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the Creative Commons Attribution 4.0 License.
| 15 Jan 2024
Methane Sulphonic Acid in East Antarctic Coastal Firn and Ice Cores and Its Relationship with Chlorophyll-a and Sea Ice Extent
Abstract. Sea ice is important for both regional and global climate, but comprehensive sea ice records are lacking pre-1978, when global-scale spaceborne observations began. Attempts to reconstruct sea ice conditions in different regions of Antarctica with the help of methane sulphonic acid (MSA) records from ice cores have had varying success, highlighting the often-regional relationship between ice core MSA and sea ice. This study uses MSA records from three firn cores and one ice core drilled on Fimbul Ice Shelf in Dronning Maud Land, East Antarctica, to investigate the relationship to satellite-derived sea ice extent (SIE) in the Southern Ocean. Chlorophyll-a concentrations, serving as a measure of phytoplankton biomass, are correlated to the MSA records to further test the MSA – SIE relationship. The relationship to both SIE and chlorophyll-a differs largely between the different firn and ice core MSA records. We find significant correlations for the MSA records from the two higher accumulation core sites to SIE and chlorophyll-a in the Weddell Sea, Western Pacific Ocean, and Ross Sea Sectors. Furthermore, the use of stacked MSA records introduced significant correlations between MSA from the lower accumulation core sites and SIE. The absence of coherent correlation patterns between the MSA records across the four investigated cores and SIE or chlorophyll-a in the Southern Ocean suggests that the Fimbul Ice Shelf MSA records are not consistent proxies for regional SIE.
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RC1: 'Comment on egusphere-2023-3156', Anonymous Referee #1, 06 Mar 2024
Review for:
Methane Sulphonic acid in East Antarctic Coastal Firn and Ice Cores and Its Relationship with Chlorophyll-a and Sea Ice Extent
Nilsson et al., 2024
Summary
The article interprets MSA records from three firn cores and one ice core from Fimbul ice shelf in the East Antarctic core. MSA (methanesulphonic acid) concentration in the atmosphere is linked to oceanic primary productivity for this reason it has been used in several studies to reconstruct past Antarctic sea ice variability. These studies however show contrasting results and at present the abundance of this compound in Antarctic snow can’t be used confidently for past sea ice reconstructions.
The aim of this manuscript is to determine whether MSA concentrations in firn and ice cores from the Fimbul ice shelf are linked to sea ice variability and chlorophyll. The study finds some positive (although weak) correlation with both sea ice and chlorophyll in the Weddell sea sector, however, the lack of coherency in the correlations across all cores evidences the unsuitability of the site for past sea ice reconstructions using MSA.
General comments
I think the article is well written and with good level of English. I appreciated the discussion on the differences among the sites resulting in big differences in MSA deposition even though the sites are quite close to each other. Maybe this is something that could be highlighted better in the manuscript.
However, I think there are several major weaknesses in this article that should be addressed before publication. First and most important, I struggle to see the novelty in this study. There have been several publications of MSA in Antarctica showing contrasting results as the authors also state in the Introduction but I feel the study lacks an in-depth discussion on why the site is unsuitable for sea ice reconstructions using MSA (atmospheric patterns? Orography? Ocean circulation?). I would also apreciate if the authors could add a discussion on how this finding relates to these previous studies. This discussion would help defining a logic for determining whether a site is suitable or not for sea ice reconstructions using MSA.
Second, the manuscript is not clear whether the aim of the study is to investigate the MSA-sea ice relationship across the entire Southern ocean or in the source area of impurities uplifted, transported and deposited in the Fimbul ice shelf. Studies using sea ice proxies (e.g. Thomas et al., 2019) define the source area of impurities transported the site, so in this context showing correlations with all sectors of the Southern ocean doesn’t make too much sense. I think the authors should at least state which of the sectors are the ones that should correlate the most given the proximity and the air mass transport. Defining the source area could be done through backtrajectory analysis or by considering main wind patterns and the lifetime of MSA in the atmosphere.
Third, the manuscript shows quite weak correlations that are computed on fairly large sectors. How can it be argued that the identified correlations are not merely spurious? Here the authors are comparing multiple records against multiple sectors of the Southern Ocean, so there is a fair amount of chance that some of the correlations will be positive, even if no relation exists. Can the authors exclude that this is the case?
