Extreme N<sub>2</sub>O accumulation in the coastal oxygen minimum zone off Peru

Kock, A.; Arévalo-Martínez, D. L.; Löscher, C. R.; Bange, H. W.

doi:https://doi.org/10.5194/bg-13-827-2016

Articles | Volume 13, issue 3

https://doi.org/10.5194/bg-13-827-2016

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

Special issue:

Biogeochemical processes, tropospheric chemistry and interactions...

https://doi.org/10.5194/bg-13-827-2016

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

Articles | Volume 13, issue 3

Research article

|

12 Feb 2016

Research article |

| 12 Feb 2016

Extreme N₂O accumulation in the coastal oxygen minimum zone off Peru

A. Kock, D. L. Arévalo-Martínez, C. R. Löscher, and H. W. Bange

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Final revised paper (published on 12 Feb 2016)
Preprint (discussion started on 08 Jul 2015)

Interactive discussion

Status: closed

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

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- Supplement

RC C4105: 'review of Kock et al.' ms.', Anonymous Referee #1, 06 Aug 2015
- AC C6025: 'Response to Review #1', A. Kock, 01 Oct 2015
RC C4234: 'bg-2015-246', Anonymous Referee #2, 12 Aug 2015
- AC C6031: 'Response to Review #2', A. Kock, 01 Oct 2015

Peer-review completion

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

ED: Reconsider after major revisions (30 Oct 2015) by S. Wajih A. Naqvi

AR by Annette Kock on behalf of the Authors (03 Nov 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Nov 2015) by S. Wajih A. Naqvi

RR by Anonymous Referee #3 (28 Nov 2015)

Suggestions for revision or reasons for rejection

Review of BG-2015-246: Extreme N2O accumulation in the coastal oxygen minimum zone off Peru

General comments:

The authors of bg-2015-246 Extreme N2O accumulation in the coastal oxygen minimum zone off Peru present a clear and comprehensive description of a very large N2O data set from coastal Peru, which is an important oceanic source region for atmospheric N2O.

The introduction provides an excellent summary of the present understanding of factors controlling N2O distributions and sets a good foundation for the interpretation of the data. In general, the methods, results/discussion, and conclusions are comprehensive, clearly written, and well-organized. The paper provides a valuable contribution to the understanding of processes controlling N2O distributions in one of the most important marine regions for N-cycling, and underscores the need for additional research efforts in coastal upwelling shelf waters where AOU is not able to adequately predict N2O concentrations.

I have only a few minor issues related to the analysis of local water masses, Figures 5 and 6, and some grammatical/technical corrections (see below) that will not greatly alter the main findings of the paper, and thus feel that it should be published with only minor revisions.

General Issues:
Lines 106-110, and 220-229: The water masses described in the introduction on lines 106-110 (EUC, SSCC, PCUC) were not mentioned later in the text, while none of the water masses discussed in the results/discussion (lines 220-229; AAIW, ESIW, ESSPW, etc.) were mentioned in the introduction. There should be some consistency between the water masses mentioned in the introduction and discussion.

Line 220-229 and Figure 5:
The locations of the water mass labels in Fig5A do not correspond well with the temperature and salinity ranges listed in the text. E.g. the salinity values of the ESPIW is listed as 34.6 between temp 10-15C (line 225), but the points on Fig5 lie around s=35. Please adjust the text or label accordingly.

Figure 5A CCW label appears to indicate a large number of data points associated with the CCW in the open ocean, contradicting the statement that CCW was not observed in the open ocean (line 228). Is the CCW label in the right location?

Line 227, the salinity of CCW is listed as 15, which is outside the salinity range of the figure. Did you mean to say the CCW salinity is 35.5? Please confirm the salinity values and modify the text and label positions accordingly.

Fig 5: water mass labels only listed on panel A, perhaps you could label both figures with the relevant water masses? This would make it easier to identify the differences between the coastal vs. open ocean water masses.

lines 220-229: Can you please also include the salinity and temperature characteristics of the STSW? ESSW? These are not listed in the text.

Lines 220-229: There is no indication whether high N2O concentrations are associated with one or more water masses, though Figure 5 shows that high N2O concentrations are associated with certain water masses more than others. I understand that N2O distributions are more closely related to O2, but a brief statement linking the N2O distributions to the water mass ages/histories/characterics might help to improve the impact of this section and Figure 5.

Figure 5 and 6 symbol selection:
I am unclear on the significance of the grey symbols, which is not described in the figure caption. I also find them difficult to distinguish from the colored points in Figure 5. I suggest possibly removing the grey symbols, or making the colored symbols larger, and including a description of what the grey symbols represent in the figure caption or as a figure legend.

Symbols are different between fig 5 and 6, but fig 6 caption suggests they are the same. Please adjust the symbols or figure captions accordingly.

I find it difficult to differentiate between the two ‘star’-shaped symbols in Figure 5, and impossible in Figure 6. I suggest changing one of the star shaped symbols to a square, and adding a legend to the figures.

