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  • 1
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    Radiostratigraphy and age structure of the Greenland Ice Sheet (2015)
    Wiley-Blackwell
    Details
    Publication Date: 2015-01-18
    Description: Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice-sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along-track, these slope fields predict the radiostratigraphy and simplify semi-automatic reflection tracing. Core-intersecting reflections were dated using synchronized depth–age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth–age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley-Blackwell on behalf of American Geophysical Union (AGU).
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  • 2
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    A new bed elevation dataset for Greenland (2013)
    In:  EPIC3The Cryosphere, 7(2), pp. 499-510
    Details
    Publication Date: 2017-10-17
    Description: We present a new bed elevation dataset for Greenland derived from a combination of multiple airborne ice thickness surveys undertaken between the 1970s and 2012. Around 420 000 line kilometres of airborne data were used, with roughly 70 % of this having been collected since the year 2000, when the last comprehensive compilation was un- dertaken. The airborne data were combined with satellite- derived elevations for non-glaciated terrain to produce a con- sistent bed digital elevation model (DEM) over the entire is- land including across the glaciated–ice free boundary. The DEM was extended to the continental margin with the aid of bathymetric data, primarily from a compilation for the Arc- tic. Ice thickness was determined where an ice shelf exists from a combination of surface elevation and radar soundings. The across-track spacing between flight lines warranted in- terpolation at 1 km postings for significant sectors of the ice sheet. Grids of ice surface elevation, error estimates for the DEM, ice thickness and data sampling density were also pro- duced alongside a mask of land/ocean/grounded ice/floating ice. Errors in bed elevation range from a minimum of ±10 m to about ±300 m, as a function of distance from an obser- vation and local topographic variability. A comparison with the compilation published in 2001 highlights the improve- ment in resolution afforded by the new datasets, particularly along the ice sheet margin, where ice velocity is highest and changes in ice dynamics most marked. We estimate that the volume of ice included in our land-ice mask would raise mean sea level by 7.36 m, excluding any solid earth effects that would take place during ice sheet decay.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 3
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    Eemian interglacial reconstructed from a Greenland folded ice core (2013)
    Nature Publishing Group
    In:  EPIC3Nature, Nature Publishing Group, 493(7433), pp. 489-494, ISSN: 0028-0836
    Details
    Publication Date: 2017-10-18
    Description: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling (‘NEEM’) ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 ± 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 4
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    Ultra-Wideband Radars for Measurements over Land and Sea Ice (2015)
    In:  EPIC3AGU Fall Meeting, San Francisco, 2015-12-14-2015-12-18
    Details
    Publication Date: 2017-10-17
    Description: We developed two ultra-wideband (UWB) radars for measurements over ice sheets in Greenland and Antarctica, and sea ice. One of the UWB radars operates over the frequency range of 150-600 MHz with a large cross-track 24-element array. It is designed to sound ice, image ice-bed interface and map internal layers with fine resolution. The 24-element-array consists of three 8-element sub-arrays. One of these sub-arrays is mounted under the fuselage and the other two are mounted under the wings of BT-67 aircraft. The measured inflight VSWR is less than 2 over the entire operating range. The fuselage sub- array is used both for transmission and reception, and wing-mounted sub-arrays are used for reception. The transmitter consists of 8-channel digital waveform generator to synthesize chirped pulses of selectable pulse width and duration. It also consists of driver and power amplifiers to increase the power level to about 1 kW and a fast transmit/receive switch that can handle more than 1 kW of peak power. Each receiver consists of a limiter, switches, low-noise and driver amplifiers, and filters to shape and amplify received signals to the level required for digitization. The digital sub-section consists of timing and control sub-system, and 24-A/D converters to digitize received signals at a rate of 1.6 GHz. The radar performance is evaluated using a delay line to simulate returns from about 2 km thick ice and the loop sensitivity is more than 215 dB. The other UWB microwave radar operates over the frequency range of 2-18 GHz in the Frequency-modulated Continuous Wave (FM-CW) mode. It is designed to sound more than 100 cm snow over sea ice and map internal layers to a depth about 25-40 m in polar firn and ice. We operated the microwave radar over snow-covered sea ice and mapped snow as thin as 5 cm and as thick as 60 cm. With an early version of the radar we mapped internal layers to a depth of 45 m with fine resolution in West Antarctica. We will discuss design considerations and present laboratory results to document radars performance including their impulse response functions. We will show the results from a field campaign over the Greenland ice sheet.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 5
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    Constraining the recent mass balance of Pine Island and Thwaites glaciers, West Antarctica, with airborne observations of snow accumulation (2014)
    Copernicus Publications on behalf of the European Geosciences Union
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Cryosphere 8 (2014): 1375-1392, doi:10.5194/tc-8-1375-2014.
