| 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.
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.