Opening PAGES

Eos, Vol. 83, No. 2 3 , 4 June 2002

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

VOLUME 83 NUMBER 23 4 JUNE 2002

PAGES 253-264

Drilling Reveals Climatic

Consequences of Tasmanian Gateway Opening

PAGES 2 5 3 , 2 5 8 - 2 5 9

One of the great stories of g e o s c i e n c e is how Gondwana broke up and the other southern continents drifted northward from Antarctica, which led to major c h a n g e s in global climate.

The recent drilling of O c e a n Drilling Project (ODP) Leg 189 addressed in detail what hap­

pened as Australia drifted away from Antarctica and the Tasmanian Gateway o p e n e d . T h e drifting contributed to the c h a n g e in global climate, from relatively warm early Cenozoic

"greenhouse" conditions to late Cenozoic "ice­

house" conditions. It isolated Antarctica from warm gyral surface currents from the north and provided the critical deepwater conduits that eventually led to o c e a n conveyor circula­

tion between the Atlantic and Pacific O c e a n s . Leg 189 continuously c o r e d sediments on foundered continental blocks (Figure 1) between Australia and Antarctica.The cores cover the early slow separation of Australia and Antarctica ( 7 0 - 4 3 Ma), the later fast sepa­

ration while a land bridge still existed ( 4 3 - 3 7 Ma), the initial current breakthrough south of the South Tasman Rise ( 3 7 - 3 3 . 5 M a ) , a n d Australia's independent fast movement north­

ward thereafter.

Early Deep S e a Drilling Project (DSDP) drilling of the Tasmanian Gateway provided a b a s i c framework of paleoenvironmental c h a n g e s associated with the opening. Kennett et al. [1975] proposed that climatic cooling and an Antarctic i c e sheet developed as the Antarctic Circumpolar Current (ACC) progres­

sively isolated Antarctica thermally. Its onset c o i n c i d e d with the onset of global cooling near the E o c e n e - O l i g o c e n e boundary at - 3 3 . 5 Ma [Kennett, 1977; Miller et al., 1 9 8 7 ] .

Leg 189 cores reflect the evolution of a tightly- integrated and dynamically-evolving system that involves lithosphere, hydrosphere, atmos­

phere, cryosphere, and biosphere.The relatively shallow water depths throughout the Cenozoic make this o n e of the few places where almost c o m p l e t e marine s e q u e n c e s , containing well- preserved skeletons of c a l c a r e o u s plankton, could b e c o r e d in the Southern O c e a n . Dating of the c o r e s depends largely on

c a l c a r e o u s and siliceous plankton and dinocysts. Environmental interpretation depends on such plankton, benthic foraminifers, dinocysts, spores and pollen, and the

sediments in which they are e n t o m b e d . Exon et al. [2001] fully d o c u m e n t the early results and the conclusions drawn from them.

D r i l l i n g

The drill sites were at 6 5 - 7 0 ° S until 33.5 Ma, and are on continental blocks that formed the Tasmanian land bridge until then.The bridge essentially closed the eastern end of the widening Australo-Antarctic Gulf (AAG).

Later, the sites drifted northward with Australia to their present latitudes of 4 2 - 4 8 ° S . T h e

I ODP Leg 189 sites DSDP site

Fig. 1. Extensive swath-bathymetry was used to map the region offshore Tasmania. Leg 189 sites are solid circles and Deep Sea Drilling Project sites are solid squares. Contours are in meters.

Eos,Vol. 83, No. 2 3 , 4 June 2002

European Stage

Western West South West South Tasmania Margin Tasman Rise Tasman Rise

2475 m 3580 m 2 7 1 0 m 42°36'S, 144°24'E 47°03'S, 145°14'E 47°09'S, 146°02'E

Site 1168 Site 1169 Site 1170

South East Tasman Rise Tasman F

2150 m 2630 m 48°30'S, 149°07'E 43°58'S, 149°56'E

Site 1171 Site 1172

i

Serravaliian Lanq.hian

Aquitanian

L Maastrichtian

Pleistocene [

Pliocene

Oligocene

Eocene

EBB

• a

nannofossil bearing clay

Clayey chalk, sandy claystone, organic bearing silty claystone/clayey siltstone

Organic and glauconite bearing silty claystone/clayey siltstone I Organic bearing, nannofossil bearing, I silty claystone/clayey siltstone

