Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn

Schlegel, Rebecca; Diez, Anja; Löwe, Henning; Mayer, Christoph; Lambrecht, Astrid; Freitag, Johannes; Miller, Heinz; Hofstede, Coen; Eisen, Olaf

doi:10.1017/aog.2019.10

Cited by 24 publications

(29 citation statements)

References 64 publications

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“…The directional dependence of the seismic velocity, i.e. the seismic anisotropy of the firn, can also be determined by acquiring seismic data in different horizontal directions (Kirchner and Bentley, 1979, 1990; Schlegel and others, 2019).…”

Section: Introductionmentioning

confidence: 99%

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

et al. 2021

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We use seismic refraction data to investigate the firn structure across a suture zone on the Amery Ice Shelf, East Antarctica, and the possible role of glacier dynamics in firn evolution. In the downstream direction, the data reveal decreasing compressional-wave velocities and increasing penetration depth of the propagating wave in the firn layer, consistent with $\sim$ 1 m firn thickening every 6 km. The boundary between the Lambert Glacier unit to the west and a major suture zone and the Mawson Escarpment Ice Stream unit to the east, is marked by differences in firn thicknesses, compressional-wave velocities and seismic anisotropy in the across-flow direction. The latter does not contradict the presence of a single-maximum crystal orientation fabric oriented 45– $90^{\circ }$ away from the flow direction. This is consistent with the presence of transverse simple shear governing the region's underlying ice flow regime, in association with elevated strain along the suture zone. The confirmation and quantification of the implied dynamic coupling between firn and the underlying ice requires integration of future seismic refraction, coring and modelling studies. Because firn is estimated to cover $\sim$ 98% of the Antarctic continent any such coupling may have widespread relevance to ice-sheet evolution and flow.

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Section: Introductionmentioning

confidence: 99%

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

et al. 2021

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“…Under creep conditions, Theile et al (2011) simulated intragranular deformations and microstructural changes and concluded a measurable impact of the crystallographic anisotropy on the densification of structurally isotropic snow. Back to polar firn conditions, recent measurements of seismic diving waves at Kohnen (Schlegel et al, 2019) indicated the necessity of quantifying the crystallographic fabric alongside with the structural fabric to evaluate uncertainties in the inversion technique giving access to the elastic moduli in firn cores.…”

Section: Introductionmentioning

confidence: 99%

On the Birth of Structural and Crystallographic Fabric Signals in Polar Snow: A Case Study From the EastGRIP Snowpack

Montagnat

Löwe²,

Calonne

et al. 2020

Front. Earth Sci.

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The role of near-surface snow processes for the formation of climate signals through densification into deep polar firn is still barely understood. To this end we have analyzed a shallow snow pit (0-3 meters) from EastGRIP (Greenland) and derived high-resolution profiles of different types of mechanically relevant fabric tensors. The structural fabric, which characterizes the anisotropic geometry of ice matrix and pore space, was obtained by X-ray tomography. The crystallographic fabric, which characterizes the anisotropic distribution of the c-axis (or optical axis) orientations of snow crystals, was obtained from automatic analysis of thin sections. The structural fabric profile unambiguously reveals the seasonal cycles at EastGRIP, as a consequence of temperature gradient metamorphism, and in contrast to featureless signals of parameters like density or specific surface area. The crystallographic fabric profile unambiguously reveals a signal of cluster-type texture already at shallow depth. We make use of order of magnitude estimates for the formation time of both fabric signals and discuss potential coupling effects in the context of snow and firn densification.

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“…To determine the subglacial water pathways, we used a simple flux routing scheme to compute subglacial water pathways as described in Humbert et al (2018). Subglacial water flow is governed by the hydraulic potential (Shreve, 1972), which can be written as…”

Section: Routing Of Subglacial Watermentioning

confidence: 99%

“…Assuming α does not change over the travel path, A 0 can be calculated. As our geophones are vertically orientated and the direct wave is a diving wave (a continuously refracted wave due to the continuous densification of the firn pack; Schlegel et al, 2019), we used pairs of traces at larger offsets (97 m and larger) from the source, which causes the ray path of the diving wave to arrive at angles closer to normal incidence.…”

Section: Determination Of the Source Amplitude Amentioning

confidence: 99%

Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

et al. 2021

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Abstract. Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

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Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn

Cited by 24 publications

References 64 publications

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

On the Birth of Structural and Crystallographic Fabric Signals in Polar Snow: A Case Study From the EastGRIP Snowpack

Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

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