How is particulate organic carbon transported through the river-fed Congo Submarine Canyon to the deep-sea?

Hage, Sophie; Baker, Megan L.; Babonneau, Nathalie; Soulet, Guillaume; Dennielou, Bernard; Silva Jacinto, Ricardo; Hilton, Robert G.; Galy, Valier; Baudin, François; Rabouille, Christophe; Vic, Clément; Sahin, Sefa; Açikalin, Sanem; Talling, Peter J.

doi:https://doi.org/10.5194/egusphere-2024-900

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https://doi.org/10.5194/egusphere-2024-900

Preprints

04 Apr 2024

| 04 Apr 2024

How is particulate organic carbon transported through the river-fed Congo Submarine Canyon to the deep-sea?

Sophie Hage, Megan L. Baker, Nathalie Babonneau, Guillaume Soulet, Bernard Dennielou, Ricardo Silva Jacinto, Robert G. Hilton, Valier Galy, François Baudin, Christophe Rabouille, Clément Vic, Sefa Sahin, Sanem Açikalin, and Peter J. Talling

Abstract. The transfer of carbon from land to the near-coastal ocean is increasingly being recognized in global carbon budgets. However, a more direct transfer of terrestrial carbon to the deep-sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the Congo Submarine Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers ~7 % of the total organic carbon from the world’s rivers. However, carbon and sediment transport mechanisms that operate in the Congo Canyon, and submarine canyons more globally, are poorly constrained compared to rivers because monitoring of deep-sea canyons remains challenging. Using a novel array of acoustic instruments, sediment traps and cores, this study seeks to understand the hydrodynamic processes that control delivery of particulate organic carbon via the Congo Submarine Canyon to the deep-sea. We show that particulate organic carbon transport in the canyon-axis is modulated by two processes. First, we observe periods where the canyon dynamics are dominated by tides, which induce a background oscillatory flow (speeds of up to 0.15 m/s) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 m/s) turbidity currents occur for 35 % of the time during monitoring periods and transport both muddy and sandy particulate organic carbon at an estimated transit flux that is more than ten times the flux induced by tides. Remarkably, organic carbon transported and deposited in the submarine canyon has a similar isotopic composition to organic carbon in the Congo River, and in the deep-sea fan at 5 km of water depth. Episodic turbidity currents, together with background tidal currents thus promote efficient transfer of river-derived particulate organic carbon in the Congo Submarine Fan, leading to some of the highest terrestrial carbon preservation rates observed in marine sediments globally.

Received: 30 Mar 2024 – Discussion started: 04 Apr 2024

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Status: final response (author comments only)

RC1:
'Comment on egusphere-2024-900', Miquel Canals, 26 Apr 2024
This is an excellent paper by all means, based on a consistent data set resulting from 4 months of in situ monitoring. It proves how efficient, fast and recurrent the transfer of large amounts of river-sourced organic carbon can be. The magnitudes involved are particularly remarkable. So far, the Congo River-canyon-deep sea fan system constitutes a rather unique study case in the world’s ocean, which hopefully will pave the way for future research in other similar systems, both present and past.
Corrections required
The caption of Fig. 3 refers to “The yellow and black horizontal lines...” while there is no yellow horizontal line in the figure (but red).

References need cross-checking. Some references cited in the main text are missing in the References section (e.g. Canals et al., 2006; Ciais et al., 2013, Friedlingstein et al., 2022,….).
Citation: https://doi.org/10.5194/egusphere-2024-900-RC1
- AC1:
  'Reply on RC1', Sophie Hage, 06 Jun 2024
  RC1: 'Comment on egusphere-2024-900', Miquel Canals, 26 Apr 2024
  This is an excellent paper by all means, based on a consistent data set resulting from 4 months of in situ monitoring. It proves how efficient, fast and recurrent the transfer of large amounts of river-sourced organic carbon can be. The magnitudes involved are particularly remarkable. So far, the Congo River-canyon-deep sea fan system constitutes a rather unique study case in the world’s ocean, which hopefully will pave the way for future research in other similar systems, both present and past.
  AC1.1: We thank you for this positive feedback on our work.
  Corrections required
  The caption of Fig. 3 refers to “The yellow and black horizontal lines...” while there is no yellow horizontal line in the figure (but red).
  
