COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP
1.6 SPECIALTY GROUP REPORT: PHYSICAL AND MECHANICAL PROPERTIES, ICE DYNAMICS AND GEOPHYSICS
A. Understanding the physical and structural properties of ice and the detailed flow and strain-rate patterns of selected regions is crucial to understanding ice-sheet dynamics and assessing future stable or unstable changes in ice sheets.
B. Ice-dynamical modeling is needed to provide the depth-age scale for deep cores. The modem dynamical state and temperature-depth profile contain important paleoclimatic information that can be extracted through modeling studies.
C. Physical and structural properties accurately reflect the deformational history of ice, which we must understand to evaluate the continuity of geochemical records; internal-shearing or folding could seriously distort records used in climatic reconstruction. Physical and structural properties also exert an important control on current rates of deformation and modem ice dynamics.
D. Accurate documentation of bulk physical and structural properties of ice is necessary for rational interpretation of chemical and other properties. Core is subject to rapid changes in most properties in response to decompression and thermal stress during recovery; these changes can include microcracking, fracturing, recrystallization, and potentially incomplete exsolution of gases, all of which could bear critically on interpretation of geochemical data.
II. Scientific Objectives
A. Log and identify stratigraphic discontinuities in core
B. Examine densification processes
C. Characterize pore close-off
D. Understand grain-growth processes
E. Determine deformation history of ice
F. Detemiine rheological properties of ice
G. Examine D.C. conductivity characteristics
H. Investigate internal layering
I. Measure 3-D velocity and strain-rate fields
J. Evaluate gas solution and dissolution mechanics and effect on ice rheology
K. Determine physical environment and boundary conditions
L. Estimate past changes in surface elevation
M. Reconstruct age-depth relationship
N. Reconstruct past climatic forcing from modem temperature - depth profile and dynamic state
O. Measure whether fifth force exists
- Acquire and examine visually smooth-surfaced core.
- Measure depth-density profile on samples selected with regard to
-Examine crystal/bubble relationships in thin sections.
-Examine closed bubbles in thin sections (complementary to total-gas content
-Measure air permeabilities.
-Model air-exchange processes and age of enclosed gas.
- Measure grain size in thin sections to determine growth rate as function of
depth and time, and identify discontinuities.
- Examine crystal/bubble relations in thin sections
- Investigate relative distributions of grain sizes, bubbles,
microparticles, and dissolved impurities (including location of impurities) on closely spaced samples from several depths.
- Measure c-axis fabrics and other fabric and texture elements (bubble elongation, grain shape, limited a-axis studies, etc) on thin sections.
- Measure c-axis fabrics through 3-component sonic/seismic measurements on core, on borehole, and from surface geophysics.
- Use laboratory and model studies to relate fabrics and textures to strain history.
- Deform samples from various depths in pressure chambers to simulate englacial conditions and determine rheological properties; at least some samples should be recovered under pressure and used as soon as possible to minimize relaxation.
- Relate these results to fabric, grain size and shape, impurity levels and states (gas, microparticle, dissolved impurities).
- Compare results to similar experiments conducted at ambient pressures.
- Measure continuous D.C. - conductivity profile on core and on ice in hole walls.
- Compare D.C. conductivity and amount and location of impurities on selected samples, using 4-contact probe.
- Measure resistivity using surface-geophysical techniques.
- Measure permitivities and losses on core at radio-echo frequencies; compare to results of detailed high-frequency ground-based radar survey.
- Use borehole-target radar experiments to constrain interval velocities and study internal reflections.
- Use downhole radar to study internal layering, possibly to include tomographic experiments between boreholes.
- Measure surface strain-rate field in vicinity and upstream of borehole.
- Measure borehole tilting, closure (caliper log), and vertical strain (repeat vertical positioning of markers in borehole).
- Use x-ray techniques to search for clathrates in pressurized ice.
- Measure gas solution and dissolution in a pressure chamber.
- Compare rheologic properties of pressurized samples before and after controlled exsolution or dissolution of gas.
- Use standard glaciological techniques to measure accumulation and 10-m temperatures in borehole vicinity and upstream.
- Measure borehole temperature in ice and about 10 m or more into subglacial materials.
- Measure thermal conductivities of core materials and combine with temperature data to determine heat flow.
- Measure sliding velocity (if any). Measure basal water pressure (if any). Conduct geological studies on englacial and subglacial rock material.
- Combine geological and isotopic results to identify basal entrapment mechanisms of rock debris.
- Measure strain rate, surface, bed, and internal-layer topography on a detailed grid encompassing the two boreholes
- Measure total-gas content of core samples and evaluate in terms of past ice sheet
- elevation changes in conjunction with isotope and model studies.
- Use all available data as inputs to models to construct ice-core
- Calculate modem mass balance, and use modeling to infer causes of
- Use borehole-temperature profile to construct past surficial
- Conduct highly accurate borehole-gravity survey.
- Measure basal topography accurately using imaging radar.
- Measure surface topography accurately using standard surveying techniques.
- Measure borehole length accurately using single-cable logging
- Cooperation (we must measure multiple properties on the same samples)
- Core storage
- Core shipment at in situ temperatures and pressures
- Cooling of processing trench to -10oC or below
- Need for two holes in many proposed experiments to cross-correlate data records and determine if serious miscorrelations exist.
V. PICO Drilling/Logging Needs
- Pressurized core barrel
- Side-wall sampling
- Ability to conduct diverted drilling for closely spaced cores through selected depths
- Accurate depth control (single-cable logging tool)
- Ability to core debris-rich ice
- Ability to core bedrock
- Ability to core unconsolidated subglacial materials
- Ability to measure vertical strain
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