Field experiments in ocean alkalinity enhancement research

Cyronak, Tyler; Albright, Rebecca; Bach, Lennart

doi:https://doi.org/10.5194/sp-2023-9

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https://doi.org/10.5194/sp-2023-9

Preprints

27 Jun 2023

| 27 Jun 2023

Status: this preprint is currently under review for the journal SP.

Field experiments in ocean alkalinity enhancement research

Tyler Cyronak, Rebecca Albright, and Lennart Bach

Abstract. This chapter focuses on considerations for conducting open-system field experiments in the context of ocean alkalinity enhancement (OAE) research. By conducting experiments in real-world marine systems, researchers can gain valuable insights into ecological dynamics, biogeochemical cycles, and the safety, efficacy, and scalability of OAE techniques under natural conditions. However, logistical constraints and complex natural dynamics pose challenges for successful field trials. To date, only a limited number of OAE field studies have been conducted, and guidelines for such experiments are still evolving. Due to the fast pace of carbon dioxide removal (CDR) research and development, we advocate for openly sharing knowledge and lessons learned as quickly as possible within the broader OAE community and beyond. Considering the potential ecological and societal consequences of field experiments, active engagement with the public and other stakeholders is essential. Collaboration, data sharing, and transdisciplinary teams are vital for maximizing the return on investment during field trials. The outcomes of early field deployments are likely to shape the future of OAE, emphasizing the need for transparent and open scientific practices.

Received: 15 Jun 2023 – Discussion started: 27 Jun 2023

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Tyler Cyronak et al.

Status: final response (author comments only)

RC1:
'Comment on sp-2023-9', Lester Kwiatkowski, 04 Jul 2023
Overview
This chapter is well-written and I have no major objections to it. My main criticism would be that it is fairly light on concrete recommendations, examples and references. I understand that this is in large part due to the infancy of this research area and the very limited number of field experiments that have been performed/published. Nonetheless, perhaps the authors could be more specific in their recommendations by utilizing related literature. For example, can the many field experiments of mCDR based on iron fertilization offer specific insights into OAE best practices?
Given that the principal goal of OAE is CDR the chapter could do with more focus on air-sea gas exchange. I would argue that the principal objective of OAE is CDR (although minimizing OA impacts on organisms may be a secondary consideration). With this in mind, perhaps the authors could do more to highlight the types of oceanographic and carbonate chemistry conditions/measurements that might measurable CDR.
Specific comments
L62-64. Will projects adding ground alkalinity “require” tracking of mineral dissolution and the fate of alkalinity? Although these are interesting related research questions, I’m not convinced it’s a prerequisite to tracking CDR which in most cases is likely to be the principal objective.
L 84-86 See previous point on the focus on alkalinity tracking as opposed to CDR. The rationale behind tracking alkalinity could perhaps be better introduced. I consider alkalinity tracking more important to assessing environmental impacts/co-benefits than CDR.
L99-103. Perhaps missing something on physical dynamics here and there importance in relation to CDR (e.g. ocean kinetic energy, mixed layer depth, waves, tides, wind speeds).
L104. “drawdown of atmospheric CO2” – OAE could also be deployed to prevent natural carbon degassing (e.g. in upwelling regions). Maybe this should be specified. The overall outcome is the same (an increase in the ocean DIC inventory).
L108-109. I wouldn’t refer to a “DIC deficit”. The deficit that results from OAE is in pCO2 and it’s this that equilibrates with the atmosphere.
L111-112. “minimize mixing” is a bit ambiguous here. While minimal mixing of different ocean water masses may be desired, “mixing” of the surface ocean by higher wind speeds/wave action will increase the rate of air-sea gas exchange and may make CDR easier to measure. Another point that could be made here, or in the signal-to-noise section, is that sites with lower buffer capacity may also better enable measurable CDR as there is greater change in CO2(aq) per unit change in alkalinity (Egleston et al., 2010; Hauck et al., 2016).
L126-127. Is this because of the dissolution timescale? I would expect the limiting factor to be the timescale of air-sea gas exchange not the dissolution timescale of minerals. Perhaps the authors could put broad ranges on these numbers.
L139. Deployment in boat wakes to maximize dissolution have also been proposed (Renforth and Henderson, 2017; He and Tyka, 2023).
L154. If the turbidity may be affected it’s not impossible that albedo might also be. I think it would be good practice in OAE tests to monitor for any change in ocean surface albedo. The ultimate aim of CDR is to affect the Earths radiative budget and limit anthropogenic warming, unintended impacts on the radiative budget through albedo changes are an uncertainty that has received limited attention.
Table 1. More detail on the potential trace metals in some of these alkalinity sources would be useful. Should phosphate be listed as a dissolution product of carbonates?
L188-190. Prevailing meteorological and oceanographic conditions may also be a consideration.
Box 1. Physics
I’d mention potential albedo impacts here.