  AC1.2: The caption will be changed to “The brown and black horizontal lines…”
  2. References need cross-checking. Some references cited in the main text are missing in the References section (e.g. Canals et al., 2006; Ciais et al., 2013, Friedlingstein et al., 2022,….).
  AC1.2: The references have been checked and will be complete in the updated version of our manuscript
  
  Citation: https://doi.org/10.5194/egusphere-2024-900-AC1
RC2:
'Comment on egusphere-2024-900', Pere Puig, 13 May 2024

Review of Hage et al., How is particulate organic carbon transported through the river-fed Congo Submarine Canyon to the deep-sea? by Pere Puig.
General comment:
This paper presents hydrodynamic and compositional data from moored oceanographic instrumentation and sediment cores to describe and characterize the carbon and sediment transport mechanisms that operate in the Congo Canyon. Even the sediment transport mechanisms in this submarine canyon has been thoroughly addressed in previous papers, particularly the role of turbidity currents, this contribution provides novel data and analyses from the particles collected by a moored sediment trap and from sediment cores, which combined with data from published literature, considerably improves the understanding of carbon transport in the Congo Submarine Canyon.
Overall, the paper is well written, but I found many restatements of the same idea that perhaps could be simply mention once, and also a mixing of contents in various sections. For instance, the Methods includes many details to previous works, which perhaps should be included in the Background section, without restating them, and there are methodological parts included in the Results section. The Methods also includes many references to Figures that are not strictly needed, since they should be introduced later in the manuscript, when it would be more appropriated to follow the reasoning of the paper.
Another general comment refers to the role of (internal) tides in the transport of particles. In several parts of the manuscript they are considered, along with the turbidity currents, as a sediment transport mechanism contributing to an efficient transfer of river-derived particulate organic carbon from the Congo River to the Congo submarine fan. However, if the residual tidal currents are directed up-canyon, they contribute to retain particles within the canyon interior, rather to promote their transfer towards the submarine fan. This concept should be clarified throughout the paper, as the same misleading statement is mentioned in several parts of the manuscript.
Specific comments:
L 54-56: I do not tend to suggest the citation of my own papers when I provide a review of a submitted manuscript, but in this case, I clearly miss here a reference to the review by Puig et al. 2024 (Annu. Rev. Mar. Sci. 2014. 6:53–77), which specifically address this aspect. The review not only includes the direct measurements during the past decades, but it goes over the published literature since the first instrumented record obtained in a submarine canyon.
L 61: I am not sure that Shepard et al. 1979 mentioned the mechanisms of cascading and upweling in their seminal book.
Figure 1A: Change the text "see B", next to the small square, by "Fig. 1B".
Subsection 3.1: This sub-section looks more like a Background section than a Methods section, as most of the presented data is already published. The Methods should simply describe, aseptically, the methodology used in the paper, so any potential reader could understand what has been done and replicate the same approach in other study areas.
L 173: There is no need to refer to Fig. 3 here, it would be better to introduce it in the Results section
L 175: Is this reference of Hamilton (1847) strictly needed? Besides, it is not included in the bibliography.
L 176-177: This sentence and the reference to Fig. 4 belongs to Results.
L 185: Are turbidity units correctly expressed, both here and in the axis of Fig. 5D? “m^-1” corresponds to units of beam attenuation coefficient (when a transmissometer connected to the CTD is used), but “sr^-1” is new to me. Besides, reporting two different units for the same variable is not common. This should be clarified.
L 233: Table 1 includes results. It should be mentioned for the first time in the Results section (when appropriated) and not here.
L 237: The same for Table 2.
L 254: The same for the references to Figs. 6 and 7.
L 261-262: The same for the references to Fig. 7 and Table 1.
L 275-280: This part of the manuscript is repetitive, and somehow irrelevant for a Results section. Most of this information (and references) could be included in a Background section and the Results section should start straight to the point, avoiding referring to previous works.
L 290: Change “yellow” by “red”
Figure 3B: Include legend identifying the line color code (similarly to Fig. 3C).
L 305-309: Needed?
L 318: It is unclear how a sediment trap can "measure" tides and turbidity currents. I assume you refer to their deposit: If so, please rephrase the sentence to avoid confusion.
L 323: Restated information in L 319. Decide where to include the reference of the height above the seabed, here or there, since both sentences are pretty close.
Figure 5D: Check the used turbidity units.
Figure 6: Overview of the data collected from the sediment trap… and the ADCP? Plot A does not belong to the sediment trap.
Figure 7: Descriptions form plot B is missing in the figure caption, and the text after B. corresponds to the content in plot C. The letter in bold used as reference previous to the text corresponding to plot D should be also included in the figure caption, since it is missing.
Subsection 4.3: This subsection (until L 389) belongs to Methods.
L 375: There is no need to refer to Fig. 8 in this methodological description.
L 405: The residual tidal flow goes up-canyon, and therefore, they retain particles in their transit from the River to the deep-sea fan, rather than transporting them.
L 423-424: You cannot assume that tides and turbidity currents have the same sediment concentration. As it is mentioned afterwards, turbidity currents are much more concentrated! So it is better not to compute and compare fluxes in this way, as it can induce confusion to the reader.
Figure 9: Very nice summary figure.
L 521-522: The list of submarine canyons included in the review by Puig et al (2014) includes more than few of them.
L 582: Up-canyon transport rates?
L584-586: Avoid using references in the Conclusions, as they should simply reflect the outputs of the paper. Such type of referencing should be included in the Discussion section.
Bibliography: There are missing references, it needs to be revised.
Sincerely,
Pere Puig

