Thermohaline Circulation and Prolonged Interglacial Warmth in the North Atlantic

Quaternary Research 58, 17–21 (2002) doi:10.1006/qres.2002.2367

Thermohaline Circulation and Prolonged Interglacial Warmth in the North Atlantic Jerry F. McManus,1 Delia W. Oppo, and Lloyd D. Keigwin Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

James L. Cullen Department of Geological Sciences, Salem State College, Salem, Massachusetts 01970

and Gerard C. Bond Lamont Doherty Earth Observatory, Palisades, New York 10964 Received October 2, 2001

tope stages (MIS) these two peak interglaciations are designated MIS 1 (Emiliani, 1955) and MIS 5e (Shackleton, 1969). Each is associated on land with deposits reflecting marine transgressions and equable environments. No equivalent global nomenclature exists for these terrestrial interglacial deposits, although the Holocene and the Eemian are widely applied as comparable terms, particularly for sediments representing the last two interglaciations in Europe. The intervening period encompassed five 20,000-yr precessional cycles characterized by alternating intervals of partial amelioration and increasingly extreme glaciation, culminating in the last glacial maximum (LGM). Throughout this ∼100,000-yr interval the North Atlantic region was characterized by dramatic climate instability on millennial timescales (e.g., Bond et al., 1993; Dansgaard et al., 1993). A return from the Holocene interglaciation to the next glaciation and its attendant variability appears likely, if not inevitable. The process and timing of this future transition remain uncertain (e.g., Broecker, 1998), although the apparent influence of 20,000-yr insolation cycles might suggest that the elapsed portion of the Holocene is approximately the expected duration of warm interglacial conditions. The last interglaciation, having run its full course of development and demise, provides the most recent geological example of the sequence of events associated with the transition to the early phase of global glaciation. In the North Atlantic region, evidence from a number of long marine, terrestrial, and ice cores provides spatial and temporal resolution of the last interglaciation and the subsequent glacial inception.

Deep-sea sediment cores provide spatially coherent evidence for the climatic and hydrographic conditions in the subpolar North Atlantic during the last interglaciation. Taken together with similarly high-resolution terrestrial sequences, these records indicate a regional climatic progression, beginning with the extreme and variable climate late in the penultimate glaciation, continuing through a relatively stable climatic optimum during the interglaciation, and concluding with the reestablishment of the markedly variable regime that characterized the last 100,000-yr glaciation. Relatively mild conditions in much of the subpolar region significantly outlasted the minimum in global ice volume, despite declining summer insolation and the cooling influence of incipient proximal glaciers. These effects were partially offset by enhanced thermohaline circulation that paradoxically increased heat transport into the region while simultaneously providing the likely moisture source for the growth of large northern ice sheets. The inception of the last glacial cycle thus provides an example of the influence of ocean circulation on regional climate. In contrast to the apparent orbital pace of the ongoing ice-sheet growth, the subsequent deterioration of surface conditions was abrupt and dramatic.  2002 University of Washington. Key Words: Eemian; thermohaline circulation; interglaciation; MIS 5. C

THE LAST INTERGLACIATION

During the Quaternary, the Earth’s climate has generally been harsher and more variable than at present. Prior to the 11,000-yr duration of the Holocene, the previous interval of comparably mild global climate occurred more than 100,000 yr ago. In the standard deep-sea δ 18 O stratigraphic sequence of marine iso-

REGIONAL RECORDS

The deep-sea cores utilized for this study were recovered from sites of relatively rapid sediment accumulation in the subpolar

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To whom correspondence should be addressed. E-mail: jmcmanus@ whoi.edu. 17

0033-5894/02 $35.00 C 2002 by the University of Washington. Copyright  All rights of reproduction in any form reserved.

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FIG. 1. Map of the North Atlantic Ocean showing locations of subpolar sediment cores discussed in the text. Shaded boundaries delineate portions of the region affected by successive climate events. C23–25 are coolings associated with the glacial inception that followed the last peak interglaciation of the Pleistocene. At most sites, proxy evidence for the cooling comes from the relative abundance of the polar species of planktonic foraminifera, N. pachyderma, sinistral-coiling. The approximate boundaries are drawn between polar foraminifera abundance values that are significant (>10%–20%) and those that are minimal (0%–5%).

