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OVERVIEW AND RECOMMENDATIONS 5 enrichment in 18O when polar glacial expansion preferentially sequesters 16O from the hydrosphere. For reasons not yet known, periodicities of ~41,000 years, reflecting the tilt cycle of the Earth's axis, dominated until about 0.8 m.y. ago, when periodicities of ~100,000 years, reflecting the shape of the Earth's orbit, began to prevail. Cycles in some pre-Neogene marine successions appear to reflect minor changes in sea-level or biotic productivity that were forced by Milankovich controls mediated by factors that remain poorly understood but may not always have entailed changes in ice volume. Certain Mesozoic lake deposits also contain evidence for Milankovitch-driven cyclicity, perhaps related to shifting monsoons or other patterns of rainfall and evaporation. Nonperiodic Cycles The most profound nonperiodic cycles of global change have been long-terms oscillations between what have been termed the ''hothouse" and the "icehouse" states for oceans and atmospheres. The term hothouse is preferred to "greenhouse" because the conditions described may not always result from greenhouse warming; the hothouse states are, however, characterized by warm polar regions and warm deep oceans. In contrast, the icehouse state entails cold (usually glacial) polar conditions and a frigid deep-sea that results from the descent of cold polar waters. The geologic record spanning the Eocene-Oligocene boundary documents the transition between a hothouse state and the icehouse state that has persisted to the present (Kennett et al., 1972). Much farther back in the geologic record, the interval spanning the Ordovician-Silurian boundary documents a similar transition, as well as the subsequent melting and retreat of glaciers and return of warmer conditions across broad regions (see Berry et al., Chapter 2). The Eocene-Oligocene Transition The recent ice age in the Northern Hemisphere constitutes only an intensification of the icehouse state that our planet entered about 34 m.y. ago, at the end of the Eocene Epoch. Fossil floras and vertebrate faunas reveal that early in Eocene time, subtropical conditions extended north of the Arctic Circle and that southeastern England and the Paris Basin (45 to 50° N) supported tropical rain forests. Fossil floras are, in fact, the most valuable indicators of terrestrial climates for the past 100 m.y. Not only does the taxonomic composition of fossil floras reflect climatic conditions, but so does leaf morphology, especially the percentage of species with smooth, as opposed to jagged or lobed, leaf margins; this percentage varies linearly in the modern world with mean annual temperature (Figure 1). Leaf morphology and cuticular structure also provide a guide to precipitation conditions. Fossil floras show that the Eocene-Oligocene climatic shift was profound at middle and high latitudes in both hemispheres. As warm-adapted floral elements disappeared from these regions, other types of vegetation, adapted to colder and drier conditions, expanded (see Christophel, Chapter 10). Climates actually did not undergo a simple shift between Early Eocene and Early Oligocene time. The tropical flora of England began to disappear at the end of Early Eocene time, as global temperatures began to cool, especially at high latitudes. By Late Eocene time, woodland savanna had already become the dominant vegetation of midcontinental North America (see Webb and Opdyke, Chapter 11). Whether the particular temporal pattern observed for North America characterized other continents remains uncertain, in part because of uncertain dating and in part because in some regions, such as Australia, a floral record is missing for much of the Eocene. It is now widely agreed that the plate tectonic separation of Australia from Antarctica was a primary trigger of climatic changes near the end of the Eocene (and continuing separation caused further climatic changes after Eocene time). This event created the
OVERVIEW AND RECOMMENDATIONS 6 incipient circum-Antarctic current, which began to isolate the Antarctic continent from warm waters flowing from the north. The resulting cooling of surface waters led to the formation of cool deep waters. Enrichment of 18O in both planktonic and deep-sea benthic foraminifera and an influx of ice-rafted sediments indicate a significant, although temporary, expansion of the East Antarctic ice sheet at this time (see Kennett and Stott, Chapter 5). North Atlantic deep water (NADW), which is less dense than South Atlantic deep water (SADW), began to form slightly later, when rifting separated Greenland from Europe, permitting Arctic waters to descend into the North Atlantic (Schnitker, 1980). There is no question that climates cooled at middle and high latitudes, but four major questions remain: Figure 1 Estimated changes in temperature in four areas over the course of Cenozoic time, based on percentages of species or fossil terrestrial plants having smooth leaf margins. Especially evident is dramatic cooling near the end of the Eocene Epoch (after Wolfe, 1978). 1. Fossil floras indicate that temperate conditions extended to high latitudes in both hemispheres during Early Eocene time. How was so much heat transported from the equator toward the poles? 2. During this very warm interval, were the tropics warmer than, cooler than, or comparable to the tropics today? 3. How did the oceanographic changes during the Eocene-Oligocene transition shut down the heat transport system that had existed previously? 4. How did biotas throughout the world respond to the environmental changes? Strong wind-driven currents cannot account for most of the meridional transport during the Eocene because, being dependent on steep thermal gradients, such currents are self-limiting. Most likely, a primary transport mechanism was the poleward flow of warm, saline subsurface water masses that formed at low latitudes. Whether the flux was sufficient to depress tropical temperatures below their modern levels remains a major question. Another is how the new system of thermohaline circulation thwarted this heat flux. Also at issue is the role of the greenhouse effect in producing the widespread warmth