interstadial transitions?

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Is ocean thermohaline circulation linked to abrupt stadial/interstadial transitions? Edward A. Boyle Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract Are the abrupt stadial/interstadial (S/IS) climate transitions observed in the Greenland ice cores also seen in marine climate records? Literature which shows that the S/IS events have a large footprint in ocean surface marine properties encompassing the entire Northern Hemisphere has been reviewed. Whether this in#uence extends to the deep circulation is more equivocal on most recent evidence. Several of the `Heinricha ice rafting debris events are clearly associated with elimination of North Atlantic source waters from the deep Atlantic. But most cores studied do not resolve the other S/IS events, except for one new record from the Bermuda Rise which shows all of the major S/IS cycles in benthic foraminiferal. Deep-sea corals can tell us how rapid these events are in the deep sea.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Paleoclimate studies of ice cores and other archives reveal dramatic decadal}century climate transitions, which raise signi"cant concerns about the predictability of climate. Central Greenland ice cores show rapid regional temperature shifts of as much as a third of the full glacial/interglacial amplitude (Grootes et al., 1993; Grootes and Stuiver, 1997). Several mechanisms have been proposed to account for these rapid climate transitions, known as stadial/interstadial (S/IS) transitions. One class of theories invokes instabilities in large continental ice sheets (MacAyeal, 1993a,b; Saltzman and Verbitsky, 1994,1996). Another class of explanations cites the inherent instability of the ocean thermohaline circulation (Broecker et al., 1990; Broecker et al., 1985). Thermohaline circulation instabilities may raise more concern for predicting future climate than do ice sheet instabilities (Broecker, 1997). Certainly, there is evidence for signi"cant climate change within the last 10,000 yr, a time of low ice volume (e.g. Keigwin, 1996). A well-established literature has explored the mechanisms responsible for ocean thermohaline circulation instabilities. The fundamental reason for this instability is that the density of polar waters is governed strongly both by temperature and salinity. Although there is a strong feedback between the atmospheric temperature and sea surface temperature, there is essentially no feedback between the saltiness of seawater and evaporation,

precipitation, and water vapor transport. Stommel (1961) used a simple box model to demonstrate that a convective seawater system could have multiple stable solutions. Subsequent studies have employed models with a wide range of complexity and sophistication to draw the same conclusion (e.g. Marotzke, 1994; Marotzke and Willebrand, 1991; Weaver and Hughes, 1994). Although it is more di$cult to prove that the real ocean has multiple stable states than it is to illustrate such behavior in model systems, no one has conversely proven that these multiple states cannot exist in the real ocean. A large and growing volume of theoretical work supports the notion that ocean thermohaline instabilities may have caused past abrupt climate changes. Although it is common to discuss changes in the thermohaline circulation as if deep-water sources can only be `ona or `o!a, the real situation is likely to be complex, with evolutions between some range of continuum states occurring gradually as well as through abrupt changes. For example, there is paleoceanographic data indicating that Glacial North Atlantic Deep-water (DADW) did not vanish entirely but became a shallow water mass with net export from the Atlantic (Boyle and Keigwin, 1987; Duplessy et al., 1988; Oppo and Lehman, 1993; Yu et al., 1996). Models show more complex behavior as well (Manabe and Stou!er, 1995,1997; Weaver et al., 1991; Rahmsdorf, 1995). The deep-water response may be dependent on the character of the forcing * meltwater discharge may elicit a di!erent oceanic response than

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iceberg discharge. There is already evidence from modern oceanography that deep convection can change on internannual and decadal time scales, and that spreading velocities of pulses of deepwater formation can be surprisingly rapid (e.g., Sy et al., 1997). Although the discussion below may seem to be dominated by `ona or `o!a thinking, this usage is just a shorthand for what clearly is a much more subtle matter, including interannual and decadal changes, wave propagation within the ocean (e.g. DoK scher et al., 1994), and changes to the property "elds. For the purposes of this paper, I will consider the circulation `changeda when it has persisted long enough to a!ect the steady-state tracer "eld (T, S, chemistry), since this is the type of evidence we most commonly use in the paleoceanographic record. Given this strong theoretical support, it may be surprising that there is little observational evidence in the literature showing a strong link between the ocean thermohaline circulation and abrupt climate transitions such as those seen in the Greenland ice cores. As it turns out, it is surprisingly di$cult to "nd paleoclimatic archives in the deep sea that can establish whether or not deep-water properties are closely linked to abrupt climate transitions on decadal-to-century time scales. The major impediment is biological stirring of the upper few centimeters of deep-sea sediments, which constitutes a low-pass "lter removing evidence for short-term events (given typical oceanic sedimentation rates of a few centimeters per thousand years). As will be seen, the only solutions to this problem are: (1) to work at sites where the accumulation rate of detritus on the sea#oor is much higher than normal, (2) to work with sediments deposited under anoxic (nonbioturbated) conditions, or (3) to seek out new high-resolution archives, such as deep-sea corals. This paper will review the evidence for the link between the S
IS transitions seen in Greenland and the deep-sea record of climate change.

