Here, Xa hem is the hemispheric mole fraction, x3' arethe measured atmospheric mole fractions of thecompound, na and nhem are the total number ofmoles in the atmosphere and hemisphere, respec-

tively, (na= 1 .77 X 1020 mol), and E, and E0- 1 are,

respectively, the latitudes at the beginning and atthe end of each incremental interval. Constant mix-ing ratios were assumed between 45.30 and theNorth Pole (11.55 ppt) and between -53.3° andthe South Pole (8.25 ppt).

18. If we change the ITCZ from 00 to 50N, the hemispher-ic means change from 11.1 to 11.3 ppt for the NHand from 8.5 to 8.6 ppt for the SH.

19. The saturation anomaly (in percent) is defined as thedeparture of the observed dissolved amount fromequilibrium:

Ag 100( pP) (3)

where pw and Pa are the partial pressures of the gasin water and air, respectively. Because inert com-

pounds like CFC-1 1 do not react with seawater, thedepartures of their surface partial pressures fromequilibrium are directly attributable to physical pro-

cesses, such as warming and cooling, advection, ormixing, and can be used under certain conditions tofactor out physical effects for gases that are notconservative in the surface ocean (15). In Eq. 1, a netsaturation anomaly is calculated by subtracting theCFC-1 1 saturation anomaly from the observedanomaly of CH3Br.

20. H. J. McLellan, Elements of Physical Oceanrography

(Pergamon, Oxford, 1965). Nitrous oxide is a strongindicator for upwelling regions and was measuredduring this cruise in both air and surface water with aGC-ECD system.

21. J. H. Ryther [Science 166, 72 (1969)1 assumed 9.9%for coastal waters including offshore areas of highproductivity and 0.1% for regions of intense upwelling.H. W. Menard and S. M. Smith [J. Geophys. Res. 71,4305 (1966)] showed that 7.5% of the ocean is shal-lower than 200 m, the classical definition of coastalwaters. Because we found positive CH3Br saturationanomalies in waters deeper than 200 m and haveincluded less intense areas of upwelling in our esti-mate, we assigned 10% of the ocean to each of theseregions.

22. S. Elliot and F. S. Rowland, Geophys. Res. Lett. 20,1043 (1993); W. Mabey and T. Mill, J. Phys. Chem.Ref. Data 7, 383 (1978).

23. One residual standard deviation from a fit of atmo-spheric data to a locally weighted, statistical smooth-ing (LOESS) algorithm was 0.6 ppt [W. S. Cleveland,J. Am. Stat. Assoc. 74, 829 (1979)].

24. E. Kossina, Inst. Meereskunde Veroff. Geogr.-Naturwiss. 9, 70 (1921).

25. The partial atmospheric lifetime mo. with respect tooceanic loss is computed from

1 AKw zks + Vk4

To HM .zk, + Kw + VkJy

with the area of the ocean A = 361 x 1012 m2, themean depth of the mixed surface water layerz = 75

m [see Y.-H. Li, T.-H. Peng, W. S. Broecker, H. G.Ostlund, Tellus B 36, 212 (1984)], the reciprocalsolubility H for each oceanic regime (Table 2),the mass of the atmosphere M = 1.77 x 1020 mol,the air-sea exchange coefficient KW (Table 2), andthe in situ degradation rate ks (1). The term fordownward transport \kp is calculated from

an in-situ degradation rate k, at an estimated ther-mocline temperture and the diffusivity D. given byLi et al.

26. Global Ozone Research and Monitoring Project(WMO Rep. 25, World Meteorological Organization,Geneva, 1991). Mellouki et al. (12) included strato-spheric losses in their estimate, whereas Zhang et al.(13) did not. We used a stratospheric lifetime of 50years from the WMO report to adjust the number ofZhang et al.

27. J. M. Lobert et al., unpublished results from Po-larstein cruise ANT Xl1-1, October to November1994.

28. R. Wanninkhof, J. Geophys. Res. 97C, 7373(1992).

29. This study was funded by the Methyl BromideGlobal Coalition and the Atmospheric ChemistryProject of NOAA's Climate and Global Change Re-search Program. We thank M. R. Nowick for elec-trical expertise, E. Saltzman for critical review, theNOAA-Pacific Marine Environmental Laboratorygroup for their support, and the helpful crew of theNOAA Ship Discoverer.

27 July 1994; accepted 27 December 1994

Iceberg Discharges into the North Atlantic onMillennial Time Scales During the Last Glaciation

Gerard C. Bond and Rusty Lotti

High-resolution studies of North Atlantic deep sea cores demonstrate that prominentincreases in iceberg calving recurred at intervals of 2000 to 3000 years, much more

frequently than the 7000- to 1 0,000-year pacing of massive ice discharges associatedwith Heinrich events. The calving cycles correlate with warm-cold oscillations, calledDansgaard-Oeschger events, in Greenland ice cores. Each cycle records synchronousdischarges of ice from different sources, and the cycles are decoupled from sea-surfacetemperatures. These findings point to a mechanism operating within the atmospherethat caused rapid oscillations in air temperatures above Greenland and in calving frommore than one ice sheet.

