The layering of polar firn [Elektronische Ressource] : investigations of the climatic impact on polar firn structure using high resolution density measurements and 3D-X-ray-microfocus-computer-tomography / Maria Hörhold

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The layering of polar firnInvestigations of the climatic impact on polar firn structure usinghigh resolution density measurements and3D-X-ray-microfocus-computer-tomographyMaria HörholdAlfred-Wegener-Institut for Polar and Marine ResearchColumbusstrasse27568 BremerhavenGermanyUniversität BremenFachbereich Geowissenschaften (FB5)Bibliotheksstrasse 128359 BremenGermanyGutachterProf. Dr. H. MillerProf. Dr. K. HuhnPrüferProf. Dr. S. KasemannProf. Dr. F. WilhelmsPromotionskolloquium3. November 2010ErklärungHiermit versichere ich, dass:• ich die beiliegende Arbeit ohne Hilfe Dritter• und ohne Benutzung anderer als der angegebenen Quellen und Hilfsmittel ange-fertigt• und die den benutzten Quellen wörtlich oder inhaltlich entnommenen Stellen alssolche kenntlich gemacht habe.ContentsContents 51 Summary 72 Zusammenfassung 113 List of Publications submitted for the Thesis 174 Introduction and Objectives 194.1 Topic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.3 Main questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.4 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Methods 315.1 Literature survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.2 Firn cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.3 High resolution gamma absorption method .

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2010
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 12 Mo

Extrait

Thelayeringofpolarfirn

Investigationsoftheclimaticimpactonpolarfirnstructure

resolutionhighandmeasurementsdensity

3D-X-ray-microfocus-computer-tomography

iaMarHörhold

Alfred-Wegener-InstitutforPolarandMarineResearch

asseusstrColumb

nevBremerha27568

manGery

BremenersitätUniv

(FB5)wissenschaftenGeoachbereichF

liotheksstrBib1asse

Bremen28359

ymanGer

using

3.

embervNo

2010

Prolloquiumomotionsk

.Prof

.rD

.F

Wilhelms

erPrüf

Kasemann

S.

.rD

.Prof

.Prof

rD.

K.

Huhn

hterGutac

.Prof

.rD

H.

Miller

Hiermitversichereich,dass:

•

•

•

Erklärung

ichdiebeiliegendeArbeitohneHilfeDritter

undohneBenutzungandereralsderangegebenenQuellenundHilfsmittelange-

tigtref

unddiedenbenutztenQuellenwörtlich

solchekenntlichgemachthabe.

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inhaltlich

entnommenen

Stellen

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Contents

Contents

ySummar1

Zusammenfassung2

3ListofPublicationssubmittedfortheThesis

ObjectivesandoductionIntr44.1Topic...................................
4.2Background................................
4.3Mainquestions..............................
4.4Stateoftheart..............................

Methods55.1Literaturesurvey.............................
5.2Firncores.................................
5.3Highresolutiongammaabsorptionmethod...............
5.3.1Missinglayers...........................
5.4Permeabilitymeasurements.......................
5.5X-ray-microfocus-computer-tomography(CT)..............
5.6MicrostructuralanalysisusingMAVI...................

DiscussionandResults6

5

5

7

11

17

1919212526

3131313233343435

39

6.1Parameteroflayering...........................39
6.2Generationoflayering..........................44
6.3Evolutionoflayeringwithdepth.....................44
6.4Layeringinmicrostructureandairpermeability.............47
6.5Layeringatdifferentsites.........................50

lusionsConc7

57

8ProblemsandOpenTopics59
8.1Methodicallimitations..........................59
8.2"Global"microstructure..........................60
8.3Convectionoftheupperfirn.......................62

9Acknowledgement

yliographBib

APublication1-Thedensificationoflayeredpolarfirn

65

76

73

BPublication2-Fromrandomdepositiontoafirncorerecord-ontheimpact
ofimpuritiesonthedensificationofpolarfirn-afirstapproach113

CPublication3-Grainsizeoflayeredpolarfirn-evolution,variabilityanda
newgraingrowthmodel153

