equatorial Atlantic: Implications for high-latitude

Transcription

1 PALEOCEANOGRAPHY, VOL. 13, NO. 1, PAGES 84-95, FEBRUARY 1998 Early Pliocene deep water circulation in the western equatorial Atlantic: Implications for high-latitude climate change Katharina Billups Earth Sciences Department, University of California at Santa Cruz A. C. Ravelo Ocean Sciences Department, University of California at Santa Cruz J. C. Zachos Earth Sciences Department, University of California at Santa Cruz Abstract. High-resolution (- 3-4 kyr) stable isotope stratigraphies from sites drilled along a depth transect on Ceara Rise (Ocean Drilling Program Leg 154, Sites 925 and 929) are used to reconstruct the deep water circulation response to the long- and short-term climate changes of the early Pliocene ( Ma). Over the long term, benthic foraminiferal carbon isotope recordshow that the vertical fil3c gradient in this region was similar to that of the late Holocene, implying a steady flux of Northern Component Deep Water (NCDW) into the deep Atlantic during most of the early Pliocene. The vertical benthic foraminiferal oxygen isotope gradient is reversed with respecto that of the late Holocene. On the basis of density constraints imposed by seawater stability along this depth transect we attribute the reversed gradiento warmer and more saline NCDW (5øC and 35.1). On orbital timescales we find that the phase relationship between fi180 and fil3c values at the deeper Site 929 differs from the late Pliocene/Pleistocene, while that at the shallower Site 925 was essentially the same. 1. Introduction A general consensus has emerged that relative to today, the early Pliocene (-5-3 Ma) was the most recent interval of sustained global warmth [e.g., Crowley, 1991; Kennett and Hodell, 1993]. Much of the evidence comes from the mid- Pliocene (-3 Ma), when high-latitude North and South Atlantic sea surface temperatures (SSTs) may have been as much as 8øC and 2ø-3øC higher, respectively, than today with little warming in the tropics [Dowsett and Poore, 1991; Dowsett et al., 1992, 1996; Cronin et al., 1991, 1993]. The distribution of these temperatures is suggestive of higher oceanic heat transport associated with more vigorous thermohaline circulation [Dowsett et al., 1992]. However, quantitatively, this mechanism alone might not be sufficient to account for the degree of observed warming because it tends to redistribute heat between hemispheres rather than adding to the total [Crowley, 1992]. In fact, it has been suggested that a reduction in latitudinal temperature gradients should, if anything, slow the thermohaline conveyor [Crowley, 1991; 1992; 1996]. More recent studies call on a combination of increased atmospheric CO2 levels and enhanced thermohaline circulation to explain Pliocene warmth [Crowley, 1996; Raymo et al., 1996]. Copyright 1998 by the American Geophysical Union. Paper number 97PA /98/97PA A large body of work exists addressing Northern Component Deep Water (NCDW) formation and the role of thermohaline circulation on glacial to interglacial timescales during the late Pliocene and Pleistocene [e.g., Boyle and Keigwin, 1982; Oppo and Fairbanks, 1987; Curry et al., 1988; Rayrno et al., 1990a; demenocal et al., 1992; Rayrno et al., 1992; Oppo et al., 1995]. However, comparatively little is known about deep-ocean circulation for the time interval immediately preceding significant northern hemisphere glaciation, primarily because of a lack of sufficiently highresolution records from key locations. Some stable isotope evidence exists for a NCDW like deep water mass at a depth of 4000 m in the western South Atlantic (Rio Grande Rise) during part of the early Pliocene, a situation similar to today [Hodell et al., 1983]. However, our understanding of early Pliocene circulation, in general, is inadequate for testing hypotheses of global and/or high-latitude warmth. Here we present benthic foraminiferal stable isotope records spanning the early Pliocene from two sites drilled along a depth transect on Ceara Rise (Ocean Drilling Program (ODP) Leg 154) (Figure 1). Sites 925 (water depth 3042 m) and 929 (water depth 4361 m) bound the modern mixing zone between NCDW and Southern Component Deep Water (SCDW) in the western equatorial Atlantic; Site 925 is dominated by relatively 12C-depleted NCDW, while Site 929 is strongly influenced by 12C-enriched SCDW (Figure 2), a feature that is clearly reflected in the fi13c values of the late Holocene benthic foraminifera Cibicidoides [Curry et al., 1988]. In 84

2 BILLUPS ET AL.' EARLY PLIOCENE THERMOHALINE CIRCULATION 85 7'N 45'W 44'W 43'W 42'W 7'N 6'N 6'N 5'N 5'N 4'N 4'N 3'N -- 3'N 45'W 44'W 43'W 42'W Figure 1. (a) Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) sites discussed in this study (Table l). (b) Locations of Sites 925 and 929 along the Ceara Rise depth transect (ODP Leg 154, from Curry et al., 1995). principle, changes in the relative fluxes of NCDW versus SCDW during the early Pliocene should be evident in the vertical stable isotope gradient if the stable isotopic composition of NCDW and the SCDW source waters can be sufficiently characterized. To separate local from global effects on the Ceara Rise records and to characterize NCDW source waters, we compare the Sites 925 and 929 stable isotope records to those of Site 606 in the North Atlantic [Keigwin, 1985] and Site 849 in the eastern equatorial Pacific [Mix et al., 1995] (Table 1). Today, Site 606 (water depth of 3000 m) monitors the core of NCDW. Any changes in NCDW source water ]3C composition during the early Pliocene should be evident in the 513C record at Site 606. Eastern equatorial Pacific Site 849 (water depth of 3851 m) records mean Pacific 13C composition. Any global ocean changes in 513C values as well as ice volume and deep water temperature are best represented by the Site 849 record [Mix et al., 1995].