The pioneering studies by Ruddiman andhis colleagues of the North Atlantic's gla-cial record demonstrated that the amountsof drifting glacial ice were closely tied to

Milankovitch-forced growth and decay ofNorthern Hemisphere ice sheets (1-3).Subsequent work on North Atlantic sedi-ment from the last glacial cycle revealed inaddition a higher frequency of iceberg dis-charges, about every 7000 to 10,000 years,

that have come to be known as Heinrichevents (4, 5). A leading explanation for theHeinrich events is surging of ice in HudsonStrait (6). Glaciological models, however,do not readily explain two recent findings.One is that five of the six Heinrich events

occurred at the culminations of progressivecoolings and were followed precipitously by

warmings to almost interglacial tempera-tures (7). The second is that the youngestHeinrich event was nearly synchronouswith rapid disintegration of the Barents icesheet (8). In this paper we report resultsfrom high-resolution analyses of deep sea

cores that bear further on mechanisms forice-rafting events in the North Atlantic.

One of these findings is the presence ofice-rafting cycles in the intervals betweenHeinrich events. We identified these cycles,defined by variations in lithic concentra-tions [see also (9)], in two cores, DSDP site609 and VM23-81 (Figs. 1 and 2). Therecords from the two cores match well (Fig.2). Because the cores are separated by nearly50 of latitude and are in different deposition-al environments, the correlation is not an

artifact of local ocean surface or sea floorprocesses. Including the Heinrich events,there are 13 ice-rafting cycles in the interval

SCIENCE * VOL. 267 * 17 FEBRUARY 1995

from 10,000 to 38,000 radiocarbon years ago,

yielding an average cycle duration of be-tween 2000 and 3000 years (Fig. 2).

The evidence of a 2000- to 3000-yearice-rafting cycle in the North Atlantic is anintriguing result for it recalls the spacing ofthe Dansgaard-Oeschger temperature cycles(10), so convincingly documented by themeasurements of 8180 in ice from the re-

cent drilling at Summit, Greenland (11).To investigate this relation further, we

compared the 518Q record in the GRIP icecore with lithic cycles in VM23-81 (Fig. 3).We first pinned the two records at the peakYounger Dryas cooling and then stretchedthe deep sea record until H4 matched theequivalent level in the ice identified byBond and others (7). Below H4 the deep sea

record was clipped and stretched again untilthe oldest peak in both records matched.Although the correlation was achievedwith fewer clippings and stretchings than in(7), it is essentially the same. Between theYounger Dryas and H2, GRIP and GISP2ages (11, 12) agree, within error, with 14Cto calendar age conversions (13). In addi-tion, a sample of Acropora palmata fromBarbados has a 14C age of -27,000 years

ago and a UfTh age of 30,500 years ago

(13), corroborating the GISP2 chronologywe infer for the marine record (Fig. 3).

Our correlation demonstrates that allbut one Dansgaard-Oeschger cycle corre-

sponds to a lithic peak in the marine record(Fig. 3), indicating a close link between theshifts in air temperatures above Greenlandand the ice-rafting cycles. Lithic peaks a, b,and c (Fig. 3), however, occur within thepuzzling interval that lacks Dansgaard-

1005

Lamont-Doherty Earth Observatory of Columbia Univer-sity, Palisades, NY 10964, USA.

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Oeschger cycles. Apparently, in that inter-val the forcing mechanism was more strong-ly imprinted in the ice-rafting signal than inGreenland ice.

Because most of the lithic cycles be-tween the Heinrich events occur at times oflowered air and sea surface temperatures, itis possible that they simply reflect the in-creased survivability of icebergs as oceansurface temperatures dropped [for example,(3, 5)]. If that were the case, the pacingsand amplitudes of the lithic cycles and ofocean surface temperatures should be simi-lar. The structure of the two records, how-ever, is clearly discordant. For example, theice-rafting record lacks the distinct longer-term rampings in sea surface coolings, suchas began about 33,000 and 38,000 14C yearsago (Fig. 3). Lithic cycles f, g, and h and thetwo below H4 clearly lag the increase in seasurface coolings as well (Fig. 3). Further-more, just below and within H3, lithic con-centrations are relatively low even thoughthe interval is saturated with the polar fora-minifera Neogloboquadrina pachyderma (leftcoiling). In contrast, in other intervals sat-urated with that foraminifera, lithic con-centrations drop abruptly to near-ambientlevels, such as in the interval containingcycles a to c, between cycles H2 and d, andbetween cycles H3 and f (Fig. 3). Becausechanges in ocean surface circulation wouldlikely be accompanied by changes in surfacetemperatures, we rule out any mechanismrequiring significant changes in iceberg tra-jectories. We conclude, therefore, that theamount of glacial ice discharged into theNorth Atlantic Ocean increased suddenlyevery 2000 to 3000 years.