I’d split point 4 into 2 points both of which are important. 1) what is the air-sea gas exchange prior to OAE? (I wouldn’t call this “natural” as it may be strongly influenced by the anthropogenic carbon) 2. Water is the residence time of water in the surface ocean/mixed layer and how does this relate to the estimated equilibration time?

Not sure what physical “risks” are being referred to here. Is this referring to local water residence times and the potential impact on MRV?

Chemistry
The authors could distinguish mean state carbonate chemistry conditions and their variability (diurnal/seasonal/interannual). Both of these will impact signal-to noise.

Biology
Are there times of the day or seasons with enhanced species/ecosystem sensitivities?

Table 2.
Some of the transboundary transport tools could be equally used for assessing surface ocean residence times

Mixed layer depth and atmospheric wind speeds may also be important parameters as they have been in iron fertilization field experiments.

Ocean biogeochemical models are also likely to be provide useful assessments of water residence time, gas exchange and potential nutrient/primary production impacts.

Section 2.5. Could this section be expanded to dual tracer techniques in general? While the technique discussed is applicable for assessing potential ecosystem cobenefits it is less applicable to CDR. Perhaps details on He/SF6 and gas exchange for example from the SOGasEx experiments would be useful (e.g. Ho et al., 2011).
Table 3. It might be worth mentioning the very high global warming potential of the 2 gases in this table as a potential limitation.
L320-335 There are many references that could be added here on the diel, seasonal and interannual variability/controls of carbonate chemistry and air sea gas exchange (e.g. Bates et al., 1998; Bates, 2002; Hagens and Middelburg, 2016; Landschützer et al., 2018; Sutton et al., 2019; Torres et al., 2021). Often these studies relate to the “time-of emergence” of anthropogenic trends but the similar signal-to-noise considerations are relevant for CDR detection.
L320-335 numerical simulations and machine learning based network design are potentially valuable tools to optimize observational networks to detect changes.

Minor points
L52. Maybe “functionality” should be “efficacy”.
L235. Should “categorizing” by characterizing?
L272. I wouldn’t refer to air-sea gas exchange as a community carbon flux (it would occur in abiotic waters).