2. Some questions E Is there any evidence for a relationship between (S/IS) transitions and the thermohaline circulation? E If so, do the amplitudes of the S/IS transitions and the thermohaline signals correspond? E What is the detailed phase relationship between thermohaline circulation and other manifestations of S/IS transitions in the earth's climate system? E Who is the chicken, and what is the egg? I will show that there is evidence in the literature for links between interstadials and thermohaline circulation, but that this evidence is far from su$cient to establish a cause-and-e!ect relationship. New evidence from Bermuda Rise sediment cores begins to "ll this gap, and the reader may come to believe that there may in fact

be a relationship between deep-water and the S/IS events. This evidence begins to speak to this question of amplitudes and phase relationships, but more detailed work will be required to answer this question. This paper will conclude on a "rmly agnostic note concerning the "nal question } do thermohaline circulation instabilities cause S/IS transitions or is it vice versa? This question may well take a decade or more of research to answer.

3. Evidence for a link between abrupt climate change in Greenland and the sea surface Numerous studies have established that there is a tight link between events in central Greenland and characteristics of the surface ocean. Following up on studies by Ruddiman et al. (1980) and Heinrich (1988), Bond et al. (1993) presented evidence for a one-to-one correspondence between S/IS cycles and northern North Atlantic surface temperatures at a site near Ireland (as re#ected in the relative abundance of the polar foraminifera left-coiling N. pachyderma) (Fig. 1). Furthermore, they called attention to the `supergroupa of smaller cycles that end with major episodes of ice rafting of glacial debris observed by Heinrich (1988). The identi"cation of the marine events and the ice core events relies to some extent on pattern matching. The marine chronology } based on a few relatively uncertain radiocarbon ages (from the lowradiocarbon tail), and oxygen isotope stratigraphy based upon tuning to Milankovitch cycles } is simply too inaccurate to show that the ice core and marine cycles are precisely coeval. Furthermore, some of the cycles are indistinct in the marine record because they occur at the limits of temporal resolution allowed for by the sedimentation rate and bioturbation. Nonetheless, few marine paleoceanographers would doubt the Bond et al. premise that the S/IS events are re#ected in the marine temperature record. Improving the accuracy and precision of the chronology is a challenge for all work in this "eld. The link between North Atlantic SST and the Greenland S/IS events is likely to be closer than could be demonstrated in the core studied by Bond et al. Lehman and Keigwin (1992) showed that a more detailed correspondence between climate events could be seen in a higher sedimentation rate core o! Norway covering the past glacial to present. More recently, Kroon et al. (1997) have presented faunal paleotemperature evidence from a core on the continental margin o! Scotland (56/-10/36, 56343N, 9319W, 1320 m) for the period extending from the last glacial maximum through to the end of deglaciation (Fig. 2). This high accumulation rate core shows a strong pattern matching the submillennial details of deglaciation as recorded by the Greenland isotope

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Fig. 1. Bond et al. (1993) V23-81 temperature proxy (% left-coiling N. pachyderma) compared to Greenland Ice Core (GISP2)
O S/IS cycles. `Heinricha ice rafted debris (IRD) peaks in V23-81 are indicated, as is the Ash Layer II. This event provides a precise time link between the two records. The original Bond time scale for V23-81 has been altered beyond the range of accurate radiocarbon dating (30 kyrBP) by stretching the time scale linearly between control points so as to match the GISP2 age for Ash Layer II and the peak of interstadial 21. V23-81 data derived from digitization of published "gure.

record. This work leaves little doubt that ocean surface temperature in the northern North Atlantic is closely linked to temperatures in central Greenland. Rasmussen et al. (1996) showed that magnetic susceptibility variations in a core on the continental margin northeast of the Faeroe Islands displayed all of the S/IS cycles (Fig. 3). Magnetic susceptibility measures the capacity of sediments to retain an externally imposed magnetic "eld, and re#ects the type, concentration, and grain size of magnetic minerals in the sediment. Although it is di$cult to link this measurement to particular environmental properties, its variability must re#ect climate factors governing the supply and redistribution of magnetic minerals to this part of the ocean. The complex of processes that govern this supply and transport must be linked to the climate #uctuations seen in central Greenland. In the low tropics, far from Greenland, Hughen et al. (1996,1998) demonstrated that there is a detailed submillennial pattern match during deglaciation between the

