the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2024
Aeolian dust and diatoms at Roosevelt Island (Ross Sea, Antarctica) over the last two millennia reveal the local expression of climate changes and the history of the Ross Sea polynya
Abstract. The pattern of atmospheric and climate changes recorded by coastal Antarctic ice core sites, and the processes they illustrate, highlight the importance of multiproxy studies on ice cores drilled from such peripheral areas, where regional to local-scale processes can be documented. Here, we present a 2000 year long record of aeolian mineral dust and diatoms windblown to the Roosevelt Island obtained from the RICE (Roosevelt Island Climate Evolution project) ice core. Mineral dust and diatoms are highly complementary at RICE since they are related to the large-scale South Pacific atmospheric circulation regime, carrying dust-rich air masses that travelled above the marine boundary layer, and local oceanic aerosol transport by low-level marine air masses, respectively. The 550–1470 CE period is characterized by enhanced mineral dust transport originating from the Southern Hemisphere continents, reduced sea-ice extent in the Eastern Ross and Amundsen Seas, and more frequent penetration of humid air masses responsible for the relative increase in snow accumulation. Around 1300 CE, in particular, in concomitance with marked El Niño-like conditions, the Ross Sea dipole reaches its maximum expression. After 1470 CE, relatively lower dust and snow deposition at RICE suggests an increase in pack ice. This period is characterized by episodes of unprecedented peaks of aeolian diatom deposition, indicating a rapid reorganization of atmospheric circulation linked to the eastward enlargement of the Ross Sea polynya, likely culminating with the opening of the proposed Roosevelt Island polynya, and to an increased influence of low-level marine air masses to the site during the Little Ice Age.
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Status: final response (author comments only)
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RC1: 'Comment on cp-2024-56', Anonymous Referee #1, 01 Oct 2024
I enjoyed reading this paper. I think it adds to the growing research in this field of science. The paper does have some limitations in that it reads more like a thesis chapter than a manuscript. This does not mean that the paper isn't scientifically significant, only that with some minor revisions it can be more succinct and highlight the results instead of 'hiding' them. My revision suggestions are:
- Check all abbreviations (e.g 142-"...Little Ice Age" should be LIA).
- Restructure the methods/results section (e.g. talk about methods first, then results, as it currently reads like a discussion)
- Reduce discussion of others' work in the Discussion section to highlight manuscript results (e.g. it reads like the introduction section)
- Reduce long sentences (e.g. if there are more than 4 commas then try to break it down into 2-3 sentences for clarity).
- Add in stable water isotope methods (e.g. 185)
I have also attached a supplement PDF with more detailed revisions. Again, I'd like to reiterate that I enjoyed this paper and look forward to seeing it in its final form.
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AC1: 'Reply on RC1', Barbara Delmonte, 14 Dec 2024
We thank the reviewer for his comments, that are mainly stylistic. We reworded some sentences as requested, and we apported all the (minor) modifications suggested in the pdf.
We note however that the main structure of this work will be rearranged according to the suggestions of rev.2.
Best Regards
Barbara Delmonte and co-authors
Citation: https://doi.org/10.5194/cp-2024-56-AC1
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RC2: 'Comment on cp-2024-56', Anonymous Referee #2, 25 Nov 2024
This is a review for the manuscript from Lagorio and co-authors. The authors present a record covering the last 2000 years for dust, diatoms, snow accumulation and water stable isotopes. Their variability in the RICE ice-core record is linked to past atmospheric circulation and ocean conditions.
I think that the interpretation the authors give is not fully convincing due to:
- Poor analogue discussions (e.g., 550-1470 CE vs 300 CE) ;
- Poor statistical evaluation and discussion of the results;
- Contradictive statements. E.g., the authors claim that conditions of reduced sea-ice in the ERS are associated with increased snow accumulation, high dust, and isotopically enriched water vapor (L150-153). However, they also claim that sea-ice is reduced in the ERS around 300 CE, when a isotopically enrichment in deuterium is not visible.