    Description: In Antarctica, uncertainties in mass input and output translate directly into uncertainty in glacier mass balance and thus in sea level impact. While remotely sensed observations of ice velocity and thickness over the major outlet glaciers have improved our understanding of ice loss to the ocean, snow accumulation over the vast Antarctic interior remains largely unmeasured. Here, we show that an airborne radar system, combined with ice-core glaciochemical analysis, provide the means necessary to measure the accumulation rate at the catchment-scale along the Amundsen Sea coast of West Antarctica. We used along-track radar-derived accumulation to generate a 1985–2009 average accumulation grid that resolves moderate- to large-scale features (〉25 km) over the Pine Island–Thwaites glacier drainage system. Comparisons with estimates from atmospheric models and gridded climatologies generally show our results as having less accumulation in the lower-elevation coastal zone but greater accumulation in the interior. Ice discharge, measured over discrete time intervals between 1994 and 2012, combined with our catchment-wide accumulation rates provide an 18-year mass balance history for the sector. While Thwaites Glacier lost the most ice in the mid-1990s, Pine Island Glacier's losses increased substantially by 2006, overtaking Thwaites as the largest regional contributor to sea-level rise. The trend of increasing discharge for both glaciers, however, appears to have leveled off since 2008.
    Description: This research was supported at UW by NSF OPP grants ANT-0631973 (B Medley, I. Joughin, E. J. Steig, and H. Conway) and ANT-0424589 (B. Medley and I. Joughin). Work at WHOI was supported by NSF OPP grant ANT-0632031 and NASA grant NNX10AP09G (S. B. Das and A. S. Criscitiello). D. H. Bromwich and J. P. Nicolas were supported by NASA grant NN12XAI29G and NSF grant ANT-1049089.
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    Airborne-radar and ice-core observations of annual snow accumulation over Thwaites Glacier, West Antarctica confirm the spatiotemporal variability of global and regional atmospheric models (2013)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 3649–3654, doi:10.1002/grl.50706.
    Description: We use an airborne-radar method, verified with ice-core accumulation records, to determine the spatiotemporal variations of snow accumulation over Thwaites Glacier, West Antarctica between 1980 and 2009. We also present a regional evaluation of modeled accumulation in Antarctica. Comparisons between radar-derived measurements and model outputs show that three global models capture the interannual variability well (r 〉 0.9), but a high-resolution regional model (RACMO2) has better absolute accuracy and captures the observed spatial variability (r = 0.86). Neither the measured nor modeled accumulation records over Thwaites Glacier show any trend since 1980. Although an increase in accumulation may potentially accompany the observed warming in the region, the projected trend is too small to detect over the 30 year record.
    Description: This research was supported at UW by NSF OPP grants ANT-0631973 (B.M., I.J., E.J.S., and H.C.) and ANT-0424589 (B.M. and I.J.) and at WHOI by ANT-0632031 (S.B.D. and A.S.C.). D.H.B. and J.P.N. were supported by NASA grant NN12XAI29G. We acknowledge the work by the CReSIS team that went into developing the snow-radar system, which was partially supported with by NASA grant NNX10AT68G and by NSF OPP grant ANT-0424589 awarded to S.P.
    Description: 2014-01-26
    Keywords: West Antarctica ; Snow accumulation ; Airborne radar ; Firn
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 7
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    A search in North Greenland for a new ice core drilling site (1997)
    In:  EPIC3Journal of Glaciology, 43(144), pp. 300-306
    Details
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 8
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    CryoSat cal/val activities on Austfonna – Part II: Ground-based measurements using a Ku-band radar, for validation of the CryoSat-2 SIRAL data (2012)
    In:  EPIC3Earth Observation and Cryosphere Science Conference, 2012-11-13-2012-11-16ESA-ESRIN, Frascati
    Details
    Publication Date: 2017-10-17
    Description: We show first results of a combined ground based and airborne validation campaign for CryoSat-2 synthetic aperture interferometric radar altimeter (SIRAL) measurements over Austfonna, Svalbard. A decade time series of glacier facies for this ice cap interpreted from 800 MHz ground-penetrating radar data clearly show there to be considerable variation within the firn pack, which needs to be captured by CryoSat-2 if we are to correctly interpret the satellite observed surface changes. The ground based radar data were obtained with a CRESIS Ku-band FMCW radar. The radar was operated to cover the frequency band of the CryoSat-2 SIRAL and the airborne version, Airborne Synthetic Aperture and Interferometric Radar Altimeter System (ASIRAS) carried by the Danish geophysics plane. Simultaneous profiles were obtained within half a day of each other with all three radars during the spring calibration/validation campaign on the Austfonna ice cap, Svalbard. The profiles, totaling approximately 200 km, span an elevation change of 400 m from the summit down to the ablation area, and cover a range of glacier facies and surface snow conditions. The ground based KU-band radar, which clearly images the near surface layering, was supported by manual snow depth sounding, 800 MHz radar, snow pits, firn cores, and borehole videos, all of which are used to validate interpretation of the CryoSat-2 data.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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