23/0A/1178 Unconformity on seismic evidence only Note: Some time breaks probably

occur in the Paleocene- Eocene sequences and near Eocene-Oligocene boundary

Fig. 2. This summary describes sediments deposited through time for all sites drilled during Leg 189, arranged from west (left) to east (right). Original color image appears at the back of this volume.

sedimentary s e q u e n c e is entirely marine, with major terrigenous siliciclastic input until the earliest Oligocene (Figure 2 ) , and it contains a wealth of micro-fossil assemblages that record conditions from the Late Cretaceous (70 Ma) until now. Drilling recovered 4 5 3 9 m of c o r e at five drill sites in water depths of 2 4 7 5 - 3 5 7 9 m, and core recovery was generally good despite Southern O c e a n storms.

The broad geological history of all five sites was comparable, with deposition of terrigenous muds in shallow marine deltas until 37 Ma, glauconitic terrigenous silts on a subsiding continental shelf until 33.5 Ma, and pelagic calcareous clays and oozes in deep water thereafter (Figure 2 ) . W h e n the Tasmanian Gateway first o p e n e d in the late E o c e n e , east­

erly flowing currents winnowed the bottom sediments on the shallow blocks and formed c o n d e n s e d s e q u e n c e s . Although sedimenta­

tion was continuous for long periods, appre­

ciable time breaks probably exist in the deltaic and shelf sediments and certainly exist in the deepwater sediments at most sites.

Three sites were west of the Tasmanian land bridge until the latest E o c e n e , and h e n c e in the AAG: Site 1168 on the west Tasmanian margin, and Sites 1169 and 1170 on the west­

ern South Tasman Rise (STR).The other two sites were always in the Pacific O c e a n and were separated from the AAG by the Tasman­

ian land bridge until the Oligocene. In Site 1171 on the southernmost STR,time breaks in the Oligocene and the late M i o c e n e c a n b e

related to the ACC, which broke through nearby Site 1172, on the East Tasman Plateau (ETP) and farther from the ACC, recovered an almost- c o m p l e t e stratigraphic section from the Late Cretaceous onward.

Two transitional s e q u e n c e s of global signifi­

c a n c e were fully cored, the Cretaceous-Tertiary boundary ( 6 5 Ma) and the E o c e n e - O l i g o c e n e boundary (33.5 M a ) ; and detailed study is underway. Site 1172 contains ODP's most southerly Cretaceous-Tertiary boundary.The boundary lies immediately above a 60-cm- thick sandy mudstone of high magnetic sus­

ceptibility in a thick, massive gray mudstone of latest Cretaceous age. The sandy mudstone is presumed to represent debris related to the Yucatan Peninsula asteroid impact, which caused the demise of the dinosaurs and many other creatures. About 4 0 c m above the mag­

netically susceptible sequence, the core changes to brown, highly bioturbated mudstone of ear­

liest C e n o z o i c age.

The E o c e n e - O l i g o c e n e boundary was c o r e d in all four d e e p sites and has no paleontologi- cally recognizable time break across it, although a depositional break may well exist in the three eastern sites.There is an upward transi­

tion over a few meters from gray late E o c e n e mudstones to greenish latest E o c e n e glauconitic siltstones, and finally to white early O l i g o c e n e chalks. The E o c e n e terrigenous sediments were laid down in water depths increasing from 100 m to 5 0 0 m , a n d under strengthening easterly flowing currents, as the Tasmanian

land bridge separated from Antarctica.

O l i g o c e n e chalks were deposited in rapidly deepening water as the region subsided to water depths e x c e e d i n g 1000 m and was cut off from terrigenous sediment sources.

T e c t o n i c s D r o v e t h e C h a n g e s

The t e c t o n i c separation of the Tasmanian region from Antarctica after 33.5 Ma, and its drift northward to its present latitude, allowed the ACC to form.This isolated Antarctica from warm northern water, and an i c e c a p formed.

Three important C e n o z o i c t e c t o n i c events c a n now b e identified by c o m b i n i n g Leg 189 results with other geophysical and geological data: P a l e o c e n e strike-slip movement on the southeast STR, which was e n d e d at 55 Ma by seafloor spreading to the south; E o c e n e conti­

nental strike-slip movement along the western boundary between STR and Antarctica, which terminated in the latest E o c e n e around 34 Ma; and early Oligocene c o l l a p s e of the conti­

nental margin around Tasmania after 33.5 Ma.

