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BIOTIC RESPONSES TO TEMPERATURE AND SALINITY CHANGES DURING LAST DEGLACIATION, GULF OF MEXICO 211 associated with variations in equatorial divergence. Other studies have examined the response of both planktonic and benthic biota. Planktonic and benthic foraminiferal assemblages were affected by salinity and circulation changes in the Red Sea and the Gulf of Aden (Locke and Thunell, 1988) and in the Mediterranean (Thunell et al. , 1977; Muerdter et al., 1984). Pedersen et al. (1988) documented the response of benthic foraminiferal populations to deglacially induced changes in surface productivity in the Panama Basin. Radiometrically dated, high-resolution work in some areas has produced highly detailed faunal and isotopic records of deglaciation. Oxygen isotopic and planktonic foraminiferal assemblage records from the Sulu Sea (Linsley and Thunell, 1990; Kudrass et al., 1991) and the Gulf of Mexico (Kennett et al., 1985; Flower and Kennett, 1990) illustrate the sensitivity of planktonic foraminifera to rapid oceanic environmental change and document the expression of the Younger Dryas cooling event. Labracherie et al. (1989) showed coherence between faunal and oxygen isotopic changes in the Indian sector of the Southern Ocean and documented a brief return to near-glacial temperatures from ~12 to 11 ka. Studies such as these integrate high-resolution faunal and isotopic evidence to define in detail the character of rapid deglacial environmental and biotic changes. The integration of Quaternary faunal and geochemical records from deep-marine sequences has demonstrated the extensive influence of North Atlantic oceanography in disparate areas of the world's oceans and has led to the concept of a "conveyor belt" mode of circulation (Broecker et al., 1985). In this model, the Quaternary ocean circulation system is paced by the formation of North Atlantic deep water (NADW), which flows as a deep current through the South Atlantic Ocean, is entrained into the circumpolar current, and flows through the Indian Ocean to the Pacific where it upwells, eventually returning to the North Atlantic via surface currents. A circuit is completed, on average, every 1000 yr. Turning the conveyor belt on or off may have controlled oceanographic and climatic changes in the late Quaternary, which influenced the Earth's biota. Ecology of Modern Planktonic Foraminifera The need to interpret fossil planktonic foraminiferal assemblages has fueled interest in the biology and ecology of modern forms. Modern planktonic foraminifera are found throughout the oceans, with diversity increasing toward the tropics. Effectively only one species is found in polar oceans, whereas about 30 species inhabit the tropics. The geographic distribution of distinctive assemblages, which follows major water mass boundaries, is shown for the North Atlantic Ocean in Figure 12.1. Plankton tow work (for example, BÃ© and Tolderlund, 1971; Tolderlund and BÃ©, 1971; Fairbanks et al., 1982) has shown that most forms live in the mixed layer (upper 100 m), Figure 12.1 Distribution of planktonic foraminiferal assemblage provinces and ocean stations in the North Atlantic (from Tolderlund and BÃ©, 1971).