Early deglacial Atlantic overturning decline and its role in atmospheric CO<sub>2</sub> rise inferred from carbon isotopes (δ<sup>13</sup>C)

Schmittner, A.; Lund, D. C.

doi:https://doi.org/10.5194/cp-11-135-2015

Articles | Volume 11, issue 2

https://doi.org/10.5194/cp-11-135-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-11-135-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 2

Research article

|

05 Feb 2015

Research article |

| 05 Feb 2015

Early deglacial Atlantic overturning decline and its role in atmospheric CO₂ rise inferred from carbon isotopes (δ¹³C)

A. Schmittner and D. C. Lund

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Final revised paper (published on 05 Feb 2015)
Supplement to the final revised paper
Preprint (discussion started on 09 Jul 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C1275: 'cp-2014-80', Anonymous Referee #1, 08 Aug 2014
- AC C1515: 'Response to referee #1', Andreas Schmittner, 20 Sep 2014
RC C1296: 'Interesting results, but interpretation and presentation needs revision', Anonymous Referee #2, 11 Aug 2014
- AC C1519: 'Response to referee #2', Andreas Schmittner, 20 Sep 2014
SC C1390: 'AMOC unlikely to have made much of a direct impact on CO2 during HS1', Eric Galbraith, 28 Aug 2014
- AC C1525: 'Response to Eric Galbraith', Andreas Schmittner, 20 Sep 2014
EC C1433: 'editor's comments after review process', Hubertus Fischer, 04 Sep 2014
- AC C1528: 'Response to editor', Andreas Schmittner, 20 Sep 2014

Peer-review completion

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

AR by Andreas Schmittner on behalf of the Authors (20 Sep 2014) Author's response

ED: Referee Nomination & Report Request started (24 Sep 2014) by Hubertus Fischer

RR by Anonymous Referee #2 (12 Nov 2014)

Suggestions for revision or reasons for rejection

I am disappointed by the response of the authors and it reflects a missed opportunity to improve the manuscript based on input provided by the reviewers and E. Galbraith.

A marked-up version with the changes identified would have been helpful for the reviewer(s).
The authors should provide the simulations requested by E. Galbraith and reviewer 2. The authors should summarize the finding of earlier freshwater model experiments in a paragraph in the introduction. The authors should summarize the literature for changes in SH westerlies in another paragraph in the introduction. The authors should remove the misinterpretation of Tschumi et al, 2011 in the next manuscript.
1) Figure 10 reveals that the simulated d13C anomalies are in the range from 0.5 to – 1.6 permil whereas the observation based range is about a factor of two smaller. This is an important finding and should be noted in the result section. Moreover, the authors should try to find a model solution where they can reduce this large mismatch. The author should clearly spell out in the abstract and the discussion that this most likely leads (or at least “may lead”) to an overestimation of the response in CO2 to an AMOC shut-down for the LGM.

2) The authors apply a cost-efficient Earth System Model of Intermediate Complexity with an energy balance atmosphere. The advantage of such an approach, e.g. in contrast to a comprehensive ESM, is that many simulations can be carried out and the sensitivity of simulated responses to initial states and parameters can be explored.

The authors should extend the simulation FW015 (or preferentially a simulation that starts from LGM conditions) to a state where the AMOC is turned on again. They may restart their simulation and add a negative freshwater anomaly at 14.7 ka to mimic the onset of NADW at the beginning of the B/A.

I appreciate that it might be more difficult to get an LGM state, but this should also not be out of reach, given that the model has been initialized for the LGM before. Further, the authors state: “Even though it would be possible to use a simulation with LGM boundary conditions its value would be low if the deep ocean circulation and carbon cycle simulation is inconsistent with observations.” and “Whereas simulations with glacial boundary conditions (lower GHG, orbital parameters, ice sheets) are easy and have been done before (e.g. in PMIP)

It is possible (or even “easy”) and the authors should run the model for an initial state with a shallower than modern AMOC to see whether they can reduce the d13C data-model mismatch and what the implications are for atm. CO2 and d13C

It is a weak argument that the LGM ocean is poorly constraint and that their model might be unrealistic for the LGM. This argument applies equally for the simulations now described. We know that the modern ocean is different from the LGM state and the early termination, e.g. from d13C and 14C data etc. Still the authors use the modern condition as a starting point for the simulation. They do not hesitate to use the results for the modern conditions to come up with their strong and, as pointed out by E. Galbraith, likely unrealistic conclusion on the early CO2 rise.

3) A paragraph is needed in the introduction that summarizes the results from earlier freshwater hosing experiments. This would allow the reader to put this work in the context of the literature. It would provide a starting point for the work done here. In particular the findings that the CO2 responses, and related carbon stock changes on land and in the ocean, are found to be very sensitive to the initial state must in my opinion be discussed upfront. Similarly, responses were found to be smaller for LGM boundary conditions than for preindustrial conditions. A point that also need to be mentioned upfront. I have already provided various references in my initial review. Another study that is of interest in this context is the work by Menviel et al., QSR, 2012 where the results of transient glacial-interglacial simulations is discussed and model outcomes are compared with proxy data. This reveals that changes in AMOC for a reasonably realistic LGM state have a much smaller impact on atm. CO2 than postulated by Schmittner and Lund.

