Pliocene and early Pleistocene

Transcription

1 PALEOCEANOGRAPHY, VOL. 16, NO. 5, PAGES , OCTOBER 2001 Millennial-scale climate change and oceanic processes in the late Pliocene and early Pleistocene Katherine Mc Intyre Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA Margaret L. Delaney and A. Christina Ravelo Ocean Sciences Department and Institute of Marine Sciences, University of California, Santa Cruz, California, USA Abstract. We generated year resolution records of oceanic processes in the North Atlantic (Ocean Drilling Program Site 983, 60ø24 N, 23ø38 W, 1983 meters water depth) for intervals the latest Pliocene ( Ma) and the earliest Pleistocene ( Ma) in order to examine the linkages between millennial-scale variations in the ocean and background glacial-interglacial climate change. Within glacial intervals we find evidence for variations similar to those observed in the late Pleistocene. We find discrete ice-rafted debris (IRD) events that reoccur every 2-5 kyr. These events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters. The timing of IRD events in the late Pliocene and early Pleistocene intervals is similar to that of Dansgaard-Oeschger cycles, but we find no IRD events comparable in timing to late Pleistocene Heinrich events. Although interglacial intervals are much more stable, we do find evidence for low-amplitude variations in deep water properties that reoccur every 2 kyr within interglacial intervals. The similarity between our late Pliocene--early Pleistocene records and late Pleistocene records implies that the mechanism driving millennial-scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late Pleistocene. 1. Introduction Records of millennial-scale climate change covering the last 150 kyr have been recovered from environments ranging from Greenland ice [Johnsen et al., 1992; Dansgaard et al., 1993] to Pacific sediments [Beh! and Kennett, 1996; Lurid and Mix, 1998]. In North Atlantic and sub-arctic sediments, millennialscale climate changes are seen as periods of cooling terminated by a very brief interval of ice-rafted debris (IRD) deposition, reduced sea surface salinity [Broecker et al., 1992; Bond et al., 1993; Rasmussen et al., 1996; Bond et al., 1997; Elliot et al., 1998], and reduced North Atlantic (NADW) formation [Oppo and Lehman, 1995; Curry and Oppo, 1997; Marchitto et al., 1998; Oppo et al., 1998; McManus et al., 1999], followed by an abrupt warming. These rapid climate events have several characteristic timescales of reoccurrence. The largest-amplitude events occurred every 6-10 kyr [Heinrich, 1988; Broecker et al., 1992; Bond et al., 1993; McManus et al., 1999], interspersed with shorter, less extreme cycles every years [Johnsen et al., 1992; Dansgaard et al., 1993; Rasmussen et al., 1996; Oppo et al., 1998; Elliot et al., 1998] and even smaller IRD events every 1500 years [Bond et al., 1997]. While the longer cycles occurred only during glacial intervals, the 1500 year climate variations continued into the present interglacial period [Bond and Lotti, 1995; Bond et al., Paper number 2000PA /01/2000PA [Bond et al., 1993] and that smaller ice sheets tinging the North Atlantic surge in response [McCabe and Clark, 1998]. Modeling of ice sheet dynamics [MacAyeal, 1993a, 1993b; Alley and MacAyeal, 1994] indicates that the binge-purge behavior could be driven by free oscillations in the flow of the Laurentide ice sheet. The emerging picture is one of small, frequent events that operate continuously throughout glacial-interglacial cycles, and larger events that require the presence of a large glacial ice sheet. A number of authors have extended the history of millennialscale events beyond the last 150 kyr and have found that the amplitude of millennial-scale climate variability is greatest during times of enlarged glacial ice sheets. Oppo et al. [1998] demonstrated that climatic variations at periods of 3.8 and 1.5 kyr persist throughout glacial-interglacial cycles from 380 to 500 ka at a site on the Feni Drift in the North Atlantic (Ocean Drilling Program (ODP) Site 980; Table 1), but that the highest amplitude variations occurred during periods of ice growth and decay. Raymo et al. [1998] found a similar pattern of millennial-scale variations in surface and deep waters at a site on the Gardar Drift in the highlatitude North Atlantic (ODP site 983) during two 41 kyr glacialinterglacial cycles in an interval around 1.3 Ma. McManus et al. [1999] extended the Feni Drift record from 0 to 500 ka, observing that IRD deposition occurs whenever C. wuellerstorfi 80 values exceed 3.5%0 and suggesting this modest increase in ice volume exceeds some ice sheet threshold condition. Wara et al. [2000] demonstrated that North Atlantic millennial-scale climatic varia- 1997], indicating that the mechanisms driving these events may be continuously at work throughout glacial-interglacial cycles. Bond