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Journal of Glaciology , Vol. 49, No.164, 2003

Icecores from sub-polar : chronology and post-depositional processesdeduced fromradioacti vity measurements

Jean Francis PINGLOT,1 Rein A.VAIKMA« E,2 Kokichi KAMIYAMA,3 Makoto IGARASHI,3 Diedrich FRITZSCHE,4 Frank WILHELMS,5 Roy KOERNER,6 Lori HENDERSON,7 Elisabeth ISAKSSON,8 Jan-GunnarWINTHER,8 Roderik S.W.VANDE WAL, 9 Marc FOURNIER,10 Patrick BOUISSET,10 Harro A.J. MEIJER11 1Laboratoire de Glaciologie et Ge ¨ ophysique del’Environnement, CNRS,BP96 ,38402 SaintMartin d’He © res Cedex,France E-mail: [email protected] 2Institute of Geology,TallinnTechnical University,7Estonia Avenue,EE-10143 Tallinn,Estonia 3Meteorological and Glaciological Section, National Institute of Polar Research,1-9-10 Kaga 1-chome,Itabashi-ku,Tokyo 173-8515,Japan 4Alfred-Wegener Institute for Polar and Marine Research,P.O.Box 600149,D-14473 Potsdam, Germany 5Alfred-Wegener Institute for Polar and Marine Research, P.O.Box 120161,D-27515 Bremerhaven,Germany 6Glaciology Section,Terrain Sciences Division,Geological Survey of Canada, 601Booth Street,Ottawa,Ontario K1A0E8 ,Canada 7Department of Earth Sciences,University of Ottawa,140 Louis Pasteur,Ottawa,Ontario K1N 6N5,Canada 8Norwegian Polar Institute,Polar Environmental Centre,N-9296 TromsÖ, 9Institute for Marine andAtmospheric Research,Utrecht University,Princetonplein 5,3584 CCUtrecht ,The Netherlands 10Institut deProtection et deSu ª rete¨ Nucle¨ aire,DPRE,SERNAT,LMRE-Orsay,Ba ª t.50 1,Bois des Rames,91400 Orsay Cedex,France 11Centre for Isotope Research,University of Groningen,Nijenborgh 4,9747 AG Groningen,The Netherlands

ABSTRACT.Theresponse ofArctic masses toclimate changeis studied usingice cores containinginformation on past climatic andenvironmental features. Interpretation ofthis informationrequires accuratechronological data. Absolute dating of ice cores from sub-polarArctic glaciersis possibleusing well-known radioactive layers deposited by atmosphericnuclear tests (maximumfallout in 1963)andthe Chernobylaccident ( 1986). Analysisof severalisotopes ( 3H, 137Cs)showsthat 3Hprovidesthe most accuratedating of the 1963maximum, as indicatedalso in comparisonwith results fromtotal-beta measure- ments (90Sr and 137Cs).Meanannual net mass balancesare derived from the datedice cores from1963up to the dateof the drillings.The 137Cs and 3Hdepositedby nuclear tests, after decaycorrection, are used todefine a melt indexfor all 1 3ice cores studied. Therelative strength ofmelting and percolation post-depositional processes isstudied onthe basis of these 137Cs and 3H deposits.

INTRODUCTION different isotopeswhen subjected topost-depositional pro- cesses, inparticular melting and percolation (and wind Thisstudy is mostlyconcerned with the datingof ice cores scouringfor the Chernobyllayer) ,asstudied byPrantl and fromArctic sub-polarglaciers and small ice caps,not others (1973).Theextent ofthese processes is estimated for includingthe coldsnow layers of the Greenlandplateau. Both eachice corestudied. After correctingfor the 1963mean meltingand percolation occur at these locationsand conse- fall-outdate or the absolutedate of eachsnow layer ,amelt quentlydating based on stratigraphy or the annualvariation indexcan be derived giving the relativemagnitude of the

View metadata, citation and similar papers at core.ac.uk brought to you by CORE ofkeyparameters provided by(stable Electronic Publication Information Center isotopes,major ionic species, etc.) meltingand percolation processes. Theseice-core chronolo- canbe veryuncertain. However ,these ice cores canbe dated giescan be used todetermine the meanannual net mass accuratelyon the basisof well-known radioactive layers ori- balance(MANMB) from1 963to the dateof the drillings. ginatingfrom past atmosphericnuclear tests (1954^74),and Thefall-out of 3H and 137Csisstudied overan archipelago nuclearaccidents (Chernobyl, 1 986).Pastvolcanic events (,N orway)and over the Arctic area.Wehaveused (e.g.LakagigarIsland, 1 783or Bezymianny^Kamtchatka, the Chernobyllayer extensively in previous studies forabso- 1956)canin some cases substantiatethe chronologyobtained lute datingof shallow ice cores (Lefauconnierand others, usingthese radioactivemarkers (Fritzsche andothers, 2002). 1994;Pinglot and others,1994,1999,200 1). Theaim of this studyis tocompare chronologies deter- minedusing different radioactivityprofiles for ice cores retrieved fromseveral glaciers in the high-Arcticarea. Arti- STUDYAREAöPREVIOUS WORK ficialisotopes from nuclear tests ( 3H and 137Cs) wereana- lyzedalong with natural isotopes (mainly 210Pb). These Wehavestudied the distributionof natural and artificial chronologiesallow us tocompare the behaviourof the radio-isotopes(from atmosphericnuclear tests andthe 149 Journal of Glaciology

Fig. 2. 137Csfall-out atTromsÖand 3Hcontent in fall-out at Isfjord Radio from atmospheric nuclear tests.

