EGU2020-7644
https://doi.org/10.5194/egusphere-egu2020-7644
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

3D imaging of subglacial lineations under the Rutford Ice Stream, West Antarctica

Rebecca Schlegel1, Adam Booth2, Tavi Murray1, Andy Smith3, Alex Brisbourne3, Ed King3, Roger Clark2, and Steph Cornford1
Rebecca Schlegel et al.
  • 1Swansea University, Geography, Swansea, UK (967901@swansea.ac.uk)
  • 2University of Leeds, School of Earth and Environment, Leeds, UK
  • 3British Antarctic Survey, Natural Environment Research Council, Cambridge, UK

There are numerous theoretical descriptions of the subglacial conditions (water availability, subglacial geology, flow dynamics) required for the formation of subglacial lineations, such as mega-scale glacial lineations and drumlins, that are known to be indicative of fast ice flow. Traditionally, mapping in de-glaciated areas, both onshore and offshore, has been undertaken using bathymetric maps, satellite data and field observations; here, lineations currently beneath the Rutford Ice Stream (West Antarctica) have been mapped using ground-penetrating radar (GPR) and seismic methods.

The Rutford Ice Stream is more than 2 km thick, of which 1.4 km are located below sea level. The ice surface speed at the grounding line is >1 m per day, and satellite observations indicate a stable ice flow over the past 30 years. The ice-bed interface is assumed to be at the pressure-melting point, while the bed can be divided into a region of soft, deforming sediment, and one of stiff, non-deforming, sediment. Long, elongated lineations, up to ~14 km, up to 150 m high, and 50-500 m wide, are found aligned in the ice-flow direction in the area of the soft sediment, within which the deposition of a drumlin was observed over a period of <10 years. Together with local erosion occurring in the same timescale, this demonstrates the temporal variability of ice stream beds.

To study the detailed architecture of the lineations, 3D grids of GPR data were acquired during the Antarctic Summer Season 2017/18, enabling 3D-processing and imaging of lineations. Using this unique dataset, in conjunction with previous publications plus data from the paleo record, we hope to better understand the possible mechanisms of formation of subglacial lineations as well as subglacial conditions at the Rutford Ice Stream.

How to cite: Schlegel, R., Booth, A., Murray, T., Smith, A., Brisbourne, A., King, E., Clark, R., and Cornford, S.: 3D imaging of subglacial lineations under the Rutford Ice Stream, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7644, https://doi.org/10.5194/egusphere-egu2020-7644, 2020

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Presentation version 2 – uploaded on 06 May 2020
changed the last slide
  • CC1: Comment on EGU2020-7644, Matteo Spagnolo, 08 May 2020

    Hi Rebecca,

    just wanted to say that this looks very interesting. I am looking forward to seeing the results from the remaining two blocks. Let me know when this evolve into a paper.

    Matteo

    • AC1: Reply to CC1, Rebecca Schlegel, 26 May 2020

      Hi Matteo,

      thanks you for your comment. More than happy to get in contact once the remaining dataset has been processed and analyzed. 

      Best

      Rebecca

  • CC2: Comment on EGU2020-7644, Mark Johnson, 08 May 2020

    I enjoyed your talk very much! I am working in SW Sweden, and it is very common to have crag-and-tails, that is, till ridges that form down flow of a bedrock bump. Is that what you see in your images? However, we all also have the opposite: till ridges ('stoss-side drumlins') that have till accumulation on the upflow side of the (usually larger) bedrock bump, and with no 'tail' on the down ice side. Have you ever seen any of these?

    • AC2: Reply to CC2, Rebecca Schlegel, 26 May 2020

      Hi Mark,

      Sorry about the late reply and thank you for your question and interest. Whether or not there is a bedrock bump situated at the upstream end of these landforms can not be proven so far, but the radar reflectivity and also the seismic impedance suggests that they are completely formed of soft sediment. These landforms have been interpreted as drumlins and MSGL. From repeated Seismics (Smith et al 2007 doi:10.1130/G23036A.1) we know that one of the Drumlins did evolve or get longer towards downstream over 7 years period, so it looks (at least for this location) like we have a downflow accumulation.

