Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World

Moser, Dorothea Elisabeth; Thomas, Elizabeth R.; Nehrbass-Ahles, Christoph; Eichler, Anja; Wolff, Eric

doi:https://doi.org/10.5194/egusphere-2023-1939

Preprints

https://doi.org/10.5194/egusphere-2023-1939

Preprints

27 Sep 2023

| 27 Sep 2023

Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World

Dorothea Elisabeth Moser, Elizabeth R. Thomas, Christoph Nehrbass-Ahles, Anja Eichler, and Eric Wolff

Abstract. Melting polar and alpine ice sheets in response to global warming pose ecological and societal risks but will also hamper our ability to reconstruct past climate and atmospheric composition across the globe. Since coastal low-elevation ice caps are crucial environmental archives but changing rapidly, the (sub-)polar research community is increasingly faced with melt-affected ice cores common in alpine settings. Here, we review the characteristics and effects of near-surface melting on ice-core records focussing on a (sub-)polar readership and make recommendations for melt-prone study regions. This review covers (1) melt layer formation; (2) identification and quantification of melt; (3) structural characteristics of melt features; effects of melting on (4) records of chemical impurities, i.e. major ions, trace elements, black carbon, and organic species, (5) stable water isotopic signatures, and (6) gas record; (7) applications of melt layers as environmental proxies.

Melting occurs during positive surface energy balance events, which are shaped by global to local meteorological forcing, regional orography, glacier surface conditions and subsurface characteristics. Meltwater flow ranges from homogeneous wetting to spatially heterogeneous preferential flow paths and is determined by temperature, thermal conductivity and stratigraphy of the snowpack. Melt layers and lenses are the most common consequent features in ice cores and are usually recorded manually or using line-scanning. Chemical ice-core proxy records of water-soluble species are generally less preserved than insoluble particles such as black carbon or mineral dust due to their strong elution behaviour during percolation. However, a high solubility in ice as observed for ions like F⁻, Cl⁻, NH₄⁺, or ultra-trace elements can counteract the high mobility of these species due to a burial in the ice interior. Stable water isotope records like δ¹⁸O are often preserved but appear smoothed if significant amounts of meltwater were involved. Melt-affected ice cores are further faced with questions about the permeability of the firn column for gas movement, and gas concentrations can be increased through dissolution and in-situ production. Noble gas ratios can be useful tools to identify melt-affected profile sections in deep ice. Despite challenges for ice-core climate reconstruction based on chemical records, melt layers are a proxy of warm temperatures above freezing, which is most sensitive in the dry snow and percolation zone.

Bringing together insights from snow physics, firn hydrology and ice-core proxy research, we aim to foster a more comprehensive understanding of ice cores as climate and environmental archives, provide a reference on how to approach melt-affected records, and raise awareness of the limitations and potential of melt layers in ice cores.

Received: 24 Aug 2023 – Discussion started: 27 Sep 2023

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Dorothea Elisabeth Moser, Elizabeth R. Thomas, Christoph Nehrbass-Ahles, Anja Eichler, and Eric Wolff

Status: closed

RC1:
'Comment on egusphere-2023-1939', Anonymous Referee #1, 29 Oct 2023

Review for:
Moser, D. E., Thomas, E. R., Nehrbass-Ahles, C., Eichler, A., and Wolff, E.: Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1939, 2023.

General comments:
This is a great review paper on melt features in cores and should be published with some very minor changes. The authors do a great job in summarizing the importance of melt features and their effect on e.g. the climatic integrity of ice cores, which is of great importance to the ice core community.
I am missing a short explanation of the effect of melt in alpine and polar ice cores at the beginning of the manuscript. Similar to lines 577 to 581. This would be useful to distinguish when the authors refer to alpine or polar cores and to make it clear to the reader if the current section is valid for the full spectrum of melt events, i.e. massive melt events on coastal cores or tiny mm-melt events on the central Greenland ice sheet.
There are also some inconsistencies using the dash separating, e.g. “line scanning” and “line-scanning” throughout the manuscript.
For a better overview of the manuscript, I would suggest changing the list from (1)-(8) in lines 91 to 94 to the corresponding section numbers in the manuscript.

