4.1 Introduction
Two types of ice were tested in this study. Alongside the samples of Greenland ice described in chapter 3 samples of laboratory ice were also tested under the same experimental conditions. Laboratory ice differs significantly from glacier ice in a number of ways. Unlike glacier ice laboratory ice has a well defined grain size and the orientation of the c-axes is completely isotropic. Laboratory ice is also produced from distilled, de-aerated and de ionized water and as such contains only minimal amounts of impurities and air bubbles. There have been numerous studies carried out to determine the mechanical response of laboratory ice under a variety of experimental conditions and consequently the behaviour of laboratory ice and the mechanisms responsible for its deformation are well documented. Nevertheless, there are a number of reasons why it was felt important to perform tests on laboratory ice alongside the samples of glacier ice. Firstly, because of the difference in its structure and composition, laboratory ice provides a useful reference frame for assessing the degree to which deformation is affected by anisotropy, grain size variations, impurities etc. Secondly since the acoustic interrogation techniques employed in this study are unique in their application to ice, a study based on laboratory ice alone would provide original data on the deformation mechanisms active. Also since the modes of deformation of laboratory ice are well documented, their typical acoustic signatures can be more easily recognized and in turn modes of deformation of glacier ice can be inferred by comparing acoustic signatures.
In this chapter the method of producing laboratory ice is outlined and the differences between this ice and glacier ice are highlighted. The quality of the samples is an important consideration and procedures for assessing ice quality are described. The method of preparing both laboratory ice and glacier ice for testing is also described together with a technique for making thin sections.
4.2 Laboratory ice
To perform a systematic investigation the results of which can be readily compared with other studies, it is necessary to have reproducible samples of comparable quality to those samples used in other investigations. The laboratory ice used in this study was
produced following the method outlined by Rist (1989) which itself was based on a method outlined by Jacka and Lile (1984). A thin section of undeformed laboratory grown ice is shown in Fig 4.1.
The ice is produced from distilled, de-aerated and de ionised water, has a mean grain size of 1mm and has an isotropic distribution of c-axes. Cylindrical samples 25mm in diameter and 65mm in length were used in the final testing programme. Jones and Chew (1981) have shown that the mechanical integrity of experimental ice samples is maintained providing there are more than 12 grains across the diameter. It is also recognized that sample length to diameter ratio should be between 2.5:1 and 3:1 (Jaeger and Cook 1979). Thus the sample size to grain size ratio of the laboratory ice is acceptable as is the length to diameter ratio.
Fig 4.1 Thin section of laboratory grown ice, grain size 1mm. The section is 40mm in diameter.
4.2.1 Laboratory ice production
Distilled water was provided by the geochemistry lab in the geological sciences department at UCL. To remove any remaining impurities and to de ionize the water it was passed through a Seradest S600 deioniser. The water was then sealed in a bell jar which was connected to a vacuum pump and de-aerated for approximately 1 hour. The de-aerated water, kept under a vacuum, was allowed to cool in the cold room until it reached 0°C
Ice seeds of the required grain size were prepared by freezing quantities of distilled de ionized water and then crushing the ice using a commercially available ice crusher. This created large angular grains of between 5 and 10mm diameter, these grains were ground to a finer size using a coffee grinder and a system of sieves were then used to separate the correct size fraction. For a grain size of 1mm a sieved fraction in the range of + 1.400mm -0.800mm was used.
With the ice seeds prepared and the water cooled to around 0°C, the seal on the bell jar was broken and an approximately equal volume of ice seeds was mixed with an equal volume of water to produce an ice/water slurry. The slurry was then spooned into a tall perspex tube, approximately 350mm long with an internal diameter of 40mm. As the tube was filled a long thin piece of steel was used to agitate and remove any large bubbles that formed. Finally, with the tube approximately 90% full, a brass cap was placed into the top of the tube and the slurry was compressed using a purpose built screw driven press. In this way excess pore water in which further bubbles may form was driven out and, on removal from the press, could be poured away. The sample was allowed to freeze thoroughly for a few days prior to removal. This was achieved by gently warming the perspex tube with the hands and allowing the cylinder of ice to slide out. On removal the samples were seen to be clear for the most part, with a hazy centre. This haze was caused by tiny air bubbles which for some reason formed along the central axis of the core, the size of the bubbles was estimated to be between 0.05 and 0.1mm from thin section observations. Schulson, Hoxie and Nixon (1989) have demonstrated that defects such as pores, bubbles or small cracks are not strength limiting features in polycrystalline ice providing they are significantly smaller than the grain size of the sample. Chemical impurities are however known to significantly affect the flow of ice and as such purity of the water and cleanliness of the equipment was an important consideration when preparing the samples.
