Title The Multiscale Structure of Antarctica Part I : Inland Ice

Faria, Sérgio H.; Kipfstuhl, Sepp; Azuma, Nobuhiko; Freitag, Johannes; Weikusat, Ilka; Murshed, M. Mangir; Kuhs, Author(s) Werner F.

低温科学, 68(Supplement), 39-59 Citation Physics of Ice Core Records II : Papers collected after the 2nd International Workshop on Physics of Ice Core Records, held in Sapporo, Japan, 2-6 February 2007. Edited by Takeo Hondoh

Issue Date 2009-12

Doc URL http://hdl.handle.net/2115/45430

Type bulletin (article)

Note I. Microphysical properties, deformation, texture and grain growth

File Information LTS68suppl_005.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP The Multiscale Structure of Antarctica Part I: Inland Ice

Sergio H. Faria -t , Sepp Kipfstuhl .. , Nobuhiko Azuma ... , Johannes Frei tag •• , Ilka Weikusat .. , M. Mangir MU fshed • , Werner F. Kuhs •

* cze. Sect. a/Crystallography, Universityo[Gotlingen. COl/illgen, GermallY ** Alfred iVegener illslirurejor Po/arand Marille Research, BremerllOl'ell, Germany *** Dept. of Mechanical Engineerillg, Nagaoka Universit), ofTec1l11ology, Nagaoka. Japa"

Abstract: The dynamics of polar ice sheets is strongly often imposes a terrible dilemma. A typical example is innuenced by a complex coupling of intrinsic structures. the disparity in the vocabularies used by geoscientists Some of these structures are extremely small, like dislo­ and materials scientists to describe the same microstruc­ cation walls and micro-inclusions: others occur in a wide IU ral features in polycrystals. In an allempt to be system­ range of scales, like stratigraphic features; and there are atic without being completely discrepant with the exist­ also those coll055al structures as large as megadunes :md ing literature. we list below some important defini tions subglacial lakes. Their signific:mce results from their in­ and acronyms used throughout the text. teractions with the ice-sheet flow and the environment We adopt here a compromise between the vocabulary through an intricate S/l'IIcl/lre-Form-£IIl1i/VI!lIIellf Imer­ used by Bunge & Schwart:er 122], Drury & Urai [37J, play (SFEI). Glaciologists are not unaware of the SFEI Humphreys & Hatherly [84J and Poirier LI 19j. It should issue. as particular details of the problem are well doc­ be noticed that this does /lot always co incide with the umented in the literature. Nevertheless. many aspects of terms in vogue in glaciology; and Ihis is done on pur­ the S FEI remain unclear and a comprehensive perspective pose: we wish to emphasize the need for a more ap­ of the problem is missing. Here we present some selected propriate vocabulary for dicussions with physicists, engi­ resul ts of a joint investigation of these structures via field­ neers and materials sc ientists, by rejecting the oblivious work. theoretical modeling. and experiments. The basic li se of vague expressions whose meanings are too often strategy is to conceive the Antarctic ice sheet as a het­ taken for granted or prone to misunderstanding and mis­ erogeneous system of structured media that interact in lise. T he terms adopted here may probably not be the best a hierarchical fashion via the SFEJ. Special emphasis is and ultimate c hoice for this purpose. but they represent at given to snow and fim structures, interactions between least an attempt towards d arity. microstructure, stratigraphy and impurities, and the inter­ play between subglacial structures and the overlaying ice. Cimhm/e hydm/e: Crystallinc compound containing guesl mo lecules enclosed in cage·likc structures made up of hydrogen-bonded water molecules. When the gueSI mole· Key words : Ice. I1rn. multiscale modeling. microstructure. cules fonn gas under standard condit ions. such compounds recrystallization, stratigraphy. subglacial environment. are also named gus hydrates. In particular. uir hydmles are formcd by atmosphcric gases (viz. mainly 02 and N2). Cloudy band: Icc stratum with turbid appearancc due to a high 1 Glossary concentration of micro-inclusions. CrysUliii/e: Crystalline domain in a solid polycrystal. Also callcd !lmil/. It should be noticed Ihc difference between SIiCceSS is reitllire: crystallitcs in polycrystalline solids (e.g. iron or icc) and the /[ is what !I'e call make of Ihe mess lI'e hare made of lose crystals in crystalline granular media (e.g. fine qu;u1Z Ihings. sand or fresh Antarctic snow). T. S. Eliot [44J (chamctcr: Agatha). p. ! 18. Cr)'j'/(liiographic lex/ure: Dircclional pal1crn of lal1ice orien­ tations in a polycrystal (cr. grain slcrcology). Also callcd T he c hoice of a consistent and unambiguous vocabulary fabric. PLO (Preferre(/ umice Oriel/lations). or simply ·'Iex· fordiscussing science in a multidiscipli nary environment ture".1 [n particular, a polycrystal with a rJndom distribu tion

t CorresponJing amhOf: sh. [email protected] I We adopl here Ihe slmodard nomenclalure of mmerials science 122. 7 I. 841. NOIice Ihm lhis is nOl the definilion of "leXlure" often invoked in the in the gladological lilerJlure. The reason for th is choke is con"enience: experience shows lhal it is rdlher easy for glaciologists 10 use the lenn "cryslallogmphic texture" 3l; a synonym for "fabric". whereas il is awkward for mOSI physicists, engineers. and male rials scientists to use the lenn "texture" in the sense frequemty employeJ in glaciology (i.e. as synonym fo r grain stereology).

-39- of lanice orientations is said to be texture-free (viz. random space-borne platforms as alternating bands and supposed ly fabric. no PLO). produced by persistelll katabatic winds. De/oI7/Uilioll-relale(1 SlrllClures: Structural features produced Micro-inclusion: Inclusion not larger than a few micrometers. and/or affectcd by deformation. e.g. dislocations. subgrain and eonsequclllly not clearly identifiable undcr optical mi­ boundarics. slip bands, str..l1igraphic fo lds. etc. croscope. e,g, dust particles. salt inclusions. microscopic DEP: Die lectric Profili ng. bubbles. etc. O;sloc(I/;oll u·ol/: Dcfonnation-rdatcd structurc consisting of MicroshC(lr: Strong, localizcd shear across a grain that expe­ dislocations arro.lnged in a two dimensional framework; the riences a highly inhomogeneous shear deformation. It cul­ precursor of a subgrain boundary (cf. subgrain). minates wi th the formation of a new. flat subgrain boundary DAfL: Dronning Maud Land. parallel to the shear plane. called micros/war boll/Il/ar)' (ef. slip bands). Dynamic gmill groll'lh (DGG}: Class of phenomenological processes of grain coarsening in polycrystals (Illrillg de­ Microslfllclllre: Collection of all microscopic dcformation· /orlll(l/iOIl. Scvcral recovery and recrystallization processes related structures. inclusions, and the orientation slCreology may be simultancously activc during DGG, all competing (cf. id.) ofa polycrystal. for the minimi:wtion of both, the stored strain energy and Mi1!,'wioll recryswlliUllioll: In full slmill-ill(Jllcefl mi!!,mliOIl the gmin-boundary energy. The essential feature of DGG reClysllIl/iz(llioll. Class of phenomenological recrystalliza­ (in comparison to other recrystalli zation processes) is the tion processes based on the elemental)' SIBM mechanism (cf. monotonic increase of the mean grain size with time. Ow­ id.). If nuclcation (cf. id.) is involvcd in the process, we ing to its dynamic nature. howevcr.thc diversificd kinctics of may call it lIucleated migralioll recryslal/iUllioll (SIBM-N), DGG can gcnerally not be compared with the simple kinctics whcre the suffix "·-N·· stands for ·"new grain·'. Otherwise. i,e. predictcd for nonnal grain growth (NGG. cr. id.). if the migration of boundaries occurs without formation of ECM: Elcctrical Conductivity Mc,tsuremcnt. new grains. we may call it ordinary migralioll recrystllilizlI­ lioll (SIBM-O). where the suffix '"-0·· stands for ··old grain··.3 EOC: EPICA-Dome C. Ml/llisclI!e slruclllre: The collection of all sons of structural EOML: EPICA-DML. fcatures obscrvcd in a matcrial body, with emphasis on tbcir Elementary SIft/ell/ro/ process: Thc fundamental oper,ltion of interactions. In ice sheets such structural features occur on structural change via recovery or recrystallization. e.g. grain the (sub-)microscale (e.g. dislocations. air hydrates), on boundary migration or subgrain rotation. Several elementary the mesoscale (e.g. cloudy bands) and on very large scales processes may combine in a number of ways to produce a (megadunes. subglacial lakes. etc.). Some of them may ap­ varicty of pilellollle/tolo!!,ica/ Slft/CII/ra! processe? (cf. id.). pear also in a range of scales (e.g. folds). Most of them are £PICA: European Project for Icc Coring in Antarctica. evolving structures that illieract with cach other as well as F(lbric: Sce cr),swl/o!!,rtlpllic Ie.Wl/re. with the icc-shcet flow and thc environment via SFEI (cf. id.). Fim: Sintcred snow that has outlastcd at Icast o nc summer. NCRIP: North·Gn:enland lee-Core Project. also abbreviatcd as Grain: Scc cr),SllIl/ile. NorthGRIP, Graill slereo!o!!,),: Spatial arrangcmcnt of grains in a polycrys­ Normal gmil! groll'lh (NGG): Phenomenologicall\.'Crystalliza­ t'll. inclUding thcir sizes and shapes (cf. crystallographic tion process of grain coarsening in polycrystals. result­ texture). ing from ""the interaction between the topological require­ ments of space-filling and the geometrical needs of (grain­ Gmin subdil'isioll: Phenomenological recovery process of for­ boundal)') surfaee·tcnsion cquilibrium·· 11351. By definition. mation of new SlIbgmill bolilldaries. It involves the progres­ grain coarsening during NGG is SWl;SliC(llIy III1i/orm and sive rotation of certain portions of the grain. called subgmins sel/-simi/ar. grain-boundary migration is e.rclllsiJ'ely driven (d. id.), as wcll as the strcllgthening of dislocation walls by minimization of the grain-boundary area (and associated through dislocation rearrangemcnt and migration in regions free energy). and the grain stercology is close to a configu­ with strong lattice curvature. If the misorientation across the ration of ··surface-tension equilibrium·· (so-called ·'foam-like new subgrain boundary increases wi th time. grain subdivi­ structure·,), Owing to these esscntial fcatures, NGG is gen· sion may give ri se to rolliliolll1!CI}'Slallizmioll (d. id.). erally regarded as a static recrystallization process (cf. re­ IIIClluioll: Locallized deposit of un dissolved chemical impuri­ crystallization) taking pl,lce before/after deformation (ef. dy­ ties observed in polar ice. e.g. bubbles. clathmte hydrates. namic grain growth). Mathematical and physical arguments brine pockets. platc~li"e inclusions. ctc. strongly suggest that the kinetics of NGG is parabolic with Meg(ullmes: Wavy snow structurc with fcw kilometers in wave­ respect to the mean grain radius.4 length and several meters in amplitude. visible from air- and

