Quaternary Science Reviews 19 (2000) 255}272

Is ocean 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 deepcirculation 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 deepAtlantic. But most cores studied do not resolve the other S/IS events, exceptfor 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 deepsea.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction precipitation, and water vapor transport. Stommel (1961) used a simple box model to demonstrate that a convec- Paleoclimate studies of ice cores and other archives tive system could have multiple stable solutions. reveal dramatic decadal}century climate transitions, Subsequent studies have employed models with a wide which raise signi"cant concerns about the predictability range of complexity and sophistication to draw the same of climate. Central Greenland ice cores show rapid re- conclusion (e.g. Marotzke, 1994; Marotzke and Willeb- gional temperature shifts of as much as a third of the full rand, 1991; Weaver and Hughes, 1994). Although it is glacial/interglacial amplitude (Grootes et al., 1993; more di$cult to prove that the real ocean has multiple Grootes and Stuiver, 1997). Several mechanisms have stable states than it is to illustrate such behavior in model been proposed to account for these rapid climate transi- systems, no one has conversely proven that these mul- tions, known as stadial/interstadial (S/IS) transitions. tiple states cannot exist in the real ocean. A large and One class of theories invokes instabilities in large conti- growing volume of theoretical work supports the notion nental ice sheets (MacAyeal, 1993a,b; Saltzman and Ver- that ocean thermohaline instabilities may have caused bitsky, 1994,1996). Another class of explanations cites the past abrupt climate changes. inherent instability of the ocean thermohaline circulation Although it is common to discuss changes in the ther- (Broecker et al., 1990; Broecker et al., 1985). Thermoha- mohaline circulation as if deep-water sources can only be line circulation instabilities may raise more concern for `ona or `o!a, the real situation is likely to be complex, predicting future climate than do ice sheet instabilities with evolutions between some range of continuum states (Broecker, 1997). Certainly, there is evidence for signi"- occurring gradually as well as through abrupt changes. cant climate change within the last 10,000 yr, a time of For example, there is paleoceanographic data indicating low ice volume (e.g. Keigwin, 1996). that Glacial North Atlantic Deep-water (DADW) did not A well-established literature has explored the mecha- vanish entirely but became a shallow water mass with net nisms responsible for ocean thermohaline circulation export from the Atlantic (Boyle and Keigwin, 1987; Du- instabilities. The fundamental reason for this instability is plessy et al., 1988; Oppo and Lehman, 1993; Yu et al., that the density of polar waters is governed strongly both 1996). Models show more complex behavior as well by temperature and . Although there is a strong (Manabe and Stou!er, 1995,1997; Weaver et al., 1991; feedback between the atmospheric temperature and sea Rahmsdorf, 1995). The deep-water response may be de- surface temperature, there is essentially no feedback pendent on the character of the forcing * meltwater between the saltiness of seawater and evaporation, discharge may elicit a di!erent oceanic response than

