the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Massive and localized export of selected marine snow types at eddy edges in the South Atlantic Ocean
Abstract. The open ocean plays a critical role in mitigating climate change by sequestering carbon dioxide (CO2) from the atmosphere for long periods of time. This carbon storage occurs over decades to millennia and relies on the physical pump that transports cold, dense, and DIC-rich waters to the deep ocean, as part of the ocean’s overturning circulation, and the biological carbon pump (BCP). The BCP encompasses a wide range of processes, from the fixation of atmospheric CO2 by phytoplankton activity to carbon sequestration in the deep ocean. Atmospheric CO2 concentrations would be about 200 ppm higher than in a world without biology, and the global climate would be much warmer by default. This study highlights the idea that BCP efficiency is enhanced by the ocean dynamics at mesoscale and submesoscale. In fact, our results suggest that frontal regions, such as those between mesoscale eddies, could lead to an important accumulation and transport of particulate organic matter (POM) from the mixed layer depth (MLD) down to depths of about 600 meters. To reach these conclusions, a multifaceted approach was applied. It included in-situ measurements and marine snow images from a BGC Argo float equipped with an Underwater Vision Profiler (UVP6), satellite altimetry data, and Lagrangian physics diagnostics. We focused our study on three intense features in marine snow distribution observed during the 17-month long float mission in the Cape Basin, southwest of Africa. These features were located in the frontal region between mesoscale eddies. Our study suggests that a particle injection pump induced by a frontogenesis-driven mechanism has the potential to enhance the effectiveness of the biological pump by increasing the depth at which carbon is injected into the water column. This work also emphasizes the importance of establishing repeated sampling campaigns targeting the interface zones between eddies. This could improve our understanding of the mechanisms involved in the deep accumulation of marine snow observed at eddy interfaces.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-1558', Anonymous Referee #1, 17 Jul 2024
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RC2: 'Comment on egusphere-2024-1558', Anonymous Referee #2, 24 Jul 2024
The authors provided an interesting and serious observational (BGC-Argo float) evidence of 3 major events of POC (marine snow) injection at great (~600m) depth. The article contributes nicely to the timely topic where physical (sub)mesoscale features can interact with the BCP. The descriptive study is serious and offer a qualitative /observational description.
I really enjoyed the straightforward reading. The overall good structured really helps the reader. In terms of science, the analysis of the POC dynamics is well established. However, the physical oceanography assessment is extremely weak and speculative. I understand that it is impossible to reach out to vertical velocities (either from floats or even models are bad at it), but the only physical diagnostic provided (besides positioning with eddies) is the FTLE, a front detection diagnostic. This is an indicator for water mass convergence (explained too late in the text). Everything else is purely speculative (relying on the extensive literature of frontogenesis of course) and/or descriptive. So I felt quite frustrated as it could have gone a bit further, especially with the Lagrangian diagnostics. I would have been more convincing if, for example, the authors provided more straightforward evidence of upwelling / downwelling. Just drawing some isopycnals on top of the MLD could intuitively help (Fig. 2, 3). More importantly, the authors have to provide (or at least try to) more convincing evidences of the physical subduction. The demonstration is now purely qualitative although there are some reachable techniques to objectively characterize subduction events from BGC argo floats (see for example: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JC012861, or https://bg.copernicus.org/articles/18/5539/2021/ ).
Lagrangian Chlorophyll a is introduced, mentioned briefly in section 3.1 but never shown (unless I missed the obvious) ! Overall, the article would benefit to better exploit the strength of Lagrangian diags. The authors should do better at explaining with those tools how they can properly rule out this is not lateral advection.
Another aspect which was disappointing, is the absence of quantitative assessment. The authors do have the carbon content and sinking rate (correct me I may be wrong), so what does stop them from providing fluxes ? If there is a good reason, then the authors should explain the reviewers.
Otherwise, it feels that the authors are a bit over-influenced by their own community, lacking a bit of prospective from the global community, as the same references are used over and over. On many occasions, they provided references of reviews, not the actual research, which should be (I hope you agree) avoided. But I guess this is the case of every working groups.
Besides this, the authors should re-work the streamlining of the abstract and introduction, but it is not a major issue. I also noted many little awkward formulations that also should be easily fixed.
Point-by-point comments:
Figure 1: It would help to highlights where the export events happened here. Although this is shown in detail in Figure 7. Figure 7: “Solid lines correspond to the eddy
662 maximum speed” Isnt this just isoline of ADT ? Please specify/correct.
