Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Ding, Ling; Ge, Tiantian; Wang, Xuchen

doi:https://doi.org/10.5194/os-15-1177-2019

Articles | Volume 15, issue 5

https://doi.org/10.5194/os-15-1177-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-15-1177-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 5

Research article

|

06 Sep 2019

Research article |

| 06 Sep 2019

Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Ling Ding, Tiantian Ge, and Xuchen Wang

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Final revised paper (published on 06 Sep 2019)
Supplement to the final revised paper
Preprint (discussion started on 10 Sep 2018)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review for Ding et al manuscript', Anonymous Referee #1, 18 Oct 2018
- AC1: 'Response to referee comments', Ling Ding, 31 Dec 2018
RC2: 'Review comments', Anonymous Referee #2, 31 Oct 2018
- AC2: 'Author response to referee 2', Ling Ding, 31 Dec 2018
AC1: 'Response to referee comments', Ling Ding, 31 Dec 2018
RC3: 'Interactive comments on “Influence of hydrodynamic mixing on the distribution of dissolved organic carbon in the East China Sea and the northwest Pacific”, by Ling Ding et al.', Anonymous Referee #3, 08 Jan 2019
- AC3: 'Response to referee comments', Ling Ding, 28 Jan 2019

Peer-review completion

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

AR by Ling Ding on behalf of the Authors (12 Feb 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Feb 2019) by Mario Hoppema

RR by Anonymous Referee #2 (19 Feb 2019)

Suggestions for revision or reasons for rejection

After the revision, I think the manuscript has been largely improved. The authors have added the DIC and 14C-DIC data, added more discussion to include both hydrodynamic mixing processes and biological processes in shaping DOC distribution, and developed better evidence to support the physical mixing point. However, there are still some minor revisions the authors should make before the final publication.

One major concern I still have is that the authors haven’t fully addressed the comment of inappropriately relying on DOC vs. temperature correlation to support physical mixing. In the response, the authors stated that when they tried excluding surface DOC to derive the correlation or correlating DOC and temperature only at specific depths, correlations were worse. I think this is probably due to the fact that physical mixing is not universally distributed to certain depth across all studied regions, but only to certain locations or different depths at different stations. I think the tones related to physical mixing solely based on DOC vs. temperature correlation should be further softened (for example, line 363-367) and the authors should at least acknowledge somewhere in the text that the correlation itself alone cannot fully support the physical mixing idea since depth co-variation cannot be excluded. And they should highlight that they derive the physical mixing more from the transection distribution plots such as Fig. 7 and Fig. 9.

Some other specific comments:
Line 24: restructure sentence, for example “In addition to biological processes that are estimated to account for 7%.....respectively, the DOC distribution is largely controlled …..”

Line 30: delete “In contrast”

Line 45: “plays” change to “play”

Line 122: “which” change to “and”

Line 125: “a branch”, whose branch? KC? Rework on the sentence.

Line 172: delete “each”

Line 257: “drop in only 300m” change to “drop within only the upper 300m”. Based on the figure, it looks like it is more or less close to 500m. Recheck on this.

Line 260: “decreased to background value? at 5000m”

Line 310: sentence confusing, Sta. Z1, Z2, Z3 are more influenced by KC? But above just said Sta. 1, 7 are more affected

Line 451: Do you mean biological degradation along lateral transport here?

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RR by Anonymous Referee #3 (02 Mar 2019)

Suggestions for revision or reasons for rejection

Dear Editor,

I carefully read the revised version of the paper and the replies by the authors.

I appreciate the efforts made by the authors to follow the reviewer’s suggestions and I think the manuscript is improved, however some of the points that I raised were not addressed and there are still some weak points that hinder the publication of the paper in the present form.

As an example, I asked to discuss DOC data from the ECS, since the values are higher than those usually observed in deep oceanic waters, but the authors just added 3 lines (P19, L 448-451), without explaining this finding.
I asked to include in this paper radiocarbon data on DOC, but the authors replied that “the distribution of 14C-DOC values in the northwestern North Pacific would be discussed in details in another paper.”
The authors state that “there were no chlorophyll data measured by CTD during the same cruise”, but at P13, L313 they write “At Stn. 1 and Z1, the subsurface DOC maximums were not related to the chlorophyll maximum (data not shown)”. This is confounding, are chlorophyll data available or not? In case they are available they should be included in the paper.

The data are new and interesting and I would like to see them published, but I think the goal of the paper should be changed.

I suggest to change the title to “Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific” and to focus the discussion on the main processes affecting DOC dynamics, since I do not think the data they used and the approach they followed are adequate to support the main goal of the paper.