tions from 200 to 600 ka were harmonics and combination tones of et al. [1999] suggested that the longer cycles represent occasional Milankovitch insolation forcing created as the Milankovitch signal amplifications of the 1500 year cycle through climatic operators propagated through a resonating Northern Hemisphere ice-oceanwith longer time constant such as large ice sheets. For the 6-10 atmosphere system. They found that ice sheet activity, as indicated kyr period Heinrich events, sedimentary evidence indicates that by ice-rafted debris abundance, seemed to be driving nonlinear these events are driven by surges of the large Laurentide ice sheet responses in sea surface temperature and deep water mass source, suggesting that ice sheet dynamics were the important player in causing sub-milankovich resonances. Copyright 2001 by the American Geophysical Union. In order to examine the connections between millennial-scale 535 climate changes in the North Atlantic and ice volume, we focused on two intervals representative of 41 kyr glacial-inter-

2 536 MCINTYRE ET AL.: MILLENIAL-SCALE CLIMATE CHANGE Table 1. Locations and Depths of North Atlantic Sites Discussed in this Study Location Site Latitude øn Longitude øw Depth rn Gardar Drift ODP ' ' 1983 Feni Drift ODP ' ' 2179 Feni Drift ODP ' ' 2173 Feni Drift V ø 16' ' 2370 Ceara Rise ODP ' ' 4356 West Africa ODP ' 21 ø02' 3070 Rockall Plateau DSDP ' ' 2311 Reference This study Oppo et al. [ 1998] Mc Intyre et al. [1999] McManus et al. [1994] Bickert et al. [1997] Tiedemann et al. [ 1994] Shackleton and Hall [1984] glacial cycles, one in the late Pliocene ( Ma; marine isotope stages (MIS) 68-70) and one in the early Pleistocene ( Ma; MIS 63-66). Maximum glacial ice volume in these older intervals was one third to one half as large as in the late Pleistocene (Figure l a), and NADW formation continued throughout glacial-interglacial cycles [Raymo et al., 1990; Bickert et al., 1997]. Glacial-interglacial cycles occurred every 41 kyr, responding quasi-linearly to variations in the amount of incoming insolation at high latitudes driven by the Earth's obliquity cycle [Raymo et al., 1989]. In contrast, late Pleistocene 100 kyr glacial-interglacial cycles appear to be driven by a very nonlinear climate response to the Earth's eccentricity cycle [Imbrie et al., 1993]. We use the different forcing-climate linkage for the older interval to examine how millennial-scale events are related to glacial-interglacial changes in climate and ice volume. The existence of late Pliocene--early Pleistocene millennial-scale cycles similar to those in the late Pleistocene implies that these changes are driven by processes independent of the very large ice sheets of the late Pleistocene and independent from the longer-term response of the climate to Milankovitch insolation forcing. In this study, we address some basic questions with respect to this issue: What is the nature of millennial-scale events in the 41 kyr glacial world? Are late Pliocene-early Pleistocene millennial-scale events and their link to ice volume similar to those during the late Pleistocene? Our data comes from a site on the Gardar drift in the highlatitude North Atlantic (ODP site 983; Table 1 and Figure 2). This site was chosen because it sat on the boundary between Glacial North Atlantic Intermediate Water (GNAIW) and Antarctic Bottom Water (AABW) at the Last Glacial Maximum [Oppo and Lehman, 1995; Shipboard Scientific Party, 1996] and was dried for 24 hours at 50øC and then weighed. We calculated the percent coarse fraction as the ratio of the dry weight of the >63 I m fraction to the dry weight of the bulk sample. We measured and 1513C of 1-3 C. wuellerstorfi >150 gm in size using a FISONS Prism III mass spectrometer. Errors (1 ( ) on replicate internal laboratory standards were +0.05%0 for 1513C and ñ0.08%0 for Errors on replicate samples were always larger, %o for 1513C and 0.10%o for 15180, Results are reported relative to Vienna pee dee belemnite (VPDB) calibrated via NBS-19. We made a quantitative split of the original sample >150 gm and counted the percent of planktonic foraminifera N. pachyderma sinistral and dextral out of,- 300 specimens. Reproducibility (1 ( ) on replicate samples was,- 10%. Foraminiferal abundances were low during glacial stage 64, and we do not present faunal abundance counts made on samples with <250 specimens. We did not measure the percent N. pachyderma in the older Pliocene interval because the sinistral form of this species had not yet become part of the assemblage. We measured the IRD/g sediment by counting the number of rock fragments >212 gm in the entire sample or a quantitative split of one fourth to one half the sample. In some samples, there were pyritized burrows and mud clasts, which we considered to be formed in situ and did not include in our counts of IRD. We chose the 212 m over the 150 m size