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  • 11
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    Online Resource
    Influence of ice crystal anisotropy on seismic velocity analysis
    International Glaciological Society ; 2014
    In:  Annals of Glaciology Vol. 55, No. 67 ( 2014), p. 97-106
    Details
    In: Annals of Glaciology, International Glaciological Society, Vol. 55, No. 67 ( 2014), p. 97-106
    Abstract: In 2010 a reflection seismic survey was carried out on the Alpine glacier Colle Gnifetti. The processed and depth-converted data could be compared to a nearby ice core, drilled almost to the bed. Comparisons showed that the depth of the P-wave bed reflection was too shallow, while the depth of the SH-wave bed reflection fitted the ice-core length well. We are now able to explain the major part of these differences using the existing crystal orientations of the ice at Colle Gnifetti. We calculate anisotropic velocities for P- and SH-waves that are usually picked for stacking and compare them with zero-offset velocities needed for the depth conversion. Here we take the firn pack at Colle Gnifetti into account for P- and S-wave analysis. To incorporate the S-wave analysis we first derive a new equation for the relationship between density and S-wave velocity from diving waves. We show that anisotropic fabrics observed at Colle Gnifetti introduce a difference of only 1% between stacking and depth-conversion velocities for the SH-wave, but 7% for the P-wave. We suggest that this difference in stacking and depth-conversion velocity for the P-wave can be used to derive information about the existing anisotropy by combining our seismic data with, for example, radar data.
    Type of Medium: Online Resource
    ISSN: 0260-3055 , 1727-5644
    URL: Article
    DOI: 10.3189/2014AoG67A002
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2014
    detail.hit.zdb_id: 2122400-6
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  • 12
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    Bed topography and subglacial landforms in the onset region of the Northeast Greenland Ice Stream
    Cambridge University Press (CUP) ; 2020
    In:  Annals of Glaciology Vol. 61, No. 81 ( 2020-04), p. 143-153
    Details
    In: Annals of Glaciology, Cambridge University Press (CUP), Vol. 61, No. 81 ( 2020-04), p. 143-153
    Abstract: The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km 2 , centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting.
    Type of Medium: Online Resource
    ISSN: 0260-3055 , 1727-5644
    URL: Article
    DOI: 10.1017/aog.2020.12
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 2020
    detail.hit.zdb_id: 2122400-6
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  • 13
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    Basal roughness of the East Antarctic Ice Sheet in relation to flow speed and basal thermal state
    Cambridge University Press (CUP) ; 2020
    In:  Annals of Glaciology Vol. 61, No. 81 ( 2020-04), p. 162-175
    Details
    In: Annals of Glaciology, Cambridge University Press (CUP), Vol. 61, No. 81 ( 2020-04), p. 162-175
    Abstract: Basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.
    Type of Medium: Online Resource
    ISSN: 0260-3055 , 1727-5644
    URL: Article
    DOI: 10.1017/aog.2020.47
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 2020
    detail.hit.zdb_id: 2122400-6
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  • 14
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    Five decades of radioglaciology — CORRIGENDUM
    Cambridge University Press (CUP) ; 2021
    In:  Annals of Glaciology Vol. 62, No. 85-86 ( 2021-09), p. 390-390
    Details
    In: Annals of Glaciology, Cambridge University Press (CUP), Vol. 62, No. 85-86 ( 2021-09), p. 390-390
    Type of Medium: Online Resource
    ISSN: 0260-3055 , 1727-5644
    URL: Article
    DOI: 10.1017/aog.2021.14
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 2021
    detail.hit.zdb_id: 2122400-6
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  • 15
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    Field ionization and electron impact mass spectra of cycloolefins
    Wiley ; 1984
    In:  Organic Mass Spectrometry Vol. 19, No. 4 ( 1984-04), p. 193-195
    Details
    In: Organic Mass Spectrometry, Wiley, Vol. 19, No. 4 ( 1984-04), p. 193-195
    Type of Medium: Online Resource
    ISSN: 0030-493X , 1096-9888
    URL: Issue
    URL: Journal
    URL: Article
    DOI: 10.1002/oms.v19:4
    DOI: 10.1002/(ISSN)1096-9888
    DOI: 10.1002/oms.1210190409
    Language: English
    Publisher: Wiley
    Publication Date: 1984
    detail.hit.zdb_id: 2197367-2
    detail.hit.zdb_id: 1472468-6
    detail.hit.zdb_id: 7414-7
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  • 16
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    Preserved landscapes underneath the Antarctic Ice Sheet reveal the geomorphological history of Jutulstraumen Basin
    Wiley ; 2021
    In:  Earth Surface Processes and Landforms Vol. 46, No. 13 ( 2021-10), p. 2728-2745
    Details
    In: Earth Surface Processes and Landforms, Wiley, Vol. 46, No. 13 ( 2021-10), p. 2728-2745