Citation: https://doi.org/10.5194/egusphere-2024-900-RC2
- AC2: 'Reply on RC2', Sophie Hage, 06 Jun 2024
  
  RC2: 'Comment on egusphere-2024-900', Pere Puig, 13 May 2024
  Review of Hage et al., How is particulate organic carbon transported through the river-fed Congo Submarine Canyon to the deep-sea? by Pere Puig.
  General comment:
  This paper presents hydrodynamic and compositional data from moored oceanographic instrumentation and sediment cores to describe and characterize the carbon and sediment transport mechanisms that operate in the Congo Canyon. Even the sediment transport mechanisms in this submarine canyon has been thoroughly addressed in previous papers, particularly the role of turbidity currents, this contribution provides novel data and analyses from the particles collected by a moored sediment trap and from sediment cores, which combined with data from published literature, considerably improves the understanding of carbon transport in the Congo Submarine Canyon.
  AC2.1: We thank you for this positive feedback on our work.
  Overall, the paper is well written, but I found many restatements of the same idea that perhaps could be simply mention once, and also a mixing of contents in various sections. For instance, the Methods includes many details to previous works, which perhaps should be included in the Background section, without restating them, and there are methodological parts included in the Results section. The Methods also includes many references to Figures that are not strictly needed, since they should be introduced later in the manuscript, when it would be more appropriated to follow the reasoning of the paper.
  AC2.2: We agree with you that some sentences could be deleted and/or streamlined so we will rephrase the manuscript accordingly. In particular, we will delete the first five sentences of sub-section 3.1 (Methods), and the first five sentences of sub-section 4.1 (Results). We will also remove all references to figures from the Methods and Conclusions sections. Finally, carbon flux calculations will be moved from Results to Methods.
  Another general comment refers to the role of (internal) tides in the transport of particles. In several parts of the manuscript they are considered, along with the turbidity currents, as a sediment transport mechanism contributing to an efficient transfer of river-derived particulate organic carbon from the Congo River to the Congo submarine fan. However, if the residual tidal currents are directed up-canyon, they contribute to retain particles within the canyon interior, rather to promote their transfer towards the submarine fan. This concept should be clarified throughout the paper, as the same misleading statement is mentioned in several parts of the manuscript.
  AC2.3: We agree with you that the role of tides could be better explained, and that the role of turbidity currents could be better highlighted as turbidity currents are the only process moving material strictly downslope to the deep-sea fan. The following sentences will be modified accordingly:
  Results sub-section 4.3: “Tides have a net residual flow that is oriented up-canyon retaining particles within the canyon before turbidity currents transport particles with a net transit direction oriented downstream towards the deep-sea.”
  Sub-section 5.1 summary as follows:
  “In summary, turbidity currents rapidly move sediment and particulate organic carbon downslope in one go at a transit flux that we estimate to be six times higher compared to the flux induced by tides. It then appears that the fine material flushed episodically by the top part of turbidity currents into the canyon is then kept in suspension and mixed in the canyon water column by tides until a new turbidity current transports particles to the deep-sea. The impact of these combined hydrodynamic processes on organic carbon composition (i.e., isotopic composition and Rock-Eval indexes) is further discussed in the next section.”
  Finally, we will add “up-canyon transport rates” to a conclusion sentence.
  Specific comments:
  L 54-56: I do not tend to suggest the citation of my own papers when I provide a review of a submitted manuscript, but in this case, I clearly miss here a reference to the review by Puig et al. 2024 (Annu. Rev. Mar. Sci. 2014. 6:53–77), which specifically address this aspect. The review not only includes the direct measurements during the past decades, but it goes over the published literature since the first instrumented record obtained in a submarine canyon.
