17 landscape]
18 Scientif ic Drilling, No. 12, September 2011
6FLHQFH5HSRUWV
Wilkes Land margin also includes sites with ultrahigh accu-
mulation rates of sediments that document the Holocene
deglaciation and subsequent climate and sedimentological
variabilit y extending over the past 10,000 y. In general, our
strategy was to core and analyze sedimentary records along
the inshore-offshore transect to constrain the age, nature,
and environments of deposition, until now only inferred from
seismic surveys of the Wilkes L and continental shelf, rise,
and abyssal plain (Escutia et al., 1997; De Santis et al., 2003;
Escutia et al., 2005).
The Expedition
IODP Expedition 318 (January–March 2010; Wellington,
New Zealand, to Hobart, Australia), occupied seven sites
(Fig. 3) across the Wilkes Land Margin at water depths be-
tween ~400 mbsl and 4000 mbsl. Together, we retrieved
~2000 meters of high-qualit y upper EoceneQuaternary
sedimentary cores (Fig. 4a, b). Sites U1355, U1356, U1359,
and U1361 are on the Wilkes Land rise, and Sites U1358,
U1360, and U1357 are on the Wilkes Land shelf. The cores
span ~53 m.y. of Antarctic histor y, revealing the history of
the Wilkes Land Antarctic margin from an ice-freegreen-
house Antarctica,” to the first cooling, to the onset and ero -
sional consequences of the first glaciation and the subse-
quent dynamics of the wax ing and waning ice sheets (Fig. 4).
Furthermore, we also were able to capture the record of the
last deglaciation in terms of thick, unprecedented “tree ring
style” records with annual to seasonal resolution taken in the
Adélie depression (U1357) (Fig. 5).
Initial studies now
also reveal details of the
tectonic history of the
Australo-Antarctic Gulf
(at 53 Ma), the onset of
the second phase of
rif ting between Australia
and Antarctica (Colwell
et al., 2006; Close et
al., 2009), ever-subsiding
margins and deepen-
ing, to the present
ocean/continent configu-
ration. Tectonic and cli-
matic change turned the
initially shallow, broad
subtropical Antarctic
Wilkes Land offshore
shelf into a deeply
subsided basin with
a narrow ice-infested
margin (Fig. 6).
Siegert, 2009). However, reports of beach gravel deposited
20 m above sea level in Bermuda and the Bahamas from
420 ka to 360 ka indicate the collapse of not only the WAIS
(6 m of sea-level equivalent, SLE) and Greenland ice sheet
(6 m of SLE), but possibly also 8 m of SLE f rom East Antarct ic
ice sources (Hearty et al., 1999). Therefore, during episodes
of global warmth, with likely elevated atmospheric CO2 con-
ditions, the EA IS may contribute just as much or more to
rising global sea level as the Greenland ice sheet. In the face
of rising CO2 levels (Pachauri, R.K., and Reisinger, A., 2007),
a better understanding of the EAIS dynamics is therefore
urgently needed from both an academic as well as a societal
point of view.
A key region for analysis of the long- and short-term beha-
vior of the EAIS is the eastern sector of the Wilkes Land mar-
gin, located at the seaward termination of the largest East
Antarctic subglacial basin, the Wilkes subglacial basin. The
base of the portion of the EA IS draining through the W ilkes
subglacial basin is largely below sea level, suggesting that
this portion of the EA IS can potentially be less stable than
other areas of the EAIS (Escutia et al., 2005). Numerical
models of ice-sheet behavior (Huybrechts, 1993; DeConto
and Pollard, 2003a, 2003b; DeConto et al., 2007; Pollard and
DeConto, 2009) provide a basic understanding of the cli-
matic sensitivity of particular Antarctic regions for early
ice-sheet formation, connection and expansion, and eventual
development of the entire ice sheet. For example, in these
models glaciation is shown to have begun in the East
Antarctic interior, discharging mainly through the Lambert
Graben to Prydz Bay. These models imply that the EAIS did
not reach the W ilkes Land margin until a later stage. These
models can only be validated through drilling and obtaining
direct evidence from the sedimentary record.
Scientic Objectives
The overall objectives of Expedition 318 were to date the
identified seismic units and to obtain long-term records of
Antarctic glaciation to better understand its relationships
with global paleoclimate and paleoceanographic changes.
Of particular interest is testing the sensitivity of the EAIS
to episodes of global warming and detailed analysis of criti-
cal periods in Earth’s climate history, such as the
Eocene Oligocene and Oligocene–Miocene glaciations, late
Miocene, Pliocene, and the last deglaciation. During these
time s, the Ant arctic c ryosphere evol ved in a step -wise fash ion
to ultimately assume its present- day configuration, charac-
terized by a relatively stable EAIS. Conceivably even more
important than the histor y of the A ntarctic glaciations are
past lessons of deglaciations and periods of exceptional
warmth. We therefore planned to core several sequences
from the Pleistocene and Pliocene that formed during inter-
glacial intervals of exceptional warmth, periods that may pro-
vide valuable information about A ntarctica’s response to
warming predicted in the centuries ahead. Furthermore,
seismic reflection and shallow coring data indicate that the
Figure 5.Example of a core section
from Holocene diatomaceous ooze,
Site U1357. Note the distinct seasonal
laminations.
cm
125
115
120
110
Scientific Drilling, No. 12, September 2011 19
activity related to the commencement of rapid sea floor
spreading in the Australia-Antarctic Basin (A A B), reported
to initiate around the same time (~50 Ma; Colwell et al.,
2006). Also, combined Site U1356 and ODP L eg 189 dinocyst
distribution patterns suggest earliest through-flow of South
Pacific Antarctic waters through the Tasmanian Gateway to
be coeval with this tectonic phase. Sedimentological and
microfossil information from this interval from Hole U1356A
also suggest some deepening during the early middle
Eocene.
At Site U1356, the upper middle Eocene to the basal
Oligocene is conspicuously missing in a ~19-m.y. hiatus at
~890 bsf (~47.9–33.6 Ma) marking unconformity W L-U3
based on dinocyst and paleomagnetic evidence. Despite
ongoing tectonic reorganizations, it appears likely that the
erosive nature of unconformity WL -U3 is notably related to
the early stages of EA IS formation. T he impact of ice-sheet
growth, including crustal and sea-level response, and major
erosion by the ice sheets, is proposed as the principal mecha-
nism that for med u ncon formi t y W L-U 3. T his is suppor ted by
the abrupt increase in benthic foraminiferal į18O values and
coeval sea-level change globally recorded
in complete marine successions (Oligocene
isotope event Oi-1; Miller et al., 1985; Coxall
et al., 2005). Progressive subsidence—the
large accommodation space created by ero-
sion in the margin (300–6 00 m of missing
strata; Eittreim et al., 1995)—and partial
eustat ic recovery allowed sediments of
early Oligocene age to accumulate above
unconformity WL-U3.
Microfossils, sedimentology, and geo-
chemistry of the Oligocene sediments
from Site U1356, at present occupying a
distal setting (i.e., lowermost rise-abyssal
plain) and immediately above unconfor-
mity WL-U3, unequivocally reflect ice-
house environments with evidence of ice-
berg activity (dropstones) and at least
seasonal sea-ice cover. The sediments,
dominated by hemipelagic sedimentation
with bottom current and gravity flow
influence, as well as biota, indicate deeper
water settings relative to the underlying
middle Eocene environments. These find-
ings imply significant crustal stretching,
subsidence of the margin, and deepening of
the Tasman R ise and the Adélie Rift Block
(ARB) between 47.9 Ma and 33.6 Ma
(Fig. 6).