Early O l i g o c e n e s u b s i d e n c e and collapse also o c c u r r e d in the Victoria Land Basin east of the rising Transantarctic Mountains [Cape Roberts Science Team, 2000] and along the coast of southeast Australia.

Sedimentation rates d e p e n d e d on local tec­

tonics, which helped control distance from on-land sources, and also on sedimentation, patterns, bypassing, and current erosion.

Apatite fission track dating [OSullivan and Kohn, 1997] suggests t e c t o n i c uplift and ero­

sion in west and east Tasmania during the late P a l e o c e n e to early E o c e n e . On the west Tasmanian margin, Site 1168 received sediment continuously from Tasmania, and sedimenta­

tion rates were moderate and fairly constant, with little change as terrigenous sedimentation gave way to dominantly c a r b o n a t e sedimenta­

tion at 33.5 Ma. On STR, Sites 1170 and 1171 were part of the tectonically-active borderland between Antarctica and Australia in the P a l e o c e n e and E o c e n e , but they were isolated from major land masses thereafter, with the local hinterland sinking and dimin­

ishing in size.Thus, sedimentation was relatively fast during terrigenous deposition in the late E o c e n e and slow thereafter. On the isolated ETP sedimentation at Site 1172 was generally slow from the Maastrichtian onward for both terrigenous and c a r b o n a t e sedimentation, suggesting that the terrigenous s o u r c e was always relatively small or distant.

The onset of fast spreading at 4 3 Ma led to increased subsidence, a n d by the late E o c e n e all the sites were swept by the devel­

oping ACC and sedimentation rates were low.

Final separation of the STR and Antarctica was associated with more rapid s u b s i d e n c e , full current flow of the ACC, and slow pelagic c a r b o n a t e deposition.

P a l e o e n v i r o n m e n t s b e f o r e t h e G a t e w a y O p e n e d

(70-37

From the latest Cretaceous until the late E o c e n e , as illustrated in Figure 3a, Australia was joined to Antarctica with the long, narrow,

Eos,Vol. 83, No. 2 3 , 4 June 2002

C Legend Warm

Drifting

Fast spreading continues TLB subsiding rapidly Pelagic carbonates on TLB AAG not restricted Southern Ocean developing Deep AAG-PP connection EAC not to Antarctica ACC in full flow TOC in existence Antarctic glaciation

Separation

Fast spreading continues TLB subsiding rapidly Glauconitic silts on TLB AAG restricted

Shallow AAP-PP connection EAC not to Antarctica ACC develops soon after Global cooling soon after TOC soon after

Onset of Antarctic glaciation

Rifting

Fast spreading starts TLB in place

Shallow marine muds on TLB AAG very restricted No AAG-PP connection EAC warms Antarctic No TOC

Fig. 3. As Australia and Antarctica separated, changes through time led to Antarctic glaciation and thermohaline oceanic circulation. AAG = Australo-Antarctic Gulf; ACC=Antarctic Circumpolar Current;

EAC = Eastern Australian Current; PP = Proto-Pacific Ocean; TLB = Tasmanian Land Bridge; and TOC

= thermohaline oceanic circulation. Original color image appears at the back of this volume.

and shallow AAG separated from the proto- Pacific O c e a n by a land bridge. A warm cur­

rent bathed eastern Australia and Antarctica,

and a weak, warm current circulated in the AAG. Marine siliciclastic sediments, largely silty clays, were deposited in a warm s e a on

broad, shallow, tranquil shelves. S e d i m e n t supply to deltas kept up with the rapid subsi­

d e n c e related to rifting, with average deposi- tional rates of 5 - 1 0 c m / k y

Marine c a l c a r e o u s and siliceous micro- fossils were preserved only sporadically in the generally poorly-oxygenated sediments, but dinocysts, spores, and pollen were always preserved.The spores and pollen show that this part of Antarctica was relatively warm with little i c e throughout this time, and it supported temperate rain forests with southern b e e c h e s and ferns—part of the

"greenhouse" world. Differences in the mud- stones indicate that the eastern AAG was more poorly ventilated than the gradually widening Tasman S e a with its western bound­

ary current, the East Australian Current (EAC).

There were also differences from north to south related to distance from the opening gateway and from major land masses.

