Estimation of particulate organic carbon export to the ocean from lateral degradations of tropical peatland coasts

Kagawa, Hiroki; Yamamoto, Koichi; Sutikno, Sigit; Haidar, Muhammad; Basir, Noerdin; Koyama, Atsushi; Kanno, Ariyo; Akamatsu, Yoshihisa; Suzuki, Motoyuki

doi:10.5194/egusphere-2024-3547

Preprints

https://doi.org/10.5194/egusphere-2024-3547

Preprints

25 Nov 2024

| 25 Nov 2024

Estimation of particulate organic carbon export to the ocean from lateral degradations of tropical peatland coasts

Hiroki Kagawa, Koichi Yamamoto, Sigit Sutikno, Muhammad Haidar, Noerdin Basir, Atsushi Koyama, Ariyo Kanno, Yoshihisa Akamatsu, and Motoyuki Suzuki

Abstract. The amount of particulate organic carbon (POC) export to the ocean due to coastal erosion and peat mass movement events on Bengkalis Island, Indonesia, was estimated. The annual flux of POC to the ocean due to coastal erosion along the research area of Bengkalis Island was estimated to be in the range of 2.06 to 7.60 tC m⁻¹ yr⁻¹. POC exports to the ocean by events of peat mass movement along the coast of the northern part of Bengkalis Island were estimated in a range of 1.43 to 5.41 tC m⁻¹, with an average increase of 2.23 tC m^-1 from 2010 to 2018. The estimation of the POC flux was carried out by combining the analysis of the peat soil and the estimation of the volume of exported peat using aerial photogrammetry and satellite imagery analysis. A linear relationship was found between the area affected by the landslide and the volume of the peat soil divided by area. Coastal erosion and peat mass movements occurred in a chain of events, confirming that peat from coastal areas was exported to the ocean. Annual export of POC from coastal erosion for 1 m was equivalent to annual carbon emissions from degraded peatlands of 0.41 to 1.52 hectares. The carbon export rate per metre from events of peat mass movement corresponds to carbon emissions produced over one year of 0.29 to 1.08 hectares of degraded peatlands. On a peatland coast with an average length of 3,152 metres, the amount of POC exported to the ocean due to events of peat mass movement was estimated to range from 4.45 to 17.1 ktC, while the POC exported due to coastal erosion was estimated to range from 6.35 to 23.9 ktC yr^-1. These lateral carbon exports on the tropical peatland coast indicate a new route of carbon export to the ocean, in addition to the common riverine discharge of organic carbon.

Received: 13 Nov 2024 – Discussion started: 25 Nov 2024

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Journal article(s) based on this preprint

27 Mar 2026

Estimation of particulate organic carbon export to the ocean from lateral degradations of tropical peatland coasts

Hiroki Kagawa, Koichi Yamamoto, Sigit Sutikno, Muhammad Haidar, Noerdin Basir, Atsushi Koyama, Ariyo Kanno, Yoshihisa Akamatsu, and Motoyuki Suzuki

Biogeosciences, 23, 2119–2154, https://doi.org/10.5194/bg-23-2119-2026,https://doi.org/10.5194/bg-23-2119-2026, 2026

Short summary

Hiroki Kagawa, Koichi Yamamoto, Sigit Sutikno, Muhammad Haidar, Noerdin Basir, Atsushi Koyama, Ariyo Kanno, Yoshihisa Akamatsu, and Motoyuki Suzuki

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3547', Anonymous Referee #1, 02 Jan 2025

