Leg 189 Synthesis: Cretaceous–Holocene History of the Tasmanian Gateway

Abstract

During Ocean Drilling Program (ODP) Leg 189, five sites were drilled in bathyal depths on submerged continental blocks in the Tasmanian Gateway to help refine the hypothesis that its opening near the Eocene/Oligocene boundary led to formation of the Antarctic Circumpolar Current (ACC), progressive thermal isolation of Antarctica, climatic cooling, and development of an Antarctic ice sheet. A total of 4539 m of largely continuous upper Maastrichtian-Holocene marine sediments were recovered with a recovery rate of 89%. The sedimentary sequence broadly consists of shallow-marine mudstones until the late Eocene, glauconitic siltstones during that time, and pelagic carbonates thereafter. The microfossils in the mudstones and siltstones are largely palynomorphs and diatoms, and those in the carbonates are largely nannofossils and foraminifers. During the Late Cretaceous, northward movement of Australia away from Antarctica commenced, forming the Australo-Antarctic Gulf (AAG). However, a Tasmanian land bridge at 70°-65°S almost completely blocked the eastern end of the widening AAG until the late Eocene; there is no evidence of extensive current circulation across the ridge until the earliest Oligocene. Prior to the Oligocene, muddy marine siliciclastic sediments were deposited in temperate seas. During the late Eocene, the northeastern AAG was warmer and less ventilated than the gradually widening southwest sector of the Pacific Ocean, which was affected by a cool northwesterly flowing boundary current-a difference that may have existed since the Maastrichtian. In the late Eocene (∼37 Ma), the Tasmanian land bridge and its broad shelves began to subside, currents swept the still-shallow offshore areas, and condensed glauconitic siltstones were deposited. Palynological and diatom evidence suggest a general cooling. The southwestern South Tasman Rise finally separated from Antarctica at the time of the Eocene/Oligocene boundary (∼33.5 Ma), the rise subsided, and the continental margin of Tasmania collapsed. The Tasmanian Gateway opened to deep water, disrupting oceanic circulation at high southern latitudes and leading to one of the major climatic shifts of the Cenozoic. Thereafter, a marked reduction in siliciclastic supply, as well as the flow of warm currents from northern latitudes, favored deposition of carbonate. At the eastern sites, deposition of Oligocene bathyal carbonates directly followed an unconformity caused by the onset of the ACC, but change was more gradual in the west. In contrast, siliceous biogenic sediments typified the Antarctic margin, now isolated from warm water by the ACC. Steady northward movement kept the Tasmanian region north of the Polar Front throughout the Neogene, and pelagic carbonates accumulated.

Full text

Exon, N.F., Kennett, J.P., and Malone, M.J. (Eds.)
Proceedings of the Ocean Drilling Program, Scientific Results Volume 189
1. LEG 189 SYNTHESIS: CRETACEOUS–
HOLOCENE HISTORY OF THE TASMANIAN
GATEWAY1
Neville F. Exon,2 James P. Kennett,3 and Mitchell J. Malone4
ABSTRACT
During Ocean Drilling Program (ODP) Leg 189, five sites were drilled
in bathyal depths on submerged continental blocks in the Tasmanian
Gateway to help refine the hypothesis that its opening near the Eocene/
Oligocene boundary led to formation of the Antarctic Circumpolar Cur-
rent (ACC), progressive thermal isolation of Antarctica, climatic cool-
ing, and development of an Antarctic ice sheet. A total of 4539 m of
largely continuous upper Maastrichtian–Holocene marine sediments
were recovered with a recovery rate of 89%. The sedimentary sequence
broadly consists of shallow-marine mudstones until the late Eocene,
glauconitic siltstones during that time, and pelagic carbonates there-
after. The microfossils in the mudstones and siltstones are largely pa-
lynomorphs and diatoms, and those in the carbonates are largely
nannofossils and foraminifers.
During the Late Cretaceous, northward movement of Australia away
from Antarctica commenced, forming the Australo-Antarctic Gulf
(AAG). However, a Tasmanian land bridge at 70°–65°S almost com-
pletely blocked the eastern end of the widening AAG until the late
Eocene; there is no evidence of extensive current circulation across the
ridge until the earliest Oligocene. Prior to the Oligocene, muddy marine
siliciclastic sediments were deposited in temperate seas. During the late
Eocene, the northeastern AAG was warmer and less ventilated than the
gradually widening southwest sector of the Pacific Ocean, which was af-
fected by a cool northwesterly flowing boundary current—a difference
that may have existed since the Maastrichtian. In the late Eocene (~37
Ma), the Tasmanian land bridge and its broad shelves began to subside,
1Exon, N.F., Kennett, J.P., and Malone,
M.J., 2004. Leg 189 synthesis:
Cretaceous–Holocene history of the
Tasmanian Gateway. In Exon, N.F.,
Kennett, J.P., and Malone, M.J. (Eds.),
Proc. ODP, Sci. Results, 189, 1–##
[Online]. Available from World Wide
Web: <http://www-odp.tamu.edu/
publications/189_SR/VOLUME/
CHAPTERS/SYNTH/SYNTH.PDF>.
[Cited YYYY-MM-DD]
2Geoscience Australia, GPO Box 378,
Canberra ACT 2601, Australia.
Neville.Exon@ga.gov.au
3Department of Geological Sciences,
University of California, Santa
Barbara, Santa Barbara CA 93106, USA.
4Integrated Ocean Drilling Program,
Texas A&M University, 1000
Discovery Drive, College Station TX
77845-9547, USA.
Initial receipt: 20 January 2004
Acceptance: 15 July 2004
Web publication: 8 September 2004
Ms 189SR-101

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 2
currents swept the still-shallow offshore areas, and condensed glauco-
nitic siltstones were deposited. Palynological and diatom evidence sug-
gest a general cooling. The southwestern South Tasman Rise finally
separated from Antarctica at the time of the Eocene/Oligocene bound-
ary (~33.5 Ma), the rise subsided, and the continental margin of Tasma-
nia collapsed. The Tasmanian Gateway opened to deep water,
disrupting oceanic circulation at high southern latitudes and leading to
one of the major climatic shifts of the Cenozoic. Thereafter, a marked
reduction in siliciclastic supply, as well as the flow of warm currents
from northern latitudes, favored deposition of carbonate. At the eastern
sites, deposition of Oligocene bathyal carbonates directly followed an
unconformity caused by the onset of the ACC, but change was more
gradual in the west. In contrast, siliceous biogenic sediments typified
the Antarctic margin, now isolated from warm water by the ACC.
Steady northward movement kept the Tasmanian region north of the
Polar Front throughout the Neogene, and pelagic carbonates accumu-
lated.
INTRODUCTION
Kennett, Houtz, et al. (1975) proposed that the climate cooled and an
Antarctic ice sheet (cryosphere) developed in late Eocene to early Oli-
gocene time as the Antarctic Circumpolar Current (ACC) progressively
isolated Antarctica thermally. Australia’s movement north from Antarc-
tica formed the Tasmanian Gateway and was considered to have trig-
gered ACC formation and the global cooling that started ice sheet
formation, initially on Antarctica and later in the Northern Hemi-
sphere. While Paleogene rifting slowly opened the Australo-Antarctic
Gulf (AAG) between the two continents, the Indian and Pacific Oceans
remained separated by the almost continuous Tasmanian “land bridge”
until the late Eocene, so no ACC existed. Circulation of warm water
from the tropics warmed Antarctica through a complex series of cli-
matic feedback mechanisms. Early drilling (Deep Sea Drilling Project
[DSDP] Leg 29) in the Tasmanian Gateway between Australia and Ant-
arctica provided a basic framework of paleoenvironmental changes as-
sociated with gateway opening (Kennett, Houtz, et al., 1975). However,
the cored sequences were of insufficient quality and resolution to more
fully test the potential interrelationships of plate tectonics, circumpolar
circulation, and global climate.
Ocean Drilling Program (ODP) Leg 189, carried out in early 2000, was
designed to assist with better understanding of the nature and timing of
changes associated with gateway opening (Fig. F1). We aim in this con-
tribution to provide a summary of the Leg 189 results that includes
some postcruise studies, with special emphasis on the papers written for
this Scientific Results volume. We present new information about the na-
ture of and processes involved in the Paleocene and Eocene warm epi-
sode, late Eocene transitional climate, and Oligocene and later cooling
(Fig. F2). Also, a large group of papers based largely on Leg 189 drilling
will appear in an American Geophysical Union Geophysical Mono-
graph (Exon et al., in press a); these are only briefly mentioned here.
Implications for petroleum prospectivity were presented by Exon et al.
(2001) and are not considered here. A detailed comparison with data
from other ODP and DSDP sites and other stratigraphic information is
provided in Exon et al. (in press b). It shows that, in nonabyssal se-
quences off southern Australia and New Zealand, the transition from si-
F1. Cretaceous–Cenozoic sedi-
mentary basins, p. 28.
1171
2000
4000
2000
150 km
Tasman
Basin
3000
4000
1000
Tasman
Basin
3000
4000
3000
281
3000
4000
3000
4000
1000
5000
0
280
146°
142°E150°
South
Indian
Basin
Sorell
Basin
282
Otway
Basin
Bass
Basin
Gippsland
Basin
Victoria
Melbourne
Tasmania
Hobart
CS
1171
1170
1169
CS
1172
1168
DSDP site
282 Cape Sorell No. 1 wellODP site
Depocenter
South Tasman Rise
East
Tasman
Plateau
04-012-1
Thin or no sedimentation,
continental margin
Deepwater
oceanic crust
B
N
E T S
S T
S
38°
S
42°
46°
F2. Global temperature changes,
p. 29.
0406080
Change in temperature (°C)
Northern
Hemisphere
ice sheet
begins
Global Temperature Change
Age (Ma)
20
First Antarctic
ice sheets?
Permanent Antarctic
ice sheet develops
Warm
Cool
04-012-2
0
8
6
4
2

