the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2025
Potential of various minerals and their biogeochemical implications for ocean alkalinity enhancement in the southeastern Arabian Sea
Abstract. Ocean alkalinity enhancement (OAE) is emerging as a promising yet largely untested marine carbon dioxide CO2 removal approach. It involves the addition of alkaline substances such as powdered minerals and aqueous hydroxide solutions to seawater, shifting the carbonate chemistry speciation towards carbonate ions so as to store more CO2. Contemporaneous studies are being carried out to evaluate the efficacy, durability, and risks associated with these substances. Given the heterogeneity in a natural ecosystem, each substance will have distinct implications on carbonate chemistry as well as biogeochemistry of a given ecosystem. Our study contributes to ongoing research by examining the response of the ocean's carbonate chemistry to the addition of various alkalinity feedstocks — including both naturally occurring and anthropogenically (industrially) produced minerals, along the southeastern coastal Arabian Sea. We tested the alkalinity (AT) generation potential and traced the associated changes in the carbonate chemistry speciations for three naturally occurring minerals: (i) olivine ((MgFe)2SiO4), (ii) kaolinite (Al2Si2O5(OH)4), and (iii) dolomite (CaMg(CO3)2), and for two anthropogenically produced minerals: (i) periclase (MgO) and (ii) hydrated lime (Ca(OH)2) of two different mineral compositions, using 300 L mesocosms. Overall, no significant changes in AT, pH and dissolved inorganic carbon (DIC) were observed for the naturally occurring minerals, suggesting the lower efficiency of these minerals to increase AT,. In contrast, the dissolution of periclase and hydrated lime increased AT (up to 16 %, which corresponds to 80 % of the total added AT) and pH by up to 0.6 units. We further demonstrate that the temporal changes in the carbon isotopic composition (δ13C) of DIC as well as the changes in the DIC concentration occurring within the mesocosms can serve as an effective and reliable proxy for tracing secondary carbonate precipitation. As loss of alkalinity via secondary precipitation diminishes the overall efficiency of the OAE approach, accurate determination of the threshold at which secondary precipitation is triggered is critical for maximizing the effectiveness of this method. We determine these thresholds and provide an assessment of various alkalinity feedstocks that could work best for OAE.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-3925', Anonymous Referee #1, 27 Aug 2025
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AC1: 'Reply on RC1', Arvind Singh, 16 Oct 2025
Reviewer #1
Mehta et al., report 300 L scale incubation experiments testing the hypothesis that alkaline minerals can be used to increase the alkalinity of seawater and used to achieve OAE i.e. increased storage of DIC in seawater. The topic is definitely of broad interest and suitable for the journal. There is a need for studies that attempt to add alkaline minerals to ‘real’ seawater to explore OAE dynamics under realistic conditions alongside studies which used more controlled laboratory conditions because results from these approaches can diverge leading to over-simplistic conclusions about aspects of OAE viability.
I am a marine chemist and whilst I have a reading knowledge of carbonate chemistry, I am not a specialist in OAE so defer to other reviewers on any interpretations I have misunderstood (I also have not worked on any aspects of the Indian Ocean so similarly apologize for my lack of regional knowledge). My main queries concern some minor requests for specific details about the experiment setup and also the relevance of the specific experiment site. It seems the experiment uses quite acidic, high pCO2 coastal seawater so I think some context about the relevance and implications of this would be informative to the reader. I think most of my queries are minor and can be addressed with a little more detail and rephrasing.
Reply: We thank the reviewer for the encouraging remarks and for the quick evaluation of our manuscript. We appreciate the reviewer’s thoughtful comments, which greatly helped us improve the manuscript.
Comments by line
Comment 1
29 “damage to our ecosystem, and in some cases, the damage seems irreversible” perhaps a little vague
Reply: We have removed the statement in the revision.
Comment 2
33 suggest ‘greenhouse gas emissions’
Reply: Thank you, we have made the correction.
Comment 3
33 “Thus, to limit this warming, we require strong reductions in greenhouse gas emissions coupled with the implementation of various carbon dioxide removal (CDR) methods” I know what the authors mean, but this is not strictly speaking correct, how much CDR would you have to do to measurably affect temperature within a decade? I don’t know if this number has been calculated, but global warming results from the greenhouse gases accumulated in the atmosphere, so unless CDR meaningfully changes the accumulated CO2 in the atmosphere it will make basically zero difference to short-term temperature dynamics, it would only affect atmospheric temperature over much longer timescales. (I know why the authors say this, but it is important to be precise, CDR is often mis-sold as ‘fixing’ global warming on short timescales, which is not correct)
Reply: Thank you for pointing this out. We completely agree with your thoughts. We did not intend to say so. We have rephrased the sentence as follow:
“Thus, to limit this warming by 2100, we require strong reductions in greenhouse gas emissions coupled with the implementation of various carbon dioxide removal (CDR) methods (Minxet al., 2018; Rau et al., 2012).”
Comment 4
60 In a sentence, maybe introduce the concept and name ‘runaway precipitation’ which is referred to as it may be useful for non-specialists
Reply: Thank you for the suggestion. We have revised the sentence as follow:
“These alkalinity sources may differ in their origin (natural or anthropogenic), dissolution kinetics, amount of alkalinity released, and their impact on chemical speciation and biological responses. Also, an excessive increase in alkalinity may induce supersaturation and subsequent precipitation of secondary mineral phases, thereby reducing both the system’s alkalinity and its CO₂ sequestration capacity—a process termed runaway precipitation (Moras et al., 2022; Hartmann et al., 2023; Suitner et al., 2024).”
Comment 5
65 “large uncertainties mainly attributed to the lack of observational data” I’m not sure this is correct, my basic understanding was that once diluted in the ocean OAE would result in changes in TA which are basically undetectable compared to background changes, so perhaps it’s inherent that models are required, but more data won’t necessarily solve the underlying problem? (See Fennel et al., 2025, The Verification Challenge of Marine Carbon Dioxide Removal)
Reply: We thank the reviewer for the suggestion. The verification of CDR method does require modeling efforts, but here we are trying to highlight the ecological consequences. We have modified the statement as follows:
“While these efforts will rely on the use of skilled models for precise quantification of net CO2 flux from atmosphere to ocean (Fennel, 2025), experimental studies remain essential to understand changes in the carbonate chemistry speciation and how marine plankton communities respond to these altered conditions.”
Comment 6
67 “for a longer period of time”… than what? Perhaps longer than bottle experiments?
Reply: We have removed the statement during revision.
Comment 7
76 “on various biogeochemical processes” perhaps a little vague
Reply: We have rephrased the section as follow:
“Because the δ¹³C signal is highly sensitive to biogeochemical processes such as photosynthesis, remineralization, carbonate precipitation and dissolution, as well as CO₂ exchange with the atmosphere (ingassing and outgassing), monitoring temporal changes in DIC concentration and its isotopic composition (δ¹³C) provides a robust means to identify the dominant biogeochemical processes operating within a mesocosm.”
Comment 8
84 Just to clarify, this means “with a grain size of ≤63µm.” all the minerals have grain sizes below this size? The size is quite important for understanding the dissolution potential.
