Edward E. Adams

Formation and character of an ancient 19-m ice cover and underlying trapped brine in an “ice-sealed” east Antarctic lake

Lake Vida, one of the largest lakes in the McMurdo Dry Valleys of Antarctica, was previously believed to be shallow (<10 m) and frozen to its bed year-round. New ice-core analysis and temperature data show that beneath 19 m of ice is a water column composed of a NaCl brine with a salinity seven times that of seawater that remains liquid below −10°C. The ice cover thickens at both its base and surface, sealing concentrated brine beneath. The ice cover is stabilized by a negative feedback between ice growth and the freezing-point depression of the brine. The ice cover contains frozen microbial mats throughout that are viable after thawing and has a history that extends to at least 2,800 14C years B.P., suggesting that the brine has been isolated from the atmosphere for as long. To our knowledge, Lake Vida has the thickest subaerial lake ice cover recorded and may represent a previously undiscovered end-member lacustrine ecosystem on Earth.

Evaluation of snow-surface energy balance models in alpine terrain

Abstract The increasing complexity of snow-cover models demands high-quality forcing data. In complex alpine terrain, both short and long wave incoming radiation components are expected to be influenced by small-scale topographic effects, i.e. shading and multiple scattering as well as long-wave irradiance from the surroundings. Not only should the latter be included in distributed energy balance models, but, because of their increasing resolution, also in meteorological models of the next generation. The energy balance at the snow-cover surface is calculated by means of different distributed energy balance models over a region of a few square kilometres, the spatial resolution being 25 m. The models include topographical effects on the radiation components of the energy balance. The region of interest is located in the Eastern Swiss Alps, around study sites of the Swiss Federal Institute for Snow and Avalanche Research SLF, Davos. The primary forcing data are taken from automatic weather stations located within the study area. To assess performance and differences of the models, two approaches are taken. First, model outputs are compared to measurements of both incoming radiation and snow surface temperature measured at automatic weather stations located on either level or inclined terrain within the region. Second, the models are used to calculate snowmelt at the beginning of the ablation period. The results are compared with changes of the snow water equivalent as measured in various spots of the modelled region, including all aspects on one elevation range. In view of the above comparison, the necessity to include small-scale topographic influences on the energy balance at the snow-cover surface as well as to consider snow surface properties and internal processes within the snow cover will be discussed. The possible implications for hydrological and meteorological models of the next generation will be addressed too.

Model for effective thermal conductivity of a dry snow cover composed of uniform ice spheres

The effective thermal conductivity of a snow cover is estimated assuming an idealized collection of uniformly packed ice spheres. An effective thermal conductivity is calculated based on the thermal resistance due to ice-grain contacts or bonds, the pore space/ice acting in series and the unobstructed pore. It is shown to depend very strongly on the snow density and intergranular bonding and, to some extent, on temperature. Conductivity tends to increase as density and the ratio of the contact radius to ice-sphere radius increase. The ice network is generally determined to be the most influential in determining the effective thermal conductitivity. Calculated results fall within the range of empirically determined values.

Metamorphism of Dry Snow as a Result of Temperature Gradient and Vapor Density Differences