0277-3791/99/$- see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 6 5 - 7 256 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 iceberg discharge. There is already evidence from modern be a relationshipbetween deep-waterand the S/IS oceanography that deep convection can change on inter- events. This evidence begins to speak to this question nannual and decadal time scales, and that spreading of amplitudes and phase relationships, but more de- velocities of pulses of deepwater formation can be sur- tailed work will be required to answer this question. This prisingly rapid (e.g., Sy et al., 1997). Although the dis- paper will conclude on a "rmly agnostic note concerning cussion below may seem to be dominated by `ona or the "nal question } do thermohaline circulation instabili- `o!a thinking, this usage is just a shorthand for what ties cause S/IS transitions or is it vice versa? This ques- clearly is a much more subtle matter, including interan- tion may well take a decade or more of research to nual and decadal changes, wave propagation within the answer. 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 3. Evidence for a link between abrupt climate change in long enough to a!ect the steady-state tracer "eld (T, S, Greenland and the sea surface chemistry), since this is the type of evidence we most commonly use in the paleoceanographic record. Numerous studies have established that there is a tight Given this strong theoretical support, it may be sur- link between events in central Greenland and character- prising that there is little observational evidence in the istics of the surface ocean. literature showing a strong link between the ocean ther- Following upon studies by Ruddiman et al. (1980) and mohaline circulation and abrupt climate transitions such Heinrich (1988), Bond et al. (1993) presented evidence as those seen in the Greenland ice cores. As it turns out, it for a one-to-one correspondence between S/IS cycles is surprisingly di$cult to "nd paleoclimatic archives in and northern North Atlantic surface temperatures at the deepsea that can establish whether or not deep-water a site near Ireland (as re#ected in the relative abund- properties are closely linked to abrupt climate transitions ance of the polar foraminifera left-coiling N. pachyderma) on decadal-to-century time scales. The major impedi- (Fig. 1). Furthermore, they called attention to the `super- ment is biological stirring of the upper few centimeters of groupa of smaller cycles that end with major episodes deep-sea sediments, which constitutes a low-pass "lter of ice rafting of glacial debris observed by Heinrich removing evidence for short-term events (given typical (1988). The identi"cation of the marine events and oceanic sedimentation rates of a few centimeters per the ice core events relies to some extent on pattern thousand years). As will be seen, the only solutions to this matching. The marine chronology } based on a few problem are: (1) to work at sites where the accumulation relatively uncertain radiocarbon ages (from the low- rate of detritus on the sea#oor is much higher than radiocarbon tail), and oxygen isotope stratigraphy based normal, (2) to work with sediments deposited under upon tuning to Milankovitch cycles } is simply too anoxic (nonbioturbated) conditions, or (3) to seek out inaccurate to show that the ice core and marine cycles are new high-resolution archives, such as deep-sea corals. precisely coeval. Furthermore, some of the cycles are This paper will review the evidence for the link between indistinct in the marine record because they occur at the the S!IS transitions seen in Greenland and the deep-sea limits of temporal resolution allowed for by the sedi- record of climate change. mentation rate and bioturbation. Nonetheless, few mar- ine paleoceanographers would doubt the Bond et al. premise that the S/IS events are re#ected in the marine 2. Some questions temperature record. Improving the accuracy and pre- cision of the chronology is a challenge for all work in this E Is there any evidence for a relationshipbetween (S/IS) "eld. transitions and the thermohaline circulation? The link between North Atlantic SST and the Green- E If so, do the amplitudes of the S/IS transitions and the land S/IS events is likely to be closer than could be thermohaline signals correspond? demonstrated in the core studied by Bond et al. Lehman E What is the detailed phase relationship between ther- and Keigwin (1992) showed that a more detailed corre- mohaline circulation and other manifestations of S/IS spondence between climate events could be seen in transitions in the earth's climate system? a higher sedimentation rate core o! Norway covering the E Who is the chicken, and what is the egg? past glacial to present. More recently, Kroon et al. (1997) have presented faunal paleotemperature evidence from I will show that there is evidence in the literature for a core on the continental margin o! Scotland (56/-10/36, links between interstadials and thermohaline circulation, 56343N, 9319W, 1320 m) for the period extending from but that this evidence is far from su$cient to establish the last glacial maximum through to the end of deglaci- a cause-and-e!ect relationship. New evidence from ation (Fig. 2). This high accumulation rate core shows Bermuda Rise sediment cores begins to "ll this gap, a strong pattern matching the submillennial details and the reader may come to believe that there may in fact of deglaciation as recorded by the Greenland isotope E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 257

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 optical re#ectance of sediment cores in the Cariaco basin temperature in the northern North Atlantic is closely and the Greenland isotope record (Fig. 4). At this site, linked to temperatures in central Greenland. bottom waters are anoxic during this period of time, so Rasmussen et al. (1996) showed that magnetic suscepti- that bioturbation does not compromise the sedimentary bility variations in a core on the continental margin record. Sedimentation rates in this basin are high due to northeast of the Faeroe Islands displayed all of the S/IS the supply of terrigenous minerals and biogenic materials cycles (Fig. 3). Magnetic susceptibility measures the capa- from local upwelling. An annual `#oatinga chronology city of sediments to retain an externally imposed mag- (of precisely known duration, but uncertain endpoints) netic "eld, and re#ects the type, concentration, and grain can be obtained from annual laminations for the deglaci- size of magnetic minerals in the sediment. Although it is ation period in this basin. Visible-light optical re#ectance di$cult to link this measurement to particular environ- variability in this sediment re#ects the relative balance mental properties, its variability must re#ect climate fac- between the two dominant sources of sediment (white tors governing the supply and redistribution of magnetic diatom frustrules re#ect more light than darker ter- minerals to this part of the ocean. The complex of pro- rigenous materials). The pattern match between the cesses that govern this supply and transport must be Greenland climate record and the Cariaco Basin sedi- linked to the climate #uctuations seen in central Green- mentary record is clear and compelling (Fig. 4a). A case land. can be made for centennial and even decadal similarities In the low tropics, far from Greenland, Hughen et al. (Fig. 4b). Measurements of the radiocarbon content of (1996,1998) demonstrated that there is a detailed submil- planktonic foraminifera from this site } coupled with the lennial pattern match during deglaciation between the match to the calendar year layer-counted chronology of 258 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

