Rhythmic variations of foraminifera sea sediments, first detected by Emiliani [1955],

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1 PALEOCEANOGRAPHY, VOL. 6, NO. 1, PAGES 1-20, FEBRUARY 1991 BENTHIC FORAMINIF RAL also IN THE OCEAN'S TEMPERATURE-SALINITY-DENSITY FIELD: CONSTRAINTS ON ICE AGE THERMOHALINE CIRCULATION Rainer Zahnl Department of Oceanography, University of British Columbia, Vancouver, Canada Alan C. Mix College of Oceanography, Oregon State University, Corvallis Abstract. Benthic data from 95 core sites are used to infer possible temperature-salinity (T-S) fields of the Atlantic and Pacific oceans at the Last Glacial Maximum (LGM). A constraint of stable density stratification yields logically consistent scenarios for both T and S. The solutions are not unique but are useful as a thinking tool. Using GEOSECS data, we solve for the modem relation- ship between 1518Owate r (15w) and salinity the deep sea: 15w (SMOW) = * S As a starting point, we assume that the slope of this equation applies to LGM conditions and predict 1518Ocalcit e ( c) gradients in equilibrium with probable T-S fields of LGM deep and bottom waters. Benthic foraminiferal data from the deep Pacific (2-4 km depth), and the bottom Atlantic (> 4 km depth), are %o lower than from the deep Atlantic (2-4 km depth) at the LGM. If the modem 15w-S slope applies, Atlantic deep and bottom waters were more dense than Pacific deep waters. This assumption would imply bottom waters both fresher (AS >0.5) and colder Now at GEOMAR, Kiel, Federal Republic of Germany. Copyright 1991 by the American Geophysical Union. Paper number 90PA /91/90PA (AT-3øC) than overlying deep waters, in conflict with other data, suggesting ice age deep water much colder than at present. It is also possible that the observed 15c gradients are an artifact of laboratory intercalibration. If Atlantic deep and bottom water values were similar to deep Pacific values, this would be consistent with the hypothesis of a stronger southern ocean versus North Atlantic source for deep-ocean ventilation at the LGM. Taking the observed gradients at face value, however, a solution could be that the LGM bw-s slope in deep and bottom waters was higher than at present, conceivably because of a stronger contribution of salt to the deep ocean via more intense sea ice freezing. This would allow Pacific deep waters and Atlantic bottom waters to have a common source, again in the Antarctic. Both would be more dense than Ariantic deep waters, even though the deep waters were much colder than at present. To better constrain these inferences drawn from the spatial distribution of benthic 15180, we must reduce scatter in the data with more high-quality measurements in high sedimentation rate cores. This is especially true at bottom water sites. Also, we must intercalibrate mass spectrometers at different isotope laboratories more accurately, to insure our isotope data are compatible. INTRODUCTION Rhythmic variations of foraminifera deep- sea sediments, first detected by Emiliani [1955],

2 Zahn and Mix: Benthic Foraminiferal primarily Shackleton, reflect 1967]. changes Intercore in global differences ice volume in the variations are commonly attributed to spatial variations of deep water temperature [Chappell and Shackleton, 1986; Labeyrie et al., 1987]. Yet we know from work on the modern ocean [e.g., Craig and Gordon, 1965] that the b 80 composition of different water masses (bw) in the ocean is quite variable. Thus changes in water mass are another variable that must be considered. Broecker [ 1986, 1989] discusses possible variations in the distribution of in the glacial oceans. He emphasizes water mass changes in surface waters relative to mean deep waters, which would affect b 80 in planktonic foraminifera. Here we take the next step and explore the use of spatial distributions of benthic foraminiferal b 80 as a tracer for both temperature and salinity in the deep sea. We start by evaluating the equilibrium frac- tionation of 15ISOcalcite (15c) in the ocean's tempera- ture-salinity-density field. Next we derive bw-s relationships for the modern ocean. To interpret the ice age data in terms of T and S, we fin:st apply the slope of the modern deep-sea bw-s relationship. We later relax this restriction and consider how the bw-s pattern may have changed in the past. Finally, we infer the range of possible ice age distributions of temperature, salinity, and density in the deep sea. A unique solution to two variables (T and S) cannot be found from measurements of one variable (bc). In spite of this, the constraint of density stratification gives insight into possible T and S distributions in the deep ocean, which has implications for the mode of deep and bottom water formation in the ice age ocean. METHODS AND DATA BASE We compile benthic data from 58 northeast Ariantic and 39 Pacific core sites for the modern and the Last Glacial Maximum (LGM, approximately 18,000 years ago) (Table 1). Most of the data used here were available from the literature. References are listed in Table 1. Some of the isotope data used here are new. These were measured at Oregon State University, on a Finnigan MAT 251 mass spectrometer. This facility is equipped with microvolume inlet and an Autoprep Systems automated carbonate device. Sample preparation follows the standard techniques [Shackleton and Opdyke, 1973], except that reactions occur at 90øC. It is a "common acid bath" system, in which up to 40 reactions occur in the same vessel. Memory between samples, less than 1% of the isotope offset between samples, is undetectable in normal marine samples. Calibration to the international Pee Dee Belemnite (PDB) scale was done primarily through the U.S. National Institute of Standards and Technology carbonate stan,,dard "NBS-20" and secondarily through "NBS- 19. Long-term reproducibility (+1(5) for and 15 3C over 1 year (1989) is 0.09 and 0.04%0, respec- tively, for a local calcite standard (n=229), and 0.04 and 0.03%0, respectively, for NBS-20 (n=71). The data used here are from the two widely used benthic genera Uvigerina and Cibicidoides. We corrected all data to the Uvigerina scale, which is thought to be close to equilibrium with sea water [Shackleton, 1974]. The correction for data from Cibicidoides to Uvigerina is +0.64%0 [Shackleton and Opdyke, 1973; Shackleton, 1974]. The error introduced here may be as large as 0.2%0 for individual samples [Mix and Fairbanks, 1985]. On average, though, the adjustment appears to be quite stable in both the Atlantic and Pacific. We have used core top data to examine the modem distribution pattern of benthic b 80. Core tops with anomalously low b 3C were rejected. We suspect these are contaminated with glacial material. The LGM is deftned as the most recent foraminiferal b 80 maximum below the last glacial-interglacial transition. This may differ slightly from published chronologies, but given the potential age errors in some of the data used here, we prefer the objective criterion of using the maximum. FORAMINIFERAL AND PALEOTEMPER AT[ IRE EQUATIONS Glacial-interglacial fluctuations of foraminiferal I5 80 combine the of temperature 8 signals changing and changing 150water (15w). TO solve for temperature, we must choose among available isotopic equilibrium equations. Figure la shows empirical 15c equilibrium predictions, defined by seven different equations (see review by Mix [1987]). The equations predict shifts in 15c of %o per 1 øc temperature change (Table 2). At temperatures above 10øC, all but the earliest equations yield nearly the same isotope values. However, at typical abyssal temperatures, i.e., below 5øC, the equations diverge. To test the palcotemperatur equations, we com- pare core top data with depth profiles pre- dicted equilibrium 15c (Figure lb). The equilibrium

