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Effects of Past Global Change on Life 12 Biotic Responses to Temperature and Salinity Changes During Last Deglaciation, Gulf of Mexico BENJAMIN P. FLOWER and JAMES P. KENNETT University of California, Santa Barbara ABSTRACT Understanding the biotic response to past global change provides insights into past and present Earth systems. Marine sedimentary records of the last deglaciation centered at 11,000 14C years ago (ka) are particularly promising in contributing to understanding and predicting the biotic effects of anthropogenic environmental changes. Fossil assemblages of plankton preserved in marine sediments represent an often underutilized source of paleoenvironmental information. Relative abundances of different species and their morphology vary dynamically in response to environmental change. Numerous studies have exploited the sensitivity of marine plankton assemblages to environmental changes in the last glacial ocean by transforming downcore relative abundances directly into quantitative estimates of surface temperature, salinity, and other parameters. In some areas with unique oceanographic characteristics, transform functions do not provide reliable surface temperature and salinity estimates during times of great environmental change in the geologic past, so-called "no analog situations." For instance, planktonic foraminiferal assemblages during the last deglaciation in the Gulf of Mexico were influenced considerably by surface water salinities much lower than at present. Detailed oxygen isotopic and faunal analyses of radiometrically dated cores from the Orca Basin, Gulf of Mexico, illustrate the response of planktonic foraminifera to temperature and salinity changes in a marginal basin that amplified deglacial environmental change. Warm-water planktonic foraminifera began to replace cold-water forms in response to early deglacial warming at 14 ka, and the euryhaline form Globigerinoides ruber dominated during an episode of low-salinity meltwater influx into the Gulf of Mexico from 14 to 11.4 ka. Warm-water forms increased in abundance at 13 ka. A brief reappearance of cool species assemblages associated with the last glacial episode documents the presence of Younger Dryas cooling between 11.4 and 10.2 ka in the Gulf of Mexico. Late deglacial
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Effects of Past Global Change on Life 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
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Effects of Past Global Change on Life 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, Bé and Tolderlund, 1971; Tolderlund and Bé, 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 Bé, 1971).
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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
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Effects of Past Global Change on Life FIGURE 12.2 Oxygen isotopic records from analyses of Globigerinoides ruber (white variety) for EN32-PC4 (left, after Broecker et al., 1989) and EN32-PC6 (right, after Leventer et al., 1982), plotted against depth in core as d18O relative to the PDB standard. The cores show the late glacial to early Holocene interval, with the meltwater spike and the Younger Dryas chronozone labeled. Ages in thousands of years are accelerator radiocarbon dates, corrected for the 14C/12C difference between atmospheric CO2 and surface water SCO2, revised after Broecker et al. (1990). Mississippi outlet, the Gulf of Mexico was particularly sensitive to circulation changes in the North Atlantic and to meltwater runoff from the Laurentide ice sheet (Prest et al., 1968; 1970; Kennett and Shackleton, 1975). We have examined undisturbed sequences marked by high sedimentation rates from the Orca Basin with abundant planktonic microfossils, thus providing a high-resolution record. Located on the continental rise 290 km south of the Mississippi Delta (Figure 12.2 ), with a maximum depth of 2400 m and a sill depth of 1800 m, the Orca Basin is filled with a hypersaline brine to a depth of 2230 m. Resulting anoxic conditions in the bottom waters provide excellent preservation, eliminate dissolution, and exclude benthic organisms that mix the sediments. These conditions preserve a pristine sedimentary sequence with undisturbed laminae, allow a high-resolution stratigraphy with a sampling interval of 100 yr, and provide a clear picture of dynamic changes in faunal assemblages unaffected by dissolution or bioturbation. 14C Chronology Accelerator radiocarbon dates from two Orca Basin cores (EN32-PC4 and EN32-PC6), recently revised by Broecker et al. (1990a), provide excellent age control over the last deglaciation (Figure 12.3). Although calibration of the 14C time scale has shown that 14C dates correspond to somewhat older U-Th ages (Bard et al., 1990), the 14C time scale is adopted here for ease of comparison with previously published chronologies. The 14C ages show no stratigraphic inversions, although recent work has shown that the radiocarbon time scale remains constant at about 10 ka for a few hundred years (Oeschger et al., 1980; Andrée et al., 1986; Becker and Kromer, 1986; Lowe et al., 1988; Bard et al., 1990). All data presented here are plotted against 14C age by extrapolation between dated samples. Stable Isotopic Records High-resolution stable isotopic records have also been generated for these two Orca Basin samples (EN32-PC4, Broecker et al., 1989; Flower and Kennett, 1990, and EN32-PC6, Leventer et al., 1982; Kennett et al., 1985). The two oxygen isotopic records are similar, measured on Gs. ruber and plotted as d18O (%o relative to the PDB belemnite standard) against depth in Figure 12.2. The main feature is the pronounced negative spike in d18O, indicating major freshwater flooding into the Gulf of Mexico. Because the isotopic composition of Laurentide glacial ice FIGURE 12.3 Plots of d18O records from analyses of Globigerinoides ruber plotted versus age for EN32-PC4 and EN32-PC6 from the Orca Basin. White and pink varieties of Globigerinoides ruber are plotted for EN32-PC4 against 14C age, as discussed in text. Note the different age scale for EN32-PC6.