Reply: Yes, all the minerals have grain sizes below this size.
Comment 9
85 Again for clarity, the ‘targeted’ TA means assuming 100% dissolution of each solid on a molar basis for alkalinity? Maybe add a sentence for clarity?
Reply: Thank you. This is a good suggestion. We have added the detail as follow:
“Alkalinity enhancement was targeted at two AT levels (assuming 100% dissolution of the added minerals), +AT ; 250 μmol kg−1 in four mesocosm using olivine, kaolinite, dolomite, hydrated lime-2 and 500 μmol kg−1 in the remaining four mesocosms using olivine, periclase, hydrated lime-1 and hydrated lime-2.”
Comment 10
85 Can the authors be a bit more specific about how the minerals were added? When adding a relatively large amount of minerals in one go, if not encouraged to disperse, it is possible that the particles – for lack of a scientific term- just stick together as a sticky mess and sink. So experiments adding dry particles, or a pre-mixed suspension of particles, can see very different dissolution dynamics (I’ve seen a student get net TA loss, or complete dissolution of alkaline minerals just by varying exactly how they added it to static bottles, so it is perhaps quite important to be clear how it was done). The mixing, or lack of, in any container is also important, was there any mixing in the tanks or are they basically static and if so what do the samples represent, i.e. is TA well mixed in the 300 L units? A few more details are maybe needed for reproducibility. Same query for the basic details, what was the salinity/temperature of the water, was this maintained during the experiment? Is the water surface seawater, or something pumped from depth, if so how was it collected? Was a mesh used to remove particles?
Reply: These are very important queries that we have clarified in the revision. We collected surface water for this experiment, and no mesh was used to remove the particles. The dry powdered minerals were added at once and gently mixed manually using teflon sticks during the addition and subsequently at regular intervals (three times a day). The mesocosms were kept in ambient/atmospheric temperature conditions (i.e. we did not control the temperature). We observed an increase in salinity with time due to evaporation. The alkalinity values were corrected for the salinity effect as described in the methods section.
Comment 11
128-130 Not sure I followed this, should it state p <0.05?
Reply: Thank you for pointing this out! This was a typo, it is p<0.05. We have made this correction.
Comment 12
142 Same query again later (for pCO2), is this normal regionally? This is very acidic (and pCO2 is very high), is this a local phenomenon due to collecting the water in a bay area (I don’t know the regional oceanography, but I would assume such low pH is usually only found in estuaries or within the most intense core of oxygen minimum zones?).
Reply: Thank you for this query. The region is very shallow and has a long continental shelf with prominent oxygen minimum zones in the subsurface waters. Studies have reported the presence of strong hypoxic region covering almost entire coastal belt of the eastern Arabian Sea (Naqvi et al., 2000). The coastal region experiences strong monsoonal winds causing the upwelling of CO2 rich subsurface water to come to the surface. Hence, these high pCO2 values are consistent with regional biogeochemistry. We have clarified these points in the revised manuscript now.
Comment 13
Figure 3 Similar query, why is pCO2 so high in the control, and why is it increasing so much in the control? Is this due to using water with high organics that are degrading, or are the tanks indoors with a high CO2 concentration in the air? I haven’t seen such high pCO2 values in this type of work so would be a bit curious to know what’s driving this as it also affects the context of the results (presumably acidic conditions favor more rapid dissolution of some minerals, and affect calcite saturation states, but if the water has high organics that may also be a key issue affecting dissolution rates).
Reply: The mesocosms were kept in a semi-enclosed space which rules out the possibility of high CO2 in the air. It is possible that high organic content might lead to slower dissolution rates of minerals (Naviaux et al., 2019). Organics can significantly influence the dissolution of minerals such as olivine and dolomite, which already exhibit inherently low dissolution rates. When organics adsorb onto their surfaces, dissolution may be further inhibited. Lower pH on the other hand should enhance the dissolution of these minerals (Gislason and Arnórsson, 1993). Identifying the dominant factor controlling the dissolution of these minerals is a limitation of the present study. Nonetheless, the degradation of the organic matter is possibly a reason for high CO2 as evidenced from the δ13C and DIC observations.
We have highlighted the point in the revised manuscript.
Comment 14
194 As above, how representative is the starting condition? If the control/initial conditions had been in equilibrium with atmospheric pCO2, then I assume saturation states would have been higher.
Reply: The starting conditions are the representative of the regional surface ocean conditions and the measured pH values at that location are consistent with those reported in literature (Gatty et al., 2022). We agree with the reviewer’s observations that if the starting conditions were in equilibrium, the pCO2 would have been lower and pH and the saturation states would have been higher.
Comment 15
205 In comparing different studies might it be useful to give the key differences, e.g. grain size, and perhaps to elaborate why small-scale mixing/bottle experiments are potentially not so useful when scaling up to ‘real’ seawater?
Reply: We completely agree with the reviewer's comment. For comparing the ecosystem responses, it is important to take into account the environmental and experimental conditions as well as grain size of the alkalinity feedstock. These comparisons are more meaningful if compared according to the timescale/duration of the experiments conducted. Laboratory experiments are excellent in examining responses under controlled environmental conditions. However, owing to the heterogeneity of natural ecosystems, small scale laboratory experiments are not efficient in capturing the community composition compared to the field mesocosms. We have added the details in the manuscript as follow:
“In contrast to these studies, microcosm and ship-based experiment by Guo et al. (2024, 2025) in the coastal waters of Tasmania and in equatorial Pacific, respectively, showed that the dissolution of olivine has limited potential for CDR at smaller time scales, but its environmental impacts are more pronounced. However, these studies differ significantly in terms of the scale, duration of experiments, and grain size of olivine added. Moreover, small-scale bottle experiments cannot fully represent the community composition observed in large scale mesocosm experiments. Therefore, direct comparison with these studies should be approached with caution.”
Comment 16
208 “the limited potential of olivine for OAE in this region” Can it be stated with certainty that the difference is regional and does not reflect experimental set up e.g. mixing dynamics and particle size?
Reply: The experimental setup was designed following the “Guide to best practices in the ocean alkalinity enhancement research” (Riebesell et al., 2023, Rodríguez et al., 2023). The grain size of olivine was selected based on the evidence from the literature on small scale laboratory experiments conducted to study the dissolution kinetics of olivine (Montserrat et al., 2017). Given this, we are certain that the limited potential of the olivine can be attributed to the regional oceanographic conditions. However, small differences might still arise from the mixing dynamics and physicochemical conditions and the timescale considered.
Comment 17
212 “these minerals and have been skeptical about costs associated with the raw materials and processing” I wasn’t clear which minerals this refers to. I assume impurities are the main concern with ‘waste’ minerals whereas mining/processing costs applies to olivine etc?
Reply: The cost here refers to the cost associated with the extraction and grinding of natural minerals such as olivine, and costs associated with the production of CaO, Ca(OH)2 and MgO.
