Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica

Goursaud, Sentia; Masson-Delmotte, Valérie; Favier, Vincent; Preunkert, Suzanne; Legrand, Michel; Minster, Bénédicte; Werner, Martin

doi:https://doi.org/10.5194/tc-13-1297-2019

Articles | Volume 13, issue 4

https://doi.org/10.5194/tc-13-1297-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-13-1297-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 13, issue 4

Research article

|

24 Apr 2019

Research article |

| 24 Apr 2019

Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica

Sentia Goursaud, Valérie Masson-Delmotte, Vincent Favier, Suzanne Preunkert, Michel Legrand, Bénédicte Minster, and Martin Werner

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Final revised paper (published on 24 Apr 2019)
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Preprint (discussion started on 04 Jul 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Review Goursaud et al.,', Elizabeth Thomas, 09 Aug 2018
- AC2: 'Authors'response', Sentia Goursaud, 17 Dec 2018
RC2: 'Review of Goursaud et al.', Daniel Emanuelsson, 29 Aug 2018
- AC1: 'Authors'response', Sentia Goursaud, 17 Dec 2018

Peer-review completion

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

AR by Sentia Goursaud Oger on behalf of the Authors (16 Jan 2019)

ED: Referee Nomination & Report Request started (25 Jan 2019) by Becky Alexander

RR by Daniel Emanuelsson (09 Feb 2019)

Suggestions for revision or reasons for rejection

“Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica” Sentia Goursaud et al.

Referee comments iteration 2
The Cryosphere Discussions
D. Emanuelsson (Referee)
d.emanuelsson@gmail.com

Age scale
* One of the introduced correlation maps shows the correlation (Fig. 2a in your response) between δ18O and sea ice concentration (SIC). This figure shows a positive correlation between δ18O and SIC close to the TA coast. That is, an increase in local SIC would result in higher δ18O values, this not only intuitively doesn’t make sense to me it also goes against the existing scientific literature for coastal cores (e.g., Küttel et al. 2012; Thomas et al. 2013). I would therefore still argue that the age scale needs to be better constrained.

* “For depths lower than 10 m w.e., Ssummer (December-January) peaks were identified (i) from nssSO4 values higher than 100 ppb, synchronous with MSA peaks (with no threshold), and (ii) for nssSO4 values higher than 200 ppb (with or without a simultaneous MSA peak). Double nssSO4 peaks were counted as one summer (e.g. 2012, 2003, and 2001). For depths higher than 10 m w.e., summer peaks were identified for nssSO4 values higher than 27 ppb.” (C51).
This seems ambiguous. You introduce certain roles for the deepest three meters of the core and others for the section that is shallower than 10 m. And you do not tell the reader why this is necessary.

* Note, that only for “test1”, where 0.54 m is included in the age-scale is the correlation between δ18O and DDU SAT significant (supplementary table 5 in your response; r=0.49, p=0.05). This layer isn’t “picked” with your criteria. However, your analysis and mine seem to indicate that it should be included. Perhaps some of the samples need to be resampled (for SO4 and MSA), additional chemical records introduced and/or the criteria redefined.
Additionally and importantly, the spatial correlation makes sense between δ18O and local SIC when the 0.54m layer was included (see Fig. 3a in my first review response).
I do think that there is a lack of progress between the review iterations for the dating part. Ultimately, this doesn’t have to be any of the authors’ fault. Perhaps the data just isn’t there at the moment to allow for an unambiguous dating of the TA core.

Additional comments
To ensure that you capture the large-scale process avoid using the 10-m winds. Replace them with 850hPa winds that aren’t as affected by surface topography.
Provide the reader with some more detail in the figure text to the introduced correlation maps. What data do you use, which significance level does the line depict, the time span of correlation, etc.

Choose a different color for the dot that indicates the TA site, as it has the same color as the part of the shading.
Is Fig.3 in your response a correlation map? Add some detail to the figure text.

You note that you have made some of the detailed corrections based on my review comments, but it seems like you have not removed the text that should have been deleted completely. Example: ”This study In this study,” (C45), “PicarroICARRO” (C46), “To summarizeIn summary, we” (C57). So please check all of the old detailed comments again. (Did you copy the text over from a document where the deleted part still lingered due to “track changes”?)