3 86 BILLUPS ET AL.- EARLY PLIOCENE THERMOHALINE CIRCULATION i i i i I _ ' ' I ' I * ' ' ' '' g"?' ' '_ GEOSECS Site 925 (3042 m) - _ Site 929 (4361m) _ "'""''"'"'""''"' Cibicides corpulentus picked from the >250 lam size fraction (Figure 3) [Billups et al., 1997]. Prior to analysis all shells were ultrasonically cleaned in methanol to remove adhering particles, gently broken into smaller pieces to ensure complete reaction, and roasted under vacuum at 375øC for 1 hour to oxidize organic contaminants. Stable isotope analyses were conducted using a VG Prism mass spectrometer equipped with a common acid bath at 90øC. All 513C and 5]80 values are calibrated to Pee Dee belemnite (PDB) via National Bureau Standard-19 and an in-house standard (Carefta Marble). On the basis of replicate analyses of standards in the size range of the samples our measurement precision is better than 0.05%0 for 5]3C and 0.08%0 for 5]80 (n = 280). Individual shell measurements indicate that the intraspecies and interspecies isotopic differences are on average no more than 0.2%0 [Billups et al., 1997] in agreement with other studies [e.g., Woodruff et al., 1980; Duplessy et al., 1984; Graham et al., 1988; Farrell, 1991]. The 5180 values reported here have not been corrected for disequilibrium calcification [e.g., Shackleton and Hall, 1984] unless otherwise noted. For evaluation of site to site gradients we smooth the raw data using a Gaussian filter to remove periods smaller than 10 kyr (e.g., Figure 5). Spectral analyses were carried out on the original unsmoothed data using standard B lackman-tukey techniques [Hays et al., 1976; Irnbrie et al., 1984]. Following convention, cross-spectral analyses are carried out after 53C of ZCO 2 multiplying the 5]80 records by -1 for reference with respect Figure 2. Water column profiles of 513C of ZCO 2 at Geochemical to interglacial conditions [e.g., Irnbrie et al., 1984]. All Ocean Sections Study (GEOSECS) stations in the western records were linearly detrended and interpolated at constant 4 equatorial Atlantic [Kroopnick, 1985]. Arrows point the respective kyr intervals (n = 354 for Site 925; n = 255 for Site 929). depths of Sites 295 (3042 m) and 929 (4361 m). 2. Methods Values in the high-resolution (-3-4 kyr) benthic stable isotope records constructed for Sites 925 and 929 on Ceara Rise represent averages of several single-shell measurements on Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, and 3. Orbital Timescale Age control of the Ceara Rise records is based on magnetic susceptibility records which were tuned to the northern hemisphere insolation curve of Laskar [1990] by Tiedernann and Franz [1997]. To test the age model, we compare the Sites 925 and 9295]80 records to eastern equatorial Atlantic Site Table 1. Site Summary and References of Deep-Sea Records Discussed in This Study Site Location Water Depth, m Reference 925 4ø12'N, 43ø29'W 3042 this study 929 5ø58'N, 43ø44'W 4361 this study KNR a 4øN, 43øW 3063 Curry et al. [1988] KNR a 5øN, 43øW 4341 Curry et al. [1988] ø20'N, 35ø30'W 3007 Kei zwin [ 1985] 607 t 41øN, 33øW 3427 Ravmo et al. [1989] ø05'N, 20ø02'W 3070 Tiedemann et al. [ 1994] ø 52'S, 7ø25'E 2532 Hodell and Venz [1992] 849 0øll'N, ll0ø31'w 3851 Mix et al. [1995] athese cores, also from Ceara Rise, were used for late Holocene values for Sites 925 and 929. bthis site was used to estimate late Holocene stable isotope values for Site 606.

