The Creep of NaCl-Doped Ice Monocrystals

Abstract

ABSTRACT. Monocrystals of ice grown from N aCI solutions (concentration 5 X 10-4 to 10-' m ol/I) have been tes ted in creep at -10°C by basal glide. The maximum resolved shear stress ra nged from 0.6 to 2.5 bar. The r esulting creep c urves show a decele ration, tha t is, the creep rate decreases with time. At the highest concentra tion the creep is essentially transie nt; the strain tends to a fixed value. This is unlike the behaviour o f simila rl y ori enta ted monocrystals of pure ice or of ice grown from solutions of oth er dopants so far r eported in the literature.The possible causes for this behaviour are discussed a nd th e implica tions for the mechanical properties of polycrystalline ice, and in particular sea ice and gla cier ice, a re described . REsuME. Flllage de lIIonocristaux de glace dopce avec NaCI. D es monocristaux de glace ob tenus p a r croissa nce a. partir d e solutions de NaCI (concen trations 5 X 10-4 a. 10-2 mol/I) o nt subit des essais de flu age a. -10°C, le pla n basal etant le pl a n d e glissem ent. La valeur maxima le de la contra in te reduite est comprise entre 0,6 et 2,5 bar. L es courbes de flu age o btenues montre n t un e decelera ti o n c'est-a.-dire une diminution de la vitesse d e d eform a ti on a u cours du temps. Aux co ncentrations les plus elevees, il y a esse ntiell ement un fluage transitoire: la deformatio n te nd vers une vale ur limite. Ce co mporlement est tres diffe re nt de celui de monocristaux de meme orie ntation de glace pure o u d e glace dopee avec d 'autres composes tel qu' il est indique dans la litterature.Les origines poss ibles de ce comportement so nt discutees et les explica tions qua nt aux proprietes mecaniques d e la glace polycristalline (en particulie r d e la glace de m er o u d e glacier) sont d ecrites.ZUSAMMENFASSUNG. Das Kriechen von NaCI-dotierten Eis-Eillkristallerz. Eis-Einkristall e, die a us NaCILosungen (Kon zentration 5 X 10-4 bis 10-2 mol /I) gezllc htet wurden, wurde n im Kriechversu ch bei -10°C durch basale Gl eitung verformt. Die maximale Schubspannung in der G leite bene erstreckte sieh von 0,6 bis 2,5 bar. Die gewonnenen Kriechkurven zeige n ein verzogertes Kriech e n , d .h. die Kriechgeschwindigkeit nimmt mit d er Z eit a b. Bei der h ochsten Konzentra tion find et man im wese ntli chen ein Dbe rga ngskriechverhalte n ; die V erformung strebt ein em festen W ert zu. Dies ist vom bish er in der Literatur beschriebenen Verhalten a hnlich orientierter Einkristalle a us reinem Eis oder von Eis a us L osungen anderer D o ti e rungsstoffe verschieden.Die moglichen Ursachen fUr di eses Verhalten we rd e n besprochen, und di e Bedeutung fUr die mecha nischen Eigensch aften von viel kristallinem Eis, im besonderen von Meereis und G letschereis, wird b eschrieben.

Deformation properties of subfreezing glacier ice: Role of crystal size, chemical impurities, and rock particles inferred from in situ measurements

“…The dependence of viscosity on fabric strength is a consequence of the strong deformation anisotropy of single ice crystals [Lliboutry and Duval, 1985]. Deviations of fabric from isotropic can plausibly account for nearly a factor of 10 variation in E [Shoji and Langway, 1988;Jun et al, 1996], and it is reasonable to suggest that this is the only important control on E in natural settings. Paterson [1991] reviewed all relevant data on the subject and concluded that crystal fabric variations cause most of the softening of the ice age ices, as noted above.…”

“…Second, the apparent dominance of dislocation creep over grain boundary sliding and diffusional creep as the mechanism responsible for GL argued against a direct grain size dependence [Duval et al, 1983]. Third, Paterson [1991] Langway, 1988]. As evidence against a grain-size-sensitive mechanism, these experiments axe weak, primarily because both data sets contain a large range of crystal fabrics, whose effects could obscure a modest, but important, grain size dependence.…”

“…Laboratory experiments have shown that single ice crystals become substantially softer if doped with acids (HF and HC1) that can substitute in the crystal lattice. One set of experiments examined the rheologic effects of an important natu-ral salt, NaC1, and found that this impurity stiffens ice single crystals [Riley et al, 1978]. The relevance of these studies to the ice sheets is unclear, as most impurities will be interstitial, if in the crystal, and the dominant acids in the ice sheets are sulphuric and nitric, and these are neutralized in the dust-rich ice age ices of Greenland.…”

Effect of Sodium Chloride on Internal Quasi-Liquid Layers in Ice Ih

“…Sodium and chloride ions are among the most common solvated impurities in liquid water. In seawater, for example, sodium chloride is present in molarities of the order of ∼0.5 and there has been a significant interest in its role on the properties of liquid water. − Similarly, the influence of sodium chloride on solid phases of water, in particular the hexagonal proton-disordered ice I h , has also attracted substantial attention. − For instance, in addition to atmospheric processes involving the nucleation of ice crystals, , sodium chloride is known to affect the mechanical properties of large ice masses on Earth, including sea ice as well as glaciers. , …”

“…8−11 For instance, in addition to atmospheric processes involving the nucleation of ice crystals, 12,13 sodium chloride is known to affect the mechanical properties of large ice masses on Earth, including sea ice as well as glaciers. 14,15 Although sodium chloride dissolves easily in the liquid form of water, it has an extremely low solubility in ice I h . As a result, when saline water is frozen to a temperature between the freezing point and the eutectic temperature, the system will consist of almost pure polycrystalline ice in equilibrium with a concentrated aqueous salt solution, also known as brine.…”