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The Isotopic Climatic Records in the Allerdbllingyounger Dryas and Postyounger Dryas Events

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The Isotopic Climatic Records in the Allerdbllingyounger Dryas and Postyounger Dryas Events GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 9, NO. 4, PAGES 557-563, DECEMBER 1995 The isotopic climatic records in the Allerid-B011ing- Younger Dryas and post-Younger Dryas events Samuel Epstein Divisionof Geologicaland Planetary Sciences, California Institute of Technology,Pasadena Abstract. The combinedisotopic records in the Greenlandice cores,in modernand ancientwood samples,and in marineforaminifera provide a climaticrecord for theAller6d-B011ing-Younger Dryasand post-Younger Dryas Periods that is not compatiblewith theusual climatic interpreta- tionof the2H/1H ratios in theGreenland ice cores. For example, the Younger Dryas was not solelya NorthAtlantic event because evidence for it is alsofound in the2H/1H record in wood sampleswhich grew on thenorth slope of Alaska. The YoungerDryas was not necessarilya time of coolingover the ice caps. Moreover,it mighthave been a warmingperiod in the temperate zonesof thenorthern hemisphere continents. A betterunderstanding of theclimatic scenarios duringthese periods of timecould be ascertainedfrom additional isotopic data in plantremains whichgrew during these intervals and which cover a widerclimatic range on the continents. Introduction lb). Five of these sampleswere dated by Acceleration Mass Spectrometer (AMS) techniques. These samples were Salix The transitionfrom the last Glacial Period to the present (willow shrubs)collected in an area borderedby north latitude InterglacialPeriod is recordedin the isotopiccomposition of 69ø 38' and 70ø 30' and west longitude151 ø 36' and 155ø 56'. the Greenlanddeep ice cores(e.g., DYE 3) [Dansgaardet al., Nonexchangeablehydrogen was extractedfrom the plant cellu- 1989]. This transitionstarts with a gradualincrease in the 8D lose [Epsteinet al., 1976]. Becauseboth the ice and plants in values[•D = (Rsample/ Rstandard - 1)x 1000 (%o), R = 2H/1H effect record the isotopic compositionof the ambient meteoric standardisStandard MeanOcean Water (SMOW)], followed b; waters [Epstein et al., 1976; White eta/., 1994], their $D a sharpincrease of the $D from -260 to -230%0(Figure l a). recordsshould be comparable. Preferential loss of cellulose in This is referredto as the Aller6d-BOllingevent. The Aller6d- old wooddue to agingdoes not affectthe $D valuesof the orig- Bolling is followed by the Younger Dryas event, which is inal cellulose [Yapp and Epstein, 1977]. The negative plant associatedwith a decreaseof the 5D value to -285%0,followed $D values(-217%o to -135%o,Figure 1) indicatethat the water by a rise to -235%0between 10.6 and 10.4 Kyr B.P. This in used by these plants must have precipitated from air masses turn is followed by a drop to -255%0and a rise to -230%0, that experienced many cycles of isotopic fractionation prior followed by the generaltrend towardthe InterglacialPeriod. to their arrival on the Alaskan north slope. The AllerOd-Bollingis generallyconsidered to be a warming TheAlaskan isotope record is plottedagainst 14C age, and event,and the YoungerDryas is a coolingevent. Broeckeret the DYE 3 ice core record [Dansgaard et al., 1989] is plotted al. [Broecker et al., 1989; Broecker and Denton, 1990] againstdepth in Figure1. The coincidenceof $D maximaand attributed the cooling to a diversion of ice sheet meltwater minimaand the similarranges of the $D valuesfor the Alaskan from the MississippiRiver to the St. LawrenceRiver drainage plant remainsand the GreenlandDYE 3 ice recordsuggests that system [Broecker eta/., 1989]. This diversion served to shut the two records are reasonablysynchronous. off the North Atlantic conveyorbelt circulationsystem which As in the case of the ice record, the Aller6d-Belling in the currently supplies an enormous amount of heat to the Alaskan wood record starts with a sharp increaseof +30%0 in atmosphereover the North Atlantic region. Consequently, the $D at approximately12 Kyr B.P., followed by a continu- "the Younger Dryas is primarily a North Atlantic and not a ous decreasefrom -150 to -220%0correlated with the Younger globalevent" [Broecker and Denton,1990, pp. 54]. Enough Dryas event at approximately 10.5 Kyr B.P. This is followed stableisotope data now existto form a basisfor reexamining by a sharprise to -135%o,followed by a post-YoungerDryas this interpretationas well as reconsideringthe significanceof oscillation extending over 1000 years, spanning a 30%0 range the stableisotope records in ice coresand in samplesof wood in $D values. The synchronousvariations in the $D records that grew duringthe late-glacialperiods. in the ice core and in the willow shrubs are impressive and indicate that the Younger Dryas event is recordedin both iso- Results and Discussion tope records and therefore was apparently not exclusively a We analyzedthe $D of a seriesof 26 well-preserved•4C- North Atlantic event. In addition, the synchronousvariations also indicate that the rate of the growth of the ice cap is rela- dated plant remainsfrom the north slope of Alaska (Figure tively uniform for at least the period between 12 and 10 Kyr B.P, suggestinga uniform temperaturein this period of time. Copyright1995 by theAmerican Geophysical Union. The smallerrange in the $D valuesrecorded in the ice core,as Papernumber 95GB02741. comparedto that recordedin the wood samplesfrom the north 0886-6236/95/95GB-02741 $10.00 slope of Alaska, is likely due to some degreeof homogeniza- 557 558 EPSTEIN: ISOTOPIC CLIMATE RECORDS IN YOUNGER DRYAS EVENTS gradual increaseinto the Holocene. This is usually explained as a rapid swing in air temperaturesover the Greenlandice sheet. There is an alternativeexplanation for the existingiso- -230•.