4 BILLUPS ET AL. EARLY PLIOCENE THERMOHALINE CIRCULATION 'B' I ' ' ' I ' ' I' I ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' 2.5 :, ' '..,,,.',,,, i' ' " ' ø' " ' * ' ""... ' : :. :',:; :,-, ' Age (Ma) Figure 3. Benthic (Cibicidoides spp.) (a) 513C and (b) fi180 records along the Ceara Rise depth transect for Sites 925 (3042 m,) and 929 (4361 m). Data represent single-shell measurements averaged from each stratigraphic level. 659 [Tiedemann et al., 1994] (Figure 4). Site 659 has been calibrated to eastern equatorial Pacific Site 846 [Tiedemann et al., 1994; Shackleton et al., 1995] and serves to bridge the Ceara Rise records to the eastern equatorial Pacific. We use this approach because Sites 659 and 925, at comparable depths in the Atlantic, are bathed by the same deep water mass. Oxygen isotope variability in early Pliocene records is distinct and can be thought of in terms of well-defined isotope stages signifying global climate change [Shackleton et al., 1995]. In the most recent numbering scheme, Shackleton et al. [1995] designate magnetic reversals with corresponding capital letters and glacial intervals with even numbers. Numbering of the isotope stages is reinitialized at each chron boundary. Sites 925 and 929 display a series of maxima that can be visually correlated to most of the early Pliocene isotope stages (Figure 4). Small but consistent discrepancies between the Sites 925 and 659 b180 peaks are apparent only in the oldest part of the record (prior to 4.55 Ma) where differences between the astronomical solutions that underlie the age models for Sites 659 [Tiedemann et al., 1994] and 925 increase [Tiedemann and Franz, 1977]. From this comparison we find that the Ceara Rise records are stratigraphically complete [ Age (Ma) Figure 4. Oxygen isotope records of Ceara Rise Sites 925 and 929 compared to eastern Atlantic Site 659 [Tiedemann et al., 1994]. Numbers refer to glacial isotope stages established by Shackleton al. [1995]. The Site 929 record has been offset by 0.9%0, and the Site 659 record has been offset by +1.8%o relative to the Site 925 record. The vertical, dashed lines represent visual correlation of fi 80 maxima between the three records.

5 88 BILLUPS ET AL. EARLY PLIOCENE THERMOHALINE CIRCULATION 4. Western Equatorial Atlantic Deep Water Circulation Benthic carbon isotope records from the Ceara Rise depth transect (Sites 925 and 929) can be used to estimate the degree of NCDW and SCDW mixing and hence the relative strength of NCDW formation only if we can constrain NCDW and SCDW source water compositions. As noted above, Site 606 in the North Atlantic approximates NCDW source water 513C values, and Site 849 in the Pacific reflects mean ocean 513C variability. The Site 606 age model, which was originally based on the Berggren et al. [1985] timescale, has been modified to fit the Shackleton et al. [1995] timescale. In addition, because the resolution is much coarser than for the Ceara Rise records, we represent the Site 606 stable isotope record by the average + 1 standard deviation rather than showing the entire record (Figure 5a). The 513C gradients recorded at Ceara Rise indicate a wellventilated deep Atlantic throughout the majority of the early Pliocene. A permanent vertical gradient of 0.3%o existed between Sites 925 and 929 after 4.2 Ma (Figure 5a). This gradient was on average equivalent to the late Holocene gradient, suggesting that NCDW formation was comparable to today. From 4.2 to 3.45 Ma, average 513C values from both sites fluctuated about the late Holocene mean. A decrease in 513C values between 3.6 and 3.2 Ma at both sites matches a similar decrease in the Site 849 record suggesting the trend was global ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' I ' ' ' I ' ' I ' ' A,, 925-,,, I,,, I,,, I,,, I,,, I,,, I,,, I,,, ' ' ' Age (Ma) Figure 5. Smoothed (19 kyr Gaussian window) benthic (Cibicidoides spp.) (a) 513C records and (b) 5180 records from Sites 925 (3042 m, dashed curve) and 929 (4361 m) along the Ceara Rise depth transect compared to eastern equatorial Pacific Site 849 [Mix et al., 1995]. Horizontal lines (dashed and solid) indicate late Holocene measurements from previous Ceara Rise cores at comparable depths [Curry et al., 1988] (Table 1). In Figure 5a the shaded box represents the 513C average plus or minus one standard deviation of North Atlantic Site 606 [Keigwin, 1985] plotted on the timescale of Shackleton et al. [1995]. In Figure 5b the Site record is offset by +0.64%0. The Sites 606 and C records show that early Pliocene North Atlantic 513C values were similar to late Holocene values, while those in the Pacific were on average 0.2%0 lower (Figure 5a). This suggests that NCDW source water composition remained at late Holocene levels between -3.2 and 4.4 Ma. The slightly lower mean oceanic 513C values as recorded at Site 849 might have been caused by higher nutrient levels [Mix et al., 1995]. More importantly, over the long term, the average Atlantic to Pacific basin-to-basin 513C gradient was large and relatively stable during the early Pliocene, indicating that NCDW was