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of current debate. On a smaller scale, ice core data are now revealing that during the past 40,000 years the climate of Greenland has changed dramatically for intervals of just 10 to 20 years (Taylor et al., 1993).

Sudden Shifts and Gradual Trends

Two issues of timing are especially difficult to resolve. One is whether important events were protracted. The other is whether protracted events were pulsatile. An incomplete geologic record can give the false appearance of suddenness for an event that was actually protracted. Similarly, an imperfect record can give the false appearance of simultaneity for physical events, such as onsets or terminations of glaciation in two or more regions. A worldwide chronology for the multiple glaciations near the end of Proterozoic time has yet to be established, for example. Events of severe extinction that appear to have been protracted or to have occurred in multiple steps warrant statistical scrutiny. A key issue is the completeness of the records of taxa before their final disappearance. Imperfect records can produce an illusion of gradual or multistep extinction for a group of taxa that actually died out simultaneously: the so-called Signor-Lipps effect.

For some major events, however, the geologic record is of sufficiently high quality to document a stepwise or pulsatile pattern. Eight steps of extinction, for example, have been identified for the Cenomanian-Turonian crisis (about 91 m.y. ago). The Ordovician crisis (about 440 m.y. ago) had two principal phases, each possibly lasting hundreds of thousands of years (see Berry et al., Chapter 2). The first pulse, at or near the Rawtheyan-Hirnantian stage boundary, coincided with glacial expansion. The second occurred within the Hirnantian (latest Hirnantian Ordovician) age, during the glacial maximum.

The geologic record documents numerous changes in the global ecosystem that spanned many millions of years. For these trends, the record, though imperfect, is too extensive to be masking a single dramatic event. We may nonetheless have difficulty in distinguishing between gradual and stepwise patterns for such trends. The classic example of this kind of trend is the climatic transition toward cooler and drier conditions on many continents between Eocene and Pleistocene times. During this time, prevailing biomes over broad regions shifted from tropical forest through savanna to grassland and steppe (see Christophel, Chapter 10; Webb and Opdyke, Chapter 11). While the terminal Eocene transition described earlier was a major early step in this trend (and was itself a complex event), the degree to which later changes were stepwise is not well established. It is, however, clear that net rates of change varied from place to place. One of the difficulties in resolving the details is in distinguishing between global changes and regional changes that resulted from such events as tectonic uplift in the American West or the Himalayan region.

The most recent geologic record offers special opportunities to establish temporal resolution for events on very short time scales. Studies of glacial varves suggest that the Younger Dryas cooling episode that interrupted deglaciation in the Northern Hemisphere between about 11,400 and 10,200 years ago developed during an interval of less than 300 years and may have ended during an interval of less than 20 years (Dansgaard et al., 1989).

The Nature of Thresholds

Even a gradual environmental change can result in a sudden change of state when a threshold is crossed. The growth and contraction of glaciers are inherently unstable processes because glaciers have a higher albedo than land. One result is that the birth of a relatively small glacier can plunge a high latitude region into an interval of widespread glaciation. The development of the Antarctic cryosphere during the Eocene-Oligocene transition is such an episode. Cooling during the Late Eocene eventually proceeded to a

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