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Temperature:δ18O relationships of planktonic foraminifera collected from surface waters
Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2003) 143^152 www.elsevier.com/locate/palaeo
Temperature:N18O relationships of planktonic foraminifera collected from surface waters Stefan Mulitza a; , Demetrio Boltovskoy b , Barbara Donner a , Helge Meggers a , Andre¤ Paul a , Gerold Wefer a a
Universita«t Bremen, Fachbereich 5 ^ Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany b Universidad de Buenos Aires, Buenos Aires, Argentina Received 11 September 2002; received in revised form 1 August 2003; accepted 5 September 2003
Abstract Most of the isotopic paleotemperature equations used for paleoceanographic reconstructions have been derived from culture experiments or inorganic precipitates of calcium carbonate. To test these equations in the modern ocean, we measured the oxygen isotope composition of planktonic foraminifera (Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma) collected from Atlantic and Southern Ocean surface waters, and added published plankton tow data from the Pacific, Indian and Arctic Oceans. The resulting species-specific regression equations of the temperature:N18 O relationships for G. ruber, G. sacculifer and G. bulloides are statistically indistinguishable. The equations derived for G. sacculifer and G. bulloides agree with relationships obtained from laboratory experiments, in which these species were cultured at pH values close to modern surface waters. The equation derived from N. pachyderma has a significantly lower slope and offset than the other three species but produces a regression equation that is nearly identical to the one for the epifaunal benthic foraminifer Cibicides sp. Our work on plankton tow and pumped samples indicates that culture-derived equations appear to be more appropriate for predicting the absolute N18 O of the species examined compared to equations derived from inorganic precipitates. However, over the oceanic temperature range, the slopes of the equations we derive for living species agree with the slopes obtained from inorganic precipitates. 5 2003 Elsevier B.V. All rights reserved. Keywords: planktonic foraminifera; O-18/O-16; paleothermometry; paleotemperature; oceanic pH
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1. Introduction The ratio of the stable oxygen isotopes
18
O and
* Corresponding author. Tel.: +49-421-218-7110; Fax: +49-421-218-3116. E-mail address: [email protected] (S. Mulitza).
O is one of the most important tools in paleoceanography and paleoclimatology. In marine sediments, the oxygen isotopic composition of planktonic foraminifera is primarily used as a proxy for the temperature (e.g., Emiliani, 1954) and isotopic composition of the ambient seawater (e.g., Duplessy et al., 1991). The use of stable oxygen isotope ratios in paleoceanography, however, is limited by a number of problems. N18 O
0031-0182 / 03 / $ ^ see front matter 5 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00633-3
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measurements on living foraminifera from surface waters (Duplessy et al., 1981) and laboratory cultures (Bemis et al., 1998) show distinct deviations with respect to the isotopic equilibrium indicated by inorganic precipitates. These deviations are presumably in£uenced by life processes including calci¢cation rate (e.g., Ortiz et al., 1996), photosynthesis (Spero and Lea, 1993), and respiration (e.g., Wolf-Gladrow et al., 1999), the so-called vital e¡ects, and the carbonate ion concentration or pH of calci¢cation (Spero et al., 1997; Bijma et al., 1999; Zeebe, 1999). Despite the advances in the understanding of vital e¡ects through culture experiments and modelling studies, an empirical calibration of oxygen isotope^temperature relationships of foraminiferal shells grown in a natural environment is important to document the entire natural variability of oxygen isotope ratios in living foraminiferal populations and to validate temperature:N18 O relationships of planktonic foraminifera calibrated in laboratory experiments. The vast majority of isotope measurements in paleoceanographic studies of the last decades have been done on the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma because these species not only are very abundant, but also have a broad geographical and temporal coverage and are among the shallowest-dwelling planktonic foraminifera. Here, we present ¢eld-based temperature :N18 O relationships for samples of these species collected in surface waters over a broad range of hydrographic conditions and compare the equations with published temperature:N18 O relationships derived from culture experiments and inorganic precipitates. The N18 O values turned out to be signi¢cantly lower than suggested by established paleotemperature equations, but are very close to the temperature :N18 O relationships of foraminifera grown under controlled laboratory conditions. However, over the oceanic temperature range, the slopes of our temperature:N18 O relationships are nearly identical to the paleotemperature equation of Shackleton (1974). This suggests that temperature changes can be reliably reconstructed with this equation, at least for the species examined here.
2. Methods and data 2.1. Sample collection and oxygen isotope measurements We have measured the oxygen isotope composition of planktonic foraminifera collected in the western tropical Atlantic, the Argentine Basin and the Southern Ocean on several cruises with the research vessels Meteor and Bara‹o De Te¡e¤. In the western tropical Atlantic, seawater was pumped with the ship’s ¢re pump from about 5 m depth and ¢ltered through a 70-Wm-mesh net. Planktonic foraminifera were picked immediately from the wet samples and washed with fresh water and ethanol. Temperature was measured continuously with the ship’s onboard temperature probe. During any single sampling period, water temperature usually varied by less than K 0.2‡C (Bleil et al., 1998). At the end of each pumping transect, a 50-ml water sample was taken for oxygen isotope analysis. Several casts of strati¢ed plankton tows (70 Wm mesh size) were taken in the Argentine Basin (Segl et al., 1994) and the Southern Ocean (Boltovskoy et al., 1996, 2000). These samples were ¢xed with 3^5% bu¡ered formaldehyde or mercury chloride to prevent biological activity. Foraminiferal tests for stable isotope analyses were picked from the wet samples at the University of Bremen and the University of Buenos Aires. Water temperature was measured with a CTD probe, a bucket thermometer (surface) or XBT launchings at the towing sites. Isotope measurements of foraminifera and seawater were performed at the University of Bremen following standard procedures (e.g., Hebbeln et al., 2000). For the sites where water samples for oxygen isotope measurements were not available, seawater N18 O was estimated from regional N18 O^ salinity relationships. For the sites where both N18 O and salinity were not measured, N18 O of sea water was estimated from a 5‡ interpolation of surface (0^50 m) N18 O of seawater from the data base of Schmidt et al. (1999) with the objective analyses described in Levitus and Boyer (1994). However, 48% of the entire data set used for our calibration consists of direct measure-
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ments of seawater N18 O. All seawater N18 O data were corrected to the PDB scale by subtracting 0.27x (Hut, 1987). 2.2. Data base We extended our data set with N18 O data of living Globigerinoides sacculifer, Globigerinoides ruber (white) and Globigerina bulloides from the Indian and Paci¢c Oceans (Duplessy et al., 1981; Kahn and Williams, 1981; Ganssen, 1983; Peeters et al., 2002) and with Neogloboquadrina pachyderma (sin.) data from the Arctic Ocean (Bauch et al., 1997) (Fig. 1). Since the vertical N18 O gradient is very high at some stations in the Arctic, we only included tows 276^340 of the Bauch et al. (1997) data set, where N18 O of seawater changes less than 0.2x over the upper 500 m of the water column to avoid potentially incorrect assignments of N18 O of seawater due to strong hydrographical changes within the tow interval. From the data of Duplessy et al. (1981), Kahn and Williams (1981), Ganssen (1983) and Peeters et al. (2002) we have only included tows that extend to the sea surface. The resulting compilation covers almost the entire present-day tem-
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perature range of the surface ocean and includes measurements of various size fractions of foraminifera larger than 150 Wm. The entire data base used in his study is available at the PANGAEA data base (www.pangaea.de). 2.3. Statistical methods The temperature:N18 O relationships for the individual species were estimated with a linear least squares regression of the form y = bx+a (Table 1). The signi¢cances of the regression parameters of the resulting linear regression equations were tested against each other for equality (null hypothesis H0 : b1 = b2 and H0 : a1 = a2 ) with a t-test (e.g., Sachs, 2002) where t is calculated with Mb1 3b2 M ^t ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 sy1;x1 ðn1 32Þ þ s2y2;x2 ðn2 32Þ 1 1 þ Qx1 Qx2 n1 þ n2 34
for the comparisons of the slopes and with Ma1 3a2 M ^t ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 P 2 2 2 sy1;x1 ðn1 32Þ þ sy2;x2 ðn2 32Þ x1 x2 þ n1 þ n2 34 n1 Qx1 n2 Qx2
Fig. 1. Locations of plankton tows and pump samples used in this study. Also included are data from Kahn and Williams (1981), Duplessy et al. (1981), Ganssen (1983), Bauch et al. (1997) and Peeters et al. (2002).