drillingteams (Tarussov,1992);andat V estfonnain 1 995and Austfonnain 1998and 1999by the JapaneseN ationalInstitute ofPolar R esearch (Watanabeand others, 2001)(Fig.1b). Complementaryice cores fromthe highArctic wererecov- ered fromDevon I ce Cap,DevonIsland, N unavut,Canada, in 1998and from Akademii N aukice cap,Severnaya Zemlya, EurasianArctic, in1 999and 2000 (F ritzsche andothers, 2002)(Table1 ).Theice cores weresampled from the surface to30^40 m depth,depending on the location,for measure- ments ofradioactivity.Tenof the ice cores wereanalyzed for both 137Cs and 3Hfromnuclear tests and 210Pb,while three cores wereanalyzed only for 137Cs and 210Pb.

ARTIFICIALAND NATURALRADIOACTIVITY

Artificialradioactivity in Arctic glaciersis mainlyrelated to the atmosphericnuclear tests conductedfrom 1 954(the beginningof atmospheric fall-out) to 1 974.The maximum radioactivityin the Arctic occurredin 1 963(UKAEA, 1957^ 97;Theodo¨rsson, 1977;IAEA,1 984)(Fig.2).Thiswas due to nucleartests conductedin September^N ovember1 961at Semipalatinsk( 50³N ,80³E; 120MtTNTeq .)andin August^ December 1962at N ovayaZemlya ( 71-73³N ,55³E; 180MtTNTeq .)(Aarkrogand others, 1994).Thelong-lived productsfrom these events are 137Cs(half-lifeof 30 .15years), 90Sr (28.15years)and 3H(12.34years) as wellas transuranic elements (notstudied inthis paper). Inorder to compare the radioactivityprofiles measured in ice cores withthe originalatmospheric signals, Figure 2 includes 137Csfall-out at TromsÖ,N orway,for1955^80(W right andothers, 1999)and 3Hfall-out,expressed inTU m, at IsfjordRadio ,Svalbard,for 1 961^76( 1TU 0.118 Bqkg^1). Fig.1.Maps of the high Arctic (a)and Svalbard (b)showing ² ice-corelocations. Bothprofiles reflect the atmosphericnuclear tests. These locationsare quite close to the high-Arcticglaciers, so they clearlyindicate transfer fromthe atmosphereto the snow Chernobylaccident) in ice cores retrieved fromhigh-Arctic layers. The 137Csrecordexhibits a first peakin 1 959,anda glaciers,in particular from Devon Island (Canada) ,Svalbard maximumpeak in 1 963.Tritium wasnot monitored in Sval- (Norway)and Severnaya Zemlya (R ussia) (Fig.1a).Radio- bardbefore 1 961. 3Halsopeaked in 1 959and 1 963.A peakof activitywas measured insamples from1 3ice cores (Table1 , 3Hfall-out(about ten times the annualnatural average) see alsofor name abreviations) containing deposits fromthe alsooccurred at Isfjord Radio in 1 972,a yearwith a high atmosphericnuclear tests (from 1954^74).Elevenof the ice annualprecipitation feature with twice the meanvalue cores weredrilled in Svalbard: SnÖ fjella (Goto-Azuma and (IAEA,1984).Asdiscussed below,this 3Hpeakin 1972has others,1995);ÐsgÔrdfonna (U chidaand others, 1996);Finster- notbeen detected instudies ofhigh-Arcticice cores fromthe walderbreen(Pinglot and others, 1997);Lomonosovfonnain Greenlandice sheet (Koideand others, 1982),MountLogan, 1997(Isaksson and others, 2001)and2000; and on Nordaust- YukonTerritory,Canada(Holdsworth and others, 1984)and landet,respectively ,atVestfonnain 1981(Punningand others, AgassizI ce Cap,Ellesmere Island,Canada (Kotzer and 1986)andAustfonna in 1985and 1987by former SovietU nion others, 2000). 150 Pinglotand others:Radioactivity measurements in sub-polarArctic ice cores

Table1.Locations of 13 ice cores retrieved from the high Arctic

Areas names Ice cores Coordinates Altitude Sources (Drillingyear) Longitude m a.s.l.

Svalbard Vestfonna Ves81(1981) 79³56’ N 19³31’E 580Punning andothers ( 1986) Ves95( 1995) 79³58’ N 21³01’E 600 Austfonna Aus85( 1985) 79³50’ N 24³08’ E 783* Aus87( 1987) 79³50.6’ N 24³08.02’ E 783* Tarussov( 1992) Aus98( 1998) 79³48’ N 24³00’ E 758 Aus99( 1999) 79³50’ N 24³00’ E 783 SnÖfjella Sno92( 1992) 79³08’ N 13³17’ E 1190Goto-Azuma and others ( 1995) ÐsgÔrdfonna Asg93( 1993)79³ 26 ’38’’ N 16³42’3’’ E1140Uchida and others ( 1996) Finsterwalderbreen Fin94( 1994)77³ 25 ’46’’ N 15³18’12’’ E 668 Lomonosovfonna Lom97( 1997)78³ 51 ’53’’ N 17³25’30’’ E1250Isaksson and others ( 2001) Lom00( 2000)78³ 51 ’48’’ N 17³25’18’’ E 1250 SevernayaZemlya AkademiiN aukice cap Sev99( 1999) 80³31’ N 94³49’ E 765F ritzscheand others ( 2002) ArcticCanada DevonIce Cap Dev98( 1998) 75³00’ N 82³00’ W 1800Koerner and Taniguchi( 1976)

* Best estimatedvalues.

