Firn Densification Research Articles

Measurement of Radio Wave Velocities in the Firn Zones of Polar Ice Sheets (Abstract only)

A common-reflection point profiling experiment to obtain electromagnetic wave velocities in the ice at Dome C at 35 MHz was carried out to a maximum antenna separation of 2 km. Four different recording systems were used for this experiment. The echoes from numerous internal reflecting horizons within the ice and the bedrock were recorded in four different ways: in A-display form on film using an oscilloscope, in intensity-modulated form using the Honeywell Visi-corder and thermal intensifier on heat-sensitive silver paper, and in both raw and signal-averaged form on magnetic tape.Travel times of oblique reflections from nearly 160 internal layers down to a depth of 2 600 m and reflections from the ice bottom were measured at each station along the profile. The average wave velocities from the surface of each internal layer were measured to obtain a continuous mean velocity vs depth profile.Velocity-density models derived from the dielectric mixture equations of Looyenga, Böttcher, Lichtenrecker, Hanai-Bruggeman and Wiener were compared against the measurements. In addition Robin's empirical relationship was also used in this study. The preliminary results show that the observed velocity depth profile for the ice column is compatible with Wiener's equation (with Formzahl = 0) and a newly derived empirical relationship (from this study) of where γi is ice density, γf firn density, ε dielectric constant of mixture, and εi dielectric constant of ice; γf>0.55 kg m−3. It appears that the electromagnetic wave velocity in the firn is 20 m MS−1 or more, which is higher than previously assumed.The above models were also compared with velocity measurements in the Devon Island bore holes (Robin 1975) and at several sites on the Ross Ice Shelf where the ice is relatively thin compared to Dome C so that the effect of firn on mean velocities is more pronounced. The results for these sites suggest again that Wiener's equation and empirical relationship from this study produce velocities compatible with the observed values.

Firn Densification: An Empirical Model

AbstractAn empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m−3, the densification rate is proportional to the mean annual accumulation times the term (ρi − ρ), where ρ is the density of the snow and ρi is the density of pure ice. The densification rate in the second stage, where 0.55 Mg m −3 < ρ < 0.8 Mg m−3, is proportional to the square root of the accumulation rate and to (ρi− ρ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.

Firn Densification: An Empirical Model

AbstractAn empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m−3, the densification rate is proportional to the mean annual accumulation times the term (ρi−ρ), whereρis the density of the snow andρiis the density of pure ice. The densification rate in the second stage, where 0.55 Mg m−3< ρ < 0.8 Mg m−3, is proportional to the square root of the accumulation rate and to (ρi−ρ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.

Comparison of theoretical and observed temperature profiles in Devon Island ice cap, Canada

Summary. A non-steady-state theoretical model is used to predict the present variation of temperature with depth in two boreholes in the Devon Island ice cap, Arctic Canada. The boreholes are 300 m apart and one of them reaches bedrock. The heat transfer equation is solved numerically with the record of past temperatures obtained from measurements of the variations of oxygen—isotope ratio with depth in the cores as surface boundary condition. The effects of ice advection, refreezing of meltwater percolating from the surface (the amount of which is recorded in the cores), heating due to firn compaction and ice deformation, and heat flow in the bedrock below the ice sheet are all included in the model. The free parameters are geothermal heat flux, present surface temperature and heat loss at the surface which depends on the depth of meltwater penetration and other factors. Agreement between observed and predicted temperature—depth profiles is very close. Latent heat released by percolating meltwater is the predominating factor in determining the temperature distribution in the upper half of each borehole. The temperature distribution is insensitive to the value of the factor used to convert oxygen—isotope ratio to temperature.

Vertical Strain-rate Measurements in an Arctic Ice Cap and deductions from them

AbstractClosely spaced measurements of diameter of thermally drilled bore holes reveal a pattern of small variations. These patterns serve to igerntify points on the bore-hole wall; thus the change in length of sections of bore hole can be gertermined as a function of time. This method has been used to measure vertical strain-rate as a function of gerpth in two bore holes near the crest of the Devon Island ice cap. The measured strain-rate, corrected for firn compaction, varies significantly with gerpth. The vertical component of velocity at the surface was gertermined freom the contraction rate of a bore hole that penetrated to the base of the ice. Comparison of this velocity with the present accumulation rate suggests that the ice cap, in the vicinity of the bore hole, is thickening slightly at present. The age of the ice at various gerpths, as calculated freom the measured vertical velocities, is in broad agreement with radio-carbon dates covering the past 6 000 years. This suggests that the flow of the ice cap has not varied significantly over this period, and thus that the present accumulation rate, which is causing thickening, is slightly above the average for the period.

