Page 428 - Global Tectonics

410 CHAPTER 13

Age δ 18 0 ( ‰ ) precipitation by warm equatorial currents. The tropical
(Ma) 5 4 3 2 1 0 coal forests of the Carboniferous, for example, formed
0 Plt at the western end of the equatorial Tethyan embay-
Plio ment in the embryonic supercontinent of Pangea (Fig.
3.9). At the present day the most extensive areas of
tropical rain forest are in the Amazon basin and the
10 N. Hemisphere ice - sheets archipelago of southeast Asia, areas warmed by the
Miocene main westward directed equatorial currents of today’s
oceans. The progressive cooling of the Earth’s climate
during the last 50 Ma, particularly in higher latitudes,
20 led to a general reduction in the amount of precipita-
tion, and an increase in aridity. Thus formerly forested
areas in high latitudes were turned to tundra, and in
Oligocene temperate latitudes to grassland. As a consequence of
the major cooling about 6 Ma, even some low latitude,
30 Antarctic ice - sheets tropical forests were converted to savannah. This is
thought to have had a profound effect on mammalian,
and, ultimately, human evolution.
During “Greenhouse Earth” conditions the oceans
40 are warm throughout, with very little deep-water circu-
Eocene Partial or ephemeral lation. As a consequence the bottom waters become

Full scale and permanent deoxygenated and there is the potential for the preserva-
tion of organic material and hence the formation of
50 black shale deposits (Sections 3.4, 5.7). In as much as
there is vertical mixing, it is probably triggered by
regional changes in the salinity, and hence the density,
Paleocene much less upwelling of nutrients compared to the
60 of seawater in the tropics. Weak circulation, and the
preservation of organic matter, meant that there was
present oceans. Thus, the overall fertility of the Creta-
ceous oceans was low, but the potential for the ultimate
70 formation of oil, from Cretaceous marine source rocks,
was high. In “Icehouse Earth” conditions cold, dense
0˚ 4˚ 8˚ 12˚
Temperature ( °C ) water forms in polar regions, sinks, and fl ows towards
the equator, thereby creating a relatively vigorous, and
Fig. 13.8 Global deep sea oxygen isotope record certainly very significant, deep water circulation. The

compiled from measurements on benthic fauna from cooling that marked the transition from a Greenhouse
numerous Deep Sea Drilling Project and Ocean Drilling to an Icehouse Earth, at about the Eocene–Oligocene
Project cores. Fitted curve is a smoothed five point boundary, probably enabled sea ice to form around

running mean. The temperature scale relates to an ice- the margin of Antarctica for the fi rst time. During
free ocean and only applies therefore to the time prior to the formation of the sea ice much of the salt content
the onset of large scale glaciation in Antarctica (approx. of the seawater is expelled, increasing the density of the
18
35 Ma). Much of the subsequent variability in the d O

records reflects changes in Antarctic and Northern seawater beneath the ice. This cold, dense water would

Hemisphere ice volume. When seawater evaporates, then sink to the ocean floor, and flow northwards, as it
16
molecules containing the lighter isotope O evaporate does at the present day.
more readily. Thus when atmospheric water vapor One of the enigmas of the late Cenozoic cooling of
18
precipitates as snow in polar regions, O depleted water the Earth is the relatively sudden build-up of ice in
becomes sequestered in the polar ice caps and the Antarctica in the Mid-Miocene (Fig. 13.8). One interest-
18
proportion of O in seawater increases (part of fi gure 2 ing and remarkable possibility is that it was caused by a

in Zachos et al., 2001, reproduced from Science 292, change in the topography of the sea floor in the extreme
686–93, with permission from the AAAS). North Atlantic, as a consequence of tectonic processes