Stable isotopes in cave ice suggest summer temperatures in east-central Europe are linked to Atlantic Multidecadal Oscillation variability

Bădăluţă, Carmen-Andreea; Perșoiu, Aurel; Ionita, Monica; Piotrowska, Natalia

doi:https://doi.org/10.5194/cp-16-2445-2020

Articles | Volume 16, issue 6

https://doi.org/10.5194/cp-16-2445-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-16-2445-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 6

Research article

|

10 Dec 2020

Research article |

| 10 Dec 2020

Stable isotopes in cave ice suggest summer temperatures in east-central Europe are linked to Atlantic Multidecadal Oscillation variability

Carmen-Andreea Bădăluţă, Aurel Perșoiu, Monica Ionita, and Natalia Piotrowska

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Final revised paper (published on 10 Dec 2020)
Preprint (discussion started on 13 Jan 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review on cp-2019-141 - Age-Depth Model should be improved', Anonymous Referee #1, 20 Feb 2020
- AC1: 'Response to reviewer #1', Carmen-Andreea Badaluta, 08 Apr 2020
RC2: 'Stable isotopes in cave ice suggest summer temperatures in East-Central Europe are linked to AMO variability', Anonymous Referee #2, 27 Mar 2020
- AC2: 'Response to reviewer #2', Carmen-Andreea Badaluta, 08 Apr 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (11 Apr 2020) by Barbara Stenni

AR by Svenja Lange on behalf of the Authors (28 Apr 2020) Author's response

ED: Referee Nomination & Report Request started (05 May 2020) by Barbara Stenni

RR by Anonymous Referee #3 (16 May 2020)

Suggestions for revision or reasons for rejection

Dear Dr. Steni,
I have reviewed the manuscript referenced below, you can find my comments on-line. I just write here a response to your questions, just in case it is useful for you:
El 05/05/2020 a las 17:00, editorial@copernicus.org escribió:
Editor's Note:
"Dear Dr. Moreno,
I am sending you a manuscript by Badaluta and co-authors on a paleoclimate record from an ice cave deposit in Romania. The manuscript is at its second round of revision and I would like to have your opinion on it.
I think that the authors have answered to the relevant questions addressed by the first two reviewers, but I also think that in their revised version they did not introduce some relevant comments on the 14C dating and on the mechanism of excluding or not a sample for dating.
I agree the authors have not yet explained which were their arguments to reject so many 14C samples (and not others). This is to me the main problem with this paper: the robustness of the age model. This issue is specially remarkable since the main idea of this study is to relate d18Oice with AMO variability, at a decadal scale. The uncertainty in the chronology does not allow such comparsion.
Another point on which I would like to have your opinion, if you feel comfortable with this, is related to the text discussing the paleoclimate interpretation, between page 6 and 7 where I found difficult to follow and where some dates should be rounded. Perhaps some parts should be moved up and down, following what is the numbering of the figures instead of going back and forth.
Regarding the paleoclimate interpretation, I find interesting the d18Oice interpretation as summer temperature. This idea would probably require more monitoring data since the authors just present precipitation data for several years, but not any d18O measured in ice formed every year. In additon, temperature measured in the cave will be important to demonstrate water freezes in early fall, thus recording the summer signal in the ice. This is not yet well-demonstrated.
Finally, the comparison with other records requires some statistics. There are many "visual observations" in the manuscript about the correlation among different proxies; that has to be supported by correlation coefficients.
Hoping you can use my comments above and those in my review.

Review of Cave ice stable isotopes suggest summer temperatures in East-Central Europe are linked to AMO variability by Bădăluță et al.
This manuscript presents an interesting record from an ice cave in Rumania where the d18O data are interpreted as summer temperature variations in response to AMO variability during last 1000 years. Although the interest of this type of records is certainly high, the manuscript presents several issues that prevent its publication in Climate of the Past. I outline here my main concerns:
1) The most important problem is the lack of a robust chronology, specially to use this record as a reconstruction of an atmospheric-oceanic phenomenon such as the AMO that changes at a decadal scale. The chronology of this ice sequence, in spite of all the efforts carried out by the authors, is unfortunately not good enough for that comparison. The age-depth model is constructed by just 4 14C dates since up to 8 14C dates had to be discarded. The authors do not explain in detail the reasons to discard those ages but Fig2 shows the difficulties to construct a reliable age model for this sequence. Even if we consider that this is the best possible age model, it certainly lacks the chronological precision and accuracy to be later compared to total solar irradiance or tree-ring chronologies. If the authors decide to keep this chronology, I suggest to use the record to discuss variability at the scale of MCA vs LIA but not use it to compare with AMO variability. I outline here two possible improvements:
- The authors state that since the cave is open many organic remains reach the cave and are incorporated into the ice (they had more than 40 samples with organics). I suggest dating more samples. And, instead of looking for larger pieces of wood or leaves that are for sure more difficult to find, use the WIOC technique that requires a very small amount of carbon in the ice to be dated. Of course, some dates will provide reversals but some others, hopefully, will support the age model and will help to draw it with more certainty.
- the other idea is using the regional summer temperature curve to maximize the correlation with the d18Oice, that is displacing the data from the ice within the chronological uncertainty until the correlation is maxima. Later, the authors can present how much towards the past or towards the present they had to move the d18Oice data and see if this is still coherent with the age model. To do that, better evidences that the d18Oice is a proxy for summer temperatures are needed (see point 2)
2) This record is presented as a summer record instead of a winter one (as A294 ice cave in Spain or Scărișoara cave in Romania). The main argument is that the water that penetrates directly from the open ceiling of the cave freezes during early fall (not late fall-early winter as the other caves do) thus preserving the summer signal in the isotopes. This is probably true, but not demonstrated in the manuscript. I include here some ideas:
- I am pretty sure the authors have some sensors in the cave recording temperatures. Is it possible to see the time of the freezing? Is it always the same every year? Is it changing?
- Have you sampled the ice body surface during last 20 years? In the manuscript there is a sentence that seems to indicate that. If so, it will be excellent to see those data and their relationship with summer temperatures of that year. The comparison during instrumental period presented in Fig. 4 is nice, but the d18Oice from FV ice cave is plotted using the chronology presented here and for the last 150 years, I am afraid there is just one 14C date.
3) Finally, I find very appealing the comparison with other records in CEE during last millennium. This comparison is mostly presented in Fig.7 and in the second part of the Discussion section. I miss there some statistics, specially about the correlation among records. Those correlations are just based on visual observation and this is not enough, I think.

Referee Report: PDF

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RR by Anonymous Referee #1 (15 Jun 2020)

Suggestions for revision or reasons for rejection

Dear Authors,

Thanks for your answers and I appreciate your effort how revised your work. I think this is undoubtedly an improved version compared to the initial one. I accept your reasoning why not modify the original depth data. I accept also the solution using a uniform age distribution from 1991 to 2016 AD for the core top in age depth modelling and satisfied also with the revised Table 1.
However, I feel some further changes and clarifications are still needed before publication. I think the data are interesting so I encourage the Authors for further revision.

Major comments:
-Section 3.2. Please give quantitatively the carbon yield needed for AMS analysis. For instance, the 0.14 mg graphite yield of GdA-5086 (FV6-4/232-235) seems to be close to the critical mass which might explain why it produced an outlying old age. In addition, in your response you agreed with my comments related to old bias for large wood (you wrote:” As correctly pointed out by the reviewer, this was the reason behind rejecting the sample at -63 cm”) however I haven’t found it in the revise manuscript. In general I suggest giving more word explaining the preselection of the samples entered or excluded from age-depth modelling. Maybe you could replace a part of section 4.1 with this issue (see next comment).
- I’m afraid the discussion of temporal differences of ice accumulation rate is a bit overinterpretation. There are rather few control points (5 including the top of the core) and the finally accepted age-depth model do not suggest any remarkable change in ice accumulation rate. To be on the safe side I’d suggest reporting only the long-term mean ice accumulation rate and discuss only its comparison to other multicentennial mean cave ice accumulation rates from the surroundings. A related point is that you reported the largest accumulation rate (0.56 cm/years) for the past 40 years. It is in strong contrast compared to experiences from other ice caves from the Carpathians (or Europe).
- some parts of the discussion should move to Methods (see detailed comments below)