fraction after measuring both in a number of samples (n = 62, Figure 3). While there are differences in the relative amplitudes of the largest IRD peaks (Figure 3a), the timing of these events is identical. Excluding the highest values to focus on the fidelity at lower abundance levels, IRD counts on samples with <5 rains >212 gm/g and <10 grains >150 gm/g correlate with a r = 65 (n = 55; Figure 3b). Andrews [2000] because high sedimentation rates at this site yield a temporally demonstrated that IRD size distributions vary between sites and expanded record. We document climatic and oceanic changes at within different depths at the same location, indicating that countthis site using a number of proxies. We counted lithic grains >212 ing from specific size fractions, particularly a larger one, may not gm/g sediment as measure of ice-rafted debris supply and be representative of discrete IRD inputs. This is clearly a concern, counted the percent planktonic foraminifer Neogloboquadrina but as we will discuss below, late Pliocene-early Pleistocene IRD pachyderma to assess variations in surface water temperatures. peaks are accompanied by drops in magnetic susceptibility and We measured the of benthic foraminifer Cibicidoides peaks in percent coarse fraction indicative of an overall change in wuellerstorfi to reconstruct global ice volume, local deep water sediment grain size and mineralogy and not sampling bias. temperature, and salinity and measured the 15 3C of this species to We generated a timescale by tying our C. wuellerstorfi examine intermediate water nutrient composition as an indicator records to the C. wuellerstorfi record at ODP Site 659 of intermediate water source. We also present the percent coarse (Figure 2), which was astronomically tuned at the precessional fraction and the shipboard magnetic susceptibility data [Ship- scale [Tiedemann et al., 1994]. On the basis of this timescale, board Scientific Party, 1996] as sedimentary proxies reflecting a sedimentation rates at Site 983 average cm/kyr in the combination of IRD input, biological productivity, and drift younger interval and are higher and more variable in the older sediment deposition. interval, cm/kyr. In both intervals these sedimentation rates yield a year resolution for the IRD >212 gm/g and 2. Methods percent coarse fraction measurements. We counted the percent N. pachyderma in samples every 10 cm, for an average temporal We sampled cm 3 of sediment every 2-5 cm over the resolution of 400 years. The temporal resolution of our C. depth intervals m composite depth (mcd) and wuellerstorfi and 1513C data ranges from,- 200 years in mcd..samples were dried in the oven at 50øC for intervals when C. wuellerstorfi are continuously present, to 5-10 a minimum of 24 hours and then weighed. We washed samples kyr during intervals when the sediment is barren of C. wuellerin a sodium metaphosphate solution and wet sieved them to storfi. Data are archived at the National Geophysical Data Center separate the >63 gm fraction of sediment. The >63 gm fraction ( ngdc.noaa. go v/paleo ).

3 MCINTYRE ET AL.: MILLENIAL-SCALE CLIMATE CHANGE Results We identified MIS in the early Pleistocene interval and MIS in the late Pliocene interval based on C. wuellerstorfi isotopes, percent coarse fraction, and magnetic susceptibility (Figure 4). Glacial intervals are characterized by high benthie 60ø foraminiferal values, low 15 3C values, the presence of IRD, high magnetic susceptibility, and low percent coarse fraction (Figure 4). There are gaps in the glacial benthic and planktonic foraminiferal records when sediments were barren of foraminifera. 45 ø Ice-rafted debris appears rapidly in the sediments as soon as C. wuellerstorfi values increase beyond 3.5% o and disappears as values decrease below this value during deglaciation (Figures 4b and 4c). Glacial percent coarse fraction is extremely low, <2%, and IRD is the only major source of coarse material 30ø (Figure 4a). High magnetic susceptibility during glacial intervals indicates that there is either a relatively greater proportion of finegrained drift materials at this time and/or a shift in the composition of drift materials toward more magnetic grains (Figure 4b). IRD events appear as abrupt changes in the magnetic susceptibility record (Figure 4b). While foraminifera are rare in glacial sediments, MIS 70, which has more continuous C. wuellerstorfi isotopic coverage than stage 64 (Figures 4c and 4d), is punctuated by frequent, brief excursions toward low and 15 3C values. 