Surface Snow Layers Research Articles

Micrometeorites from Central Antarctic snow: The CONCORDIA collection

We recovered micrometeorites from surface snow layers near the French-Italian station CONCORDIA. The unique weather and isolation conditions of Dome C allowed us to recover micrometeorites that are much better preserved than those extracted from blue ice fields. We have identified a new population of friable fine-grained micrometeorites; the absence of such particles in previous collections can be explained by their destruction by mechanical processes. In contrast to previous collections of micrometeorites, the particles from CONCORDIA Collection are characterized by a high content of Fe-sulfides and an undepleted CI elemental abundance pattern of their fine-grained matrix. These features suggest that micrometeorites from Dome C snow have endured much lower alteration from terrestrial weathering, unlike the micrometeorites recovered from near the margin of the Antarctic ice sheet (Adélie Land). The CONCORDIA particles have well constrained terrestrial ages and, given the low Dome C precipitation rate, the central regions of Antarctica provide a unique opportunity to search for particles from historical meteor showers.

The Micrometeorite Program at Dome C

We launched a program to recover cosmic dust (micrometeorites) from Dome C surface snow near the French-Italian station CONCORDIA. The Dome C snow is uniquely shielded from both morainic debris and aeolian dust within the micrometeorite size range ( ≥ 25 μ m). The average temperature at Dome C ranges from -20°C to -75°C throughout the year. Once trapped in a cold and clean snow, the micrometeorites are expected to stay much better preserved from mechanical constraints and less altered by terrestrial weathering than those of previous collections (mainly from Antarctic blue ice fields). We carried out 2 field expeditions in January 2000 and January 2002 using a new collection technique at Dome C. We successfully recovered micrometeorites from surface snow layers. By contrast with previous collections of micrometeorites, the particles from the CONCORDIA-Collection have suffered very little from terrestrial weathering. They have well constrained terrestrial ages. Their periods of fall overlap the ones of the particles found in the stratospheric interplanetary dust collections (IDPs) made by NASA. Because the precipitation rate is extremely low at Dome C, it is technically possible to exploit large areas (several tens of m 2 year) of annual snow layers formed up to 3 centuries ago, which allows searching for cometary particles from historical meteor showers.

The origin of sea salt in snow on Arctic sea ice and in coastal regions

Abstract. Snow, through its trace constituents, can have a major impact on lower tropospheric chemistry, as evidenced by ozone depletion events (ODEs) in oceanic polar areas. These ODEs are caused by the chemistry of bromine compounds that originate from sea salt bromide. Bromide may be supplied to the snow surface by upward migration from sea ice, by frost flowers being wind-blown to the snow surface, or by wind-transported aerosol generated by sea spray. We investigate here the relative importance of these processes by analyzing ions in snow near Alert and Ny-Ålesund (Canadian and European high Arctic) in winter and spring. Vertical ionic profiles in the snowpack on sea ice are measured to test upward migration of sea salt ions and to seek evidence for ion fractionation processes. Time series of the ionic composition of surface snow layers are investigated to quantify wind-transported ions. Upward migration of unfractionated sea salt to heights of at least 17cm was observed in winter snow, leading to Cl- concentration of several hundred µM. Upward migration thus has the potential to supply ions to surface snow layers. Time series show that wind can deposit aerosols to the top few cm of the snow, leading also to Cl- concentrations of several hundred µM, so that both diffusion from sea ice and wind transport can significantly contribute ions to snow. At Ny-Ålesund, sea salt transported by wind was unfractionated, implying that it comes from sea spray rather than frost flowers. Estimations based on our results suggest that the marine snowpack contains about 10 times more Na+ than the frost flowers, so that both the marine snowpack and frost flowers need to be considered as sea salt sources. Our data suggest that ozone depletion chemistry can significantly enhance the Br- content of snow. We speculate that this can also take place in coastal regions and contribute to propagate ODEs inland. Finally, we stress the need to measure snow physical parameters such as permeability and specific surface area to understand quantitatively changes in snow chemistry.

Rate of evolution of the specific surface area of surface snow layers.

The snowpack can impact atmospheric chemistry by exchanging adsorbed or dissolved gases with the atmosphere. Modeling this impact requires the knowledge of the specific surface area (SSA) of snow and its variations with time. We have therefore measured the evolution of the SSA of eight recent surface snow layers in the Arctic and the French Alps, using CH4 adsorption at liquid nitrogen temperature (77 K). The SSA of fresh snow layers was found to decrease with time, from initial values in the range 613-1540 cm2/g to values as low as 257 cm2/g after 6 days. This is explained by snow metamorphism, which causes modifications in crystal shapes, here essentially crystal rounding and the disappearance of microstructures. A parametrization of the rate of SSA decrease is proposed. We fit the SSA decrease to an exponential law and find that the time constant alpha(exp) (day(-1)) depends on temperature according to alpha(exp) = 76.6 exp (-1708/7), with Tin kelvin. Our parametrization predicts that the SSA of a snow layer evolving at -40 degrees C will decrease by a factor of 2 after 14 days, while a similar decrease at -1 degrees C will only require 5 days. Wind was found to increase the rate of SSA decrease, but insufficient data did not allow a parametrization of this effect.

Isotope studies of inner snow layers in a temperate region

AbstractPost‐depositional change of isotope concentration was investigated in a temperate snow pack. Daily snow pit studies were carried out at Moshiri, Hokkaido, for about a week in April 1998. During this time the snow layers were at the melting point. Isotope analysis was conducted on snow particles and the liquid water in between the particles every 5 cm in depth, and on the precipitation and the runoff water (from the bottom of the snow pack) collected during the observation period. It was found that the oxygen isotope concentration in the liquid part was smaller by about 2‰ than the solid part of the wet snow. The difference between the isotope concentrations of the liquid and the solid was low in the surface snow layer, but increased at the bottom. The results suggested that isotope fractionation took place between the liquid and the solid, when liquid water (formed by melting at the snow surface layer) flowed groundwards. Copyright © 2002 John Wiley & Sons, Ltd.

Evolution of the specific surface area and of crystal morphology of Arctic fresh snow during the ALERT 2000 campaign

Metamorphism of surface snow results in a decrease in specific surface area (SSA), which causes the release of adsorbed trace gases. This process contributes to the impact of the snowpack on atmospheric chemistry and must be quantified. The SSA evolution of fresh snow layers was measured in February and April 2000 during the ALERT 2000 campaign at Alert (Canadian Arctic: 82°29′94″N, 62°20′55″W). Measurements were made using CH 4 adsorption at 77 K. Numerous photomacrographs of snow crystals were also taken to interpret observed SSA decreases. Five snowfalls were studied. Four snowfalls consisted of fine snow called diamond dust, whose initial SSA ranged from 1460 to 770 cm 2/g, and decreased to ∼500 cm 2/g in winter, and to ∼300 cm 2/g in spring after 10 days. The fifth one consisted of rimed dendritic crystals, whose SSA decreased from 1540 to 450 cm 2/g in 3 windy days. The continuous growth of surface hoar at the surface of the snowpack, diluted surface diamond dust and contributed to their SSA decrease. Our data indicate that the total surface area of surface snow layers is sufficient to sequester adsorbable gases. The example of HNO 3 shows that the release of this gas by SSA decrease may indeed lead to a significant increase in its atmospheric concentration.

Photochemical Production of Carbon Monoxide in Snow

Carbon monoxide concentrations were measured over an 8-day period (March 1997) in freshly fallen snow samples collected at Mount Sonnblick (Austria). Diurnal changes were system- atically observed in the snow, with higher CO during daytime, indicating that light-dependent CO production processes are active in surface snow layers. Mean daytime CO concentrations in snow varied significantly from one day to another and were found to be well correlated with the daily mean atmospheric CO concentrations. Thus, the more polluted the air mass during the snow fall, the more CO was produced through photolytical processes. Furthermore closed chamber experiments were made, giving similar observations: rapid increases of CO concentration occurred within the chamber when the snow was exposed to sunlight. In the dark, however, CO concentrations decreased. CO fluxes between the surface of the snowpack and the atmosphere were estimated at 0.6 ppb/day which may be significant in the local CO budget but not on a global scale. Laboratory experiments showed that CO was rapidly formed when melted snow samples were exposed to the light from a solar simulator and that the initial CO formation rate was strongly correlated with the concentrations of Total Organic Carbon in the melted snow samples. This correlation indicates that organic com- pounds present in snow precipitation are the major substrate for the photochemical CO production observed in freshly fallen snow. Recent studies have reported high formaldehyde concentrations in the snowpack and we suggest HCHO photolysis to be partly responsible of CO production in snow.

