Antarctic Ice from EPICA Dronning Maud Land and artificial creep test Ice

Ice, microstructures, subgrain boundaries, recrystallization, flow, deformation. - The primary objective of this thesis is the investigation of microstructures obtained from samples from the EPICA Dronning Maud Land ice core from Antarctica. The goal is to gain understanding of deformation processes an deformation-related recrystallization mechanisms using these structures. The structures are visualized with the new microstructure mapping method using the preferred sublimation along defect regions in the crystal. This method enables observation in high resolution as well as overview over a significant sample volume. In order to provide unambiguous proof of their deformational origin and to offer interpretation and characterization, experimental reproduction of the microstructural features are performed using creep tests. Subgrain boundaries and grain-boundary morphology are identified as the most direct effects of deformation and recrystallization processes, which are still easily observable. They can be used additionally to the conventional parameters (grain size, crystal-orientation distribution) to determine these mechanisms. Different sbugrain-boundary types observed in experimentally deformed samples as well as in natural ice indicate several formation processes. Results obtained from this new and novel data suggest a profound reconsideration of the classical tripartition of recrystallization regimes described in the literature in ice sheets. Instead, dynamic recrystallization in two of its forms (rotation recrystallization and strain-induced migration recrystallization) dominates the microstructure evolution in all depth regions of the EDML ice core. Results of systematic microstructure analysis of creep-test samples demonstrate ...thesi

