Grain-size evolution controls the accumulation dependence of modeled firn thickness (original) (raw)
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Snow/firn densification in polar ice sheets
低温科学= Low …, 2009
A sophisticated physical model of the dry snow/fi m densification process in ice sheets is proposed. Macroscopicnlly, snow and rim undergo vertical uniaxial compression wi th non-zero deviatoric stresses and strain ra tes. The present mathematical description of dcnsificntion includes dilatancy and "force-chain" eITects in snow and develops previous concepts of ice-pnrticlc rearrangement by grainboundary sliding and sintering by power-law creep under overburdcn pressure. Both densification mechanisms work together during the firs t snow stage until the closest pncking of ice grains is reached at criticnl dcnsi ties of 0.7-0.76 and the fim stage controlled only by thc dislocation creep sets on. In addition to the ice-grain coordination number and the slope of the radial distribution func tion, a new structural parameter is introduced to lIccount for gram bonding (agglomcr.ttion) effects. The model is constrained and validllted on direct stereological observations of ice core struc tures and a representat ive set of snow/fim densitydepth profiles coveri ng a wide range of present-day cl imatic conditions (-57.5 to-10 °C and ice accumulation at 2.15 to 330 cm yr-1). Si mple eq uat ions arc derived fo r pred icting the depth of pore closure in lirn and the ice lIge at close-ofT. The paleocli matic evolut ion of quasi-stationary density-depth profi les and close-off characteristics at Vostok Station (East Anta rcticll) are simu lated and discussed.
Grain Growth in Polar Ice: I. Theory
Journal of Glaciology, 1986
Many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood. Here we develop the theory of grain growth in ice that is not deforming rapidly, and in the succeeding paper we use this theory to explain observations from glacial ice. In the absence of significant strain energy, the driving force for grain growth arises from grain-boundary curvature. Grain growth is slowed by the interaction of grain boundaries with extrinsic materials (microparticles, bubbles, and dissolved impurities). If the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material, then the grain-boundary velocity is determined by the velocity characteristic of the extrinsic material (low-velocity regime). If the driving force is large enough to cause separation, then boundaries migrate more rapidly than the extrinsic material (high-velocity regime) but the net driving force is reduced through transient pinning by the...
Modelling of the densification of polar firn: characterization of the snow-firn transition
T he tra nsform ation of dry snow to firn is described by th e tra nsiti on b etween d en sification by d eformationless res tacking a nd d ensificati on by p ower-l aw creep. The obse rved decrease with temperature of the dens it y at the snow-firn transiti on seem s to res ult from th e competition betwee n g rain-bo und a r y sliding a nd power-l aw creep. Th ese two densification processess occ ur concurrenLl y in snow, a lthoug h there a re probably micro-regions in which sliding a lone occ urs. Validation of a geometrical densification model developed for ce ra mics has been obta i ned from d ensificatio n d a ta from several Anta rctic a nd Greenl a nd sites a nd from the cha racterization of the structure of pola r firn .
Journal of Geophysical Research, 2000
To improve understanding of the deformation properties of subfreezing polycrystalline glacier ice and, in particular the role of crystal size, chemical impurities, and rock particle impurities, we analyze in situ strain rates of the basal layers of Meserve Glacier, Antarctica. Strain rates were monitored on the walls of a subglacial tunnel (where down flow shear stress is relatively uniform) and ice properties were measured (texture, fabric, and impurity content). We propose a simple empirical model describing strain rate variations due to variations in crystal size and impurity content, and we use all relevant Meserve data to constrain model parameters. We conclude that there is a direct dependence of strain rate on crystal size, which reflects an important role for a grain-size-sensitive deformation mechanism such as grain boundary sliding or diffusion. Chemical impurities are found to enhance the grain-size-sensitive deformation and are found to be an important control on strain rate variations in the very impure ices of Meserve Glacier. However, the per molar sensitivity of strain rate to chemical impurity content is shown to be very low, such that in the ice age ices of the Greenland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement, though we cannot exclude chemical enhancements as high as 1.3 there. Our analyses detect no direct rheologic effect of rock particles in the Meserve ices, which suggests that rock content is not directly responsible for the low viscosity of dirty basal layers.
Densification of layered firn in the ice sheet at Dome Fuji, Antarctica
Journal of Glaciology, 2016
ABSTRACTIn order to better understand the densification of polar firn, firn cores from the three sites within ~10 km of Dome Fuji, Antarctica, were investigated using surrogates of density: dielectric permittivities εv and εh at microwave frequencies with electrical fields in the vertical and horizontal planes respectively. Dielectric anisotropy Δε (=εv − εh) was then examined as a surrogate of the anisotropic geometry of firn. We find that layered densification is explained as a result of complex effects of two phenomena that commonly occur at the three sites. Basically, layers with initially smaller density and smaller geometrical anisotropy deform preferentially throughout the densification process due to textural effects. Second, layers having a higher concentration of Cl− ions deform preferentially during a limited period from the near surface depths until smoothing out of layered Cl− ions by diffusion. We hypothesize that Cl− ions dissociated from sea salts soften firn due to ...
