Study On Fabric And Microstructure Of Blue Ice Cores At The Margin Of Dalk Glacier,East Antarctica

The Antarctic and Greenland ice sheets have become the most important contributors to sea level rise due to global warming.Ice sheets are formed by the accumulation of snow,which truns into firn and ice during the process of densification.Large scale anisotropy formed in ice during glacial flow,which is called fabric.Antarctic ice flows slower at the dome and faster at the margin of ice sheet,which also speeds up the melting of ice sheets and ice shelves.The fabric and microstructure of natural ice will change greatly under the influence of deformation during its movement from dome to the edge of ice sheet.In addition,the evolution of ice fabric in the ice sheet margin region is affected by temperature,impurity content and microstructure,resulting in depth and spatial changes.Therefore,it is of great significance to identify the characteristics of fabric and microstructure of ice sheet in the high-velocity region.Which is beneficial for improving the ice flow model,better understanding of the comparison between the findings in polar ice cores and the results of laboratory experiments,and revealing the mechanism of glacier evolution.In this study,ice core drilling was carried out at the shear margin near Dalk Glacier,East Antarctica,one of the key research regions of Chinese National Antarctic Research Expedition.Ten shallow one-meter-long ice cores were drilled from the front margin of Dalk Glacier(named LC)and the end of its lateral shear margin(named IC),and sixteen one-meter-long ice cores(named IW and OIW)from the near-bottom layer at the end of the lateral margin of the glacier.Fabric,microstructure,stable water isotopes and the electric conductivity of these ice cores were analyzed.Firstly,fabric and microstructure of ice sheet were studied and discussed in detail in this paper.Secondly,the spatial variations of fabric and microstructure of shallow blue ice cores were investigated.The influence of subglacial bedrock topography and surface ice velocity,and the formation mechanism were discussed.Finally,by studying the fabric and microstructure characteristics of the blue ice at the near-bottom layer,the shear stretching effect of subglacial bedrock and the dating results of the ice cores were discussed.The main conclusions are as follows:(1)The fabric and microstructure evolution trends of the three LC ice cores are similar among each other,but the depth variations are significant.Indicating that the ice layer on the raised subglacial terrain has less horizontal disturbance,and the ice core has obvious multidecadal variation characteristics.The study of IC ice cores shows that the fabric and microstructure of shallow ice cores have obvious spatial variations during the ice flows towards the exposed bedrock.The microstructure of the IC4 ice core shows completely different characteristics between the left part and the right part of the IC4 thin sections,indicating the existence of folding ice layers at the end of the glacier.(2)At the junction of multiple ice grains,there are microbubbles within the crystal lattice,which grow along the stretching direction of the ice crystals.Microbubbles in ice grains are the key factors affecting the growth of subgrain boundaries.Microbubbles connect with large bubbles at the grain boundaries and form subgrain boundaries.The study of ice fabric in IC ice cores shows that the shallow blue ice in this region forms a two-maxima cluster fabric,which is consistent with the experimental study of ice fabric under shear stress in laboratory and the study of ice fabric at shear margin under natural conditions.(3)In this study,it was observed that large bubble channels appeared at the grain boundary of the upper part of the ice cores.The formation mechanism is the relaxation of ice and surface sublimation and ablation by solar radiation.The analysis of the spatial variations of the fabric,microstructure,stable water isotope δ18O and surface ice flow velocity of LC and IC ice cores shows that the evolution of the parameters of IC2-IC4 and IC5-IC7 ice cores has a good regularity,and the extrusion of ice flow by the subglacial topography leads to the decrease of ice grain size and the weakening of the strength of ice fabric,while the surface ice flow velocity has a much stronger effect on ice fabric and microstructure.The effect of surface ice velocity on ice fabric and microstructure is much smaller than that of subglacial topography.(4)Study on microstructure of IW and OIW ice cores near the bottom of ice sheet shows that the grain size and circularity of OIW ice cores are smaller than those of IW ice cores,and the OIW ice cores experienced stronger shear stress since subglacial bedrock had a stronger effect on the ice layers of OIW drill sites.In the ice thin sections parallel to the ice flow direction,grains and bubbles show obvious shear stretching morphology,and both grains and bubbles have obvious shape preferred orientations.In the ice thin sections perpendicular to the flow direction,grains did not show shear stretching,and grain circularity is significantly larger than that of the sections parallel to the ice flow direction.This indicates that there is a compressed effect toward the basal planes of the ice grains.IW and OIW ice cores mainly had a single-maxima cluster fabric,which is different from the multi-maxima cluster fabric of shear margin ice of mountain glaciers.This indicates that the ice in the sampling region of the Dalk Glacier is subjected to a more uniform shear stress than that of the complex mountain glaciers.(5)Hexagonal plate-like bubbles were found in the blue ice cores.The bubbles were distributed in various angles,even perpendicular to each other.These bubble cavities,known as hexagonal plate-like inclusions in deep ice cores,usually occur under 600-meter-depth,and become more frequent at depths of up to 1,000 meters.In this study,there are many hexagonal plate-like cavities in IW blue ice cores.It is speculated that the original accumulation depth of the ice is more than 1,000 meters,and the age of the ice is nearly 100,000 years.In the fast-flow regions near the basal layer of ice sheet,ice grains and bubbles are influenced by the shear stress of the bedrock,and the trapped air within bubbles has a potential possibility to mix with modern air.Therefore,when searching for old ice in blue ice areas,it is necessary to investigate fabric and microstructure of ice to identify the properties of ice layers,which is beneficial for finding high-quality old blue ice cores.