Chemical Impurities and Physical Properties of Polar Ice

The Greenland Ice Sheet and the Antarctic Ice Sheet are the largest reservoirs of freshwater on our planet. They consist of ice which formed over thousands of years out of the precipitation and due to natural densification of snow. As such, polar ice represents a unique archive of the past climate. The large ice masses flow under their own weight causing a transport of ice from the inlands towards the oceans. Numerical flow models are used to simulate the ice dynamics, i. a., in order to project future contributions of the ice sheets to the rising sea level. The viscoplastic deformation of ice on the micro-scale involves similar mechanisms as the deformation of other poly-crystalline materials. The shear of individual crystals is accompanied by recrystallization and the development of characteristic microstructures. Thus, structural-geological concepts and methods can be applied to natural ice samples in order to study their deformation state and active physical processes. Chemical impurities are deposited in snow and ice during the precipitation and reflect the aerosol composition of the atmosphere. As such they play an important role for the reconstructions of the climate of the past. Despite their extremely low concentrations, impurities influence many physical properties of ice, in particular also the deformation rate. The concentrations of the different impurities vary with depth and these variations are correlated with heterogeneities in the flow-velocity profile. This results in the development of localized shear zones in ice. A better understanding of the mechanisms is necessary in order to implement the effect of impurities into the flow models. The presented thesis addresses the relationship between the chemical impurities and the physical properties of polar ice. A central challenge of the project is to understand in which form impurities integrate in ice and where they are located in the bulk, respectively how they interact with it during deformation and recrystallization. A combination of methods for the microstructural analysis was applied to natural ice material from ice cores. The distribution and composition of impurities was analyzed using a confocal cryo-Raman microscope. Within the scope of this thesis, new routines for the acquisition and data processing were developed. The results reveal a complex interplay between impurities, crystal structure and localized deformation in ice. On the one hand, high impurity concentrations cause higher strain rates occurring together with small grain sizes. On the other hand, the deforming ice matrix affects the distribution of impurities and possibly also their chemical composition. The portion of dissociated components in ice could be estimated only qualitatively and through the comparison to the chemical analysis of the meltwater. However, the Raman-spectroscopy data clearly suggest that microscopic inclusions of second phase are present in solid ice in significantly higher concentrations, compared to liquid water. Mixing and chemical reactions of impurities is promoted through the deforming ice matrix. Our image of “ice as a frozen archive” could be replaced by “ice as an effective reactor”, depending on the spatial scales and time spans referred to. The resulting implication for the chrono-stratigraphic integrity of ice-core records may still be positive, because the reaction products often posses lower diffusion rates. A universal mechanism for the impurity effect on ice deformation could not be identified. The localized deformation seems to be in fact an intrinsic property of ice produced by the mechanical anisotropy and triggered by the varying impurity concentrations. However, many questions regarding the form and effect of impurities in ice remain open for future investigations.Der Grönländische und der Antarktische Eisschild sind die größten Süßwasserreservoirs unseres Planeten. Das darin enthaltene Eis bildete sich über Jahrtausende aus dem Niederschlag und durch natürliche Verdichtung von Schnee, und stellt damit ein einzigartiges Klimaarchiv dar. Die großen Eismassen fließen unter ihrem eigenen Gewicht und verfrachten so Eis vom Landesinneren in die Ozeane. Die Eisdynamik wird mithilfe von numerischen Fließmodellen simuliert, u. a. um den zukünftigen Beitrag von Eisschilden zum Meeresspiegelanstieg zu prognostizieren. Die viskoplastische Verformung von Eis auf der Mikroskala involviert ähnliche Mechanismen wie die Deformation anderer polykristalliner Materialen. Die Scherung einzelner Kristalle wird durch Rekristallisation und Bildung bestimmter Mikrostrukturen begleitet. Daher können Konzepte und Vorgehensweisen der Strukturgeologie auf natürliche Eisproben angewandt werden, um ihren Deformationsstatus und die daran beteiligten Prozesse zu studieren. Chemische Spurenstoffe im Schnee und Eis setzen sich mit dem Niederschlag ab und spiegeln die Zusammensetzung der in der Atmosphäre enthaltenen Aerosole wider. Als solche spielen sie eine wichtige Rolle für paläoklimatische Rekonstruktionen. Trotz ihrer extrem niedrigen Konzentrationen beeinflussen Spurenstoffe viele physikalischen Eigenschaften von Eis, insbesondere auch das Deformationsvermögen. Konzentrationschwankungen diverser Spurenstoffkomponenten mit der Tiefe korrelieren mit Heterogenitäten im Fließgeschwindigkeitsprofil und scheinen die Bildung von lokalisierten Scherzonen zu begünstigen. Ein besseres Verständnis dieser Zusammenhänge und der dahinterstehenden Mechanismen ist notwendig um den Effekt von Spurenstoffen realistisch in Fließmodelle implementieren zu können. Die vorgelegte Arbeit befasst sich mit dem Zusammenhang zwischen chemischen Spurenstoffen und physikalischen Eigenschaften von polarem Eis. Eine zentrale Herausforderung ist es zu verstehen, in welcher Form und wo Spurenstoffe in der Eismatrix integriert sind, bzw. wie sie mit ihr während der Deformation und Rekristallisation interagieren. Natürliches Eismaterial aus Eiskernbohrungen wurde mittels einer Kombination verschiedener Methoden zur Mikrostrukturanalyse untersucht. Die Verteilungen und Zusammensetzungen von Spurenstoffen wurden mittels eines konfokalen Kryo-Raman-Mikroskops analysiert. Im Rahmen dieser Arbeit wurden neue Routinen zur Messung und Datenerfassung entwickelt. Die Ergebnisse zeigen ein komplexes Zusammenspiel zwischen Spurenstoffen, Kristallstruktur und lokalisierter Deformation von Eis. Einerseits verursachen hohe Spurenstoffkonzentrationen höhere Deformationsraten, die von feinkörniger Kristallstruktur begleitet werden. Andererseits wird die Verteilung und möglicherweise Zusammensetzung der Spurenstoffe durch die Deformation des Eises beeinflusst. Der Anteil dissoziirter Stoffe im Eis konnte nur qualitativ und indirekt durch den Vergleich der Ramananalyise mit den Ergebnissen einer chemischer Analyse von Schmelzwasser geschätzt werden. Die Ramanspektroskopischen Messungen deuten allerdings klar darauf hin, dass mikroskopische Einschlüsse sekundärer Phasen im Eis einen erheblich höheren Anteil bilden, als im flüssigen Wasser. Das Mischen von Spurenstoffen und chemische Reaktionen zwischen ihnen werden durch die Eisdeformation begünstigt. Unser Bild von “Eis als gefrorenes Archiv” könnte durch “Eis als effektiver Reaktor” ersetzt werden, je nachdem welche zeitlichen und räumlichen Spannen gemeint sind. Die darausfolgende Auswirkung auf die chrono-stratigraphische Integrität von Eiskerndaten ist insgesamt jedoch positiv, da die Reaktionsprodukte oft niedrigere Diffusionsraten besitzen. Ein universeller Mechanismus für die Auswirkung von Spurenstoffen auf die Eisdeformation konnte nicht vollständig aufgedekt werden. Vielmehr scheint Deformationslokalisierung als Folge der mechanischen Anisotropie eine intrinsische Eigenschaft von Eis zu sein, die durch die Variation in Spurenstoffkonzentrationen getriggert wird. Viele Fragen hinsichtlich der Form und Wirkung von Spurenstoffen im Eis stehen allerdings noch offen für zukünftige Forschungsprojekte