  AC2.4: Thank you for suggesting this paper which is clearly relevant to our work. We will add a reference to the paper in the Introduction
  L 61: I am not sure that Shepard et al. 1979 mentioned the mechanisms of cascading and upwelling in their seminal book.
  AC2.5: We will remove Shepard et al. 1979 from the reference list.
  Figure 1A: Change the text "see B", next to the small square, by "Fig. 1B".
  AC2.5: “see B” will be replaced with “Fig. 1B” in Figure 1A.
  Subsection 3.1: This sub-section looks more like a Background section than a Methods section, as most of the presented data is already published. The Methods should simply describe, aseptically, the methodology used in the paper, so any potential reader could understand what has been done and replicate the same approach in other study areas.
  AC2.6: We agree with you that the first half of sub-section 3.1 (Methods) were not needed and will thus be deleted, particularly because this part is already stated in the Settings section (2.1).
  L 173: There is no need to refer to Fig. 3 here, it would be better to introduce it in the Results section
  AC2.7: Reference to Fig. 3 will be removed L173
  L 175: Is this reference of Hamilton (1847) strictly needed? Besides, it is not included in the bibliography.
  AC2.8: This citation will be removed from previous L175.
  L 176-177: This sentence and the reference to Fig. 4 belongs to Results.
  AC2.9: This sentence will be removed.
  L 185: Are turbidity units correctly expressed, both here and in the axis of Fig. 5D? “m^-1” corresponds to units of beam attenuation coefficient (when a transmissometer connected to the CTD is used), but “sr^-1” is new to me. Besides, reporting two different units for the same variable is not common. This should be clarified.
  AC2.10: Thanks for noticing this, the turbidity units will be corrected to m-1 both in the text and in Fig. 5
  L 233: Table 1 includes results. It should be mentioned for the first time in the Results section (when appropriated) and not here.
  AC2.11: Table 1 will be removed from L233 and will only be referred to in the Results section
  L 237: The same for Table 2.
  AC2.12: Table 2 will be removed from L237 and will only be referred to in the Results section
  L 254: The same for the references to Figs. 6 and 7.
  AC2.13: Figs. 6 and 7 will be removed from L254 and will only be referred to in the Results section.
  L 261-262: The same for the references to Fig. 7 and Table 1.
  AC2.14: Fig. 7 and Table 1 will be removed from L 261-262 and will only be referred to in the Results section
  L 275-280: This part of the manuscript is repetitive, and somehow irrelevant for a Results section. Most of this information (and references) could be included in a Background section and the Results section should start straight to the point, avoiding referring to previous works.
  AC2.15: We agree and we will thus remove L 275-279.
  L 290: Change “yellow” by “red”
  AC2.16: “yellow” will be replaced with “brown”
  Figure 3B: Include legend identifying the line color code (similarly to Fig. 3C).
  AC2.17: ”black line” and “brown line” will be added to the figure legend.
  L 305-309: Needed?
  AC2.18: We agree that these sentences were not strictly needed, so they will be removed.
  L 318: It is unclear how a sediment trap can "measure" tides and turbidity currents. I assume you refer to their deposit: If so, please rephrase the sentence to avoid confusion.
  AC2.19: This sentence will be modified as follows:
  “Sedimentary deposits associated with tides and turbidity currents measured by the ADCP are recorded in the sediment trap deployed just below the ADCP instrument”
  L 323: Restated information in L 319. Decide where to include the reference of the height above the seabed, here or there, since both sentences are pretty close.
  AC2.20: We will delete the first instance of this information from L323.
  Figure 5D: Check the used turbidity units.
  AC2.21: Turbidity units will be changed to m-1
  Figure 6: Overview of the data collected from the sediment trap… and the ADCP? Plot A does not belong to the sediment trap.
  AC2.22: We will add “from the ADCP and the sediment trap” to Figure 6 title.
  Figure 7: Descriptions form plot B is missing in the figure caption, and the text after B. corresponds to the content in plot C. The letter in bold used as reference previous to the text corresponding to plot D should be also included in the figure caption, since it is missing.
  AC2.23: Thanks for noticing those mistakes, the caption will be modified accordingly:
  “Measurements made on trap and seabed samples collected in the Congo Canyon and Channel, with previous data from Congo River and lobe. A. Total organic carbon content (TOC) against grain size D90 in micrometres. B. Relative 14C enrichment (relative to year of sample collection in 2019; F14R) against D90 in micrometres. C. Relative 14C enrichment (relative to year of sample collection in 2019; F14R) against carbon stable isotope ratios (δ13C). River data are taken from Spencer et al. (2012) and Hemingway et al. (2017) and are averaged between samples collected in one year (labelled 2008, 2011, 2012 and 2013). D. Hydrogen index against oxygen index derived from Rock-Eval pyrolysis/oxidation. Lobe data are taken from Baudin et al. (2017). “
  Subsection 4.3: This subsection (until L 389) belongs to Methods.
  AC2.24: This subsection will be moved to the Methods section (new sub-section 3.4)
  L 375: There is no need to refer to Fig. 8 in this methodological description.
  AC2.25: Reference to Fig. 8 will be removed from this description.
  L 405: The residual tidal flow goes up-canyon, and therefore, they retain particles in their transit from the River to the deep-sea fan, rather than transporting them.
  AC2.26: We will add a new sentence specifying this aspect to the Result section (4.3):
  “Tides have a net residual flow that is oriented up-canyon retaining particles within the canyon before turbidity currents transport particles with a net transit direction oriented downstream towards the deep-sea.”
  We will also replace the word “transport” with “fate” at L405
  Finally, we will rephrase our subsection 5.1 summary as follows:
  “In summary, turbidity currents rapidly move sediment and particulate organic carbon downslope in one go at a transit flux that we estimate to be ten times higher compared to the flux induced by tidesIt then appears that the fine material flushed episodically by the top part of turbidity currents into the canyon is then kept in suspension and mixed in the canyon water column by tides until a new turbidity current transports particles to the deep-sea. The impact of these combined hydrodynamic processes on organic carbon composition (i.e., isotopic composition and Rock-Eval indexes) is further discussed in the next section.”
  L 423-424: You cannot assume that tides and turbidity currents have the same sediment concentration. As it is mentioned afterwards, turbidity currents are much more concentrated! So it is better not to compute and compare fluxes in this way, as it can induce confusion to the reader.
  AC2.27: We agree and we will tone this down by deleting L420-424. We will state that turbidity currents have a transit flux at least 6 times higher compared to the flux induced by tides, based on our estimated fluxes from Fig. 8 only.
  Figure 9: Very nice summary figure.
  AC2.28: Thank you for this positive comment.
  L 521-522: The list of submarine canyons included in the review by Puig et al (2014) includes more than few of them.
  AC2.29: Thank you for this relevant suggestion, we will add a reference to Puig et al. (2014) at L521-522.
  L 582: Up-canyon transport rates?
  AC2.30: “up-canyon” will be added to previous L582.
  L584-586: Avoid using references in the Conclusions, as they should simply reflect the outputs of the paper. Such type of referencing should be included in the Discussion section.
  AC2.31: References will be removed from the conclusions.
  Bibliography: There are missing references, it needs to be revised.
  AC2.32: The reference list was checked and will be complete in the updated version of our manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2024-900-AC2
RC3:
'Comment on egusphere-2024-900', Lina Madaj, 10 Jun 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-900/egusphere-2024-900-RC3-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-900-RC3
- AC3: 'Reply on RC3', Sophie Hage, 25 Jun 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-900/egusphere-2024-900-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-900-AC3

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Short summary

Climate projections require to quantify the exchange of carbon between the atmosphere, land and oceans, yet the land-to-ocean flux of carbon is difficult to measure. Here, we quantify the carbon flux between the second largest river on Earth and the ocean. Carbon in the form of vegetation and soil is transported by episodic submarine avalanches in a 1000 km-long canyon at up to 5 km of water depth. The carbon flux induced by avalanches is at least ten times greater than that induced by tides.


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