Comment 18
234 “reflecting the natural prevalence of the degradation” This hints at an answer to some earlier queries, it would be useful to somewhere clarify what the dynamics are in this coastal region, as I am inferring that the specific site has high organic loadings and is a CO2 outgassing region which affects the context about calcite saturation states, DIC dynamics and OAE viability.
Reply: Thank you. We have provided the explanation under the comments 12 and 13. The details have also been added to the manuscript.
Comment 19
235 (as per 240) This seems perhaps a little speculative without any biological parameters from the experiment. I haven’t worked in this region, but often I would expect when coastal seawater was enclosed there would be a bloom and some DIC decline over a few days, here it seems the opposite with DIC increasing suggesting there was no phytoplankton bloom and that the experiment is characterized by organic carbon degradation and the release of CO2 under control conditions. It would be useful to have some context about this.
Reply: We agree that in such enclosed experiments a phytoplankton bloom is expected initially causing the DIC to decrease. However, our observations do not support this pattern, likely because the respiration/organic matter remineralization dominated over the photosynthesis causing the DIC to increase. This might be because the water was already organic rich when we started the experiment.
Comment 20
260 As previous comments, some context would help understand how representative this seawater is of the area, how much of the coastline for example experiences low pH and high pCO2 comparable to the control herein?
Reply: The subsurface region of coastal northeastern Arabian Sea is oxygen deficient zone characterized by subsurface anoxia and strong seasonal upwelling patterns. Naqvi et al. (2000) and Gupta et al. (2021) reported suboxic (0.5- 4.5 µM) to anoxic (0.0-0.5 µM; ranges taken from Rixen et al., 2020) conditions along the coastal regions of eastern Arabian Sea. With regards to the pH and pCO2 ranges, there are currently no comprehensive datasets from this region. A few sporadic observations exist and are consistent with our observations of pH (Gatty et al., 2022). We have added this in the revised manuscript.
Supplement
Comment 21
Thanks for sharing data, the file download works, however please note multiple datasheets in file are not always machine readable, and it would be better to include units in the headers and avoid empty cells (ambiguous), consider ‘not applicable’ ‘not measured’ etc
Reply: We thank the reviewer for the helpful suggestions. We have incorporated the suggestions and made suitable changes.
References:
Minx, J. C., Lamb, W. F., Callaghan, M. W., Fuss, S., Hilaire, J., Creutzig, F., Amann, T., Beringer, T., Garcia, W. de O., Hartmann, J., Khanna, T., Lenzi, D., Luderer, G., Nemet, G. F., Rogelj, J., Smith, P., Vicente, J. L. V., Wilcox, J., & Dominguez, M. del M. Z. (2018). Negative emissions—Part 1: Research landscape and synthesis. Environmental Research Letters, 13(6), 063001. https://doi.org/10.1088/1748-9326/aabf9b
Rau, G. H., McLeod, E. L., & Hoegh-Guldberg, O. (2012). The need for new ocean conservation strategies in a high-carbon dioxide world. Nature Climate Change, 2(10), 720–724. https://doi.org/10.1038/nclimate1555
Naqvi, S. W. A., Jayakumar, D. A., Narvekar, P. V., Naik, H., Sarma, V. V. S. S., D’Souza, W., Joseph, S., & George, M. D. (2000). Increased marine production of N2O due to intensifying anoxia on the Indian continental shelf. Nature, 408(6810), 346–349. https://doi.org/10.1038/35042551
Gislason, S. R., & Arnórsson, S. (1993). Dissolution of primary basaltic minerals in natural waters: Saturation state and kinetics. Chemical Geology, 105(1), 117–135. https://doi.org/10.1016/0009-2541(93)90122-Y
Naviaux, J. D., Subhas, A. V., Dong, S., Rollins, N. E., Liu, X., Byrne, R. H., Berelson, W. M., & Adkins, J. F. (2019). Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter. Marine Chemistry, 215, 103684. https://doi.org/10.1016/j.marchem.2019.103684
Riebesell, U., Basso, D., Geilert, S., Dale, A. W., & Kreuzburg, M. (2023). Mesocosm experiments in ocean alkalinity enhancement research. State of the Planet, 2-oae2023, 1–14. https://doi.org/10.5194/sp-2-oae2023-6-2023
Iglesias-Rodríguez, M. D., Rickaby, R. E. M., Singh, A., & Gately, J. A. (2023). Laboratory experiments in ocean alkalinity enhancement research. State of the Planet, 2-oae2023, 1–18. https://doi.org/10.5194/sp-2-oae2023-5-2023
Montserrat, F., Renforth, P., Hartmann, J., Leermakers, M., Knops, P., & Meysman, F. J. R. (2017). Olivine Dissolution in Seawater: Implications for CO2 Sequestration through Enhanced Weathering in Coastal Environments. Environmental Science & Technology, 51(7), 3960–3972. https://doi.org/10.1021/acs.est.6b05942
Gupta, G. V. M., Jyothibabu, R., Ramu, C. V., Yudhistir Reddy, A., Balachandran, K. K., Sudheesh, V., Kumar, S., Chari, N. V. H. K., Bepari, K. F., Marathe, P. H., Bikram Reddy, B., & Vijayan, A. K. (2021). The world’s largest coastal deoxygenation zone is not anthropogenically driven. Environmental Research Letters, 16(5), 054009. https://doi.org/10.1088/1748-9326/abe9eb
Rixen, T., Cowie, G., Gaye, B., Goes, J., do Rosário Gomes, H., Hood, R. R., Lachkar, Z., Schmidt, H., Segschneider, J., & Singh, A. (2020). Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean. Biogeosciences, 17(23), 6051–6080. https://doi.org/10.5194/bg-17-6051-2020
Gatty, S., Mt, L., A., P., & Narshivudu. (2022). Hydrographical studies along the coastal waters off Mangalore and Padubidri, southwest coast of India.
Citation: https://doi.org/10.5194/egusphere-2025-3925-AC1
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AC1: 'Reply on RC1', Arvind Singh, 16 Oct 2025
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RC2: 'Comment on egusphere-2025-3925', Anonymous Referee #2, 03 Sep 2025
Review of ‘Potential of various minerals and their biogeochemical implications for ocean alkalinity enhancement in the southeastern Arabian Sea’.
This manuscript presents an analysis of natural and anthropogenic mineral alkalinity generation in seawater in coastal India. The authors examine minerals that have been discussed as potential alkalinity sources in the literature and novel minerals as well, for that I commend the authors, as the efficacy of mineral addition and its effects on the local carbon chemistry (speciation) needs to be examined. Additionally, they use carbon isotopes as a measure of precipitation/dissolution which is novel and should be explored further. However, the manuscript currently lacks clarity, direction, and sufficient analysis of the results. The discussion is too broad in scope, with unsupported generalisations about applicability, biological effects, and regional significance. At present, this manuscript is not mature enough for publication and requires major revisions to be suitable for Biogeosciences.
Major Comments:
This study is not well-motivated, it is not clear what the main objective of this study is. On reading the paper the authors state that they aim to identify the alkalinity generation potential of minerals, the threshold of secondary precipitation, and the effect on bgc processes. However, the authors do not address these aims sufficiently. In my view, they can only describe the alkalinity generation potential of these minerals, and this has limitations based on the length of the experiment.