Specific comments:
Figure 1: I would prefer to see the scale explanation at the beginning of the caption.
Figure 2: the map of Antarctica (1) is misleading for the figure, as the largest part of Antarctica is the plateau, which is not the main message of the figure.
Line 132: “4 mm of melt” please specify if this is related to snow, firn, ice, or water equivalent.
Line 136-139: “Atmospheric rivers, especially during winter”. Neff 2018 describes atmospheric rivers during summer and Wille et al 2019 both summer and winter. Please rephrase the sentence “especially during winter”.
Line 184: please specify if coastal or central Antarctica.
Line 198 and 230: “Infiltration beyond the current year is a source of uncertainty” implies that most melt is in the current year’s snow layer. Line 230 states that melt does not tend to form at the surface but 50-100 cm deep, i.e. below the current year’s snow layer. The two sentences seem to contradict each other, please clarify.
Line 315 and 316: here it is not entirely clear if you refer to the snowpack or polar firn for the introduction of this section. Also, do you not analyze melt in deep ice because it does not penetrate that deep, or do you not analyze the effects of these old melt events? Please clarify.
Table 1: there is a dash and a word missing at the very bottom left of page 14.
Figure 5: a, b, and c are missing in the figure.
Line 499: is this “pre-melting” term the same as used, e.g., in line 181, or should it mean before/prior to melting?
Line 527: does “preserved” refer to a spatial sense in the snowpack? Please specify.
Line 663: do we not know exactly how many? Why is the number smaller or equal to 5?
Line 668: “frequently” makes it sound like a quasi-annual phenomenon, which is not the case. Please change the wording.
Line 670: “higher-altitude”, higher than in central Greenland? Please specify.
Line 712” with larger intensity, and at higher elevation further inland.” Change to: ”… with larger intensity, at higher elevation, and further inland”.

Citation: https://doi.org/10.5194/egusphere-2023-1939-RC1
- AC1: 'Reply to Comments of Reviewer 1', Dorothea Elisabeth Moser, 19 Jan 2024
  
  Dear Editorial Team of The Cryosphere, dear Reviewer 1,
  Thank you for giving us the opportunity to respond to the reviewer comments for the manuscript “Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World”. We appreciate the effort that you and the reviewers dedicated to reviewing our paper and providing valuable comments, which have helped to improve the manuscript. We have carefully considered the suggestions by Reviewer 1 and address each of them in point-by-point responses (italic blue) in the file attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1939-AC1
RC2:
'RC2 comment', Veijo Pohjola, 22 Nov 2023

Introduction
The preprint 2023-1939 by Moser et al fills a current gap in the literature on the physics and chemistry of ice in the perspective of retaining climatic and environmental information out from coring ice fields that temporarily are effect by melt. Standard textbooks such as Takeo Hondoh´s book “Physics of Ice Core record” from 2000 are usually focused in cold firn ice cores (melting phenomena are mentioned in Hondoh´s book, and maybe worth a reference in this manuscript!), and the development of firn modeling and analytical technologies have opened new lanes of possibilities in extracting interesting and important information out from periodically melting / warm firn ice fields. This preprint is a result of a large, brave and commendable effort to collect and present a benchmark of where this field of science is today. With the current warming and its accelerated effect, it is of interest to retrieve information from these ice fields before the information gets to washed out by percolating melt and rain water. This work can such play an important role as a standard piece for new and old students in this field.

General comments
Since this is a review article I will focus at the organization of this work. My first reading of the preprint made me somewhat confused of where the review was aiming at. The general organization: 1 Introduction; 2 From melting to melt-affected ice cores; 3 Manifestations of melt in ice-core records; 4 Current applications of melt layers as proxies; 5 Conclusions and outlook makes sense from 1 to 3. Section 4 hang a bit outside the general structure, and could be added into section 3, or even placed into 2.3.1. Maybe assess the general organization again. The illustrations are another topic that caught my eye. Potentially there could be hundreds of illustrations, since the review cites a large amount of work. From what I can see the figures are used to illustrate important principles and key information in the review. The illustrations are perhaps also used to break to long sections of text, into more color and different texture to please the readers eye. If so, pages 6 -12 are lacking illustrations, and such are a bit more dense and “heavy” to get through. Try to anchor the figures better in the text; to take the opportunity to explain the information in figures and expand the relevance of the figures in the section. With this, the illustrations will bring in more information and will be used as to bring in additional knowledge, apart from the aesthetical value.

Further comments:
1. How is the structure are lined up in the abstract as compared in the introduction, and further connected to the different sections. First, should the seven points in the abstract represent the different sections, and such display a “table of content”? Second, should these items of content be similar in the abstract and in the rows 90-95 in the introduction?
2. Could you consider to walk the reader through the arguments for the chosen organization of the manuscript after your presentation of aims and objectives?
3. The ice chemistry section could be separated between particulate and ionized content. The analytic methods and the information out of these parameters are quite different, and they also bring different information, and behave differently during wet events.
4. The section of water isotopes is fairly short in comparison to the chemistry section. You could argue that the basic information is available in a number of texts, and refer to more basic literature of this sort, perhaps begin with the pioneering work by Dansgaard.

