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
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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RC2: 'Comment on egusphere-2025-4555', Anonymous Referee #2, 05 Dec 2025
The authors present observations from a year-long field trial of alkalinity enhancement in a salt marsh and use these to evaluate the feasibility of the method in a similar coastal system. The experimental design appears to have been made with care and attention to detail. The paper concludes that large scale deployment of OAE in a saltmarsh system is unlikely to be viable, partly due to the modest benefits that are extrapolated from this small-scale trial to a larger deployment. These results provide needed in-situ context for rapidly evolving discussions about deploying OAE at scale. There is some incomplete reasoning that could be expanded upon and I have made suggestions for changes to some of the figures and data presentation.
Major comments:
Scaling to annual values – this seems to be a bit liberal – consider working with daily, or seasonal rates that may be more appropriate to your experimental results?
Figures – it is difficult to follow the results describing the difference (or similarity) between pore water and surface water (and rising and lowering tide) with the axes limits in the current figures. Consider using a range that better suits the data, even if it means changes within a figure (i.e., subplots with different ranges).
Section 3.4 – the last line of this section, which indicates that positive (outgassing) fluxes were observed throughout the experiment, and that olivine treatments were associated with larger fluxes than basalt and the control, seems like an important result and should perhaps be included in the abstract?
Section 3.5 – the fact that the outwelling (advection) of both DIC and TA is far larger than the local fluxes (pore water to surface water, surface water to atmosphere) seems important, but slightly buried in the manuscript? Would this suggest that a trial with much larger quantities of minerals would be needed to get the right ‘signal to noise’ in these advection dominated (or tidally-influenced) systems?
Minor Comments –
Abstract – the first line states that ‘OAE is a carbon dioxide removal strategy aimed at reducing atmospheric CO2’ this goes without saying – i.e., it is the definition of CDR.
OAE and TA are defined several times and only rarely used. Define early in the text and be consistent with the use of the shortform thereafter.
Line 200 – put the ‘(39.5)’ after ‘salinity’ at the beginning of this sentence.
Line 255 – ‘TA:DIC ration of lowering tide RESEMBLE the ratios of control surface water samples’ – can this be quantified – it is not possible to see this from the figures that are referred to (with the current axis limits).
Line 257 – remove ‘throughout the year’
Line 295 – unclear what ‘globally’ means here?
Citation: https://doi.org/10.5194/egusphere-2025-4555-RC2 -
AC2: 'Reply on RC2', Isabel Mendes, 22 Dec 2025
We thank the reviewer for their thorough evaluation and constructive comments on our manuscript. We appreciate the positive feedback to the experimental design and implementation, and the value of our experimental results for ongoing discussions about large-scale OAE measures. We will carefully consider the reviewer’s recommendations to elaborate on sections of the manuscript where reasoning was incomplete and to improve the clarity of figures and data presentation, when the manuscript is going to be revised.
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AC2: 'Reply on RC2', Isabel Mendes, 22 Dec 2025
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RC3: 'Comment on egusphere-2025-4555', Anonymous Referee #3, 15 Dec 2025
This manuscript details a study on the application of olivine and basalt to salt marsh sediments to evaluate their effectiveness for ocean alkalinity enhancement (OAE). The research is highly relevant and timely, given the rarity of field trials in OAE. While the overall design and duration of the experiment make the study interesting, there are issues in data processing that need to be resolved. The study would also benefit from clearer contextualisation and the inclusion of additional data, which is available according to the methods section. There are currently several claims in the text that I believe are not supported by data.
Major comments:
The manuscript currently presents data on total alkalinity (TA), pH, and calculated concentrations of dissolved inorganic carbon (DIC). It is difficult to draw conclusions about the system's functioning and the impact of the added minerals from these parameters alone. However, according to the methods section, samples for nutrients and metals were also collected (L94-95). Including these data would greatly help interpret seasonal trends and understand natural TA production in the area. Any solid-phase data that might be available would be equally valuable. If data on the permanence/spread of/fate of the added minerals are not available, I suggest the authors still discuss this to the best of their ability, to understand the added minerals’ contribution to the sedimentary TA release.
The statistical treatment of the data and the presentation of results from statistical tests should be improved. Please ensure that summary tables for any relevant statistical tests, such as ANOVAs, are presented (at least in the supplementary materials). Also, consider the suitability of the statistical tests for the data. From certain parts of the text (e.g., L276-278), it appears that ANOVAs were conducted for each month. If this is the case, the tests do not permit conclusions about the overall impact of olivine and basalt over the year (only for each specific month), and the obtained p-values could be less reliable due to multiple comparisons. Furthermore, ANOVAs are generally not appropriate for time series data. I suggest that the authors consider using alternative statistical models, such as linear mixed models.
In general, the importance of the results needs to be scrutinised. There appears to be an issue with equation 10, which inflates the outwelling values. Throughout the text, there are statements about results from treatment being different, without it being clear whether the differences are statistically significant (in most cases, the figures suggest they are not). Because of these issues in data interpretation, the authors conclude that olivine and basalt drive OAE in this natural system. Based on the data, I currently do not agree that this claim is justified. If data do not show significant differences between treatments, please discuss why (since trends often are seen in laboratory experiments) – is it a question about natural variations masking the OAE, are the minerals dissolving too slowly, or is the TA lost somehow?
Detailed comments:
L14-15: Please modify this statement to better represent the data. While the TA increased immediately after the addition of olivine and basalt (L14-15), this effect was transient, and no difference was observed between treatments after a few months.
L79-82: Did the addition of frames cause the plots to be constantly submerged by 2-3 cm of water, even though the soil/sediment would normally have been exposed ~70% of the time? If so, I would have expected the porewater chemistry to change substantially over the course of the experiment compared to “normal” conditions.
Sections 2.4-2.5: If I understand these sections correctly, it is assumed that no exchange of CO2 occurs while the soil/sediment is exposed. This is not correct, see e.g., Faber et al. (2012) and Migné et al. (2016).
L83-86: Why were different amounts of olivine (0.5 cm layer) and basalt (1 cm layer) added? What was the chemical/mineralogical composition of the rocks? This information is needed for a meaningful comparison of the OAE potential between olivine and basalt.
L92-93: In the treated plots, the Rhizons were inserted into the mineral layer, is that correct? How did the mineral layer evolve over time? Were mineral grains mixed into deeper layers by animals? Were grains flushed away? Overgrowth by algae?
L106-107: Is such a high accuracy really possible? Is it not supposed to be ±3.6 µM (±0.0036 mM)?
L148-149: Did the added mineral layer have the same porosity as the underlying sediment? If not, why was a 2 cm layer used for the porosity calculation?
Section 2.6: Equation 10 is written in a very unclear way. I assume that “DIC/TA” represents “DIC” or “TA”, but it could also mean the DIC concentration divided by the TA concentration. To avoid confusion, use a more generic form in the equation, e.g., C, and explain in the text that C is the concentration of DIC or TA. Furthermore, the outwelling calculation does not seem correct. Neubauer and Anderson (2003) based their calculations on hourly data of “marsh-produced DIC”; which in turn was calculated as the difference between the DIC concentration in the riverine input and the DIC concentration in one of the salt marsh creeks. In equation 10, the value used is not an excess DIC or TA concentration (produced in the marsh), but instead the total measured DIC or TA concentration. This assumes that water with 0 µM DIC enters the marsh, where ~2500 µM DIC is added and then exported to the ocean. The outwelling results are thereby highly overestimated.
Section 2.7: Both DIC and CO2 are used and compared throughout the manuscript. To facilitate comparison, I suggest that (1) the sediment-water flux is shown as DIC, and (2) that the water-atmosphere CO2 flux is given in mols instead of grams.
L219-221: It is unclear whether the numbers given across the different cases represent average values or the variation (or range) of values for the specified time points.
L244-245: Please add a reference here; anaerobic respiration processes produce both DIC and TA.
Section 3.5: Were any statistical tests done on the outwelling results? Based on Figure 6, I highly doubt that the outwelling of TA and DIC from the treated plots was generally higher than from the controls.
Section 3.6: Here, water-atmosphere CO2 fluxes are presented, but they are consistently described as “mass”. Please correct.
L350-354: This passage is hard to follow and seems to be partly incorrect. The reoxidation of sulfide consumes the TA that was produced during sulfate reduction, but this is not the main reason for the differences in TA:DIC ratio between porewater and overlying water in marsh systems. While considerable amounts of DIC are produced in the porewater, lowering the ratio, the overlying water is often mixed with water from other sources with a TA:DIC ratio >= 1 (Reithmaier et al., 2023).
L370-373: Are these differences between treatments statistically significant? Figure 5 shows a considerable spread in the data. Do you still see trends if these error bars are considered?
L401-404: Based on the data presented and considering the statistics, I do not believe that this can be concluded.
L406: Previously, “BF” has been used rather than “Durubas”.
Figure 2: Please add uncertainties to values that are averages of multiple replicates.
Figure 4: I suggest the authors add a horizontal line at TA:DIC = 1.
Figure 6: Panel b is on top of panel a; consider changing this for a more logical flow.
Figure 7: According to the text, the graphs do not show the mass but the fluxes – the y-axis titles should be corrected accordingly. Please add uncertainties on the bars.
Figure S5: The unit for the DIC is µmol/kg, whereas µM (µmol/L) is used for TA in the main text. Please be consistent and ensure that the TA:DIC ratios are calculated using the same units.
References
Faber, P. A., Kessler, A. J., Bull, J. K., McKelvie, I. D., Meysman, F. J. R., and Cook, P. L. M.: The role of alkalinity generation in controlling the fluxes of CO2 during exposure and inundation on tidal flats, Biogeosciences, 9, 4087–4097, https://doi.org/10.5194/bg-9-4087-2012, 2012.
Migné, A., Davoult, D., Spilmont, N., Ouisse, V., and Boucher, G.: Spatial and temporal variability of CO2 fluxes at the sediment–air interface in a tidal flat of a temperate lagoon (Arcachon Bay, France), J. Sea Res., 109, 13–19, https://doi.org/10.1016/j.seares.2016.01.003, 2016.
Reithmaier, G. M. S., Cabral, A., Akhand, A., Bogard, M. J., Borges, A. V., Bouillon, S., Burdige, D. J., Call, M., Chen, N., Chen, X., Cotovicz, L. C., Eagle, M. J., Kristensen, E., Kroeger, K. D., Lu, Z., Maher, D. T., Pérez-Lloréns, J. L., Ray, R., Taillardat, P., Tamborski, J. J., Upstill-Goddard, R. C., Wang, F., Wang, Z. A., Xiao, K., Yau, Y. Y. Y., and Santos, I. R.: Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes, Nat. Commun., 14, 8196, https://doi.org/10.1038/s41467-023-44037-w, 2023.
Citation: https://doi.org/10.5194/egusphere-2025-4555-RC3
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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.