Exploring the use of compound-specific carbon isotopes as a palaeoproductivity proxy off the coast of Adélie Land, East Antarctica

Ashley, Kate E.; Crosta, Xavier; Etourneau, Johan; Campagne, Philippine; Gilchrist, Harry; Ibraheem, Uthmaan; Greene, Sarah E.; Schmidt, Sabine; Eley, Yvette; Massé, Guillaume; Bendle, James

doi:https://doi.org/10.5194/bg-18-5555-2021

Articles | Volume 18, issue 19

https://doi.org/10.5194/bg-18-5555-2021

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

https://doi.org/10.5194/bg-18-5555-2021

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

Articles | Volume 18, issue 19

Research article

|

14 Oct 2021

Research article |

| 14 Oct 2021

Exploring the use of compound-specific carbon isotopes as a palaeoproductivity proxy off the coast of Adélie Land, East Antarctica

Kate E. Ashley, Xavier Crosta, Johan Etourneau, Philippine Campagne, Harry Gilchrist, Uthmaan Ibraheem, Sarah E. Greene, Sabine Schmidt, Yvette Eley, Guillaume Massé, and James Bendle

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Final revised paper (published on 14 Oct 2021)
Supplement to the final revised paper
Preprint (discussion started on 23 Apr 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 Ashley et al.', Anonymous Referee #1, 26 May 2020
- AC1: 'Response to reviewer 1', Kate Ashley, 02 Jul 2020
  - RC3: 'Response to author response', Anonymous Referee #1, 03 Jul 2020
    - AC4: 'Response to R1', Kate Ashley, 03 Oct 2020
RC2: 'Re: Fatty acid carbon isotopes: a new indicator of marine Antarctic paleoproductivity?', Anonymous Referee #2, 04 Jun 2020
- AC2: 'Response to reviewer 2', Kate Ashley, 02 Jul 2020
RC4: 'Interactive comment on “Fatty acid carbon isotopes: a new indicator of marine Antarctic paleoproductivity?” by Kate Ashley et al.', Anonymous Referee #3, 03 Jul 2020
- AC3: 'Response to reviewer 3', Kate Ashley, 20 Sep 2020

Peer-review completion

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

ED: Reconsider after major revisions (04 Oct 2020) by Markus Kienast

AR by Kate Ashley on behalf of the Authors (29 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Dec 2020) by Markus Kienast

RR by Anonymous Referee #4 (27 Jan 2021)

Suggestions for revision or reasons for rejection

I was not one of the prior reviewers but it appears that the authors have thoroughly revised in response to the prior reviewers' comments.

While reservations remain about the generalist biomarkers and their inferences, the work is carefully reported and merits publication, as it usefully adds data to the conditions and questions posed.

A few minor issues are noted:
Line 127-53 – please attend to superscripts (13) and subscripts (FA and FAME) and italicization (n) in the track changes text this has been incompletely formatted, and elsewhere in the ms (line 359-361)

Line 160 – fully extract. The method was sonication, the HBIs are unlikely to be fully extracted. Please remove word ‘fully’. The IS doesn’t replicate analyte, but reporting its extraction efficiency would be appropriate. Query: As noted on lines 188 the standard was added after extraction for FAs, was it added before extraction for HBIs, i.e. were these separate extractions as stated or is this incorrectly stated? Please check and correct if needed.

Free-fatty acids only. As noted by a prior reviewer, bound FAs as well as free fatty acids would be pertinent to production and this is not satisfactorily resolved by simply stating the paper studies free FAs. I suggest adding a statement that the release of C16FAs is likely to be incomplete using the sonication approach, and there are references to cite on the likely extraction efficiency if this was not assessed here. Extraction of bound FAs by saponification would likely release more C16FA. Please include a brief indication of the limitations of the methodology, based on what can be found in the literature, such as might inform future studies.

Conclusion ‘producers…would require further elucidation’ – as the prior reviewer comments indicate this claim is overworked, it’s unlikely to be resolvable, or more-importantly generalizable, for such a non-specific compound. Rephrase to not suggest the information instead that can be gleaned from general biomarkers in combination with evidence for producers.

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RR by Anonymous Referee #5 (07 Feb 2021)

Suggestions for revision or reasons for rejection

The paper by Ashely et al presents a fairly high resolution record of fatty acid concentrations and delta 13C values of the most abundant ones (C18FA and C24FA), from offshore Antarctica, in an attempt to find out if this can be used to reconstruct changes in productivity. This productivity is important as a carbon sink, via the biological pump.
Overall, the data are fine and should be published. The discussion also brings up valid arguments but appears a little limited, as are the presented data.
I have given more detailed comments in an annotated pdf, but here some main concerns:
1. It would be useful to also show the TOC profile, as well as bulk 13C, to get a sense of the carbon burial rate, and to place the FA results in a broader sedimentological framework. Expression of the FA concentration on a TOC basis would be useful.
2. Have the authors considered measuring sterol concentrations, and sterol 13C contents, instead of FAs? Because sterols are clearly of phytoplankton original while FAs can also have a bacterial source. Please comment (we chose for FA, and not sterols, because...)
3. A potential bacterial source of especially C18FA should be discussed (this would be an argument against FAs..)
4. A discussion about the apparently low concentration of unsaturated FAs is in order.
5. I find the apparent correlation between the C24 FA data and diatom species abundance not convincing; this needs to be made more clear.
6. Please include also the other FA 13C data, if not for the discussion in this particular paper then for future reference for others who might measure these on other locations. The C22FA appears to be about equally abundant as the C24FA (looking at the GC trace in the supplement); the same is valid for the C16FA (which is of course a very generic lipid).

Referee Report: PDF

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ED: Reconsider after major revisions (08 Feb 2021) by Markus Kienast

AR by James Bendle on behalf of the Authors (23 May 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (26 May 2021) by Markus Kienast

RR by Sarah Feakins (04 Jun 2021)

ED: Publish subject to technical corrections (08 Jun 2021) by Markus Kienast

AR by James Bendle on behalf of the Authors (24 Jun 2021) Author's response Manuscript

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Short summary

We explore the potential for the use of carbon isotopes of algal fatty acid as a new proxy for past primary productivity in Antarctic coastal zones. Coastal polynyas are hotspots of primary productivity and are known to draw down CO₂ from the atmosphere. Reconstructions of past productivity changes could provide a baseline for the role of these areas as sinks for atmospheric CO₂.