Archival processes of the water stable isotope signal in East Antarctic ice cores

36 Pages·2017·7.09 MB·English

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Archival processes of the water stable isotope signal in East
Antarctic ice cores
MathieuCasado1,2,AmaelleLandais1,GhislainPicard3,ThomasMünch4,5,ThomasLaepple4,
BarbaraStenni6,GiulianoDreossi6,AlexeyEkaykin7,8,LaurentArnaud3,ChristopheGenthon3,
AlexandraTouzeau1,ValerieMasson-Delmotte1,andJeanJouzel1
1LaboratoiredesSciencesduClimatetdel’Environnement-IPSL,UMR8212,CEA-CNRS-UVSQ-UPS,GifsurYvette,
France
2UniversitéGrenobleAlpes/CNRS,LIPHY,F-38000Grenoble,France
3UniversitéGrenobleAlpes/CNRS,LGGE,38400Grenoble,France
4AlfredWegenerInstituteHelmholtzCentreforPolarandMarineResearch,TelegrafenbergA43,14473Potsdam,Germany
5InstituteofPhysicsandAstronomy,UniversityofPotsdam,Karl-Liebknecht-Str.24/25,14476Potsdam,Germany
6DAIS,Ca’FoscariUniversityofVenice,Venice,Italy
7ArcticandAntarcticResearchInstitute,St.Petersburg,Russia
8InstituteofEarthSciences,St.PetersburgStateUniversity,Russia
Correspondenceto:MathieuCasado([email protected])
Abstract.TheoldesticecorerecordsareobtainedfromtheEastAntarcticplateau.Waterisotopesrecordsarekeytorecon-
structing past climatic conditions over the ice sheet and at the evaporation source. The accuracy of climate reconstructions
dependsonknowledgeofalltheprocessesaffectingwatervapour,precipitationandsnowisotopiccompositions.Fractionation
processesarewellunderstoodandcanbeintegratedinRayleighdistillationandisotopeenabledclimatemodels.However,a
5 quantitativeunderstandingofprocessespotentiallyalteringthesnowisotopiccompositionafterthedepositionisstillmissing.
Inlowaccumulationsites,suchasthosefoundinAntarctica,thesepoorlyconstrainedprocessesarelikelytoplayasignificant
roleandlimittheinterpretationofisotopiccomposition.
Here, we combine observations of isotopic composition in the vapour, the precipitation, the surface snow and the buried
10 snowfromDomeC,adeepicecoresiteontheEastAntarcticPlateau.Attheseasonalscale,wesuggestasignificantimpactof
metamorphismonsurfacesnowisotopicsignalcomparedtotheinitialprecipitationsignal.Particularly,insummer,exchanges
of water molecules between vapour and snow are driven by the sublimation/condensation cycles at the diurnal scale. Using
highlyresolvedisotopiccompositionprofilesfrompitsinfiveAntarcticsites,weidentifycommonpatterns,despitedifferent
accumulationrates,whichcannotbeattributedtotheseasonalvariabilityofprecipitation.Altogether,thedifferenceinthesig-
15 nalsobservedintheprecipitation,surfacesnowandburiedsnowisotopiccompositionconstituteevidencesofpost-deposition
processesaffectingicecorerecordsinlowaccumulationareas.