Review of “Estimation of particulate organic carbon export to the ocean from
lateral degradations of tropical peatland coasts” by Kagawa et al. for consideration in EGUsphere.
Kagawa et al. estimate the amount of particulate organic carbon (POC) export to the ocean due to coastal erosion and peat mass movement events on Bengkalis Island, Indonesia, using aerial photogrammetry and satellite imagery analysis. The topic of this study is interesting and important. Nonetheless, I have three major concerns on the current manuscript.
First, to my understanding, this study is more like a study of remote sensing or GIS, rather than a biogeochemical study. The major works involved in this study is about feature (e.g. vegetation, and topography) recognition based on UVA and satellite images. Few biogeochemical analysis has been involved or revealed in this study. Maybe a journal of remote sensing is more suitable to this manuscript.
Second, I am a bit worrying about the novelty of this study. The findings in this study depends strongly on the specific conditions of topography, vegetation, climate, tide and coastal wave. I don’t think the POC loss rates due to coastal erosion at the current study site can be used as a reference for estimating the coastal POC loss rates in other places. So I am wondering whether this study has provided a vital or reliable implication for understanding global land-ocean POC fluxes. By the way, the authors should give a better discussion on the implications of this study.
Third, an analysis on the environmental controls (land use change, climate change, see level rise?) of the interannual variation of peat mass movement and the POC export from land to the ocean is important to improve the novelty of this study, and will make this study better fit the scope of Biogeosciences. Unfortunately, I have not seen any analysis on the drivers of the peat mass movement and the POC loss.
Specific comments:
The Introduction section has not been organized well. The authors using a lot words to describe the importance and formation of peatland, however, the introduction on coastal erosion, in particular the coastal erosion of peat, is very weak. Moreover, the specific aims of this study should be provided in the last paragraph of the Introduction section.
Fig. 5: The current figure caption is lengthy. A figure caption should be like “Flowchart used in this study to *****”
L140-144: Why not include more satellite in different times? Is there any Google Earth image or satellite images for recent years after 2018?
Fig. 12: What are the P01-P04 represent? Are they soil cores from different locations of the study area? Please provide a map of the soil collection sites.
Fig. 16: Why the unit of POC export rate per unit length is tC m-1, rather than tc m-1 yr-1?

Citation: https://doi.org/10.5194/egusphere-2024-3547-RC1
- AC1: 'Reply on RC1', Hiroki Kagawa, 06 Jan 2025
  
  Dear RC1,
  Thank you very much for taking the time to review our manuscript and for providing insightful and constructive comments.
  
  We greatly appreciate your valuable feedback, which will undoubtedly help improve the quality of our work.
  We are currently addressing your comments in detail and will provide a revised version of the manuscript along with a point-by-point response to each of your suggestions in due course.
  We truly value your guidance and expertise in this process.
  Thank you again for taking your time and effort for us.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC1
- AC3: 'Reply on RC1', Hiroki Kagawa, 04 Feb 2025
  
  Dear RC1,
  Thank you very much for taking the time to review our manuscript and for providing insightful and constructive comments. We greatly appreciate your valuable feedback, which will undoubtedly help us improve the quality of our work.
  We are currently in the process of addressing your comments and preparing a point-by-point revision plan. We will submit our responses along with specific revision proposals for each of your suggestions in due course.
  We truly value your guidance and expertise throughout this process. Thank you again for your time and effort.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC3
RC2:
'Comment on egusphere-2024-3547', Anonymous Referee #2, 23 Jan 2025
Kagawa et al. estimated the POC flux due to coastal erosion and peat mass movement events on Benglakis Island by integrating peat soil analysis, satellite imagery, and aerial photography. This study quantitatively estimates carbon fluxes driven by specific landscape evolution processes in organic carbon-rich terrains, offering valuable insights into Earth’s carbon cycle dynamics.
The manuscript attempts to balance the spatial analysis of coastal erosion with the importance of POC fluxes generated through the erosion of coastal peatlands. However, its current structure is fragmented. The Introduction emphasizes organic carbon export, while the Methods, Results, and Discussion focus primarily on the techniques and findings related to eroded areas and volumes. This disconnect results in a lack of coherent structure and clear focus, making it difficult to identify the manuscript’s central message. Furthermore, the discussion does not sufficiently explore the significance of coastal erosion in the context of the carbon cycle.
My comments are as follows:
Abstract:

I recommend rephrasing the abstract to follow a logical flow: background, objective, methods, results, and conclusions. Currently, the abstract jumps directly into specific findings without providing sufficient context or framing the significance of the study.
The unit abbreviations in the abstract are inconsistent and confusing. Please define all units clearly and maintain consistent terminology throughout.
Introduction:
Major concern:
While the introduction focuses on the important role of peatland erosion carbon cycling, the paper seems to be centered on using spatial data and machine learning to get barren land area estimation and land displacement due to erosion. I suppose it would be better you emphasized more on coastal erosion in the introduction section.

The introduction generally talks about land erosion in carbon cycles, however, the object of this study is the lateral exported OC from peatland through coastal erosion. I suppose it would be better to emphasize more on the role of coastal erosion in carbon cycling, and the possible fate of these carbon in marine environment. I would also recommend authors emphasize the importance of peatland as it has been done in the introduction.

The Introduction is not well structured, I recommend authors improve the logical connections between each paragraph, and each paragraph should have a topic sentence or idea to follow.