Vertical strain-rate measurements in an arctic ice cap and gerductions freom them

AbstractClosely spaced measurements of diameter of thermally drilled bore holes reveal a pattern of small variations. These patterns serve to igerntify points on the bore-hole wall; thus the change in length of sections of bore hole can be gertermined as a function of time. This method has been used to measure vertical strain-rate as a function of gerpth in two bore holes near the crest of the Devon Island ice cap. The measured strain-rate, corrected for firn compaction, varies significantly with gerpth. The vertical component of velocity at the surface was gertermined freom the contraction rate of a bore hole that penetrated to the base of the ice. Comparison of this velocity with the present accumulation rate suggests that the ice cap, in the vicinity of the bore hole, is thickening slightly at present. The age of the ice at various gerpths, as calculated freom the measured vertical velocities, is in broad agreement with radio-carbon dates covering the past 6 000 years. This suggests that the flow of the ice cap has not varied significantly over this period, and thus that the present accumulation rate, which is causing thickening, is slightly above the average for the period.

Liquid Brine in Ice Shelves

Holes drilled into thin areas of the Brunt Ice Shelf encounter a layer of liquid brine less than 1 m thick approximately at sea-level. Assuming the brine to be moving horizontally, analysis of its effects on thermal equilibrium gives an estimate of steady-state annual brine flow that is in good agreement with the value deduced from a percolation model. The effect of firn density on percolation rates is such that the slope of an active brine layer increases rapidly as ice thickness increases. However, the heat transport model predicts that brine layers are unlikely to be active in both very thick and very thin ice shelves.

Liquid Brine in Ice Shelves

Holes drilled into thin areas of the Brunt Ice Shelf encounter a layer of liquid brine less than 1 m thick approximately at sea-level. Assuming the brine to be moving horizontally, analysis of its effects on thermal equilibrium gives an estimate of steady-state annual brine flow that is in good agreement with the value deduced from a percolation model. The effect of firn density on percolation rates is such that the slope of an active brine layer increases rapidly as ice thickness increases. However, the heat transport model predicts that brine layers are unlikely to be active in both very thick and very thin ice shelves.

Analysis of a 20 m. Firn Pit on the Kesselwandferner (Ötztal Alps)

AbstractThe firn stratification in the accumulation area of an alpine glacier was studied in a 20 m. deep, vertical pit dug in the summer of 1963 on the Kesselwandferner (3,240 m. above sea-level in the Ötztal Alps). The firn is traversed by ice layers up to 25 cm. in thickness which have been formed by the damming up and freezing of melt water. Firn density increases with depth. At a depth of 13 m. (8 yr. old firn) the density reaches 0.80 g./cm.3as a mean value of one annual net accumulation. Excluding the ice layers, the firn reaches a density of 0.82 g./cm.3in a 10 yr. old layer at a depth of 16 m. The firn stratification in relation to seasons was studied by analyses of the pollen content in the lower part of the pit, beginning with the horizon “late summer 1954”. The late summer horizons obtained from the firn stratification were confirmed by the pollen analyses.

Analysis of a 20 m. Firn Pit on the Kesselwandferner (Ötztal Alps)

AbstractThe firn stratification in the accumulation area of an alpine glacier was studied in a 20 m. deep, vertical pit dug in the summer of 1963 on the Kesselwandferner (3,240 m. above sea-level in the Ötztal Alps). The firn is traversed by ice layers up to 25 cm. in thickness which have been formed by the damming up and freezing of melt water. Firn density increases with depth. At a depth of 13 m. (8 yr. old firn) the density reaches 0.80 g./cm.3 as a mean value of one annual net accumulation. Excluding the ice layers, the firn reaches a density of 0.82 g./cm.3 in a 10 yr. old layer at a depth of 16 m. The firn stratification in relation to seasons was studied by analyses of the pollen content in the lower part of the pit, beginning with the horizon “late summer 1954”. The late summer horizons obtained from the firn stratification were confirmed by the pollen analyses.

Glaciological Observations on the Ross Ice Shelf Near Scott Base, Antarctica

AbstractOn the Ross Ice Shelf near Scott Base the annual surface snow accumulation is about 18 g.cm.−2. The increase of firn density with depth is greater than normal, due to horizontal compaction. Where the bottom of the ice shelf has a density less than 0.8 g.cm.−3the shelf becomes brine-soaked by vertical infiltration. The bottom of the shelf is melting, at about 44 g.cm.−2yr.−1where the bottom is impermeable ice and 73 g.cm.−2yr.−1where the bottom is permeable firn. Surface velocities at 40 points within 30 km. of Scott Base vary between 4 and 840 m.yr.−1. From the pattern of velocities, ice thicknesses and surface features, a transition zone is delineated, separating the Ross Ice Shelf from the “McMurdo Ice Shelf”.

