Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in
the eastern tropical North Atlantic

Fiedler, Björn; Grundle, Damian S.; Schütte, Florian; Karstensen, Johannes; Löscher, Carolin R.; Hauss, Helena; Wagner, Hannes; Loginova, Alexandra; Kiko, Rainer; Silva, Péricles; Tanhua, Toste; Körtzinger, Arne

doi:https://doi.org/10.5194/bg-13-5633-2016

Articles | Volume 13, issue 19

https://doi.org/10.5194/bg-13-5633-2016

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

Special issue:

Hydrography, biogeochemistry, and biology of "dead-zone eddies"...

https://doi.org/10.5194/bg-13-5633-2016

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

Articles | Volume 13, issue 19

Research article

|

10 Oct 2016

Research article |

| 10 Oct 2016

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

Björn Fiedler, Damian S. Grundle, Florian Schütte, Johannes Karstensen, Carolin R. Löscher, Helena Hauss, Hannes Wagner, Alexandra Loginova, Rainer Kiko, Péricles Silva, Toste Tanhua, and Arne Körtzinger

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Final revised paper (published on 10 Oct 2016)
Preprint (discussion started on 26 Jan 2016)

Interactive discussion

Status: closed

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

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RC1: 'Review-BG-2016-23', Anonymous Referee #1, 09 Mar 2016
- AC1: 'Final Author Response', B. Fiedler, 20 May 2016
RC2: 'Oxygen Utilization and Downward Carbon Flux in an Oxygen-Depleted Eddy in the Eastern Tropical North Atlantic', Anonymous Referee #2, 19 Mar 2016
- AC2: 'Final Author Response', B. Fiedler, 20 May 2016

Peer-review completion

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

ED: Reconsider after major revisions (03 Jun 2016) by Leticia Cotrim da Cunha

AR by B. Fiedler on behalf of the Authors (15 Jul 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Jul 2016) by Leticia Cotrim da Cunha

RR by Anonymous Referee #2 (09 Aug 2016)

Suggestions for revision or reasons for rejection

The revised manuscript by Fiedler et al. shows substantial improvement in the quality of the text as well as the figures. On top of the already discussed relevance of the results, the new version of the paper is more pleasant to read and better organized. In particular, the Authors have added to the revised manuscript precious information that strengthens their working hypothesis, reorganized the information in a clearer way and substantially improved the quality of the figures.
I am therefore pleased with the revisions as well as with the Author's Response that has clarified most of the questions regarding the first version of the manuscript.

I have a few specific and technical comments (listed below).
The specific comments reconnect with the Author's Comment (AC) 1.4 regarding the sporadic upwelling at the rim of the eddy and the Author's Comment (AC) 15 regarding the vertical structure of the eddy.
Technical comments regard the English language: even though the writing has been largely improved there are still some issues. I encourage the Authors to double-check the manuscript again for typos and to rephrase some unclear passages in which the information is summarized in a confusing way. I have focused on some of those passages given their key role in the paper.

I suggest that the manuscript is accepted after minor revisions are done.

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Specific comments:

1) Sporadic upwelling (AC 1.4, General Comments)
I appreciate the detailed explanation given by the Authors in their comment AC 1.4 regarding the submesoscale upwelling in the eddy. However, in Section 3.2 of the revised paper I haven't found the same explanation as clear as it is in the extended Authors Comment 1.4, since the summary-sentence referring to the paper by Karstensen et al. (2016) is confusing.
Page 12 lines 26-29: “As such, this finding is interpreted as being a signature of a vertical flux event related to submesoscale processes and stratification, which on the one side isolate the core and prevent oxygen supply while in parallel support vertical nutrient flux at the eddy rim (Karstensen et al., 2016).”
It is not clear what “vertical flux event related to submesoscale processes and stratification” means, and it is not clear who “isolate the core and prevent oxygen supply” and who “support vertical nutrient flux”: “isolate”, “prevent” and “support” refer to a plural subject but cannot be done interchangeably by both stratification and upwelling.
In my opinion this is a crucial passage for understanding how the biological activity in mixed layer of the ACME is sustained and how this can be reconciled with the isolation of the core. Therefore, I suggest to rephrase the above-mentioned sentence to distinguish between stratification and submesoscale vertical fluxes in a clear way, highlighting their contrasting but simultaneous work.
If my understanding is correct, on one side there is stratification that isolates the core and prevents oxygen supply in the core and on the other side there are submesoscale sporadic upwelling events at the rim of the eddy that feed the mixed layer with nutrients, justifying the combination of tracer concentrations measured above the core. I have not been able to find a final version of the cited paper by Karstensen et al. since it seems to be currently in discussion on Biogeosciences with major revisions requested, therefore I can only try to understand its conclusions at the present stage. I kindly ask the Authors to point me to the final manuscript if available.
The following is a quick attempt to rephrase the sentence and should be considered by the Authors as a mere suggestion: “The combination of tracers concentrations measured in the mixed layer of the eddy is interpreted as the signature of a submesoscale vertical flux event. On one side stratification isolates the core and prevents oxygen supply, on the other side submesoscale upwelling at the eddy rim supports the vertical nutrient flux in the mixed layer (Karstensen et al., 2016).”
The fact that the upwelling is supposed to happen at the rim of the eddy could also be mentioned in the Conclusions section (page 18 line 27), Eg. “…are caused by upwelling events at the rim of the eddy…”.