Detailed comments:
Line 29-42: The authors provide some numbers of OC stock of peatland, I would recommend making a comparison to global soil stock to emphasize OC stock in peatland is important. Besides, the net radiative force seems not to be useful information in this context.
Line 47: Does peat fire a major concern of your study? I would delete irrelevant information.
Line 49: Is the number reported by Ludwig et al., 1996 consistent with the number reported by Galy et al., 2015? The units are switching between the global and local studies, no meaningful comparison was made to emphasize the importance of local export of POC from land.
Fig. 2 Does the ocean’s water level always remain lower than the peaty debris fan? Based on Figure 1, seawater appears to reach the peat layer. Please clarify this.
Methods:
Major concern:
While the methods section provides detailed information, its length can mislead readers into thinking the paper focuses on land classification and edge detection techniques rather than POC flux estimation. I suggest moving detailed methodological descriptions to the supplementary materials to improve focus.

The image data used in this study come from sources with varying resolutions. Since accurate volume estimation is critical, assessing the uncertainty associated with the methods is essential. Although the authors conducted an uncertainty analysis for data with differing resolutions, have they cross-compared data from the same time window but with different resolutions? This could help validate the accuracy of the method.

Detailed comments:
Please combine Fig. 3 and Fig. 4.
Please associate Table 1 with Fig. 5, such as writing in the captions.
Results and discussion
Major concerns:
This section primarily presents results rather than engaging in insightful discussion, it puts a lot of effort into presenting the results on land evolution and estimating the eroded volume. It briefly touches on POC export to the ocean through coastal erosion in Section 4.4 but lacks a deeper discussion.

In Section 4.4, there is insufficient comparison to meaningful reference values that could emphasize the significance of the rate or amount of organic carbon (OC) exported via erosion. Additionally, whether these localized findings have broader implications for peatland coastal erosion globally is unclear. Another important consideration is the timescale: how do they align when comparing CO₂ emissions from peatland degradation to exported OC? It is also important to note that peat-derived OC may oxidize during transport before being buried on the ocean floor. This oxidation could lead to additional CO₂ release into the atmosphere, meaning that exported OC is not equivalent to the amount ultimately buried. My suggestion is to focus on comparisons with data from other regions around the globe to help readers understand the significance of the reported values. Following this, discuss the potential fate of the exported OC and its role in the global carbon cycle.

Fig. 12 Please use color-coded axis labels for different data.
Line 476-477: This statement is not right, it primarily depends on the sediment accumulation rate.
R²
Citation: https://doi.org/10.5194/egusphere-2024-3547-RC2
- AC2: 'Reply on RC2', Hiroki Kagawa, 24 Jan 2025
  
  Dear RC2,
  
  Thank you very much for taking the time to thoroughly review our manuscript and for providing us with your insightful and constructive feedback.
  
  We are especially grateful for your detailed comments, which have highlighted key weaknesses and areas for improvement in our work.
  
  Your observations and suggestions have provided us with valuable guidance to refine the structure, focus, and clarity of our manuscript.
  
  We are currently working diligently to address each of your comments in detail.
  
  A revised version of the manuscript, along with a point-by-point response to your suggestions, will be submitted in due course.
  
  Your expertise and thoughtful critique have been instrumental in helping us strengthen our study, and we truly value the time and effort you have dedicated to this review process.
  
  Thank you again for your support and guidance.
  
  Kind regards,
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC2
- AC4: 'Reply on RC2', Hiroki Kagawa, 04 Feb 2025
  
  Dear RC2,
  Thank you very much for taking the time to thoroughly review our manuscript and for providing us with your insightful and constructive feedback.
  We are especially grateful for your detailed comments, which have highlighted key weaknesses and areas for improvement in our work. Your observations and suggestions have provided us with valuable guidance to refine the structure, focus, and clarity of our manuscript.
  In response to your feedback, we have carefully reviewed each of your comments and prepared a point-by-point reply along with our own revision plan. Please find attached our comments and proposed revisions based on our approach to addressing the issues raised.
  At this stage, we are presenting our planned revisions rather than a revised version of the manuscript. We would greatly appreciate your feedback on our proposed changes before proceeding with the final revision.
  Your expertise and thoughtful critique have been instrumental in helping us strengthen our study, and we truly value the time and effort you have dedicated to this review process.
  Thank you again for your support and guidance.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC4

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3547', Anonymous Referee #1, 02 Jan 2025