A heat conduction equation for the determination of the temperature profile in a snowpack is developed. The magnitude of the temperature gradient tends to increase as the snow surface is approached, with local minima through layers of high snow density and local maxima above and below these layers. Calculations are made of the difference in varor density in the pore and over the ice grain surfacps which border the pore. In the presence of sufficient temperature and temperature gra~ient, faceted crystals will develop near the top of the pore, as ice is sublimed away from the surfaces in the lower region. There will be a reduction in the percentage of rounded grains as the faceted form develops. The process is demonstrated to be enhanced at warm temperatures and large temperature gradients in low density snow. I NTRODUCTI 0 t~ Temperatures at the base of a seasonal snowpack usually remain just below O·C throughout the winter. Snow is warmed from below, as the ground gives up heat accumulated during the warmer months. If this geothermal heating coincides with cooler ambient air temperatures, characteristic of the winter months, a temperature gradient is established across the snowpack. . A variation in vapor pressure, resultlng from the temperature differential, will cause a mass flux of vapor from the zone of high vapor pressure (the deeper warmer region) to the zone of lower vapor pressure. Since dry snow, in which this mechanism is most effective, consists of a solid and a vapor phase of.the same material, the transfer can act as though lt partly takes place through the solid matrix itself, as a hand-to-hand transfer of mass (Yosida and Kojima (1950), described in Akitaya 1974). Yapor will evaporate from the top of an ice grain, diffuse across the pore and deposit as a solid on the bottom of the grain above. The ice molecules on the top of this grain will sublime and redeposit in a similar fashion. When snow of low to medium density is subjected to the correct combination of temperature and temperature gradient, angular flat-surfaced crystals, known as depth hoar, will develop at the expense of those grains with a more rounded configuration. This effect is most prevalent in low-density snow at relatively warm temperatures subjected to large temperature gradients. Snowpack development in which faceted crystal growth predominates is known as temperature gradient metamorphism. At low temperature gradients, rounded grains are observed to form, in a process known as equitemperature metamorplli srn. Col beck (1982) refers to the crystals which develop as a result of equitemperature metamorphism as the equilibrium growth form. In the proper conditions this equilibrium form may be other than spherical. Colbeck terms the faceted crystals which develop as a result of temperature gradient metamorpnism the kinetic growth form. A satisfactory value for the temperature gradient at which the transition from the equilibrium to the kinetic growth form takes place has not been established; this is because of the dependency on temperature and snow density. The point at which this transition occurs is an important topic which requires further investigation. This paper deals with processes involved during kinetic growth. Two general types of depth hoar are recognized (Akitaya 1974): a solid type, which develops under smaller temperature gradients, and a skeleton type, which predominates under large temperature gradients. Skeleton-type depth hoar is composed of large faceted crystals, which are poorly bonded together and form a weak snow 1 ayer. The soli d type cons is ts of sma 11 faceted crystals which do not show the marked reduction in strength characterized by the skeleton type. Akitaya (1974), in what is perhaps the most complete experimental work carried out on the topic of temperature gradient metamorphism, has shown that pore size, initial ice-grain size and geometry affects depth-hoar development. A large pore will enhance the rate of growth and the size of the crystal which will develop. This is in agreement with Marboutys (1980) observation that fine-grained snow with a density in excess of approximately 350 kg 013 subjected to a large temperature gradient will develop depth hoar of a solid type. Lower density snow, which has a large pore size, tends toward developing the skeleton type in the presence of a sufficient temperature gradient. Faceted crystals grow toward the direction of higher temperature in the snow. The direction of crystal growth Was observed to be dependent on the thermal gradient and not on gravity (Akitaya 1974).

A statistical validation of the snowpack model in a Montana climate

Abstract Recently, a computer model has been developed by the Swiss Federal Institute for Snow and Avalanche Research that simulates the evolution of a natural snow cover. Using common meteorological parameters as input, SNOWPACK predicts characteristics such as snowpack temperature and density, in addition to snow microstructure and layering. An investigation was conducted to evaluate the effectiveness of SNOWPACK in a Montana climate. A weather station was constructed in the Bridger Mountains near Bozeman, Montana, to provide the meteorological parameters necessary to run SNOWPACK. Throughout the 1999–2000 winter, weekly snow profiles were performed in undisturbed snow to provide a benchmark for the model output. Density, grain size, and crystallography were recorded on 10-cm intervals over the full snow depth, and the temperature profile was monitored with a thermocouple array. Finally, the meteorological parameters were input into SNOWPACK, and a statistical comparison was performed comparing the predicted snowpack to the observational data. Snowpack temperatures are predicted reasonably accurately by SNOWPACK. The modeled and observed densities correlated well, but the model typically underestimates snowpack settlement. Comparison of grain size and shape was problematic due to different definitions utilized by the model and observer, but still demonstrated some agreement.

Grain boundary ridge on sintered bonds between ice crystals

Well-sintered snow examined in a scanning electron microscope revealed a newly observed morphological structure that protrudes into the pore space along ice grain boundaries. We have termed this a “grain boundary ridge.” Grain boundary diffusion is a sintering process that occurs at the interface of two crystals, whereby mass migrates from the center of the contact to the surface of the bond. Since mass tends to sublimate from sharp features toward smaller curvature surfaces through vapor diffusion, a ridge developed by grain boundary diffusion will readily sacrifice mass to the surrounding ice surfaces. A mass balance between vapor and grain boundary diffusion based on the observed geometry is considered. This analysis indicates grain boundary diffusion may play a far more significant role than generally acknowledged. While this study was restricted to ice, it may have implications for other crystalline materials.