optical re#ectance of sediment cores in the Cariaco basin and the Greenland isotope record (Fig. 4). At this site, bottom waters are anoxic during this period of time, so that bioturbation does not compromise the sedimentary record. Sedimentation rates in this basin are high due to the supply of terrigenous minerals and biogenic materials from local upwelling. An annual `#oatinga chronology (of precisely known duration, but uncertain endpoints) can be obtained from annual laminations for the deglaciation period in this basin. Visible-light optical re#ectance variability in this sediment re#ects the relative balance between the two dominant sources of sediment (white diatom frustrules re#ect more light than darker terrigenous materials). The pattern match between the Greenland climate record and the Cariaco Basin sedimentary record is clear and compelling (Fig. 4a). A case can be made for centennial and even decadal similarities (Fig. 4b). Measurements of the radiocarbon content of planktonic foraminifera from this site } coupled with the match to the calendar year layer-counted chronology of

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Fig. 2. Kroon et al. (1997) Barra Fan foraminiferal summer sea surface temperature estimates compared to central Greenland (GISP2) ice core
O during the most recent deglaciation. Core age scale based on radiocarbon dates converted to calendar years.

the GISP2 ice core } allowed Hughen et al. to construct a detailed estimate of atmospheric radiocarbon variability during this period. These data are relevant in the context of the thermohaline circulation, and will be mentioned further on. For the moment, this record can be taken as persuasive evidence for a polar-to-tropical footprint of the abrupt climate changes seen during deglaciation. Another record testifying to the large footprint of the rapid climate events was found in the Santa Barbara Basin, southern California (Behl and Kennett, 1996, Fig. 5). In this record, a `bioturbation indexa (Bottjer and Droser, 1991) is estimated by examining the sediment layering, which varies between fully annually laminated during periods of anoxia within the basin to fully bioturbated during periods of higher oxygen. The basin has a shallow sill (475 m) and changes in the oxygen content re#ect either changes in the oxygen concentration of the in#owing thermocline water or changes in oxygen consumption within the basin. The time scale is established by radiocarbon dates through marine oxygen isotope stage 2 (MIS2) and by correlation with the SPECMAP based oxygen isotope chronology through MIS4. The record shows all but one of the Greenland interstadial events at the approximately correct time (within the uncertainty of the respective chronologies).

Perhaps even more dramatically, Schulz et al. (1998) recently demonstrated that the footprint of the S/IS events extends to the Arabian Sea (core SO90-136KL, 2337N, 66330E, 568 m) (Fig. 6). The measurement showing this variability in this case is the organic carbon content of the sediments. This tracer re#ects a balance between the varying carbon #ux to the sea#oor created by changes in upwelling intensity, preservation on the sea#oor as it depends on bottom-water oxygen and other factors, and dilution by terrigenous debris. The dating is based dominantly on oxygen isotope correlation, and as for the other records before, the chronology is not accurate enough to establish precise timing matches compared to the ice core record. However, in a nearby core (not shown), the Arabian Sea cycles are rooted into the ice core chronology by the presence of the Toba Ash layer (&70 kyr BP), which has an excess sulfate signal in the GISP2 ice core (Zielinski et al., 1996). Nonetheless, the pattern matching of the cycles and supercycles is su$cient to make a convincing case for the occurrence of these events in the tropical Indian Ocean. These latter three records establish that the S/IS cycles are likely to have a footprint encompassing much of the Northern Hemisphere. In another work just submitted, Sachs and Lehman (1999) report data on the alkenone index (surface temperature proxy) from a high

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Fig. 3. Rasmussen et al. (1997) magnetic susceptibility from a core near the Faeroe Islands (ENAM93-21) compared to central Greenland (GISP2) ice core
O S/IS cycles (numbered). Note that Ash Layer II is found between interstadials 14 and 15 in both the sediment core and in the Greenland ice core (Ram, Donarummo & Sheridan, 1996), con"rming the proposed correlation near the bottom of the record.

accumulation rate core on the (subtropical) Bermuda Rise. This record also shows a strong correlation with the Greenland record, and establishes that temperatures in the subtropics respond to the same events. Finally, Brook and Blunier and coworkers (Blunier et al., 1998; Brook et al., 1996) note that a strong linkage between the methane concentration of atmospheric air bubbles trapped in the Greenland and Antarctic ice cores also demonstrates the large-scale impact of the interstadial events. Methane is emitted from wetlands, mainly in the tropics. The occurrence of S/IS variability in CH suggests that the tropics are fully  involved in many of the S/IS events. Bender et al. (1994) also point out that the largest interstadial events also appear to be recorded in the Antarctic ice core record, although the relative timing of these suggests that the events occurred in Antarctica before Greenland (Blunier et al., 1998). In summary, ample evidence links the ocean surface to the ice core S/IS events. But what happens to the deep circulation of the ocean during these events?