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Table 1 Regression parameters for the used equations
Globigerinoides ruber Globigerinoides sacculifer Globigerina bulloides Neogloboquadrina pachyderma Cibicidoides wuellerstor¢
b
a
n
Q
c
gx2
34.44 34.35 34.70 33.55 33.60
14.20 14.91 14.62 12.69 12.75
91 68 21 49 117
77.93 74.84 29.26 64.77 67.69
0.93 1.06 1.24 1.17 0.77
676.58 545.21 46.51 536.85 971.04
for the intercept, where b and a are the slope and intercept of the regression equations, Q is the residual sum of squares, s2 is the variance and n is the number of observations. The calculated values of t and critical values of t (at the 5% error level) are given in Table 2.
3. Results We calculated regression equations for the relationship between temperature and foraminiferal N18 O (expressed as di¡erence to the N18 O of the ambient seawater) for the following species : Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma (Fig. 2, Table 1). All species show lower N18 O values than predicted by equations derived from inorganic precipitates (Fig. 2) (O’Neil et al., 1969; Shackleton, 1974; Kim and O’Neil, 1997). Conversely, all derived regression equations imply
lower isotopic temperatures than indicated by the equations mentioned above. The slopes and o¡sets of the regression equations for G. ruber, G. sacculifer and G. bulloides are relatively close. For example, the N18 O of both G. sacculifer and G. ruber changes by 0.22x/‡C and that of G. bulloides by 0.21x/‡C. The regression derived for N. pachyderma has a much lower slope and predicts a N18 O change of about 0.28x/‡C. A comparison of the equations with a t-test indicates that the equations for G. ruber, G. sacculifer and G. bulloides are not di¡erent at a 0.05 error probability (Table 2). By contrast, the same test statistic indicates a signi¢cantly lower slope and intercept for the N. pachyderma equation compared to the other equations derived here. The scatter around the regression equations (1c standard deviation of residuals) varies between 0.93 for G. ruber and 1.24‡C for G. bulloides (Table 1). The relationships derived here generally show very low deviations from equations derived for
Table 2 Results of t-test for regression coe⁄cient and intercept of the derived regression equations
Globigerinoides sacculifer, this work Globigerina bulloides, this work Neogloboquadrina pachyderma, this work Cibicidoides wuellerstor¢, core top (Duplessy et al., 2002) Globigerinoides sacculifer, culture (Erez and Luz, 1983) Globigerina bulloides, culture (Bemis et al., 1998)
tslope tintercept tslope tintercept tslope tintercept tslope tintercept tslope tintercept tslope tintercept
Globigerinoides ruber, this work
Globigerinoides sacculifer, this work
Globigerina bulloides, this work
Neogloboquadrina pachyderma, this work
0.56/1.98 1.59/1.98 1.20/1.98 1.02/1.98 5.16/1.98 2.86/1.98 5.96/1.98 3.66/1.98 0.75/1.98 6.48/1.98 0.25/1.98 0.33/1.98
^ ^ 1.46/1.99 0.62/1.99 4.26/1.98 3.81/1.98 5.04/1.98 5.08/1.98 1.08/1.99 4.21/1.99 0.28/1.99 0.64/1.99
^ ^ ^ ^ 4.34/2.00 3.28/2.00 5.83/1.98 4.93/1.98 0.33/2.02 4.49/2.02 0.04/2.05 0.45/2.05
^ ^ ^ ^ ^ 0.32/1.98 0.12/1.98 4.17/1.99 6.87/1.99 0.76/2.00 0.39/2.00
Values are t (left) and critical value of t (right). Regression coe⁄cient and intercept were tested for equality. If t exceeds the critical value of t (bold values) the null hypothesis of equality (H0 : b1 = b2 , H0 : a1 = a2 ) is rejected at the 5% error level.
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Fig. 2. Temperature versus Nc3Nw for towed and pumped specimens of Globigerinoides sacculifer (A), Globigerinoides ruber (B), Globigerina bulloides (C) and Neogloboquadrina pachyderma (D) and comparison with the paleotemperature equation of Shackleton (1974) based on inorganic precipitates of O’Neil et al. (1969) (dashed line). The bold line is the regression line for each of the species. The plot includes data from Duplessy et al. (1981), Kahn and Williams (1981), Ganssen (1983), Bauch et al. (1997) and Peeters et al. (2002).
cultured foraminifera (Bemis et al., 1998; Spero et al., 2003), in which the controlled pH has been close to present-day oceanic surface waters (Fig. 3). On the other hand, all relationships show considerably lower values than those predicted from cultured Globigerinoides sacculifer without any control of the carbonate system (Fig. 3) (Erez and Luz, 1983). We also compare the regression parameters to a linear equation derived from N18 O measurements of the benthic foraminifer Cibicides sp. recently compiled by Duplessy et al. (2002) (Fig. 4). The relationships derived for Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bul-
loides have signi¢cantly higher slopes and intercepts than that of Cibicides sp. (Table 2). By contrast, the regression equation of Neogloboquadrina pachyderma is indistinguishable from that of Cibicides sp.