• tope recordsfor the Aller6d-B011ing-YoungerDryas and post- Younger Dryas events. An alternativescenario is suggestedby the excessdeuterium values(d = •SD-8•5180, where d = excessdeuterium) for the DYE 3 ice samplescovering the transition from the Younger -260 - Dryas to the Preboreal[Dansgaard eta/., 1989]. As suggested by Dansgaard et al. [1989], these data show that the marine -270 - sourceof moisturefor the Younger Dryas in the polar regions was warmer than that for the Preboreal. They also suggested -28O - that this change was causedby a shift of a "dominatingmois- I 1780 1790 1800 1810 ture source in the subtropical part of the Atlantic Ocean" Depth in ice core (meters) [Dansgaardeta/., 1989, pp. 533] and presumablyby a warmer marine source of the moisture. Petit et al. [1991] show that I I I I the major effect on the magnitude of the excessdeuterium in B x - -140 the Antarctic snow is directly related to the sea surfacetemper- ature of the source of the moisture. /,x• / \ The effectof surfaceocean temperatures on the •SDrecord of _/x// • /•, ,/ x\ X/-'x Ii.--160 the ice caps is illustratedin Figure 2. This figure showsthe / b..,'x\•_xx/x , I Rayleigh-typecurves, similar to thosepublished by Yapp and x ,X Epstein [1977] that illustrate the relationship between iso- - X ,0' x- -180 I x/ topic compositionand temperatureof condensationfor precip- I / itation originatingfrom oceanwater with a fid of 0%0,but I x/ x -200 with different initial oceansurface temperatures (20 ø, 10ø and I I - IO/x 0øC). These curvesillustrate the effect of ocean surfacetem- peratureon the relationshipbetween temperature and the iso- -220 topic composition in the ice caps. For example, arrow A in Figure2 showsthat the fid recordin the ice cap will remain 8 9 10 11 12 constantat-300%0, in spite of a cooling of 10øC on the ice Years x103 BP cap, if at the sametime the marine sourceof water feedingthis ice cap coolsby approximately20øC. On the other hand,ar- Figure 1. (a) The õD recordof the 14C-datedwillow shrub row B showsthat an increasein fid of approximately130%o, samplesfrom the north slopeof Alaska, as comparedto (a) an normally indicating a warming trend, will be recordedon the exact copy of an enlargedversion of the publishedDYE 3 ice core data from Greenland[Dansgaard et al., 1989]. The •SD ice cap if the overlying air temperature remains constant, valuesof the ice coresare plottedagainst depth of the ice core while the temperatureof the ocean'ssurface decreases from 20ø sampleson the ice cap. (b) The points marked with crosses to 0øC. It is the unique and transienttemperature history of have a suggestedanalytical uncertainty of +160 years,whereas midlatitudeocean surfaces during the Aller6d-B011ing-Younger the open circles in Figure lb have been determinedby the Dryas and post-YoungerDryas that could play an unusualbut AMS techniquewith an error of _+80years. important role in determining the short-term isotopic varia- tions on the ice caps at that time. These•SD values recorded in the ice capdepend on thediffer- tion that must have taken place on the ice cap at this great ence in temperaturebetween the air massesover the ice and the depth in the core. marine source of moisture. If simultaneous cooling of the The good correlationbetween the fid recordsof the wood ocean'ssurface and the atmosphereover the ice caps(DYE 3) is and the ice core betweenthe period 12.3 to 10 Kyr B.P. sug- assumedto be responsiblefor a largedecrease in •SDon the ice, gestssimilar variationsin their respectivemarine sourcesand certain circumstanceshave to exist. For example, assume an cooling historiesof the air massesproviding the moisturefor oceantemperature of 20øCand a temperatureover the ice cap of thesesites. It is difficult to imaginethe distantAtlantic Ocean -8øC. The cooling history of an air massfeeding the ice cap as the source of precipitation for the north slope of Alaska will resultin a fid valueof -200%0for the snowof the ice cap, becausethe fid valuesof the northslope shrubs are systemati- accordingto Figure 2. If the ocean temperatureremains at cally ß-80%o higher than thoseof the ice in the DYE 3 core in 20øC,but the temperatureof the ice capdrops to -15øC,the •SD Greenland, which borders the Atlantic Ocean. It seems more of the snow on the ice cap will decreaseto -265%0,a changeof reasonable to assume that the Pacific Ocean off the western -65%0. However, if the oceantemperature decreases by 10øC North American continent was the dominant, ultimate source and the temperatureon the ice cap dropsto -13'C, there should of the rain and snow that fell on the north slopeduring that be no changein the •SDof the snowon the ice cap.
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