4. Ocean thermohaline circulation and abrupt climate change: paleodata 4.1. Deglaciation The work cited above demonstrates that S/IS events are seen in marine records and have a broad footprint. But only the Cariaco Basin atmospheric C record is speci"cally relevant to changes in the thermohaline circulation. Although northern and subtropical North Atlantic surface temperatures appear to match the interstadial events in detail, the absence of a strong salinity constraint prevents us from assessing surface density conditions relevant to deep-water formation. In the modern North Paci"c, deep water does not form because the surface waters are fresh and cannot become su$ciently dense to replace their underlying deep waters, even when cooled to their freezing point (Warren, 1981). We do not know whether this situation occurred within the cold events of the S/IS cycles. Duplessy et al. (1991) have attempted to estimate salinity during some of these

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Fig. 4. (a) Hughen et al. (1996) Cariaco Basin grayscale measurements (visible light re#ectance from split core surface) compared to Greenland S/IS cycles for the most recent deglaciation. Sediment core age scale is based on counting annual laminations during this period, with alignment to the tree ring radiocarbon calibration (Stuiver et al., 1998) to anchor the #oating annual chronology. (b) Cariaco Basin light-lamina varve thickness on expanded time scale to show similar &25 yr and &130 yr events in Greenland and the tropical Atlantic.

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Fig. 5. Behl and Kennett (1996) Santa Barbara Basin smoothed bioturbation index compared to Greenland S/IS cycles. Core time-scale is based on radiocarbon dates in the younger interval and oxygen isotope stratigraphy in the older interval.

Fig. 6. Schulz et al. (1998) Arabian Sea organic carbon #uctuations compared to Greenland S/IS cycles.

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events (by correcting foraminiferal oxygen isotope values for temperature changes estimated by faunal population temperature estimates). This work suggests that the last glacial maximum salinity of much of the northern North Atlantic is lower relative to mean ocean salinity. The accuracy of this method is insu$cient for con"dently assessing whether past surface waters were su$ciently dense to sink to great depth. Unless a much more accurate paleosalinity method can be found, evidence for changes in the thermohaline circulation must be sought in deep-sea proxies, not in surface proxies. Recently, it has been suggested that the paleoecology of dino#agellate cysts may provide a new paleosalinity indicator (De Vernal et al., 1994). The properties most frequently used to infer changes in oceanic thermohaline circulation patterns are the carbon isotope and cadmium content of deep-sea bottom-dwelling (benthic) foraminifera. These tracers indicate changes in water mass chemical properties involved in the oceanic biogeochemical cycles. The distinct contrast between North Atlantic and Antarctic deep-water source compositions allows us to estimate mixing percentages of these sources at core sites. However, at best this information only gives information on deep-water spreading patterns, not on the absolute rate of ventilation. The C/C ratio of the atmosphere, the C/C contrast between surface and deep waters, and the age-corrected C of Th-dated deep-sea corals have been used to a lesser extent. C has the advantage of providing information on rates as well as patterns, although sample availability limitations have hindered this approach (limited available sample size for benthic foraminifera, and the di$culty of "nding coral specimens of the appropriate age). Other types of indicators have seen some use as well: advection of Antarctic diatoms (Jones and Johnson, 1984), grain size (velocity-driven size-sorting properties of sediments) (McCave et al., 1995; Bianchi and McCave, 1999), and the Pa/Th ratio of sediments (Yu et al., 1996). Boyle and Keigwin (1987) and Keigwin et al. (1991) reported Younger Dryas age reductions in the percentage of lower NADW above the deep Bermuda Rise (4400 m, based upon benthic
C and Cd/Ca evidence). Other deglacial events were also seen in the Cd data. Sarnthein et al. (1994) questioned this "nding because they did not see a Younger Dryas event in their eastern North Atlantic cores. Many of those cores did however show a strong
C minimum at &13.3C kyr (&15.7 calendar kyr), where a high benthic Cd event is also evident in the Bermuda Rise Cd record at the same time. Boyle (1995) pointed out that many of the cores in the Sarnthein compilation were inappropriate for "nding the Younger Dryas because they were either (a) of too low a sedimentation rate to resolve the event, (b) sampled too coarsely to reliably locate the event, or (c) located in places where a Younger Dryas event associated with