4) A paragraph is needed in the introduction that summarizes the results from earlier wind stress experiments on deglacial CO2. The available results suggest a weak influence on changes in SH westerlies on atmospheric CO2 for the deglacation. Thus, the results presented by the authors fall well within the range of earlier quantitative studies and do not provide much new insight. There is even one study available with the UVIC model (d’Orgeville et al., GRL, 2010) which is not mentioned either.
See for example Menviel et al., 2008, Tschumi et al, PO, 2008, d’Orgeville etal, 2010, Lee et al., PO, 2011, Voelker and Köhler, PO, 2014) for wind and CO2, or also work by England and Sijp and by Böning et al., NatGeo, 2008 etc. considering physics only.

The authors should condense the text and figures on SH westerlies as it is a confirmation of earlier work.

5) Abstract, line 3, and line 22-25: The authors focus here on SO wind changes as the only mechanisms for SO circulation changes. The text gives a wrong impression. While it is reasonably clear from the literature that the sensitivity of atm. to realistic wind stress and wind position changes is fairly weak, there are other mechanisms that may have been responsible for deep ocean circulation changes in relation to the Southern Ocean atmospheric window. It is clear from the literature that SO changes did take place, e.g. as indicated by opal records, the d13C and D14C records. The sentence “We propose that the observed early deglacial rise in atmospheric CO2 and the decrease in _13CCO2 may have been caused by an AMOC induced decline of the ocean’s biologically sequestered carbon storage without the need to invoke changes in Southern Hemisphere winds.” gives the impression that SO changes played no role at all. As mentioned above, it is not a new finding that SO wind changes played a small role. The more general statement, that there is no need to invoke SO changes to explain atm. CO2 is can hardly be reconciled with the available proxy evidence. In particular, as this MS makes no statement on the role of SO D14C and on SO opal fluxes. If the authors wish to talk about SO wind in the abstract, they should do this in a clean statement, e.g .” Our results confirm the findings of earlier model studies that changes in SO winds likely played a small role for the deglacial CO2 rise.”

6) Line 243 ff and line 283 The misinterpretation of the work by Tschumi et al., 2011 is to be removed.
Tschumi et al., 2011 quantify the influence of SO ventilation changes on CO2 and d13C, but they do not postulate that SO wind changes are responsible for ventilation changes. They clearly state this in their paper at several places. For example, at the beginning of the discussion:
“Second, changes in deep ocean ventilation are generated by varying the magnitude of the wind stress in the Southern Ocean by more than a factor of two. This is a convenient technical tuning mechanism to adjust the deep ocean ventilation age in the model. However, we stress the fact that these variations in SO wind strength exceed the range of realistic changes. The atmospheric CO2 response to realistic variations has been found to be rather modest (Tschumi, 2008; Menviel et al., 2008).

7) Line 267: The authors state that a deep ocean state estimate of the global LGM circulation and carbon cycle that is consistent with sedimentary _13CDIC reconstructions does not exist. While of course no model is perfect, there exist simulations of LGM conditions including isotopes. For example, Menviel et al., QSR, 2012 provide a set of simulations with different LGM states and their simulated d13C anomalies for the LGM as shown in their Figure 9 compare reasonably well with reconstructions. As mentioned above, simulated changes in atm. CO2 in response to changes in AMOC are relatively small and do not explain the initial deglacial CO2 rise.

8) The authors state in the discussion section when dealing with the d13C model-data mismatch: “
„the model would overestimate
changes in volume fluxes and perhaps carbon isotopes even if a complete AMOC collapse
did occur during HS1.“ They should spell out that this applies not only to the volume flux and carbon isotopes but also to atmospheric CO2 as indicated by previous studies.

Further comments:
line 133: Sentence is confusing. “Because biologically sequestered, organic carbon is isotopically light (_13Corg = -20‰) its loss increases deep ocean _13CDIC by 0:06 permil (Fig. 3f) and its gain decreases _13CDIC (by _ 0:3 permil) in the surface ocean and _13 135 CCO2 by _ 􀀀0:25‰in the atmosphere (Fig. 1d).”

My understanding is:
- The decrease in global DOC inventory as shown in Figure 3a tends to decrease d13C of DIC, atm. CO2 and land C as isotopically-light carbon is moved from DOC to the other carbon reservoir.
-The increase in land carbon storage tends to increase d13C in the atmosphere and ocean
-The decrease in the efficiency of the biological pump tends to decrease d13C in the atm. and the surface ocean and in DOC and POC and to increase d13C in the deep ocean.
The result of these three processes is a slight increase in the mean ocean signature of DIC.