4. Discussion 4.1. Comparison of species-speci¢c equations derived from plankton tow and pump samples The equation derived from Neogloboquadrina pachyderma has a signi¢cantly lower slope and
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Fig. 3. Comparison of the oxygen isotope data for Globigerinoides sacculifer and Globigerina bulloides compiled in this study with culture-derived data and equations for these species. (A) Nc3Nw versus temperature for Globigerinoides sacculifer compiled in this study (open circles) and cultured by Erez and Luz (1983) (¢lled circles) and regression equations for both data sets (bold lines). The dashed line denotes the equation derived by Spero et al. (2003) from pH-controlled culture experiments with G. sacculifer. (B) Comparison of the oxygen isotope data for G. bulloides compiled in this study (¢lled circles) with the culture-derived equation (13-chambered shell) of Bemis et al. (1998) (open circles) and regression equations for both data sets.
intercept than the equations of Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides. Hence, the N18 O change with temperature is more pronounced at low temperatures, as predicted from thermodynamics and observed for inorganic precipitates (e.g., O’Neil et al., 1969). Our data further indicate that the regression equations derived from Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides are statistically indistinguishable. This does not necessarily mean that the temperature:N18 O relationships of these species are not di¡erent in other experiments. The statistical equality of the equations may also be due to a high variability of the residuals in our data compilation. The scatter around the regression equations is much higher than the internal error of the measurements (V0.07x = V0.3‡C). One reason for the scatter might be errors in N18 O of seawater for that part of the data set for which N18 O was estimated. However, the standard deviation of the residuals for that subset is only about 0.01‡C higher than for those samples for which N18 O of seawater was measured. This indicates that errors in the estimation of N18 O of seawater are not the main reason for the high variability in our data set. Another important factor might be the size
range of the measured foraminifera. Most of the data included in this work were derived from foraminifera in the size range between 150 and 400 Wm. Previous works indicate that smaller foraminifera in this range can be about 0.1^0.2x light-
Fig. 4. Comparison of temperature:N18 O relationships of Neogloboquadrina pachyderma and benthic Cibicides sp. (Duplessy et al., 2002) and regression equations for both relationships.
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er than larger ones (e.g., Bauch et al., 1997; Bemis et al., 1998; Peeters et al., 2002). A better control of the size fraction might result in a slight reduction of the scatter. However, at least for Globigerinoides sacculifer and Globigerinoides ruber, di¡erences related to size are not statistically signi¢cant (Duplessy et al., 1981). Recently, the pH of seawater was identi¢ed as an important factor a¡ecting the oxygen isotope composition of foraminiferal calcite (Bijma et al., 1999; Zeebe, 1999). Maps of surface water pH (Brewer et al., 1995) show variations between 8.27 in the tropical Atlantic and a minimum of 8.18 in northern high latitudes. Assuming a decrease of 1.1x per pH unit increase (Zeebe, 1999), variations in surface water pH might lead to systematic di¡erences of about 0.1x in our data set, but since the variations of surface water pH are largely correlated with temperature, we do not expect that variations in ambient pH increase the variability in our data. Another important factor are vital processes that can modify the pH in the microenvironment of the foraminiferal shell itself. JUrgensen et al. (1985) and Wolf-Gladrow et al. (1999) have shown that variations in respiration, calci¢cation and symbiont photosynthesis can lead to considerable modi¢cations of the pH in the vicinity of the foraminiferal shell on the order of K 0.3 pH units with respect to the pH of the ambient seawater. This di¡erence would translate into a N18 O variability of about K 0.3x. Calci¢cation at different light levels, for example during the day or at night, could thus have a noticeable in£uence on the oxygen isotope composition of foraminiferal shells for the symbiont-bearing species Globigerinoides ruber and Globigerinoides sacculifer. Therefore, it seems likely that foraminiferal populations are associated with a variability in their oxygen isotope composition that is not related to hydrography. If vital e¡ects are the major source of variability observed here, it will be di⁄cult to go beyond this work and to validate the generally very small di¡erences between laboratory-derived equations and those obtained for foraminifera from the open ocean, because the microenvironment and the light level of calci¢cation cannot be su⁄ciently controlled in the ¢eld.
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4.2. Comparison to equations derived from culture experiments and inorganic precipitates For the species examined here, culture-derived regression equations have been published for Globigerinoides sacculifer (Erez and Luz, 1983; Spero et al., 2003) and Globigerina bulloides (Bemis et al., 1998). The relationships derived for G. sacculifer and G. bulloides in this work are indistinguishable from those published by Spero et al. (2003) and Bemis et al. (1998) (Fig. 3). In both experiments, the pH of the culture vessel was controlled to be close to the oceanic pH. By contrast, the equation of Erez and Luz (1983) has a signi¢cantly higher intercept than equations derived here (Table 2). It seems reasonable that the observed o¡set between our equation and that of Erez and Luz (1983) is due to di¡erent carbon chemistries in the culture experiments and the present-day open ocean. As mentioned by Bemis et al. (1998), the carbonate system was not controlled during the experiments of Erez and Luz (1983). For this reason, it is possible that atmospheric carbon dioxide migrated into the culture water during the experiments, causing pH to decrease and to subsequently increase the N18 O of culture-grown foraminifera relative to those from the surface ocean. All species examined here show lower N18 O values than indicated by the often used paleotemperature equations of Shackleton (1974) and Kim and O’Neil (1997). The pH e¡ect can also explain the deviations from the synthetic carbonate equations. Zeebe (1999) calculated the critical pH (viz. the pH where carbonate begins to precipitate) for the experiments of Kim and O’Neil (1997) as 7.8. Note that open ocean pH values in the tropics are close to 8.27 (Brewer et al., 1995). Assuming a decrease of 1.1x per unit pH increase (Zeebe, 1999), this di¡erence of 0.4^0.5 pH units alone could account for 0.4^0.5x of the observed 0.4^0.7x di¡erence between Kim and O’Neil (1997) and our regression equations at the upper temperature limit. It has been suggested that planktonic foraminifera have species-speci¢c slopes in their temperature:N18 O relationships (e.g., Bemis et al., 1998). However, in the data set compiled here, the slopes
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agree very closely with the slopes observed for inorganic precipitates (O’Neil et al., 1969; Shackleton, 1974) and are not necessarily species-dependent. Whether the slopes of culturegrown foraminifera are indeed statistically signi¢cantly di¡erent remains to be proven. 4.3. Comparison to benthic foraminifera Most benthic foraminiferal oxygen isotope values are reported on the ‘Uvigerina scale’. Assuming that the benthic foraminifer Uvigerina calci¢es in isotopic equilibrium, Shackleton (1974) extended the synthetic paleotemperature equation of O’Neil et al. (1969) to cold, bottom water temperatures and validated the equation with measurements of Uvigerina peregrina from core tops. Shackleton (1974) also observed a negative disequilibrium o¡set of Cibicides relative to Uvigerina. To account for this e¡ect, Cibicides N18 O data are usually corrected to the Uvigerina scale by adding 0.64x. On the other hand, Bemis et al. (1998) suggested that Cibicides might also calcify in isotopic equilibrium and that the heavier N18 O values of Uvigerina are due to calci¢cation at lower porewater pH, because Uvigerina is an infaunal species, whereas Cibicides mainly lives on the sediment. Comparison of the Neogloboquadrina pachyderma data with the values for Cibicides recently compiled by Duplessy et al. (2002) shows an excellent agreement indeed (Fig. 4). The resulting regression equations for both species are nearly identical and statistically indistinguishable (Table 2). Both species show a mean slope of 0.28x/‡C, which is in perfect agreement with the slope of Shackleton’s (1974) relationship in the temperature range between 13 and 32‡C. Hence, the use of Shackleton’s (1974) equation seems to be justi¢ed for the species examined here, if relative temperature changes are to be reconstructed. The question as to why the N18 O of Uvigerina and Cibicides are di¡erent remains. Local pH gradients in the upper centimeters of the sediment seem to be too low in most places to explain the mean di¡erence of 0.64 (e.g., Wenzho«fer et al., 2001). On the other hand, Zahn et al. (1986) have shown that the N18 O di¡erence between Uvi-
gerina and Cibicides is slightly higher in areas with high accumulation of organic carbon and supposedly higher vertical gradients in porewater pH. Again, it might be that the pH in the vicinity of the shell is important for the recorded N18 O calcite and not the overall pH gradient in the porewater that sets the di¡erence between the species. Future microprobe studies with living benthic foraminifera will help resolve this problem.