Record of EAIS Variability
Drilling at continental rise Site U1356
also recovered a thick section of Oligocene
“Pre-Glacial” Regional Unconformity WL-U3
and the Timing, Nature, and Consequences
of the First Major Phase of EAIS Growth
Prior to the expedition, the prominent unconformity
WL -U3 had be en interpret ed to separat e pre-gla cial (Eo cene)
strat a from (Oligocene) glacial-influenced deposits (Escutia
et al., 1997, 2005). Drilling and dating of WL -U3 at continen-
tal rise Site U1356 and shelf Site U1360 (Fig. 3) confirmed
that this surface represents major erosion related to the
onset of glaciation at ~34 Ma (early Oligocene), with immedi-
ately overly ing deposits dated as 33.6 Ma (Fig. 7). Below
unconformit y WL-U3 at Site U1356, we recovered a record
that is late early to early middle Eocene in age and that inclu-
des peak greenhouse conditions and likely some of the early
Eocene hyperthermals (Fig. 8). We infer subtropical shal-
low-water depositional environments for this section based
on dinocysts, pollen and spores, and the chemical index of
alteration, among other indicators. A hiatus spanning ~2 m.y.
separates the lower Eocene from the middle Eocene record
at Site U1356 according to dinocyst and magnetostrati-
graphic evidence. This hiatus may be related to tectonic
Figure 6. Conceptual illustration of tectonic, geological, sedimentological, and climatic
evolution of the Wilkes L and margin since the middle early Eocene (~54 Ma). U3, U4, and U5
refer to seismic unconformities WL-U3, WL-U4, and WL-U5, respectively. Oi-1 = Oligocene
isotope Event 1, CPDW = Circumpolar Deep Water, ACSC = Antarctic Circumpolar Surface
Water, UCPDW = Upper Circumpolar Deep Water, LCPDW = Lower Circumpolar Deep
Water, AABW = Antarctic Bottom Water. MTD = mass transport debris flows.
500 km
Antarctic Counter Current
Proto-Leeuwin Current
Wilkes Shelf
5348 Ma
0
200
400
Wilkes Shelf
150 Ma
0
800
1600
Rifting ~50 Ma
X
0
250
400
Icebergs and
seasonal sea ice
Sea level
Sea level
Wilkes Shelf
33 Ma
(Oi-1)
onset of glaciation
Wilkes Shelf
3015 Ma
0
1
2
U3
U3
U4
U3
U4
U5
U3
U3
U5
ACSC
X
X
X
X
CPDW
Sea level
Sea level
X
AABW
UCPDW
X
X
Current coming out of page
Current going into page
Wilkes Land
U3
Deepwater production
Wilkes
Contourites and turbidites
Contourites and turbidites
MTDs, contourites, and turbidites
Neritic/Upper slope deposits
Basement
Wilkes Land
Wilkes Land
Wilkes Land
LCPDW
ACSC
CPDW
Depth (m) Depth (m)
Depth (m)
Depth (km)
20 Scientific Drilling, No. 12, September 2011
6FLHQFH5HSRUWV
Ocean open cold-water taxa, with variable abundances of
sea-ice -associated diatoms were recovered, indicating a
high-nutrient, high-productivity sea-ice-influenced set ting
throughout the Neogene. Combined sedimentological and
microfossil information indicates the ever-increasing
influence of typical Antarctic Counter Current surface
waters and intensifying AABW flow. Furthermore, the
preser vation of calcareous microfossils in several intervals
indicates times when bottom waters were favorable to the
preservation of calcium carbonate. These observations point
to a very dynamic ice -sheet/sea-ice regime during the late
Miocene through the Pleistocene. Detailed postcruise
studies in sediments from the late Neogene will provide a
history of glacial-interglacial climate and paleoceano-
graphic variability, including a history of AABW produc-
tion that can be linked to sea-ice variations in this margin.
Ultrahigh Resolution Holocene Record of
Climate Variability
Coring at Site U1357 yielded a 186-m section of contin-
uously laminated diatom ooze as well as a portion of the
underlying Last Glacial Maximum diamict. Based on much
shorter piston cores recovered from adjacent basins and
banks, the onset of marine sedimentation during the degla-
cial interval began bet ween 10,400 y and 11,000 y ago. T he
site was triple cored, providing overlapping sequences that
will aid in the construction of a composite stratigraphy span-
ning at least the last 10,000 y. The Site U1357 sediments are
unusual for Antarctic shelf deposits because of their high
accumulation rate (2 cm y
-1), lack of bioturbation, and excel-
lent preservation of organic matter as well as calcareous,
opaline, phosphatic, and organic fossils. The sediments are
profoundly anoxic, with levels of H2S as high as 42,000 ppm
at 20 mbsf. Larger burrowing organisms are completely
to upper Miocene sediments (Figs. 4, 8) indicative of a rela-
tively deep-water, sea-ice-in fluenced setting. Oligocene to
upper Miocene sediments are indicative of episodically redu-
ced oxygen conditions either at the seafloor or within the
upper sediments prior to ~17 Ma. From t he late early Miocene
(~17 Ma) onward, progressive deepening and possible inten-
sification of deep-water flow and circulation led to a transi-
tion from a poorly oxygenated low-silica system (present
from the early to early middle Eocene to late early Miocene)
to a well-ventilated si lica-enriched system a kin to the modern
Southern Ocean. This change coincides with one of the
major regional unconformities in the Wilkes Land margin,
unconformity WL-U5, which represents a ~3 m.y. latest
Oligocene–early Miocene hiatus (Figs. 4, 8). This unconfor-
mity marks a change in the dominant sedimentary proces -
ses at this site, which are dominated by mass transport pro-
cesses below the unconformity and by hemipelagic, turbidity
flow, and bottom-current deposition above.
A complete record with good recovery of late Miocene
to Pleistocene deposits was achieved at continental rise
Sites U1359 and U1361 (Figs. 4, 9), drilled on levee deposits
bounding turbidity channels. We successfully dated the
seismic units between unconformities WL-U6 and W L -U8,
and the sedimentological, wireline logging, and magnetic
susceptibility data exhibit relatively high amplitude varia-
tions, indicating strong potential for this record to reveal
EAIS dynamics down to orbital timescales (100 k.y. and
40 k.y. cyclicity) (Fig. 10). This cyclicity likely documents
the successive advances and retreats of the ice- sheet and
sea-ice cover, as well as the var ying intensity of cold saline
density flows related to bottom water production at the
Wilkes Land margin (e.g., high-salinity shelf water flowing
from the shelf into the deep ocean to form Antarctic
Bottom Water [AABW ]). In general, typical Southern
Figure 7. Multichannel seismic reflection profile across Site U1356 showing regional unconformities WL-U3, WL-U4, and WL-U5. Red rectangle =
approximate penetration achieved at Site U1356.
WL-U3
WL-U4
WL-U5
WL-U5b
5.0
5.5
6.0
6.5
Site
U1356
GA229-07
SP 7374
10 km
Levee
Levee
Channel
Channel
Two-way traveltime (s)
Shotpoint
8500.0 8400.0 8300.0 8200.0 8100.0 8000.0 7900.0 7800.0 7700.0 7600.0 7500.0 7400.0 7300.0 7200.0 7100.0 7000.0 6900.0 6800.0 6700.0 6600.0 6500.0 6400.0 6300.0
Scientific Drilling, No. 12, September 2011 21
Miocene
Pliocene
elddimetalearly
Pleist.
late
25 m m.y.-1
69 m m.y.-1
37 m m.y.-1
Age
(Ma)
Lithology
Hole U1361A
Depth (mbsf)
1H
2H
3H
4H
5H
6H
7H
8H
9H
10H
11H
12H
13H
14H
15H
16H
17X
18X
19X
20X
21X
22X
23X
24X
25X
26X
27X
28X
29X
30X
31X
32X
33X
34X
35X
36X
37X
38X
39X
40X
41X
0
50
100
150
200
250
300
350
400
024 6 8101214
Lith. unit
I
IIA
IIB
Total depth = 386.31 mbsf
WL-U8
WL-U7
Seismic
reector
WL-U6
Core
recovery
Diatom (t)
Diatom (b)
Radiolarian (t)
Rad iola rian (b)
Magnetostratigraphy
Foraminifer (o)
Foraminifer (y)
Clay/Silt enotsemiL ezoo lissofonnaN
Lithology
Diatom ooze
Change in sedimentation
rate
Figure 9. Age model for Hole U1361A.
