The Effects of Flowline Length Evolution on Chemistry-Delta O-18 Profiles from Penny Ice Cap, Baffin Island, Canada

Home , Agassiz Ice Cap, Barnes Ice Cap, Penny Ice Cap

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2002 The ffecE ts of Flowline Length Evolution on Chemistry-Delta O-18 Profiles from Penny Ice Cap, Baffinsl I and, Canada David A. Fisher

Roy M. Koerner

Gregory A. Zielinski

Cameron P. Wake

Christian M. Zdanowicz

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Repository Citation Fisher, David A.; Koerner, Roy M.; Zielinski, Gregory A.; Wake, Cameron P.; Zdanowicz, Christian M.; Bourgeois, Jocelyne C.; Mayewski, Paul Andrew; and Grummet, Nancy, "The Effects of Flowline Length Evolution on Chemistry-Delta O-18 Profiles from Penny Ice Cap, Baffinsl I and, Canada" (2002). Earth Science Faculty Scholarship. 145. https://digitalcommons.library.umaine.edu/ers_facpub/145

This Conference Proceeding is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Earth Science Faculty Scholarship by an authorized administrator of DigitalCommons@UMaine. For more information, please contact [email protected]. Authors David A. Fisher, Roy M. Koerner, Gregory A. Zielinski, Cameron P. Wake, Christian M. Zdanowicz, Jocelyne C. Bourgeois, Paul Andrew Mayewski, and Nancy Grummet

This conference proceeding is available at DigitalCommons@UMaine: https://digitalcommons.library.umaine.edu/ers_facpub/145 Annals of Glaciology 35 2002 # InternationalGlaciological Society

The effectsof flowline length evolution onchemistry^ ¯18O profiles fromP ennyIceCap, Baffin Island, Canada

David A. FISHER,1 Roy M. KOERNER,1 Gregory A.ZIELINSKI, 2 Cameron P.WAKE,3 Christian M.ZD ANOWICZ, 1 Jocelyne C.BOUR GEOIS, 1 Paul A.MAYEWSKI, 2 Nancy GRUMMET4 1Terrain Sciences Division, Geological Survey of Canada, 601Booth Street,Ottawa,Ontario K1A0E8 ,Canada E-mail: [email protected] 2Climate Studies Center,University of Maine,Institute for Quaternary Studies,Orono,ME 04469-5790,U.S.A. 3Glacier Research Group,Morse Hall,University of New Hampshire,Durham, NH 03824-3525,U.S.A. 4School of Earth Sciences,Stanford,Braun Hall Building 320,45 Serra Mall,Stanford,CA94305-21 15,U.S.A.

ABSTRACT.Theisotopic and chemical signatures for ice-age and Holocene ice from Summit, Greenland,and Penny Ice Cap,Baffin Island, Canada, are compared. The usual pattern oflow d18O, high Ca2+ and high Cl^ is presented inthe Summit records, butPenny Ice Caphas lower than present Cl ^ inits ice-ageice. Asimple extensionof the Hansson model(Hansson, 1 994)is developedand used tosimulate these signatures.The low ice- age Cl^ fromPenny Ice Capis explainedby having the ice-ageice originatingmany thou- sandsof kminlandnear the centre ofthe Laurentideice sheet andmuch further fromthe marinesources. Summit’sflowlinesall start closeto the present site. ThePenny Ice Cap early-Holocene d18O’shadto be corrected tooffset the Laurentidemeltwater distortion. Theanalysis suggests that presently the Summit andPenny Ice Capmarine impurity ori- ginatesabout 500 km away,andthat presently PennyI ce Capreceives asignificantamount oflocalcontinental impurity .