5. Conclusions Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma, caught from surface waters, generally show lower N18 O values than predicted by the paleotemperature equations of Shackleton (1974) (expanded from O’Neil et al., 1969) and Erez and Luz (1983). Since our data agree with regression equations derived from culture experiments (Bemis et al., 1998; Spero et al., 2003), in which pH is controlled to be close to present-day surface waters, it is likely that the pH of calci¢cation is the reason for the deviations. This implies that culture-derived and pH-controlled equations give more reliable results for ecological applications (e.g., for the calculation of habitat depths) than the equations given by Shackleton (1974) and Erez and Luz (1983). However, it must also be noted that the slopes of the equations derived here agree with the slopes indicated by paleotemperature equations from inorganic precipitates (O’Neil et al., 1969; Shackleton, 1974), which suggest that relative temperature changes can be reliably predicted by these equations over the temperature range examined here (31.8 to 31.1‡C). Slopes and intercepts of species-speci¢c regression equations derived for Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides are statistically not signi¢cantly di¡erent within the data set compiled here. Since the variability is high in our data set, only future comparisons between laboratory-derived equations can show whether the relationships between these species are indeed statistically indistinguishable or whether the use of species-speci¢c equations is justi¢ed.
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Acknowledgements We thank M. Segl, B. Meyer-Schack and W. Bevern for help with sample preparation and oxygen isotope analyses. Thanks to J. Bijma, C. Ru«hlemann, H. Spero, and R. Zeebe for discussion and B. Bemis and J. Erez for making their data available. J.-C. Duplessy, F. Peeters and an anonymous referee provided constructive comments on an earlier version of the manuscript. This work was funded by the Deutsche Forschungsgemeinschaft (DFG Research Center Ocean Margins) and the Bundesministerium fu«r Bildung und Forschung (DEKLIM E, Grant 01 LD 0019 to S.M.). References Bauch, D., Carstens, J., Wefer, G., 1997. Oxygen isotope composition of living Neogloboquadrina pachyderma (sin.) in the Arctic Ocean. Earth Planet. Sci. Lett. 146, 47^58. Bemis, B., Spero, H.J., Bijma, J., Lea, D.W., 1998. Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations. Paleoceanography 13, 150^160. Bijma, J., Spero, H.J., Lea, D.W., 1999. Reassessing foraminiferal stable isotope geochemistry: Impact of the oceanic carbonate system (experimental results). In: Fischer, G., Wefer, G. (Eds.), Use of Proxies in Paleoceanography ^ Examples from the South Atlantic. Springer, Berlin, pp. 489^512. Bleil, U., Cruise Participants, 1998. Report and preliminary results of METEOR Cruise M38/2, Recife^Las Palmas. Ber. Fachbereich Geowiss. 95, 126 pp. Boltovskoy, E., Boltovskoy, D., Correa, N., Brandini, F., 1996. Planktic foraminifera from the southwestern Atlantic (30‡^60‡S): species-speci¢c patterns in the upper 50 m. Mar. Micropaleontol. 28, 53^72. Boltovskoy, E., Boltovskoy, D., Brandini, F., 2000. Planktonic foraminifera from southwestern Atlantic epipelagic waters: abundance, distribution and year-to-year variations. J. Mar. Biol. Assoc. U.K. 80, 203^213. Brewer, P.G., Glover, D.M., Goyet, C., Shafer, D.K., 1995. The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater. J. Geophys. Res. 100, 8761^8776. Duplessy, J.C., Blanc, P.L., Be¤, A.W.H., 1981. Oxygen-18 enrichment of planktonic foraminifera due to gametogenic calci¢cation below the euphotic zone. Science 213, 1247^ 1250. Duplessy, J.C., Labeyrie, L., Juillet-Leclerc, A., Maitre, F., Duprat, J., Sarnthein, M., 1991. Surface salinity reconstruction of the North Atlantic Ocean during the last glacial maximum. Oceanol. Acta 14, 311^324.
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Duplessy, J.C., Labeyrie, L., Waelbroeck, C., 2002. Constraints on the ocean oxygen isotopic enrichment between the last glacial maximum and the Holocene. Paleoceanographic implications. Quat. Sci. Rev. 21, 315^330. Emiliani, C., 1954. Depth habitat of some species of pelagic foraminifera as indicated by oxygen isotope ratio. Am. J. Sci. 252, 149^158. Erez, J., Luz, B., 1983. Experimental paleotemperature equation for planktonic foraminifera. Geochim. Cosmochim. Acta 47, 1025^1031. Ganssen, G., 1983. Dokumentation von ku«stennahem Auftrieb anhand stabiler Isotope in rezenten Foraminiferen vor Nordwest-Afrika. ‘Meteor’-Forschungsergebnisse 37C, 1^46. Hebbeln, D., Marchant, M., Wefer, G., 2000. Seasonal variations of the particle £ux in the Peru-Chile current at 30‡S under ‘normal’ and El Nino conditions. Deep Sea Res. 47, 2101^2128. Hut, G., 1987. Consultants group meeting on stable isotope reference samples for geochemical and hydrological investigations, Report to the Director General, International Atomic Energy Agency, Vienna, 42 pp. JUrgensen, B.B., Erez, J., Revsbech, N.P., Cohen, Y., 1985. Symbiotic photosynthesis in a planktonic foraminiferan, Globigerinoides sacculifer (Brady), studied with microelectrodes. Limnol. Oceanogr. 30, 1253^1267. Kahn, M., Williams, D.F., 1981. Oxygen and carbon isotopic composition of living planktonic foraminifera from the northeast Paci¢c Ocean. Palaeogeogr. Palaeoclimatol. Palaeoecol. 33, 47^69. Kim, S.T., O’Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope e¡ects in synthetic carbonates. Geochim. Cosmochim. Acta 61, 3461^3475. Levitus, S., Boyer, T.P., 1994. World Ocean Atlas, Temperature, vol. 4. US Department of Commerce, Washington, DC, 117 pp. O’Neil, J.R., Clayton, R.N., Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys. 51, 5547^5558. Ortiz, J.D., Mix, A.C., Rugh, W., Watkins, J.M., Collier, R.W., 1996. Deep-dwelling planktonic foraminifera of the northeastern Paci¢c Ocean reveal environmental control of oxygen and carbon isotopic disequilibria. Geochim. Cosmochim. Acta 60, 4509^4523. Peeters, F.J.C., Brummer, G.J.A., Ganssen, G., 2002. The effect of upwelling on the distribution and stable isotope composition of Globigerina bulloides and Globigerinoides ruber (planktic foraminifera) in modern surface waters of the NW Arabian Sea. Global Planet. Change 34, 269^291. Sachs, L., 2002. Angewandte Statistik. Springer, Berlin, 889 pp. Schmidt, G.A., Bigg, G.R., Rohling, E.J., 1999. Global seawater oxygen-18 database. http://www.giss.nasa.gov/data/ o18data. Segl, M., Cruise Participants, 1994. Report and preliminary results of METEOR-Cruise M29/1, Buenos Aires^Montevideo. Ber. Fachbereich Geowiss. 58, 94 pp. Shackleton, N., 1974. Attainment of isotopic equilibrium be-