DPublication4-Theimpactofaccumulationrateontheanisotropyandair
permeabilityofpolarfirnatahighaccumulationsite193

EPublication5-LatticeBoltzmannmodelingoftheairpermeabilityofpolar
233firn

ySummar1

Thepolarfirnstructureisinvestigatedintermsoflayeringusinghighresolutionden-
sitymeasurementsof19firncoresandX-ray-microfocus-computer-tomographyimage
analysisof6surfacefirncores.Theimpactoflocalclimateconditionssuchasannual
meantemperatureandaccumulationrateonthegeneration,structureandevolutionof
thelayeringisstudied.Wefindatallsitesahighvariabilityindensityandmicrostruc-
tureduetothelayeredcharacterofthefirn.Thestandarddeviationofthemeasured
physicalpropertiesisusedasaproxytoparameterizethelayeringandtocomparethe
degreeoflayeringatdifferentsites.Mainresultsare:

1.Thedifferentsitescanbedistinguishedbythedegreeoflayering,i.e.variabilityin
densityandmicrostructure.Thevariabilityingrainsizeincreaseswithdecreas-
ingannualmeantemperature,accumulationrateandmaximumtemperaturegra-
dient.Siteswithloweraccumulationrateandannualmeantemperatureshow
highervariabilityandthusahigherdegreeinlayeringintheupperfirncolumn
thansiteswithhigheraccumulationrateandannualmeantemperature.This
means,thatasitefromthehighEastAntarcticPlateaucanbeexpectedtoshow
higherlayeringcomparedtositesfromcoastalregionsofAntarctica.

2.Thelayersseemtobecreatedrandomlyatthesurface.Inthisstudyaseasonal
variabilitywithconstantfrequencyindensitycouldnotbedetectedwithinthe
presenteddata.Possiblereasonscanbeahighvariationinaccumulationrate,
thatcansuperposeaseasonalityindeposition,orlargehorizontaldiscontinuity,
whichmeans,thatapunctualfirncoresitewouldnotcapturealllayers.However,

7

withthedatainvestigatedinthisstudyitcanbehypothesized,thatsurfacefirn
stratigraphydoesnotshowaseasonallayering.

3.Thedensityvariabilityisnotcontinuouslydecreasingwithdepth,butshowsa
ratherrapiddropintheupper10-20meterdepth,whichisfollowedbyasecond
maximumindensityvariability.Thisleadstoincreaseddensityvariabilityatthe
depthintervalofthefirn-ice-transition.

4.Thefrequencyandamplitudeofthedensityvariabilityischangingwithdepth.
Eventhoughthereisnoseasonalsignaturewithinthenear-surfacevariability,in
thedepthintervaloftheseconddensityvariabilitymaximum,thedensityfluctua-
tionsshowaseasonalfrequencyatmediumtohighaccumulationratesites.

5.Thedensityvariabilitydevelopsacorrelationwiththeconcentrationofthecalcium
ionwithdepth.Atthesurfacenocorrelationisevident.Atthedepth-intervalof
theseconddensityvariabilitymaximumthecalciumconcentrationandthedensity
variabilityshowaremarkablyhighcorrelation.Togetherwiththeshiftinfrequency
ofthedensityvariability,thisindicatesanimpactof(seasonalvarying)impurities
onthedensificationofthefirn.

6.Thesummaryoftheresults2-5indicates,thattheseasonalstratigraphyinice
corerecords,whichisassumedtobeformedbyacontinuousdepositionatthe
surfaceandyieldsthebasisofpaleo-climatestudies,isnotnecessarilyformed
atthesurfacebydepositionalmechanisms,butbydensificationandsnowmeta-
morphismdeeperdowninthefirncolumn,influencedbyimpurities.

