Spatial and Temporal Variations of Particulate Organic Carbon Sinking Flux in Global Ocean from 2003 to 2018
Round 1
Reviewer 1 Report
This papers uses satellite remote sensing to examine spatial and temporal variations of particulate organic carbon (POC) in the ocean over the past couple of decades. The authors examine several methods and models for using the satellite data and decide on the best on for their question. They then use that to examine the fields in question using historical data to examine key regions and consider processes that drive POC variability.
This is an interesting and important topic that is definitely worth further study. The results are potentially quite interesting. However, the paper in its present form has some issues, and so will need some potentially significant modifications before it could be published.
The first question is why has this work been submitted to the Remote Sensing rather than an oceanographic journal? Although it is based on remote sensing data, the results are focussed on the ocean and ocean particulate fluxes (which is the biggest section of the manuscript). So the audience that may really want to see the results may miss it.
The writing, while generally technically correct, has lots of run-on sentences and other grammatical issues that make the work hard to read and follow in parts. It is also written too conversationally in parts (i.e. it is exciting, etc.). There are also a fair number of typos (e.g. La Nia in the abstract) that need to be corrected. So a good pass through to improve the English and focus the explanations and discussions would really help.
The introduction is too short and does not do enough work to put the present work in context. There are many more studies that examine POC and carbon sinking fluxes, including through satellite data. These need to be discussed, with their results highlighted, to place the present work. Of the (only) 16 papers mentioned in the introduction, only 2 are more recent than 2015. The same lack of link to the recent literature continues through the manuscript – this need to be fixed. Also, given the climate implications, why not some discussion and referenced links to the IPCC reports?
The authors want to highlight the importance of satellite methods. Fine. But don’t do it at the cost of negatively impinging other approaches when not needed/justified. For example, the authors seem very negative to towards models and their estimates. Sure, models have their issues and errors, just like other approaches. But a number of biogeochemical models have been run in recent years, at high resolutions, and given good results. This needs to be incorporated into the literature review, and used to compare with the authors’ results. Also, totally disagree with the statement about modellers overlooking the importance of coastal waters when building models. There are many models that resolve the shelves well and are used for shelf-basin exchange studies. Including those with carbon components at resolutions of ¼ and higher.
Figures: Color contour figures would be clearer and easier to read using discrete contouring. Also, what are the black areas at high latitudes in figure like 2-4. Given the area changes between figures, I assume it is then not sea-ice?
In computing POC sedimentation, are ocean currents not considered? Also why use a 1-radian topographic product when that is a lot higher in resolution than your satellite products? In fact, what is their resolution?
The discussion of the results needs to be better integrated into the rest of the literature, with more comparison of the authors results with other studies. It feels like the satellite fields are being discussed but their oceanographic meaning is not brought out and linked to the state of knowledge.
The links to El Nino are potentially interesting, but is the data binned only for the tropical Pacific? And again, the literature on this topic is ignored.
For the high latitude changes, are both the northern and southern hemisphere considered together? If so, why, especially given the differences in sea evolution between the hemisphere? The discussion on reduction of sea-ice doesn’t feel integrated with the results, and again, many studies are considering changes in Arctic Sea and changes in productivity and carbon export. Yet they aren’t mentioned at all.
Author Response
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Reviewer 2 Report
This article compared the published methods and adopted the most suitable set to estimate global particle organic carbon (POC) flux from 2003-2018. The criteria for algorithm evaluation were the long-term POC flux records from the two tropical time-series stations- HOT and BATs. Based on the calculation results, the authors present spatial and temporal variations of global POC flux. It fits the scope of this journal- Remote sensing applications.
Major Comments:
This study has synthesized satellite-driven data, but the assessment for the estimation can be improved more.1a. The authors mentioned that high POC fluxes exist in the high-latitude and coastal areas (L17-20, L188-190). However, only tropical in situ POC fluxes from low flux regions have been adopted to examine the estimated results. If the authors add other in situ POC flux data from high flux regions, the estimation will be more reliable. For instance, the measured POC fluxes in the East China Sea and the Beaufort Sea are the handy data from the high POC flux region (Hung et al., 2016; Sallon et al., 2011).
1b. The time-series stations provide long-term records which are appropriate for examining the trend of the estimated temporal variation. If the trends of the time-series stations support the results of this study, that will improve the credibility of this estimation.
1c. The authors pointing out the importance of data accuracy at L134-141. If the estimated results also include the error range, the readers will get a better picture of this study.
1d. The interpretation for the abnormal POC flux has not fully fitted in El Nino and La Nina periods (Fig.7; L306-310, L341-345). i.e. the lowest value at the end of 2007. The time lag is missing. How long will particles take to sink from the surface to the bottom of the ocean euphotic layer?
This manuscript contains different published methods and the results may be changed by using different methods. It will be better to mention or compare new methods from the cited authors especially.
2a. Table 1 lists several formulas for calculating e-ratio, how about the later e-ratio formulas? i.e. Dunne et al., 2007; Laws et al., 2011.
2b. It is good to see the authors list the estimated POC flue results from different methods in Table 3. Still, a lot of estimations could be added to confirm this study result. i.e. Henson et al., 2015; Laufkötter et al., 2016
2c. The authors should provide a rational reason for this new estimation since the global POC flux has been calculated in different methods.
Minor Comments:
It is very confusing to read a manuscript without clear writing. The important terms should be consistent i.e. POC flux represents the POC flux through the bottom of the euphotic layer. It is hard to understand some sentences when the abbreviation has not been used properly. i.e. According to our calculations, the total NPP of the coastal continental shelf with a water depth less than 2000m accounts for only 18.1% of the word, while the annual POC accounts for 29.5% of the world, and the proportion of POC reaching the seabed further increases to 90.1% (L320-322).
