14 12 10 8 6 4 2 0 »bottom water temperature [°C] Mg/Ca

Cenozoic average 012345 S18O[%o]

12 8 4 0 Bottom-water temperature [ Ice-free ocean

Figure 4.25 Size changes in planktic foraminiferans from high and low latitudes during the last 70 Ma, compared to temperature profiles generated from oxygen isotope data and Mg : Ca ratios. Three phases are recognized, a first (65-42 Ma) with dwarf taxa, a second (42-12 Ma) with moderate-sized taxa, and a third (12 Ma to present) with large-sized taxa. Size increases are correlated with intervals of global cooling. (Courtesy of Daniela Schmidt.)

Figure 4.26 Climate change through time illustrated together with changes in sea level and fluctuations in the intensity of volcanicity. (Based on various sources.)

2 On the other hand, greenhouse oceans were more stable, and better stratified with surface waters ranging in temperature from 12 to 25°C with deep-water temperatures between 10 and 15°C. Slow bottom currents carried little oxygen and productivity was generally low.

Extinctions were associated with the transitions between these oceanic states.

In addition to these major climatic fluctuations a series of major extinction events, some associated with extraterrestrial causes, clearly prompted major climate change over several million years. Such events caused major taxo-nomic extinctions together with major restructuring of the marine and terrestrial ecosystems. Generally, greenhouse biotas were most susceptible to extinction; their species were more specialized and thus more exposed to environmental change.

Consequences for evolution

Microevolution is obvious in many fossil lineages (Benton & Pearson 2001) although the link between speciation events and climatic change is more controversial. Generally, marine plankton show gradual evolution whereas marine invertebrates and vertebrates display a pattern of punctuated equilibria (see p. 121). Moreover, it is probable that narrowly-fluctuating, changing environments host persistent gradualistic evolution whereas widely-fluctuating environments host morphological stasis (Sheldon 1996). This resistance to morphological change is clear in a number of lineages such as Ordovician trilo-bites and Pliocene mollusks (see p. 123).

Short-term climatic fluctuations, for example those associated with Milankovitch cycles, can clearly disrupt and promote the reassembly of both marine and terrestrial communities. In some cases they can drive local extinctions and radiations, for example in the conodont and graptolite faunas of the Silurian (see p. 434).

Climate surely drives larger-scale aspects of evolution. For example, the Cambrian explosion (see p. 249) - marked by the diversification of skeletal organisms and the appearance of reef-building organisms and the first predators - is associated with increasingly warm climates and higher sea levels. On land the radiation of early terrestrial tetrapods in the Early Carboniferous and the diversification of large flying insects in the first extensive forests, in cooler climates and more exposed land areas, have been correlated with high levels of atmospheric oxygen (Berner et al. 2000).

Some of the largest events of all, such as the appearance of entire new biotas and grades of organization, for example the origin of life itself, the development of photosynthesis and the appearance of the metazoans, may also be associated with climate change. The first two events have been associated with a stable Archaean crust and relatively cooler climates, which are favorable for carbon-based life to evolve. Metazoans appeared and diversified after the decay of the near global ice sheets of "snowball Earth" (Box 4.10), whereas skeletal organisms radiated during the greenhouse climates and higher oxygen levels of the Early Cambrian.

Biological feedbacks_

If climate drives evolution, could life itself drive climate change? Few people doubt that humans can affect the climate, and everyone is aware of how the industrialized nations are

