scholarly journals Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

2009 ◽  
Vol 55 (191) ◽  
pp. 461-472 ◽  
Author(s):  
Ilka Weikusat ◽  
Sepp Kipfstuhl ◽  
Sérgio H. Faria ◽  
Nobuhiko Azuma ◽  
Atsushi Miyamoto
Keyword(s):  
Crystal Orientation ◽  
Strain Energy ◽  
Strong Influence ◽  
Subgrain Boundary ◽  
Statistical Data ◽  
Ice Core ◽  
Mechanical Anisotropy ◽  
Classical View ◽  
Dronning Maud Land ◽  
Observable Feature

AbstractSubgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.

scholarly journals Acoustic velocity measurements for detecting the crystal orientation fabrics of a temperate ice core

2021 ◽  
Vol 15 (7) ◽  
pp. 3507-3521
Author(s):  
Sebastian Hellmann ◽  
Melchior Grab ◽  
Johanna Kerch ◽  
Henning Löwe ◽  
Andreas Bauder ◽  
...  
Keyword(s):  
Acoustic Waves ◽  
Crystal Orientation ◽  
Ice Core ◽  
Point Contact ◽  
Ice Cores ◽  
Swiss Alps ◽  
Mechanical Anisotropy ◽  
Similar Amount ◽  
Core Samples ◽  
Ultrasonic Measurements

Abstract. The crystal orientation fabric (COF) in ice cores provides detailed information, such as grain size and distribution and the orientation of the crystals in relation to the large-scale glacier flow. These data are relevant for a profound understanding of the dynamics and deformation history of glaciers and ice sheets. The intrinsic, mechanical anisotropy of the ice crystals causes an anisotropy of the polycrystalline ice of glaciers and affects the velocity of acoustic waves propagating through the ice. Here, we employ such acoustic waves to obtain the seismic anisotropy of ice core samples and compare the results with calculated acoustic velocities derived from COF analyses. These samples originate from an ice core from Rhonegletscher (Rhone Glacier), a temperate glacier in the Swiss Alps. Point-contact transducers transmit ultrasonic P waves with a dominant frequency of 1 MHz into the ice core samples and measure variations in the travel times of these waves for a set of azimuthal angles. In addition, the elasticity tensor is obtained from laboratory-measured COF, and we calculate the associated seismic velocities. We compare these COF-derived velocity profiles with the measured ultrasonic profiles. Especially in the presence of large ice grains, these two methods show significantly different velocities since the ultrasonic measurements examine a limited volume of the ice core, whereas the COF-derived velocities are integrated over larger parts of the core. This discrepancy between the ultrasonic and COF-derived profiles decreases with an increasing number of grains that are available within the sampling volume, and both methods provide consistent results in the presence of a similar amount of grains. We also explore the limitations of ultrasonic measurements and provide suggestions for improving their results. These ultrasonic measurements could be employed continuously along the ice cores. They are suitable to support the COF analyses by bridging the gaps between discrete measurements since these ultrasonic measurements can be acquired within minutes and do not require an extensive preparation of ice samples when using point-contact transducers.

scholarly journals Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric

2007 ◽  
Vol 1 (1) ◽  
pp. 1-10 ◽  
Author(s):  
O. Eisen ◽  
I. Hamann ◽  
S. Kipfstuhl ◽  
D. Steinhage ◽  
F. Wilhelms
Keyword(s):  
Crystal Orientation ◽  
Ice Core ◽  
Pulse Length ◽  
Vertical Plane ◽  
Radio Echo Sounding ◽  
Core Profile ◽  
Dronning Maud Land ◽  
Ice Sheet Dynamics ◽  
Drilling Site ◽  
Orientation Feature

Abstract. The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle to an increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2025–2045 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile, synthetic RES data, and a RES profile with different electromagnetic polarisation azimuths. The reflector's magnitude shows maximum values for polarisation parallel to the nearby ice divide and disappears for polarisation perpendicular to it, identifying the orientation of the girdle to lie in the vertical plane parallel to the ice divide. Observations allow us to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics and modeling.