Specific comments
Line 26-28: In this sentence the authors first refer the sea ice in the Southern ocean and then to MSA being a proxy of regional sea ice. Please modify the sentence to be consistent to either hemispheric or regional sea ice.
Line 31-32: please explain how sea ice is a facilitator of DMS production.
Line 38: MSA is not typically used for reconstructing sea ice in Arctic ice cores. Please see Osman et al., (2019). In their study they use MSA records from the Greenland ice sheet to reconstruct subarctic productivity that changed in relation to oceanic circulation, rather than Arctic sea ice decline in the industrial era.
Line 59: “There are now longer records”: with respect to which ones specifically?
Line 65: “and chlorophyll-a concentrations IN the Southern Ocean”
Line 104: can you describe the thickness of the layers?
Line 137: I cannot find this citation in the bibliography
Figure 5: please make clearer that the first 5 boxplots are for summer and the other 5 are for winter. My suggestion is two curly brackets which contain summer and winter boxplots.
Line 247-248: please move up “from the BI and KM cores”
Line 260: this section diverts from the scope of the study I suggest to remove it
Lines 331-332: Please state why it is still consistent to stack them together
Lines 397-400: can you split up this sentence? It is very long to read
Lines 402-404: I don’t think that positive correlations in the Ross sea sector is of any significance as it’s outside the source area of moisture transported to the Fimbul ice shelf. I also find hardly significant the correlation found in the Indian ocean given that westerly winds transport air masses eastward (see for instance Fig 2 of Clem et al., 2020). Please acknowledge this when stating that MSA records correlated with sea ice and chlorophyll in this sectors.
Bibliography
Clem, K. R., Fogt, R. L., Turner, J., Lintner, B. R., Marshall, G. J., Miller, J. R., & Renwick, J. A. (2020). Record warming at the South Pole during the past three decades. Nature Climate Change, 10(8), 762–770. https://doi.org/10.1038/s41558-020-0815-z
Osman, M. B., Das, S. B., Trusel, L. D., Evans, M. J., Fischer, H., Grieman, M. M., Kipfstuhl, S., McConnell, J. R., & Saltzman, E. S. (2019). Industrial-era decline in subarctic Atlantic productivity. Nature, 569(7757), 551–555. https://doi.org/10.1038/s41586-019-1181-8
Thomas, E. R., Allen, C. S., Etourneau, J., King, A. C. F., Severi, M., Winton, V. H. L., Mueller, J., Crosta, X., & Peck, V. L. (2019). Antarctic sea ice proxies from marine and ice core archives suitable for reconstructing sea ice over the past 2000 years. Geosciences (Switzerland), 9(12). https://doi.org/10.3390/geosciences9120506
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AC1: 'Reply on RC1', Carmen P. Vega, 16 Apr 2024
Response to Referee 1 comments:
The authors would like to thank the referee for the time taken to thoroughly review and comment the manuscript. We write our responses below, following each comment made by the referee (RC1).
General comments
RC1: “I think the article is well written and with good level of English. I appreciated the discussion on the
differences among the sites resulting in big differences in MSA deposition even though the sites are quite close to each other. Maybe this is something that could be highlighted better in the manuscript.”
Response: We will highlight the differences in MSA deposition as suggested by the referee; we agree that this did not have the emphasis it should in the manuscript.
RC1: “However, I think there are several major weaknesses in this article that should be addressed before
publication. First and most important, I struggle to see the novelty in this study. There have been several publications of MSA in Antarctica showing contrasting results as the authors also state in the Introduction but I feel the study lacks an in-depth discussion on why the site is unsuitable for sea ice reconstructions using MSA (atmospheric patterns? Orography? Ocean circulation?).”
