Page 424 - Global Tectonics

406 CHAPTER 13

Mg/Ca ratio for seawater is calculated (Hardie, 1996). If the Earth’s surface was entirely covered by an
During the resulting periods of low Mg/Ca ratio, asso- ocean, the resulting westerly directed, equatorial ocean
ciated with high sea level stands, nonskeletal carbonates current, and the intermediate latitude easterly directed,
are composed of (low magnesium) calcite, and marine circumpolar currents, would bracket irregular “gyres,”
evaporites are characterized by late forming KCl circulating clockwise in the northern hemisphere, and
(sylvite), and an absence of Mg salts. By contrast, the anticlockwise in the southern hemisphere. In this situ-
periods of high Mg/Ca ratio are characterized by non- ation the world-encircling equatorial and circumpolar
skeletal carbonate deposits composed of high magne- currents would tend to inhibit the transfer of heat by
sium calcite and aragonite (a polymorph of calcite), and surface currents from low to high latitudes, and the
marine evaporites in which MgSO 4 formed during the temperature gradient between the equator and the

final stages of evaporation. The former periods have poles would be accentuated. As a consequence, sea ice
been termed periods of “calcite seas,” and are thought might form in the polar oceans. However, land masses
to be associated with high pCO 2 and high surface tem- with north–south trending shorelines in low and inter-

peratures; i.e. a “Greenhouse Earth” such as that which mediate latitudes, will deflect the equatorial and cir-
probably characterized the Cretaceous. The periods of cumpolar currents, to the right in the northern
high Mg/Ca ratio have been designated as periods of hemisphere, and to the left in the southern hemisphere,
“aragonite seas.” These appear to correlate with times thereby intensifying the gyres, and transferring heat
of low pCO 2 , and low surface temperatures, and include from the tropics to higher latitudes by means of western
ice ages, i.e. an “Icehouse Earth”. boundary currents. In this scenario the temperature
Variations in pCO 2 in the atmosphere in the geologic gradient is reduced. A classic example at the present day
past are thought to have been largely due to the outgas- is the Gulf Stream of the western North Atlantic, which
sing of CO 2 from volcanic activity. Thus eustatic changes warms the air above the ocean in the extreme North
in sea level, changes in seawater chemistry, and varia- Atlantic, thereby ameliorating the climate of Iceland
tions in the concentration of CO 2 in the Earth’s atmo- and northwest Europe. The opening or closing of gate-
sphere in the past might all be related to variations in ways for the equatorial or circumpolar currents, as a
the rates of sea fl oor spreading and plume activity. result of continental drift, can, therefore, have
pronounced effects on the Earth’s climate (Smith &
Pickering, 2003).
During the past 200 Ma the supercontinent of Pangea
13.1.2 Changes in oceanic has progressively rifted apart. The resulting fragments
circulation and the have drifted across the face of the globe, such that a
continuous tropical seaway, the neo-Tethys, was formed,
Earth’s climate and subsequently closed, and a southern ocean gradu-
ally opened up around Antarctica (Figs 13.2–13.7). By
Two of the most signifi cant influences on the Earth’s the mid-Cenozoic, a complete southern circumpolar

climate are the concentration of greenhouse gases in current came into existence, which isolated and insu-
the atmosphere (Sections 5.7, 13.1.1), and the extent, lated Antarctica, and was probably instrumental in trig-

distribution, and bottom topography of the oceans. gering the first major build-up of the Antarctic ice cap
The confi guration of the ocean basins affects the trans- (Kennett, 1977).
port of heat in the oceans, by surface currents and At the beginning of the Mesozoic Era, 250 Ma ago,
deep-water circulation, thereby affecting the tempera- the supercontinent of Pangea extended from pole to
ture and moisture content of the atmosphere over pole (Fig. 13.2), without extensive polar landmasses in
oceanic areas. Surface currents are essentially wind either hemisphere. Strong western boundary currents
driven, and, therefore, largely determined by the circu- off the eastern shores of Pangea would have trans-
lation of the atmosphere. The rotation of the Earth, ported warm water to high latitudes, preventing the
and the concentration of incoming solar radiation formation of ice-sheets and warming the east-facing
within the tropics, produces surface easterly (trade) coasts relative to the west. The interior of the supercon-
winds at low latitudes, westerlies at intermediate tinent would have had strong seasonal extremes. By
latitudes, and easterlies at high latitudes (greater 160 Ma (Fig. 13.3) a low latitude east–west seaway had
than 60°). started to open up, between what is now North America