scholarly journals Acoustic velocity measurements for detecting the crystal orientation fabrics of a temperate ice core

2021 ◽  
Author(s):  
Sebastian Hellmann ◽  
Melchior Grab ◽  
Johanna Kerch ◽  
Henning Löwe ◽  
Andreas Bauder ◽  
...  
Keyword(s):  
Acoustic Waves ◽  
Crystal Orientation ◽  
Ice Core ◽  
Point Contact ◽  
Ice Cores ◽  
Swiss Alps ◽  
Mechanical Anisotropy ◽  
Similar Amount ◽  
Core Samples ◽  
Ultrasonic Measurements

Abstract. The crystal orientation fabrics (COF) in ice cores provides detailed information, such as grain size and distribution and the orientation of the crystals in relation to the large-scale glacier flow. These data are relevant for a profound understanding of the dynamics and deformation history of glaciers and ice sheets. The intrinsic, mechanical anisotropy of the ice crystals causes an anisotropy of the polycrystalline ice of glaciers and affects the velocity of acoustic waves propagating through the ice. Here, we employ such acoustic waves to obtain the seismic anisotropy of ice core samples and compare the results with calculated acoustic velocities derived from COF analyses. These samples originate from an ice core from Rhonegletscher, a temperate glacier in the Swiss Alps. Point-contact transducers transmit ultrasonic p-waves with a dominant frequency of 1 MHz into the ice core samples and measure variations of the travel times of these waves for a set of azimuthal angles. In addition, the elasticity tensor is obtained from laboratory-measured COF and calculate the associated seismic velocities. We compare these COF-derived velocity profiles with the measured ultrasonic profiles. Especially in the presence of large ice grains, these two methods show significantly different velocities since the ultrasonic measurements examine a limited volume of the ice core whereas the COF-derived velocities are integrated over larger parts of the core. This discrepancy between the ultrasonic and COF-derived profiles decreases with an increasing number of grains that is available within the sampling volume and both methods provide concise results in presence of a similar amount of grains. We also explore the limitations of ultrasonic measurements and provide suggestions for improving their results. These ultrasonic measurements could be employed continuously along the ice cores. They are suitable to support the COF analyses by bridging the gaps between discrete measurements, since these ultrasonic measurements can be acquired within minutes and do not require an extensive preparation of ice samples when using point-contact transducers.

scholarly journals Direct evidence for radar reflector originating from changes in crystal-orientation fabric

2007 ◽  
Vol 1 (1) ◽  
pp. 1-16 ◽  
Author(s):  
O. Eisen ◽  
I. Hamann ◽  
S. Kipfstuhl ◽  
D. Steinhage ◽  
F. Wilhelms
Keyword(s):  
Crystal Orientation ◽  
Ice Core ◽  
Pulse Length ◽  
Radio Echo Sounding ◽  
Conductivity Profile ◽  
Core Profile ◽  
Dronning Maud Land ◽  
Ice Sheet Dynamics ◽  
Drilling Site ◽  
Orientation Feature

Abstract. The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle- to increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2020–2030 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile and synthetic RES data. Our observations allow to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics. As the conductivity profile of the EPICA shows no distinctive peak at this depths, we exclude changes in conductivity as the reflector origin. This is supported by application of numerical forward modelling of electromagnetic wave propagation, based on the conductivity profile, which is able to reproduce nearby reflections, but fails to reproduce this one. Because of background noise, the permittivity profile based on dielectric does not show prominent signals at these depths. We therefore interpret the observed reflector to originate from this change in crystal fabric.

scholarly journals Dielectric anisotropy as indicator of crystal orientation fabric in Dome Fuji ice core: method and initial results

2021 ◽  
pp. 1-12
Author(s):  
Tomotaka Saruya ◽  
Shuji Fujita ◽  
Ryo Inoue
Keyword(s):  
Crystal Orientation ◽  
Horizontal Plane ◽  
Ice Core ◽  
Vertical Plane ◽  
Vertical Direction ◽  
Core Sample ◽  
Dielectric Anisotropy ◽  
Principal Axes ◽  
Polycrystalline Ice ◽  
Initial Results

Abstract Polycrystalline ice is known to exhibit macroscopic anisotropy in relative permittivity (ɛ) depending on the crystal orientation fabric (COF). Using a new system designed to measure the tensorial components of ɛ, we investigated the dielectric anisotropy (Δɛ) of a deep ice core sample obtained from Dome Fuji, East Antarctica. This technique permits the continuous nondestructive assessment of the COF in thick ice sections. Measurements of vertical prism sections along the core showed that the Δɛ values in the vertical direction increased with increasing depth, supporting previous findings of c-axis clustering around the vertical direction. Analyses of horizontal disk sections demonstrated that the magnitude of Δɛ in the horizontal plane was 10–15% of that in the vertical plane. In addition, the directions of the principal axes of tensorial ɛ in the horizontal plane corresponded to the long or short axis of the elliptically elongated single-pole maximum COF. The data confirmed that Δɛ in the vertical and horizontal planes adequately indicated the preferred orientations of the c-axes, and that Δɛ can be considered to represent a direct substitute for the normalized COF eigenvalues. This new method could be extremely useful as a means of investigating continuous and depth-dependent variations in COF.

scholarly journals Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO2 changes during the past millennium

Tellus B ◽  
2005 ◽  
Vol 57 (1) ◽  
pp. 51-57 ◽  
Author(s):  
URS SIEGENTHALER ◽  
ERIC MONNIN ◽  
KENJI KAWAMURA ◽  
RENATO SPAHNI ◽  
JAKOB SCHWANDER ◽  
...  
Keyword(s):  
Ice Core ◽  
Atmospheric Co2 ◽  
Supporting Evidence ◽  
The Past ◽  
Dronning Maud Land ◽  
Past Millennium

scholarly journals Glacial–interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-δ<sup>15</sup>N measurements

2013 ◽  
Vol 9 (3) ◽  
pp. 983-999 ◽  
Author(s):  
E. Capron ◽  
A. Landais ◽  
D. Buiron ◽  
A. Cauquoin ◽  
J. Chappellaz ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Detailed Comparison ◽  
Last Deglaciation ◽  
Depth Study ◽  
Dronning Maud Land ◽  
The Last Deglaciation ◽  
Correct Estimation ◽  
Lock In

Abstract. Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of δ15N of N2 in air trapped in ice core, assuming that δ15N is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-δ15N measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. Our δ15N profiles reveal a heterogeneous response of the firn structure to glacial–interglacial climatic changes. While firn densification simulations correctly predict TALDICE δ15N variations, they systematically fail to capture the large millennial-scale δ15N variations measured at BI and the δ15N glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. New constraints of the EDML gas–ice depth offset during the Laschamp event (~41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model–δ15N data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the δ15N profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.

scholarly journals Simulation of the Antarctic ice sheet with a three-dimensional polythermal ice-sheet model, in support of the EPICA project. II. Nested high-resolution treatment of Dronning Maud Land, Antarctica

2000 ◽  
Vol 30 ◽  
pp. 69-75 ◽  
Author(s):  
A. Savvin ◽  
R. Greve ◽  
R. Calov ◽  
B. Mügge ◽  
K. Hutter
Keyword(s):  
High Resolution ◽  
Shear Deformation ◽  
Ice Core ◽  
Geographic Origin ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Climate History ◽  
Antarctic Ice Sheet ◽  
Drill Site ◽  
Dronning Maud Land

AbstractThe modern dynamic and thermodynamic state of the entire Antarctic ice sheet is computed for a 242 200 year paleoclimatic simulation with the three-dimensional polythermal ice-sheet model SICOPOLIS. The simulation is driven by a climate history derived from the Vostok ice core and the SPECMAP sea-level record. In a 872 km × 436 km region in western Dronning Maud Land (DML), where a deep ice core is planned for EPICA, new high-resolution ice-thickness data are used to compute an improved bedrock topography and a locally refined numerical grid is applied which extends earlier work (Calov and others, 1998). The computed fields of basal temperature, age and shear deformation, together with the measured accumulation rates, give valuable information for the selection of a drill site suitable for obtaining a high-resolution climate record for the last glacial cycle. Based on these results, a possible drill site at 73°59′ S, 00°00′ E is discussed, for which the computed depth profiles of temperature, age, velocity and shear deformation are presented. The geographic origin of the ice column at this position extends 320 km upstream and therefore does not leave the DML region.

scholarly journals "EDML1": a chronology for the EPICA deep ice core from Dronning Maud Land, Antarctica, over the last 150 000 years

2007 ◽  
Vol 3 (3) ◽  
pp. 475-484 ◽  
Author(s):  
U. Ruth ◽  
J.-M. Barnola ◽  
J. Beer ◽  
M. Bigler ◽  
T. Blunier ◽  
...  
Keyword(s):  
Time Scales ◽  
Internal Consistency ◽  
Time Scale ◽  
Ice Core ◽  
Ice Flow ◽  
Volcanic Events ◽  
Dronning Maud Land

Abstract. A chronology called EDML1 has been developed for the EPICA ice core from Dronning Maud Land (EDML). EDML1 is closely interlinked with EDC3, the new chronology for the EPICA ice core from Dome-C (EDC) through a stratigraphic match between EDML and EDC that consists of 322 volcanic match points over the last 128 ka. The EDC3 chronology comprises a glaciological model at EDC, which is constrained and later selectively tuned using primary dating information from EDC as well as from EDML, the latter being transferred using the tight stratigraphic link between the two cores. Finally, EDML1 was built by exporting EDC3 to EDML. For ages younger than 41 ka BP the new synchronized time scale EDML1/EDC3 is based on dated volcanic events and on a match to the Greenlandic ice core chronology GICC05 via 10Be and methane. The internal consistency between EDML1 and EDC3 is estimated to be typically ~6 years and always less than 450 years over the last 128 ka (always less than 130 years over the last 60 ka), which reflects an unprecedented synchrony of time scales. EDML1 ends at 150 ka BP (2417 m depth) because the match between EDML and EDC becomes ambiguous further down. This hints at a complex ice flow history for the deepest 350 m of the EDML ice core.

scholarly journals New constraints on the gas age-ice age difference along the EPICA ice cores, 0–50 kyr

2007 ◽  
Vol 3 (3) ◽  
pp. 527-540 ◽  
Author(s):  
L. Loulergue ◽  
F. Parrenin ◽  
T. Blunier ◽  
J.-M. Barnola ◽  
R. Spahni ◽  
...  
Keyword(s):  
Ice Core ◽  
Phase Relationship ◽  
Ice Cores ◽  
Climatic Conditions ◽  
Ice Age ◽  
Age Difference ◽  
Last Deglaciation ◽  
Last Glacial Period ◽  
Polar Ice Sheets ◽  
Dronning Maud Land

Abstract. Gas is trapped in polar ice sheets at ~50–120 m below the surface and is therefore younger than the surrounding ice. Firn densification models are used to evaluate this ice age-gas age difference (Δage) in the past. However, such models need to be validated by data, in particular for periods colder than present day on the East Antarctic plateau. Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We also use the structured 10Be peak, occurring 41 kyr before present (BP) and due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the Δage during this event. Our results seem to reveal an overestimate of the Δage by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and temperature scenarios do not entirely resolve this discrepancy. Although the exact reasons for the Δage overestimate at the two EPICA sites remain unknown at this stage, we conclude that current densification model simulations have deficits under glacial climatic conditions. Whatever the cause of the Δage overestimate, our finding suggests that the phase relationship between CO2 and EDC temperature previously inferred for the start of the last deglaciation (lag of CO2 by 800±600 yr) seems to be overestimated.

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