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 3
liciclastic to carbonate sedimentation generally occurred somewhere in
the late Eocene or early Oligocene. In the Australian sector of the Ant-
arctic margin, the Eocene–Oligocene transition is from nonmarine clas-
tics to shallow glaciomarine siliciclastics and diatomaceous sediments.
Overview of Drilling Results
During Leg 189, 4539 m of nearly continuous sediment core was re-
covered from five deepwater sites off Tasmania (Table T1), ranging in
age from Late Cretaceous (75 Ma) to present day. The general character
of the drilled sequences is summarized in Figures F3 and F4. The Paleo-
gene sediments vary considerably among sites (Fig. F3), reflecting their
varied locations with respect to the pre-Oligocene Tasmanian land
bridge between Australia and Antarctica, their meridional position, and
their tectonic differences. At that time, all the sites were located in high
southern latitudes of 60°–70°S, compared to their present latitudes of
42°–48°S. There are also differences in the carbonates of the post-separa-
tion Oligocene and younger sequences.
In the Paleogene, the western Site 1168 was located in the narrow
and tranquil AAG, west of the land bridge and connected to the Indian
Ocean. This sequence on the west Tasmanian margin (cored to 883
meters below seafloor [mbsf]) recovered upper Eocene shelf mudstone
and an almost continuous sequence of Oligocene and younger chalk
and ooze.
The other sites were located in the Pacific Ocean and east of the cul-
mination of the Tasmanian land bridge. This sector of the southwest Pa-
cific ocean was relatively narrow and constricted during the Paleogene.
Site 1170 on the western South Tasman Rise (STR) (cored to 780 mbsf)
contains middle to upper Eocene shelf mudstone and lower Oligocene
and younger chalk and ooze. This site lay closer to the developing ACC
than Site 1168, and the ACC caused erosion or nondeposition of much
of the middle Oligocene. At nearby Site 1169 (cored to 264 mbsf), mid-
dle Miocene and younger ooze was recovered.
Site 1171 on the southernmost STR (cored to 959 mbsf) consists of an
upper Paleocene to upper Eocene shelf mudstone sequence and an al-
most complete upper Oligocene and younger chalk and ooze sequence.
Hiatuses of latest Eocene and Oligocene age reflect increased bottom
water flow. The Neogene section is relatively continuous, apart from a
late Miocene hiatus. Site 1172 on the East Tasman Plateau (ETP) was far-
ther from the ACC. An Upper Cretaceous to upper Eocene shelf mud-
stone sequence was recovered, along with Oligocene and younger chalk
and ooze. Hiatuses were identified in the lowermost Paleocene, middle
Paleocene, lower middle Eocene, and lowermost Oligocene.
Figure F3 compares the four deepest penetrating sites. As usual in
ODP drilling, recovery was poorest in the deepest section. Upper Maas-
trichtian mudstones were cored only at Site 1172. Paleocene mudstones
were cored at Sites 1171 (upper Paleocene only) and 1172 (a 75-m se-
quence with a major hiatus between the Danian and late Paleocene).
Complete sequences of Eocene mudstones were cored at Sites 1171
(~600 m) and 1172 (~200 m), the thin sequence on the ETP probably
representing its remoteness from source areas. Middle and upper
Eocene mudstones were cored at Site 1170, and upper Eocene mud-
stones were cored at Site 1168 west of Tasmania. The thick (~300 m),
primarily Oligocene sequence at Site 1168 grades eastward into rapidly
thinning deepwater Oligocene chalks. Miocene deepwater calcareous
ooze is thickest at Site 1168 (~300 m) and thinnest at Site 1171 (~170
T1. Leg 189 sites, p. 36.
F3. Sequences drilled, p. 30.
080160
080160
080160
600
700
200
300
400
500
600
700
800
900
Nannofossil ooze/chalk
Foraminifer ooze/chalk Clayey ooze/chalk
Oligocene
Miocene
Pliocene
Pleistocene Pleistocene
Miocene
Oligocene
Pliocene
Eocene
Eocene
Pliocene
Miocene
Eocene
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Cretaceous
Maastrichtian
middle
Eocene
200
300
400
500
600
700
Pleistocene
Glauconitic siltstone
H
H
H
H
H
H
H
late
Paleocene
Paleocene
late
Eocene
Oligocene
04-012-3
NGR (API)
NGR (API)
NGR (API) NGR (API)
Depth (mbsf)
Claystone
Hiatus
H
Siliceous ooze
Silty claystone/clayey siltstone
Western
Tasmania Margin
Site 1168
Western South
Tasman Rise
Site 1170
South
Tasman Rise
Site 1171
East
Tasman Plateau
Site 1172
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
080
F4. Time stratigraphy and sedi-
ment facies, p. 31.
European
Stage
60
55
50
65
45
40
10
15
20
25
30
35
1
2
3
4
0
5
m
e
l
e
PliocenePleist.MioceneOligoceneEocenePaleocene
Age (Ma)
Epoch
Site 1169
West South
Tasman Rise
3580 m
47° 03´S, 145° 14´E
Site 1168
Western
Tasmania Margin
2475 m
42° 36´S, 144° 24´E
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
West South
Tasman Rise
2710 m
47° 09´S, 146° 02´E
Site 1170
South
Tasman Rise
2150 m
48° 30´S, 149° 07´E
Site 1171
East
Tasman Plateau
2630 m
43° 58´S, 149° 56´E
Site 1172
m
l
e
m
l
e
l
l
e
e
L
Calabrian
Piacenzian
Zanclean
Messinian
Tortonian
Serravallian
Langhian
Burdigalian
Aquitanian
Chattian
Rupelian
Priabonian
Bartonian
Lutetian
Selandian
Danian
Ypresian
Thanetian
Maastrichtian
Cret.
Cretaceous
70
767 m
04-012-4
884 m
246 m
780 m
959 m
Foraminifer-bearing or siliceous-
bearing nannofossil ooze/chalk
Nannofossil ooze/chalk
Clayey nannofossil ooze-chalk/
nannofossil-bearing clay
Clayey chalk, sandy claystone, organic-bearing
silty claystone/clayey siltstone
Organic-bearing, nannofossil-bearing,
silty claystone/clayey siltstone
Unconformity on seismic
evidence only
Note: Some time breaks probably
occur in the Paleocene-
Eocene sequences and
near the Eocene/Oligocene
boundary
Organic- and glauconite-
bearing silty claystone/clayey siltstone

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 4
m). Pliocene oozes are remarkably consistent in thickness (~70 m) at all
sites. Pleistocene oozes are thickest at the deepwater STR Site 1170 and
thinnest at the shallower, current-swept STR Site 1171. As outlined in
the Leg 189 Initial Reports volume (Exon, Kennett, Malone, et al., 2001)
and a brief synthesis in Eos (Exon et al., 2002) drilling showed that in
the Tasmanian-Antarctic region there were three phases of sedimenta-
tion, clearly related to plate tectonic configuration and its influence on
changes in ocean circulation and global climate. Depositional rates at
Leg 189 sites varied considerably during the three phases and from loca-
tion to location (Fig. F5). Warm “greenhouse” conditions persisted dur-
ing the Late Cretaceous, Paleocene, and Eocene and were associated
with deposition of shelf mudstone, the depositional rate of which (~2–5
cm/k.y.) matched the rapid subsidence on the rifting continental mar-
gins. In the late Eocene (37–33.5 Ma), early separation occurred be-
tween Australia and Antarctica in shallow water and the currents began
to flow in the Tasmanian Seaway. This led to a second phase of deposi-
tion, consisting of slow sedimentation (<1 cm/k.y.) of glauconitic silt-
stone, which failed to keep up with subsidence of the margin. Rapid
subsidence at the eastern sites commenced in the latest Eocene. By the
earliest Oligocene (33.5 Ma) the seaway had opened substantially, and
the third phase of deposition commenced, with slow deposition (1–2
cm/k.y.) of deepwater pelagic carbonates as Australia moved northward.
Water depths no longer significantly increased following the Oligocene
because subsidence rates had fallen off. The third phase covered both
intermediate “doubthouse” conditions (33.5–15 Ma) and “Icebox” con-
ditions (15–0 Ma).
OVERVIEW OF CONTRIBUTIONS TO THE
SCIENTIFIC RESULTS VOLUME
Twelve papers appearing in this volume add significant knowledge to
the Cenozoic paleoenvironmental development of the region. These
contributions are briefly summarized as follows.
Stickley et al. (this volume) summarize the Late Cretaceous to Qua-
ternary biostratigraphy and calibrate this with magnetostratigraphy.
Their age models for Sites 1168, 1170, 1171, and 1172 (see Fig. F5) inte-
grate information from calcareous, siliceous, and organic walled micro-
fossils. These data provide the necessary chronologic foundation upon
which all other research papers from Leg 189 depend. The study is a
unique synthesis of latest Cretaceous to Quaternary biostratigraphy in
the Australian-Antarctic region, using all key microfossil groups.
Robert (this volume) presents a data report on bulk and clay mineral
assemblages for the entire sequences at all sites except Site 1169. He
provides detailed data tables and informative graphs and background
information for Robert (in press). Differences in clay mineral assem-
blages are related to source material, regional tectonics, weathering, and
erosion. At the eastern sites (1170–1172) a threefold split in bulk sedi-
ment mineralogy matches lithologic changes: lower upper Eocene and
older siliciclastics (terrigenous minerals), uppermost Eocene–Oligocene
transition (terrigenous minerals plus some biogenic carbonate), and
Oligocene and younger carbonates (biogenic carbonate minerals). At
Site 1168 the terrigenous–carbonate transition is gradual, with terrige-
nous minerals common until the lower middle Miocene. The clay min-
eral assemblages do not differ from east to west as much as the bulk
F5. Sedimentation rate curves,
p. 32.
04-012-5
A
g
e
(
Ma
)
Depth (mbsf)
60 70
cm/k.y.
0.5
1.0
2.05.0
Oligocene Eocene Paleocene
late late late earlyearlyearlyearly middle Late
Cret.Plio.
latee.
Pleist.
l.
Miocene
middle
Site 1171
South Tasman Rise
Site 1170
West South
Tasman Rise
Site 1172
East Tasman
Plateau
Site 1168
Western Tasmania
Margin
0
200
400
600
800
1000
40 5020 30010

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 5
mineralogy. Smectite is mostly dominant, with variable subordinate
proportions of kaolinite and illite. Chlorite is commonly present at
Sites 1170–1172.
Latimer and Filipelli (this volume) provide a data report on Eocene
to present sediment geochemical results from Site 1171. Fe, Al, and Ti
concentrations and elemental ratios (carbonate free) were measured to
identify changes in metal sources and terrigenous inputs. Export pro-
duction was studied using P and Ba concentrations and P/metal and Ba/
metal; higher values represent higher production. There are major
changes at ~260–290 mbsf (Eocene–Oligocene transition), where silici-
clastic sediments grade upward into pelagic carbonate sediments. P/Ti
and Ba/Ti ratios indicate large export production increases, the ratios
changing from negligible in the Eocene to ~6–10 g/g in the Oligocene.
The ratios declined gradually to ~4 g/g in the Pliocene–Pleistocene.
Williams et al. (this volume) compare Southern Ocean and global
dinoflagellate cyst index events for the Late Cretaceous to Neogene.
They use Leg 189 sites for much of the Southern Ocean control, and
these sites have the benefit of detailed independent age control, prima-
rily from magnetostratigraphy and, to some extent, from planktonic
foraminifers and calcareous nannofossils. Williams et al. carefully docu-
ment stratigraphic ranges of the dinocysts with abundant line drawings
of taxa and photomicrographs provided to assist with identification, a
useful resource for the international community of dinocyst workers.
Brinkhuis, Sengers, et al. (this volume) describe latest Cretaceous to
earliest Oligocene (and Quaternary) dinoflagellate cysts from Site 1172
on the East Tasman Plateau, providing a standard reference for dinocyst
biostratigraphy for these latitudes during the latest Cretaceous through
the Oligocene. The Maastrichtian to earliest Oligocene record is well
represented, with the exception of much of the early and some of the
late Paleocene. Dinocyst species are largely endemic and relatively cool
water, representing the “Transantarctic Flora,” or are bipolar types. Un-
til the early late Eocene, the assemblages are indicative of shallow-
marine to restricted-marine, prodeltaic conditions. By middle late
Eocene times, slow glauconitic sedimentation became established, re-
flecting the deepening of the Tasmanian Gateway. An associated nota-
ble turnover in dinocyst associations reflects a change from marginal
marine to more offshore conditions. Organic microfossils are virtually
absent in the Oligocene and Neogene pelagic carbonates.
Brinkhuis, Munsterman, et al. (this volume) provide an important
overview paper on late Eocene, Oligocene, Miocene, and Quaternary di-
noflagellate cyst distributions at Site 1168 west of Tasmania and illus-
trate the main trends in palynomorph distribution. The dinocyst
species are largely cosmopolitan with some low-latitude taxa and, un-
like those at Site 1172 to the southeast, the assemblages do not contain
endemic Eocene Antarctic taxa. The general palynomorph distributions
suggest relatively warm waters, an initially restricted shallow-marine
setting, and deepening and initiation of open-ocean conditions in the
Oligocene.
Sluijs et al. (this volume) describe dinoflagellate cysts from the
Eocene–Oligocene transition, particularly for Site 1172 on the East Tas-
man Plateau and Sites 1170–1171 on the South Tasman Rise, and com-
pare the results with broader shipboard information from Site 1168
west of Tasmania. At Sites 1170–1172, three distinctive dinocyst assem-
blages indicate relatively rapid stepwise environmental changes, from a
prodeltaic to a deeper marine pelagic setting. The Antarctic endemic as-