References
Bates, N. R.: Seasonal variability of the effect of coral reefs on seawater CO2 and air—sea CO2 exchange, Limnology and Oceanography, 47, 43–52, https://doi.org/10.4319/lo.2002.47.1.0043, 2002.
Bates, N. R., Takahashi, T., Chipman, D. W., and Knap, A. H.: Variability of pCO2 on diel to seasonal timescales in the Sargasso Sea near Bermuda, Journal of Geophysical Research: Oceans, 103, 15567–15585, https://doi.org/10.1029/98JC00247, 1998.
Egleston, E. S., Sabine, C. L., and Morel, F. M. M.: Revelle revisited: Buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity, Global Biogeochem. Cycles, 24, GB1002, https://doi.org/10.1029/2008GB003407, 2010.
Hagens, M. and Middelburg, J. J.: Attributing seasonal pH variability in surface ocean waters to governing factors, Geophys. Res. Lett., 43, 2016GL071719, https://doi.org/10.1002/2016GL071719, 2016.
Hauck, J., Köhler, P., Wolf-Gladrow, D., and Völker, C.: Iron fertilisation and century-scale effects of open ocean dissolution of olivine in a simulated CO 2 removal experiment, Environ. Res. Lett., 11, 024007, https://doi.org/10.1088/1748-9326/11/2/024007, 2016.
He, J. and Tyka, M. D.: Limits and CO₂ equilibration of near-coast alkalinity enhancement, Biogeosciences, 20, 27–43, https://doi.org/10.5194/bg-20-27-2023, 2023.
Ho, D. T., Sabine, C. L., Hebert, D., Ullman, D. S., Wanninkhof, R., Hamme, R. C., Strutton, P. G., Hales, B., Edson, J. B., and Hargreaves, B. R.: Southern Ocean Gas Exchange Experiment: Setting the stage, Journal of Geophysical Research: Oceans, 116, https://doi.org/10.1029/2010JC006852, 2011.
Landschützer, P., Gruber, N., Bakker, D. C. E., Stemmler, I., and Six, K. D.: Strengthening seasonal marine CO 2 variations due to increasing atmospheric CO 2, Nature Climate Change, 8, 146–150, https://doi.org/10.1038/s41558-017-0057-x, 2018.
Renforth, P. and Henderson, G.: Assessing ocean alkalinity for carbon sequestration, Reviews of Geophysics, 55, 636–674, https://doi.org/10.1002/2016RG000533, 2017.
Sutton, A. J., Feely, R. A., Maenner-Jones, S., Musielwicz, S., Osborne, J., Dietrich, C., Monacci, N., Cross, J., Bott, R., Kozyr, A., Andersson, A. J., Bates, N. R., Cai, W.-J., Cronin, M. F., De Carlo, E. H., Hales, B., Howden, S. D., Lee, C. M., Manzello, D. P., McPhaden, M. J., Meléndez, M., Mickett, J. B., Newton, J. A., Noakes, S. E., Noh, J. H., Olafsdottir, S. R., Salisbury, J. E., Send, U., Trull, T. W., Vandemark, D. C., and Weller, R. A.: Autonomous seawater pCO₂ and pH time series from 40 surface buoys and the emergence of anthropogenic trends, Earth System Science Data, 11, 421–439, https://doi.org/10.5194/essd-11-421-2019, 2019.
Torres, O., Kwiatkowski, L., Sutton, A. J., Dorey, N., and Orr, J. C.: Characterizing Mean and Extreme Diurnal Variability of Ocean CO2 System Variables Across Marine Environments, Geophysical Research Letters, 48, e2020GL090228, https://doi.org/10.1029/2020GL090228, 2021.
Citation: https://doi.org/10.5194/sp-2023-9-RC1
- AC1: 'Reply on RC1', Tyler Cyronak, 19 Sep 2023
  
  We appreciate the reviewer’s constructive comments and suggestions. Based on these and other reviewer comments, we will incorporate more concrete and specific recommendations throughout the manuscript.
  We agree that measuring oceanic CO₂ uptake via air-sea gas exchange following alkalinity enhancement is one of the many important considerations to determine the success of an OAE operation. Since Ho et al. (this issue) provides a detailed discussion of the topic we have opted to not expand our discussion on it but refer to this chapter throughout our manuscript.
  We will address specific comments when we upload a revised version of this manuscript.
  
