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?

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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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:

1. 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.
2. 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.

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