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 ...
Mass-balance and dynamic measurements carried out on glacier de Saint Sorlin since 1957 provide a good opportunity to study the dynamics of this glacier. Ice-flow analysis shows that dynamic changes have been important over the last 40 years and that these changes are not consistent with the concepts usually used in glacier modelling. Present velocities are larger than the 1960 velocities, although the thickness decreased everywhere (10^30 m in the ablation zone). A simple numerical ice-flow model which does not include longitudinal stress gradients has been used to investigate these phenomena. This model allows us to infer the sliding velocity from observed surface and calculated deformation velocities. We conclude that: (1) the sliding velocity cannot be described by Weertman analysis or empirical relations which link the sliding to the thickness and surface slope; (2) the inferred sliding velocity is uniform over at least half of the glacier; and (3) there is no clear link between the sliding process and the quantity of water coming from surface ablation. Furthermore, it may not be reasonable to calibrate model flow parameters from geometry changes because the surface geometry is relatively insensitive to velocity changes over some decades.
A water table appearing every summer where the ice begins, at a gerpth 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 contains from 0 to 0.6% water. The average water content increases gradually with the gerpth because of the heat of gerformation. 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 gerpth of 33 m, a foliation of sedimentary origin gradually gervelops in the ice. Its dip increases regularly to a gerpth of 145 m. At 145 m it jumps sudgernly freom 20° to 40°, then at 170 m freom 40° to 65°, which can be explained 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, shows seasonal fluctuations between 30 and 50 m, then jumps freom a diameter of 1 or 2 mm to 10 or 20 mm between 50 and 80 m. Between 180 and 187 m, the ice is mager of large crystals (3-10 cm diameter; the figure, however, is probably inexact due to a recrystallization of the samples).The very strong sub-vertical orientation of the optic axes of the firn crystals disappears quickly, and freom 66 m on, in ice with large crystals, a fabric of multiple maxima appears (generally, 3 or 4 directions, forming a triangle or a rhombus). On the other hand, in the small crystals that form bands parallel to the plane of foliation, only one direction of preferential orientation can be seen, or two close to one another. Crystals of intermediate size (10 to 50 mm) generally have two directions of preferred orientation at an angle of approximately 50° to one another. No matter how big the crystals are, the angle between the most commonc-axis orientation and the vertical does not change freom 60 to 170 m gerpth.
Glacial mass-balance reconstruction for a long-term time-scale requires knowledge of the relation between climate change and mass-balance fluctuations. A large number of mass-balance reconstructions since the beginning of the century are based on statistical relations between monthly meteorological data and mass balance. The question examined in this paper is: are these relationships reliable enough for long-term time-scale extrapolation? From the glacier de Sarennes long mass-balance observations series, we were surprised to discover large discrepancies between relations resulting from different time periods. The importance of the albedo in relation to ablation and mass balance is highlighted, and it is shown that it is impossible to ignore glacier-surface conditions in establishing the empirical relation between mass-balance fluctuations and climatic variation; to omit this parameter leads to incorrect results for mass-balance reconstruction in the past based on meteorological data.
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