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BIOTIC RESPONSES TO TEMPERATURE AND SALINITY CHANGES DURING LAST DEGLACIATION, GULF OF MEXICO 214 was about -30 to -40%o (Shackleton, 1977), the spike centered at 12 ka has been interpreted as an influx of glacial meltwater. The deglacial meltwater spike is superimposed on the general decrease in δ18O due to deglacial warming and the effect of reduced continental ice volume. The meltwater spike is followed immediately by an episode of increased δ18O (Figure 12.3) correlative with the Younger Dryas cooling (Flower and Kennett, 1990). Oxygen isotopic records for the two cores are plotted versus age in Figure 12.3. The meltwater spike began at about 14 ka, reached a peak at 12 ka, and ended abruptly at 11.4 ka. The cessation of meltwater influx to the Gulf of Mexico was followed immediately by an episode of higher δ18O that lasted from 11.4 to 10.2 ka, indicating some combination of higher salinity and cooler sea-surface temperatures. Cool surface water foraminiferal assemblages in EN32-PC4 (Flower and Kennett, 1990; this chapter) and in EN32-PC6 (Kennett et al., 1985) confirm the presence of the Younger Dryas event in the Gulf of Mexico. The rapid onset of the Younger Dryas following the cessation of meltwater influx lends support to the hypothesis of a meltwater trigger in the North Atlantic shutting down the conveyor belt and causing an oceanically controlled cooling (Broecker et al., 1989; Broecker et al., 1990b; Flower and Kennett, 1990). A diversion of low-salinity meltwater away from the Mississippi toward the St. Lawrence system might have upset the density-driven production of NADW and disrupted the heat pump in the North Atlantic, plunging the region into the Younger Dryas cool episode. This hypothesis requires a reintroduction of meltwater to the Gulf of Mexico at the end of the Younger Dryas, as the ice front receded and southern outlets were reexposed. Support comes from a rapid decrease of 1.2%o within 200 yr centered at 10.2 ka in EN32- PC4 (Broecker et al., 1989; Flower and Kennett, 1990); Spero and Williams (1990) also found evidence for seasonal low- salinity events at 9.8 ka based on isotopic analyses of single foraminifera from EN32-PC6. An oxygen isotopic shift of 1%o occurs over 500 yr in EN32-PC6 at 10 ka (Figure 12.2). In both cores, however, oxygen isotopic values are much less negative than those of the main meltwater spike. Further, the oxygen isotopic record for EN32-PC4 derived from the pink form of Gs. ruber shows no evidence for the Younger Dryas event or for a reintroduction of meltwater to the Gulf of Mexico at 10ka (Figure 12.2). Since this form is favored during summers in the North Atlantic and the Gulf of Mexico (Bé and Tolderlund, 1971; Tolderlund and Bé, 1971; Deuser and Ross, 1989; Flower and Kennett, 1990), it should have recorded an increase in meltwater flux ~10 ka, because meltwater flow from the continent almost certainly would have peaked during summer months. The constancy of the δ18O values is a complication possibly explained by continued summer meltwater flux during the Younger Dryas without an increase at its conclusion (Flower and Kennett, 1990). Faunal Response to Temperature and Salinity Changes in the Gulf of Mexico Planktonic foraminiferal assemblages were very sensitive to the temperature and salinity changes in Gulf of Mexico surface waters associated with rapid deglacial climatic shifts. Past work in the North Atlantic has shown faunal migrations in response to changing surface water circulation (Duplessey et al., 1981; Ruddiman and McIntyre, 1981). Our high-resolution work in the Gulf of Mexico documents the deglacial faunal response not only to changing surface water temperatures, but also to other environmental stresses including the influence of low-salinity meltwater from the Laurentide ice sheet to the north. Relative abundance changes in two Orca Basin cores (EN32-PC4 and EN32-PC6) are presented here for five temperature- and/or salinity-sensitive species of planktonic foraminifera (Figures 12.4 and 12.5); a more complete treatment is given in Flower and Kennett (1990). Species sensitive to surface water temperatures were identified by their association with late Quaternary glacialinterglacial cycles in the Gulf of Mexico (Kennett and Huddlestun, 1972; Malmgren and Kennett, 1976). The warmest surface water indicators include Globorotalia menardii and Pulleniatina obliquiloculata. Cold water forms include Globorotalia inflata, Globigerinafalconensis, and Globigerina bulloides. The δ18O stratigraphy derived from Gs. ruber (white variety) is shown