  Citation: https://doi.org/10.5194/sp-2023-9-AC1
RC2:
'Comment on sp-2023-9', Will Burt, 21 Jul 2023

Overall I found this chapter to be well-written and quite comprehensive. As someone at more advanced stages of planning such a field trial, I found some concepts to be somewhat obvious, but this is likely just my personal circumstance. For someone at the early stages of considering or even planning a field trial, I see this being a very valuable guide. I recognize the challenging in presenting such a guidebook at such an early stage, as there are little-to-no lessons/examples to build from. Overall, I have a series of minor comments but feel this manuscript 'does the trick' as a best-practices guide, at least at this early stage.
General comments are as follows:
Little mention of emissions accounting and associated logistics are discussed here, but gaining an accurate picture of the LCA behind a proposed OAE deployment could be an critical learning objective. Not sure if this deserves mention in a sentence, or perhaps an entire section. Perhaps this is discussed more in another part of the Best Practices guide, but i think it should be mentioned more in this chapter.
Given how early-stage we are as a community, one general note of caution is about using language like 'essential'. Generally the authors do a good job avoiding this, but one example in on line 334. Good baselining of a region is deemed 'esssential', but that this may take years to do. Perhaps this idea of 'good baselining' is intentionally vague, and ultimately I agree that baselining is important, but I feel that the assertion that years of baselining is required can create a signifiant barrier to progress. In my view, some level of baselining is done prior to a field experiment, but understanding seasonal and/or interannual changes in the region could occur during the progression at a proposed pilot site (i.e. after initial trial(s) have occurred). In contrast, line 219 states that it may or may be a 'priority' to measure trace metals or nutrient concentrations. I think labeling things as 'priorities' is a better way to go.
It may be worth recognizing early in the document that resources will quickly become limiting in these early field trials if attempting to 'do it all'. The authors mentioned this when establishing main goals of the experiment, but explicitly stating that each of these 'potential overarching goals' listed are large and complex endeavors might be wise. The best example of this of course is the community engagement piece of a field trial, very easy to say, but very challenging to do properly, and way out of the wheelhouse of academic/industry researchers that often lead experimental development.
The article only has essentially 1 figure, albiet a very useful one showing conceptual dye tracer work. Given context of the material a lack of figures is understandable, but more of these idealized figures could have been useful. One example could be a visual showing a theroteical alkalinity plume and how one might most effectively sample that plume (using different sensors/platforms)?
I have a number of relatively minor suggestions/comments, but overall I commend the authors of a job well done. Suggestions, line by line, are given below:
Line 38: LCA can be a critical logistical contraint.
Line 59: may not be appropriate for a project to only focus on one of these goals. Seems like responsible (and publicly acceptable) research may want to prioritize having at least elements of 3+ of these larger goals.
Line 64: 'require' tracking both the dissolution PLUS the fate of the dissolved alkalinity...I think it's feasible that a project focus on the dissolution and not try and track the fate. Again, words like prioritize feel more appropriate here.
Line 68: residence time of the receiving waters is an important consideration (perhaps covered by dilution and advection)
Section 2.1: important to note that these alkalinty types fall along a spectrum, and are not binary between rocks that dissolve slowly and electrochemical techniques. Some rocks/minerals dissolve very quickly and thus are probably closer to electrochemical than to olivine. This is important because measuring in-situ dissolution will be very challenging when using a rapidly dissolving mineral in the water column. This is discussed more later in the document (e.g. line 165-166), and in the tables, but at this early point it reads like just 2 types and nothing in between.
Line 104: just a note that 'tracking the added alkalinity' and 'observing drawdown of atmospheric CO2' are very different things. I'd arge they are major challenges 2 and 3. Later in the paragraph its stated 'this is likely to be the main scientific concern'. I feel these need to be split out because directly measuring the CO2 uptake is probably not going to be main concern early on when the alkalinity addition rates are small.
Line 109: we aren't generating a 'DIC deficit' right that is then equilibrated, right?
Lines 118-129: some of this felt a bit obvious, but may be a biased view
Line 140-142: this feels obvious but is important. Lets not waste too much time/resources on scenarios that aren't going to be applicable down the road. Two lines later the authors suggest using 'barriers' to avoid rapid loss of ground material...to me its not an open system anymore and that is a problematic for applying results down the road.
Line 155: authors could suggest that 'guardrails' are put in place that explicitly state what types of results would generate a 'pause' in the experiment. That seems to be well received as an idea.
Table 1: I feel it should read "natural or manufactured magnesium-derived alkalinity sources", rather than just Mg(OH)2. Just putting Mg(OH)2 limtis you to 'reagent grade' Mg(OH)2, which I doubt will be used too often at field-study scale. There are many 'manufactured' Mg(OH)2 products just like the lime-derived sources (these would have trace nutrients and metals just like the CaO). Also, you could put Mg(OH)2 and CaO etc. as their own rows, but stikcing to Mg-dervied and lime-derived might be easier.
Line 195/Box1: i feel there are geological considerations too. Sediment type, underlying geology, in some cases must play a role? Also, water depth is quite important. No explicit mention of 'natural variability' in the system. No mention of 'prior data', how much exists to date is an important consideration.
Table 2 seems well written and quite comprehensive. In my experience, some communities want more thought beyond planktonic and benthic species...as challenging as that can be.
Line 261-263: A little confused. Aren't small-scale experiments still directly transferable to to natural systems, and they help us learn prior to larger scale deployments. I suppose in some cases the early field trials may not be as transferable, but i'd advise against that as much as is reasonable.
Line 271: The dual tracer example/technique is very nicely presented and will be very helpful. I wonder if there are other specific examples like this that could be added/explored. I can't think of any just now but worth a thought.
Line 323: as mentioned at the outset. 'essential' multi-year baselining (even if not written exactly like that) could be a siginificant barrier. Could a suggestion be made that some baselining must occur, and can/should continue at the early stages of a site development (because the alkalinity additions will eventually fully flush, leaving the system ready for decent baselining again? A fairly repetitive statement like this is on line 334-335.
Line 379-380 : I appreciate how the authors provide suggestions alongside actual ideas/examples. For example, the idea of a centralized data platform is backed up by two potential ideas for where such a platform could be housed.

