Biology Reference
In-Depth Information
et al. 2002; 2.3-4 times according to Passalia 2009, based on stomatal anal-
ysis of gymnosperm leaves from Patagonia of Aptian-Cenomanian age; see
table E.1 below). If MAT increases by approximately 2°C with each doubling
of CO
2
, this would give a temperature warmer by at least 8°C-12°C (fi g. 3.4).
Oxygen isotope measurements of ocean bottom waters, indeed, do show
temperatures 15°C-20°C warmer than the present 1°C-2°C. Computer
simulations further predict a reduced north-south meandering of the polar
jet streams and less amplitude between the troughs (low pressure systems)
and ridges (high pressure systems) as the fronts moved across the conti-
nents. These physical and climatic features suggest a Cretaceous world with
warm, equable conditions extending along low thermal gradients between
the equator and the poles, with less seasonality and regional differentiation
of climate. As a consequence of this, there were many species with extensive
geographic ranges, and vegetation was abundant and diverse, as revealed,
for example, by stable isotope studies of Late Cretaceous dinosaur tooth
enamel and fi sh scales from Montana and Alberta (Fricke et al. 2008).
Still to be resolved is the possibility of the presence, extent, and location
of glaciers in the Cretaceous. The evidence as reviewed in Hay 2008 (see
chap. 3) consists of (1) early modeling results (e.g., Barron et al. 1981) that
raised, and continue to raise, the possibility of extended periods of subfreez-
ing temperatures over extensive polar areas; (2) fl uctuations in sea level
during the Cretaceous unlikely attributable solely to movement of the land
or reduction in ocean basin volume; and (3)
18
O ratios seeming to require
the presence of large volumes of ice. The uncertainties in the estimates still
allow for ice volume ranging from continental glaciation on Antarctica to
limited sea ice along the coasts of Antarctica and in the Arctic. Paleobo-
tanical evidence is most extensive for the north polar regions, and there the
extent and diversity of Late Cretaceous vegetation is diffi cult to reconcile
with widespread, sustained freezing and glaciation.
Oxygen isotope and other evidence reveals that toward the end of the
Middle Cretaceous, atmospheric CO
2
concentration began to decline as
plate movement and associated volcanism waned. Also, with uplift and less
input of new crustal material into the ocean basins, the epicontinental seas
of North and South America began to drain from the interiors. This con-
tributed to an expansion of continental climates characterized by greater
seasonal variation. Temperatures decreased toward the K/T boundary from
their mid-Cretaceous highs, then began to increase in the Paleocene owing
to enhanced volcanic activity along the rift between Greenland and Europe,
and then more suddenly to an explosive emission of methane gas from the
Norwegian Sea. At about 55 Ma, temperatures reached their maximum for
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