~ R eco\'ery and recrystallization are complex physicat phenomena that are better understood if deo:)lnposed in a hierarchy of structural processes or mechanisms. here qual ified as "'elemelllary"" and ··phenomenological:· A somewhal similar hierarchical scheme for recrystallizaJion has fonnerly been proposed by Drury & Urai 1371. but wilh Ihe eJlpre5sions ·"elemenwrylphenomenological process·· replaced respectively by ·"basic process·· and "·mech­ anism··, We f""or here the qualitiers ""elementary/phenomenological"· (against the ··process/mechanism·· scheme) because these qualilicrs f"ciliWle the visualization of Ihe hierarchy and leal·e us free 10 use Ihe tenns ··process·· and ··mechanism·· as synonyms. 3The definition adopted here is based on the concepl of ""grain-boundary migmtion recrystal lization·· originally described in the pioneering work by Beck and Sperry 11 11. Notice that this delinition is not identical to that used by Poirier 11191 or Humphreys & Hatherly 1841. and it is also quite distinct from some loose connotations invoked in the glaciological lilerJture. The lemlS SrBM-N and SrBM-O are not standard in the lilera1Ure. bUlthey are nt:v.:nheless adopled h.:re because Ih.:y d.:scribe quile precisely the kind of infomlalion oblJined from micrm;copic analyses of ice core s<:ct ions. There is unfonunately no one-to-one relation between SIBM·NISIBM·O and the expre5sions ""multiple/single subgrain SIBM·· used e.g. in 1841. 4 As discussed by Smith I t351. the imerest in NGG comes from the fact that its kinet ics depends solely on the propenies of the migl':lting boundaries and is otherwise independent of the medium or it>; defonnation history. This means that the theory underlying the NGG kinetics is not restricted to polycryst:lls: sim ilar coarsening phenomena are also observed in fooms. some tissues. and many other cellular media.

-40- Nue/calion: Class of phenomenological recrystallization pro­ ifiers cQllIinuousltliscon/ i/llIOII~' and conli/llwlldi.\·CQllIilllllll. cesses involving the formation of new IIflciei (viz. tiny strain­ used 10 specify. respectively. the spatial homogeneity and free new grains). Two types of nucleation mechanisms can tcmporal conti nuity of the recrystalli7.ation process. These be identified. here call ed "pseudo-" and "classical nucle­ classifications are. however. not always unique and arc there· ation". During c/assica/llllc/e(l/ioll a cluster of atoms spon­ fore of limited use. taneously form a new nudeus under the action of thermally­ RQ/(l/ion recry.\·wlliwliol1: Phenomenological recrystalli7.ation activated fluctuations. Despite numerous mentions to it in process responsible for the formati on of new grain oolllld­ the glaciological literature. it is currently aknowledged that ufies. It proceeds from the mechanism of grain m/){lil'ision. thi s mechanism is ccrtainly not re levant for polar ice.~ Dur­ and as such it involves the progressive rotation of subgrains ing pselldo-nllcie(l/ion a special combination of elementary as well as the migmtion of subgrain boundaries through re­ recrystallization processes (e.g. SIBM. subgrain rotation and gions with lattice curvature. Noti ce that this recrystallization growth) takes place lI'ilhill a slIllIfl crplO.liille regioll with process docs not require signifi cant mi grati on of pre-existing high storcd strain energy. givi ng rise to a little strain-free new grain boudaries. in contrast to migmtion recrystallization. b grain call ed psemio-lludells. If pseudo-nucleation occurs SFE/: Structure-fonn-environment interplay. Genemlly. the naturally in polar icc. it mosllikely happens at grain bound· c nvironment influences the form: changes in the fonn affect aries and other zones of hi gh stored strain energy. e.g. at air the environment; environmental changes act on the structu re; bubbles and solid inclusions. the structure modulates the form evolution: the evolving form Oriell/mion slereology: Spatial arrangemcnt of lattice orienta­ ;I!ters the structure. In the case of glaciers and icc sheeh it tions in a polycrystal. i.e. the combination of grail! slereol­ dcnotcs the intc ra ctio n .~ betwcen the deforming icc. its mul­ og)' and cryslO.llogruphic lextllfe. tiseale structure. and the environment. Phellomenological slmctuml procesj': Any combination of el­ SIBM: Seeslraill-induced boundury migralion. ementary structural processes that gives rise to general SIBM-N/SIBM-O: See migration recr),stallizmiolJ. chllllges in the structu re of the polycrysta l (cL elemelllaf), Slip /xIlIl/~': Series of parallel layers of in tense slip acti vi ty Slruclllra[ process). Examples of phenomenological pro­ and high amount of intracrystalline lanice dcfects (espe­ cesses are nucleati on ll!ld grain subdi vision. ciall y dislocations). Slip bands in ice appear always in Plale-like inc/usioll (PU ): Microscopic. flat cavity with hexag­ groups parallcl to the basal planes and arc indicative of onal symmetry lying on the basal plane of its hosting icc a nearly homogeneous shear deformation of the respective crystallite. i.e. a thin negati ve crystal. PUs are usually filled grain (cf. microshear). wi th air. appear in a wide range of tcmperatures. and arc sup­ Slorell .\·/raill ellergy: Fmction of the mechanical energy ex­ posed to be produced by some kind of stress relaxation within pended during defonnation that is stored in the material in the iee7 (98. 105 1. di verse types of intracrystalline lanice defects. e.g. di sloca­ PO/YKoniUlliOI1: Special type of reeovcry mechanism for the tions, stacking faults. subgrai n boundari cs. etc. fonnation of lill bounduries. It is a particular case of grain Slmin·inducell bOll/ulary migratiO/1 (SIBM): Elemcntary re- subdivision (cf. id.). by restricting it to tilting (bending) of crystalli zati on proccss of grain boundary motion driven by crystallographic planes. In icc, polygoniz.1Iion is often used minimization of the stored strain e nergy. It involves the mi­ in referrcnee to the bending of basal planes. gration of a grain boundary towards a region of high stored RecOI'er),: Release of the stored strain ellergy by any Ihenno­ strain energy. The migrating boundary heals the hi ghl y ener­ mechanical process of microstructural change other than re­ getic lanice defects in that rcgion. therefore promoting a net cryst;tllizatio n (d. id.). The qualifiers dYI1(jmic and SIOlic reduction in the total stored strain energy of the polycrystal. dcnote recovery phcnomena occurring during and prior/ajrer Subglacial slruclure: Any structural fcature underncath the icc. dcfomlation. respectively. Frequentl y (especiall y under dy­ ranging from till and rocks to channels and lakes. namic conditions). rccovcry and recrystallization coexist and may cven be complementary (e.g. during the development Subgmill: Sub-domain of a grJ in. delimited by a slIbgmill of new subgrJin boundaries). so that the distinction between OOllll(/(iry and chantctcrizcd by a lattice orientation that is them is sometimes \'ery difficult. si milar. but not identical. to that of the rest o f the grain. In ice, th e fanice misorientation across a subgrain boundary is Recrys/(ii/iullioll: Any re-orientation of the lattice causcd by limited 10 a fcw degrees (usu. ca. < 5°; although this limit gmin boundary migmtion and/or formation of new grain is somewhat arbitrary 11581). boundaries' (d. recovery). The qualifiers (/ynlllllic and slatic denote recrystallization phenomena occurring dllring Texlure: sec crysllIl!ogmlJhic Ie.rlllre. and prior/ajrer deformation. respecth·cly. Further classifica­ Tilloolll1ll(lry: Speciallype of subgrain boundary in which the tion schemes ofte n invoked in the literature include the qual - misorientation a)(is is tangential to the boundary interface.