Citation: https://doi.org/10.5194/sp-2023-9-RC2
- AC2: 'Reply on RC2', Tyler Cyronak, 19 Sep 2023
  
  e thank the reviewer for their constructive comments and suggestions. Overall we agree it is difficult to make concrete recommendations based on the few OAE field trials that are still ongoing. In our revisions we will make more concrete recommendations based on all of the reviewer's comments. We will also address all minor comments in the manuscript revision. Some general areas we will address are highlighted below.
  We will add more discussion on life cycle assessment and the roles it can play in determining the efficacy of the OAE projects and highlight the, pointing the reader towards other discussions and studies (e.g. Foteinis et al. 2023).
  We will change some of the language, especially when using words like ‘essential’ to more accurately describe research ‘priorities’.
  We agree that resources can become limiting and it is important to focus field trials on specific goals and outcomes. Collaboration with experts in aspects like community engagement that are outside the expertise of researchers will be an important part to the success of OAE and we will further highlight this in our revisions.
  
  Citation: https://doi.org/10.5194/sp-2023-9-AC2
CC1:
'Comment on sp-2023-9', Grace Andrews, 25 Jul 2023

This chapter provides a decent starting point for those considering undertaking an OAE field trial. I believe it nicely conveys the breadth of considerations and there is only one major piece of the puzzle missing from the discussion, namely logistics. I also believe it would benefit from some clearer definitions around terms like “field trial” and “full-scale”. I detail these two concerns below followed by some minor comments. In general, this chapter should be published following the appropriate revisions.

Comment 1: Logistics – This chapter focused primarily on scientific considerations with respect to field trials but appropriately acknowledged permitting and stakeholder engagement as well. However, there was no discussion of logistics and the significant role that plays in shaping field trials. For example, I believe proximity to a marine institute (for land-based approaches) or access to a research cruise (for open ocean) is often essential to execute the type of science program advocated for here. In both cases, the location of these kinds of scientific resources plays a defacto but significant role in dictating where one does a field trial.