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 Perhaps even more dramatically, Schulz et al. (1998) a detailed estimate of atmospheric radiocarbon variabil- recently demonstrated that the footprint of the S/IS ity during this period. These data are relevant in the events extends to the Arabian Sea (core SO90-136KL, context of the thermohaline circulation, and will be men- 2337N, 66330E, 568 m) (Fig. 6). The measurement tioned further on. For the moment, this record can be showing this variability in this case is the organic taken as persuasive evidence for a polar-to-tropical carbon content of the sediments. This tracer re#ects footprint of the abrupt climate changes seen during de- a balance between the varying carbon #ux to the sea#oor glaciation. created by changes in upwelling intensity, preservation Another record testifying to the large footprint of the on the sea#oor as it depends on bottom-water oxygen rapid climate events was found in the Santa Barbara and other factors, and dilution by terrigenous debris. Basin, southern California (Behl and Kennett, 1996, Fig. The dating is based dominantly on oxygen isotope 5). In this record, a `bioturbation indexa (Bottjer and correlation, and as for the other records before, the chro- Droser, 1991) is estimated by examining the sediment nology is not accurate enough to establish precise timing layering, which varies between fully annually laminated matches compared to the ice core record. However, during periods of anoxia within the basin to fully bio- in a nearby core (not shown), the Arabian Sea cycles turbated during periods of higher oxygen. The basin are rooted into the ice core chronology by the presence has a shallow sill (475 m) and changes in the oxygen of the Toba Ash layer (&70 kyr BP), which has an content re#ect either changes in the oxygen concentra- excess sulfate signal in the GISP2 ice core (Zielinski et al., tion of the in#owing thermocline water or changes 1996). Nonetheless, the pattern matching of the cycles in oxygen consumption within the basin. The time and supercycles is su$cient to make a convincing case for scale is established by radiocarbon dates through marine the occurrence of these events in the tropical Indian oxygen isotope stage 2 (MIS2) and by correlation with Ocean. the SPECMAP based oxygen isotope chronology These latter three records establish that the S/IS through MIS4. The record shows all but one of the cycles are likely to have a footprint encompassing much Greenland interstadial events at the approximately cor- of the Northern Hemisphere. In another work just sub- rect time (within the uncertainty of the respective chrono- mitted, Sachs and Lehman (1999) report data on the logies). alkenone index (surface temperature proxy) from a high E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 259

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 4. Ocean thermohaline circulation and abrupt climate Rise. This record also shows a strong correlation with the change: paleodata Greenland record, and establishes that temperatures in the subtropics respond to the same events. 4.1. Deglaciation Finally, Brook and Blunier and coworkers (Blunier et al., 1998; Brook et al., 1996) note that a strong The work cited above demonstrates that S/IS events linkage between the methane concentration of atmo- are seen in marine records and have a broad footprint. spheric air bubbles trapped in the Greenland and Antarc- But only the Cariaco Basin atmospheric C record is tic ice cores also demonstrates the large-scale impact of speci"cally relevant to changes in the thermohaline circu- the interstadial events. Methane is emitted from wet- lation. Although northern and subtropical North Atlan- lands, mainly in the tropics. The occurrence of S/IS tic surface temperatures appear to match the interstadial variability in CH suggests that the tropics are fully events in detail, the absence of a strong salinity constraint involved in many of the S/IS events. Bender et al. (1994) prevents us from assessing surface density conditions also point out that the largest interstadial events also relevant to deep-water formation. In the modern North appear to be recorded in the Antarctic ice core record, Paci"c, deepwater does not form because the surface although the relative timing of these suggests that the waters are fresh and cannot become su$ciently dense events occurred in Antarctica before Greenland (Blunier to replace their underlying deep waters, even when et al., 1998). cooled to their freezing point (Warren, 1981). We do not In summary, ample evidence links the ocean surface to know whether this situation occurred within the cold the ice core S/IS events. But what happens to the deep events of the S/IS cycles. Duplessy et al. (1991) have circulation of the ocean during these events? attempted to estimate salinity during some of these 260 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