To investigate further the origin of thelithic cycles, we measured the abundancesof 15 lithic grain types (14) in DSDP 609and VM23-81 at a resolution of between300 and 500 years. We found distinct peaksin three types; fresh, basaltic glass derivedfrom Iceland (15), grains with hematitecoatings (mostly quartz and feldspar) de-rived from any of the extensive red beddeposits in the region (Fig. 1), and detritalcarbonate grains that came mainly fromcarbonate rocks in eastern and northernCanada (5, 16) (Fig. 1). The peaks riseabove an ambient background composedmostly of quartz and feldspar and less thanabout 10% of grains with hematite coatings.

In addition to the peaks in detrital car-bonate at the levels of the Younger Dryasand Hl, H2, and H4 already documented(5), we found a narrow peak of carbonategrains at the level of H3 (Fig. 4). This is incontrast to the results of earlier work atlower resolution, which suggested that H3lacked detrital carbonate grains in the east-em North Atlantic (5). Compositions oflithic grains in a high sedimentation ratecore, EW9303-3 1, on the turbidite-free

1006

crest of Orphan Knoll in the southwesternLabrador Sea (Fig. 1) are further evidencethat detrital carbonate-bearing ice exitedthe Labrador Sea during the Younger Dryasand during Heinrich events 1, 2, 3, and 4(Fig. 5) [see also (17)]. Isotopic results ofGrousset et al. (18) pointed to sources inEurope for sediment in H3, but their bulkanalyses may reflect the compositions ofnon-ice-rafted fine-grained material. Hein-rich event 3 differs from the other Heinrichevents, however, in that it has an unusuallylow concentration of lithic grains.

The other two grain types, basaltic glassand hematite-coated clasts, form a series ofprominent compositional cycles that corre-spond directly to all but one of the lithicpeaks (Fig. 4). The increases in ice-rafteddebris from specific sources are further evi-

dence against ocean surface coolings as thesole cause of the lithic cycles. More impor-tantly, the evidence points to nearly syn-chronous changes in the rates of icebergdischarges from the sources of the basalticglass and hematite-coated grains. In Hein-rich events 1, 2, and 4, the abundances ofbasaltic glass and hematite-coated grainsfirst increase, then drop sharply to low val-ues at the levels where detrital carbonategrains begin to increase (Fig. 4). The pic-ture that emerges is of recurring increases inthe discharges of icebergs from the sourcesof basaltic glass and hematite-coated grains;at the time of Heinrich events 1, 2, and 4,sediment deposited from those icebergs ap-parently was diluted to trace amounts bythe huge additions of sediment from iceexiting the Labrador Sea.

Gulf ofFig. 1. Map showing the modern circulation Hudson Strait St. Lawrencein the North Atlantic Ocean and NordicDenmark Strait 75-55,56 VM116-2403Labrador Seas (dashed arrows), locations of VM28-14 -cores discussed, and the distribution of rock 2types that are potential sources of detrital car- 22 - -

bonate and hematite-coated grains (data Zo A L

from 28) that we have identified in the lithic 0° 10 10 50 100cycles in Fig. 4. The histograms show thenumber frequency of percentages of hema- % hematite-coated grainstite-coated grains from 63 to 150 mm acrossin samples from the indicated cores. For VM28-14, grains were counted in samples containing each of thepeaks in basaltic glass (Fig. 5). For the cores near Hudson Strait, grains were counted in samples every 10cm beginning 60 cm above H1, then from between H1 and H2, and from between H2 and H4(?). In coresfrom the Gulf of St. Lawrence, grains were counted in samples from within and between the "brick redlayers" (29); the uppermost of those layers has an AMS radiocarbon age of 13,300 years (24). The sampledinterval probably extends to close to Termination 1. As explained in the text, we measured the abundancesof hematite-coated grains in these places as an aid in identifying possible sources of those grains inice-rafted sediment in the eastern North Atlantic.

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The Icelandic source of the basaltic glass in that core are much larger than in the ago, H2, and H3 that are consistent withis confirmed by the presence of correlatives eastern North Atlantic (Fig. 5). In addition, the geochemistry of rocks in the rift zone inof the peaks in dark glass in a core much microprobe analyses on basaltic glass from Iceland, perhaps mainly in the northerncloser to Iceland, VM28-14, and by evi- VM28-14 indicate compositions at the lev- part of the island (20). Although it is pos-dence that peak abundances of basaltic glass els of the Younger Dryas, HI, 16,500 years sible that the dark glass at the level of the