      As far as the radar data, a crag and tail, with a bedrock bump at the upstream end, looks less likely, as it looks like the whole landform is composed of soft sediment. I guess a rock cored drumlin, might look like a drumlin composed of only soft sediment from the radar, but I would assume that seismic reflections would pick up the bedrock bump and we haven’t seen anything like this. But this is a really interesting thought, I will have another close look at the seismic data to make sure I didn’t miss that. Also analyzing the remaining radar datasets include the upstream end of a very prominent and big landform, so it will be really interesting to see what that looks like in more detail.

       

      Thank you for that idea, would be nice to have a chat, once the  remaining dataset has been processed

      Best wishes from Germany

      Rebecca

      • CC3: Reply to AC2, Mark Johnson, 26 May 2020

        Rebecca thanks. An additional concern is this--r´crag-and-tails and rock cored drulins are common in SW Sweden, where the glacier was not an ice stream, nor has it been thought of as a fast-flowing ice lobe. Most of these long MSGLs that get imaged from modern ice sheets are on ice streams, but in Sweden, we see these similar forms in a different setting. Not sure what to make of that! Thanks again for your response!

Presentation version 1 – uploaded on 30 Apr 2020
  • CC1: Comment on EGU2020-7644, Steven Franke, 01 May 2020

    Hi Rebecca,

    I envy you for your high-resolution data set : )
    Also, Donald Duck is a nice choice!

    I don't yet fully understand what exactly the values of the envelope of the bed reflection are.
    Is ist the integrated return signal power (every sample) of the bed in a certain window (that's what I assume)?

    I am wondering if the envelope value might also depend on the orientation of the radar antenna and the beam geometry in relation to the orientation of lineations on the bed. So, some sort of directional dependent roughness that could produce different return patterns. Is the length of your envelope constant or variable and does the reflection pattern of the bed look the same at intersections of your profiles?

    I am curious about these things because I am also working a bit on the analysis of bed reflection properties and I am wondering what causes the differences in reflectivity as well (next to the real bed properties).

    Cheers,

    Steven

    • AC1: Reply to CC1, Rebecca Schlegel, 06 May 2020

      Hi Steven,

      Thanks for your message. 
      I am jealous of the NEGIS data you god, the images look amazing. Congrads on your paper as well!

      Really good question, I thought about including some of this but tried to keep the presentation short, but here it comes: 

      Yes the envelope is the sum of amplitudes over a certain time window, which is fixed and encloses the whole bed reflection. 

      The dataset presented here is acquired using a ground based radio echo sounder, while data acquisition is perpendicular to ice flow, which I think is the most ideal, when it comes to linear features. Due to the consistency of most of the features along flow (at least landforms) I expect the dependency of radar orientation to landform orientation to be neglectable. But yes the different kind of topography we see might influence the reflection pater, e.g. upstream and downstream area. But the slopes of the topography are so low, that I don't expect a major effect. In other parts of the data, I see changing reflectivity along landforms (not shown here) which then suggests these changes to be independent from the radar orientation. 
      There was a study about the sensitivity of radar signals with a certain wavelength to different RMS highs (McGregor et al), which stated that data comparable to ours (3MHz) should not experience a significant reflectivity decrease from features of less than 10m RMS hight. So small scale topography should not be a major problem, and anything bigger 10m RMS we should see in the data-especially after 3D migration. A plot of e.g. slope vs. Reflectivity or azimuth vs. Reflectivity does not show a visible correlation or trend either. 
      Regarding intersections, we didn't manage to acquire intersections, the same year, but data acquired a year later looks very similar. 

      So exclude effects of the radiation patter, due to frequency we are using and the flat topography, I did investigate effects that might influence the signal, like interference or a possible englacial cause for this pattern of envelope, but this is still ongoing

      Hope that makes sense?

      Would be nice to have a chat with you after EGU?

      Thanks
      Rebecca