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 6
semblage was replaced by a more cosmopolitan offshore assemblage at
~35.5 Ma and by an even further offshore assemblage at ~34 Ma.
Wei et al . (this volume), in a brief data report on the Paleogene cal-
careous nannofossil biostratigraphy of Leg 189, list the distribution of
nannofossils at the various sites and summarize the occurrence of nan-
nofossil datums. The nannofossil assemblages are particularly impor-
tant in establishing the biostratigraphy of Oligocene carbonate
sequences. They also provide sporadic but valuable ages and environ-
mental information for the pre-Oligocene siliciclastic sequences.
Pfuhl and McCave (this volume) built integrated age models for the
early Oligocene to early Miocene (30–14 Ma) at four sites, comparing
biostratigraphy, magnetostratigraphy, stable isotope records, carbonate
content, and weight percent sand. They show that the Marshall Para-
conformity (named in New Zealand) forms a hiatus (~33–30 Ma) at the
eastern sites but is essentially absent at Site 1168. At the two eastern-
most sites (1171 and 1172), the Oligocene/Miocene boundary is marked
by a condensed section or hiatus (~24–23 Ma). There is a problematic
mismatch of the Mi-1 event (~24 Ma) at Sites 1168 and 1170.
Ennyu and Arthur (this volume) provide a data report on oxygen
and carbon stable isotope records of Miocene planktonic and benthic
foraminifers and fine-fraction carbonate from Sites 1170 and 1172, as
background for interpretations provided in Ennyu and Arthur (in
press).
McGonigal and Wei (this volume) provide a data report on Miocene
calcareous nannofossil biostratigraphy containing species range charts,
a tabulation of key biohorizons, a summary of nannofossil zones and
datums, and plates of photomicrographs. Although diversity and bio-
stratigraphic resolution were greatest at Site 1168, a solid integrated bio-
stratigraphy was constructed at all sites by incorporating the results
from other microfossil groups and magnetostratigraphy.
Stant et al. (this volume) report on the Quaternary nannofossil bio-
stratigraphy of four sites: two north and two south of the present-day
Subtropical Front. Their study indicates that movement of the front in
the Quaternary and late Pliocene influenced the distribution of
warmth-loving Discoaster and large Gephyrocapsa species. In addition,
discoasters survived longer (until 1.95 Ma) east of Tasmania at Site 1172
than west of Tasmania (until 2.51 Ma), suggesting that the East Austra-
lian Current warmed the eastern waters. An early Pleistocene hiatus en-
compasses the entire Helicosphaera sellii Zone, as it does at many other
DSDP and ODP sites in the region.
TECTONIC EVOLUTION
The Cretaceous through Eocene tectonic history of this region is sim-
ilar to that of other margins of Antarctica. East-west rifts between Aus-
tralia and Antarctica, a result of northwest–southeast oblique extension,
may have formed as early as the latest Jurassic (Willcox and Stagg,
1990). In the Early Cretaceous, east Gondwana was still intact and the
Tasmanian region lay deep within present-day Antarctica, southeast
Australia, and the continental block of Lord Howe Rise, Campbell Pla-
teau, and New Zealand (LCNZ) (Fig. F6). Ocean currents are inferred to
have flowed west and north of Australia and east of the LCNZ continen-
tal block. Early in the Late Cretaceous, rifting caused marine transgres-
sion into the AAG from the west, and seafloor spreading commenced
between ~95 Ma (Veevers, 1986) and ~83 Ma (Sayers et al., 2001). A
F6. Setting of the Tasmanian re-
gion during the Late Cretaceous,
p. 33.
Antarctica
ETP
W-STR
E-STR
?
?
95 Ma
Australia
Site
282
Site
281
Site
1169
Site
1168 Site
1172
Site
1170
60°
S
65°
70°
75°
140° E145° 150° 155°
0 100 200 300
km
Site
1171
95 Ma
TASZ
TASZ
04-012-6
Lord Howe Rise and
Campbell Plateau
Marginal marine rift sediments