Glaciological Observations on the Ross Ice Shelf Near Scott Base, Antarctica

AbstractOn the Ross Ice Shelf near Scott Base the annual surface snow accumulation is about 18 g.cm.−2. The increase of firn density with depth is greater than normal, due to horizontal compaction. Where the bottom of the ice shelf has a density less than 0.8 g.cm.−3 the shelf becomes brine-soaked by vertical infiltration. The bottom of the shelf is melting, at about 44 g.cm.−2 yr.−1 where the bottom is impermeable ice and 73 g.cm.−2 yr.−1 where the bottom is permeable firn. Surface velocities at 40 points within 30 km. of Scott Base vary between 4 and 840 m.yr.−1. From the pattern of velocities, ice thicknesses and surface features, a transition zone is delineated, separating the Ross Ice Shelf from the “McMurdo Ice Shelf”.

The Use of the Rammsonde as an Instrument for Determining the Density of Firn

Abstract It has been found that in the cold firn of the Greenland Ice Sheet a direct relationship exists between the specific gravity of a layer and its “resistance to penetration” as measured with a Rammsonde. Corrections which should be applied to the measured “resistance” to allow for the frictional resistance of the walls of the hole on the Rammsonde tube are discussed. In certain circumstances these corrections can be estimated with reasonable accuracy for tube lengths up to three metres. The statistical method of obtaining the relationships between “resistance” and specific gravity is briefly outlined, and the effect of the size of the snow grains is also considered. It is shown that under these particular conditions it is possible, from measurements of “resistance”, to estimate the specific gravity of a firn layer with a standard error of 0.04, when the specific gravity of the sample is in the range 0.3 to 0.57.

The Use of the Rammsonde as an Instrument for Determining the Density of Firn

AbstractIt has been found that in the cold firn of the Greenland Ice Sheet a direct relationship exists between the specific gravity of a layer and its “resistance to penetration” as measured with a Rammsonde. Corrections which should be applied to the measured “resistance” to allow for the frictional resistance of the walls of the hole on the Rammsonde tube are discussed. In certain circumstances these corrections can be estimated with reasonable accuracy for tube lengths up to three metres. The statistical method of obtaining the relationships between “resistance” and specific gravity is briefly outlined, and the effect of the size of the snow grains is also considered. It is shown that under these particular conditions it is possible, from measurements of “resistance”, to estimate the specific gravity of a firn layer with a standard error of 0.04, when the specific gravity of the sample is in the range 0.3 to 0.57.

Features of the Firn on Upper Seward Glacier St. Elias Mountains, Canada

Particular attention is devoted to diagenetic changes within firn, leading to an increase in density and producing structures such as ice layers, lenses, and glands. Density increases irregularly downward in the Seward firn, with layers of exceptionally low density underlying impermeable horizontal ice masses. The mean rate of density increase with depth in the first 50 feet is 0.0054 per foot, being more rapid-0.01 per foot-in the first 20 than in the last 30 feet, where it is 0.0023 per foot. The rate of density increase in time varies with depth and with the season. Mean density of the 1948-1949 firn layer increased by 0.10 in ten weeks during the summer of 1949, but mean density of the 1946-1947 layer at a depth of 15-18 feet increased only 0.021 in a year. On the upper Seward, a geo-physically temperate glacier, the increase of firn density is caused about 25 per cent by meltwater freezing and about 75 per cent by compaction-settling within the zone of annual chilling, the upper 40-50 feet. Horizontal layers and lenses of ice in firn are formed by freezing of meltwater that has percolated downward and collected above or moved laterally along impermeable horizons. An estimated 20-25 per cent of the Seward firn is converted to solid ice by this process before passing beyond the depth of annual chilling. Ice glands-crude vertical columns of ice within the firn-are likewise attributed to the freezing of meltwater that has been concentrated along channels extending downward into the firn. The process is pictured as occurring in two stages: first the development of a water-rich condition without much freezing, followed by freezing of the entire mass as a unit, rather than as a process of incremental growth produced by alternate meltwater invasions and repeated freezings along a single channel. As ice layers and lenses are thickest and most numerous near ice glands and extend horizontally from them, a common origin is indicated. These bodies of ice form within the firn during spring when the winter's chilled layer is being ameliorated by the heat given up through freezing of the meltwater that has percolated downward from the surface.