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1 Introduction
Iceisanaturalarchiveofpastclimatevariations.Thechemicalandphysicalcompositionsofice,andtheairbubblestrapped
insideit,areusedaspaleoclimateproxies(JouzelandMasson-Delmotte,2010). Overlargeicesheets,thewaterisotopesin
icecorescanprovidereconstructionsofhistorictemperaturesasfarbackasthelastglacialperiodinWestAntarctica(upto60
5 000yearsago),(WAISDivideProjectmembers,2013),andthelastinterglacialperiodinGreenland(120,000yearsago)(North
GreenlandIceCoreProjectmembers,2004;NEEMCommunitymembers,2013).InEastAntarctica,lowaccumulationrates
enablethereconstructionofpastclimatesoverseveralinterglacialperiods,e.g.420000yearsatVostok(Petitetal.,1999),720
000yearsatDomeF(Kawamuraetal.,2017)and800000yearsatDomeC(EPICA,2004,2006).Eventhoughreconstruc-
tionsfromicecoresfromGreenland andWestAntarcticadonotextendasfarbackasfromEastAntarctica,highresolution
10 analyses of these cores provide very fine temporal resolution from which the seasonal cycle can be resolved (Vinther et al.,
2010;Markleetal.,2017).Seasonalvariationsarealsoimprintedinthesnowisotopiccompositionofhighaccumulationsites
incoastalareasofAntarctica(Morgan,1985;Masson-Delmotteetal.,2003;Kütteletal.,2012).Forlowaccumulationsitesas
foundontheEastAntarcticPlateau,thereisnoconsensuswhethericecorerecordscanrevealtheclimaticsignalatresolutions
finerthanmultidecadal(Baronietal.,2011)ornot(Ekaykinetal.,2002;Poletal.,2014;Münchetal.,2016).Ekaykinetal.
15 (2002)analysedmultiplepitsfromVostokandidentifiedlargespatio-temporalvariationscausedbypost-depositionprocesses
associatedwithsurfacetopographyandwindinteractions.Thesephenomenacreatestrongnon-climatevariabilityinasingle
core,whichcallsforstackingisotopiccompositionprofilesfromseveralsnowpitstorevealthecommonclimaticsignal.Still,
ontheEastAntarcticPlateau,aclearseasonalcycleisdepictedintheisotopiccompositionoftheprecipitation(FujitaandAbe,
2006; Landais et al., 2012; Stenni et al., 2016) and of the surface snow (Touzeau et al., 2016). So far, whether this seasonal
20 cycleisarchivedornotinburiedsnow,andthus,whetherstackinganarrayofsnowpitsenablesustoincreasethesignalto
noiseratioanddepictaclimaticrecordattheseasonalscalefromwaterisotopicsignalisanimportantopenquestion(Ekaykin
etal.,2014;Altnauetal.,2015;Münchetal.,2016,2017).
Severalstudieshavefocusedonunderstandinghowtheclimaticsignalisarchivedintheisotopiccompositionofsnowand
25 iceontheEastAntarcticPlateau.SincetheearlyworksofDansgaard(1964)andLoriusetal.(1969),therelationshipbetween
ice isotopic composition and local temperature has been attributed to the distillation associated with the successive conden-
sation events on the path from the initial evaporation site to the deposition site (Criss, 1999). Nevertheless, the relationship
betweenisotopiccompositionandsurfacetemperatureisnotconstantthroughtimeandspace,duenotablytoprocesseswithin
the boundary layer (Krinner et al., 1997), the seasonality of the precipitation between glacial and interglacial periods (Sime
30 etal.,2009),variationsinairmasstransporttrajectories(Delaygueetal.,2000;Schlosseretal.,2004)andintheevaporation
conditions (Vimeux et al., 1999). For East Antarctica, the glacial-interglacial isotope-temperature relationship appears quite
close to the spatial gradient, but its validity for inter-annual variations (Schmidt et al., 2007) and warmer than present-day
conditions(Simeetal.,2009)isunclear.
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In addition, under the exceptionally cold and dry conditions of East Antarctic drilling sites, the contribution of post-
deposition processes to the isotopic composition of the snow cannot be neglected at the atmosphere/snow interface (Town
et al., 2008; Sokratov and Golubev, 2009). Indeed, the relationship between temperature and isotopic composition of the
surface snow is different from the one found in the precipitation (Touzeau et al., 2016). It has been recently evidenced that
5 summer exchanges between snow and water vapour at the surface significantly affect the isotopic composition of the snow
bothinGreenland(Steen-Larsenetal.,2014)andontheEastAntarcticPlateau(Ritteretal.,2016).Inthisstudy,weintend
to evaluate the different contributions to the snow isotopic composition signal in order to explain how the climatic signal is
archivediniceisotopiccomposition.Therefore,weusedDomeCasanopenairlaboratorytostudythedifferentcontributions
tothesurfacesnowisotopiccomposition,combining:(1)directprecipitationinput,(2)blowingsnow,(3)exchangeswiththe
10 atmosphericvapourand(4)exchangewiththefirnbelowthesurface;aspresentedinFig.1.
