PALEOCEANOGRAPHY, VOL. 11, NO. 5, PAGES 563-577, OCTOBER 1996 Origin of the first global meltwater pulse following the last glacial maximum PeterU. Clark,l Richard B. Alley, 2Lloyd D. Keigwin,3Joseph M. Licciardi,• SigfusJ.Johnsen, 4'•and Huaxiao Wang 6 Abstract. Well-datedsea level recordsshow that the glacioeustaticrise followingthe lastglacial maximumwas characterizedby two or possiblythree brief intervalsof rapid sealevel rise separat- ing periodswith muchlower rates. Thesevery highrates of sealevel rise indicate periods of ex- ceptionallyrapid deglaciation of remainingice sheets.The LaurentideIce Sheetis commonlytar- getedas the sourceof the first, andlargest, of the meltwaterpulses (mwp-IA between-14,200 (12,200•4C years B.P.) and 13,700 years ago (11,700 •4C years B.P.)). In alloceanic records of deglaciationof the formernorthern hemisphere ice sheetsthat we review, only thosefrom the Gulfof Mexico and the Bermuda Rise show evidence oflow ;5•80 values at the time of mwp-IA, identifyingthe southernLaurentide Ice Sheetas a potentialsource for mwp-IA. We questionthis sourcefor mwp-IA, however,because (1) ice sheetmodels suggest that this sectorof the ice sheet contributedonly a fraction(<10%) of the sealevel needed for mwp-IA, (2) meltingthis sector of the ice sheetat the necessaryrate to explainmwp-IA is physicallyimplausible, and (3) ocean modelspredict a muchstronger thermohaline response to theinferred freshwater pulse out of the MississippiRiver into theNorth Atlanticthan is recorded.This leavesthe AntarcticIce Sheetas the only otherice sheetcapable of deliveringenough sea level to explainmwp-IA, butthere are currentlyno well-datedhigh-resolution records to documentthis hypothesis. These conclusions suggestthat reconstructionsof the LaurentideIce Sheetin the ICE-4G model,which are con- strainedto matchthe sealevel record,may be too low for time periodsyounger than 15,000years ago.Furthermore, ;5•80 records from the Gulf of Mexico show variable fluxes of meltwater from the southernmargin of the LaurentideIce Sheetwhich can be tracedto the openingand closing of eastwarddraining glacial-lake outlets associated with surgingice sheetbehavior. Thesevariable fluxesthrough eastern outlets were apparentlysufficient to affectformation of North Atlantic DeepWater, thus underscoring the sensitivityof thisprocess to changesin freshwaterforcing. Introduction et al., 1990a; Edwards et al., 1993; Blanchonand Shaw, 1995]. Thesevery high ratesof sealevel rise indicateperiods of excep- Well-dated sea level recordsshow that the glacioeustaticrise tionally rapid deglaciationof remainingice sheets.The source(s) followingthe last glacialmaximum (approximately 21,000 years of these rapid rates of sea level rise, or "meltwaterpulses" ago) (calendaryears are representedas years and radiocarbon [Fairbanks, 1989], however, has not been established. In the yearsare represented as•4C years B.P.) was characterized bytwo caseof the first, and largest,of the meltwaterpulses, several dif- or possiblythree brief intervalsof rapid sealevel rise separating ferent ice sheetshave been implicated. The FennoscandianIce periodswith muchlower rates (Figure 1) [Fairbanks,1989; Bard Sheetmay have beenresponsible [Birchfield et al., 1994] because it occurs"downwind" of the Nordic seaswhere large amountsof •DepartmentofGeosciences, Oregon State University, Corvallis. heat are released when active in the formation of North Atlantic 2EarthSystem Science Center and Department ofGeosciences, Penn- Deep Water (NADW). The BarentsIce Sheetmay havebeen re- sylvaniaState University, University Park. sponsiblebecause it was largely groundedbelow sea level and 3WoodsHole Oceanographic Institution, Woods Hole, Massachu- thereforesusceptible to the rapid collapsesuggested by the first setts. 4NiehlsBohr Institute, Department of Geophysics, University of Co- meltwater pulse [Lindstromand MacAyeal, 1993]. Many re- penhagen,Denmark. searchers,however, have targetedthe LaurentideIce Sheetas the 5Alsoat ScienceInstitute, Department of Geophysics, University of primary sourcefor the first meltwaterpulse for two reasons:(1) Iceland,Reykjavik. The Laurentidewas the largestice sheetand thus most likely to 6LawrenceLivermore National Laboratory, IAvermore, California. deliver sucha large volumeof meltwater,and (2) oxygeniso- topesin deep-seacores from the Gulf of Mexico and midlatitude Copyright1996 by the AmericanGeophysical Union. North Atlanticrecord an intervalof low fi•80 at the time of the Papernumber 96PA01419. first meltwaterpulse suggesting that glacialmeltwater discharged 0883-8305/96/96PA-01419512.00 down the MississippiRiver and spreadas a low-salinityplume 563 564 CLARKET AL.: ORIGIN OF THE FIRSTGLOBAL MELTWATER PULSE acrossthe temperateNorth Atlantic [e.g., Keigwin