the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Feb 2026
Technical note: Verification of coastal enhanced weathering by tracking the dissolution of alkaline minerals: Theory and laboratory tests with olivine
Abstract. Coastal Enhanced Weathering (CEW) is a marine Carbon Dioxide Removal (CDR) method that adds ground alkaline minerals to shallow regions of the ocean in order to increase seawater alkalinity, i.e., its capacity for storing atmospheric CO2 as bicarbonate. While CEW is promising with regard to cost and scalability, it is an uncontained, “open-system” style of CDR, and presents significant challenges to effective measurement, reporting, and verification (MRV) of the process. In particular, quantifying how much alkalinity is released from an added amount of mineral is challenging as the minerals dissolve and release alkalinity over wide spatial and long temporal scales. Such quantification is further complicated by the fact that dissolved alkalinity is rapidly diluted below detectable concentrations. Here, we propose an approach to measure alkalinity formation that relies on solid sediment tracers to track mineral grains underwater and quantify how much they have dissolved over time. The amount of dissolution at any given point in time is proportional to the amount of alkalinity released. Thus, the approach aims to overcome the near impossible detection of alkalinity accumulation in the dissolved phase by tracking the loss of alkaline material in the solid phase. We describe a test of the fundamental aspects of this method including the measurement of the mineral and tracer content of a sediment sample, and the extent to which those measurements correlate with changes in seawater chemistry. We found that olivine dissolution significantly increased alkalinity in seawater, compared to control incubators. X-Ray Diffraction (XRD) was able to quantify the change in the olivine in the sediment (albeit with large relative errors) and automated particle counting methods were able to enumerate tracer particles when illuminated under UV-A light. These results serve as a proof-of-principle (and starting point) to further explore a promising way to measure alkalinity addition to the ocean resulting from CEW deployments.
Status: open (until 25 Apr 2026)
- RC1: 'Comment on egusphere-2026-929', Anonymous Referee #1, 27 Mar 2026 reply
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RC2: 'Comment on egusphere-2026-929', Anonymous Referee #2, 17 Apr 2026
reply
This paper proposes a new approach for measuring mineral dissolution in coastal enhanced weathering (CEW) applications, using UV fluorescent tracer particles. The study uses seawater incubations with the tracer and olivine+quartz (treatment) and quartz (control). The tracers did not prove useful in measuring dissolution rates, however, as methodological issues affected their stability (size fractionation) over the course of the experiment. The authors make two main conclusions: 1) the tracer method is a promising tool for measuring mineral dissolution (despite the fact they were not ultimately useful in this particular experiment); and 2) olivine dissolution and alkalinity generation occurred in the treatments (based on alkalinity measurements, and inferred loss of olivine mass). The paper is overall well-written and the experiments explained clearly. I admire the risk taken here to explore a new methodology, and respect that “methodology papers” are a lot of work! That said, the data fundamentally do not support the proposed method – nor do they indicate a clear failure / dead-end. The paper could be improved with new data, resolving the methodological issues, to better assess the methodology.
- Major issues:
- The main issue is that the key aim of the paper – that tracers can facilitate quantification of mineral dissolution – is not demonstrated. I understand that this is a proof of concept, and completely welcome the unexpected results, but the authors’ conclusions are fundamentally flawed. L430: “we conclude that using UV fluorescence as a traceable characteristic in tracer particles is effective and could be suitable for operational deployment of this dissolution quantification approach”. And “Preliminary indications are that this method could possess sufficient resolution to meaningfully measure mineral dissolution, and corresponding alkalinity addition, in deployment scenarios.” But they have not shown that this approach works to quantify dissolution at all; in fact, they don’t use it in their results. They make this claim in reference to the fact that they were able to see the particles via UV fluorescence, which is not surprising, and was presumably known a priori. This is a great idea, and I appreciate the well-designed experiment. But the results as presented seem like an early experiment, prior to the necessary trial and error to get data of sufficient quality to test the hypothesis. The data simply do not answer the posed question.
- In addition to their key aim, they also use mineral mass measurements to estimate olivine dissolution at the end of the experiment. While this would have been complementary to the tracer-based dataset, it is not particularly novel or useful on its own. Also, they do not provide absolute masses –only relative percentages – so it’s not clear to me that there was not absolute loss, or loss across the different minerals from experimental artifacts. They acknowledge this potential later in the discussion (L506)
- Fundamental questions about the proposed method that are not adequately discussed:
- The paper broadly equates mineral dissolution with alkalinity addition. E.g., L73, “This loss of alkaline mineral would be used to infer the amount of alkalinity added to the seawater.” There are many ways alkalinity may be lost in sediments on short time scales following dissolution, with the consequence of no alkalinity added to the water column. (See Geerts et al review for discussion). The authors discuss this later, and suggest using a site- and mineral-specific alkalinity release factor. However, that factor is unknown given challenging MRV -- which the paper’s method is supposed to help with. (Plus, given different diagenetic environments, you might need an alkalinity release factor for every microenvironment your feedstock is dispersed to.) So there’s some circular logic here, and the issue at hand remains – we wouldn’t know the site specific alkalinity release factors required to use this method in the field.
