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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RC1: 'Comment on egusphere-2025-3925', Anonymous Referee #1, 27 Aug 2025
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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
Citation: https://doi.org/10.5194/egusphere-2025-3925-RC1 -
RC2: 'Comment on egusphere-2025-3925', Anonymous Referee #2, 03 Sep 2025
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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
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