Temperature
Free gradient
atmosphere
Eddy
diffusion
Precipitation input
Atmospheric Sublimation
condensation
boundary Ri P
layer v v
Exchange Snow
Ri with the precipitation
P
vapour
Molecular
Surface Ri P diffusion
snow 0 sat
Interstitial
vapour Ri
S
Molecular diffusion
in the interstitial air
Deeper Ri
snow N
Figure1.Schematicofthedifferentcontributionstothesnowisotopiccomposition(Ri standsforthecompositionofisotopeiinthephase
X
X: P stands for precipitation, V stands for vapour, S stands for surface snow, N stands for the deeper snow, 0 stands for the vapour at
equilibriumwiththesnow):abovethesurface,boththeprecipitationandthesublimation/condensationcyclescancontributetothesurface
composition;intheopen-porousfirnbelowthesurface,icecrystalscanexchangewiththeairinthepores,influencedornotbywindpumping.
Deeperinthefirn,moleculardiffusionintheinterstitialairaffectsthesnowisotopiccomposition.
AtasiteasfaralongthedistillationpathasDomeC(andingeneralontheEastAntarcticPlateau),onlyasmallamountof
precipitablewaterisavailable(Ricaudetal.,2014)andprecipitationissparseandirregular(Genthonetal.,2015).Asaresult,
accumulation of snow at Dome C is not a homogeneous process affecting a flat surface, and the snow deposition is patchy
15 andstronglydependantofthesurfaceroughness(GrootZwaaftinketal.,2013;Liboisetal.,2014;Picardetal.,2016a).As
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aresult,asmallbutsignificantcontributiontotheannualmassbalancecomesfromsublimation/condensationdirectlyatthe
surface(Genthonetal.,2017).Indeed,duringtheAustralsummer,negativefluxesupto 0.3mmw.e.month 1,associated
−
−
with sublimation, are observed whereas during the Austral winter, positive fluxes up to 0.1 mm w.e. month 1, associated
−
with condensation, are found. Integrated, these contributions for a site such as Dome C add up to 10% of the total annual
5 accumulation.Eventhoughtheamountofwaterintroducedbythiscontributionislimited,weexpectasignificantimpacton
theisotopicbudgetofthesnow.
Atthediurnalscale,thisasymmetryoftemperatureconditionsduringcondensation/sublimationcyclesisalsoobservedin
summer:eventhoughthesunnevergoesdown,theradiativebudgetcreatesasignificanttemperaturecycle(upto16 Cperday)
◦
10 duringwhichthevapourisotopiccompositionevolvesinparallelwiththesnowsurfacetemperature(Casadoetal.,2016;Ritter
etal.,2016).Assublimationoccursathighertemperaturethancondensation,thediurnalcycleresultsinanon-neutralisotopic
budget.Inaddition,thedynamicoftheatmosphericboundarylayerisdifferentduringthedayandduringthenight:duringthe
day,activeconvectionenablesturbulentmixingthroughouttheboundarylayerwhileduringthenight,theconditionsaremuch
morestratifiedasillustratedbytheimportanttemperaturegradientsthatareobserved(Casadoetal.,2016;Vignonetal.,2017).