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6 .o Zahn and Mix: Benthic Foraminiferal A /5 SOeee minus õ Owater P_ T..OTEMPERATURE EOUATIONS AFTER: a) MCCREA [1950] b) EREZ AND LUZ [1983] c) O'NEm ET AL. [ 1969] d) SHACKLETON [1974] e) EeST IN ET AL. [1953] 0 C do [1965] g) HORmE O ^ [1972] 5'80, z ( %o PDB ) a 1: te [ 6 Fig. 1. (a) Empirical relationships between 518Ocalcite ( ) and water temperature. (b) Vertical profries 5c, using temperature and S18Owater profiles from GEOSECS stations 29 and 115 (data from (Sstlund et al. [1987a]). Crosses are benthic 5180 values (Uvigerina scale) from Northeast Ariantic core tops. Best fit is with equation b. B profiles for northeast Atlanti core sites are computed from vertical temperature and Sw sections of GEOSECS stations 29 and 115 [Ostlund et al., 1987a]. The bw values were transformed from the Standard Mean Ocean Water (SMOW) scale to the PDB carbonate scale by subtracting 0.27%0 from the data tabulated on the SMOW scale [Hut, 1987]. Offsets between predicted values and core top data (Figure lb) point either to error in calibrating the equations low temperatures or to isotopic disequilibrium in Uvigerina. The best fit to the core top data below 2 km depth is with the equation of Erez and Luz [1983]. This equation was calibrated between 14øC and 30øC, with cultured planktonic foraminifera. We will use this equation to estimate equilibrium 5c. Our conclusions would not change had we chosen one of the other equations, however. The fit of the Erez and Luz equation to core top benthic 5180 values at water depths < 2 km is poor. Predictions are systematically less than the measured values (Figure lb). At these depths, most of the core sites used here are near the Strait of Gibraltar and are influenced by 180-enriched Mediterranean outflow waters. The hydrographic profiles of GEOSECS stations 29 and 115, which we use to compute the vertical 5c profile in Figure 1, are far from Gibraltar. They are not strongly influenced by Mediterranean waters. Thus the mismatch of core top data and the predicted water column values above 2 km depth does not indicate failure of the Erez and Luz [ 1983] equation but rather a water mass effect. For the Pacific, we use T and Sw data from GEOSECS stations 306, 322, and 345 [Ostlund et al., 1987a]. Here, the Erez and Luz equation predicts 5c lower than some of the core-top data (see Figure 5 below). We cannot prove why this mismatch occurs. We speculate, however, that biological mixing of foraminiferal shells from late glacial core sections to the core tops is more severe at some of the Pacific core sites we have used. In the Pacific, foraminiferal shells are most common in carbonaterich glacial sediments and less common in core tops where carbonate contents are strongly reduced. The opposite is u'ue of many Atlantic sites. Thus there is a higher chance for Pacific core tops to be contami- nated with high 5 80 glacial material due to bioturba- tion. Birchfield [ 1987] reached a similar conclusion. TABLE 2. Slope and Temperature Predictions Using Different Paleotemperature Equations Equation Slope T øc a McCrea [1950] Epstein et al. [1953] Craig [1965] O'Neil et al. [1969] Horibe and Oba [ 1972] Shackleton [1974] Erez and Luz [1983] a calculated for 15w of +0.27%o (SMOW) and 15c of +3.5%o (PDB).