More recently (26April 1 986),the Chernobylaccident bothartificial and natural isotopes, without any possible also spread 137Csallover the Northern Hemisphere glaciers discrimination.In Svalbard glaciers and in other locations (Pourchetand others, 1988).Bothnuclear tests andthe inthe Arctic, artificialand natural radioactivity are of Chernobylaccident occurred in the Northern Hemisphere equivalentmagnitudes. This explains why the total-beta- andmost Arctic ice capsand glaciers received the corres- countingtechnique is notalways valid for the detectionof pondingfall-out (Pinglot and others, 1994). the 1963or Chernobyl layers. Naturalradioactivity comes from 210Pb(half- of 22 Inorder to properly quantify 137Cs and 210Pb, we used years),adecayproduct of 238U.When the parentisotope high-resolutiongamma-ray spectrometry .Our equipment (238U) disintegrates to 226Ra, 222Rn (anoble gas) escapes from isdesignedto detect verylow levels of radioactivity,includ- the .This isotope, after severalshort-lived disintegration inga 20%high-purity Ge (N-type) detector,withan anti- processes, givesrise to 210Pband tends toreach a secularequi- Comptonscintillation detector (Pinglotand Pourchet,1 994). libriumin the . 7Be( 53.6days)originating from Thedetection levelsfor 137Cs and 210Pbare4 and1 0mBq, cosmic rays,is anothernatural isotope found in glaciers. respectively,for3 daymeasurements witha 97.5%confi- Bothartificial and natural isotopes are deposited on the dencelevel. 137Cs and 210Pbaremeasured atthe same time. snowsurface mainlyby washout and to a lesser extent by dryfall-out (Pinglot and others, 2001).

Table 2.Depths (mw .e.)ofthe 1963 layer,from 3H and 137Cs, SAMPLING ANDANALYTICALMETHODS for ten ice cores,with correspondingdepth and dating differences

Allice-core samples werecollected using shallow or deep electromechanicaldrilling equipment. Sub-samples for Ice SampleIsotopes 1963depth Depth difference Dating 137 3 radioactivitymeasurements camefrom the surface downto cores length min. max.mean Cs to Hdifference about40 m(maximumdepth corresponding to the first fall- mw.e. mw.e.m w.e.m w.e.m mw.e.years outfrom atmospheric nuclear tests) .Thelength of each samplevaried from 5^200 cm, dependingon the ice-core Ves81 0.54 3H5.676.2 15.940. 670.5 11.56 137 locationand the measured isotope(T able2) .Inorder to 0.91 Cs 6.006.9 16.45 Ves95 1.20 3H10.861 2.061 1.460.8 10.721 .99 obtainan age-scaleequivalent, snow depths wereconverted 0.91 137Cs 11.831 2.521 2.17 todepths expressed inmetres ofwater equivalent (m w.e.), Aus85 0.37 3H9.429. 799. 600.80 0. 721 .64 usingthe densificationof snowwith depth. 0.88 137Cs 9.881 0.761 0.32 3 The 3Hanalysiswas conducted on melted sub-samples Aus98 0.22 H15.751 5.971 5.860. 120.110.25 0.44 137Cs 15.751 6.1915.97 (5cm long),followedby liquid-scintillation counting .The Aus99 0.37 3H15.701 6.071 5.880.00 0.00 0.00 1997Lomonosovfonna profile was measured usinglow-level 0.37 137Cs 15.701 6.071 5.88 proportionalcounters, forwhich technique a sample Sno92 0.13 3H12.061 2.1812.120.500.44 0.94 137 amountof only 5 mL suffices. Therefore,the spatialreso- 1.76 Cs 11.681 3.441 2.56 Asg93 0.38 3H 9.02* 10.04* 9.53 0* 0* 0* lutionof this specific profilecould be higher. 0.38 137Cs 9.021 0.049. 53 Total-betameasurements werecarried out on melted Lom97 0.04 3H12.881 3.081 2.98^0. 10^0.07^0.2 1 samples filtered throughion-exchange papers (Delmas and 0.35 137Cs 12.731 3.081 2.90 90 Lom00 0.38 3H8.869.24 9. 050.00 0.02 0. 11 Pourchet,1977;Pinglotand Pourchet,1979)andinclude Sr, 137 137 210 0.38 Cs 8.489.67 9. 07 Cs and Pbcations,which are insoluble particulates. All 3 3 90 Dev98 0.07 H8.188.328.25 ^0. 34^0. 19^0.79 the above-describedisotopes emit betarays ( H and Sr 0.12 137Cs 8.008. 128.06 arepure beta emitters) .Thetotal-beta-radioactivity meas- urement ofsnow samples fromice cores is the amountof * Best estimatedvalues. 151 Journal of Glaciology

Fig.3.Radioactivity at time of fall-out ( 137Cs and 3H), and at 210 time of measurement (totalbeta and Pb,respectively,1986 Fig.4.Radioactivity at time of fall-out ( 137Cs and 3H), and at and1993)vsdepth for Devon (a ,b)and Austfonna (c, time of measurement (totalbeta and 210Pb,respectively,1988 3 d). Hmeasurements from Devon Island (thinline) ar enot and 1993)vs depth for three ice cores from Austfonna (a^d). continuous.The arrow indicates the equivalent 1954 computed The arrow indicates the equivalent 1954 computed year. year.