0
200
400
600
800
1000
020 30 40 60
Hiatus
(9.54.74 Ma)
Hiatus/
condensed interval
(23.1217.5 Ma)
47R-CC
46R-CC
1R-2,140 cm
Hiatus
(47.933.6 Ma)
95R-1
94R-CC
Hiatus
(51.90 Ma to 51.06 Ma)
101R-1,100
100R-1,100
~37 m m.y.-1
~67 m m.y.-1
~30 m m.y.-1
~24 m m.y.-1
Miocene Oligocene Eocene
lmelme
Plio.-
Pleist. Paleoc.
le
53.8 Ma
~100 m m.y.-1
Hole U1356A
Age
(Ma)
Diatom (t)
Diatom (b)
Radiolarian (t)
Radiolarian (b)
Palynomorph (t)
Palynomorph (b)
Magnetostratigraphy
Foraminifera (o)
Foraminifera (y)
Nannofossil (o)
Nannofossil (y)
0301
Palynomorph (y)
50
Depth (mbsf)
Core
recovery
1R
2R
3R
4R
5R
6R
7R
8R
9R
10R
11R
12R
13R
14R
15R
16R
17R
18R
19R
20R
21R
22R
23R
24R
25R
26R
27R
28R
29R
30R
31R
32R
33R
34R
35R
36R
37R
38R
39R
40R
41R
42R
43R
44R
45R
46R
47R
48R
49R
50R
51R
52R
53R
54R
55R
56R
57R
58R
59R
60R
61R
62R
63R
64R
65R
66R
67R
68R
69R
70R
71R
72R
73R
74R
75R
76R
77R
78R
79R
80R
81R
82R
83R
84R
85R
86R
87R
88R
89R
90R
91R
92R
93R
94R
95R
96R
97R
98R
99R
100R
101R
102R
103R
104R
105R
106R
Total depth = 1000.08 mbsf
I
VI
V
IV
III
II
VII
VIII
IX
X
WL-U5
WL-U4
WL-U3
Lith. unit
Seismic
reector
XI
Lithology
Clay/Silt Sand
Nannofossil ooze
Sandy mud Diamict
Limestone
Lithology
Figure 8. Age model for Hole U1356A.
22 Scientific Drilling, No. 12, September 2011
6FLHQFH5HSRUWV
The IODP Expedition 318 Scientists
C. Esc utia (Chief Sci entist), H. Brink huis (Chief Scie ntist),
A. K laus (Staff Scientist), J.A.P. Bendle, P.K. Bijl, S.M.
Bohaty, S.A. Carr, R.B. Dunbar, J.J. Gonzàlez, A. Fehr, T.G.
Hayden, M. Iwai, F.J. Jimemez-Espejo, K. K atsuki, G.S.
Kong, R.M. McKay, M. Nakai, M.P. Olney, S. Passchier, S.F.
Pekar, J. Pross, C. Riesselman, U. Röhl, T. Sakai, P.K.
Shrivastava, C.E. Stickley, S. Sugisaki, L. Tauxe, S. Tuo, T.
van de Flierdt, K. Welsh, T. Williams, M. Yamane .
References
Close, D.I., Watts, A.B., a nd Stagg, H.M.J., 200 9. A marine geophysi -
cal st udy of the Wilkes Land rifted continental margin,
Antarctica. Geophys. J. Int., 177(2):430 450, doi:10.1111/j.
1365-246X.2008.04066.x.
Colwell, J.B., Stagg, H.M.J., Direen, N.G., Bernander, G., and
Borisova, I., 2006. T he struct ure of the conti nenta l margin
off W ilkes Land and Terre A delie C oast , East Antarctica . In
Futterer, D.K., Damaske, D., Kleinschmidt, G., Miller, H.,
and Tessensohn, F. (Eds.), Antarctica: Contributions to
Global Earth Sciences, Berlin (Springer-Verlag), 327–340.
excluded from this ecosystem, yet the regu-
lar occurrence of benthic foraminifers sug-
gests that some oxygen is present at the
sediment-water interface. These sediments
provide an excellent sample set for geomicro-
biology and sedimentary geochemistry st u-
dies. In fact, the upper 20 m of one of the
three holes was intensively sampled for inte-
grated pore water and microbiological stu-
dies.
A paramount achievement from a paleocli-
matic standpoint was the retrieval of this
continuously laminated deposit (Fig. 5). Spot
checks of laminae from top to bot tom
of the split Hole U1357A sections suggest
that paired light-dark laminae sets range in
thickness from ~1 cm to 3 cm. Based on
radiocarbon dating of a piston core taken ear-
lier from this site (Costa et al., 2007), our
own preliminary secular paleomagnetic find-
ings, and the thickness of the deposit combi-
ned with the expected age at its base, it is
very likely each laminae pair represents
one year. If supported by our shore-based
research, this will be the first varved sedi-
mentary sequence extending through the
Holocene recovered from the Southern
Ocean. Analysis at the annual timescale will
permit us to examine decadal to subdecadal
variabilit y in sea ice, temperature, and wind
linked to the Southern Annual Mode (SAM ),
Pacific Decadal Variability, and possibly
ENSO. We will also be able to address ques-
tions regarding rates of change during the
Hypsithermal Holocene neoglacial events and the time
immediately following the first lift-off and pull-back of ice at
the end of the last glacial interval. In addition, we now have
an excellent opportunity for ultrahigh resolution correlation
to the nearby Law Dome Ice Core, one of the most important
Holocene ice cores in A ntarctica.
Acknowledgements
We thank the captain and crew of the JOIDES Resolution,
the IODP 318 operation superintendent, ice pilot, weather-
man, all technicians, and videographer who were instrumen-
tal in the success of Expedition 318. They allowed and as-
sisted us in drilling, documenting, sampling, and onboard
sample analyses during Expedition 318. Numerous people at
IODP-TAMU as well as USIO provided their dedicated effort
supporting us, including preparation of the expedition and
publication of the proceedings. T he USIO curatorial team
provided us with their able support during the sampling
party at College Station. We also thank the co -PIs of the
Proposal 482 and APL 638 and the master of the R/ V
Astrolabe for his support.
Figure 10. Downhole geophysical logs, Hole U1361A. HSGR = total spectral gamma ray,
MAD = moisture and density, IDPH = deep induction phasor-processed resistivity, IMPH =
medium induction phasor-processed resistivity, PWS-X = x-direction caliper.
Depth (mbsf)
Depth (mbsf)
0
50
100
150
200
250
300
350
400
0
50
100
150
200
250
300
350
400
0.5 1.2(Ωm)
IMPH
0.5 1.2(Ωm)
IDPH
(m s-1)
Pass 1
(m s-1)
Lab, PWS-X
1500 2000
1500 2000
Gamma ray (HSGR)
0 100(gAPI)
Hole diameter
10 30(inch)
Bulk density (MAD)
(g cm
-3
)
1.2 2
Density
1.3 2(g cm-3)
Porosity
90 30(%) (m s-1)
Pass 2
1500 2000
Core
recovery
1H
2H
3H
4H
5H
6H
7H
8H
9H
10H
11H
12H
13H
14H
15H
16H
17X
18X
19X
20X
21X
22X
23X
24X
25X
26X
27X
28X
29X
30X
31X
32X
33X
34X
35X
36X
37X
38X
39X
40X
41X
Resistivity
Velocity
Scientific Drilling, No. 12, September 2011 23
Costa, E., Dunbar, R.B., Kr yc, K.A ., Mucciarone, D.A., Brach feld, S.,
Roark , E.B., Manley, P.L., Murray, R.W., and Leventer, A.,
2007. Solar forcing and El Niño- Southern Oscillation
(ENSO) influences on productivity cycles interpreted from
a late Holocene high -resolution marine sediment record,
Adélie Drift, East A ntarctic margin. In Cooper, A.K., and
Ray mond, C. R., a nd the ISA ES Editor ial Team . (Eds.),
Antarctica: A Keystone in a Changing World - Proceedings for
the Tenth International Symposium on Antarctic Earth
Sciences. USGS Open File Rep., 2007–1047. Washington, DC
(The National Academies Press), 1–6, doi:10.3133/of2007-
1047.sr p036 .
