Eisen, Olaf
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Item-typ:Veröffentlichung, Imprints of Ice Dynamics and Atmospheric Signals on the internal structure of Antarctic Ice as seen via Radar(2011-10-24); ; ; Different electromagnetic reflection methods can operate from satellites, airplanes or ground vehicles to illuminate the surface and the inside of ice sheets across varying spatial scales. The backscattered signal is formed by micro-physical ice properties, many of which in turn are influenced by mechanisms operating on a macro-scale: the alignment of crystal orientation fabric (COF) depends on the specific strain regime and the initial impurity loading; the pattern in internal layering is imprinted by accumulation and the surrounding flow regime; the brightness of the bottom reflection over ice-shelves depends on the melting or refreezing of platelet ice which is susceptible to changing ocean currents and grounding line positions. The study is subdivided in five chapters which have been or will be published separately. All studies focus on the link between these small-scale features and their large-scale expressions. The synthesis of the different methods improves the capability of remote sensing to deliver variables from which the current state of the cryosphere can be determined. The study contributes to assimilating geophysical data into the coming generation of ice-dynamic models which improve the understanding of ice-sheet histories and prognose their future behaviour. The starting point is the appearance of the radio-echo free zone (EFZ), which is a feature-less band observed above the ice--bed interface in many radargrams across Greenland and Antarctica. The comparison of the EFZ onset with optical ice-core images yields a connection between the mm-cm scale disturbances in the core's stratigraphy and the disappearance of radar reflection horizons. This is evidence that ice flow can disturb internal ice layering, which hampers the derivation of a coherent age--depth scale and indicates a changing flow behaviour with depth. A polarimetric radar survey in the same study area shows that backscattered power varies with the horizontal orientation of the antennas (i.e. with the polarization plane). Extrema in backscatter for a constant antenna angle change in direction at 900~m depth. By using different scattering models I differentiate between competing mechanisms for the observed anisotropy, namely a vertically varying COF or ellipsoidal air bubbles. The analysis suggests that the effect from a varying COF is superior. Radar polarimetry thus is capable to infer the principal components of COF, and with it the principal components of the corresponding stress--strain regime. Potentially, the different modes in COF variations are related to climate signals. The detection of anisotropic COF is an important factor in terms of anisotropic ice flow, which in turn influences the shape in internal layering especially near ice divides. The third study combines satellite data and GPS measurements with airborne and ground-based radar surveys to characterize a potential drill site with respect to internal layering, accumulation, and the topography of surface and bedrock. Particular attention is given to the upwarping of internal layering beneath the divides, which is a consequence of a nonlinear and anisotropic rheology. All datasets are used as input for a two-dimensional, anisotropic flow model which estimates an age--depth distribution. Likely, ice from the last glacial is present at larger depths. Ice at intermediate depths seems suitable for studies targeting the last few thousand years. Based on the structure of internal layering, the ice divides and the surrounding flow regime were stable over the last 10 000 years. The investigated ice ridge is surrounded by ice shelves. Surface velocities as well as the transition of grounded and floating ice are mapped from satellite images. This is used in the last two studies. In a collaborative project, the most landward freely floating line of ice shelves, and the most seaward line where ice flow is still influenced by the bedrock are mapped from optical satellite imagery around the Antarctic perimeter. The simultaneous derivation of coastal elevation as a basis for the derivation of flow velocities is a starting point for mass balance estimates, in which ice flow, but also sub-ice shelf melting are important parameters. The latter is treated in a case study. Using the continuity equation in a steady-state approximation, it is possible to spatially map the sub-ice shelf melt rates. A new approach is presented which circumnavigates the interpolation of ice-thickness data by applying the continuity equation only along profile lines.Dissertation318 90 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Flow characteristics of the Northeast Greenland Ice Stream(2026-02-20); ; ; ; In recent years, increasing global temperatures have been shown to be causing the widespread shrinking of ice sheets and glaciers. In particular, rates of ice mass loss from the Greenland Ice Sheet have been accelerating, with around 50% of this mass imbalance being attributed to the dynamical discharge of fast-flowing ice streams and outlet glaciers to the ocean. In order to quantify and predict the future changes of ice sheet mass loss, the accurate representation of ice dynamics in ice sheet models is required. Specifically, the subglacial processes which underpin the fast flow of ice streams are poorly understood, in addition to the temporal and spatial stability of ice streams, which are central factors of the dynamic behaviour of past, present, and future ice sheets. This thesis explores the impacts of subglacial topography and basal conditions on the ice flow dynamics of Greenland’s largest ice stream (the Northeast Greenland Ice Stream; NEGIS), through the analysis of radio-echo sounding data. My focus is on the interactions between subglacial geomorphology and ice flow dynamics, to provide deeper insight into the processes which govern this fast-flowing outlet of the Greenland Ice Sheet and its evolution, as well as the formational processes of subglacial topography on multiple spatial scales. Here, I find that the basal conditions and subglacial geomorphology of the NEGIS evolve spatially downstream, and are more heterogenous than previously thought, which gives insight into the topographic and geologic controls on the NEGIS and provides new constraints on the subglacial geology. The change from upstream regions of softer sediment to rougher, hard bedrock downstream, as well as a trough channelling the ice flow, is likely to enable its propagation so far into the interior of the ice sheet. In addition, the finding of mega-scale glacial lineations (MSGLs) at the onset of the NEGIS was made possible with swath radar imaging, which produces