Specific comments:
Page 2 lines 41-43: reference is needed for AMO.
Page2 line11: Stoffel et al. 2009 (https://doi.org/10.1016/j.yqres.2009.03.002 ) could be also cited as an example when changes in cave ice accumulation rate was used as indicator of past climate variability.
page2 line 19: Bella and Zelinka 2018 (https://doi.org/10.1016/B978-0-12-811739-2.00029-2 ) should be also cited supporting the statement that “The Carpathian Mountains host several ice caves”
page2 lines 31-35 These statements are supported by the air temperature monitoring of Focul Viu Ice Cave (Perşiou et al. (2007) Preliminary data on air temperature in Focul Viu Ice Cave (Bihor Mts, Romania). In: Zelinka, J. (ed.) Proceedings of the 2nd International Workshop on Ice Caves, Liptovský Mikuláš, pp.62-64.).
page4 lines16 please consider rephrasing “dating of organic remains and 1100 cal BP at 4.86 m below surface (extrapolation).” to “dating of organic remains (Table 1) and extrapolated as 1100 cal BP at 4.86 m below surface.”
page4 line 16-18. This sentence should go to section 3.1.
page5 line 13 Please rephrase this sentence. It is a bit strange reading that records show “stable values” when you write about stable isotope ratios. Perhaps you could write that there is no trend.
page5 lines 27-32 Station description should go to Section 3.3. Maybe you could mark these stations in Fig1a.
page5 lines 36-40 This is mainly methodological description. I think it should also go to Section 3.3.
page6 lines 1-2 I think the statement about extreme high temperatures and AMO could be omitted since extreme high temperatures were not assessed in this study.
page6 line 7-8 How could enhanced ice melting lead to high δ18O values? Maybe this needs more explanation.
page6 line 11 Please, add “from the Eastern Carpathians” between “anomalies” and the citation. In addition, please, keep in mind that the cited study developed summer temp reconstruction only from 1163 AD so you should not cite it for cold or warm periods before this date.
page6 line 21. “solar influence” mentioned at the end of this sentence is somehow astounding. The explanation is only in lines 35-38. Maybe some rearranging could be useful here.
page6 line 26 Please consider inserting “in the ice core chronology” between the words “uncertainty” and “between”
page 6 line 39-40 The first period mentioned as low and high δ18O values overlap. Please check the dates.

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ED: Reconsider after major revisions (20 Jun 2020) by Barbara Stenni

Dear Dr. Bădăluță and Dr Perşoiu
The second version of your manuscript have been reviewed by the Ref1 and by another referee and both have indicated “reconsidered after major revisions”.
There are three major drawbacks in my opinion and following both referees:
1. The dating
2. The comparison with other records
3. the structure of the manuscript
Regarding the point 1. In the revised manuscript both referees found, and I agree with them, that you did not explain very well the reasons for rejecting a so high number of 14C data. These parts should be really improved. The lack of a robust chronology has a real implication to the second major drawback, the comparison with other records at a decadal scale for being able to look at the AMO variability. In the text you are using dates rounded at half a decade and with the uncertainty of the chronology that you have, I think that this is not possible. Perhaps the comparisons should be limited at the multidecadal-centennial time scale. One referee suggests two solutions to overcome this problem: further 14C dating with another technique or using the regional temperature records to maximize the correlation with the d18Oice over their common period.
Regarding the point 2. In all the comparisons with other records you never use any statistics and I personally found difficult to follow all the dates in the text and at the same time looking at the different figures to follow the visual observations, that in some cases did not match at all. I am referring to figures 4, 6 and 7 and, in particular, to the following sections: page 6 lines 10-15; page 6, lines 38-42; page 7 lines 5-6. Several times I found that the periods you were referring to cold or warm periods were not evident from the figures at all. Please, check the dates!!! It is difficult to follow without some statistics. By the way, how did you define the 50-year lag of the d18Oice behind the AMO?
Regarding the point 3. There are several times in the manuscript, some paragraphs or sentences that seem to be in the wrong place. For example, at a certain point you are referring to the solar influence without explaining nothing at all before. The figure 6 and the its explanation comes after the figure 7. Some parts of the text in the discussion should be moved in section 3 (see comments by the referees). At a certain point you are referring to the solar minima and maxima without referring to any figure and without introducing your reasoning. All these weaknesses and lack of continuity make the reading very difficult.
Moreover, apart from what reported above, there are other questions, issues highlighted also by the referees, that would need your attention. You can find them in the referee’s reports. Please, look at the them carefully. One of this, is the ice accumulation rate. There is not any associated figure and the last sentence of the paragraph (page 4, lines 34-36) as well as at page 4 line 27 (“The high accumulation rate …”) does not agree with the results reported above (page 4, lines 19-21). One of the referees found that these data are overinterpreted, and I agree on this, or at least I was not able to follow.
One of the referees found that you did not demonstrate in the manuscript that the d18Oice of the FV cave is a summer temperature records contrary to what happen in other ice caves. The referee would suggest including in the manuscript the cave recording temperatures to see the time of the freezing. I think that more explanations and reasoning on the ice forming processes should be needed.
In the figure 4 the Summer temperatures of the stations show an increasing trend after 1980 AD. In the text (page 5, line 34) it is wrongly reported.
The spatial correlation map (figure 5) does not show any correlation in your area (see green arrow in the figure). Please, explain better. Perhaps I miss the point.
In the text you are saying that you compared your FV record to the TRW record by Popa and Kern (2009) but in the figure 7 the TRW record is by Seim et al., 2012). Please, may you check?
The sentence at page 6 lines 35-37 seems a bit an overinterpretation if you are not dealing with all the above points and explain better your reasoning.
At page 7 line 19 you are referring to millennial scale variability. Please rewords, the length of your record is about 1000 years.
Although I found your paper interesting, and I would really have liked to see it published I, found that at the present state this solution is not so easy. I think that there is still a lot of work to be done to improve this manuscript for being accepted in Climate of the Past, considering that we are already at the second round of revision. Nevertheless, I think that the ice cave deposits would deserve more attention in the paleoclimate community, particularly in regions where other paleoclimate archives are lacking.
So, what I am proposing, if you are still interested in publishing your manuscript in Climate of the Past, is that you really deal with ALL the comments above (some are from me) as well as all the referee comments. What I need to see, it is a real restructured manuscript, something more than a major revision. Otherwise I cannot accept the manuscript. When you will resubmit your revised version, I will send it again to at least one of the previous referees, and obviously there is no guarantee that the manuscript will be accepted at the end. I don't want to waste your time but the manuscripts in its present form is not acceptable and needs more working.
I kindly ask you to carefully check all relevant citations and references and add those pointed out by the referees. After restructuring your manuscript, you will probably need to rewrite the abstract and the conclusions.