aglacierwith negative (^23³ C) 10mtemperature (Devon RESULTS IceCap)will be comparedwith profiles from Austfonna, a DevonIce Cap and Austfonna sub-polarglacier .Incold glaciers the 10mtemperature is negative,closely representing the meanannual tempera- Asareference, the radioactivityprofiles of anice corefrom ture. Inthe accumulationarea of sub-polar glaciers, the 152 Pinglotand others:Radioactivity measurements in sub-polarArctic ice cores

10mtemperature is generallyabout 0³ C orslightly nega- tive.I tis negativein the ablationarea. Forthe DevonIce Capice core,there is excellentagree- ment betweentotal-beta 137Cs and 3Hmeasurements, the 1963peak of artificialradioactivity occurring at almost the same depth(Fig .3aandb) .The1963horizon was generally assignedto depths wherethere wasa clearpeak of radio- activityfor either 3H or 137Cs.F orat least twoice cores (Aus87and V es81)the identificationof the 1963horizon needs specialattention. This 1963date was fixed in the fol- lowingway: after the largeatmospheric nuclear tests in1 961 and1 962there wasa moratorium,with no other tests for some years.Even if there isnota clearmaximum, one can considerthat whenthe radioactivity,after rangingat a givenlevel, falls to a lowvalue at shallower depth, then the 1963horizon is quite closeto this depth. The 3Hprofileis notcontinuous, and extrapolated valueswere included for estimating the fall-out.Thus the exactlocation of the 1963maximum is estimated tooccur atdepths of8.18^8.32m w.e.(Table2 ).The 137Csprofilealso showsthe 1959peak. As willbe shown later forother ice cores (Figs4^6 ),the beginningof atmospheric fall-out in 1954is wellmarked only for the Devonice core(Koerner andT aniguchi,1976).Theinitial increase of 137Cs compared to 3Hhasalready been pointed out by Koide and others (1982)andHoldsworth and others (1984). Forthe 1985Austfonna ,the radioactivityprofiles (Fig.3candd) includetotal beta, 137Cs, 210Pb and 3H. For the 137Csprofile, a clearmaximum occurs at9 .88^10.76m w.e. depth,indicating the 1963maximum. This is inclose agree- ment withthe 1963 3Hmaximum,which is, however,slightly shallowerthan the 137Csmaximum.The 3Hatomsare con- stituents ofthe watermolecule and, compared to the 137Cs particulates,much of the radioactivity(atoms) contributing to the 3Hmaximumwas not propagated downwards. The 137Csprofile shows another maximum at about 1 3.5mw.e., possiblyrepresenting the 1959fall-out (Fig .3c). Thetotal-beta profile (Fig .3d)cannotbe used fordating . Theclear maximum corresponds to fall-out that occurred wellbefore 1 963,as measured from 137Cs.I tis closeto the bottomof the ice coreand does not represent anyartificial radioactivity.Thismaximum corresponds to a veryhigh Fig.5.Radioactivity at time of fall-out ( 137Cs and 3H), and at level of 210Pb,as shownon the profile(Fig .3d).Verysimilar time of measurement (totalbeta and 210Pb,respectively,1982 disturbed total-betaprofileswere also measured forAustfonna and1993)vsdepth for three ice coresfromVestfonna (a^c).The 1987(Fig.4d)and V estfonna1 981(Fig.5c) .This 210Pb arrow indicates the equivalent 1954 computed year. increase is notsupported by long-lived parents ( 238U, 226 Ra).Thetotal-beta values involve beta activity from ence rangesfrom 0^0 .72m w.e.corresponding to 0^ 1.6years 210Pb,but also the accompanyingalpha and beta activities 210 210 210 ofnet accumulation,while the atmosphericsignals indicate from the Pbdaughters ( Bi and Po)and to a lesser 137 3 226 214 that both Cs and Hfall-outpeaks occurred at the same extent fromradioactive dust particles ( Ra and Pb from 137 238 210 time (1963;Fig .2).Thisconfirms that the Cs fall-out Uparent).Sucha large Pbincrease alsooccurs inallice experiences downwardmigration due to melting and perco- cores retrieved fromAustfonna ( 1985,1987and 1 998),except 210 137 3 lation.T otal-betaand Pbprofilesat Aus87 (Fig .4d)and forthe 1999ice core.Although the Cs and Hprofilespro- Aus85(Fig .3d)are disturbed. Thisdemonstrates that the videvaluable dating information at Austfonna and for ten 210 absolutedating of ice cores inthis sub-polarice capdepends otherice-core locations(Pinglot and others, 1994),the Pb onthe isotopemeasured: the 3Hprofilegives a better chron- andtotal-beta profiles reveal a strongscavenging process ology than 137Cs. dueto melting and infiltration for these particulate-com- posedelements, asfor ionic species (Goto-Azumaand Vestfonna others, 1993).Chronologiesbased on the radioactivedecay ofa givenisotope (e.g . 210Pb)are generally corrupted by Samplesfrom two ice cores fromV estfonnawere also ana- post-depositionalprocesses. lyzed for 137Cs and 3H(Fig.5aandb) . Along the 137Csprofiles,the 1963maximum is locatedat Forthe 1981Vestfonnaice core(Fig .5b),there is aclear greaterdepth than in the 3Hprofiles(Fig. 4a andb,Table2 ). deformationof the originalatmospheric signal both for Forthree ice cores fromAustfonna, the 1963depth differ- 137Cs and 3H. Thetentative depth given for the 1963peak is 153 Journal of Glaciology

Fig. 7. 137Cs and 3Hfall-out ratio from atmospheric nuclear tests,determined from1963mean time of fall-out and absolute date of snow layers,respectively.

atmosphericsignal while the 137Csprofileshows large tem- poralvariations. The1963layer at Lom00 ( 9.05m w.e.)isnot asdeepas the Lom97ice core( 12.98m w.e.)(Table2) .Asdis- cussed below,the low 3Hfall-outat Lom00 most probably indicatesthat the originalprecipitation is low,asis the MANMB. The 210Pbprofile(Fig .6b)at Lom97 indicates a generaldecrease ofactivitywith depth. However ,datingof this ice corefrom 210Pbdecay does not reproduce the 1963 horizon.