15 ø -60 ø -45 ø -30 ø - 15 ø -60 ø -45 ø -30 ø -15 ø 60 ø 45 ø 30 ø 15 ø _ Figure 2. Location of sites discussed in this study. Sites locations and depths are described in Table 1. Shaded lines are the 2000 m isobath. Interglacial stages have relatively stable benthic foraminiferal isotopic values with small variations of 0.4%o in and 1513C (Figures 4c and 4d). There is only one major interglacial IRD peak that occurs during interglacial stage 65 ( Ma) (Figure 4b). This event ended in a 6 kyr excursion in most proxies toward glacial conditions (Figure 4) and, combined with the observation that percent coarse fraction is low throughout stage 65, suggests that this was an anomalously cold interglacial interval. Interglacial percent coarse fraction is higher and magnetic susceptibility is lower than in the glacial period, and these records are roughly anticorrelated (Figure 4a). The interglacial increase in foraminiferal diversity and decrease in our cold end-member species, N. pachyderma sinistral, argues for increased productivity, which then dilutes the fine-grained magnetic material at depth. Transitions into and out of glacial stages are generally coincident among proxies with three exceptions. During glacial onset, as the benthic foraminiferal begins to increase, 15 3C increases to a maximum around 1%0 and then decreases once IRD appears in the 3 0, sediment (Figures 4b-4d). During deglaciation the benthic foraminiferal 15 3C reaches interglacial values 5 kyr before reaches its minimum values (Figures 4c and 4d). The appearance of IRD lags the beginning of the glaciation by 4 kyr, and IRD disappears 10 kyr before full interglacial conditions take hold (Figure 4b). During the termination of glacial stage 64, the percent i I i i i i iv. pachyderma sinistral increases in conjunction with increasing percent coarse fraction (Figures 4a and 4e), as the sinistral form becomes the cold end-member species. About 20 kyr later, the percent N. pachyderma sinistral values begin to decrease, the Figure 1. Late Pliocene and early Pleistocene intervals discussed percent N. pachyderma dextral increases, and other species come in this study. (a) Benthic foraminiferal at equatorial Atlantic to dominate the assemblage (Figure 4e) at full interglacial conditions. Ocean Drilling Project (ODP) Site 659 over the last 3 Myr [Tiedemann et al., 1994] with interval discussed in this paper highlighted by the shading, (b) at equatorial Atlantic ODP 4. Discussion Site 659 for the interval discussed in this study with glacial stages marked, and (c) at high-latitude northeastern Atlantic ODP In the following sections, we draw upon these records to make Site 983 for the late Pliocene and earliest Pleistocene (this study). several observations about late Pliocene-early Pleistocene climate

4 538 MCINTYRE ET AL.' MILLENIAL-SCALE CLIMATE CHANGE 25O ' = IRD >212um/g E : 200 o I RD> 150u m/g y= x (r2=0.65, n=55) Q 150 E 100 E 1 o e,! 50 A OO - IO Depth (mcd) IRD >212 gm/gram bulk sediment Figure 3. Comparison of ice-rafted debris (IRD) size fractions. (a) The number of IRD >212 Frn/g bulk sediment (solid circles) and of IRD > 150 Frn/g bulk sediment (open circles) versus meters composite depth. Note the different y scales. (b) The number oflrd > 150 Frn/g bulk sediment versus IRD >212 Frn/g bulk sediment for values of 0-5 IRD >212 Frn/g bulk sediment. and the climate system in general. First, we note that the relationship between the appearance of IRD in the sediments and ice volume is very much like that observed for the late Pleistocene, indicating that this linkage has existed throughout 41 kyr and 100 kyr glacial periods. Second, we discuss the environmental conditions surrounding the first appearance of N. pachyderma sinistral in the North Atlantic, a biostratigraphic datum for the North Atlantic. Third, we discuss the relationship between glacial IRD input and deep water formation in the North Atlantic. In particular, we demonstrate that while North Atlantic and sub-arctic thermohaline overturning continued through late Pliocene-early Pleistocene glacial periods, there was still substantial variability in deep water source at our site associated with IRD inputs. Finally, we describe how glacial IRD events in the late Pliocene and earliest Pleistocene are comparable in structure and timing to Dansgaard- Oeschger events as they are expressed in late Pleistocene sediments, and we note that there is some indication of variability on 1-2 kyr timescales within interglacial intervals. waters and the positioning of a cold frontal system south of our Gardar Drift site (Figures 4b-4d). When the front is south of our site, IRD input events came and went, but foraminiferal abundances remained low (Figure 4b), indicating that major IRD inputs are episodic, while the frontal system continually holds its position south of this site throughout maximum glaciation. Thus, during our time intervals, the location of the Arctic front dictated where IRD could be deposited in the sediments, while a distant process, such as surging at ice sheet margins, may have controlled the appearance and timing of IRD peaks in the sediment. The large IRD peaks we observe during glacial onset and deglaciation support this view, suggesting the movement