Ice Accumulation Rate Research Articles

Characteristics of bubble volumes in firn-ice transition layers of ice cores from polar ice sheets

The air-bubble formation process has been studied experimentally by using five ice cores from the Greenland and Antarctic ice sheets. Bubble volumes in firn-ice samples were measured by a classical method based on Boyle Mariotte's law for an ideal gas. It was found that the bubble volume varies with depth as a function of bulk density in the firn-ice transition layer, which is represented by an exponential function of firn density. Air bubbles start to form rapidly at a bulk density of 0.763–0.797 Mg m-3. This density (ρib) seems to be correlated with the ice temperature in the ice sheets; ρibincreases with a decrease in the ice temperature. Vbshows the maximum value in the density range 0.819–0.832 Mg m-3. The corresponding porosity of the density ranges between 0.110 and 0.097. This porosity does not seem to correlate with ice temperature or accumulation rate at the coring site. These characteristics of firn densities probably affect the amount of entrapped air in glacier ice (total air content) in polar ice sheets.

Characteristics of bubble volumes in firn-ice transition layers of ice cores from polar ice sheets

The air-bubble formation process has been studied experimentally by using five ice cores from the Greenland and Antarctic ice sheets. Bubble volumes in firn-ice samples were measured by a classical method based on Boyle Mariotte's law for an ideal gas. It was found that the bubble volume varies with depth as a function of bulk density in the firn-ice transition layer, which is represented by an exponential function of firn density. Air bubbles start to form rapidly at a bulk density of 0.763–0.797 Mg m-3. This density (ρib) seems to be correlated with the ice temperature in the ice sheets; ρib increases with a decrease in the ice temperature. Vb shows the maximum value in the density range 0.819–0.832 Mg m-3. The corresponding porosity of the density ranges between 0.110 and 0.097. This porosity does not seem to correlate with ice temperature or accumulation rate at the coring site. These characteristics of firn densities probably affect the amount of entrapped air in glacier ice (total air content) in polar ice sheets.

Characteristics of bubble volumes in firn-ice transition layers of ice cores from polar ice sheets

The air-bubble formation process has been studied experimentally by using five ice cores from the Greenland and Antarctic ice sheets. Bubble volumes in firn-ice samples were measured by a classical method based on Boyle Mariotte's law for an ideal gas. It was found that the bubble volume varies with depth as a function of bulk density in the firn-ice transition layer, which is represented by an exponential function of firn density. Air bubbles start to form rapidly at a bulk density of 0.763–0.797 Mg m-3. This density (ρib) seems to be correlated with the ice temperature in the ice sheets; ρib increases with a decrease in the ice temperature. Vb shows the maximum value in the density range 0.819–0.832 Mg m-3. The corresponding porosity of the density ranges between 0.110 and 0.097. This porosity does not seem to correlate with ice temperature or accumulation rate at the coring site. These characteristics of firn densities probably affect the amount of entrapped air in glacier ice (total air content) in polar ice sheets.

Polar fallout of radionuclides 32Si, 7Be and 210Pb and past accumulation rate of ice at Indian station, Dakshin Gangotri, East Antarctica

The concentrations of radionuclides 32Si, 7Be, 210Pb and 137Cs have been measured in snow samples from the shelf ice near the Indian Station, Dakshin Gangotri, East Antarctica. The annual fallout of cosmic-ray produced 32Si — the first measurement of its kind in Antarctica — has been estimated to be 2·34 × 10 −5 dpm cm −2y −1 which corresponds to a global production rate of 32Si of 0·75 × 10 −4 atoms cm −2y −1 if the half-life of 32Si is 140 y. The fallout of the other natural radionuclides 7Be and 210Pb are estimated to be 4·2 and 1·86 × 10 −2 dpm cm −2 y −1, respectively. Based on the depth profile of 210Pb and 137Cs concentrations in the shallow ice core samples, the accumulation rate of ice has been estimated to be 13·5 g cm −2 y −1 (0·3 my −1) over the past few decades.

Surface water temperature changes in the high latitudes of the southern hemisphere over the Last Glacial‐Interglacial Cycle

A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a “dissolution assemblage”. Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is ± 1°C in the −1 to +10 °C summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43°S and 55°S) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods.