North Atlantic (Fig. 1). New measurements of the abundance of the polar planktonic foraminifera Neogloboquadrina pachyderma sinistral (N. pachyderma s.) from several of the cores were added to previously published records (McManus et al., 1994, 1999; Oppo et al., 1997, 2001; Chapman and Shackleton, 1998, 1999). These records allow a robust assessment of the geographic extent of climate changes within the peak interglaciation and during the transition associated with the glacial inception. Modern and coretop abundances of N. pachyderma s. vary inversely with sea-surface temperature (SST), providing a dynamic range of approximately 8◦ C between 0% and 100% (e.g., Kohfeld et al., 1996). Oxygen isotope data from the benthic foraminifera Cibicidoides at the sites serve to anchor the records stratigraphically, and the associated carbon isotope data provide indicators of ventilation by intermediate and deep water masses. Ice-core data from Summit, Greenland (Dansgaard et al., 1993; Grootes et al., 1993) and pollen data from Grande Pile, France (Woillard, 1978, 1979) are used for comparison to the ice sheet and the European continent. Five of the deep-sea cores extend through the entire 135,000–100,000 yr B.P. interval considered here (Fig. 2) as does the sequence at Grande Pile, which contains within it the Lure interglaciation, a possible Eemian-equivalent (Woillard, 1979). Several additional deep-sea records include the early glacial inception at 115,000–100,000 yr. B.P. The combined records indicate a regionally coherent climatic progression, with a spatially extensive peak in conditions followed by the increasing influence of a

FIG. 2. Correlation of climate indices in the North Atlantic region. All records include the climatic oscillations C23 and C24 associated with the glacial inception. Some sediment cores did not penetrate the entire peak interglaciation and the recorded response to C25 is spatially limited. Shaded interval coincides with cooling influence of insolation minimum and growth of northern ice growth, leaving thermohaline circulation (THC) as primary regional forcing of continued warmth.

PROLONGED NORTH ATLANTIC INTERGLACIAL WARMTH

series of climatic oscillations associated with the onset of the last glaciation. CLIMATIC PROGRESSION

Following the initial decay of ice sheets that had grown during the penultimate glaciation, the H11 catastrophic iceberg discharge (Heinrich, 1988) preceded rapid warming throughout the subpolar ocean (Fig. 2). Prior to H11, the North Atlantic was characterized by marked deglacial variability (Oppo et al., 2001). The amplitude of climatic variability at the same locations during the subsequent warm interval, continuing through the high sea-stand, was diminished relative to both the preceding and following glacial intervals. By mid MIS 5e, regional SSTs were near modern values (CLIMAP, 1984; Cortijo et al., 1994, 1999; Oppo et al., 1997; Chapman and Shackleton, 1998). Although SST values varied locally during this optimum, often as a trend and occasionally in a single step, no large regional oscillations occurred. Based on N. pachyderma s. abundance, EW37JPC was the coolest subpolar site (Fig. 2), displaying a trend from 2% to 10%, equivalent to a range of approximately 1◦ C. As ice sheets began to grow, the region generally stayed warm, although a millennial-scale oscillation brought a cooling, C25, that was significant at the far northern and western subpolar Atlantic locations, and more subtle in the east and in Europe (Fig. 1). This episode is probably correlative with the Woillard event in Grande Pile (Kukla et al., 1997). Following C25, the entire region returned briefly to warm conditions, despite the already reduced insolation (Berger et al., 1996) and the continued increase of substantial ice volume. Ice growth would tend to increase the salinity of surface waters (Duplessy and Shackleton, 1985). The sequestration of freshwater in the growing ice sheets would alter the hydrologic cycle, limiting runoff, decreasing the associated buoyancy flux (Schmitt, 1998), and thus enhancing the density effect of evaporative cooling of saline waters advected northward. During this ice growth, the benthic carbon isotopes (δ 13 C) shift to higher values (Fig. 2), suggesting a reinvigoration of the thermohaline circulation (THC) following a transient decrease that is also evident in Cd/Ca and sedimentary proxies for deep circulation (Keigwin et al., 1994; Adkins et al., 1997). The compensating northward flow of thermocline waters (Gordon, 1986) would have increased the heat transport into the region. As recognized by Ruddiman and McIntyre (1979; Ruddiman et al., 1980), the proximity of such a warm ocean to growing ice sheets presented an ideal moisture source. The subpolar sea surface remained warm until this increase in benthic δ 13 C and its implied enhancement of THC had run its course, well into stadial substage MIS 5d. Throughout this transitional interval, benthic records show that the Pacific underwent a steady decrease in δ 13 C (Shackleton et al., 1983; Mix et al., 1995), thus supporting a circulation, rather than a mean ocean, origin for the Atlantic signal. Although atmospheric CO2 remained high during this interval (Barnola et al., 1987) and cannot be ruled out as an addi-