of a zone of maximum melting over the site (Figure 4b) N. pachyderma Abundances and the N. pachyderma Acme Zone While the history of N. pachyderma is not the major focus of this paper, we can briefly touch upon the role that changing environments in the high-latitude North Atlantic may have played 4.1. Appearance of IRD in the Sediments and Ice Volume in the evolution of this species. The switch in dominance of N. Within the two intervals, IRD appeared in the sediment when- pachyderma dextral to N. pachyderma sinistral at 1.8 Ma ever C. wuellerstorfi 6 80 values exceeded %0, close to the ( Ma in our record) (Figure 4e) is a foraminiferal stratimaximum C. wuellerstorfi 6180 reached within 41 kyr glacial- graphic datum for the North Atlantic [Berggren et al, 1995]. Our interglacial cycles in the late Pliocene and early Pleistocene (Figure data indicate that the transition occurred during glacial stage 64 2). This relationship between IRD and C. wuellerstorfi 80 is (Figure 4), consistent with earlier work by Berggren et al. [1995]. comparable to the ice volume threshold proposed by McManus et The transition is very rapid as N. pachyderma appeared between al. [1999] for the late Pleistocene and observed in the early two samples with an age difference of <1 kyr in this chronology Pleistocene by Raymo et al. [1998]. The records we present (Figure 4e). It appeared in conjunction with a peak in IRD in the indicate that this threshold existed as far back as the late Pliocene. sediments at this site (Figures 4b and 4e), suggesting a linkage While the exact physical process controlling this threshold behav- between glacial icebergs, the southward movement of the Polar ior is not known, for the intervals we are describing, there is good front, and the arrival of this cold end-member species into the evidence to suggesthat it represents a movement of cold frontal Atlantic. Moreover, this switch occurred just after interglacial systems over this site. stage 65, which we have interpreted as an anomalously cool The presence of IRD in the sediments accompanied by low interglacial period (Figure 4). At a nearby site on the Feni Drift foraminiferal abundances correlates with the presence of Arctic (Figure 5) the rise of N. pachyderma sinistral occurred at the surface waters in the modem Nordic Seas [Johannessen et al, beginning of a,- 200 kyr interval of frequent IRD inputs during 1994] and has been used to map the areal extent of colder polar both glacial and interglacial intervals, suggesting that cold waters surface waters at the Last Glacial Maximum [Ruddiman and frequently occupied the high-latitude northeastern Atlantic during Mcintyre, 1981]. Following these correlations, the late Pliocene- this time period (Figure 5b) [Mc Intyre et al., 1999]. The early Pleistocene Gardar Drift data indicate that the small continuous interval of cooling, in conjunction with the freshwater increase in ice volume represented by the 3.5%0 threshold was inputs implied by the IRD data, may have helped establish N. accompanied by the southward movement of cold sub-arctic pachyderma sinistral as the cold water end-member species, a

5 1 MCINTYRE ET AL.: MILLENIAL-SCALE CLIMATE CHANGE 539 õ ' 16263, , 6 0,,., /////-,,, oo 400 o o- 800,-" b v o _ i_-z-_y:.--;::.;--.:;_.:,.;,3- oo o. 3o o -=':--'::=----='-=; / / 0"9' e. -- % s in is tra l % dextra I i! i Figure 4. High-resolution data from the early Pleistocene (MIS 63-66) and the late Pliocene (MIS 69-70). (a) The percent coarse fraction (Shaded line) and the magnetic susceptibility (solid line) with marine isotope stages marked along the top, (b) the number oflrd >212 [[m/g bulk sediment (on log and linear scales), (c) C. wuellerstorfi fi 80, (d) C. wuellerstorfi fi 3C, and (e) the percent N. pachyderma sinistral (solid circles) and dextral (open circles). The small numbers are the time between peaks in kyr. Magnetic susceptibility data are from Shipboard Scientific Party [1996]. niche it had occupied in other ocean since the Miocene [Hooper and Weaver, 1987] Deep Water Variations The record of variations in deep water chemistry in the late Pliocene and early Pleistocene preserved in C. wuellerstorfi fi 80 and fi 3C values indicates an interglacial circulation similar to that of the modem ocean, while glacial circulation is different. The Gardar Drift had the highest interglacial C. wuellerstorfi 80 and 3C values observed in the world ocean in the late Pliocene and early Pleistocene (Figure 6) [Mc Intyre et al., 1999] indicating that dense NADW source waters bathed this site during interglacial intervals. During the onset of glaciation, increasing C. wuellerstorfi fi 3C values indicate an increase the volume or in the preformed