    Abstract: The landscape of Antarctica, hidden beneath kilometre‐thick ice in most places, has been shaped by the interactions between tectonic and erosional processes. The flow dynamics of the thick ice cover deepened pre‐formed topographic depressions by glacial erosion, but also preserved the subglacial landscapes in regions with moderate to slow ice flow. Mapping the spatial variability of these structures provides the basis for reconstruction of the evolution of subglacial morphology. This study focuses on the Jutulstraumen Glacier drainage system in Dronning Maud Land, East Antarctica. The Jutulstraumen Glacier reaches the ocean via the Jutulstraumen Graben, which is the only significant passage for draining the East Antarctic Ice Sheet through the western part of the Dronning Maud Land mountain chain. We acquired new bed topography data during an airborne radar campaign in the region upstream of the Jutulstraumen Graben to characterise the source area of the glacier. The new data show a deep relief to be generally under‐represented in available bed topography compilations. Our analysis of the bed topography, valley characteristics and bed roughness leads to the conclusion that much more of the alpine landscape that would have formed prior to the Antarctic Ice Sheet is preserved than previously anticipated. We identify an active and deeply eroded U‐shaped valley network next to largely preserved passive fluvial and glacial modified landscapes. Based on the landscape classification, we reconstruct the temporal sequence by which ice flow modified the topography since the beginning of the glaciation of Antarctica.
    Type of Medium: Online Resource
    ISSN: 0197-9337 , 1096-9837
    URL: Issue
    URL: Article
    DOI: 10.1002/esp.v46.13
    DOI: 10.1002/esp.5203
    Language: English
    Publisher: Wiley
    Publication Date: 2021
    detail.hit.zdb_id: 1479188-2
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  • 17
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    Determining the age distribution of Colle Gnifetti, Monte Rosa, Swiss Alps, by combining ice cores, ground-penetrating radar and a simple flow model
    International Glaciological Society ; 2013
    In:  Journal of Glaciology Vol. 59, No. 213 ( 2013), p. 179-189
    Details
    In: Journal of Glaciology, International Glaciological Society, Vol. 59, No. 213 ( 2013), p. 179-189
    Abstract: Ice cores from cold Alpine glaciers may provide unique paleoclimate information from non-polar latitudes. We explore the three-dimensional internal age distribution of the small cold glacier saddle (Colle Gnifetti, Monte Rosa, Italy/Switzerland) to compare the age/depth relations from four local deep ice cores. Tracking isochronous reflection horizons detected by ground-penetrating radar (GPR) among the core locations reveals consistent dating up to 80 years BP. This approach is confined to recent ages, due to the lack of clear reflections below the firn/ice transition. We attempt to overcome this limitation by including a two-dimensional flow model adapted to the GPR-derived surface accumulation and ice thickness distribution. Modeled and GPR isochrones are compared, indicating agreement in shape but featuring a potential offset of 0–3.5 m. The modeled isochrones are interpolated to the core array with ages assigned according to the ice-core datings. The resulting age distribution is consistent up to 110 years BP, with age uncertainties increasing from 7 to 〉 80 years in the lower half of the ice. This combination of methods is novel for Alpine sites and may be adapted for spatial extrapolation of ice properties other than age.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.3189/2013JoG12J072
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2013
    detail.hit.zdb_id: 2140541-4
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  • 18
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    Improved method to determine radio-echo sounding reflector depths from ice-core profiles of permittivity and conductivity
    International Glaciological Society ; 2006
    In:  Journal of Glaciology Vol. 52, No. 177 ( 2006), p. 299-310
    Details
    In: Journal of Glaciology, International Glaciological Society, Vol. 52, No. 177 ( 2006), p. 299-310
    Abstract: We present a technique that modifies and extends down-hole target methods to provide absolute measures of uncertainty in radar-reflector depth of origin. We use ice-core profiles to model wave propagation and reflection, and then cross-correlate the model results with radio-echo sounding (RES) data to identify the depth of reflector events. Stacked traces recorded with RES near the EPICA drill site in Dronning Maud Land, Antarctica, provide reference radargrams, and dielectric properties along the deep ice core form the input data to a forward model of wave propagation that produces synthetic radargrams. Cross-correlations between synthetic and RES radargrams identify differences in propagation wave speed. They are attributed to uncertainties in pure-ice permittivity and are used for calibration. Removing conductivity peaks results in the disappearance