Logistics can manifest in other ways too that may be obvious but weren’t stated here: If your OAE approach is tied to wastewater discharge or desalinization, then you’re constrained to areas with these plants. If your OAE approach is fundamentally about putting sand on coastlines then you need to work on stretches of coastline that have roads and truck access, or have deep enough water that they are accessible by dredge so that you can actually place the material. If your reactor process consumes lots of energy, you may want to think about the local grid before you start building. The list goes on.

Comment 2: Definitions – I believe this chapter would benefit from a more thorough discussion of what is actually meant by “field trial” versus “full-scale.” The authors loosely define field experiment as “the addition of alkalinity to a natural system in ways that simulate planned OAE deployments” but I find this vague. What is the difference between something that “simulates” a deployment and an actual deployment? Is a field trial defined by the size of the intervention – total added alkalinity, length of time alkalinity is added, both? What is the line? Is “small” or “big” different for different kinds of OAE interventions?

Perhaps “field trial” is defined by the extent of the rigorous, in-depth science program? Maybe it is defined by the project’s owners or goals (e.g. academic or NGO research, industry R&D, carbon credit sales) regardless of the breadth of the science program?

Alternatively, is “field trial” defined by the permit as in many jurisdictions there are specific “research permits” that can be sought?

A combination of above? Who gets to decide and why? What are the implications? What is precedent from other areas of CDR, CCS?

Anyway, I think this is important because depending on how we define “field trial” that of course can change the guidance on how to conduct one. As an example, if field trials inherently have to be small-scale, the authors should provide some thinking on how to define “small” for any given intervention and it fundamentally changes the discussion about potential environmental impacts associated with field trials. Statements such as “field experiments evaluating CDR approaches carry the risk of unintended consequences and impacts over vast spatial scales, so appropriate scaling (e.g., starting small) is necessary” (pg 20) are no longer appropriate. Rather, the small-scale nature of field trials is then key to their value in that they can evaluate the potential for impact without meaningful risk of causing significant impact over larger spatial or temporal scales themselves. I think that is an important point about starting small that I would love to see emphasized in the text.

As a side note, do the authors distinguish between “field trials” and “field experiments”? Both are used interchangeably in the text but the common definition of a field trial is a test of a product or device in the environment while the common definition of a field experiment is simply an experiment conducted in the natural environment (but not necessarily of a product; the product here being the OAE intervention). If field trials and field experiments are different then how does that change the guidance in this chapter for one versus the other? In the introduction the authors define ‘field experiment’ as “the addition of alkalinity to a natural system in ways that simulate planned OAE deployments” but elsewhere in the text make statements such as “to make results more broadly applicable, field experiments should attempt to mimic real world alkalinity application scenarios” (pg 6) and “field experiments will presumably mimic plans for real world OAE deployments.” These statements seem redundant with the authors given definition.

Minor Comments:

L64: Projects adding alkaline materials to the ocean are not “required” to track the dissolution of those materials, it may be sufficient to track one or the other.

L70: This section starts off as “addressing public concern”. The framing should be broader here. Not all members of the public will be concerned; some will be supportive, some will be neutral but they must all be given an opportunity to be informed and have their questions answered. Same comment L358.

L71: The field site (e.g. state and/or nation-level boundaries, for example) is a factor in determining permitting requirements but so is the source of alkalinity and method of introduction.

L109: Not a DIC deficit but a CO2 deficit & and also not necessarily a deficit, OAE also works when outgassing is prevented (L114).

L120: Also important for minimizing environmental impact and in many cases, meeting regulatory requirements.

L153: I would argue that best practice should include testing to insure there are no “other contaminants.”

L150-179: I think this page would benefit from a discussion on how potential environmental impact changes with the scale of the field trial and also how to evaluate risk. For example, permits in the US require documents like Environmental Assessments which consider both the potential impact but also the likelihood that the impact is sustained, efforts to minimize the impact, efforts to monitor the impact, and whether the risk outweight the benefit of the project. Also, many potential impacts of OAE additions that are referenced in this chapter (e.g. turbidity for coastal enhanced weathering and extreme localized changes in carbonate chemistry for aqueous additions) are evaluated in standard permitting pathways (e.g. beach renourishment and wastewater discharge, respectively), so regulatory compliance will help insure these potential impacts are addressed. In summary, impact is nuanced and a risk-benefit framework is generally used. I would advocate for this chapter to provide guidance on risk-benefit analysis. Foteinis et al., 2023 also has some nice thinking on this.