lower NADW would not be expected. More recently, Bond et al. (1997) have shown a clear benthic
C minimum of Younger Dryas age in a North Atlantic core o! Ireland (V29-191, 543N, 173W, 2370 m), as well as another strong minimum associated with the `H1a Heinrich ice rafted debris (IRD) event (Fig. 7). This evidence indicates that there were thermohaline circulation events both at the Younger Dryas and at &16 calendar kyr (probably associated with the H1 IRD event). Hughen et al. (1998) examined the implications of the Cariaco Basin C record for changes in deep ocean circulation. When ocean circulation slows down, more C remains in the atmosphere (and surface ocean). The C changes that their data specify appear to require a major slowdown in ocean ventilation at the beginning of the Younger Dryas and then a resumption before the event ends. This evidence also shows that the deep ocean circulation is not immune to deglacial climate changes. 4.2. Marine Isotope Stage (MIS) 2 and 3 Are the MIS 2 and 3 S/IS events seen in the thermohaline circulation? The current literature provides hints that at least the largest/longest of the events appears to have left a mark, but there is no proof of a strong association between thermohaline circulation and the S/IS events. However, this situation represents absence of proof rather than proof of absence; the sedimentary records that have been examined in the published literature have insu$cient sedimentation rate (or poor benthic foraminiferal abundances). These cores could not be expected to show a strong signal even if a full amplitude event had occurred in the geochemical properties of the bottom water at the site. At the end of this survey, some new data from a high sedimentation rate site with consistent benthic foraminiferal abundances (Bermuda Rise) will be presented that shows a strong association between proxy water chemistry and the interstadial events. Vidal et al. (1997) obtained carbon isotope records from cores NA87-22 (55329.8N 14341.7W, 2161 m) and SU90-08 (43303.1N 3032.5W, 3080 m) (Fig. 8). These moderate-resolution sediment cores are in the region where the Heinrich IRD events are clearly evident. In these cores, pronounced light planktonic foraminiferal
O features (in left-coiling N. pachyderma) are seen that coincide with `Heinricha IRD events H1}H5. This signal presumably arises from the melting of isotopically depleted icebergs. There are indications of brief
C minima at the same time for some of the events. NA87-22 records clear events for H1, H4, and H5 with hints at H3. SU90-08 shows a strong event at H1 with hints of events at H3, H4, and H5. This evidence supports a ventilation minimum associated with the IRD events, but it does not indicate ventilation changes associated with the other S/IS events.

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Fig. 7. Bond et al. (1997)
C data from a North Atlantic sediment core showing depleted benthic
C during Younger Dryas period.

Fig. 8. Vidal et al. (1997) benthic (C. wuellerstorx)
C data from two northern North Atlantic sediment cores showing evidence for
C events associated with `Heinricha IRD events.

Similarly, in a study of core near the Portuguese margin (SO75-26KL, 373N49.3N, 9330.2W, 1099 m), Zahn et al. (1997) also found light
C events in benthic foraminifera during three IRD events equivalent to H1, H2, and H4 (Fig. 9). Curry and Oppo (1997) studied cores in the western Equatorial Atlantic (EW9209-1, 53N 433W 4056 m) that record planktonic O events and benthic C events which qualitatively match the pattern of the largest stadial interstadial events (Fig. 10). However, once

again, the sedimentation rates of these cores are insu$cient to reliably record the shorter interstadials, even if they had been fully expressed as bottom-water
C minima at the core sites. A more convincing case for the existence for briefer events in benthic foraminiferal
C is given by the Southern Ocean (Atlantic sector) data from Charles et al. (1996), (Fig. 11). In this study, large-amplitude highfrequency benthic C changes are seen that resemble the

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Fig. 9. Zahn et al. (1997) benthic (C. wuellerstorx)
C data from a core o! the Portuguese margin showing evidence for
C events associated with `Heinricha IRD events.

pattern of interstadials. Although this core is not itself highly resolved enough to match all of the S/IS events one-for-one, it provides strong evidence for millennial signals in the deep-sea
C record. As the authors point out, the origin of the
C signal in cores from this region is not entirely straightforward. Benthic
C record variability at this site cannot be explained by the simple mixing of North Atlantic and Paci"c deep-water end members: the
C values of benthic foraminifera in this region are lighter than either of the supposed endmembers. Hence this artifact of uncertain origin complicates their interpretation of this signal as a direct re#ection of changes in NADW. In summary, the existing literature shows that at least some of the major North Atlantic ice rafting (`Heinricha IRD) events and stronger S/IS cycles are associated with benthic foraminiferal
C events indicative of lower NADW percentages at several sites. Although there is plausible evidence that high-frequency variability reminiscent of the Greenland interstadials reaches the Southern Ocean, there (so far) has been no evidence demonstrating a tight link between deep-water behavior and S/IS events. This situation re#ects absence of proof,

not proof of absence. The cores studied could not have recorded events of this duration. In view of the importance of establishing whether there is a link between the ocean thermohaline circulation and the S/IS events, e!ort is being directed at coring highaccumulation sites where su$cient temporal resolution may be obtained. This e!ort is one of the main goals of the International Marine Global chanGES program (IMAGES program, the marine component of PAGES). On the "rst cruise of this program undertaken on the French research vessel Marion Dufresne in 1995, many long piston cores were taken for this purpose. In particular, three sediment cores averaging 47 m length were taken on the Bermuda Rise, an area where the average accumulation rate during this interval is '25 cm 1 kyr. Calcium carbonate records at this site show transitions closely resembling the ice core S/IS sequences (Boyle, 1997; Keigwin et al., 1994; Keigwin and Jones, 1989). Although work on these cores is in its early phases, interstadial 8 and its previous stadial period (proximate in time to `Heinricha IRD event 4) have been studied in moderate detail for benthic foraminiferal cadmium variability in this core and in another core (KNR31 GPC5)

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Fig. 10. Curry and Oppo (1997) benthic
C data from an equatorial Atlantic sediment core showing larger `supercyclea events associated with Greenland S/IS cycles.