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 7
northwest–southeast, left lateral Tasmanian-Antarctic Shear Zone
(TASZ) absorbed the relative motion of the two continents west of Tas-
mania, and AAG waters transgressed southward along the TASZ. Spread-
ing propagated eastward, being fully under way west of Tasmania by the
middle Eocene (Royer and Rollet, 1997), but the Tasmanian-Antarctic
land bridge in the east allowed little to no water exchange between the
AAG and the proto-southwest Pacific.
In the Late Cretaceous (~75 Ma), continental breakup and seafloor
spreading began between Australia and the LCNZ (Cande and Stock, in
press). Rifting propagated northward east of Australia, forming the Tas-
man Sea, and final breakup off northeastern Australia took place in the
Paleocene (~60 Ma) (Gaina et al., 1999). Thereafter, major ocean cur-
rents could flow along the eastern coasts of Australia and Tasmania, the
ETP and STR, and along the Antarctic margin to the south. However,
the Tasmanian land bridge separating the AAG from the Pacific Ocean
remained essentially intact until the latest Eocene. When a deepwater
passageway developed between South America and Antarctica, to com-
plete the Southern Ocean oceanographic circuit, remains disputed.
Barker and Burrell (1977, 1982) argued that a deepwater pathway could
not have developed in Drake Passage until close to the Oligocene/
Miocene boundary. In contrast, Lawver and Gahagan (1998, 2003) sug-
gested that the passageway opened somewhat later than the Tasmanian
Gateway but no later than the early Oligocene, allowing the ACC to be-
come established by then.
Leg 189 drill sites were located on four continental tectonic blocks:
Site 1168 in the Sorell Basin on the west Tasmanian margin, Sites 1169
and 1170 in the Ninene Basin on the western STR block, Site 1171 in a
small strike-slip basin on the central STR block, and Site 1172 on the
ETP. The drill testing of seismic profiles has helped interpretation of the
local tectonics (Hill and Exon, in press). According to Royer and Rollet
(1997), the ETP rifted from Tasmania and the STR as part of Tasman Sea
break-up in the Late Cretaceous (95 Ma), although Site 1172 subsided
only slowly until the late Eocene.
Apatite fission track dating (O’Sullivan and Kohn, 1997) indicates a
period of uplift and erosion near the Paleocene/Eocene boundary on
the western and eastern margins of Tasmania. Between the eastern and
central STR blocks near Site 1171, deformation ceased along the Balleny
Fracture Zone at ~55 Ma, dating breakup between the southeastern STR
and Antarctica. Northwest–southeast strike-slip movement along the
west Tasmanian margin (Site 1168) ended in the middle Eocene (~43
Ma), when fast spreading between Australia and Antarctica transferred
strike-slip movement to the north–south Tasman Fracture Zone on the
west STR margin (Site 1170). Continent-continent movement ended
only when Antarctica cleared the STR at the end of the Eocene (~34
Ma). However, the STR continued to move northward along the Tasman
Fracture Zone relative to the western spreading center, finally clearing it
in the early Miocene (~20 Ma). Heat from the passing spreading center
caused uplift along the margin. At all Leg 189 sites the water started to
deepen somewhat around the middle/late Eocene boundary (~37 Ma)
(Hill and Exon, in press), but this may be attributable largely to a de-
crease in sedimentation rates rather than accelerated subsidence. A very
rapid period of subsidence occurred near the Eocene/Oligocene bound-
ary.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 8
CRETACEOUS–MIDDLE LATE EOCENE HISTORY:
BEFORE GATEWAY OPENING
Before the Tasmanian Gateway opened, rifting and associated hinter-
land uplift and erosion allowed rapid deltaic sedimentation in the rifts.
Evidence from seismic reflection profiles, drilling, and dredging indi-
cates that as much as 4000 m of Cretaceous to Eocene, largely deltaic
sediments were deposited in the offshore Tasmanian region (Exon et al.,
1997; Hill et al., 1997, 2001; Hill and Exon, in press). For Leg 189 drill
sites, late Maastrichtian through early late Eocene deposition (~75–36
Ma) was shallow-marine deltaic and siliciclastic. Moderately high depo-
sitional rates (~5–10 cm/k.y.) kept up with subsidence and compaction.
Calcareous micropaleontological and palynological evidence indicates a
cool to warm temperate climate through the late Maastrichtian to the
late Eocene (Brinkhuis, Sengers, et al. and Brinkhuis, Munsterman,
et al., both this volume).
Cretaceous (Maastrichtian) sediments were recovered only at Site
1172 on the ETP, where they are 70 m thick. The microflora indicate a
humid and seasonally cool climate. Smectite completely dominates the
clays (Robert, in press), suggesting a warm climate and extreme chemi-
cal weathering in the source area. The lower, thicker facies is dark clay-
stone and silty claystone, which is essentially noncalcareous and rarely
bioturbated (Exon et al., in press b). Dinocysts, spores, pollen, and dia-
toms are common, and the environment is interpreted as generally
highly restricted and paralic. Occasional more marine beds contain
molluskan debris, planktonic foraminifers, and nannofossils. The up-
per, thinner facies comprises brown, paralic, sideritic sandstone and
sandy mudstone that contain 20%–55% sand consisting largely of ei-
ther siderite micronodules or quartz and glauconite. The Cretaceous/
Tertiary boundary is not preserved, and an iridium anomaly is lacking
(Schellenberg et al., in press).
By the Paleocene, areas of continental crust that had been thinned
by Cretaceous rifting—the future Bass Strait, the South Tasman Saddle
between Tasmania and the STR, parts of the TASZ between Antarctica
and the STR, and the East Tasman Saddle between Tasmania and the
ETP—had subsided and were near sea level. Thereafter, very limited in-
terchange of shallow-marine waters could have occurred between the
AAG and the Pacific Ocean. Although plate tectonic reconstructions
such as those of Royer and Rollet (1997), Lawver and Gahagan (2003),
and Cande and Stock (in press) suggest the presence of a shallow-
marine connection by the middle Eocene (Fig. F7A), the sedimentary
evidence from Leg 189 suggests that this connection must have been
very limited. Even in the late Eocene, the contrast between the poorly
oxygenated, relatively warm shallow-marine waters of the AAG (Site
1168) and the better oxygenated, relatively cool shallow-marine waters
of the southwest Pacific Ocean (Sites 1170–1172) suggests a lack of sig-
nificant interchange between southern Indian and Pacific Ocean waters
even at the shallowest depths.
Throughout the Paleocene and Eocene, water circulation in the AAG
was probably a sluggish clockwise gyre, with contributions of some
warmer waters from lower latitudes in the Indian Ocean. In contrast,
and modifying precruise assumptions, micropaleontological evidence
(Brinkhuis, Sengers, et al., this volume), with support from climate
modeling, suggests that the eastern sites were influenced by cooler wa-
ters transported northwestward as a western boundary countercurrent.
A
A A G
E A C
Proto - Pacific
130E
50 S
60 S
70 S
160 E
170 E
180
170 W
50 S
60 S
150W
70S
140 W
120 E
140E
150 E
F7A. Middle Eocene situation,
p. 35.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 9
Evidence for glaciation is completely lacking at Leg 189 sites, with both
marine and terrestrial microfossils indicating temperate conditions. Rel-
atively diverse late Eocene calcareous nannofossil assemblages indicate
slightly warmer conditions in the Tasmanian Gateway sector of the
Southern Ocean than at comparable latitudes elsewhere, although most
of the sites lack warmth-loving discoasters. The nannofossil diversity
and the dinocyst and diatom assemblages confirm the absence of sea-
sonal sea ice over the shelf.
Paleocene and Eocene organic-rich mudstones were deposited on a
highly restricted, moderately tranquil broad shelf near the rift opening
between Antarctica and Australia. Benthic foraminiferal assemblages in-
dicate deposition at shelf depths. The lack of sedimentary characteris-
tics indicating turbulence suggests deposition below wave base (which
may have been shallow in the prevailing equable climatic conditions)
and largely free of appreciable current or tidal influences.
Paleocene
The Paleocene was cored at Site 1172 on the ETP, and probably at Site
1171 on the STR. At Site 1172, a thin Danian (lowermost Paleocene) se-
quence is disconformably overlain by a thicker upper Paleocene se-
quence. The lowermost Danian (6 m thick) disconformably overlies the
uppermost Maastrichtian and is brown noncalcareous muddy glauco-
nitic quartz sandstone and sandy mudstone, deposited in paralic condi-
tions. The disconformably overlying uppermost Paleocene (75 m thick)
is dark glauconitic quartz-bearing mudstone; the water shoaled through
time but remained paralic. Offshore dinocysts are much more common
in the upper Paleocene than during the Danian. The microflora indicate
a relatively warm and humid but weakly seasonal climate. Smectite
continues to completely dominate the clays (Robert, in press), suggest-
ing a warm climate and extreme chemical weathering.
At Site 1171, we follow Röhl et al. (in press a) in distinguishing a 44-
m-thick upper Paleocene section that consists of laminated dark mud-
stone with almost no sand or carbonate. Sporomorphs are abundant,
but other diagnostic fossils are rare; diverse pollen and spore assem-
blages indicate a strong terrigenous influence. The microflora and
abundant smectite suggest a relatively warm, humid, but weakly sea-
sonal climate. Brinkhuis, Sengers, et al. (this volume) and Röhl et al.
(in press a) suggest that very shallow marine conditions prevailed. How-
ever, in the late Paleocene, Site 1171 had more restricted marine condi-
tions than Site 1172.
Early–Middle Eocene
The tectonic setting in the middle Eocene is shown in Figure F7A.
Lower and middle Eocene siliciclastic sediments were cored at Sites
1170–1172:
1. Site 1172: lower Eocene = 70 m thick; middle Eocene = 180 m
thick;
2. Site 1171: lower Eocene = 145 m thick; middle Eocene = 420 m
thick; and
3. Site 1170: lower Eocene = 50 m thick; middle Eocene = 210 m
thick.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 10
At all sites in the early and middle Eocene, pervasive pollen and spore
assemblages and abundant and continuous low-diversity assemblages of
dinocysts indicative of eutrophic and brackish surface waters point to
highly restricted nearshore conditions. Sporadic occurrences of well-
preserved calcareous nannofossils suggest that their rarity is due to lim-
ited access to the restricted coastal setting and to high sedimentation
rates, rather than to dissolution. Water mass ventilation was generally
poor to limited, judging by the high organic carbon content, limited bio-
turbation, and benthic foraminiferal assemblages dominated by aggluti-
nated forms and nodosariids.
Distinct cycles in physical properties, sediment type, and microfossil
assemblages are well documented in middle and upper Eocene sedi-
ments at Site 1172 (Röhl et al., in press b) and are also evident at Sites
1170 and 1171. The cycles are between dark, poorly bioturbated, dino-
cyst-rich but nannofossil-poor sediments lacking glauconite and
lighter, dinocyst-bearing, and more nannofossil-abundant, bioturbated
sediments containing glauconite. Röhl et al. (in press b) conclude that
the sediment cycles were produced under the influence of orbital per-
turbations of the Earth relative to the sun (Milankovitch cycles), which
affected sea level and climate and, in turn, changed siliciclastic sedi-
ment supply, upwelling and nutrient supply, and associated bottom wa-
ter ventilation. White (in press) argues that cycles observed in
geochemical parameters in lower to middle Eocene sediments at all sites
resulted from the influence of glacioeustasy in these very shallow ma-
rine environments.
At Site 1172, the plant microflora suggest a cool and uniformly hu-
mid climate in the hinterland, whereas abundant smectite suggests a
warm climate and intense weathering. Exon et al. (in press b) showed
that the lowermost Eocene (35 m) is dark, noncalcareous, variably glau-
conitic, quartz-rich shallow-marine mudstone with abundant siderite
micronodules toward the top, suggesting paralic deposition. The overly-
ing lower to middle Eocene dark, noncalcareous mudstone (83 m) is
more fossiliferous and slightly more marine. The overlying middle
Eocene is dark, noncalcareous diatomaceous mudstone (69 m), repre-
senting very different shelfal conditions in which siliceous organisms
thrived and were preserved. The upper middle Eocene is somewhat cal-
careous, dark, diatom-bearing to diatomaceous mudstone (64 m) con-
taining more sand. Deposition probably occurred on an open
continental shelf. Overall, the Eocene trend was toward more open ma-
rine conditions as waters became slightly deeper, with less reducing
conditions with time, as indicated by increasing carbonate and diatom
components in the sediments in addition to changes in dinocyst assem-
blages (Brinkhuis, Sengers, et al., this volume).
At Site 1171, marine mudstones contain dinocysts, indicating re-
stricted, eutrophic, and neritic conditions throughout, with open ma-
rine taxa being relatively rare (Shipboard Scientific Party, 2001d). The
microflora indicate a relatively warm, humid, but weakly seasonal early
Eocene climate and a cooler and uniformly humid middle Eocene. The
lower Eocene (45 m) consists of greenish noncalcareous and quartz-rich
mudstone. The mixed clay mineral assemblage suggests intense erosion
of steep relief areas (Robert, in press). The middle Eocene (142 m) is
greenish gray mudstone with nannofossil and carbonate content mod-
erate at the bottom, negligible in the middle, and moderate at the top.
The return to dominance of smectite suggests a warm climate and de-
creasing erosion and intense weathering in the hinterland. At Site 1170,
middle Eocene dark mudstones (241 m) contain abundant dinocysts

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 11
that indicate somewhat restricted, euphotic, neritic conditions (Ship-
board Scientific Party, 2001c). Some microfloral evidence suggests a rel-
atively cool, humid climate, but smectite dominates the clays,
suggesting warm climate and intense weathering in the hinterland. The
nannofossil distribution suggests periods of somewhat more open ma-
rine, less restricted conditions.
In the Eocene, both the AAG and the southern Proto-Pacific Ocean
were under the influence of temperate climate, but the currents were
warmer in the AAG than in the proto-Pacific (Fig. F7A). Ventilation of
the waters increased toward the developing seaway on the Pacific side
of the Tasmanian land bridge. At Site 1170, laminations are periodically
absent and there is some bioturbation. Farther east, the well-biotur-
bated shelf sediments at Site 1171 and other evidence indicate more
ventilated conditions and the absence of an oxygen minimum zone.
Within the open Pacific Ocean at Site 1172, the water mass was better
ventilated than at either of the sites to the west.
LATE EOCENE–OLIGOCENE HISTORY:
THE GATEWAY OPENS
Late Eocene
Site 1172 contains only 11 m of upper Eocene sediment deposited on
the ETP shelf and small hiatuses (Stickley et al., submitted [N1]) caused
by current action and nondeposition. As the water deepened from shelf
to upper slope depths and current action increased, the sediment
changed from diatomaceous mudstone to glauconitic siltstone. Smec-
tite is the dominant clay. Site 1171 contains ~80 m of sediment laid
down in a local basin on the southern STR, almost all being diatoma-
ceous mudstone overlain by a few meters of glauconitic quartz-bearing
sandstone. Smectite is the dominant clay at this site also. The water was
deepening and current activity increasing on this part of the former
land bridge and at Site 1170 in the Ninene Basin. Site 1170 contains
~50 m of upper Eocene sediments: marine mudstone overlain by glau-
conitic diatomaceous siltstone. Illite and smectite alternate as domi-
nant clays; the illite content suggests proximity to an area of active
tectonism (Robert, in press).
Site 1168 contains ~130 m of upper Eocene siliciclastic mudstone de-
posited in a restricted embayment on the broad west Tasmanian shelf in
the AAG, which becomes increasingly more open marine upcore. Un-
like the situation at other sites, spores are much more abundant than
dinocysts, suggesting closer proximity to land during deposition. Also,
the dinocyst taxa are largely cosmopolitan with some low-latitude
forms, and (unlike those at Site 1172 to the southeast) the assemblages
generally lack endemic high-latitude taxa, suggesting relatively warmer
conditions (Brinkhuis, Munsterman, et al., this volume). The lower-
most sequence consists of sandy mudstone, possibly largely nonmarine.
Kaolinite dominates the clay assemblage, suggesting a source region
marked by tectonic activity and intense chemical weathering (Robert,
in press) or perhaps a change in source rocks. The bulk of the sediments
are laminated, organic-rich, pyritic shallow-marine to paralic mud-
stone, indicating sluggish circulation and poor ventilation. Kaolinite
decreases upcore and smectite increases, suggesting reducing tectonic
activity but continuing chemical weathering (Robert, in press). Anoxic
to dysoxic depositional environments extended up onto the continen-

