Ice-Gouging Topography of the Exposed Aral Sea Bed

Round 1

Reviewer 1 Report

Ice gouging or scouring is an important geomorphological phenomenon and a major hazard in high-latitude to mid-latitude seas and large lakes.  The research detailed in this paper utilises satellite imagery to map the occurrence and density of ice gouge structures across approximately 50,000km² of the former Aral Sea bed, and uses UAV and field work to examine some gouges in more detail.

This work is of international interest and potentially provides information on ice gouging frequency and processes in relatively shallow basins.  The work could be published, but needs a number of improvements.  The following major changes need to be made:

1.      The discussion and conclusions need significant work.  The authors argue that the preserved ice gouges formed at depths of 2-5m, on the basis of a comparison with the Caspian Sea, which has a similar climate.  However, they also argue that preservation is a factor of protection from wave erosion, and of rapid reduction in water height in the Aral Sea.  Wave erosion is controlled by wave base, which is related to wind fetch and wind strength, and as the Caspian Sea is far larger than the Aral Sea, wave base is likely to differ significantly. The rate of reduction in water height is likely to be important, but only if these gouges last more than one year – the maximum drop quoted is 1750px per annum, but the gouges are suggested to form below this.  Elsewhere the authors suggest that the silty bottom sediment aids preservation – but does this differ from the Caspian Sea? This flaw in the central argument needs addressing as a priority.

2.      Much useful information is missing.  In particular, a map or DEM (SRTM?) showing the bottom topography of the Aral Sea is essential, given the discussion of gouge preservation and water depth, and might help the authors refine their argument.  Other figures need major improvement. The location map has no scale and needs the extent of Aral Sea in 1960 marked, plus a delineation of the area mapped. Rivers also needed, as are more placenames.

 

3.      There are gaps in the review of literature. There is an extensive literature on the North American Great Lakes which is not included; much is behind a paywall, but Daly (2016) is available on open access – further references are given therein and should be used.  There is also literature on the Beaufort Sea, that I’m less familiar with.

 

4.      The authors have used UAV surveys to construct photomosaics and DEMS of the exposed gouges.  It appears that 5 different polygons were surveyed, but only one DEM is shown, and photomosaics from two polygons only.  More of this data needs to be presented here.  In addition, a hillshading of the DEM(s) (at a low angle with high vertical exaggeration) would be a better way to illustrate these features.  One possibility is to use a programme like Relief Visualization Toolbox (a free resource developed for archaeological use; reference below).

 

Comments line by line below:

Line 34  “temperate” – “mid-latitude” preferable, there is no single temperate climate – change throughout text

Line 48  Delete “shows”

Line 49  XX should be 20^th

Section 2.1 Preceding Studies – further Great Lakes literature needs to be included. Daly (2016) provides a useful summary of C-Core work; consider the Beaufort Sea literature too.

Line 57  should be: “in the Northern Caspian Sea”

Line 67 – leave out initial

Line 81 – reword – not sure what is meant by “back up to” – before 1960?

Line 83  change “it” to “they”

Lines 87- 88        “West Aral”, “East Aral” and “North Aral” should all have “Sea” included. If this is too long, you could always use an acronym – WAS, EAS and NAS?

Figure 3:  This needs an outline of the Aral Sea in 1960 to show full extent. No scale bar is provided – add.

Line 97  change “grown up to” to “increased to”

Line 104               change “broke up” to “was broken up”

Line 112 – change “names” to “locationS” – but they are not shown on any map, as far as I can see – include.

Lines 127-132     Please indicate on your location map the exact area covered – it’s not clear whether you looked at the whole lake basin, or mostly the northeast.

Line 134 – change “immensely rise” to “immensely increase”

Line 136 – change “when, to our assumption” to “when we assume”

Line 200 onwards – “the Aral Sea bottom” – this should be “sea bed” or “bed”, not bottom – change throughout

Line 207 – “another maximum” – do you mean a secondary peak? – change

Line 212 – “makes an evidence of” – change to “indicates”

Line 221 –“most of them are sharp” – not sure what you mean here – do you mean they have angular changes in direction? – in which case, they are not curves

Figure 7 – indicate which polygon this image is taken from

Lines 244-245     Sediments – you mention a trial pit here, but there is no description of the type of sediment these features are cut into.  This would be useful to include – is there any difference between polygons or across the sea bed that you are aware of?

