Michèle Lemmens

Flow-induced mixing in the GRIP basal ice deduced from the CO2 and CH4 records

This paper documents a larger degree of mixing in ice near the bottom of an ice sheet than described, or suspected, previously. It shows, thanks to favourable circumstances due to CO2 and CH4 production underneath the ice, that flow-induced mixing within the basal ice has taken place at the scale of a few centimeters in the GRIP core. Such a mechanism must be considered when interpreting the ice properties in the bottom part of ice sheets and must be taken into account as a potential process of layer disruption in the low levels of the Central Greenland ice cores.

Subglacial carbonate deposition: An isotopic study of a present-day case

Abstract 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.

Stable isotopes in the basal silty ice preserved in the Greenland Ice Sheet at summit; environmental implications

Modelling ice sheet behaviour in the context of climatic changes depends on initial and boundary conditions which can be better defined by studying the composition of basal ice. This study deals with basal ice reached by deep drilling at Summit in Central Greenland (GRIP core). The isotopic composition of this ice indicates that ice formed at the ground surface in the absence of the ice sheet largely contributed to its formation. The basal silty ice is a remnant of a growing stage of the ice sheet, possibly the original build up.

Very low oxygen concentration in basal ice from Summit, central Greenland

Oxygen concentration as low as 3% has been detected in the basal silty ice of the GRIP core. Such values were never observed in ice from ice sheets. They are most probably the consequence of organic matter oxidation in ice developed in a peaty deposit when the Greenland Ice Sheet was not present at the site. Flow-induced mixing has further incorporated this ice into glacier ice during the ice sheet build up. The part of the local ice component in the mixing process diminishes with the distance from the bed. This is the process which explains the oxygen profile.

Reconstruction of basal boundary conditions at the Greenland ice sheet margin from gas composition in the ice

Abstract Modelling the response of the Greenland Ice Sheet to a temperature increase is an essential step towards estimating the climatic changes that could affect a large area of the Northern Hemisphere in the near future and is heavily dependent on accurate evaluation of the boundary conditions at the ice-bedrock interface. Here we use a study of gas composition in the basal ice from West Greenland. This study shows that changes in basal thermal zones occur towards the border in the marginal zone of the ice sheet. First, slight melting at crystal boundaries and vein water squeezing occurs when the ice could still be below the pressure melting point. Then, where the melting point is reached and more meltwater is produced, sliding by regelation occurs. Closer to the margin, partial freezing and ice accretion takes place, the melting point being maintained at the bed because of the latent heat release. Pressure effects induce a certain amount of decoupling. Such changes in basal flow conditions can in turn promote ice sheet thinning at the margin. A temperature increase of the air along the edge of some ice sheet areas in West Greenland, where decoupling occurs, would thus give rise to higher ablation rates and would be likely to lead to fast ice retreat.

Relations entre concentration en cations dissous et debit de l'emissaire du glacier de Tsijiore Nouve (Valais)

Abstract Meltwaters were collected in the proglacial zone of the Tsijiore Nouve glacier near the snout. In the bilogarithmic coordinate system used, concentration of each major cation (Na, K, Ca, Mg) is a negative linear function of discharge. These relationships are explained by the effects of hydrologic conditions on cationic diffusion into the water from a sorbed state on the fine detritic fraction, and not by a mixture of ground water rich in dissolved cations with meltwaters of very weak concentration.

De l'exportation du sodium et du potassium par les eaux de fusion d'un glacier alpin en region calcaire

Summary Glacial meltwater samples were collected at the border of the Tsanfleuron glacier in Switzerland. After filtration in the field in order to avoid the influence of desorption on their chemical composition, the samples were analyzed for the four major cations by a atomic absorption spectrophotometer. The alkalines sorbed on suspended and bottom sediments were also determined. The Na and K content of the meltwater is mainly explained by desorption which is particularly favored when hydrological conditions are such that a large spreading of water prevails in the outwash zone. The transport of these alkaline cations sorbed on suspended particles is not to be neglected. A difference in Na and K quantities sorbed on the suspended load and on the bottom sediments was noticed. It can be related to the influence of the dissolved cations.