Bare Ice Surface Research Articles

The role of snow on microwave emission and scattering over first-year sea ice

Investigates the geophysical and thermodynamic effects of snow on sea ice in defining the electromagnetic (EM) interaction within the microwave portion of the spectrum. The authors combine observational evidence of both the physical and thermodynamic characteristics of snow with direct measurements of scattering and emission at a variety of frequencies. They explain their observational results using various "state-of-the-art" forward scattering and emission models. Results show that geophysical characteristics of snow effect emission above about 37 GHz and above 5 GHz for active microwave scattering. They understand these effects to be driven by grain size and its contribution to volume scattering in both passive and active interactions within the volume. With snow cover, the Brewster angle effect is not significant and there is a gradual rise in emission from 10 to 37 GHz. They find emissivity to be dominated by direct emission from saline ice through the snow layer. Hence, the influence of grain size is small but the trend is clearly a drop in total emission as the grain size increases. They find that the role of the volume fraction of snow on emission and scattering is a complex relationship between the number density of scatterers relative to the coherence of this scattering ensemble. At low volume fractions, they find that independent scattering dominates, resulting in an increase in albedo and the extinction coefficient of the snow with frequency. The thermodynamic effects of snow on microwave scattering and emission are driven by the role that thermal diffusivity and conductivity play in the definition of brine volumes at the ice surface and within the snow volume. Prior to the presence of water in liquid phase within the snow volume, they find that the indirect effects are dominated by an impedance matching process across the snow-ice interface. They find that the complex permittivity at the snow-ice interface is considerably higher than over the bare ice surface.

Late Wisconsinan erosion and eolian deposition, Summer Island area, Pleistocene Mackenzie Delta, Northwest Territories: optical dating and implications for glacial chronology

The Summer Island area of the Pleistocene Mackenzie Delta, western Canadian Arctic, is thought to have been last glaciated during the Early Wisconsinan (>35 ka), with a suggested Late Wisconsinan (~13 ka) glacial limit located ~80–100 km to the south. Cryostratigraphic observations in the Summer Island area identify eolian and fluvial sediments overlying an erosion surface. On Summer Island, eolian sand occurs within sand wedges, the tops of which are contiguous with an eolian sand sheet deposited on a formerly bare ice surface. This paleo-ice surface is attributed to fluvial erosion, and that in turn is attributed to proglacial meltwater activity. The age of erosion is inferred to be shortly before ~14 ka, determined by optical dating (using infrared stimulation of luminescence from feldspar) of the eolian sand wedges. This ~14 ka meltwater erosion is significantly north of the generally accepted limit of Late Wisconsinan glaciation in this coastal area, suggesting that ice last covered the whole of Richards Island near the end of the Late Wisconsinan, rather than in the Early Wisconsinan, as previously thought.

Equilibrium line and mean annual mass balance of Finsterwalderbreen, Spitsbergen, determined by in situ and laboratory gamma-ray measurements of nuclear test deposits

In order to determine the equilibrium line (EL) and the annual net mass balance over the accumulation area of Finsterwalderbreen, Spitsbergen, Svalbard (by detection of the 1962–63 radioactive peak from the 1961–62 atmospheric nuclear tests), we collected 14 ice cores, at elevations of 445–730 m, in the springs of 1994 and 1995. The corresponding samples were melted and filtered for laboratory gamma spectrometry. In the accumulation area, the 1962–63 radioactive layer is found well below the surface. The mean annual accumulation is not invariably related to altitude. The EL, averaged to 545 in a.s.l., leads to an accumulation area ratio of 0.3 and indicates a strong negative balance. The 584 Bq m−2 mean137 Cs deposition rate (1954–74 nuclear tests) for eight ice cores in the accumulation area is nearly twice the 340 Bq m−2 mean Svalbard value obtained from six other glaciers (at time of deposition). An in situ gamma-ray detector was lowered down each borehole, and137 Cs levels were recorded. The counting rate is proportional to the apparent deposition rate and the specific activity. The laboratory measurements perfectly match the in situ determinations. In the ablation area, a dust layer and the associated nuclear test deposits are concentrated close to the bare ice surface of the glacier, under the winter snow layer and present maximum 137 Cs and 210 Pb contents. The dust layer acts like a filter for radioactive materials removed from the glacier and its basin by melting and water flow. The original specific activities and deposition rates at a given location are enhanced by adsorption of additional radioactivity on the dust particles. A linear relationship exists between 137 Cs and 210 Pb deposition rates. This process is almost constant for all studied ice cores. The apparent 137 Cs deposition rate for seven ice cores in the ablation area is 465 Bq m−2 (at date of measurement: 1 July 1995).