Line 13: If you are referring to climate change then it should be followed by “uptaking and sequestering anthropogenic CO2 from the atmosphere”
Line 14-16: The sentence reads weirs. I would say that the storage relies on ocean pump. Period. Then then the pumps consist of the PCP and BCP…
Line 17: BCP -> Organic carbon right ?
I personally find that the first lines 13-18 are a very verbose and a long way to present the article. It really sounds like an introduction. A lengthy definition should not stand in an abstract. I suggest re-working the abstract. This article is more like a process study. The physical pump is off-topic, as well as the climate change aspect. Go straight to your point. What’s novel about the article is the physical-biological interaction. Emphasize this instead.
Just an example:
“ The transfer of organic carbon from the upper to the deep ocean forms the long-term storage of carbon of biological origin. This ‘biological carbon pump’ is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~200 ppm relative to a world without export flux. “
…
“However, the BCP interactions with physical features like eddies have been largely overlooked. This study highlights…” or something like that
Line 18: Certainly not !! What you are investigating is the export “potential” or something like that. You are not quantifying the export flux, neither the export efficiency (e-ratio), or even less the BCP efficiency.
Read for example:
10.1029/2021GB007083
Or
10.1111/gcb.17124
Line 23: “Lagrangian diagnostics”: would delete “physics”
Line 37: “and operates worldwide” weird formulation… I would remove.
Line 40: Wrong citation. Overall biased and very influenced literature. I recommend to zoom out a bit to the outside of your community. Boyd et al. certainly did not come up with this number. This number is from the bible of biogeochemistry: Sarmiento, J. L. & Gruber, N. Ocean Biogeochemical Dynamics Ch. 8 (Princeton Univ. Press, Princeton, 2006).
Line 45: I would rephrase this more straightforwardly:
“Large particles of POC (>500μm) are called marine snow,…”
Line 47: quite painful to see all this extensive, creative and quantitative science attributed to a single review paper.. please use original studies on top.
Rate = speed ?
Lines 53-55: un-worthy repetition. Just say in situ imagery have been proved a useful tool to measure sinking speeds. Period.
Line 57: Organic carbon. Migration pump out of topic.
Line 75: I would rather use “interact” rather than “coupled”
Line 125: “various biogenic matter such as POC (Particulate Organic124 Carbon), marine snow and fecal pellets” weird phrasing… fecal pellets and marine snow is not POC ? To me all this is organic particulate detritus. Therefore it’s all POC. I would just say “organic/biogenic detritus” or just “POC”. This is confusing.
Line 137. This is just a comment. Nothing to answer here. I acknowledge that this instrument, the UVP, is a valuable imagery system. But at this extreme level of post-processing, and classification, one could however wonder if the data is still data. But it seems to work !
Line 218: convoluted ! Please use something more straightforward like: “Due to intense mesoscale activity, water masses exhibit intense spatio-temporal variability in the studied area”
Line 258: “evidenced by”
Line 259-260: try using a more direct formulation. E.g.: “Also, a noticeable increase in bbsr indicates greater concentrations of small… ”
Line 290: No need for “indeed” here.
Line 298: Rvmax is never defined… at least I did not find it.
Line 305. Please stick to “export event”, export column sounds odd. Or say “the high abundance of particulate carbon in the water column…associated with the three export events”
Line 347: “which can be used as a proxy of water masses convergence (i.e. frontal zones)” this should have been explained way before ! Same in Line 359.
Line 348: “consistent with the model analysis proposed by.. ” you have to explain a bit better than this, or remove. What is the take-home message of this paragraph ? you suggest it is detritus from phytoplankton plus coagulation due to physical convergence ? It should be stated clearly.
Line 431: Statement poorly supported by results. turbulence ? It’s the first time that it is used, you cannot drop that like this in the conclusion !! plus specify vertical or horizontal. I guess you want to say horizontal trapping (convergence) here ? This should be better assessed with lagrangian chl-a. Also very annoying that you are concluding about primary production although you just provide an indicator for biomass (chl-a). Also coagulation/aggregation processes are purely speculative. Or prove me wrong.
Line 435: abusive use of “coupling” throughout the text. Coupling is connoted. I have nothing against but would rather use “interaction” or so.
Figure 8: MLD is constant throughout eddies really ? Nice but very speculative… the isopycnal slope is not very well demonstrated… I would try to provide at least more evidence of this (knowing that vertical velocities are out of reach). You can always provide some more detailed analysis in the supplement.
Line 447 : Exactly why I would have expected some quantitative numbers here.
Citation: https://doi.org/10.5194/egusphere-2024-1558-RC2
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