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OVERVIEW AND RECOMMENDATIONS 8 Chapter 3). Near the end of the Ordovician Period, upward advection of toxic anoxic waters associated with expansion of the oxygen minimum zone may have caused major extinctions of midwater planktonic graptolites, while cooling also eliminated marine taxa (see Berry et al., Chapter 2). Value of High Latitude Biotas Stratigraphic records at high latitudes are often critical to understanding patterns and causes of global climatic change. They also document the radiation of cold-adapted biotas during icehouse intervals and the extinction of these biotas during transitions to the hothouse state. It is enlightening to compare the middle to late Cenozoic transition from warm-adapted to cold-adapted terrestrial biotas at northern and southern high latitudes (see Askin and Spicer, Chapter 9). Apparently because higher taxa in the north spread from seasonally arid middle-latitude regions, angiosperms that occupied the new cold climates of the Northern Hemisphere were all deciduous or capable of dormancy. Antarctica, in contrast, became a center of evolutionary innovation as it grew increasingly isolated with the rifting apart of Australia and South America. Here evergreen rain forests prevailed in relatively warm coastal areas. Both northern and southern polar regions suffered drastic declines in floral diversity during the climatic cooling trend that began in the Middle Eocene. Today, only mosses and lichens grow along ice-free margins of Antarctica. In cold temperate and boreal zones of the Northern Hemisphere, the mixed coniferous forest that is widespread today became well established early in the Miocene at a time of widespread nonglacial climates. Grasses assumed a prominent role in floras of northern high latitudes near the Miocene- Pliocene transition (about 5.3 m.y. ago), when the taiga and tundra expanded dramatically. Unidirectional Shifts Whereas most large-scale environmental transitions in Earth history have been reversed after an interval of time, others have represented unreversed net secular trends. The composition of ancient soils, for example, points to a buildup of atmospheric oxygen from about 1% of the present atmospheric level (PAL) at about 2200 m.y. ago to about 15% PAL at about 1900 m.y. ago (see Knoll and Holland, Chapter 1). This shift, which may have been affected by a complex feedback system involving the marine geochemistry of iron and phosphorus, must have dramatically increased the production of nitrates and thus permanently altered patterns of productivity in the oceans. Carbon-13 enrichment of carbonates and buried organic matter during the interval between 850 and 580 m.y. ago probably reflects accelerated burial of organic carbon, especially near the end of this interval. Thus, it may also reflect an increase in the partial pressure of atmospheric oxygen, as may a contemporaneous shift in the isotopic composition of marine sulfates. New data continue to support the hypothesis that atmospheric oxygen levels increased both at the beginning and at the end of the long Proterozoic Eon and had important consequences for biological evolution. RATES OF TRANSITION During the past few years, new evidence from high-resolution stratigraphy has revealed that many important environmental changes and biotic responses were more sudden than previously believed. The most notable example is the group of events that ended the Cretaceous Period. Most of these appear to have occurred in a crisis, perhaps measured in months rather than years, that many experts believe resulted from the impact of a comet or a meteoriteâor from two or more related crises of this type. Evidence of this event, and of other sudden (though less dramatic) changes in the global ecosystem, has led to a resurgence of catastrophism as a paradigm to explain some fraction of the change in the earth system through time. How large a shift toward catastrophism is justified is a matter