the dominating source of deep water in the Atlantic. In several intervals, the Site C recor displays brief negative excursions that would effectively diminish the equatorial Atlantic-Pacific gradient (see raw data, Figure 3a, at 3.66 and 3.75 Ma; see raw data at 3.37 Ma only for Site 925). Individual shell analysis of Cibicidoides from these intervals reveals a wide range of 513C values (up to 1%o) in a single assemblage of 5-10 shells, with only the epibenthic species Cibicidoides wuellerstorfi recording extremely low 513C values (not shown). In a few cases the 513C values of individual Cibicidoides wuellerstorfi are even lower than 513C values observed at Site 849. This pattern is more consistent with calcification in an 13C depleted microenvironment such as that associated with variable amounts of organic matter accumulating at the sediment-water interface (e.g., the "organic fluff layer" of Mackensen et al. [1993]) than with deep water circulation induced 513C variability. While it is remotely possible that these negative 513C excursions represent a SCDW signal, we do not place significance on such intervals in terms of water mass distribution. In sum, permanent 513C gradients between Sites 925, 929, and 849 provide the most convincing evidence for a steady production of NCDW throughouthe early Pliocene. These results agree well with the relatively high 5 3C values between and 3.5 M a found at 4000 m water depth at Deep Sea Drilling Project Site 518 (30øS, 38øW) [Hodell et al, 1983]. Given the lack of evidence for significant increases NCDW source water or global ocean 3C composition, we interpret the relatively high 513C values in the deep equatorial Atlantic during the early Pliocene to indicate a NCDW flux that was at least as strong as today. By extension we infer that thermohaline circulation was relatively vigorous and that its potential for transporting heat poleward was at least comparable to today. 5. Ice Volume and Deep Water Temperatures Benthic foraminiferal 5 80 data provide a record of early Pliocene ice volume and deep water temperature change. Comparison of the Ceara Rise Sites 925 and records with Site 849 reveals several importantrends (Figure 5b). At Site 925, 5180 values decreased from 4.7 to 4.3 Ma, paralleling the trend at Site 849, suggesting a reduction in ice volume and/or deep water warming. Between 4.2 and 3.7 Ma the 5180 gradient between Sites 925 and 929 was reversed relative to the late Holocene. In addition, over the long term, 5 80 values do not drift, suggesting on average a relatively stable Antarctic ice sheet between 4.2 and 3.7 Ma. Between 3.7 and 3.35 Ma, Sites 925 and records overlap. Furthermore, Site 925 parallels Site 849 in well-defined

6 BILLUPS ET AL. EARLY PLIOCENE THERMOHALINE CIRCULATION BW BW C 41 CI i/ ii:i i'j :'i , iii:i0! 0.15 ii::ii::10 i '... ;;; " ' ',... ;I... i -200 frequency (cycles/4 kyr) frequency (cycles/4 kyr) Figure 6. Cross-spectral analysis of 513C values (y variable) against values (x variable) for (a) Site 925 and (b) Site 929. Shading highlights the positions of the primary Milankovitch frequencies (41 and 23 kyr). The solid, horizontaline indicates nonzero coherence at the 80% level. At Site 925, 513C values are coherent and essentially in phase with -5 80(Figure 6a). At 929, 3C values are coherent with - 80 values and lag by 150 ø at the 23 kyr period (Figure 6b). Note thathe x axis for the Site 929 record (Figure 6b) extends to the Nyquist frequency in this comparison. stepwise 5180 increases toward 3.3 Ma, indicatingrowth of continental ice. The reversed vertical 5180 gradient at Ceara Rise between 4.2 and 3.7 Ma suggests that temperature/salinity properties of NCDW and/or SCDW source waters were differenthan today or for the last 3.7 Ma. On average, Site values were 0.2%0 higher than at Site 929 during this interval. In addition, several maxima in the Site record exceed late Holocene values by as much as 0.4%0, while Sites 929 and 849 maxima remain well below the late Holocene reference line. These observations suggest that changes in the deeper Atlantic were tightly coupled with mean ocean 5180 variability recorded in the deep Pacific. In contrast, the 5180 values recorded at Site 925 are higher than expected from globally lower ice volume during the early Pliocene. As we shall show below, these relatively high 5180 values are best explained by an increase in NCDW temperature and salinity. 6. Time Series Analysis In the deep Atlantic, benthic foraminiferal 13C/12C ratios primarily reflect local water mass carbon chemistry as affected by changes in the relative strength of thermohaline circulation. For example, late Pliocene/Pleistocene circulation in the Atlantic is characterized by the presence of a nutrientdepleted (high 13C/12C) water mass of northem Atlantic origin during interglacial intervals and a more nutrient-rich (low 13C/ 2C) water mass originating in the Southern Ocean during glacial periods [e.g., Reymo et el., 1990a; Oppo et el., 1990; Curry et el., 1988]. As a result, relatively high benthic foraminiferal 513C values tend to be in phase with low 5180 values, while low 513C values are in phase with high 51SO values. This water mass effect is superimposed on whole ocean 513C variability because of the partitioning of carbon between the marine and terrestrial reservoirs on glacial to interglacial timescales [Sheckleton, 1977] which also results in maximum 13C enrichment of foraminiferal calcite during peak interglacial periods adding to the circulation effect. To characterize glacial to interglacial deep water circulation changes, we examine the cross-spectral relationships between Ceara Rise 513C and-5180 values. Cross-spectral analysis of the 513C and-51so values shows that at the shallower Site 925 the variables are coherent and essentially in phase at the obliquity (phase = 2 kyr + 3 kyr)

