Abstracts of papers on physical properties (excluding electrical), and clathrates

This file contains abstracts of GRIP papers whose primary content concerns physical properties (stratigraphy, fabrics, crystal size, mechanical properties), or clathrates. Papers are listed alphabetically by first author. You can go straight to the abstract you want.

Alley, R.B., Gow, A.J., Johnsen, S.J., Kipfstuhl, J., Meese, D.A. & Thorsteinsson, T. 1995. Comparison of deep ice cores. Nature, 373, 393-394.

Corresponding author: Richard Alley, Earth System Science Center, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.

No abstract with this paper, but summary follows. The paper reports comparative studies carried out on the visible stratigraphy and crystal fabrics of the GRIP and GISP2 cores. Both cores contain structures that could represent inverted strata. Depths of highly-inclined layers, and of small z-folds, are given for each core. Simple-shear features seen at GRIP, currently at the ice divide, suggest recent or current divide migration. This study cannot by itself invalidate or confirm the climate instabilities reported for the Eemian period.

Castelnau, O., Thorsteinsson, T., Kipfstuhl, J., Duval, P. & Canova, G.R. 1996. Modelling fabric development along the GRIP ice core, central Greenland. Annals of Glaciology, 23, 194-201.

Corresponding author: Olivier Castelnau, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP 96, 38402 Saint-Martin-d'Heres Cedex, France.

The preferred c-axes orientation (fabric) observed in cold polar ice is induced by intracrystalline slip only when the grain-boundary migration rate is low enough (i.e. corresponding to grain growth or rotation recrystallisation regimes). Fabrics reflect the entire thermomechanical history of the ice and strongly influence its mechanical behaviour. Large viscoplastic anisotropy is always associated with pronounced fabrics. We use a viscoplastic self-consistent (VPSC) polycrystal deformation model to calculate fabric development. In this model, stress- and strain-rate fields are not uniform within the polycrystal and both equilibrium and compatibility conditions are fulfilled. We compare fabrics measured on thin sections along the GRIP core (central Greenland) with those calculated down to a depth of 2800 m. Behaviours predicted by uniform stress and uniform strain bounds are presented for comparison. Predictions of the VPSC model are in close agreement with measurements within the upper 650 m, which corresponds to the entire grain-growth zone. Deeper down, the simulated fabric strength appears to be too high. A simple calculation shows that this discrepancy may be fully attributed to the effects of rotation recrystallisation.

Dahl-Jensen, D., Thorsteinsson, T., Alley, R. & Shoji, H. 1997. Flow properties of the ice from the Greenland Ice Core Project ice core: The reason for folds? Journal of Geophysical Research, 102, 26831-26840.

Corresponding author: Dorthe Dahl-Jensen, Geophysics Department, University of Copenhagen, Juliane Maries Vej 30, DK2100 Copenhagen, Denmark.

Long-term deformation tests on ice from the Greenland Ice Core Project (GRIP) deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample. The deformation tests are performed by uniaxial unconfined compression at -16 degrees C with an octahedral compression stress of 3 bars. The ice samples are compressed for 1/2 to 3 years until the tertiary strain rate is reached. It is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions. All samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations. The results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression. When tertiary creep is reached, the crystal size is smaller in the cold ice than in the warm. The ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2-3 times more rapidly in compression. The tertiary steady state crystal size is increased by 50% and the ice crystals have oriented more favorably for the applied compression in the warm ice, which is believed to be the reason why the strain rates are greater here than in the cold ice. In the bottom 200 m of the GRIP ice core, zones are observed with folds on the scale of 1-8 cm. An investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones. The folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures.

Hvidberg, C.S., Dahl-Jensen, D. & Waddington, E.D. 1997. Ice flow between the Greenland Ice Core Project and Greenland Ice Sheet Project 2 boreholes in central Greenland. Journal of Geophysical Research, 102, 26851-26859.

Corresponding author: Christine Schott-Hvidberg, Geophysics Department, University of Copenhagen, Juliane Maries Vej 30, DK2100 Copenhagen, Denmark.

The ice flow between the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) boreholes in the Summit region of central Greenland is modeled with a steady state finite element model. The model calculates the free ice sheet surface and the coupled ice flow and temperature fields along the flow line between the boreholes. Three-dimensional effects are expressed in terms of the divergence of the flow and included in the coupled stress and temperature fields. In addition to ice-core data, several geophysical surface programs have provided data that are used to constrain the model. The modeled isochrones resemble the bedrock structure along the flow line but rise at the divide. The rise at the divide is not seen in internal layers found by surface radio echo sounding. An ice particle, originally deposited at the surface, moves through different stress and temperature regimes. A set of trajectory lines are followed from the surface to the boreholes, in order to follow the variation of stress deviators, temperature, and deformation along with the ice movement. These variations along a trajectory line constitute a history of the corresponding ice layer. The difference between the history of ice layers found in the GRIP and GISP2 boreholes is mainly due to the shear stress, which is 3-4 times higher for GISP2 ice than for GRIP ice. Furthermore, the GRIP ice experiences a nonzero longitudinal stress deviator much longer than the GISP2 ice.