an unprecedentedly high-resolution Digital Elevation Model of the subglacial bed. The presence of MSGLs beneath relatively slow ice flow velocities (<60 m yr-1), also places a new boundary condition on their formational processes, and raises questions surrounding their use as an indicator of very high (>100 m yr-1) ice flow velocities when reconstructing palaeo-ice sheets. This illustrates the importance of high-resolution mapping of the ice base, in order to understand both past ice flow, and the in-situ formation of subglacial landforms.Dissertation32 28 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Evidence of anthropogenic climate effects in snow and firn of East Antarctica? Characterization of low accumulation areas using multiparameter-analysis from snow and firn cores(2021-09-21); ; ; Antarctic ice masses are a unique climate archive, but are also strongly affected by global climate change. Field data from the East Antarctic Plateau are necessary for both, studying the signal formation of climate proxies and validating results from remote sensing. This work provides data and analysis of snow cores, sampled on a traverse between Kohnen Station and Plateau Station during the Antarctic summer of 2016/17. X-ray CT was used to determine the density and stratigraphic properties, then the cores were analyzed for stable water isotopes and major ions. Multiple snow cores per sampling location allow a more representative determination of the investigated parameters. Along the traverse route, the mean surface snow density is 355 kg m-3 and shows a lower dependence on temperature and accumulation rate than assumed. Modeled values show a significant discrepancy of about -10% from the measured density. In this work, the first dataset on the spatial distribution of crusts in polar snow is presented. Contrary to the assumption of finding more crusts in locations with lower accumulation rates, the total number of crusts per meter decreases with decreasing accumulation. The results suggest a relationship between the number of crusts and the logarithmic accumulation rate. Cycles in δ18O around Kohnen Station can still be interpreted as seasonal signals, but below an accumulation rate of 50 kg m-2a-1 they are no longer suitable for dating the snowpack on short time scales. A comparison with ECHAM6-wiso validates the model trend along the traverse, but shows a constant offset in δ18O. Modeled snow profiles with precipitation values from ECHAM6-wiso and a diffusion model represent the measured profiles well at 1-2 m depth. However, at the very surface, redeposition and sublimation appear to contribute significantly to (postdepositionally) shaping of the δ18O signal. The change in surface snow density between samples from 2005/06 and samples from this study can be attributed to different volume errors in sampling, but mean δ18O values over the recent decades show an increasing trend. While these values are within the range of natural variability, they may be early indications that climate proxies in the snow of the East Antarctic Plateau have already recorded the increase of the global temperature.Dissertation446 183 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Towards understanding the signal formation in polar snow, firn and ice using X-ray computed tomography(2018-02-07); ; ; Polar ice cores act as a unique archive of the Earth's climate system. However, due to logistic constraints, the representativity of these ice-core records cannot be estimated directly. One possible remedy is to analyze the spatial variability in polar snow and firn and combine the results with an improved understanding of the formation of paleoclimatic ice-core signals and their evolution with depth. Here, X-ray computed tomography is applied as a non-destructive method that yields information on stratigraphy and microstructure in polar snow and firn. The results are used to contribute to both subtopics of this indirect approach for estimating representativity. New methods for sampling the snowpack as well as the detection and alignment of coherent signals in spatially-distributed datasets are presented. They are applied to analyze spatial variability in the snowpack both on the local (trench studies in Greenland and East Antarctica, distances up to 100 m) and the regional scale (450 km traverse through North Greenland). The matching algorithm is validated using randomly generated profiles with the same statistical properties as the original data. Snow and firn density as markers of stratigraphy are determined by two-dimensional radioscopic imaging, the water-isotopic d18O signal is used for age dating. The results show that regionally a significant share of the stratigraphic density signal persists over hundreds of kilometers. Locally, there is a strong directional influence of the wind with a much larger homogeneity of the snowpack along the main wind direction. As density is an important input parameter for remote sensing and surface-mass-balance estimates, representative profiles or mean values of snow and firn density are required. Such a profile is provided for the upper two meters of the North Greenland snowpack. On the local scale, the estimation of representative densities for certain areas of interest (such as the footprint of an altimeter) is complicated by the directional dependence of the stratigraphic variability. As the density layering is significantly impacted by melting of the snow surface, melt features dating to the warm Greenlandic summer of 2012 are analyzed in detail. A large heterogeneity of these features is quantified, which does not only affect remote measurements (where ice layers act as reflectors for electromagnetic waves) but also strongly influences the ability to interpret single-core melt records. Methodological advances in the three-dimensional computed tomography of polar firn allow the creation of a first extensive dataset of direct firn-microstructure measurements. Three ice cores that represent different extremes of the temperature and accumulation ranges are analyzed throughout the lock-in zone, the depth range where pores are sealed from the atmosphere. The fundamental lock-in process is a determining factor for the gas-age-ice-age difference, which can be on the order of several 1,000 years. Thus, accurate estimates of this value are of particular importance for the interpretation of phase relationships between ice and gas records. The dataset is used to show that the critical porosity of pore enclosure is a climate-independent constant, a finding that is corroborated by percolation theory. Incorporation of this result significantly influences the dating of trace-gas records, reducing mismatches with other climate proxies by up to more than 1,000 years. Furthermore, it is demonstrated why previous measurements yielded misleading results.Dissertation392 154