4.2.2 Preparation of laboratory ice for testing and quality control
After removal from their moulds the 40mm diameter samples were machined down to 25mm diameter on a miniature lathe. The ends of the sample were also smoothed and made parallel. Cylinders of ice were removed from their tubes and machined as close as was practically possible to the test time. Since the tests themselves were usually of the order of several hours, removal and machining of the sample was done the day before with the samples being stored overnight in a storage freezer at -40° C. Samples were stored in small sealed plastic bags with a quantity of ice scraped from the outside of the pressure vessel. This prevented sublimation of the sample during the brief storage period.
A number of observations were made to assess the quality of the laboratory ice sample prior to testing. A visual inspection of the sample was carried out, any samples containing large visible voids, bubbles or visible cracks was rejected. The sample was accurately measured and weighed and the bulk mean density was calculated. The density of the samples was fairly consistent at 915 + 1 kgm ^, and can be regarded as a measure of the small amount of air included in the sample. Measurements were also made of the meltwater conductivity of undeformed samples. Values were comparable to that of distilled, de-aerated and de ionized water (0.50 to 0.75 ^mhos/cm at 20°C) which indicated that no contamination had taken place during the production of the sample. Occasionally thin sections were taken off the ends of newly prepared samples as a further method of assessing the quality, however this was a time consuming method and it was soon realised that any sample that was not up to standard would be revealed by visual inspection, density or conductivity measurements.
4.3 Glacier ice preparation.
The glacier ice tested was acquired from the Greenland Ice-core Project. In all 13 samples were made available for testing from various palaeoclimatological and structural horizons within the ice sheet. Since completion of the coring programme
in July 1992, the ice has been stored in cold rooms maintained at approximately -3(PC
at the Geophysics Institute in Copenhagen. A full list of samples, depths and the horizons from which they originate is given in table 3.1. Sections were cut from the core that were large enough in cross section to accommodate the 25mm diameter core barrel and long enough ( =» 90mm) to fit in the coring jig and at the same time allow
approximately 70mm of core to be taken. The samples were then wrapped in bubble wrap and packed in a large insulated box, where they were surrounded with chemical refrigeration elements and scraps of snow and ice from the cold room. The time taken to travel between the Copenhagen cold rooms and those at UCL was no longer than 6 hours, whilst this method of transporting ice is regularly used for journeys of 24 hours or more. On arrival at UCL the samples were removed from the insulating box and transferred to a storage freezer maintained at -40°C.
Cores of 25nun diameter were cut from these samples using a purpose built coring barrel mounted on a Bosch power drill. The power drill was in turn mounted on a coring stand in which the ice could be clamped. The entire assembly was temporarily housed inside one of the cold rooms at UCL, however once coring of the samples was completed the coring stand had to be removed due to limitations in cold room space. The ends of the cored sample were then machined flat and parallel on the miniature lathe in the same manner as the laboratory ice. The samples were then sealed in small air tight plastic bags together with a quantity of snow to prevent sublimation of the core. These bags were then stored in the storage freezer at about -40°C prior conditioning of the sample (see section 5.3). No cracking of the sample was observed during sample preparation.
After preparing the core a small block of Greenland ice remained, this was used to produce thin sections of undeformed samples (see chapter 3). Small scraps were also melted down and measurements made of the melt water conductivity. Values of conductivity varied between 7.5 and 14. l^mhos and, though significantly higher than the values obtained for laboratory ice, they did not show any systematic variation with palaeotemperature or any physical ice properties. Finally, after testing, a visual inspection was made of the sample noting any interesting features and thin sections were made.
4.4 Thin section preparation
Thin sections were produced using a technique adapted from the technique used at the Alfred Wegener Institute in Bremerhaven (H.Oerter pers. comm). A disc approximately 1cm in length was taken from the desired region of the core (usually the central region) and mounted on a glass slide using water cooled to 0°C. The top face of the disc was then worked smooth on a base sledge microtome housed in the cold rooms at UCL. This smooth face was then held against a second glass slide and
lightly forced against this slide using a G-clamp. Care must be taken not to be too heavy handed with the G-clamp as this would cause the disc of ice to fracture. Using a pipette, water at 0°C is laid onto the second slide around the ice and, since the water freezes in a short period of time, the ice is adhered to the glass slide. Providing it is clamped strongly enough no water will pass underneath the disc of ice, but the disc will be firmly frozen in place about its rim. The first glass slide used can be easily removed by resting it on a piece of brass or some other metal which is kept at room temperature and brought into the cold room solely for this purpose. The upper surface can then be microtomed to a thickness of approximately 200/>im. The advantage of this method over that described by Rist (1989) is that both sides of the finished section are worked smooth with the microtome resulting in a clearer thin section and no cyano acrylate adhesive is used, this was found to form bubbles between the thin section and the glass slide. To prevent sublimation of the slides, they are stored in small sealed plastic bags together with a quantity of waste ice in storage freezers maintained at -40°C.