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. E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 261

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. 262 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 events (by correcting foraminiferal oxygen isotope values lower NADW would not be expected. More recently, for temperature changes estimated by faunal population Bond et al. (1997) have shown a clear benthic C temperature estimates). This work suggests that the last minimum of Younger Dryas age in a North Atlantic core glacial maximum salinity of much of the northern North o! Ireland (V29-191, 543N, 173W, 2370 m), as well as Atlantic is lower relative to mean ocean salinity. The another strong minimum associated with the `H1a Hein- accuracy of this method is insu$cient for con"dently rich ice rafted debris (IRD) event (Fig. 7). This evidence assessing whether past surface waters were su$ciently indicates that there were thermohaline circulation events dense to sink to great depth. Unless a much more accu- both at the Younger Dryas and at &16 calendar kyr rate paleosalinity method can be found, evidence for (probably associated with the H1 IRD event). changes in the thermohaline circulation must be sought Hughen et al. (1998) examined the implications of the in deep-sea proxies, not in surface proxies. Recently, it Cariaco Basin C record for changes in deepocean has been suggested that the paleoecology of dino#agel- circulation. When ocean circulation slows down, more late cysts may provide a new paleosalinity indicator (De C remains in the atmosphere (and surface ocean). Vernal et al., 1994). The C changes that their data specify appear to require The properties most frequently used to infer changes in a major slowdown in ocean ventilation at the beginning oceanic thermohaline circulation patterns are the carbon of the Younger Dryas and then a resumption before isotope and cadmium content of deep-sea bottom-dwell- the event ends. This evidence also shows that the deep ing (benthic) foraminifera. These tracers indicate changes ocean circulation is not immune to deglacial climate in water mass chemical properties involved in the oceanic changes. biogeochemical cycles. The distinct contrast between North Atlantic and Antarctic deep-water source com- 4.2. Marine Isotope Stage (MIS) 2 and 3 positions allows us to estimate mixing percentages of these sources at core sites. However, at best this informa- Are the MIS 2 and 3 S/IS events seen in the thermoha- tion only gives information on deep-water spreading line circulation? The current literature provides hints patterns, not on the absolute rate of ventilation. The that at least the largest/longest of the events appears to C/C ratio of the atmosphere, the C/C contrast have left a mark, but there is no proof of a strong between surface and deepwaters, and the age-corrected association between thermohaline circulation and the CofTh-dated deep-sea corals have been used to S/IS events. However, this situation represents absence of a lesser extent. C has the advantage of providing in- proof rather than proof of absence; the sedimentary re- formation on rates as well as patterns, although sample cords that have been examined in the published literature availability limitations have hindered this approach (lim- have insu$cient sedimentation rate (or poor benthic ited available sample size for benthic foraminifera, and foraminiferal abundances). These cores could not be ex- the di$culty of "nding coral specimens of the appropri- pected to show a strong signal even if a full amplitude ate age). Other types of indicators have seen some use as event had occurred in the geochemical properties of the well: advection of Antarctic diatoms (Jones and Johnson, bottom water at the site. At the end of this survey, some 1984), grain size (velocity-driven size-sorting properties new data from a high sedimentation rate site with consis- of sediments) (McCave et al., 1995; Bianchi and McCave, tent benthic foraminiferal abundances (Bermuda Rise) 1999), and the Pa/Th ratio of sediments (Yu et al., will be presented that shows a strong association between 1996). proxy water chemistry and the interstadial events. Boyle and Keigwin (1987) and Keigwin et al. (1991) Vidal et al. (1997) obtained carbon isotope records reported Younger Dryas age reductions in the percentage from cores NA87-22 (55329.8N14341.7W, 2161 m) and of lower NADW above the deepBermuda Rise (4400 m, SU90-08 (43303.1N3032.5W, 3080 m) (Fig. 8). These based upon benthic C and Cd/Ca evidence). Other moderate-resolution sediment cores are in the region deglacial events were also seen in the Cd data. Sarnthein where the Heinrich IRD events are clearly evident. In et al. (1994) questioned this "nding because they did not these cores, pronounced light planktonic foraminiferal see a Younger Dryas event in their eastern North Atlan- O features (in left-coiling N. pachyderma) are seen that tic cores. Many of those cores did however show a strong coincide with `Heinricha IRD events H1}H5. This signal C minimum at &13.3C kyr (&15.7 calendar kyr), presumably arises from the melting of isotopically de- where a high benthic Cd event is also evident in the pleted icebergs. There are indications of brief C min- Bermuda Rise Cd record at the same time. Boyle (1995) ima at the same time for some of the events. NA87-22 pointed out that many of the cores in the Sarnthein records clear events for H1, H4, and H5 with hints at H3. compilation were inappropriate for "nding the Younger SU90-08 shows a strong event at H1 with hints of events Dryas because they were either (a) of too low a sedi- at H3, H4, and H5. This evidence supports a ventilation mentation rate to resolve the event, (b) sampled too minimum associated with the IRD events, but it does not coarsely to reliably locate the event, or (c) located in indicate ventilation changes associated with the other places where a Younger Dryas event associated with S/IS events. E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 263

