Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 52

that is important. Urey argued, on thermodynamic grounds, that the formation of
crystalline CaCO 3 (calcite) - with dissolved calcium in sea water and which is taken
up by forams and corals - should involve some isotopic fractionation. The idea was
that the fraction of 18 O incorporated into the calcite should decrease with increasing
water temperature, essentially because the heavier nucleus slows down the vibrational
modes of molecules with 18 O- compared to 16 O-containing water molecules, which
then have a thermodynamic advantage in the process of calcite formation. However,
to exploit this isotopic effect for palaeothermometry one has to correct for the global
extent of ice caps and glaciers at the time in question. This is because water molecules
with the normal, lighter 16 O preferentially evaporate compared to 18 O, which requires
more energy that goes with higher temperatures; it is this last that underpins the
theory behind 18 O dendrochronology mentioned above. So, using 18 O in shells for
palaeothermometry is only really effective when there is little ice on the planet. When
there is a lot of ice (such as now, with Antarctica and Greenland) 18 O in shells is a
better indicator of the amount of ice worldwide than global temperature. (We shall
return to this 18 O ice issue shortly.)
So, because ice caps take water molecules with a mix of oxygen isotopes out of
the oceanic pool of water molecules, ice caps complicate things. However, from the
late Cretaceous (65 mya) to the late Miocene (5 mya), the Earth had negligible ice
caps and terrestrial ice (such as on mountain tops). Consequently 18 O palaeother-
mometry is an applicable palaeoclimatic tool for this geological window. There is a
complication in that different plankton species, although they all float in the top few
hundred metres of the ocean, live at different depths below the surface (hence pos-
sibly at different temperatures), and they often change habitat depth with the seasons.
Some species, for example, keep to the upper layers because they live symbiotically
with photosynthesising algae. Not knowing the detailed life cycles of all the long-
extinct creatures means that some inferences need to be made, with some inadvertent
erroneous assumptions. Consequently, in these palaeoclimatic studies researchers
customarily deduce surface temperature for a given multi-species sample from the
species that yields the lowest 18 O fraction, presumably corresponding to the habitat
closest to the surface.
Since the mid-1980s, however, models of carbon dioxide greenhouse warming have
confronted 18 O data from fossil planktonic (floating) Foraminifera with the so-called
cool-tropics paradox. In stark contrast to many global climate models, the planktonic
18 O data seemed to suggest that 50 mya tropical ocean surfaces were about 10 ◦ C
cooler than they are now, even though this was a time when the carbon dioxide level
was almost certainly much higher than it is today, and the Arctic was warm enough
for crocodiles and giant monitor lizards.
The controversy was resolved largely due to an analysis of planktonic Foraminifera
from the late Cretaceous to the late Eocene (67-35 mya). This was conducted by
Paul Pearson (at the University of Bristol in the UK) and co-workers in 2001, and
does much to lay this troubling cool-tropics paradox to rest. Because planktonic
Foraminifera, while alive, float at or near the surface, researchers had assumed that
their 18 O concentration reflects the temperature of the sea surface. But Pearson and
company, doing isotopic analyses of unusually well-preserved samples of pristine
shells selected with the help of electron microscopy, concluded that the surpris-
ingly high 18 O level of traditional samples is a misleading consequence of extensive