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 12
tal shelf. Above this mudstone sequence are shallow-marine sandstones
and mudstones containing quartz and sponge spicules derived from
nearby beaches and banks (Exon et al., in press b).
In the transitional upper Eocene interval, sedimentation changes up-
hole from more muddy to more sandy, with increased glauconite and
quartz, reflecting an increase in bottom currents and winnowing. At
Sites 1170, 1171, and 1172 the upward gradation is from mudstone into
glauconitic siltstone. Despite evidence for general winnowing and the
presence of hiatuses in the glauconitic siltstone, some levels containing
angular quartz indicate episodes marked by little reworking. The glau-
conitic sediments are strongly bioturbated and were deposited in well-
oxygenated bottom waters. Stepwise changes in dinocyst, pollen, and
spore assemblages indicate environmental changes and deepening wa-
ter (Sluijs et al., this volume). Diatoms and benthic foraminifers indi-
cate deepening from shallow to deeper neritic or possibly uppermost
bathyal environments.
At Site 1172, dinocyst assemblages continue to be dominated by en-
demic forms (e.g., Brinkhuis, Sengers, et al., this volume). Of course,
Southern Ocean stable isotopic records generally indicate progressive
cooling through the middle and late Eocene (Shackleton and Kennett,
1975; Stott et al., 1990; Kennett and Stott, 1990). During the late
Eocene at Site 1171 and, especially, Site 1170, cooler continental condi-
tions are indicated by increased illite relative to smectite, suggesting a
reduction in continental chemical weathering. Pollen and spore records
suggest diverse and cool temperate late Eocene plant communities in
the hinterland. Floras were dominated by Nothofagus and podocarps
with an understory of ferns, similar to a floral assemblage of similar age
in the Weddell Sea sector of Antarctica (Mohr, 1990). Although the late
Eocene pollen assemblages indicate cooling, they also show that the
Tasmanian part of the Antarctic margin was still relatively warm com-
pared to the distinctly cooler Oligocene. This contrasts with the Prydz
Bay margin far to the west, where clear evidence exists for late Eocene
glaciation close to sea level (Barron et al., 1991a, 1991b; Cooper and
O’Brien, 2004). There is also convincing evidence for early Oligocene
growth of a significant ice sheet on at least parts of East Antarctica (Za-
chos et al., 1996).
Eocene–Oligocene Transition
Various lines of evidence suggest that Antarctica and the South Tas-
man Rise separated fully during the Eocene–Oligocene transition (Fig.
F7B). All four deep sites contain a fairly continuous record over this in-
terval until the earliest Oligocene, after which there are hiatuses at all
sites except Site 1168 until ~30 Ma (Pfuhl and McCave, this volume;
Fuller and Touchard, in press). In the early Oligocene, deposition at the
Pacific sites changed to open-water pelagic carbonates. This lithologic
change reflects the shift from siliciclastic to biogenic sedimentation,
from a poorly to a well-oxygenated benthic environment, from tranquil
to moderately dynamic environments, and from relatively warm to
cool climatic conditions. This paleoenvironmental change in the
oceans was the most profound of the entire Cenozoic (Kennett, 1977;
Zachos et al., 1993). An early Oligocene cooling and dissolution episode
is recorded widely in deep-sea carbonates and is associated with the
well-known positive oxygen isotopic shift (Oi-1) at ~33 Ma (e.g., Shack-
leton and Kennett, 1975; Miller et al., 1991; Zachos et al., 1994).
B
AAG
A C C
50 S
130 E
120 E
60 S
70 S
80S
80 S
70W
60S
150 W
70S
140 W
160E
140E
170 E
180
F7B. Eocene/Oligocene boundary
situation, p. 35.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 13
In the earliest Oligocene, similar open-ocean conditions began to de-
velop on both sides of the former Tasmanian land bridge. We argue that
a shallow-water proto-ACC was established at the time of final separa-
tion and that the cool countercurrent that had reached Sites 1170–1172
from the southeast no longer did so (Fig. F7B). Currents continued to
circulate clockwise in the AAG and westward along the Antarctic coast
in the Pacific Ocean. By the late Oligocene, nearly all of the former land
bridge south of Tasmania had submerged. The Tasmanian Gateway
south of the STR was hundreds of kilometers wide and continuing to
widen, and water depths were abyssal. The ACC, flowing from the west
and accommodating an ever-increasing circumpolar flow, was effective
in all water depths and eroded and dissolved older sediments. The
Drake Passage, south of South America, may have opened to deep water
in the early Oligocene (Lawver and Gahagan, 1998, 2003) or at the Oli-
gocene/Miocene boundary (Barker and Burrell, 1977). The expansion of
the Antarctic cryosphere during the middle and late Cenozoic, and its
effect of strengthening thermohaline circulation at deep and intermedi-
ate water depths, contributed to very widespread deep ocean erosion
and the formation of hiatuses.
Why was there such a change from siliciclastic to carbonate sedimen-
tation at the Eocene/Oligocene boundary rather than early in the late
Eocene? A very broad, shallow Australian-Antarctic continental shelf
had been supplied with siliciclastic sediments since early in the Creta-
ceous. Although rifting, subsidence, and compaction had commenced
then, sedimentation had kept up, and shallow-marine sediments were
deposited rapidly until the end of the middle Eocene. Australia and
Antarctica were almost completely separated when fast spreading began
in the middle Eocene (~43 Ma), and this could be expected to increase
the rate of subsidence. In the Tasmanian region, slower siliciclastic sedi-
mentation continued in deepening but largely shelfal water at Sites
1170, 1171, and 1172 in the late Eocene until the Eocene/Oligocene
boundary (~33.5 Ma), some 10 m.y. after fast spreading started. We sug-
gest that the slower sedimentation resulted from current winnowing,
bypassing, and probably also falling sediment supply. Subsidence
curves (Hill and Exon, in press) suggest faster subsidence at the Eocene/
Oligocene boundary in the Tasmanian region, like that which formed
the Victoria Land Basin in nearby Antarctica (Cape Roberts Science
Team, 2000). Such subsidence would have rapidly reduced the area of
potential erosion in the Tasmanian region and drastically reduced sedi-
ment supply. However, only in the earliest Oligocene did pelagic car-
bonate sedimentation rapidly replace the siliciclastic and diatomaceous
sedimentation at the eastern sites. The change in the biogenic compo-
nent of sedimentation, from diatomaceous in the Eocene to calcareous
in the Oligocene, must have been related to the changes in oceanogra-
phy and latitude. At the western Site 1168, the transition began at the
same time but continued through the entire Oligocene.
The Eocene/Oligocene change to carbonate sedimentation probably
was also related to contemporaneous climatic cooling, which would
have greatly reduced rainfall, and thus weathering and erosion, reduc-
ing siliciclastic supply. Thereafter, slow deposition of pelagic carbonate
completely dominated off southern and eastern Tasmania and was in-
creasingly important west of Tasmania. The sequence of changes in the
sediments over the transition is remarkably consistent over the STR and
ETP, as determined from our deep cored sequences. Differences in detail
are clearly related to individual setting at the time of deposition (such
as latitude) and proximity to the ocean and landmasses. Sequences

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 14
from elsewhere on the Antarctic margin show a similar drastic reduc-
tion in siliciclastic sedimentation and increase in biogenic sedimenta-
tion during the Eocene–Oligocene transition. The earliest Oligocene is
often marked by an increase in biogenic sediments or components in
otherwise relatively slowly deposited siliciclastic sediments, including
diamictites (Diester-Hass and Zahn, 1996; Kennett and Barker, 1990;
Salamy and Zachos, 1999). However, outside the Tasmanian region, the
biogenic component is usually biogenic silica (diatoms) rather than bio-
genic carbonate (calcareous nannofossils and foraminifers). On the
shallow (probably neritic) northwest margin of the Weddell Sea, car-
bonate-free diatom ooze was deposited during the earliest Oligocene,
suggesting significant cool-water upwelling (Barker, Kennett, et al.,
1988). On the margins in Prydz Bay and the southern Ross Sea, diatoms
became an important component in diamictites (Barron et al., 1991a,
1991b; O’Brien, Cooper, Richter, et al., 2001).
Evaluation of the sedimentary sequences cored in the Tasmanian
Gateway region (Stickley et al., this volume) suggests opening of the
Tasmanian Gateway to cool shallow-water flow occurred during the lat-
est Eocene, with intensifying current flow toward the Eocene/Oligocene
boundary. This was followed in the earliest Oligocene by expansion of
the Antarctic cryosphere and deepwater interchange between the
southern Indian and Pacific Oceans. This interchange heralds the ACC
in this part of the Southern Ocean. Although planktonic microfossils in
the Leg 189 cores indicate climatic cooling, there is no evidence of gla-
ciation in these sequences. Indeed, the calcareous nannofossil assem-
blages suggest somewhat warmer conditions at equivalent latitudes
elsewhere in the Southern Ocean (Wei and Wise, 1990; Wei and Thier-
stein, 1991; Wei et al., 1992).
The late Eocene glauconitic siltstones in the sites closest to Antarctica
are overlain, with little gradation, by ooze and chalk of early Oligocene
age, whereas near western Tasmania there is more gradation upward
into the Oligocene. From the earliest Oligocene onward, sedimentation
at the eastern Leg 189 sites was completely dominated by deposition of
nannofossil ooze. Sedimentation rates of these oozes were faster than
those of the glauconitic silts. At the eastern sites, Oligocene rates were
slower than those of the lower and middle Eocene siliciclastic sedi-
ments, but later rates were comparable. In contrast, at Site 1168 rates
were slower across the Eocene/Oligocene boundary but comparable in
the upper Eocene siliciclastic and upper Oligocene to lower Miocene
marly sequences. Although the age of the base of the carbonates re-
quires better constraint, existing stratigraphic data suggest deposition
commenced at ~30 Ma (Stickley et al., submitted [N1]), following the
oxygen isotope shift, which is dated at ~33.5 Ma. The isotopic shift rep-
resents major cooling and the initial major cryospheric development of
East Antarctica (Shackleton and Kennett, 1975; Miller et al., 1991; Wei,
1991; Zachos et al., 1994) and major expansion of the psychrosphere
with its deep ocean circulation (Kennett and Shackleton, 1976). On
northwest Tasmania there is an alpine glacial tillite, dated palynologi-
cally as latest Eocene or earliest Oligocene (Macphail et al., 1993). In
summary, the synchronous commencement of biogenic carbonate de-
position appears to reflect major tectonic, climatic, and oceanographic
changes that affected broad regions in the Southern Ocean near Tasma-
nia. At the eastern Leg 189 sites, these changes created a more dynamic,
well-ventilated ocean with increased upwelling and higher surface wa-
ter biogenic productivity, which increased rates of sedimentation of cal-
careous nannofossils and diatoms and decreased preservation of

