Page 427 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 409

and northwest Africa, as a consequence of the fi rst benthic microfossils (Shackleton & Kennett, 1975), and

phase of rifting of the supercontinent. This was initi- the first major build-up of ice on Antarctica, coincided
ated about 180 Ma ago. Thus the “Tethyan embayment” with these developments, and appear to mark a transi-
in Pangea (Fig. 13.2) was extended to the west to facili- tion from a Greenhouse to an Icehouse Earth. The
tate a circum-global equatorial current. This meant that change in oxygen isotope values is particularly pro-
some tropical waters were heated to a higher tempera- nounced and well documented, and is essentially coin-
ture before turning northwards and southwards to cident with the Eocene–Oligocene boundary (Fig. 13.8).
warm higher latitudes. In this way the whole Earth This is also the time of the opening of the gateway
became warmer and the temperature gradient from the south of Tasmania (Exon et al., 2002). The full opening
equator to the poles was further reduced. of the Drake Passage is less well constrained, but was
The separation of Antarctica from Africa, which probably shortly after this (Livermore et al., 2004).

started about 165 Ma ago, was the first stage in the Oxygen isotope ratios and a drop in sea level of 40 m
break-up of Gondwana (Fig. 13.4). This was followed suggest that during the early Oligocene the volume of
at about 125 Ma by the rifting apart of South America ice in Antarctica built up to perhaps as much as one-half
and Africa, which started in the south and propagated of its present volume. This and subsequent increases in
northwards. This, coupled with the complex fracture ice volume, and changes in sea level, gave rise to an
zone pattern in the equatorial Atlantic region, due to emergence of land areas, and a major reduction in the
transform faulting, meant that the gateway between area of shallow seas on continental crust (cf. Figs 13.6,
the North and South Atlantic did not open up until 13.7).
about 95 Ma (Fig. 13.5) (Poulsen et al., 2001). The Following a period of warming and deglaciation in
initial changes in the deep-water circulation, resulting the late Oligocene (Fig. 13.8), additional major increases
from the opening of this gateway, may explain the in the volume of ice on Antarctica, and associated drops
“anoxic event” that produced the widespread black in sea level, are thought to have occurred in the mid-
shales in adjacent areas at that time (Poulsen et al., Miocene and at the end of the Miocene. The drop in
2001). By 95 Ma India had separated from Antarctica sea level associated with the increase in ice volume at
and a major Southern Ocean was opening up south 6 Ma may explain the isolation, and subsequent desicca-
of Africa and India. However, in the late Cretaceous, tion, of the Mediterranean Sea, as a result of the expo-
and even in the early Cenozoic (Fig. 13.6), the circum- sure of the sill at the Strait of Gibraltar (Van Couvering

equatorial current still existed, and the surface water et al., 1976), and would have restricted the flow of water
in the high latitude oceans was still very much warmer through the ocean gateway between North and South
than it is today. America. However, additional tectonic movements
Throughout the Cenozoic, Africa, India, and Austra- were required before a complete land bridge formed,
lia continued to drift northwards, away from Antarctica, about 3 Ma ago; as determined from the interchange of
thereby enlarging the southern and Indian Oceans, and mammals between North and South America (Mar-
ultimately forming the Alps and the Himalayas as a shall, 1988). The gradual formation of the Isthmus of

result of the collision of Africa and India with Eurasia Panama would have led to the intensification of the
(Section 10.4.1). By 30 Ma (Fig. 13.7), the Tethyan Gulf Stream, and ultimately, perhaps, to the formation
seaway was effectively closed, and the Southern Ocean of the northern hemisphere ice-sheets (Haug &
completely encircled Antarctica, as a result of the Tiederman, 1998).
opening of gateways south of Tasmania, and in the The warm waters of the Gulf Stream would have
Drake Passage, south of South America. These results given rise to more warm and moist air, and hence more
of continental drift, gave rise to major changes in the precipitation, in relatively high latitudes in the North
near surface oceanic circulation. There was no longer a Atlantic area. The geographic distribution of ice sheets
complete circum-equatorial current, and a pronounced is determined not only by cold ambient temperatures,
circum-polar current was established in the Southern but also by the availability of precipitation. The Plio-
Ocean. Thus the equatorial water became less warm, Pleistocene ice-sheets of the northern hemisphere were
and Antarctica was insulated from the warmer water restricted to Greenland, northern North America, and
circulating in the major southern hemisphere gyres of northwest Europe for this reason. Similarly, the occur-

the Pacific, Atlantic, and Indian Oceans. A change in rence of tropical rain forests is determined not only
oxygen isotope ratios in the tests of planktonic and by high temperatures, but also by the delivery of