Response: The referee is correct that this study is not particularly novel regarding analysis or interpretation. However, it presents a completely new data set which we believe is a valuable contribution from a continent that has limited field data. We think that this study contains data that are highly relevant for the understanding of atmospheric deposition and accumulation for coastal sites in East Antarctica. These areas are sensitive to any changes in atmospheric and oceanic circulation and therefore will be the first places where such influences can be detected. Furthermore, some of the study sites are located on ice rises, an additional component that might be relevant for the interpretation of the MSA data. Altogether, our data set differs from previous work that mainly report MSA in ice cores collected from glaciers at the coast or at inland locations in Antarctica (Thomas et al. 2019). Finally, we want to emphasize that this is an area of DML where research activities, including drilling of new coastal ice cores, are increasing and thus any previous field data that could help both the selection of drill sites and general planning are important to highlight.
RC1: “I would also appreciate if the authors could add a discussion on how this finding relates to these previous studies. This discussion would help defining a logic for determining whether a site is suitable or not for sea ice reconstructions using MSA.”
Response: We agree with the referee, and we will consider this comment so the results of the manuscript can be interpreted within the context of previous work on MSA done in Antarctica, and therefore, highlight why the manuscript results are relevant.
RC1: “Second, the manuscript is not clear whether the aim of the study is to investigate the MSA-sea ice
relationship across the entire Southern Ocean or in the source area of impurities uplifted, transported and deposited in the Fimbul ice shelf. Studies using sea ice proxies (e.g. Thomas et al., 2019) define the source area of impurities transported the site, so in this context showing correlations with all sectors of the
Southern ocean doesn’t make too much sense. I think the authors should at least state which of the sectors are the ones that should correlate the most given the proximity and the air mass transport. Defining the source area could be done through backtrajectory analysis or by considering main wind patterns and the
lifetime of MSA in the atmosphere.
Response: The referee has a good point here. The aim of the study was to assess the suitability of cores drilled in FIS to use MSA as proxy (or not) of SIE in the area. Therefore, we will rewrite the discussion and emphasize the correlations with the sectors that are relevant to this part of DML. In addition, we will thoroughly discuss the most likely MSA source regions to FIS based on existing literature. New back-trajectory calculations are not necessary here. Defining the specific source area of MSA was not an objective of the paper. However we appreciate the remark, and we will consider it in any further investigations.
RC1: Third, the manuscript shows quite weak correlations that are computed on fairly large sectors. How can it be argued that the identified correlations are not merely spurious? Here the authors are comparing multiple records against multiple sectors of the Southern Ocean, so there is a fair amount of chance that some of the correlations will be positive, even if no relation exists. Can the authors exclude that this is the case?
Response: The referee is correct. We will rework this part of the analysis and focus on the regions that are relevant to FIS according to previous work on the main atmospheric patterns in the area. We will investigate whether it is possible with a subdivision of the relevant sectors to FIS and analyze the MSA-SIE relationship in more detail. We also agree that multiple comparisons can indeed be an issue when several similar inferences are made, potentially generating false positives (false 0-hypothesis rejections). However, we do not think that in this particular case we need to prove the causation, as a general physical link between sea ice variability, MSA emission and its deposition is known to exist. The derived correlations are therefore more an indication of a potential for deriving quantitative reconstructions of sea ice for the area as well as a motivation for future research in this direction. We also don’t consider the correlations, where found statistically significant, to be weak. R^2 of the order of 0.3-0.4 is indicative of correlations of 0.5 - 0.6 which is quite a typical value in climate proxy analyses, often considered to be sufficiently good for making a quantitative reconstruction of the sought climate variable. We actually find the results promising for continuing the work on the subject with ice materials from the area.
Specific comments
Response: Generally, for the specific comments, we will include the referee comments and suggestions in a new version of the manuscript unless it is explicitly mentioned below that it won’t be done, along with an argumentation.
RC1:
“Line 26-28: In this sentence the authors first refer the sea ice in the Southern ocean and then to MSA being a proxy of regional sea ice. Please modify the sentence to be consistent to either hemispheric or regional sea ice.
Line 31-32: please explain how sea ice is a facilitator of DMS production.”
Response: We will include a concise explanation, since is not the scope of the manuscript and the mechanism can be found in literature elsewhere.
RC1:
“Line 38: MSA is not typically used for reconstructing sea ice in Arctic ice cores. Please see Osman et al., (2019). In their study they use MSA records from the Greenland ice sheet to reconstruct subarctic productivity that changed in relation to oceanic circulation, rather than Arctic sea ice decline in the
industrial era.
