4:45 PM - 5:00 PM
[SCG49-06] Development of microstructures in the Dome Fuji ice core and influence of impurities on ice-sheet ice rheology
★Invited Papers
Keywords:ice-sheet ice rheology, crystal orientation fabric
An ice sheet is a mass of polycrystalline ice consisting of compressed snow layers, which flows from inland to coastal areas under their own weight. Behaviors of ice sheets closely relate with the sea level rising, so the understanding of ice sheet flow mechanism is important in the prediction of future climate change. Furthermore, ice sheets keep ancient air and aerosol within ice crystals at greater depth. By analyzing various components along the deep ice core, we can extract paleoclimate records. In this study, we used a 3035 m long Dome Fuji ice core drilled at a dome summit in East Antarctica.
In the ice core analysis, crystal orientation fabric (COF) is one of the most important factors determining the behaviors of ice sheet deformation and flow. It is generally accepted that dislocation creep is the dominant deformation mechanism in ice sheets. In particular, uniaxial compression dominates the deformation at the dome area. Therefore, the c-axes of ice crystals concentrate toward the core axis at greater depth. Various studies of ice cores have indicated the relationship between microstructures and climate change. For example, the grain size in glacial periods becomes smaller than that in interglacial periods due to highly concentrated impurities. However, the cause of COF fluctuations is still unclear. In previous studies, the orientation of the c-axis is commonly determined from thin sections using automated crystal fabric analyzers. It is difficult to detect the small fluctuation of COF in the thin section measurement due to the difficulty of measuring conditions.
To investigate the small fluctuations of COF, we conducted dielectric tensor measurements using the Dome Fuji ice core. Polycrystalline ice is known to exhibit macroscopic anisotropy in permittivity depending on the formation of COF. By measuring the dielectric properties of ice cores, we can estimate the degree of c-axis concentration toward the core axis. Our results revealed the fluctuations of COF associated with climate change. In particular, the cluster strength is highly affected by the concentration of chloride ions and dust particles. We found a positive correlation with the concentration of chloride ions and an inverse correlation with the amount of dust particles. Chloride ions are known to increase point defect and promote, which will result in active plastic deformation. Our results are consistent with this trend. On the other hand, the influence of dust particles is ambiguous with either softening or hardening. Solid particles restrict grain growth and maintain the fine grains via Zenner pinning effect, while they impede dislocation movement via Orowan hardening.
At greater depth in ice sheets, very old paleoclimate proxies are preserved in ice, so obtaining oldest-ice is required in international ice-core community. However, the physical conditions of oldest ice are little known. Therefore, detailed investigation of oldest ice is the important subject.
In this presentation, we discuss the rheology of ice-sheet ice with a focus on the relationship between the variations of COF and impurities, and physical conditions of the oldest ice.
In the ice core analysis, crystal orientation fabric (COF) is one of the most important factors determining the behaviors of ice sheet deformation and flow. It is generally accepted that dislocation creep is the dominant deformation mechanism in ice sheets. In particular, uniaxial compression dominates the deformation at the dome area. Therefore, the c-axes of ice crystals concentrate toward the core axis at greater depth. Various studies of ice cores have indicated the relationship between microstructures and climate change. For example, the grain size in glacial periods becomes smaller than that in interglacial periods due to highly concentrated impurities. However, the cause of COF fluctuations is still unclear. In previous studies, the orientation of the c-axis is commonly determined from thin sections using automated crystal fabric analyzers. It is difficult to detect the small fluctuation of COF in the thin section measurement due to the difficulty of measuring conditions.
To investigate the small fluctuations of COF, we conducted dielectric tensor measurements using the Dome Fuji ice core. Polycrystalline ice is known to exhibit macroscopic anisotropy in permittivity depending on the formation of COF. By measuring the dielectric properties of ice cores, we can estimate the degree of c-axis concentration toward the core axis. Our results revealed the fluctuations of COF associated with climate change. In particular, the cluster strength is highly affected by the concentration of chloride ions and dust particles. We found a positive correlation with the concentration of chloride ions and an inverse correlation with the amount of dust particles. Chloride ions are known to increase point defect and promote, which will result in active plastic deformation. Our results are consistent with this trend. On the other hand, the influence of dust particles is ambiguous with either softening or hardening. Solid particles restrict grain growth and maintain the fine grains via Zenner pinning effect, while they impede dislocation movement via Orowan hardening.
At greater depth in ice sheets, very old paleoclimate proxies are preserved in ice, so obtaining oldest-ice is required in international ice-core community. However, the physical conditions of oldest ice are little known. Therefore, detailed investigation of oldest ice is the important subject.
In this presentation, we discuss the rheology of ice-sheet ice with a focus on the relationship between the variations of COF and impurities, and physical conditions of the oldest ice.