Equilibrium line and mean annual mass balance of Finsterwalderbreen, Spitsbergen, determined by in situ and laboratory gamma-ray measurements of nuclear test deposits

In order to determine the equilibrium line (EL) and the annual net mass balance over the accumulation area of Finsterwalderbreen, Spitsbergen, Svalbard (by detection of the 1962–63 radioactive peak from the 1961–62 atmospheric nuclear tests), we collected 14 ice cores, at elevations of 445–730 m, in the springs of 1994 and 1995. The corresponding samples were melted and filtered for laboratory gamma spectrometry. In the accumulation area, the 1962–63 radioactive layer is found well below the surface. The mean annual accumulation is not invariably related to altitude. The EL, averaged to 545 in a.s.l., leads to an accumulation area ratio of 0.3 and indicates a strong negative balance. The 584 Bq m−2 mean137 Cs deposition rate (1954–74 nuclear tests) for eight ice cores in the accumulation area is nearly twice the 340 Bq m−2 mean Svalbard value obtained from six other glaciers (at time of deposition).An in situ gamma-ray detector was lowered down each borehole, and137 Cs levels were recorded. The counting rate is proportional to the apparent deposition rate and the specific activity. The laboratory measurements perfectly match the in situ determinations. In the ablation area, a dust layer and the associated nuclear test deposits are concentrated close to the bare ice surface of the glacier, under the winter snow layer and present maximum 137 Cs and 210 Pb contents. The dust layer acts like a filter for radioactive materials removed from the glacier and its basin by melting and water flow. The original specific activities and deposition rates at a given location are enhanced by adsorption of additional radioactivity on the dust particles. A linear relationship exists between 137 Cs and 210 Pb deposition rates. This process is almost constant for all studied ice cores. The apparent 137 Cs deposition rate for seven ice cores in the ablation area is 465 Bq m−2 (at date of measurement: 1 July 1995).

Strain in the ice sheet deduced from the crystal-orientation fabrics from bare icefields adjacent to the Sør-Rondane Mountains, Dronning Maud Land, East Antarctica

AbstractStructural analyses of ice collected from the bare ice surface in the region of the Sør-Rondane Mountains were carried out. Crystal-orientation fabrics and the disposition of surface cracks were investigated to determine the stress/strain configuration in the ice sheet near the mountains. Single-maximum fabric patterns with the axis of the maximum roughly perpendicular to the flow line on the horizontal plane were observed. It was deduced from the observations that the ice exhibits a fabric pattern indicating that the ice sheet is subjected to vertical shear strain between the ice flow and the nunataks.

Strain in the ice sheet deduced from the crystal-orientation fabrics from bare icefields adjacent to the Sør-Rondane Mountains, Dronning Maud Land, East Antarctica

AbstractStructural analyses of ice collected from the bare ice surface in the region of the Sør-Rondane Mountains were carried out. Crystal-orientation fabrics and the disposition of surface cracks were investigated to determine the stress/strain configuration in the ice sheet near the mountains. Single-maximum fabric patterns with the axis of the maximum roughly perpendicular to the flow line on the horizontal plane were observed. It was deduced from the observations that the ice exhibits a fabric pattern indicating that the ice sheet is subjected to vertical shear strain between the ice flow and the nunataks.

Meteorite infall as a function of mass: Implications for the accumulation of meteorites on Antarctic ice

Abstract— Antarctic meteorites are considerably smaller, on average, than those recovered elsewhere in the world, and seem to represent a different portion of the mass distribution of infalling meteorites. When an infall rate appropriate to the size of Antarctic meteorites is used (1000 meteorites 10 grams or larger/km2/106 years), it is found that direct infall can produce the meteorite accumulations found on eight ice fields in the Allan Hills region in times ranging from a few thousand to nearly 200 000 years, with all but the Allan Hills Main and Near Western ice fields requiring less than 30 000 years. Meteorites incorporated into the ice over time are concentrated on the surface when the ice flows into a local area of rapid ablation. The calculated accumulation times, which can be considered the average age of the exposed ice, agree well with terrestrial ages for the meteorites and measured ages of exposed ice. Since vertical concentration of meteorites through removal of ice by ablation is sufficient to explain the observed meteorite accumulations, there is no need to invoke mechanisms to bring meteorites from large areas to the relatively small blue‐ice patches where they are found. Once a meteorite is on a bare ice surface, freeze‐thaw cycling and wind break down the meteorite and remove it from the ice. The weathering lifetime of a 100‐gram meteorite on Antarctic ice is on the order of 10 000 ± 5000 years.