The results section is strongly improved, but the discussion should be deeply reworked, since most of it is not supported by the data. I still think that the authors do not have strong evidences that mixing and hydrodynamic processes are the main processes affecting DOC distribution in the study area. As reported in my previous comments, I do not think that the good linear regression between DOC and temperature can be used to support that mixing affects DOC distribution, unless data from the same depth range are compared or surface samples are removed from the regression, but, as stated by the authors, in these cases the correlation is lost, supporting that the correlation is mainly driven by the similar shape of the vertical profiles for both parameters.

The authors report in many parts of the paper that Kuroshio water is characterized by high-salinity whereas the south-flowing Oyashio Current is characterized by low salinity, salinity would therefore be a better tracer for water masses, why the authors do not use salinity to follow the mixing? Did they look at correlation between DOC and Salinity? Did they try to apply an Optimum MultiParameter (OMP) water mass analysis as done by other authors? (Catalá, et al., 2015, Global Biogeochemical Cycles, DOI: 10.1002/2014GB005048; Catalá, et al., 2015, Nature communication DOI: 10.1038/ncomms6986; Catalá, et al., 2018, Progress in Oceanography, https://doi.org/10.1016/j.pocean.2018.05.002).

The authors also state that the good correlation between DOC and temperature, reported in figure S2, is a supporting evidence for their data, but they also report that the four stations used for the correlation were in the same water mass. In my opinion, this correlation clearly shows that temperature along the water column cannot be used as a tracer of the water masses. Density alone cannot be used as a tracer of water masses as well, since it is mostly related to the structure of water column, water masses are usually identified by their salinity and temperature.

The authors should describe the main water masses found in the study area and they should clearly identify them in the T/S graphs.

I find interesting the use of DIC14 to estimate the concentration of DOC due to conservative mixing, but I suggest to clearly report in the paper how the calculation was done.

Some suggestions, specific concerns, and questions are provided below.

ABSTRACT
P1, L18-20 “DOC[…] and its distribution and behavior play important roles in carbon cycling and biogeochemical processes in the ocean.” This sentence is not well written. What does DOC behavior mean?

INTRODUCTION
“microbial respiration processes, the DOC pool in the ocean also indirectly contributes to the sink of atmospheric CO2.” Respiration cannot contribute to the sink of atmospheric CO2, in contrast it will represent a source of CO2.

METHODS
Information about CRM are still missing. The author should report the average value ± standard deviation and the number of CRM analyzed in the period of the analysis.

RESULTS
Section 3.1 Hydrography
The physical properties of the main water masses occurring in the study area should be described in this section and the water masses should be identified in the Ɵ/S graphs.

P10, L240 “DOC concentrations decreased to much lower levels …” Than??

P11, L246 I think Fig. 5c should be cited not 5b.

P12, L272-273, “Export of DOC from the shelf water to the slope was also limited because most of the bioavailable DOC had been respired in the shelf waters and this could be the case, as we observed a statistically positive correlation between DOC and water temperature (R2 = 0.82, p
274 < 0.001) for the stations in the ECS (Fig. 6a).” Why and how does the positive correlation between DOC and temperature support that the bioavailable DOC had been respired in the shelf waters?

P13, L296 ” To further demonstrate the influence of different water mass mixing processes on the hydrological properties, Figure 7 compared the transectional distributions of density”. Density mainly gives information on the vertical structure of the water column. I think salinity would be more appropriated to trace water mixing. What does “transectional distributions” mean?

P13, L “The cross-section density plot (Fig. 7a) showed that the water mass in the studied area was composed of mixed Kuroshio and shelf waters.” I do not understand how density can show this mixing and there is no information on the figure.

P13, L299-301 “It appeared likely that the influences of Kuroshio intermediate water (500-800 m) on the bottom water at station Z4 and Stn. 11 brought low concentrations of DOC, high concentrations of DIC and low 14C values of DIC. This intrusion of Kuroshio intermediate water diluted the DOC at Stn. 11 and Z4 (Figs. 7b-d)”. As above reported, I do not find this information in the figure. What I can see is that DOC accumulates in the mixed layer at all the stations and, as expected, it decreases below the pycnocline.

P13, L302-302 “However, it appears that this upwelling intrusion had almost no effect on the surface water (<100 m depth) for the shelf stations.” This supports that mixing is not the main driver of DOC distributions at these stations.

P13, L305-310. This paragraph is not clear to me and as above reported, the information reported in Figure 7 do not support this part of the discussion.