et al., 1991; 0 Charles and Fairbanks, 1992; Fairbanks et al., 1992; Lehman and Keigwin, 1992]. This interpretationhas figured prominently Barbados in recentmodeling of the deglaciationof the global ice sheets New Guinea (ICE-4G) [Peltier, 1994], whereby the LaurentideIce Sheet is 25 constrainedto losenearly 20 m of sealevel equivalentduring the first meltwaterpulse between 15,000 and 14,000years ago. In this paper,we presentseveral lines of evidencewhich sug- gestthat the LaurentideIce Sheetcould not have beenthe pri- marycontributor tothis first meltwater pulse. Although •5180 re- 50 mwp-IB cordsclearly identify meltwaterflowing down the Mississippi River, our evidencesuggests that this dischargelikely only ac- counted for a fraction of the total sea level rise. Furthermore, deep-searecords of meltwaterand icebergdischarge from the_ 75 Gulf of St. Lawrence and Hudson Strait, or areas that se•;,ed as the other primary outletsof freshwaterto the North Atlantic in mwp-IA additionto the MississippiRiver, showevidence of significantly reduceddischarge at the time of the first meltwaterpulse. Be- cause marine records near the former Fennoscandian and Barents 100 Ice Sheetsalso showno signatureof accelerateddeglaciation at the time of the first meltwaterpulse, records of deglaciationof the AntarcticIce Sheetshould be examinedfor possibleevidence of its role in this prominentsea level event. 125 20 15 10 Sea Level Record (ka) Assuminga 105m riseof globalsea level I•etween21,000 and 6000 yearsago [Peltlet, 1994],the rateof sealevel rise would be Figure 1. Coral recordsof sealevel datedby U•'h from Barba- 7 m/1000 years if all ice sheetshad melted unitbrmly. Corals dos [Bard et al., 1990a, 1993] and New Guinea [Edwardset ai., dated by U/Th recordingsea level rise at Barbados[Fairbanks, 1993]. Two periodsof rapid rise of sea level are identifiedas 1989; Bard et al., 1990a, 1993], New Guinea[Edwar&' eta/.,' mwp-IA and mwp-IB. 1993], and Tahiti [Bard et al., 1995], however, show that be- tween 15,000 and 10,000 yearsago, the postglacialsea level his- tory was punctuatedby two brief intervalsof rapid ratesof sea level rise,which Fairbanks [ 1989] referredto asmeltwater pulses northernhemisphere ice sheetswere involved,this eventshould (mwp's) IA and lB (Figure l). Accordingto the Barbadosrec- have left a clear signalin the North Atlantic Ocean,which re- ord, sea level during the first meltwater pulse (mwp-IA) rose about24 m between14,690 + 25 yearsago (12,600 + 460 ceivedmost of the meltwaterand icebergsfrom theseice sheets. yearsB.P.) and 13,730 + 100years ago (11.720 + 400inc years Here we examinerecords for evidenceof an abruptdeglaciation event from the northernhemisphere ice sheetsat the time of B.P.), with mostof this sea level rise (19 m) occurringhfter 14,230+ 100years (12,200 + incyears B.P.) [Bard et al., 1990a] mwp-IA. (Figure 1; Table l). Blanchonand Shaw [1995] arguedthat changesfrom a monospecificAcropora palmata reef framework Laurentide Ice Sheet to a deeper-watercoral reef framework constrain13.5 m of sea level rise in <290 + 50 yearsbeginning at or shortlyafter 14,230 + 100 yearsago. On the basisof this estimate,meltwater was in- The LaurentideIce Sheetwas the largestof the former ice troducedto the globaloceans during mwp-IA at an averagerate sheets. Most meltwaterand icebergsfrom the LaurentideIce of-4.5 cm/yr, correspondingto a freshwaterflux of-•0.52 Sv Sheetdrained to theNorth Atlantic through three outlets: Hudson (Sverdrup)(1 Sv = 106m 3 s-l),for about 300 years. The Fair- Strait, the Gulf of St. La'arence,and the MississippiRiver banksand Bard data give a best-estimatesea level rise of 19 m (Figure2). The oxygenisotope record in boththe Gulf of Mex- over 500 years(3.8 cm/yr), or 0.43 Sv. Thusthe freshwaterflux ico [Leventeret al., 1982] andthe BermudaRise [Keigwinet al., of mwp-IA is in the vicinity of 0.5 Sv by eithercalculation, while 1991]show a spikeof low•5•80 at the time of mwp-IA(Figure the estimateof sealevel riseranges from 13.5 to 19 m. 3b), leading several researchersto suggestthat the meltwater pulseoriginated from the southernmargin of the LaurentideIce Sheet through the MississippiRiver to the Gulf of Mexico [Keigwinet al., 1991; Fairbankset al., 1992] [seealso Emiliani, Evidence From Northern Hemisphere Ice Sheets 1976; Emiliani et al., 1978]. Extractinga meltwater flux of 0.5Sv from the 2 x 10•2 m 2 or The rapidity and magnitudeof mwp-IA strongly suggests less sectorof the LaurentideIce Sheet draining to the Gulf of abruptcollapse of someportion(s) of remainingice sheets.If the Mexico would requirethat the entire sectorbecame an ablation CLARK ET AL.: ORIGIN OF THE FIRST GLOBAL MELTWATER PULSE 565 Table 1. Agesof SamplesConstraining the Timing of MeltwaterPulse IA Sample Depth,am