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tween ocean water and the benthonic foraminifera genus Uvigerina: Isotopic changes in the ocean during the last glacial. Colloq. Int. C.N.R.S. 219, 203^209. Spero, H.J., Lea, D.W., 1993. Intraspeci¢c stable isotope variability in the planktic foraminifera Globigerinoides sacculifer: Results from laboratory experiments. Mar. Micropaleontol. 22, 221^234. Spero, H.J., Bijma, J., Lea, D.W., Bemis, B.B., 1997. E¡ect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes. Nature 390, 497^500. Spero, H.J., Mielke, K.M., Kalve, E.M., Lea, D.W., Pak, D.K., 2003. Multispecies approach to reconstructing eastern equatorial Paci¢c thermocline hydrography during the past 360 kyr. Paleoceanography 18(1), 1022, doi:10.1029/ 2002PA000814
Wenzho«fer, F., Adler, M., Kohls, O., Hensen, C., Strotmann, B., Boehme, S., Schulz, H.D., 2001. Calcite dissolution driven by benthic mineralization in the deep-sea: In situ measurements of Ca2 , pH, pCO2 and O2 . Geochim. Cosmochim. Acta 65, 2677^2690. Wolf-Gladrow, D.A., Bijma, J., Zeebe, R.E., 1999. Model simulation of the carbonate chemistry in the microenvironment of symbiont bearing foraminifera. Mar. Chem. 64, 181^198. Zahn, R., Winn, K., Sarnthein, M., 1986. Benthic foraminiferal N13 C and accumulation rates of organic carbon: Uvigerina peregrina group and Cibicidoides wuellerstor¢. Paleoceanography 1, 27^42. Zeebe, R., 1999. An explanation of the e¡ect of seawater carbonate concentration on foraminiferal oxygen isotopes. Geochim. Cosmochim. Acta 63, 2001^2007.
PALAEO 3215 24-11-03
Temperature:N18O relationships of planktonic foraminifera collected from surface waters Stefan Mulitza a; , Demetrio Boltovskoy b , Barbara Donner a , Helge Meggers a , Andre¤ Paul a , Gerold Wefer a a
Universita«t Bremen, Fachbereich 5 ^ Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany b Universidad de Buenos Aires, Buenos Aires, Argentina Received 11 September 2002; received in revised form 1 August 2003; accepted 5 September 2003
Abstract Most of the isotopic paleotemperature equations used for paleoceanographic reconstructions have been derived from culture experiments or inorganic precipitates of calcium carbonate. To test these equations in the modern ocean, we measured the oxygen isotope composition of planktonic foraminifera (Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma) collected from Atlantic and Southern Ocean surface waters, and added published plankton tow data from the Pacific, Indian and Arctic Oceans. The resulting species-specific regression equations of the temperature:N18 O relationships for G. ruber, G. sacculifer and G. bulloides are statistically indistinguishable. The equations derived for G. sacculifer and G. bulloides agree with relationships obtained from laboratory experiments, in which these species were cultured at pH values close to modern surface waters. The equation derived from N. pachyderma has a significantly lower slope and offset than the other three species but produces a regression equation that is nearly identical to the one for the epifaunal benthic foraminifer Cibicides sp. Our work on plankton tow and pumped samples indicates that culture-derived equations appear to be more appropriate for predicting the absolute N18 O of the species examined compared to equations derived from inorganic precipitates. However, over the oceanic temperature range, the slopes of the equations we derive for living species agree with the slopes obtained from inorganic precipitates. 5 2003 Elsevier B.V. All rights reserved. Keywords: planktonic foraminifera; O-18/O-16; paleothermometry; paleotemperature; oceanic pH
16
1. Introduction The ratio of the stable oxygen isotopes
18
O and
* Corresponding author. Tel.: +49-421-218-7110; Fax: +49-421-218-3116. E-mail address: [email protected] (S. Mulitza).
O is one of the most important tools in paleoceanography and paleoclimatology. In marine sediments, the oxygen isotopic composition of planktonic foraminifera is primarily used as a proxy for the temperature (e.g., Emiliani, 1954) and isotopic composition of the ambient seawater (e.g., Duplessy et al., 1991). The use of stable oxygen isotope ratios in paleoceanography, however, is limited by a number of problems. N18 O
0031-0182 / 03 / $ ^ see front matter 5 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00633-3
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measurements on living foraminifera from surface waters (Duplessy et al., 1981) and laboratory cultures (Bemis et al., 1998) show distinct deviations with respect to the isotopic equilibrium indicated by inorganic precipitates. These deviations are presumably in£uenced by life processes including calci¢cation rate (e.g., Ortiz et al., 1996), photosynthesis (Spero and Lea, 1993), and respiration (e.g., Wolf-Gladrow et al., 1999), the so-called vital e¡ects, and the carbonate ion concentration or pH of calci¢cation (Spero et al., 1997; Bijma et al., 1999; Zeebe, 1999). Despite the advances in the understanding of vital e¡ects through culture experiments and modelling studies, an empirical calibration of oxygen isotope^temperature relationships of foraminiferal shells grown in a natural environment is important to document the entire natural variability of oxygen isotope ratios in living foraminiferal populations and to validate temperature:N18 O relationships of planktonic foraminifera calibrated in laboratory experiments. The vast majority of isotope measurements in paleoceanographic studies of the last decades have been done on the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma because these species not only are very abundant, but also have a broad geographical and temporal coverage and are among the shallowest-dwelling planktonic foraminifera. Here, we present ¢eld-based temperature :N18 O relationships for samples of these species collected in surface waters over a broad range of hydrographic conditions and compare the equations with published temperature:N18 O relationships derived from culture experiments and inorganic precipitates. The N18 O values turned out to be signi¢cantly lower than suggested by established paleotemperature equations, but are very close to the temperature :N18 O relationships of foraminifera grown under controlled laboratory conditions. However, over the oceanic temperature range, the slopes of our temperature:N18 O relationships are nearly identical to the paleotemperature equation of Shackleton (1974). This suggests that temperature changes can be reliably reconstructed with this equation, at least for the species examined here.
