Ilka Hamann

One-to-one coupling of glacial climate variability in Greenland and Antarctica.

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth’s climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard–Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard–Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard–Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

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.

Application of a continuum-mechanical model for the flow of anisotropic polar ice to the EDML core, Antarctica

We present an application of the newly developed CAFFE model (Continuum-mechanical, Anisotropic Flow model based on an anisotropic Flow Enhancement factor) to the EPICA ice core at Kohnen Station, Dronning Maud Land, Antarctica (referred to as the EDML core). A one-dimensional flow model for the site is devised, which includes the anisotropic flow law and the fabric evolution equation of the CAFFE model. Three different solution methods are employed: (1) computing the ice flow based on the flow law of the CAFFE model and the measured fabrics; (2) solving the CAFFE fabric evolution equation under the simplifying assumption of transverse isotropy; and (3) solving the unrestricted CAFFE fabric evolution equation. Method (1) demonstrates clearly the importance of the anisotropic fabric in the ice column for the flow velocity. The anisotropic enhancement factor produced with method (2) agrees reasonably well with that of method (1), even though the measured fabric shows a girdle structure (which breaks the transverse isotropy) in large parts of the ice core. For method (3), we find that the measured fabric is reproduced well by the model down to ∼2100m depth. Systematic deviations at greater depths are attributed to the disregard of migration recrystallization in the model.

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.