~ Ca1culations show [23. 841thm classical nucleation recrystallization is extremely unlikely to occur in single-phase mat.:rials. owing to the high energies required for Ihe creation and growth of classical nuclei. except if stmng chemical drivi ng forces are present. which is clearly not the ca~e fo r polar icc. 61he prelix "p:;eudo-" is used here to emphasi'-e Ih,11 Ihis nucleus is usually mut'h greater than the nucleus fo m":d by classical nucle'lt ion but st ill small enough to be strai n-free. It shou ld be nOliced that the distinction between pseudo·nucleation and a combination of SlBM·Q with rotation recrys­ tallization is basically a maner of scale: in the laner case the new crystallite is large enough to inheri t a considerable amount of internal structures from Ihe parenl gr~in. 7Plate_like inclusions should not be ~"Onfused with Tyndall ligures: the laner are negative crystals produced in ice by internal mehing and tilled with liquid water and vapor [1041. gin C<)!\t rastlO the delinit ion adopted here. some aUlhor.; reserve the lem1 ""recryslalli1.alion" solely for those processes driven by the stored sirain energy. therefore excluding e.g. normal grain growlh (NGG. cr. id. ) from its delinition. Other aUlhor.; (especiall y in the older litemture) loosely use "recrystallization" as a synonym fo r SIBM-N (cf. migration recrysta ll ization).

-41- Twisl bOlllulary: Special type of subgrain boundary in which This means that whereas the diversit y of structures in the mi sorientation axis is orthogonal 10 the boundary imer­ an ice sheet may provide a wealth of information about face. its now and interaction with the environment. a proper lVind cruSI: Hard. thin layer of snow with hi gh mass density, understanding of thi s information requires knowledge of produced on th e icc-sheet surface by strong, persistent winds the mechani sms of genesis and evolution of each of these combined with appropriate conditions of humidity, tempera­ structures. To the despair of the traditional ice-sheet mod­ ture. insolation. etc. eler, such mechanisms are often coupled in a so intricate manner as to require special models to describe their re­ 2 lee sheets and the environment ciprocal interactions. Owing to the complications mentioned above. the The jif!>"I llay or ~·o. we al/ poillle(11O ollr cOlllllries. The study of the lIIultiscale Slfl/Clure of polar ice sheets has Ihird or fO llnh day. we were poiming 10 ollr cominellls. only recently received proper attention [25, 38. 43, 49, 8y Ihefiflll dll),. we were lIware of 0111)' olle Ear/I!. 51. 60. 64, 65, 99, 101 , 126.1 34.137, 142. 144J. This Prince Sultan bin Salmon ai-Saud. Saudi Arabian work discusses the importance of multiscale modeling for astronaut. Quoted by Carl Sagan I 125J. p. 139 modern polar research and highlights some past and cur­ Today it is common knowledge that Antarctica and rent issues on this topic. II is iII/elided 10 be lleilher a Greenland are covered by immense ice sheets. both general review IIor (I focused repol"/ Oil a Sf)ecijic iI!I'es­ huge ice masses of continental size. There is also in­ figalioll. Rather. it presents a selection of glaciologically disputable evidence that other equally large ice sheets relevant results from recent studies carried out by th e au­ have existed in the past. overlaying extensive parts of thors themselves with collaborators inside and outside the North Americ:1. Europe and Asia about 20-100 thou­ glaciological scene. Our main objective with such a sum­ mary of our own work is to examine the rise ofmultiscale sand years ago [4, 132 ]. By the time of maximal vol­ ume. these frozen giants might have contained more than modeli ng as an autonomous branch of research. based on two times the actual amount of ice found on Earth (ca. our own recent experiences in th is field. In this context. the literature cited here is neither complete nor represen­ 29 x 106 km3). The withdrawal of such a colossal amount tative, but rather illustrative only. It may be biased by the of ice cannot be explained simply by melting: the ice had to have nowed away over the millennia, creeping slowly personal opinions and interests of the authors. like a very viscous nuid towards the ocean, until disin­ tegrating itself into countless icebergs (see Part II 194]). 3 Modeling At present. polar ice continues this saga, nowing from the remaining ice sheets in Antarctica and Greenland at a DOII'I gel illmil'ell ill plirlilil problems. bill always lak.e pace from several to many meters per year 19, 79J. fliglll 10 wllere tllere is l/ free dew orer tile wllole si ngle great problem. el'ell iflhis dell' is sliIi 1101 (/ ciear olle. The waxing and waning of polar ice sheets render them an essential pan of Eanh's environmental system. owing Ludwig Wiugenstci n 11 611. p. 23 to their ability to interact with the atmosphere and the hydrosphere on a global scale. Thi s interaction is con­ As it happened in most field s of science. also the early re­ spicuous in contemporary environmental issues, like sea­ search in the dynamics of glaciers and ice sheets was per­ level ri se and global warming. and remains recorded in formed by individual, multifaceted scienti sts that could deep ice. viz. in layers of dust. aerosol s and isotopes once conduct field meJlsurements and laboratory experiments deposited on the snow surface and later buried and com­ with the same ability as to develop theories and models, pacted by the burden of subsequent snowfalls. The result­ and still apply these models to practical problems [1.61. ing stratification makes ice sheets also unique archives 151 J. This primordial approach. simpli stic and romantic of Earth's climate in Ihe past hundreds of thousands of in its essence. was soon replaced by the more efficient years [46, 47.117, 157]. dichotomy "experiment (fi eld work) & theory (model­ The basic coupling between the aClive interaction of ing)" , each pan performed by distinct individuals, which ice sheets with the environment and their passive record­ is still today the paradigm of the academia and policy ing of the past climate is high lighted by their (JYl!amic makers. The greatest advantage of this dissociation is 1I1111liscale SI I"IIC/Ure. composed of structural features that it opened the doors of glaciology to the talented the­ rangi ng from kilometer-long megadunes and subglacial oretici an . who was interested in ice modeling but was not lakes l64. 133j to SUb-microscopic dislocati on walls and fond of the perils and discomforts of the field work on ice micro-inclusions [92. 111]. In other words. the strata in (Fig. la). an ice sheet vary with depth not only in thickness and im­ As a natural con seq uence of the ri se of scientific com­ purity content. but also in crystallographic tex tureY grain puting, during the last four decades numerical modeling stereo logy, and other structural features that depend upon of ice sheets has established itself as an indispensable the deposition and deformation history. branch of polar research. as a complement to th eoretical A severe complication to the interpretation of such modeling. Initially. computer simulations were si mple structures is the fact that most of them evolve with lime. and restricted to highly idealized situalions [108, 1201.

9See the definition of "crystatlogmphic texlUre" in the Gtossary in Sec!. I.

-42- comput ....elenee

qUirit~lIv, global cllmM. eslinliltes mo''',

(a) An:haic (b) Old

ilo.,.1 cllm •• models

physk.of cond.., .. d m.tt..

mathem'Hcel computlr scl,nc:. phy.ilc.

(c) New

Figure I: Schematic illustratiOIl ofmodelillg strategies. Although feedbacks do exist in each orgmrigrwlI between the several branches of mOlleling. the.l·e are not indudn/ here for simplicity. (a): Archaic strategy, IISel/ in fhe early fimes of ice-sheet //Iodeling. prior to ~·c i elllific complltillg. III the absence of /llImeriwl simI/la/ions. ollly higflly idealiZell problems cOl/ld be Iackled III tlris IJrimordial ~·lage all tasks were pe/fomrell by ollly olle modeler. (b): Stra/egy atlopted after tire emergence of scientific compllti/lg. Initially, nllmericalmodels were relatively simple and idealized. bllt they soon developed illto realistic simulatiolls that split lIumerical mOl/elillg illto '"IJrimltly" lIlId ··appliel/". III prillciple there are three types of modelillg tasks, bllt oftell tlrey are performed by just /11'0 modelers. (c): The lIewest strawgy for ice-sheet modeling. Notice t/rat IIIl1ltiscale structural 1II00/clillg provides support IlOt ollly to theoretical ice-sheet modelillg, but also 10 appliel/ IIIlIIrerics. In prillcilJle there (Ire fourtYIJes of mOl/elilig tasks, bill often tlrey (Ire perjormel/ by just three modelers.