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 and the extent of ancient ice caps. Oxygen has three stable isotopes, the lightest being 16O, then 17O, and the heaviest 18O. The ratio of 18O : 16O is used in most geological investigations. When calcite is precipitated from seawater the ratio of 18O : 16O increases with temperature. This ratio is also standardized 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 1%o in A18O 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 interfere 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 preferentially found in water vapor and thus rainfall. During glacial episodes, snow and ice can act as reservoirs for 16O, thus depleting the world's oceans of that isotope. Thus during ice ages the oceans are characterized by higher amounts of 18O. This simple model has formed the basis for our understanding 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 gases into the atmosphere. Global climate warming will affect the plants and animals of the cold temperate and polar regions as climate zones move about 100 km per century towards the poles (Wilson 1992). Nevertheless, a number of models for long-term climatic change have also involved the role of feedbacks from biological organisms. For example, the Gaia hypothesis is an attractive model that treats the Earth as a living system. The constant interaction between the Earth's living organisms, the atmosphere and the oceans helps keep the planet in check. The idea is certainly not new. James Hutton (1726-1797), the father of geology, once described the Earth as a kind of superorganism. But there were times in the Earth's history, the Day After Tomorrow ice age of snowball Earth (see p. 112) or the sustained hot climates of the Cretaceous world, when the Earth's climate seemed to be out of control. Nevertheless some climate change can be modeled by Gaia - some of the most marked during the Pre-cambrian (Fig. 4.28). The diversification of photosynthesizers together with consumers from the Early Proterozoic onwards, hiked oxygen levels concomitant with declines in greenhouse gases. Such models promote the vital effects of life as a stabilizing influence on the planet's climate, reducing the otherwise steady rise in the Earth's surface temperatures. In the same way the extensive coal swamps and forests of the later Paleozoic may also have contributed to an interval of cooler climate as diversifying land plants mediated atmospheric oxygen levels, predicting the importance of modern rain forests as a climatic buffer.

There is no doubt that life on planet Earth is resilient and despite the extremes of climate change through deep time may have, through biological feedbacks, been able to conserve and control its own environment.

Box 4.10 Snowball Earth

Strong evidence suggests that a number of Late Neoproterozoic ice ages were of global extent (Hoffman et al. 1998). The occurrence of tillites in close association with carbonates in near-equatorial positions has suggested to Paul Hoffman and his colleagues that during these intervals the Earth was virtually covered by ice. These data supported a model first developed by Brian Harland in the 1960s, subsequently christened "snowball Earth" by Joe Kirschvink in the 1980s. But paleomagnetic data for low-latitude ice is not the only line of evidence for a global snowball. The majority of these glacial deposits are overlain by so-called cap carbonates. These rocks suggest deposition in extreme greenhouse conditions, under an atmosphere of high concentrations of carbon dioxide and seawater supersaturated with calcium carbonate. Such conditions were promoted by the high temperatures required to kick the Earth out of its "snowball" state (Fig. 4.27). The incredible buildup of the greenhouse gas, carbon dioxide, in the atmosphere was a direct consequence of a lack of liquid water and the cessation of weathering processes; this buildup essentially saved the surface of the planet from an eternal frozen state. The glacial deposits and the cap carbonates, however, are also strongly depleted in the 13C isotope; this suggests very little biological productivity was in progress that could have removed the lighter 12C isotope, causing preferential enrichment of the heavier 13C stable isotope. And, finally, banded ironstone formations (BIFs) are a feature of the snowball interval suggesting the existence of an anoxic, stratified ocean system. Some BIFs are even associated with ice-rafted dropstones. Not everyone, of course, agrees with this hypothesis; some have suggested a milder "slushball Earth" and some even deny the possibility of global ice sheets altogether. But, surely these "freeze-fry" episodes had an important influence on the mode of organic evolution. Biological evolution would certainly have continued, not least associated with active volcanic vents deep under the ice and in other extreme environments. However evidence for metazoan life seems to appear directly after snowball Earth.

Figure 4.27 Snowball Earth scenario. (a) Continents are near the equator, increasing precipitation removes CO2 from the atmosphere, and with falling temperatures ice begins to spread from the poles. (b) Ice continues to spread with temperatures further reduced by the albedo (reflection of solar energy) effect. (c) Atmospheric CO2 increases due to volcanic activity, prompting a reversal in temperatures. (d) Greenhouse conditions return and the ice sheets recede. (Courtesy of Jorgen Christiansen and Svend Stouge.)

Figure 4.27 Snowball Earth scenario. (a) Continents are near the equator, increasing precipitation removes CO2 from the atmosphere, and with falling temperatures ice begins to spread from the poles. (b) Ice continues to spread with temperatures further reduced by the albedo (reflection of solar energy) effect. (c) Atmospheric CO2 increases due to volcanic activity, prompting a reversal in temperatures. (d) Greenhouse conditions return and the ice sheets recede. (Courtesy of Jorgen Christiansen and Svend Stouge.)

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