Technical corrections:
Remove extra brackets where citations are listed inside brackets throughout the text; e.g. Line 56.
Line 58: Add spaces between references
Line 156: add space between ‘gaseswere’
Line 222: Suggest writing ‘carry relatively fresh water’, since S=34.8 is not fresh water.
Line 240: Replace ‘high nitrate to phosphate’ with ‘low nitrate to phosphate’.
Line 337: add space between ‘(Hu et al., 2015)which’

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RR by Anonymous Referee #1 (11 Dec 2015)

Suggestions for revision or reasons for rejection

I have revised for a second time the manuscript now titled: “ Extreme N2O accumulation in the coastal oxygen minimum zone off Peru by Annette Kock, Damian L. Arévalo-Martínez1, Carolin R. Löscher, Hermann W. Bange.

The manuscript has been improved and I reaffirm that this work contains high quality data that should be published to the scientific community. It continues to have some aspects within the writing that prevents the rapid interpretation of the main results, this problem starts in the abstract. For example the comparison between “Open ocean N2O profiles” (line 12) and continental shelf N2O profiles (line 15-16) should be described in a way that clarifies the difference between these two areas. A further important point, for example, is …several hundred nanomoles per litre at selected… (line 18) the concentration should be put in nM to make a comparison with N2O profiles in the open ocean.

Explain why or what physical or biogeochemical process could be functioning on the continental shelf and state out why “relationship between ΔN2O and AOU could not be observed (Line 20)”. Also the abstract fails to include the suggestion of which process is occurring and what is responsible for the extreme accumulation of N2O

The introduction is very long and a considerable amount of the text is devoted to describing the processes responsible for N2O cycling, however the results do not focus upon biogeochemical processes. I suggest that further oceanographic context is given to the introduction and I would also summarise the text (4 pages¡¡).

Line 40- onwards. The text is poorly discussed and vague, it should outline the potential outcomes as a result of processes which involve N2O cycling in the presence of various levels of oxygen.

Line 90. The sentence is not complete, a verb is missing… consumption starts is not yet well determined, however (Cornejo and Farias, 2012; Zamora et al., 2012)……

Line 96 This sentence is not entirely accurate. There have been many recent studies in N2O cycling since Farias et al., 2009, including those of Zamora et al 2012, Cornejo & Farias, 2012, Arevalo-Martinez et al., 2015, etc.

Despite improvements in the oceanographic context since the first version, it still is not sufficient. Please remember that the study zone includes complicated oceanic and coastal circulation at the equatorial band, as well as marked/high intra-seasonal variability. The question comes to mind if the sampling took place during active or relaxed upwelling events or another oceanographic processes, it would be expected that changes in the concentration of N2O would occur during these phenomenon.

Materials and methods: Which nutrients were measured? Include a description regarding how the mixed layer was estimated.

The results section is poorly organised (maybe is due to the combination of results and discussion , for example between lines 204 – 212 the north and south distribution is described; east and west oxygen distribution, and also the nutrient distribution. I suggest that the text should start with the description of water masses and preformed oxygen and nutrient levels within these water masses, also the levels of nitrous oxide associated with these water masses should be presented, for example in the form of a T-S diagram (offshore and inshore) .

Line 2008-2009. This sentence is incorrect, I believe it should say that the upper limit, or the oxycline rose up/ascended, or that it became shallower/more shallow. Due to the coastal upwelling, the depth of the upper OMZ boundary significantly decreased towards the coast, with a well oxygenated mixed layer of ~50 m in the open ocean, and a mixed layer depth of less than 5 m on the shelf.

Between the lines 233-234, it is understood that the oxygen minimum zone is a separate/its own separate/distinct water mass off Peru; however it is widely accepted by oceanographers that it is a water mass extending from the equator to 40°S, as would be understood in the text: a broad N2O maximum at the depth of the oxygen minimum was found at the northern and southern periphery of the oxygen minimum zone where the minimum oxygen concentrations did not fall below 5 μM. I think the text requires a much broader oceanographic setting.

What is the meaning of: “A bilinear ΔN2O/AOU relationship”

Line 265 and following lines: The explanation is not convincing, it could be a result of mixing, there are differences between relaxed and active upwelling events, mesoscale mechanisms,

Line 337 reflected in the distribution of different functional gene abundances (Loescher et al., 2014). Does this refer to microorganisms? It gives the idea that the genes are free in the seawater.

Finally, the conclusion does not really give conclusions; instead it is a discussion or implies specific points; what is the conclusion or what is the new information presented in this investigation?

The manuscript has typographical errors and spaces must be included after full stops, semicolons and commas (see examples from reference).

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ED: Publish subject to minor revisions (Editor review) (15 Dec 2015) by S. Wajih A. Naqvi

AR by Annette Kock on behalf of the Authors (25 Jan 2016) Author's response Manuscript

ED: Publish as is (27 Jan 2016) by S. Wajih A. Naqvi

AR by Annette Kock on behalf of the Authors (27 Jan 2016) Author's response Manuscript

Short summary

We measured the nitrous oxide (N₂O) distribution in the water column in the oxygen minimum zone off Peru, an area with extremely high N₂O emissions. Our data show very variable and often very high N₂O concentrations in the water column at the coast, which lead to high N₂O emissions when these waters are brought to the surface. The very high N₂O production off Peru may be caused by high nutrient turnover rates together with rapid changes in the oxygen concentrations.