Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using <i>δ</i><sup>13</sup>C

Morée, Anne L.; Schwinger, Jörg; Ninnemann, Ulysses S.; Jeltsch-Thömmes, Aurich; Bethke, Ingo; Heinze, Christoph

doi:https://doi.org/10.5194/cp-17-753-2021

Articles | Volume 17, issue 2

https://doi.org/10.5194/cp-17-753-2021

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

https://doi.org/10.5194/cp-17-753-2021

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

Articles | Volume 17, issue 2

Research article

|

06 Apr 2021

Research article |

| 06 Apr 2021

Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using δ¹³C

Anne L. Morée, Jörg Schwinger, Ulysses S. Ninnemann, Aurich Jeltsch-Thömmes, Ingo Bethke, and Christoph Heinze

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Final revised paper (published on 06 Apr 2021)
Supplement to the final revised paper
Preprint (discussion started on 14 Feb 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 comments', Anonymous Referee #1, 26 Mar 2020
- AC1: 'Author Comment to RC1', Anne Morée, 18 May 2020
RC2: 'Reviewer's comments', Anonymous Referee #2, 05 Apr 2020
- AC2: 'Author Comment to RC2', Anne Morée, 18 May 2020

Peer-review completion

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

ED: Reconsider after major revisions (19 May 2020) by Laurie Menviel

AR by Anne L. Morée on behalf of the Authors (12 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 Dec 2020) by Laurie Menviel

RR by Anonymous Referee #1 (08 Jan 2021)

RR by Anonymous Referee #2 (14 Jan 2021)

Suggestions for revision or reasons for rejection

The authors have addressed most of my comments and the overall structure is now easier to follow. However, it could still use a few changes to improve clarity, especially in the Discussion session. See below some suggestions on this and other minor points.
Note that pages and line numbers refer to the track-changes version of the manuscript.

Abstract: remove the last statement “on which we include a detailed discussion” as it is unnecessary here.

Page 5, lines 30-31: it would be appropriate to cite at least one earlier reference and especially one that is based on the analysis of the sedimentary record, rather than only numerical simulations - e.g. Kohfeld et al. (2005)
Kohfeld, K.E., Le Quéré, C., Harrison, S.P. and Anderson, R.F., 2005. Role of marine biology in glacial-interglacial CO2 cycles. Science, 308(5718), pp.74-78.

Page 7, line 20: I still don’t find “theoretical approach” the right definition here and a somewhat confusing way to call this. These are “offline calculations of the biological pump efficiency” or something along these lines.

Page 7, lines 30-32: this sentence is a little long. Break up or at least add a comma between “mechanisms” and “obtained”.

Page 8, line 1: figure should be sequential, but here you reference figure 7. But this is probably fine if you at least state that this figure is in Appendix A (here and everywhere else in the text afterward).
That said, I am not sure that you need both a Supplementary Material and an Appendix?

Page 11, line 12: I just find this reference to a “theoretical framework” unnecessarily confusing again here. All you need to say really is something like: […] and discuss the LGM-PI changes by exploring the efficiency of the biological pump (Sect. 3.3).
Similarly in other parts of the text, as noted before.

Page 9, Line 28: “weaker” rather than “slower”.

Page 10, line 3: the meaning of the first part of the sentence (“Besides radiocarbon aging of in particular SSW”) is unclear, so please rephrase.

I think that supplementary Figure S2 is useful, but it doesn’t seem to be referenced anywhere in the text.

Page 15, line 7: as well as

Page 14 onwards: the current “Discussion” session (4) is clearer as a self-standing section, but a little heavy as it is and could use some breaking up into subsections from line 15 onwards.
I would suggest, for instance, renaming the whole section something like “LGM model-data biases” and add a couple more subsections to break this up a little. So the current subsection 4.2 could keep the same title but become 4.4 and before that the physical/water mass changes could be highlighted in one subsection (4.1) and then separate the biological mechanisms and DIC discussion from page 15 line 8 (4.2).
Or split it in a different way if you think it is more appropriate, but add at least two additional break points here.
I am saying this because this discussion is interesting and useful in highlighting several important results from these simulations, but it is hard to follow and the reader would struggle with it as is.

Page 18, line 5 onwards: it may be useful to refer to, as another example, this newly accepted paper: Zhu et al. (2021), GRL, Assessment of equilibrium climate sensitivity of the Community Earth System Model version 2 through simulation of the Last Glacial Maximum. Where the LGM simulation is used to show biases that would also affect simulations used for future projections with the same model (even though the focus is on equilibrium climate sensitivity and cloud feedbacks).

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ED: Publish subject to minor revisions (review by editor) (17 Jan 2021) by Laurie Menviel

Dear Dr. Morée,

I have received 2 reviews of your manuscript, which are included in your manuscript record page. Based on the reviews, your manuscript may be suitable for publication after minor revisions.

I have also re-read your manuscript carefully, and I attach a few comments in the pdf. There are three points I would like you to consider carefully during this revision:
1) The simulated changes in oceanic circulation in the Atlantic Ocean seem relatively small, and there is no shoaling (or even a slight deepening) of NADW at the LGM compared to PI. The LGM ACC is stronger, does that mean the Southern Ocean upwelling is stronger at the LGM? In any case, given the text in section 3.2.1, it should be made clearer in the Abstract, Discussion and Conclusion, that stronger changes in oceanic circulation could also provide a better model-data agreement, and given little changes in oceanic circulation a Beff of 75% is needed.

2) Some clarifications might also be needed regarding the LGM to PI shift in mean d13C: the mean oceanic d13C was about 0.34 permil lower at the LGM than PI (e.g. Peterson et al., 2014). Given that the LGM and PI atmospheric d13CO2 are relatively similar, this shift arises from a lower terrestrial carbon reservoir at the LGM, and potential imbalances between weathering and sedimentation. As the LGM oceanic state is forced with -6.5 permil d13CO2, this shift is not included, which is fine. However, caution has to be taken while comparing model and data d13C, as the model values will of course be higher than the data. I am saying this because I am worried that when adjusting the Beff, this was not taken into account, which would lead to an “over-adjustment” of the Beff.

3) Previous LGM model-data comparisons, and particularly the ones using d13C, are completely ignored (e.g. Tagliabue et al. 2009; Hesse et al., 2011; Gebbie 2014; Schmittner & Somes, 2016 (this one is discussed a bit); Menviel et al., 2017, Muglia et al., 2018; Menviel et al., 2020). A paragraph in the Introduction would have been nice, but a few sentences putting your results in the context of these studies definitely need to be added in the discussion.

I look forward to receiving your revised manuscript.
Best regards,
Laurie Menviel

Comments to the Author: PDF

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AR by Anne L. Morée on behalf of the Authors (12 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Feb 2021) by Laurie Menviel

AR by Anne L. Morée on behalf of the Authors (16 Feb 2021)

Short summary

This modeling study of the Last Glacial Maximum (LGM, ~ 21 000 years ago) ocean explores the biological and physical changes in the ocean needed to satisfy marine proxy records, with a focus on the carbon isotope ¹³C. We estimate that the LGM ocean may have been up to twice as efficient at sequestering carbon and nutrients at depth as compared to preindustrial times. Our work shows that both circulation and biogeochemical changes must have occurred between the LGM and preindustrial times.

Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using δ13C

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Interactive discussion

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Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using δ¹³C