Line 59: “There are now longer records”: with respect to which ones specifically? Line 65: “and chlorophyll-a concentrations IN the Southern Ocean”
Line 104: can you describe the thickness of the layers? Line 137: I cannot find this citation in the bibliography
Figure 5: please make clearer that the first 5 boxplots are for summer and the other 5 are for winter. My suggestion is two curly brackets which contain summer and winter boxplots.
Line 247-248: please move up “from the BI and KM cores”
Line 260: this section diverts from the scope of the study I suggest to remove it Lines 331-332: Please state why it is still consistent to stack them together
Lines 397-400: can you split up this sentence? It is very long to read
Lines 402-404: I don’t think that positive correlations in the Ross sea sector is of any significance as it’s outside the source area of moisture transported to the Fimbul ice shelf. I also find hardly significant the correlation found in the Indian ocean given that westerly winds transport air masses eastward (see for
instance Fig 2 of Clem et al., 2020). Please acknowledge this when stating that MSA records correlated with sea ice and chlorophyll in these sectors.”
References
Thomas et al. Geosciences 2019, 9(12), 506; https://doi.org/10.3390/geosciences9120506
Citation: https://doi.org/10.5194/egusphere-2023-3156-AC1
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AC1: 'Reply on RC1', Carmen P. Vega, 16 Apr 2024
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RC2: 'Comment on egusphere-2023-3156', Anonymous Referee #2, 16 Mar 2024
There has been an increasing interest in reconstructing past sea ice extent (SIE) in polar oceans using proxies derived from glacier ice core records. Methane sulphonic acid (MSA) is one of such proposed proxies, but its relationship with SIE remains controversial at best. This paper tests the validity of using Antarctic coastal firn and ice core records of MSA as a proxy for SIE in the Southern Ocean. MSA records were derived from previously reported data from three firn cores and one ice core retrieved from the Fimbul Ice Shelf in East Antarctic. Historical SIE and chlorophyll-a (Chl a) concentrations in the Southern Ocean were retrieved from satellite data. The results show that the MSA-SIE or MSA-Chl relationships are incoherent even though the firn and ice cores were collected from a relatively small region (~200 km from each other), hence not supporting the use of the MSA records as an SIE proxy. The paper is fairly well organized and written, and the methodology clearly explained.
Although I see some value of cautioning the use of firn/ice core MSA as an SIE proxy, I think the formulation of the hypothesis and interpretation of the data as presented herein are overly simplistic and thin. While multiple ice core (bio-)geochemical proxies have shown some promise in helping reconstructing historical SIE, it is unlikely that any one of them would be universally applicable at any site; instead the solution will most likely rely on a combination of them. This is due to the fact that there are complex processes happening before an SIE-related proxy is preserved in firn/ice cores: production/emission to the air, transport inland, dry/wet deposition to the surface, and post-depositional physical and chemical changes. Different proxies will have different sensitivities to these processes. Instead of focusing only on MSA, the paper would have been much stronger if other proxies (e.g., bromine enrichment) are examined for comparisons as well. For instance, would a similar incoherence also exist in the correlation (or lack thereof) between other proxies and SIE?
Another major challenge is the spatial scale of SIE. In the context of this paper, what would be the regionality of the SIE that is most likely to be reflected by MSA in the firn/ice cores? The authors divide the Southern Ocean into five sea sectors simply based on longitudes. While such divisions might make sense geographically when talking about regional sea ice trends, they appear to me to be totally arbitrary for the present work (or at least the appropriateness is not elaborated in the paper). It would have made much more sense to divide source regions of MSA based on prevailing wind patterns etc (e.g., via back trajectory analysis).
Furthermore, there should also be some discussion on how “conservative” MSA is as it is formed from DMS oxidation over the MIZ/ocean, and transported and deposited in firn and glacial ice. How stable is MSA with respect to chemical reactions (e.g., photochemistry, reactivity with aerosols) and physical processes (e.g., dry and wet deposition, percolation in snow/ice) during those process? What’s its lifetime?