Factors controlling nitrate in ice cores: Evidence from the Dome C deep ice core

In order to estimate past changes in atmospheric NOx concentration, nitrate, an oxidation product of NOx, has often been measured in polar ice cores. In the frame of the European Project for Ice Coring in Antarctica (EPICA), a high‐resolution nitrate record was obtained by continuous flow analysis (CFA) of a new deep ice core drilled at Dome C. This record allows a detailed comparison of nitrate with other chemical trace substances in polar snow under different climatic regimes. Previous studies showed that it would be difficult to make firm conclusions about atmospheric NOx concentrations based on ice core nitrate without a better understanding of the factors controlling NO3− deposition and preservation. At Dome C, initially high nitrate concentrations (over 500 ppb) decrease within the top meter to steady low values around 15 ppb that are maintained throughout the Holocene ice. Much higher concentrations (averaging 53 ppb) are found in ice from the Last Glacial Maximum (LGM). Combining this information with data from previous sampling elsewhere in Antarctica, it seems that under climatic conditions of the Holocene, temperature and accumulation rate are the key factors determining the NO3− concentration in the ice. Furthermore, ice layers with high acidity show a depletion of NO3−, but higher concentrations are found before and after the acidity layer, indicating that NO3− has been redistributed after deposition. Under glacial conditions, where NO3− shows a higher concentration level and also a larger variability, non‐sea‐salt calcium seems to act as a stabilizer, preventing volatilization of NO3− from the surface snow layers.

Loss of volatile acid species from upper firn layers at Vostok, Antarctica

Significant natural artifacts in ice chemical records have been pointed out in recent preliminary glaciochemical studies carried out in central Antarctic areas with very low snow accumulation rates (generally less than 5 g cm−2 yr−1). Several deep drilling operations are underway in these regions for long‐term paleoclimatic reconstructions. A detailed glaciochemical study has been carried out at Vostok Station in order to investigate post deposition changes of ion concentrations in the snow and firn layers. The results show that, in general, concentration profiles of species such as Cl, F, and NO3, partly deposited as gases, exhibit a rapid decrease in the first few meters, indicating that a fraction, sometimes major, of these compounds is expelled back in the atmosphere after deposition. Some redeposition process of the gases is likely in the upper firn layers. Surprisingly, a similar effect is found for methanesulfonate (MS), suggesting that this compound could have a gaseous component in central Antarctic regions. The data also show that Cl, F, NO3, and MSA may be slowly but significantly displaced in the firn layers by high sulfuric acid levels of volcanic origin. The drastic changes observed in the surface snow layers may severely question current interpretations of certain chemical data recovered in these areas and point out an urgent need for new field and laboratory experiments on the air‐to‐ice transfer processes prevailing under central Antarctic conditions.

Different post-depositional processes of NO3– in snow layers in East Antarctica and on the northern Qinghai-Tibetan Plateau

AbstractThrough comparison of snow-pit NO3– profiles from central East Antarctica and the northern Qinghai-Tibetan Plateau (QTP), we conclude that NO3– peaks in the uppermost surface snow layers in central East Antarctica are not related to an atmospheric signal and that they need to be accounted for by post-depositional effects. Such effects, however, are not found in the snow-pit NO3– profiles from the northern QTP. NO3– can be deposited as a gas (HNO3–) or as a neutral salt, particularly by reaction with ammonia to form NH4NO3, or fixed by sea salt or terrestrial dust (Mulvaney and others, 1998). Thus, a difference in speciation between NO3– in snow layers in East Antarctica and at the northern QTP is suggested as the reason for the different post-depositional processes of NO3– in the two areas.

