The control of hydrogen sulfide on benthic iron and cadmium fluxes in the oxygen minimum zone off Peru

Plass, Anna; Schlosser, Christian; Sommer, Stefan; Dale, Andrew W.; Achterberg, Eric P.; Scholz, Florian

doi:https://doi.org/10.5194/bg-17-3685-2020

Articles | Volume 17, issue 13

https://doi.org/10.5194/bg-17-3685-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Ocean deoxygenation: drivers and consequences – past, present...

https://doi.org/10.5194/bg-17-3685-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 13

Research article

|

15 Jul 2020

Research article |

| 15 Jul 2020

The control of hydrogen sulfide on benthic iron and cadmium fluxes in the oxygen minimum zone off Peru

Anna Plass, Christian Schlosser, Stefan Sommer, Andrew W. Dale, Eric P. Achterberg, and Florian Scholz

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Final revised paper (published on 15 Jul 2020)
Supplement to the final revised paper
Preprint (discussion started on 15 Oct 2019)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review of the manuscript from Plass et al. BG 2019-390', Edouard Metzger, 01 Dec 2019
- AC1: 'Response to referee 1 Edouard Metzger', Anna Plass, 17 Feb 2020
RC2: 'Fe flux from reducing sediments off Peru', Michael Staubwasser, 03 Feb 2020
- AC2: 'Response to referee 2 Michael Staubwasser', Anna Plass, 17 Feb 2020

Peer-review completion

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

ED: Reconsider after major revisions (18 Feb 2020) by S. Wajih A. Naqvi

ED: Reconsider after major revisions (18 Feb 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Anna Plass on behalf of the Authors (27 Feb 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Mar 2020) by S. Wajih A. Naqvi

RR by Edouard Metzger (20 Mar 2020)

Suggestions for revision or reasons for rejection

Dear colleagues,

The efforts made to introduce more nuance to their statements regarding uncertainties of flux calculations from different methods used here is appreciable. However, I think some more work should be done.

For instance, suggestions made by Michael Staubwasser to show better pore water profiles by zooming around the interface and try to fit more than two points are crucial here. The arguments advanced to stick on 2 points modelling are, in my opinion, not really convincing.
argument 1. I am not sure that saying "many others did, we should do" is very scientific.
In addition, looking back to papers from Sundby and collaborators (Sundby et al, 1986 and Westerlund et al 1986), it seems that they do not calculate fluxes out of porewaters. In the second one, they compare overlying water and the first layer of porewater to predict flux direction, they do not go further.
argument 2
the authors say that "advanced curve fitting methods would fail to capture sharp gradients..." in order to justify why they do a simple 2 points fitting to calculate diffusive flux. I agree that there is no problem using linear regressions with pour-resolved vertical profiles. The problem here is that between a 2-points and a 3-points regression, there is a big gap in terms of modelling validity.
argument 3
Indeed, if bottom water concentration is low so it does not affect the gradient, however the first porewater concentration does. The latter concentration is the result of a certain volume of porewater mixed altogether during sampling and extraction. We can imagine that some of it had oxygen, some of it sulphide and the final concentration is the result of thermodynamic equilibium, not only due to mixing porewaters of different concentrations.
This is better said by Warnken et al., 2001 as follows:
"The diffusive fluxes of Fe agreed with measured fluxes at station 1, but diffusive flux estimates of Fe at stations 2 and 3 were greater than the measured chamber fluxes, possibly indicating that pore water Fe concentration gradients did not accurately represent actual interfacial concentration gradients at these stations. This could be due to less steep gradients in the upper mm of sediment suggesting that the 1 cm sampling resolution used here was not adequate. Accounting for the reduced slope very near the sediment–water interface would result in lower calculated diffusive fluxes that would possibly agree more closely with measured fluxes. Additionally, rapid oxidative processes at or near the sediment–water interface could also reduce the amount of Fe released from the sediments, as concentration increases during the incubation experiments conducted at stations 2 and 3 were reduced when compared to station 1."
Having a third point, helps to show that the trend is maintained despite interface processes.
I would suggets the authors to simply explain that linear regressions were made because of scarcity of data due to sharpness of gradients at a centimetre resolution profiling. In addition, I would put a warning about the precision of numbers obtained.
Another point that was not fully addressed is the uncertainties on incubation calculations. You must have errors over 100 % with a R2 of 0.5. You should look in detail what are uncertainties and which regression has statistical significance.

All this should lead to a discussion about how we can do better. As it is, it gives me the idea that since 1986 no analytical progress was made to improve spatial resolution and to limit incubation time and increase incubation replicates. Maybe that is the case, but it has to be said somewhere

Best regards

Edouard Metzger

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ED: Reconsider after major revisions (23 Mar 2020) by S. Wajih A. Naqvi

ED: Reconsider after major revisions (23 Mar 2020) by Katja Fennel (Co-editor-in-chief)

AR by Anna Plass on behalf of the Authors (01 Apr 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Apr 2020) by S. Wajih A. Naqvi

RR by Anonymous Referee #2 (19 May 2020)

ED: Publish subject to minor revisions (review by editor) (22 May 2020) by S. Wajih A. Naqvi

ED: Publish subject to minor revisions (review by editor) (26 May 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Anna Plass on behalf of the Authors (26 May 2020) Author's response Manuscript

ED: Publish as is (05 Jun 2020) by S. Wajih A. Naqvi

ED: Publish as is (09 Jun 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Anna Plass on behalf of the Authors (12 Jun 2020) Manuscript

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Short summary

We compare the cycling of Fe and Cd in sulfidic sediments of the Peruvian oxygen minimum zone. Due to the contrasting solubility of their sulfide minerals, the sedimentary Fe release and Cd burial fluxes covary with spatial and temporal distributions of H₂S. Depending on the solubility of their sulfide minerals, sedimentary trace metal fluxes will respond differently to ocean deoxygenation/expansion of H₂S concentrations, which may change trace metal stoichiometry of upwelling water masses.