Lower oceanic <i>δ</i><sup>13</sup>C during the last interglacial period compared to the Holocene

Bengtson, Shannon A.; Menviel, Laurie C.; Meissner, Katrin J.; Missiaen, Lise; Peterson, Carlye D.; Lisiecki, Lorraine E.; Joos, Fortunat

doi:https://doi.org/10.5194/cp-17-507-2021

Articles | Volume 17, issue 1

https://doi.org/10.5194/cp-17-507-2021

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

https://doi.org/10.5194/cp-17-507-2021

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

Articles | Volume 17, issue 1

Research article

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25 Feb 2021

Research article | Highlight paper |

| 25 Feb 2021

Lower oceanic δ¹³C during the last interglacial period compared to the Holocene

Shannon A. Bengtson, Laurie C. Menviel, Katrin J. Meissner, Lise Missiaen, Carlye D. Peterson, Lorraine E. Lisiecki, and Fortunat Joos

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

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RC1: 'Some details missing', Anonymous Referee #1, 29 Jun 2020
- AC1: 'Response to Reviewer', Shannon Bengtson, 30 Sep 2020
RC2: 'Review of “Lower oceanic d13C during the Last Interglacial compared to the Holocene” by Bengston et al.', Anonymous Referee #2, 13 Jul 2020
- AC2: 'Response to Reviewer', Shannon Bengtson, 30 Sep 2020

Peer-review completion

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

ED: Reconsider after major revisions (05 Oct 2020) by Marit-Solveig Seidenkrantz

AR by Shannon Bengtson on behalf of the Authors (12 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Oct 2020) by Marit-Solveig Seidenkrantz

RR by Anonymous Referee #1 (27 Oct 2020)

Suggestions for revision or reasons for rejection

Review on Bengstson 2020 CP V2

The draft has significantly improved and the comments of the the reviewer have been well considered. I have only a few minor issues to be dealt with.

- Throughout main text, SI text and Fig 1: To address atmospheric carbon dixode concentration I believe it has to be addressed as „CO2“ in „ppm“ (not pCO2).

- Intro:
- Please specify on which criteria the definition of your end of the penultimate glaciation and of the last glacial inceptions are based on. You give quite a number of references here, but only in Govin et al 2015 I already find 4 definitions, depending on the variable of choice. To me, it seems like 129-116 ka BP is based on sea level crossing the 0m line according to Dutton and Lambeck 2012.
- PI defined as 1850-1900: This is a problem for d13CO2, since the 13C Suess Effect is already well visible in this time-window, e.g. see Bauska et al. (2015), doi: 10.1038/ngeo2422
- Holocene CO2: The plotted spline falls from 268 to 260 ppm (by 8ppm, the text mentions 5 ppm) in the early Holocene, before rising to ~277 ppm (by +17 ppm, not the mentioned +18ppm) at 2 ka BP. Or you stick to your rise by 18 ppm which would end at 278ppm, but change when this is reached. Maybe also spend a few words on the small-scale variability seen during the last 2 kyr or so.
- CH4: I believe CH4 reached 700ppb in both HOL anf LIG.
- Fig 1c. Please plot EDC dD on the most recent age model AICC2012 as the other ice core records, which is available (for example) here: https://doi.pangaea.de/10.1594/PANGAEA.824891. Note, that your calculation of temperature out of dD of EPICA Dome C ignores any necessary sea level correction, e.g. see Parrenin et al 2013, Eq 1-3 in the SI, doi: 10.1126/science.1226368

- Section 2
- lines 100f: Here, LIG covers 130-118 ka and the HOL 8-2 ka, while the grey shaded bare in Fig 1 cover different time periods. Please explain here why a different time window is chosen. I believe this issue was already covered in the last round of review, so I am not sure, why I am still confused here. It might be enought to refer to section 3.1 for more details on the chosen time windows.

- Results
- Table 3: Please add information, if these regional summaries contain any contributions from the size of the regional water masses or if this is based on the pure point-data. Since volume-weighted averaging is done in section 3.2 this was probably not done here, so a brief notion (non-volume weighted) in the caption would be enough.
- Fig 6: Please use the same scale for the x axis for LIG and the Holocene, right now the Holocene time axis is half as large as that of the LIG (1.5 kyr in the Holocene are the same as 3 kyr in the LIG).
- line 271. Please refer to Fig 6 at the end of the sentence.