On determining ice accumulation rates in the past 40,000 years using in situ cosmogenic 14C

Radiocarbon is produced in situ in ice by nuclear spallations of oxygen by cosmic ray neutrons. As the firn accumulates, it acquires a predictable concentration of in situ14C, inversely proportional to the rate of accumulation. Most of this production occurs when the amount of overlying ice is less than (2–3) Λ, where Λ is the absorption mean free path for cosmic radiation in ice, about 150 g · cm−2, i.e. within the top 10 m. In most accumulation areas, this is firn. In situ produced 14C is added to the firn as it accumulates, and is not expected to be lost by diffusion. During the firnice transition, atmospheric CO2 is trapped, adding 14CO2 to the ice. The signature of in situ14C is however not obliterated since ∼ 60% of in situ14C is instantly oxidized to 14CO in the ice. We discuss the results available to date and propose that this in situ14CO can be used to determine ice accumulation rates back to 40,000 yrs in the past.

Air-Ice-Ocean Feedback Mechanisms and Ice Oscillation on Millennial Time Scales

Air-ice-ocean feedback mechanisms, which are not conventionally incorporated within either climate or glacial models, are investigated to illustrate their potential role in generating ice advance/retreat on the time scale of 103–104 years; i.e. for examining the internal causes for the ice oscillation.Three main feedback loops are found from a coupled air-ice-ocean model developed in this paper: (a) ice advance → lower air temperature → ice freezing → ice advance; and (b) ice advance → higher ocean temperature → ice melting → ice retreat; (c) ice advance/retreat → modification of evaporation rate → change of ice accumulation rate and sea-level height → ice advance/retreat. The relative strength of the three feedback mechanisms determines the characteristics of the modes: growing or decaying, oscillatory or non-oscillatory. The solutions show the generation of growing oscillatory modes with the time scale of 103–104 years in certain parameter ranges.

Air-Ice-Ocean Feedback Mechanisms and Ice Oscillation on Millennial Time Scales

Air-ice-ocean feedback mechanisms, which are not conventionally incorporated within either climate or glacial models, are investigated to illustrate their potential role in generating ice advance/retreat on the time scale of 103–104 years; i.e. for examining the internal causes for the ice oscillation. Three main feedback loops are found from a coupled air-ice-ocean model developed in this paper: (a) ice advance → lower air temperature → ice freezing → ice advance; and (b) ice advance → higher ocean temperature → ice melting → ice retreat; (c) ice advance/retreat → modification of evaporation rate → change of ice accumulation rate and sea-level height → ice advance/retreat. The relative strength of the three feedback mechanisms determines the characteristics of the modes: growing or decaying, oscillatory or non-oscillatory. The solutions show the generation of growing oscillatory modes with the time scale of 103–104 years in certain parameter ranges.

Modelling the NW India monsoon for the last 40 000 years

Modeling of the monsoon requires at least a hemispheric model. At present such models require inputs such as ice cover and other parameters. The model used in the present work uses only irradiance and volcanic aerosol estimates, modeling the ice cover by first modeling the glacial volume and area. The net annual continental ice accumulation rate must be intergrated to obtain the global glacial volume. The integration may be done analytically and combined with a global volcanicity index time-series to give a volcanically modulated glacial model. This combined integral model matches both the general variation of glacial volume and the short-term events as well. This model is then combined with the calculated irradiance to drive a hemispheric heat-budget model yielding monthly hemispheric surface temperatures. The model matches most known climatic events seen in field data. The model also yields monthly evaporation values for the hemisphere that can be used to estimate the rainfall, much of which is monsoonal. The match with the history obtained from the pollen record is fairly good. Assuming that the N-S component of the northern Arabian Sea summer windfield is proportional to the annual contrast of hemispheric temperature and that the W-E component is proportional to the N-S temperature gradient, one may then model the Late Pleistocence and Holocence wind direction and strength. Preliminary estimates are consistent with much of the field data.

