the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jun 2023
Surface dynamics and history of the calving cycle of the Astrolabe glacier (Antarctica) derived from optical imagery
Abstract. The recent calving of the Astrolabe glacier (Terre Adélie, East Antarctica) in November 2021 presents an opportunity to better understand the processes leading to ice fracturing. Optical satellite imagery is used to retrieve the calving cycle of the glacier since 2000 by mapping the ice front location. A recent archive of high resolution optical images from Sentinel-2 is used to measure the ice motion and the ice strain rates for the period 2017–2021 in order to document fractures and rift evolution. These observations are compared with sea ice extent and concentration measurements. We found that a significant change in the sea ice melting periodicity at the vicinity of the Astrolabe glacier occurred in the last decade (2011–2021) with respect to previous observations (1979–2011). After 2011, the occurrence of consecutive years of high sea-ice concentration at the vicinity of the glacier seems to have favored the ice tongue spatial extension. This lead to an unprecedentedly observed extension of the ice tongue until November 2021. The analysis of strain rate time series revealed that the glacier dislocated suddenly in June 2021 in the middle of the winter before releasing an iceberg of around 20 km2 in November 2021 at the onset of sea ice melting season. These observations suggest that although the presence of sea ice favors glacier extension, its buttressing effect may not be sufficient to prevent fracture opening.
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RC1: 'Comment on egusphere-2023-736', Anonymous Referee #1, 28 Jul 2023
This manuscript reports on the recent calving dynamics of Astrolabe Glacier, a small glacier in Terre Adélie Land, East Antarctic. The manuscript reports of an anomalous advance of the ice tongue which the authors link to strong landfast sea-ice conditions, but they also highlight that fractures can develop during the winter. Sea-ice buttressing is a potenitally important but understudied topic and the detailed inter-annual observations presented in this manuscript have the potential to be useful for the wider community. However, I think major revisions are required here before publication can be considered. My concerns are detailed in the below comments:
Major comments
“Unprecedented advance”: The manuscript states in a few locations that there was an unprecedented advance of the ice tongue up to November 2021. This may well still prove to be correct, but I cannot be certain of this because the study does not show the full time series of available satellite imagery. The manuscript looks in detail between 2000-2022, but what about the earlier satellite imagery? I am pretty sure there is available imagery going back to the 1970s e.g. 1997 radarsat mosaic, ~1992 ERS data, Landsat4/5 late 1980s, Landsat 1 early 1970s, maybe even the ARGON mosaic from 1963 (Kim et al., 2007) or even aerial photography from the 1940s. I have not checked for each of those satellites, but there is certainly a cloud-free Landsat-1 image from the early 1970s. I appreciate that there are gaps between these images, but they are still important to include to help determine if this really is an unprecedented advance. The ice-front extent from these images should be added to Figure 5a.
Sea-ice concentrations: As I understand, the authors extract mean sea ice concentrations from a 4,000km box surrounding the glacier, they infer periods of multi-year landfast sea-ice conditions form this analysis. I am not sure this is the case. The dataset the authors use is not the best for determining Landfast sea-ice conditions because the pixels are course and they do not extend all the way to the coastline. Indeed, a quick look at the MODIS imagery shows that actually the glacier is sea-ice free for large portions of nearly every summer, including the period of ice tongue advance. So the claim that multi-year fast ice has forced the advance of the ice tongue is incorrect. It appears that there is no MYFI at any stage over the period 2011-2021. I think a useful figure for any revised manuscript could be a multi-year panel showing a MODIS image showing the sea-ice conditions at the sea-ice minima (roughly March) every year from 2000 (or at least the period of anomalous advance 2011-2022), this would enable the reader to better visualise the sea-ice conditions. (see MODIS imagery on NASA Worldview website; link below).
This is important, the other studies mentioned in the manuscript (Miles et al., 2017 – Porpoise Bay, Gomes-Fell et al., 2020 – Victoria Land and Christie et al., 2022 – Antarctic Peninsula and other studies) link the advance of outlet glaciers to persistent multi-year fast ice, and when the fast ice finally breaks away the ice shelves/tongue calve instantly. A different process is clearly operating here and the differences between the studies mentioned above and the processes operating here should be discussed in any revised version. The increased sea-ice concentrations in the 4,000 km box are clear in the summer post 2011 and Figure 5b is convincing, but the mechanism that the authors hypothesize is driving the changes to the iec tongue needs to be much more clear.
It is also essential that the authors detail the sea-ice concentrations in the methods section of the paper, it has been completely missed out. Including where you have actually extracted the data i.e. show the 4,000 km box on a figure.
Glacier acceleration and ice break off: 2021: In general I was a little confused in this section, are you refereeing to an acceleration of the entire glacier? Or is this just a local acceleration on the ice tongue near the ice-front?