The introduction is broad, and at times repetitive. It provides extensive general background on ocean carbon uptake and acidification but does not sufficiently build toward the specific knowledge gap or objectives of this study. It should be shortened and focused on the rationale for testing these minerals, the novelty of applying δ¹³C, and how the study builds on (or differs from) previous work.
It is not clear whether stirring or mixing was applied during mineral additions. If minerals were added without agitation, it is unsurprising that phases such as olivine and dolomite showed limited alkalinity release. This should be clarified, as it affects interpretation of dissolution rates.
The manuscript makes broad claims regarding applicability to the Arabian Sea, secondary precipitation thresholds, and biological impacts without adequate evidence. These should either be substantiated with additional data and literature or removed.
The study appears to replicate Moras et al. (2022) with the addition of δ¹³C. However, Moras et al. conducted longer experiments. How can the authors conclude that secondary precipitation, olivine/dolomite dissolution, or CO₂ in-gassing do not occur beyond the 9-day timeframe used here? Without this, the conclusions about long-term implications are not justified.
Minor comments:
Abstract
14 – increases by percent here are confusing, are you saying the A_T increased by 80% of it’s potential? Which corresponds to a 16% increase in the total A_T? this might be simpler to say, increases from X to Y µmol/kg.
15-18 – While measuring the d13C is novel and interesting in this field, I’m not sure it’s best use is in monitoring for secondary precipitation, could this not be determined through less ‘relative’ measures, i.e., turbidity/TSS, and/or observed visually?
Introduction
28 – What is the effect on calcifiers?
29 – Vague statement on damage to ecosystem, needs to be clarified.
38 – Repeat sentence on ocean sequestering ¼ of CO2.
66 – Longer period of time compared with? many bottle sample analyses for example last many weeks, i.e., Moras et al., 2022.
67 – There are a number of studies from outside the North Atlantic, e.g., Ferderer et al., 2022, completes microcosm studies in Australia, Guo et al., 2025 looks at different A_T source materials on Pacific waters and phytoplankton communities.
Methods
84 – Should state what the other half of hydrated lime-2 is here
85 – It would be interesting to note how much (g) of each mineral was needed to reach 250/500 µmol/kg.
89 – Which BGC process were tested for?
115 – Why did you use pH for pyCO2SYS instead of DIC? pH is the least robust measurement to use in CO2SYS.
125 – ‘smoothening’ should be smoothing
127 – should p not be < 0.05 rather than < 0.5 (<0.5 is not significant)
Results
145 – p values are now <0.05?
161 – if you use the initial DIC you get a pCO2 of ~830, which is still high, but lower than the pH-derived value.
Discussion
189 – Should reference Moras 2022/2024 here alongside Takahashi and Koishi
192 – I don’t think you can compare high pH in mesocosms to OA mitigation, there is no dilution happening here and only limited CO2 uptake over 9 days, so it is unsurprising that the pH stays elevated
194 – I think it’s a stretch to say these results can be applied across the region.
204 – Guo’s results support the other results cited here, in that olivine has the potential but it’s limited by dissolution timescale.
Figure 4, panel a – There should be a panel for in-gassing, which should overlap with the remineralisation panel.
233 – Would clustering in quadrant 4 not also indicate uptake of CO2? Particularly considering this only occurs in your Hydrated-lime and periclase experiments?
235 – Without any measurements of the phytoplankton community composition I don’t think you can make conclusions about impact on biology. Are there any measurements of the Arabian Sea coastal phytoplankton community you can draw on here?
245 – How does this provide evidence for OA mitigation?
255 – Which socioeconomic benefits?
This research does have the potential to mature into a publishable study, and I recognise that the present submission is an important first step in that direction. I hope my comments, though critical, will be constructive and helpful in guiding the revision of this manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-3925-RC2 -
AC2: 'Reply on RC2', Arvind Singh, 16 Oct 2025
Reviewer #2
Review of ‘Potential of various minerals and their biogeochemical implications for ocean alkalinity enhancement in the southeastern Arabian Sea’.
This manuscript presents an analysis of natural and anthropogenic mineral alkalinity generation in seawater in coastal India. The authors examine minerals that have been discussed as potential alkalinity sources in the literature and novel minerals as well, for that I commend the authors, as the efficacy of mineral addition and its effects on the local carbon chemistry (speciation) needs to be examined. Additionally, they use carbon isotopes as a measure of precipitation/dissolution which is novel and should be explored further. However, the manuscript currently lacks clarity, direction, and sufficient analysis of the results. The discussion is too broad in scope, with unsupported generalisations about applicability, biological effects, and regional significance. At present, this manuscript is not mature enough for publication and requires major revisions to be suitable for Biogeosciences.
Reply: We thank the reviewer for giving valuable time for critically reviewing our manuscript and providing constructive comments and suggestions. We have addressed the concerns in detail in the revised manuscript and provided a detailed point-by-point response to the specific comments below.
Major Comments:
Comment 1
This study is not well-motivated, it is not clear what the main objective of this study is. On reading the paper the authors state that they aim to identify the alkalinity generation potential of minerals, the threshold of secondary precipitation, and the effect on bgc processes. However, the authors do not address these aims sufficiently. In my view, they can only describe the alkalinity generation potential of these minerals, and this has limitations based on the length of the experiment.
Reply: We sincerely thank the reviewer for this valuable comment and for highlighting an important aspect of the study. A variety of OAE methods have been proposed in recent years; however, only a handful of experimental studies exist for each approach. This scarcity of systematic experimental work leaves significant knowledge gaps and limits our confidence in the scalability and real-world applicability of these methods. Furthermore, the diversity of alkaline minerals available for OAE—with varying dissolution kinetics, chemical compositions, and potential secondary effects—adds to the uncertainty. Many of these minerals, despite their large natural abundance and theoretical CO2 sequestration potential, have not yet been experimentally evaluated. Their behavior may also differ substantially depending on environmental settings and ecosystem characteristics.
In addition to the uncertainty around mineral dissolution efficiency, the potential for secondary mineral precipitation and the subsequent influence of OAE on marine biogeochemical processes remain poorly constrained. These aspects are critical for understanding both the stability of added alkalinity and the ecological implications of OAE interventions.
Motivated by these knowledge gaps, the present study was designed to (1) quantify the alkalinity generation potential of selected alkaline minerals, (2) identify the threshold for secondary precipitation, and (3) assess the potential influence of mineral dissolution on biogeochemical processes within a controlled mesocosm setting. As the reviewer correctly noted, the first objective has been addressed directly through our experimental observations. To investigate the second objective, we employed a novel isotopic approach using the stable carbon isotopic composition (δ¹³C) of dissolved inorganic carbon (DIC), which serves as an effective tracer of carbonate system dynamics. Details of this approach and its interpretation have been elaborated in the Discussion section.
