the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2024
Biogeochemical Layering and Transformation of Particulate Organic Carbon in the Tropical Northwestern Pacific Ocean Inferred from δ13C
Abstract. Particulate organic carbon (POC) serves as the main carrier of the biological pump and determines its transmission efficiency, yet the transformation processes of POC remain incompletely understood. This study reports the vertical distribution of POC, dissolved inorganic carbon (DIC), δ13C-POC, and δ13C-DIC in the tropical Northwestern Pacific Ocean (TNPO). The research identified three distinct biogeochemical layers governing POC transformation: the POC rapid synthesis-degradation layer (RSDL, 0–300 m), the net degradation layer (NDL, 300–1,000 m), and the stable layer (SL, 1,000–2,000 m). From the top to the bottom of the RSDL, δ13C-POC decreased by an average of 2.23 ‰, while the carbon-to-nitrogen ratios (C:N) increased by an average of 2.3:1, indicating the selective degradation of POC. In the NDL, δ13C-POC and δ13C-DIC exhibited a significant negative correlation (r = 0.43, p < 0.05), indicating a net transformation of POC to DIC. In the SL, POC proved to be resistant to degradation, with POC exhibiting the highest C:N (15:1 on average) and the lowest δ13C-POC (average -27.71 ‰).
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3467', Anonymous Referee #1, 13 Jan 2025
While this paper represents new data on POM elemental and stable isotope composition (which is always welcome) and the interpretation given appears fairly senseful, though largely speculative. But because the data set is limited to ‘classical’ parameters of POM and does not provide more specific data about POM composition (e.g. isotopic composition of specific components) the paper falls short in substantially improving our insights in the fate of POM in the oceanic water column with regard to existing literature.
A further major shortcoming of this paper is the one-dimensional (surface to deep) approach used when interpreting the data, despite the apparent complexity of ocean currents and counter currents in the studied area. No use is made of T-S, nutrient data to inform on mixed layer depth, DCM position and to identify major water masses and possible impacts of advection processes on observed profiles.
The method section should be more detailed, since no information on sample preservation, standards, references used, corrections applied .. is given.
Citation: https://doi.org/10.5194/egusphere-2024-3467-RC1 -
AC1: 'Reply on RC1', Xuegang Li, 16 Jan 2025
We sincerely thank you for your valuable time and insightful comments on our manuscript. We have carefully considered each of the issues raised and made corresponding revisions to the manuscript. Detailed responses to all comments are provided in the attached document titled "Response to RC1". Your feedback has been instrumental in improving the clarity and scientific rigor of our study, and we greatly appreciate your constructive suggestions.
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RC2: 'Reply on AC1', Anonymous Referee #1, 17 Jan 2025
I appreciate the fact that a T-S diagram has been provided. The depth scale in color gradient is not really necessary as it renders the reading of the graph more difficult and since in any case isopycnals are shown. So, either remove the depth scale or consider plotting another parameter instead of depth (DIC, d13C, or may be try plotting nitrate or phosphate, for which you have data).
You mention NPSSW, which does not appear in the T-S plot; do you mean NTPSSW?
In order to address my concern about absence of discussion how water masses might possibly affect the vertical distributions of studied parameters, I wonder whether you could indicate the position of the different water masses present at each of the stations (vertical profiles in Figs 2 and 4).
In Fig. 1 you show an arrow marked ‘NGCUC’ along the PNG coast. It was not identified and discussed.
Citation: https://doi.org/10.5194/egusphere-2024-3467-RC2 -
AC2: 'Reply on RC2', Xuegang Li, 18 Jan 2025
We would like to thank you for your quick and valuable feedback on our manuscript.
In response to your second round of comments, we have carefully considered each point raised and made the necessary revisions to the manuscript. For detailed information regarding the specific changes, please refer to the attached document titled "Response to RC2."
Your insights have significantly contributed to refining our study, and we are grateful for your thoughtful recommendations.
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AC2: 'Reply on RC2', Xuegang Li, 18 Jan 2025
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RC2: 'Reply on AC1', Anonymous Referee #1, 17 Jan 2025
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AC1: 'Reply on RC1', Xuegang Li, 16 Jan 2025
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RC3: 'Comment on egusphere-2024-3467', Anonymous Referee #2, 26 Feb 2025
General comments:
This manuscript provides valuable data on the isotopic and elemental composition of particulate organic matter in the tropical Pacific Ocean and utilizes comparisons with the dissolved inorganic pool and dissolved oxygen to infer transformation and degradation of organic matter over depth. However, the authors mainly discuss the POM data in terms of its vertical distribution, despite the data spanning almost 2000 m and there likely being influences of water masses on all the parameters measured. The authors discuss some differences in the water column structure and characteristics of the different stations due to the currents in the study area, but no data is presented demonstrating these differences. I see that water mass information has been added to some figures based on a previous comment, however without reading an updated manuscript version I cannot confirm if discussion has been added about this. Additionally, there is no mention of the possibility of variations in d13C-POC values due to photosynthetic variations within the upper water column. As POM is mainly produced by phytoplankton in the upper ocean, this is one other potential explanation for some of the variations observed. Further, many statements are made without evidence and should either be bolstered with evidence/data or altered to clarify that these are inferences from the data available; please see the examples listed in the “specific comments” section below. Lastly, the methods section needs more detail throughout, though I am aware that a previous comment also mentioned this and it has likely been addressed already -- please disregard my comments below where appropriate. Overall, the manuscript presents valuable data, but the discussion of the data and the interpretation (at least in the manuscript version that I have seen) could be improved.