INTRODUCTION THEHANSSON MODEL

TheSummit (Greenland) records areused here torepresent Sincean extendedHansson model is centralto these differ- atypicalN orthern Hemisphere site whoseflow geometry has ences, wewill now review that model.As Figure 4 shows, notchanged much over the last 50kyr .TheSummit records continentaland marine impurity is throwninto the airwith arecompared to those fromPenny Ice Cap,Baffin Island, ^3 aninitial air concentration Cair(0)(kgimpuritym air), Canada,whose flowline origin is notso well constrained in andthis is verticallyaveraged up to the topof the tropo- the past.Figure 1 showsthe locationsof the Summit site sphere (thickness H).Thisinitial injection of impurity is (72³34’ N, 37³37’ W;3232m a.s.l.),PennyI ce Cap( 67³15 ’ N, reducedmainly by rain precipitation over its lifetime, but 65³45’ W;1900m a.s.l.)andBarnes I ce Cap,Baffin Island, finallyat high latitudes bysnow ,whichdrops a remnant of Canada.The background map of Figure 1 showsthe maxi- the originalinjected amountonto the ice cap.With time t mum (about22 kyr BP)reconstructed ice coverof N orth (t 0beingthe injectiontime) the impurityloses mass in ˆ America(Fisher andothers, 1985)assuminga non-deforming proportionto the concentration(see Fig.4): bedunder Hudson Bay .Figure2 showsa suite ofSummit variables: d18Ofromthe GreenlandI cecore Project(GRIP) dCair t ice core(J ohnsenand others, 1997)and the ions,calcium … † ªCair t ; 1 2+ ^ + dt ˆ ¡ … † … † (Ca ),chlorine(Cl )andammonium (NH 4 ), from the GreenlandI ce Sheet Project2 (GISP2)ice core(Mayewski ªt whichintegrates immediatelyto C t C 0 e¡ , andothers, 1997).Figure3 showsthe same suite ofvariables air… † ˆ air… † where ª (1/time) is the proportionalityrate constantfor the fromthe 1995Penny ice core(Fisher andothers, 1998; givenimpurity . ª is takenas aconstantfor a givenimpurity . Zdanowiczand others, 2000)forthe bottom-most 15mof Amore intuitivequantity related to ª is survivaltime ½ 1/ core.Comparing Figures 2 and3 ,it isobviousthat Cl pro- ˆ ª (units oftime) ,whichis specific tothe impurity.For videsthe majordifference in ice-age signatures: Penny I ce example,sea-salt particles tend tobe removed faster than Caphas lower Cl forthe ice agethan for the Holocene,while smallcontinental-dust particles. Theequation for C t Summit hasmuch higherCl inthe ice age. air… † becomes: Thedashed lines inFigure 1connectingKeewatin Dome (``KK’’)tothe present positionof Penny(` `P’’)andBarnes t=½ C t C 0 e¡ : 2 (``B’’)Ice Capsare the longestpossible flowlines for ice in air… † ˆ air… † … † these tworemnant ice masses. With this reconstruction, FoxDome (``FF’’)is infacta longridge. Down-ridge flow Over most ofthe water/impuritycycle from source to canoccur ,sosuch longflowlines are possible. removal,most ofthe impurityis removedby wet processes. 150 Fisher and others:Chemistry^ d18Oprofiles from Penny Ice Cap

Fig.1.Map of the reconstructed ice coverover North America at the Last Glacial Maximum (from Fisher and others,1985) assumingthebed under Hudson Bay is not deforming.The present positions of the Laurentide remnant ice caps,Penny and Barnes,are indicated by a``P’’and ``B’,’ respectively,and the longest possible flowline to them is shown as the thick gray dashed line joining them to the Keewatin flow centre denoted ``KK’’.The position of the Summit(Greenland) cores is marked ``S’.’ Summitis already virtually at the top of the flowline.A flowlinefrom Summitto the west coast of Greenland (Pa ª kitsoq, ``PS’’)(Reehand others,1991)is marked for comparison. Since there is not very much lateral room for expansion of the Green- land ice,the Summitflowline origins must always have been close to the present position.

