Study of an Ice Core to the Bedrock in the Accumulation zone of an Alpine Glacier

Abstract

ABSTRACT. A water table appea ring every summer where the ice begins, at a depth of approximately 30 m, accelerates the transformation of firn into ice during the summer (80% of the ice formed every year appears in less than 2 months ). The ice formed in this way con tains from 0 to 0.6 % water. The average water content increases gradually with the depth because of the h eat of deformation. But, near bedrock, between 180 and 187 m, the permeability of the blue ice is such that the water content drops (0.3 % as Compared to 1.3 % between 160 and 180 m ) .From a depth of 33 m , a foliation of sedime ntary origin gradually develops in the ice. I ts dip increases regularly to a d epth of 145 m. At 145 m it jumps suddenly from 20° to 40°, then at 170 m from 40° to 65°, which can be expl a in ed by old modifications in the bergschrund. This foliation disappears near bedrock ( 180-187 m ), where there are no bubbles in the ice.The average size of an ice crystal increases slowly in the firn, sh ows seasonal fluctu a ti ons between 30 and 50 m, then jumps from a diameter of I or 2 mm to 10 or 20 mm between 50 and 80 m. Between 180 and 187 m, the ice is made of large crystals (3-10 cm diameter; the figure, however, is probably inexact due to a recrystallization of the samples) .The very strong sub-ve rti ca l orientation of the optic axes of the firn crys tals disappears quickly, and from 66 m on, in ice with large crystals, a fabric of multiple maxima appears (genera ll y, 3 or 4 directions, forming a tri a ngle or a rhombus) . On the other h and, in the small crystals that form bands parallel to the plane of foliation, only one direction of preferential orie ntation can be seen, or two close to one another. Crystals of intermed iate size ( 10 to 50 mm ) generally have two directions of preferred orie nta tion at an a ngle of approximately 50° to one another. No matte r h ow big the crystals a re, the angle be twee n th e most common c-ax is orientation and the vertica l does not ch ange from 60 to 170 m d epth.R ESU,"L Etude d'une caroUe de glace jllsqll'au lit dons la zone d'accumlllation d'WI glacier tempire.Un ni veau aq uifere appa ra issa nt chaque ete au contact de la gl ace, vers 30 m d e profondeur, a ccelere la transformation d u neve e n g lace durant l'ete (80% de la glace formee chaq ue a nnee apparait en 2 mois) . La glace a insi fo rm ee renferme d e 0 it 0,6°{, d 'ea u liquid e. La teneur moye nn e e n ea u liquide a ugm e nte progressivem e nt a vec la profondeur, par su ite de la chaleur d e deformation. M a is a u contact du lit, e ntre 180 et 187 m , la permeab ilite d e la glace ble ue est tell e que la te neur en eau devi ent faible (0,3 % contr'e 1,3 % entre 160 et 180 m ) .Dans la gl ace, it partir d e 33 m d e profondeur, se developpe progressivement une foliation d'origine sedimenta ire. Son pendage eroit regu lierement jusqu'it 145 m d e profondeur. A 145 e t '70 m, il passe brusqueme nt de 20° it 40° puis d e 40° it 65°, ce qu 'on explique par d'aneiennes modifi ca tions de la rim aye. Ce tte ...

Water-Content of Glacier-Ice: Limitations on Estimates from Velocity Analysis of Surface Ground-Penetrating Radar Surveys

“…Studies published since that date also fall within this range. The highest values all result from remote radar studies, while measurements on cores tend to show water-contents around 0.6-1.7% (Raymond and Harrison, 1975;Vallon et al, 1976). Measurements using radar will integrate all water bodies smaller than the radar resolution, typically of ,decimeters, and will therefore be comprised of both water contained within the vein network and larger water bodies, channels and linked fractures.…”

Spatial variability in water content at the cold‐temperate transition surface of the polythermal Storglaciären, Sweden

“…The methods used to estimate the water content in ice could be divided into thermodynamic and remote sensing methods. The thermodynamic methods involve the use of an adiabatic calorimeter on retrieved ice cores [ Dupuy , 1970; Vallon et al , 1976; Duval , 1977] or in situ calorimetric measurements, where the water content is determined from the propagation speed of an artificial freezing front in the ice and from the boundary condition at the freezing front [ Hutter et al , 1990; Zryd , 1991]. The remote sensing techniques often use radar wave velocity analysis [ Macheret et al , 1993; Moore et al , 1999; Murray et al , 2000a] or the backscattered power in radargrams [ Bamber , 1988; Hamran et al , 1996; Macheret and Glazovsky , 2000] to estimate water content.…”

Physical properties of the P96 ice core from Penny Ice Cap, Baffin Island, Canada, and derived climatic records

“…The fabric of the superimposed ice near the surface shows a preferred vertically oriented c axis distribution (Figure 6). Since a similar c axis distribution was found in firn near the surface of temperate glaciers [ Vallon et al , 1976], it is plausible that the c axis distributions in the firn are kept in the superimposed ice by growth of the firn grains soaked in meltwater [ Shumskii , 1964]. Here, the growth has been explained in terms of the discrepancy of crystal sizes [ Colbeck , 1973].…”

“…As a result, the average crystal size increases with the time the snowpack is soaked in meltwater [ Colbeck , 1973]. That part of the snowpack that is soaked until late in the melt season and, therefore, for longer consequently has the largest grains [ Jonsson and Hansson , 1990; Vallon et al , 1976]. This relationship can be used to detect annual layers in the ice.…”