6 540 MCINTYRE ET AL.' MILLENIAL-SCALE CLIMATE CHANGE 8O o Figure 5. The percent N. pachyderma sinistral and IRD/g at two sites in the high-latitude North Atlantic. (a) The percent N. pachyderma sinistral at Site 983 on the Gardar Drift (solid circles) and at Site 981 on the Feni Drift (open circles) and (b) the IRD >212 [[m/g bulk sediment at Site 983 (solid circles) and the IRD > 150 [[m/g at Site 981 (open circles). The Feni drift data are from Mc Intyre et al. [1999]. 15 3C value of NADW, but at the first appearance of IRD in the Overall, we lack sufficient spatial coverage of the intermediate sediments this strong NADW signal became extremely variable or and deep ocean during late Pliocene and early Pleistocene to fully disappeared entirely. After IRD deposition began in MIS 70, C. explain the distribution of water masses we have found thus far, but wuellerstorfi and 15 3C values fluctuated between higher we can make one essential point with respect to the millennialand 15 3C values indicative of NADW (Figures 4c and 4d) and scale variability in deep water formation. Whichever scenario [8 13 lower and 15 3C values, %0 and %0, respec- causes the lowering of 150 and 15 C values, the response of deep tively. After IRD deposition began in MIS 64, a rapid decrease in water formation to IRD inputs is the same in our intervals as it is in both and 1513C values and a drop in foraminiferal abundances the late Pleistocene. The presence of ice-rafted debris in the accompanied the first appearance of IRD (Figures 4b-4d). The sediments is accompanied by changes in the C. wuellerstorfi sparse C. wuellerstorfi and 15 3C values thereafter and 15 3C values indicative of a change in the deep water remained low until IRD deposition ceased at the end of MIS 64 found at the Gardar Drift. These data show that even as early as 1.9 (Figures 4b-4d). Thus, for the intervals described here, when Ma, the Gardar Drift site was at a depth and location sensitive to NADW was present, there was no IRD in the sediment. When climatically driven variations in deep water. Furthermore, our data them was IRD in the sediment, the C. wuellerstorfi isotopic from the late Pliocene and early Pleistocene indicate that millensignature at the Gardar Drift suggests the increased presence of nial-scale IRD input events exist even though the deep water a low and 15 3C water mass. circulation pattern in the North Atlantic appears to have been quite One possible source of the low and 15 3C signal at the different. In this time period, nutrient-depleted waters characteristic Gardar Drift is an increased admixture of either Antarctic deep or of NADW occupied depths below 2000 m throughouthe North intermediate water, but such a conclusion is not borne out by other Atlantic, suggesting that NADW formation continued in glacial data from this time period. ODP site 929 at 4356 m depth on the periods [Raymo et al., 1989, 1990; Bickert et al., 1997; Mc Intyre Ceara Rise has monitored glacial AABW entering the North et al., 1999]. Thus it appears that the background state of North Atlantic for at least the last 2.6 Myr [Bickert et al., 1997]. Atlantic thermohaline circulation can vary considerably, while Contemporaneous C. wuellerstorfi and 15 3C values at the millennial-scale changes in circulation continue independently. deep Ceara Rise and at the Gardar Drift should be the same if both sites are bathed in AABW, but this is not the case. MIS 64 Gardar 6b). MIS 70 Gardar Drift 1513C values were close to those at the deep Ceara Rise, but Gardar Drift values were always higher by 0.5% o (Figure 6b). These inconsistenciesuggest that the water mass represented by the low and 15 3C values at the Gardar Drift was not AABW. Nor do we observed a comparable water mass at other intermediate depth sites in the Atlantic, at 2157 m on the Feni Drift (Figure 6) [Mc Intyre et al., 1999] or at 2311 m at the Rockall Plateau [Shackleton and Hall, 1984], where we would also expect to see upwelled AABW. Even a site in the Caribbean used to monitor GNAIW for the last 1 Myr [de Menocal et al., 1992] had substantially higher values than those at the Gardar Drift. Overall, the evidence suggests that the water mass at the Gardar Drift is not AABW and may not be Antarctic Intermediate Water but that it also was not seen at other intermediate depth sites within the North Atlantic. An alternate theory is that the low isotopic values we observe at the Gardar Drift are indicative of a localized water mass formed in the Nordic Seas or on nearby Iceland shelves. Mc Intyre et al. [1999] suggested that continuous sediment drifting at the Gardar Drift throughout glacial-interglacial intervals indicates that there was some continued overflow from the Nordic Seas into the Atlantic during late Pliocene-early Pleistocene glacial periods. The persistence of glacial C. wuellerstorfi values at the Gardar Drift higher than those at other sites in the Atlantic would confirm that there was still some thermohaline overturning in the Nordic Seas feeding these overflow waters. Periodic lowering of glacial Gardar