7.Thelayeringisdisplayedinthemicrostructure,butshowsaverycomplexpattern
astheresultofthecombinedeffectofinitiallayering,sinteringandshort-term
conditions:climatelocalinchangesa)Thelayeringiscreatedrandomlyatthesurface,initiatingacertaincombi-
nationofdensityandmicrostructure.Withineachdepthintervalthelayering

showsalinearcorrelationindensityandgrainsize,sothatthegrainsize
couldbeparameterizedviadensity.Thetrendofthiscorrelationhowever,
differsforthedifferentsitesandchangeswithdepth.
b)Thefirnlayersallundergotherathergradualprocessofsintering-thein-
creaseindensityandgrainsize,whichisdeterminedbytheaccumulation
rateandannualmeantemperatureofasite.Alongtermtrendindensity
andgrainsizeisobserved,describingthedensificationandgraingrowthof
depth.withnfirthec)Short-termchangesinlocalclimateconditions,suchasvariationsinac-
cumulationrateortemperaturegradientatthesurfacesuperposetheini-
tiallayeringandthegradualsintering,bychangesinthemetamorphismof
thesnow.Increasedordecreasedexposuretonearsurfacetemperature
gradientsorwind-ventilationcausesmoreorlesscoarseningormoreor
lesspronouncedanisotropywithinthestructure.Thecoarseningleadstoa
short-termincreaseinporesizeorairpermeabilitywithdepthuntilamaxi-
mumat2-4meterdepths.Thisisobservedespeciallyatthefirncoresfrom
lowaccumulationratesitesinAntarcticaandatthefirncorefromtheGreen-
landsite.Accordingly,themostcoarsenedstructuresintermsofporesize
canbeexpectedatsiteswithlowaccumulationrate.Thesefindingsimply,
thatventilationoftheuppermostfirnlayerscanbeexpectedtobelargestat
lowaccumulationratesites.

8.Thecombinationofinitiallayering,sinteringandcoarseningatthesurfacein-
ducesanoveralldiverseevolutionofmicrostructurevariability.Therelationsbe-
tweendensity,microstructureandairtransportpropertiesareinfluencedbyeach
oftheseconditionsverydifferently.Whereasthefirnlayershowanegative,lin-
eartrendinthedensity-grain-sizerelationship,thesinteringshowsalineartrend
aswell.Butthesinteringshowsatrendwithoppositesign,andthecoarsen-
ingshowsanon-linearpattern,whichischangingwithdepth.Thisleadstoa

complexpatternofdensity-microstructurerelationship.Thereforeasimplepa-
rameterizationofmicrostructuralcharacteristics,suchasspecificsurfacearea
andgrainsize,orairpermeability,withdensityisnotstraightforwardinpolarfirn.

9.Neverthelessthemicrostructurefromtheseverydifferentsitesandverydiverse
metamorphicstatesshowssomesurprisinglywelldefinedrelationships:thespe-
cificsurfaceareaofthefirnsamples,whichcanbedeterminedbyseveralmeth-
ods,canbedescribedbijectivefromthemeasuredchordlength(averageinter-
sectionoftheicephasewithaline).Thisstrengthenstheassumption,thatoptical
propertiesofsnowandfirncanbedescribedbyaneffectiveradiusofspheres,
showingthesamespecificsurfaceareaasthemeasuredsample.Foranyappli-
cationswheretheeffectiveice-air-interfaceofthefirnisimportant,(air-exchange,
chemicalinteractions,interactionswithopticalproperties,microwaves),theeasy-
to-measure-specificsurfaceareaissufficienttoobtaintheeffectiveradiusofthe
firnstructure.Thisradiuscaneasilybeincludedintograingrowthmodelsorgrain
sizemodelingfromremotesensingsurfaceobservationssuchastheModerate
(MODIS).adiometerSpectrorImagingresolution

Furthermore,astructuremodelindexiscalculated,whichdescribestherelation
ofcurvatureandsurfaceofthefirn.Thisindexseemstobeawelldefinedfunc-
tionofporosityforallinvestigatedfirnsamplesofthisstudy.Thisindicates,that
despitethediversepatterninoriginallayeredmicrostructurecreatedatthesur-
face,sinteringandcoarsening,topologicalsimilarstructuresdevelopduringthe
wholefirnmetamorphismprocess.

2Zusammenfassung

DieStrukturdespolarenFirnsalsporösesgeschichtetesMediumwirdmitde