Coxall, H.K., W ilson, P.A., Päli ke, H., L ear, C.H., and Backman, J.,
2005. Rapid stepwise onset of A ntarctic glaciation and
deeper calcite compensat ion in the Paci fic Ocean. Nature,
433(7021):53–57, doi:10.1038/nature03135.
DeConto, R.M., and Pollard, D., 2003a. A coupled cl imate–ice sheet
modeli ng approach to the early Cenozoic history of t he
Ant arctic ice sheet. Palaeogeogr., Palaeoclimatol.,
Palaeoecol., 198(1–2):39– 52, doi:10.1016/S0031- 0182(03)
00393-6.
DeConto, R.M., and Pollard, D., 200 3b. Rapid Cenozoic glaciation of
Antarctica induced by declining atmospheric CO2. Nature,
421(6920):245–249, doi:10.1038/nature01290.
DeConto, R., Pollard, D., and Ha rwood, D., 20 07. Sea ice feedback and
Cenozoic evolution of Ant arctic climate and ice sheets.
Palaeoceanography, 22(3):PA3214, doi:10.1029/200 6PA
001350.
De Sant is, L ., Brancolini, G., and Donda, F., 2003. Seismo -
stratigraphic a nalysis of the W ilkes Land Continental
Margin (East Antarctica): influence of glacially driven proc-
esses on the Cenozoic deposition. Deep Sea Res. Par t II,
50(8 –9):1563–1594 , doi:10.1016/S0967- 06 45(03)0 00 79 -1.
Eitt reim, S.L ., Cooper, A.K., a nd Wannesson, J., 1995. Seismic strati-
graphic evidence of ice-sheet advances on the Wilkes Land
margin of Antarctica. Sediment. Geol., 96(1–2):131–156,
doi:10.1016/0037-0738(94)00130-M.
Escutia, C., De Santis, L., Donda, F., Dunbar, R.B., Cooper, A.K .,
Brancolini, G., and Eittreim, S.L., 2 005. Cenozoic ice sheet
history from East Antarctic Wilkes Land continental mar-
gin sediments. Global Planet. Change, 45(1– 3):51–81,
doi:10.1016/j.gloplacha.2004.09.010.
Escutia, C., Eit treim, S. L., and Cooper, A.K., 1997. Cenozoic sed imen-
tation on the Wilkes L and continental r ise, A ntarctica. In
Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution
and Processes. Proc. Int. Symp. Antarct . Earth Sci.,
7:791–795.
Florindo, F., and Siegert, M., 2 00 9. Antarctic Climate Evolution.
Developments in Earth and Environmental Sciences, Vol. 8:
Amsterdam, The Netherla nds (Elsevier).
Gradstein, F.M., Ogg, J.G., a nd Smith, A ., 2004. A Geologic Time Scale
2004: Cambridge (Cambridge University Press).
Hear ty, P.J., Kindler, P., Cheng, H., and Edwards , R.L., 1999. A +20 m
middle Pleistocene sea-level highstand (Bermuda and
the Bahamas) due to partial collapse of A ntarct ic ice.
Geology, 27(4):375–378, doi:10.1130/0091-7613(1999)027
<0375:MMPSL>2.3.CO;2.
Huber, M., Brinkhuis, H., Stickley, C.E ., Döös, K., Sluijs, A., Warnaa r,
J., Schellenberg, S.A., a nd Willia ms, G.L., 200 4. Eocene cir-
culation of the Southern Ocean: was Antarctica kept warm
by subtropical waters? Paleoceanography, 19(4):PA4026,
doi:10.1029/2004PA001014.
Huybrechts, P., 1993. Glaciological modelling of the Late Cenozoic
East A nta rctic Ice Sheet: st abil ity or dynamism? Geograf.
Ann., 75(4):221–238m, doi:10.2307/521202.
Intergovernmental Panel on Climate Change (I PCC), The AR4
Synthesis Report 2007: http://www.ipcc.ch/
Kennett, J. P., 1977. Cenozoic evolution of Antarct ic glaciation, the
circum-Antarctic Ocean, and their impact on global pale-
oceanography. J. Geophys. Res., 82(27):3843– 3860,
doi:10.1029/JC082i027p038 43.
Miller, K.G., Aubry, M.-P., Kahn, M.J., Mel illo, A.J., Kent, D.V., and
Bergg ren, W.A ., 1985. Oligocene –Miocene biostrat igraphy,
magnetostratigraphy, and isotopic stratigraphy of the west-
ern Nor th At lant ic. Geology, 13(4):257–261, doi:10.1130/009
17613(1985)13<257:OBM AIS>2.0.CO;2.
Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B., and Bohaty, S.,
2005. Ma rked decline in atmospheric carbon dioxide con-
centrat ions during the Paleogene. Science, 309(573 4):60 0
603, doi:10.1126/science.1110063.
Pollard, D., and De Conto, R .M., 2009. Modelling West Antarct ic ice
sheet growth and collapse t hrough the past five million
years. Nature, 458(7236):329–332, doi:10.1038/
nature07809.
Wise, S.W., Jr., Breza, J.R ., Ha rwood, D.M., and Wei, W., 1991.
Paleogene glacial histor y of Antarctica. In Müller, D.W.,
McKenzie, J. A., and Weissert, H. (E ds.), Controversies in
Modern Geology: Evolution of Geological Theories in
Sedimentology, Earth History and Tectonics: Cambridge
(Cambridge University Press), 133–171.
Zachos, J.C., Dickens, G.R., a nd Zeebe, R.E., 2 008. An early Cenozoic
perspect ive on greenhouse war ming and ca rbon-cycle
dynamics. Nature, 451(7176):279–283, doi:10.1038/
nature06588.
Zachos , J.C., Flower, B.P., and Paul, H., 19 97. Orbitally paced climate
oscillations across the Oligocene/Miocene boundar y.
Nature, 388(6642):567–570, doi:10.1038/41528.
Authors
Carlota Escutia, Instituto Andaluz de Ciencias de la Tierra,
CSIC-Universidad de Granada, Campus de Fuentenueva s/n,
18002 Granada, Spain, e-mail: cescutia@ugr.es.
Henk Brinkhuis, Biomarine Sciences, Institute of
Environmental Biology, Laboratory of Palaeobotany and
Palynology, Utrecht University, Budapestlaan 4, 358 4 CD
Utrecht, The Netherlands, e-mail: h.brinkhuis@uu.nl.
Adam Klaus, Staff Scientist/Expedition Project Manager,
United States Implementing Organization, Integrated Ocean
Drilling Program, Texas A&M Universit y, 1000 Discovery
Drive, College Station, TX 77845-9547, U.S.A ., e-mail:
aklaus@iodp.tamu.edu.
and the IODP Expedition 318 Scientists
Related Web Link
http://publications.iodp.org/preliminary_report/318/
http://www.stratigraphy.org/
... At Site U1360, however, earliest Oligocene glacimarine sediments lie at around 90 m above unconformity WL-U3. This suggests that progressive tectonic subsidence, the large accommodation space created by erosion in the margin (300À600 m of missing strata on the shelf; Eittreim et al., 1995), and partial eustatic recovery (Stocchi et al., 2013) allowed Eocene sediments, in addition to those from the early Oligocene, to accumulate above unconformity WL-U3 on the continental shelf while a hiatus formed at the distal U1356 (Escutia and Brinkhuis, 2014). The continuous presence of reworked middle-late Eocene dinocyst species in Oligocene sediments from Site U1356 supports unabated submarine erosion of the Antarctic shelf late Eocene strata . ...