As there is no consensus in the literature that MSA can be used an SIE proxy, the argument presented in this paper, based on rather simplistic and potentially problematic analysis, that it might not be a promising proxy offers little new insight. Therefore, I cannot recommend the acceptance of the paper for publication, at least not in the present form.
Should the authors want to revise and resubmit it, I’d encourage them to address the aforementioned issues by perhaps providing a more thorough analysis of the appropriateness (or lack thereof) of using MSA as an SIE proxy, based not only on the limited data from this study, but also other relevant studies on the very topic.
Citation: https://doi.org/10.5194/egusphere-2023-3156-RC2 -
AC2: 'Reply on RC2', Carmen P. Vega, 16 Apr 2024
Response to Referee 2 comments:
The authors would like to thank the referee for the time taken to thoroughly review and comment the manuscript. We write our responses below, following each comment made by the referee (RC2).
RC2: “There has been an increasing interest in reconstructing past sea ice extent (SIE) in polar oceans using proxies derived from glacier ice core records. Methane sulphonic acid (MSA) is one of such proposed proxies, but its relationship with SIE remains controversial at best. This paper tests the validity of using Antarctic coastal firn and ice core records of MSA as a proxy for SIE in the Southern Ocean. MSA records were derived from previously reported data from three firn cores and one ice core retrieved from the Fimbul Ice Shelf in East Antarctic. Historical SIE and chlorophyll-a (Chl a) concentrations in the Southern Ocean were retrieved from satellite data. The results show that the MSA-SIE or MSA-Chl relationships are incoherent even though the firn and ice cores were collected from a relatively small region (~200 km from each other), hence not supporting the use of the MSA records as an SIE proxy. The paper is fairly well organized and written, and the methodology clearly explained.
Although I see some value of cautioning the use of firn/ice core MSA as an SIE proxy, I think the formulation of the hypothesis and interpretation of the data as presented herein are overly simplistic and thin. While multiple ice core (bio-)geochemical proxies have shown some promise in helping reconstructing historical SIE, it is unlikely that any one of them would be universally applicable at any site; instead the solution will most likely rely on a combination of them. This is due to the fact that there are complex processes happening before an SIE-related proxy is preserved in firn/ice cores: production/emission to the air, transport inland, dry/wet deposition to the surface, and post-depositional physical and chemical changes. Different proxies will have different sensitivities to these processes. Instead of focusing only on MSA, the paper would have been much stronger if other proxies (e.g., bromine enrichment) are examined for comparisons as well.”
Response: The referee mentions that the work is “overly simplistic and thin”, that “it is unlikely that any one of them would be universally applicable at any site” when referring to geochemical proxies for SIE and suggests a “combination of them”. We agree that none or those proxies would be universally applicable at any site. However, to move science forward it is necessary to have as much data as possible, especially if the data come from remote polar regions. Combined records are the ideal, however, in regions where data are scarce also individual records are valuable as they pave the way to more extensive studies, should these first (limited) attempts of data analysis proven to be prospective. We believe that despite the fact that we have not provided new analytical methods or interpretations, it is important to bring out field data from a region that has not been studied before to the scientific community. These data can greatly influence the decision making regarding where new ice cores will be drilled in coastal Antarctica. In fact, there are already such ongoing initiatives in this part of DML. In the case of this study, bromine analyses were not even considered. The S100 ice core was sampled and analysed almost 25 years ago when analytical methods where not as advanced as today. The scientific aim with the FIS cores presented in this work did not require, total bromine analysis. However, as more recent literature suggests (Burgay et al. 2023) it would be relevant to consider/implement the analysis of total bromine in any further ice core work in the coastal regions of DML.
RC2: “For instance, would a similar incoherence also exist in the correlation (or lack thereof) between other proxies and SIE?”
Response: We cannot unfortunately answer this question given the FIS data available.
