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BIOTIC RESPONSES TO TEMPERATURE AND SALINITY CHANGES DURING LAST DEGLACIATION, GULF OF MEXICO 210 warming fostered the appearance of Holocene assemblages at 10.2 ka. Further warming occurred at 6 ka. Faunal assemblage analysis offers an independent approach to understanding paleoenvironmental change. Information contained in census data bears on entire communities throughout the year and is not limited to certain times and/or depths. By comparison, oxygen isotopic analysis of the pink form of Gs. ruber, an inferred summer dweller in the Gulf of Mexico, does not show evidence for Younger Dryas cooling. Faunal changes further demonstrate that the onset of Younger Dryas cooling was very rapid, occurring in less than 200 yr. The integration of high-resolution faunal and geochemical records of the last deglaciation helps constrain climate models of the geographic extent and rapidity of climatic changes. Faunal analysis also has the potential for resolving smaller-scale events and perturbations that cannot be confirmed by geochemical methods but are essential to an understanding of climate systems. INTRODUCTION The chapters presented in this volume explore the close linkages between global environmental and biotic changes in the geologic past. The sensitivity of the Earth's biota to Phanerozoic environmental events is illustrated by a range of responses, including migrations, extinctions, evolutionary turnover, and morphological variation. Little is known, however, about the potential responses of the modern biota to recent and future anthropogenically forced environmental changes. Understanding the faunal response to past global change provides insights into past and present Earth systems, and can assist with the prediction of the effects of global warming, consequent sea-level rise, changes in oceanic and atmospheric circulation patterns, and climatic variability. Prediction of the biotic response to global change requires an understanding of the faunal response to similar changes in the recent geological past. Excellent records exist of the changes associated with the last major climatic transition, from the last glacial episode (oxygen isotope stage 2) to the Holocene, centered at ~11 ka. Marine sedimentary sequences of this age are accessible, have been extensively cored, are well-preserved, and provide continuous records. As a result, much is known about global environmental changes during this time. Marine microfossils in deep-sea sedimentary sequences are often abundant enough for statistically reliable faunal analysis and geochemical work. Much information about past environments has been inferred from microfossil assemblages composed of extant species whose ecology is well known. These assemblages not only respond to major environmental changes inferred by using independent approaches, but also reflect smaller-scale changes that are not resolved by geochemical and other methods. Morphological responses of marine plankton to environmental change are large and have also been underutilized. First exploited primarily as a stratigraphic tool and termed "the new paleontology" by Emiliani (1969), the analysis of ecophenotypic variation has important applications to paleoenvironmental problems (for a summary, see Kennett, 1976). Considerable work on Quaternary marine sediments over the past two decades has highlighted the sensitivity of marine plankton communities to major oceanographic changes in the North Atlantic associated with the transition from the last glacial episode to the Holocene. A selection of papers representing work on different plankton groups and using different approaches is discussed here. Foraminifera (Imbrie and Kipp, 1971; Duplessey et al., 1981; Ruddiman and McIntyre, 1981; Kellogg, 1984), radiolaria (Imbrie and Kipp, 1971; Morley and Hays, 1979), and coccolithophorids (McIntyre et al., 1976) followed deglacial shifts in water mass locations and geochemical parameters in the North Atlantic. Ruddiman and McIntyre (1981) traced the latitudinal migrations of planktonic foraminifera associated with the North Atlantic polar front from 15 to 9 ka. Boltovskoy (1990) showed how planktonic foraminiferal assemblages followed temperature changes in the western equatorial Pacific. Deglacial oceanographic changes in the North Pacific and the Bering Sea produced changes in community structure in diatoms (Sancetta, 1979) and radiolaria (Morley and Hays, 1983; Heusser and Morley, 1985). The CLIMAP project fully exploited the sensitivity of marine plankton to environmental change and utilized empirical transfer functions to translate downcore plankton assemblages into quantitative estimates of temperature, salinity, and other parameters for the glacial ocean at 18 ka (CLIMAP, 1976; Cline and Hays, 1976; McIntyre et al., 1976; Moore et al., 1980). Recent work has also focused on the biotic response to other water mass property changes in addition to temperature. Molfino and McIntyre (1990) interpreted abundance changes of the deep-dwelling Florisphaera profunda relative to other coccolithophorids over the past 20,000 yr in terms of nutricline depth changes in the equatorial Atlantic