(Fig. 12). The two nearby cores are correlated by matching their %CaCO and optical re#ectance  (CaCO +white) records. This correlation between the  two cores is strengthened by observation of an IRD peak presumed to be `Heinricha IRD event H4 in both cores (see below). There is a correspondence with the high %CaCO period linked to IS8 having  lower benthic Cd than the preceding low %CaCO  section. We do not know whether the high %CaCO  and IS8 events are precisely in phase, phase di!erences of several centuries or even a millennium are possible with present chronological constraints. In a lower resolution study of core KNR31-GPC5, Keigwin and Boyle (1999) have shown that there is a one-to-one correspondence between the major stage 3 S/IS cycles (as represented by %CaCO in the Bermuda Rise sediments), the 
O of planktonic foraminifera (representing mainly temperature changes with some in#uence of local salinity and global ice volume), and benthic foraminiferal
C (representing changes in the percentage of nutrientdepleted NADW at the site relative to nutrient enriched AABW (Fig. 13). We also observed two episodes of ice rafting in the Bermuda Rise cores that correspond to `Heinricha IRD events H4 and H5. These IRD

events on the Bermuda Rise should be coeval with the IRD events seen at higher latitudes, hence providing chronological tie lines between cores covering a broad region of the North Atlantic. The amplitude of the Cd/Ca (0.06 mol/mol) and
C (&0.6) signals are consistent with the modern day correlation of the two tracers. When both tracers are consistent, we may expect that both are una!ected by artifacts peculiar to each one. Finally, we have more persuasive evidence of a close link between the deep thermohaline circulation and the progression of S/IS events seen in the Greenland ice cores. Future high-resolution work on these Bermuda Rise cores will show just how close this link is. Work at other high accumulation rate sites is also needed to understand the three-dimensional characteristics of this deep-sea signal: is the I/IS signal con"ned only to the mixing of lower NADW and AABW in sensitive transition zones such as the Bermuda Rise, or does this signal pervade the entire deep Atlantic? We must also struggle with the chicken and egg problem: does the deep thermohaline signal cause the rapid climate shifts or is it just a response to climate events generated by some other mechanism?

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Fig. 11. Charles et al. (1996) RC11-83 benthic
C data showing multiple millennial scale benthic
C events in the Atlantic sector Southern Ocean resembling the pattern of Greenland S/IS events.

4.3. How fast can the deep thermohaline circulation change? Although the evidence cited above provides some evidence that the chemical characteristics of the deep Atlantic ocean can change on millennial } perhaps even centennial } time scales, the Greenland ice core data tell us that climate transitions can be even more rapid, extending down to a few decades or less. Can the deep Atlantic circulation change as rapidly as that? Given the sparseness of evidence documenting millennial deep-sea change, it is not surprising that evidence for decadal deep-sea changes is hard to "nd. One clear example for rapid changes in the deep sea comes from the study of deep-sea corals. Deep-sea corals, occurring from shallow waters down to 4000 m or greater (but most commonly found in the depth range 200}2000 m), are roughly 10}20 cm in size, and deposit successive layers of aragonite during a life cycle of about one century. They incorporate uranium and C from the seawater they grow in, and hence can be used to derive an absolute chronology from Th dates and provide an estimate of the C ventilation time of the deep water. Because the coral is solid, no bioturbational blurring compromises the "delity of the record. The temporal resolution of a coral climate record is limited only by practical consid-

erations of sampling and chronological re"nement. Very little paleoclimatological work has been undertaken on these archives. Early work of Emiliani et al. (1978) explored the isotopic characteristics of these organisms and demonstrated substantial disequilibria from inorganic carbonate. More recently, deep-sea corals were revived as archives by Smith et al. (1997). In a recent paper by Adkins et al. (1998), several coral specimens from the North Atlantic Ocean (383N 603W 1784 m) were found to have grown 15.4 kyr ago, several of which displayed older radiocarbon ages for their most recently grown carbonate compared to that deposited earlier, by up to 670 yr. The only logical explanation for this inverse Ponce-de-Leon e!ect (aging faster than normal) is that the water the coral was growing in switched from a higher-radiocarbon source to an older low-radiocarbon source. Cd/Ca data from one of these specimens supports a rapid transition from initial high-NADW percentage water to "nal low-NADW percentage water occurring within less than &100 yr (the approximate age of the coral specimen when it ceased growing) (Fig. 14). The increase in the C age from the youngest to oldest segment mainly re#ects the replacement of a percentage of NADW by AABW (as seen in the Cd/Ca data), not the actual radioactive aging of a stagnant water column. This is the "rst data that show that the chemical

