Reviewer Comments to Author

The authors measured a suite of biogeochemical proxies including multi-biomarkers in surface sediments from an East China Sea shelf transect to investigate the sources and reactivity of OM. This paper provides valuable insights into OM transformation and carbon cycling across the land-ocean interface. With the benefit of multiproxy, terrestrial and marine derived OM, vegetation sources and degradation conditions were tried to be examined. Overall, this paper provides a valuable package of data with insights into OM transformation and carbon cycling across the land-ocean interface. While I support this manuscript for publication after a major revision, I do have some comments/concerns they need to address.

Major comments

1.The authors mention that this transect provides an ideal area to investigate the hydrodynamic-driven redistribution of OC. However, the manuscript lacks detailed descriptions of the hydrodynamic conditions. A more comprehensive introduction and explanation of how hydrodynamic data were measured or utilized would strengthen the manuscript.

Regarding Figure 1 and hydrodynamic description: Hydrodynamics and currents are also important components in this paper, especially in the ocean part. Even though this figure 1 is presented in 3-D, it does not properly show the ‘spatial range (i.e., approximate depth and range) of key current (KC, TWC, CDW…)’. Additionally, the sampling stations are difficult to identify. I recommend revising Figure 1 to improve clarity and informational value.

2. This paper includes a nice dataset of biogeochemical parameters, and I believe this could be beneficial for other researchers working on the East China Sea shelf region. I recommend the authors include a summary table (maybe in supplementary) listing the proxies used, their function or roles in this study, and relevant references. This would be particularly helpful for readers who are less familiar with these parameters.

3.Are there other possible terrestrial OM transport pathways in the study area (e.g., smaller rivers, submarine groundwater discharge, etc.)? A brief discussion of these factors would be helpful.

Large river estuaries typically exhibit substantial seasonal variability. Could this variability introduce potential bias in the terrestrial OM input due to changes in both its quantity and composition?

The sediment samples used in this study were collected in 2009 and 2010, which is approximately 15 years ago. Do the authors anticipate any bias due to this time gap, such as the impacts of human activities (e.g., dam construction, population growth, industrialization) or climate change over the intervening years? 

4. It may not be straightforward, but is it possible for the authors to estimate the degradation or transport time of OM by combining geochemical evidence from the sediments with the known distances (in km) along the transect?

Minor comments

Line 36: “whereas ~ in sediment” I feel this sentence suddenly popped up, but phosphorus is not a MAIN topic dealt in this paper. I recommend removing this part.

Line 46: sometimes “land-ocean” used but sometimes “land-sea”. Please unify the vocabulary, unless the author wants to categorize something different (if so, please explain).

Line 50: Does C/N in this paper is OC/TN? Or TC?TN? Maybe it’s better to clarify when it first appears. (I saw this information in line 146).

Line 71: a few ppm: in this ms, per mil (‰) kept used. I recommend sticking to the same unit.

Line 125: 0-2 cm sediment samples were collected by which device? Boxcore? Multicore?

Line 143, 145: Inorganic carbon removing part is written twice.

Line 170-174: Does Chl-a were measured with a YSI sonde and from the filter? Please clarify which method you used in this paper.

Line 177 and Figure 6 : N/C ratio? or C/N ratio?

Line 220: 16.5 +_0.5 (n=##) please provide the number of samples used for this calculation.

Line 249: Section 3.2, description of stable isotopic values is too short. You can write down more, or I recommend considering this section within section 3.1., as your stable isotopic values were also measured in bulk.

Figures and Tables

Fig.2 and 3

- please provide error bars

Fig. S1

-I recommend drawing a stacked-bar graph, instead of this.

Overall graphs

-Some minor ticks represent a number with decimal places. Please adjust the number of minor ticks so it represents integers.