15
In the top first metres of the snowpack, isotope exchanges involved during snow metamorphism and diffusion within the
porousmatrixadditionallyaffecttheisotopiccompositionofthesnow(Langway,1970;Johnsen,1977;WhillansandGrootes,
1985; Calonne et al., 2015). The diffusion length associated with these processes depends on the firn ventilation, the snow
density, porosity and tortuosity and the exchange rate between the atmospheric water vapour and the surface snow (Johnsen
20 etal.,2000;Gkinisetal.,2014).Thiswiderangeofprocesseshamperstheinterpretationoftheisotopicsignal.Inparticularit
isnotclearhowmuchoftheoriginalsignalacquiredduringtheformationoftheprecipitationisconservedduringtheburialof
thesnow(Münchetal.,2017).
RecentstudieshavefocusedonthemonitoringoftheisotopiccompositionofthesnowpackontheEastAntarcticPlateau
25 (Touzeauetal.,2016),oftheprecipitation(FujitaandAbe,2006;Landaisetal.,2012;Stennietal.,2016),andoftheatmo-
sphericwatervapour(Casadoetal.,2016;Ritteretal.,2016);exploringtheirlinkstoclimaticparametersandtheimplications
for post-deposition processes during the archiving of the climatic signal by the snow isotopic composition. Here, we study
theisotopiccompositionofthecontinuumbetweenatmosphericvapour,precipitation,surfaceandburiedsnow.Todoso,we
combinedifferentdatasets fromDomeC,ontheEastAntarcticPlateau,basedonpublishedstudiesandnewresults,inorderto
30 qualitativelycharacterisethedifferentprocessesaffectingsurfacesnowisotopecompositionfromdiurnaltoannualtimescales.
Wefirstreportandcomparethedifferentmethodologiesappliedforsamplingsurfacesnow,snowpits,precipitationandwater
vapourintheatmosphere(Section2).Then,wedelivertheresultsfromdifferentstudiesincludingsurfacesnowmeasurements
over several years, precipitation measurements, vapour and snow measurements and snow pits sampling (Section 3) before
summarisingourkeyconclusions.
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2 Sites,materialandmethods
2.1 Site
The East Antarctic Plateau is a high elevation area, over 2500 metres above sea level (ma.s.l.) covered with snow and ice
spreading across most of the eastern continental part of Antarctica (Fig. 2). The East Antarctic Plateau is characterised by
5 meanannualtemperaturesbelow 30◦C andaccumulationbelow80kg.m−2.yr−1,asillustratedinFig.2.
−
Annual mean surface
air temperature (°C) 0o
-5 3 0o W 30oE
-10
-15 6o0W Kohnen 60o
E
-20
-25
oW South Pole 90
90 Eo
-30 Vostok
S2
Dome C
80oS
-35
-40 120Wo 12o0E
70oS
-45
-50 150oW 1 5 0o E
180oW
Figure2.MapofAntarcticahighlightingtheEastAntarcticPlateau(greyline=contourof2500ma.s.l.elevation)indicatingthelocation
ofthesamplingsites(solidsquares)includedinthiswork.Coloursindicatetheannualmeansurfaceairtemperatureat2mmodifiedfrom
theERA-interimdatasetfrom1979to2009(NicolasandBromwich,2014).
This study mainly focuses on Dome C, the site of the permanent station Concordia, enabling year-long operations and
thus giving the rare opportunity to study the entire seasonal cycle of isotopic composition. This site is characterised by low
accumulation(27mmw.e.peryear)andlowtemperature( 52.4 C).WepresenttheconditionsfoundatDomeCcompared
◦
−
10 tootherdeepicecoresitesontheEastAntarcticPlateau:Kohnen,VostokandAmundsen-ScottSouthPolestations,andthe
point S2, which is one of the drilling sites of the campaign Explore-Vanish joining Dome C and Vostok (see Fig. 2). These
locationsspanalargerangeofclimaticconditionsoftheEastAntarcticPlateauasillustratedonTable1.