of related synthetic reflections and enables the unambiguous relation of electric signatures to RES features. We find that (i) density measurements with g-attenuation or dielectric profiling are too noisy below the firn–ice transition to allow clear identification of reflections, (ii) single conductivity peaks less than 0.5 m wide cause the majority of prominent reflections beyond a travel time of about 10 µs (~900m depth) and (iii) some closely spaced conductivity peaks within a range of 1–2m cannot be resolved within the RES or synthetic data. Our results provide a depth accuracy to allow synchronization of age–depth profiles of ice cores by RES, modeling of isochronous internal structures, and determination of wave speed and of pure-ice properties. The technique successfully operates with dielectric profiling and electrical conductivity measurements, suggesting that it can be applied at other ice cores and drill sites.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.3189/172756506781828674
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2006
    detail.hit.zdb_id: 2140541-4
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  • 19
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    Characteristics and small-scale variability of GPR signals and their relation to snow accumulation in Greenland’s percolation zone
    International Glaciological Society ; 2008
    In:  Journal of Glaciology Vol. 54, No. 185 ( 2008), p. 333-342
    Details
    In: Journal of Glaciology, International Glaciological Society, Vol. 54, No. 185 ( 2008), p. 333-342
    Abstract: We investigate snowpack properties at a site in west-central Greenland with ground-penetrating radar (GPR), supplemented by stratigraphic records from snow pits and shallow firn cores. GPR data were collected at a validation test site for CryoSat (T05 on the Expéditions Glaciologiques Internationales au Groenland (EGIG) line) over a 100 m × 100 m grid and along 1 km sections at frequencies of 500 and 800 MHz. Several internal reflection horizons (IRHs) down to a depth of 10 m were tracked. IRHs are usually related to ice-layer clusters in vertically bounded sequences that obtain their initial characteristics near the surface during the melt season. Warm conditions in the following melt season can change these characteristics by percolating meltwater. In cold conditions, smaller melt volumes at the surface can lead to faint IRHs. The absence of simple mechanisms for internal layer origin emphasizes the need for independent dating to reliably interpret remotely sensed radar data. Our GPR-derived depth of the 2003 summer surface of 1.48 m (measured in 2004) is confirmed by snow-pit observations. The distribution of IRH depths on a 1 km scale reveals a gradient of increasing accumulation to the northeast of about 5 cm w.e. km −1 . We find that point measurements of accumulation in this area are representative only over several hundred metres, with uncertainties of about 15% of the spatial mean.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.3189/002214308784886207
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2008
    detail.hit.zdb_id: 2140541-4
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  • 20
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    Characterizing the glaciological conditions at Halvfarryggen ice dome, Dronning Maud Land, Antarctica
    International Glaciological Society ; 2013
    In:  Journal of Glaciology Vol. 59, No. 213 ( 2013), p. 9-20
    Details
    In: Journal of Glaciology, International Glaciological Society, Vol. 59, No. 213 ( 2013), p. 9-20
    Abstract: We present a comprehensive approach (including field data, remote sensing and an anisotropic ice-flow model) to characterize Halvfarryggen ice dome in coastal Dronning Maud Land, Antarctica. This is a potential drill site for the International Partnerships in Ice Core Sciences, which has identified the need for ice cores covering atmospheric conditions during the last few millennia. We derive the surface topography, the ice stratigraphy from radar data, and accumulation rates which vary from 400 to 1670 kg m −2 a −1 due to preferred wind directions and changing surface slope. The stratigraphy shows anticlines and synclines beneath the divides. We transfer Dansgaard–Johnsen age–depth scales from the flanks along isochrones to the divide in the upper 20–50% of the ice thickness and show that they compare well with the results of a full-Stokes, anisotropic ice-flow model which predicts (1) 11 ka BP ice at 90% of the ice thickness, (2) a temporally stable divide for at least 2700–4500 years, (3) basal temperatures below the melting point (−12°C to −5°C) and (4) a highly developed crystal orientation fabric (COF). We suggest drilling into the apices of the deep anticlines, providing a good compromise between record length and temporal resolution and also facilitating studies of the interplay of anisotropic COF and ice flow.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.3189/2013JoG12J134
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2013
    detail.hit.zdb_id: 2140541-4
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