AkademiiNauk ice cap

Inorder to extend our study of the spatio-temporalvari- ationsof the MANMB andthe globalmass budgetto other glaciersand ice capsin the highArctic, 137Cs and 210Pb profileswere also measured inan ice corefrom Akademii Naukice cap(Fig .6c).The1963peak for 137Csiswell-defined between1 5.55and 1 6.73m w.e.This is inagreement with the 1956Bezymianni volcanic layer studied byF ritzsche and others (2002).Previously,Vaikma « eandothers (1980)ana- lyzed 3Hatthe nearbyV avilovice capand concluded that Fig.6.Radioactivity at time of fall-out ( 137Cs and 3H), and at 210 the correspondingprofile was disturbed. The1954computed time of measurement (totalbeta and Pb,respectively,1998 137 year of Csfall-out does not correspond to this absolute and 2000)vs depth for two ice cores from Lomonosovfonna (a, date.Therefore it couldbe concluded that meltingalso b)and for an ice core from Akademii Nauk ice cap (c).The occurs atthe summit ofAkademii N aukice cap,which is arrow indicates the equivalent 1954 computed year. supportedby positive air temperatures recordedby an auto- matic weatherstation here insummer 2000(Hagen and based on the 3Hprofileas the 137Csprofileshows a strongly Melvold,200 1).Vaikma « eandothers (1981)evenreported disturbed signal.The depth difference rangesfrom 0. 60^ temperature of+10^15³C atSevernaya Zemlya. 0.73m w.e.and corresponds to about 1 .7and1 .9years of net accumulationfor V estfonna1 981and1 995,respectively INTERPRETATION ANDDISCUSSION (Table2) .Notethat the summit ofV estfonna( 580^ 600ma.s.l.)is lowerthan Austfonna ( 758^783m a.s.l.),so Chronology highermelting is more likelyat Vestfonna. Theice cores fromthe Arctic areahave been dated using Lomonosovfonna radioactivitymeasurements. Themost accuratedetection ofthe 1963peak of artificialradioactivity is obtainedfrom Thesame analyseswere conducted for two ice cores from 3Husingthe proportional-countingtechnique, and then Lomonosovfonna,drilled in 1 997(Lom97 )and2000 from 137Cs.This is dueto the higherresolution of samples (Lom00)(Table1 ),locatedabout 1 50m fromeach other (Fig . subjected to 3Hanalysis,0 .04and 0. 07m w.e.for the Lom97 6aandb) .Forthe 1997core, there aresimilar trends for 137Cs andDev98 ice cores, respectively(T able2) .Gamma spec- and 3Handthe 1963peak is well-defined.The detailed 3H trometry of 137Csneeds samples withhigher mass and profile(preliminary ,asall samples arenot yet analyzed) length,compared to 3H analysis. 3Hdates arealso better wasdetermined fromthe analysisof 5 cm resolutionsamples. than 137Csbecausethe 1963horizons are much stronger for Forthe coredrilled in 2000, the 3Hprofilereflects the 3Handlargely remain at the originalfall-out depth. There 154 Pinglotand others:Radioactivity measurements in sub-polarArctic ice cores

Table 3. 137Cs and 3Hfor13ice cores:MANMB from1963 to dateofdrilling;fall-out from atmospheric nuclear tests (corrected for 1963 mean fall-out date and the absolute date of each snow layer);and fall-out ratio (fall-out at absolute date vs fall-out in1963)

Ice cores 137Cs 3H MANMB Depth1963 Depth abs. Ratio (1963/abs.)MANMB Depth1963 Depth abs. Ratio (1963/abs.) m w.e. Bq m^2 Bq m^2 m w.e. TUm TUm

Ves81 0.35 224 398 1.78 0.33 4595 7381 1.61 Ves95 0.38 369 444 1.20 0.36 5688 7842 1.38 Aus85 0.47 434 626 1.44 0.44 6505 7825 1.20 Aus87 0.45 269 357 1.33 n.d. n.d. n.d. n.d. Aus98 0.46 407 526 1.29 0.45 5440 6200 1.14 Aus99 0.44 293 346 1.18 0.44 4319 4718 1.09 Sno92 0.47 259 308 1.19 0.47 n.d. n.d. n.d. Asg93 0.31 418 555 1.33 0.31 n.d. n.d. n.d. Fin94 0.17 461 907 1.97 n.d. n.d. n.d. n.d. Lom97 0.36 343 398 1.16 0.36 3709 4301 1.16 Lom00 0.23 279 342 1.23 0.23 2979 3893 1.31 Sev99 0.45 549 709 1.29 n.d. n.d. n.d. n.d. Dev98 0.23 310 365 1.18 0.24 4862 4158 0.86