Underthis wet-process assumption,the survivaltime is approximatedby: 18 »air H Fig.2.Summitice-core records:(a) d Ofromthe GRIP core ½ ; 3 (Johnsen and others,1997);(b)Ca 2+concentration (ppb) ˆ »water WP … † (mass)in GISP2cor e(Mayewski and others,1997);(c) where H isthe activevertical mixing height in the troposphere ^ + ^1 Cl concentration (ppb)in GISP2ice core;(d)NH 4 con- (about10km), P is the averageprecipitation rate (mwatera ) centration (ppb)in GISP2core . overthe impurity/watercycle, and »air, »water arestandard densities. W,the scavengingratio ,relates the airconcentra- ^3 tion Cair (kg m )tothe concentrationin the precipitationin ^1 epoch,Hansson ( 1994)calculatesrelative concentrations rainor snow/ ice,say Cice (kgimpurity (kg ice)), i.e. andfluxes using variables relative to the modernones that »airCice aredenoted by an asterisk, i.e. W , C , A , ½ and t , W : 4 ¤ air¤ ¤ ¤ trans¤ ˆ C … † ©¤, C¤ , A¤, C¤ 0 and A¤.Thusthe relativeair concentra- air ice air… † Over aspecific site (onan ice cap),wherethe accumu- tioncomes directlyfrom Equation ( 2): ^1 ^2 ^1 lationrate is A (m a ),the impurityflux ©wet (kg m a ) is: t=½ Cair Cair 0 e¡ Cair 0 1 n=k t =½ » … † … † e… ¡ † ¤ ¤ ; 6 ice ˆ t¤=½¤ ˆ … † ©wet CiceA»ice Cair AW : 5 Cair¤ Cair¤ 0 e¡ Cair¤ 0 ˆ ˆ »air … † … † … † where n t=t¤ isthe relativetransit time and k ½=½ ¤ is Incold, dry places at the end-pointsof impuritycycles, ˆ ˆ the relativesurvival time. Both n and k arespecies-specific. there is undoubtedlydry fallout, which is notcovered expli- Therelative ice concentrationfrom Equation ( 4)is: citlyby Equation ( 5).Becausethe drycomponentof ice-cap C C W impurityflux is omitted, the theoryunderestimates the total ice air ; 7 flux.However ,asdiscussed below,taking © © is C¤ ˆ C¤ W … † total ˆ wet ice air ¤ notserious andin any case iswhatHansson does. andthe relativewet fluxfrom Equation ( 5)is: Rather thantry topredict past impurityconcentrations andfluxes absolutely by estimating the actualprocess vari- ©wet CairWA : 8 ables W, Cair, A, ½ and ttrans (totaltransit time) foreach ©wet¤ ˆ Cair¤ W ¤A¤ … † 151 Fisherandothers:Chemistry^ d18Oprofiles fromPenny Ice Cap

Fig.4.Cartoon of the Hansson model,relating ice-core geo- chemical concentration and flux towater-cycle variables and impurity-specific variables.There are two broad input cate- goriesof impurity:marine and continental. Asterisks indicate present values.

½ » =» H= WP W ¤P ¤ W ¤A¤ k … air water† … † ; ˆ ½ ˆ » =» H = W P ˆ WP ˆ WA ¤ … air¤ water¤ † ¤ … ¤ ¤† 9 … † whereit isassumed that » =» H … air water† 1 : »¤ =»¤ H ˆ … air water† ¤ Thetroposphere thickness mayhave been smaller during Fig.3.Penny Ice Capice-cor erecords:(a) d18O(Fisher and colderepochs, but this isoffset bythe higherair density ,so others,1998);(b)Ca 2+ concentration (ppb)(mass) (Fisher the equationimmediately above is notunreasonable .We ^ + and others,1998);(c)Cl concentration (ppb);(d)N H assume here, asdidHansson ( 1994),that W ¤=W 1. 4 ˆ concentration (ppb). TheHansson model is largelydriven by accumulation rate A atthe snow-outsite . A inthe modelcomes directly from d andits relationshipto Summit accumulationrates (Fig.5).Therethe backgroundassumption is that d’s and USING THEMODEL accumulationrate inthe ice coreboth depend on site airtem- perature. A is further assumed tobe proportional to the aver- Equations( 6^8)constitute the Hanssonmodel. In Hansson agesource-to-site averageprecipitation rate P. This may (1994) n,the relativetransit time, israther insensitive, being workas well as it appearsto ,becauseHansson ’smodeldoes presently 1andduring the coldest partof the ice age0 .5, notexplicitly include dry fallout. Thesite’ s A isnodou bt more reflectingthe ideathat increasedstorminess andhigher variablethan the cycleaverage P,becausethe high-latitude- windspeeds decrease the transit time (Hansson,1994).This site temperatures aremore variablethan the averagetem- ofcourse assumes the distancefrom source tosite is not perature overthe broadlatitude band of the wholewater changedvery much. This was taken as true forall species. cycle.The model including only wet capture actually simu- Therelative survival time k ½=½ ¤,however,is seen to lates the dryin the followingmanner .Colderperiods of time ˆ varyover a much largerrange: from the present 1toabout inthe ice cores havemore negative d18O,and,as shown 5orlarger during the ice age(Hansson, 1 994).Thisis below,the site’sinferred relativeaccumulation rate A=A¤ is because k islargelydefined by the accumulationrate over lower.Becausethe site hasa greatertemperature variability the water/impuritycycle, and that is stronglycontrolled by thanthe wholewater cycle, the changeto a coldperiod is the watercontent ofthe atmosphere,which is verysensitive more extreme atthe site thanfor the wholecycle, so that totemperature. From Equation( 3)we obtain: A=A¤ < P=P ¤.From Equations( 6^9)wecan see that the 152 Fisher and others:Chemistry^ d18Oprofiles from Penny Ice Cap