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 15
organic carbon. Open-ocean planktonic diatoms replaced neritic dia-
toms, reflecting this deepening and also suggesting initiation of limited
coastal upwelling on the STR and Tasmania during the earliest Oli-
gocene (Stickley et al., this volume). Furthermore, associated cooling
of the Antarctic and Australian continents apparently decreased weath-
ering rates and transport of siliciclastic sediments to the margins. In ad-
dition, subsidence rapidly reduced land areas, which also became more
remote from the depocenters on the continental margins, dramatically
decreasing the transport of siliciclastic sediment to those depocenters.
The environment of deposition was thus transformed from the late
Eocene to the earliest Oligocene from siliciclastic to deep-sea carbonate
sediments. At the relatively nearshore western Site 1168 there was a
long transition, extending from the earliest Oligocene until the early
Miocene.
OLIGOCENE AND YOUNGER HISTORY:
SUBSIDENCE AND FLIGHT NORTHWARD
Oligocene
During the Oligocene, Antarctica and the South Tasman Rise sepa-
rated further (Fig. F7C). By the late Oligocene, the ACC was well estab-
lished at all water depths south of the STR and currents also moved
through the South Tasman Saddle between Tasmania and the STR.
Much of the early Oligocene (~33–30 Ma) at all sites except Site 1168 is
represented by a hiatus considered to be equivalent to the regional Mar-
shall Paraconformity (Pfuhl and McCave, this volume) and to have
been caused by initiation of the ACC. At Site 1168, farther to the north
and to the west of Tasmania, the interval usually represented by the
Marshall Paraconformity is represented only by an interval of reduced
sedimentation rates (Pfuhl and McCave, this volume). In spite of Oli-
gocene cooling, conditions remained temperate in the vicinity of Tas-
mania and the South Tasman Rise. By this time the development of the
proto-ACC prevented a countercurrent like that of the late Eocene from
flowing northward across the South Tasman Rise. As a result, the warm
East Australia Current began to influence the Tasmanian region.
Relatively thin deepwater Oligocene chalks at Sites 1170–1172,
where current action greatly compressed the section, grade westward
into the thick (~300 m) marly Oligocene sequence at Site 1168. A brief
hiatus seems to be present at the abrupt Eocene/Oligocene lithologic
boundary at Sites 1170–1172 (Shipboard Scientific Party, 2001c, 2001d,
2001e; Fuller and Touchard, in press), although it can be clearly dated
only at Site 1172 (Stickley et al., submitted [N1]). At the current-swept
southern sites (1170 and 1171) the late Oligocene unconformity, com-
mon in much of the Southern Ocean, is well developed. Overall, the as-
semblages suggest well-ventilated cool temperate conditions and
bathyal water depths at Sites 1170–1172. At Site 1172, the Oligocene se-
quence is ~20 m of pale foraminifer-bearing nannofossil chalk with
some thin, greenish glass-bearing mudstone horizons (Shipboard Scien-
tific Party, 2001e). Nannofossils and planktonic foraminifers dominate,
but palynomorphs are largely absent. Diatoms and nannofossils (and
dinocysts present in one sample) indicate relatively warm well-venti-
lated conditions and bathyal water depths. CaCO3 increases upcore
from 55 to 85 wt%, with a rapid decline in siliciclastic debris, siliceous
organisms, and organic walled palynomorphs as open-ocean and oxi-
C
E A C
A C C
40S
130 E
50S
120E
80S
80S
70S
60 S
160 E
170 E
170W
160 W
150 W
140 W
140 E
180
F7C. Early Oligocene situation,
p. 35.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 16
dizing conditions were established. At Site 1171, the Oligocene se-
quence is only ~10 m thick, and at Site 1170 it is ~60 m thick. At both
sites it consists of pale foraminifer-bearing nannofossil chalk.
At Site 1168, the Oligocene sequence is represented by ~310 m of
multicolored calcareous mudstone (Shipboard Scientific Party, 2001b).
The ~40-m-thick lower Oligocene sequence consists of varicolored silty
claystone, clayey siltstone, and sandy claystone with <20 wt% CaCO3.
The ~270-m-thick upper Oligocene sequence is more calcareous (<40
wt% CaCO3). Robert (in press) shows that quartz, clay, and biogenic cal-
cite are roughly subequal; illite again predominates over kaolinite, indi-
cating reduction of relief but ongoing intense weathering. Calcareous
nannofossils and planktonic foraminifers dominate, and molluskan
fragments are present. Benthic foraminifers increased in diversity be-
cause oxygenation increased while the water depth deepened from ner-
itic to upper bathyal. Dinocysts increasingly dominated over spores and
pollen as the sea level rose and distance from land increased. Condi-
tions were cool to warm temperate. This was a quiet, restricted, rela-
tively oxygen poor environment. In contrast to the other sites, there is
no abrupt lithologic change at the Eocene/Oligocene boundary, but
rather a steady increase in water depth, a steady decrease in sand frac-
tion (mainly quartz), and an increase in carbonate dominated by calcar-
eous nannofossils through the Oligocene.
Comparison with the Antarctic Margin
Oligocene carbonates are common in the Tasmanian region because
of the interplay of tectonics, climate, and oceanography. The sequences
at the southerly Sites 1170 and 1171 have a markedly different Eocene–
Oligocene sediment transition compared with nearby parts of Antarc-
tica. We do not know of Antarctic margin sectors that experienced pe-
lagic carbonate deposition in the earliest Oligocene. The Antarctic
margin was marked by deposition of biosiliceous sediments or more
slowly accumulating siliciclastic sediments with an increased siliceous
biogenic component. Why did the environment near the Tasmanian
margin apparently favor biogenic carbonate preservation and relatively
low biosiliceous productivity? Here, even Eocene siliciclastic sediments
generally contain a better record of better preserved calcareous nanno-
fossils and foraminifers than elsewhere.
These observations suggest that different climatic regimes existed
near the Tasmanian and Antarctic margins during the Eocene and Oli-
gocene. The earliest Oligocene was a time of major cryospheric expan-
sion in the southern Indian Ocean sector and in the southern Ross Sea.
But in the Tasmanian region, biogeographic evidence from calcareous
nannofossils, as well as lack of any evidence for glaciation, indicate that
conditions were slightly warmer than elsewhere, even during the Oli-
gocene.
Why are carbonates preserved off Tasmania during the Oligocene but
not on the nearby Antarctic margin? We hypothesize that warmer sur-
face waters were carried southward from the subtropics, along the east-
ern margin of Australia by the East Australian Current, and southward
around western Australia into the Australo-Antarctic Gulf. The begin-
ning of constriction of the Indonesian Seaway in the Oligocene (Hall,
1996) would have increased southward flow of warm waters along the
east Australian margin. These subtropical waters would have been rela-
tively saline and thus would have helped promote production of deep
waters. Hence, this sector of the margin may have operated in an anti-

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 17
estuarine mode (Berger et al., 1996), marked by downward flux of deep
waters and inward flow of surface waters, as in the modern North Atlan-
tic. In this case, upwelling of nutrient-rich waters is diminished and car-
bonate accumulation is favored over biosiliceous accumulation.
The Antarctic margin was already separated from warm waters by the
onset of the ACC. There was strong carbonate dissolution at shallow
water depths and high biosiliceous accumulation, and the margin may
have operated in estuarine mode, marked by upwelling of nutrient-rich
deep waters and outflow of surface waters. There, carbonate dissolution
is favored by the upwelling of old, deep, low-alkalinity, high-pCO2 wa-
ters like those in the modern North Pacific Ocean.
A major strengthening of oceanic thermohaline circulation occurred
at the climatic threshold of the Eocene–Oligocene transition. This re-
sulted largely from the major cooling and cryospheric development of
Antarctica (Kennett and Shackleton, 1976). This cooling, in turn, led to
increased onshore aridity and a major reduction of freshwater flow to
the surrounding continental margin, which is reflected by the marked
reduction in transport of siliciclastic sediments to the Tasmanian mar-
gin. Surface waters near the margin would have increased in salinity. A
major positive feedback almost certainly would have resulted, with fur-
ther strengthening of bottom water production and expansion of the
oceanic psychrosphere (deep-ocean circulation). Thus, the delivery of
high-salinity surface waters to the Tasmanian margin, caused by its
plate tectonic setting, may well have enhanced bottom water produc-
tion and, in turn, increased carbonate biogenic accumulation.
Neogene History
Neogene sedimentation at Leg 189 sites on the STR and the Tasma-
nian margin was completely dominated by nannofossil oozes with a
significant foraminiferal component. Pelagic carbonate sedimentation
was largely continuous, except during the late Miocene and earliest
Pliocene, at a number of sites. Miocene deepwater calcareous ooze is
thickest at Site 1168 (~300 m) and thinnest at Site 1171 (~170 m).
Pliocene oozes are remarkably consistent in thickness (~70 m) at all
sites. Pleistocene oozes are thickest at the deepwater STR Site 1170 and
thinnest at the shallower, current-swept STR Site 1171. The Miocene–
Pliocene transition is missing at Sites 1169 and 1171, and the lower up-
per Miocene is missing at Site 1168. Otherwise, the lower and upper
Miocene and the Pliocene to Quaternary appear to be largely complete
sequences. The uppermost Miocene hiatus may have resulted from in-
creased thermohaline circulation associated with Antarctic cryosphere
expansion at that time (Hodell et al., 1986). Altogether, the Neogene
sediments cored during Leg 189 provide a fine suite of sequences depos-
ited in cool temperate and subantarctic water masses of the Southern
Ocean. These represent a treasure chest for high-resolution Neogene pa-
leoclimatic and biostratigraphic investigations of the Southern Ocean.
The Neogene carbonates exhibit changes that record changing environ-
mental conditions in response to the northward movement of the STR,
Tasmania, and the ETP from Antarctica and shifting positions of the
Subtropical Convergence and the Subantarctic Front. The pelagic car-
bonates accumulated at relatively low rates (~1–2 cm/k.y.) typical of the
open ocean. Relatively low diversity benthic foraminiferal assemblages
indicate deposition in abyssal depths under generally well ventilated
conditions characteristic of the Antarctic Circumpolar Current region.
Other than a small, pervasive clay fraction, siliciclastic sediments are

