Pervasive diffusion of climate signals recorded in ice-vein ionic impurities

Ng, Felix S. L.

doi:https://doi.org/10.5194/tc-15-1787-2021

Articles | Volume 15, issue 4

https://doi.org/10.5194/tc-15-1787-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-15-1787-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 4

Research article

| Highlight paper

|

13 Apr 2021

Research article | Highlight paper |

| 13 Apr 2021

Pervasive diffusion of climate signals recorded in ice-vein ionic impurities

Felix S. L. Ng

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Final revised paper (published on 13 Apr 2021)
Supplement to the final revised paper
Preprint (discussion started on 07 Sep 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review of the paper „Pervasive diffusion...” by Felix Ng', Anonymous Referee #1, 02 Oct 2020
- AC1: 'Initial response to Reviewer 1', Felix Ng, 24 Oct 2020
- AC2: 'Initial response to Reviewer 2', Felix Ng, 26 Oct 2020
- AC3: 'Final Author Response to all reviews', Felix Ng, 29 Nov 2020
RC2: 'Review of "Pervasive diffusion of climate signals recorded in ice-vein ionic impurities” by Felix S. L. Ng', Alan Rempel, 09 Oct 2020
- AC2: 'Initial response to Reviewer 2', Felix Ng, 26 Oct 2020
  - RC3: 'grain size evolution and the fate of impurities', Alan Rempel, 29 Oct 2020
    - AC3: 'Final Author Response to all reviews', Felix Ng, 29 Nov 2020
      - AC4: 'Final Author Response to all reviews [postcript]', Felix Ng, 01 Dec 2020

Peer-review completion

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

ED: Publish subject to minor revisions (review by editor) (16 Dec 2020) by Nanna Bjørnholt Karlsson

AR by Felix Ng on behalf of the Authors (23 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to revisions (further review by editor and referees) (12 Feb 2021) by Nanna Bjørnholt Karlsson

AR by Felix Ng on behalf of the Authors (12 Feb 2021) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Feb 2021) by Nanna Bjørnholt Karlsson

RR by Eric Wolff (03 Mar 2021)

Suggestions for revision or reasons for rejection

This is a really impressive and thorough paper. It starts by examining an idea that has certainly caused a lot of interest and concern in the ice core community (cited 87 times), but which seems inconsistent with observed data. Despite this, the mismatch with data has never been explained, and this paper represents a real advance in that it explores the implications of the idea with additional considerations, and starts a discussion of the possible ways of reconciling theory and data. It’s quite a tough read for a non-mathematical reader, but with enough simpler explanations that its interesting implications can be understood. I did not review the first version of this paper but I have been asked to look particularly at the new section of text (section 3.4).

Firstly on the inclusion of section 3.4 at all – I think the exchange in the interactive discussion between the author and reviewer 2 was important and it is definitely worthwhile making clear the assumptions that Rempel made about grain size and whether they are reasonable. For this reason I think the first half of section 3.4 should definitely be there, although it may be possible to smooth its edges a little. The second half, where the implications of a non-uniform grain size are explored, is an interesting new angle, and I think is correct. It isn’t strictly to the point for this paper and might seem a little distracting, but as it would probably not warrant a separate paper on its own I agree that it should be here, as long as it is clearly explained (which needs some work).

I am therefore suggesting some mainly minor changes (chiefly clarifications) but otherwise I certainly recommend publication.

In the discussion it was clear that Rempel felt that the justification for neglecting the curvature term in the 2001 paper was clear, while the present author clearly feels it was not. I do not think that the motivation of Rempel et al (2001) on that point is particularly important for readers of this paper and I would suggest some minor wording changes so that this is not a point. In addition this paper should stand without readers needing to look at the discussion, which will just be a distraction. I therefore suggest a rewording of lines 430-434, and that there should be a single and full citation to the discussion without continually referring to it.

Line 431: I suggest “In this connection, in the Interactive Discussions of our manuscript (give proper reference according to TCD style to the necessary discussion comments) it was clarified that Rempel et al. (2001) neglected the Gibbs–Thomson effect from the liquidus relation based on an assumption that the vein radii rv were spatially uniform – the justification for this being an anticorrelation between mean grain size and impurity loading, which has been observed in ice-core records”. I would suggest removing “As explained in RC2” in line 434 as it is obvious this is a continuation of discussing what was in the comment.

Line 442 and following, where you discuss the need for dg^2 to be proportional to C_B: there is actually another reason why this is unlikely. The actual freezing point depression, and therefore the equilibrium value of C (the vein concentration) is dependent on the entire mix of chemicals in the liquid phase. The evolution of grain size will also somehow be dependent on different chemicals interacting with grain boundaries. It is vanishingly unlikely that the way the different chemicals combine to control freezing point is the same as the way they control grain size. Thus even if the proportionality was true for one mix of chemicals, it would not be true for a different mixture. This is a kind of extension of your reason (iii). I realise this is a detail and I don’t insist that you add it but it might be something else to consider.

For lines 460 onwards, you have not (as in previous sections) given a layperson’s explanation of what is occurring here, and I found it hard to work out exactly why this creation of peaks is occurring. I think I got it, so I will give an explanation of my own: if I am right you should include something similar so that those not wanting to follow the maths can still understand the mechanism. I think the argument is: <<Smaller grain size implies for a given C_B, more vein length and (by equation 4) lower rv. This in turn implies that the Gibbs-Thomson effect in eq 6 is stronger (more freezing point depression) and therefore for a given ice temperature, the solute effect must be lower, ie c must be smaller in the ice with smaller grain size. This leaves a concentration gradient and causes diffusion that raises C_B in the area with small grains at the expense of the surrounding ice.>>. It might also be worth spelling out that the effect (again from eq 4) is that rv increases, and presumably diffusion only continues until rv has reached the size it is in the surrounding ice (I think eq 4 then tells us that C_B will reach an asymptotic value related to the square of the ratio of d_g inside and outside the perturbation). It would actually be helpful if Fig 12 included the evolution of rv – could this be added?

Finally in the conclusions, the paper contrasts impurities dissolved in the veins, and impurities in the grains or grain boundaries. Remember that they can also be in the veins but not dissolved. As an example at Dome C at -50 degrees, sulphuric acid would be dissolved (well above the eutectic of -70) but NaCl would almost certainly have precipitated out somewhere around -23. This adds a further complication because in my example, NaCl could dissolve back into the veins at the warmer temperatures at depth.

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ED: Publish subject to minor revisions (review by editor) (03 Mar 2021) by Nanna Bjørnholt Karlsson

AR by Felix Ng on behalf of the Authors (05 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (05 Mar 2021) by Nanna Bjørnholt Karlsson

AR by Felix Ng on behalf of the Authors (05 Mar 2021) Author's response Manuscript

Short summary

Current theory predicts climate signals in the vein chemistry of ice cores to migrate, hampering their dating. I show that the Gibbs–Thomson effect, which has been overlooked, causes fast diffusion that prevents signals from surviving into deep ice. Hence the deep climatic peaks in Antarctic and Greenlandic ice must be due to impurities in the ice matrix (outside veins) and safe from migration. These findings reset our understanding of postdepositional changes of ice-core climate signals.