Note:n.d., notdetermined. isgenerallya stronger migrationof 137Cscomparedto 3H, Earlier studies (Zagorodnovand others, 1990)reported asshownbelow (Fig .7).Thedepth differences forthe 1963 MANMB valuesat the summit ofAkademii N aukice cap peak,based on 137Cs and 3H, areindicated in T able2, rangingfrom 0. 20^0.30m w.e.a ^1 comparedto our deter- togetherwith the correspondingage differences. Minimum minationof 0.45m w.e.a ^1. andmaximum depths correspondto the lengthof samples representing the 1963layer .While this layeris welldefined 137Cs and 3Hfall-out andmelt indexes for 3H, the 137Cslayercorresponding to 1 963was in some cases estimated tobe just beforethe decrease ofactivityfol- Both 137Cs and 3Hfall-outfrom atmospheric nuclear tests lowingthe nuclear-test moratorium.However ,foreach ice (1954^74)overArctic glaciershave been determined (Table core,the depthdifferences for1 963is lowerthan the length 3).Thetransport mechanisms (residence time) of 3H and 3 137 ofthe corresponding H or Cssample (Table2) . 137Csthroughthe atmosphere arealmost the same (Pourchet 137 3 Thedifferences indating obtained using Cs and H andPinglot, 1 979),sothey do not affect the locationof the areclose to zero for several ice cores andmay be notsignifi- 1963peak in the cores. Assuminga constantMANMB at cantfor the otherice cores.Themaximum difference may eachice-core location,as already demonstrated forthe extendup to 2 yearsat V estfonnaand slightly opposite periodfrom 1 963to the dateof the drillings(Pinglot and (^0.8year) for Devon I ce Cap.This may be dueto the dis- others, 1999),the 137Cs and 3Hactivityprofiles have been 3 continuous Hsampling,with one sample in the 1963layer decay-corrected,taking into account the precise dateof the that wasnot analyzed. The total-beta activity profile was snowlayers. This will enable us tocompute the fall-outof 3 used toestimate the probabledepth of the 1963layer for H bothisotopes in two ways .Thehypothesis is that the exact (i.e. 8.18^8.32m w.e.). 1963peak did not propagate downward. Even if 1963is not the exactmean date of atmospheric nuclear tests fall-out, Meanannual net mass balances (MANMB) the melt indexstudied is representative ofthe strength of meltingand percolation processes. Allice cores wereretrieved fromthe summits ofthe studied Inthe first correctionfor decay ,weassume that the mean ice capswith very low horizontal velocities and terrain dateof fall-out for all samples is 1963.Thisis areasonable slopes.The thinning effect ofdeeper ice layers(down to assumption,given that the totalperiod of atmospheric nuclear 40minthis study) isnegligible,and we can use the chron- testing extends equallybefore and after 1963.Forthe second ologyresults todetermine the MANMB. correction,each sample radioactivity value is corrected to TheMANMB valuesrange from 0. 17(Fin94)to takeinto account the absolutedate of the snowlayers. F rom 0.47 mw.e. a^1 (Aus85and Sno92 )(Table3 )from1 963to the the dateof drilling,datingis givenby the equivalentdepth drillingdates (1981^2000).Apartfrom a MANMB value (mw.e.)dividedby the MANMB, aspreviouslydetermined. determined forDevon I ce Capby Koerner andT aniguchi Bothcorrections takeinto account the dateof allmeasure- (1976),earlierMANMB valuesdetermined forother Arctic ments, extendingfrom 1 983^2000(T able3 ).Thedepth of the locationsdo not agree with our determinations, due to diffi- layercorresponding to the year1954(first significantincrease culties ininterpreting the stratigraphyand total-beta profiles. ofartificial radioactivity in the Arctic) wasthen determined Thisis particularlytrue forice cores fromN ordaustlandet andincluded on allprofiles (Figs 3^6 ). (Vestfonnaand Austfonna) ,whereMANMBs werepre- Thereis ageneraldisagreement betweenthe 1954com- viouslymisinterpreted ,asshown by Pinglot and others, 2001. putedequivalent depth and the first increase ofisotopes. TheMANMBs ofLom97 and Lom00 ice cores (1250 The 137Csprofilesapparently propagated to greater depths ma.s.l.)are0. 36and 0. 23m w.e.a ^1,respectively.Theseare thanthe 1954equivalent depth. This clearly demonstrates much lowerthan the previous0.82 m w.e.a ^1 valuedeter- the post-depositionalprocesses dueto melting and percola- minedfor a 1000m a.s.l.site (Gordiyenkoand others, 1981). tionduring successive summers. Periodsof meltingduring 155 Journal of Glaciology

Fig. 9. 3H vs 137Csfall-out from atmospheric nuclear tests at eight Arctic ice-core locations.

