|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.|
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
Change “content” to “concentration”
Page 8 line 29
Delete “visited during the cruise”
Page 9 Line 5
should “analyses” be “the analysis”?
Change “CO2” to “CO2”
Page 10 line 30
Change “pCO2sea.” to “pCO2sea.”
“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?
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”.
Change “in case it is mixed with” to “in the case when it mixes with”