Pauer, F., Kipfstuhl, J. & Kuhs, W.F. 1995. Raman spectroscopic study on the nitrogen/oxygen ratio in natural ice clathrates in the GRIP ice core. Geophysical Research Letters, 22, 969-971.

Corresponding author: Frank Pauer, Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Germany.

We have carried out Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core. We present new N2/O2 measurements that markedly differ from previous measurements of the Dye-3 ice core: the N2/O2 ratio we observe is much closer to atmospheric. This has new implications for the interpretation of gas distributions in ice sheets and the reconstruction of past atmospheric conditions.

Pauer, F., Kipfstuhl, J. & Kuhs, W.F. 1997. Raman spectroscopic and statistical studies on natural clathrates from the Greenland Ice Core Project ice core, and neutron diffraction studies on synthetic nitrogen clathrates. Journal of Geophysical Research, 102, 26519-26526.

Corresponding author: Frank Pauer, Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Germany.

We present the results of Raman spectroscopic experiments on air clathrates in the GReenland Ice Core Project (GRIP) deep ice core, which differ markedly from previous measurements on the Dye 3 ice core. The N-2/O-2 ratio we observe is much closer to the atmospheric value. This has new implications for the interpretation of gas distributions in ice sheets. Raman spectroscopic scans to determine the N-2/O-2 ratios on different planes through a clathrate, in which the two axes of the scans are perpendicular to each other, give no indication of fractionation effects on the N-2/O-2 concentrations within a clathrate specimen. The frequency shift of the N-2 and O-2 peaks due to decomposition of a clathrate to an air bubble is shown qualitatively. From their peak integrals there is no indication of different retransformation rates to air bubbles between the oxygen and the nitrogen contents of clathrates. In air bubbles resulting from clathrate decomposition, the N-2/O-2 ratio shows similar values to those observed in clathrates and present atmospheric values. Statistical studies on the size, shape, and number concentration of clathrates are intended to give an estimate of the total amount of gas occluded in the clathrates. We present preliminary results obtained from 27 samples in a depth range between 1100 and 3000 m. The first neutron powder diffraction experiments reveal an overall degree of filling of 96.4% for a clathrate at a pressure of 449 bars and the existence of a type I phase at 1311 bars with an overall degree of filling of 107.5%.

Thorsteinsson, T., Kipfstuhl, J., Eicken, H., Johnsen, S. & Fuhrer, K. 1995. Crystal size variations in Eemian-age ice from the GRIP ice core, Central Greenland. Earth and Planetary Science Letters, 131, 381-394.

Corresponding author: Thorsteinn Thorsteinsson, Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Germany.

Continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the GRIP ice core from Central Greenland. The ice in this interval is at present considered to originate from the Eemian interglacial period. The record reveals that the crystal size in ice older than 100,000 yr is highly dependent on climatic conditions at the time of snowfall. This dependence shows up as a strong correlation between delta18O values and crystal size throughout the Eemian, as well as a negative correlation between crystal size and several soluble and insoluble impurities. Although high-resolution impurity records are available from selected parts of the Eemian ice, the study is not conclusive on which impurities are most effective in slowing grain growth. It is shown that the normal grain-growth process, commonly observed in the upper few hundred metres of polar ice sheets, does not yield grain sizes compatible with observed ones at this depth in the ice sheet, even in those parts of the Eemian ice where impurity drag effects are not present. Polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain-size variations. The relevance of possible flow disturbances of the GRIP Eemian climatic record for the results presented is discussed briefly.

Thorsteinsson, T. 1996. Textures and fabrics in the GRIP ice core, in relation to climate history and ice deformation. Berichte zur Polarforschung, 205. Bremerhaven: Alfred-Wegener-Institut fur Polar- und Meeresforschung, 146 pp.

Corresponding author: Thorsteinn Thorsteinsson, Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Germany.

Within the framework of the joint European Greenland lcecore Project (GRIP) a 3029 m long ice core was drilled in Central Greenland in the years 1989-1992. Studies of isotopes and various atmospheric constituents in the core have revealed a detailed record of climatic variations reaching more than 100 000 years back in time. The results indicate that Holocene climate has been remarkably stable and have confirmed the occurrence of rapid climatic variation during the last ice age (the Wisconsin). Climatic instability observed in the core part believed to date from the Eemian interglacial has not been confirmed by other climate records.

The GRIP core offers a unique possibility to study the growth, rotation and recrystailization of polar ice at an ideal location, covering a time span of more than 100.000 years. This information is obtained by a comprehensive thin section study of crystal sizes and c-axis orientations along its entire length. The results confirm earlier, basic observations on deep ice cores and have led to new insights. A significant variation of crystal size with climatic parameters is shown to persist to a great depth in the core; the development of a strong crystalline anisotropy in the ice sheet is also demonstrated. Considerable insight is obtained into the rheological properties of the ice sheet from these studies.