agemodern values. The A=A¤ providedabove will give k inthe model.The value of n is estimated asin Hansson (1994)byassigning n 1formodern accumulation (i.e. for ˆ ¯¤ ^34.83%) and n 0.5forthe ice-ageice (i.e. for ¯¤ ˆ ˆ ˆ ^42.00%)andlinear in between. Thus the keymodel variables n=k and A=A¤ neededfor using Equations ( 6^8)for Summit appearin Figure5a.

PennyIce Cap site

Theaccumulation^ d18Orelationshipfor P ennyI ce Capis 0:117 ¯ ¯ assumed tohave the same slope(i.e . A A¤e … ¡ ¤†) as Sum- ˆ ^1 mit, buthas different modernaverages, i.e . A¤ 0.37 m ice a 18 ˆ and¯¤ ^24.23%. n 1for modern d O ^24.23%, n 0.5 ˆ 18 ˆ ˆ ˆ forthe ice-age d O ^34.00% andlinear in between. The ˆ PennyI ce Cap n=k and A=A¤ functionsappear in Figure 5b .

THEDIST ANCEFR OM SOURCETOSITE,AND THE EFFECTS ON n=k

InHansson’ smodel, t isthe traveltime takenfor an impur- ityto travel from its source tothe ice-capflowline origin, and t¤ is the present traveltime tothe modernice cap.Thus n t=t¤ isthe relativetravel time. n is 1presently andfor ˆ the Greenlandsites onlyslightly different duringthe ice age( 0.5)becausealthough the storminess wassomewhat more extreme the sources wereprobably a little further away,dueto increased sea ice andsnow cover . Theearly part of the Holoceneat high latitudes wasthe warmest, andsaw sea ice andma jorice coverretreat most quickly(K oernerand Fisher ,1990;Dyke and others, 1996). Forexample, during the earlyH olocenethe melt layeringwas ata maximumon Agassiz I ce Cap,Ellesmere Island,Canada. One wouldexpect travel times todecrease quicklyin the early Holocenebecause the continentaland marine sources were suddenlycloser then duringthe ice age.Thatcom binedwith possiblestormier conditionswould lead one to expect shorter traveltimes n < 1duringthe earlyH olocene. Duringthe ice age,the flowlineorigins for Summit may havebeen slightly further away,butthe source distancemay Fig.5.The past accumulation rate A with respect to present havebeen 2^3 times removed,because of seaice andsnow 18 cover.Forthe Pennycore ice, whoseflowline origins during A¤ vs d O,andthe Hansson model variable n=k, where n is the ice ageare postulated to have been possibly thousands of the relative transit time, t=t¤,for agiven type of impurity and kilometers fromthe present site (Fisher andothers, 1998), k is the relative survival time, ½=½ ¤.Asterisks indicate present values.(a)M easured values for the GRIP core (Dahl- the distanceto source, and hence travel time, mayhave been Jensen, 1993);(b)inferred values for the Penny Ice Capcore verymuch longer,becausethe site itself hasbeen removed. 18 Toallowexplicitly for the distanceto source beingdifferent using the same A^d Orelationship. forsites likeSummit andPenny Ice Capwe change the defi- nition of n slightlyto: modeledice concentrationsand fluxes would be too large, t becausewe have used the site accumulationsbased on the n D ; 18 d O,whichoverestimate the cycletemperature swings.This ˆ t¤ overestimationcompensates forthis ``wet’’model’slackof where D X=X¤, X beingthe distancefrom the source to ˆ explicitdry fallout, which becomes more importantfor low- the flowlineorigin for ice ofa givenage in the core,and t=t¤ accumulationperiods such asthe Late-glacial. is the relativetravel time fora fixeddistance as in the original Hanssonmodel. X¤ is the moderndistance. This change Summit site mainlyaffects Equation( 6),wherenow n=k is replacedby the slightlyaugmented version n=k D: Asmentionedabove, the A=A relationshipcomes fromthe … † ¤ n t A W empiricalrelationship found between d18Oandthe meas- D D : k ˆ t A W uredannual accumulation rate A (derivedfrom annual ¤ ¤ ¤ layerthickness) forSummit (Dahl-Jensen andothers, 1993): Thevariables that determine the modelpredicted con- 0:117 ¯ ¯ centrations are: t=t¤, A=A¤, t¤=½ ¤ and D X=X¤. A A¤e … ¡ ¤ † ; ˆ ˆ Variousruns ofthe modelwill be made using the above ^1 where A¤ 0.23 m ice a and ¯¤ ^34.83% arethe aver- variables.The important variable A=A¤ (andthe derived ˆ ˆ 153 Fisherandothers:Chemistry^ d18Oprofiles fromPenny Ice Cap