P a l e o e n v i r o n m e n t s a s t h e G a t e w a y O p e n e d ( 3 7 - 3 3 . 5 M a )

In the late E o c e n e ( 3 7 - 3 3 . 5 Ma), as illustrated in Figure 3b, fast sea-floor spreading was mov­

ing Australia rapidly northward away from Antarctica. The AAG was widening and deep­

ening, and the Tasmanian land bridge and its broad shelves started to subside. Warm, shal­

low currents no longer r e a c h e d the Antarctic margin, and c o o l shallow currents penetrated the newly-formed Tasmanian Gateway from the west and started to circulate around Antarctica, providing positive f e e d b a c k for further cooling.These currents swept the still-shallow offshore areas s o that clays were no longer deposited, and glauconitic silts were deposited very slowly as c o n d e n s e d s e q u e n c e s (<1 c m / k y ) . Palynological and other e v i d e n c e suggests that c o n s i d e r a b l e fluctuations in temperature were superimposed on general cooling, and the a m o u n t of upwelling also fluctuated. Calcareous micro- fossils are rare, but diatoms and foraminifers indicate that there was minor deepening in the latest E o c e n e at s o m e sites.

The most c o n s p i c u o u s c h a n g e s of the entire 70-million-year history of this region o c c u r r e d over the E o c e n e - O l i g o c e n e transi­

tion, w h e n Australia and A n t a r c t i c a finally s e p a r a t e d . T h e c h a n g e s were from w a r m to c o o l climate; from shallow to d e e p water deposition; from poorly-ventilated b a s i n s to well-ventilated o p e n s e a ; from dark silici­

clastic to light pelagic c a r b o n a t e deposition;

from micro-fossil a s s e m b l a g e s d o m i n a t e d by dinoflagellates to those d o m i n a t e d by c a l c a r e o u s pelagic microfossils; a n d from organic-rich to organic-poor s e d i m e n t a t i o n .

P a l e o e n v i r o n m e n t s

a f t e r S e p a r a t i o n (33.5 M a O n w a r d s )

From the early Oligocene onwards, everything c h a n g e d . As illustrated in Figure 3c, Australia was separating rapidly from Antarctica, with o p e n o c e a n between.Warm currents from the tropics were completely cut off from s o m e parts of Antarctica by the developing ACC,

Eos, Vol. 83, No. 2 3 , 4 June 2002

now with both shallow and d e e p circulation, leading to global cooling and s o m e formation of ice sheets. However, the warm EAC, and warm waters flowing down the western Australian margin and into the Southern Ocean, kept the Tasmanian region relatively warm, resulting in c a r b o n a t e deposition rather than the deposi­

tion of siliceous diatoms that marks much of the Antarctic margin.

In the cores from the offshore Tasmanian region, and even in the Cape Adare region on the conjugate Antarctic margin [Hayes et al, 1975],there is no sign of early Oligocene glacia­

tion despite the presence of mountain glaciers on Tasmania. Although the cores contain no evidence of land vegetation, plant material may have b e e n deposited and then oxidized.

Despite the northward movement of the Tasmanian region, the micro-fossils and clay minerals indicate that the Oligocene and Neo- gene were cooler than the E o c e n e . Much of the land bridge had subsided beneath the o c e a n , so there was a smaller hinterland to supply sed­

iment. Furthermore, the colder o c e a n provided less moisture, decreasing precipitation and ero­

sion. As a result, far less siliciclastic sediment was eroded and transported from the land, and generally slow deposition of deep-water pelagic sediments (~1 cm/ky) set in on the starved Antarctic and Tasmanian margins.

The Tasmanian Gateway continued to open, strengthening and widening the ACC, and fur­

ther isolating Antarctica from warm water. I c e sheets like the present sheets covered East Antarctica from about 15 Ma [Kennett, 1 9 7 7 ] . This intensified global cooling and thermoha­

line circulation, and the "icehouse" world had arrived. However, temperatures and current activity fluctuated, and dissolution and erosion varied over time. T h e movement of the Tasmanian region northward kept it north of the o c e a n i c Polar Front, and pelagic c a r b o n a t e continued to a c c u m u l a t e slowly in d e e p waters. In the late Neogene, Australia was progressively moving into the drier mid- latitudes. Along with global climate c h a n g e

associated with i c e s h e e t expansion in high latitudes, this led to massive aridity on Australia, and in s o m e Leg 189 s e q u e n c e s there is a c o n c o m i t a n t increase in dust blown from Australia after 5 Ma.