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Fig. 12. Keigwin and Boyle (1999) Bermuda Rise (squares) KNR31-GPC5%CaCO and (circles) MD95-2036 optical re#ectance (top) and benthic  Cd/Ca data (bottom) showing a match between a deep-water signal and Greenland climate for one S/IS cycle (IS8 and preceding stadial).

characteristics of the deep sea can change in less than a century; climate change in the deep sea is apparently as rapid as it is in the atmosphere over Greenland.

5. Needs for the future: high-resolution drift and continental margin deposits, more deep-sea corals, and better time control From the previous discussion, it is evident that we need a better three-dimensional perspective on the relationship between deep-sea circulation and S/IS

transitions. This goal will require more data from high accumulation rate deep-sea sites, which in practice requires more data from focussed drift deposits and continental margin deposits. Continental margin sites are more problematical (turbidites and other sedimentary disturbances being harder to avoid and detect), but a determined e!ort with abundant AMS C age controls (for samples younger than about 40 kyr) can make signi"cant progress on this problem. It is also evident that more work on deep-sea corals will be required to answer questions concerning decadal deep-sea variability. Deep-sea coral sampling so far has

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Fig. 13. Keigwin and Boyle (1999) Bermuda Rise (KNR31-GPC5) stage 3%CaCO , benthic
C, IRD, and planktonic
O (G. ruber) data from  Bermuda Rise core KNR31-GPC5.

been a hit-or-miss proposition, with samples coming inadvertently from dredges carried directed at unrelated purposes. Even with a purposeful coral sample collection e!ort, it would be hard to control the age distribution of the specimens recovered. For the immediate future, we can envision receiving tantalizing anecdotes about short periods of time, not long continuous climate records, from these archives. However, with a determined e!ort and some luck, those anecdotes can illuminate key climate transitions and help us evaluate timing of deep-sea events relative to surface climate changes.

Progress on the chicken/egg problem will require a better chronology for the diverse climate records from land and sea. During most of MIS 3, the time scales that have been constructed have an accuracy of a few thousand years. In other words, correlations (such as that between Greenland I/IS and atmospheric methane cycles and the Santa Barbara Basin bioturbation index or the Bermuda Rise %CaCO record) could be in error by as  much as a whole S/IS cycle! The precision of correlation is sometimes better; for example, ice core records can be correlated between the northern and southern hemi-

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Fig. 14. Adkins et al. (1998) deep-sea coral data showing a transition from lower Cd, higher C waters to higher Cd, lower C waters during the lifetime of a single deep-sea coral.

spheres using global atmospheric methane variability (e.g. Blunier et al., 1998; Brook et al., 1996). But it is di$cult to link di!erent types of records (e.g. ice core and marine sediment cores) with this level of con"dence. In time, improvements in the precision and accuracy of C dating may help for sediments younger than about 35}40 kyr, although this task will also require better understanding of changes in surface water C relative to the atmosphere (i.e. reservoir age corrections, Bard et al., 1994). Th dating of surface and deep-sea corals should help with this problem. The path towards improved temporal resolution may be slow and unpredictable, but in time an improved millennial-precise chronology should be achievable. Consider some examples: (1) The ash from the volcanic explosion on the Indonesian island Toba has been reported in sediment cores from Indonesia (Ninkovich et al., 1978) to the Arabian Sea (Schulz et al., 1998), and the excess sulfate from the eruption is reported in Greenland ice (Zielinksi et al., 1996). Because ash and sulfate fall out of the atmosphere

rapidly, this marker provides an isochronous time line precise to one year (!) wherever it is found (the precision of course also depends on the temporal resolution of the archive). (2) Studies of the paleomagnetic intensity record in high-resolution marine sediments are suggesting that the brief `Laschampa magnetic minimum (and other #uctuations as well) may be found in some marine sediment cores (Laj et al., in press) as well as be re#ected in the Be concentration of ice (Yiou et al., 1997). Here again we have a time marker, resolvable to perhaps a thousand years or better, that applies wherever indicators of past magnetic intensity can be found } on continents, in ice cores, and marine sediments. (3) The occurrence of the `Heinricha IRD events as far south as subtropical waters (see above) also provides a precise tie line linking marine records over much of the North Atlantic. As tie lines such as these are found and linked into other climate markers (ice core methane, etc.), the precision and accuracy of our time scales will improve. Achieving this task is a challenge for the next decade.