Sediment Melt-Migration Dynamics in Perennial Antarctic Lake Ice

Abstract We examined sediment melt-migration dynamics in the ice cover of Lake Fryxell, Taylor Valley, McMurdo Dry Valleys, Antarctica, using a combination of laboratory experiments, field observations, and modeling. The specific objectives were to determine the thermal conditions required for sediment melt and how sediment migration rates vary with meteorological forcings and ice microstructure. These characteristics are relevant to the influence of climate change on lake ice structure and ecosystem processes in polar regions. Sediment began melting through laboratory ice at −2 °C in simulated summer conditions, with warmer ice producing faster melt rates. An energy balance model, supported by our laboratory experiments, demonstrated that subsurface sediment can melt down to an equilibrium depth of ∼2 m in two years. Field experiments and modeling revealed that surficial sediment melts at about half the rate of subsurface sediment because of heat losses to shallow, cold ice and the cold, dry atmosphere. Gravity flow of sediment along grain boundaries was pronounced in laboratory ice warmer than −1 °C. This mechanism produced a flux of 0.1 g m−2 hr−1, a significant value relative to published benthic sedimentation rates for these lakes indicating an important sediment sorting mechanism.

Ice crystals grown from vapor onto an orientated substrate: application to snow depth-hoar development and gas inclusions in lake ice

A laboratory experiment was conducted in which new ice crystals were nucleated from the vapor phase onto large existing ice crystals obtained from Antarctic lake ice. Flat, smooth ice-crystal surfaces were prepared, with c axes oriented either vertically or horizontally. When these were subjected to a supersaturated vapor environment, multiple individual crystals nucleated onto the substrates adopting the same crystallographic orientation as the parent. A dominant grain-growth scenario for kinetic-growth metamorphism in snow, which in some ways is analogous to the oriented morphologies in lake ice, is hypothesized. In the lake-ice-growth scenario, optimally oriented crystals will grow at the expense of those less preferentially positioned. The proposed dominant grain-growth theory for snow is in agreement with the observed decrease in the number of grains and the proximal similarity of crystal habit in kinetic-growth metamorphism in snow. Similarly, kinetic crystal growth on the interior of gas inclusions in Antarctic lake ice will also acquire the crystallographic orientation of the substrate ice. These small-faceted interior crystals significantly influence light scattering and penetration in the lake-ice cover.

A constitutive theory for snow as a continuous multiphase mixture

Abstract Snow on the ground is viewed in this formulation, as a saturated two-phase granular material comprised of small grains of ice with interstitial pores filled by a single vapor. The snow is considered as a continuous mixture in which the ice and vapor constituents are themselves treated as individual but interacting continua. Mathematical modeling of the Snow is accomplished using a relatively recent continuum theory for mixtures where the individual constituents are physically separate. This approach considers the volume fraction occupied by each constituent as an additional kinematic variable. Therefore, in addition to the balance equations for mass, linear momentum, angular momentum and energy, usually applied in continuum mechanics, an equation which accounts for changes in the volume fraction, called the balance of equilibrated force, is included. Balance equations for each constituent as well as for the mixture are considered. The immiscible nature of the constituents allows constitutive equations to be developed which depend only on those variables which pertain to that constituent. Exchange between the ice and vapor is aecounted for by interaction terms which enter the theory through the balance equations for the constituents. Forms for these interaction terms are used which guarantee that the entropy inequality is not violated. A one-dimensional analysis of an isothermal homogeneous snow cover suddenly subjected to a colder surface temperature reveals a thermodynamically active zone associated with a large temperature gradient, initially located near the top surface, but which moves downward with time in a wavelike fashion decreasing in intensity. Slight differences in constituent temperatures are calculated during the more active transient phase in conjunction with a decrease in snow density.

A microstructural dry-snow metamorphism model for kinetic crystal growth

Historically, dry-snow metamorphism has been classified by the thermal environment and thermodynamic processes in a snowpack. Snow experiencing predominantly macroscopic isothermal conditions develops different microstructure than snow subjected to large temperature gradients. As such, much previous research has been categorized by and limited to specific thermal conditions. The current research expands a generalized approach for the movement of heat and mass to include a snow crystal kinetic growth model. An existing spiral defect propagation theory for kinetic growth on simple faceted geometry is utilized. Primary crystal habit as a function of temperature is incorporated. A model of heat and mass transfer through an ice and pore structure is coupled with phase-change thermodynamics during kinetic growth. A kinetic growth microstructure model is developed and integrated into heat and mass transfer representations, which are solved using finite-difference techniques. The kinetic morphology model approximates frequently observed hopper-type crystals. The snow microstructure is allowed to change at every step, resulting in a transient description of kinetic growth metamorphism. Variable kinetic growth rates are demonstrated based on temperature and on crystallographic orientation relative to a temperature gradient. Crystals preferentially aligned with the temperature gradient have significantly higher growth rates, supporting previous observations of predominant crystal habits developing under temperature gradient conditions. Grain-size dispersion increase with time is demonstrated and supported experimentally in the literature. A dominant grain growth theory based on crystallographic orientation that has been previously postulated is supported. A broad range of metamorphic geometric parameters and thermal conditions may now be simulated with a single model.