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Effects of Past Global Change on Life
while some are associated with thermocline depths, and a few are found as deep as 600 m. Shallow-dwelling species tend to be spinose, whereas deeper-dwelling forms are nonspinose.
Time-series sampling through the year by plankton tows and sediment traps (Curry et al., 1983; Thunell et al., 1983a; Deuser and Ross, 1989) has demonstrated a seasonal succession of planktonic foraminiferal communities. Different depth habitats and seasonal preferences are reflected in the stable isotopic and trace metal chemistry of their calcium carbonate tests (Thunell et al., 1983b; Deuser and Ross, 1989). Shallower-dwelling species can be distinguished from deeper forms on the basis of oxygen and carbon isotopic composition, other geochemical differences, and morphology. Summer-dwelling forms are marked by lower oxygen isotopic values. Morphology also varies between different environments. For example, test size and pore diameter of Orbulina universa in the Indian Ocean increase toward lower latitudes (Bé et al., 1973).
Laboratory culture work has demonstrated the temperature and salinity tolerances of certain planktonic foraminiferal species (for example, Bijma et al., 1990). Stable isotopic analysis of forms cultured under different growth conditions has furthered understanding of the effect of light, temperature, and nutrient availability on the isotopic composition of the shell (Spero and DeNiro, 1987; Spero and Williams, 1988, 1990). A valuable summary of recent work on the biology and ecology of planktonic foraminifera is provided by Hemleben et al. (1988).
The Last Deglaciation
The oceanwide trend of rising temperatures associated with the last deglaciation was accompanied by a poleward expansion of warm-water faunal provinces. Changes in surface circulation returned tropical and subtropical waters and their associated planktonic foraminiferal assemblages to the Gulf of Mexico, where they replaced transitional zone assemblages (Kennett and Huddlestun, 1972; Sidner and Poag, 1972; Brunner, 1982). Average sea surface temperature (SST) rose from 22 to 24°C (CLIMAP, 1976).
The general trend of rising temperatures and higher sea-level was punctuated by rapid increases, but included a brief cool interval, the Younger Dryas. Named after the reappearance of the dryas flower that flourished in northern Europe during the late glacial, the Younger Dryas is shown as an intense cooling episode in marine records of the North Atlantic region, and in terrestrial records including northern Europe. Faunal, isotopic, and sedimentological work has also documented its occurrence in areas across the Northern Hemisphere. An oceanic cool interval correlative with the Younger Dryas has recently been documented in the Sulu Sea (Linsley and Thunell, 1990; Kudrass et al., 1991), the Gulf of California (Keigwin and Jones, 1990), the western North Pacific (Chinzei and Oba, 1986; Chinzei et al., 1987; Kallel et al., 1988), and the Gulf of Mexico (Kennett et al., 1985). High-resolution work in the Gulf of Mexico highlights the speed of faunal and isotopic changes associated with the Younger Dryas episode (Flower and Kennett, 1990). A brief cooling during deglaciation in the Indian sector of the Southern Ocean (Labracherie et al., 1989) seems to precede the Younger Dryas cooling by 1000 yr. If the dates for this cooling are correct, it implies a diachronism between the Southern Ocean and the Northern Hemisphere for this brief reversal during general deglaciation.
Greenland ice core work has suggested that the Younger Dryas is bracketed by rapid climatic transitions of less than 300 yr. Dansgaard et al. (1989) have suggested that this cool interval ended in about 20 yr and involved a temperature increase of about 7°C, although Fairbanks (1989, 1990) has questioned the interpretation of the oxygen isotopic record and the chronology, and instead suggests that the Younger Dryas was an interval of slower sea-level rise. Two distinct meltwater-induced rises in sea-level, centered at 13 to 11 ka and 10 to 7 ka, are separated by an episode of slower sea-level rise between 11 and 10 ka, correlative with the Younger Dryas.
Because the source of deglacial sea-level rise was largely meltwater derived from the Laurentide and Fennoscandian ice sheets, oceanic biota near continental outlets and in marginal basins were affected directly by the influx of low-salinity fresh water. Planktonic foraminiferal assemblages in the Gulf of Mexico (Kennett et al., 1985; Flower and Kennett, 1990; this chapter) responded to this influx of meltwater from the Laurentide ice sheet. Periodic freshwater input to the Mediterranean in the late Quaternary also had an influence on foraminiferal assemblages. Sapropel layers in the eastern Mediterranean inferred to accumulate during episodes of freshwater influx are usually characterized by a distinctive "sapropel-related" assemblage (Thunell et al., 1977; Muerdter et al., 1984). Rapid faunal changes associated with deglacial freshwater influx should also be apparent in the Black Sea and the Caspian Sea, as a result of the wasting of the Fennoscandian ice sheet. High-resolution biostratigraphic work in such marginal basins will expand our understanding of the faunal response in areas especially sensitive to global change.
RESULTS AND DISCUSSION
In this chapter, data are summarized on the response of key members of the planktonic foraminiferal community to deglacial warming and to freshwater influx to the Gulf of Mexico. As a semienclosed basin influenced by the