Thebest estimates for 137Cs and 3Hfall-outare deter- minedfrom the 1963decay-corrected values, as the decay correctionof fall-out,applied for each snow-layer date, leads tooverestimated values.As alreadyspecified, MANMB at eachice-core locationis almostconstant for periods extend- ingfrom 1963^86and from 1986to the drillingdates (Pinglot andothers, 1999).From this importantfeature for ice cores fromSvalbard, 137Cs and 3Hfall-outis studied inrelation to the MANMB foreach core location (Fig .7aandb) ,evenif the allperiods are not the same. 137Csfall-outfrom the nucleartests forall studied cores Fig. 8. 137Cs (a) and 3H(b)f all-out from atmospheric nuclear spansfrom 224 (V es81)to549 Bq m ^2 (Sev99)(Fig. 8a) . tests vs MANMBfor13high-Arctic ice cores. Altogether,the mean 137Csfall-outvalues for the 13Arctic and1 1Svalbardice cores are355 Bq m ^2 ( 95 Bq m^2) and § 341Bq m^2 ( 81Bq m^2),respectively.Forfourice cores near § warmersummers inthe early1 950shave been reported the summit ofAustfonna, the fall-outvalues extend from (Fisher andKoerner ,1994;FÖ rland and others, 1997).This 269^434Bq m^2.Fall-outvalues are 3 10and549 Bq m ^2, isalsoin accordance with the 210Pbprofile(Fig .3d),show- respectively,forthe DevonIce Capand Akademii N auk inga majoractivity peak at greater depths, ashas been ice capcores (Table3) .Thefall-out apparently tends to detected forother Austfonnaice cores (Pinglotand others, decrease fromeastern tomore western Arctic locations.This 1994). 137Cs and 3Hfall-outwill beoverestimated asaresult agreeswith the 137Csfall-out in the Arctic predicted by ofthe overestimationof the ageof deeper samples. Figure7 Wrightand others (1999)using a geographicalinformation indicatesboth the 137Cs and 3Hfall-outratio, as determined system tocombine 137Csfall-outand data. either froma decaycorrection of fall-outcomputed for 1 963 The mean 3Hfall-outfor the same 1954^74periodis orforthe absolutedate of eachsnow layer . 4762 1126TU mforeight ice cores andspans from 2979^ § Thisfall-out ratio represents amelt-index equivalent. 6505TU m(Fig.8b).Forcomparison,the 3Hfall-outmeas- Insteadof a fall-outratio ideally equal to 1 forcold glaciers uredfor the IsfjordRadio coastal station in Svalbard (IAEA, andice caps,the present fall-outratios are close to 1 .2forice 1984)forthe period1 961^76is 2793TUm. Thisis inaccord- cores fromLomonosovfonna (Lom97 ),DevonI ce Cap(Dev98 ) ancewith the 3Hdatafrom ice cores, asthe 3Hfall-outat andSnÖ fjella (Sno92 )(Table3 ).Forthe othercores studied, IsfjordRadio (mean annual precipitation is 0.444m) was the respective fall-outratio increases, rangingup to about 2 notrecorded before 1 961.The 3Hfall-outincreases with forlower-altit udecores fromV estfonna(V es81)andFinster- MANMB (r 0.56),froma previousstudy in Greenland ˆ 137 walderbreen(Fin94 ),at580 and 668 m a.s.l.,respectively . (Merlivatand others, 1973).Anull Csfall-outcorresponds Thefall-out ratio is generallyhigher for 137Cs than for 3H toabout a 1000TU m 3Hfall-out(Fig .8b).Thiscorresponds (fivecores) ,exceptfor V es95and Lom00. This melt-index tothe natural 3H fall-out ( 50 TUma^1)over20 years, the 3 ¹ studyalso demonstrates that percolationis less importantfor periodwhen artificial Hwasalso deposited ( 1954^74). 3H than 137Cs. As the 137Csprofile at Lom00 core reveals Forall ice cores, there shouldbe proportionality largevariations, perhaps the correspondingcorrected fall- between 137Cs and 3Hfall-outfrom atmospheric nuclear outs werenot determined accurately.Thisdemonstrates that tests. Thisrelationship prevails and exhibits a 0.69correla- fora givenice-core location,the studyof the fall-outof radio- tioncoefficient (Fig .9).The 3Hfall-outat Lomonosovfonna activitycan be used todetermine the relativestrength ofthe (Lom97and Lom00) ,3709(estimated value,as some meltingand percolation processes. Thereis evidencethat samples werenot analyzed) and 2979 TU m, respectively,is cores fromcold glaciers are less subjected tothese processes. lowcompared to the otherice cores (meanof 49 13TUm) However,allcores fromsub-polar glaciers experience large (Fig. 8b). post-depositionalprocesses, asdescribed byKoerner (1997) 137Csfall-out in the Lom97and Lom00 ice cores, 343 andTarussov( 1992). and 279 Bq m^2,respectively,is closeto the meanvalue of 156 Pinglotand others:Radioactivity measurements in sub-polarArctic ice cores