Moreover, δ¹³C (DIC) provides valuable insights into concurrent biogeochemical processes such as photosynthesis, respiration, and organic matter degradation, which collectively influence the mesocosm’s carbon cycling. By integrating δ¹³C (DIC) observations with changes in DIC concentrations, we were able to infer the interplay between mineral dissolution and biological activity. While the duration of the experiment naturally limits the temporal scale of these interpretations, the results nonetheless contribute to a broader understanding of the mechanistic links between OAE reactions and marine biogeochemistry.
In the revised manuscript, the paragraph:
“Here we present a mesocosm experiment designed to investigate the following (i) the alkalinity generation potential of naturally occurring minerals (olivine, kaolinite and dolomite) and anthropogenically produced minerals (periclase and hydrated lime) in the coastal water of the southeastern Arabian Sea (ii) define the threshold of alkalinity addition so as to prevent secondary precipitation and (iii) effect of mineral addition on various biogeochemical processes.”
has been revised to
“With this motivation, we conducted a mesocosm experiment in the coastal water of the southeastern Arabian Sea to investigate the alkalinity generation potential of various alkalinity feedstocks. These include naturally occurring minerals (olivine, kaolinite and dolomite) and anthropogenically produced minerals (periclase and hydrated lime). We aimed to determine the threshold level of alkalinity addition that triggers secondary precipitation. We propose that temporal shifts in the stable isotopic composition of carbon (δ13C) of DIC (δ13CDIC)and DIC concentration can serve as reliable indicators of the onset of secondary precipitation. Because the δ13CDIC signal is highly sensitive and strongly influenced by biogeochemical processes such as photosynthesis, remineralization, carbonate precipitation and dissolution, tracking the temporal changes in the δ13CDIC together with the DIC concentration can provide a robust means of identifying the dominant biogeochemical processes occurring within a mesocosm. Hence, based on this approach, we aimed to identify the dominant biogeochemical processes occurring within the mesocosms.”
Yes, indeed there is a limitation based on the duration, but we would like to point out that no additional seawater was added in between the experiment. As soon as water level dropped by 1/3rd of initial volume, we terminated the experiment, due to which it could not be sustained for longer timeframe.
Comment 2
The introduction is broad, and at times repetitive. It provides extensive general background on ocean carbon uptake and acidification but does not sufficiently build toward the specific knowledge gap or objectives of this study. It should be shortened and focused on the rationale for testing these minerals, the novelty of applying δ¹³C, and how the study builds on (or differs from) previous work.
Reply: Thank you for this comment. We have revised the introduction. The knowledge gaps are now described as follow:
“Numerous approaches to OAE have been proposed and are actively being explored; such as the spreading powdered minerals, raining aqueous solutions of hydroxides (i.e., water jets containing a slurry of alkaline minerals pumped into the air from the ships), and electrochemical alkalinity generation (Kheshgi, 1995; Hartmann et al., 2013; Renforth and Henderson, 2017; Bianchi et al., 2024). These alkalinity sources may differ in their origin (natural or anthropogenic), dissolution kinetics, amount of alkalinity released, and their impact on chemical speciation and biological responses. Also, an excessive increase in alkalinity may induce supersaturation and subsequent precipitation of secondary mineral phases, thereby reducing both the system’s alkalinity and its CO₂ sequestration capacity—a process termed runaway precipitation (Moras et al., 2022; Hartmann et al., 2023; Suitner et al., 2024). While it is important to consider the broader consequences of mineral addition, the primary challenge lies in identifying the suitable alkalinity feedstocks (Guo et al., 2025a) and determining the threshold of alkalinity addition to prevent runaway precipitation (Moras et al., 2022; Hartmann et al., 2023; Suitner et al., 2024). Laboratory experiments and offshore mesocosm studies consistently indicate that runaway precipitation results in a net loss of alkalinity (Moras et al., 2022; Hartmann et al., 2023; Suitner et al., 2024). While these studies rely on the use of skilled models for precise quantification of net CO2 flux from the atmosphere to the ocean (Fennel, 2025), experimental studies remain essential to understand the changes in the carbonate chemistry speciation and how biogeochemistry will respond to these changing conditions.”
The objectives are revised as follow:
“With this motivation, we conducted a mesocosm experiment in the coastal water of the southeastern Arabian Sea to investigate the alkalinity generation potential of various alkalinity feedstocks. These include naturally occurring minerals (olivine, kaolinite and dolomite) and anthropogenically produced minerals (periclase and hydrated lime). We aimed to determine the threshold level of alkalinity addition that triggers secondary precipitation. We propose that temporal shifts in the stable isotopic composition of carbon (δ13C) of DIC (δ13CDIC)and DIC concentration can serve as reliable indicators of the onset of secondary precipitation. Because the δ13CDIC signal is highly sensitive and strongly influenced by biogeochemical processes such as photosynthesis, remineralization, carbonate precipitation and dissolution, tracking the temporal changes in the δ13CDIC together with the DIC concentration can provide a robust means of identifying the dominant biogeochemical processes occurring within a mesocosm. Hence, based on this approach, we aimed to identify the dominant biogeochemical process occurring within the mesocosms.”
Comment 3
It is not clear whether stirring or mixing was applied during mineral additions. If minerals were added without agitation, it is unsurprising that phases such as olivine and dolomite showed limited alkalinity release. This should be clarified, as it affects interpretation of dissolution rates.
Reply: We thank the reviewer for the comment. We would like to clarify that the stirring was applied to mineral addition during the addition and all the tanks were stirred at regular intervals (3 times a day). This has now been clarified in the manuscript.
Comment 4
The manuscript makes broad claims regarding applicability to the Arabian Sea, secondary precipitation thresholds, and biological impacts without adequate evidence. These should either be substantiated with additional data and literature or removed.
Reply: We would like to emphasize that we did acknowledge this limitation in the Discussion section of the original version, noting that our observations are based on a single site and should therefore be interpreted with caution (Line 259: “These thresholds might not be representative of the entire coastal Indian Ocean and may vary spatially. Hence, these thresholds must be applied with caution.”). In the revised version, we have further refined the discussion to avoid making broad generalizations and to more explicitly highlight the site-specific nature of our findings.
We have also highlighted that “the present study lays a foundation for the future CDR research in this region as there is no existing literature in terms of experimental studies from the Indian Ocean on mCDR especially OAE and further in situ experiments are needed for better assessment and more conclusive findings” (line no 261 – 273, and 288-290).
Comment 5
The study appears to replicate Moras et al. (2022) with the addition of δ¹³C. However, Moras et al. conducted longer experiments. How can the authors conclude that secondary precipitation, olivine/dolomite dissolution, or CO₂ in-gassing do not occur beyond the 9-day timeframe used here? Without this, the conclusions about long-term implications are not justified.
Reply: We wish to underscore that the present study is not a replication of Moras et al. (2022); rather, it differs substantially in several critical aspects. Moras et al. (2022) focused on studying the dissolution kinetics of minerals using filtered seawater, which removed the biological component. Consequently, the observed changes were entirely chemical, allowing their experiments to be sustained over longer durations. In contrast, our study added minerals directly to unfiltered seawater to capture both physicochemical and biogeochemical changes that are likely to occur in a natural ecosystem. Daily sub-sampling reduces the water volume, and over time, the remaining water no longer reflects the initial conditions. Since no seawater was replenished during the experiment, this naturally limits the experimental duration.