Title: I feel it could be made more impactful if it included something related to sea-ice, the process that the authors hypothesize is driving change here. Very few glaciologists would recognize Astrolabe Glacier and I fear most would simply skip over the manuscript based on the current lengthy title. I think the title needs to include something relating to sea-ice to entice a much wider readership. Perhaps something along the lines of: ‘Anomalous advance of the Astrolabe Glacier Tongue driven by more persistent sea-ice conditions 2011-2021’. (Or something similar)
Cause of shift in sea-ice conditions: The shift in sea-ice conditions appears is around 2011. This more or less coincides with the calving of the Mertz Glacier tongue just round the corner from Astrolabe. The calving of the Mertz Glacier fundamentally changed the local sea-ice conditions (see Tamura et al., 2012; Campagne et al., 2015 and several others). Feel free to ignore, but maybe worth briefly investigating and would make an interesting discussion point.
Figures
Figure 1: There is no satellite image of the entire glacier at any point in the manuscript. This is an essential requirement and It would be make sense for this to be in figure 1. I would recommend replacing the DEM with a Landsat or Sentinel-2 image. You could also add all the mapped ice-fronts to this figure.
Figure 5: I think panel (a) is a nice figure, but you could consider extending the ice-front position change back to the 1970s. It is difficult to judge panel (c) because I cannot really distinguish any changes from the color scheme, please consider amending this.
New figure suggestion: A multi panel figure showing a MODIS image of the glacier every sea-ice minima (roughly e.g. Late Feb to March)
Minor comments
Numerous incidents of Figures or references not bracketed
Line 19-23: Not sure, I buy this argument. Most of the ice shelves in West Antarctica are actually quite small. There have also now been at least a dozen of East Antarctic focussed studies on individual glaciers. Please revise.
Line 32-44: I think it would be nice to explicitly define Landfast sea-ice here and how it differs to sea-ice, some readers of The Cryosphere will not know this. Then clearly go through the mechanisms in which Landfast sea-ice can promote ice shelf advance/delay calving/stabilize ice tongues, but also how sea-ice on the open ocean can be important (e.g. buffering ocean swell) using examples from the literature. This will help set up the paper. A recent review paper on Landfast sea-ice and the references within could be a useful start here: (see Fraser et al., 2023; ‘Antarctic Landfast Sea Ice: A Review of Its Physics, Biogeochemistry and Ecology’).
Line 43-45: This is an excellent question.
Line 49 – “Landfast sea-ice melting” is awkward because the sea-ice can be melting but still present. Perhaps “sea-ice free conditions” or similar is better. Please revise throughout manuscript.
Line 58: What is wrong with radar images during this time period?
Line 103: “ice tongue surface” – this figure does not show changes to the ice tongue surface.
Line 187: “Figure ?”
Line 209: is there any bathymetry data available to confirm this? Are there any pinning points?
Line 216: There is little to no MYFI here.
Line 223-240: There is some discussion of sea-ice trends from 1979-2016, but what about 2016-2023? There has been an exceptional decline in sea-ice, particularly this year. As a reader, I am interested what the results from this study might mean for a future with considerably less sea-ice?
Line 229-231: Please investigate the availability of imagery; I believe that there are imagery dating back to the early 1970s for the vast majority of East Antarctica that has been used in other studies.
Line 235: “while before 2010, 1 to 2 images per year at most are available during summer”. This is not correct. MODIS allows a daily viewing of this glacier since 2000, the resolution is sufficient enough to see at least the major calving events.
MODIS link: https://worldview.earthdata.nasa.gov/?v=1524288.6381649177,-2038092.853452241,1794076.1394911779,-1918924.853452241&p=antarctic&t=2019-03-16-T07%3A45%3A59Z
Citation: https://doi.org/10.5194/egusphere-2023-736-RC1 -
AC1: 'Reply on RC1', Floriane Provost, 13 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-736/egusphere-2023-736-AC1-supplement.pdf
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AC1: 'Reply on RC1', Floriane Provost, 13 Nov 2023
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RC2: 'Comment on egusphere-2023-736', Anonymous Referee #2, 11 Aug 2023
This manuscript adds to the literature on an important topic in glaciology: controls on glacier calving. Specifically, the authors construct a time-series of ice-front change and related variables (ice-flow velocity, strain rates, and sea-ice conditions) for Astrolabe Glacier in East Antarctica to better understand the causes of several calving events during the record. The general approach is useful, and the questions addressed are interesting. I think there is a fundamental issue with the analysis related to the treatment of sea ice that is important to address, and the paper could use some editing and polishing.
