Nobuhiko Azuma

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

A flow law for anisotropic ice and its application to ice sheets

A new formulation of the flow law of polycrystalline ice was derived for an rbitrary c-axis orientation fabric pattern. The rate factor of the flow law is given by a geometrical factor that can be calculated with c-axis fabric data and stress condition. A theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field. According to this flow model of polycrystalline ice, if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain, whereas simple shear makes the ice body softer with strain. This may significantly affect ice sheet dynamics.

Microstructure mapping: a new method for imaging deformation- induced microstructural features of ice on the grain scale

This work presents a method of mapping deformation-related sublimation patterns, formed on the surface of ice specimens, at microscopic resolution (3-4 mm pixel -1 ). The method is based on the systematic sublimation of a microtomed piece of ice, prepared either as a thick or a thin section. The mapping system consists of an optical microscope, a CCD video camera and a computer-controlled xy-stage. About 1500 images are needed to build a high-resolution mosaic map of a 4.5 9 cm section. Mosaics and single images are used to derive a variety of statistical data about air inclusions (air bubbles and air clathrate hydrates), texture (grain size, shape and orientation) and deformation-related features (subgrain boundaries, slip bands, subgrain islands and loops, pinned and bulged grain boundaries). The most common sublimation patterns are described, and their relevance for the deformation of polar ice is briefly discussed.

Ice-fabrics study in the upper 1500 m of the Dome C (East Antarctica) deep ice core

Abstract A study of c-axis orientations in the upper 1500m of the Dome C (East Antarctica) deep ice core has been carried out using an automatic ice-fabric analyzer (AIFA). Twenty-nine vertical and a few horizontal thin sections from different depths in the core have been analyzed. Several statistical parameters describing fabric strength and fabric shapes have been calculated from the c-axis orientation data. The fabric diagrams display a near-random c-axis orientation distribution in the uppermost parts of the ice sheet. A tendency of c-axis rotation towards a broad single-maximum fabric is observed in the lowest part of the studied interval. The fabric development at Dome C thus appears typical for ice-sheet summit and dome sites. The fabric development at Dome C is compared with the fabric evolution in the Dome F and GRIP ice cores, and data on crystal size obtained with image-analysis techniques are presented. Studies of misorientation angles between the c axes of neighbouring crystals reveal little evidence for polygonization, but microscopic observations show that sub-grain boundaries are present in half of the grains at any depth.

A flow law for anisotropic polycrystalline ice under uniaxial compressive deformation

Abstract The deformation behavior of the individual grains which constitute polycrystalline ice has been observed in situ during plane strain deformation using thin, columnar-grained specimens. The relationship between the c-axis orientation and the strain of individual grains was examined. It was found that the strain of an individual grain was proportional to the geometric factor for the crystallographic orientation of the grain (the Schmid factor for the slip direction), to the local mean strain surrounding the grain and inversely proportional to the local mean Schmid factor. Moreover, the local mean strain was proportional to the fourth power of the local mean Schmid factor. From these results it is derived that the strain rate for steady state flow of polycrystalline ice is proportional to the fourth power of the mean value of the Schmid factor of each grain in the aggregate.

Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

Subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.

The Hans Tausen drill: design, performance, further developments and some lessons learned

Abstract In the mid-1990s, excellent results from the GRIP and GISP2 deep drilling projects in Greenland opened up funding for continued ice-coring efforts in Antarctica (EPICA) and Greenland (NorthGRIP). The Glaciology Group of the Niels Bohr Institute, University of Copenhagen, was assigned the task of providing drilling capability for these projects, as it had done for the GRIP project. The group decided to further simplify existing deep drill designs for better reliability and ease of handling. The drill design decided upon was successfully tested on Hans Tausen Ice Cap, Peary Land, Greenland, in 1995. The 5.0m long Hans Tausen (HT) drill was a prototype for the ~11m long EPICA and NorthGRIP versions of the drill which were mechanically identical to the HT drill except for a much longer core barrel and chips chamber. These drills could deliver up to 4m long ice cores after some design improvements had been introduced. The Berkner Island (Antarctica) drill is also an extended HT drill capable of drilling 2 m long cores. The success of the mechanical design of the HT drill is manifested by over 12 km of good-quality ice cores drilled by the HT drill and its derivatives since 1995.

Subgrain boundaries in Antarctic ice quantified by X-ray Laue diffraction

Ice in polar ice sheets undergoes deformation during its flow towards the coast. Deformation and recrystallization microstructures such as subgrain boundaries can be observed and recorded using high-resolution light microscopy of sublimation-edged sample surfaces (microstructure mapping). Subgrain boundaries observed by microstructure mapping reveal characteristic shapes and arrangements. As these arrangements are related to the basal plane orientation, full crystallographic orientation measurements are needed for further characterization and interpretation of the subgrain boundary types. X-ray Laue diffraction measurements validate the sensitivity of different boundary types with sublimation used by microstructure mapping for the classification. X-ray Laue diffraction provides misorientation values of all four crystal axes. Line scans across a subgrain boundary pre-located by microstructure mapping can determine the rotation axis and angle. Together with the orientation of the subgrain boundary this yields information on the dislocation types. Tilt and twist boundaries composed of dislocations lying in the basal plane, and tilt boundaries composed of nonbasal dislocations were found. A statistical analysis shows that nonbasal dislocations play a significant role in the formation of all subgrain boundaries.

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling

Global cooling in intermediate glacial climate with northern ice sheets preconditions climatic instability with bipolar seesaw. Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.

Evolution of ice crystal microstructure during creep experiments

Ice of polar ice sheets consists of compacted layers of snow from the past 100000 to several 100000 years which preserves a variety of palaeo-atmospheric parameters and therefore is an important information source to study the past climate. Knowledge of mechanical properties of ice is of vital importance for the interpretation and dating of ice core records and modeling of ice sheet flow.Interpretation of deformation effects in polar ice samples is complicated by the fact that initial properties of samples and physical parameters are unknown and changing from layer to layer. Furthermore, interaction with other processes occurs and cannot easily be distinguished from substantial deformation effects. For example, inherited attributes of the ice such as inclusions significantly influence grain growth behaviour. Laboratory experimental creep tests help to improve the understanding on flow and deformation behaviour and processes in polycrystalline ice.During flow, various processes on the atomic scale are conducting the deformation and producing or promoting strain. As these processes are acting on the atomic scale, they are difficult to observe directly in deformed polycrystalline ice. However, they leave behind certain structures on the microscopic scale indicating deformation mechanisms. The microstructure mapping method enables detailed observation and recording of many kinds of microstructures such as grain boundaries, sub-grain boundaries and slip lines.Analysis of samples from uni-axial compression creep tests with small grained and bubble-free isotropic ice (stress: 0.2 to 0.6MPa, strain: 0.5 to 8.6%, temperature:-5°C and -20°C) reveal a strain dependence of sub-grain boundary density, which reaches a steady value together with the achievement of constant secondary strain rate at ca. 2% strain. Strain shape, measured by the perimeter ratio, the first time applied for ice, also depends on strain, which clearly demonstrates the increasing influence of strain induced grain boundary migration.