F u t u r e S t u d i e s

Work is continuing a p a c e on the material from Leg 189 to build o n the early results recorded in Exon et al [ 2 0 0 1 ] . Comparisons with s e q u e n c e s drilled elsewhere on the Antarctic margin—for example, ODP Leg 188 in Prydz Bay [O'Brien et al,2001; Cape Roberts Science 7eaA77,2000]—will further improve our understanding of m o m e n t o u s changes in the Earth's history and s o m e of the constraints on m o d e r n climates. Post- cruise studies and c o m p a r i s o n s will better define and explain regional similarities and differences in tectonism, sedimentation, and climate. Initial studies of physical properties, wireline logs, and micro-fossils all show that climatic cycles of varying length are present throughout the entire s e q u e n c e , and the ongo­

ing studies will better define Milankovitch and other cycles. In the N e o g e n e pelagic carbon­

ates, the excellent preservation and high depositional rates will allow detailed isotope studies to determine surface and bottom water temperatures through time. We c a n now build unique Southern O c e a n correlations between various micro-fossil groups: c a l c a r e o u s nanno- fossils, planktonic foraminifers, diatoms and radiolarians, and dinocysts, spores, and pollen.

A c k n o w l e d g m e n t s

This research used samples and data provided by the O c e a n Drilling Program, which is spon­

sored by the U.S. National S c i e n c e Foundation (NSF) and participating countries under the management of Joint Oceanographic Institutions ( J O I ) Incorporated. Funding for this research was provided by U.S. and international agencies.

A u t h o r s

Neville Exon, Jim Kennett, Mitch Malone, Henk Brinkhuis, George Chaproniere, Atsuhito Ennyu, Patrick Fothergill, Michael Fuller, Marianne Grauert, Peter Hill, Tom Janecek, Clay Kelly, Jennifer Latimer, Kristeen McGonigal, Stefan Nees, Ulysses Ninnemann,Dirk Nuernberg, Stephen Pekar, Caroline Pellaton, Helen Pfuhl, Christian Robert, Ursula Rohl,

Stephen Schellenberg, Amelia Shevenell, Catherine Stickley, Noritoshi Suzuki, Yannick Touchard, Wuchang Wei, and Tim White For m o r e information, c o n t a c t either Neville Exon, G e o s c i e n c e Australia, GPO B o x 378, Can­

berra, Australia 2601; E-mail: Neville.Exon@

ga.gov.au; or J i m Kennett, Department of Geological S c i e n c e s , University of California, S a n t a B a r b a r a , USA; E-mail: k e n n e t t @ geol.ucsb.edu

R e f e r e n c e s

Cape Roberts S c i e n c e Team, Studies from the Cape Roberts Project, Ross Sea, Antarctica. Initial Report on CRP-3,Terra Antarctica 7 , 1 / 2 , 2 0 0 0 .

Exon, N. F, et al., Proc. O c e a n Drill. Program Initial Rep., 189, O c e a n Drill. Program, College Station, Tex., 2 0 0 1 .

Hayes, F, et Initial Rep. Deep Sea Drill. Proj., 28, 1017 pp., U S . Government Print. Off ice, Washing­

ton, D.C., 1975.

Kennett, J. P, C e n o z o i c evolution of Antarctic glacia­

tion, the Circum-Antarctic Ocean, and their impact on global paleoceanography, J. Geophys. Res., 82, 3 8 4 3 - 3 8 5 9 , 1 9 7 7 .

Kennett, J. P, et al.,Initial Rep. Deep Sea Drill.

Proj., 29,1197 pp., U.S. Government Print. Office, Washington, D.C., 1975.

Miller, K. G., R. G. Fairbanks, and G. S. Mountain,Ter­

tiary oxygen isotope synthesis, sea-level history and continental margin erosion, Paleoceanogr., 2, 1 - 1 9 , 1 9 8 7 .

O'Brien, PE., et al., Proc. Ocean Drill. Prog. Initial Rep., 188, O c e a n Drilling Program, College Station,Tex., 2 0 0 1 . 0'SuIlivan,PB.,and B.PKohn, Apatite fission track

thermochronology of Tasmania, Aust. Geol.Surv.

Org. Record 1997/35,61 pp.,Australian Geological Survey Canberra, 1997.