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 mar- again, the sedimentation rates of these cores are insu$- gin (SO75-26KL, 373N49.3N, 9330.2W, 1099 m), Zahn cient to reliably record the shorter interstadials, even if et al. (1997) also found light C events in benthic they had been fully expressed as bottom-water C foraminifera during three IRD events equivalent to H1, minima at the core sites. H2, and H4 (Fig. 9). Curry and Oppo (1997) studied cores A more convincing case for the existence for briefer in the western Equatorial Atlantic (EW9209-1, 53N433W events in benthic foraminiferal C is given by the 4056 m) that record planktonic O events and benthic (Atlantic sector) data from Charles et al. C events which qualitatively match the pattern of the (1996), (Fig. 11). In this study, large-amplitude high- largest stadial interstadial events (Fig. 10). However, once frequency benthic C changes are seen that resemble the 264 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

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 not proof of absence. The cores studied could not have highly resolved enough to match all of the S/IS events recorded events of this duration. one-for-one, it provides strong evidence for millennial In view of the importance of establishing whether there signals in the deep-sea C record. As the authors point is a link between the ocean thermohaline circulation and out, the origin of the C signal in cores from this region the S/IS events, e!ort is being directed at coring high- is not entirely straightforward. Benthic C record accumulation sites where su$cient temporal resolution variability at this site cannot be explained by the simple may be obtained. This e!ort is one of the main goals of mixing of North Atlantic and Paci"c deep-water end the International Marine Global chanGES program members: the C values of benthic foraminifera in this (IMAGES program, the marine component of PAGES). region are lighter than either of the supposed endmem- On the "rst cruise of this program undertaken on the bers. Hence this artifact of uncertain origin complicates French research vessel Marion Dufresne in 1995, many their interpretation of this signal as a direct re#ection of long piston cores were taken for this purpose. In particu- changes in NADW. lar, three sediment cores averaging 47 m length were In summary, the existing literature shows that at least taken on the Bermuda Rise, an area where the average some of the major North Atlantic ice rafting (`Heinricha accumulation rate during this interval is '25 cm 1 kyr. IRD) events and stronger S/IS cycles are associated with Calcium carbonate records at this site show transitions benthic foraminiferal C events indicative of lower closely resembling the ice core S/IS sequences (Boyle, NADW percentages at several sites. Although there is 1997; Keigwin et al., 1994; Keigwin and Jones, 1989). plausible evidence that high-frequency variability remi- Although work on these cores is in its early phases, niscent of the Greenland interstadials reaches the South- interstadial 8 and its previous stadial period (proximate ern Ocean, there (so far) has been no evidence in time to `Heinricha IRD event 4) have been studied in demonstrating a tight link between deep-water behavior moderate detail for benthic foraminiferal cadmium varia- and S/IS events. This situation re#ects absence of proof, bility in this core and in another core (KNR31 GPC5) E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 265