7 90 BILLUPS ET AL.: EARLY PLIOCENE THERMOHALINE CIRCULATION Table 2. Summary of Late Pliocene/Pleistocene (0-2.5 Ma) and Early Pliocene ( Ma) 813C (y) Versus -8180(x) Phase Relationships Site 925 Site 929 Site 925 Site 929 Obliquity, deg Late P liocene/pleistocene (0-2.5 Ma) Precession, deg a 34 Early Pliocene ( Ma) Late Pliocene/Pleistocene summary from Bickert et al. [1997]. anegative values indicate a lead of y versus x. during the early Pliocene. As such, increases bl3c values at Site 929 reflect either maximum relative NCDW flux and/or minimum relative SCDW flux. A lag in b 3C with respect to interglacial conditions suggests that the mixing front of NCDW/SCDW reached its southernmost extension during initial stages of ice sheet expansion. We speculate that relatively strong NCDW flux may have contributed to global ice volume fluctuations by delivering relatively warm water and hence atmospheric moisture to polar latitudes. However, this hypothesis cannot be tested without better control on climate variability on orbital timescales in the North Atlantic and the Southern Ocean. In sum, Ceara Rise 813C records provide convincing evidence that NCDW was the dominant deep water mass to a depth of 3000 m in the equatorial Atlantic during the early Pliocene. Global carbon reservoir changes dominate the shallow Site C variability on orbital timescales. However, changes in the relative contribution of NCDW are evident at the deeper Site 929 (4300 m) that are linked to glacial-interglacial oscillations on precessional timescales. and precessional period (phase = 1.2 kyr _+ 1.7 kyr) (Figure 6a). These relationships are very similar to those at Site 849 [Mix et al., 1995], suggesting that the Site C variability was controlled primarily by changes in mean ocean 8 3C. We rule out a deep water circulation control on the occurrence of high 813C values with low 8 80 values and vice versa at Site 925 because of the lack of evidence for a strong SCDW influence on Site 925 as discussed above (e.g., the 7. Early Pliocene NCDW Temperature/Salinity As noted above, between 4.2 and 3.7 Ma the Site values are on average 0.2%0 higher than those at the deeper Site 929 with maxima as much as 0.4%0 higher than late Holocene. In contrast, Site maxima never exceed late Holocene values in agreement with Site 849 in the Pacific (e.g., Figure 5b). Assuming that Sites 849 and 929 are representative of the global mean, the relatively high 8180 vertical 813C gradient between Sites 925 and 929 is stable; values at Site 925 (leading to a reversed 8180 gradient) suggest Figure 5a). At the deeper Site 929 the 8 3C record is coherent with at the 23 kyr periodicity (Figure 6b) and lags by 165 ø that the temperature and salinity of NCDW were different than today. Reversal of 8180 values along a vertical depth transect (phase = 10 kyr _+ 2 kyr). The obliquity peak in the 8 3C record reflects either warmer, relatively saline water underlying is shifted toward slightly higher frequencies (corresponding to a period of about 35 kyr). Thus the coherence between and 813C at the obliquity period may be artificial. These relationships are not consistent with a transfer of carbon between reservoirs as inferred by the in phase behavior of the cooler, less saline water or the reverse [Kennett and Stott, 1987; Woodruff and Savin, 1991]. Distinguishing one from the other requires consideration of salinity, temperature, and density relationships. The 180/160 ratio and the salinity of seawater both depend on evaporation and vapor transport two variables observed at Site 849. At the obliquity period, processes [Craig and Gordon, 1965] and are therefore 8 3C variability at Site 929 may have been influenced by local water mass effects not linearly related to ice volume empirically related in the open ocean. Calcite 8180 values reflecthe 180/160 ratio, and thus the salinity, of the seawater fluctuations. These observations are different from those of as well as the ambient water temperature [Urey, 1947]. For the late Pliocene to Pleistocene at Ceara Rise (Table 2). For example, late Pliocene/Pleistocene (0-2.5 Ma) 8 3C values at both Ceara Rise Sites are essentially in phase at the obliquity period and lag by 89 ø (Site 925) and 34 ø (Site 929) at the precessional period (Table 2) [Bickert et al., 1997]. The inphase relationships on Ceara Rise at the obliquity