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6. Speculations on controls on ocean thermohaline circulation, mixing coe7cients, and the bipolar seasaw What controls the thermohaline overturning rate of the ocean? Beginning with the ocean model studies of Bryan (1986,1987), this question has often been answered with the model-based observation that the vertical mixing rate in the ocean controls the rate of deep- and bottom-water production. As vertical di!usion mixes heat downwards from the surface ocean, it creates `opportunitiesa at high latitudes for creating water that is su$ciently dense to sink down and replace the less dense warm water. The faster the ocean mixes vertically, the more frequently deep-water formation opportunities arise at high latitudes. Conversely cooler glacial tropics may di!use less heat downwards and result in diminished deep-water formation opportunities. This seems a rather simple answer for what seems to be a very complicated process. Deep-water formation only occurs in very limited regions for brief periods (Warren, 1981); in some regions, deep water does not form for years on end. Perhaps the answer is misleadingly simple, because vertical mixing in the ocean itself is not a simple matter, and it is certainly not homogeneously distributed in the real ocean as models assume it is. Microscale structure and deliberate tracer release experiments have shown that vertical di!usion in ocean basins away from rugged topography is an order of magnitude less than the canonical value required by whole ocean balances (Polzin et al., 1997a). Recently, it has been shown that vertical mixing occurs much faster over rugged topography than over smooth sea#oor. It is thought that tidal interactions with the rugged topography may account for this observation. In this case, then the ocean-wide ocean mixing is accomplished near boundaries, mid-ocean ridges, and seamount complexes, and then laterally diffused and advected into the ocean interior. Munk and Wunsch (1998) have considered the matter and conclude that vertical mixing in the ocean is accomplished by roughly equal contributions from tidal activity interacting with rough topography (e.g. see Polzin et al., 1997b) and the interaction of wind with the Circumpolar Current. What does this perspective contribute to the study of temporal changes in the ocean thermohaline circulation? To date, paleoceanographers have largely focussed on surface processes that might change the rate of deepwater formation. For example, formation of a low-salinity layer in the northern North Atlantic is expected to reduce the formation of NADW. However, if the overall thermohaline circulation is controlled neither by vertical mixing and nor by the local surface conditions, then a reduction of NADW formation will have to be made up by an increase in deep-water production elsewhere, either in the Antarctic or North Paci"c. Wang et al. (1999) observed this behavior in a simpli"ed ocean circulation

model; when they created low-salinity conditions that hindered `NADWa production, `AABWa production increased. A similar e!ect was previously observed in a model study by Stocker et al. (1992). It is worth noting that Broecker (1998) examined evidence on the timing of climate change in the Northern and Southern Hemisphere and suggested that the climate system may be operating as a `bipolar seesawa. If vertical mixing rates do not change much and hence require roughly constant deep-water formation rates, does this requirement drive the polar/bipolar seesaw? Must vertical mixing rates and the overall thermohaline circulation overturning be constant? Could not the vertical mixing rate in the ocean not vary over time? In fact, there are at least two reasons why vertical mixing should vary temporally. First, tidal dissipation by the earth is presently dominated by dissipation in shallow shelf waters. During a glacial maximum, when most of these shallow waters have turned into dry land, more of the tidal dissipation should have occurred in the deep sea. Second, interactions of wind with the Circumpolar Current is the other major mechanism creating vertical mixing. Most models for glacial climate seem to call for greater wind speeds over the Southern Ocean during glacial times; if so, vertical mixing would also increase in the Southern Ocean. Both major mechanisms in#uencing vertical mixing would then be presumed to call for an increase in vertical mixing during glacial times. By the logic of Bryan (1986,1987), this increase in vertical mixing calls for increased deep-water formation rates. Present evidence does not support this argument for an increase in the overturning of the ocean during glacial times. C data from pairs of planktonic and benthic foraminifera in ocean sediments (Broecker, 1989; Broecker et al., 1988; Shackleton et al., 1988) indicate that the overturning time of the ocean increased somewhat during glacial time (Adkins and Boyle, 1997). Perhaps this evidence is not conclusive. C in the ocean is in#uenced by other factors, such as gas exchange with the sea surface and surface boundary conditions (Broecker et al., 1991). Perhaps a simple interpretation of the C age of the deep ocean (as re#ecting the rate of physical overturning of the ocean) is not warranted (Campin et al., 1999. Issues such as these will be among the matters that studies of the relation between climate change and ocean circulation must address in the future.

Acknowledgements I thank Ray Bradley for inviting me to make this presentation at the PAGES Open Science Meeting and Frank Old"eld for his patience with the manuscript preparation. Thanks to Rainer Zahn, Chris Charles, and Bill Austin for sending their data for replotting.

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