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AR by Svenja Lange on behalf of the Authors (07 Oct 2020) Author's response

ED: Publish subject to minor revisions (review by editor) (16 Oct 2020) by Barbara Stenni

Dear Dr. Bădăluță and Dr Perşoiu
I appreciated the work that you have done for restructuring the manuscript and I decided not to send the last version to another referee round. I understand from your comments that the chronology cannot be further improved. Nevertheless, the chronological uncertainties and limitations should be clearly stated in the conclusions (see my comments below). As I said in my previous comments, I think that the ice cave deposits should deserve more attention in the paleoclimate community.

In my opinion there are still some minor comments to resolve before being accepted. Please, see the list below:

Although in your answer to editor and referee 2 comments about solar irradiance, you said that you are going to remove it from the text there are still several places where the solar induced changes are nominated. These are in the abstract (page line 21, page 7 line 12, 25 and 34). Please, change the sentences accordingly. I would also remove from fig 7 the record of solar irradiance by Steinhilber, as well as the reference itself. Pay attention that changing the figure 7 will necessarily need some changes in the text when referring to the single panels of the figure 7.

Page 3, line 38: The sentence on “old wood effect” should be better linked/explained to the 2 sentences before….. otherwise it is not completely clear…

Page 4, line 20: ….. As expected ….. Maggi et al (2008). Please, explain better and make a link to the “old wood effect”.

Page 5, line 18: may you check the distance between Ghețar and the drilling site? Here is 10 km south of the cave and at page 3 (line 22) is 15 km SE.

Page 5, line 33: here I would suggest referring to a figure showing the entire record and the d18O values as well, so figure 6 or 7. In the manuscript I am still finding some difficulties in the figure sequence….

Page 6, line 3: The figure should be 3a and not 3b.

Page 6, line 15: “ …. the last four decades …”. The figure 4 stops at 2000 AD.

Page 6, lines 35-37: Please, refer to fig. 7.

Page 7, Conclusion: please add or include in a sentence the limitations in the past reconstructions due to the chronology. For example, saying at line 32, … “Given the strong relationship” sounds too much. I would introduce here the possible limitation linked to chronological uncertainties.

Page 15, fig. 2 caption: Please add what the two-colour envelope is meaning.

Page 17, fig. 4 caption: please add “summer (JJA)” before air temperature.

Page 20, fig. 7 caption: after removing panel a, re-arrange the caption accordingly.

Technical comments:
Page 3, line 2: change acve into cave …
Page 3, line 19: check the English, perhaps there should be a comma before and after the words “considering also the annual layering”.
Page 4, line 19: delete “it” at the beginning of the line.
Page 4, line 32: add “the” before “ …. short-term”.
Page 5, line 8: check the English “…. a possibly” … sounds not correct.
Page 7, line 9: add a space before “The …”.
Page 8, line 9: change CBD to CAB.
Page 8, line 15: pay attention to figure panels after removing the Steinhilber record from Fig. 7.
Page 8, line 24: three referees instead of two.
Page 13, lines 30-31: remove Steinhilber.

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AR by Carmen-Andreea Badaluta on behalf of the Authors (20 Oct 2020) Author's response Manuscript

ED: Publish subject to technical corrections (28 Oct 2020) by Barbara Stenni

AR by Carmen-Andreea Badaluta on behalf of the Authors (29 Oct 2020) Author's response Manuscript

Short summary

We present a reconstruction of summer temperature for the last millennium in east-central Europe that shows little summer temperature differences between the Medieval Warm Period and the Little Ice Age on centennial scales as well as the fact that well-expressed minima and maxima occurred synchronously with periods of low and high solar activity, respectively. Furthermore, summer temperatures fluctuated with a periodicity similar to that of the Atlantic Multidecadal Oscillation.