Review of “Estimation of particulate organic carbon export to the ocean from
lateral degradations of tropical peatland coasts” by Kagawa et al. for consideration in EGUsphere.
Kagawa et al. estimate the amount of particulate organic carbon (POC) export to the ocean due to coastal erosion and peat mass movement events on Bengkalis Island, Indonesia, using aerial photogrammetry and satellite imagery analysis. The topic of this study is interesting and important. Nonetheless, I have three major concerns on the current manuscript.
First, to my understanding, this study is more like a study of remote sensing or GIS, rather than a biogeochemical study. The major works involved in this study is about feature (e.g. vegetation, and topography) recognition based on UVA and satellite images. Few biogeochemical analysis has been involved or revealed in this study. Maybe a journal of remote sensing is more suitable to this manuscript.
Second, I am a bit worrying about the novelty of this study. The findings in this study depends strongly on the specific conditions of topography, vegetation, climate, tide and coastal wave. I don’t think the POC loss rates due to coastal erosion at the current study site can be used as a reference for estimating the coastal POC loss rates in other places. So I am wondering whether this study has provided a vital or reliable implication for understanding global land-ocean POC fluxes. By the way, the authors should give a better discussion on the implications of this study.
Third, an analysis on the environmental controls (land use change, climate change, see level rise?) of the interannual variation of peat mass movement and the POC export from land to the ocean is important to improve the novelty of this study, and will make this study better fit the scope of Biogeosciences. Unfortunately, I have not seen any analysis on the drivers of the peat mass movement and the POC loss.
Specific comments:
The Introduction section has not been organized well. The authors using a lot words to describe the importance and formation of peatland, however, the introduction on coastal erosion, in particular the coastal erosion of peat, is very weak. Moreover, the specific aims of this study should be provided in the last paragraph of the Introduction section.
Fig. 5: The current figure caption is lengthy. A figure caption should be like “Flowchart used in this study to *****”
L140-144: Why not include more satellite in different times? Is there any Google Earth image or satellite images for recent years after 2018?
Fig. 12: What are the P01-P04 represent? Are they soil cores from different locations of the study area? Please provide a map of the soil collection sites.
Fig. 16: Why the unit of POC export rate per unit length is tC m-1, rather than tc m-1 yr-1?

Citation: https://doi.org/10.5194/egusphere-2024-3547-RC1
- AC1: 'Reply on RC1', Hiroki Kagawa, 06 Jan 2025
  
  Dear RC1,
  Thank you very much for taking the time to review our manuscript and for providing insightful and constructive comments.
  
  We greatly appreciate your valuable feedback, which will undoubtedly help improve the quality of our work.
  We are currently addressing your comments in detail and will provide a revised version of the manuscript along with a point-by-point response to each of your suggestions in due course.
  We truly value your guidance and expertise in this process.
  Thank you again for taking your time and effort for us.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC1
- AC3: 'Reply on RC1', Hiroki Kagawa, 04 Feb 2025
  
  Dear RC1,
  Thank you very much for taking the time to review our manuscript and for providing insightful and constructive comments. We greatly appreciate your valuable feedback, which will undoubtedly help us improve the quality of our work.
  We are currently in the process of addressing your comments and preparing a point-by-point revision plan. We will submit our responses along with specific revision proposals for each of your suggestions in due course.
  We truly value your guidance and expertise throughout this process. Thank you again for your time and effort.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC3
RC2:
'Comment on egusphere-2024-3547', Anonymous Referee #2, 23 Jan 2025
Kagawa et al. estimated the POC flux due to coastal erosion and peat mass movement events on Benglakis Island by integrating peat soil analysis, satellite imagery, and aerial photography. This study quantitatively estimates carbon fluxes driven by specific landscape evolution processes in organic carbon-rich terrains, offering valuable insights into Earth’s carbon cycle dynamics.
The manuscript attempts to balance the spatial analysis of coastal erosion with the importance of POC fluxes generated through the erosion of coastal peatlands. However, its current structure is fragmented. The Introduction emphasizes organic carbon export, while the Methods, Results, and Discussion focus primarily on the techniques and findings related to eroded areas and volumes. This disconnect results in a lack of coherent structure and clear focus, making it difficult to identify the manuscript’s central message. Furthermore, the discussion does not sufficiently explore the significance of coastal erosion in the context of the carbon cycle.
My comments are as follows:
Abstract:

I recommend rephrasing the abstract to follow a logical flow: background, objective, methods, results, and conclusions. Currently, the abstract jumps directly into specific findings without providing sufficient context or framing the significance of the study.
The unit abbreviations in the abstract are inconsistent and confusing. Please define all units clearly and maintain consistent terminology throughout.
Introduction:
Major concern:
While the introduction focuses on the important role of peatland erosion carbon cycling, the paper seems to be centered on using spatial data and machine learning to get barren land area estimation and land displacement due to erosion. I suppose it would be better you emphasized more on coastal erosion in the introduction section.

