the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 May 2025
Sources, Reactivity and Burial of Organic Matter in East China Sea Sediments, as Indicated by a Multi-geochemical Proxy Approach
Abstract. Large-river estuaries and adjacent seas play an important role in material exchanges and interactions in the land-ocean continuum and thereby impact global marine biogeochemistry. Due to the highly dynamic and complex transport and transformation processes of organic matter (OM), its distribution, sources, and reactivity in this region, especially under the multiple pressures of intense human activities and climate change, are not fully understood. An East China Sea transect, ranging from the mouth of the Changjiang River to the Okinawa Trough (OT), was selected to investigate the sources and reactivity of OM in surface sediments using multi-geochemical proxies. Carbon (C) to nitrogen (N) ratios and stable isotopic signatures, total hydrolyzable amino acids, neutral sugars and organic carbon (OC)-normalized total lignin-phenol indicated that OM in nearshore surface sediments derived primarily from terrestrial vascular plants, while offshore OM was dominantly derived from marine production. In the estuary vegetation mainly consisted of gymnosperms, whereas nonwoody angiosperms were dominant in offshore regions. Hydrodynamics, i.e., Changjiang Diluted Water and the Kuroshio Current markedly impacted sediment characteristics along this transect. The degree of OM degradation increased seaward, and sedimentation rates and OC burial fluxes were highest in Changjiang prodeltaic sediments and lowest in the OT. These results based on multi-biomarkers will advance our understanding of OC sources and burial during transport and deposition processes from estuaries to the deep oceans.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1976', Anonymous Referee #1, 13 Jun 2025
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.
Citation: https://doi.org/10.5194/egusphere-2025-1976-RC1 -
AC1: 'Reply on RC1', Xihua Yu, 08 Jul 2025
Publisher’s note: the supplement to this comment was edited on 15 July 2025. The adjustments were minor without effect on the scientific meaning.
Thank you for offering us an opportunity to improve the quality of our submitted manuscript. We greatly appreciate the reviewers' constructive and insightful comments.
Your valuable suggestions have played a crucial role in improving the quality of our manuscript. In particular, your Comment 4 in the major revision section of the article has been especially enlightening to us. In this response, we have addressed all comments and suggestions. Please find our changes and responses in the attached PDF. We hope you will consider these satisfactory.
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AC1: 'Reply on RC1', Xihua Yu, 08 Jul 2025
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RC2: 'Comment on egusphere-2025-1976', Anonymous Referee #2, 26 Jun 2025
In this study a set of surface samples, taken along a transect from the Changjiang outer estuary into the Okinawa Trough, were investigated with the main focus on the contribution of terrestrial (plant + non-plant) and marine organic matter. In addition to d13C and lignin which are used for end-member mixing, e.g. quantification of terrestrial vs. marine organic matter contribution, σ15N, TOC, TN, grain sizes, neutral sugar and amino acid content and spectral distributions are measured. This study is thus interesting due to its multi-proxy approach. A similar approach was used by Cowie, G., et al. (2014) (Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope. Biogeosciences, 11(23), 6683–6696. doi:10.5194/bg-11-6683-2014.), and could thus be an important paper to compare the results with. It is interesting that despite a different major question the results are somewhat similar and stress the importance of grain size for organic matter accumulation. There may of course be more studies of a similar kind in the literature.
In the data presentation many of the results are shown but I really miss a Figure of the % terr as in Figure S2. This is a very informative Figure as it relates different variables such as clay content and % terr as well as lignin-phenol. Plant % would also be a good variable to be included in an additional Figure. The authors should include a Figure of the results of the end-member model with some of the relevant other variables.
The introduction to neutral sugars and amino acid is quite short and should be enlarged with some reference to the use of these indicators (see comment to Lines 364-365).
Lines 73-75: In contrast to the previous introduction to d13C, the introduction to d15N is rather short. In addition the range of terrestrial d15N values is much wider and a d15n of 0 ‰ is certainly not representative. A very comprehensive overview is available by “Kendall, C., Elliott, E. M. & Wankel, S. D. (2007). Tracing anthropogenic inputs of nitrogen to ecosystems, in Stable Isotopes in Ecology and Environmental Science. edited by R. H. Michener and K. Lajtha, pp. 375–449, Blackwell Publishing.” But there are many more studies which suggest that less polluted and large rivers would have a d15N signal of about 4 ‰, whereas polluted rivers can have a much higher σ15N and pristine mountain rivers a lower d15N value than 4 ‰ (Voss, M., Deutsch, B., Elmgren, R., Humburg, C., Kuuppoo, P., Pastuszak, M., Rolff, C. & Schulte, U. (2006). River biogeochemistry and source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments. Biogeosciences Discussions, 3, 475–511.).
Line 90: AA are not really molecular biomarkers as they are not very specific indicators of certain organisms but are ubiquitous in living organisms. They have, therefore, often been called biogeochemical indicators as they delineate degradation pathways (as the authors also describe).
Lines 118-120: a short description of the impact of currents on the study area is missing rather than just reporting their presence.
Line 178: C/N instead of N/C
Line 300: Glu is missing in the Figure caption of Figure 5.
Lines 323-324: imprecise: clay minerals are possibly not the sinks for AA but clay minerals may have organic coatings or fine organic matter may be transported with the clay fraction?
Line 331-333: The increase of σ15n from the prodelta in offshore direction could be due to a reduced contribution of terrestrial material (see remark above).
Lines 364-365: the use of Asp/Gly vs. Ser+Thr as a source indicator should be explained with reference in the methods section or introduction.
Lines 421ff: When the TOC, TN and THAA contents are related to grain size and sorting rather than degradation processes it is feasible that THAA% and DI are not related as the material degrades as it is transported offshore or to greater depths. Further, the much lower SR and the decrease in offshore direction matches the degradation as reflected in the DI. These aspects are discussed later but some of the discussion can be deleted by combining these aspects.
Line 465: see earlier comments on “molecular biomarkers” and change the term here too.
Citation: https://doi.org/10.5194/egusphere-2025-1976-RC2 -
AC2: 'Reply on RC2', Xihua Yu, 08 Jul 2025
Publisher’s note: the supplement to this comment was edited on 15 July 2025. The adjustments were minor without effect on the scientific meaning.
Thank you for offering us an opportunity to improve the quality of our submitted manuscript. We greatly appreciate the reviewers' constructive and insightful comments.
Your valuable suggestions have played a crucial role in improving the quality of our manuscript. In particular, your comments concerning the introduction of δ15N in the article and the relationship between sediment particle size and organic matter have been especially enlightening to us. In this response, we have addressed all the comments and suggestions. Please find our changes and responses in the attached PDF. We hope you will consider our revisions satisfactory.
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AC2: 'Reply on RC2', Xihua Yu, 08 Jul 2025
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