ALBERT

Description: A method has been developed for measuring N2O concentrations in the air extracted from the bubbles contained in ice cores. The air extraction is performed by cutting the ice into very small pieces with a rotating knife, in a controlled atmosphere. The N2O concentrations are measured by gas chromatography. The complete original procedure will be discussed, and the results of the different experimental tests given, with a discussion of the uncertainties. This method has been used to perform about 40 measurements on Antarctic ice samples. Ten air samples from the D57 core date approximately from the beginning of the seventeenth and twentieth centuries. The others were taken from the Dome C core and date from the Holocene and the period around the Last Glacial Maximum. The D57 results are in agreement with those of Pearman and others (1986), leading to a similar pre-industrial N2O level (270-290 ppb volume). Furthermore, our Dome C results suggest that during the Last Glacial Maximum atmospheric N2O content was not drastically different from the recent period.

Description: There is at present a need to develop a better technique for measuring the rate of icing on structures such as, for example, overhead transmission lines. For aircraft and helicopter icing, the most widely used method of measurement is the rotating cylinder. However, for measuring the icing of structures, this method is difficult to apply and also less accurate due to lower wind velocities. Different approaches are now being developed using fixed cylinders.Icing tests were conducted with fixed and rotating cylinders in a wind tunnel. The rate of icing was obtained through measurements of volume, accretion cross-section and time of deposition. Tests were made using five different liquid water contents and droplet diameter spectra, and four cylinder diameters, keeping the wind velocity and temperature constant. The rate of icing is presented as a function of the diameters of the fixed and rotating cylinders for each of the liquid water contents tested. Results indicate that at lower wind velocities the accretion rate is overestimated for the smaller rotating cylinders. This difference is probably due to the variation of the collection efficiency with diameter. From these results it is suggested that the rate of ice accretion on structures should be based on at least two fixed cylinders of different small sizes in order to take into account the effect of the collection efficiency.

Description: There is at present a need to develop a better technique for measuring the rate of icing on structures such as, for example, overhead transmission lines. For aircraft and helicopter icing, the most widely used method of measurement is the rotating cylinder. However, for measuring the icing of structures, this method is difficult to apply and also less accurate due to lower wind velocities. Different approaches are now being developed using fixed cylinders. Icing tests were conducted with fixed and rotating cylinders in a wind tunnel. The rate of icing was obtained through measurements of volume, accretion cross-section and time of deposition. Tests were made using five different liquid water contents and droplet diameter spectra, and four cylinder diameters, keeping the wind velocity and temperature constant. The rate of icing is presented as a function of the diameters of the fixed and rotating cylinders for each of the liquid water contents tested. Results indicate that at lower wind velocities the accretion rate is overestimated for the smaller rotating cylinders. This difference is probably due to the variation of the collection efficiency with diameter. From these results it is suggested that the rate of ice accretion on structures should be based on at least two fixed cylinders of different small sizes in order to take into account the effect of the collection efficiency.

Description: A method has been developed for measuring N2O concentrations in the air extracted from the bubbles contained in ice cores. The air extraction is performed by cutting the ice into very small pieces with a rotating knife, in a controlled atmosphere. The N2O concentrations are measured by gas chromatography. The complete original procedure will be discussed, and the results of the different experimental tests given, with a discussion of the uncertainties.This method has been used to perform about 40 measurements on Antarctic ice samples. Ten air samples from the D57 core date approximately from the beginning of the seventeenth and twentieth centuries. The others were taken from the Dome C core and date from the Holocene and the period around the Last Glacial Maximum. The D57 results are in agreement with those of Pearman and others (1986), leading to a similar pre-industrial N2O level (270-290 ppb volume). Furthermore, our Dome C results suggest that during the Last Glacial Maximum atmospheric N2O content was not drastically different from the recent period.

Description: The thickness of glaciers in High-Mountain Asia (HMA) is critical in determining when the ice reserve will be lost as these glaciers thin but is remarkably poorly known because very few measurements have been made. Through a series of ground-based and airborne field tests, we have adapted a low-frequency ice-penetrating radar developed originally for Antarctic over-snow surveys, for deployment as a helicopter-borne system to increase the number of measurements. The manoeuvrability provided by helicopters and the ability of our system to detect glacier beds through thick, dirty, temperate ice makes it well suited to increase greatly the sample of measurements available for calibrating ice thickness models on the regional and global scale. The Bedmap Himalayas radar-survey system can reduce the uncertainty in present-day ice volumes and therefore in projections of when HMA's river catchments will lose this hydrological buffer against drought.