2. Methods and data 2.1. Sample collection and oxygen isotope measurements We have measured the oxygen isotope composition of planktonic foraminifera collected in the western tropical Atlantic, the Argentine Basin and the Southern Ocean on several cruises with the research vessels Meteor and Bara‹o De Te¡e¤. In the western tropical Atlantic, seawater was pumped with the ship’s ¢re pump from about 5 m depth and ¢ltered through a 70-Wm-mesh net. Planktonic foraminifera were picked immediately from the wet samples and washed with fresh water and ethanol. Temperature was measured continuously with the ship’s onboard temperature probe. During any single sampling period, water temperature usually varied by less than K 0.2‡C (Bleil et al., 1998). At the end of each pumping transect, a 50-ml water sample was taken for oxygen isotope analysis. Several casts of strati¢ed plankton tows (70 Wm mesh size) were taken in the Argentine Basin (Segl et al., 1994) and the Southern Ocean (Boltovskoy et al., 1996, 2000). These samples were ¢xed with 3^5% bu¡ered formaldehyde or mercury chloride to prevent biological activity. Foraminiferal tests for stable isotope analyses were picked from the wet samples at the University of Bremen and the University of Buenos Aires. Water temperature was measured with a CTD probe, a bucket thermometer (surface) or XBT launchings at the towing sites. Isotope measurements of foraminifera and seawater were performed at the University of Bremen following standard procedures (e.g., Hebbeln et al., 2000). For the sites where water samples for oxygen isotope measurements were not available, seawater N18 O was estimated from regional N18 O^ salinity relationships. For the sites where both N18 O and salinity were not measured, N18 O of sea water was estimated from a 5‡ interpolation of surface (0^50 m) N18 O of seawater from the data base of Schmidt et al. (1999) with the objective analyses described in Levitus and Boyer (1994). However, 48% of the entire data set used for our calibration consists of direct measure-
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ments of seawater N18 O. All seawater N18 O data were corrected to the PDB scale by subtracting 0.27x (Hut, 1987). 2.2. Data base We extended our data set with N18 O data of living Globigerinoides sacculifer, Globigerinoides ruber (white) and Globigerina bulloides from the Indian and Paci¢c Oceans (Duplessy et al., 1981; Kahn and Williams, 1981; Ganssen, 1983; Peeters et al., 2002) and with Neogloboquadrina pachyderma (sin.) data from the Arctic Ocean (Bauch et al., 1997) (Fig. 1). Since the vertical N18 O gradient is very high at some stations in the Arctic, we only included tows 276^340 of the Bauch et al. (1997) data set, where N18 O of seawater changes less than 0.2x over the upper 500 m of the water column to avoid potentially incorrect assignments of N18 O of seawater due to strong hydrographical changes within the tow interval. From the data of Duplessy et al. (1981), Kahn and Williams (1981), Ganssen (1983) and Peeters et al. (2002) we have only included tows that extend to the sea surface. The resulting compilation covers almost the entire present-day tem-
145
perature range of the surface ocean and includes measurements of various size fractions of foraminifera larger than 150 Wm. The entire data base used in his study is available at the PANGAEA data base (www.pangaea.de). 2.3. Statistical methods The temperature:N18 O relationships for the individual species were estimated with a linear least squares regression of the form y = bx+a (Table 1). The signi¢cances of the regression parameters of the resulting linear regression equations were tested against each other for equality (null hypothesis H0 : b1 = b2 and H0 : a1 = a2 ) with a t-test (e.g., Sachs, 2002) where t is calculated with Mb1 3b2 M ^t ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 sy1;x1 ðn1 32Þ þ s2y2;x2 ðn2 32Þ 1 1 þ Qx1 Qx2 n1 þ n2 34
for the comparisons of the slopes and with Ma1 3a2 M ^t ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 P 2 2 2 sy1;x1 ðn1 32Þ þ sy2;x2 ðn2 32Þ x1 x2 þ n1 þ n2 34 n1 Qx1 n2 Qx2
Fig. 1. Locations of plankton tows and pump samples used in this study. Also included are data from Kahn and Williams (1981), Duplessy et al. (1981), Ganssen (1983), Bauch et al. (1997) and Peeters et al. (2002).
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Table 1 Regression parameters for the used equations
Globigerinoides ruber Globigerinoides sacculifer Globigerina bulloides Neogloboquadrina pachyderma Cibicidoides wuellerstor¢
b
a
n
Q
c
gx2
34.44 34.35 34.70 33.55 33.60
14.20 14.91 14.62 12.69 12.75
91 68 21 49 117
77.93 74.84 29.26 64.77 67.69
0.93 1.06 1.24 1.17 0.77
676.58 545.21 46.51 536.85 971.04
for the intercept, where b and a are the slope and intercept of the regression equations, Q is the residual sum of squares, s2 is the variance and n is the number of observations. The calculated values of t and critical values of t (at the 5% error level) are given in Table 2.
3. Results We calculated regression equations for the relationship between temperature and foraminiferal N18 O (expressed as di¡erence to the N18 O of the ambient seawater) for the following species : Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma (Fig. 2, Table 1). All species show lower N18 O values than predicted by equations derived from inorganic precipitates (Fig. 2) (O’Neil et al., 1969; Shackleton, 1974; Kim and O’Neil, 1997). Conversely, all derived regression equations imply
lower isotopic temperatures than indicated by the equations mentioned above. The slopes and o¡sets of the regression equations for G. ruber, G. sacculifer and G. bulloides are relatively close. For example, the N18 O of both G. sacculifer and G. ruber changes by 0.22x/‡C and that of G. bulloides by 0.21x/‡C. The regression derived for N. pachyderma has a much lower slope and predicts a N18 O change of about 0.28x/‡C. A comparison of the equations with a t-test indicates that the equations for G. ruber, G. sacculifer and G. bulloides are not di¡erent at a 0.05 error probability (Table 2). By contrast, the same test statistic indicates a signi¢cantly lower slope and intercept for the N. pachyderma equation compared to the other equations derived here. The scatter around the regression equations (1c standard deviation of residuals) varies between 0.93 for G. ruber and 1.24‡C for G. bulloides (Table 1). The relationships derived here generally show very low deviations from equations derived for
Table 2 Results of t-test for regression coe⁄cient and intercept of the derived regression equations
Globigerinoides sacculifer, this work Globigerina bulloides, this work Neogloboquadrina pachyderma, this work Cibicidoides wuellerstor¢, core top (Duplessy et al., 2002) Globigerinoides sacculifer, culture (Erez and Luz, 1983) Globigerina bulloides, culture (Bemis et al., 1998)
tslope tintercept tslope tintercept tslope tintercept tslope tintercept tslope tintercept tslope tintercept
Globigerinoides ruber, this work
Globigerinoides sacculifer, this work
Globigerina bulloides, this work
Neogloboquadrina pachyderma, this work
0.56/1.98 1.59/1.98 1.20/1.98 1.02/1.98 5.16/1.98 2.86/1.98 5.96/1.98 3.66/1.98 0.75/1.98 6.48/1.98 0.25/1.98 0.33/1.98
^ ^ 1.46/1.99 0.62/1.99 4.26/1.98 3.81/1.98 5.04/1.98 5.08/1.98 1.08/1.99 4.21/1.99 0.28/1.99 0.64/1.99
^ ^ ^ ^ 4.34/2.00 3.28/2.00 5.83/1.98 4.93/1.98 0.33/2.02 4.49/2.02 0.04/2.05 0.45/2.05
^ ^ ^ ^ ^ 0.32/1.98 0.12/1.98 4.17/1.99 6.87/1.99 0.76/2.00 0.39/2.00
Values are t (left) and critical value of t (right). Regression coe⁄cient and intercept were tested for equality. If t exceeds the critical value of t (bold values) the null hypothesis of equality (H0 : b1 = b2 , H0 : a1 = a2 ) is rejected at the 5% error level.
