Liste des publications (1995) |
Abstract : 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.
Abstract : A better knowledge of boundary conditions near a grounding line is critical for understanding the behaviour of ice shelves and floating glaciers. We show here that significant information can be gained from a high resolution isotopic and textural study of marine ice accreted at the bottom of a floating glacier near its grounding line. Two different types of marine ice have been found. Type 1 is bubble-and debris-free ice with properties which, we believe, can be explained by intrusion of brackish water in open basal fissures. Closing of the fissures by progression of a freezing front from the sides is precluded and filling by frazil ice is favoured. Type 2 is made of thin clear ice and debris layers which are thought to have formed when a subglacial water-filled sediment enters into contact with sea water and is subjected to freezing under a double diffusion process. The paper also stresses that, in a dD-d18O diagram, the alignment of marine ice samples on a mixing line does not, necessarily, imply a mixture of continental water and sea water in varying proportions.
Abstract.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.
Antarctic climate during the Pliocene has been the subject of considerable debate. One view holds that, during part of the Pliocene, East Antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains. An alternative viewpoint argues for the development of a stable polar ice sheet by the middle Miocene, which has persisted since then. Here we report the discovery of buried glacier ice in Beacon valley, East Antarctica, which appears to have survived for a least 8.1 million years. We have dated the ice by 40Ar/39Ar analysis of volcanic ash in the thin, overlying glacial till which, we argue, has undergone little (if any) reworking. Isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet. We suggest that stable polar conditions must have persisted in this region for at least 8.1 million years for this ice to have avoided sublimation.