Svalbardice cores (341Bqm ^2),indicatingthat winderosion decrease, asseen below30 m depth.Thisindicates that dating maybe low ,but cannotexplain the surprisinglylow based on the 210Pbhalf-life ( 22years) may be impossible for MANMB atthe Lomonosovfonnasites atthe summit ofthis most ice cores fromsub-polar glaciers. ice cap.Analysis of three ice cores fromthe Lomonosovfonna domedid not show the presence ofthe Chernobylsignal, indicatingthat some windscouring occurs atthis location. CONCLUSIONS However,the Chernobylevent was present inice cores Datingof Arctic ice cores basedon the 1963maximum of locatedat lower altitudes (1044and 1 173m a.s.l.).Aclose artificialradioactivity originating from atmospheric nuclear examinationat the relationshipbetween 137Cs and 3H fall- tests wasinitially performed using the total-betatechnique. out(Fig. 9 )revealsthat boththe Lom97and Lom00 ice Thissensitive (andrelatively inexpensive) method is valid cores experienceabout 25% lower 3Hfall-outthan the forice cores retrieved fromcold glaciers, such asthose in meanvalues. This may be dueto snow and 3Hsublimation, high-ArcticCanada, central Greenland and . F or asdescribed byWinther andothers (1998). ice cores retrieved fromsub-polar glaciers that experience The1986Chernobyl layer meltingand percolation in summer ,the best datingis accomplishedusing 3H and 137Cs.Theoriginal fall-out depth 137Csfall-outfrom the Chernobylaccident has been success- ofthe 1963maximum is better preserved forthe 3H profiles fullydetected inabout50 Svalbard ice cores (Lefauconnier formost ice cores. Arctic ice cores havebeen successfully andothers, 1994;Pourchet and others, 1996;Pinglot and datedfrom both atmospheric nuclear tests andthe Cher- others,1994,1997,1999,200 1).More recently,wedetermined nobylaccident based on 137Csmeasurements inSvalbard, the Chernobylfall-out for the Devonand Akademii N auk high-ArcticCanada (Devon Island) and Severnaya Zemlya. ice caps.The 137Csfall-outvaries from 0^47 Bq m ^2 for the The 137Cs and 3Hactivityprofiles have been decay-corrected Svalbardice cores.Wind scouringhas been determined tobe either forthe mean1 963fall-out date or forthe precise dates the mainfactor explaining this variability,asthe fall-out ofthe snowlayers. A fall-outratio, or melt index,has been fromChernobyl only occurred during a fewdays (Pinglot derivedfrom both fall-out values and represents the extent andothers, 2001).Thefall-out on DevonIsland is 5Bqm ^2 ofmelting and percolation for all 1 3ice cores studied. fortwo ice cores.Thisvalueis ofthe same orderas the Sval- Forseverallocations, the 210Pbprofilesindicate a clear bardand Greenland values (Dibb, 1989).On Severnaya mixingof seasonal layers for certain chemicalelements. Zemlya, the 137Csfall-outis about1 Bqm ^2,asdetermined Radioactivitymeasurements confirmthat the seasonal froma shallowice coreretrieved in2000 .Thislow value is signalmay not be preserved insub-polar Arctic glaciers. closeto the detection limit ofourlow-level gamma spectro- On the otherhand, the Lomonosovfonna1 997 3H profile meter.Itwouldappear however to be valid,given that the withhigh spatial resolution indicates the conservationof correspondingmass balancefor 1 986^2000is 0.52m w.e. sub-annualsignals to at least some extent. Continuationof comparedto 0. 45m w.e.for 1 963^2000(T able3 ). the analysisof the 1970’ssamples willreveal how far the semi-annual 3Hprecipitationsignal is conservedin this 210 Dating from Pb profiles core.The dating method based on the 210Pbexponential decrease withdepth (expressed inwaterequivalent) is pos- Both 137Cs and 210Pbwere simultaneously analyzed by gamma siblyvalid for selected high-altitudesites inSvalbard. spectrometry.The 210Pbprofilesfor Aus85, Aus87 and V es81 Theseradioactivity measurem ents provideaccess tokey ice cores (Figs3d, 4d and 5c) indicate a strong 210Pb parameters forthe studyof Arctic glaciers.Theabsolute chron- increase between1 8and24 m w.e.Samples from other ice ologyof ice cores is the most importantfeature. In relation to cores fromAustfonna, Vestfonnaand Spitsbergen were also possibleclimate warmingand -level rise, the MANMB analyzedfor 210Pb.The corresponding 210Pbprofiles are withthe associatedspatio-temporal variations ( 1963^86and includedin Pinglot and others (1994).Other ice cores from 1986to the dateof drilling) can be studied usingradioactive Svalbard(Asg93 and Sno92 )havealso been analyzed for horizons.The total mass budgetof the accumulationarea of 210Pb(Suzuki and F ujii,1 992;Suzuki and others, 1995). the studied glaciersis determined fromthe altitudinalgradient Thereis noexponential decay starting fromthe surface ofthe MANMB, whichis alsoincorporated in mass-balance down,for any profile. The 210Pbprofile(Fig .6b)forLom97 models.Radioactivitymeasurem ents alsoprovide a better indicatesa generaldecrease ofactivity with depth. How- understandingof post-depositional processes, such asmelting ever,the datingof this ice core,based on 210Pbdecay,does andpercolation as well as windscouring ,whichaffect the dis- notcorrectly position the 1963radioactive layer from atmos- tributionof both natural and artificial isotopes. phericnuclear tests. ForAkademii N aukice cap,aformerlydetermined 210Pb profile(Zagorodnov and others, 1990)canbe closely com- ACKNOWLEDGEMENTS paredto the newprofile (Fig .6c).Insteadof a regulardecrease of the 210Pbactivity with depth, the activityis almoststable Thisstudy was funded by several E Ucontracts, including (andeven increases withdepth in the first 10m) fromthe sur- contractN o.ENV04-CT97-0490: ` `Theresponse ofArctic facedown to about 2 5mw.e.Then the activityregularly ice masses toclimate change’’(ICEMASS),coordinatedby decreases, asexpected. This unusual feature was already J.O.Hagen.Comments andadvice from all colleagues of the reported byZagorodnov and others (1990).Ata depthof InternationalArctic ScienceCommittee^Mass balanceof about20 m w.e.,there is evena highlevel of 210Pbactivity . Arctic Glaciers andIce sheets inrelation to the Climateand Theinterpretation may be that 210Pbparticulates travel Sea-levelchanges (IASC^MA GICS)group were very fruitful. downwardinto the deeper firnlayers due to summer melting Logisticaland financial support respectively from the Nor- andpercolation. Once the firnlayers reach a densityclose to wegianP olarInstitute andInstitut Franc°aisde la R echerche 0.9,the migrationof 210Pbstops andits activitybegins to et dela T echnologiePolaires was greatly appreciated. H. 157 Journal of Glaciology

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MSreceived 18March 2002 and accepted in revised form 6January 2003 158