Fig. 2. Lithic and foraminiferal concentrations, in Vema 23 - 081 DSDP site 609numbers of grains >150 pum per gram of sediment, -and the percentage of lithic grains measured relative AMS 140 datesto the sum of lithic grains and foraminifera >150 pm 10 YD i10in sediment from DSDP 609 and VM23-81 (Fig. 1).The records from the two cores face in to emphasize +the correlation. Radiocarbon ages given by a (+) and Hithe age models are from (5, 7). Both cores reveal 15- -15prominent cycles in lithic concentrations at the levels aof Heinrich layers and in-between these levels as well. J bThe concentrations of foraminifera and the percent- cages of ice-rafted debris were originally part of the & 20 20definition of Heinrich events in (4, 5, 30). Low foram 0 H2zones, which are so prominent in DSDP 609, are less - ddramatic in VM23-81 however, and are absent at the 0 e6level of Heinrich event 4. The percentages of lithic ° 25 H3 2grains delimits the Heinrich events as they were orig- >inally defined (30), through either a decrease in foram-*iniferal concentration, an increase in lithic concentra- 3 30tion, or a combination of both. The record of lithic 9gpercentages does not, however, delimit all of the ice- hrafting cycles. Hence, lithic concentrations are the ibest indicator of ice-rafting events. 3s H435

0 2.5 0 500 1 4 2 100 50 10 0103 formcwg % nlc grains 103 lItNc gruns/g 102 lIthc gralna/g % lihc grains 103 foramslg

Fig. 3. Comparison of the oxygen Summit ice core (GRIP) VM23-081isotope record and age model forthe GRIP ice core, Summit, Green-land (11), with measurements of 12 -1-2 *_------Y10- --lithic concentrations (as in Fig. 2) _;and percentages of the planktic_foraminifera Neogloboquadrina 1 -16 1 Hi--pachyderma (left coiling), a proxy i--15for surface water temperatures, in _aVM23-81 (7). That foraminifera to- 0 Interstadial bday lives in waters <100C and a,20 number

ccomprises about 95% ofthefauna -- e 2 -20at summer temperatures of <5C. 0 --24 -n _---- H2------Age model for the marine record is-the same as in Fig. 2. Coring sites S 24 , -.- --d--are located in Fig. 1. Cycles be- 'S '28 D -2------5------tween the Heinrich events are given 0 4 H3-----e--- -

letters to aid their description in the ¢ 28 etext. We find a striking match be- ° _32-f5 =tween the lithic concentration cy- E _ 6 306 ~~~~~~~~~~~~~~~~~~~0cles and the temperature cycles in - ----g------- 0

the ice core; the match of the lithic o7 32cycles to the ocean surface tem- < 36.peratures, however, is much poor- 8 3er. We included the GISP2 time -8

-4-3scale derived from layer counting to 3 -4041,000 years for comparison with 10the GRIP time scale and the 14C -

time scale. The GISP2 time scale 11

was transferred into the GRIP 4record at the sharp interstadialboundaries, which are precisely lo - 0cated in both ice core records (11, -36 -38 -40 -42 0 0.5 1 0 50 10012), and then the ages were inter- a 180 (per mil) 103 lithic grains/g % N. pachyderma (sp.)polated between those boundaries.The progressive difference in ages, reaching about 10% at 40,000 calendar years ago, is consistent with error estimated for the ice core dating (31).

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Younger Dryas came from glacial icebergs reveals no evidence of ash falls at the time Atlantic during the last glaciation. One ofand sea ice covered with debris from the of each Dansgaard-Oeschger cycle (12). these cores is VM28-14 in the southerneruption which produced the Vedde Ash To narrow the sources of hematite-coat- Denmark Strait (Fig. 1). This coring site(15), a similar origin is hardly likely for ed grains we measured the abundances of monitors the passage of icebergs from Eastevery one of the earlier cycles (21). Also, those grains in cores that lie along the likely Greenland, Svalbard, around the Arcticthe sulfate record from the GISP2 core major iceberg trajectories into the North Ocean, and western Europe, as suggested by

Fig. 4. Measurements of the VM23-81 DSDP 609abundances of three grain types,detrital carbonate, basaltic glass, 10 - ---- YD -10and hematite-coated grains, in e vsediments from DSDP 609 and CVM23-81 (located in Fig. 1). Agemodels are from Fig. 2. Lithic cy- H1dles labeled as in Fig. 2. Peak 15 15abundances in both basaltic glass ------a --- -------

and hematite-coated grains oc- bcur at the peak concentrations of

c

lithic grains in each lithic cycle in .2 20both cores. At the Heinrich events A H2and the Younger Dryas, peak --------

abundances in a third grain type, - d -

detrital carbonate, are present as e -250 25 25

well. We found a narrow peak in 0'5 -- -H3

detrital carbonate at the level of XH3 in both cores, which did notappear in much coarser analyses ------- f Ldone previously (5). To measure 30- -- - - 30these grain types we first embed- 9ded a small amount of the 63- to h i150-pgm fraction from each sam-ple in glycerin on a glass slide. Us- 35 -35ing a petrographic microscope we . H4,line-counted 200 lithic grains toobtain a measure of the percent-age of each tracer by number. We rX- m ,'~,--,T XF.r lT-' lag o echtrce b nmbr.We0 10 0 10 0 10 0 1 0 20 5 15 0 10 2 4chose a relatively small grain size010 0 0 0010 25 50 02 4

% detrital % hematite- % basaltic 10 lithic % dltrital % hamatite- % basaltic103 llthicfor greater accuracy in petro- carbonate coated grains glass gralnslg carbonate coated grains glasa grainsiggraphic identification; we foundthat no compositional bias was introduced from counting smaller grains, however, by comparing the results with grain counts in the >1 50-pgm fraction acrosscompositional cycles in VM23-81.

