Abstract
A model for the isotopic composition in
δ
D and
δ
18
O of ice formed by refreezing at the glacier sole is developed. This model predicts relatively well the distribution of points ...representing samples from basal layers of an Arctic and an Alpine glacier on a
δ
D–
δ
18
O diagram. The frozen fraction which is the part of the liquid that refreezes can be determined for each basal ice layer. This may have implications on the study of the ice–water system at the ice–rock interface.
One of the important questions in ice-core studies is the extent to which isotope measurements made on ice loaded with bedrock debris can be trusted when interpreting the climate record from ice-core ...data. Data that shows that the isotopic characteristics of basal ice containing debris from beneath the Greenland ice sheet can be understood only if isotope exchange between hydroxyl-bearing minerals and water is taken into account is presented.
The main objective of AMICS (Antarctic ice-sheet dynamics and climatic change: Modelling and Ice Composition Studies) is to contribute to the international research effort leading to an improved ...understanding of the dynamic behaviour of the Antarctic ice sheet resulting from climatic change, through a better knowledge of the internal ice dynamics and the ice sheetâs interactions with the subglacial environment. To clarify the dynamic interactions between the ice sheet and the subglacial environment a new thermomechanical ice-sheet model was developed, including higher-order stress gradients. Such a model is capable of properly simulating the ice flow in areas characterized by complex basal interaction (ice streams, subglacial lakes, ...). As a contribution to the EPICA project, the model is imbedded in a large-scale model of the Antarctic ice sheet to determine the chronology and the origin of the ice within the EPICA DML ice core. A comprehensive effort to improve our understanding of the physical processes at the interface between a frozen lake and a cold-based glacier explained the complex formation of the lake ice cover. It showed how sediments become trapped in lake ice and how this lake has contributed to the formation of the basal ice layer of the adjacent damming glacier. Moreover, an isotopic model has been elaborated for basal freeze-on associated with subglacial upward flow of pore water and tested against two Antarctic outlet glaciers by studying the dD-d18O relationships in the basal ice layers of these glaciers. Investigation of the isotopic composition of the deepest part of the Vostok ice core shows the build-up of a highly deformed basal ice layer. Therefore, Lake Vostokâs behaviour has been reassessed from new and existing ice core data. In view of these results, the higher-order model was applied to the area surrounding subglacial Lake Vostok. This showed for the first time that the surface flattening and turning of the ice flow across the lake are a direct consequence of the lack of friction at the ice-water interface, and that subglacial lakes can be at the origin of enhanced ice flow and the onset of continental ice streams. A detailed numerical investigation of such streams demonstrated that a large variability in glacier response can be expected when interaction with subglacial water flow is considered. Speedup and slowdown of such large flow features may be a result of ice piracy of neighboring streams. The AMICS project has demonstrated that basal processes play an important - if not crucial - role in the ice flow of the vast interior of the ice sheet, a zone which was previously thought of being unconditionally stable. Subglacial interactions determine the onset of fast-flowing areas such as ice streams, which has its consequence for the stability of the Antarctic ice sheet with changing climate.
The 3623 m long Vostok ice core is divided into two main parts: (1) from the surface to a depth of 3310 m, the ice layers are undisturbed and may be used to reconstruct the climatic record over the ...past 420 kyr; (2) from 3538 m to the bottom, the core is made up of accreted ice formed by freezing of the lake. In between, the glacier ice is disturbed by ice dynamics, as shown, for example, by inclined volcanic-ash layers. Microparticle concentrations and distributions as well as ionic and isotopic content were used to subdivide this third section. The identification of layers with bedrock material provides clues as to the entrainment processes at sub-freezing temperatures. Dust concentration and size distribution as well as ionic content are comparable with values found in ice of glacial and interglacial periods. Below 3450 m depth, however, the mode of the volume size distribution clearly shifts from 2.1 mu m to 3.4 mu m in diameter. Particles as large as 30 mu m in diameter are observed and cannot have an aeolian origin. From microscopic observations, we suggest that they originate from the bedrock and represent glacial flour entrained in a shear layer up to 89 m from the glacier sole. This process most likely occurred when the ice sheet was grounded before flowing over the lake.
Experiments on progressive unidirectional freezing are conducted to determine the evolution in δD and δ
18O of successive water samples and ice layers taken during the course of freezing. Results ...indicate that this evolution takes place, in a δD–δ
18O diagram, along a straight line with a characteristic slope. This slope, different from that due to the precipitation effect, gives a finger-print of the occurrence of a freezing or of a melting–refreezing process in the studied reservoir.
Results from a detailed profile in a 5.54 m multi-year sea-ice core from the rift area in the southern part of George VI Ice Shelf are presented. Stratigraphy, stable isotopes and Na content are used ...to investigate the growth processes of the ice cover and to relate them to melting processes at the bottom of the ice shelf. The thickest multi-year sea ice in the sampling area appears to be second-year sea ice that has survived one melt season. Combined salinity/stable-isotope analyses show large-scale sympathetic fluctuations that can be related to the origin of the parent water. Winter accretion represents half of the core length and mainly consists of frazil ice of normal sea-water origin. However, five major dilution events of sea water, with fresh-water input from the melting base of the ice shelf reaching 20% on two occasions, punctuate this winter accretion. Two of them correspond to platelet-ice production, which is often related to the freezing of ascending supercooled water from the bottom of the ice shelf. Brackish ice occurs between 450 and 530 cm in the core. It is demonstrated that this results from the freezing of brackish water (Jeffries and others, 1989) formed by mixing of normal sea water with melted basal shelf ice, with dilution percentages of maximum 80% fresh water.
A detailed dielectric profiling (DEP) conductivity profile (σ∞) 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 μS m−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 μS m−1). On a steady increase of the σ∞ values down the sequence are superimposed large fluctuations “inphase” with other variables measured in the core such as δ18O, debris content, or gas compositions in CO2 and CH4. 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 σ conductivity by CO2 and CH4 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. NH4+ 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 NH3 in the ice. This proposition is supported by the NH4+ profile in the basal ice sequence. Using calibration curves from higher up in the core, it is shown that σ is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice. The origin of the NH4+ 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.
A freezing slope, distinct from that of precipitation, is displayed on a delta D- delta O-18 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.
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