Variable reactivity of particulate organic matter in a global ocean biogeochemical model

Aumont, Olivier; van Hulten, Marco; Roy-Barman, Matthieu; Dutay, Jean-Claude; Éthé, Christian; Gehlen, Marion

doi:https://doi.org/10.5194/bg-14-2321-2017

Articles | Volume 14, issue 9

https://doi.org/10.5194/bg-14-2321-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Progress in quantifying ocean biogeochemistry – in honour...

https://doi.org/10.5194/bg-14-2321-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 9

Research article

|

05 May 2017

Research article |

| 05 May 2017

Variable reactivity of particulate organic matter in a global ocean biogeochemical model

Olivier Aumont, Marco van Hulten, Matthieu Roy-Barman, Jean-Claude Dutay, Christian Éthé, and Marion Gehlen

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Final revised paper (published on 05 May 2017)
Supplement to the final revised paper
Preprint (discussion started on 27 Sep 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of “A reactivity continuum of particulate organic matter in a global ocean biogeochemical model”', John Dunne, 08 Oct 2016
- AC1: 'Response to review by John Dunne', Olivier Aumont, 29 Nov 2016
RC2: 'Review', Anonymous Referee #2, 04 Nov 2016
- AC2: 'Response to the review by Referee 2', Olivier Aumont, 02 Dec 2016

Peer-review completion

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

ED: Reconsider after major revisions (05 Dec 2016) by Arne Winguth

AR by Olivier Aumont on behalf of the Authors (10 Jan 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Jan 2017) by Arne Winguth

RR by John Dunne (24 Jan 2017)

RR by Anonymous Referee #2 (02 Feb 2017)

Suggestions for revision or reasons for rejection

I thank the authors for their reply to my comments. Again, I have tried understand how exactly the reactivity model works, and how the labilities are computed. Although some things have now become clearer, I think that the model description could still be improved. Given this new approach to model particle properties in the water column, and the good fit to particle data, I would really suggest to spend even a bit more time on explaining the method more clearly. My basic concerns or suggestions are the following:

(1) In some cases, units can be very helpful for the reader: e.g. for "k" (p. 4, line 29 and later; per day?), for lambda (per day?), "a" (lifetime; days?), etc.

(2) It would, to my opinion, also be helpful to distinguish more clearly between "k", "overbar(k)", lambda, lambda*, "overbar(lambda)" etc. In fact, I would suggest to provide a table that explains all these parameters, their definitions, units, and values (if applicable).

(3) Probably much of my initial confusion during the first review came from the fact that the title of the paper is "A reactivity continuum of particulate organic matter", whereas the authors seem to have a multi-G model (Eqn. 2 discretized into 15 lability classes for both sPOC and bPOC), with subgrid scale parameterizations via the Gamma function within each lability class. Correct? If this is the case, the title is a bit misleading.

(4) Eqn. 8 is still difficult to understand for me: are these equations solved within each vertical grid box? More general, the interplay between vertical and lability discretization is a bit unclear to me. How does sinking exactly impact the lability distribution?

(5) Further, with regard to the last term of Eqn. 8 (second line), i.e. k_i f(T) ... : how does this term connect to the later statement (p. 7, line 11-12): "A mean k̄ is inferred from that lability distribution which is then used in PISCES to model the decomposition of POC"? Is POC in each class decaying, or do you compute an average k, and then dissolve average POC from it?

Minor comments:

p. 2, lines 24-25 "POC concentrations tend to be strongly underestimated in the deep ocean, which suggests that an excessive loss of POC is predicted in the mesopelagic zone." - Here a reference would be good to motivate why it could be helpful to include the new processes in any model.

Eqn. 9 Is the mixed layer the only domain where mixing is taken into account for lability?

p. 6, line 25 "each pool" - each lability class?

p. 7, line 33: "rate parameter a" - better "lifetime parameter a"?

p. 10, lines 30-32: Could this indicate either a too slow decrease of lability with depth, or the lack on an increase of particle sinking speed with depth?

Figure 6, caption: what is GOC?

Figure 10: How was "b" computed? In particular: over which vertical domain?

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ED: Publish subject to minor revisions (Editor review) (14 Feb 2017) by Arne Winguth

AR by Olivier Aumont on behalf of the Authors (17 Feb 2017) Author's response Manuscript

ED: Publish as is (01 Mar 2017) by Arne Winguth

AR by Olivier Aumont on behalf of the Authors (15 Mar 2017)

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Short summary

The marine biological carbon pump is dominated by the vertical transfer of particulate organic carbon (POC) from the surface ocean to its interior. In this study, we explore the impacts of a variable composition of this organic matter using a global ocean biogeochemical model. We show that accounting for a variable lability of POC increases POC concentrations by up to 2 orders of magnitude in the ocean's interior. Furthermore, the amount of carbon that reaches the sediments is twice as large.