period during the Pleistocene are attributed to glacial suppression of NADW [Bickert et al., 1997] consistent with previous studies [e.g., Rayrno et al., 1990a]. While it is difficult to discuss the source of the 35 kyr peak in the 8 3C record, the lag of the example, an increase in benthic 8180 values can indicat either 180 enrichment at the sea surface in source regions of deep water formation or calcification at lower temperatures. However, along any depth transect, because density constraints to maintain seawater stability must be satisfied, we should be able to approximate from benthic 8180 values the temperature-salinity signals of the two water masses. By assuming that seawater density of a shallower water mass (Site 925) was not higher than that of an underlying deeper water mass (Site 929), we can compute the specific 813C record with respect to at the precessional frequency temperature-salinity combination required to produce a 0.2%0 is enigmatic. One possible explanation involves shifts in the mixing boundary between NCDW and SCDW. Today, Site 929 is higher equilibrium calcite value at Site 925 with respecto Site 929. For this exercise, we assume that conditions at Sites 929 and 849 are representative of the global mean which limits the located within the mixing zone of NCDW and SCDW, while Site 925 is closer to the core of NCDW. On the basis of the 813C records it appears that very similar conditions existed extento which the 8180 signal at Site 929 can be influenced by salinity changes. Site 929 temperature and salinity are estimated by assigning 0.3%0 of the 0.43%0 difference relative

8 BILLUPS ET AL.' EARLY PLIOCENE THERMOHALINE CIRCULATION 91 Table 3. Late Holocene and Early Pliocene Temperature and Salinity Summary From Ceara Rise Sites 925 and 929 Temperature, a øc Salinity b 15180cc, %0 tj365½) c Late Holocene Site Site Early Pliocene ( Ma) Site Site acore top reconstructions using the paleotemperature equation of Erez and Luz [ 1982] with calcite values (15 8Occ) corrected for disequilibrium calcification (+0.64%0 [Shackleton and Hall, 1984]) and seawater values (15 SO w) adjusted to Pee Dee belemnite (PDB) (-0.27%0 [Hut, 1987]). bsa!inities are in situ for the late Holocene and derived for the early Pliocene using the modem 151SOw-salinity relationship for the deep Atlantic from Zahn and Mix [1991] (see text). CThe c 3650 corresponds to a mean depth of 3650 m between Sites 925 and 929. to late Holocene to lower ice volume [e.g., Kennett and Hodell, 1993] and the remaining 0.13%o to 0.6ø-0.7øC warmer bottom/deep water temperatures (2.7øC). An ice volume decrease of this magnitude yields a 30 m higher sea level (0.01%o m -1 sea level [Fairbanks and Matthews, 1978])and a 0.27 decrease in global ocean salinity (34.60). Using these parameters (2.7øC and 34.6; Table 3), seawater density (expressed as 1J3560) at Site 929 is computed to be [UNESCO, 1981]. The temperature, salinity, and density relationships are illustrated in a sigma-t diagram along with isopleths of equilibrium calcite (818Oc c) (Figure 7). The temperature and salinity that is consistent with the average Site value of 3.03%0 (between 3.7 and 4.2 Ma) as well as the Site 929 target density (44.07) corresponds to the intersection of the 3.03%0 isopleth with the isopycnal. The intersection occurs at a temperature of-5øc and a salinity of (Figure 7 and Table 3). Note that these represent minimum estimates, as lower densities at Site 925 (e.g., following the 3.0%0 isopleth upslope) would result in a stable water column at higher temperatures and salinities. We calculated the b 80 cc isopleths using the paleotemperaturequation of Erez and Luz [ 1982] (equation 1) and the relationship between the b 80 value of seawater (8 80,) and salinity in the modern deep Atlantic derived by Zahn and Mix [1991] (equation 2): PT = (818Occ- 8 8Ow) (8 8Occ - 8 8Ow) 2 8 8Ow = 1.53Salinity ( ) (2) We choose the Erez and Luz equation because it yields the best possible agreement between core top reconstructions and in situ temperature measurements in the deep ocean as previously demonstrated by Zahn and Mix [1991] and by the Ceara Rise core top results in this study (Table 4). The 818Ow-salinity relationship of the modern deep Atlantic is used as a first approximation, and the sensitivity of Site 925 temperature- Table 4. Modern Hydrography and Core Top Reconstructions at Ceara Rise Sites 925 and 929 a b Temperature Salinity tj in Situ, a øc w 15 80cc Temperature (SMOW) c Core Top d Core Top, e øc Site Site ageosecs station 39 for Site 929' station 40 for Site 925. bthe c 3650 corresponds to a mean depth of 3650 m between Sites 925 and 929. CThe Standard Mean Ocean Water (SMOW) w value was calculated from the in situ salinity using the deep water 151SOw-salinity 8 relationship (15 O w = 1.53Salinity ) from Zahn and Mix [1991]. dthe 15 8Occ values are from Curry et al. [1988] adjusted for disequilibrium calcification by +0.64%0 [Shackleton and Hall, 1984]. ecore top temperatures were calculated using the equation of Erez and Luz [1983], using w values relative to PDB (-0.27%0, [Hut, 1987]).