4A. Ash layers on the bare ice surface in Antarctica(Abstracts of Papers Presented at the Spring Meeting of the Society)

4A. Ash layers on the bare ice surface in Antarctica(Abstracts of Papers Presented at the Spring Meeting of the Society)

Volcanic Ash Layers in Bare Ice Areas near the Yamato Mountains, Dronning Maud Land and the Allan Hills, Victoria Land, Antarctica

Dirt layers of tephra were found on the bare ice surface in the Meteorite Ice Field near the Yamato Mountains, Dronning Maud Land, and near the Allan Hills, Victoria Land, Antarctica. The grain-size analyses of volcanic ash fragments show that the mean grain size in the Allan Hills region is larger than that in the Yamato Mountains region. This fact indicates that the volcanic sources of the dirt layers in the Yamato Mountains region are farther away than those in the Allan Hills. Their constituent fragments are well-sorted and composed mainly of volcanic glass shards with minor amounts of crystal fragments. Glass shards of the tephra from the Yamato Mountains region have a composition of tholeiitic andesite which is low in alkali and high in iron but not so enriched in titanium, and the associated crystal fragments consist of calcic plagioclase, subcalcic clinopyroxene, orthopyroxene and magnetite. The nature of island arc tholeiite of the tephra indicates that its source is some volcano in the South Sandwich Islands. On the other hand, the tephra from the Allan Hills region is composed of glass shards of trachybasaltic composition and crystal fragments of titanaugite calcic plagioclase, kaersutite, olivine, rhönite and titanomagnetite. A young volcano of the McMurdo volcanic group is suggested as a possible source of this tephra.

Volcanic Ash Layers in Bare Ice Areas near the Yamato Mountains, Dronning Maud Land and the Allan Hills, Victoria Land, Antarctica

Dirt layers of tephra were found on the bare ice surface in the Meteorite Ice Field near the Yamato Mountains, Dronning Maud Land, and near the Allan Hills, Victoria Land, Antarctica. The grain-size analyses of volcanic ash fragments show that the mean grain size in the Allan Hills region is larger than that in the Yamato Mountains region. This fact indicates that the volcanic sources of the dirt layers in the Yamato Mountains region are farther away than those in the Allan Hills. Their constituent fragments are well-sorted and composed mainly of volcanic glass shards with minor amounts of crystal fragments.Glass shards of the tephra from the Yamato Mountains region have a composition of tholeiitic andesite which is low in alkali and high in iron but not so enriched in titanium, and the associated crystal fragments consist of calcic plagioclase, subcalcic clinopyroxene, orthopyroxene and magnetite. The nature of island arc tholeiite of the tephra indicates that its source is some volcano in the South Sandwich Islands.On the other hand, the tephra from the Allan Hills region is composed of glass shards of trachybasaltic composition and crystal fragments of titanaugite calcic plagioclase, kaersutite, olivine, rhönite and titanomagnetite. A young volcano of the McMurdo volcanic group is suggested as a possible source of this tephra.

The sensitivity of a one‐dimensional thermodynamic sea ice model to changes in cloudiness

A thermodynamic sea ice‐lead model is used to assess the importance of cloud cover changes to modeled ice thickness. For regions of either permanent multiyear ice or seasonal sea ice, the cloud amount variations have relatively little impact. However, for regions where the presence of summer ice is variable from year to year, the predicted ice thickness is strongly dependent on cloud cover. In general, with a snow covered surface, decreased cloud leads to surface cooling while increased cloud gives rise to a surface warming. For a melting bare ice surface, the reverse occurs. The ice model response time is too long for interannual variations in cloud amount to explain interannual variations in ice thickness and extent. Nevertheless, the implication of the results is that numerical modeling of sea ice distribution requires accurate cloud data or cloud prediction and that trends in cloud cover may lead to significant perturbations in sea ice extent and thickness.