7 .. Zahn and Mix: Benthic Foraminiferal 8 ]80 WATER MASS PATI'E S THE DEEP SEA OF 8w AND 8c IN The distributions of b]80 in seawater (bw) and salinity (S) are both controlled mostly by evaporation and precipitation [Dansgaard, 1964; Craig and Gordon, 1965; Joussaume et al., 1984]. Within each region 8w is linearly related to water mass salin- ity (Figure 2). The slope of the 8w-S relationship, however, varies between 0.1 (for tropical surface waters) and 0.6 (for high-latitude surface waters) [Craig and Gordon, 1965]. This reflects greater Rayleigh distillation of precipitation in high latitudes, which preferentially removes 80 from high-latitude fresh waters. Without additional mechanisms such as freezing of seawater, the zero-salinity intercept in the 8w-S plot gives the 8 80 value of the mean regional fresh water diluent of seawater. I. 1 it, average 8 80 composition of precipitation and fiver J NORTHATLANTIC_DEEP WATER runoff in the high-latitude North Atlantic is about - 0 /WEDDELL /l[ PACIF[ _.AN_._.D,,,I,ND.I_. _, _OCF_ ' 21%o (SMOW)[Craig and Gordon, 1965; 0stlund [SURFACE / 7 DEEP WATER (Abw/AS = -1.5 in Figure 2) thus yielding a "fictitious" zero-salinity intercept of 8180, much lower than the 8180 value of the true freshwater dilu- ent. Water masses from the southern ocean and the North Atlantic are the principal sources of ventilation in the modem deep ocean. This was probably also true (at varying mixing ratios) in the glacial ocean [Curry and Lohmann, 1985; Curry et al., 1988; Duplessy et al., 1988; Keigwin, 1987; Oppo and Fairbanks, 1987]. Mixing between these end-member water masses is most easily seen in the Atlantic Ocean which represents the geographic connection between the deep and bottom water source areas in the high-latitude North Atlantic and the southern ocean. Using GEOSECS 8w and salinity data [0stlund et al., 1987a], we evaluate 8w-S relationships for the Atlantic modem deep and bottom waters (Figure 3). Because of their close similarity, we combine the 8w-S equations from Figure 3 into one single deep-ocean equation: 8w(SMOW)=l.529*S (r2=0.95, n=110) (1) Again, note the relatively negative zero-salinity intercept, below -50%0(SMOW). In contrast, the ] ANTARCTIC BOTTOM WATER -11.,.,.,. i SALINITY Fig. 2. Regression lines and slopes (s) for 8w and salinity. Line A, North Atlantic surface waters. Line B, tropical surface waters [after Craig and Gordon, 1965]. Line C, Mediterranean Waters [Stahl and Rinow, 1973]. Dashed line is for deep and bottom water samples in the Atlantic (see also Figure 3). This is not true in polar regions, where salinity is influenced by freezing of sea ice. Freezing rejects salt, but causes almost no 8 80 fractionation [Craig and Gordon, 1965]. Thus southern ocean waters have a relatively wide range of salinities ( ) even though they have virtually the same 8w (about- 0.2%0 versus SMOW) (Figure 2) [Craig and Gordon, 1965; Weiss et al., 1979; Jacobs et al., 1985]. For deep water, mixing of water masses derived from sea ice freezing with other water masses results in high slopes of the 8w-S relationship [] 2-4kin (A) A >4kin (B) SALINITY A: $, = 1.45 * S (r 2 = 0.95) B: $w = 1.68 * s (r 2 = 0.96) Fig. 3. Relationships between 8]SOwater and salinity in Atlantic deep waters (open squares, line A) and bottom waters (open triangles, line B). Data from Ostlund et al. [ 1987a].

8 oo Zahn and Mix: Benthic Foraminiferal 80 and Hut. 1984; Osttund et at., 1987b]. The esti- mated f80 of precipitation and meltwaters in the Southern Ocean ranges from about -13%o to -50%0 (SMOW) [Weiss et al., 1979; Jacobs et at., 1985]. At its most negative extreme, this approaches equation (1)'s intercept value of-53.18%o. The contribu- tion of 180-depleted Antarctic ice sheet meltwaters to Antarctic Bottom Water, however, is less than 0.1% of the total [Weiss et al., 1979]. Thus the low inter- cept of 180 in equation (1) must reflect mixing of North Atlantic Deep Water with a freeze-enhanced Antarctic end-member, rather than a true freshwater dituent [Craig and Gordon, 1965]. We will use the slope of equation (1) as a starting point to relate salinity to f w in the glacial ocean. It is important to note the limits in this assumption, however. Using equation (1) for the past oceans assumes that the role of freezing and salt rejection (as a fractional contribution to water masses) is the same as at present. We relax this constraint later in the paper, to explore the possibility of varying slope of the deep-sea f w-s relationship. In addition, the role of intermediate waters of different w is neglected. For example, Craig and Gordon [ 1965] note that modem Pacific deep waters have slightly lower salinities than expected from simple mixing between NADW and AABW (Figure 2). They suggesthat this offset may be explained by an admixture of lower salinity intermediate waters. Birchfield [1987] disagreed. Arguing that Craig and Gordon's Antarctic end-member was too low in 180, he inferred that only two end-members from these lines reflect changes not directly related to storage of 80-depleted ice on land, i.e., due to local are needed. The general point that intermediate waters could have had different w-s patterns than modem deep water, however, remains. We cannot yet quantify the importance of intermediate waters in the glacial ocean, but it appears that they were better ventilated than now [Boyle, 1988; Oppo and Fairbanks, 1987; Zahn et al., 1987; Kallel et al., 1988]. Thus it is probable that the deepwater w-s relationship was different in the glacial ocean. We will illustrate the effects of varying f w-s relationships on the reconstruction of T and S from foraminiferal fi 80 in the final section of this study. DISTRIBUTION OF BENTHIC FORAMINIFERAL b 80:MODERN AND LGM In the modem Atlantic, North Atlantic Deep Water (NADW) forms a fi180 maximum between 2 and 3 km depth with w values >+0.2%0 (SMOW). Antarctic Bottom Water (AABW) is evident in low 80 water below 4 km depth (Figure 4a). This pattern is similar in the western and eastern basins of the North Atlantic. The isotopic offset between deep and bottom waters is slightly less in the east, because the Mid-Atlantic Ridge forms a topographic barrier to AABW [Metcalf et al., 1964]. Using published data sets of w and temperature [Ostlund et al., 1987a] and applying the paleotemperature equation of Erez and Luz [1983], we esti- 18 mate fi Ocalcite ( 5c) values for the deep Atlantic and Pacific oceans (Figure 4b and Table 3). In the Atlantic, the decrease of fiw from NADW to AABW goes along with a decrease in temperature. The net result is similar tic values for Atlantic deep and bottom water masses. In the Pacific, a slight (- 0.1%o) increase in predicted fie occurs in bottom waters, due mostly to lower temperatures. Consistent with equilibrium predictions, below- 2 km water depth, the Atlanticore top benthic fi180 values reach a roughly constant level of %o (Figure 5 and Tables 1 and 3). At the LGM, benthic fi180 is higher everywhere. This reflects the com- bined effects of increased global ice volume and decreased water temperature (Figure 6 and Table 4). The mean Atlantic increase of 1.7%o is larger than the mean Pacific increase of 1.5%. In Figure 6, hypothetical vertical tic profiles for the LGM assume that modem water mass patterns apply. We account for ice volume by shifting the modem equilibrium fi 80 profiles by +1.3%o, consistent with recent data on sea level [Fairbanks, 1989]. Deviations of the data changes in temperature or f w of the water mass. Most of the measured benthic tic data from the LGM are significantly heavier than would be predicted by a 1.3%o ice volume effect (Figure 6). In the Atlantic, 15c values below 4 km approach those predicted and are on average -0.2%0 lower than the deepwater tic mean between 2 and 4 km. A cross section (Figure 7) shows that the deep water tic maximum is most obvious north of about 10øN. In the Pacific, the data do not reach true bottom water depths. The latitudinal cross section hints at a deepwater maximum of benthic tic to the north of 40øN (Figure 7). At present, however, we cannot con- clude that any statistically significant fi 80 gradients exist between glacial Pacific deep and bottom waters. The Atlantic offset between deep and bottom waters is statistically significant at the 99% level (t test for difference between means) if there are no systematic measurement errors between depth intervals. The