2) Eddy vertical structure (AC 15, Specific Comments)
I suggest that the Authors specify also the depth of the lower bound of the eddy core.
This could fit for example after page 11, lines 31-32 where the mixed layer depth is specified, and would help the reader to interpret the figures. Alternatively, horizontal dashed lines could be added to the plots (Figures 3,4,5,6) to highlight the depth of the core region.

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Technical comments:

1) page 3 line 21: “about 2 to 3 ACME were generated each year” - I suggest either to add in which years or to use “are generated” as a general statement

2) page 4 line 27: “we did not had” should be “we did not have”

3) page 4 line 30: “opportunistic Argo float data” - “opportunistic” is not the right adjective, probably “suitable” or “appropriate”

4) page 5 line 4: “the marine carbonate system functioning on low-oxygen eddies” - “on” should be “in”

5) page 5 line 27: “This was also conducted outside of the eddy” - given the previous sentence, it's not clear what “This” refers to, maybe better to start with “CTD casts were also performed outside of the eddy”

6) page 6 lines 1-2: “This points out that these stations were more at the rim of the eddy, rather than in the surrounding water representing typical background conditions.” - I find this sentence confusing, I would rephrase it, for example: “This points out that these stations were more at the rim of the eddy rather than in the surrounding water, therefore they do not represent typical background conditions.”

7) page 8 lines 28-30, page 9 line 1: “At the same time, subsequent sinking of particulate matter combined with an efficient isolation of the core from surrounding waters hinders oxygen ventilation.” - I don't understand this statement: how does the sinking of particulate matter hinder oxygen ventilation? Is “hinders” (singular) only referring to the “efficient isolation”? Maybe this could be rephrased to better express how the sinking and the isolation contribute to lower the oxygen in the eddy core.

8) page 9 line 27: Data is a plural noun, “was” should be “were”

9) page 11 line 22: “westward propagation from the shelf into the open.” - This sentence misses a final noun, eg. “ocean” or “sea” or "waters"

10) page 11 line 22: “being” is not needed: This hypothesis is further corroborated […]

11) page 11 lines 26-27: numbers in brackets should be accompanied with the unit (years)

12) page 12 line 12: when talking about the decrease “of about 57.0 umol/kg to suboxic levels”, an initial shelf value and a precise final value should be specified in the text “of about 57.0 umol/kg, from X to Y”

13) page 12 lines 22-23: “associated with the passage of a former ACME passing the observatory” - passing should be substituted, eg. “near”, “in the region of”, “in correspondence of”

14) page 13 line 14: “This value is very much in contrast to the regional background condition” - This sentence should be rephrased, eg. “This value is in clear contrast with the regional background condition”

15) page 17 line 23: missing “those”: “…show higher values than those found…”

16) page 18 line 25: “continuing” should be substituted with “continuous” or “persistent”

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ED: Publish subject to minor revisions (Editor review) (10 Aug 2016) by Leticia Cotrim da Cunha

AR by B. Fiedler on behalf of the Authors (22 Aug 2016) Author's response Manuscript

ED: Publish as is (30 Aug 2016) by Leticia Cotrim da Cunha

AR by B. Fiedler on behalf of the Authors (09 Sep 2016)

Short summary

Oxygen-depleted mesoscale features in the open eastern tropical North Atlantic, which are formed in the Mauritanian upwelling region, were discovered recently. This study examines biogeochemical structure and magnitudes of related processes within these isolated water masses. We found very low oxygen concentrations and strongly enhanced acidity at near-surface depth. Oxygen utilization and downward carbon export were found to exceed known values for this ocean region.