Major comment: Treatment of sea ice
The paper links the calving behaviour of Astrolabe Glacier to sea-ice forcing, which is presented fairly generally in the abstract. However, most of the rest of the paper refers to this analysis as addressing “landfast sea-ice forcing.” Land-fast sea ice is a specific sea-ice configuration that is attached to land, which may mean that it provides more buttressing potential than freely floating sea ice. There are indeed several papers, cited in this manuscript for comparison, that attempt to address the role of land-fast sea ice in calving and glacier behaviour. However, this study only quantifies sea ice, not land-fast sea ice. Looking at sea-ice extent and concentration is not equivalent to assessing the presence of land-fast sea ice, and it means that the analysis cannot be as directly compared to studies that assess land-fast sea ice. Instead, the differences between the studies and the implications for differing mechanisms should be explored in more detail.
I also have trouble seeing the connection between proposed physical mechanisms for ice-tongue stabilization and the analyses performed, particularly in regard to sea-ice extent. The area over which sea-ice extent is assessed is listed as being 4000 square kilometers, but the area chosen is never shown or justified. It is unclear to me how the authors determined the area over which sea-ice extent should matter to the behaviour of the ice tongue. Sea-ice concentration is taken from a pixel in a sea-ice product that covers the ice tongue, but this area is also not shown in the paper, and it is not clear whether it is centered on the ice tongue or whether all areas in the pixel are likely to affect the ice tongue. These decisions should be clearly justified and the areas shown in the paper.
Still, the fact that there is some correlation between these variables and ice-tongue behaviour is likely to be interesting. However, that correlation is not quantified. The authors claim that it is well-correlated, and there does seem to be some evidence of that in Figure 5, but it is very difficult to interpret the data from the very small panels in the figure. It would be helpful to perform the correlations, perhaps between sea-ice extent and the trend in ice-front position, for example, to better quantify the relationship.
Finally, the term “melting” appears to be used incorrectly in regard to sea ice. It seems to be used synonymously with a decrease in sea-ice concentration or extent. While this can be due to melting, these variables may also change due to sea-ice advection. Since sea-ice melting does not appear to be assessed in the study, it would be better to use a more general term.
Other comments:
The manuscript is generally fairly clearly written, but there are typos and grammar issues throughout the manuscript that should be addressed. For example, hyphens between compound nouns acting as adjectives are used inconsistently, and there are many spots where verb tenses don’t match the noun form. In line 8, “lead” should be “led.” I am also accustomed to the term “transverse” rather than “transversal” being used for strain rates, but that may just be a convention I’m not familiar with.
Section 2.1.2: It would be helpful to have some indication of estimated error in the velocity correlations
Section 3.2: It would be helpful to have some more explanation in this section. I think that the GNSS measurements were averaged over the whole year to match the satellite-derived measurements, but that wouldn’t be possible with the bamboo stakes, if I’ve understood the methods correctly. It doesn’t necessarily make sense to compare a small time slice, taken to be ground-truth, to measurements averaged over a longer period of time, but perhaps that’s what the comments on lack of seasonal variation are trying to address. It’s also not always reasonable to compare point measurements to those averaged over a large spatial area. Finally, I can’t quite figure out what the last two sentences in this section are trying to say. I suspect all the analyses discussed in this section are reasonable, but I can’t quite figure that out based on what is written.
Lines 158-169 says: “It can be noted that compressional strain rates are measured from 2017 to 2020 at the terminus of the glacier tongue with strain rate larger than 0.001 day−1 while it is
not observed anymore in 2021.” Strain rates are usually positive in extension, so compressional strain rates could not, by definition, be larger than 0. Is this an absolute value, or is there a different convention being used here?
Figures:
There are several spots where figures are not referenced correctly. E.g. in section 2.1.1, it seems like the references to Fig. 1 should refer to Fig. 2, and in section 3.4, Fig. ?? should be corrected to the figure number.
Figure 2: It would be helpful if the y-axes were the same in panels e-g to facilitate comparison between panels.
Figure 4: I don’t find Fig. 4a very helpful, because the panels are too small to clearly see what is discussed in the text. Consider showing just a few panels that are necessary to the analysis, and shifting the rest of the panels in larger form to supplementary information. I think Fig. 4b is a very clever way to display the information.
In general, it would be helpful to have more labels in the figures that correspond to what is discussed in the text. For example, the discussion talks about the “main rift” and refers us to Fig. 3 on line 194. I can make some guesses based on Figure 3 about what the main rift is, but I would rather have a label or two that helps me know exactly what the authors are referring to. It would be helpful to have those labels in Figure 4, as well.
Citation: https://doi.org/10.5194/egusphere-2023-736-RC2 -
AC2: 'Reply on RC2', Floriane Provost, 14 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-736/egusphere-2023-736-AC2-supplement.pdf
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AC2: 'Reply on RC2', Floriane Provost, 14 Nov 2023
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