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OVERVIEW AND RECOMMENDATIONS 7 of the Early Eocene. If mean global temperature was well above that of the modern world, greenhouse warming is perhaps the most likely cause. If warmer high latitudes were accompanied by cooler equatorial conditions, then heat transport may have been the dominant control on the latitudinal temperature gradient. More generally, one can ask how the secular changes in the greenhouse capacity of the atmosphere have interacted with increasing solar luminosity, continental geographies, and orogenic uplift to produce the significant climatic oscillations recorded throughout geologic time. The Younger Dryas Cooling Events in the North Atlantic during the earliest stages of deglaciation between 14,000 and 11,400 years ago represent another example of change in thermohaline circulation. The North Atlantic is part of a circuit that extends to the Pacificâthe so-called great conveyor belt (Broecker and Denton, 1989). North Atlantic deep water flows southward and is entrained in the Antarctic Current. It then passes into and through the Indian Ocean and the South Pacific to the North Pacific, where it upwells and returns by surface currents to the North Atlantic. There it loses heat that warms the climate of northern Europe and sinks again. During the emergence from the most recent glacial maximum, there was a brief expansion of glaciers between 11,400 and 10,200 years ago known as the Younger Dryas event. The cause of this reversal was in part a change in the flow of meltwater from North America to the ocean: Surface salinities declined in the North Atlantic because meltwater was diverted from the Mississippi drainage eastward to the St. Lawrence. Mixture with these buoyant waters reduced the density of the waters flowing northward into the North Atlantic and interrupted the formation at the surface of relatively dense NADW until the meltwater pulse had largely ended. Shifts in the isotopic composition of fossil planktonic foraminifera document this sequence of events, especially in the Gulf of Mexico region (see Flower and Kennett, Chapter 12). Plankton assemblages responded to changes in the salinity of surface waters. The Younger Dryas event illustrates how even a relatively small-scale perturbation of thermohaline circulation can have global oceanographic and climatic consequences. The Terminal Ordovician Transition Studies of events at the close of the Ordovician Period, about 440 m.y. ago, illustrate how it is also possible to interpret general causes and consequences of major climatic fluctuations for pre-Cenozoic intervals (see Berry et al., Chapter 2). These events are associated with one of the largest mass extinctions of all time. The stratigraphic record offers evidence that continental glaciation began at this time in the supercontinent of Gondwanaland and that the buildup of glaciers lowered sea- level by at least 50 m. Paleomagnetic data and environmental reconstructions reveal that Gondwanaland was moving over the South Pole. Marine fossils indicate a transition to a hothouse state: Hirnantian fauna, which had previously been restricted to cool water masses of the deep-sea and to high latitudes, expanded over broad regions of the ocean, replacing warm-adapted taxa that became extinct. Shifts to Hothouse Intervals A shift in the opposite direction, from the icehouse to the hothouse state, is associated with the development of warmer polar regions and deep ocean waters that are warm, sluggish, and dysaerobic to anoxic. Such a shift will automatically eliminate much life in the deep-sea. During the mid-Cretaceous highstand of sea-level, anoxic conditions extended upward into the deep portions of epicontinental seas, and it was during this interval that environmental perturbations produced pulses of biotic destruction that constitute the Cenomanian-Turonian mass extinction (see Kauffman,