RC2: “Another major challenge is the spatial scale of SIE. In the context of this paper, what would be the regionality of the SIE that is most likely to be reflected by MSA in the firn/ice cores? The authors divide the Southern Ocean into five sea sectors simply based on longitudes. While such divisions might make sense geographically when talking about regional sea ice trends, they appear to me to be totally arbitrary for the present work (or at least the appropriateness is not elaborated in the paper). It would have made much more sense to divide source regions of MSA based on prevailing wind patterns etc (e.g., via back trajectory analysis). “
Response: The referee has a good point, and this was also noted by Referee 1. We will focus the correlation analysis between MSA and SIE considering the current literature on source regions of MSA and transport patterns to FIS instead of presenting the correlations using the regions described in the manuscript.
RC2: “Furthermore, there should also be some discussion on how “conservative” MSA is as it is formed from DMS oxidation over the MIZ/ocean, and transported and deposited in firn and glacial ice. How stable is MSA with respect to chemical reactions (e.g., photochemistry, reactivity with aerosols) and physical processes (e.g., dry and wet deposition, percolation in snow/ice) during those process? What’s its lifetime?
Response: Is the referee suggesting that we demonstrate that the MSA signal in the FIS cores is well preserved? We will include a brief discussion on that so that there is no doubt about it, following the discussions presented in Vega et al. 2016 and 2018, and references therein in relation to the FIS and S100 cores. If the referee means the atmospheric processes related to oxidizing DMS to MSA we consider this beyond the scope of our paper.
RC2: “As there is no consensus in the literature that MSA can be used an SIE proxy, the argument presented in this paper, based on rather simplistic and potentially problematic analysis, that it might not be a promising proxy offers little new insight. Therefore, I cannot recommend the acceptance of the paper for publication, at least not in the present form.”
Response: We would appreciate if the referee could give us more insight into why, from his/her point of view, the analysis presented in the paper can be problematic. This would help us to understand the issue in its substance and correct it accordingly. Otherwise, we can immediately argue in response that the correlation analysis and regression methods are widely used in all kinds of scientific literature and considered to be robust if applied correctly (as any other method of data analysis). We also do not consider that manuscript analysis is simplistic, at least not from the point of view of the methods used, as they are essentially sufficient for the purpose of our manuscript. We hope that modifications we plan to make to the manuscript will be sufficient to prove the value of the presented study.
RC2: “Should the authors want to revise and resubmit it, I’d encourage them to address the aforementioned issues by perhaps providing a more thorough analysis of the appropriateness (or lack thereof) of using MSA as an SIE proxy, based not only on the limited data from this study, but also other relevant studies on the very topic.”
Response: We thank the suggestion of the referee, and we will include relevant work that has been recently published on the topic in other to strengthen the discussion; however, we cannot include additional data, such as total bromine, since it has not been measured in the cores.
The main aim of our paper is to provide a new data set from a coastal region of Antarctica, and we believe that this will be highly relevant for future studies in the area and thus contribute to improve understanding of the complexities interpreting MSA in snow and ice forward.
References
Burgay, F., Fernández, R. P., Segato, D., Turetta, C., Blaszczak-Boxe, C. S., Rhodes, R. H., Scarchilli, C., Ciardini, V., Barbante, C., Saiz-Lopez, A., and Spolaor, A.: 200-year ice core bromine reconstruction at Dome C (Antarctica): observational and modelling results, The Cryosphere, 17, 391–405, https://doi.org/10.5194/tc-17-391-2023, 2023.
Vega, C. P., Schlosser, E., Divine, D. V., Kohler, J., Martma, T., Eichler, A., Schwikowski, M., and Isaksson, E.: Surface mass balance and water stable isotopes derived from firn cores on three ice rises, Fimbul Ice Shelf, Antarctica, The Cryosphere, 10, 2763–2777, https://doi.org/10.5194/tc-10-2763-2016, 2016.
Vega, C. P., Isaksson, E., Schlosser, E., Divine, D., Martma, T., Mulvaney, R., Eichler, A., and Schwikowski-Gigar, M.: Variability of sea salts in ice and firn cores from Fimbul Ice Shelf, Dronning Maud Land, Antarctica, The Cryosphere, 12, 1681–1697, https://doi.org/10.5194/tc-12-1681-2018, 2018.
Citation: https://doi.org/10.5194/egusphere-2023-3156-AC2
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AC2: 'Reply on RC2', Carmen P. Vega, 16 Apr 2024
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