Fig. 5. Lithic cycles in VM23-81 (from Fig. 2) com EW9303-31 VM28-14 VM23-81pared with abundances of grain types measured in a Orphan Knoll Denmark Strait Eastern North Atlanticcore from the southern part of Denmark Strait and Depth (cm) Depth (cm)

200 100 0 300 200 100 0from Orphan Knoll, an elevated block of crust in the 10 _30 200 100____southwestern Labrador Sea (Fig. 1). Accelerator ra- 10 - -------- -------------- -YD--- =10diocarbon ages in Orphan Knoll from Zurich; in VM28- - 514 the upper four accelerator dates are from S. Leh-man (32) and the rest are accelerator dates from Zur- -.--ich. A reservoir correction of 400 years has been 15 -asubtracted from the 140 measurements. Peak abun- +,dances in basaltic glass in VM28-14 match each lithic Age/depth 4. b

peak in VM23-81, demonstrating a remarkable de- 20 . model c 20gree of coherency between the two records. In the -- *-----------Orphan Knoll core, our measurements of detrital car- Age/deptbonate confirm the presence of a source of detrital 0 mo Ae/ 25carbonate-bearing icebergs in the Labrador Sea, in- , 25- -25

cluding that for H3. -' ---t-L

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~f0305- I_30

35- ,( ------- -------- H4--- 3

0 10 20 0 10 20% detrital % hematite-carbonate coated grains

'0 20 40 60 0 1% basaltic glass 103 lithic grains/g

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M~ i---- __M mow i W~ w, .INUMUMH,.-,,.,Mayo~. ffill--mkatsklm=M 1wmmw wahom' boil~' WO

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the present-day circulation (Fig. 1) and re-constructions of the full glacial circulation(1-3, 22). Cores HU89-9 and HU75-55, inthe western Labrador Sea off Hudson Strait,lie beneath a second major pathway, that ofLaurentide ice exiting Hudson Strait to-gether with any ice from northern Canadaand Greenland (Fig. 1). Finally, sitesVM17-181, VM17-203, and VM16-240, inand just offshore of the Gulf of St. Lawrence(Fig. 1), monitor the passage of icebergscalved from another large outlet of Lauren-tide ice (23).

The maximum abundances of hematite-coated grains are about 10% in the southernDenmark Strait and in the Labrador Sea(Fig. 1), but they are much greater, up to95%, in and near the Gulf of St. Lawrence.Hence, because the percentages of hema-tite-coated grains in each lithic peak inVM23-81 and DSDP site 609 are wellabove 10% (Fig. 4), discharge of ice fromthe Gulf of St. Lawrence must have in-creased during each lithic cycle (24). Sim-ilarly, in the core from Orphan Knoll, hem-atite-coated grains exceed 10% at levelscorrelative with some, but not all of thecycles in the eastern North Atlantic (Fig.5). Ice from the Gulf of St. Lawrence per-haps did not drift to Orphan Knoll, and redbeds in Newfoundland could have suppliedthe required number of hematite-coatedgrains (Fig. 1).

Our compositional data do not rule outiceberg discharges from the Greenland, Bar-ents, and Scandinavian ice sheets. We havelimited our analyses to a few key and easilyrecognizable grain types. In addition, sedi-ment-laden icebergs from more northerlysources may not have reached the easternNorth Atlantic.

The results of this study suggest thatevery 2000 to 3000 years, coincident withthe Dansgaard-Oeschger coolings, ice with-in the Icelandic ice cap and probably withinor near the Gulf of St. Lawrence underwentnearly synchronous increases in rates ofcalving. Hence, it is improbable that thecalving cycles were forced by an internalprocess analogous to the surging of a glacier.Instead, the cycles must reflect the opera-tion of climate upon unstable ice, probablywithin ice streams or ice shelves. Thosecycles in which the Dansgaard-Oeschgercoolings lead the increases in ice-rafted de-bris (Fig. 3) suggest that iceberg dischargesoccurred when air temperatures droppedacross a critical threshold.