More recently, however, the increasing complexity of number of relevant structures in ice sheets. Distinct struc­ programs and applications has induced a spl itting of nu­ tures could be found on di verse scales and many of them merical modeling into so-called "primary" and "applied": could evolve according to their own laws. often forming the former is supposed to denote the development of a coupled system. new simulation programs and algorithms, while the lauer All these results doomed the traditional theoretical describes the application of established simulation pro­ modeler, who could not cope with the complexity of grams and algorithms to realistic boundary-value prob­ evolving structures in multiple scales. It transpired that a lems (Fig. Ib). On many occasions applied numerical fundamental reorgan ization of the model ing strategy was modelers employ programs developed by themselves in needed, with the emergence of a particular kind of mod­ an earlier period of their careers, when they were engaged eling, not for the enti re ice sheet itself. but for its intrin­ in primary numerical modeling. Modelers that ma nage to sic structures. The main task of this particular branch of act a.lone in both fields simultaneously and with a. cer­ ice-sheet modeling, from now on named IIII/Itisc:ale ~· t/"llC­ tain degree of success are becoming scarce. owing to the tllral lIIodelillg, should be 10 solve completely the prob­ stren uous effort to conform to the increasing standards of lem of evolution and muJ\iscale interac tion of distinct soph istication and real ism req ui red for modern numerical structures in ice sheets, by using theoretical and, when simulations. necessary, also numerical ingredients. Consequently, it Curiously, the sophistication of the newest ice-sheet should provide support not only to theoretical ice-sheet simulations produced an interesting effect upon theoret­ modeling, but also directly to ap plied ice-sheet simula­ ical model ing, which culminated with a complete rear­ tions (Fig, Ic). A simple example of the first kind of sup­ rangement of the modeling hierarchy: ever more realistic port is the modeling of crystallographic tex ture. which applications set off a quest for not only more elaborate gives rise to anisotropic ice-sheet lIIot/els. The second theoretical models, but also more refined field data .. . and type of support is ill ustrated e.g. by the modeling of sub­ soon ittumed Ollt thai this quest was revealing a growing glacial till deformation. which after being combined with

-43- an appropriate model of ice-sheet dynamics leads to an example of the class of cuupletl ice-sheellllodeis. ... i Today. multiscale structural modeling is a prolific field _3~ --..... - in glaciology. Models of fabric evolution [7. 55. 67. 68. - [02. [411. snow and fim metamorphism 12. 6. 28. 321. - dislocation dynamics 130.821. recrystallization L51. 101. <=,- 137.153] and subglacia[ hydrology [18. 25. 49J are just some of many examples (see also Pall II [94]). Stud­ -- ies of the muitisca[e interactions between these structures themselves and/or the environment are. nevenheless, not very frequent. In the sequel we examine some potentia[ly significant types of multiscale structural interaction and present some recent research results that are directly or indirectly related to them. Figure 2: Parchy snow slllface (1/ Drolllling Ma/ld Lalld. AII/(Irctic(I. The layered Slructure ill the center is the rell/­ Before we embark on practical examples. however. II(//It of (1/1 old barchall-like dUlle disfigure(1 by we(l/h­ it should be remarked that the emergence of multiscale ering. The steps producell by wind erosiOIl reveal the structural modeling should not be misinterpreted as an­ slralijiell slructure of the dune. dillracierizell by allemllle other triumph of specialization, but rather the opposite. layers of dellse. fille-grained SIIOW (wind C/"IISIS) and less It is indeed a specific research field. but its main role is COIllIJaCI. coarse-grallle{/lIItllerial. Tlte wi(illt of lite cen­ that of intermediator. which tailors concepts from theo­ Iral,Jarl oflhe illlage correspollt/s 10 ca. 30111. retica[ modeling and condensed matter physics to suit the needs of appl ied ice-sheet modeling and the findings from The surface of the Antarctic ice sheet consists basi­ field observations. Thus. it promotes multidiscip[inarity cally of such a patchwork of different types of compacted and union against specialization and isolation, for it [inks snow, discontinuously accumulated during snowfall and fiel ds that would otherwise remain poorly connected. drifting events over numerous years (Fig. 2). The origins of the various granular compositions found in these mu l­ 4 Polar ice formation and climate records tiscale surface structures, marked by contrasts in grain size and shape, texture, porosity. impurity and moisture / dose(/ Illy eyes. content, are still not fully understood, owing to the in­

There was (I $olllld like 111(11 oj the gemle closillg oj a tricate chemical and metamorphic processes taking place portal (IS big as the j'k)~ Ihe greal door oj helll'en being in the snowpack. Indeed. air-snow exchanges of trace closed softly. It \\IllS (I grand AH- IVHOOM. compounds affect considerably the chemistry of the snow / o/Jened Illy eyes--lmd (III the sell was ice-nine. cover and the lower atmosphere. and consequently inter­ The moiSI green eanh was il bille-\\'hite pearl. fere with the snow metamorphism [34, 75J. In addition to the evident consequences for the climate records. such The .l"liy dllrliencli. Homsisi. the SIIII. beellme a sickly yellow bal/. liny (/lui crud changes in the chemistry and metamorphism can alter the surface albedo and the permeability of snow. with sig­ The .I"~)' was fil/ed with WOn/IS. The WOr/IIS II'ere nificant implications for the validation of radar backscat­ IOnuuloes. ter signals (ERS SAR. Radarsat. CryoSat) detected from Kurt Vonncgul. Jr. 1154J. p. 174 the ice sheet surface during altimetry/interferometry sur­ veys [159]. Cl imate conditions initially determine the size and shape Subsequent snow compaction caused by the increas­ of snow crystals -the first structures of interest-as well ing overburden of new snow layers leads to the forma­ as the concentration of dust, aerosols and other trace com­ tion of a porous material called fim (Fig. 3). In dry po­ pounds deposited on/within the snow crystals that accu­ lar regions. the fim zone extends from the near surface mulate on the ice sheet surface [34. 58]. Isotopes ratios down to 50-100m depth. with a gradual mass density of precipitation (HDJ60 and Hj 8 0 relative to H1GO) are increase from about 0.2g/cm3 to 0.8g1cm3. In the top commonly used as proxies for the temperature at the time 10-20m. air is exchanged with the atmosphere mainly of snow formation. even though seasonal variations are through forced convection in response to pressure gradi­ usua[(y lost by diffusive mixing 131j. Some types of ents at the surface l13. 29]. Below this depth. gas trans­ snow crystals may drift long distances under the action port occurs predominantly via diffusion driven by water of strong winds. and accumulate selectively in a variety vapor density gradients [28J. Allhecritica[ depth for pore of surface patterns on multiple size scales, ranging from close-off (namedfim-ice trallsitioll depth). the connected nimsy wind crusts up to vast megadunes 164]. These mu[­ pore space separates into isolated bubbles occupying ap­ tisca[e surface stnlctures are then further modified by the proximately 10% of the bubbly ice volume [139]. These rapid metamorphism of snow, triggered by direct exposi­ bubbles represent a unique archive for the reconstruction tion to insolation, wind, moisture and temperature gradi­ of past changes of the atmospheric composition, provided ents 128. 32. 63]. thei r ages can be correctly determined.

-44- Figure 3: DifJerem aspect!; of the jim structllre. Left: 3D-reconstruction via computer TOmography of a cllbicjim sample (side length 5111111) from 10 m depth. £DML jim core 833. Computer TOmogra!Jhy is a lJo\l'e1flll tool for stlu/ying the TOpologic(ll and geometrical pro!Jerties of the pore network al1(/ ice skeleton. like e.g. porosity. tortuosity. neck size (/is­ tributioll. etc. It is also very useful ill sflu/ies of pore-SfJace anisotropy (ll/d in !Jrovidillg realisric boundary cOI1(/irions for gas percolarion srudies. Right: Microstrucrure lIIapping phoromicrograph of a jim salllple from 80 m deprh. EDML jim core 837 (image whfrl! ca. 5 111m: from [93 J). 11/ COl!/rasr /0 cOlllpllfer tomography. the microstruc/llre mapping metho(f is especially convenient for studying rhe microstl'llcture of the ice skeleton (viz.. grain boulldarie!:;. (ieformatioll-iIUJuced structures. etc.).

Due to a lack of absolute dating tools, the age of an ice a reliable dating of ice and gas inclusions 1961, Both is­ layer is usually estimated by a mixture of several meth­ sues are strongly connected to the struc tural properties ods 196. 113]. including theoretical models combined of snow and tim, as well as their accum ulation rate and with layer counting and records of chemical components metamorphism. wi th seasonal cycles (e.g. sodium, calcium or sulphate). Below the tim-ice transition, bubbles decrease in sil.e Some ice horizons containing volcanic events can be also and the enclosed gas pressure increases with depth [97. cross-correlated with other well-dated archives to give 127. 129]. When the pressure in the bubbles becomes absolute markers for the age scale L80J. It should be no­ high enough (dependi ng on ice temperature), they start ticed. however, that gas inclusions are always younger to convert into crystalline compounds called c/mfml/e than the surrounding ice matrix: air in a rim column hydrate!>' ]91, 100, 128]. Clathrate hydrates in polar down to 50--- 100 m depth is stil l in exchange with the at­ ice are essentially composed of oxygen and nitrogen mosphere by connected pathways in the pore network. molecules confined in water-molecule cages. Interest­ Hence. ice underneath the rim-ice transition depth can ingly, not all bubbles transform into clathrates simulta­ be a few thousands of years older than the air entrapped neously: there exists a bubble-clarlmlfe Trallsition ZOlle, in it [10, 12. 122l- A correct interpretation of climatic which generally spans several hundreds or meters in records in the upper part of ice cores depends therefore on depth. Within this wne, there occurs a depth-dependent the k.nowledge of the li nk between atmospheric composi­ gas fractionation, with N2-enriched bubbles co-existing tion and the climatic signal stored in ice. together with with 02'enriched clathrates r86. 87], This fractio na-

1CIOpm

Figure 4: Cla[hrate-hydrate SfruCfl/reS obsen'e(i at difJerent (ie",hs ill the NGRIP dee" ice core. From left to right: prilllmy hydrate (polycrys[al/ille sphelVid). rough sphelVid metalllorphosing inro a lIIulri/aceted plVrrusion; polyhedral l'/wpe: slllooth globule ({IVII! [ 114 lJ.