In the upper 700 m of the core, the average area of ice crystals is found to increase linearly with time, in accordance with a well known grain-growth law. This is the regime of normal grain growth. Below 700 m, grain size does not increase any further and keeps a nearly constant average diameter of 4 mm. The stop in grain growth is probably due to the fragmentation of crystals under increasing strain (polygonization). The polygonization regime covers the lower part of the Holocene and the entire Wisconsin ice (1625-2790 m), with smaller crystals in the latter (typically 2-3 mm). The difference is probably due to a heightened impurity content in the ice age ice. The impurities tend to segregate to grain boundaries and slow their migration, and thereby the grain growth rate.

In the Eemian ice (2790-2865 m), a continuous record of crystal size has revealed a surprisingly strong correlation of this parameter with various climatic parameters measured on the core, such as isotopes, impurity content and electric conductivity. Large crystals (1-2 cm) are found in ice from warm periods, small crystals (3-4 mm) in ice from cold stages. Although several studies have now suggested that the Eemian sequence might not be in stratigraphic order, it is likely that the result described here will prove to be a valid one: that outlines of past climatic variation can be inferred from a study of crystal size changes.

Closer to the bottom, the growth of very large crystals is observed. This is likely due to extensive recrystallization at relatively high temperatures near the bottom (-10 degrees C), combined with the effects of great age. The crystal size -isotope-impurity covariation disappears in the lowest 100 m of the core.

The results from the crystal fabric measurements are compatible with the ice flow regime expected at Summit, which is positioned at the top of a dome on the Central Greenland ice divide. The c-axis orientation gradually and progressively changes from a random pattern near the surface to a strong vertical single maximum fabric at 2200 m depth. A rapid strengthening of the fabric is not observed at the Holocene-Wisconsin transition. Enhanced flow of Wisconsin ice is thus not expected at Summit. The formation mechanism of the preferred fabric is believed to be through c-axis rotation by intracrystalline dislocation glide, under the influence of vertical compressive stress. In the coarse grained ice within and below the Eemian, the fabric weakens, due to either recrystallization mechanisms or different stress conditions in the lowest part of the ice sheet or both.

There are clear indications that the vertical single maximum fabric hardens the ice against vertical compression. This hardening is evident in deformation tests carried out on samples from the core. Softening of the ice against horizontal shear is also expected, especially in fine-grained layers. The c-axis measurements reveal fabric contrasts between fine-grained and coarsegrained layers, which are likely to result in rheological differences.

The climatic interpretations from the core assume that the layer sequence in the core is continuous and undisturbed by irregular flow patterns. Studies of the visual stratigraphy in the ice core have revealed that significant flow distortion has occurred between 2850 m and 2950 m depth, an interval which includes the lowest 1/4 of the Eemian sequence. Distortions have not been found in the upper 3/4 of the Eemian ice. Highly inclined layering occurs in a 1 m increment 30 m above the Eemian, indicating that the timescale below 2750 m depth, beyond 100 kyr, cannot be considered certain. Thin section study of the inclined layering and of other flow distortions reveals that these features exert a marked influence on the ice fabric, by rotating c-axes out of the vertical position. The presence of overturned folds below 2850 m depth suggests that simple shear is, or has recently been active in the lowest part of the ice sheet.

Thorsteinsson, T., Kipfstuhl, J. & Miller, H. 1997. Textures and fabrics in the GRIP ice core. Journal of Geophysical Research, 102, 26583-26599.

Corresponding author: Thorsteinn Thorsteinsson, Alfred-Wegener-Institut fur Polar- und Meeresforschung, Columbusstrasse, D-27568 Bremerhaven, Germany.

A comprehensive study of textures and fabrics has been carried out on the Greenland Ice Core Project (GRIP) ice core. Crystal sizes and c axis orientations have been measured on thin sections with conventional techniques, yielding new information on the growth, rotation and ; recrystallization of ice crystals in the Greenland Ice Sheet. Normal grain growth is found to persist to a depth of 700 m in the core, where the onset of polygonization due to increasing strain prevents a further increase in grain size in the Holocene ice. Smaller crystals are observed in the Wisconsin ice, larger crystals are found in the Eemian ice, and the crystal size is found to vary with climatic parameters in these periods. This dependence, which probably results from variable impurity content in the ice, persists to a depth of 2930 m. Coarse-grained ice, probably resulting from rapid growth of crystals at comparatively high temperatures, is found in the lowest 100 m of the core. The data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet. A significant strengthening is not observed at the Holocene-Wisconsin transition. The fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m. The evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear. Evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth. Stretched fabrics in coarse-grained ice in the lowest 100 m could be due to tensional stresses; this ice is unlikely to be undergoing any significant horizontal deformation at the present time.