In the list of authors, is there something missing about “jin Wang”? (L5) The affiliation should not include the author’s e-mail address. (L7) I don’t understand the reason for using “Particulate Organic Carbon” and “particulate organic carbon”. (L12, 35, 94) The definition of the high-latitude should be consistent. There are two definitions for the high-latitude in the abstract: a) above 40 degrees and b) 60~90 degrees. (L18, 22) Another different definition for the high-latitude is above 45 degrees. (L189) Is there something missing about “La Nia” ? (L21) There should be a citation for the mentioned published article. (L59-61) The articles that have been cited in the manuscript should be consistent with the content. i.e. L 58-63 “Up to now, there have been several studies of the spatial and temporal variations of POC at large scales by remote sensing data [2, 17, 18]. For example, Stramska analyzed the seasonal and regional variability of POC concentration in the Barents Sea from 1998 to 2014 using satellite observations. However, most of these analyses are aimed at local sea areas, such as the Indian Ocean and the northern Pacific Ocean, the analysis of large-scale and long-time series in global ocean is insufficient.” However, reference #2 is for the Southern Ocean and #17 is for the global ocean. Reference #22 seems to use N instead of C. (L90-92) What does “the POC calculation models” mean? (L118) “it’s obvious that POC output ratio is only 10% or even lower in the low-latitude ocean, but in the high-latitude ocean, the output ratio is higher than 30%.” (L182-183) Does it mean that the highest e-ratio is in the mid-latitude ocean? Table 3, Law et al., 2000 should be Laws et al., 2000. Any citation for “One part of the POC returns to the sea surface through air-sea exchange to participate in the circulation of CO2 in the atmosphere;”? (L211-212) In Figure 5, the title of y-axis is POC in the middle one, but the caption is POC flux. Table 4 is confusing. What does the total value stand for? total average POC flux per meter square? The total area of continental margins should be 3.6Í107 km2 instead of 4.8Í107 km2 according the cited article (Liu et al., 2000). I can’t find the definition of “offshore region” with water depth <= 2000m in the same article.
Author Response
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Reviewer 3 Report
The article entitled: “Spatial and Temporal Variations of Particulate 2 Organic Carbon Sinking Flux in Global Ocean from 3 2003 to 2018” refers to very important and useful aspect on carbon dioxide flux between the ocean and the atmosphere. The advantage of this article is application of several models for POC flux estimation.
However, a minor revision is needed in case of the section Results and English grammar in general.
Specific comments:
The aim of the study should be clarified. Certain confusion arises from the citing of references in section of Results. It seems to be a little bit as Results and Discussion. Please check the sentences in lines, e.g.: 134-141, 149-159, 202-205, 245-250. Maybe, it would be better to join both the sections Results and Discussion. In Table 2, the used codes should be explained.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Thanks for these responses.
Response 1:
Through the research literature, we added the in situ data of POC flux in the Besufort sea, and obtained the near shore data mentioned in (Hung et al., 2013, 2016) by email (thanks to the author of these articles). After preprocessing, we added the POC flux data of these two sea areas. (L103-105)
We are very so excited to find that the overall correlation of the model is improved after the increase of the measured data in the high flux area (as shown in Table 2). We also use the in situ data of high POC flux area to verify the satellite inversion results, and find that they have a high correlation, R-square is over 0.6.
We thank the reviewers for their suggestions. The reliability of our research results is better guaranteed.
Reply:
I am happy to know the result has been improved but also surprised to know that the renewed data were not included in the revised version.
Comment 6: Is there something missing about “La Nia” ? (L21)
Response 6: Yes, It's a spelling mistake, thank you again.
Reply:
Please check L467-468.
Response 8:
[1] We have revised the inaccurate cititations you mentioned above. (L71-73)
[2] The question about Reference #22, now it is #45. We refered to the relevant literature and founded that the concept of "new productivity" was first put forward by (Dugdale et al., 1967). From the source of carbon, a small part of the carbon used in phytoplankton photosynthesis is replenished from the atmosphere, while most of it is recycled carbon in the ocean, so the process of photosynthesis through carbon replenishment in the atmosphere is called "new production", and the process of photosynthesis through the circulation of carbon obtained from the ocean surface is called "reproduction. The total primary productivity is the sum of re productivity and new productivity. However, in terms of the form of carbon, there is no difference between the two kinds of carbon. It is impossible to distinguish the two kinds of productivity directly with carbon. Therefore, Dugdale et al. expressed the proportion of new productivity and re-productivity in the way of new nitrogen (mainly nitric nitrogen, which is supplemented by the outside of the true light layer) and recycled nitrogen (mainly ammonia nitrogen). The main way to obtain new nitrogen was to output micro particles at the bottom of the true light layer.
Reply:
We have very different understanding with the definition of new productivity. The authors mentioned “The process of photosynthesis through atmospheric carbon supplementation is called “new production”, while the process of obtaining carbon for photosynthesis by ocean surface circulation is called “reproduction”, total primary productivity is the sum of them [45] . Eppley et al. used a large number of measured data to prove that under the premise of stable marine productivity system, the output ratio of POC in the surface layer of the ocean is approximately equal to the ratio of new productivity to total productivity [23].” (L110-115)
The first sentence of the abstract from #23 reference is as follows: Primary production in the oceans results from allochthonous nutrient inputs to the euphotic zone (new production) and from nutrient recycling in the surface waters (regenerated production). If the definition of new productivity has been changed I am not sure about which one we should follow in the present.
Author Response
Thank you for your kind advice, please see the attachment.
Author Response File: Author Response.docx