- Discussion
- line 318. Most recent paper on the geological CH4 release is missing (Dyonisius et al2020, doi: 10.1126/science.aax0504): This might replace the Petrenko et al 2017 paper.
- line 320: d13C of CO2 from volcanism has been assumed to be -2 to -8 permil in Roth and Joos (2012), which is NOT close to zero as suggested here. Please revise the discussion of this issue.

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RR by Anonymous Referee #2 (26 Nov 2020)

Suggestions for revision or reasons for rejection

The revised version of the manuscript is a major improvement on the initial submission, which itself was already a solid contribution. Overall, the authors have done a superb job addressing the initial round of concerns, particularly those regarding: 1) age model consistency, 2) the lack of a d13C mass balance estimate, and 3) the depth coverage of the compiled records. While the last point remains an issue given the very few records below 3 km depth in the Pacific, it is now much easier for the reader to assess this limitation given the addition of Figure 3. It is also very useful to have an estimate of the amount of terrestrial organic matter that would be required to yield lower mean oceanic d13C during the LIG, even with the assumption of closed system behavior.

Two other issues, regarding AMOC variability and the driver of lower oceanic d13C, were also addressed but in a less complete way. In regard to the first issue, the authors use a temporal and spatial averaging that make it difficult to see any differences in AMOC between the early and late LIG, despite clear evidence for differences in subpolar North Atlantic d13C. For example, Figure 7 of Hodell et al. (2009) shows a ~1 per mil offset between early and late MIS 5e d13C records at depths shallower than 2 km. Such a large change points to marked shift in the endmember d13C of NADW, which was likely associated with weakened AMOC early in MIS 5e. Note that this is not just centennial-scale variability but rather changes in d13C that span several millennia. In Figure S2 of the revised manuscript, however, there is little apparent difference between the early and late MIS 5e results. It appears that part of the problem is that the mean d13C values in Figure S2 are based on an average of all d13C results from the Atlantic basin, including light d13C data in the South Atlantic that reflect the influence of southern source watermasses. The limited difference in mean d13C between the two time intervals is also likely due to the use of overlapping windows (125-120 ka and 128-123 ka). If the spatial domain were instead limited to the North Atlantic between 1.5 and 3.5 km, for example, and the time windows were non-overlapping, would the resulting mean d13C values still be the same? It is also important to note that most of the LIG data are from the eastern Atlantic, where AMOC perturbations to the d13C tracer field will be less obvious than in the western Atlantic. Given these issues, it would be helpful if the authors used different spatial and temporal constraints for their analysis, or if they were to add caveats to the existing results to alert the reader to potential issues.

Lastly, the authors have added a substantial amount of discussion regarding the driver of lower mean ocean d13C during the LIG. The additions regarding ocean-atmosphere gas exchange and terrestrial carbon storage are very helpful, in particular the details about peatlands and permafrost. Yet after walking through the various possibilities, the authors return to their original statement about a long-term imbalance between isotopic fluxes to and from the lithosphere. Do the authors believe the LIG vs. Holocene contrast is simply part of a longer term trend that these two time windows happen to capture? If this were the case, we should expect to see earlier interglacial intervals to have even more depleted mean ocean d13C that MIS 5e. Is there evidence to support such a trend?

And a couple final details…

Line 144: ‘Intermediate’ typically is reserved for depths less than 1000 m, consider using ‘deep’ instead.

Line 260: Check the slopes listed here, they seem to be reversed from those in the figure.

Line 321: The d13C of mantle carbon is ~ -7 per mil, which is quite different than zero. Do you mean that volcanic CO2 has a d13C similar to the atmosphere?

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ED: Publish subject to minor revisions (review by editor) (26 Nov 2020) by Marit-Solveig Seidenkrantz

AR by Shannon Bengtson on behalf of the Authors (14 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Dec 2020) by Marit-Solveig Seidenkrantz

AR by Shannon Bengtson on behalf of the Authors (24 Dec 2020) Manuscript

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Short summary

The last interglacial was a warm period that may provide insights into future climates. Here, we compile and analyse stable carbon isotope data from the ocean during the last interglacial and compare it to the Holocene. The data show that Atlantic Ocean circulation was similar during the last interglacial and the Holocene. We also establish a difference in the mean oceanic carbon isotopic ratio between these periods, which was most likely caused by burial and weathering carbon fluxes.

Lower oceanic δ13C during the last interglacial period compared to the Holocene

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Lower oceanic δ¹³C during the last interglacial period compared to the Holocene