P13, L313 “At Stn. 1 and Z1, the subsurface DOC maximums were not related to the chlorophyll maximum (data not shown) and could not accumulate in the developed stratification water column, as inferred from the σt distribution (Fig. 7a).” This sentence makes no sense to me. What does “could not accumulate in the developed stratification water column” mean?

P13, L314 What does “fixed sinking POC” mean?

P14, L316-317. I do not see any DOC subsurface maximum. Between station 7 and Z3 the pycnocline is deeper, DOC accumulates in the mixed layer and this can easily explain the observed DOC distribution. There is no data supporting the role of POC in this paper.

P15, L341-342. How can the authors explain “the relatively large spatial variations for DOC concentration among these stations, especially in the upper 1500 m (Fig. 3b and Fig. 4b).”. No hypothesis is reported.

P15, L360-367. As reported in the general comments, this part of the discussion is not supported by the data.

P16, L370-371. “It can be observed that the distributions of DOC, DIC and 14C-DIC were clearly associated with different water masses, as identified by the potential water density (σ0). “As reported in the general comments, density alone cannot be used to identify the different water masses.

P16, L372- 375 “Higher levels of DOC were associated with lower DIC concentrations, and high 14C-DIC values were found in lower density waters (σ0 < 25.5, water mass A), while lower levels of DOC were associated with higher DIC concentrations, and low 14C-DIC values occurred in denser waters (water mass C and water mass D at σ0 > 27.1) (Fig. 8).” As I reported many times this finding can be easily explaining by biological processes and water column stratification. The surface waters (with the lowest density) are characterized by the highest DOC concentration (due to biological production of DOC) low DIC concentration and high 14C-DIC values due to the exchanges with the atmosphere. This is something observed everywhere in the oceans.

P17, L392-394. “The relatively higher DOC level in the upper 200 m at Sta K2 was influenced by the northeastward-flowing Kuroshio, which carries a subtropical warm and high-salinity water mass in the upper layers, as demonstrated in Fig. 2b and Fig. S1”. Fig 2b does not show DOC data and looking at table S1, St. K2 is not characterized by DOC concentration higher than at stations A8 and B8.

P16-17, L383-412 All this paragraph should be rewritten since it is confusing and not supported by the data.

LP17, L402-403. The authors in many parts of the paper state that the Kuroshio water is characterized by high salinity values. I do not therefore understand how they can write “It can be observed that the Kuroshio, which carries relatively high DOC, dominated stations B9, B8 and A1-b from ~200 to 1500 m depth.” Since station B8 and B9 are clearly characterized by a minimum of salinity between 500 and 1000 m.

P17 L403-407. The occurrence of Oyashio water, characterized by low salinity, is visible at St. B2 between ~100 and ~700 m (Fig. 9a), so I do not understand how the authors can state “In contrast, the Oyashio, which carries low salinity, low DOC but high DIC concentrations, and low 14C-DIC values in the subarctic intermediate water, influenced the entire water column at Sta B2”.

Figure 2, 4 and 8. I recommend to clearly identify on the Ɵ/S graphs the water masses occurring in the study area.

Figure 3 the same x-axis should be used for the graphs

Figure 4, it seems that the correlation between DOC and temperature is investigated taking into consideration all the data, as reported in my general comments this correlation cannot support that mixing and hydrodynamic processes are the main drivers of DOC distribution. This figure should be removed.

Figure 7. -Density does not give any information about water masses, I suggest to replace density with salinity.

Figure 7 and 9. The numbers are really small, as well as the points where data are available. I would add the contour lines and the name of the main water masses occurring in the study area.

Figure 9 it is not possible to discriminate between black squares and black circles.

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RR by Anonymous Referee #1 (05 Mar 2019)

ED: Reconsider after major revisions (05 Mar 2019) by Mario Hoppema

AR by Ling Ding on behalf of the Authors (27 Jun 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (28 Jun 2019) by Mario Hoppema

ED: Publish subject to minor revisions (review by editor) (22 Jul 2019) by Mario Hoppema

Dear Dr. Ding and co-authors,

Thank you for your revised manuscript, which has much improved. I think you generally answered the questions and comments by the referees satisfactorily. I read through the manuscript and listed my comments below. Your manuscript is almost through. Please submit a new version of your manuscript in which my comments are accounted for.

L90-93 “In the deep ocean, a 14 μM decrease in DOC concentrations occurs along the abyssal circulation pathway from the North Atlantic to the North Pacific Ocean due to differences in thermohaline circulation patterns (Hansell and Carlson, 1998).” I do not understand what is contended here. And I think the decrease of DOC from the North Atlsntic to the North Pacific is due to the worldwide circulation combined with DOC degradation, correct? Please modify this sentence.

