The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons

Hutchinson, David K.; Coxall, Helen K.; Lunt, Daniel J.; Steinthorsdottir, Margret; de Boer, Agatha M.; Baatsen, Michiel; von der Heydt, Anna; Huber, Matthew; Kennedy-Asser, Alan T.; Kunzmann, Lutz; Ladant, Jean-Baptiste; Lear, Caroline H.; Moraweck, Karolin; Pearson, Paul N.; Piga, Emanuela; Pound, Matthew J.; Salzmann, Ulrich; Scher, Howie D.; Sijp, Willem P.; Śliwińska, Kasia K.; Wilson, Paul A.; Zhang, Zhongshi

doi:https://doi.org/10.5194/cp-17-269-2021

Articles | Volume 17, issue 1

https://doi.org/10.5194/cp-17-269-2021

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

https://doi.org/10.5194/cp-17-269-2021

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

Articles | Volume 17, issue 1

Review article

| Highlight paper

|

28 Jan 2021

Review article | Highlight paper |

| 28 Jan 2021

The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons

David K. Hutchinson, Helen K. Coxall, Daniel J. Lunt, Margret Steinthorsdottir, Agatha M. de Boer, Michiel Baatsen, Anna von der Heydt, Matthew Huber, Alan T. Kennedy-Asser, Lutz Kunzmann, Jean-Baptiste Ladant, Caroline H. Lear, Karolin Moraweck, Paul N. Pearson, Emanuela Piga, Matthew J. Pound, Ulrich Salzmann, Howie D. Scher, Willem P. Sijp, Kasia K. Śliwińska, Paul A. Wilson, and Zhongshi Zhang

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Final revised paper (published on 28 Jan 2021)
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Preprint (discussion started on 18 May 2020)
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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: 'The Eocene-Oligocene transition', Anonymous Referee #1, 13 Jun 2020
- AC1: 'Response to Referee 1', David Hutchinson, 29 Aug 2020
RC2: 'EOT review', Anonymous Referee #2, 02 Jul 2020
- AC2: 'Response to Referee 2', David Hutchinson, 29 Aug 2020

Peer-review completion

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

ED: Reconsider after major revisions (07 Sep 2020) by Ran Feng

AR by David Hutchinson on behalf of the Authors (13 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Oct 2020) by Ran Feng

RR by Anonymous Referee #3 (07 Nov 2020)

Suggestions for revision or reasons for rejection

I was asked to comment on the revised version of the manuscript by Hutchinson and colleagues. I have read the revision along with authors’ response to the comments provided by the two previous reviewers. In general, I feel that authors have done an excellent job addressing reviewers’ comments and improving the presentation of the manuscript.

In this study, Hutchinson et al. conducted a thorough and up-to-date review on the EOT study. The review includes marine and terrestrial proxy data, model simulations, and model-data comparisons, as well as paleogeography and forcing mechanisms. Even though I have been working on this topic for a while, I could still get some new information from this review. Re-defining and clarifying the terminology used in the EOT study in Section 1 is also very helpful to eliminate the ambiguity of the terminology used in current studies. Thus my overall assessment on this review is very positive and I’d like to see this review officially published after technical corrections/minor revisions. Below I provide some minor comments/suggestions for authors’ consideration in their revision, which are mostly for clarification and correcting typos.

Comments and Suggestions:

1. Low-latitude TEX86 records: It is pretty clear that some of the low latitude TEX86 records showing large amplitude of EOT cooling is problematic. Liu et al. (2009) made it clear that the signal, especially from Site 803 and 998, may be related to non-thermal factors, and suggested that low-latitude cooling should be on the order of 2-3C. I hope these problematic TEX86 data were not used in authors’ data-model comparisons (Fig. 8). Otherwise, it would affect authors’ assessment on the model performance. The data points based on these TEX86 records could also be removed from Fig. 3 in order not to mislead readers.
2. Authors correctly discussed influencing factors for individual temperature indicators, but I feel the completeness of the individual records also matters in order to derive reliable temperature changes across the EOT. For instance, the available 336 UK’37 SST record only has a couple of data points from a snapshot time interval. Thus it may not be representative of the mean SSTs for the broadly defined Late Eocene and Early Oligocene periods. As currently data resolution in many SST records is not very high and some key intervals are missing due to sediment hiatus, this could largely contribute to the uncertainty in the estimates of SST changes for the two intervals. Authors could add one or two sentences somewhere to address this issue.
3. The time interval to represent post-EOT: Authors used a broad time interval between 33.9 Ma and 30 Ma to represent post-EOT. However, based on the relatively well resolved 1404 SST record, SST values between 29 Ma and 31 Ma could reach the late Eocene level, thus the mean SST over this broad time interval could potentially underestimate post-EOT temperature changes. I would suggest using a time interval broadly consistent with the defined interval for the EOGM, 33.9-33.16 Ma for post-EOT. On the other hand, I recognize that this approach could reduce the number of records to be qualified for this requirement. If the number is significantly reduced, I would suggest discarding my suggestion here, but perhaps it is better to mention in the text that the shorter time interval is more appropriate but due to practical reasons not adopted. Lastly, I note that 1404 SST record was not used in Fig. 3, which could fit the data gap around 40N.
4. CO2 changes across the EOT: Authors mentioned the intriguing rebound of CO2 in Pearson et al. (2009) record. Indeed, such a feature is quite common in alkenone-based CO2 records reconstructed from Southern Ocean. Pagani et al. (2011) suggested that oceanic changes in the Southern Ocean due to opening gateways could bias the long-term CO2 trend, and the reconstruction from tropical sites 925/929 is more representative of atmospheric CO2 changes across the EOT. Also recently Zhang et al. (2020, GCA, 281, 118-134, Fig. 10) have adjusted the assumed b values to allow the reconstructed CO2 values falling into a reasonable range. I’m not sure whether it is worth for authors to discuss this in the text, but it does provide a slightly better justification for the CO2 changes used in model experiments (~300 ppm from data and halving in model), as tropical sites provide larger CO2 changes across the EOT than southern Ocean sites.

Other minor ones:

1. Table 1: EOGM should be 33.65 Ma to ~33.16 Ma in order to be consistent with the ~490 kyr duration. A typo here? Fig. 1 also suggests 33.16 Ma.
2. Line 325, Meridional Overturning Circulation: Authors could briefly mention/summarize the various timing of the onset of AMOC suggested in previous studies.
3. Line 496: The UK’37 index saturates at ~29C for the Muller et al. calibration (1C makes slight difference).
4. Line 508-509: see my previous comment # 1.
5. Line 519: I believe they suggest an apparent decrease in CO2 over that period, not increase.
6. Line 576-578. I feel that the 2C surface cooling is OK for tropics, but for high latitude southern hemisphere, the cooling should be >5C. I recognize that some of the records could not resolve the detailed structure of cooling, but if only 2C cooling is assumed at the first step in the Southern Ocean, then additional cooling has to take place at the second step? Perhaps rephrase this sentence?
7. Line 721: I think this is related to the completeness of the SST record. The same might be also true for Site 277 and 1090 where data points representing the EOGM interval are generally missing.
8. Line 924-926: So the shallow Barents Sea gateway re-opened some time later? Is there any geological evidence for that?

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RR by James C. Zachos (16 Nov 2020)

ED: Publish subject to minor revisions (review by editor) (16 Nov 2020) by Ran Feng

AR by David Hutchinson on behalf of the Authors (17 Nov 2020) Author's response Manuscript

ED: Publish as is (18 Nov 2020) by Ran Feng

AR by David Hutchinson on behalf of the Authors (20 Nov 2020) Author's response Manuscript

Short summary

The Eocene–Oligocene transition was a major climate cooling event from a largely ice-free world to the first major glaciation of Antarctica, approximately 34 million years ago. This paper reviews observed changes in temperature, CO₂ and ice sheets from marine and land-based records at this time. We present a new model–data comparison of this transition and find that CO₂-forced cooling provides the best explanation of the observed global temperature changes.