Description: Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams. Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

Description: Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams.Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

Description: Both in the interior of West Antarctica and on Ross Ice Shelf the ice column is dominated by ice with a distinct preferred c-axis orientation. An understanding of the dynamics of the West Antarctic ice sheet requires that we know the gross crystalline structure that characterizes each of its glaciological regimes (i.e. ice sheet, ice stream, and ice shelf). An important question is whether the strong fabric of the interior ice is preserved when this ice passes through the zone in which it is accelerated from sheet flow to stream flow, a zone that must be marked by strong longitudinal extension. Using generalized inverse techniques we have inverted seismic-reflection travel times observed at Upstream Β camp (on Ice Stream B) to obtain the gross crystalline structure of the ice column at that location. We find that the strong fabrics of the interior are indeed destroyed; only a slight preferred orientation remains. The evidence at Upstream Β camp is particularly strong because shear waves of both polarizations, which are particularly sensitive indicators of anisotropy, were analyzed as well as P-waves.

Description: Detailed seismic short-refraction profiling was conducted on Ice Stream Β (UpB) during the 1983–84 austral summer. A new high-resolution data logger, developed at the University of Wisconsin, recorded both compressional- and shear-wave arrivals. We report here on P-wave and S-wave profiles recorded along a line parallel to the axis of the ice stream. Source-receiver separations up to 720 m yielded seismic velocity-depth curves to below the firn-ice transition zone (slightly greater than 30 m at UpB).For the compressional-wave profile, geophones were separated by 2.5 m, which yielded a velocity-depth curve with a granularity of ∼1 m. The corresponding density-depth curve agrees well with direct density measurements obtained from a core extracted nearby (Alley and Bentley 1988, this volume). Discontinuities in the velocity gradient do not appear at the “critical densities” as they did at Byrd Station, Antarctica, and elsewhere (Kohnen and Bentley 1973, Robertson and Bentley 1975).Two shear-wave profiles were recorded, both with geophone spacings of 5 m, one with longitudinal polarization (SV) and the other with transverse polarization (SH). There is a marked difference in velocity between the SH and SV waves, particularly in the shallow firn. We suggest that a strong vertical shape-and-bonding fabric in the shallow firn, as observed in cores collected at UpB, would account for this disparity.

Description: The air found in polar ice becomes isolated from the atmosphere during the transformation of snow into ice. The total gas content, V, measured on selected ice samples depends theoretically on the atmospheric pressure and temperature prevailing at the ice-formation site and also on the volume, Vc, of the firn pores at the time they become isolated from the atmosphere in newly formed ice.In spite of scattered individual values, a clear decrease in mean v-results has previously been observed for increasing elevations at the ice-formation site. Assuming that the pores close off at atmospheric pressure, this decrease primarily reflects the pressure-elevation gradient and typically only about 10% of the decrease can be attributed to the temperature-elevation gradient or changes in Vc.Measurements of the total gas content of Antarctic ice cores are potentially the most direct and sensitive indicator of the elevation at which the ice was formed and provide a unique tool for reconstructing the past elevation of the ice-sheet surface. Nevertheless, to make such a reconstruction with confidence, we need first to evaluate the individual contributions of the different parameters which should influence Vc. Along this line, a stage was reached with the semi-empirical observation of a quasi-linear correlation between mean Vc and temperature, although the data points on which the correlation is based are relatively scarce. Furthermore, confidence in the paleo-interpretation of V-measurements is dependent also on an understanding of the scatter in individual V-values generally observed for a given ice-core section.New measurements of V are presented, which permit a better and more detailed description to be made of the changes in V with atmospheric pressure and elevation under approximately present-day conditions. As a first step, scattering in the values of individual V-measurements of a single core section is investigated. The experimental method used for this work is basically the same as the one used for establishing the relationship between V and elevation, as given by Raynaud and Lebel (1979). Generally measurements performed on the same horizontal layer (about 3 cm thick) reproduce the same result within the experimental uncertainty (which is ±1%). On the other hand, measurements performed on adjacent horizontal layers may show V-differences as high as 20-25%. This confirms that the variability of the V measurements that occurs over short intervals in the core is not due to experimental uncertainties and should mainly reflect changes in Vc.The main reason for making this new set of measurements was to increase significantly the number of sites investigated in the study of the relationship between V and elevation. The work of Raynaud and Lebel (1979) was based on six sites. The results for 14 sites (in Antarctica, Greenland, and on Mount Logan in Canada) are now available, with elevations ranging from about 340 to 5300 m and temperatures in the range −12.4° to −55.4 °C. The results confirm a general decrease in Vc with temperature. But at least one site shows mean V-values which are clearly incompatible with this general trend.Finally, before proceeding further with studies of the distribution of V versus site parameters, we need to correct our V-measurements for the loss of gas which occurs when cutting the ice samples (and consequently opening air bubbles at the sample surface). We have developed, for this purpose, a technique which calculates random distributions of bubbles in the samples and counts the cut bubbles. This calculation depends on bubble shape and size as well as on sample volume and V. In the case of the samples used in the present study, between 3 and 10% of gas is typically lost during cutting. Bubble-size measurements have yet to be made before an accurate correction can be applied to the measurements presented here.