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Table1.Climaticconditionsatthedifferentsitesusedinthisstudy(Alley,1980;Petitetal.,1982;WendlerandKodama,1984;Oerteretal.,
2000;Ekaykinetal.,2002;vanAsetal.,2007;Lazzaraetal.,2012;Caseyetal.,2014;Genthonetal.,2015;Touzeauetal.,2016;Laepple
etal.,2016)
Altitude AWSmean 10mfirn Accumulation Meanwind
Site Location
(ma.s.l.) temperature( C) temperature( C) (kg.m 2.yr 1) speed(m.s 1)
◦ ◦ − − −
Kohnen 75.0 S-0.1 E 2892 -42.2 -44.5 62-73 4.5
◦ ◦
Vostok 78.5 S-106.8 E 3488 -55.2 -57 21 5.1
◦ ◦
S2 76.3 S-120 E 3229 NA -55.1 21 NA
◦ ◦
DomeC 75.1 S-123.3 E 3233 -52.4 -54.3 27 3.3
◦ ◦
SouthPole 90 S-0 E 2835 -49.3 -50.8 80 4.1
◦ ◦
2.2 Surfacesnowandprecipitationsampling
Precipitation and surface snow have been regularly sampled at Dome C by different teams since 2008. Here, we report new
measurementsofprecipitationandsurfacesnowtogetherwithpreviouslypublisheddatafromStennietal.(2016)andTouzeau
et al. (2016) (see Table 2). Because different teams were in charge of the different sampling activities, the protocols differ
5 betweentheyears.
Thesamplingprotocolofthe2011campaign(SUNITEDC)hasbeenpreciselydescribedbyTouzeauetal.(2016):theupper
firstmillimetresofsnow(1to5mm)weregatheredevery1-2weeksusingametallicbladeoverasurfaceof20by20cm.This
leadstosamplesofapproximately20mL.Thesamplingareaswererandomlypicked,providedthesurfacewasflat.
10
DuringtheNIVOproject(from2013to2016),thesurfacesnowwasgatheredbysamplingroughly15mmofsnowwitha
corning flask over a surface of 20 by 10cm. This led to samples of approximately 50mL. The sampling areas of the NIVO
projectwerechosenrandomlyina100by100m"cleanarea"neartheAtmosphericShelterinparallelwithdensityandspecific
surfacearea(SSA)measurements(seesection2.4). Toaddresstheproblemofspatialvariability,twosamplesseparatedby10
15 to 50m were gathered during each collection and we present here the average value of the two samples. In addition, during
summer2013/14,regularsamplingsofsurfaceandsub-surfacesnowwereperformeddailyforalmost2months.Thesurface
samplesweregatheredusingacorningflaskfrom0to3cmdepth.Thesub-surfacesamplesweregatheredbythesametool
from 3 to 6cm depth. In 2014/15, an additional sampling took place within the GLACIOLOGIE project twice a day from
December2014toJanuary2015nearthelocationoftheinletusedforwatervapourmonitoring(Seesection2.4andCasado
20 etal.(2016))followingthesameprotocol (15mmthicksamplesgathereddirectlyinthesnow).
Sampling of surface snow and isotopic composition of precipitation were carried out in parallel by the Italian winterover
crews(programPRE-REC).Precipitationsampleshavebeencollectedallyearroundonan80by120cmwoodentablestand-
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ing1mabovethegroundlevel800mfromConcordiaStationfrom2008to2011.Thesampleswerecollectedat1a.m.UTC
everydayiftheamountwassufficient. AsalreadymentionedbyStennietal.(2016),afewepisodesofblowingsnowmight
affect the precipitation gathered on the plate. The surface snow samples were gathered from an adjacent wooden plate of
80by120cmatgroundlevel.Iftheamountofsnowonthissecondtablewassufficient,snowsampleswerecollected.Both
5 precipitationandsurfacesnowsamplesweresealedintodate-labelledplasticbagsandpreservedinafrozenstateuntildelivery
andmeasurementinItaly.Fortheprecipitationsamples,theprotocolisdetailedbyStennietal.(2016).Itisimportanttonote
thattheprotocolofsurfacesnowsamplingfromthePRE-RECcampaigndiffersgreatlyfromtheprotocolsfromtheNIVOand
SUNITEDCprogramsduetothepresenceofthewoodplate.