- It seems to me that sometimes the interpretation of the results is cherry-picking driven, rather than statistically driven. For example, I was confused on the kind of proxy they used for describing El Nino-like conditions. First SOI was used, then the laminated sediment record from Laguna Pallcacocha. The authors discuss only their results when the two reconstructions agree (e.g., 1000-1500 CE and around 300 CE). However, what about when they do not agree (e.g., 0-300 CE, or 500-700 CE?) Which is the proxy that should be used? And why?
Also, the paper requires an extensive work of revision especially due to its length, useless discussions of topics that are not relevant and confusion in the organization of the different paragraphs (e.g., mixing Methods and Results, introducing sub-chapters in the introduction etc.…). For these reasons, my suggestion is to reject the manuscript in its current form.
Please find below some general and specific comments.
General comments:
- I think that an overall revision of the structure of the manuscript structure should be undertaken. For example, section 1.1 is a mix between methods (description of the properties of the ice core) and introduction (what information can be retrieved from the core). Similarly, the results should not be presented together with the Methods, but on a distinct section. In the current form, the Method and Results section is a continuous back and forth to showing results and discussing methods, making the reading hard. I added some specific comments below to guide the authors in rearranging the structure of the manuscript. Also, the introduction alone covers 1/3 of the overall length of the manuscript, this is too much, and it is probably more suited for a thesis rather than for a scientific paper. For this reason, I suggest polishing and reducing the length of the intro.
- I believe that the manuscript can be further improved by adding some more (basic) statistics. For example, the identification of periods with high/low dust and their comparisons it is not supported by any statistical tests.
- References to climate periods such as the Roman Warm Period or the Medieval Climate Anomaly is completely irrelevant for this manuscript as these periods refer to regional rather than global climate changes. Indeed, the authors discussed the ice-core record based on differences in the dust concentrations, so why referring to periods with no relevance for Antarctic cores? I would remove any reference to these periods as it is absolutely not relevant, and misleading.
- The authors identify as a main period for discussion and data interpretation the one between 550-1470. That’s a correct choice. However, they decide not to discuss what happened before 550 in a distinct paragraph as they did for the period 1470-1990. Why? They only briefly discuss a sort of climatic analogue around 300 CE (whose interpretation is poorly convincing)
- The quality of some figures (e.g., Fig. 5) is extremely poor at a magnification factor of 1.
- The manuscript will benefit from a scheme, showing the conditions leading to the different sea-ice conditions to happen (e.g., ENSO, dust, water stable isotopes, accumulation and sea-ice condition).
SPECIFIC COMMENTS
L26: the authors refer here to the Ross Sea dipole as it is something well known. Please rephrase.
L27: an increase in pack-ice, where?
L28: what do you mean by unprecedented? I would rather be more specific saying “unprecedented over the last 2000 years”.
L37-59: I think that most of the text reported here is not relevant for the paper. I would delete this part and add information that can help the reader to have a clear framework of the topic of the paper. The classification among MCA, RWP etc, is in my opinion also wrong as Antarctica has not been influenced by these climatic periods (Neukom et al., 2019). The introduction should be more specific towards the questions the paper aims to address. As it is written is probably good for a thesis or a book chapter, but not for a scientific paper.
Neukom, Raphael, et al. "No evidence for globally coherent warm and cold periods over the preindustrial Common Era." Nature 571.7766 (2019): 550-554.
L71-83: details regarding the ice-core drilling should be included in the Method section, not in the introduction.
L79: can you provide some numbers? Which is the accumulation rate at RICE? Which is the accumulation rate at other sites?
L84 – 104: I am not fully convinced that all this long description of trace elements is relevant for the objective of the paper. I suggest to strongly summarize this section to the most relevant aspects that can guide the reader for a clear interpretation of your data. For example, what you write afterwards for diatoms (L105-136) goes more into this direction, albeit it can be still summarized avoiding useless details.
L107: repetition of “material”, choose other terms to improve the flow
L172: be specific when mentioning how many samples were measured for dust (i.e., not saying >400, but the actual number). Similarly, specify what “subset” means and therefore how many samples were analyzed for diatoms counting and identification.
L189: can you specify quantitatively the “pronounced variability”, for example by including a relative standard deviation?
L186-L193: this is a mix of results and methods. The results must go in a different section (if you want together with the Discussion, but not with the method).
L192: an average without a standard deviation does not give a lot of information to the reader regarding the dust variability. Please add std. Dev. Or coefficient of variation.