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 18
absent throughout the Neogene, except in the lower Neogene of Site
1168, which is the site closest to a present land mass. Nannofossil oozes
are conspicuously pure white on the STR in the lower Neogene, which
corresponds to a period when the STR was well clear of the siliciclastic
influences of Antarctica and yet had not come under the late Neogene
influence of increasing aridification and associated dustiness of Austra-
lia. Diatoms are present consistently throughout the Neogene carbon-
ates but exhibit a distinct increase in abundance and diversity after the
middle Miocene. This almost certainly reflects an increase in upwelling
within the Southern Ocean at that time in response to the well-known
expansion of the Antarctic cryosphere. A marked increase in carbonate
ooze deposition during the early Pliocene at Sites 1169 and 1170, on
the southwestern STR, is not observed at other sites, suggesting local
concentrations of calcareous nannofossils rather than any regional
trend like that in the southwest Pacific (Kennett and von der Borch,
1986). During the latest Neogene, planktonic foraminifers become
much more important relative to calcareous nannofossils. This may re-
flect increased winnowing by deep currents and/or a decrease in relative
production of calcareous nannofossils compared to planktonic fora-
minifers.
Postcruise investigations of Leg 189 Neogene sequences are leading
to a significant increase in understanding of paleoclimatic and pale-
oceanographic history of the Southern Ocean. Upper Neogene sections
have been satisfactorily spliced from multiple cores from four of the
sites (Sites 1168 and 1170–1172) to provide essentially continuous
paleoclimatic records. Pervasive sedimentary cycles are apparent
throughout the entire Neogene, based on observations of the sediment
record and changes in the physical properties of the sediments. Investi-
gations of clay assemblages suggest relatively warmer conditions during
the early Neogene until ~15 Ma. After that, clay assemblages show in-
creases in chlorite, illite, and/or kaolinite, suggesting general regional
cooling and Antarctic glacial expansion (Robert, this volume).
Sequences cored during Leg 189 have provided stable isotopic
records with the highest chronologic resolution so far of the early Mio-
cene (Ennyu and Arthur, this volume, in press) and the middle Mio-
cene (Shevenell and Kennett, in press) from the Southern Ocean. These
well-dated stable isotopic records clearly exhibit the well-known major
oxygen isotopic shift of the middle Miocene at ~14 Ma as well as re-
gional ocean circulation changes (at depths >1500 m) commensurate
with the middle Miocene global climate transition (16.8–12 Ma). Re-
gional oxygen and carbon isotopic trends have been considered to sup-
port hypotheses relating middle Miocene cooling and Antarctic
cryosphere expansion to reorganization of ocean circulation and re-
lated changes in meridional heat flux (Shevenell and Kennett, in press).
Kelly and Elkins-Tanton (in press) describe an occurrence in a single
sample of microtektites from the upper middle Miocene–lowermost
Pliocene of Site 1169. Although precise biostratigraphic dating of depo-
sition is not possible, by using major element composition they at-
tribute the origin of the microtektites to the HNa Australite field
considered to be of late Miocene age (~10.2 Ma) (Bottomley and Koe-
berl, 1999).
During the late Neogene, clays become increasingly important in the
pelagic carbonates, in part because of increasing dust transport from
Australia. A distinct influx of kaolinite at several sites, including the
southern STR, during the late Pliocene and Quaternary probably reflects
increasing southeastward wind transport of relict clays from an increas-

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 19
ingly arid Australia. The uppermost Neogene sediments at Site 1172,
which is downwind from Tasmania, exhibit distinct cycles in clay abun-
dance in the biogenic oozes, almost certainly in response to glacial–
interglacial oscillations in Australian continental aridity. This increase
in aridity was almost certainly linked with a cooling trend during the
late Neogene. The disappearance in the Tasmanian region of large ge-
phyrocapsids above the middle Pleistocene (Stant et al., this volume) is
consistent with this long-term trend toward cooler conditions. Stant et
al. (this volume) also showed from the distribution of warm-loving dis-
coasters in Pliocene sediments the presence of a subtropical watermass
front (~45°S), south of which the discoasters did not extend during the
late Pliocene. Furthermore, their distribution indicates the stronger in-
fluence of the East Australia Current compared with the Leeuwin Cur-
rent in transporting subtropical waters to the region during the late
Pliocene.
The Leg 189 sites also provided opportunities for studies of Quater-
nary paleoclimatology. Nürnberg et al. (in press) used geochemical
proxy data from four Leg 189 sites to reconstruct the regional history of
glacial and interglacial changes near the subtropical convergence in the
last 500 k.y. There is a complex story of variations in paleoexport pro-
duction, terrigenous flux, sea-surface temperature, and movements of
water masses and oceanographic fronts through time. Each glacial pe-
riod and each interglacial period was different from the others; how-
ever, the authors did find that interglacial periods were commonly
times of lower productivity and that their deposits contained less terrig-
enous matter than those of glacial periods, indicating that the subtropi-
cal convergence was south of most sites during most interglacials.
Malone et al. (2004) used a diffusion-advection model to calculate
the glacial–interglacial change in bottom water d18O from pore water
oxygen isotopic profiles at Sites 1168 and 1170. The results indicate
that Circumpolar Deep Water temperatures were –0.2°C (Site 1170) and
–0.5°C (Site 1168) at the Last Glacial Maximum. Since the last glacial
maximum, d18O changed by 1.0‰–1.1‰ (±0.15‰) and bottom water
temperatures increased by ~1.9° and ~2.6°C, respectively, at the sites.
DISCUSSION AND CONCLUSIONS
As noted in Exon, Kennett, Malone, et al. (2001), the Leg 189 drill
sites, in 2463–3568 m water depths, have tested, refined, and extended
the hypothesis that climatic cooling and an Antarctic ice sheet (cryo-
sphere) developed in late Eocene to early Oligocene times, as the ACC
progressively isolated Antarctica thermally (Kennett et al., 1975). This
has led to improved understanding of Southern Ocean evolution and
its relationship to Antarctic climatic development. The relatively shal-
low region off Tasmania is one of the few locations where well-
preserved and almost complete marine Cenozoic sequences can be
drilled at present-day latitudes of 40°–50°S and paleolatitudes of up to
70°S. The Oligocene and younger sequences are carbonate rich and not
deeply buried and are hence suitable for stable isotopic investigations.
The broad geological history of all the sites is comparable, although
there are important pre-Miocene differences between Site 1168 in the
AAG and the sites in the Pacific Ocean, as well as from north to south.
The drill sites are on submerged continental blocks extending to 600
km south of Tasmania. These blocks were at polar latitudes in the Late
Cretaceous when Australia and Antarctica were still united, although

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 20
rifts had developed associated with commencement of slow separation
and northward movement of Australia. In all, 4539 m of core was re-
covered with average recovery of 89%. The deepest core hole penetrated
960 m beneath the seafloor. The entire sedimentary sequence cored is
marine and contains varied microfossil assemblages that record condi-
tions from the Late Cretaceous (Maastrichtian) to the late Quaternary.
Until the earliest Oligocene, terrestrially derived siliciclastic sediments
predominate at all sites. During the Oligocene there was an abrupt
change to pelagic carbonate deposition at the eastern sites, but silici-
clastic debris remained important into the early Miocene at Site 1168.
The record in the cores indicates that the Tasmanian land bridge al-
most completely blocked the eastern end of the widening AAG during
both the slow-spreading phase and the fast-spreading phase (starting at
~43 Ma) until the late Eocene. Drilling evidence, especially that from
clay minerals, and other geological and geophysical evidence points to
a number of tectonic events during the Cenozoic in the Tasmanian re-
gion:
1. Paleocene north–south strike-slip movement within the STR, ter-
minated at ~55 Ma by seafloor spreading south of STR;
2. Uplift and erosion on the Tasmanian margins near the Paleo-
cene/Eocene boundary;
3. Termination of northwest–southeast strike-slip movement west
of Tasmania when fast spreading began at ~43 Ma;
4. Eocene (post ~43 Ma) north–south strike-slip movement along
the western boundary between the STR and Antarctica, terminat-
ing in the latest Eocene at ~33.5 Ma; and
5. Early Oligocene subsidence of the STR and collapse of the conti-
nental margin around Tasmania.
The early Oligocene subsidence and collapse also occurred in the Vic-
toria Land Basin east of the rising Transantarctic Mountains (Cape Rob-
erts Science Team, 2000) and along the Otway coast on mainland Aus-
tralia, northwest of Tasmania.
Prior to the late Eocene, marine siliciclastic sediments (largely mud-
stones at Leg 189 sites) were deposited in a temperate sea on broad,
shallow, tranquil shelves. There was little or no circulation of marine
waters between the AAG and the Pacific Ocean. Sediment supply kept
up with subsidence despite the rifting, drifting, and compaction during
largely deltaic deposition. Dinocysts, spores, and pollen are ever
present. Reducing conditions in the often organic-rich sediments
helped ensure that especially calcareous and, to some extent siliceous,
microfossils were preserved only sporadically. The spores and pollen in-
dicate that this part of Antarctica was temperate (with little ice) during
this time and supported rain forests with southern beeches and ferns—
part of the Late Cretaceous to Eocene “Greenhouse” world. During the
late Eocene the sequences still document marked differences between
east and west, when the eastern AAG was warmer and more poorly ven-
tilated than the gradually widening Pacific Ocean. Hence, marine circu-
lation across the former land bridge must still have been very limited.
Microfossil biogeography suggests that the east Tasmanian region (Sites
1170–1172) was influenced by a northwestward-flowing cool counter-
current during the Eocene (Fig. F7A). This circulation pattern may well
have been in operation from the Maastrichtian, when the proto-Pacific
Ocean first existed east of Australia and north of Antarctica (Cande and
Stock, in press), until the beginning of the Oligocene.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 21
By the late Eocene (37 Ma), the Tasmanian land bridge had largely
separated from Antarctica and the bridge and its broad shelves began to
subside. Surface currents affected the deepening shelves. These swept
the still-shallow offshore areas, and glauconitic siltstones were depos-
ited slowly as condensed sequences. Palynological and diatom evidence
suggest that there were fluctuations in temperature superimposed on a
general cooling and that the amount of upwelling also fluctuated in re-
sponse to the changing oceanic circulation. Calcareous microfossils re-
mained rare. Benthic foraminifers and other evidence indicate that the
sites began to deepen slightly at ~37 Ma, although shelf depths contin-
ued. Final separation of the southwestern tip of the STR from Antarctica
occurred at ~33.5 Ma, leading to profound changes in sediment deposi-
tion, climate, and ocean history (Fig. F7B).
By the earliest Oligocene, bathyal pelagic carbonates were being de-
posited at the eastern sites and marls at Site 1168. The developing Ant-
arctic Circumpolar Current cut off warm currents from the north,
leading to cooling and some ice sheet formation. These events contrib-
uted to global cooling. Conditions were significantly cooler in the Tas-
manian offshore region, and there is no positive evidence of terrestrial
vegetation in the sediments, although vegetation then existed on Tas-
mania. However, almost all organic matter deposited during the early
Oligocene would have been oxidized in well-ventilated waters.
There were several reasons for the change to pelagic carbonate depo-
sition. Much of the land bridge had subsided beneath the ocean, so
there was a smaller hinterland to supply sediment. Furthermore, the
colder ocean provided less moisture and, hence, decreasing precipita-
tion and erosion. Therefore, far less siliciclastic sediment was trans-
ported from the land. The reduced flow of detrital organic matter ended
the earlier reducing conditions in the sediments and ensured that cal-
careous organisms were now preserved. Generally slow deposition of
deepwater pelagic sediments was initiated. Currents from the north
kept the Tasmanian region relatively warm, supporting carbonate depo-
sition rather than the siliceous biogenic deposition that marks much of
the Antarctic margin. In the Tasmanian region, and even in the Cape
Adare region on the conjugate Antarctic margin, there is no sign of
widespread glaciation during the early Oligocene.
The Drake Passage probably opened in the Oligocene, and the Tasma-
nian Seaway continued to open. In the late Oligocene (Fig. F7C) and
Neogene, the ACC strengthened and widened, strongly isolating Ant-
arctica from warm-water influences. At ~15 Ma, the east Antarctic cryo-
sphere evolved into ice sheets comparable to those of the present day.
This intensified global cooling and thermohaline circulation. The “Ice-
house” world had arrived, but temperatures and current activity fluctu-
ated and dissolution and erosion varied over time. The steady
northward movement of the Tasmanian region kept sedimentation
north of the Polar Front, and pelagic carbonate continued to accumu-
late in deep waters at average rates of 1–2 cm/k.y. Australia’s movement
northward into the drier mid-latitudes, along with the global climate
change associated with high-latitude ice sheet expansion, led to mas-
sive aridity in Australia and an increase in windblown dust abundance
at Site 1172 after 5 Ma.
Comparisons with sequences drilled elsewhere on the Antarctic mar-
gin are improving our understanding of these momentous changes in
Earth history and some of the constraints on modern climates. We sug-
gest that if Australia had not broken away from Antarctica and moved
northward, the Earth might not have experienced its Cenozoic ice ages.