9 Zahn and Mix: Benthic Foraminifera LATITUDE 0 ø 10 ø 20 ø 30 ø 40 ø 50 ø 60øN A 0 o 10 o 20 ø 30 ø ø 60ON B Fig. 4. Modem distributions of (a), and (b) t ½ in the North Atlantic (data from Ostlund et al. [1987a]; equation of Erez and Luz [1983]). NADW and AABW can be seen in hw but not in hc. The decrease of f is offset by lower temperatures depth. offset between Atlantic and Pacific deep waters, with Pacific values lower than Atlantic values by %0 at the LGM, are also significant at the 99% level. Before we interprethe isotope data in terms of water mass changes, however, we must evaluate potential geological and analytical problems that may limit our conclusions. INTEGRITY OF BENTHIC 5180: BIOTURBA- TION AND MASS SPECTROMETER CALIBRATION A possibl explanation for intercore differences of stratigraphic records is to assume smoothing of signals by bioturbation [Berger and Heath, 1968; Depth Level, km TABLE 3. Hydrography and Oxygen Isotope Signal of Modem Water Masses In Sire Temperature, øc Salinity 15w, %0 SMOW 15c, %0 PDB a North Atlantic: (Geosecs Stations 29, 115) > Central Pacific: (Geosecs Stations 306, 322, 345) > a calculated using the seawater- equilibrium equation of Erez and Luz [1983]

10 1o Zahn and Mix: Benthic Foraminiferal j180 I MODERNI NE-ATLANTIC (%o PDB) PACIFIC 80 (%. PDB) i ß i. i [] CAMBRIDGE ' + KIEL 2 2 ß O LDGO GIF S. YVETTE x WHOI 3 3 $ OSU Fig. 5. Modem depth distributions of benthic foraminifera16180 (PDB scale, normalized to Uvigerina). Symbols identify laboratories. Solid curves are predicted equilibrium 6c (equation of Erez and Luz [1983]). Values for temperature and 6w are from GEOSECS stations 29 and 115 in the Atlantic and stations 306, 322 and 345 in the Pacific [ stlund et al., 1987a]. I LAST GLACIAL MAXIMUM i NE-ATLANTIC PACIFIC 5 'SO (%o PDB) 5 '80 (%o PDB) i i. i 0 N 000 [21 2 o 4 [] CAMBRIDGE O GIF S. YVETTE + KIEL ß LDGO $ OSU a WHOI 5 Fig. 6. Depth distributions of benthic foraminiferal j180 (PDB scale, normalized to Uvigerina) at the Last Glacial Maximum. Symbols identify laboratories. Solid curves are the same as in Figure 5 except increased by 1.3%o to account for ice volume change. 6

11 Zahn and Mix: Benthic Foraminiferal 5180 Water Depth, km 2-4 >4 Whole-basin mean c TABLE 4. Benthic 5180(%0 PDB) at Northeast Atlantic and Pacific Core Sites M o d ½ r n _ Last Glacial Maximum A j a st n 5180 a St n LGM-Rec Northeast Atlantic 3.21 (b) (cm 1000 yr 4) Pacific > Whole-basin mean c st, lo standardeviation; n, number of core sites per depth horizon. a Uvigerina scale of b Without value of 2.89%0 from core END066-10GGC (see Table 1). c Weighted mean CLIMAP Project Members, 1984]. Bioturbation works most efficiently where sedimentation rates are low. Sedimentation rates at the core sites from > 2 km water depth used here vary between 1 and 8 cm kyr -1(Tables 1 and 4). Thus we might hypothesize thathe scatter in our 180 profiles may come in part from differential smoothing as a function of sedimen- tation rate. ILAST GLACIAL MAXIMUM I S LATITUDE N [] CAMBRIDGE o GIF s. YVETTE + KIEL ß LDGO ß OSU t, WHOI 6 6!.'." o/o0 (PDB) "'::... > 5.0 o/o0 (PDB) Fig. 7. Meridional crossections of benthic foraminiferal 5 80 at the Last Glacial Maximum (PDB scale, normalized to Uvigerina). Symbols identify laboratories. Relatively low values are found at depths below km.

12 12 Zahn and Mix: Benthic Foraminifera A numerical bioturbation model (similar to that of Peng et al. [ 1979]) predicts that smoothing becomes important at sedimentation rates below 4 cm kyr -1 (Figure 8). We assume a 10 cm mixed layer (probably an overestimate), with diffusivity of 60 cm 2 kyr -1, and constant downcore abundances of benthic foraminifera. The model predicts that core tops are affected more than glacial maximum samples. Upward mixing of high glacial shells is moreffective than downward mixing of low4180 Holocene shells. This is because downward mixing is limited to one mixing depth. There is no clear correlation between the model predictions and the core data. The scatter of the data does not decrease with increasing sedimentation rates (Figure 8). Changes in the abundance of benthic foraminifera glacial and interglacial sediments would modify the results shown here. Based on the available data, however, we conclude that biomrbation is unlikely to be the sole cause of scatter in the data. Another potential bias may result from improper intercalibration of mass spectrometers. We use data from six isotope laboratories (Table 1). To check for consistency between laboratories, we averaged LGM values from the northeast Atlantic and Pacific between 2 and 4 km depth. In this interval there is no trend with water depth, and for the Atlantic, data are available from all six laboratories. For the Atlantic, four of the laboratories give mean values within one standard deviation (+ 0.17%o)of the mean. At the extremes, two laboratory means differ by nearly 0.4%0 from each other (Table 5). The Pacific deepwater mean value (Table 4) is dominated b[8 data from only two laboratories (Table 5). The 15 O gradient between the oceans, if calculated on the basis of data from these two laboratories, is only 0.05%0, i.e., at the border of analytical detection. Thus, we cannot exclude the possibility that the observed intraoceanic and interoceanic gradients are, at least to some extent, an artifact of calibration offsets between the laboratories. This does not prove that true calibration offsets exist. Our comparison is based on relatively few samples, and it is not based on analyses of aliquots of the same carbonate sample ß i - i - i - i - ß - i. w - [ LGM m/nu MODERN I l o. ß i - i - i - E 4.5[ o 4.0[ ß LAST GLACIAL MAXIMUM [] CAMBRIDGE o Gig s. YVETTE + KIEL ALDGO ß OSU AWHOI O 3.0 ' + MODERN SEDIMENTATION RATES ( cm 1000 yr 4) Fig. 8. Modem and Last Glacial Maximum benthic foraminiferal and glacial-interglacial 1518 O shifts at depths > 2 km compared to sedimentation rates. Solid curves represent numerically predicted values if bioturbated layer is 10 cm thick, with diffusivity of 60 cm 2 ky-1.