on the same plot for comparison. Abundances of cool-water forms were generally higher, whereas warm-water forms were low during the late glacial through the early part of the meltwater spike from 20 to 13 ka, when surface water temperatures were low, nearly 2°C cooler at the glacial maximum (CLIMAP, 1976). Warm-water forms dominated from the later part of the meltwater spike beginning at 13 ka through the Holocene, except for a brief return of cold-water forms during the Younger Dryas. Globigerinoides ruber (Figures 12.4e and 12.5e) was the dominant species in fossil assemblages throughout the late glacial and the Holocene, with abundances ranging from 20 to 70% but usually averaging 30 to 40%. In both EN32-PC4 and EN32-PC6, Gs. ruber averaged about 35% during the late glacial, reached maximum abundances of 70% during the early part of the meltwater spike, dropped rapidly to 30% at the cessation of meltwater influx, and increased slightly into the late Holocene. Globorotalia inflata (Figures 12.4a and 12.5a) is an indicator species for the temperate/subarctic zone in the modern North Atlantic ocean and a clear marker of Quaternary glacial episodes in the Gulf of Mexico (Kennett
BIOTIC RESPONSES TO TEMPERATURE AND SALINITY CHANGES DURING LAST DEGLACIATION, GULF OF MEXICO 215 and Huddlestun, 1972; Malmgren and Kennett, 1976). This species showed high abundances during the last glacial maximum, disappeared near the beginning of the meltwater spike at 14 ka, and reappeared briefly at 11.4 ka. Figure 12.4 Percentage frequency variations of selected environmentally-sensitive species of planktonic foraminifera in the >150 µm size fraction in EN32-PC4 from Orca Basin: (a) Globigerinoidesfalconensis, (b) Globorotalia inflata, (c) Pulleniatina obliquiloculata, (d) Neogloboquadrina dutertrei, (e) Globigerinoides ruber, plotted against corrected 14C age. Also shown are the δ18O record (f) of Globigerinoides ruber (white variety) and foraminiferal subzones of Kennett and Huddlestun (1972). Subzones Y2, Y1, Y1A, and Z2 correspond to the late glacial, the meltwater spike, the Younger Dryas, and the early Holocene, respectively. Globigerina falconensis (Figures 12.4b and 12.5b), a cold-water species in the Gulf of Mexico (Kennett and Huddlestun, 1972; Malmgren and Kennett, 1976), showed relatively high frequencies during the late glacial, decreased during the later part of the meltwater interval from 13 to 11.5 ka, increased between 11.0 and 10.0 ka, and decreased to its lowest frequencies in the Holocene. Pulleniatina obliquiloculata (Figures 12.4c and 12.5c), a warm-water species in the Gulf of Mexico, was absent during the late glacial, appeared during the early part of the meltwater spike at about 13.7 ka, and showed sporadically high abundances between 12.7 and 11.7 ka, after which it decreased slowly to a minimum ~10.2 ka. It then increased in steps at 9.8 and 8.7 ka. Globorotalia menardii (not figured), a tropical/warm subtropical species in the Gulf, was absent during the late glacial, was present sporadically during the meltwater spike, was a consistent component after 9.8 ka, and underwent a further increase at 5.5 ka. Neogloboquadrina dutertrei (Figures 12.4d and 12.5d), a marginally warm-water species in the Gulf of Mexico (Kennett and Huddlestun, 1972; Malmgren and Kennett, 1976), exhibited moderate frequencies during the late glacial. Abundances increased at 13 ka and generally remained high until 11.3 ka, decreased between 11.3 and 10.2 ka, and then increased to Holocene values. Changing relative abundances of planktonic foraminifera with well-known environmental preferences follow closely the history of deglacial temperature and salinity changes. Late glacial assemblages until about 14 ka included Globorotalia inflata and Globigerina falconensis (Figures 12.4 and 12.5). The reappearance of a warmwater fauna at about 13 ka in the Gulf of Mexico corresponded to an increase in meltwater influx, but preceded its peak. Low salinities in the early part of the meltwater spike favored the euryhaline Gs. ruber. This association is supported by independent observations, which showed that Gs. ruber tolerates lower salinities than other planktonic species (as low as 22%o; Bijma et al., 1990). Field observations in Barbados (Hemleben et al., 1987) suggested that all planktonic foraminiferal species except Gs. ruber descend to higher- salinity waters in response to periodic appearances of low-salinity lenses derived from the Amazon River. Maximum abundances of Gs. ruber were reached at 13.5 ka and preceded the peak of the meltwater spike at 12 ka marking lowest salinities.
