par Souchez, Roland 
;Lemmens, Michèle
Référence Palaeogeography, palaeoclimatology, palaeoecology, 51, 1-4, page (357-364)
Publication Publié, 1985-10
;Lemmens, Michèle Référence Palaeogeography, palaeoclimatology, palaeoecology, 51, 1-4, page (357-364)
Publication Publié, 1985-10
Article révisé par les pairs
| Résumé : | Different types of calcite deposits may be formed in the subglacial environment. Patchy coatings on polished and striated bedrock pavements occur on outcrops recently exposed by retreating temperature glaciers; coated pebbles and thin limestone crusts in fissures are present in recent basal till exposures. Freezing of meltwater is the main process susceptible to explain most of these carbonate deposits. Partial freezing is accompanied by a rejection of ions from the growing ice and a concentration increase in the residual water, eventually until saturation is reached. Glacier sliding over a protuberance, by pressure-melting on the stoss side and refreezing on the lee side, is likely to be responsible for subglacial carbonate deposition. A fixed bedrock protuberance will give rise to fluted and furrowed coatings on its lee side. A mobile protuberance like a pebble at the ice-bedrock interface, because it is susceptible to rotate while in traction, will have a coating on most of its surface. A present-day case in the Swiss Alps is studied by a comparison of the isotopic composition of calcite deposits, ice and meltwater from the glacier. This study indicates that melting of the basal ice layer, not of glacier ice, is at the origin of the initial water giving rise to the precipitate. Since the basal ice layer is formed in the marginal zone of the glacier, subglacial carbonate deposition is a marginal feature in this case. © 1985. |