Figure 11 – this is very hard to read – I can’t make out the dates of the various lake levels easily, nor the location of the lines.  Consider highlighting every 5^th year with a thicker coloured line? Or excluding some years? The legend is also hard to follow – colours should be selected to grade in tone from light to dark, with darkest tones indicating highest concentrations – currently this is not the case.

Figs 13-15  References for Baydaratskaya images needed.

Lines 321-328 Again, where are references to the N American Great Lakes? The Beaufort Sea?

Table 3: should include some Great Lakes data, e.g. that in Daly (2016).

Line 341               Change “proved” to “shown”

Line 355 on         You argue that ice gouges will form at similar depths in the Caspian Sea and Aral Sea because they have similar climates.  I have some issues with this assumption:

Firstly, no climate data is presented for the Caspian Sea.  A single reference, that I have been unable to access, appears to relate to the Aral Sea only (21). 

Secondly, the formation of ice ridges and stamukha is related to duration and thickness of ice cover, but again, no figures are given for the Caspian Sea.  In any case, ice cover thicknesses do not necessarily decide gouging depth – ice cover thickness is similar for Lake Erie, but gouge depths of up to 25m occur, and depth of ice ridges is controlled primarily by AFDDs (accumulated freezing degree-days), rather than climate specifically (Daly 2016). 

Thirdly, the authors point out that gouge preservation (as opposed to formation) is controlled by wave action – gouges above wave base are unlikely to be preserved. Since wave base is decided partly by wind fetch, the size of the lake and wind direction are critical.  Thus using the Caspian Sea, an order of magnitude larger than the Aral Sea and with a far greater wind fetch, as a comparison is risky.  It is entirely possible that ice gouges are preserved in much shallower water in the Aral Sea.

Lines 360-362     But what about wave erosion in this zone? How deep is wave base?

Lines 363-364 – why? See Daly 2016 for Lake Erie, a similarly sized lake, with similar climate

Lines 369-371     This seems as important a point as climate – but there is no information the bottom topography of the Aral Sea.

Lines 372-374     But if the rate of lake level fall was 1750px per year, the previous season’s ice gouges would still be below the water level, but above wave base and removed during the following summer – if they formed at 2-5m depth….

Lines 407-408     So are ice gouges removed by erosion (as you stated earlier) or covered by sedimentation. In which case, what is the rate of sedimentation in the Aral Sea? What was it? It would need to be 1250px a year to cover the average gouge, which is high….. . 

 

References:

 

Daly, S.F., 2016. Characterization of the Lake Erie Ice Cover.  ERDC/CRREL TR-16-5; available at: http://acwc.sdp.sirsi.net/client/en_US/default/index.assetbox.assetactionicon.view/1049346?rm=%E2%80%A2COLD+REGIONS+0%7C%7C%7C1%7C%7C%7C2%7C%7C%7Ctrue

 

Relief Visual Toolbox: available at - https://iaps.zrc-sazu.si/en/rvt#v

 

Author Response

Dear reviewer! Please find our response in file below. We were recommended to use the "Track Changes" function in Microsoft Word, but it made the manuscript rather complicated. Please, use "modified document" option to see the final version.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript explains the origin and provides details on the morphology spectacular linear landforms present at the Aral Sea bed, an understudied feature attributed by the authors to the ice gouging, or scouring. The authors had made an effort to conduct fieldworks in the area and to combine the results with the remotely sensed data.

The reasoning provided by the authors is convincing and the divine origin of these features can finally be refuted. This is the major outcome of the study, along with some sporadic data on the morphology of these landforms, and a scheme of scour density distribution. Owing to this fact, the manuscript should be substantially enhanced in order to be accepted for publication.