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147
Fig. 2. Temperature versus Nc3Nw for towed and pumped specimens of Globigerinoides sacculifer (A), Globigerinoides ruber (B), Globigerina bulloides (C) and Neogloboquadrina pachyderma (D) and comparison with the paleotemperature equation of Shackleton (1974) based on inorganic precipitates of O’Neil et al. (1969) (dashed line). The bold line is the regression line for each of the species. The plot includes data from Duplessy et al. (1981), Kahn and Williams (1981), Ganssen (1983), Bauch et al. (1997) and Peeters et al. (2002).
cultured foraminifera (Bemis et al., 1998; Spero et al., 2003), in which the controlled pH has been close to present-day oceanic surface waters (Fig. 3). On the other hand, all relationships show considerably lower values than those predicted from cultured Globigerinoides sacculifer without any control of the carbonate system (Fig. 3) (Erez and Luz, 1983). We also compare the regression parameters to a linear equation derived from N18 O measurements of the benthic foraminifer Cibicides sp. recently compiled by Duplessy et al. (2002) (Fig. 4). The relationships derived for Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bul-
loides have signi¢cantly higher slopes and intercepts than that of Cibicides sp. (Table 2). By contrast, the regression equation of Neogloboquadrina pachyderma is indistinguishable from that of Cibicides sp.
4. Discussion 4.1. Comparison of species-speci¢c equations derived from plankton tow and pump samples The equation derived from Neogloboquadrina pachyderma has a signi¢cantly lower slope and
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Fig. 3. Comparison of the oxygen isotope data for Globigerinoides sacculifer and Globigerina bulloides compiled in this study with culture-derived data and equations for these species. (A) Nc3Nw versus temperature for Globigerinoides sacculifer compiled in this study (open circles) and cultured by Erez and Luz (1983) (¢lled circles) and regression equations for both data sets (bold lines). The dashed line denotes the equation derived by Spero et al. (2003) from pH-controlled culture experiments with G. sacculifer. (B) Comparison of the oxygen isotope data for G. bulloides compiled in this study (¢lled circles) with the culture-derived equation (13-chambered shell) of Bemis et al. (1998) (open circles) and regression equations for both data sets.
intercept than the equations of Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides. Hence, the N18 O change with temperature is more pronounced at low temperatures, as predicted from thermodynamics and observed for inorganic precipitates (e.g., O’Neil et al., 1969). Our data further indicate that the regression equations derived from Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides are statistically indistinguishable. This does not necessarily mean that the temperature:N18 O relationships of these species are not di¡erent in other experiments. The statistical equality of the equations may also be due to a high variability of the residuals in our data compilation. The scatter around the regression equations is much higher than the internal error of the measurements (V0.07x = V0.3‡C). One reason for the scatter might be errors in N18 O of seawater for that part of the data set for which N18 O was estimated. However, the standard deviation of the residuals for that subset is only about 0.01‡C higher than for those samples for which N18 O of seawater was measured. This indicates that errors in the estimation of N18 O of seawater are not the main reason for the high variability in our data set. Another important factor might be the size
range of the measured foraminifera. Most of the data included in this work were derived from foraminifera in the size range between 150 and 400 Wm. Previous works indicate that smaller foraminifera in this range can be about 0.1^0.2x light-
Fig. 4. Comparison of temperature:N18 O relationships of Neogloboquadrina pachyderma and benthic Cibicides sp. (Duplessy et al., 2002) and regression equations for both relationships.
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er than larger ones (e.g., Bauch et al., 1997; Bemis et al., 1998; Peeters et al., 2002). A better control of the size fraction might result in a slight reduction of the scatter. However, at least for Globigerinoides sacculifer and Globigerinoides ruber, di¡erences related to size are not statistically signi¢cant (Duplessy et al., 1981). Recently, the pH of seawater was identi¢ed as an important factor a¡ecting the oxygen isotope composition of foraminiferal calcite (Bijma et al., 1999; Zeebe, 1999). Maps of surface water pH (Brewer et al., 1995) show variations between 8.27 in the tropical Atlantic and a minimum of 8.18 in northern high latitudes. Assuming a decrease of 1.1x per pH unit increase (Zeebe, 1999), variations in surface water pH might lead to systematic di¡erences of about 0.1x in our data set, but since the variations of surface water pH are largely correlated with temperature, we do not expect that variations in ambient pH increase the variability in our data. Another important factor are vital processes that can modify the pH in the microenvironment of the foraminiferal shell itself. JUrgensen et al. (1985) and Wolf-Gladrow et al. (1999) have shown that variations in respiration, calci¢cation and symbiont photosynthesis can lead to considerable modi¢cations of the pH in the vicinity of the foraminiferal shell on the order of K 0.3 pH units with respect to the pH of the ambient seawater. This di¡erence would translate into a N18 O variability of about K 0.3x. Calci¢cation at different light levels, for example during the day or at night, could thus have a noticeable in£uence on the oxygen isotope composition of foraminiferal shells for the symbiont-bearing species Globigerinoides ruber and Globigerinoides sacculifer. Therefore, it seems likely that foraminiferal populations are associated with a variability in their oxygen isotope composition that is not related to hydrography. If vital e¡ects are the major source of variability observed here, it will be di⁄cult to go beyond this work and to validate the generally very small di¡erences between laboratory-derived equations and those obtained for foraminifera from the open ocean, because the microenvironment and the light level of calci¢cation cannot be su⁄ciently controlled in the ¢eld.