Fig.7.Penny Ice Capchlorine vs uncorrected d18O for early- Holocene and ice-age ice.The early-Holocene d18O values have probably been affected by meltwater from the wasting Laurentide ice cap and consequently need correcting.For cor- Fig.6. Acartoon explaining the extension to the Hansson rected version of this figure see Figure 8b. model. X is the average model distance from source to the start oftheflowline. X can be changed either by moving the impurity source or by moving the start of the flowline.For Summitthe The d18Ovaluesshould be corrected backto a ``zero-SMOW start of the flowline can at most be afew tens of km from the (StandardMean Ocean W ater) source ocean’’sothat the present drill site.For Penny Ice Capthe Late-glacial flowlines empiricalrelationships between d and A arenot confused could be 2000 km from the present site (see Fig.1). X¤ is the bythe meltwater effect. Thisis, inretrospect, notvery present distance. importanthere forthe Summit databut has a majoreffect forthe PennyI ce Cap( d18O, Cl/Cl*)pairs.The correction 18 n=k)willcome from the measured d18Odependencetaken comes fromexamination of the d Oandmelt-feature fromthe Summit coreand shown in Figure 5a and b .As recordfrom Agassiz I ce Cap(Fisher andothers, 1995)and byexamining the elevation-correctedtemperature record before,the (distance constant)relative travel time t=t¤ will be1.0^0.5.Thepresent traveltime normalizedto the ``survival forthe Summit Holocene.Bothstrongly suggest that the time’’ofthe impurityis veryspecies-specific andwe will use warmest partof the Holocenewas the earliest partright after the Hanssonvalues as ourguide for the marine(Cl ^) and the the ice-agetermination at 1 1550 BP (calendar).TheAgassiz 2+ melt-feature recordclearly shows this, whereasthe records continentalspecies (Ca ).Inkeeping with the simplicity of 18 the Hanssonmodel, D,forPenny I ce Cap,willbe a function fromall the northernsites havea d Omaximum around7^9 ofthe ice-core d18O,withthe maximumbeing assigned to the kyrand more negative d’sfromthat dateto the transition. most negativeice-age d18O,andthe modernvalue of 1 to Thisoffset of d andactual early-H olocenetemperature maxi- modern d18O.Inbetween Dwillvary linearly .Themaximum mum, whichis probablycaused by the source-watercontami- D duringthe ice ageis calledD ISTinFigures 8 and9 below. nation,has been corrected byassuming that the Agassiz d ForSummit, DISTis closeto 1 ,becausethe ice-core site is trend 0^7.5kyris continuedright back to the early-Holocene alreadywithin a fewtens ofkm ofall the flowlineorigins for maximum suggestedby the Agassizmelt-fea ture trend. The squares inFigure 8b showwhat happens to the PennyI ce Cap that core(see Fig.1).Inthe PennyI ce Capcore, however , * DISTdependingon the impurityis either about5 (formarine (d, Cl/Cl )datapairs after this correctionhas been applied to sources) or2.5(forthe more remote continentalsources) (see the early-Holocene d’s.Therelationship now appears to be Fig.6).Thisis becausethe PennyI ce Capcore flowline origins single-valued.Similarly ,the Summit pairsare given in Figure 8a,but this looksvirtually identical to the uncorrectedversion, mayhave been deep inside the Laurentideice sheet during * the ice age.After extensiveexperimentation, these were becauseof the shapeof the ( d, Cl/Cl )relationship.Figure 8 foundto be the valuesthat best fit the ( d18O, Cl) and (d18O, presents the marine-impurityrepresentati veCl, and the con- Ca)pairs plots using the series ofFigures 2 and3 forSummit tinentalCa is givenin Figure 9 forboth sites. andPenny I ce Cap,respectively .

RESULTSANDDISCUSSION CORRECTING THEEARLY-HOLOCENE d18O FOR ICE-AGE SPIKED SOURCE WATER Runningthe extendedHansson model as presented using Figure7 presents the Cl/Cl * vs d18Oforthe PennyIce Cap the fullrange of variables allows one to optimize the core.The relationship between these twowould seem tobe model-predicted ( d, Cl/Cl*) and (d, Ca/Ca*)pairs.I twas double-valued,but we suggest that this isdueto early-Holo- foundthat the Hanssonmodel in the extendedform is cene``d18Ocontamination’’ofthe surfacewater of the source neededto produce the ``odd’’lowCl/ Cl * valuesfor the Penny oceansby fresh low- d meltwater fromthe wastageof the IceCapcore and also best fit the PennyIce Capcontinental Laurentideand F ennoscandianice sheets (Fisher,1992). Ca/Ca* record. 154 Fisher and others:Chemistry^ d18Oprofiles from Penny Ice Cap

Fig.8.Measured and modeled relationship between marineimpurity Cl^ and d18Ofor Penny Ice Capand Summitice cores. Fig.9.Measured and modeled relationship between continental- (a)The Summitcor eresults can be fitted best with no signifi- 2+ 18 cant difference in path length (DIST 1)and amodern mar- impurity Ca and d OforPenny Ice Capand Summitice cores. ˆ ine transit time 2.7 times the survival time for the injected salts. (a)The Summitcore results can be fitted best with no significant differencein path length (DIST 1)and amodern contin- The Summitmarine-impurity concentration in glacial times is ˆ 5^10times the present value.(b)The Penny Ice Capcor e ental transit time 5times the survival time for the injected results need significantly longer flowlines during the Late- impurity.The Summitmarine-impurity concentration in glacial glacial in order to model the lower marine salts during glacial times is 30^100times the present value.(b)The Penny Ice Cap times (i.e.DIST 5).The modern marine-salt transit time is core results need significantly longer flowlines during the Late- ˆ as above,which is reasonable.Glacial marine concentrations glacial in orderto modelthesomewhat highercontinental impurity during glacial times (i.e.DIST 2.5).The modern contin- are lower than modern values,which is unique and the starting ˆ observation for this whole extension. ental impuritytransit time isclosetothat from Summit,i.e.1.75^ 5timesthe modern survival time.Glacial continental concentrations are 2^15times higher than modern values.That the value Summit marine of DISTis 5for marine salts and 2.5 for continental impurities The Summit (d, Cl/Cl*)pairsover the last 40kyrare is to be expected, because the marine sources,being closer at pres- shownin Figure 8a assquares, andone can clearly see the ent,willbecome relatively much further away from aremote ice-ageCl is upto 1 0times higherthan typical Holocene flowline start than asource already veryfaraway (see Fig.6). values.Using the A=A¤ and n=k functionsfrom Figure 5a, the datapairs can be fit withno significant change in the of Ca/Ca* variationis much greater,being 460 larger Summit flowlineorigin, i.e. DIST 1,andwith a modern duringthe last Late-glacial.Both the ( d,marine) and( d, ˆ marine-impurity relativetravel time t¤=½¤ 2.7.Thissug- continental)impurity pairs span about 40 kyrand can be ˆ gests the marine-impurity source isrelativelynearby .DIST explainedwith DIST 1. ˆ 1means onlythat there isnotmuch roomto extend the ˆ lengthof the flowlinethrough Summit. PennyIce Cap marine * Summit continental