13 .. Zahn and Mix: Benthic Foraminiferal j Table 5. Mean Benthic j180 at the LGM From Core Sites Between 2 and 4 km Water Depth Northeast Atlantic Pacific Laboratory a 518 O b st n A c 5180 b st n Cambridge Gif sur Yvette Kiel OSU WHOI Mean A c st, 1 o standard deviation; n, number of data points. a For explanation of laboratory codes see Table 1. b Uvigerina scale of 5 0 (%. PDB). c Deviation from mean. measured at different laboratories. A more thorough intercalibration between the laboratories is needed before corrections to the data can be made. It appears that at least some of the variability observed in Figures 5, 6, and 7 comes from true differences in the core samples analyzed. Given the noise level in individual measurements, our discussion will emphasize average values within deepwater (2-4 km) and bottom water (> 4 km) core sites (Table 4). Because of questions about intercalibration, however, the difference in mean 5c between the glacial Atlantic and Pacific deep waters and Atlantic deep and bottom waters is questionable. We will interpret the 5½ patterns considering two options: that the measured gradients are real, or that there is no gradient. FORAMINIFERAL 5180 IN THE OCEAN'S TEMPERATURE-SALINITY-DENSITY FIELD Foraminifera monitors the combined variations of water temperature and bw. Unfortunately, bw is not uniquely linked to salinity. Thus, measurements of benthic 5180 on their own cannot uniquely reconstruct both temperature and salinity in the past. The 5 80 gradients in benthic foraminifera, however, can narrow the field of acceptable solutions, if we include the following requirements and assumptions: (1) temperatures of the source waters must not fall below the freezing point of seawater; (2) in situ temperatures are used to calculate 5c(not potential temperatures as there are no pressure effects on foraminiferal 5c);(3) the deepest water masses form at high latitudes and have the lowest temperatures; (4) the global salt budget must balance; and (5) any vertical profile must have either constant or increasing density with increasing water depth. The first and second points are physical constraints. Note that the freezing constraint applies to source waters at the sea surface. By the time these waters mix into the deep-sea, ambient heat fluxes require that they are well above the freezing point [Mix and Pisias, 1988]. The third point is an assumption. It is not foolproof, as warm salty bottom waters are conceivable, but it appears to be reasonable for the LGM. The fourth and fifth points are physical constraints which cannot be violated, but they do not yield unique solutions. Because we do not have full coverage of the glacial oceans, one could always argue that the salt budget balances somewhere outside our field of measurements. For the density constraint, stable water mass stratification may come from a wide range of T-S combinations. The major purpose of points 4 and 5 is to eliminate unrealistic scenarios that would create an unstable water column, or make it impossible to balance the salt budget. Several water mass patterns are possible. Each would be consistent with the vertical and horizontal distributions of benthic 5 80 at the LGM. We show these on traditional temperature-salinity (T-S) diagrams. Also plotted are 5c equilibrium isolines on the diagrams. To do this we initially convert salinity to Sw using equation (1) and use the seawater-calcite equilibrium equation of Erez and Luz [1983]. Density is computed from the international equation of state of seawater [UNESCO, 1981] and expressed in conventional sigma (o) units. We calculate density at the 4 dbar level (04, approximately 4 km water

14 14 Zahn and Mix: Benthic Foraminiferal $ 80 depth) to eliminate pressur effects on seawater density at depth. Note that the diagram axes are AT and AS, rather than T and S. We avoid absolute T-S estimates, because these depend on knowing the ice volume effect well. In each diagram, T, S, 15c, and o4 are given in comparison to a reference water mass. We wish to emphasize the spatial variations in the signals, rather than the absolute glacial-interglacial changes. The difference calculations are relatively insensitive to small errors in the choice of a reference water mass. We use a hypothetical mean Atlantic deep water as the reference. The modem T-S values of this reference water mass are T=2.5øC and S=34.9. For the glacial maximum, we set the deepwater reference temperature to 0øC [Duplessy et al., 1980; Chappell and Shackleton, 1986; Labeyrie et al., 1987], and salinity to 35.9 to accommodate lower glacial sea levels. Positive AT, AS, Abc, or Ao4 numbers indicate higher T, S, bc, or o4 values with respecto the reference water mass. The Atlantic-Pacific Gradient of Benthic i 180 at Depths 2-4 Ion Pacific deep waters today are colder (AT = -1.2), less saline (AS = -0.2) and slightly more dense (Ao4 = +0.05) than deep waters of the North Atlantic (Figure 9a and Table 3). At the same time, benthic obtained from core top samples is essentially M 0 D E R N LAST GLACIAL MAXIMUM LAST GLACIAL MAXIMUM,, option 1 option 2 +1 o..-,+1 I ATLANTtC- =... I ß ' ["'PACIFIC. :."'-/f.." t '.-' PACIFIC.-'" +1 t, :- - o.- /. ß C o.. 5"' t.." "P" A ''"'"".'... ' V 9."/..."..,.5". i" B ' 1-1 t".?""-"...,., -'" '"' '-' '11' C t! A SALINITY EQUILIBRIUM FRACTIONATION LINES FOR 180 calcite (AS A 18Ocalcit ½ )... DELTA DENSITY ISOLINES ((h) Fig. 9. Temperature, salinity, and water mass density (Ao4, dotted lines) anomalies of deep waters relative to NADW reference. Solid lines show equilibrium bc anomalies (as A&). Positive numbers for all properties indicate higher values than the Atlantic deep watereference. (a) Modem: North Atlantic deep water (solid square) and North Pacific deep water (solid triangle). The Pacific is colder (AT=-1.2), less saline (AS=-0.2), and slightly more dense (Ao4=+0.05) than the Atlantic. The c values are similar in both oceans (Abc=0). (b) Last Glacial Maximum, option 1: Benthic foraminiferal 80 in the deep North Pacific is lower than in the deep Noah Atlantic (A c=-0.2). Open triangle shows anomalies if AS stayed at its modem value (AT=-0.4, Ao4 = ). Open circle shows anomalies if salinity was the same in Atlantic and Pacific deep waters (AT=+0.8, Ao4=-0.13). Both scenarios predict that Atlantic deep waters were more dense than Pacific deep waters a the LGM. (c) Last Glacial Maximum, option 2: Benthic foraminiferal 80 in the deep North Pacific is the same as in the deep North Atlantic (Abc=0). Solid pentagon shows anomalies if AS was the same as today (AT=-0.4, Ao4=-0.03). Dashed arrow showshift of Pacific waters if their salinity was the same as in the Atlantic (open pentagon). In this case the Atlantic and Pacific property fields were identical (AT=O, Ao4---0). Scenarios for LGM assume - salinity slope same as today.