Although the Heinrich events wereoriginally thought to be the dominant fea-tures of the North Atlantic's deep searecord, our findings demonstrate that theyare superimposed on the rhythm of theclimate-forced ice-rafting events. The Hemn-rich events reflect another, slower rhythmtied to the massive discharges in Hudson

Strait. An unexpected result of our compo-sitional measurements is the evidence thatthe two rhythms may be linked. Each dis-charge of detrital carbonate-bearing icefrom Hudson Strait lags just slightly thedischarge from the other two sources (Fig.4). Perhaps sudden Dansgaard-Oeschgercoolings triggered the discharges in HudsonStrait at nearly the same time as dischargesfrom the other sources. Alternatively, thetrigger could have been the rise of sea levelthat must accompany each 2000- to 3000-year flood of icebergs into the ocean (25).The sea level mechanism is consistent withthe slight lag in the discharges from HudsonStrait. Moreover, the amount of ice enteringthe ocean each time might have been quitelarge if other ice sheets farther north dis-charged icebergs at the same time. In eithercase, a binge-purge model can explain theslower pacing of Heinrich events as a con-sequence of the time it took for HudsonStrait ice to recover from a massive dis-charge and reach an unstable condition inwhich another discharge might be triggered(6).

Our findings also bear on the question ofwhat caused the sudden changes in glacialclimates revealed by the Dansgaard-Oeschger cycles. Ocean circulation modelssuggest that injections of fresh water intothe North Atlantic can alter the ocean'sheat conveyor and cause abrupt shifts inNorth Atlantic sea surface and air temper-atures (26). A possible source of fresh wateris the discharge of icebergs from surging icesheets. Yet the iceberg discharges we haveidentified, including those from HudsonStrait, appear to have been triggered byclimate or a climate-related mechanismrather than vice versa. Perhaps a recurringinternal process in another ice sheet dis-charged enough ice each time to alter theNorth Atlantic's heat conveyor, and theresulting ocean surface coolings triggeredthe iceberg discharges we have identified. Ifso, a large ice sheet, perhaps in Greenland,in the Barents Sea, or in Scandinavia, musthave collapsed every 2000 to 3000 years. Itis also possible, though, that the Dansgaard-Oeschger coolings reflect a recurring cli-mate-driven mechanism that triggered dis-charges from ice sheets, and the impact ofmeltwater on the heat conveyor only am-plified the temperature shifts. In this regardit is interesting that climate shifts withpacings not unlike those we have identifiedfor the ice rafting events appear to bepresent in a number of Holocene climaterecords, including advances of mountainglaciers in both hemispheres (27).

REFERENCES AND NOTES

1. W. F. Ruddiman, Geol. Soc. Am. Bull. 88, 1813(1977).

2. W. F. Ruddiman and A. Mcintyre, Palaeogeogr.

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Palaeoclimatol. Palaeoecol. 35, 145 (1981).3. F. W. Smythe Jr., W. F. Ruddiman, D. N. Lumsden,

Mar. Geol. 64,131 (1985).4. W. S. Broecker et al., Chim. Dyn. 6, 265 (1990).5. G. Bond et al., Nature 360, 245 (1992).6. D. R. MacAyeal, Paleoceanography 8, 767 (1993);

ibid., p. 775; R. B. Alley and D. R. MacAyeal, ibid. 9,503 (1994).

7. G. Bond et al., Nature 365, 14 (1993).8. J. F. Bischof, Mar. Geol. 117, 35 (1994).9. Lithic concentration is defined as numbers of lithic

grains > 150 pum per gram of sample. Our measure-ments of lithic concentrations from DSDP 609 differfrom earlier ones made in the same core (4). For theearlier measurements, samples were washed to pre-serve all foraminifera; this unavoidably left much ofthe fine lithic fraction in aggregates coarser than 150gpm. Our revisions to the lithic data were made byrewashing and recounting samples containing fine-fraction aggregates. The correlation of the DSDP 609lithic record with that from VM23-81, in which aggre-gates of fine fraction are rare, attests to the validity ofthe lithic records in both cores.

10. W. Dansgaard, H. G. Clausen, N. Gunderstrup, S. J.Johnsen, G. Rygner, in Greenland Ice Core: Geo-physics, Geochemistry, and the Environment, C. C.Langway, H. Oeschger, W. Dansgaard, Eds. (Geo-phys. Monogr. 33, American Geophysical Union,Washington, DC, 1985), pp. 71-76.

11. W. Dansgaard et al., Nature 364, 218 (1993).12. P. A. Mayewski et al., Science 263, 1747 (1994).13. E. Bard et al., Radiocarbon 3, 191 (1993).14. The grain types counted are: quartz, feldspar, volca-

nic rock fragments (acidic and basaltic where thedistinction could be made), metamorphic rock frag-ments, carbonate rock fragments, siliclastic rockfragments, chert, glauconite, mafic grains (horn-blende, pyroxene, and olivene), dark basaltic glassand palagonite, clear rhyolitic glass, and hematite-coated grains. Hematite-coated grains are definedas grains with 5% or more hematite staining. Theabundances of hematite-stained grains do not in-clude counts of rhyolitic and basaltic glass and red-dish basalt clasts which could come from oxidizedflow tops.

15. M. N. Bramlette and W. H. Bradley, U.S. Geol. Surv.Prof. Pap. 196-A, 1 (1941); G. P. L. Walkeretal., Site115, Init. Rep. Deep Sea Drill. Proj. Red 12, 361(1972); W. F. Ruddiman and L. K. Glover, Geol. Soc.Am. Bull. 83, 2817 (1972); J. Sjoholm et al., J. Quat.Sci. 6, 159 (1991).

16. J. T. Andrews and K. Tedesco, Geology 20, 1087(1992); J. T. Andrews, H. Erlenkeuser, K. Tedesco,A. Aksu, A. J. T. Jull, Quat. Res. 41, 2(1994).

17. The peak in grains of detrital carbonate we haveidentified at the level of H3 cannot be as old as theevent at -29,000 years assigned tentatively to H3 ina core from the flank of Orphan Knoll (19).

18. F. E. Grousset et al., Paleoceanography 8, 175(1993).

19. C. Hillaire-Marcel, A. DeVernal, G. Bilodeau, G. iNu,Can. J. Earth Sci. 31, 63 (1994).

20. K. Gronvold, personal communication.21. Moreover, eruptions correlating so precisely with the

Dansgaard-Oeschger cycles are extremely unlikely.22. Although ice may have drifted east as well as west of

Iceland, Ruddiman (1) regarded the enormousamounts of ice-rafted debris in the vicinity of VM28-14 as evidence that ice along both routes convergedin the southern part of the Denmark Strait. Ruddiman(1) also discounted the importance of ice flowingdirectly southward from Scandinavia into the subpo-lar North Atlantic owing to the strength of the WestWind Drift.

23. G. H. Denton and T. J. Hughes, in The Last Great IceSheets (Wiley, New York, 1981).

24. We have an AMS radiocarbon age (from the Univer-sity of Arizona lab) for the top of the uppermost"brick red till" in VM 17-203 (29) (Fig. 1) of 13,300years (with a 400-year reservoir correction subtract-ed from the 14C measurement), suggesting that thislayer is correlative with the peak in abundance ofhematite-coated grains at -13,500 years in DSDPsite 609 (Fig. 4).

25. G. C. Bond and R. Lotti, Eos (Fall Suppl.) 75, 54(1994).

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26. W. Broecker et al., Clim. Dyn. 6, 265 (1992); W. S.Broecker, Nature 372, 421 (1994); L. D. Keigwin andS. J. Lehman, Paleoceanogr. Currents 9, 185(1994); D. Paillard and L. Labeyrie, Nature 372, 162(1994); S. Rahmstorf, ibid., p. 82.

27. G. H. Denton and W. Karlen, Quat. Res. 3, 155(1973); W. Dansgaard et al., in Climate Processesand Climate Sensitivity, J. E. Hansen and T. Taka-hashi, Eds. (Geophys. Monogr. 29, American Geo-physical Union, Washington, DC, 1984), pp. 288-298; F. Rothlisberger et al., Verlag Sauerlander (ATVerlag, Aarau, Switzerland, 1986); M. Magny, Quat.Res. 40,1 (1993).

28. J. K. A. Habicht, in Paleoclimate, Paleomagnetism,

and Continental Drift, M. K. Horn, Ed. (AmericanAssociation of Petroleum Geologists, Tulsa, OK,1979); R. H. Dott Jr. and R. L. Batten, Evolution ofthe Earth (McGraw-Hill, New York, 1981).

29. J. R. Conolly and M. Ewing, Nature 208,135 (1965);F. T. Barranco Jr., W. L. Balsam, B. C. Deaton, Mar.Geol. 89, 299 (1989).

30. H. Heinrich, Quat. Res. 29, 142 (1988).31. R. B. Alley et al., Nature 362, 527 (1993).32. S. Lehman, personal communication.33. We thank G. Denton, D. MacAyeal, R. Alley, and T.

Sowers for comments on the manuscript, and weacknowledge the able assistance of J. Zweibel, W.Tayler, and T. McMichaels in our analyses. We also

thank Tappan Zee High School students V. Tongand W. Wang who volunteered their time to helpprepare samples. J. Andrews and M. Kirby kindlygave us samples from cores near Hudson Strait. Thisproject was funded in part by grants from the Nation-al Science Foundation and from the National Ocean-ic and Atmospheric Administration. Samples were

provided by Lamont-Doherty Earth Observatory (L-DEO) Deep-Sea Sample Repository which is sup-

ported by the NSF and the ONR, and through theassistance of the Ocean Drilling Program. This isL-DEO contribution #5309.

17 October 1994; accepted 28 December 1994

Resonant Tunneling in the Quantum HallRegime: Measurement of Fractional Charge

V. J. Goldman* and B. Su

In experiments on resonant tunneling through a "quantum antidot" (a potential hill) in thequantum Hall (QH) regime, periodic conductance peaks were observed as a function ofboth magnetic field and back gate voltage. A combination of the two periods constitutesa measurement of the charge of the tunneling particles and implies that charge deficiencyon the antidot is quantized in units of the charge of quasi-particles of the surrounding QHcondensate. The experimentally determined value of the electron charge e is 1.57 x 10- 19coulomb = (0.98 ± 0.03)e for the states v = 1 and v = 2 of the integer QH effect, andthe quasi-particle charge is 5.20 x 10-20 coulomb = (0.325 + 0.01)e for the state v =

1/3 of the fractional QH effect.

