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156 J. A. CRAME & B. R. ROSEN

physical isolation of Antarctica. Although

largely separated from the other southern conti-

nents by the end of the Cretaceous Period, there

were still intermittent terrestrial and shallow-

water connections between both eastern

Antarctica and southeasternmost Australia, and

the Antarctic Peninsula and southernmost

South America (Crame 1999, fig, IB) (Fig. 1).

Full, deep-water separation of the South

Tasman Rise from Antarctica was probably not

achieved until the late Early Eocene (50 Ma BP),

by which time the continent must have been

almost completely isolated by deep water. The

first indications of global cooling occured in the

interval 50-40 Ma BP, and there were significant

2°C temperature falls in both the late Middle

Eocene and Middle-Late Eocene boundary

(Stott et al 1990). These were followed by a

major cooling event at approximately the

Eocene-Oligocene boundary (37 Ma BP) where

both surface and bottom waters may have

decreased by as much as 5°C in 75-100 ka

(Lazarus & Caulet 1993; Crame 1999; and

references therein).

The precise time of opening of the Drake

Passage is still uncertain. Whereas marine

geophysical evidence indicates that seafloor

spreading began in the early Late Oligocene

(28 Ma BP), it may have been as late as 23.5 ±

2.5 Ma (Oligocene-Miocene boundary) before

full deep-water circulation was established

(Barker et al 1991). On the other hand, there is

some micropalaeontological evidence to suggest

that there was an opening, at least at shallow and

intermediate depths, as early as 36 Ma BP

(Diester-Haas & Zahn 1996), and a taxonomic

assessment of mid-Eocene mammal faunas from

Seymour Island (northern tip of the Antarctic

Peninsula) shows that they had been isolated

from a South American ancestral stock since at

least the Early Eocene (54-51 Ma BP) (Wood-

burne & Case 1996). In any event it is apparent

that a full deep-water, circum-Antarctic current

system and polar frontal zone had been estab-

lished by the Middle Miocene (15 Ma BP)

(Lazarus & Caulet 1993) (Fig. 1). This was the

time of development of a latitudinal tempera-

ture gradient very similar to that seen today

(Loutit et al 1983; Kennett et al 1985).

Nevertheless, it should also be emphasized

that other Cenozoic tectonic events almost

certainly contributed to significant global

cooling. Of particular importance was the

progressive northward movement of the

Africa/Arabia landmass that led to the constric-

tion of the Tethyan Ocean and its eventual

closure in the Early Miocene (20 Ma BP),

followed by the Early-Middle Miocene

(20-15 Ma BP) collision of the Australian plate

with Indonesia (Kennett 1977; Kennett et al

1985; see below). The net effect of these changes

was to shift oceanic circulation from predomi-

nantly equatorial to strongly meridional (i.e.

north-south) or gyral; this was especially so in

the Pacific Ocean (Kennett et al 1985; Grigg

1988). The transport of warm waters into high-

latitude regions is thought to have led to

increased levels of precipitation there which in

turn contributed to the gradual build-up of

glacial ice (see also below).

Even Late Neogene palaeogeographical

changes are thought to have had profound

climatic implications. For example, the gradual

uplift of the Central American Isthmus (CAI,

i.e. the Isthmus of Panama), which occurred

over the interval of 13-1.9 Ma BP, began to have

a major effect on oceanographic circulation

patterns by about 4-6 Ma BP (Coates et al 1992;

Haug & Tiedemann 1998). At this time the

Central American Seaway had shallowed to

<100 m and the Gulf Stream was beginning to

deflect warm, shallow waters northward along

the eastern US seaboard (Fig. 2). Although this

undoubtedly led to some warming in northern

hemisphere mid- to high latitudes, it is apparent

that, as a result of evaporation of surface waters

in the relatively narrow North Atlantic Ocean

by trade winds, this water would have been

slightly hypersaline. When it reached the north-

ern high latitudes it began to descend in both the

Norwegian and Labrador seas to form North

Atlantic Deep Water (NADW). This then

spread into both the South Atlantic and central

Pacific to initiate a major 'conveyor belt' of

deep-ocean circulation (Haua; & Tiedemann

1998) (Fig. 2).

Further closure of the CAI saw the North

Atlantic thermohaline circulation system inten-

sify by 3.6 Ma BP, and the Arctic Ocean effec-

tively isolated from the warm Atlantic waters. It

is this isolation from the oceans to the south that

led to the drastic temperature decline in the

north polar regions and the eventual onset of

glaciation at 2.5 Ma BP (Stanley 1995). Changes

in the Earth's obliquity amplitude fluctuations

between 3.1 and 2.5 Ma BP may also have been

an important contributory factor (Haug &

Tiedemann 1998). A further important effect of

the closure of the CAI was to reverse the flow of

water through the Bering Strait, and this in turn

influenced Pliocene-Pleistocene patterns of

North Atlantic thermohaline circulation

(Shaffer & Bendtsen 1994; Marincovich 2000).

It can be concluded that, throughout the

Cenozoic Era, a series of tectonic changes

occurred that led directly to global cooling. First
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