15 Zahn and Mix: Benthic Foraminiferal j the same in both oceans (3.2 to 3.3%0; Tables 3 and 4). This is illustrated in Figure 9a by the 5c equilibrium fractionation line, which passes through both the Pacific and Atlantic water masses. At the LGM, the 5 80 excursion relative to modem is largest in benthic foraminifera from the North Atlantic (average of 1.8%o). This exceeds changes observed at Pacific core sites by about 0.2%0 (Table 4; see also Duplessy et al. [1980], Duplessy [1982], and Shackleton et al. [1983]), yielding an interoceanic A 5c value of-0.2%0 at the LGM. This gradient may be questionable due to inaccurate intercalibration of mass spectrometers (see above). Figure 9b shows the line of acceptable solutions for T-S of Pacific deep waters for a 15c value 0.2%0 lower than the Atlantic. If we maintain the modem salinity contrast between oceans (AS = -0.2), then the Pacific-Atlantic temperature offset decreased to AT =-0.4 (open triangle in Figure 9b). This is less than half of the modern interoceanic temperature difference. If this scenario is correct, it reversed the density contrast between the deep Pacific and deep Atlantic, with Ate4 = The deep Atlantic would have been denser that the Pacific, because of the extra glacial cooling of the Atlantic coupled to its higher salinity. If however Pacific Jc values were the same as Atlantic 15c values (Abc= 0), then Pacific water would lie along the same 5c isoline as Atlantic waters. If the modem salinity and temperature contrast between both oceans were maintained, the density of Pacific and Atlantic waters would be more similar than today (Ate4 = +0.03; closed pentagon in Figure 9b), because of the steeper slopes of the isopycnals under low, glacial temperatures. This is not the only acceptable scenario, however. In this first scenario, we assumed that the interoceanic salinity gradient was the same as today. What if we assume instead that LGM salinity was the same in the Atlantic and Pacific Oceans (i.e., AS = 0)? The measured Pacific 15c values would require deep water in the Pacific to be warmer than in the Atlantic (AT = +0.8, open circle in Figure 9b). Note that the increase in deep Pacific salinity relative to the mean ocean would require lower salinities elsewhere for the salt budget to balance. As in the previous case, deep Pacific water would be less dense than deep North Atlantic water. Here, the interoceanic density contrast would be even larger (Ate4 =-0.13) than that predicted in the first scenario. If, however, Abc= 0 and AS = 0, then AT = 0 and A( 4 = 0. That is, if there was no 15c gradient between both oceans, and if salinity was the same for Pacific and Atlantic waters, both oceans had identical temperatures and densities (open pentagon in Figure 9c). Other scenarios are also possible. If inferred Pacific waters were falling along the A 5c isoline of- 0.29'00 relative to the Atlantic reference water mass, a solution with deep Pacific densities as great as or greater than Atlantic densities would not be possible without changing the slope of the 5w-S relationship as it would require deep Pacific temperatures below freezing. The inference of higher-density waters in the Atlantic at the LGM would suggest a separate source of cold and/or salty deep waters to the Atlantic. This problem disappears if the observed 5c gradient between both oceans were an artifact of mass spectrometer intercalibration. If Pacific and Atlantic 5c values were similar, this would be more consistent with recent inferences from carbon isotope and cadmium data of stronger southern ocean bottom water sources, and weaker NADW sources at the LGM [Boyle and Keigwin, 1982; Broecker, 1986; Oppo and Fairbanks, 1987], and perhaps replacement of NADW with a North Atlantic intermediate water source [Boyle, 1988]. However, we will return to this question following discussion of isotopic variability within the North Atlantic and show that if Pacific 5c values were to be 0.2%0 lower than Atlantic values, a reasonable change in the 5w-S relationship can also give an acceptable solution. The Vertical Distribution of Benthic i5180 in the North Atlantic Stable water mass stratification between deep (2-4 km) and bottom (>4 km) waters in the modem Atlantic is due primarily to a decrease of temperature with depth. As shown in Figure 10a, AT = -0.3 (bottom-deep Atlantic temperatures). This temperature decrease would increase Jc, but it is offset by a slight decrease in salinity with depth (AS = -0.03). The net effect is a small density increase (Ate4 = +0.03) with depth and benthic 180 essentially the same at deep and bottom water sites in today's Atlantic (Figure 10a and Table 3). At the LGM, Atlantic benthic is increased by 'oo relative to the modem. The vertical prof'fie in the northeast Atlantic indicates a slight 151 O decrease of about 0.2%0 from deep to bottom water sites (Figures 6 and 7, and Table 4). As noted above, this gradient can be questioned, due to the low number of analyses in the bottom waters and possible problems with interlaboratory calibration of the mass spectrometers. Given this uncertainty, we