Table2.Summaryofthedifferentcampaignsofsurfacesnowandprecipitationsamplingspresentedhere.
Resolution
Project Location Years Reference
(days)
SUNITEDC(French) Surfacesnow 2011 7 (Touzeauetal.,2016)
Precipitation 2008to2011 1 Partiallyin(Stennietal.,2016)
PRE-REC(Italian)
Surfacesnow 2012and2014 7 Thisstudy
Surfacesnow 2013to2016 3
NIVO(French) Thisstudy
Sub-surface 11/2013to01/2014 1
GLACIOLOGIE(French) Surfacesnow 12/2014to01/2015 1 Thisstudy
10 2.3 Snowpitssampling
WepresentprofilesofisotopiccompositionsampledinsnowpitsatDomeC:twounpublishedprofilesfromthefirstprelimi-
narycampaignatDomeCin1978andtwonewsnowpitprofilesobtainedin2014/15,dug50mapartinparallelwithsurface
snowsamplingandvapourmonitoring.Foroneofthem,snowtemperatureanddensityprofileswereestablished.Thesamples
were taken in plastic flasks and analysed later on in the laboratory. To extend the results to other sites of the East Antarctic
15 Plateau, we compare the isotopic profiles to other snow pit samplings performed through several campaigns over different
sitesofEastAntarcticawhichwererealisedandanalysedbydifferentteams.
In addition to the Dome C snow pits, two new isotopic composition profiles from Kohnen were extracted from trenches,
followingthemethodologyreportedinMünchetal.(2016)butuptoadepthof3.6mandaverticalresolutionof3cm.Both
20 profiles are separated by approximately 500m well beyond the decorrelation scale of the stratigraphic noise, to enable eval-
uation of the spatial variability in the profiles of isotopic composition. A large number of snow pits from Vostok station are
presentedhereaswell,whichwerepreviouslydescribedinEkaykinetal.(2002,2004)andEkaykinandLipenkov(2009).We
combinetheresultsfromsixsnowpitswithdepthsvaryingfrom2.5mto12mandaminimumresolutionof5cm.Inaddi-
tion,snowpitsfromtheExplore-Vanishcampaignareincludedcomprisingone3.5mdeepsnowpitfromVostok,one2.6m
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Table3.Summaryofthedifferentsnowpitspresentedinthisstudy.
Resolution Number
Station Years Reference
(cm) ofpits
2001to2015 2to5 6 Ekaykinetal.(2002,2004,2009)
Vostok
2012/13 3 1 Touzeauetal.(2016)
Kohnen 2014/15 3 2 Thisstudy
1977/78 1to3 2 Thisstudy
DomeC 2012/13 3 1 Touzeauetal.(2016)
2014/15 1.5to5 2 Thisstudy
S2 2012/13 3 1 Touzeauetal.(2016)
1978 2 1 Jouzeletal.(1983)
SouthPole
1989/90 1.1 1 Whitlowetal.(1992)
deepfromS2andone2mdeepfromDomeC,allofthemincludingtripleisotopiccompositions(δ18O,δ17O andδD)pub-
lishedinTouzeauetal.(2016).Finally,weincludetwosnowpitsfromtheSouthPole(Jouzeletal.,1983;Whitlowetal.,1992).
Wedonotexpectanyimpactofthesamplingtechniquewhencomparingthedifferentsnowpitsconsideringthesimilarity
5 oftheprotocolsofallthesnowpitsamplings.