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 events on the Bermuda Rise should be coeval with matching their %CaCO and optical re#ectance the IRD events seen at higher latitudes, hence pro- + (CaCO white) records. This correlation between the viding chronological tie lines between cores covering two cores is strengthened by observation of an IRD a broad region of the North Atlantic. The amplitude peak presumed to be `Heinricha IRD event H4 in of the Cd/Ca (0.06 mol/mol) and C(&0.6) both cores (see below). There is a correspondence signals are consistent with the modern day correlation of with the high %CaCO period linked to IS8 having the two tracers. When both tracers are consistent, we lower benthic Cd than the preceding low %CaCO may expect that both are una!ected by artifacts peculiar section. We do not know whether the high %CaCO to each one. and IS8 events are precisely in phase, phase di!erences Finally, we have more persuasive evidence of a close of several centuries or even a millennium are possible link between the deepthermohaline circulation and the with present chronological constraints. In a lower resolu- progression of S/IS events seen in the Greenland ice tion study of core KNR31-GPC5, Keigwin and Boyle cores. Future high-resolution work on these Bermuda (1999) have shown that there is a one-to-one correspond- Rise cores will show just how close this link is. Work at ence between the major stage 3 S/IS cycles (as represent- other high accumulation rate sites is also needed to ed by %CaCO in the Bermuda Rise sediments), the understand the three-dimensional characteristics of this O of planktonic foraminifera (representing mainly deep-sea signal: is the I/IS signal con"ned only to the temperature changes with some in#uence of local salinity mixing of lower NADW and AABW in sensitive and global ice volume), and benthic foraminiferal C transition zones such as the Bermuda Rise, or does this (representing changes in the percentage of nutrient- signal pervade the entire deep Atlantic? We must also depleted NADW at the site relative to nutrient enriched struggle with the chicken and egg problem: does the deep AABW (Fig. 13). We also observed two episodes of thermohaline signal cause the rapid climate shifts or is it ice rafting in the Bermuda Rise cores that correspond just a response to climate events generated by some other to `Heinricha IRD events H4 and H5. These IRD mechanism? 266 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

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 erations of sampling and chronological re"nement. Very change? little paleoclimatological work has been undertaken on these archives. Early work of Emiliani et al. (1978) ex- Although the evidence cited above provides some evid- plored the isotopic characteristics of these organisms and ence that the chemical characteristics of the deepAtlantic demonstrated substantial disequilibria from inorganic ocean can change on millennial } perhaps even centen- carbonate. More recently, deep-sea corals were revived as nial } time scales, the Greenland ice core data tell us that archives by Smith et al. (1997). In a recent paper by climate transitions can be even more rapid, extending Adkins et al. (1998), several coral specimens from the down to a few decades or less. Can the deepAtlantic North Atlantic Ocean (383N603W 1784 m) were found circulation change as rapidly as that? to have grown 15.4 kyr ago, several of which displayed Given the sparseness of evidence documenting millen- older radiocarbon ages for their most recently grown nial deep-sea change, it is not surprising that evidence for carbonate compared to that deposited earlier, by up to decadal deep-sea changes is hard to "nd. One clear 670 yr. The only logical explanation for this inverse example for rapid changes in the deep sea comes from the Ponce-de-Leon e!ect (aging faster than normal) is that study of deep-sea corals. Deep-sea corals, occurring from the water the coral was growing in switched from a high- shallow waters down to 4000 m or greater (but most er-radiocarbon source to an older low-radiocarbon commonly found in the depth range 200}2000 m), are source. Cd/Ca data from one of these specimens supports roughly 10}20 cm in size, and deposit successive layers of a rapid transition from initial high-NADW percentage aragonite during a life cycle of about one century. They water to "nal low-NADW percentage water occurring incorporate uranium and C from the seawater they within less than &100 yr (the approximate age of the grow in, and hence can be used to derive an absolute coral specimen when it ceased growing) (Fig. 14). The chronology from Th dates and provide an estimate of increase in the C age from the youngest to oldest the C ventilation time of the deepwater. Because the segment mainly re#ects the replacement of a percentage coral is solid, no bioturbational blurring compromises of NADW by AABW (as seen in the Cd/Ca data), not the "delity of the record. The temporal resolution of the actual radioactive aging of a stagnant water col- a coral climate record is limited only by practical consid- umn. This is the "rst data that show that the chemical E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 267