Postdepositional change in snowpack nitrate from observation of year‐round near‐surface snow in coastal Antarctica

Postdepositional loss of nitrate in near‐surface snowfall is well known, with mean levels of nitrate in ice cores of around 20 to 80 ng g−1, while nitrate in surface snow may occasionally reach over 300 ng g−1. This has been explained as reemission of nitrate (as nitric acid) during aging, via processes that are not yet clear. However, clear seasonal cycles remain in nitrate profiles from higher accumulation rate ice cores and, across Antarctica, the mean concentration of nitrate is remarkably similar despite widely varying deposition conditions, marking nitrate out as quite different to other major ice core species. This paper examines the year‐round deposition of nitrate at the snow surface at a coastal Antarctic site and discusses the degree and timing of nitrate loss. At Halley Station, Antarctica, the mean concentration of near‐surface snow was 96 ng g−1 over a 2.6 year daily sampling period, while the mean concentration in newly accumulated snow was 79 ng g−1. At the end of this period, a shallow core integrating the sampling period had a mean nitrate concentration of 65 ng g−1 taken over 2 full years of accumulation. Nitrate concentrations in the surface layer were in general highest during the summer period reaching 400 ng g−1 with a mean of about 150 ng g−1 in each of December, January, and February, and lowest during the winter with a mean of around 50 ng g−1 in June. Fresh snow data from Neumayer Station shows a similar seasonal signal, with a mean nitrate concentration of 77 ng g−1, while year‐round aerosol data shows total nitrate (particulate and gas phase) in the air is at a minimum in April to June and reaches a maximum in late November [Wagenbach et al., this issue], slightly out of phase with snowfall nitrate. The observation of a reduction in high nitrate concentrations in new snowfall over a few days does not appear to be general, and at Halley, there is evidence of both uptake and loss of nitrate in the surface snow layer, possibly indicating an equilibrium with changing air concentrations. However, there is attenuation of the nitrate signal over the longer period, with concentrations in the ice core taken at the end of sampling never reaching values seen in the upper surface layer.

Preliminary investigation of palaeoclimate signals recorded in the ice core from Dome Fuji station, east Dronning Maud Land, Antarctica

ABSTRACT An ice core to 2503.52 m depth was drilled during 1995 and 1996 at 77°19′01″S, 39°42′12″ E (3810 m a.s.l.), at the summit of Dome Fuji, East Antarctica. Climatic and environmental conditions were observed at the coring site throughout the 2 years of wintering operation. Meteorological and glaciological observations made clear the present surface processes of mass balance. The chemistry of the surface snow layers was observed for investigation of the atmospheric environment reflected in snow deposits. Core analyses have been carried out in Japan. Permeability and density measurements show that air bubbles are completely closed off by densification processes at about 98 m depth. The vertical profile of δ 18O suggests that the core covers two warm stages and one cold stage from the surface down to 1800 m depth. The transition from the cold to the warm period affects the vertical distribution of ice-crystal size. The chemical constituents in the ice core show large differences between warm and cold periods. A depth–age relationship is presented and a study made of palaeoclimatic and core signals.

Assessment of Spatial Variability of Major-Ion Concentrations and DEL Oxygen-18 Values in Surface Snow, Upper Fremont Glacier, Wyoming, U.S.A.

One hundred samples were collected from the surface of the Upper Fremont Glacier at equally spaced intervals defined by an 8,100 m2 snow grid to assess the significance of lateral variability in major-ion concentrations and del oxygen-18 values. For the major ions, the largest concentration range within the snow grid was sodium (0.5056 mg/l) and the smallest concentration range was sulfate (0.125 mg/l). Del oxygen-18 values showed a range of 7.45 per mil. Comparison of the observed variability of each chemical constituent to the variability expected by measurement error indicated substantial lateral variability within the surface-snow layer. Results of the nested ANOVA indicate most of the variance for every constituent is in the values grouped at the two smaller geographic scales (between 506 m2 and within 506 m2 sections). Calcium and sodium concentrations and del oxygen-18 values displayed the largest amount of variance at the largest geographic scale (between 2,025 m2 sections) within the grid and ranged from 14 to 26 per cent of the total variance. The variance data from the snow grid were used to develop equations to evaluate the significance of both positive and negative concentration/value peaks of nitrate and del oxygen-18 with depth, in a 160 m ice core. Solving the equations indicates that both the nitrate and del oxygen-18 ice-core profiles have concentration/value trends that exceed the limits expected from lateral variability. Values of del oxygen-18 in the section from 110-150 m below the surface consistently vary outside the expected limits and possibly represents cooler temperatures during the Little Ice Age from about 1810 to 1725 A.D.