Minor comments:
Abstract
Comment 6
14 – increases by percent here are confusing, are you saying the A_T increased by 80% of it’s potential? Which corresponds to a 16% increase in the total A_T? this might be simpler to say, increases from X to Y µmol/kg.
Reply: We thank the reviewer for the suggestion. We have rephrased the sentence as follows:
“In contrast, dissolution of periclase and hydrated lime substantially increased AT by up to 16%, representing about 80% of the total added alkalinity, and elevated the pH by up to 0.6 units”.
These minerals were tested under different targeted alkalinity levels, and providing absolute concentrations for each would make the sentence lengthy and confusing. Since the percentage increase relative to the target addition is comparable for both cases, we believe expressing the change in percentage terms provides a clearer and more consistent representation of the results.
Comment 7
15-18 – While measuring the d13C is novel and interesting in this field, I’m not sure it’s best use is in monitoring for secondary precipitation, could this not be determined through less ‘relative’ measures, i.e., turbidity/TSS, and/or observed visually?
Reply: The stable carbon isotopic composition (δ¹³C) provides a robust and quantitative method for determining both the initiation and progression of secondary precipitation. While the onset of large precipitation can be visually observed, tracking its continuation and eventual termination over time is not feasible through physical inspection alone. In controlled mesocosm experiments, and particularly in potential future deployment sites, isotopic measurements may offer a more reliable and objective approach where visual observations may be impractical. Furthermore, δ¹³C measurements also provide valuable insights into the dominant biogeochemical processes occurring within the system, such as photosynthesis, respiration, and organic matter degradation. With advances in analytical techniques, isotopic measurements are now more accessible and straightforward than in the past.
Introduction
Comment 8
28 – What is the effect on calcifiers?
Reply: The high pCO2 in water causes decrease in the saturation state and decreases the pH of water making it corrosive to calcium carbonate and thus leads to reduced calcification rates and increased dissolution of existing shells, particularly in the upper water column where majority of marine organisms thrive. We have rephrased the text as follow:
“The ocean acidification causes a substantial change in marine carbonate chemistry by lowering the calcium carbonate saturation state (Ω) and pH of the seawater, making it corrosive to calcium carbonate, thus reducing the calcification rates and increasing the dissolution of existing shells of marine calcifiers (Feely et al., 2004)”
Comment 9
29 – Vague statement on damage to ecosystem, needs to be clarified.
Reply: We have removed the sentence.
Comment 10
38 – Repeat sentence on ocean sequestering ¼ of CO2.
Reply: We have removed the sentence.
Comment 11
66 – Longer period of time compared with? many bottle sample analyses for example last many weeks, i.e., Moras et al., 2022.
Reply: The sentence was removed during the revision. We would however like to clarify that longer term here refers in context to sustaining natural microbial communities under near natural conditions. Bottle experiments are difficult to sustain with natural communities. As described above, Moras et al. (2022) had a different experimental setup which used filtered seawater.
Comment 12
67 – There are a number of studies from outside the North Atlantic, e.g., Ferderer et al., 2022, completes microcosm studies in Australia, Guo et al., 2025 looks at different A_T source materials on Pacific waters and phytoplankton communities.
Reply: Here we are referring to the mesocosm studies. Ferderer et al. (2022) conducted microcosm studies (as mentioned by reviewer too), and Guo et al. (2025) used 500 mL Nalgene bottles with the experiment duration lasting for only 48 hours.
Methods
Comment 13
84 – Should state what the other half of hydrated lime-2 is here
Reply: We have added the detail in the sentence.
“The minerals included were olivine, kaolinite, dolomite, periclase, and two distinct compositions of hydrated lime: hydrated lime-1 (70% Ca(OH)2 and 30% CaCO3) and hydrated lime-2 (54% Ca(OH)2 and 46% CaCO3)”
Comment 14
85 – It would be interesting to note how much (g) of each mineral was needed to reach 250/500 µmol/kg.
Reply: This is a good suggestion. We have added the table to the data sheet uploaded in the online repository.
Comment 15
89 – Which BGC process were tested for?
Reply: Here we are using the δ13CDIC to identify the dominant processes occurring within the mesocosm. These include photosynthesis, respiration (organic matter degradation), carbonate dissolution, and secondary carbonate precipitation. The details of biogeochemical processes are added to the discussion section and Figure 4.
Comment 16
115 – Why did you use pH for pyCO2SYS instead of DIC? pH is the least robust measurement to use in CO2SYS.
Reply: We thank the reviewer for this important observation. We used the combination of pH and total alkalinity as inputs for CO2SYS because this pairing is both practical and widely adopted in similar OAE and mesocosm studies (e.g., Ferderer et al., 2025, England and Bach, 2024), which facilitates direct comparison with existing datasets. While we acknowledge that pH can be less robust than DIC under certain conditions, in our experimental setup pH was measured with high-precision electrodes and frequent calibration, ensuring reliable and internally consistent data.
Comment 17
125 – ‘smoothening’ should be smoothing
Reply: We have made the correction.
Comment 18
127 – should p not be < 0.05 rather than < 0.5 (<0.5 is not significant)
Reply: We thank the reviewer for pointing it out. We have made the correction.
Results
Comment 19
145 – p values are now <0.05?
Reply: We apologize for the confusion. We have made the correction above. The p value indicated here is correct.
Comment 20
161 – if you use the initial DIC you get a pCO2 of ~830, which is still high, but lower than the pH-derived value.
Reply: The high pCO2 in this region is attributed to the regional biogeochemistry and has been discussed in detail under comment 12 (#Reviewer 1).
Discussion
Comment 21
189 – Should reference Moras 2022/2024 here alongside Takahashi and Koishi
Reply: We thank the reviewer for the suggestion. Moras et al. (2022) was already cited, we have now also added Moras et al. (2024).
Comment 22
192 – I don’t think you can compare high pH in mesocosms to OA mitigation, there is no dilution happening here and only limited CO2 uptake over 9 days, so it is unsurprising that the pH stays elevated
Reply: We thank the reviewer for the comment. We agree with the reviewer that based on the timescale of experiment the CO2 uptake (ingassing) is likely limited. Based on our findings, we propose that if such high pH conditions are sustained for a longer time frame, they could potentially contribute to mitigating ocean acidification. We do not claim that this method will mitigate OA, rather we highlight the potential towards OA mitigation. Further extensive studies with longer experimental duration are required to accurately assess its efficiency. We have now added these details to the manuscript.
Comment 23
194 – I think it’s a stretch to say these results can be applied across the region.
Reply: We agree with the reviewer’s statement. We have added the following text for clarity:
“Through our experiment, we demonstrate the potential thresholds for alkalinity additions for periclase and hydrated lime. Owing to the heterogeneity of the coastal ecosystems, however, these thresholds might not be representative of the entire coastal Indian Ocean and may vary spatially. Hence, these thresholds must be applied with caution. Site-specific assessment should be carried out to fully understand the response of OAE to the local and regional ecosystem variability.”