A Review of the Microstructural Location of Impurities in Polar Ice and Their Impacts on Deformation

Insoluble and soluble impurities, enclosed in polar ice sheets, have a major impact on the deformation behaviour of the ice. Macro- and Micro-scale deformation observed in ice sheets and ice cores has been retraced to chemical loads in the ice, even though the absolute concentration is negligible. And therefore the exact location of the impurities matters: Allocating impurities to specific locations inside the ice microstructure inherently determines the physical explanation of the observed interaction between chemical load and the deformational behaviour. Both, soluble and non-soluble impurities were located in grain boundaries, triple junctions or in the grain interior, using different methods, samples and theoretical approaches. While each of the observations is adding to the growing understanding of the effect of impurities in polar ice, the growing number of ambiguous results calls for a dedicated and holistic approach in assessing the findings. Thus, we here aim to give a state of the art overview of the development in microstructural impurity research over the last 20 years. We evaluate the used methods, discuss proposed deformation mechanisms and identify two main reasons for the observed ambiguity: 1) limitations and biases of measurement techniques and 2) the physical state of the analysed impurity. To overcome these obstacles we suggest possible approaches, such as the continuous analysis of impurities in deep ice cores with complementary methods, the implementation of these analyses into established in-situ ice core processing routines, a more holistic analysis of the microstructural location of impurities, and an enhanced knowledge-transfer via an open access data base

Deriving micro- to macro-scale seismic velocities from ice-core c axis orientations

One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135ms−1 for P-wave and 200ms−1 for S-wave velocity compared to the earlier framework (estimated 1% difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45ms−1 for the alpine ice core and 59ms−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185ms−1 for P-wave and 280ms−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281ms−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200ms−1 for P wave and more than 200ms−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50ms−1 per 10cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding

The nature and timing of Late Quaternary glaciation in southernmost South America

The timing and extent of former ice sheet fluctuations can demonstrate leads and lags during periods of climatic change and the forcing factors responsible, but this requires robust glacial chronologies. Patagonia, in southern South America, offers a well preserved record of glacial geomorphology over a large latitudinal range that is affected by key climatic systems in the Southern Hemisphere, but establishing the timing of ice advances has proven problematic. This thesis targets five southernmost ice lobes that extended from the former Patagonian Ice Sheet during the Quaternary; from north to south: the Río Gallegos, Skyring, Otway, Magellan and Bahía Inútil – San Sebastián (BI-SSb) ice lobes. The region is chosen because there is ambiguity over the age of glacial limits, which have been hypothesised to relate to different glacial cycles over hundreds of thousands of years but yield cosmogenic nuclide exposure data dominantly < 50 ka. This contradiction is the focus of the thesis: was the sequence of glacial limits deposited over multiple glacial cycles, or during the last glacial cycle? A new geomorphological map is used to reconstruct glacial limits and to help target new dating. Cosmogenic nuclide depth-profiles through glacial outwash are used to date glacial limits whilst accounting for post-depositional processes. These reveal that limits of the BI-SSb lobe hypothesized to date from MIS 12 (ca. 450 ka) and 10 (ca. 350 ka) were actually deposited during the last glacial cycle, with the best-dated profile giving an MIS 3 age of ca. 30 ka, indicating an extensive advance prior to the global Last Glacial Maximum (gLGM). A glacial reconstruction indicates that this may not have been unique to the BI-SSb lobe, and a compilation of published dates reveals that similar advances during the last glacial cycle indicate related forcing factors operating across Patagonia and New Zealand