Firstly, the manuscript is to be restructured, some parts moved to Introduction, some to a new Site Description section, repetitions and expletive figures deleted. Some sections in their present form are not needed, or can be reduced in size without harming the readers impression.

Secondly, the authors should clearly define the spatial extent of their investigation. What is the area of investigation, for which the ice scours were manually deciphered ? What is the total number being deciphered ? Their distribution across sizes and directions, and a bivariate analysis can be used to relate both variables to each other. Geostatistics can be used to relate these parameters to certain uniform areas and their geological/geomorphic features. Detailed explanations on the representativeness of the uniform areas (Lines 158-163) should be given, and their limits are to be shown on a relevant figure.

Thirdly, curiously, no discussion for the Figure 11 which is one of the major outcomes of the study. Why such ice scour distribution ? The dominant winds in February are from NE, and it is the north-eastern Aral Sea region where the ice scours are the most abundant. This should be explained in detail. No bathymetric chart is given to relate these features to at least former sea depth (though this can appear to be less informative owing to a rapid sea level decline). Sea depth can be used as yet another variable in the multivariate analysis of ice scour parameters.

Fourthly, the terminology is to be consistent with the one currently being used in the literature; this should be corrected throughout the manuscript. Moderate English polishing is required, and some changes are already proposed in an attached revision file.

Finally, this manuscript is a novel essay on the landforms that are currently understudied, sheding light on its origin and morphology. A transition from an essay to a scientific publication will require some extra effort, to make this light more vivid and the science behind the manuscript text more sound and solid. Further comments can be found in the attached pdf. This paper should be subject to major revision, and thence reconsidered for publication.

Comments for author File: Comments.pdf

Author Response

Dear reviewer! Please find our response in file below. We were recommended to use the "Track Changes" function in Microsoft Word, but it made the manuscript rather complicated. Please, use "modified document" option to see the final version.

Author Response File: Author Response.docx

Reviewer 3 Report

This is an excellent paper which greatly expands the known examples of ancient ice scours in Pleistocene glacial deposits and the famous example of modern scours Racetrack Playa in California. 

The authors are unaware of: 

Eyles, N., Meulendyk, T., 2008. Ground‐penetrating radar study of Pleistocene ice scours on  a glaciolacustrine sequence boundary. Boreas 37, 226-233.

Lorenz, R.D., Jackson, B.K., Barnes, J.W., Spitale, J., Keller, J.M., 2011a. Ice rafts not sails: Floating the rocks at Racetrack Playa. Am. J. Phy. 79, 37-42.

Lorenz, R.D., Jackson, B.K., Barnes, J.W., Spitale, J.N., Radebaugh, J., Baines, K.H., 2011b. Meteorological conditions at Racetrack Playa, Death Valley National Park: implications for rock production and transport. Jl. of App. Meteorology and Clim. 50, 2361-2375.

            Eden, D. and Eyles, N. 2002 Case study of an iceberg scour at Scarborough Bluffs,                     Ontario and implications for pipeline engineering. Canadian Geotechnical Journal. 39,                 519-34.

            Eden, D., and Eyles, N. 2001. Description and numerical model of a Pleistocene iceberg             scour at Scarborough Bluffs, Ontario, Canada. Sedimentology 48, 1079-1022.

 

Author Response

Dear reviewer! Thank your enthusiastic review and some interesting literature references. We were already familiar with some of them, other became bright findings. We added some of them to the literature review. 

We were recommended to use the "Track Changes" function in Microsoft Word, but it made the manuscript rather complicated. Please, use "modified document" option to see the final version.

Round 2

Reviewer 2 Report

The authors had made a great work at significantly enhancing the manuscript, which fully merits publication and can be accepted in its present form. I would still suggest to remove some figures, though expressively spectacular, as they are graphical overkill; but this is left at authors discretion. The bathymetric chart could have been labelled in the same way as your Figure 4 is, and the title of Section 5.1 slightly changed to 'Origin and development of the Aral Sea ice gouging topography'; this is however at authors discretion also. Congratulations on the excellent work and happy holidays! :)