Low <i>p</i>CO<sub>2</sub> under sea-ice melt in the Canada Basin of  the western Arctic Ocean

Kosugi, Naohiro; Sasano, Daisuke; Ishii, Masao; Nishino, Shigeto; Uchida, Hiroshi; Yoshikawa-Inoue, Hisayuki

doi:https://doi.org/10.5194/bg-14-5727-2017

Articles | Volume 14, issue 24

https://doi.org/10.5194/bg-14-5727-2017

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

https://doi.org/10.5194/bg-14-5727-2017

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

Articles | Volume 14, issue 24

Research article

|

20 Dec 2017

Research article |

| 20 Dec 2017

Low pCO₂ under sea-ice melt in the Canada Basin of the western Arctic Ocean

Naohiro Kosugi, Daisuke Sasano, Masao Ishii, Shigeto Nishino, Hiroshi Uchida, and Hisayuki Yoshikawa-Inoue

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Final revised paper (published on 20 Dec 2017)
Preprint (discussion started on 21 Apr 2017)

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Status: closed

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

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RC1: 'bg-2017-148 rewiev', Anonymous Referee #1, 05 Jun 2017
- AC1: 'Reply to reviewer #1', Naohiro Kosugi, 31 Jul 2017
RC2: 'On the process that leads to the subsurface (30-50m) pCO2 minimum in the Canada Basin', Anonymous Referee #2, 03 Jul 2017
- AC2: 'Reply to reviewer #2', Naohiro Kosugi, 31 Jul 2017

Peer-review completion

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

ED: Reconsider after major revisions (08 Aug 2017) by Jack Middelburg

AR by Naohiro Kosugi on behalf of the Authors (19 Sep 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Sep 2017) by Jack Middelburg

RR by Wei-Jun Cai (15 Oct 2017)

Suggestions for revision or reasons for rejection

I am glad to see the authors have reorganized the manuscript structure and present the more data in depths deeper than 50m. The data of 0-180m water column are more convincing for reporting the subsurface low pCO2 in the Canada Basin, though data are available at only three stations. I would like to support the publication of this paper with moderate improvement.

Main points:
p.9, the text below equation 10 should be expanded and made clear. This is the most interesting part of the paper. The authors have improved this argument, but I feel it is still not quite clear. Overall, I agree that the low pCO2 in water of S=29.3-33.1 is partly a result of previous biological removal of CO2 when the water was still in surface during the early part of the summer and partly a continuous biological removal in the subsurface. I think this is what the authors trying to say and I believe this is enough (though say further work is needed to separate the two). I appreciate the authors’ effort trying to separate the two, but the approach they took with Preformed nDIC may not be reliable. It is possible that the higher nDIC in the surface is an artifact caused by a non-zero river DIC. In the definition of nDIC one assumes a 0 DIC in the freshwater (e.g., rainwater), which is not the case for the Arctic River. A single endmember normalization (nDIC = DIC/S*35) will lead to an artificially higher DIC in low salinity water (See Frii et al. paper below). I do not know how serious this would be (you can check nTA to see if your nTA is invariable with salinity). I would prefer you either should do a full mixing model or drop this. However if you choose to keep it, at least admit the potential problem here.
Friis, K., A. Kortzinger, and D. W. R. Wallace. 2003. The salinity normalization of marine inorganic carbon chemistry data. Geophys. Res. Lett. 30.

On gas exchange rate and time.
4.2, lines, 8-12, Please also verify if you really calculated half time. It appears to me a 100 days half time is a bit too long in a 15m water depth (See Fig. 4 insert in Cai et al. 2010).

4.2 15-18, true, gas exchange is slow, but it was September, nearly 3 months after the ice melt. What maintains the pCO2 at a very low level must also be the relatively high biological production in the CSW as some nutrient is always there in the newly input water from the Pacific in the shelf area there and a relatively deep mixed layer. As the authors recognized that this is in great contrast with those BCW and CBW. Thus this point should be mentioned too. Perhaps, in line 8, modify “This was because net CO2 exchange…” to
“This was because of both a relatively still high biological production and slow net CO2 exchange…”

On data availability
Regarding: Most of data used in this paper are available from the JAMSTEC Data Site for Research Cruises (http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr13-5 06_leg1/e).
I noticed that neither pCO2 nor DIC is available online. These are really the main data used in this paper. Please make them available as soon as possible.

Abstract: the first part should be shortened as it only confirms what was report before, e.g., Cai et al. (2010) and Elsa et al. 2013. This will leave space to expand the discussion on the subsurface maximum.

Other minor points
Page 2 line 6
Delete “e.g.” before the citation

Line 14
Change “content” to “concentration”

Page 8 line 29
Delete “visited during the cruise”

Page 9 Line 5
should “analyses” be “the analysis”?

Line 9
Change “CO2” to “CO2”

Page 10 line 30

Change “pCO2sea.” to “pCO2sea.”

Line 10
“Here, our frequent observations facilitated classification of the water masses and their origins; this in turn explains the biological production in the subsurface layer of the Canada Basin.”

What does “this” refer to? observations? classification of the water masses? Or the subsurface maximum Chla?

line 26-27
Should not use salinity to indicate depths at the same time in your discussion. Easy to get the readers confused.
For example, “the water above S=29.3” could be understood as “the water with salinity above (higher than) S=29.3” or “the water above the depth where the salinity is 29.3”.

Line 18
Change “in case it is mixed with” to “in the case when it mixes with”

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RR by Anonymous Referee #1 (24 Oct 2017)

ED: Publish subject to minor revisions (review by editor) (27 Oct 2017) by Jack Middelburg

AR by Naohiro Kosugi on behalf of the Authors (10 Nov 2017) Author's response Manuscript

ED: Publish subject to technical corrections (15 Nov 2017) by Jack Middelburg

AR by Naohiro Kosugi on behalf of the Authors (16 Nov 2017) Manuscript

Short summary

Recent variation in air–sea CO₂ flux in the Arctic Ocean is focused. In order to understand the relation between sea ice retreat and CO₂ chemistry, we conducted hydrographic observations in the Arctic Ocean in 2013. There were relatively high pCO₂ surface layer and low pCO₂ subsurface layer in the Canada Basin. The former was due to near-equilibration with the atmosphere and the latter primary production. Both were unlikely mixed by disturbance as large sea-ice melt formed strong stratification.

Low pCO2 under sea-ice melt in the Canada Basin of the western Arctic Ocean

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Low pCO₂ under sea-ice melt in the Canada Basin of the western Arctic Ocean