Laughlin (1) has explained the exactnessof the Hall conductance quantization in theinteger quantum Hall effect (QHE) (2) as a

consequence of a combination of the gauge

invariance of the electromagnetic field andthe quantization of the charge of the cur-

rent carriers, the electrons. He considered a

gedanken experiment in which a two-dimensional electron system (2DES) formsa finite length cylinder with magnetic fieldB normal to the surface of the cylinderand additional magnetic flux (D threadedthrough the cylinder parallel to its axis.Laughlin showed that if (i) disorder is suf-ficiently small so that it does not destroythe Landau quantization of the electronicstates, (ii) the chemical potential lies inthe mobility gap between two Landau lev-els, and (iii) temperature T is small as com-

pared to cyclotron energy, then adiabaticchange of (D by )0, the flux quantum, isstrictly equivalent to the transfer of one

electron per Landau level from one edge ofthe cylinder to the other. This implies thatthe Hall conductance of such a 2DES sam-

ple is quantized exactly to ie2/h, where i isthe number of Landau levels below p. and his Planck's constant. This gedanken exper-iment was adapted to Corbino geometryand elaborated by Halperin (3).

In a seminal work (4), Laughlin related

1010

the fractional QHE (5) at Landau levelfilling v = 1/(2k + 1) (k is an integer) tothe fractional quantization of the charge ofelementary charged excitations (quasi-particles) of that state in units of e* = e/(2k+ 1). Halperin (6) recognized that Laugh-lin's quasi-particles could be described byfractional statistics, and Kivelson andRocek (7) have shown that fractionallycharged quasi-particles must obey fractionalstatistics.

It has been argued (8) that fractionalquantization of Hall conductance impliesthat the quasi-particles have a charge thatis a rational fraction of the electroncharge. However, it was subsequently rec-

ognized that quantization of Hall conduc-tance is a property of the condensate; forexample, the fractional QHE states can beunderstood within the composite fermiontheory (9), without explicit considerationof quasi-particles at all. Several experi-ments have been attempted to determinethe charge of quasi-particles (10). Howev-er, either their interpretations were am-

biguous or the measurement can be relatedto the quantization of Hall conductance.Several theoretical works (11, 12) haveconsidered resonant tunneling (RT) inthe QHE regime, which is, in some sense,a microscopic implementation of Laugh-lin's gedanken experiment. Experimentalwork was reported on RT in confinedquantum dots (13) and antidots (14), allin the integer QHE and without back

SCIENCE * VOL. 267 * 17 FEBRUARY 1995

gates. However, we are not aware of ex-

perimental or theoretical works explicitlypredicting quantization of charge deficien-cy on an antidot.

Here we report experiments on RTthrough states magnetically bound on a

lithographically defined potential hill("quantum antidot") in the integer andfractional QHE regimes. We see quasi-periodic RT conductance peaks as a func-tion of both the magnetic field B andback gate voltage VBG. We find that a

combination of these two measurementsfor a given v QHE state constitutes a

direct measurement of the charge of thetunneling particles. Our results imply thatthe charge deficiency on the antidot isquantized in units of the charge of thequasi-particles of the surrounding QHEcondensate q. Using this technique, we

have measured the electron charge as q =

1.57 X 10`9C = (0.98 + 0.03)e at v =

1 and v = 2 and the quasi-particle chargeas q = 5.20 X 10-20 C = (0.325 ± 0.01)eat v = 1/3.

Samples were fabricated from very lowdisorder GaAs heterostructure materialdescribed in (15). The antidot-in-a-con-striction geometry was defined by standardelectron-beam lithography on a pre-

etched mesa with ohmic contacts (Fig. 1).The sample was then chemically etched,and Ag front gates were deposited in theetched trenches; samples were mountedon sapphire substrates with In, whichserves as the back gate. The two frontgates were contacted independently andwere used to vary v between the gates (ina given B) and to balance the two barriersin the RT regime. We prepared the 2DESwith a density n 1 X 1011 cm-2 and

2 X 106 cm2 V-l s` by exposing the

sample to red light. Experiments were per-formed in a dilution refrigerator with sam-

ple probe wires filtered at millikelvin tem-

peratures, so that the total electromagnet-ic background at the sample's contacts isbelieved to be <2 [iV (root mean square).The four-terminal magnetoresistance ofthe samples R(2 3;1 4) (see Fig. 1) was mea-

sured with a lock-in amplifier with an

excitation of -2 [tV.The etched front gates and antidot cre-

Department of Physics, State University of New York,Stony Brook, NY 11794, USA.

*To whom correspondence should be addressed.

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