積雪中における押し出し流の形成

Translatory flow is observed in a ground water runoff during a rainstorm. Layered snow cover is considered to play the same role as soil layer for water runoff. The purpose of this paper is to report the translatory flow phenomena of snow meltwater in a snowpack. Studies on the snowmelt runoff in a snowpack were carried out at eastern Canada and Hokkaido. In the case of Canada, the hydrograph of snowmelt is separated into “old water” (meltwater in the lower snowpack) and “new water” (meltwater percolated from surface snow layer) by the concentration of NO3-. The concentration of NO 3- in meltwater in the lower snowpack is estimated to be higher than that in meltwater generated from the surface snow layer. Separated “old water” is the major component of early snowmelt runoff during a day. This quick response of the meltwater in the lower snowpack requires a translatory flow mechanism in the snowpack. “New water” is the major component of the recession limb of the hydrograph. The same phenomena were observed at Moshiri, Hokkaido, during the 1988 snowmelt. Two peaks on the meltwater hydrograph were observed. The first peak is composed of the meltwater in the lower snowpack; the major component of the second peak is the meltwater percolated from the surface snow layer. The translatory phenomenon in a snowpack is not observed when the depth of snow cover is not so thick.

A snowpack evolution model based on the mass and heat balance method

A numerical model, based on the heat balance method, was developed to simulate snowpack evolution at a given location, as a function of meteorological conditions: air-temperature, wind speed, precipitation and incident short-wave radiation.The model takes into account three major processes, (1) the increase in the amount of snowpack by precipitation, (2) melting of the surface snow layer as a result of energy balance and (3) drainage of water from the snowpack. The model was tested with data collected at TAKADA and NAGAOKA, JAPAN. The results show the ability of the model to simulate the day-to-day changes of the snow mass with reasonable accuracy. Emphasis was placed on the characteristics unique to this and other warmer climatic regions.

Snow-accumulation variability from seasonal surface observations and firn-core stratigraphy, eastern Wilkes Land, Antarctica

AbstractAnnual accumulation records were derived from two firn cores drilled at GD03 (69° 00’S, 115° 30’E, 1835m) and GD15 (69° 00’S, 130° 48’E, 2155m), which are separated by 750 km along the 2000 m contour in eastern Wilkes Land. The accumulation records, spanning the period 1930–85, show that annual accumulation has increased by 25% from 1960 to 1985, resulting in the highest accumulation rates in the 55 year records. Annual accumulation layers were identified in the firn cores using a firn-stratigraphic model (based on the observed characteristics and variability of the seasonal snow-surface layer) together with seasonal oxygen-isotope ratios. The accumulation records support other evidence for a similar recent increase across Antarctica. The greater increase over the long-term mean for 1930–85 in eastern Wilkes Land was observed at GD15 which is located on a northeast aspect slope near Terre Adèlie. It is suggested that the increase is the result of changes in the general atmospheric circulation pattern which have produced a higher annual frequency of precipitation events.

Snow-accumulation variability from seasonal surface observations and firn-core stratigraphy, eastern Wilkes Land, Antarctica

AbstractAnnual accumulation records were derived from two firn cores drilled at GD03 (69° 00’S, 115° 30’E, 1835m) and GD15 (69° 00’S, 130° 48’E, 2155m), which are separated by 750 km along the 2000 m contour in eastern Wilkes Land. The accumulation records, spanning the period 1930–85, show that annual accumulation has increased by 25% from 1960 to 1985, resulting in the highest accumulation rates in the 55 year records. Annual accumulation layers were identified in the firn cores using a firn-stratigraphic model (based on the observed characteristics and variability of the seasonal snow-surface layer) together with seasonal oxygen-isotope ratios. The accumulation records support other evidence for a similar recent increase across Antarctica. The greater increase over the long-term mean for 1930–85 in eastern Wilkes Land was observed at GD15 which is located on a northeast aspect slope near Terre Adèlie. It is suggested that the increase is the result of changes in the general atmospheric circulation pattern which have produced a higher annual frequency of precipitation events.