Mass Balance of the Greenland Ice Sheet at Dye 3

AbstractThe mass balance of the Greenland ice sheet at Dye 3 is estimated on the basis of observations of ice thickness, accumulation rate, surface velocities, and surface strain-rates. The calculations indicate a rate of increase of surface elevation of 3 cm/year, with 95% confidence limits of −3 cm/year and +9 cm/year. Previous estimates of the mass balance of the Greenland ice sheet by the same method reported large imbalances; these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth-averaged quantities. The most reliable observations indicate that the interior regions of the Greenland ice sheet are at present thickening at a rate of a few centimetres per year; a contributing cause for this may be the slow thinning of a bottom layer of relatively soft Wisconsin ice.

Mass Balance of the Greenland Ice Sheet at Dye 3

AbstractThe mass balance of the Greenland ice sheet at Dye 3 is estimated on the basis of observations of ice thickness, accumulation rate, surface velocities, and surface strain-rates. The calculations indicate a rate of increase of surface elevation of 3 cm/year, with 95% confidence limits of −3 cm/year and +9 cm/year. Previous estimates of the mass balance of the Greenland ice sheet by the same method reported large imbalances; these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth-averaged quantities. The most reliable observations indicate that the interior regions of the Greenland ice sheet are at present thickening at a rate of a few centimetres per year; a contributing cause for this may be the slow thinning of a bottom layer of relatively soft Wisconsin ice.

Observing Polar-Ice Variability

The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.

Observing Polar-Ice Variability

The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.

Ice accumulation rate in Changme-Khangpu glacier, Sikkim

Vertical profiles of137Cs and210Pb have been determined in a 9 m column of ice from accumulation zone of Changme-Khangpu glacier in north Sikkim valley.137Cs activity varies from 4 to 22 dpm/ L. In many samples210Pb occurs at a level of 20 to 65 dpm/ L which is much higher than the expected fallout value.137Cs and210Pb activities correlate well with each other but not with the dust content. Possibility of210Pb production in the nuclear explosions is discussed. Several peaks appear in the depth profile of137Cs and210Pb which can be matched with Chinese atmospheric nuclear explosions with some phase difference if a uniform ice accumulation rate of 0.7 m per year is assumed since 1969.

A numerical study on cyclic behaviour of polar ice sheets

Possible cyclic behaviour of polar ice sheets is studied with a numerical ice-flow model. The model includes a calculation of bedrock adjustment and temperature field in the ice sheet. Basal water is traced and affects ice-mass discharge. Relaxation oscillations occur only for low ice-accumulation rates (less than 5 cm ice depth per year). For larger ice-accumulation rates, 2 steady-state regimes exist, depending on temperature: an ice sheet frozen to the bottom, and an ice sheet subject to basal melting every where. The volume of the latter is considerably smaller (about 65% of that of the “cold sheet”). In general, the mean basal ice temperature decreases with increasing ice accumulation rate. This is due to the advection term in the heat equation (which has been ignored in earlier studies of ice-sheet surging).

Response of the antarctic ice sheet to a climatic warming: A model study

AbstractIt is generally believed that the increasing CO2 content of the atmosphere will lead to a substantial climatic warming in the polar regions. In this study the effect of consequent changes in the ice accumulation rate over the Antarctic Ice Sheet is investigated by means of a numerical ice flow model. In the model runs, temperature increases linearly with time during 100 years, and is kept constant afterwards.The results indicate that a climatic warming will probably lead to a sea‐level lowering of some tens of centimetres in the next centuries. This is because for Antarctic conditions the increase in snow accumulation exceeds the increase in melting. This estimate does not take into account the effects of possible surging of parts of the Antarctic Ice Sheet and the response of the Greenland Ice Sheet (which may be quite different).

Effect of irregular fluctuations in Antarctic precipitation on global sea level

One of the reasons for the continuing interest in the global sea level is that secular variations may be caused by climatic changes. Such a change could, for example, be an atmospheric warming due to CO2 accumulation. Changes in the amount of ice in the major ice sheets will be reflected in secular variations of sea level; it has, for example, been suggested that ice-shelf thinning may change the drainage of parts of the Antarctic Ice Sheet1. Attempts to monitor climatic change by measuring global sea level will, however, be complicated by random fluctuations of the ice volume in the major ice sheets, themselves the consequence of random variations in the ice accumulation rate. Precipitation rates are highly variable, and this also applies to Antarctica2, which stores most of the continental ice mass. By means of a simple model for ice flow in the Antarctic, together with proxy data on precipitation variability derived from ice cores, I show that long-term sea-level variations with a standard deviation of roughly 5 cm are to be expected on this account. This ‘climatic noise’ is comparable in magnitude with many of the secular effects now being sought.