Figure8b shows Cl/ Cl plottedagainst d forglacial and Holocenetime. Early-Holocene dshavebeen corrected, as Bycomparison, the continental-dustindicator ,Ca/Ca * (Fig. described above,to remove the early-Holocenesurface 9a),fits best (DIST 1,asbefore)with t¤=½¤ 4^5, sug- waterLaurentide doping. The A=A¤ and n=k functionsof ˆ ˆ gestinga more remote source forthese impurities.Therange the corrected d areas inFigure 5b. In order for the model 155 Fisherandothers:Chemistry^ d18Oprofiles fromPenny Ice Cap to fit the (d,marine) datapairs for Penny I ce Cap,DIST Abetter estimate of X¤ wouldcome from considering ˆ contin 5 and t¤=½¤ 2.5.Thelatter iscloseto the Summit value,as the NorthAmerican ( X¤ ) and Asian (X¤ )distances sepa- ˆ na as wouldbe expected for a species-specific constant.D IST 5 rately,althoughthis introducesmore unknownquantities. ˆ isneededto produce ice-age Penny Ice CapCl/ Cl *’s smaller Xna¤ 2000 Xas¤ 2000 thanrecent ones.All othernorthern sites havehigh marine- DIST ‡ Wtna ¡ Wtas 2:5 ; impurityconcentrations in their ice-ageice. One would ˆ Xna¤ ‡ ‡ Xas¤ ˆ expectBarnes I ce Capto share aPennyI ceCap-likesigna- ture (Zdanowiczand others, 2002).Thissignature,as Figure where Xna¤ and Xas¤ arethe present distances toNorth Amer- Wt Wt 6illustrates, suggests that duringthe Laurentidemaximum icanand Asian sources and na and as arethe Late-gla- (alsoshown in Fig.1)the flowlineorigin for the PennyIce cialrelative weights (contributions) .If,for the sakeof Wt Wt X Capcore ice was5 times further awayfrom the relatively argument, na and as areequal and as¤ is 10 000 km, X nearbyocean sources. then na¤ is onlyabout 600 km presently and2600 km at Thissimple additionto the modelallows one to estimate the Last GlacialMaximum. This seems intuitivelytoo the distanceto the sources, atleast insome first-order sense. small.This intuitive unease could be remedied withrel- Supposethe moderndistance to the marinesource is denoted, ativelylarger contributions of North Americansources end- ingwith the first continentalexample above. Xmarine¤ .Lookingat Figure 1 ,the highestand furthest origin pointfor P ennyIce Capice wouldbe K eewatinDome Thesearguments are very simple andbased on avery (``KK’’),whichis about2000 km from the BaffinIsland east simple model.Thatthey seem tomake simple sense, possibly coast.During the GlacialMaximum, when the flowlineori- means that toa first orderthey and the extendedHansson ginwas deep inside the mainice sheet, the maximumdis- modelare correct. tanceto source wouldbe ( Xmarine¤ +2000)km.So: X X¤ 2000 DIST marine ‡ 5 ; ACKNOWLEDGEMENTS ˆ X¤ ˆ Xmarine¤ ˆ and thus X¤ 500km, or about 5³ of latitude. This Helpfulfeedback from M. Hanssonwas appreciated. Help- marine ˆ implies the marinesource areafor the Baffincore sites fulandconstructive reviewsfrom K. Goto-Azumaand J .P. includesBaffin Bay ,DavisStrait, andthe northeastAtlantic Steffensen werealso very useful. boundedby Labrador ,West Greenlandand N ewfoundland. Thisconstitutes arelativelylocal source forsea salts andother marineimpurity .Thesame localsources providemost ofthe REFERENCES marineimpurity for Summit. Dahl-Jensen,D .,S. J.Johnsen, C. U.Hammer,H.B. Clausenand J .Jouzel. PennyIce Cap continental 1993.Past accumulation rates derived from observed annual layers in theGRIP ice core from Summit, centralGreenland. In Peltier,W.R., * ed.Ice inthe climate system .Berlin, etc.,Springer-V erlag,5 17^532.