L93 “confirmed DOC export” What kind of export? From where?

L131 I would like to propose not to use abbreviations like NP for North Pacific. It reduces the readability and is not necessary for text space reasons in an online paper

Section 2.1. There is quite some overlapping info in this section and in the Introduction. Please reduce this by either deleting it from the Introduction, or from this section. I guess that while you present such a section, most double texts from the Introduction should be deleted.

Table 1: Is depth the sampling depth or bottom depth?

L226, 227 “end” I think you mean “end member” here, correct?
L250-251 “In comparison, the concentrations of DOC in the KE region were much lower (43-65 μM) and showed large spatial variations among the stations above 1000 m (Fig. 3b).” This sentence is not logically composed. Change to: The concentrations of DOC in the KE region were much lower (43-65 μM) than in the ECS, and above 1000 m the stations showed large spatial variations.

L287 Please do not use OC here, but write out full. Also at other places in the text.

L287 “comprised 45-50 % DOC to the ECS in 2009 and 2015.” I am not sure what you mean with this contention. Is this percentage of the input (per year?) or the percentage of the total amount present in the ECS?

L288 Please do not use OM here, but write out full. Also at other places in the text.
L288 … with 14C ages of around 1,000 years

L290 caused (instead of: influenced)?
L291 “and fluxes offshore.” It is not clear to me what is meant. Please rephrase.

L291 the DOC concentration (not just DOC)
L296-300 I think the sentence must be split into two. The second part could say something like: In this respect we observed a…

L319 delete: could
L324-326 “As mentioned above, the statistically significant positive correlation between DOC and water temperature indicated the hydrodynamic mixing has important influences on DOC distribution.” This is not correct and should be changed/deleted. The referees indicated that this correlation is most likely due to co-variation along depth. And above it was written that this may indeed explain this correlation.

L354 I think the title should be: Processes that influence the DOC profiles in the Kuroshio Extension

L358 “during spring was low in the KE region” Is that always the case, or only in a particular year? In which year? Such info must be added here.

L372, 373 Define chl; write chlorophyll
L387-389 “These observed correlations of DOC concentrations and hydrographic variables indicate the physical water mixing played important roles on the DOC distribution in the KE region.” It is not clear to me if you mean Temp and DIC when you write hydrographic variables. For DIC this contention does not hold. Actually, one would expect a negative correlation of DOC and DIC indeed, if degradation of DOC occurs. I do agree, however, that the magnitude of DIC changes as shown in Figure 6 is too large to explain the DOC changes. So please bring more nuance into this discussion.

L408-409 “which supplied the environment for DOC accumulation in the surface layer.” This is grammatically not correct. I suggest something like: which is beneficial for DOC accumulation in the surface layer.

L419 delete: fresh
L438 define NPIW

L465 delete: old
L468 “could be due to the differences in thermohaline circulation patterns” This is not a correct description of what you mean, I think. It is due to the general circulation pattern of the world ocean, with young water in the N Atlantic and much older water in the N Pacific.

L469 … who presented a trend in the deep-ocean DOC …

L720, 723 Delete the abbreviations here, because they have no use in the caption.
Figure 1: In the text the rivers Yangtze and Yellow river are mentioned. It would be helpful to indicate their locations in this figure.

L728 delete: plot (because “diagram” says the same)
L730 lines (not: lined)

L741-742 I suggest: Note in panel b that the depth below 1500 m is on a different scale. (same for L753-754)

L746 delete: Field-observed
L750 Please add SMW

L766 in caption: (a) is salinity, not density
L769-737 Please modify the text to mend the density issue

Thank you and best wishes
Mario Hoppema

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AR by Ling Ding on behalf of the Authors (31 Jul 2019) Author's response Manuscript

ED: Publish as is (01 Aug 2019) by Mario Hoppema

AR by Ling Ding on behalf of the Authors (06 Aug 2019) Manuscript

Short summary

Dissolved organic carbon (DOC) is the largest OC pool in the ocean. Its dynamics are largely influenced by hydrodynamic mixing of different water masses in the East China Sea and Kuroshio Extension. Water from Kuroshio intrusion could dilute DOC in the shelf break of the ECS, and spatial variations of DOC in upper 700 m in the KE were mainly influenced by mixing of the Kuroshio and Oyashio currents. The study provides updated and useful information for understanding DOC cycles in the ECS and KE.