Table 1: Naturally-occurring brucite (as opposed to synthetic MgOH2) contains other elements as well but the two are not distinguished here.

Box 1, Biology: I would add the consideration of whether there are culturally or commercially important species present. Not that the presence of commercially important species would necessarily be an issue, but one might want to be sure to have a focus on that in the monitoring program.

L333: Im not sure how practical it is to suggest that baselining occur “over long periods of time”. This is probably another factor that should be incorporated into site selection, i.e. select locations with a wealth of pre-existing scientific data either in the peer-reviewed literature and/or proximity to publically available (often government run) stations/buoys such as the USGS or NOAA monitoring networks in the US.

L371: I would include a call to publish data in open source, peer-reviewed journals whenever possible.

Citation: https://doi.org/10.5194/sp-2023-9-CC1
- AC3: 'Reply on CC1', Tyler Cyronak, 19 Sep 2023
  
  We thank the reviewer for their thoughtful comments and will address them in our revisions, with come overall considerations highlighted here. We will also address all minor comments in the manuscript revision.
  Overall, logistics will be critical in determining the type of project that can be completed and the scientific plan that needs to be implemented. We will add a paragraph that discusses how logistics need to be considered for various OAE approaches and how those considerations could impact the scientific planning process.
  We will tighten our definitions, especially concerning the terms ‘field trial’ and ‘field experiment’. Broadly, we agree with the reviewer's comment that a field trial has the connotation that a product is being tested with the intention of deployment in the environment.
  
  Citation: https://doi.org/10.5194/sp-2023-9-AC3
RC3:
'Comment on sp-2023-9', Alex Poulton, 28 Jul 2023

This is a well written overview of the techniques, pros and cons of open field experiments. It will provide a valuable resource to those planning open field experiments. There are a few minor issues that should be addressed.
Ln 62-63: Is it worth also highlighting the need to monitor the sinking speeds of the (ground) particulate material?
Ln 158-159: Another important consideration to highlight is the potential for bio-accumulation in higher trophic level organisms (especially those of commercial importance).
Box 1: Chemistry, ‘or micronutrients’ – some of these micronutrients are also potentially toxic at high concentrations.
Table 2: typo in carbonate chemistry conditions row – how much rather than high much.
Table 2: macronutrient and micronutrient rows – Beyond the potential pathway for assessment of the basic methods for determining concentrations of these elements, should the table highlight experimental methodology to allow the ‘assessment of whether the designated system is prone to’ macronutrient or micronutrient fertilization via OAE.
Ln 237: ‘ideally’ – this seems an understatement; surely in any OAE experimental work that needs to understand the efficacy and impact of OAE, two carbonate chemistry parameters should be measured as standard.
Ln 268: ‘additional alkalinity’ – do you mean excess alkalinity?
Fig 2. Differences between panels (a) and (b) are too subtle and it is difficult to distinguish. What about putting the panels together and emphasizing the differences in some way?

Citation: https://doi.org/10.5194/sp-2023-9-RC3
- AC4: 'Reply on RC3', Tyler Cyronak, 19 Sep 2023
  
  We thank the reviewer for their positive feedback and will address all of their minor comments in our revised manuscript.
  
  Citation: https://doi.org/10.5194/sp-2023-9-AC4

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Short summary

Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (CDR) approach. Publicly funded research projects have begun, and philanthropic funding and start-ups are collectively pushing the field forward. This rapid progress in research activities has created an urgent need to learn if and how OAE can work at scale. This chapter of the Best Practices Guide to OAE research focuses on field experiments.


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