... These include the CRP-3 (Florindo et al., 2005;Galeotti et al., 2012;Passchier et al., 2013) and, possibly, CIROS-1 (Roberts et al., 2003) drillholes in the Ross Sea, ODP Site 696 in the Weddell Sea (Carter et al., 2017;Ehrmann and Mackensen, 1992) and ODP Site 739 in the Prydz Bay (O'Brien et al., 2001;Passchier et al., 2013Passchier et al., , 2017. In addition, sediments recovered from the eastern Wilkes Land margin at Site U1360 attest to ice sheet advance to the coast/continental shelf by the earliest Oligocene (33.6 Ma; Escutia and Brinkhuis, 2014;Escutia et al., 2011). All these records provide evidence for a major glacial expansion across the EOT. ...
... Location map and chronostratigraphic extent of drill sites in Wilkes Land, from Integrated Ocean Drilling Program (IODP) Expedition 318. Adapted fromEscutia and Brinkhuis (2014). ...
Chapter
Antarctica underwent a complex evolution over the course of the Cenozoic, which influenced the history of the Earth’s climate system. The Eocene-Oligocene boundary is a divide of this history when the ice-free ‘greenhouse world’ transitioned to the ‘icehouse’ with the glaciation of Antarctica. Prior to this, Antarctica experienced warm climates, peaking during Early Eocene when tropical-like conditions existed at the margins of the continent where geological evidence is present. Climate signals in the geological record show that the climate then cooled, but not enough to allow the existence of significant ice until the latest Eocene. Glacial deposits from several areas around the continental margin indicate that ice was present by the earliest Oligocene. This matches the major oxygen isotope positive shift captured by marine records. On land, vegetation was able to persist, but the thermophylic plants of the Eocene were replaced by shrubby vegetation with the southern beech Nothofagus, mosses and ferns, which survived in tundra-like conditions. Coupled climate–ice sheet modelling indicates that changing levels of atmospheric CO2 controlled Antarctica’s climate and the onset of glaciation. Factors such as mountain uplift, vegetation changes, ocean gateway opening and orbital forcing all played a part in cooling the polar climate, but only when CO2 levels reached critical thresholds was Antarctica tipped into an icy glacial world.
... The Aurora Subglacial Basin (ASB) (Fig. 1) contains 3.9 m of sea level equivalent ice that is drained by the Totten Glacier and its tributaries that terminate at the Sabrina Coast, East Antarctica (Young et al., 2011;Wright et al., 2012;Greenbaum et al., 2015, Morlighem et al., 2020. The Sabrina Coast (115°to 121°E, 67°S) is located on the Wilkes Land continental margin (Young et al., 2011), which formed as Antarctica rifted from Australia in the mid-Cretaceous (Cande and Mutter, 1982;Escutia et al., 2011). As a major drainage outlet of the ASB, sediments deposited on the Sabrina Coast shelf likely contain historical records of glacial evolution in the ASB (Gulick et al., 2017;Montelli et al., 2020). ...
Article
The Aurora Subglacial Basin (ASB) catchment contains 3-5 m of sea-level equivalent ice volume that drains to the Sabrina Coast, East Antarctica via the Totten Glacier system. Observed thinning and retreat of Totten Glacier indicate regional sensitivity to oceanographic and atmospheric warming. Paleoclimate studies of climatically sensitive catchments are required to understand the evolution of the East Antarctic Ice Sheet (EAIS) and its outlet glacier systems. Recent seismic and sediment studies from the Sabrina Coast document the evolution of the EAIS in the ASB catchment, suggesting that the region has long been sensitive to climatic changes. This study presents new palynological and biomarker data from Sabrina Coast continental shelf sediments. Detailed palynological records were obtained from four short jumbo piston cores (JPC; NBP14-02 JPC-30, -31, -54 and -55), enabling reconstructions of regional vegetation and environments prior to and during Cenozoic EAIS development. The Sabrina Flora is dominated by angiosperms, with Gambierina spp. often exceeding 40% of the assemblage, and diverse Proteaceae, Battenipollis spp., Forcipites spp., Nothofagidites spp., fern, and conifer palynomorphs indicative of an open shrubby ecosystem. Excellent preservation and frequent occurrence of Gambierina spp. clusters suggest that a majority of the Sabrina Flora assemblage is penecontemporaneous with sedimentation; however, some uncertainties remain whether this sedimentation occurred in the Late Cretaceous or the Paleogene. Despite that uncertainty, high abundances of Gambierina spp. and Battenipollis spp., in combination with relatively low (<10%) Nothofagidites spp. abundances indicate that the Sabrina Flora is unique in Antarctica. Evaluation of biomarkers finds evidence for penecontemporaneous and reworked components. The penecontemporaneous C30 n-alkanoic acids have ẟ¹³C values of -30.2 ± 0.5‰, consistent with ẟ¹³C values in an open canopy woodland or shrubby open vegetation. Their hydrogen isotope (ẟD) values of -215 ± 4.5‰, indicate precipitation isotopic composition (ẟDprecip) of -130‰, similar to coastal snow in the same region today. Together, Sabrina Flora palynomorph and plant wax data suggest a drier, more open coastal vegetation in the Aurora Basin of East Antarctica rather than the closed rainforest vegetation often described from other parts of Antarctica for the Cretaceous to Paleogene. To directly compare records from the circum-Antarctic, additional long sedimentary records with improved biostratigraphic constraints are required. Such records will enable identification of regional climate gradients or micro-climates, and allow assessment of the environmental conditions and mechanisms driving observed differences.
Article
Full-text available
Terrestrial organic carbon (TerrOC) acts as an important CO2 sink when transported via rivers to the ocean and sequestered in coastal marine sediments. This mechanism might help to modulate atmospheric CO2 levels over short- and long timescales (103 to 106 years), but its importance during past warm climates remains unknown. Here we use terrestrial biomarkers preserved in coastal marine sediment samples from Wilkes Land, East Antarctica (~67°S) to quantify TerrOC burial during the early Eocene (~54.4 to 51.5 Ma). Terrestrial biomarker distributions indicate the delivery of plant-, soil- and peat-derived organic carbon (OC) into the marine realm. Mass accumulation rates of plant- (long-chain n-alkane) and soil-derived (hopane) biomarkers dramatically increase between the earliest Eocene (~54 Ma) and the early Eocene Climatic Optimum (EECO; ~53 Ma). This coincides with increased OC mass accumulation rates and indicates enhanced TerrOC burial during the EECO. Leaf wax δ 2H values indicate that the EECO was characterised by wetter conditions relative to the earliest Eocene, suggesting that hydroclimate exerts a first-order control on TerrOC export. Our results indicate that TerrOC burial in coastal marine sediments UOB Open could have acted as an important negative feedback mechanism during the early Eocene, but also during other warm climate intervals.
Article
Knowledge regarding the response of the East Antarctic Ice Sheet to glacial–interglacial climatic cycles in the late Pleistocene is critical to understanding the global climate system and projections of future sea level rise. Here, we observed notable glacial–interglacial cyclicity in magnetic properties, bulk detrital Sr–Nd isotopes, and Fe/Ti ratios over the previous 530 kyr in three well-dated gravity cores from the continental rise offshore of Prydz Bay (East Antarctica). Our results show that Antarctic continental sources with more Ti-rich magnetite, less radiogenic epsilon neodymium (εNd), and higher Fe/Ti ratios were predominant during glacials in comparison with interglacials. Specifically, the εNd amplitude through MIS 11–5 differs from that in the remainder of the records, which is also expressed in the magnetic coercivity cycles with subdued patterns. Following source identification on the basis of the detrital Sr–Nd distribution, we recognize two main (rock type) sources and infer two types of ice drainage flow pattern (“flank” and “channelized”), which follow different pathways in the Lambert Glacier–Amery Ice Shelf system (LG-AISS). The first follows an eastern path connecting the Ingrid Christensen Coast (flank), while the second follows a central channel via the LG-AISS (channelized) during MIS 11–5. Regular dynamics on glacial–interglacial timescales, manifested by changes in magnetic coercivity, are closely related to the modeled Antarctic ice volume and ice sheet movement, in which the second channelized pathway during MIS 11–5 corresponds to a 340-kyr-long episode with contiguous warmer-than-present Antarctic interglacials (MIS 11, 9, 7, and 5). Our records thus provide the clearest evidence so far of variable patterns of ice sheet dynamics during the late Pleistocene in the Prydz Bay sector of East Antarctica, which coincided with similar variation of ice drainage during the late Miocene–early Pliocene at around 1.13 Ma (ODP188). Similar ice drainage changes in these two periods imply that major ice flow reconfiguration can be triggered repeatedly by abrupt changes from a stable warm period to a cold one. The presented data not only reveal glacial–interglacial cyclicity in ice sheet advance and retreat in the meridional direction, but also implicate latitudinal adjustment (lateral) within a thin elongated drainage basin of the LG-AISS.