Change the header for section 2 to “Material and Methods”.
“Every core piece was weighted and its length measured in order to produce a density profile” change to “weight and length were measured for all the core sections in order to calculate the TA core’s density profile”?
“Methane sulfonate” change to “Methane sulphonic acid” (C46).

References
Küttel M, Steig EJ, Ding Q, et al (2012) Seasonal climate information preserved in West Antarctic ice core water isotopes: relationships to temperature, large-scale circulation, and sea ice. Clim Dyn 39:1841–1857. doi: 10.1007/s00382-012-1460-7
Thomas ER, Bracegirdle TJ, Turner J, Wolff EW (2013) A 308 year record of climate variability in West Antarctica. Geophys Res Lett 40:5492–5496. doi: 10.1002/2013GL057782

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RR by Anonymous Referee #3 (21 Feb 2019)

Suggestions for revision or reasons for rejection

Review of timescale in Goursaud et al.

I was asked to evaluate the timescale of the TA firn core presented in Goursaud et al. and thus will limit my comments on the manuscripts to that. I am pleased to find such a detailed review by Dr. Emanuelsson and a thorough response from the authors.

Coastal firn cores are challenging to date because the seasonal cycle in many different parameters is not as clockwork as the more interior cores. This challenge is reflected in the TA core where the year 2008 had to be added in based on comparisons with measured accumulation rates at the site, rather than from the firn core records themselves.

The GLACIOCLIM stake network is quite useful for dating the TA core. It is exceedingly rare to have an independent constraint on annual accumulation and it should be utilized. One issue is whether there should be an additional year added at the beginning of the record, which would then impact the entire timescale. The authors rejection of a year at ~0.68m is well justified in my view. The accumulation comparison does not support it, and a year here is less compelling in the MSA and nssSo4 records than for the 2008 addition. Thus, I support the authors that no year at ~0.68m is most likely. However, I do not think that it would be impossible to have a year at ~0.68m, and thus I would like to ensure that the discussion in the review and author response is directly included in the main text and supplement of the paper – i.e. this is an important exchange and should not be isolated to the review and author response. Including a year at ~0.68m does make sense from an d18O perspective – the minimum value at ~0.68m would indicate an annual cycle at any depth in the core (though it is worth remembering that these measurements are very near the surface where the snow is not dense and diffusion may not have smoothed individual storm isotopic values).

The tests of the various iterations of the timescale were quite interesting (in the author comment). I would encourage all possibilities of the timescale with the four uncertain years to be considered. While this is additional work, it will also form a robust analysis of the impact of the timescale uncertainty and provide an upper limit of the possible relationship between d18O and SAT which is focus of the manuscript.

I have two main questions about the timescale:

-1) Why are there no additional chemical and particle data sets presented to help interpret ambiguous years. Calcium, magnesium, particle counts, and liquid conductivity might be quite useful, and have become standard ice core measurements. If these data were collected and are not helpful, that should be stated. If they were not collected, that should be indicated and a reason given.

-2) The seasonal dating has large uncertainty. I didn’t not see good support for the timing of peaks in MSA and nssSO4. That is, how do you know they are actually January 1 of a year and not lagged by month or two. Further, the peaks are quite broad and it is unclear to me whether there is a consistent method to define the annual pick. For instance, the 2012 pick is considerably different than surrounding years.

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ED: Publish subject to minor revisions (review by editor) (27 Feb 2019) by Becky Alexander

AR by Sentia Goursaud Oger on behalf of the Authors (12 Mar 2019) Author's response Manuscript

ED: Publish as is (14 Mar 2019) by Becky Alexander

AR by Sentia Goursaud Oger on behalf of the Authors (18 Mar 2019) Author's response Manuscript

Short summary

We report new water stable isotope records from the first highly resolved firn core drilled in Adélie Land and covering 1998–2014. Using an updated database, we show that mean values are in line with the range of coastal values. Statistical analyses show no relationship between our record and local surface air temperature. Atmospheric back trajectories and isotopic simulations suggest that water stable isotopes in Adélie provide a fingerprint of the variability of atmospheric dynamics.