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tional influence, the early and therefore asynchronous decline in sea-surface temperatures observed at high southern latitudes (e.g., Martinson et al., 1987) is inconsistent with greenhouse forcing and supports the THC mechanism, which may result in antiphase relationships near the two poles (Broecker, 1998). Similarly strengthened THC is also associated with glacial initiation in a coupled atmosphere-ocean-ice paleoclimate model experiment (Wang and Mysak, 2000). Only late in the ice-sheet growth cycle did the entire subpolar North Atlantic cool significantly (Fig. 2), despite the presence of northern hemisphere ice (e.g., Ruddiman and McIntyre, 1979; Karabanov et al., 1998). This first regional climatic expression of the onset of the 100,000-yr glaciation was rapid and dramatic. Event C24 influenced the entire region shown in Fig. 1, with much of the subpolar ocean undergoing a cooling on the order of 4◦ C, approximately half the last glacial–interglacial (G-IG) temperature contrast (McIntyre et al., 1976). At the same time, the Melisey I event in Grande Pile records the first major decline of the closed-canopy forest. It is also likely that a similar cooling affected the atmosphere over Greenland, although the C24 event may not be faithfully expressed in the existing GRIP and GISP2 ice cores. This event, C24-Melisey I, peaked rapidly at 107,000 yr, only after the ice-sheet growth reached a temporal local maximum, and thus occurred well into MIS 5d, thousands of years after insolation had already bottomed out and begun to rise (Fig. 2). Another warming followed, beginning within MIS 5d, and was followed in turn by the cooling event C23 at ∼102–103,000 yr. The warm interval corresponds to Greenland interstadial 24 (Dansgaard et al., 1993), and the cold C23 corresponds to the Montaigu event in Grande Pile (Woillard, 1978). These oscillations complete the sequence of events within MIS 5d and the transition into MIS 5c. The stadial substage MIS 5d was therefore not a uniformly cold interval in this region. Rather, it was characterized initially by warm conditions at most subpolar locations and subsequently by a series of suborbital climate oscillations with only a relatively brief cold maximum. Similarly, the regional warmth that accompanied the beginning of the MIS 5c substage was also interrupted by the C23-Montaigu millennial-scale oscillation. After the dramatic G-IG contrast in SST of ∼8◦ –10◦ C, variability within the interglaciation prior to C25 was limited to ∼1◦ –2◦ C. The cooling during C25 was more substantial, and during C24 the entire region cooled on the order of ∼4◦ C, with some locations likely to have cooled even more. Rapid variability of similar amplitude characterized most of the subsequent 100,000 yr (Bond et al., 1993; McManus et al., 1994), suggesting that amplitudes of suborbital variability during the glaciation were consistently larger than that of the peak interglaciation, and approximately half the total glacial–interglacial change. The shift to higher amplitudes in these oscillations as glaciers grew and sea level fell during the MIS 5e/5d transition is consistent with an ice-volume threshold in the amplification of the climate response (Chapman and Shackleton, 1999; McManus et al., 1999).

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DURATION OF EQUABLE CLIMATE

REFERENCES

In light of the millennial-scale variability associated with the glacial inception, it is necessary to define carefully the beginning and end point of the peak interglaciation before estimating the duration of specific interglacial conditions. For example, the duration of the Eemian (Lure) in Grande Pile, although not well known, is generally considered to be the time between the initial growth of the closed forest and the sharp decline of those same forests during Melisey I (Woillard, 1979). Similarly, the marine “Eemian” can be considered to constitute the interval of warmth that followed rapidly after the H11 iceberg discharge of the penultimate deglaciation and persisted until the major cooling throughout the region during C24. Orbital tuning of the benthic stratigraphy to an insolation target (Martinson et al., 1987) provides a chronology that indicates that this warm North Atlantic interglacial interval may have lasted nearly 20,000 yr (Fig. 2). Based on this chronology, peak warm conditions were established by 125,000 yr and continued until C24 at 107,000– 108,000 yr, a total of 17,000–18,000 yr. Calculations based on the different assumption that sedimentation rates were constant at each site yield a range of estimates (16,000–24,000 yr) bracketing 20,000 yr. Applying a third approach using 230 Thxs profiling at site V28-82 yields a similar estimate that the warmth lasted approximately 20,000 yr (McManus et al., 1998a, 1998b; J. McManus, unpublished data). All of these estimates are close to twice the elapsed duration of the Holocene, and all of them are the approximate length of an entire cycle of the presumed forcing and/or pacing of precession. An additional climate influence beyond insolation is required to account for this persistence of warm conditions. Reinvigorated THC is an internal mechanism that provides such an influence, although it may result in regional rather than global prolongation of interglacial warmth.

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CONCLUSIONS

The last interglaciation was an extended interval of equable and relatively stable climate in the North Atlantic. Regional warmth was prolonged relative to the duration of the MIS 5e minimum in global ice volume, continuing well into the glacial growth of MIS 5d. Despite the glacial initiation following the decline of summer insolation, the subpolar North Atlantic remained relatively warm for a total of approximately 20,000 yr. Enhanced thermohaline circulation provided the heat transport that helped prolong the interglacial warmth, while at the same time providing an ideal moisture source for ice-sheet growth. In contrast to the many intervals when the interaction of climate and THC present a “chicken and egg” puzzle, the last glacial inception is a clear instance of the direct influence of THC on the regional climate. The subsequent deterioration of surface conditions was abrupt and dramatic, on the order of half the full glacial–interglacial contrast in SST.

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