Chapter
The Miocene to Pliocene (Neogene) occurred between 23.04 and 2.58 million years ago and includes intervals of peak global warmth where Earth’s average surface temperature was up to 8℃ warmer than present. Major cooling steps also occurred, across which Antarctica’s ice sheets advanced to the continental shelf for the first time and sea ice expanded across the Southern Ocean. Knowledge of Antarctic environmental change and ice sheet variability through this dynamic period in Earth history has advanced over the past 15 years. Major field and ship-based efforts to obtain new geological information have been completed and significant advances in numerical modelling approaches have occurred. Integration of ice proximal data and coupled climate-ice sheet model outputs with high-resolution reconstructions of ice volume and temperature variability from deep sea δ¹⁸O records now offer detailed insight into thresholds and tipping points in Earth’s climate system. Here we review paleoenvironmental data through key episodes in the evolution of Neogene climate to include the Miocene Climatic Optimum (MCO), Middle Miocene Climate Transition (MMCT), Tortonian Thermal Maximum (TTM), Late Miocene Cooling (LMC), and Pliocene Warm Period (PWP). This review shows that Antarctica’s climate and ice sheets remained dynamic throughout the Neogene. Given the analogous nature of warm episodes in the Miocene and Pliocene to future projections, the environmental reconstructions presented in this chapter offer a stark warning about the potential future of the AIS if warming continues at its current rate. If average global surface warming above pre-industrial values exceeds 2℃, a threshold will be crossed and AIS instabilities would likely be irreversible on multi-century timescales.
Chapter
The past three decades have seen a sustained and coordinated effort to refine the seismic stratigraphic framework of the Antarctic margin that has underpinned the development of numerous geological drilling expeditions from the continental shelf and beyond. Integration of these offshore drilling datasets covering the Cenozoic era with Antarctic inland datasets, provides important constraints that allow us to understand the role of Antarctic tectonics, the Southern Ocean biosphere, and Cenozoic ice sheet dynamics and ice sheet–ocean interactions on global climate as a whole. These constraints are critical for improving the accuracy and precision of future projections of Antarctic ice sheet behaviour and changes in Southern Ocean circulation. Many of the recent advances in this field can be attributed to the community-driven approach of the Scientific Committee on Antarctic Research (SCAR) Past Antarctic Ice Sheet Dynamics (PAIS) research programme and its two key subcommittees: Paleoclimate Records from the Antarctic Margin and Southern Ocean (PRAMSO) and Palaeotopographic-Palaeobathymetric Reconstructions. Since 2012, these two PAIS subcommittees provided the forum to initiate, promote, coordinate and study scientific research drilling around the Antarctic margin and the Southern Ocean. Here we review the seismic stratigraphic margin architecture, climatic and glacial history of the Antarctic continent following the break-up of Gondwanaland in the Cretaceous, with a focus on records obtained since the implementation of PRAMSO. We also provide a forward-looking approach for future drilling proposals in frontier locations critically relevant for assessing future Antarctic ice sheet, climatic and oceanic change.
Article
Full-text available
Antarctic continental ice masses fluctuated considerably during the Oligocene “coolhouse”, at elevated atmospheric CO2 concentrations of ∼600–800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7–24.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 ∘C ± 5.2 ∘C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5–24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states.
Article
Antarctic cryosphere has significant impact on the global climate system by influencing the ocean currents, the atmosphere, and the sea level for long term durations. Antarctic Ice Sheet (AIS) has evolved from temporary to permanent ice sheet during Oligocene (∼32 Ma). Throughout its evolution, it witnessed severe climatic conditions leading to several phases of retreat and advancements. Major climatic events were directly associated with the evolution of AIS. Paleocene Eocene Thermal Maximum (PETM), Early Eocene Climatic Optimum (EECO, ∼44.9 Ma), Eocene Oligocene Boundary (Oi1 event, ∼34 Ma), Oligocene-Miocene Boundary (Mi1 event ∼25Ma), Mid Miocene Climatic Optimum (MMCO; ∼15Ma), Miocene-Pliocene Boundary (∼3Ma) and Mid-late Pleistocene Transition (MPT, after ∼1.25 Ma) are the major reported global climatic events. This work summarises these events and critically reviews the role of various factors in the advancement and retreat of AIS and its coupled response to the global climate change including future global challenges. Existing knowledge gaps and challenges are outlined for each of the climatic events and priorities for future research are suggested.
Article
Full-text available
The rapidly thinning Totten Glacier on the Sabrina Coast, East Antarctica, is the primary drainage outlet for ice within the Aurora Subglacial Basin, which could destabilize under the current atmospheric warming trend. There is growing need for direct geological evidence from the Sabrina Coast to frame late twentieth century Totten melting in the context of past warm climate analogs. Addressing this need, sediment archives were recovered from two sites on the Sabrina Coast slope and rise that record changes in terrigenous sedimentation and primary productivity in the region over glacial cycles since the mid‐Pleistocene transition (MPT). This research presents physical properties, grain size, diatom abundance and assemblages, and geochemical analysis from the two sites to determine how the processes that control sedimentation change between glacial and interglacial phases. The stratigraphic sequences in both cores record cyclic variations in physical properties and diatom abundances, which radiocarbon and biostratigraphic chronologies reveal as 100 Kyr glacial‐interglacial cyclicity. During glacials, terrigenous sediment deposition is enhanced by advanced grounded ice on the shelf, while primary productivity is restricted due to permanent summer sea ice extending past the continental slope. During interglacials, pelagic sedimentation suggests high surface productivity associated with contractions of regional sea ice cover. Comparison with post‐MPT slope records from Wilkes Land and the Amundsen Sea shows that this pattern is consistent in slope sediments around the margin. The higher‐amplitude variations in Antarctic ice volume and sea ice extent post‐MPT ensure that these signals are pervasive around the Antarctic margin.
Preprint
Full-text available
Antarctic continental ice masses fluctuated considerably in size during the elevated atmospheric CO2 concentrations (~ 600–800 ppm) of the Oligocene “coolhouse”. To evaluate the role of ocean conditions to the Oligocene ice sheet variability requires understanding of past ocean conditions around the ice sheet. While warm ocean conditions have been reconstructed for the Oligocene Wilkes Land region, questions arise on the geographical extent of that warmth. Currently, we lack data on surface ocean conditions from circum-Antarctic locations, and ice-proximal to ice-distal temperature gradients are poorly documented. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from the Deep Sea Drilling Project (DSDP) Site 274, offshore the Ross Sea continental margin. This site offshore Cape Adare is ideally located to characterise the Oligocene regional surface ocean conditions, as it is situated between the colder, ice-proximal Ross Sea continental shelf, and the warm-temperate Wilkes Land Margin in the Oligocene. We improve the existing age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse dinoflagellate cyst assemblages and lipid biomarkers (TEX86) to reconstruct surface paleoceanographic conditions during the Oligocene (33.7–25.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 °C ± 5.2 °C) surface ocean conditions at Site 274 throughout the Oligocene. Increasingly similar oceanographic conditions between offshore Wilkes Land margin and Cape Adare developed towards the late Oligocene (26.5–25.4 Ma), likely in consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. To maintain marine terminations of terrestrial ice sheets in a proto-Ross Sea with as warm offshore SST as our data suggests, requires a strong ice flux fed by intensive precipitation during colder orbital states in the Antarctic hinterland, but with extensive surface melt of terrestrial ice during warmer orbital states.