Minor comments
Li 12-14. Rephrase the sentence “Since coastal…..in alpine settings”.
Li 14. Does not this review also include warming firn outside alpine settings?
Li 97. On this row there is a short aim and objective, that can be expanded as commented in major comments above, if we, has the meaning authors, that line is badly posed, since I would presume it is the reader who wants to learn…
Li 128-131. May the sentence of temperature increase related to heat content be too trivial in The Cryosphere?
Li 133-149. This section on heat from advected air masses over ice fields is a bit exhaustive, and takes focus off the ice properties. Consider to shorten/cut this part.
Li 184. Typo? “Therefor”
Li 635-639. I would guess that gas content and gas evacuation capacity is dependent on how the ice crystal boundaries are configured in ice matrix, and how the effective porosity varies in the firn/ice matrix.

Citation: https://doi.org/10.5194/egusphere-2023-1939-RC2
- AC2: 'Reply to Comments of Reviewer 2', Dorothea Elisabeth Moser, 19 Jan 2024
  
  Dear Editorial Team of The Cryosphere, dear Veijo Pohjola,
  Thank you for giving us the opportunity to respond to the reviewer comments for the manuscript “Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World”. We appreciate the effort that you dedicated to reviewing our paper and providing valuable comments, which have helped to improve the manuscript. We have carefully considered the suggestions and address each of them in point-by-point responses (italic blue) in the file attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1939-AC2

Status: closed

RC1:
'Comment on egusphere-2023-1939', Anonymous Referee #1, 29 Oct 2023

Review for:
Moser, D. E., Thomas, E. R., Nehrbass-Ahles, C., Eichler, A., and Wolff, E.: Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1939, 2023.

General comments:
This is a great review paper on melt features in cores and should be published with some very minor changes. The authors do a great job in summarizing the importance of melt features and their effect on e.g. the climatic integrity of ice cores, which is of great importance to the ice core community.
I am missing a short explanation of the effect of melt in alpine and polar ice cores at the beginning of the manuscript. Similar to lines 577 to 581. This would be useful to distinguish when the authors refer to alpine or polar cores and to make it clear to the reader if the current section is valid for the full spectrum of melt events, i.e. massive melt events on coastal cores or tiny mm-melt events on the central Greenland ice sheet.
There are also some inconsistencies using the dash separating, e.g. “line scanning” and “line-scanning” throughout the manuscript.
For a better overview of the manuscript, I would suggest changing the list from (1)-(8) in lines 91 to 94 to the corresponding section numbers in the manuscript.

Specific comments:
Figure 1: I would prefer to see the scale explanation at the beginning of the caption.
Figure 2: the map of Antarctica (1) is misleading for the figure, as the largest part of Antarctica is the plateau, which is not the main message of the figure.
Line 132: “4 mm of melt” please specify if this is related to snow, firn, ice, or water equivalent.
Line 136-139: “Atmospheric rivers, especially during winter”. Neff 2018 describes atmospheric rivers during summer and Wille et al 2019 both summer and winter. Please rephrase the sentence “especially during winter”.
Line 184: please specify if coastal or central Antarctica.
Line 198 and 230: “Infiltration beyond the current year is a source of uncertainty” implies that most melt is in the current year’s snow layer. Line 230 states that melt does not tend to form at the surface but 50-100 cm deep, i.e. below the current year’s snow layer. The two sentences seem to contradict each other, please clarify.
Line 315 and 316: here it is not entirely clear if you refer to the snowpack or polar firn for the introduction of this section. Also, do you not analyze melt in deep ice because it does not penetrate that deep, or do you not analyze the effects of these old melt events? Please clarify.
Table 1: there is a dash and a word missing at the very bottom left of page 14.
Figure 5: a, b, and c are missing in the figure.
Line 499: is this “pre-melting” term the same as used, e.g., in line 181, or should it mean before/prior to melting?
Line 527: does “preserved” refer to a spatial sense in the snowpack? Please specify.
Line 663: do we not know exactly how many? Why is the number smaller or equal to 5?
Line 668: “frequently” makes it sound like a quasi-annual phenomenon, which is not the case. Please change the wording.
Line 670: “higher-altitude”, higher than in central Greenland? Please specify.
Line 712” with larger intensity, and at higher elevation further inland.” Change to: ”… with larger intensity, at higher elevation, and further inland”.

Citation: https://doi.org/10.5194/egusphere-2023-1939-RC1
- AC1: 'Reply to Comments of Reviewer 1', Dorothea Elisabeth Moser, 19 Jan 2024
  
  Dear Editorial Team of The Cryosphere, dear Reviewer 1,
  Thank you for giving us the opportunity to respond to the reviewer comments for the manuscript “Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World”. We appreciate the effort that you and the reviewers dedicated to reviewing our paper and providing valuable comments, which have helped to improve the manuscript. We have carefully considered the suggestions by Reviewer 1 and address each of them in point-by-point responses (italic blue) in the file attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1939-AC1
RC2:
'RC2 comment', Veijo Pohjola, 22 Nov 2023

