A freezing slope, distinct from that of precipitation, is displayed on a δD–δ18O diagram by basal ice in different circumstances. However, if the subglacial reservoir allowed to freeze is mixed in ...the course of time with an input having a lighter isotopic composition, basal ice cannot be distinguished from glacier ice in terms of slope. Such a situation is encountered at the base of Grubengletscher and is indicated by a computer simulation using the open-system model of Souchez and Jouzel (1984). Suggested implications for the paleoclimatic interpretation of deep ice cores recovered from the bottom part of polar ice sheets are given.
A detailed dielectric profiling (DEP) conductivity profile ( sigma arrow left ) measured in the 6 m of the basal silty ice sequence from the Greenland Ice Core Project (GRIP) ice core (Summit, ...Central Greenland) is presented and compared to previous multi-parametric studies. DEP conductivities span the whole glacial-interglacial range observed higher up in the GRIP core (9-25 mu S m super(-1)). Values in the bottom meter of the sequence reach the level of some of the highest peaks from Holocene volcanic layers in the core (33 mu S m super(-1)). On a steady increase of the sigma arrow left values down the sequence are superimposed large fluctuations "inphase" with other variables measured in the core such as delta super(18)O, debris content, or gas compositions in CO sub(2) and CH sub(4). Analysis of the type and strength of intercorrelations shows that the controlling variable for the DEP signal must be closely related to the gas content and composition of the ice. Plausible candidates for this causality link are investigated. Enhancing of the sigma conductivity by CO sub(2) and CH sub(4) encaged in the ice lattice as gas hydrates is ruled out since these are nonpolar clathrates of structure I, known as having negligible impact on the orientational stability of the water molecules under ac currents. NH sub(4super+) is proposed as the best candidate since it has been shown to enhance DEP conductivities by introducing Bjerrum defects in the ice lattice and since it could have been initially present partly as gaseous NH sub(3) in the ice. This proposition is supported by the NH sub(4super+) profile in the basal ice sequence. Using calibration curves from higher up in the core, it is shown that sigma is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice. The origin of the NH sub(4super+) signal is discussed in the light of organic acid profiles (formate, acetate, and oxalate). It appears that the most likely source is local degradation of biological residues, which supports the hypothesis that part of the basal ice was formed locally, in the absence of the present-day ice sheet.
Polar ice composition and global change. Polar ice has been recently widely used to reconstruct environmental conditions of the past. For example, the oxygen or hydrogen isotopie composition of the ...ice can give the temperature of the site of formation and the carbon dioxide content in the occluded bubbles, the composition of the atmosphere at the time of occlusion.
Ice composition can independantly be used to better define the initial and boundary conditions, namely the conditions at the ice-bedrock interface, of polar ice sheets. Such an investigation is important for the modelling of the climate system.
Ice composition can also be used to collect information on regulation mechanisms affecting the cryosphere-ocean system.
Our research team is deeply involved into the two last approaches.
The basal ice layer of Alpine glaciers, the thickness of which can reach more than one metre, generally appears as a sequence composed of layers of bubble-poor ice 0.5-3 cm thick and dirt layers ...containing dispersed rock fragments. Locally, debris-free bubbly ice is also present in the sequence. This basal layer exists on the lee side of rock protuberances or along the marginal part of the glacier. Three geochemical characteristics shed some light on its origin:
(a)
The high calcium content of basal ice sliding over limestones from the north side of Glacier do Tsanfleuron can be successfully predicted from considerations of the phase relations in regelating subglacial solutions in approximate equilibrium with subglacial calcite deposits.
(b)
The (Na + K)/(Ca+Mg) ratio of bubble-poor basal ice from Glacier de Tsijiore Nouve which flows over gneissic rocks is significantly higher than this ratio in other samples taken in the same environment (glacier ice, melt water). This may be explained by refreezing of squeezed water, the ionic composition of which is governed by selective diffusion of ions during squeezing and/or by the water being forced through mud layers as demonstrated by us at the base of Glacier d'Argentière (Souchez and Lorrain. 1975).
(c)
The Lead-210 activity per kg of basal ice from the south side of Glacier de Tsanfleuron is greater than that of the glacier ice immediately above, which indicates that it is younger despite percolation effects. This can be explained if recent ice accretion has occurred at the base of relatively old glacier ice. Accretion of blowing snow has been found to occur under the southern margin of the glacier.
Isotope studies show that the Vostok ice core consists of ice refrozen from Lake Vostok water, from 3539 meters below the surface of the Antarctic ice sheet to its bottom at about 3750 meters. ...Additional evidence comes from the total gas content, crystal size, and electrical conductivity of the ice. The Vostok site is a likely place for water freezing at the lake-ice interface, because this interface occurs at a higher level here than anywhere else above the lake. Isotopic data suggest that subglacial Lake Vostok is an open system with an efficient circulation of water that was formed during periods that were slightly warmer than those of the past 420,000 years. Lake ice recovered by deep drilling is of interest for preliminary investigations of lake chemistry and bedrock properties and for the search for indigeneous lake microorganisms. This latter aspect is of potential importance for the exploration of icy planets and moons.
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