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 deepsea can change in less than transitions. This goal will require more data from high a century; climate change in the deep sea is apparently as accumulation rate deep-sea sites, which in practice re- rapid as it is in the atmosphere over Greenland. quires more data from focussed drift deposits and conti- nental margin deposits. Continental margin sites are more problematical (turbidites and other sedimentary 5. Needs for the future: high-resolution drift and disturbances being harder to avoid and detect), but a de- continental margin deposits, more deep-sea corals, termined e!ort with abundant AMS C age controls (for and better time control samples younger than about 40 kyr) can make signi"cant progress on this problem. From the previous discussion, it is evident that we It is also evident that more work on deep-sea corals need a better three-dimensional perspective on the will be required to answer questions concerning decadal relationshipbetween deep-seacirculation and S/IS deep-sea variability. Deep-sea coral sampling so far has 268 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

  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 Progress on the chicken/egg problem will require inadvertently from dredges carried directed at unrelated a better chronology for the diverse climate records from purposes. Even with a purposeful coral sample collection land and sea. During most of MIS 3, the time scales that e!ort, it would be hard to control the age distribution of have been constructed have an accuracy of a few thou- the specimens recovered. For the immediate future, we sand years. In other words, correlations (such as that can envision receiving tantalizing anecdotes about short between Greenland I/IS and atmospheric methane cycles periods of time, not long continuous climate records, and the Santa Barbara Basin bioturbation index or the from these archives. However, with a determined e!ort Bermuda Rise %CaCO record) could be in error by as and some luck, those anecdotes can illuminate key cli- much as a whole S/IS cycle! The precision of correlation mate transitions and helpus evaluate timing of deep-sea is sometimes better; for example, ice core records can be events relative to surface climate changes. correlated between the northern and southern hemi- E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 269

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 rapidly, this marker provides an isochronous time line (e.g. Blunier et al., 1998; Brook et al., 1996). But it is precise to one year (!) wherever it is found (the precision di$cult to link di!erent types of records (e.g. ice core and of course also depends on the temporal resolution of the marine sediment cores) with this level of con"dence. In archive). (2) Studies of the paleomagnetic intensity record time, improvements in the precision and accuracy of in high-resolution marine sediments are suggesting that C dating may helpfor sediments younger than about the brief `Laschampa magnetic minimum (and other 35}40 kyr, although this task will also require better #uctuations as well) may be found in some marine sedi- understanding of changes in surface water C relative to ment cores (Laj et al., in press) as well as be re#ected in the atmosphere (i.e. reservoir age corrections, Bard et al., the Be concentration of ice (Yiou et al., 1997). Here 1994). Th dating of surface and deep-sea corals should again we have a time marker, resolvable to perhaps help with this problem. The path towards improved a thousand years or better, that applies wherever indi- temporal resolution may be slow and unpredictable, but cators of past magnetic intensity can be found } on in time an improved millennial-precise chronology continents, in ice cores, and marine sediments. (3) The should be achievable. Consider some examples: (1) The occurrence of the `Heinricha IRD events as far south as ash from the volcanic explosion on the Indonesian island subtropical waters (see above) also provides a precise tie Toba has been reported in sediment cores from In- line linking marine records over much of the North donesia (Ninkovich et al., 1978) to the Arabian Sea Atlantic. As tie lines such as these are found and linked (Schulz et al., 1998), and the excess sulfate from the into other climate markers (ice core methane, etc.), the eruption is reported in Greenland ice (Zielinksi et al., precision and accuracy of our time scales will improve. 1996). Because ash and sulfate fall out of the atmosphere Achieving this task is a challenge for the next decade. 270 E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272