(NA TO ThePenny I ce Cap( d, Ca/Ca )pairs(Fig 9b) have a more ASI SeriesI: Global Environmental Change 1 2.) typicalGlacial to Holocene signature, with 5^1 0times higher Dyke,A. S., J.Hooperand J .M.Savelle.1 996.A historyof sea ice in the Cain the ice age.Thedata pairs can befit with t¤=½¤ between CanadianArctic Archipelago basedon postglacial remains of the Bowhead 2.5and4 (forSummit it is4^5)andD IST 2.5insteadof the Whale (Balenamysticetus ). Arctic, 49(3),235^255. ˆ Fisher,D.A., N.Reeh and K. Langley .1985.Objective reconstructions of the marinevalue of 5 .Assuggestedby Figure 6, this differencein LateWisconsinanLaurentide ice sheet. Ge¨ ogr.Phys. Quat. , 39(3),229^238. DISTis ageometric effect ofthe continentalsources being Fisher,D.A.,R.M .Koernerand N .Reeh.1995.Holoceneclimatic records from muchfurther away.Theaugmentation of the distanceto these AgassizIce Cap ,EllesmereIsland ,N.W.T.,Canada. Holocene, 5(1),19^24. sources bythe Late-glacialflowlines originating at KK in Fisher,D.A. and12others .1998.Penny Ice Cap cores, Baffin Island,Canada, andthe Wisconsinan F oxeDome connection: two states of HudsonBay Figure1 is the same asforthe marinesalts, i.e.2000 km. If ice cover. Science, 279(5351),692^695. the present averagedistance to modern continental source Hansson,M. E.1994.The Renlandice core: a NorthernHemisphere record areas is X¤ then, asabove,the glacialdistance is ( X¤ ofaerosolcomposition over 1 20000 years. Tellus, 46B(5),390^418. contin contin 18 +2000)km and Johnsen,S. J. and14others . 1997. The d Orecordalong the Greenland Ice CoreProject deep ice core and the problem of possible Eemian climatic X Xcontin¤ 2000 instability. J.Geophys.Res. , 102(C12),26,397^26,410. DIST ‡ 2:5 ; Koerner,R.M .andD .A. Fisher.1990.A recordof Holocene summer climate ˆ X¤ ˆ X¤ ˆ contin froma Canadianhigh-Arctic ice core. Nature, 343(6259),630^631. and thus X¤ 1333km, or about 1 3³of latitude. This Mayewski,P .A. and 6 others.1997.Majorfeatures and forcing of high-latitude contin ˆ implies the modernsource regionincludes most ofeastern NorthernHemisphere atmospheric circulation using a 110,000-year-long andcentral N orthAmerica down to latitude 50³ and that glaciochemicalseries. J.Geophys.Res. , 102(C12),26,345^26,366. Reeh,N .,H.Oerter ,A. Letre ¨ guilly,H.Miller andH.- W.Huberten.1 991.A duringthe ice agethe ( X¤ + 2000) 3333km puts the contin ˆ new,detailedice-age oxygen- 18recordfrom the ice-sheet margin in central sources 433³of latitudeaway ,asfarsouth as 30³N andwell WestGreenland. Palaeogeogr.,Palaeoclimatol., Palaeoecol.,Ser.GlobalPlanet. southof the marginof the Laurentideice sheet. Thesedis- ChangeSect . , 90(4),373^383. tances forPenny I ce Capdust-source areasmust beconsid- Zdanowicz,C. M.,G.A. Zielinski, C.Wake,D .A. Fisher andR. M. Koerner. 2000.AHolocenerecord of atmospheric dust depositionon the Penny Ice ered minimums, becausethe Asiansources with Xcontin¤ in Cap, Baffin Island,Canada. Quat. Res., 53, 62^69. the 10000^20000 km rangewere excluded in the abovecal- Zdanowicz,C., D.Fisher,I.Clark and D .Lacelle.2002. An ice-marginal 18 culation.In fact the GlacialMaximum flowline origins for d Orecordfrom Barnes Ice Cap, Baffin Island,Canada. Ann.Glaciol. , the PennyI ce Capwould have been geometrically closer to 35 (seepaper in this volume). the Asiansources.

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