9 92 BILLUPS ET AL.' EARLY PLIOCENE THERMOHALINE CIRCULATION so-depleted northern hemisphere ice sheets, would be different than at present. This limits our ability to accurately 14 - characterize the b18ow-salinity for high-latitude surface and 12 _ deep water. Nevertheless, by assuming fixed b18ow-salinity ratios, we can still estimate the temperature and salinity combination required to produce the 0.2%0 offset in equilibrium calcite between the two Ceara Rise sites. To evaluate the sensitivity of the Site 925 paleotemperatures to different b18ow-salinity relationships, we vary the slope and intercept of (2) using the modern hydrography of water masses ranging from the East Greenland Seas to the western equatorial Atlantic [Fairbanks et al., 1992] 2 - (Table 5). In this test the coolest temperature (3.39øC) and lowest salinity (34.76) for NCDW are obtained using the b18ow-salinity relationship of Slope Water and the warmest -2 - temperatures and highest salinities using the relationship of 34.! ! Salinity Figure 7. Sigma-t diagram showing isopycnals (solid contours) at a East Greenland surface water (7.41øC and 35.81). All other equations yield temperatures and salinities within 1 øc and 0.1 of values derived from the modern deep water b18ow-salinity regression. Hence all equations produce the same result: in the depth of 3650 m (mean depth between Sites 925 and 929) and isopleths early Pliocene, NCDW was warmer than today and, consistent (dashed contours) of of equilibrium calcite (1518Occ). See text for with modern deep water hydrography, warmer than SCDW. details on equations used to derive these relationships. The inset box gives the averaged Ma 5 80 values (corrected by +0.64%0 to This finding is robust and does not appear to hinge on a account for disequilibrium calcification [Shackleton and Hall, 1984]). particular choice of equations. The temperature and salinity of Site 925 that yield a stable water column The evidence for warmer NCDW is significant as it implies with respecto Site 929 correspond to the intersection of the that the high northern latitudes were warmer as well. isopycnal with the 3.03%0 isopleth (arrows). The c 3650 of water at Site Although, there are no direct sea surface temperature 929 (44.07) was reconstructed assuming a 30 m change in sea level and a 0.6øC increase in deep water temperatures (see text). reconstructions for high-latitudes prior to 3 Ma, evidence does exist for greater warmth during the mid-pliocene (3 Ma) at middle to high latitudes of both hemispheres [Dowsett et al., 1996]. Warmer and more saline waters in the Nordic Seas may salinity values is specified to other illsow-salinity parameters have been a natural consequence of reduced Arctic sea ice cover below. [Rayrno et al., 1990b]. Higher temperatures and salinities have potential implications for the rate of early Pliocene NCDW formation. Admittedly, there are several uncertainties associated with these calculations. A minor uncertainty is the absolute magnitude of the ice volume effect versus temperature change with respecto late Holocene assigned for Site 929. Because ice volume affects both Ceara Rise records equally, this type of error does not impact relative estimates of temperature and salinity. A larger uncertainty involves the choice of the 518 Ow_salinity relationship which, with warmer surface temperatures and in the absence of large proportionally more Today, net evaporation of 0.35 Sv (1 Sv = 106 m 3 s -1) in the North Atlantic drives the export flux of 20 Sv of NADW [Broecker, 1991]. In general, higher temperatures and salinities in this region would tend to increase this flux. As a consequence, northward surface flow must increase accordingly to conserve water mass balance. Today, the deep water flowing south has a temperature of-2-3øc, while water flowing north Table 5. Sensitivity of Northern Component Deep Water Paleotemperatures and Salinities to the b 8Ow-salinity Relationship Water Mass a rn b fi180 w (SMOW) Temperature, øc Salinity Deel Atlantic East Greenland Slol e Water Sargasso Sea Western Eel. Atlantic Atlantic Surface Water Salinity is bt8ow + b/m' rn = slope; b = intercept. The target density is afairbanks et al. [1992], except the global surface water, which is from Ostlund et al. [1987].

