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The Multiscale Structure of Antarctica Part I: Inland Ice

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The Multiscale Structure of Antarctica Part I: Inland Ice The Multiscale Structure of Antarctica Part I: Inland Ice S´ergio H. Faria ∗† , Sepp Kipfstuhl ∗∗ , Nobuhiko Azuma ∗∗∗ , Johannes Freitag ∗∗ , Ilka Weikusat ∗∗ , M. Mangir Murshed ∗ , Werner F. Kuhs ∗ ∗ GZG, Sect. of Crystallography, University of Gottingen,¨ Gottingen,¨ Germany ∗∗ Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany ∗∗∗ Dept. of Mechanical Engineering, Nagaoka University of Technology, Nagaoka, Japan Abstract: The dynamics of polar ice sheets is strongly often imposes a terrible dilemma. A typical example is influenced 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 tural features in polycrystals. In an attempt to be system- range of scales, like stratigraphic features; and there are atic without being completely discrepant with the exist- also those collossal structures as large as megadunes and ing literature, we list below some important definitions subglacial lakes. Their significance 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 Structure–Form–Environment Inter- used by Bunge & Schwarzer [22], Drury & Urai [37], play (SFEI). Glaciologists are not unaware of the SFEI Humphreys & Hatherly [84] and Poirier [119]. It should issue, as particular details of the problem are well doc- be noticed that this does not always coincide with the umented in the literature. Nevertheless, many aspects of terms in vogue in glaciology; and this is done on pur- the SFEI remain unclear and a comprehensiveperspective 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- results of a joint investigation of these structures via field- neers and materials scientists, by rejecting the oblivious work, theoretical modeling, and experiments. The basic use 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 use. The terms adopted here may probablynot be the best a hierarchical fashion via the SFEI. Special emphasis is and ultimate choice for this purpose, but they represent at given to snow and firn structures, interactions between least an attempt towards clarity. microstructure, stratigraphy and impurities, and the inter- play between subglacial structures and the overlaying ice. Clathrate hydrate: Crystalline compound containing guest molecules enclosed in cage-like structures made up of hydrogen-bonded water molecules. When the guest mole- Key words: Ice, firn, multiscale modeling, microstructure, cules form gas under standard conditions, such compounds recrystallization, stratigraphy, subglacial environment. are also named gas hydrates. In particular, air hydrates are formed by atmospheric gases (viz. mainly O2 and N2). Cloudy band: Ice stratum with turbid appearance due to a high concentration of micro-inclusions. 1 Glossary Crystallite: Crystalline domain in a solid polycrystal. Also called grain. It should be noticed the difference between Success is relative: crystallites in polycrystalline solids (e.g. iron or ice) and the It is what we can make of the mess we have made of lose crystals in crystalline granular media (e.g. fine quartz things. sand or fresh Antarctic snow). T. S. Eliot [44] (character: Agatha), p. 118. Crystallographic texture: Directional pattern of lattice orien- tations in a polycrystal (cf. grain stereology). Also called The choice of a consistent and unambiguous vocabulary fabric, PLO (Preferred Lattice Orientations), or simply “tex- for discussing science in a multidisciplinary environment ture”.1 In particular, a polycrystal with a random distribution †Corresponding author: [email protected] 1 We adopt here the standard nomenclature of materials science [22, 71, 84]. Notice that this is not the definition of “texture” often invoked in the in the glaciological literature. The reason for this choice is convenience: experience shows that it is rather easy for glaciologists to use the term “crystallographic texture” as a synonym for “fabric”, whereas it is awkward for most physicists, engineers, and materials scientists to use the term “texture” in the sense frequently employed in glaciology (i.e. as synonym for grain stereology). of lattice orientations is said to be texture-free (viz. random space-borne platforms as alternating bands and supposedly fabric, no PLO). produced by persistent katabatic winds. Deformation-related structures: Structural features produced Micro-inclusion: Inclusion not larger than a few micrometers, and/or affected by deformation, e.g. dislocations, subgrain and consequently not clearly identifiable under optical mi- boundaries, slip bands, stratigraphic folds, etc. croscope, e.g. dust particles, salt inclusions, microscopic DEP: Dielectric Profiling. bubbles, etc. Dislocation wall: Deformation-related structure consisting of Microshear: Strong, localized shear across a grain that expe- dislocations arranged in a two dimensional framework; the riences a highly inhomogeneous shear deformation. It cul- precursor of a subgrain boundary (cf. subgrain). minates with the formation of a new, flat subgrain boundary DML: Dronning Maud Land. parallel to the shear plane, called microshear boundary (cf. slip bands). Dynamic grain growth (DGG): Class of phenomenological processes of grain coarsening in polycrystals during de- Microstructure: Collection of all microscopic deformation- formation. Several recovery and recrystallization processes related structures, inclusions, and the orientation stereology may be simultaneously active during DGG, all competing (cf. id.) of a polycrystal. for the minimization of both, the stored strain energy and Migration recrystallization: In full strain-induced migration the grain-boundary energy. The essential feature of DGG recrystallization. Class of phenomenological recrystalliza- (in comparison to other recrystallization processes) is the tion processes based on the elementary SIBM mechanism (cf. monotonic increase of the mean grain size with time. Ow- id.). If nucleation (cf. id.) is involved in the process, we ing to its dynamic nature, however, the diversified kinetics of may call it nucleated migration recrystallization (SIBM-N), DGG can generally not be compared with the simple kinetics where the suffix “-N” stands for “new grain”. Otherwise, i.e. predicted for normal grain growth (NGG, cf. id.). if the migration of boundaries occurs without formation of ECM: Electrical Conductivity Measurement. new grains, we may call it ordinary migration recrystalliza- tion (SIBM-O), where the suffix “-O” stands for “old grain”.3 EDC: EPICA-Dome C. Multiscale structure: The collection of all sorts of structural EDML: EPICA-DML. features observed in a material body, with emphasis on their Elementary structural process: The fundamental operation 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- 2 variety of phenomenological structural processes (cf. id.). pear also in a range of scales (e.g. folds). Most of them are EPICA: European Project for Ice Coring in Antarctica. evolving structures that interact with each other as well as Fabric: See crystallographic texture. with the ice-sheet flow and the environment via SFEI (cf. id.). Firn: Sintered snow that has outlasted at least one summer. NGRIP: North-Greenland Ice-Core Project, also abbreviated as Grain: See crystallite. NorthGRIP. Grain stereology: Spatial arrangement of grains in a polycrys- Normal grain growth (NGG): Phenomenological recrystalliza- tal, including their 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- Grain subdivision: Phenomenological recovery process of for- boundary) surface-tension equilibrium” [135]. By definition, mation of new subgrain boundaries. It involves the progres- grain coarsening during NGG is statistically uniform and sive rotation of certain portions of the grain, called subgrains self-similar, grain-boundary migration is exclusively driven (cf. id.), as well as the strengthening of dislocation walls by minimization of the grain-boundary area (and associated through dislocation rearrangement and migration in regions free energy), and the grain stereology 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 with time, grain subdivi- structure”). Owing to these essential features, NGG is gen- sion may give rise to rotation recrystallization (cf. id.). erally regarded as a static recrystallization process (cf. re- Inclusion: Locallized deposit of undissolved
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