6. Speculations on controls on ocean thermohaline model; when they created low-salinity conditions that circulation, mixing coe7cients, and the bipolar seasaw hindered `NADWa production, `AABWa production in- creased. A similar e!ect was previously observed in What controls the thermohaline overturning rate of a model study by Stocker et al. (1992). It is worth noting the ocean? Beginning with the ocean model studies of that Broecker (1998) examined evidence on the timing of Bryan (1986,1987), this question has often been answered climate change in the Northern and Southern Hemi- with the model-based observation that the vertical mix- sphere and suggested that the climate system may be ing rate in the ocean controls the rate of deep- and operating as a `bipolar seesawa. If vertical mixing rates bottom-water production. As vertical di!usion mixes do not change much and hence require roughly constant heat downwards from the surface ocean, it creates `op- deep-water formation rates, does this requirement drive portunitiesa at high latitudes for creating water that is the polar/bipolar seesaw? su$ciently dense to sink down and replace the less dense Must vertical mixing rates and the overall thermoha- warm water. The faster the ocean mixes vertically, the line circulation overturning be constant? Could not more frequently deep-water formation opportunities the vertical mixing rate in the ocean not vary over time? arise at high latitudes. Conversely cooler glacial tropics In fact, there are at least two reasons why vertical may di!use less heat downwards and result in diminished mixing should vary temporally. First, tidal dissipation deep-water formation opportunities. by the earth is presently dominated by dissipation in This seems a rather simple answer for what seems to be shallow shelf waters. During a glacial maximum, when a very complicated process. Deep-water formation only most of these shallow waters have turned into dry land, occurs in very limited regions for brief periods (Warren, more of the tidal dissipation should have occurred in the 1981); in some regions, deepwater does not form for deepsea. Second, interactions of wind with the Circum- years on end. Perhaps the answer is misleadingly simple, polar Current is the other major mechanism creating because vertical mixing in the ocean itself is not a simple vertical mixing. Most models for glacial climate seem to matter, and it is certainly not homogeneously distributed call for greater wind speeds over the Southern Ocean in the real ocean as models assume it is. Microscale during glacial times; if so, vertical mixing would also structure and deliberate tracer release experiments have increase in the Southern Ocean. Both major mechanisms shown that vertical di!usion in ocean basins away from in#uencing vertical mixing would then be presumed to rugged topography is an order of magnitude less than call for an increase in vertical mixing during glacial the canonical value required by whole ocean balances times. By the logic of Bryan (1986,1987), this increase in (Polzin et al., 1997a). Recently, it has been shown that vertical mixing calls for increased deep-water formation vertical mixing occurs much faster over rugged topogra- rates. phy than over smooth sea#oor. It is thought that tidal Present evidence does not support this argument for interactions with the rugged topography may account for an increase in the overturning of the ocean during glacial this observation. In this case, then the ocean-wide ocean times. C data from pairs of planktonic and benthic mixing is accomplished near boundaries, mid-ocean foraminifera in ocean sediments (Broecker, 1989; ridges, and seamount complexes, and then laterally dif- Broecker et al., 1988; Shackleton et al., 1988) indicate that fused and advected into the ocean interior. Munk and the overturning time of the ocean increased somewhat Wunsch (1998) have considered the matter and conclude during glacial time (Adkins and Boyle, 1997). Perhaps that vertical mixing in the ocean is accomplished by this evidence is not conclusive. C in the ocean is in- roughly equal contributions from tidal activity interac- #uenced by other factors, such as gas exchange with the ting with rough topography (e.g. see Polzin et al., 1997b) sea surface and surface boundary conditions (Broecker et and the interaction of wind with the Circumpolar al., 1991). Perhaps a simple interpretation of the C age Current. of the deepocean (as re #ecting the rate of physical over- What does this perspective contribute to the study of turning of the ocean) is not warranted (Campin et al., temporal changes in the ocean thermohaline circulation? 1999. Issues such as these will be among the matters that To date, paleoceanographers have largely focussed on studies of the relation between climate change and ocean surface processes that might change the rate of deep- circulation must address in the future. water formation. For example, formation of a low-salin- ity layer in the northern North Atlantic is expected to reduce the formation of NADW. However, if the overall Acknowledgements thermohaline circulation is controlled neither by vertical mixing and nor by the local surface conditions, then I thank Ray Bradley for inviting me to make this a reduction of NADW formation will have to be made up presentation at the PAGES Open Science Meeting and by an increase in deep-water production elsewhere, either Frank Old"eld for his patience with the manuscript prep- in the Antarctic or North Paci"c. Wang et al. (1999) aration. Thanks to Rainer Zahn, Chris Charles, and Bill observed this behavior in a simpli"ed ocean circulation Austin for sending their data for replotting. E.A. Boyle / Quaternary Science Reviews 19 (2000) 255}272 271

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