-45- tion process is still not perfectly understood. even though 5 Microstructure it is cmcial for a correct interpretation of paleoclimatic records [12. 122, 1381. Trace gases such as CH" and CO2 [... [the I'e,)' renn structure implies interrelation between are also expected in clathrate hydrates, often combined parr;,- wilh reference 10 {/ whole. with N2 and 0 2 [95J. Cyri l Stanley Smith [1361. p. 3 Below the bubble-clathrate transition zone, the lasting clathnlte hydrates continue to evolve along the millennia .. llhe whole is 1101 merely rhe sum of irs parts. II is Ihis. in a strange fashion (Fig. 4): from rough polycrystalline and lIIuch more Iholl Ihis. For ir is 1101 (I bundle ofJXIf/S spheroids over faceted and/or slender shapes towards usu­ but {/II org{/llizlltioll of fUlrtS. of partS ill rheir ml/rual ally isometric single-crystals [91. 115, 130, 152]. The armllgemenr. filling olle wilh (lilother. ill w/r(lt Arislotle activation energies and driving forces causing such cease­ c(l/ls "a single Wid ill(lil'i~'ible prillciple of IIl1it)' '' 1... 1 less metamorphoses going from air bubbles into various D'arcy Wcntwonh Thompson [1411. p. 114 forms of air hydrates are still not completely understood and their determination may shed new li ght on the inter­ At temperatures and pressures prevailing on Earth's sur­ actions between deep polar ice and air inclusions, face, ice possesses a hexagonal crystalline structure Studies of climate records in the lowest tens of me­ called ice III , which is characterized by a wurtzite-like lat­ ters of deep ice cores are usually impaired by dynamic tice of oxygen atoms stabilized by statistically distributed recovery and recrystallization, flow disturbances (fold­ protons that form covalent and hydrogen bonds 159, 811. ing, etc.) and interactions with subglacial features. Ad­ For our purposes, the major features of thi s lattice ditionally, regardless of careful low-temperature stonlge, structure are: its hexagonal symmetry -which defines relaxation structures quickly emerge throughout deep ice the well-known b{/sal, prismatic and pymmidal plalles cores. They consist mostly of polygonal cavities call ed (Fi g. 6)- and the ex istence of two categories of basal plate-like illdusiOl1!i (PUs; Fig. 5) 172]. In spite of planes, called glide set and shuffle sel [118]. Interest­ the paucity of studies about these objects, recent spec­ ingly. the planes of the glide set have the ability to fit troscopic observations 1105] revealed that plate-like in­ over one another in a very peculiar way, which resem­ clusions are negative crystals frequently filled with 0 2- bles well the close packing of metals and allows so the enriched air. A beller understanding of PU formation dissociation of dislocations on such planes into Shockley is urging in order to clarify its relation to diffusion pro­ partial dislocarions separated by a stacking fault 166, 821. cesses in polar ice. Unfortunately, many glaciologists in­ This dissociation is only possibl e because the energy of sist on ignoring these relaxation features, by regarding a stacking fault lying on II b;lsal plane of the glide set is them as futile artifacts. very low. Thus, basal dislocations are expected to sta­ bilize into ribbon-like structures that severely restrict the dislocation-glide motion 10 the basal plane, since cross slip to other planes would require a constriction of the extended dislocation. From this succinct description we conclude that the peculiar properties of dislocations in ice, and in part icular the low stacking-faulr energy of its basal planes, endow ice crystals with a strong plas­ tic anisotropy [82, 1181. In simpler words. ice crystals deform easily by (Jislocafioll creep I'ia basal slip. c

1 , : ! -:.,.~--- ," "', \V :'1$ :\ , ' . : \ .~ -.:- - -'''. -~ --.- - --". .~, '\ .-- -- .. '!' , ""'.'

Figure 5: Plate-like indusiom' in Ihe EPICA Dome C Basal Prismatic Pyramidal (EDC) core. Top: NI/Ille/VIIS plate-like illelusiol1s (PLls) ill all EDe sample from 511m deplh. The wi(fth of the Figure 6: Sketch of (lIIIOllocrystal/ille, hexagol!al prism image is 2.5111111. Bottom: Time series of micrographs of ice. Three families of c/)'slal/ogmphic planes are illlli­ showing rhe gradual decomposirions ofa PLI. DIJrillg fhe cared. Basal plalles (0001 ) fie orlhogollallo the c axis. firsr days rhe PLI becomes rhicker and loses rhe IJOlyg­ which is rhe maill axis of optical (lIId cryslaflogmphic ollal shape. Filially, it collapses illlo mic/vsC01Jic: cal'i­ ~ymlllelry of Ihe cry.~tal. The prismatic faces {loIo} lieslbubbles. The width of each image is 30011m. are parallel /0 lire c (uis. while pyramidal planes. e.g. {lOll }. CI'OSS tlte bulk of lite prism (from (50]),

-46- Figure 7: Microslrtlef/lml feaf/lres of AlI/arClie ice rel'ealet/ by Ihe melho

The conjecture that the creep of polycrystalline ice features. Together with slip bands they constilute the sheets should also be dominated by intracrystalline basal most relevant examples of what is sometimes called the slip has been supported by the extrapolation of results deformatioll-related micros/mctllres of polar ice, from labora/ory tests 18, 21,88] and by reasonable argu­ As in other geological materials, the microstructllre of ments concerning the cl)'stallographic tex/llre evoilltioll polar ice evolves over centuries and millennia under the in polar ice [39]. Additionally, it has received recent sup­ action of continual deformation and thermally activated port from measurements of dislocatioll density via X-ray processes like dynamic recovery and recrystallization, diffraction [78j and most particularly from the direct vi­ Duri ng several decades glaciologists have devised a sim­ sualization of slip bWl(is in ice core samples from Green­ ple model for the evolution of the microstructure in po­ land and Antarctica [53, 92, 149, 156[. As explained in lar ice sheets. It is based on the assumption that a single, Sect. I, sl ip bands are series of parallel layers of intense universal hierarchy of three thermally activated processes slip activity and relatively high amount of intracrystalline should control the microstructural evolution of polar ice la!lice deffects (di slocations, etc,), In naturally deformed throughout the ice sheet [3, 39, 40, 55, 74, I 13, 1 [6, 1481, polar ice they consist excl usively of basal planes and are Succinctly, according 10 this three-stage //IOflei the mi­ so faint, owing to the low strain rate, that their direct vi­ crostruclUre of polar ice in the upper layers of an ice sualization requires the use of special microscopy tech­ sheet should evolve under the regime of "normal grain niques, like e,g, microstructure mapping [92j (Fig, 7). growth" (NGG), being counterbalanced after some hun­ Because of the strong plastic anisotropy of the ice 1:lt­ dreds of meters depth by grain splitting via "polygoniza­ tice .md the need of topological compatibility between tion", whi le in the deepest hundreds of meters of the ice neighboring grai ns, the slow deformation of polar ice is sheet, where the ice temperature raises above ca. - 10°C, the microstructure evolution should be dominated by dy­ very i"holl/ogeneolls (Fig. 7). Not only does the stress vary immensely from grain to grain in the polycrystal. namic recrystallization with nucleation of new grains (S IBM-N).IO but also the deformation inside each grain is inhomoge­ neous, with the marginal region (the "//Ian/Ie") experienc­ One of the great virtues of the three-stage model is ing more deformation than the cent ral part (the "core") undoubtedly its simplicity. It provides a satisfactory fit of the grain (Fig. 7b), Thus, the deformation of polar of the average grain size data of most polar ice cores ice is characterized by the continual rearranging of dislo­ and promises to modelers an ice sheet consisting in great cations into disloearioll walls, forming slIbgraill bound­ part (excluding the messy bollom layers where SIBM-N aries which provide evidence for graill subdivision by is supposed to be active) of harmonious grains grow­ shearing, twisting and bending (sometimes called "poly­ ing like bubbles of froth, without being considerably af­ gonization") under the action of local stress concentra­ nicted by stress concentrations or deformation inhomo­ tions [53,77, 158). According to recent observations of geneities, except for the sporadic punishment of "poly­ the EDML deep ice core [92, 931, such subgrain bound­ goni),.:Ition", applied to those eager grains that happened aries seem to occur throughout the ice sheet and interact to grow too much, Nevertheless, an increasing number of markedly not only with grain boundaries but also with scientists is coming 10 the conclusion that the three-stage pores, bubbles, clathrates, dust and other climate-related model may be more than just simple, namely an o~'er- lOSee Glossary in Sect I.