10 BILLUPS ET AL.' EARLY PLIOCENE THERMOHALINE CIRCULATION 93 is -18øC. This leads to a net loss of heat from the South Atlantic to the North Atlantic [e.g., Crowley, 1992]. Therefore thermohaline circulation is not considered to add to the total heat budget; it only serves to redistribute heat between the hemispheres [Crowley, 1992]. However, with a warming of the North Atlantic, perhaps related to sea ice albedo feedbacks, the total heat budget increases [e.g., Raymo et al., 1990b]. Thus we speculate, on the basis of these temperature-salinity reconstructions, that the flux of NCDW was larger than today and may have also contributed to relative southern hemisphere and/or global warmth. 8. Implications and Speculations Several aspects of early Pliocene climate can be addressed with the Ceara Rise records including ice volume and deep water temperatures, Southern Ocean surface water temperatures, and thermohaline circulation. Benthic 5180 records from the Pacific indicate that early Pliocene ice volume was lower and deep water was slightly warmer than today [Mix et al., 1995; Shackleton et al., 1995, Hodell and Venz, 1992; Hodell and Warnke, 1991; Kennett and Hodell, 1993]. At Site O values are on average %o lower than late Holocene values, a shift which is larger than the shift recorded in the Southern Ocean (-0.3%o at Site 704 [Hodell and Venz, 1992]), at Ceara Rise Sites 925 (-0.07%o) and 929 (-0.43%o), and in the eastern tropical Atlantic (+0.13%o at Site 659 [Tiedemann et al., 1994]). The implications of this can be realized by considering the horizontal 51SO gradients for the time interval between 4.2 and 3.7 Ma (characterized by the reversed 51SO gradient on Ceara Rise) relative to late Holocene gradients (Figure 8). During the late Holocene, benthic 51SO values increase from the equatorial Atlantic to the Southern Ocean and Pacific by roughly 0.5%o, reflecting a deep water temperature gradient of-2.5 øc between the ocean basins (Figure 8a). However, between 4.2 and 3.7 Ma, only the Sites SO gradient approaches that observed during the late Holocene. At 3000 m water depth, the Atlantic to Pacific 51 O gradient is notably smaller (Figure 8b). In fact, Southern Ocean 51SO values are higher than those in the Pacific as recognized by Mix et al. [1995], a finding that is difficult to reconcile given our current understanding of deep water circulation. One possible explanation for the relatively high Southern Ocean benthic 5180 value is that it reflects the contribution of warm and saline NCDW, rather than cooling of the Southern Ocean. This would be more consistent with evidence for Southern Ocean warmth during the early Pliocene [Hodell and Venz, 1992; Hodell and Warnke, 1991]. For example, South Atlantic sea surface temperatures as inferred from planktonic foraminiferal 5180 values were -1.5øC warmer than today (Site 704 [Hodell and Venz, 1992]), and terrestrial evidence from East Antarctica indicates temperatures <3øC higher than today [Marchant and Denton, 1996]. This explanation is appealing because it provides a mechanism for warming in the Southern Ocean without invoking a significant change in the strength of thermohaline circulation. Furthermore, the export of heat from the southern to the northern hemisphere that occurs in surface water flow [Crowley, 1992; Gordon, 1986] would be, at least in part, compensated for by an increase in the amount of heat supplied to the southern hemisphere by relatively warmer 200(} o 20O A Atlantic late Holocene Ma O N Latitude S Pacific (110øW) Figure 8. Interocean comparison of $ 80 averages (a) Late Holocene (4-0 kyr) (see Table 1 for references and depth information) and (b) Ma. In Figure 8b, relatively high $ 80 values as compared to the eastern equatorial Pacific Site 849 are evident at Sites 659, 925, and 704. Site 704, in particular, displays $ 80 values higher than those at Site 849, which cannot be explained by simple deep water cooling (see text). deep water upwelling in the circumpolar Antarctic [Gordon, 1981]. We speculate that positive feedbacks involving southern hemisphere atmospheri circulation and northern hemisphere poleward heat transportogether with sea ice feedbacks may have played an important role in maintaining early Pliocene climate. Ocean circulation model runs by Toggweiler and Samuels [1993] suggest that the westerly wind stress in the latitude band of the Drake Passage "pulls" NCDW to the surface. The magnitude of upwelling is determined by the latitudinal position of the axis of maximum westerly wind stress, where northward movement of this axis results in weakening of the thermohaline overturn. During the early Pliocene between 4.2 and 3.7 Ma it appears that the axis of maximum westerly wind stress remained in a favorable southerly position to pull NCDW into the Southern Ocean. Reconstructions of the latitudinal movement of the Polar Front Zone based on percent calcium carbonate measurements [Hodell and Warnke, 1991] (corrected for the difference in timescales between Sites 704 and 925/929) support a more southerly position between-4.2 and 3.7 Ma. Moreover, fluctuations in the flux of relatively warm NCDW upwelling in the Southern Ocean may have provided a negative feedback on the position of the axis of maximum westerly wind stress by enhancing the moisture content of the polar atmosphere, favoring precipitation and ultimately ice growth. As evidenced by lagging Site C values with respect to interglacials, the relative flux of NCDW into the Southern Ocean may have been largest during intervals of ice growth. Moisture feedbacks may have eventually weakened in response to the growth of the northern hemisphere ice sheet. Eventually, heat flow into the Nordic seas as well as heat export into the southern hemisphere may have been reduced in conjunction with more permanent Arctic sea ice cover. = I 0

11 94 BILLUPS ET AL.: EARLY PLIOCENE THERMOHALINE CIRCULATION 9. Conclusion Ceara Rise stable isotope records support earlier studies suggesting that the early Pliocene was an interval of highlatitude warmth. Moreover, the Ceara Rise records provide for the first time evidence of a relatively vigorous thermohaline circulation between -3.2 and 4.7 Ma. Movements of the mixing front between NCDW and SCDW as recorded at Site 929 suggest that the relative NCDW flux was strongest during stages of ice growth. Moreover, the presence of a reversed gradient indicates that deep waters in the Atlantic at m depth were at least 1ø-2øC warmer than today even though the absolute magnitude of the paleotemperatures is difficult to constrain. We speculate that increased temperatures and salinities in the source regions of deep water formation may provide a mechanism for relatively strong NCDW export flux. 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