Comment 24
Reply: 204 – Guo’s results support the other results cited here, in that olivine has the potential but it’s limited by dissolution timescale.
Reply: We thank the reviewer for suggestion. We have added the detail.
“In contrast to these studies, microcosm and ship-based experiment by Guo et al., (2024, 2025) in the coastal waters of Tasmania and in equatorial Pacific, respectively, showed that the dissolution of olivine has limited potential for CDR at smaller time scales, but its environmental impacts are more pronounced.”
Comment 25
Figure 4, panel a – There should be a panel for in-gassing, which should overlap with the remineralisation panel.
Reply: We thank the reviewer for the insightful comments. We have modified the figure accordingly.
Comment 26
233 – Would clustering in quadrant 4 not also indicate uptake of CO2? Particularly considering this only occurs in your Hydrated-lime and periclase experiments?
Reply: We thank the reviewer for the query and agree with the observation. The observed changes in the Hydrated-lime and periclase can be attributed to both ingassing and degradation. We have now incorporated the changes in the figure and the text.
Comment 27
235 – Without any measurements of the phytoplankton community composition I don’t think you can make conclusions about impact on biology. Are there any measurements of the Arabian Sea coastal phytoplankton community you can draw on here?
Reply: We appreciate the reviewer’s insightful comment. We agree that, in the absence of direct measurements of phytoplankton community composition, it is not possible to make definitive conclusions regarding biological impacts. Our interpretations are therefore presented cautiously and are based on indirect evidence of biogeochemical processes that may help infer potential responses of the phytoplankton community to OAE. As noted in the manuscript, this is the first study of its kind from this region, and currently, no comparable datasets or measurements are available to substantiate these interpretations. In order to fill these knowledge gaps, we are also participating in the “Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP)” (Bach et al., 2024) for the assessment of the response of phytoplankton community in this region.
Comment 28
245 – How does this provide evidence for OA mitigation?
Reply: We thank the reviewer for the query. We would like to clarify that we do not strongly advocate that OAE can lead to OA mitigation. However, based on the results, we speculate that addition of certain minerals like hydrated lime and periclase, can keep the pH elevated for a longer time thereby having the potential to mitigate OA. This has also been discussed under comment 22.
Comment 29
255 – Which socioeconomic benefits?
Reply: As described in the introduction section- “Coastal regions have been identified as the most viable sites for CDR deployment, as near-coast operations are economically favourable from a resource perspective (Renforth and Henderson, 2017; He and Tyka, 2023). Considering the vast expanse of the northern Indian Ocean and the extensive Indian coastline (∼1.1×104 km), this region may emerge as a promising candidate for large-scale CDR deployment in the future scenario”, the socioeconomic benefits here refers to the potential economic advantages owing to logistical feasibility, which can further lead to economic growth and social well-being.
Comment 30
This research does have the potential to mature into a publishable study, and I recognise that the present submission is an important first step in that direction. I hope my comments, though critical, will be constructive and helpful in guiding the revision of this manuscript.
Reply: We thank the reviewer for the critical and insightful comments. These were indeed quite helpful for us to revise the manuscript.
References
Moras, C. A., Bach, L. T., Cyronak, T., Joannes-Boyau, R., & Schulz, K. G. (2022). Ocean alkalinity enhancement – avoiding runaway CaCO3 precipitation during quick and hydrated lime dissolution. Biogeosciences, 19(15), 3537–3557. https://doi.org/10.5194/bg-19-3537-2022
Moras, C. A., Cyronak, T., Bach, L. T., Joannes-Boyau, R., & Schulz, K. G. (2024). Effects of grain size and seawater salinity on magnesium hydroxide dissolution and secondary calcium carbonate precipitation kinetics: Implications for ocean alkalinity enhancement. Biogeosciences, 21(14), 3463–3475. https://doi.org/10.5194/bg-21-3463-2024
Guo, J. A., Strzepek, R. F., Yuan, Z., Swadling, K. M., Townsend, A. T., Achterberg, E. P., Browning, T. J., & Bach, L. T. (2025). Effects of ocean alkalinity enhancement on plankton in the Equatorial Pacific. Communications Earth & Environment, 6(1), 270. https://doi.org/10.1038/s43247-025-02248-7
Ferderer, A., Chase, Z., Kennedy, F., Schulz, K. G., & Bach, L. T. (2022). Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community. Biogeosciences, 19(23), 5375–5399. https://doi.org/10.5194/bg-19-5375-2022
Kheshgi, H. S. (1995). Sequestering atmospheric carbon dioxide by increasing ocean alkalinity. Energy, 20(9), 915–922. https://doi.org/10.1016/0360-5442(95)00035-F
Renforth, P., & Henderson, G. (2017). Assessing ocean alkalinity for carbon sequestration. Reviews of Geophysics, 55(3), 636–674. https://doi.org/10.1002/2016RG000533
Bianchi, R., Abbate, S., Lockley, A., Abbà, A., Campo, F., Varliero, S., Grosso, M., & Caserini, S. (2024). Evaluating rainbowing for ocean alkalinity enhancement. Environmental Research Communications, 6(9), 095003. https://doi.org/10.1088/2515-7620/ad707b
Hartmann, J., West, A. J., Renforth, P., Köhler, P., De La Rocha, C. L., Wolf‐Gladrow, D. A., Dürr, H. H., & Scheffran, J. (2013). Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. Reviews of Geophysics, 51(2), 113–149. https://doi.org/10.1002/rog.20004
Hartmann, J., Suitner, N., Lim, C., Schneider, J., Marín-Samper, L., Arístegui, J., Renforth, P., Taucher, J., & Riebesell, U. (2023). Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO2 storage. Biogeosciences, 20(4), 781–802. https://doi.org/10.5194/bg-20-781-2023
Suitner, N., Faucher, G., Lim, C., Schneider, J., Moras, C. A., Riebesell, U., & Hartmann, J. (2024). Ocean alkalinity enhancement approaches and the predictability of runaway precipitation processes: Results of an experimental study to determine critical alkalinity ranges for safe and sustainable application scenarios. Biogeosciences, 21(20), 4587–4604. https://doi.org/10.5194/bg-21-4587-2024
Fennel, K. (2025). The Verification Challenge of Marine Carbon Dioxide Removal. Annual Review of Marine Science. https://doi.org/10.1146/annurev-marine-032123-025717
Feely, R. A., Sabine, C. L., Lee, K., Berelson, W., Kleypas, J., Fabry, V. J., & Millero, F. J. (2004). Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans. Science, 305(5682), 362–366. https://doi.org/10.1126/science.1097329
He, J., & Tyka, M. D. (2023). Limits and CO2 equilibration of near-coast alkalinity enhancement. Biogeosciences, 20(1), 27–43. https://doi.org/10.5194/bg-20-27-2023
Ferderer, A., Schulz, K. G., Willis, A., Baker, K. G., Chase, Z., & Bach, L. T. (2025). Carbonate chemistry fitness landscapes inform diatom resilience to future perturbations. Science Advances, 11(38), eadu8024. https://doi.org/10.1126/sciadv.adu8024
England, P. I., & Bach, L. T. (2025). Influence of wave action on applications of olivine‐based Ocean Alkalinity Enhancement on sandy beaches. Geophysical Research Letters, 52,e2025GL114922. https://doi.org/10.1029/2025GL114922
Bach, L. T., Ferderer, A. J., LaRoche, J., & Schulz, K. G. (2024). Technical note: Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP). Biogeosciences, 21(16), 3665–3676. https://doi.org/10.5194/bg-21-3665-2024
Citation: https://doi.org/10.5194/egusphere-2025-3925-AC2
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AC2: 'Reply on RC2', Arvind Singh, 16 Oct 2025
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- 1
Potential of various minerals and their biogeochemical implications for ocean alkalinity enhancement in the southeastern Arabian Sea
Mehta et al., report 300 L scale incubation experiments testing the hypothesis that alkaline minerals can be used to increase the alkalinity of seawater and used to achieve OAE i.e. increased storage of DIC in seawater. The topic is definitely of broad interest and suitable for the journal. There is a need for studies that attempt to add alkaline minerals to ‘real’ seawater to explore OAE dynamics under realistic conditions alongside studies which used more controlled laboratory conditions because results from these approaches can diverge leading to over-simplistic conclusions about aspects of OAE viability.