Figure 1: Reference to Schmitt et al., 2012 seems missing here.

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RR by Anonymous Referee #1 (25 Nov 2014)

ED: Publish subject to minor revisions (review by Editor) (09 Dec 2014) by Hubertus Fischer

Dear authors

Your manuscript has now been seen by the two initial reviewers again, which diverge in their recommendation how to proceed. While referee #2 is essentially satisfied with your changes, referee #1 still asks for substantial and important additions to the manuscript.

Although an extended modeling framework with LGM boundary conditions and an extended timeframe as requested by referee #1 in his point 2) would clearly strengthen the paper, I can understand that this may be beyond the scope of this paper at this point, however, it should be clearly tackled in future research and this could be mentioned in the conclusions.

Moreover, as a truely glacial run was out of reach at this point, I think the comparison with observations can only be qualitative, i.e., the amplitudes in observed d13C changes cannot be compared quantitatively. That is to say that common trends (d13C goes up/down both in the model and data) can be made, but comparisons of the amplitude of these changes in tenth of a permille are not really quantitatively meaningful due to the starting boundary conditions as outlined by referee #1 and Eric Galbraith in his previous comment. A good quantitative correspondence may be fortuitous. This is not to say that the results in your paper are wrong but the wording should be carefully chosen and the statements better qualified with respect to this in the entire text. I give many examples below, where such a qualification or change in wording is advisable.

Having said that, I strongly agree with referee #1 that your revisions did not sufficiently address many of the referee’s other points, such as for example an in-depth discussion of previous work and their results on CO2 and d13C changes in response to freshwater hosing experiments. Referee #1 provides a detailed list in the re-review with necessary changes (points 1, 3, 4, 5, 6, 7, 8) that I would ask you to address point by point. Final acceptance of the paper is dependent on adequately addressing these points and I would strongly encourage you to do this extra work as I think your paper is an important contribution to this research question and should be published in CP in its final form.

Specific comments

Based on the fact that you start with HOL boundary conditions, your experiment is closest to the 8.2 kyr event. Can you make a statement based on your model, how large a potential freshwater forcing was at the 8.2 k event given that the CO2 drop was small (Indermühle et al., Nature 1999; Elsig et al., Nature 2009)?

Line 16: is this overestimation in d13C in the North Atlantic due to the starting boundary conditions?

Line 35: As stressed by referee #1 the citation of Tschumi here is misleading

Line 43: cite the basic papers on the bipolar seesaw by Stocker&Johnsen, Paleocean 2003, EPICA community members, Nature 2006, Blunier et al., Nature 1998) here.

Add a thorough discussion on previous work in the introduction

Line 99-101: Not only the mean level may be different but also the amplitudes of changes. This should be clearly stressed here and discussed throughout the manuscript

Line 119. Here and throughout the manuscript. The paper by Parrenin is only a minor update of previous CO2 measurements. Please cite also Monnin et al., Science 2011 and Marcott et al., Nature 2014

Line 120-121. Can you specify where this decrease in efficiency occurs?

Line 120-131: state that these numbers are dependent on the boundary conditions

Line 133-139: state that these numbers are dependent on the boundary conditions and that the quantitative agreement with ice core measurements may be fortuitous

Line 160-167: dependent on the boundary conditions

Line 181-186: using other boundary conditions the agreement would be most likely different for the same freshwater forcing, please qualify your statements

Line 190-195: here you argue that the difference in model and observations may be due to the model boundary conditions, at other instances, where they agree, you do not.

Line 206-208: agreement dependent on the boundary conditions

Line 236-237: I wonder in how far this is dependent on the boundary conditions

Line 243-246: That’s not what Tschumi et al. want to say

Line 268-269: See and discuss references given by referee #1

Line 275-278: This statement confused me as your AMOC goes down to 2 Sv, please clarify

Line 282-283. See comment above on Tschumi et al.

Line 314: see comment on Parrenin paper above

Line 390: please explain FeL

Figure caption 1: see comment on Parrenin paper above

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AR by Andreas Schmittner on behalf of the Authors (05 Jan 2015) Author's response Manuscript

ED: Publish as is (08 Jan 2015) by Hubertus Fischer

AR by Andreas Schmittner on behalf of the Authors (08 Jan 2015)

Short summary

Model simulations of carbon isotope changes as a result of a reduction in the Atlantic Meridional Overturning Circulation (AMOC) agree well with sediment data from the early last deglaciation, supporting the idea that the AMOC was substantially reduced during that time period of global warming. We hypothesize, and present supporting evidence, that changes in the AMOC may have caused the coeval rise in atmospheric CO2, owing to a reduction in the efficiency of the ocean's biological pump.

Early deglacial Atlantic overturning decline and its role in atmospheric CO2 rise inferred from carbon isotopes (δ13C)

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Early deglacial Atlantic overturning decline and its role in atmospheric CO₂ rise inferred from carbon isotopes (δ¹³C)