-47- .fimpiijicclliol1. The basic argument is that a more care­ of recovery. recrystallization and related thermally acti­ ful consideration of the ice microstructure reveals that vated processes. Unfortunately. in situ rates of recovery (d. 123, 71. 84, 140]; see also the detailed definitions in and other thermally activated processes are very difficult the Glossary in Sect. I): to evaluate for polar ice, not only because of the inho­ I. NGG cannot strictly tllke place in a material un­ mogeneity of the deformation on the microscale, but also dergoing deformation. since the necessary ""fO,un­ because of potential relaxation effects taking place in the like structure" is usually not preserved in this case. ice core. It is believed that such undesirable relaxation ef­ Thus. the usual filling of the average grain size ver­ fects can be minimized by appropriate core storage con­ sus age in pohr ice cores with a parabolic NGG­ ditions (temperatures below - 50°C are currently pursued type law (e.g. the Burke- Turnbull- Hillen law) as standard) and early preparation of thin sections (sup­ may have no physical sign ificance. posed to hinder relaxation via surface pinning effects). There is. however, no ultimate conclusion about the effi­ 2. Polygonization is just one of a number of grain­ ciency of these measures. especi;llly for the lowest pans subdivision processes that may occur in an inho­ of deep ice cores. mogeneously deforming polycryslal. More com­ plex subdivision processes cannot be explained by 6 Stratification grain size arguments only (as done in the simplest versions of the three-stage model). but rather by UI!(/ollbted/y we hm'e 110 qllestiolls 10 ask which are stress inhomogeneities on the grain scale. Whether IIIUlI1sll·erab/e. \I 'to IUlist trust the perfectiOiI of the cre­ one can trace a direct relation between mean grain (l/ioll.fo jar, as 10 beliel'e that II"lul/el'er curiosity the or­ size and stress inhomogeneities on the grain scale der oj things has {j]\"(/kelled ill Ollr //Iind.f. the olr/er oj remains an open question. thillgs call s(lti4y. [ ... J lIatlife is iI/feildy. ill its jonus (llId 3. Dynamic recrystallization need not necessarily in­ ten(/ellcies. de~'cribillg its 011'11 desigll. volve the energet icall y expensive nucieution of Ralph Waldo Emerson 145J. p. 7 new grains. as already demonstrated by Beck & Sperry Ill] more than a half-century ago. Con­ As discussed in the previous sections, the formation of sequently, the assertion thai dynamic recrystalliza­ polar ice from snow is a lengthy and intricate process tion becomes significant only above a criticaltem­ that lakes many centuries and entails a series of multi­ peralure of ca. _ lOoC should be reconsidered. scale interactions between various snow structures. the Recent microscopic in vesti gations have also cast doubt lower atmosphere. and the lirn layer. In view of the di­ on the validity of the three-stage model. For instance. ve rsi ty of structures and chemical reactions involved in Kipfstuhl and others 154, 92, 93. 1581 studied the mi­ the densification process, as well as the strong inhomo­ crostructure of the whole EDML deep ice core in mi­ geneity of the upper snow layers. it is not without lillIe croscopic resolution and concluded that grain subdivi­ surprise that we realize how ordered and regular the ice sion and migration recrystallization (S IBM-O) are signi f­ layers underneath the lim- ice transition are. More fas­ icantly active in diverse depths at the EDML site. includ­ cinating is the fact that this stratified structure. charac­ ing the firn layers. Addi tionally. Faria and others 156. 57] terized by chemical and microstructural variations with investigated the microstructures of certain deep strata of depth, persists throughout the ice sheet (with the excep­ "soft ice" found in the EDML deep ice core and discov­ tion of the mixed strata near the bedrock). ered a curious grain stereology that was attributed to the Methods for visualizing the stratigraphy of polar ice ex istence of wel l-developed microshear boundaries liS]. sheets are numerous and well known by any glaciolo­ Now. if we consider the fact that there is no reason to re­ gist. In fact. almost all properties measured in polar ice gard the EDML core as extraordinary - actually. it has reveal some kind of stratification: (di-)electric contrasts been extracted from a much less exotic site than most measured via ECM or DEP,II variations in the concentra­ deep ice cores, since it was not extracted from a dome­ tion of isotopes or impurities (e.g. dust or soluble traces), then we are led to the conclusion that the structures ob­ variations in grain size ... all reveal some particular type served in the EDML core may well be representative of of stratified stru cture /46. 47, 146. 162]. many sites in Antarctica. Another useful method to visualize isochronous strata From the arguments above we can infer that although is radio-echo sounding, in which the detected inter­ most microstructural features of polar ice are related to nal renections are related to horizons of dielectric con­ deformlltion. their evolution is strongly innuenced by trast 1106. 134]. The origins of such internal reflections chemical impurities and thermally activated processes. are still matter of debate. with variations in density, acid­ Thus. with respect to the deformation history. the polar ity and texture being the most common causes 143, 65, ice microstructure may be regarded at best as a quali­ 131]. The main advantage of radio-echo sound ing is that tative '"fading record" only [541- The "memory persis­ it provides a three-dimensional representation of a large tence"" of this fading record depends on a seri es of fac­ portion of the ice sheet, which is useful for ice-sheet now tors. ranging from stress and impurity content to the rates sllldies. On the other hand, its shortcomings are the low

11 See Glossary in Sec!. t.

-48- - - '- ---. J-.. ----_.------­ r~ if" I I :~'

Figure 8: LinesC(lI111er images of fWO EDML (feel' ice core pieces from disrillc/ deplhs. Recall rll(l1 rhe images are PIV­ (II/ced by light scallerjllS from abore. agaillsf a (lark background; cOl/seqllell/ly rhey aplJUlr as "egolil'e IJicrures of rhe core. Brighler regiolls ilUlicare stronger light scarrering due to (l higher cOllcenrrarioll of impurilies. Ti,e rop of the cores i.f fa rhe left. Top: IIpper 50cm of WI ice core piece/rom 855111 (Iep,l!, The "granulated" appearance offlle core is ,llIe to

Ihe ligh, scattering by air bubbles. Bottom: IIP1Jf!r 50cIII of (lli ice core piece [mill 243411/ depth. NOle the inclined wavy ~· tralll (lnd Ihe sliddell challge ill layer inclillatioll after Ihe firsl 10 cm. wilh sume indicatioll of imermixing.

resolution of the data and the complete lack of internal nating with the formation of shear zones and extruded layering signal from deeper ice, within what is called the strata [48, 56, 76. 83, 112. 1431. The possible causes echo-free :,one of the ice sheet. of this now enhancement are apparently not unique, al­ In the last years. a modern version of the old method though all enhancement mechanisms seem 10 be ulti­ of visual stratigraphy has brought new impulse for strati­ mately related to interactions between microstructure and graphic studies. The essential idea of modern ice-core impurities under special conditions of stress, impurity linescanners is simply to scan the whole ice core using a content and temperature. high-resolution CCO-camera with enhanced contrast op­ During the last years. the interest in the interaction be­ tion 1142. 1441. The resulting image reveals internal lay­ tween ice-sheet stratigmphy and now has increased sig­ ering of polar ice due to variations in the light scattering nificantly. Several models have been applied to simula­ properties of the core (Fig. 8). The information obtained tions of Antarctic sites 138. 126. 155J. with the intention from linescan studies is two-dimensional. in contrast to of reproducing at least some of the typical effects of stnll­ radio-echo sounding. but it is much more localized (lim­ Hkation. These simulations often take for granted that the ited 10 the diameter of the core) and has incomparably high nuidity ("softness") of an ice layer is mainly caused higher resolution. As a consequence, it reveals details by a relatively wong single maximum texture ("fabric"). of the ice sheet flow that could not be achieved by other However, not all soft ice layers can be explained in methods, including wow ing. mixing and folding of strata, terms of suitable crystallographic textures. One inter­ even in the deepest parts of the ice core. esting example is given by the EDM L soft ice layer de­ In spite of the long tradition of stratigmphic studies scribed in [56. 57J. A noticeable closure of the EDML and the Vllst literature on the subject, often have polar ice borehole starting abruptly at 2385 III depth indicated a strata been regarded as mere passive structures record­ sudden change in ice rheology, i.e. a soft layer. Fab­ ing past climate changes and ice flow tendencies. The ric analysis showed no conspiclloll s difference between recognition that stmtification may also have an actiJ'e ef­ the crystallographic textures above and inside the layer. fect upon the rheological properties of polar ice. and con­ whereas the microstructure mapping images revealed a sequently that it does not only record but also interact striking change in the grain stereology. It was concluded with the ice now. can nevertheless be traced back at least that the high impurity content and temperature. combined to many decades ago [83, 112. 113. 1431. For instance, with the typically low deviatoric stress of ice sheets. ac­ deep ice layers from the cold, dry. windy ice ages are tivated within that layer a microstrain mechanism known particularly rich in cloudy bands. viz. layers with an un­ as micIVsileal' lI5, 36, 85J. This means that the usual de­ usually high concentration of micro-inclusions and other formation regime of dislocation creep has been enhanced chemical impurities [73. 142]. The gTllins in such bands within that particular layer by a special accommodation are generally much smaller than those in the surround­ mechanism, which involves the formation of microshear ing ice and the crystallographic texture ("fabric") is of­ boundaries (Fig. 9) and small amounts of grain boundary ten stronger. Observations of tunnel and borehole clo­ sliding [1401. sure/tilting rates suggest that the microstructure of such This accommodation mechanism via microshear has layers tends to evolve towards a pronounced enhance­ the ability to release stress concentrations and strain in­ ment of the ice now. especially at warmer layers. culmi- compatibilities. without severe impact on the crystallo-

-49- 2395m

b \~ ., \r-'- ., ) .. --~~~--~ )

" '\--'-...,--.1.... P1 '\0 --1-...... G' G' ------• (a) Original structure (b) Stress concemr.l1ion (e) Subgrain fonnmion

"-"H_ G> , r~; f--'---r-'-T-+---i;"' ) \ P1

.. -.. -~----.--- • ------~• --- • (d) First path inactivation (el Complete inactiwtion (I) Micr(lshear reactivation