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 22
ACKNOWLEDGMENTS
This research used samples and/or data provided by the Ocean Drill-
ing Program (ODP). ODP is sponsored by the U.S National Science
Foundation (NSF) and participating countries under management of
Joint Oceanographic Institutions (JOI), Inc. We recognize and ack-
nowledge with thanks the vitally important input of all the scientists
involved in ODP Leg 189: sedimentologists, biostratigraphers, paleo-
magnetists, physical properties specialists, wireline loggers, and
geochemists. We draw heavily on the results of their labors published in
the “Leg Summary” chapter (Shipboard Scientific Party, 2001a) of the
Initial Reports volume. Lorri Peters of IODP edited the text in detail. Pe-
ter Webb of Ohio State University and especially Alan Cooper of Stan-
ford University are thanked for their thoughtful reviews of the paper,
and John Firth of IODP provided further comments.
Exon publishes with the permission of the CEO of Geoscience Aus-
tralia, and Kennett acknowledges and appreciates funding from JOI/US-
SAC (U.S. Science Advisory Committee) and the National Science
Foundation (Marine Geology and Geophysics).

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 23
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LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 24
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LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 27
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N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 28
Figure F1. Locality map showing Cretaceous–Cenozoic sedimentary basins of southeast Australia, after
Exon et al. (in press a). Stipple indicates depocenters with >1500 m of Cretaceous and Cenozoic sedimen-
tary section. Also shown are ODP and DSDP sites and bathymetry. CS = Cape Sorell No. 1 petroleum explo-
ration well, ETS = East Tasman Saddle, STS = South Tasman Saddle, NB = Ninene Basin. Since Eocene or
Oligocene times, strong easterly currents have swept shallower South Tasman Rise (STR) areas, the southern
Tasmanian margin, and the South Tasman Saddle between Tasmania and the STR, reducing sedimentation
rates.
1171
2000
4000
2000
150 km
Tasman
Basin
3000
4000
1000
Tasman
Basin
3000
4000
3000
281
3000
4000
3000
4000
1000
5000
0
280
146°
142°E150°
South
Indian
Basin
Sorell
Basin
282
Otway
Basin
Bass
Basin
Gippsland
Basin
Victoria
Melbourne
Tasmania
Hobart
CS
1171
1170
1169
CS
1172
1168
DSDP site
282 Cape Sorell No. 1 wellODP site
Depocenter
South Tasman Rise
East
Tasman
Plateau
04-012-1
Thin or no sedimentation,
continental margin
Deepwater
oceanic crust
B
N
E T S
S T
S
38°
S
42°
46°

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 29
Figure F2. Global temperature changes from oxygen isotopes of deep-sea benthic foraminifers (after Crow-
ley and Kim, 1995).
0406080
Change in temperature (°C)
Northern
Hemisphere
ice sheet
begins
Global Temperature Change
Age (Ma)
20
First Antarctic
ice sheets?
Permanent Antarctic
ice sheet develops
Warm
Cool
04-012-2
0
8
6
4
2

N.F. E
XON
ET
AL
.
L
EG
189 S
YNTHESIS
: H
ISTORY
OF
THE
T
ASMANIAN
G
ATEWAY
30
Figure F3. Leg 189 sequences drilled. NGR = natural gamma radiation.
080160
080160
080160
600
700
200
300
400
500
600
700
800
900
Nannofossil ooze/chalk
Foraminifer ooze/chalk Clayey ooze/chalk
Oligocene
Miocene
Pliocene
Pleistocene Pleistocene
Miocene
Oligocene
Pliocene
Eocene
Eocene
Pliocene
Miocene
Eocene
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Cretaceous
Maastrichtian
middle
Eocene
200
300
400
500
600
700
Pleistocene
Glauconitic siltstone
H
H
H
H
H
H
H
late
Paleocene
Paleocene
late
Eocene
Oligocene
04-012-3
NGR (API)
NGR (API)
NGR (API) NGR (API)
Depth (mbsf)
Claystone
Hiatus
H
Siliceous ooze
Silty claystone/clayey siltstone
Western
Tasmania Margin
Site 1168
Western South
Tasman Rise
Site 1170
South
Tasman Rise
Site 1171
East
Tasman Plateau
Site 1172
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
080

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 31
Figure F4. Leg 189 time stratigraphy and sediment facies diagram.
European
Stage
60
55
50
65
45
40
10
15
20
25
30
35
1
2
3
4
0
5
m
e
l
e
Pliocene Pleist.MioceneOligoceneEocenePaleocene
Age (Ma)
Epoch
Site 1169
West South
Tasman Rise
3580 m
47° 03´S, 145° 14´E
Site 1168
Western
Tasmania Margin
2475 m
42° 36´S, 144° 24´E
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
West South
Tasman Rise
2710 m
47° 09´S, 146° 02´E
Site 1170
South
Tasman Rise
2150 m
48° 30´S, 149° 07´E
Site 1171
East
Tasman Plateau
2630 m
43° 58´S, 149° 56´E
Site 1172
m
l
e
m
l
e
l
l
e
e
L
Calabrian
Piacenzian
Zanclean
Messinian
Tortonian
Serravallian
Langhian
Burdigalian
Aquitanian
Chattian
Rupelian
Priabonian
Bartonian
Lutetian
Selandian
Danian
Ypresian
Thanetian
Maastrichtian
Cret.
Cretaceous
70
767 m
04-012-4
884 m
246 m
780 m
959 m
Foraminifer-bearing or siliceous-
bearing nannofossil ooze/chalk
Nannofossil ooze/chalk
Clayey nannofossil ooze-chalk/
nannofossil-bearing clay
Clayey chalk, sandy claystone, organic-bearing
silty claystone/clayey siltstone
Organic-bearing, nannofossil-bearing,
silty claystone/clayey siltstone
Unconformity on seismic
evidence only
Note: Some time breaks probably
occur in the Paleocene-
Eocene sequences and
near the Eocene/Oligocene
boundary
Organic- and glauconite-
bearing silty claystone/clayey siltstone

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 32
Figure F5. Sedimentation rate curves (courtesy of Kristeen McGonigal, after Stickley et al., this volume).
04-012-5
A
g
e
(
Ma
)
Depth (mbsf)
60 70
cm/k.y.
0.5
1.0
2.05.0
Oligocene Eocene Paleocene
late late late earlyearlyearlyearly middle Late
Cret.Plio.
latee.
Pleist.
l.
Miocene
middle
Site 1171
South Tasman Rise
Site 1170
West South
Tasman Rise
Site 1172
East Tasman
Plateau
Site 1168
Western Tasmania
Margin
0
200
400
600
800
1000
40 5020 30010

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 33
Figure F6. Setting of the Tasmanian region within Gondwana during the Late Cretaceous (95 Ma) using the
plate tectonic reconstruction of Royer and Rollet (1997). The figure shows the Tasmanian-Antarctic Shear
Zone (TASZ) and areas of rift sedimentation. W-STR = west South Tasman Rise, E-STR = east South Tasman
Rise, ETP = East Tasman Plateau.
Antarctica
ETP
W-STR
E-STR
?
?
95 Ma
Australia
Site
282
Site
281
Site
1169
Site
1168 Site
1172
Site
1170
60°
S
65°
70°
75°
140° E145° 150° 155°
0 100 200 300
km
Site
1171
95 Ma
TASZ
TASZ
04-012-6
Lord Howe Rise and
Campbell Plateau
Marginal marine rift sediments

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 34
Figure F7. Changes through time in the Tasmanian Gateway region as Australia and Antarctica separated.
The changes contributed to Antarctic glaciation and thermohaline oceanic circulation (TOC). Maps after
Exon et al. (2002) and based on Cande et al. (2000). A. 43.7 Ma (middle Eocene). B. 33 Ma (Eocene/Oli-
gocene boundary. C. 26 Ma (early Oligocene). AAG = Australo-Antarctic Gulf, TLB = Tasmanian land bridge,
PP = proto-Pacific Ocean, EAC = Eastern Australian Current, N.Z. = New Zealand. (Figure shown on next
page.)

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 35
Figure F7 (continued). (Caption shown on previous page.).
AAG fast spreading starts
TLB in place
Shallow marine muds on TLB
AAG very restricted
No AAG-PP connection
EAC warms Antarctic
No TOC
04-012-7
Fast spreading continues
TLB subsiding rapidly
Pelagic carbonates on TLB
No AAG
Southern Ocean developing
Deep AAG-PP connection
EAC not to Antarctica
ACC in full flow
TOC in existence
Antarctic glaciation
C
B
A
C. Early Oligocene
B. Eocene/Oligocene boundary
A. Middle Eocene
Fast spreading continues
TLB subsiding rapidly
Glauconitic silts on TLB
AAG restricted
Shallow AAP-PP connection
EAC not to Antarctica
ACC forming
Global cooling soon after
TOC soon after
Onset of Antarctic glaciation
AAG
E A C
A C C
A A G
E A C
A C C
Proto - Pacific
40S
130 E
50 S
120E
50 S
130 E
120 E
130E
50 S
60 S
70 S
60S
70 S
80S
80 S
80 S
80S
70S
60 S
160 E
170 E
170 W
160 W
150 W
140 W
70W
60 S
150 W
70 S
140 W
160 E
170 E
180
170 W
50 S
60 S
150W
70 S
140 W
120 E
140E
150 E
160E
140E
170 E
180
140 E
180

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 36
Tab l e T1. ODP Leg 189 sites.
Site Location
Water
depth
(m)
Depth
cored
(mbsf)
Tot al
recovered
(m)
Tot al
recovery
(%)
Oldest sediment
cored
189-
1168 42º36.6’S, 144°24.8’E 2464 883.5 1191 93 upper Eocene
1169 47º03.9’S, 145°14.2’E 3568 246.3 225 91 middle Miocene
1170 47º09.0’S, 146°03.0’E 2407 779.8 1027 87 middle Eocene
1171 48º30.0’S, 149°06.7’E 2148 958.8 995 82 upper Paleocene
1172 43º75.6’S, 149°55.7’E 2622 776.5 1100 92 Maastrichtian

N.F. EXON ET AL.
LEG 189 SYNTHESIS: HISTORY OF THE TASMANIAN GATEWAY 37
*Dates reflect file corrections or revisions.
CHAPTER NOTES*
N1. Stickley, C.E., Brinkhuis H., Schellenberg S.A., Sluijs, A., Röhl, U., Fuller, M.,
Grauert, M., Huber, M., Warnaar J., and Williams, G.L., submitted. Timing and
nature of the opening of the Tasmanian Gateway: the Eocene/Oligocene transi-
tion of ODP Leg 189 sites. Paleoceanography.