the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Sep 2025
Evaluating ocean alkalinity enhancement for carbon dioxide removal: evidence from a one-year saltmarsh field experiment
Abstract. Ocean alkalinity enhancement is a promising carbon dioxide removal (CDR) strategy aimed at reducing atmospheric carbon dioxide (CO2) concentrations. To evaluate its effectiveness and potential biogeochemical impacts, field experiments under natural conditions are essential. We report results from a one-year in-situ experiment conducted in the saltmarsh pioneer vegetation zone at Ria Formosa coastal lagoon, Portugal. The experiment comprised replicate deployments of olivine and basalt (treatments), and untreated control sites. Total alkalinity (TA) responded immediately to the treatments, with pore water 1.5 to 2.3 mM higher than the control. High concentrations of CO2 in pore water led to an increase of dissolved inorganic carbon (DIC) higher than TA. Continuous CO2 degassing from the saltmarsh soil was observed, with the treatments prompting higher CO2 fluxes than control. Carbon was laterally exported to the ocean (outwelling), following the trend of excess TA production. This effect was most pronounced during the first seven months after deployment, with basalt producing the best results. These findings provide critical insights into the temporal dynamics and efficacy of alkalinity enhancement in coastal vegetated systems.
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RC1: 'Comment on egusphere-2025-4555', Anonymous Referee #1, 26 Sep 2025
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AC1: 'Reply on RC1', Isabel Mendes, 28 Oct 2025
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Overview
The manuscript “Evaluating ocean alkalinity enhancement for carbon dioxide removal: evidence from a one-year saltmarsh field experiment” tests basalt and olivine for their suitability for OAH in-situ at a saltmarsh. While the manuscript presents an interesting and comprehensive dataset, some of the calculations rely on very simplified approaches e.g.CO2 emissions are calculated based on TA and pH measurements, and outwelling based on concentration differences between inside and outside the plots. This information is still valuable, but authors have to be more careful when presenting and comparing this data. See comments below.
We appreciate the reviewer’s careful evaluation and valuable comments. CO2 concentrations were indeed calculated from measured total alkalinity (TA) and pH values, using the CO2SYS program, which is a well-established and widely used standard in marine carbonate chemistry when direct pCO2 measurements are unavailable. This approach enables comparability with numerous previous studies.
The outwelling calculations were based on the concentration differences between the surface water inside the plots (which interacted with porewater during high tide) and the incoming tidal channel water representing the open-ocean endmember. This method follows approaches used in previous studies (e.g., Wang et al., 2016; Santos et al., 2019) and allows for estimating the material flux from the saltmarsh to the adjacent coastal waters. We acknowledge the reviewer’s concern and will ensure that the limitations and assumptions of these calculations are clearly stated in the revised manuscript.
General comments
Your abstract/discussion and title are not well aligned. Introducing CO2 fluxes and outwelling independently seems disconnected. Put them always into perspective to OAH.
We thank the reviewer for this valuable observation. Having read the title, abstract and discussion, we agree that these sections could be better connected. We will revise them accordingly, including possible adjustments to the title, and rewrite some sentences in the abstract and discussion to better integrate them in the context of OAH.
Calculating CO2 fluxes based on calculated pCO2 from alkalinity and pH seems like a stretch. Saltmarshes likely have high organic alkalinity messing up co2sys calculations and pH measurements tend to be very unreliable. You could make a sensitivity analysis to see how calculated pCO2 values change when assuming organic alkalinity to be 1-5% (remove from TA when calculating CO2) and accounting for the pH precision (use pH plus and minus precision). Then you could report a range for pCO2 and corresponding fluxes. Same for calculated DIC.
We thank the reviewer for this valuable comment. Calculating pCO₂ from total alkalinity (TA) and pH using the CO2SYS program is a well-established and widely applied approach in marine carbonate chemistry when direct pCO₂ measurements are not feasible. We acknowledge that this method can be subject to uncertainties, particularly in low-alkalinity or organic-rich systems where non-carbonate (organic) alkalinity contributes to TA. This may lead to an overestimation of calculated pCO₂ and, an underestimation of DIC.
However, our study site is characterized by relatively high TA and pH, where such effects are expected to be of minor influence. Similar studies have shown that in such buffered systems, CO2SYS calculations agree well with direct pCO₂ measurements (e.g., Abril et al., 2015, Biogeosciences). In our study, we compare different treatments with untreated control boxes, both placed in the same area. Thus, any potential bias in absolute pCO₂ values would be consistent across treatments and would not affect the relative differences between them.
Regarding pH measurements, we used a high-precision pH sensor (SenTix® 940-3, accuracy of ±0.004) that was calibrated monthly with three standard buffer solutions (pH 4: YSI‐381, pH 7: YSI‐3822, and pH 10: YSI‐3823) before fieldwork. The buffer solutions have an accuracy of 0.002 units at 25°C according to the manufacturer's data instructions. They are compatible with National Institute of Standards and Technology pH standards. To ensure accuracy and minimize sensor drift, the probe was allowed to equilibrate in ambient surface water prior to each measurement, and it was carefully cleaned between samples to prevent cross-contamination. We are therefore confident in the reliability of our pH data and the robustness of our derived pCO₂ estimates.
Outwelling/lateral flux calculations ignore porewater/groundwater fluxes, which are a major drivers of carbon outwelling in saltmarshes. Your calculations are still interested but be more specific in the abstract and discussion. E.g., “Saltmarshes were source of TA at low tide as indicated by elevated TA concentrations inside the plots compared to external seawater.” Right now abstract reads as if you did ecosystem scale measurement. To report outwelling in mmol/m2/d when you only calculated for such a short amount of time is not valid. You have to report in per hour and always add “at ebb tide”.
We thank the reviewer for this valuable comment. We agree that our flux calculations are based on short-term (approximately quarter of the day) measurements during ebb tide and do not capture continuous or groundwater-driven fluxes. However, groundwater seepage is not expected to influence our study site. In the Ria Formosa lagoon, groundwater seepage occur only in a few places (Leote et al., 2008). They were recognised by short pulses of low salinity in a tidal channel by the authors (unpublished). At the Esteiro Ancao backbarrier section, they were not recognised higher than 0.6 m below mean tidal level and showed a higher salinity than the outflowing tidal waters (Schönfeld and Mendes, 2021). These settings differ markedly from our sampling area, where no evidence of groundwater inputs has been reported.
Our approach is consistent with previous studies that estimated tidal outwelling from concentration differences between marsh and adjacent coastal waters (e.g., Wang et al., 2016; Santos et al., 2019). While porewater fluxes were not directly used, the surface water sampled during ebb tide had equilibrated with porewater during high tide, as also described in Wang et al. (2016). Thus, the measured concentrations already reflect the integrated influence of porewater exchange on the surface-water composition that is later exported during ebb tide. This surface-water export represents the operational definition of “outwelling” applied in comparable saltmarsh studies.
We will revise the Abstract and Discussion to clarify that our study represents discrete, monthly measurements of surface-water fluxes at ebb tide, rather than ecosystem-scale fluxes. The term “at ebb tide” will be used in places where it is important for the understanding.
Comments by line
L15 Set this into context of the basalt/olivine addition or remove.
We will change the sentence accordingly.
L16-17 Why did control have lower CO2 fluxes despite higher TA?
We thank the reviewer for the comment and would like to clarify that the control plots did not have higher TA. In fact, the treatment plots showed higher TA values compared to the control. Correspondingly, the control exhibited lower TA and lower CO₂ fluxes. We will rephrase the respective sentence in the manuscript to accurately reflect these findings and avoid this misunderstanding.
L17-18 Was outwelling different between treatments?
Yes, outwelling differences among treatments were observed during the first 7 months of the experiment. During this period, DIC outwelling was 3.5 % (3.9 mmol m-2 h-1) higher in the treatments than in the control, with the coarse-grained olivine (5.2 mmol m-2 d-1) and basalt (4.8 mmol m-2 d-1) treatments showing the highest values. Similarly, TA outwelling was 2.6 % higher than in the control, with the highest differences (3.3 %) also recorded in the coarse-grained olivine and basalt treatments (both 4.3 mmol m-2 d-1).
Corresponding clarifications and supporting details will be added to the Abstract and Discussion sections to emphasize these treatment-related differences in outwelling.
L24 Change to “might” be necessary
We appreciate the suggestion. However, we prefer to retain the phrasing “are deemed necessary,” as there is broad consensus among stakeholders and experts that carbon dioxide removal (CDR) is necessary to compensate for residual, unavoidable CO₂ emissions. This conclusion is also supported by the IPCC, which states that CDR will be required to achieve net-zero CO₂ emissions.
L49 km2 superscript
Accepted.
L47 – 64 This should be under Methods: 2.1 Study site
We thank the reviewer for the suggestion and will incorporate this information into the Methods section (Section 2.1, Study site).
L64-70 Reduce methodological details. Describe aims and hypothesis.
We will revise this section to reduce methodological details. A new paragraph will be added at the end of the Introduction, to clearly describing the study’s aims and hypotheses, as suggested by the reviewer.
L71 Not entire caption in bold. Nice map!
Thank you! We will do the captions in regular letters and only the “Figure 1” in bold.
L91 Remove minus before ” - ml”
Accepted.
L91 For which parameters which vials/beakers?
We will clarify this information in the manuscript by specifying in brackets which vials were used for each parameter. Specifically, 20 ml vials were used for alkalinity measurements to ensure samples could be stored chilled and airtight, while 100 ml vials were used for probe direct measurements and for collecting water for nutrient and trace metal analyses.
L93 Why porewater extracted so shallow? Top 1 cm likely mixture of porewater and water sitting on top.
The porewater was extracted from the top 1 cm because this corresponds to the thickness of the substrate layer deployed in the experiment. Sampling within this layer was essential to capture the direct effects of material dissolution on porewater chemistry. Below this depth, biological activity and other geochemical processes could already influence the chemical composition, potentially masking the specific effects of the added material. For methodological coherence, the same sampling depth was used the control boxes. We will add a clarifying sentence to the manuscript to explain this rationale.
L97 remove ) after YSI-381
Accepted.
L98 Need accuracy of instrument not the buffer solution.
Accepted. The brand and type (SenTix® 940-3), and the accuracy (±0.004) of the pH sensor will be added in the Water sampling and on-site measurements description section.
L106 What is precision?
The reproducibility, accuracy and linearity of the alkalinity titration method can be obtained from Mendes et al. (2025), Supporting Information S2, as mentioned in the manuscript. Verification of the method over a period of three days yielded an accuracy of 1.64%, an inter-day precision of 1.96%, and a linearity of 0.995.
L109 Report constants.
Accepted. We added the constants to the Manuscript.
L190 Split section in smaller paragraphs. Some for rest of manuscript to improve readability.
Accepted. The section will be divided in smaller paragraphs to improve readability, as were other parts of the manuscript.
L211 Remove variability and comma before from.
Accepted.
Fig 3. I wonder of delta TA (treatment – control) would be more informative. Maybe you could add two more subplots. For panel a, could you use a shorter y scale. It is very hard so see differences. Same for all other figures. Would adapt y axis to data of each plot.
Round to significant digits throughout results.
The delta TA (treatment – control), referred in this study referred as the excess of alkalinity, is available in the Supplement Figure S1 (surface water) and S2 (pore water) to complement the information presented in Fig. 3. In these supplementary figures, the differences between the treatments and the control are more clearly visible. Because we aimed to directly compare the same parameters in the surface and pore water, we considered using the same y-scale the most effective way to visualize these differences.
In addition, all data throughout the manuscript have been rounded to the appropriate number of significant digits.
L314-316 This is interesting and could be mentioned in abstract.
We appreciate the reviewer’s suggestion. A new sentence will be added to the abstract mentioning that the olivine treatments produced more CO2 than the control, while basalt produced the best results.
L322-330 Repetition of results. Shorten.
We shorten the paragraph and took out the repetition of the Results.
L330 Or is the alkalinity decrease just caused by substrate being washed away over time. If not over the surface maybe over porewater fluxes.
The saltmarsh vegetation, together with the installed frames, prevented the substrate from being washed away during the experiment. Over time, after approximately one year, the mineral layer became patchily covered with newly deposited mud. However, quarterly sediment sampling confirmed that the substrate remained in place and was still present after one year.
L385-399 Outwelling would not be order of magnitude higher if fluxes from high tide would be accounted for. Cannot compare your fluxes to other sites that measured ecosystem scale outwelling. Focus more on the differences between treatments than on actual numbers.
We appreciate the reviewer’s comment. Our study does not represent an ecosystem control, as it was based on measurements from 15 small experimental boxes, located in the pioneer vegetation zone of the salt marsh. Accordingly, the corresponding paragraph will be revised to emphasize the differences between treatments and the control site.
L400-401 Important finding should be in the abstract.
We appreciate the reviewer’s comment and agree with this observation. Accordingly, we will add a sentence to the abstract highlighting that no significant differences were observed between the olivine and basalt treatments or among grain sizes in total alkalinity enhancement.
References:
Abril, G., Bouillon, S., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Ochieng Omengo, F., Geeraert, N., Deirmendjian, L., Polsenaere, P., and Borges, A. V.: Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters, Biogeosciences, 12, 67–78, https://doi.org/10.5194/bg-12-67-2015, 2015.
Leote, C., Ibánhez, J.S., Rocha, C.: Submarine groundwater discharge as a nitrogen source to the Ria Formosa studied with seepage meters, Biogeochemistry, 88, 185–194. https://doi.org/10.1007/s10533-008-9204-9, 2008.
Santos, I. R., Burdige, D. J., Jennerjahn, T. C., Bouillon, S., Cabral, A., Serrano, O., Wernberg, T., Filbee-Dexter, K., Guimond, J. A., and Tamborski, J. J.: The renaissance of Odum's outwelling hypothesis in 'Blue Carbon' science, Estuarine, Coastal and Shelf Science, 255, 107361, https://doi.org/10.1016/j.ecss.2021.107361, 2021.; Santos et al., 2019.
Schönfeld, J. and Mendes, I.: Environmental triggers of faunal changes revealed by benthic foraminiferal monitoring, Estuarine, Coastal and Shelf Science, 253, https://doi.org/10.1016/j.ecss.2021.107313, 2021.
Wang, Z. A., Kroeger, K. D., Ganju, N. K., Gonneea, M. E., and Chu, S. N.: Intertidal salt marshes as an important source of inorganic carbon to the coastal ocean, Limnology and Oceanography, 61, 1916-1931, https://doi.org/10.1002/lno.10347
Citation: https://doi.org/10.5194/egusphere-2025-4555-AC1
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AC1: 'Reply on RC1', Isabel Mendes, 28 Oct 2025
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- 1
Overview
The manuscript “Evaluating ocean alkalinity enhancement for carbon dioxide removal: evidence from a one-year saltmarsh field experiment” tests basalt and olivine for their suitability for OAH in-situ at a saltmarsh. While the manuscript presents an interesting and comprehensive dataset, some of the calculations rely on very simplified approaches e.g..CO2 emissions are calculated based on TA and pH measurements, and outwelling based on concentration differences between inside and outside the plots. This information is still valuable, but authors have to be more careful when presenting and comparing this data. See comments below.
General comments
Your abstract/discussion and title are not well aligned. Introducing CO2 fluxes and outwelling independently seems disconnected. Put them always into perspective to OAH .
Calculating CO2 fluxes based on calculated pCO2 from alkalinity and pH seems like a stretch. Saltmarshes likely have high organic alkalinity messing up co2sys calculations and pH measurements tend to be very unreliable. You could make a sensitivity analysis to see how calculated pCO2 values change when assuming organic alkalinity to be 1-5% (remove from TA when calculating CO2) and accounting for the pH precision (use pH plus and minus precision). Then you could report a range for pCO2 and corresponding fluxes. Same for calculated DIC.
Outwelling/lateral flux calculations ignore porewater/groundwater fluxes, which are a major drivers of carbon outwelling in saltmarshes. Your calculations are still interested but be more specific in the abstract and discussion. E.g., “Saltmarshes were source of TA at low tide as indicated by elevated TA concentrations inside the plots compared to external seawater.” Right now abstract reads as if you did ecosystem scale measurement. To report outwelling in mmol/m2/d when you only calculated for such a short amount of time is not valid. You have to report in per hour and always add “at ebb tide”.
Comments by line
L15 Set this into context of the basalt/olivine addition or remove.
L16-17 Why did control have lower CO2 fluxes despite higher TA?
L17-18 Was outwelling different between treatments?
L24 Change to “might” be necessary
L49 km2 superscript
L47 – 64 This should be under Methods: 2.1 Study site
L64-70 Reduce methodological details. Describe aims and hypothesis.
L71 Not entire caption in bold. Nice map!
L91 Remove minus before ” - ml”
L91 For which parameters which vials/beakers?
L93 Why porewater extracted so shallow? Top 1 cm likely mixture of porewater and water sitting on top.
L97 remove ) after YSI‐381
L98 Need accuracy of instrument not the buffer solution.
L106 What is precision?
L109 Report constants.
L190 Split section in smaller paragraphs. Some for rest of manuscript to improve readability.
L211 Remove variability and comma before from.
Fig 3. I wonder of delta TA (treatment – control) would be more informative. Maybe you could add two more subplots. For panel a, could you use a shorter y scale. It is very hard so see differences. Same for all other figures. Would adapt y axis to data of each plot.
Round to significant digits throughout results.
L314-316 This is interesting and could be mentioned in abstract.
L322-330 Repetition of results. Shorten.
L330 Or is the alkalinity decrease just caused by substrate being washed away over time. If not over the surface maybe over porewater fluxes.
L385-399 Outwelling would not be order of magnitude higher if fluxes from high tide would be accounted for. Cannot compare your fluxes to other sites that measured ecosystem scale outwelling. Focus more on the differences between treatments than on actual numbers.
L400-401 Important finding should be in the abstract.