The reconstruction of the Laurentide Ice Sheet of North America from sea level data: Method and preliminary results

Usually, ice sheets are reconstructed using the flow law of ice and a number of assumptions regarding ice sheet temperature, accumulation rate, and basal sliding resistance. In this paper, an alternative and independent reconstruction method is given. The weight of the ice sheets deformed the earth's surface and its geoid. The amount of deformation, which can be determined from presently emerged shorelines, is related to the thickness of the overlying ice burden. Therefore elevation and age data of sea levels from North America during the past 17,000 yr can be inverted to determine the history of the most recent North American ice sheet (Laurentide ice sheet), assuming the earth behaves as a viscoelastic material. Because of errors in the data and a number of simplifying assumptions, quadratic programing methods were necessary to insure that the predicted ice thicknesses of the past were always greater than at present. The predicted ice sheet is relatively thin (2000–3000 m), which suggests that it was very dynamic and probably not in a steady state. But poor resolution indicates that other ice thickness models, perhaps of greater thickness, may fit the sea level data as well. The fit to the data is, in general, good, and it indicates that a large proportion of the North American sea level data can be explained by glacial isostatic processes. However, at regions distant from the ice sheet (e.g., Florida) the fit is poor, because the assumption that water‐loading effects can be ignored is no longer valid. For regions close to a fluctuating ice sheet margin (e.g., Connecticut) the fit to the data suffers because of the assumption that the areal extent of the ice sheet remained fixed through time. Methods are developed which obviate both of these unrealistic assumptions so that future inversion calculations will be more realistic. Furthermore, proglacial lake deleveling data can be used to extend the data set throughout the interior of North America so that the entire perimeter of the ice sheet is close to the constraining data. The error of fit and resolution may thus be improved considerably.

Icing Rate on Stationary Structures Under Marine Conditions

AbstractIcing on stationary structures is an increasingly serious problem as off-shore drilling operations in the sub-polar regions becomes more popular. Since this problem is less complicated than icing on a ship, an attempt was made to calculate accretion rate using existing data.The rate of ice accumulation R can be calculated from R = CfCcF where F is the mass flux of the water drops and Cf and Cc are the proportions of spray frozen on the surface and coefficient of capture of drops, respectively. Cc can be close to unity for larger drops such as sea spray. Although many other factors may contribute, Cf seems to be a strong function of the air temperature.Mass flux can be written as Vr3 dr where n(r) is the number of drops of radius r in unit volume, V is the wind velocity, ρ is the density of water; n(r) is a function of wind velocity and height of observation. For a stationary structure, the mass flux is primarily dependent upon the wind speed.The ice accretion rate R, calculated using published data on the size distribution of sea spray and using a capture efficiency Cc of I agrees surprisingly well with the diagrams given by previous authors for icing on ships.

Icing Rate on Stationary Structures Under Marine Conditions

Abstract Icing on stationary structures is an increasingly serious problem as off-shore drilling operations in the sub-polar regions becomes more popular. Since this problem is less complicated than icing on a ship, an attempt was made to calculate accretion rate using existing data. The rate of ice accumulation R can be calculated from R = C f C c F where F is the mass flux of the water drops and C f and Cc are the proportions of spray frozen on the surface and coefficient of capture of drops, respectively. Cc can be close to unity for larger drops such as sea spray. Although many other factors may contribute, C f seems to be a strong function of the air temperature. Mass flux can be written as Vr3 dr where n(r) is the number of drops of radius r in unit volume, V is the wind velocity, ρ is the density of water; n(r) is a function of wind velocity and height of observation. For a stationary structure, the mass flux is primarily dependent upon the wind speed. The ice accretion rate R, calculated using published data on the size distribution of sea spray and using a capture efficiency Cc of I agrees surprisingly well with the diagrams given by previous authors for icing on ships.