Drift values in conjunction with IRD in the sediments could then be the result of a change in the mode of deep water formation in response to icebergs at the surface. Brine formation on nearby Icelandic shelves or in the Nordic Seas has been suggested to explain low benthic foraminiferal and 15 3C values at sites in the Nordic Seas and in the high-latitude Atlantic in the late Pleistocene [IS'dal et al., 1997, 1998; Dokken and Jansen, 1999]. An admixture of dense waters created via brine formation from surface waters with some component of meltwater 8 would explain the lowered 15 O values at the Gardar Drift during MIS 64 and 70. Under such a scenario, the origin of the low 15 C values could be a signature inherited from surface waters or from poorly ventilated deep waters from the Nordic Seas [Mc Intyre et al., 1999] Millennial-Scale Variations Drift values were very close to those at the Ceara Rise (Figure Throughouthe late Pliocene-early Pleistocene glacial intervals 6a), but Gardar Drift 1513C values were substantially lower (Figure we sampled, there are discrete peaks in IRD/g (Figure 4b). This

7 MC INTYRE ET AL.' MILLENIAL-SCALE CLIMATE CHANGE o o : [ee &e b Figure 6. Benthic foraminiferal 6 80 and 613C for multiple North Atlantic sites. (a) C. wuellerstorfi 6180 and 613C for Sites 983 (open circles), 981 (dashed line), and 929 (solid line) during marine isotope stages and (b) the C. wuellerstorfi 6180 and 6 3C at Sites 983 (open circles), 981 (dashed line), and 929 (solid line) during marine isotope stages Shaded bars indicate intervals when IRD was present in the sediments at Site 983. The Site 981 data are from Mc Intyre et al. [1999]. The Site 929 data are from Bickert et al. [ 1997]. IRD appeared rapidly, in <1 kyr, and most peaks last several kyr, the late Pliocene and early Pleistocene. The 2-4 kyr timing of although peak duration may be lengthened by bioturbation. The events is also comparable to the timing of events described by percent N. pachyderma increased for,.o 1.5 kyr prior to IRD events, Oppo et al. [1998]. The primary difference between glacial IRD suggesting a cooling of surface waters preceding these IRD events inputs in our intervals and those in the late Pleistocene that we (Figure 4e). When C. wuellerstorfi were sufficiently abundanto find no clear evidence for IRD events comparable to late Pleistomake isotopic analyses, IRD events were accompanied by short- cene Heinrich events. While the 5-10 kyr spacing of IRD events term decreases in C. wuellerstorfisotopic values, which we have within stage 70 is close to the temporal spacing of late Pleistocene suggested are associated with reorganization of deep water masses Heinrich events [Heinrich, 1988; Broecker et al., 1992; Bond et al., (Figures 4c and 4d). IRD input events have distinct periodicities in 1993; McManus et al., 1999], the amplitudes of these events are both glacial intervals reconstructed. IRD events reoccurred every smaller. The lack of Heinrich-scale IRD events in our late Pliocene 4 kyr within glacial stage 64 (Figure 4b) and were more chaotic and early Pleistocene intervals supports the idea that they are a during glacial stage 70, reoccurring every 5-10 kyr (Figure 4b). feature of larger late Pleistocene ice sheets. Duringlacial stage 70, low C. wuellerstorfi 6 3C peaks associated Given the evidence for millennial-scale variability during late with IRD events were interspersed with similar peaks that reoccur Pleistocene interglacials [Bond et al., 1995, 1997; Oppo et al., every 3 kyr (Figure 3b). While not an obvious cycle in the IRD 1998], we would like to be able to make similar observations in our record, it is possible that these deep water shifts represent other older time intervals. As in the Pleistocene, the amplitude of such IRD events that do not reach this site but do affect the formation of variations is small, but we have some hints of ongoing variability. deep waters that feed this site. After IRD inputs cease during deglaciation, C. wuellerstorfi These discrete IRD events, with their accompanying changes in maxima reoccurred every 3-4 kyr until minimum 6 80, i.e., surface and deep waters, resemble the expression of Dansgaard- minimum ice volume, was established. This suggests that while Oeschger temperature cycles as they are seen in late Pleistocene the cold frontal systems had passed north of this site, some sediments [Bond et al., 1993; Bond and Lotti, 1995; Rasmussen et variability was still occurring in the zone of deep water formation. al., 1996; Elliot et al., 1998]. The timing of these events is During interglacial stage 63 (Figure 4c), 0.4%0 increases the C. comparable to the longer cycles within glacial stage 3 [Dansgaard wuellerstorfi 6 80 occurred every,.o2 kyr, suggesting a,-o2øc et al., 1993], which is similar in ice volume to glacial maxima in warming. A smaller decrease in 6 3C accompanied thes events.