16 l0 Zahn and Mix: Benthic Foraminiferal MODERN "... ATLAN+),_ ' BOTToMIC I3 ½ LAST GLACIAL MAXIMUM option 1 I'' ' AN¾I'C./.'! WA '_.-' '"'" t I A SALINITY EQummanm FRACnOSAZ OS Lmms FOR 815OCalci (AS A815 Ocaici )... D.LTA DENSITY ISOL ES ( o4 ) LAST GLACIAL MAXIMUM option 2 C... ß XTLANTf/:: /Lff'" /... DE.WA 1.].t...'... / o'.:.atlanti "q ß..o.... WAT.E Fig. 10. Same as Figure 9, but for Atlantic deep and bottom waters. (a) Modem: North Atlantic deep water (solid square) and bottom water (solid triangle). The bottom waters are colder (AT= -0.3), less saline (AS=-0.03) and slightly more dense (Ac 4=+0.03) than deep waters. The 8c values are similar at both depths (ASc=0).(b) Last Glacial Maximum, option 1: Assumes 8w-S slope same as today, and no difference between 8c in deep and bottom waters. If the deep-bottom water salinity gradient was the same as at present (open triangle), the vertical density gradient would be lower (Ac 4 = +0.01), due to the lower reference temperature. To maintain vertical stability equal to today, T and S gradients must increase (open circle, AT=-0.9, AS=-0.12, Ac 4=+0.03). (c) Last Glacial Maximum, option 2: Assumes bottom water benthic 8c is 0.2%0 less than in deep water. If 8w-S slope was the same as today, neutral stability requires bottom waters to be colder and less saline than overlying deep waters (AT=-2.9, AS=-0.56). Stable stratification would require even larger gradients. Alternatively, a steeper 8w-S slope (bold dashed lines) would allow for more reasonable hydrographic properties of bottom water (open circle, AT=-0.9, AS=-I.2, Ac 4=+0.03), while maintaining lower 8c in bottom water. In this case, Pacific deep waters could have T-S properties close to Atlantic bottom waters. discuss the implications of two possible scenarios: first one with no 8c gradient between Atlantic deep and bottom waters, and second one with the 0.2%08c gradient as measured. In the first scenario (Figure 10b), we assume that bc is the same at the deep and bottom water sites. If so, the T-S gradients between deep and bottom waters in the LGM North Atlantic could have been essentially the same as today (open triangle in Figure 10b). In this scenario the density gradient between deep and bottom waters would have been lower than today (Ac 4 = +0.01). This results from changes in the slope of isopycnals under the glacial conditions of lower temperatures. This lower-density stratification, if correct, would presumably allow for more vertical mixing between deep and bottom waters than today. In contrast, the well-documented benthic 813C decrease from deep to bottom water sites at the LGM [Curry and Lohmann, 1985; Curry et al., 1988] is generally though to reflect less active mixing (hence greater density contrast) between deep and bottom waters in this area. To have a vertical density contrast at LGM equal to today's (still assuming no c offset between deep and bottom water), bottom water must have been both freshet (AS = -0.12) and colder (AT = -0.8) than the deep water (open circle in Figure 10b). This is possible, given reconstructions of stronger influence of southern ocean water in the deep glacial Atlantic [e.g. Boyle and Keigwin 1982, 1985/1986; Oppo and_ Fairbanks, 1988]. It does require, however, that this water mass was much colder than at present, at

17 Zahn and Mix: Benthic Foraminiferal least -0.8øC if our assumed temperature of 0øC for the North Atlantic deep water temperature at LGM is correct. This may be an acceptable scenario. In the second scenario, we take the measured benthic foraminiferal 5 80 gradient of -0.2%0 between deep and bottom water sites at face value. This would require a thermohaline structure of the deep Atlantic ocean quite differenthan at present. If the 5w-S slope was 1.5, as it is now, to maintain neutral buoyancy bottom waters must have been much colder (AT = -2.9) and fresher (AS = -0.57) than the overlying deep waters (open triangle in Figure 10c). Even larger temperature and salinity differences would be needed to achieve the stable stratification neexled to preserve a benthic 5 80 gradi- ent. This scenario, however, would either put bottom waters below the freezing point (which is impossible) or require deep waters to approach modem temperatures, thus requiring an ice volume effect on 5 80 of nearly 1.7%o (which violates the sea level constraint of Fairbanks [1989]). This appears to be an unacceptable solution. Either the Atlantic deepbottom water gradient at LGM or the assumptions going into the T-S-o4- c plot are wrong. A solution could be found if the slope of the 5w-S relationship was greater at LGM than at present. This would be possible if the contribution of salt from sea ice freezing to the deep ocean water masses was more important at the LGM. Source water temperatures in today's southern ocean are close to the freezing point and could not have been much colder at the LGM. A plausible way to maintain higher bottom water densities over the cold glacial deep waters would be to increase the southern ocean salt content relative to its 5w. More freezing would inject more salt into the source waters, and thus increase their density, without changing the end- member Sw. This relatively salty, 80-depleted bot- tom water, after mixing with deep waters, would increase the slope of the deep ocean 5w-S relationship. If this slope doubled, to a glacial value of 3.0 (dashed lines in Figure 10c), water mass stability could occur with reasonable T-S properties in bottom waters (AT = -0.9, AS = -0.15; open circle in Figure 10c). A steeper 5w-S relationship would also solve a problem with the possible Atlantic-Pacific deep water 5c gradient discussed above and illustrated in Figure 9. Using the modem 5w-S slope, we inferred that Atlantic deep and bottom waters were much more dense than Pacific deep waters at the LGM (if the 5c offset between both oceans is real). This would require different sources of Pacific deep water and Atlantic bottom water. If the Sw-S slope was 3.0 instead of 1.5, this problem goes away. Bottom waters in the Atlantic could have had T-S properties similar to the deep Pacific. These waters would have been slightly fresher and colder than the diminished NADW sources, but more dense consistent with their position as bottom water in the Atlantic. A likely source for both water masses would be the Antarctic. SUMMARY AND CONCLUSIONS Craig [1965, p. 173] stated, "It is obvious that a detailed knowledge of Pleistocene variations in isotopic composition of the ocean is very important for the understanding of the causes of glaciation". Broecker [1986, p. 133] repeated this statement and added that "unfortunately a knowledge of 80 distri- bution in the glacial ocean is currently beyond our grasp". We pick up the task here, and attempt to constrain the distribution of 5 80 within the deep sea. A complete picture remains beyond our grasp, but with new data we have made some progress. We do not arrive at a unique solution to the distri- bution of 5 80 in the glacial ocean. Instead, we have used hypothetical distributions of Swater ( Sw) in the ocean's temperature and salinity fields as a thinking tool to examine the logical consequences of possible circulation schemes in terms of density distributions. Finding true relationships between 5w and S in the past will be very difficult. It depends on rates and patterns of evaporation, precipitation, runoff, the ratio of sea ice freezing to deepwater formation rates, and the mixing patterns of water masses within the deep sea. For simplicity, we have not considered intermediate water masses. Some of these water masses at present have slightly different 5w-S relationships than deep waters (Figure 2), so variations in their isotope budgets could add complexity. We find that glacial maximum 5180 values of benthic foraminifers ( 5c) from deep Atlantic sites (2-4 km depth) are on average higher than those from Atlantic bottom water sites (>4 km depth) or from the deep Pacific. This finding, though consistent with other studies, may in part reflect problems of interlaboratory calibration. It can, however, be interpreted in terms of reasonable water mass characteris- tics. Considering the constraint added by density stratification, it appears that changes in the slope of the 5w-S relationship are critical to making sense of ice age 5c patterns. If we assume that the measured 5c