L193: please change “estimation” to “estimate”, and provide a standard deviation (or CV) for WAIS. Comparing to average values can provide some information, but says nothing regarding the variability of the signal at the two sampling sites.
L203: rephrase in “20-80% and 10-50% for FPP% and CPP%, respectively”. Rephrase in the same way at L204.
L212: why not plotting the dust in the same Figure?
L215: before you introduced dust fluxes (L192), why here you are back to discuss dust concentration? Please explain in the text the rationale behind this choice.
L215: how did you calculate the beginning and the end of the prolonged dust periods at RICE? You compared the results with changes in the deuterium isotopes that were determined using a minimum threshold parameterisation to achieve a minimised residual error. Did you used the same method? If yes, please be more specific al L214, otherwise please apply the same approach and compare the time series using similar statistical approaches.
L219: A box plot showing the distribution of dust concentration (or fluxes) in the three periods (0-500; 500-1500;1500-1900) that the authors identified, would be helpful to immediately identify differences in the three regimes. Also, I think that saying something like: “dust levels remain below/above average for about XX% of the time” does not bring anything. If you want to discuss the statistical differences among the different periods, you should rather introduce some basics statistics (e.g., t-test).
L221: “spanned.... spanning”, reformulate to enhance flow
L225: I don’t understand why you don’t show the values for the Holocene? I suggest adding the corresponding plots to the SM. Or at least provide an average value for the Holocene. Written in this way, it does not bring anything to the discussion of the results...
L225: why don’t saying: out of the XX% identified diatoms, >98.5% were.... In this way the reader knows the fraction of identified and unidentified diatoms.
L237-243: this is method, again mixed after presenting results. This part should be included when you discuss how you measured diatoms as a proof of the robustness of your method (e.g., after L185).
L286: please use the same names in the text as they are in the figure. MCA in the text is MWP in the Figure. Choose one acronym and stick to this, although I am skeptical in defining these periods for sampling locations that were not affected by them
L288: how much is this negative correlation? Can you provide the value here and associated p-value?
L299: NA should be Na
L300: can you briefly explain what do you mean in terms of “polynya activity”: how is it measured? I would also be consistent with the y-label in Figure 5, where it is reported “polynya efficiency”.
L304: dust fluxes are actually decreasing after reaching a peak in 900 CE
L309-L343: I think that while interesting, this part can be significantly shortened and linked to the previous paragraph. If you want to keep the sub-paragraph (not recommended), change 3.2.2 to 3.2.1.
L343: but you also compared ssNa from TALDICE, not just water stable isotopes.
L344-L353: If I have correctly understood, high dust values at RICE correspond to low sea-ice conditions in ERS, while high ssNa at TALDICE corresponds to enhanced sea-ice conditions (max pack ice). The authors indicate that this can be associated with ENSO conditions, as inferred by the high SOIpr values peaking in 1300 CE. The authors also observe an analogue condition at around 300 CE, where SOIpr values are peaking. However, besides the peak in the dust flux at RICE, which may correspond to low sea-ice conditions in ERS (however, no increase in the deuterium is observed), ssNa at Taldice it is at its lowest, suggesting low pack ice conditions. Also, the SOIpr values persist at high values even before 300 CE, while dust fluxes at RICE decrease and deuterium is not enriched (as they claim as a needed condition for reduced sea-ice, L150-153). The authors do not discuss these inconsistencies, but rather they put their interpretation in front of the data (i.e., they use another proxy for ENSO that seems to work better). Also, according with the proxy used in Fig. 5d, El Nino like conditions are also around 600 CE, but not significant increase in the dust is observed: why is this not discussed? Without statistics it is hard to give an interpretation of the results.
L378: describing what polynyas are here is a bit too late. Why don’t you anticipate this discussion earlier in the text, and explaining what do you mean for polynya activity/efficiency?
L382: is there in literature any figure of diatoms collected far from the source? It would be nice to cite it or provide a simple comparison. However, this is a minor thing.
L479: I question the “unprecedented low dust input". Have you calculated the mean between 0-500 and between 1500-1940 for dust concentration and checked for statistical differences?
Citation: https://doi.org/10.5194/cp-2024-56-RC2
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