Figure 9: Fonll(l/iOl1 of subgraill-boulld(llY "bridges" 011(1 moi/1faillOIlCe of rhe "brick-wall pal/ern" Ilia micros/war. Top (image): Typical /ayerel/ structure of grains ("brick-wall pattern") ill the soft ice stratulII of the EPICA-DML ice core. MallY grains ([re block- or S-~·Iwped. with IIIOSI hOI/ill/aries orientel/ nearly parallel or orthogol/al to the local ~· rraligr(jphy. Lollg chaills of grain bOllndaries following the local stratigraphy are also (Illite COl11mOIl. being /requclllly cOllllee/ed by subgrain boundary "bridges". III this image we see three ofsuch subgraill bou/U/ary "bridges" ill early stage offormatioll (illdicate(1 by the arrows). Notice also the fainl. but visible. slip blllu/s ill tlte large S-slwpe(/ graill ill the allier. The scale bar stands for I mm. Bottom (illustration sequence): Simplified cartoon of the "micro-sli(Je m'alanches" associated with microshear. For sim­ plicity. grains are schematically represenred by recf(lllgies and the grain shifts relatel/ TO micro-slide have been I/{'glectel/ (Olhenvise more complex geometries would be needed to m'oid incompatibility between the grains). Therefore. this car­ toon is not il!lended fa €.\plain Ihe mechanism of microsltcar ;lSelf (which illl'olves rather more compJicare geometries . .5ubgraill raullion. erc.; ([ /56J). bur nuher simply the concept of "micra-slide al'alanches ", which has liar beell (l(/drcssel/ in l/ctai/ in /56/. (a): We consit/er a pol),cryS/{llJille regioll of Ihe soft ice layet: subjected to localJizel/ shear stress. (b): Due to a particular combination of high impurity content. loll' stress (/lid hig" temperature. certain grain bOlllu/aries seem TO I1m'e porellfial to slitle. We identify two graill bO/llu/ary paths (thick lilies marketl by PI and P2 ) that are active for slillillg. The el/(/s of these IUlths (a/ the blocking graim' G I. G2 01/(/ G3) become points of stress col/centration. (c): Suppose that microshear occllrs first ill G3. Thlls. a subgrain (microshear) boundary is formed. which slowly evohes into a grain bOllllda/)' as tlte micro-slide comilllles. The stress concellfratioll is now trallsferred to G I (/1/{/ G2. (d): Assuming that G2 is the!irst grain to u/Ulergo lIIicroshear alollg the path PI, Ihellihe whole stress COllcellfratioll at GI (//1(1 G2 is released II/lVllgh a microscopic stress discharge along PI. The pa/II P2 becomes 11011' inactive. (e): Further miclV-slide (/I'alollches I/'(II/sfer the stress cOllcellfr(lfions 10 other regions of the I'o/)'crys/(l/. making tlte path PIa/so il/aClil'e. 8y comparillg (a) and (e) Il'e call lIolice 11011' tlte "brick-wall" pal/em has beell enlwncetl. (I): the lack of actill;/Y of tlte p(lfhs ,,/lOll'S grain boundaries to migrate. weakenillg rhe "brick-wall" /Jaffei'll. However, fll l'th er slress (/ischarges elsewhere lIIay SOOIl or later activate olle of the paths agaill. ill this case, P2, so that a new cycle of slidillgs ~· tarts.

-50- graphic texture evolution via dislocation creep, by means cians awoke from their clean studies of ice sliding over of interminent microscopic stress discharges (or "micro­ hard bedrock. and realized that life at the su bglacial bed slide avalanches") in different points of the soft ice layer was less pristine than they had thought; that metres-thick (Fig. 9). Additionally, the formation of microshear bands layers of subglacial till sometimes exist [ ... ], and that this generates a feedback to the microstructure, seeing that material deforms:' Not only the rheology of till . but also they contribute to the formation of long grain bound­ the effect of the subglacial hydrology has been neglected ary paths that facilitate further grain boundary sliding. 12 in this manner. These paths (or chains) of grain boundaries endow the Hydrological models describing the dynamics of sub­ microstructure with a "brick-wall" appearance that is glacial channels and lakes exiSled already in the early considerably stable and typical of the microshear mecha­ I 970s [107. 124]; theories of till rheology appeared some nism (d. [15,361). years later [16, 17,241. Attempts to incorporate thedefor­ From the di scussion above we realize that stratifica­ Illation of subglacial till into glacier and ice-sheet models tion is involved in a number of multiscale interactions followed shortly after [5 . 33. 89J. but the debate about the with the microstructure and the ice-sheet flow. Many of appropriate till rheology (plastic versus viscous: the old these interactions are still not well underslOod. and many dilemma of realism versus si mplicity) persists today. On others probably remain to be discovered. A typical ex­ the other hand. only in the last few years have hydrologi­ ample is the conjecture of mesoscale slmin. proposed cal models been successfully coupled with models of ice­ few years ago by Faria & Kipfstuhll531. According to sheet dynamics. undoubtedly one of the greatest achieve­ this conjecture, the stratification of the ice sheet could ments of multi scale model ing [60] . possi bly induce a sort of deformation inhomogeneity on Latest fin dings of a number of subglacial lakes and the mesoscale, viz. on the scale of several centimeters, channels underneath Antarctica have caught the interest which is spatially larger than the microstructural inhomo­ of the scientific community and the lay public [19. 123. geneities on the grain scale, but still sufficiently small 10 133J. Ac tually, this is not as surprising as it may seem become homogenized on the large scale relevant for ice­ at first sight. for it is well known that thick ice masses sheer flow. Be that as it may, further research on this and (> 1 km thick). like those covering Antarctica and Green­ other types of multiscale interactions is needed in order land, insulate well \he bed from the polar atmosphere and to elucidate the real importance of stratification for the enhance internal strain heating. Warm-based conditions dynamics of polar ice sheets. can therefore be achieved. where basal ice is at pressure­ mel ting point and liquid water can accumulate under the 7 Subglacial structures ice. The main concern about the unexpectedly large (and still increasing) number of lakes and channels discovered Flifse hopes life more dangerous Iflllll fears. in Antarctica comes from the fact that high subglacial wa­ 1. R. R. Tolkien [150J ter pressures over significant portions of the ice-sheet bed (character: Sador "Labada]"'). p. 41 can decouple the ice from the underlying bedrock sub­ strate, reducing or even el iminating basal friction. and Muc h has been said here about the illlrillsic structures of pemlilling high rates of ice now (up to several kilometers ice sheets. There are. however. many eXlrillsic strtU:lllres per year). Indeed. there are Strong evidences that simi ­ that are crucial for the behavior of large polar ice masses. lar flow phenomena have occurred in the past, sometimes Most of them He beneath the ice and are therefore called with dramatic consequences for the env ironment [14,27]. subgJacial srruclllres. These comprise sediment and till The consequences of the recently discovered subglacial layers, topographic features (hills, valleys. etc.), meltwa­ structures underlying the Antarctic ice sheet are still di f­ ter channels and lakes. From the point of view of mod­ ficult to work out, although it is evident from the above eling, intrinsic structures affecl the constitutive laws of discussion that processes operating underneath a large ice ice. while extrinsic, subglacial structures have a deci sive mass can sometimes have a greater innuence on ice now effect upon interface/boundary conditions. than those operating within it [18.25]. During long time modelers tried 10 avoid the complica­ Additionally, it should be remarked that besides the tions caused by the subglacial environment. They used to interplay between subglacial stru ctures and ice dynam­ subsume all effects of subglacial interactions into a pre­ ics me ntioned above. there exists also a reciprocal in­ scribed field of sliding velocity of ice over hard bedrock. terplay between subglacial dynamics and ice structures. which should vanish whenever the temperature at the The laller has been recently observed in subglacial lake base of the ice sheet is below the pressure-melting point. discharges promoted by the overburden pressure of the Among other shortcomings. such treatmellls were incom­ overlying ice [26. 160J. which indicate that the dynamics patible with many real examples of warm-based ice that of subglacial hydmulic systems can be quile sensitive to experiences alternating cycles of slow and fast now. such the mechanical properties of the ice cover. as glacier/ice-sheet surges and ice streams. As neatly Exciting news in the context of Antarctic subglacial stated by Fowler [62]: "IL is not so long since theoreti- hydrology has been the recent discovery of meltwater

lZlt should be rem~rk.ed that no ev idence of superplastic now could be found. which would require much larger amounts of gmin boundary slid ing and grain switchi ng (cf. 141.69.70. 11 9. t40]).