Article
Full-text available
Marine deposits at +20 ± 3 m on the tectonically stable coastlines of Bermuda and the Bahamas support the hypothesis of a partial collapse of the Antarctic ice sheet during the middle Pleistocene. Beach sediments fill a sea cave at +22 m in Bermuda, and horizontal, fenestrae-filled beds crop out on platforms at two sites as high as +21 m in Eleuthera, Bahamas. Carbonate beach sands are bound by an early generation of isopachous fibrous cement that is characteristic of a phreatic marine environment. Amino acid racemization and TIMS (thermal-ionization mass spectrometry) dates constrain the age of the deposits to between 390 and 550 ka, while proxy evidence supports a correlation with oxygen isotope stage 11. This direct geologic evidence of a 20% decrease in polar ice during the middle Pleistocene has important implications for the stability of ice sheets during warm interglaciations.
Article
Full-text available
1] The extent and thickness of Antarctic sea ice have important climatic effects on radiation balance, energy transfer between the atmosphere and ocean, and moisture availability. This paper explores the role of sea ice and related feedbacks in the Cenozoic evolution of Antarctic climate and ice sheets, using a numerical climate model with explicit, dynamical representations of sea ice and continental ice sheets. In a scenario of decreasing Cenozoic greenhouse gas concentrations, our model initiates continental glaciation before any significant sea ice forms around the continent. Once variable ice sheets are established, seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet's control on regional temperature and low-level winds. Although the expansion of sea ice has significant climatic effects near the coast, it has only minimal effects in the continental interior and on the size of the ice sheet. Therefore the Cenozoic appearance of Antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of Paleogene and Neogene glaciation. The influence of the East Antarctic Ice Sheet on sea ice, Southern Ocean surface temperatures, and winds has important implications for ocean circulation, the marine carbon cycle, and the development of the West Antarctic Ice Sheet. The sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long-term stability of Antarctic Ice Sheets. Citation: DeConto, R., D. Pollard, and D. Harwood (2007), Sea ice feedback and Cenozoic evolution of Antarctic climate and ice sheets, Paleoceanography, 22, PA3214, doi:10.1029/2006PA001350.
Conference Paper
In 2001 and 2002, the Australian Government acquired approximately 9000 km of high-quality geophysical data over the margin of East Antarctica between 110-142 degrees E that provide a sound framework for understanding the geology of the region. The data comprise 36-fold deep-seismic, gravity and magnetic data and non-reversed refraction/wide-angle reflection sonobuoys recorded along transects that extend from the lower continental slope out to oceanic crust at a spacing along the margin of approximately 90 km. The continental slope is underlain by a major rift basin beneath which the crust thins oceanwards through extensive faulting of the rift and pre-rift sedimentary section and by mainly ductile deformation of the crystalline crust. Outboard of the margin rift basin, the 90 to 180 km wide continent-ocean transition zone is interpreted to consist primarily of continental crust with magmatic components that can account for the lineated magnetic anomalies that have been interpreted in this zone. The thick sedimentary section in the COT zone is floored by dense lower crustal or mantle rocks indicating massive (>10 km) thinning of the lower and middle crust in this zone. The boundary between the margin rift basin and the COT is marked by a basement ridge which potential field modelling indicates is probably composed of altered/serpentinised peridotite. This ridge is similar in form and interpreted composition to a basement ridge located in a similar structural position at the inboard edge of the COT on the conjugate margin of the Great Australian Bight. On both margins, the ridge is probably the product of mantle up-welling and partial melting focussed at the point of maximum change/necking of crustal thickness. Integrated deep-seismic and potential field interpretations point very strongly to the boundary between unequivocal oceanic crust and largely continental crust of the continent-ocean transition as lying in very deep water, and considerably seaward of most previous interpretations (often based on inadequate seismic data or magnetic data only). We consider the continent-ocean boundary to be well-constrained from 124-131 degrees E and unequivocal from 131-140 degrees E, but open to debate in the sector from 110-124 degrees E. There is a strong degree of pre-breakup symmetry between the conjugate margins of southern Australia and East Antarctica east of about 120 degrees E. In addition to the crustal symmetry,there is also a strong correlation in seismic character between the margins, which allows us to date the major unconformities as probably of base Turonian, Maastrichtian and early Middle Eocene age.
Article
Numerical modeling studies of ancient Antarctic ice sheets have relied on empirical parameterizations based on modern climatologies for their surface mass balance forcing. An alternative approach, using a Global Climate Model (GCM) asynchronously coupled to a dynamical ice sheet model, has been developed, tested, and applied to the early glacial history of Antarctica. The coupled GCM-ice sheet model was used to test the sensitivity of the coupled atmosphere-ocean-cryosphere system to evolving Cenozoic boundary conditions, including paleogeography, atmospheric carbon dioxide, changing orbital parameters, and changes in ocean heat transport. The asynchronous coupling scheme enables long (106 year) integrations, simulating not only ice sheet inception, but subsequent ice sheet variability over orbital timescales. Our results suggest that the combination of declining Cenozoic atmospheric carbon dioxide and an orbital configuration producing cold austral summers triggered snow/ice albedo and height-mass balance feedbacks that allowed a continental-scale East Antarctic Ice Sheet (EAIS) to form in a relatively sudden transition. In the model, the CO2 threshold for glacial inception is between ~3 and 2.5 present. The simulated early Oligocene ice sheets exhibit extreme variability in response to orbital forcing. Changes in ocean heat transport, like those assumed to have occurred in response to the opening of Southern Ocean gateways (Tasmanian and Drake Passages) are shown to have a smaller effect than that expected in the transition from a "greenhouse" to "icehouse" climate, having only a minor effect on the timing of major glaciation. In our model, the opening of the Drake Passage is a potential trigger for glacial inception, but only within a narrow range of atmospheric CO2, reinforcing the importance of pCO2 as a fundamental boundary condition for Cenozoic climate change.
Article
Experiments were performed with a 3-D model of the Antarctic Ice Sheet to determine the ice sheet geometries to be expected under various kinds of climatic conditions and the physical mechanisms that may be involved. The results support the concept of a stable East Antarctic Ice Sheet with respect to a climatic warming, and point to the glaciological difficulties involved to explain an ice-free corridor over the Pensacola and Wilkes subglacial basins. The latter event is a crucial element in the "waxing and waning ice sheet hypothesis' and would require a temperature rise of between 17 and 20K above present levels. For a temperature rise of less than 5K, the model actually predicts a larger Antarctic Ice Sheet than today as a result of increased snowfall, whereas the West Antarctic Ice Sheet was found not to survive temperatures 8-10K above present values. Furthermore, basal temperature conditions in these experiments point to the difficulties involved in raising the ice-sheet base to the pressure melting point over the large areas necessary to consider the possibility of sliding instability. Based on these findings, it appears difficult to reconcile a highly variable East Antarctic Ice Sheet with the modest warmings recorded in, for instance, the deep sea records for the late Neogene. -from Author
Article
Deep-sea drilling in the Antarctic region (Deep-Sea Drilling Project legs 28, 29, 35, and 36) has provided many new data about the development of circum-Antarctic circulation and the closely related glacial evolution of Antarctica. The Antarctic continent has been in a high-latitude position since the middle to late Mesozoic. Glaciation commenced much later, in the middle Tertiary, demonstrating that near-polar position is not sufficient for glacial development. Instead, continental glaciation developed as the present-day Southern Ocean circulation system became established when obstructing land masses moved aside. During the Paleocene (t=~65 to 55 m.y. ago), Australia and Antarctica were joined. In the early Eocene (t=~55 m.y. ago), Australia began to drift northward from Antarctica, forming an ocean, although circum-Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t=55 to 38 m.y. ago) the Southern Ocean was relatively warm and the continent largely nonglaciated. Cool temperate vegetation existed in some regions. By the late Eocene (t=~39 m.y. ago) a shallow water connection had developed between the southern Indian and Pacific oceans over the South Tasman Rise. The first major climatic-glacial threshold was crossed 38 m.y. ago near the Eocene-Oligocene boundary, when substantial Antarctic sea ice began to form. This resulted in a rapid temperature drop in bottom waters of about 5°C and a major crisis in deep-sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom water activity over wide areas of the deep ocean basins, creating much sediment erosion, especially in western parts of oceans. A major (~2000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage. During the Oligocene (t=38 to 22 m.y. ago), widespread glaciation probably occurred throughout Antarctica, although no ice cap existed. By the middle to late Oligocene (t=~30 to 25 m.y. ago), deep-seated circum-Antarctic flow had developed south of the South Tasman Rise, as this had separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in southern hemisphere deep-sea sediment distribution patterns. The next principal climatic threshold was crossed during the middle Miocenc (t=14 to 11 m.y. ago) when the Antarctic ice cap formed. This occurred at about the time of closure of the Australian-Indonesian deep-sea passage. During the early Miocene, calcareous biogenic sediments began to be displaced northward by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. Since the middle Miocene the East Antarctic ice cap has remained a semipermanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t=~5 m.y. ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling, a rapid northward movement of about 300 km of the Antarctic Convergence, and a custatic sea level drop that may have been partly responsible for the isolation of the Mediterranean basin. Northern hemisphere ice sheet development began about 2.5-3 m.y. ago, representing the next major global climatic threshold, and was followed by the well-known major oscillations in northern ice sheets. In the Southern Ocean the Quaternary marks a peak in activity of oceanic circulation as reflected by widespread deep-sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice-rafted debris.
