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.

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.

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?