Firn profile changes in response to extreme 21st-century melting at Devon Ice Cap, Nunavut, Canada
Journal of Glaciology, 2013
Estimating regional-scale rates of glacier mass change from repeat airborne and satellite altimetry requires assumptions about the density of the material being added to, or removed from, the glacier surface. This may vary in both space and time, especially in a period of warming climate such as has occurred in Arctic Canada since 2005. Here we compare firn properties measured on 20 shallow cores recovered from elevations of 1400–1900 m a.s.l. on Devon Ice Cap in 2012 with equivalent measurements from 34 cores recovered from the same locations between 2004 and 2011. The average density of the top 2.5 m of the firn column has increased by 13–80%. As a result, measured rates of density increase in the top 1.25 mw.e. of the firn layer have likely thinned this layer at rates of 0.021–0.168 m a−1 over the period 2004–12. These thinning rates are significant relative to local rates of surface height change measured by repeat airborne laser altimetry between 2004 and 2012 (−0.077 to −0.252...
In situ measurements of Antarctic snow compaction compared with predictions of models
Journal of Geophysical Research, 2010
We describe in situ measurements of the compaction of Antarctic snow. At three different sites in Antarctica, the rate of compaction was measured hourly, over various depth intervals, for up to two years. These measurements show that compaction at each of the sites occurs through slow, viscous deformation of the snowpack, with no significant contribution from sudden collapse of weak layers. The measured rates of compaction at the coldest site exhibit a strong seasonality, consistent with a temperature-dependent sintering mechanism having activation energy of 70 kJ mol −1. At the two warmer sites, activation energies of 80 and 120 kJ mol −1 provide slightly better agreement with the observations. Published models of snow compaction underestimate the temperature sensitivity. A good match to our observations is provided by a semi-empirical model, based on rate equations for lattice-diffusion (Nabarro-Herring) creep of material around pores, combined with normal grain growth. This model also provides a theoretical basis for a widely used empirical model of snow compaction. The rate coefficient for lattice-diffusion inferred from our measurements is considerably higher than published values, however, and other creep mechanisms cannot be ruled out.
Journal of Glaciology, 2014
Evaluation of how accurately snowpack models can capture hydrological processes in firn is critical to determining how well they will simulate future glacier mass-balance changes. Here we compare simulations using the Crocus snowpack model with the evolving firn stratigraphy recorded in 14 cores drilled at four elevations in the accumulation zone of Devon Ice Cap, Canada, during the 2004–12 period of rapid summer warming. Simulations were forced with a combination of surface observations and reanalysis data. Simulations resulted in positive model bias in near-surface density, and negative bias in density at depth compared to observations. Results point to the importance of incorporating heterogeneous percolation in firn in order to improve the representation of meltwater flow, better reproduce observed firn density and temperature profile evolution, and improve simulations of glacier mass balance during periods of climate warming.
Grain Growth in Polar Ice: II. Application
Journal of Glaciology, 1986
Grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials (microparticles, bubbles, and dissolved impurities) are characterized fully. Dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice. The high concentration of soluble impurities in Wisconsinan ice from the Dome C (Antarctica) ice core (and perhaps other ice cores) probably causes the small grain-sizes observed in that ice. Microparticles have little effect on grain growth in ordinary ice. In ice layers that appear dirty owing to concentrations of volcanic tephra (such as in the Byrd Station (Antarctica) ice core) or of morainal material, micro particles reduce grain-growth rates significantly. The relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks, so grain growth in firn is limited by boundary migration rather than by neck growth. Bubbles formed by po...
2000
State-of-the-art thermomechanical models of the modern Greenland ice sheet and the ancient Laurentide ice sheet that covered Canada at the Last Glacial Maximum (LGM) are not able to explain simultaneously the observed forms of these cryospheric structures when the same, anisotropy-enhanced, version of the conventional Glen flow law is employed to describe their rheology. The LGM Laurentide ice sheet, predicted to develop in response to orbital climate forcing, is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface-geomorphological and solid-earth-geophysical constraints. We show that if the Glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress^strain-rate relation, then the aspect ratios of both the modern Greenland ice sheet and the Laurentide ice sheet, that existed at the LGM, are simultaneously explained with little or no retuning of the flow law.