Article
The Wilkes Land margin of East Antarctica, conjugate to the southern Australian margin, is a non-volcanic rifted margin that formed during the Late Cretaceous. During 2000–01 and 2001–02, Geoscience Australia acquired ∼10 000 line km of seismic reflection and refraction, magnetic anomaly and gravity anomaly data over the margin. We have used the seismic data to estimate the sediment thickness along the margin. The data reveal a deep (>11 km) rift basin that contains over 9 km of sediments seaward of the Totten Glacier, western Wilkes Land. The limits of oceanic and continental crust that underlies the thick post-rift and the sig-nificantly thinner pre-and syn-rift sediments are equivocal. Seismic reflection data suggest a 30–100 km wide continent–ocean transition zone (COTZ) along the margin. The COTZ ex-tends over 400 km seaward of the shelf break off the eastern Wilkes Land/Terre Adélie sector of the margin. This seaward salient, referred to here as the Adélie Rift Block, is associated with anomalously shallow bathymetry, an atypical continental margin free-air gravity edge-effect anomaly, and an absence of seafloor spreading related magnetic anomalies. Off the central and western Wilkes Land margin, the COTZ extends ∼200 km from the shelf break and encompasses the magnetic anomaly previously interpreted as Chron 34y. It is clear, however, that this is not a seafloor spreading anomaly since oceanic crust was not emplaced in the Australia–Antarctic Basin until after 83 Ma. Integrated gravity and magnetic anomaly mod-elling indicates that the magnetic anomalies are likely to be caused by ridges of serpentinized mantle peridotites exhumed during rifting. Process-oriented gravity modelling indicates that the Wilkes Land margin lithosphere is characterized by a relatively high effective elastic thickness (T e) of ∼30 km, whereas preliminary models of the southern Australian margin are characterized by a lower average T e of ∼15 km. This contrast between the two margins is inter-preted to reflect changing lithospheric rigidity since breakup in the Late Cretaceous. Whereas the southern Australian margin was heavily sediment-loaded during the Late Cretaceous but largely sediment starved throughout the Tertiary, the Wilkes Land margin was less extensively sedimented during the Late Cretaceous and early Cenozoic but loaded by thick sediments from the Late Oligocene to Middle Miocene. This contrast in loading histories allows discrete estimates of T e to be constrained rather than average estimates. We interpret this to suggest that the T e of stretched lithosphere increases through time following rifting. Rifted continental margins surround almost the entirety of the basins of the Atlantic, Indian and Arctic Oceans and parts of the west-ern Pacific Ocean. Although they differ greatly in their tectonic evolution, subsidence and uplift history and deep-structure, two end-member margin types have been recognized: volcanic and non-volcanic (e.g. Eldholm et al. 1995). Volcanic margins are charac-terized by seaward-dipping reflector sequences, a relatively narrow continent–ocean transition zone (COTZ) and high-velocity (P-wave velocity > 7.2 km s −1) lower crustal bodies. Non-volcanic rifted margins are characterized by tilted fault blocks and half-grabens, a relatively broad COTZ and, on some margins, exhumed mantle rocks. The 5500 km long conjugate rifted margins of Wilkes Land, East Antarctica and the southern margin of Australia have been identified as examples of non-volcanic margins by a number of authors (e.g. Sayers et al. 2001; Colwell et al. 2006). Weissel & Hayes (1971, 1972) interpreted magnetic anomaly data from the southeast Indian Ocean to indicate that breakup between Wilke's Land, Antarctica and Australia occurred during the Eocene.
Article
Multichannel seismic reflection data were collected from the Wilkes Land continental margin and particularly focusing on the lower rise area, by the international WilkEs basin GlAcial history (WEGA) project. Existing, lower-resolution seismic data also are used to understand the relationship between the basement and the sedimentary cover and to correlate the evolution of the rise and the shelf.On the basis of the acoustic character and the internal geometry, we recognise nine different sequences (named WL-S1–WL-S9, from the deepest to the shallowest) within the sedimentary section. Sequence WL-S1 represents syn-rift sediments deposited within tilted blocks of sedimentary, crystalline and volcanic basement. Sequences WL-S2–WL-S9 represent the post-rift sediments that show quite a complex architecture: sequence WL-S2 is a well-developed wedge downlapping onto the break-up unconformity; sequences WL-S3–WL-S7 are parts of a margin prograding/aggrading sequence during which large sediment ridges were deposited on the slope and rise. In particular, sequences WL-S5–WL-S7 began with the deposition of a turbidite fan system (sequence WL-S5), that developed in a braided channel–levee complex (WL-S6–WL-S7), dominated by downslope flows. Indirect correlation of the sequences WL-S4–WL-S6 with DSDP 269 about 600 km north of the study area suggests an early late Miocene age. Sequences WL-S8 and WL-S9 represent the smoothing and filling of the pre-existing morphology, in a generally low-energy environment with reduced bottom current activity. During this time, most of the sediment is deposited on the shelf and slope, while the rise is generally starved.Differences in downslope current energy and in sediment load are interpreted to reflect variation of the terrigenous input from the shelf to the rise through time. We suggest that channel–levee systems on the rise reflect an increase in sediment supply from the continental shelf during the Early Miocene, marking the growth of wet-based, temperate ice sheet on the near continent. The youngest sequences on the rise and shelf reflect decreased sediment supply to the rise that may be related to the transition from a temperate to a polar glacial system.
Article
The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene “climatic optimum” (14–10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14–10 Ma). Sequence WL-S9 forms under a polar regime during the Pliocene–Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope-forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability.Validation of our inferences about the nature and timing of Wilkes Land glacial sequences can be achieved by deep sampling (i.e., using IODP-type techniques). The most complete record of the long-term history of glaciation in this margin can be obtained by sampling both (1) the shelf, which contains the direct (presence or no presence of ice) but low-resolution record of glaciation, and (2) the rise, which contains the distal (cold vs. warm) but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the presumed “onset” of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing links between the proximal and the distal records of glaciation to be established. Additionally, the eastern segment of the Wilkes Land margin may be more sensitive to climate change because the East Antarctic Ice Sheet (EAIS) is grounded below sea level. Therefore, the Wilkes Land margin is not only an ideal location to obtain the long-term EAIS history but also to obtain the shorter-term record of ice sheet fluctuations at times that the East Antarctic Ice Sheet is thought to have been more stable (after 15 Ma-recent).