Specific comments:
Methods section: More detail is needed on the dissolved oxygen methods in Lines 105-106. Since the method is very common, just a brief description would be fine. The POC/d13C-POC/PN section should mention blank corrections for POC and d13C-POC values. All sections on chemical measurements (DO, POC/PN, DIC, isotopes, chl-a) should include some mention of instrument calibration and types and sizes of standards used.
Line 112: I believe the per mil should be a percent sign for the POC/PN concentration accuracy (±0.8)
Section 3.1: This section includes important information on the differences in water column characteristics between the five stations. However, more information would be helpful for the reader to follow the authors’ interpretation of the data. Namely, the euphotic depth and mixed layer depth should be reported for each station, if possible. If photosynthetically available radiation (PAR) was not measured via a CTD sensor, some other metric would still be helpful (like the primary productivity zone based on chlorophyll concentrations, if possible; https://doi.org/10.1073/pnas.1918114117) . A figure including the chlorophyll concentration profiles would also help the reader envision the differences/similarities between the different sites.
Section 3.2: All interpretation of the d13C values of POC within this section is related to degradation of POC. However, there is potential for the magnitude of photosynthetic carbon isotope fractionation to vary over depth within the euphotic zone (https://doi.org/10.1073/pnas.2304613121), and thus production (as well as degradation) can affect the d13C values of POC. Separating out Figure 3A by station for just the RSDL (0-300m) would be one way of investigating this possibility – selective degradation of amino acids and carbohydrates, as described by the authors, should present itself as a positive relationship between POC concentration and d13C-POC values, whereas a different relationship or lack of a relationship may be an indicator that something else is driving the variations – potentially something relating to production of POC or different patterns of degradation than what is being described.
Line 150 mentions the POC concentration averaged over “the surface to the deep chlorophyll maximum layer (DCM, 0-150 m)” – this makes it sound like the DCM at all stations is 150 m, which is not true based on the associated dataset. Clarification of why 150 m was chosen would be helpful
Line 191: are there any studies from this region with biochemical measurements that could be used for comparison?
Sections 3.2 and 3.3: As mentioned in the general comments section above, there is a lot of discussion of the data in terms of variation over depth but little consideration of water masses affecting the observations. For example, Lines 200-202 mention a small increase in DIC over depth due to a small amount of POC degradation, but DIC concentrations are also impacted by the temperature of those waters and where in the surface ocean they originated from.
Lines 223-225: smaller variation in DIC vs POC d13C values is also due to the much larger size of the DIC pool in comparison to POC, as the authors previously note in Lines 75-77. This should be added here as well.
Lines 239-240: degradation of POC, regardless of which portions of the POC pool are degraded, should decrease DIC d13C values because of the large difference in the d13C values of POC and DIC due to photosynthetic carbon isotope fractionation during the production of POC (i.e., POC d13C values <<< DIC d13C values, so even degradation of isotopically heavy POC is going to lower the d13C value of the DIC pool). Discussion within this section should be altered to reinterpret the data within this context.
Some examples of statements made without evidence:
Line 159-161: change “which enhanced” to “which likely enhanced” as heterotrophic microbial activity is not reported here, unless another study (one of the citations?) has shown this in this region. If so, please make this clear in the way the references are presented
Lines 200-202: the wording of this sentence is not accurate to what was measured – there is no evidence that DIC concentration increased “slowly” because no rates were measured
Line 233-234: is there evidence that the synthesis rate of POM is greater than the degradation rate above the DCM? Rates of NPP would be evidence of this but otherwise I would change the wording here.
Lines 246-247: “the photosynthesis rate to slow”, “the respiration rate … was still very fast”; no rate measurements are reported here
Lines 249-250: “the rate of chemosynthesis of organic carbon was lower than the degradation rate of POC” -- again no measurements of POC degradation rates or chemoautotrophic fixation of OC were made
Line 268: no evidence is presented about the proportion of refractory lipids in POC. This is an inference due to suggestions from the presented data, not an observation
Technical corrections:
Throughout the manuscript: d13C values should always be referred to as “d13C values of…” not just “d13C of…”
Line 44: change “the generally accepted…” to “one generally accepted…”
Line 69: change “molecules” to “concentration”
Line 84: change “respiration of heterotrophic organisms” to “respiration by…”
Line 100: change “was” to “were”
Line 150: use nonbreaking space between “150” and “m” so the unit does not separate from the number
Citation: https://doi.org/10.5194/egusphere-2024-3467-RC3 -
AC3: 'Reply on RC3', Xuegang Li, 03 Mar 2025
We deeply appreciate the time and effort you have dedicated to reviewing our manuscript and providing thoughtful and constructive feedback. Your comments have been invaluable in helping us refine our work and strengthen its scientific rigor. We have carefully addressed each of your concerns and made the necessary revisions to the manuscript. A detailed point-by-point response to your comments is provided in the attached document titled "Response to RC3." Thank you once again for your expertise and guidance, which have significantly improved the quality of our study.
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AC3: 'Reply on RC3', Xuegang Li, 03 Mar 2025
Data sets
Vertical layering and transformation of particulate organic carbon in the tropical Northwestern Pacific Ocean waters based on δ13C Detong Tian, Xuegang Li, Jinming Song, Jun Ma, Huamao Yuan, and Liqin Duan https://doi.org/10.6084/m9.figshare.26197808
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