the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jun 2026
Unveiling carbonate dissolution in coastal sediments and its influence on seawater buffering capacity with δ13CDIC and 224Ra–228Th disequilibria
Abstract. Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO2 accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO3) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO3 dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the 224Ra–228Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ13CDIC) were employed to trace DIC sources in porewater. The results showed that CaCO3 dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO3 dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO2 accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO2 content, the TA supplied by the CaCO3 dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO3 dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO2 elevation.
- Preprint
(1507 KB) - Metadata XML
-
Supplement
(927 KB) - BibTeX
- EndNote
Status: open (until 06 Sep 2026)
- RC1: 'Comment on egusphere-2026-3442', Anonymous Referee #1, 09 Aug 2026 reply
-
RC2: 'Comment on egusphere-2026-3442', Anonymous Referee #2, 31 Aug 2026
reply
General comments
This study pairs two tracers (224Ra and 228Th disequilibrium for benthic exchange; d13C-DIC for the source of DIC) at three sites in Sansha Bay, and reports that CaCO3 dissolution supplies a quarter to half of the benthic DIC efflux. They show that dissolution does not simply track the intensity of organic carbon degradation, and that the associated alkalinity buffers acidification caused by remineralisation. I think it is a well written and important paper. The tracer combination is a good idea and, as far as I know, has not been applied in this setting. The radium work is careful, the arithmetic is internally consistent, data and code are archived. My comments are about strengthening the paper rather than questioning the approach. The main gap is that the saturation state is never calculated, even though every ingredient is there. Beyond that, the flux magnitudes deserve a clearer benchmark, and the headline number needs an uncertainty.
The subject of the paper is CaCO3 dissolution, but Ω_calcite and Ω_aragonite appear nowhere. You have porewater TA and DIC at 1 cm resolution, so this is a short calculation. It matters, because the reported porewater endpoints do not obviously permit the dissolution you infer. Running them through PyCO2SYS (T = 18 C, S = 30), TA 2326 / DIC 2184 gives Ω_calcite = 2.9 and TA 3081 / DIC 3007 gives 2.4. Both supersaturated. Only layers where DIC exceeds TA, as at SSW-B above 15 cm, fall below Ω_calcite = 1. This should be discussed and interpreted.
Your microenvironment argument may well resolve this (Wang et al., 2025, L&O Lett. do show shell dissolution at Ω > 1). Another reading would be inspired by Hu and Burdige (2007), who proposed coupled dissolution and reprecipitation for exactly this isotopic signature, which produces heavy porewater DIC with little net carbonate loss. A porewater Ca profile, if possible, would be enlightening and settle the question directly.
You report F_DIC = 624 mmol m-2 d-1 at SSW-A, that is 228 mol C m-2 yr-1. That is well above independent measurements: the global coastal benthic DIC efflux of Krumins et al. (2013), 126 Tmol yr-1, corresponds to roughly 13 mmol m-2 d-1, and the highest in-situ chamber values I can find for organic-rich systems are 18 to 78 mmol m-2 d-1 (Rassmann et al., 2020) or around 100 to 120 (Fan et al., 2024). That said, the reported values are consistent with the 224Ra literature, the method paper you follow, Cai et al. (2015), reports Pearl River Estuary DIC fluxes spanning two orders of magnitude, from -38 to 8500 (plus or minus 16000!) mmol m-2 d-1, and Hong et al. (2017) give 26 to 1000 mmol m-2 d-1 for the Jiulong River estuary. I would make this comparison explicitly and discuss it, including the fact that the largest 224Ra-derived fluxes carry very large uncertainties.
No uncertainty is propagated onto f_TOC, f_CaCO3 or the headline range. A Monte Carlo over the endmember distributions (shells n = 9, TOC n = 27) is straightforward and would strengthen the paper. For instance, how sensitive are the results to the d13C signatures and their uncertainties? The measured TA flux gives an independent estimate. Inverting F_TA/F_DIC = (2r + 0.14)/(r + 1) with your 0.95, 0.70, 0.73 gives r = 0.77, 0.43, 0.46, i.e. a CaCO3 share of 44/30/32%. Presenting both sets of estimates would provide a better uncertainty envelope.
Minor comments
- Missing metadata: sampling month or season is never given, and temperature, salinity and porosity are never reported. All three are needed to reproduce the calculations.
- Anaerobic pathways are never mentioned. In organic-rich mariculture mud this needs a sentence, since sulfate reduction with sulfide burial gives ΔTA/ΔDIC of about 1 rather than the 0.14 you assume.
- CO2SYS constants: Lehrter et al. (2012) is a benthic flux study, not a source of K1 and K2. Please name the parameterisation actually used. The canonical program citation is Lewis and Wallace (1998).
- "Liner mixed-effects" (line 400): change to "linear".
- "rarely influenced by benthic inputs" (line 472): change to "little influenced".
- "sediments … are in great heterogeneity" (line 96): change to "are highly heterogeneous".
References cited in this review
Cai, P., Shi, X., Hong, Q., Li, Q., Liu, L., Guo, X., and Dai, M.: Using ²²⁴Ra/²²⁸Th disequilibrium to quantify benthic fluxes of dissolved inorganic carbon and nutrients into the Pearl River Estuary, Geochim. Cosmochim. Acta, 170, 188–203, https://doi.org/10.1016/j.gca.2015.08.015, 2015.
Fan, L.-F., Kang, E.-C., Natividad, M. B., Hung, C.-C., Shih, Y.-Y., Huang, W.-J., and Chou, W.-C.: The role of benthic TA and DIC fluxes on carbon sequestration in seagrass meadows of Dongsha Island, J. Mar. Sci. Eng., 12, 2061, https://doi.org/10.3390/jmse12112061, 2024.
Hong, Q., Cai, P., Shi, X., Li, Q., and Wang, G.: Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from ²²⁴Ra/²²⁸Th disequilibrium, Geochim. Cosmochim. Acta, 198, 338–359, https://doi.org/10.1016/j.gca.2016.11.002, 2017.
Hu, X. and Burdige, D. J.: Enriched stable carbon isotopes in the pore waters of carbonate sediments dominated by seagrasses: Evidence for coupled carbonate dissolution and reprecipitation, Geochim. Cosmochim. Acta, 71, 129–144, https://doi.org/10.1016/j.gca.2006.08.043, 2007.
Humphreys, M. P., Lewis, E. R., Sharp, J. D., and Pierrot, D.: PyCO2SYS v1.8: marine carbonate system calculations in Python, Geosci. Model Dev., 15, 15–43, https://doi.org/10.5194/gmd-15-15-2022, 2022.
Krumins, V., Gehlen, M., Arndt, S., Van Cappellen, P., and Regnier, P.: Dissolved inorganic carbon and alkalinity fluxes from coastal marine sediments: model estimates for different shelf environments and sensitivity to global change, Biogeosciences, 10, 371–398, https://doi.org/10.5194/bg-10-371-2013, 2013.
Lehrter, J. C., Beddick, D. L., Devereux, R., Yates, D. F., and Murrell, M. C.: Sediment-water fluxes of dissolved inorganic carbon, O₂, nutrients, and N₂ from the hypoxic region of the Louisiana continental shelf, Biogeochemistry, 109, 233–252, https://doi.org/10.1007/s10533-011-9623-x, 2012.
Lewis, E. and Wallace, D. W. R.: Program developed for CO₂ system calculations, ORNL/CDIAC-105, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, https://doi.org/10.2172/639712, 1998.
Rassmann, J., Eitel, E. M., Lansard, B., Cathalot, C., Brandily, C., Taillefert, M., and Rabouille, C.: Benthic alkalinity and dissolved inorganic carbon fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes, Biogeosciences, 17, 13–33, https://doi.org/10.5194/bg-17-13-2020, 2020.
Wang, H., Teevan-Kamhawi, F., and Rebernik, O.: Harnessing nature's buffer: Assessing the role of bivalve shells in coastal alkalinity regeneration, Limnol. Oceanogr. Lett., 10, 774–781, https://doi.org/10.1002/lol2.70033, 2025.
Citation: https://doi.org/10.5194/egusphere-2026-3442-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 112 | 34 | 9 | 155 | 17 | 12 | 11 |
- HTML: 112
- PDF: 34
- XML: 9
- Total: 155
- Supplement: 17
- BibTeX: 12
- EndNote: 11
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Dear Authors,
This study investigates the importance of calcium carbonate in sediments for regulating pH and carbonate buffering capacity in the water column at a highly productive land–sea interface. The authors have also quantitatively evaluated these effects in three sites, including a reference site, a low CaCO3 site, and a high CaCO3 site. Overall, this is a good study. However, the description of the overall study framework could be further improved, and the limitations regarding the application and interpretation of the results are not sufficiently clear. My major comments are as follows:
1. The overall framework of the experimental design should be described more clearly. The abbreviations for the three sites should also be clearly defined, particularly which site represents the reference site, which site has CaCO3-rich sediments, and which site represents the fishpond site. It would be helpful to include a flowchart of the experimental design showing the three sites and the experiments conducted at each site.
2. This study uses quantitative approaches and provides quantitative results; however, the presentation of the results is not sufficiently quantitative. A summary table should be included to clearly present and compare the major differences among the three sites.
3. A conceptual diagram should be included to summarize the major findings and mechanisms proposed in this study. Chemical equations are also needed in the diagram.
4. How do productivity and water residence time affect the results of this study? Are the findings applicable to other environments? For example, would similar effects be expected in high-latitude or low-productivity regions? The applicability and limitations of the findings should be more clearly described and discussed.
Reference:
The following study may be relevant to the discussion and could be considered as a reference:
Organic–inorganic carbon coupling shapes carbon dioxide fluxes in seagrass ecosystems