8 542 MCINTYRE ET AL.: MILLENIAL-SCALE CLIMATE CHANGE The amplitude and timing of these events is comparable to those nance and spatial distribution in order to reconstruct the long-term observed by Oppo et al. [1998] during the late Pleistocene at the evolution of individual ice sheets. Feni Drift. While the evidence is limited, it does hint that millennial-scale variations in climate occurred throughout glacial- We sought to examine the nature of millennial-scale variability in the 41 kyr world because it lacked the nonlinearity of 100 kyr interglacial cycles in the intervals studied here and that this process glacial forcing and response. As presented in this study and by is one that has been continuously at work for at least the last 2 Myr. Raymo et al. [ 1998], rapid, episodic glacial inputs oflrd appear to 5. Conclusions be a feature of 41 kyr glacial-interglacial cycles. While the areal extent of such events needs to be verified by future work, the foremost conclusion has to be that such events occurred in the Overall, the late Pliocene and early Pleistocene glacial-interglapresence of enlarged Northern Hemisphere ice sheets regardless of cial cycles investigated here are similar in many ways to those in the mean, long-term state of the climate. These events cannot be the late Pleistocene. As ice volume increased, surface water cooled, purely a function of large late Pleistocene ice sheets nor of the and icebergs appeared in the North Atlantic. Within glacial nonlinear responses of these ice sheets to incoming solar insolaintervals, inputs of ice-rafted debris were episodic in nature and tion. Instead, the climatic variability represented by IRD events is their timing was comparable to that of Dansgaard-Oeschger events driven by some component of the system that is consistent in the late Pleistocene. As in the late Pleistocene, the presence of throughout at least the last 2 Myr. While it could be related to IRD in the sediments was accompanied by reorganizations of deep instabilities inherent in ice sheets of any size, the similarity in the water masses. Interglacial intervals were more stable, with no periodicity of these events regardless of ice sheet size may indicate evidence for IRD, but with there was still deep water variability the importance of forcing outside the ice sheet itself. comparable in amplitude and timing to interglacial variations observed in the late Pleistocene. While the overall structure of Acknowledgments. Ocean Drilling Program samples were supplied events during glacial-interglacial cycles in our time period is through the assistance of the National Science Foundation. This manuscript similar to cycles in the late Pleistocene, we find no strong evidence was improved by conversations with J. T. Andrews and the comments of for IRD events comparable in timing or amplitude to late Pleisto- two anonymous reviewers. At UC Santa Cruz, this research was improved through discussions with L. Anderson, M. Wara, and K. Faul. We thank R. cene Heinrich events, which leads us to suspect that Heinrich Hoag, H. Lao, E. Torlyn, and C. Wilson for sample preparation and M. events may be a function of longer late Pleistocene glaciations and Flower and D. Neenan for sample preparation and analysis. This work was larger late Pleistocene ice sheets. This second finding would be funded by NSF OCE to M.L.D. and by NSF OCE to strengthened by future work comparing variations in IRD prove- A.C.R. References Alley, R. B., and D. R. MacAyeal, Ice-rafted debris associated with binge/purge oscillations of the Laurentide ice-sheet, Paleoceanography, 9, , Andrews, J.T., Icebergs and iceberg rafted detritus (IRD) in the North Atlantic: Facts and assumptions, Oceanography, 13, , Behl, R. J., and J.P. 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