The origin, composition and behaviour of basal ice at Nigardsbreen, Norway
The basal ice of temperate-type glacier Nigardsbreen is described in terms of a sediment facies model-after Lawson (1979). The ice is characterised using data obtained from sedimentological, chemical and stable isotopic analyses. These indicate that ice nearest the glacier bed is distinguishable from the ice above descriptively and genetically. In particular, the measurement of paired Oxygen and Hydrogen isotope contents, in accordance with theory proposed by Jouzel and Souchez (1982), provides a means of separating the basal facies ice component, formed by subglacial/basal regelation processes, from ice formed by 'normal' non-fractionating firnification processes. In the latter case the basic isotope content is close to that of the original precipitation whilst in the former this precipitation (meteoric) relationship is modified when ice is subjected to more or less predictable levels of fractionation during regelation. Some 1-1.5m of the basal ice, both early and late in the melt season, was found to originate from regelation. Within the basal facies ice, both stratified debris-laden layers comprising fine gravel-dominated debris up to concentrations of 1kg/l (c30% by volume) and clear intercalated ice types, reflect the regelation processes involved in debris entrainment and the freezing on of water. Thick (up to 40cm) layers of clean macrocrystalline 'regelation1 ice are considered to have been recrystallised. Coisotopic analysis may be used to highlight the fact that mixing of waters of differing origins occurs at the ice-bed interface and that the prediction of the extent of refreezing proposed by Jouzel and Souchez appears untenable. Major cation chemical analysis, undertaken to obtain data for use in the Souchez and Tison (1981) model of basal ice formational processes, which purports to discriminate between ice formed from water squeezed through the basal ice and water flowing, or ponded, at the bed, proved inconclusive. It appears that considerable desorption from clay minerals occurs and that squeezing of basal waters through the stratified facies may occur. In the absence of a clear statement of mineralogical composition in the basal debris clay fraction, which was attempted at Nigardsbreen, the Souchez and Tison model is seen to require development before it will reveal new information about basal processes. Measurements of temperature, sliding velocity and strain within subglacial cavities, in conjunction with observations on changes in sedimentological and isotopic conditions throughout the basal facies zone, are used to test and vindicate a descriptive model of the character, origin and evolution of the basal ice. The model is in turn proposed for further test.