2.4 Atmosphericandsnowsurfacemonitoring
WatervapourisotopiccompositionhasbeenmeasuredatDomeCin2014/15(Casadoetal.,2016).Toreducethenoise,the
datasetwasaveragedtohourlyresolutionandcoversapproximatelyonemonth.Inparalleltowatervapourisotopiccomposi-
tionmonitoring,surfacesnowwassampledoncetotwiceaday.Foraperiodof27hours,thesurfacesnowwassampledevery
10 hourtoevaluatethediurnalcycleofboththevapourandthesnowisotopiccomposition(seesection2.2).
We use temperature, wind speed and humidity measurements from the 45m meteorological profiling system described by
Genthonetal.(2013).ThetemperatureandhumidityobservationsareperformedusingventilatedthermohygrometersHMP155
andarethereforefreeofradiationbiases(Genthonetal.,2011).Thetemperaturereanalysisproduct(ERAinterim)hasbeen
15 comparedtoventilatedautomaticweatherstationdata(AWS)fromGenthonetal.(2013)andwefoundagoodagreementat
theseasonalscaleandfairlygoodagreementattheeventscale(notshownhere).Windspeedanddirectionaremeasuredusing
Young 05103 and 05106 aerovanes. Snow surface temperature is measured with a Campbell scientific IR120 infrared probe
located2mabovegroundlevel.
20 Inthefield,snowmetamorphismisdifficulttoquantifyduetothelargenoisecreatedbythespatialvariability,requiringa
largenumberofsampleseveryday.Therefore,weincludegrainindexobservations(Picardetal.,2012)obtainedbysatellite
measurements using passive microwave satellite data. Picard et al. (2012) argue that the grain index is an indicator of the
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coarsening of snow grains and show its increase in summer to be anti-correlated with the integrated summer precipitation
amount.Whenavailable,weincludeSurfaceSpecificArea(SSA)measurementsalsoasanindicatorofmetamorphism(Libois
etal.,2015).Theseopticalmethodsarecompletedwithsnowobservations.Frostdepositionwasmonitoredwithatimelapse
ofthegrowthofhoaratthesurface(seethevideoathttps://vimeo.com/170463778).Animageprocessingscriptwasusedto
5 characterisethegrowthofafewcrystalsatthesurfaceofasastruga.
2.5 Modellingapproaches
To investigate the impact of post-deposition processes, it is necessary to present how the surface snow isotopic composition
differs from the initial precipitation signal formed during the Rayleigh distillation. Here, we make use of the Rayleigh-type
Mixed Cloud Isotope Model (MCIM) developed by Ciais and Jouzel (1994) which computes the Rayleigh distillation along
10 the air mass trajectories. The model includes microphysical properties of clouds and in particular takes into account mixed
phaseconditions.ItistunedwithtriplesnowisotopiccompositionmeasuredalongatransectfromTerraNovaBaytoDomeC
(Landaisetal.,2008).Oneofthemainuncertaintiesinthismodelisthesupersaturation.Indeed,JouzelandMerlivat(1984)
evaluatetheimpactonkineticfractionationduringthesnowformationparametrisedfromthesupersaturation.Thetuningof
the supersaturation has been proven suitable to evaluate the variations of isotopic composition at Dome C (Winkler, 2012).
15 Thiswillprovideacomparisonbetweenthespatial(atthescalesfrom10to1000km)andthetemporalslopeoftheisotopic
compositionofprecipitationattheseasonalscale.Itwillalsobeusedtoquantifytheimpactofpost-depositionprocesseson
the surface snow isotopic composition by providing a reference for the precipitation isotopic composition, for days with no
precipitation.
3 Resultsanddiscussions
20 Inthissection,wereviewtheresultsfromthedifferentdatasets,illustratingthedifferentstepsofthearchivingoftheclimatic
signalbythesnowisotopiccomposition,fromtheprecipitationtotheburiedsnow(seeFig.1). Asweexpecttheprecipitation
inputtobetheprimarycontribution,wewillfirstcomparethevariationsofprecipitationisotopiccompositionbasedondata
fromStennietal.(2016)andadditionalnewdata,tosurfacesnowisotopiccomposition.Then,wewillevaluatetheexchanges
withtheatmosphere:firstatthediurnalscalebyreportingparallelmeasurementsofwatervapourandsurfacesnowisotopic
25 composition; and second, at the day-to-day and at the seasonal scales the difference between surface snow and precipitation
isotopiccompositiontothemeteorologicalconditionsandthegrainindex.Finally,wewillcomparetheisotopiccomposition
ofthesurface,sub-surfaceandburiedsnowtoevaluatetheexchangeswithinthefirn.