-51- at the bOllom of the EDML deep borehole at Dronning water-saturated sediment. This fac t added to the recent Maud Land. Antarctica [35, 109[. Large amounts of sub­ findings of numerous lakes and complex hydrological glacial water invaded the lowest 170 m of the 2774.15 III systems beneath the Antarctic ice sheet show that the risk depth borehole and obstructed the hole by freezing at ca. of in:ldvertently meeting pristine subglacial water at the 2600m depth. The source of all this water is still un­ bottom of future boreholes is still not quite predictable. clear: no indication of water could be recognized in the It is now widely recognized that the Antarctic subglacial radar-echo sounding data (F. Wilhelms and D. Steinhage. environment is very vulnerable to contamination, espe­ personal communication), whereas the lowest two me­ cially by harmful chemicals as HCFC- 14Ib. We have at ters of the EDML deep ice core consist of a rather un­ the moment no means to estimate the damage that could usual ice that shows very low electrical conductivity and be caused by a potential contamination of the Antarctic is essentially free of air hydrates or other impurities [90J. hydrological system ... and to rely on the hope that this No trace of geological sediments could be detected either may not happen, or if it happens, that the contamination (F. Wilhelms. personal communication). will be restricted to a small. isolated sector. is so far to­ A sample of the EDML subglacial water (SGW) was tally unfounded. Indeed, false hopes are more dangerous collected from the bottom of the hole in January 2006. It than fears. froze almost immediately during its way up to the sur­ face, due to the pressure-temperature shock (pressure 8 Conclusion and temperature changes of ca. 25 MPa and - 40cC. re­ spectively, in less than one hour). X-ray and Raman £s iSI eill herrUches Gefiihl. (lie £illheillichkeil eilles investigations of the SGW sample [103[ revealed that KOlIIl'lexes \'011 £rscheimlllgell ZII erkellllell. die der at least a substantial part of it consists of clathrate hy­ direklell silll1licilell tt'ahmcllll1img a/:; 8l11rZ gelrel1l1le (Irates with structure type 1/ (sJ/ hydrates). After sev­ Dillge ene/reillen. 13 eral investigations. the drilling nuid densifier used in Albert Einstein (42). in a letter to Marcel Grossmann. the EDML drilling program, viz. hydrochlorofluorcar­ 14 Apri1190J. bon l4lb (HCFC- 141 b, also abbrevi:lted as R- 141 b), has been identified as the main sll-hydrate former. It is con­ The importance of polar ice sheets for the dynamics of jectured that the HCFC-14lb has reacted with the SGW Earth's climate is evidenced not only by their active in­ in the borehole during drilling [103]. terplay with the atmosphere and the hydrosphere (e.g. It should be remarked that HCFC-14Ib is known by sea-level rise, global warming), but :llso by their passive chemical engineers as an effective sll-hydrate former at recording of the global past climate. The basic coupling any temperature below 8.6°C and pressure higher than between these two processes, viz, active environmen­ 42 kPa [20. 1101. Consequently, the use of HCFC-14 l b tal interplay and passive paleoclimate recording. is re­ as densifief for deep drilling raises several concerns. For­ vealed by the multiscale structure of the ice sheet. which mer investigations by Talalay & Gundestrup ll45j indi­ comprises a series of dynamic structural features rang­ cate that HCFC-14Ib has the ability to '"bond", to a lim­ ing from megadunes and subglacial lakes (up 10 several ited extent, to ice chips within the borehole, thereby in­ kilometers in size) to sub-microscopic features like dislo­ creasing their mass density. Over a long period of time cation clusters and micro-inclusions. such chips may sink to the boltom of the borehole to The significance of these structures is four-fold: form slush. which may contribute 10 the sticking of the drill when the driller ignores the possibility of a slushy L They enhancelinhibitthe ice now. bouom, e.g. after a long period of pause such as a long 2. They determine the integrity of the climatic record. winter of inactivity. In addition, the observed formation of HCFC-14l b clathrate hydrate explains the abrupt ter­ 3. They serve as qualitative indicators of the ice-now mination of a major fraction of drill runs in the warm ice, history. where the drill suddenly blocks completely and a bright white material is found between ice core and core barrel 4. They serve sometimes as complementary records or in the chip transport system of the drill. of past climate. From the arguments above we conclude that the use of Clearly. items 3 and 4 show that the genesis and evo­ HCFC-141 b as drilling nuid densifier should be avoided lution of such structures are caused by the ice-sheet de­ and prospective new drilling liquids should be screened formation :lnd the action of the environment, whereas for their ability to form clathrate hydrates when in con­ items I and 2 tell us that the multiscale structure of an tact with ice or meltwater. ice sheet provides feedbllcks to the ice deformation and The case of the EDML borehole reminds us that field its interactions with the environment. Thus. an intri­ surveys using radar-echo sounding and similar methods cate Slmcrure-Forlll-Enl';rollment IlIIel]J/ay (SFEI) takes are still not fully adequate for the identification of small place in polar ice sheets, which is fUriher complicated by subglacial structures. like narrow channels or layers of the reciprocal interaction of structures on different scales

13Tmns1.: II is a ""OIulerfllljeeli"g I(} re("(J8"jze 'he ""ily (}j tI c"mplex (}j tlppetlrw,("c," wltich. /() dire,'1 $e"s~ eXl'eriettL·c)". tlPIX'tI' to be qllile S"l'arltle tllillgo·.

-52- (consider for instance the interplay between flow, temper­ ajoint European Science FoundationlEuropean Commis­ ature, impurity content, grain stereology and crystallo­ sion scientific programme, funded by the EU (EPICA­ graphic texture, as described in Sect. 6 for soft ice layers MIS) and by national contributions from Belgium, Den­ in grounded ice, as well as in Part II [94J for ice shelves). mark , France, Germany. Italy. the Netherlands. Norway, Glaciologists and climatologists are not unaware of the Sweden. Switzerland and the United Kingdom. The main SFEI issue. as particular aspects of the problem are well logistic support was provided by IPEV and PNRA (at documented in the literature. e.g. the relationship be­ Dome C) and AWl (at Droll ningMaud Land). This is tween ice-sheet flow and crystallographic texture. or the EPICA publication no. 234. role of the pore space geometry of firn (porosity. tortu­ osity. etc.) on the transport of climatic tracers into deep References ice. Actuall y. it can be said that, in a certain sense, any study of polar ice structures invariably represents an at­ [1 [ J. L. R. Agassiz. "The glacial theory and its recent tempt to understand some aspect of the SFEI. Notwith­ progress". Edinburgh New Philos. J.. 33. 1842. pp. standing, many features of the SFEI remain unclear and 217- 283. a comprehensive perspective of the problem is lacking. In this work we discussed many examples of multi scale [2[ R. B. Alley. "Fim dcnsification by grain boundary structures and interactions that support the view of the sliding: a first model". J. Phys. (Paris). 48(Cl), 1987. Antarctic ice sheet as a heterogeneous mixture of struc­ pp.249- 256. tured media that interact in a hierarchical fashion via the r3] R. B. Alley. "Flow-Iaw hypothesis for ice-sheet mod­ SFEL This view is based on the concept that the usual in­ elling" , J. Glacio!.. 38, 1992. pp. 245- 256. terpretation of structures on diverse scales as ;'quite sep­ arate things" is in fact misleading. Most of these struc­ [41 R. B. Alley, The Two-Mile Time Machine, Princeton tures are so intimately coupled that their dynamics can University Press, Princeton. 2000. only be understood when they are collectively regarded as a "unity of a complex of appearances", that is. as a [5[ R. B. Alley. D. D. Blankenship, S. T. Rooney and IIIlIlriscale srmcrure. This does not mean th;l\ we should C. R. Bentley, "Till beneath Ice Stream 8. 4. A cou­ discard the specialized studies of ideally isolated struc­ pled ice- till flow model.". J. Geophys. Res .• 92(89), tures. but rather that the time has come to complement 1987. pp. 8931 - 8940. them with a unifying. more comprehensive perspective. [6[ L. Arnaud, 1. M. Barnola and P. Duval. ;'Physical The quesl fo r a comprehensive perspective of the SFEI modeling of the densification of snowlfirn and ice in problem should not be considered an exercise of self­ the upper part of polar ice sheets", T. Hondoh (Ed.) indu lgence, innocence, 'lrrogance or vagueness. Rather. Physics of Ice Core Records. Hokkaido University it is a necessary requisite to make a sensible advance in Press. Sapporo, 2000, pp. 285- 305. a subject that is becoming increasingly multidisciplinary. As discussed in [52[. geoscientists see in Ihe flow of ice 17[ N. Azuma and K. Goto-Azuma. "An anisotropic sheets a unique oportunity to study the natural deforma­ flow law for ice-sheet ice and its implications". Ann. tion of polycrystals on size- and timescales far beyond Glaciol.. 23. 1996. pp. 202- 208. those achieved experimentally. physicists and materials scientists regard ice as a useful ideal material for the [8] N. Azuma and A. Higashi. "Formation processes of ice fabric pallems in ice sheets" , Ann. Glacio!., research of diverse physical processes, crystallograpers 6, wonder at the complex structures of air hydrates enclosed 1985, pp. 130-134. in polar ice, and there are many other examples. To ex­ ]9[ J. L. Bamber, D. G. Vaughan and I. loughin. ploit all these possibilities in a systematic manner. we "Widespread complex now in the interior of the have to have multi scale scientists able to coordinate and Antarctic ice sheet". Science. 287. 2000. pp. 1248- harmonize all specific lines of research with a wide view 1250. of the whole problem. [JOJ 1. M. Barnola. D. Raynaud. Y. S. Korotkevich and Acknowledgements C. Lorius, <;Vostok ice core provides 160.000-year record of atmospheric carbon dioxide", Nature. 394. The authors are grateful to the Chief Editor (T. Hondoh) 1987. pp. 738-743. for h.is invitation to prepare this article and to K. Kida­ [II [ P. A. Beck and P. R. Sperry. "Strain induced grain hashi for her kind assistance. We thank also G. Durand boundary migration in high purity aluminum". 1. App!. for providing a suitable 15TE>' template and an anony­ Phys .• 21. 1950, pp. 150-152. mous referee for useful suggestions. Some authors (SHE SK. JF and WFK) acknowledge partial funding from the [121 M. L. Bender. "Orbital tuning chronology for the priority program SPP-1158 of the Deutsche Forschungs­ Vostok climate record supported by trapped gas com­ gemeinschaft (DFG). This work is a cOlltribution to the position". Earth Planet. Sci. Let! .. 204. 2002. pp. 275- European Project for Ice Coring in Antarctica (EPICA). 289.

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