Page 124 - Introduction to Paleobiology and The Fossil Record

PALEOECOLOGY AND PALEOCLIMATES 111

Box 4.9 Paleotemperature: isotopes to the rescue?

Is it possible to find out how hot or cold the Earth really was in the past? Stable oxygen isotopes
can be extremely useful as paleothermometers but also in assessing the salinity of ancient oceans
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and the extent of ancient ice caps. Oxygen has three stable isotopes, the lightest being O, then O,
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and the heaviest O. The ratio of O : O is used in most geological investigations. When calcite is
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precipitated from seawater the ratio of O : O increases with temperature. This ratio is also stan-
dardized with respect to standard mean ocean water (SMOW) or the Peedee belemnite standard
(PDB), Belemnitella americana from the Cretaceous Peedee Formation in South Carolina. A shift of
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1‰ in Δ O values represents a change in temperature of about 4–5°C. Unfortunately, not all shells
are precipitated in equilibrium with surrounding seawater; the vital effects of some organisms inter-
fere with the process. Moreover diagenesis can also affect isotope data. For these reasons corals,
calcareous algae and echinoderms do not give good results; on the other hand brachiopods, bivalves
and foraminiferans have yielded useful data. In addition, the lightest isotope is generally preferen-
tially found in water vapor and thus rainfall. During glacial episodes, snow and ice can act as reser-
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voirs for O, thus depleting the world’s oceans of that isotope. Thus during ice ages the oceans are
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characterized by higher amounts of O. This simple model has formed the basis for our understand-
ing of climate change over the last 1 myr and the relationships of such changes to Milankovitch
cycles (see p. 36).
A dataset of oxygen isotopes is available for time series analysis at http://www.
blackwellpublishing.com/paleobiology/.

burning fossil fuels and pumping greenhouse some climate change can be modeled by Gaia
gases into the atmosphere. Global climate – some of the most marked during the Pre-
warming will affect the plants and animals of cambrian (Fig. 4.28). The diversifi cation of
the cold temperate and polar regions as climate photosynthesizers together with consumers
zones move about 100 km per century towards from the Early Proterozoic onwards, hiked
the poles (Wilson 1992). Nevertheless, a oxygen levels concomitant with declines in
number of models for long-term climatic greenhouse gases. Such models promote the
change have also involved the role of feed- vital effects of life as a stabilizing infl uence on
backs from biological organisms. For example, the planet’s climate, reducing the otherwise
the Gaia hypothesis is an attractive model steady rise in the Earth’s surface temperatures.
that treats the Earth as a living system. The In the same way the extensive coal swamps
constant interaction between the Earth’s living and forests of the later Paleozoic may also
organisms, the atmosphere and the oceans have contributed to an interval of cooler
helps keep the planet in check. The idea is climate as diversifying land plants mediated
certainly not new. James Hutton (1726–1797), atmospheric oxygen levels, predicting the
the father of geology, once described the Earth importance of modern rain forests as a
as a kind of superorganism. But there were climatic buffer.
times in the Earth’s history, the Day After There is no doubt that life on planet Earth
Tomorrow ice age of snowball Earth (see p. is resilient and despite the extremes of climate
112) or the sustained hot climates of the Cre- change through deep time may have, through
taceous world, when the Earth’s climate biological feedbacks, been able to conserve
seemed to be out of control. Nevertheless and control its own environment.