The introduction generally talks about land erosion in carbon cycles, however, the object of this study is the lateral exported OC from peatland through coastal erosion. I suppose it would be better to emphasize more on the role of coastal erosion in carbon cycling, and the possible fate of these carbon in marine environment. I would also recommend authors emphasize the importance of peatland as it has been done in the introduction.

The Introduction is not well structured, I recommend authors improve the logical connections between each paragraph, and each paragraph should have a topic sentence or idea to follow.

Detailed comments:
Line 29-42: The authors provide some numbers of OC stock of peatland, I would recommend making a comparison to global soil stock to emphasize OC stock in peatland is important. Besides, the net radiative force seems not to be useful information in this context.
Line 47: Does peat fire a major concern of your study? I would delete irrelevant information.
Line 49: Is the number reported by Ludwig et al., 1996 consistent with the number reported by Galy et al., 2015? The units are switching between the global and local studies, no meaningful comparison was made to emphasize the importance of local export of POC from land.
Fig. 2 Does the ocean’s water level always remain lower than the peaty debris fan? Based on Figure 1, seawater appears to reach the peat layer. Please clarify this.
Methods:
Major concern:
While the methods section provides detailed information, its length can mislead readers into thinking the paper focuses on land classification and edge detection techniques rather than POC flux estimation. I suggest moving detailed methodological descriptions to the supplementary materials to improve focus.

The image data used in this study come from sources with varying resolutions. Since accurate volume estimation is critical, assessing the uncertainty associated with the methods is essential. Although the authors conducted an uncertainty analysis for data with differing resolutions, have they cross-compared data from the same time window but with different resolutions? This could help validate the accuracy of the method.

Detailed comments:
Please combine Fig. 3 and Fig. 4.
Please associate Table 1 with Fig. 5, such as writing in the captions.
Results and discussion
Major concerns:
This section primarily presents results rather than engaging in insightful discussion, it puts a lot of effort into presenting the results on land evolution and estimating the eroded volume. It briefly touches on POC export to the ocean through coastal erosion in Section 4.4 but lacks a deeper discussion.

In Section 4.4, there is insufficient comparison to meaningful reference values that could emphasize the significance of the rate or amount of organic carbon (OC) exported via erosion. Additionally, whether these localized findings have broader implications for peatland coastal erosion globally is unclear. Another important consideration is the timescale: how do they align when comparing CO₂ emissions from peatland degradation to exported OC? It is also important to note that peat-derived OC may oxidize during transport before being buried on the ocean floor. This oxidation could lead to additional CO₂ release into the atmosphere, meaning that exported OC is not equivalent to the amount ultimately buried. My suggestion is to focus on comparisons with data from other regions around the globe to help readers understand the significance of the reported values. Following this, discuss the potential fate of the exported OC and its role in the global carbon cycle.

Fig. 12 Please use color-coded axis labels for different data.
Line 476-477: This statement is not right, it primarily depends on the sediment accumulation rate.
R²
Citation: https://doi.org/10.5194/egusphere-2024-3547-RC2
- AC2: 'Reply on RC2', Hiroki Kagawa, 24 Jan 2025
  
  Dear RC2,
  
  Thank you very much for taking the time to thoroughly review our manuscript and for providing us with your insightful and constructive feedback.
  
  We are especially grateful for your detailed comments, which have highlighted key weaknesses and areas for improvement in our work.
  
  Your observations and suggestions have provided us with valuable guidance to refine the structure, focus, and clarity of our manuscript.
  
  We are currently working diligently to address each of your comments in detail.
  
  A revised version of the manuscript, along with a point-by-point response to your suggestions, will be submitted in due course.
  
  Your expertise and thoughtful critique have been instrumental in helping us strengthen our study, and we truly value the time and effort you have dedicated to this review process.
  
  Thank you again for your support and guidance.
  