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Effects of Past Global Change on Life 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 d18O due to deglacial warming and the effect of reduced continental ice volume. The meltwater spike is followed immediately by an episode of increased d18O (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 d18O 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 d18O 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 d18O 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
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Effects of Past Global Change on Life 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 d18O 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. 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. 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.
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Effects of Past Global Change on Life 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 d18O 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 d18O (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.
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Effects of Past Global Change on Life FIGURE 12.6 Percentage ratio (percent pink:percent white) for the pink and white varieties of Globigerinoides ruber is plotted for EN32-PC4 from the Orca Basin against 14C age, as discussed in text. Also shown are the d18O 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. ruber during the meltwater spike also cannot be translated into temperature because of the low-salinity overprint. However, modern field observations (Hemleben et al., 1987) suggest that most planktonic foraminifera probably migrated to deeper waters below the relatively fresh surface waters that were perhaps colder. The end of the low-salinity meltwater interval was marked by an important reappearance of cold-water forms characteristic of the last glacial episode. A brief reappearance of Gr. inflata occurs within a few centimeters in both cores and is dated at 11.4 ka. This species is today associated with the transition zone in the North Atlantic between the subtropical and subpolar surface water masses. Its reappearance in the Gulf of Mexico represents a major shift in the position of this boundary and is inferred to mark the onset of the Younger Dryas. The speed of the biotic response was remarkable, occurring in less than 200 yr. The reappearance of Gr. inflata was followed by increased relative abundances of Gg. falconensis and decreased abundances of N. dutertrei, until about 10.2 ka. These changing abundances mark an interval between 11.4 and 10.2 ka of very cold, followed by cool, surface water conditions. The correspondence of this cool interval with the Younger Dryas centered in the North Atlantic region shows that oceanic cooling extended to the Gulf of Mexico. Its presence in the Gulf of Mexico (Kennett et al., 1985; Flower and Kennett, 1990; this chapter), the Sulu Sea (Linsley and Thunell, 1990; Kudrass et al., 1991), and the Gulf of California (Keigwin and Jones, 1990) suggests that expressions of the Younger Dryas occur throughout the Northern Hemisphere, if not worldwide. The end of the Younger Dryas is marked by faunal and isotopic changes over less than 500 yr. Declining cool-water assemblages coincide with a decrease in d18O and are followed by increases in warm-water assemblages. The reappearance of consistent Gr. menardii, usually taken as marking the beginning of the Holocene, occurs at 9.8 ka. It is accompanied by large increases in Pu. obliquiloculata and N. dutertrei. Further increases in the warm-water species Gr. menardii and Pu. obliquiloculata along with a decrease in the marginally warm-water species N. dutertrei occur at 5.5 ka and distinguish a warmer subzone in the late Holocene. This warmer late Holocene assemblage is not accompanied by any change in d18O of Gs. ruber, underlining the importance of independent faunal analysis in addition to geochemical methods in the investigation of paleoenvironmental change. CONCLUSIONS Oxygen isotopic and faunal analyses of high-resolution, radiometrically dated sediment sequences in the Gulf of Mexico demonstrate that planktonic foraminiferal communities were sensitive to deglacial environmental changes, including rapid temperature and salinity changes. Fossil assemblages reflect deglacial warming and low-salinity meltwater influx from the Laurentide ice sheet, the rapid onset and conclusion of the Younger Dryas, and stepwise warming into the Holocene. Rapid migrations characterized the dynamic response of oceanic biota and water masses in an area that amplified deglacial environmental change. Warm-water planktonic foraminifera including Pulleniatina obliquiloculata and Neogloboquadrina dutertrei began to replace cold-water forms at 14 ka as the glacial species Globorotalia inflata disappeared in response to early deglacial warming. Simultaneously, the euryhaline
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Effects of Past Global Change on Life form Globigerinoides ruber increased to its greatest abundances of 70% during a period of low-salinity meltwater influx. Warm-water forms increased in abundance at 13 ka as the cool-water species Globigerina falconensis decreased in response to warmer SSTs. A brief reappearance of the glacial species Gr. inflata at the expense of warmwater forms at 11.4 ka marks a rapid, temporary migration of cold surface water into the Gulf of Mexico. This event is followed immediately by an interval of increased abundances of Gg. falconensis, and decreased abundances of N. dutertrei and Pu. obliquiloculata, and heralds the beginning of the Younger Dryas cooling in the Gulf of Mexico. Late deglacial warming at about 10.2 ka fostered the appearance of warm-water Holocene assemblages including Gr. menardii. Further warming at 5.5 ka distinguishes a warmer subzone in the late Holocene. The euryhaline species Gs. ruber bloomed during the early portion of the meltwater spike. After surface waters had warmed sufficiently at 13 ka, the low-salinity tolerant species N. dutertrei also showed higher abundances due to some combination of lower salinities and warmer temperatures. Lowest salinities at 12 ka favored the pink form of Gs. ruber. There is no faunal evidence that surface waters were cooled directly by meltwater influx. In fact, warm-water assemblages are present during the interval of lowest salinity. However, field observations suggest that most planktonic foraminifera probably migrated to deeper waters below the relatively fresh surface waters that were perhaps cooler. These results demonstrate the need for further high-resolution work on the response of oceanic fauna to rapid environmental changes associated with deglaciation, including temperature and salinity. 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Representative terms from entire chapter: