the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jan 2026
Impacts of water advection and CO2 exchanges on the carbon dioxide removal potential of ocean alkalinity enhancement
Abstract. Ocean alkalinity enhancement is a carbon dioxide removal strategy with high CO2 uptake potential and rather low cost. Long term modelling studies have focused on this strategy, but most laboratory experiments focus on shorter term with strong advection, which may not be representative of natural systems. Hence, the long-term fate of alkalinity is yet to be addressed. Also, the role of CO2 ingassing is still largely overlooked. In a new setup, 6-month experiments using solid Ca(OH)2 and Mg(OH)2, and liquid NaOH have been conducted with a constant supply of CO2. Bottles were either kept stagnant or exposed to slow advection. All experiments revealed lower measured TA compared to expected, but bottles subjected to advection led to nearly twice as much TA generation in some cases and significantly faster CO2 ingassing rates. This led to a quicker equilibration, decreasing the low CO2 period, lately identified as a potential threat to living organisms, and a decreased critical alkalinity period (CAP) for CaCO3 formation. Yet, CaCO3 precipitated in most bottles, either as needles with Ca(OH)2 or as “broccolis” with Mg(OH)2. Such broccoli resulted most likely from precipitation on slowly dissolving Mg(OH)2 particles. Once the CAP was over, conditions remained stable with no redissolution of the CaCO3 precipitates. Using NaOH revealed buffering by Mg(OH)2 precipitation, which ultimately redissolved in the bottles exposed to advection. Finally, the bottles exposed to advection represented open ocean conditions with wind speeds of ~3 m s-1, which, given the calm state of the ocean, may serve as a lower threshold for implementation in global models.
Competing interests: Jens Hartmann is consulting the Planeteers GmbH.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6144', Anonymous Referee #1, 11 Feb 2026
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RC2: 'Comment on egusphere-2025-6144', Anonymous Referee #2, 01 Mar 2026
General comments
The manuscript addresses an interesting topic; the potential of ocean alkalinity enhancement in removing CO2 using laboratory experiments. However, in its current form, the manuscript requires substantial revision before it can be considered for publication.
The introduction would benefit from a more comprehensive and critical engagement with the existing literature. At present, it does not sufficiently synthesize prior work or clearly articulate how this study advances the field. Expanding the literature review, deepening the discussion of competing models, and broadening the citation base beyond a limited subset of recurring studies would significantly strengthen the manuscript. Currently the manuscript relies on a relatively small subset of studies, including multiple self-citations.
More broadly, while the authors introduce new experimental variables that move in a promising and appropriate direction, the methodological rigor with which these experiments were conducted and documented is currently insufficient to clearly demonstrate advancement beyond previous studies. As presented, it remains unclear whether the modifications—such as extended experimental duration and altered mixing conditions—translate into meaningful new insights or improved constraints on the processes under investigation. Strengthening the experimental design, documentation, and quantitative analysis would help clarify the extent to which this study advances our understanding of these issues.
Several aspects of terminology and conceptual framing require refinement. In multiple instances, terms are used in ways that may not align with their standard definitions. Improving precision in terminology and clarifying key concepts would enhance the scientific rigor of the manuscript. For example, “advection” and “laboratory stirring” are not the same thing. Similarly, phrases such as “due to the non-dissolution of CaCO3 in seawater” is both unclear and strange.
The experimental design and quality control procedures also require clearer documentation. Additional methodological detail and justification would strengthen confidence in the robustness and reproducibility of the results. There are certain aspects of the methodology that left me wondering if these experiments were done in the most rigorous way possible. For example, I was shocked to read that that MilliQ was added to compensate for evaporation. Why MilliQ and not seawater? How much was added? No documentation of this effect is presented. There are also no controls to compare to. These are all serious problems.
Another thing about the structure of the manuscript, I am unclear why the figures are placed in an Appendix rather than integrated into the main text. Presenting figures within the main manuscript would improve readability and allow readers to more easily follow the argument.
The section discussing wind speed and advection rate is a promising step toward connecting laboratory experiments with real-world conditions. However, this analysis currently feels incomplete. It would be valuable to fully develop this approach—potentially by solving the mentioned equations and presenting a figure that explicitly relates the experimental mixing conditions to realistic environmental scenarios. This would substantially strengthen the broader relevance of the study.
Detailed comments
Line 15 The terms “needles” and “broccolis” are informal and not standard terminology for crystal morphology. Please revise using conventional mineralogical or crystallographic terms.
Line 37 The description of “runaway precipitation” needs clarification. This term typically refers to the extent of reaction and the resulting decrease in total alkalinity, rather than necessarily implying an increase in precipitation rate over time. Please clarify whether there is direct evidence for changing reaction rates.
How was it determined that two hours of UV exposure was sufficient to prevent microbial growth? Were control experiments performed? Providing more detail on these points would strengthen the methodological rigor of the study.
Line 101 Please clarify the reference to “Carl Roth” (e.g., supplier name and location).
Line 123 The values reported here appear to represent variability rather than analytical error. Were samples analyzed in replicate? Please report the standard deviation (or other relevant metric) of replicate analyses.
Line 151 The discussion of quality control procedures is difficult to follow. Please clarify how analytical precision, accuracy, and reproducibility were assessed. I have no idea what the authors are talking about here.
Line 290 The statement regarding previous studies and the use of stir bars may need revision. Not all prior studies relied on stir bars, and the use of a shaking table may not constitute a substantial methodological innovation.
Citation: https://doi.org/10.5194/egusphere-2025-6144-RC2
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Dear authors,
Please find attached my reviewer comments.
Best regards