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3.1 Precipitationandsurfacesnowisotopiccompositiontimeseries
3.1.1 Precipitationisotopiccomposition
Inthissection,wepresentprecipitationisotopiccompositiondataatDomeCfromStennietal.(2016)depicting3complete
annualcyclesfrom2008to2010,completedbyanew,unpublisheddatasetfrom2011(Fig.3).
5
AtDomeC,theisotopiccompositionofprecipitationshowslargevariabilityattheday-to-dayscaleandaregularseasonal
cycle.Insummer,wesystematicallyobserveprecipitationδ18O above-40‰,whereasinwinter,δ18O valuesbelow-65‰
p p
aresystematicallyobserved.Attheseasonalscale,theisotopiccompositionofprecipitationisrelativelywellcorrelatedtothe
localtemperaturewithaslopeof0.46‰ C 1 (R2=0.65,n=1111),thisslopeissimilartotheoneobtainedbyStennietal.
◦ −
10 (2016) for the years 2008 to 2010 of 0.49 0.02 ‰◦C−1 (R2=0.63, n=500). No lag between temperature and isotopic
±
compositionvariationsisobserved.
Comparedtootheryear-longprecipitationtimeseriesontheEastAntarcticPlateau,thisslopeislowerthanatDomeF(0.78
‰ C 1 with R2=0.78 (Fujita and Abe, 2006)) and higher than at Vostok (0.26 ‰ C 1 with R2=0.58 (Touzeau et al.,
◦ − ◦ −
15 2016)).
3.1.2 Surfacesnowisotopiccomposition
Here,wepresentmeasurementsofsurfacesnowisotopiccompositionatDomeCfromDecember2010toJanuary2016(Fig.3)
combiningresultsfromTouzeauetal.(2016)withnewdatapresentedforthefirsttimeinthisstudyfromthePRE-REC,NIVO
andGLACIOprojects.Thedatasetincludesthreecompleteannualcyclesofsurfacesnowisotopiccomposition(in2011,2014
20 and2015)andpartofthe2012cycle,withtherespectivetemperaturevariationsandtheprecipitationevents(fromreanalysis
products,ERA-interim).
First,wefocusontheimpactoflocalspatialvariability(below1km)ofthemeasurements.Indeed,significantdifferences
canbefoundinthesnowisotopiccompositionfordistancesrangingfrom1to1000m.Todisentanglethelocalspatialvari-
25 abilityfromthetemporalvariationsofthesurfacesnowisotopiccomposition(Fig.3),wecomparetheduplicatemeasurements
realisedduringtheyear2014intheprojectNIVO(redshadeinFig.4andseeSection2.2).Wefoundthatthespatialuncer-
taintyofthesurfacesnowisotopiccompositionmeasurementsis3.4‰forδ18O from2standarddeviationscalculatedwith
s
theduplicatesontheNIVOsamples(randomlypickedwithin50m).Forthatyear,severalsetsofmeasurementsareavailable
from3differentprograms(PRE-RECinblue,NIVOinredandGLACIOLOGIEinblack).EventhoughthePRE-RECsam-
30 pling is more affected by heavy snowfall events due to the wood plate, apart from a limited number of outliers (5 over 59),
the dataset is most of the time representative of the surface snow and not of the precipitation. This provides an independent
test to validate our estimation of the spatial variability. Strong differences are visible at the event scale, in particular during
some events in March, May and June. During these events, the very low values of the PRE-REC sampling are in agreement
10