I am a marine chemist and whilst I have a reading knowledge of carbonate chemistry, I am not a specialist in OAE so defer to other reviewers on any interpretations I have misunderstood (I also have not worked on any aspects of the Indian Ocean so similarly apologize for my lack of regional knowledge). My main queries concern some minor requests for specific details about the experiment setup and also the relevance of the specific experiment site. It seems the experiment uses quite acidic, high pCO2 coastal seawater so I think some context about the relevance and implications of this would be informative to the reader. I think most of my queries are minor and can be addressed with a little more detail and rephrasing.
Comments by line
29 “damage to our ecosystem, and in some cases, the damage seems irreversible” perhaps a little vague
33 suggest ‘greenhouse gas emissions’
33 “Thus, to limit this warming, we require strong reductions in greenhouse gas emissions coupled with the implementation of various carbon dioxide removal (CDR) methods” I know what the authors mean, but this is not strictly speaking correct, how much CDR would you have to do to measurably affect temperature within a decade? I don’t know if this number has been calculated, but global warming results from the greenhouse gases accumulated in the atmosphere, so unless CDR meaningfully changes the accumulated CO2 in the atmosphere it will make basically zero difference to short-term temperature dynamics, it would only affect atmospheric temperature over much longer timescales. (I know why the authors say this, but it is important to be precise, CDR is often mis-sold as ‘fixing’ global warming on short timescales, which is not correct)
60 In a sentence, maybe introduce the concept and name ‘runaway precipitation’ which is referred to as it may be useful for non-specialists
65 “large uncertainties mainly attributed to the lack of observational data” I’m not sure this is correct, my basic understanding was that once diluted in the ocean OAE would result in changes in TA which are basically undetectable compared to background changes, so perhaps it’s inherent that models are required, but more data won’t necessarily solve the underlying problem? (See Fennel et al., 2025, The Verification Challenge of Marine Carbon Dioxide Removal)
67 “for a longer period of time”… than what? Perhaps longer than bottle experiments?
76 “on various biogeochemical processes” perhaps a little vague
84 Just to clarify, this means “with a grain size of ≤63µm.” all the minerals have grain sizes below this size? The size is quite important for understanding the dissolution potential.
85 Again for clarity, the ‘targeted’ TA means assuming 100% dissolution of each solid on a molar basis for alkalinity? Maybe add a sentence for clarity?
85 Can the authors be a bit more specific about how the minerals were added? When adding a relatively large amount of minerals in one go, if not encouraged to disperse, it is possible that the particles – for lack of a scientific term- just stick together as a sticky mess and sink. So experiments adding dry particles, or a pre-mixed suspension of particles, can see very different dissolution dynamics (I’ve seen a student get net TA loss, or complete dissolution of alkaline minerals just by varying exactly how they added it to static bottles, so it is perhaps quite important to be clear how it was done). The mixing, or lack of, in any container is also important, was there any mixing in the tanks or are they basically static and if so what do the samples represent, i.e. is TA well mixed in the 300 L units? A few more details are maybe needed for reproducibility. Same query for the basic details, what was the salinity/temperature of the water, was this maintained during the experiment? Is the water surface seawater, or something pumped from depth, if so how was it collected? Was a mesh used to remove particles?
128-130 Not sure I followed this, should it state p <0.05 ?
142 Same query again later (for pCO2), is this normal regionally? This is very acidic (and pCO2 is very high), is this a local phenomenon due to collecting the water in a bay area (I don’t know the regional oceanography, but I would assume such low pH is usually only found in estuaries or within the most intense core of oxygen minimum zones?).
Figure 3 Similar query, why is pCO2 so high in the control, and why is it increasing so much in the control? Is this due to using water with high organics that are degrading, or are the tanks indoors with a high CO2 concentration in the air? I haven’t seen such high pCO2 values in this type of work so would be a bit curious to know what’s driving this as it also affects the context of the results (presumably acidic conditions favor more rapid dissolution of some minerals, and affect calcite saturation states, but if the water has high organics that may also be a key issue affecting dissolution rates).
194 As above, how representative is the starting condition? If the control/initial conditions had been in equilibrium with atmospheric pCO2, then I assume saturation states would have been higher.
205 In comparing different studies might it be useful to give the key differences, e.g. grain size, and perhaps to elaborate why small scale mixing/bottle experiments are potentially not so useful when scaling up to ‘real’ seawater?
208 “the limited potential of olivine for OAE in this region” Can it be stated with certainty that the difference is regional and does not reflect experimental set up e.g. mixing dynamics and particle size?
212 “these minerals and have been skeptical about costs associated with the raw materials and processing” I wasn’t clear which minerals this refers to. I assume impurities are the main concern with ‘waste’ minerals whereas mining/processing costs applies to olivine etc?
234 “reflecting the natural prevalence of the degradation” This hints at an answer to some earlier queries, it would be useful to somewhere clarify what the dynamics are in this coastal region, as I am inferring that the specific site has high organic loadings and is a CO2 outgassing region which affects the context about calcite saturation states, DIC dynamics and OAE viability.
235 (as per 240) This seems perhaps a little speculative without any biological parameters from the experiment. I haven’t worked in this region, but often I would expect when coastal seawater was enclosed there would be a bloom and some DIC decline over a few days, here it seems the opposite with DIC increasing suggesting there was no phytoplankton bloom and that the experiment is characterized by organic carbon degradation and the release of CO2 under control conditions. It would be useful to have some context about this.
260 As previous comments, some context would help understand how representative this seawater is of the area, how much of the coastline for example experiences low pH and high pCO2 comparable to the control herein?
Supplement
Thanks for sharing data, the file download works, however please note multiple datasheets in file are not always machine readable, and it would be better to include units in the headers and avoid empty cells (ambiguous), consider ‘not applicable’ ‘not measured’ etc