18 18 Zahn and Mix: Benthic Foraminiferal 5180 gradients in the glacial ocean are real and if the modem relationship of 15w and salinity in the deep sea applies to the last glacial maximum, Atlantic deep and bottom waters must have been more dense than those in the Pacific. This density distribution would seem to require a major source of high density glacial deep water in the Atlantic and a separate source of deep water somewhere in the Pacific. This is inconsistent with circulation patterns inferred from carbon isotope distributions. If we assume that ice age 15c was the same at Atlantic and Pacific core sites or if the 5w-S slope was higher than it is now, this conflict goes away. In both cases, Pacific deep waters and Atlantic bottom waters could have had a common source in the southern ocean. A weak northern source of glacial Atlantic deep waters (or mixing with intermediate waters) is permitted. We tentatively conclude that sea ice formation contributed more salt to the glacial deep ocean than today. If true, this would change the salinity distribution within the ocean [Broecker and Peng, 1987], which has implications for paleo-co2 and other geochemical budgets. At present, however, the interpretations are limited both by precision and accuracy in the isotopic measurements. More analyses are needed at bottom water sites. Different laboratories must intercalibrate their mass spectrometers more accurately by sharing standards and techniques. In principle, these problems can be solved, and a better view of ice age circulation can be achieved. Acknowledgments. We thank Michaela Knoll for her advice on physical oceanographic procedures. R.Z. appreciates discussions with Laurent Labeyrie during the Third International Conference on Paleoceanography in Cambridge, UK, September 1989, where parts of this study were presented. Reviews by Christina Ravelo and an anonymous reviewer were very helpful. Support for this study came from NSERC-Canada and the National Science Foundation (OCE and ATM ). REFERENCES Berger, W.H., and G.R. Heath, Vertical mixing in pelagic sediments, J. Mar. Res., 26, ,1968. Birchfield, G.E., Changes in deep-ocean water/5180 and temperature from the last glacial maximum to the present, Paleoceanography, 2, , Boyle, E.A., Cadmium: Chemical tracer of deepwater paleoceanography, Paleoceanography, 3, , Boyle E.A., and L.D. Keigwin, Deep circulation of the North Atlantic over the last 200,000 years: Geochemical evidence, Science, 218, , Boyle, E.A., and L.D. Keigwin, Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: Changes in deep ocean circulation and chemical inventories, Earth Planet. Sci. Lett., 76, , 1985/1986. Broecker, W.S., Oxygen isotope constraints on surface ocean temperatures, Quat. Res., 26, , Broecker, W.S., The Salinity contrast between the Atlantic and Pacific oceans during glacial time, Paleoceanography, 4, , Broecker, W.S., and T.-H. Peng, The oceanic salt pump' Does it contribute to the glacial-interglacial difference in CO2 content? Global Biochemical Cycles, 1, , Broecker, W.S., D.M. Peteet, and D. Rind, Does the ocean-atmosphere system have more than one stable mode of operation?, Nature, 315, 21-26, Chappell, J., and N.J. Shackleton, Oxygen isotopes and sea level, Nature, 324, , CLIMAP Project Members, The last interglacial ocean, Quat. Res., 21, , Craig, H., The measurement of oxygen isotope paleotemperatures, in Stable Isotopes in Oceanic Studies and Paleotemperatures, edited by E. Tongiorgi, pp , Consiglio Nazionale Delle Ricerche, Laboratorio Di Geologia Nucleare, Pisa, Craig, H., and L.I. Gordon, Deuterium and oxygen- 18 variations in the ocean and marine atmosphere, in Stable Isotopes in Oceanic Studies and Paleotemperatures, edited by E. Tongiorgi, pp , Consiglio Nazionale Delle Ricerche, Laboratorio Di Geologia Nucleare, Pisa, Curry, W.B., and G.P. Lohmann, Reduced advection into Ariantic Ocean deep eastern basins during the last glaciation maximum, Nature, 306, , Curry, W.B. and G.P. Lohmann, Carbon deposition rates and deep water residence time in the equatorial Atlantic throughout the last 160,000 years, in The Carbon Cycle and Atmospheric C02: Natural Variations Archean to Present, Geophys. Monogr. Ser., vol. 32, edited by E.T. Sundquist and W.S. Broecker, pp , AGU, Washington, D.C., 1985.

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