  Kind regards,
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC2
- AC4: 'Reply on RC2', Hiroki Kagawa, 04 Feb 2025
  
  Dear RC2,
  Thank you very much for taking the time to thoroughly review our manuscript and for providing us with your insightful and constructive feedback.
  We are especially grateful for your detailed comments, which have highlighted key weaknesses and areas for improvement in our work. Your observations and suggestions have provided us with valuable guidance to refine the structure, focus, and clarity of our manuscript.
  In response to your feedback, we have carefully reviewed each of your comments and prepared a point-by-point reply along with our own revision plan. Please find attached our comments and proposed revisions based on our approach to addressing the issues raised.
  At this stage, we are presenting our planned revisions rather than a revised version of the manuscript. We would greatly appreciate your feedback on our proposed changes before proceeding with the final revision.
  Your expertise and thoughtful critique have been instrumental in helping us strengthen our study, and we truly value the time and effort you have dedicated to this review process.
  Thank you again for your support and guidance.
  Kind regards,
  
  Hiroki Kagawa
  
  Yamaguchi University
  
  Citation: https://doi.org/10.5194/egusphere-2024-3547-AC4

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (05 Feb 2025) by Olivier Sulpis

AR by Hiroki Kagawa on behalf of the Authors (12 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (31 Mar 2025) by Olivier Sulpis

RR by Anonymous Referee #1 (02 Apr 2025)

RR by Anonymous Referee #2 (23 May 2025)

Suggestions for revision or reasons for rejection

The authors have clearly made considerable efforts to improve this manuscript, and their revisions are appreciated. However, the overall length remains excessive, and the manuscript is still challenging to follow due to structural and thematic imbalance.
As previously noted, the title, abstract, introduction, and conclusion frame the study around organic carbon fluxes associated with peatland degradation, particularly in the context of coastal erosion and peat mass movement events (PMMs). While this framing is appropriate and compelling, the core sections of the manuscript — particularly the Methods, Results, and Discussion — remain disproportionately focused on geomorphic analysis and technical methodology, with relatively limited engagement in the biogeochemical implications of carbon flux and feedback.
The Discussion section lacks depth in interpreting the carbon-related findings. It remains largely descriptive and does not sufficiently explore the implications of particulate organic carbon (POC) export for regional or global carbon budgets, nor does it address the fate of exported carbon in marine systems — both of which are key to supporting the stated objectives of the study.
The manuscript appears to incorporate two primary themes:
1. Coastal erosion and geomorphic processes [focusing on remote sensing methods];
2. Carbon flux and biogeochemical feedback [focusing on fluxes and potential feedback].
While both are relevant and scientifically valuable, the current version does not balance or integrate them effectively. If the authors intend to retain the current framing around carbon fluxes, I would strongly recommend shifting some of the detailed geomorphic and remote sensing methodology to the Supplementary Information. This would create space in the main text for a more substantive discussion on carbon processes, transport pathways, uncertainties, and potential climatic implications.
Focusing the main text more sharply on carbon dynamics would greatly improve the manuscript’s coherence and ensure that it delivers on the promises made in the abstract and introduction.

Major comments
Introduction:
This section has been extended from 6 paragraphs to 15 paragraphs; I appreciate authors’ effort demonstrating a strong command of the literature and addresses a novel and timely topic—lateral carbon export from tropical peatlands due to coastal erosion and peat mass movements (PMMs). However, it is overly long and reads more like a review article than a focused opening to a research paper. The structure lacks a clear narrative flow, with the main research problem and objective introduced too late. Numerous detailed global case studies, while informative, are not well-integrated into the tropical context and could be more effectively summarized. To improve clarity and impact, the introduction should be significantly shortened, the research gap and objectives stated earlier, and the content reorganized to highlight the study’s relevance in a more concise, reader-friendly manner.
I break your introduction into 6 sections: paragraph 1-2; 3 and 11-12; 4-6 and 9-10; 7-8; 13-14; 15. I provide my comments for these sections as below:
Paragraphs 2: Authors are focusing on tropical peatlands, especially coastal types, their formation, and distribution in Southeast Asia. However, it has a sudden switch from formation history to carbon loss due to fire and drainage without a transition. What is the reason blending these two points in one paragraph.
Paragraphs 3 and 11-12: These paragraphs introduce key physical degradation processes—coastal erosion and peat mass movement events (PMMs)—and articulate the study’s research gap and objective. Paragraphs 11 and 12 emphasize the scarcity of tropical PMM research. Paragraph 14 and 15 highlight the implications for carbon export and present the study aim. Can authors focus on introducing PMMs and erosion, and emphasizing the research gap with concluding the study’s objective?
Paragraphs 4-6 and 9-10: It is valuable for establishing a precedent and broader relevance. However, the level of detail is disproportionate relative to the study’s tropical focus. I would recommend authors condense these paragraphs into a single, comparative overview, and focus on drawing contrasts relevant to tropical PMMs and erosion. I would also suggest authors make sure each example is explicitly linked to the central theme-the poorly constrained role of lateral degradation in tropical systems.
Paragraphs 7-8: I would recommend integrate only those aspects of coastal wetland vulnerability that directly relate to peatland systems or carbon loss via erosion. Please remove or significantly truncate the Amazon research review unless framed as an example of regional under representation in peatland degradation studies.
Paragraph 13-14: I would suggest authors to reframe the discussion to just emphasize the hypothesis that tropical peat erosion and PMMs represent a potentially significant and under-quantified pathway of terrestrial carbon export to the ocean.
Detailed comments
Line 77: I recommend rephrase, authors can describe the global distribution of peatlands and then just refer to Fig.1. For example: (see Fig. 1).
Line 80-84: The global area statistics can be simplified, by the way, can you provide a comparison with an area general reader know the size, so it will be more intuitive to understand the large numbers. I see you are introducing the fraction to Earth’s total land, I think it is not necessary to introduce Earth’s land area, besides, you are using a different unit.
I would also recommend authors to rephrase this section, it repeated a lot of information that is already provided in the previous sentence.
Line 85-86: Rephrase is recommended for clearly introducing the two classifications.
Line 89: (kyBP) is not the right unit here.

Figures:
Figure 1: Caption did not explain the squares in different color and labels, and these labelled panels seem not be introduced in the Introduction section. Please add details.

I did not provide detailed comments for other sections due to the overall assessment.

Hide

ED: Reconsider after major revisions (26 May 2025) by Olivier Sulpis

AR by Hiroki Kagawa on behalf of the Authors (30 Jun 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Jul 2025) by Steven Bouillon

RR by Anonymous Referee #2 (11 Aug 2025)

Suggestions for revision or reasons for rejection

Thank you for the authors’ efforts in revising the paper; it has been greatly improved. I have the following concerns that should be addressed:
Line 18: I noticed that the authors use ha as the land area unit throughout the manuscript. I recommend converting this to the International System of Units (SI), especially since volumes are reported in m³.
Line 57: This paragraph should be combined with the preceding paragraph.
Line 61: Please add references.
Table 1: This table should be moved to the Supplement, as it is not well explained in the manuscript. The authors should then rephrase lines 46–48 accordingly.
Materials and Methods section (pages 8–20): I recommend moving parts of the Materials and Methods to the Supplementary Material. The main text could contain a concise summary of the Materials used in this study, with detailed descriptions placed in the Supplement. For example, sections 3.1.1 to 3.1.5 could be moved, along with their associated tables and figures, to avoid providing unnecessary detail in the main manuscript.
Line 587: Please rephrase. I believe the intent is to point out that the environment becomes more anoxic with depth due to the saturation of seawater. Following depth from the surface downward, conditions become progressively more anoxic. Please also find a suitable reference to support this statement. I also have difficulty following the authors’ point here: according to Figure 16, OC concentration generally decreases below ~1 m depth, but OC is still present, so the cumulative OC stock should increase. Please clarify what is meant by “cumulative OC content,” as this is not reflected in the figure. In profile P02, OC concentration increases at 2 m depth, yet dry density also increases at this depth; thus, this cannot be explained solely by oxic conditions associated with water presence. The compression of soils is not reflected in other depths. Given that the paper focuses on OC stock and lateral flux rather than mechanisms of OC preservation, I recommend removing the explanation of possible mechanisms for OC variation with depth.
Statistical validation: For the regressions presented in the manuscript, please provide statistical validation measures, such as t-tests.
Line 624: Please delete “TOM.”
Line 647: Please delete “blue carbon.”
Lines 624–632: I do not recommend introducing DOC or comparing the fate of POC and DOC, as no data are presented to support this. Furthermore, the discussion does not sufficiently explore this aspect. Please focus instead on discussing the possible fate of POC, considering both source and sink processes.
Section 4.6: I recommend expanding the discussion of flux and comparing the estimated values to other processes. The use of OC yield is valuable, as it suggests that eroding peatlands have a high potential to release large amounts of OC. However, it is also important to present flux values in absolute terms (e.g., kg C yr⁻¹).

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ED: Reconsider after major revisions (14 Oct 2025) by Steven Bouillon

AR by Hiroki Kagawa on behalf of the Authors (18 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (03 Jan 2026) by Steven Bouillon

Dear Hiroki and co-authors,

Apologies for the delay in reaching a decision on your revised version.

I have now gone through your author replies and revised version. Based on this, I feel sufficient progress has been made in clarifying aspects raised in the last review, and that the current version is overall sufficiently transparent to publish your manuscript in BG, pending some minor revisions detailed below.
Please reach out to me directly if you have any questions on the points listed below.

Best regards
Steven Bouillon

Introduction: the estimates of global carbon stocks in tropical peatlands refer to literature from some time ago, there are several studies with updated estimates. See for example:
Dargie et al. (2017); https://www.nature.com/articles/nature21048
Ribeiro et al. (2020), https://doi.org/10.1111/gcb.15408
I would recommend to include some more recent estimates and references.

In response to one of the review comments, you have now replaced 'cumulative OC content' and 'carbon stock' with the term 'soil organic carbon density' - but this is in my interpretation ambiguous and not appropriate.
Carbon density is a term regularly used in the literature, but refers to a quantity of carbon per unit of volume, whereas you use it to describe a quantity per unit of surface area (i.e. a stock), which will make things more confusing.
You mention also in your RC2-Response6 that you would change "carbon concentration" to "carbon content per unit volume" - but this is exactly what the definition of carbon density is.
Hence, please use correct terminology. Figure 18 for example represents carbon stocks and not carbon densities.
This also needs to be corrected in Table 1, where you mention SOCD is the carbon stock as a function of depth. Stocks are expressed per unit of surface area, densities are expressed per unit of volume; hence this definition is confusing. Check throughout the text, Tables (e.g. Table 5 and 6), and Figures where the term SOCD is currently used.

Regarding Table 1, I feel that adding units for each of the parameters or terms would clarify things a lot.

Reconsider whether all Figures are really necessary in the main text. Figure 14 is a good example: while these data are relevant as background information, they are only briefly referred to (L499) and not really discussed further. These can be moved towards the supplementary information.

Figure 7: A figure without a black background would be more suitable. Please also make this Figure more accessible in terms of color use. See guidelines here:
https://www.biogeosciences.net/submission.html#figurestables
The color gradient used in Figure 7 renders it difficult or impossible to read for people with color vision deficiencies. The link on the BG website provides some links to resources that can resolve this.

Figure 9: readability would be improved by replacing the red & blue with e.g. open and closed symbols, and/or using different line types for the A1 and A2 data series.

Figure 12: here too, see suggestions on improving accessibility by using alternative colors and/or different line types, in particular for the lower panel of this Figure.

Figure 17, 18, 20, F2, F3, I1: same suggestions as above, please review accessibility of this Figure in terms of color use and consider alternative ways to improve.

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AR by Hiroki Kagawa on behalf of the Authors (16 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Jan 2026) by Steven Bouillon

AR by Hiroki Kagawa on behalf of the Authors (28 Jan 2026) Manuscript

Journal article(s) based on this preprint

27 Mar 2026

Estimation of particulate organic carbon export to the ocean from lateral degradations of tropical peatland coasts

Hiroki Kagawa, Koichi Yamamoto, Sigit Sutikno, Muhammad Haidar, Noerdin Basir, Atsushi Koyama, Ariyo Kanno, Yoshihisa Akamatsu, and Motoyuki Suzuki

Biogeosciences, 23, 2119–2154, https://doi.org/10.5194/bg-23-2119-2026,https://doi.org/10.5194/bg-23-2119-2026, 2026

Short summary

Hiroki Kagawa, Koichi Yamamoto, Sigit Sutikno, Muhammad Haidar, Noerdin Basir, Atsushi Koyama, Ariyo Kanno, Yoshihisa Akamatsu, and Motoyuki Suzuki

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Nov 2024	89	18	4	111
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Feb 2025	58	8	5	71
Mar 2025	39	4	0	43
Apr 2025	52	5	2	59
May 2025	60	2	1	63
Jun 2025	83	4	8	95
Jul 2025	98	4	0	102
Aug 2025	139	3	1	143
Sep 2025	405	6	1	412
Oct 2025	124	12	2	138
Nov 2025	94	13	1	108
Dec 2025	92	24	3	119
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Feb 2026	68	8	2	78
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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Short summary

This study elucidated the carbon export that has never been highlighted before, such as coastal peat mass movements and coastal erosion in tropical peatlands. The flux of POC exported to the ocean in the northern part of Bengkalis Island, Indonesia was estimated to range from 2.06 to 7.60 tC m^-1 yr^-1 due to coastal erosion and from 1.43 to 5.41 tC m^-1 due to peat mass movement events. A significant amount of carbon is continuously released to the ocean.


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