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