Introduction
The preprint 2023-1939 by Moser et al fills a current gap in the literature on the physics and chemistry of ice in the perspective of retaining climatic and environmental information out from coring ice fields that temporarily are effect by melt. Standard textbooks such as Takeo Hondoh´s book “Physics of Ice Core record” from 2000 are usually focused in cold firn ice cores (melting phenomena are mentioned in Hondoh´s book, and maybe worth a reference in this manuscript!), and the development of firn modeling and analytical technologies have opened new lanes of possibilities in extracting interesting and important information out from periodically melting / warm firn ice fields. This preprint is a result of a large, brave and commendable effort to collect and present a benchmark of where this field of science is today. With the current warming and its accelerated effect, it is of interest to retrieve information from these ice fields before the information gets to washed out by percolating melt and rain water. This work can such play an important role as a standard piece for new and old students in this field.

General comments
Since this is a review article I will focus at the organization of this work. My first reading of the preprint made me somewhat confused of where the review was aiming at. The general organization: 1 Introduction; 2 From melting to melt-affected ice cores; 3 Manifestations of melt in ice-core records; 4 Current applications of melt layers as proxies; 5 Conclusions and outlook makes sense from 1 to 3. Section 4 hang a bit outside the general structure, and could be added into section 3, or even placed into 2.3.1. Maybe assess the general organization again. The illustrations are another topic that caught my eye. Potentially there could be hundreds of illustrations, since the review cites a large amount of work. From what I can see the figures are used to illustrate important principles and key information in the review. The illustrations are perhaps also used to break to long sections of text, into more color and different texture to please the readers eye. If so, pages 6 -12 are lacking illustrations, and such are a bit more dense and “heavy” to get through. Try to anchor the figures better in the text; to take the opportunity to explain the information in figures and expand the relevance of the figures in the section. With this, the illustrations will bring in more information and will be used as to bring in additional knowledge, apart from the aesthetical value.

Further comments:
1. How is the structure are lined up in the abstract as compared in the introduction, and further connected to the different sections. First, should the seven points in the abstract represent the different sections, and such display a “table of content”? Second, should these items of content be similar in the abstract and in the rows 90-95 in the introduction?
2. Could you consider to walk the reader through the arguments for the chosen organization of the manuscript after your presentation of aims and objectives?
3. The ice chemistry section could be separated between particulate and ionized content. The analytic methods and the information out of these parameters are quite different, and they also bring different information, and behave differently during wet events.
4. The section of water isotopes is fairly short in comparison to the chemistry section. You could argue that the basic information is available in a number of texts, and refer to more basic literature of this sort, perhaps begin with the pioneering work by Dansgaard.

Minor comments
Li 12-14. Rephrase the sentence “Since coastal…..in alpine settings”.
Li 14. Does not this review also include warming firn outside alpine settings?
Li 97. On this row there is a short aim and objective, that can be expanded as commented in major comments above, if we, has the meaning authors, that line is badly posed, since I would presume it is the reader who wants to learn…
Li 128-131. May the sentence of temperature increase related to heat content be too trivial in The Cryosphere?
Li 133-149. This section on heat from advected air masses over ice fields is a bit exhaustive, and takes focus off the ice properties. Consider to shorten/cut this part.
Li 184. Typo? “Therefor”
Li 635-639. I would guess that gas content and gas evacuation capacity is dependent on how the ice crystal boundaries are configured in ice matrix, and how the effective porosity varies in the firn/ice matrix.

Citation: https://doi.org/10.5194/egusphere-2023-1939-RC2
- AC2: 'Reply to Comments of Reviewer 2', Dorothea Elisabeth Moser, 19 Jan 2024
  
  Dear Editorial Team of The Cryosphere, dear Veijo Pohjola,
  Thank you for giving us the opportunity to respond to the reviewer comments for the manuscript “Review article: Melt-Affected Ice Cores for (Sub-)Polar Research in a Warming World”. We appreciate the effort that you dedicated to reviewing our paper and providing valuable comments, which have helped to improve the manuscript. We have carefully considered the suggestions and address each of them in point-by-point responses (italic blue) in the file attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1939-AC2

Dorothea Elisabeth Moser, Elizabeth R. Thomas, Christoph Nehrbass-Ahles, Anja Eichler, and Eric Wolff

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https://doi.org/10.5194/egusphere-2023-1939-supplement

Dorothea Elisabeth Moser, Elizabeth R. Thomas, Christoph Nehrbass-Ahles, Anja Eichler, and Eric Wolff

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Short summary

Increasing temperatures worldwide lead to more melting of glaciers and ice caps, even in the polar regions. This is why ice core scientists need to prepare for analysing records affected by melting and refreezing. In this paper, we present a summary of how near-surface melt forms, what structural imprints it leaves in snow, how various signatures used for ice-core climate reconstruction are altered, and how we can still extract valuable insights from melt-affected ice cores.


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