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4.2. Comparison to equations derived from culture experiments and inorganic precipitates For the species examined here, culture-derived regression equations have been published for Globigerinoides sacculifer (Erez and Luz, 1983; Spero et al., 2003) and Globigerina bulloides (Bemis et al., 1998). The relationships derived for G. sacculifer and G. bulloides in this work are indistinguishable from those published by Spero et al. (2003) and Bemis et al. (1998) (Fig. 3). In both experiments, the pH of the culture vessel was controlled to be close to the oceanic pH. By contrast, the equation of Erez and Luz (1983) has a signi¢cantly higher intercept than equations derived here (Table 2). It seems reasonable that the observed o¡set between our equation and that of Erez and Luz (1983) is due to di¡erent carbon chemistries in the culture experiments and the present-day open ocean. As mentioned by Bemis et al. (1998), the carbonate system was not controlled during the experiments of Erez and Luz (1983). For this reason, it is possible that atmospheric carbon dioxide migrated into the culture water during the experiments, causing pH to decrease and to subsequently increase the N18 O of culture-grown foraminifera relative to those from the surface ocean. All species examined here show lower N18 O values than indicated by the often used paleotemperature equations of Shackleton (1974) and Kim and O’Neil (1997). The pH e¡ect can also explain the deviations from the synthetic carbonate equations. Zeebe (1999) calculated the critical pH (viz. the pH where carbonate begins to precipitate) for the experiments of Kim and O’Neil (1997) as 7.8. Note that open ocean pH values in the tropics are close to 8.27 (Brewer et al., 1995). Assuming a decrease of 1.1x per unit pH increase (Zeebe, 1999), this di¡erence of 0.4^0.5 pH units alone could account for 0.4^0.5x of the observed 0.4^0.7x di¡erence between Kim and O’Neil (1997) and our regression equations at the upper temperature limit. It has been suggested that planktonic foraminifera have species-speci¢c slopes in their temperature:N18 O relationships (e.g., Bemis et al., 1998). However, in the data set compiled here, the slopes
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agree very closely with the slopes observed for inorganic precipitates (O’Neil et al., 1969; Shackleton, 1974) and are not necessarily species-dependent. Whether the slopes of culturegrown foraminifera are indeed statistically signi¢cantly di¡erent remains to be proven. 4.3. Comparison to benthic foraminifera Most benthic foraminiferal oxygen isotope values are reported on the ‘Uvigerina scale’. Assuming that the benthic foraminifer Uvigerina calci¢es in isotopic equilibrium, Shackleton (1974) extended the synthetic paleotemperature equation of O’Neil et al. (1969) to cold, bottom water temperatures and validated the equation with measurements of Uvigerina peregrina from core tops. Shackleton (1974) also observed a negative disequilibrium o¡set of Cibicides relative to Uvigerina. To account for this e¡ect, Cibicides N18 O data are usually corrected to the Uvigerina scale by adding 0.64x. On the other hand, Bemis et al. (1998) suggested that Cibicides might also calcify in isotopic equilibrium and that the heavier N18 O values of Uvigerina are due to calci¢cation at lower porewater pH, because Uvigerina is an infaunal species, whereas Cibicides mainly lives on the sediment. Comparison of the Neogloboquadrina pachyderma data with the values for Cibicides recently compiled by Duplessy et al. (2002) shows an excellent agreement indeed (Fig. 4). The resulting regression equations for both species are nearly identical and statistically indistinguishable (Table 2). Both species show a mean slope of 0.28x/‡C, which is in perfect agreement with the slope of Shackleton’s (1974) relationship in the temperature range between 13 and 32‡C. Hence, the use of Shackleton’s (1974) equation seems to be justi¢ed for the species examined here, if relative temperature changes are to be reconstructed. The question as to why the N18 O of Uvigerina and Cibicides are di¡erent remains. Local pH gradients in the upper centimeters of the sediment seem to be too low in most places to explain the mean di¡erence of 0.64 (e.g., Wenzho«fer et al., 2001). On the other hand, Zahn et al. (1986) have shown that the N18 O di¡erence between Uvi-
gerina and Cibicides is slightly higher in areas with high accumulation of organic carbon and supposedly higher vertical gradients in porewater pH. Again, it might be that the pH in the vicinity of the shell is important for the recorded N18 O calcite and not the overall pH gradient in the porewater that sets the di¡erence between the species. Future microprobe studies with living benthic foraminifera will help resolve this problem.
5. Conclusions Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma, caught from surface waters, generally show lower N18 O values than predicted by the paleotemperature equations of Shackleton (1974) (expanded from O’Neil et al., 1969) and Erez and Luz (1983). Since our data agree with regression equations derived from culture experiments (Bemis et al., 1998; Spero et al., 2003), in which pH is controlled to be close to present-day surface waters, it is likely that the pH of calci¢cation is the reason for the deviations. This implies that culture-derived and pH-controlled equations give more reliable results for ecological applications (e.g., for the calculation of habitat depths) than the equations given by Shackleton (1974) and Erez and Luz (1983). However, it must also be noted that the slopes of the equations derived here agree with the slopes indicated by paleotemperature equations from inorganic precipitates (O’Neil et al., 1969; Shackleton, 1974), which suggest that relative temperature changes can be reliably predicted by these equations over the temperature range examined here (31.8 to 31.1‡C). Slopes and intercepts of species-speci¢c regression equations derived for Globigerinoides ruber, Globigerinoides sacculifer and Globigerina bulloides are statistically not signi¢cantly di¡erent within the data set compiled here. Since the variability is high in our data set, only future comparisons between laboratory-derived equations can show whether the relationships between these species are indeed statistically indistinguishable or whether the use of species-speci¢c equations is justi¢ed.
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Acknowledgements We thank M. Segl, B. Meyer-Schack and W. Bevern for help with sample preparation and oxygen isotope analyses. Thanks to J. Bijma, C. Ru«hlemann, H. Spero, and R. Zeebe for discussion and B. Bemis and J. Erez for making their data available. J.-C. Duplessy, F. Peeters and an anonymous referee provided constructive comments on an earlier version of the manuscript. This work was funded by the Deutsche Forschungsgemeinschaft (DFG Research Center Ocean Margins) and the Bundesministerium fu«r Bildung und Forschung (DEKLIM E, Grant 01 LD 0019 to S.M.). References Bauch, D., Carstens, J., Wefer, G., 1997. Oxygen isotope composition of living Neogloboquadrina pachyderma (sin.) in the Arctic Ocean. Earth Planet. Sci. Lett. 146, 47^58. Bemis, B., Spero, H.J., Bijma, J., Lea, D.W., 1998. Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations. Paleoceanography 13, 150^160. Bijma, J., Spero, H.J., Lea, D.W., 1999. Reassessing foraminiferal stable isotope geochemistry: Impact of the oceanic carbonate system (experimental results). In: Fischer, G., Wefer, G. (Eds.), Use of Proxies in Paleoceanography ^ Examples from the South Atlantic. Springer, Berlin, pp. 489^512. Bleil, U., Cruise Participants, 1998. Report and preliminary results of METEOR Cruise M38/2, Recife^Las Palmas. Ber. Fachbereich Geowiss. 95, 126 pp. Boltovskoy, E., Boltovskoy, D., Correa, N., Brandini, F., 1996. Planktic foraminifera from the southwestern Atlantic (30‡^60‡S): species-speci¢c patterns in the upper 50 m. Mar. Micropaleontol. 28, 53^72. Boltovskoy, E., Boltovskoy, D., Brandini, F., 2000. Planktonic foraminifera from southwestern Atlantic epipelagic waters: abundance, distribution and year-to-year variations. J. Mar. Biol. Assoc. U.K. 80, 203^213. Brewer, P.G., Glover, D.M., Goyet, C., Shafer, D.K., 1995. The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater. J. Geophys. Res. 100, 8761^8776. Duplessy, J.C., Blanc, P.L., Be¤, A.W.H., 1981. Oxygen-18 enrichment of planktonic foraminifera due to gametogenic calci¢cation below the euphotic zone. Science 213, 1247^ 1250. Duplessy, J.C., Labeyrie, L., Juillet-Leclerc, A., Maitre, F., Duprat, J., Sarnthein, M., 1991. Surface salinity reconstruction of the North Atlantic Ocean during the last glacial maximum. Oceanol. Acta 14, 311^324.
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