- Sediment diagenesis and geomorphology considerations: “A perfect tracer will decrease in diameter at the same rate as the dissolving mineral particles do in order to maintain matching hydrodynamic properties, and therefore transport characteristics, to the mineral particles over the full dissolution period.” The decrease in diameter will depend on the mineralogical properties of the material and diagenetic environment. Given different mineralogies, I would think it's virtually impossible that the tracer will shrink at the same rate as the feedstock. It isn’t explained until the caption of Figure 1 that “the scenario of this experiment where an imperfect non-shrinking tracer was used.”.
- Minor issues:
- I’m curious why such small amounts of sediment were used (7.5 g = 0.1 to 3 mm depth)? Maybe explain.
- Figure 1 caption is far too long; the information should be part of the main text.
- Throughout the manuscript, I suggest caution with the assumption that dissolution equals alkalinity release, and adding words like “initial” or “preliminary” to indicate there may be subsequent losses. Instead of saying, e.g., “alkalinity formation”, it’d be better to caveat with “initial alkalinity release”.
- L41: I’m not aware of any CEW projects or research suggesting brucite for CEW (rather, for water column dissolution; brucite on sediments likely has ecology concerns). Similarly L89: lime is not appropriate for benthic CEW.
- L60: perhaps centuries, realistically?
- L63: sources?
- L421: the reason for choosing a high temperature should be explained in the methods.
- L423: Why would you expect this high level of dissolution? Citation?
Citation: https://doi.org/10.5194/egusphere-2026-929-RC2 - Major issues:
Data sets
Data for "Technical note: Verification of coastal enhanced weathering by tracking the dissolution of alkaline minerals: Theory and laboratory tests with olivine." Alexander Milde and Lennart Bach https://zenodo.org/records/18665602
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- 1
General comment:
The technical note by Milde and Bach investigates the challenges of MRV in the OAE context. They approached this topic by exploring a mineral tracer, that should be able to help quantifying the amount of olivine dissolution and related alkalinity generation. This is an interesting and straight-forward approach, that could advance the field of MRV. In their study, the authors conducted batch experiments to investigate the usefulness of the tracer, i.e. Silicon Carbide.
While an alkalinity increase was clearly detected in their experiments, the tracer evaluation failed due to an increase in tracer counts after sediment incubation, meaning a decrease in tracer particle size. This was unexpected as no change in number or size of the tracer particles should have occurred and the authors attributed the changes to reactions during tracer shipping and storage and related high temperatures. As these challenges can always occur, I was surprised to only detect the ‘failure’ of the experiments when reading the main text and that it was not mentioned in the abstract. My first reaction was to recommend rejection of this manuscript as the experiments couldn’t evaluate the effectiveness of the tracer method and the other approaches to define alkalinity generation and CO2-sequestration by the authors (e.g. via XRD) contained large uncertainties. Nevertheless, in times as urgent climate change mitigation actions are required, I came to the conclusion, that this manuscript should be published anyway, as it will advance the research field in the sense that the experienced challenges can hopefully be avoided in future studies and mistakes avoided. Also failed experiments should be published in order to advance science on a timely manner. Therefore, I recommend publication of this manuscript after moderate revisions (see below) and only if the authors incorporate the fact, the results and reasons of the failed experiment in the abstract.
Specific comments:
Lines 147-148: Something is wrong in this sentence, I don’t understand: ‘…, side to err to as….’?
Line 149ff: I understand that this is just a first theoretical approach, but how well does one know the dispersal area after 1 year (or several) of mineral deployment? I assume this could create quite large uncertainties in the assessment as the identification of the regional distribution could be quite challenging at different scales and settings? I would recommend to do some sensitivity analyses to identify the impact on MRV based on these uncertainties.
Line 184: Why is there serpentinite in the treatment group and not olivine? Or do you mean the ultramafic rock addition with serpentinite? Please clarify.
Line 206ff: If understood correctly, there was no shaking of the bottles. I wonder if particle movement, as it is common in coastal settings, would lead to particle abrasion and also abrasion of the tracer. By this, adding another level of uncertainty to the potentially inert character of the tracer. Have you tested the robustness of the tracer with respect to grain collision?
Line 218-219: I would add a comment or reference here, that you also conducted XRD analyzes as I was missing this information when reading this paragraph.
Line 551: Could you give examples of these areas? I find it challenging to come up with places, where mineral particles would not be subject to resuspension.
Figure 2: Please add again a description of what the control and S1-4 is composed of (either legend or caption). I was searching this, when wondering what the causes could be for the increases in TA, pH and aragonite saturation in the control treatment. Is this discussed anywhere? This needs to be incorporated in the main text as it might indicate that the tracer is not as unreactive as initially thought or that reactions in the sand are inducing these geochemical changes. Are you sure it’s really only composed of quartz? Please clarify these points in your revision.