Combined Technologies Allow Rapid Analysis of Glacier Changes

PAGES 2 5 3 , 2 6 0 - 2 6 1

Monitoring of glacier c h a n g e s plays an important role within the Global Climate Observing System (GCOS) [Haeberli et al, 2000], and Landsat imagery has proven to b e a useful tool for monitoring glacier c h a n g e s over large and remote areas [Aniya et al., 1996; Li et al, 1998] .An accurate glacier map can b e obtained by simple segmentation of a ratio image from Thematic Mapper (TM) c h a n n e l s 4 and 5

[Bayr et al, \994, Jacobs et al, 1997; Paul, 2 0 0 2 ] . Individual glaciers were recently derived within a Geographic Information System (GIS) using a vector layer with glacier basin boundaries.

Glacier changes were calculated and visual­

ized by processing sequential images within a fully automated work flow.

This method was originally developed for the new Swiss Glacier Inventory 2000 (SGI 2000), which used a high-resolution digital elevation model (DEM) for ortho-rectification and retrieval of glacier parameters, a n d a digitized glacier inventory from 1973 for delineation of glacier basins [Paul et al, in press, 2 0 0 2 ] . In many glacierized regions of the world that lack information about glacier extent or changes, neither a high-resolution DEM nor a digitized glacier inventory is accessible.This study presents the possibilities of glacier change documentation

in the southern part of the Tyrolean Alps, Austria, without using such information.

Digital overlay of TM-derived glacier maps from 1985 and 1999 clearly exhibit changes that could not b e revealed by in-situ measure­

ments of length changes at the glacier terminus.

While small glaciers often suffer a d e c r e a s e in area around the whole perimeter, glaciers larger than 5 km² also shrank significantly after b e c o m i n g separated from formerly adja­

c e n t streams or an increasing area of rock outcrops within the glacier.

I m a g e S e l e c t i o n

With the TM4/TM5 ratio image method, all debris-free glacier ice, as well as snow, is classified as "glacier" and, as a c o n s e q u e n c e , snow fields adjacent to a glacier would hide the real glacier perimeter. Thus, the most important task for glacier studies with Landsat TM and other optical sensors is to find a

Eos, Vol. 83, No. 23, 4 June 2002

European Stage

Western Tasmania Margin

2475 m 42°36'S, 144°24'E

Site 1168

West South Tasman Rise 3580 m 47*03'S, 145°14'E

Site 1169

West South Tasman Rise

2 7 1 0 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 4 3E5 8 ' S , 149°56'E

Site 1172

Serravailian

GBBi

Oligocene

ffij

L L i J LLU

Foraminifer-bearing or siliceous bearing nannofossil ooze/chalk Nannofossil ooze/chalk

H

nannofossil bearing clay

Clayey chalk, sandy claystone, organic bearing silty claystone/clayey siltstone

Organic and glauconite bearing silty claystone/clayey siltstone Organic bearing, nannofossil bearing, silty claystone/clayey siltstone

23/OA/1178 Unconformity on seismic evidence only Note: Some time breaks probably

occur in the Paleocene- Eocene sequences and near Eocene-Oligocene boundary

Page 2 5 8

Fig. 2. This summary describes sediments deposited through time for all sites drilled during Leg 189, arranged from west (left) to east (right).

Eos, Vol. 83, No. 23, 4 June 2002

Drifting

F a s t s p r e a d i n g c o n t i n u e s T L B subsiding rapidly P e l a g i c c a r b o n a t e s on T L B AAG not restricted

S o u t h e r n O c e a n developing D e e p A A G - P P c o n n e c t i o n E A C not to A n t a r c t i c a A C C in full flow T O C in e x i s t e n c e Antarctic glaciation

Separation

F a s t s p r e a d i n g c o n t i n u e s T L B subsiding rapidly Glauconitic silts on T L B AAG restricted

Shallow A A P - P P c o n n e c t i o n E A C not to A n t a r c t i c a A C C d e v e l o p s s o o n after Global cooling s o o n after T O C s o o n after

O n s e t of Antarctic glaciation

Rifting

F a s t s p r e a d i n g s t a r t s T L B in p l a c e

Shallow m a r i n e m u d s on T L B AAG v e r y restricted No A A G - P P c o n n e c t i o n E A C w a r m s Antarctic No T O C

23/OA/1262

Fig. 3. As Australia and Antarctica separated, changes through time led to Antarctic glaciation and thermohaline oceanic circulation. AAG = Australo-Antarctic Gulf; ACC = Antarctic Circumpolar Current;

EAC = Eastern Australian Current; PP = Proto-Pacific Ocean; TLB = Tasmanian Land Bridge; and TOC

= thermohaline oceanic circulation.