Origins and variations of nitrate in south polar precipitation

South polar firn cores spanning the last millennium have been analyzed to determine the nitrate background level of high‐latitude precipitation and its temporal variations. The resulting data reveal no evidence of a positive correlation between solar activity (11‐year solar cycle, low solar activity time periods, and solar proton events) and the NO3 content of south polar snow. These data therefore suggest that NOx production in the upper stratosphere, mesosphere, and thermosphere does not contribute significantly to the antarctic NO3 budget. This study of the NO3 content of high latitude precipitation suggests a major contribution by lightning (from a third to a half of the total) and by NOx produced in the lower stratosphere (approximately a third from N2O oxidation and to a lesser extent galactic cosmic rays) to the NO3 budget of this background atmosphere, the remaining portion being related to the present NOx surface sources of the southern hemisphere. For the first time, our data point to a decrease of NO3 content when very large amounts of sulfuric acid are present in South Pole snow layers. This observation suggests a possible interaction (via the hydroxyl radical) between S‐ and N‐derived species when large quantities of SO2 are injected by volcanic eruptions. Although the deposition mechanism of HNO3 in South Pole snow layers is far from well understood, our data suggest that only a minor amount of this compound is deposited directly on surface snow layers. Finally, the role played by particles (volcanic ash, terrestrial impurities, or ice particles) to enhance the uptake of odd nitrogen from the atmosphere by heteregeneous processes is discussed. In particular, it is suggested that in late winter, under certain meteorological conditions leading to ice particle formation, a significant uptake of HNO3 from the lower stratosphere can occur.

On radiative effects of anthropogenic aerosol components in Arctic haze and snow

The Arctic aerosol is strongly enriched by anthropogenic pollution which may cause significant modifications of the Arctic climate. A radiation model with an optically interactive snow surface layer is developed to investigate the effect of anthropogenic elemental carbon (EC) and sulphuric acid in the Arctic atmosphere. The model, based on the delta-Eddington method, has 490 wavelength intervals covering solar and terrestrial radiation domains. Computations were made for aerosols-containing natural components and cases contaminated by anthropogenic EC and sulphuric acid, both categories with dry and moist conditions. Comparisons of model results with measurements in the Arctic of direct and total irradiance agree well. During mid-spring averaged conditions, the increase in the daily mean solar warming is found to be 2 or 3 times lower than earlier estimates. The previously expected surface cooling can be offset by direct to diffuse conversion in the haze and by snow contamination. In the infrared, the haze located in a temperature inversion can increase the upwelling irradiance by about 1%. This earth-atmosphere column cooling is caused by deliquescent aerosol material. In contrast, hygroscopic but non-deliquescent compounds like sulphuric acid result in internal exchange of radiative energy. In haze layers, the local cooling rate nearly offsets the daily averaged mid-spring local solar heating rate. More significant net warming due to EC particles takes place in dry and optically thin aerosol layers typically found above the Arctic boundary layer during pollution episodes. DOI: 10.1111/j.1600-0889.1987.tb00101.x

Observations on the Relation of the Shortwave Reflectivity of Recently Deposited Snow to its Physical Properties

Abstract The integrated albedo for solar radiation in the 0.4–0.7 μm wavelength range was measured near noon over a wet snow cover before and after a new snowfall. Observed values were compared with those estimated from measurements of surface density, air permeability, and the total-to-diffuse-flux ratio by means of five models described in the literature and by using empirical correlations to estimate grain size. The models yield widely divergent results. The model with the best apparent performance shows an rms error of 0.02 with no particular bias. With one exception, however, the remaining models yield over-estimates. Albedos calculated from samples taken from differing surface layers and variation of the prediction errors with the snow accumulation pattern were compared. Results suggest that a major source of error is the large depth interval sampled by the surface layer measurement technique as compared with the thickness of the surface layer of snow in which the bulk of absorption and scattering o...