Oceanic CO<sub>2</sub> outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach

Lacroix, Fabrice; Ilyina, Tatiana; Hartmann, Jens

doi:https://doi.org/10.5194/bg-17-55-2020

Articles | Volume 17, issue 1

https://doi.org/10.5194/bg-17-55-2020

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

https://doi.org/10.5194/bg-17-55-2020

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

Articles | Volume 17, issue 1

Research article

|

06 Jan 2020

Research article |

| 06 Jan 2020

Oceanic CO₂ outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach

Fabrice Lacroix, Tatiana Ilyina, and Jens Hartmann

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Final revised paper (published on 06 Jan 2020)
Supplement to the final revised paper
Preprint (discussion started on 25 Apr 2019)

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

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RC1: 'Review of bg-2019-152', Anonymous Referee #1, 10 Jun 2019
- AC1: 'Author response to 'Review of bg-2019-152'', Fabrice Lacroix, 18 Jul 2019
RC2: 'Review on Lacroix et al. Oceanic CO2 outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modelling approach', Anonymous Referee #2, 19 Jun 2019
- AC2: 'Author response to Anonymous Referee #2', Fabrice Lacroix, 19 Jul 2019

Peer-review completion

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

ED: Reconsider after major revisions (22 Jul 2019) by Jack Middelburg

AR by Fabrice Lacroix on behalf of the Authors (21 Aug 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Aug 2019) by Jack Middelburg

RR by Anonymous Referee #1 (14 Oct 2019)

Suggestions for revision or reasons for rejection

Review of “Oceanic CO2 outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modelling approach”

by Fabrice Lacroix et al.

In this new version, the authors made the text noticeably more concise and to the point. The discussion is better organized. This makes the paper much clearer and less cumbersome to read. The novelties from this study, and how the latter can be compared to previous work is now easy to grasp.

In my previous review, I already acknowledged that this is a thorough piece of work improving our understanding of C cycling at the global scale along the land-ocean-atmosphere continuum. I think its quality now reaches Biogeosciences’ standards, but still have some minor comments that could be addressed.

Minor comments:

L9p1. “riverine exports leads to” -> “riverine exports lead to”

L10p1. “results from a source from inorganic carbon loads” -> “results from inorganic carbon loads”

L11-12p1. “… and a slight model desiquilibrium” -> This is still not clear if you haven’t read the whole paper, and also not the most important conclusion. Maybe just indicating that the sink is mainly caused by uptake and resulting alkalinity production is sufficient.

L19-23p1. “… as well as to consider the long-term…” -> The sentence could be split in 2 here, since it is very long, and presents 2 distinct conclusions of the study, which are not directly related.

L23p7. “Pfert,catch was the input from agricultural application of fertilizers, manure and inorganic matter” -> precise that this is the amount of P reaching surface freshwaters due to surface runoff on agricultural lands within the catchment. It is not totally clear when reading that it is not the amount of fertilizer applied to the agricultural lands.

“P exports due to soil perturbations were not considered” -> This doesn’t seem totally accurate, since Pfert,catch from Beusen et al. (2016) does account for disequilibrium in soils.

L6p4. The 2 last paragraphs of the Introduction could be more smoothly connected, e.g. by saying that the present study aims at filling the listed knowledge gaps above, or part of them…

L2p6. It is still not totally clear why the authors decided to scale the runoff. Why would MPI-ESM perform less well than the model from Fekete et al. (2002)? It would seem more logical to scale the runoff to the freshwater inputs used in the ocean model (from OMIP). However, this is probably a detail to the whole study in comparison to other assumptions.

L10p6. “The Beusen et al. (2005) describes” -> “Beusen et al. (2015) describe”

L18-20p8. “We did not consider particulate inorganic phosphorus exports…” -> Even though there are no PIP inputs to the ocean model, this sentence is a bit confusing, since Fe-P exports are estimated. This could be more detailed by stating that for the ocean model inputs, only the desorbed fraction is considered, and the remaining is assumed to be non-bioavailable in the ocean.

L20p10. “and correcting errors in the model” -> this is very vague and maybe not necessary to add here.