BIOTIC RESPONSES TO TEMPERATURE AND SALINITY CHANGES DURING LAST DEGLACIATION, GULF OF MEXICO 216 Figure 12.5 Percentage frequency variations of selected environmentally sensitive species of planktonic foraminifera in the >150 µm size fraction in EN32-PC6 from Orca Basin: (a)Globigerinoidesfalconensis, (b) Globorotalia inflata, (c) Pulleniatina obliquiloculata, (d)Neogloboquadrina dutertrei, (e) Globigerinoides ruber, plotted against corrected 14C age. Also shown are the δ18O record (f) of Globigerinoides ruber (white variety) and foraminiferal subzones of Kennett and Huddlestun (1972). Subzones Y2, Y1, Y1A, Z2 and Z1 correspond to the late glacial, the meltwater spike, the Younger Dryas, the early Holocene, and the late Holocene, respectively. N. dutertrei also displays an association with low-salinity, but its increase in abundance during lowest salinities from 13 to 12 ka could be due to surface water warming, because other warm-water species increased at the same time. This association was found previously in Gulf of Mexico cores (Kennett and Shackleton, 1975; Thunell, 1976) and in the North Atlantic and Mediterranean (Ruddiman, 1969; Bé and Tolderlund, 1971; Thunell et al., 1977; Thunell, 1978; Loubere, 1981). Increased abundances of N. dutertrei were found associated with some but not all Quaternary sapropel layers in the eastern Mediterranean, inferred to have been triggered by periodic low-salinity events (for a summary, see Muerdter et al., 1984). Surface waters in the early part of the meltwater spike are thought to have remained relatively cool until 13 ka, too cool for N. dutertrei to have proliferated like the more opportunistic Gs. ruber. Deglacial warming at 13 ka and lowest salinities at 12 ka were marked by increased abundances of warm-water species, decreased abundances of cool-water species, and continued elevated frequencies of low-salinity tolerant species. Neogloboquadrina dutertrei and Pulleniatina obliquiloculata show simultaneous increases in both cores, while Gg. falconensis decreases and Gr. inflata disappears (Figures 12.4 and 12.5). Surface waters in the later part of the meltwater spike were then warm enough to support N. dutertrei in addition to Gs. ruber. Our data further show that the lowest salinities during deglaciation favored the pigmented form of Gs. ruber over the white form. A plot of the pink:white Gs. ruber percentage ratio for EN32-PC4 shows a peak coincident with the meltwater spike in δ18O (Figure 12.6). The reason for this is unknown and may involve different salinity tolerances, a longer summer season favoring the pink form, or other environmental conditions favorable to pigmentation. The latter may include an association with a symbiont that induces coloration. There is no faunal evidence to indicate that surface waters were cooled directly by the meltwater influx itself, as suggested from modeling studies (Oglesby et al., 1989; Overpeck et al., 1989). Although the planktonic foraminiferal assemblages cannot be translated directly into temperatures, they do indicate warm surface waters during the interval of lowest salinity. The isotopic values of Gs.
