Constraints on ocean circulation at the Paleocene–Eocene Thermal  Maximum from neodymium isotopes

Abbott, April N.; Haley, Brian A.; Tripati, Aradhna K.; Frank, Martin

doi:https://doi.org/10.5194/cp-12-837-2016

Articles | Volume 12, issue 4

Clim. Past, 12, 837–847, 2016

https://doi.org/10.5194/cp-12-837-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Climatic and biotic events of the Paleogene

Clim. Past, 12, 837–847, 2016

https://doi.org/10.5194/cp-12-837-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 4

Research article 07 Apr 2016

Research article | 07 Apr 2016

Constraints on ocean circulation at the Paleocene–Eocene Thermal Maximum from neodymium isotopes

April N. Abbott et al.

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Final revised paper (published on 07 Apr 2016)
Preprint (discussion started on 30 Jun 2015)
Supplement to the preprint

Interactive discussion

Status: closed

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

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SC C1216: 'The Introduction', Gerald Dickens, 31 Jul 2015
RC C1318: 'Data validation and interpretation', Anonymous Referee #1, 12 Aug 2015
RC C1483: 'Data interpretation and presentation', Anonymous Referee #2, 26 Aug 2015
AC C1756: 'Response to Reviewers', Brian Haley, 23 Sep 2015

Peer-review completion

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

ED: Reconsider after major revisions (06 Oct 2015) by André Paul

AR by Brian Haley on behalf of the Authors (16 Oct 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Nov 2015) by André Paul

RR by Anonymous Referee #1 (11 Dec 2015)

Suggestions for revision or reasons for rejection

The text and the figures were improved for the clarity although more efforts will be required to demonstrate that the proposed hypothesis is supported by the data.

Now the authors acknowledge the eNd offsets between the sediment leachates and the fish teeth/debris, possibly because of different uptake or preservation of eNd records. Consequently, they decided to use the trend of eNd changes of leachates and of fish eNd, instead of absolute eNd values. With this treatment, temporal coverage of reconstructed bottom water eNd values is further uneven with different archives, and it becomes ambiguous which size of variability can be considered as sign of circulation changes. I asked several points about the relationship between the proposed circulation changes and data described in section 3 in my previous review (ex. specific comments 5 and 6). The authors answered them but I am sorry to say that the responses are not enough precise.

I think that careful overhaul revision, in particular for section 3, Figures 2 and 3, is required.
Taking Figure 3 as an example, I explain my concern because this is the key figure of this work that summaries the main finding.
I appreciate that the authors added eNd values in Figure 3. However there is no explanation how the reconstructed bottom water eNd values are calculated. Since temporal coverage of different records is uneven, the considered period and data points to estimate the indicated eNd values should be explained. When eNd data are not available (“n/a” in Figure 3), I do not know how the authors determined the circulation patterns shown with the black arrows. The circulation pattern is very similar between “pre-PETM” and “PETM”, which does not correspond to the hypothesis of ocean circulation changes as a trigger of negative carbon isotope excursion. One noticeable eNd change is a lower value (eNd of -6.0) during the “trigger” period (age range?) of site 1220 in the Pacific. But this low value can be rather interpreted to more contribution of southern origin water in the Pacific, which is contradictory with a shift of deepwater formation zone from the southern ocean to the North Pacific related to the PETM. Please clarify which data support the hypothesis throughout the whole text.

Figure 2: It is necessary to show the trend of eNd variability based on fish teeth separately from the leachate results (Figure 2) as already suggested by reviewer #2. The authors discarded this suggestion because the figure became too busy. However, the difficulty can be overcome by using different eNd scale adapted to each oceanic basin. The main message of Figure 2 is the different timing of d13C and eNd variability. It is not compulsory to use the same eNd scale for the three basins.

Figure 3: Since bathymetry constrain is discussed in the text, it will be helpful to add bathymetry as background in the map. It is not clear how the starting points of the black arrows are determined and what are the expected eNd values of the area of staring points.

In the answer to the reviewers, the authors stated that there was no evidence for the presence of volcanogenic material in the sediments. It is necessary to clarify how the absence of the phase was examined.

In the revised version, the authors indicate that acetic acid leachates had comparable eNd values with HH leachates within 0.5 e-unit. This information is important and should be added in Table 2.

Table 1. I do not think that core sites are all “IODP” (mostly DSDP and ODP).

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ED: Publish subject to minor revisions (review by Editor) (01 Feb 2016) by André Paul

AR by Brian Haley on behalf of the Authors (18 Feb 2016) Author's response Manuscript

ED: Publish as is (06 Mar 2016) by André Paul

Short summary

The Paleocene-Eocene Thermal Maximum (PETM) was a brief period when the Earth was in an extreme greenhouse state. We use neodymium isotopes to suggest that during this time deep-ocean circulation was distinct in each basin (North and South Atlanic, Southern, Pacific) with little exchange between. Moreover, the Pacific data show the most variability, suggesting this was a critical region possibly involved in both PETM triggering and remediation.