L26p11. The origin of some of the parameters is still not totally clear even though the range is discussed (0.003 d-1 degradation rate for tDOM). Is this somehow calibrated? Discussion on this deserves to be refined in 7.2, since due to high C:P ratio, the degradation of tDOM contributes significantly to the estimated CO2 “riverine-induced” outgassing.

L1p12. Even though the physical modelling part is not the scope of this paper, I think it would be useful to have more information on the hydrology/hydrodynamics inputs and characteristics. E.g., is the meteorology from one specific year used for all simulated years?

L9p12. “REF was simulated” -> “REF was run”

L17p12. “was simulated” -> redundant in the sentence.

L6p13. “0.8 – 4 TgP yr-1” -> To my understanding, the upper limit of the range does not only account for weathering. It is confusing to compare it to the weathering estimates in text, while it was not included in Table 2.

L21p17. “river specie” -> “river species”

L23p17. “and to a framework” -> “and constitutes a framework”

L3p19. “REF and R” -> “REF and RIV”

L1-3p22. Precise that the processes described here are what happens in the model.

L11p31. “fixed a N:P ratio for” -> “a fixed N:P ratio of”

L11p31. “While N:P ratios…” -> “N:P ratios…” + precise that “we are incapable” of assessing spatially-variable global N:P ratios for river exports within the present framework (other existing global frameworks do assess variable N:P ratios).

L20-22p33. I would end the conclusion by the local importance of riverine inputs, since this is more “impressing” than these 10%, especially that these are fraught with large uncertainties.

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ED: Publish subject to minor revisions (review by editor) (17 Oct 2019) by Jack Middelburg

AR by Fabrice Lacroix on behalf of the Authors (04 Nov 2019) Author's response Manuscript

ED: Publish subject to technical corrections (11 Nov 2019) by Jack Middelburg

Dear Dr. Lacroix:

Thank you for submitting your second revision to Biogeosciences. I have read your paper and I am happy to inform you that your paper is now accepted for publication pending some minor technical corrections and some additional editing to improve the writing.

With best regards, Jack Middelburg, Associate Editor

All through:
- be precise if you refer to inorganic or organic carbon
- use ‘on the other hand’ only if there is one the hand as well

p. 1., l.6: … long-term fate of …. carbon in the ocean. We…
p. 2, l. 4: revise: it is not logical to write in the ocean…. are exported offshore…
p.2 l. 10-12: merge this floating sentence with paragraph below
p.2, l. 15 inorganic carbon?
p.2, l. 33 reformulate: it previous degradation in rivers to something like degradation during transit in rivers or alike
p. 3, l. 22: Although prior degradation
p.3 l. 12: delete even
p.3, l. 24: the impact of riverine nutrients…
p.7, l. 25: allochthonous
p. 10, l.13: diffusion of…
p. 10, l. 28, 29: total Alk includes carbonate…
p. 11, l. 26-32: be clear when you have to write rate constant and rate. Now it is unclear.
p.12, l. 20: delete first
p.12, l. 23: delete ocean
p. 14, Table entry: the Alk release given is too low. Given that DIC=Alk in rivers (as you state yourself in the text), it should be about 36 and not 18.8 Tmol y-1. Also are you presenting Tmol Alk or Tmol C in this table?
p. 19, Figure 3. Why do you present DIC as well as Alk in this figure if they are basically the same but for the unit (again, Tmol equivalents or Tmol C?).
p. 21, line 7: increased with respect to what?
p. 21, line 11-12: rephrase, is unclear as written
p. 26, line 9: on the one hand…
p. 36, lines 16-19: do rewrite these. There is no eq. (16) in the text, uptake of nitrate, but not of ammonium, increases alkalinity, there is no eq. (14).
p. 36, line 21: there is no equation D1.
p. 37, line 16: should D2 not be replaced with 11?

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AR by Fabrice Lacroix on behalf of the Authors (19 Nov 2019) Author's response Manuscript

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Short summary

Contributions of rivers to the oceanic cycling of carbon have been poorly represented in global models until now. Here, we assess the long–term implications of preindustrial riverine loads in the ocean in a novel framework which estimates the loads through a hierarchy of weathering and land–ocean export models. We investigate their impacts for the oceanic biological production and air–sea carbon flux. Finally, we assess the potential incorporation of the framework in an Earth system model.

Oceanic CO2 outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach

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Oceanic CO₂ outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach