Reply to comments on EGUSPHERE-2023-2611

RC1: 'Comment on egusphere-2023-2611', Andrew Dickson, 05 Mar 2024 (https://doi.org/10.5194/egusphere-2023-2611-RC1)

I apologize for the time it took me to get to writing this review. I found this to be an interesting manuscript introducing what is likely to become a real problem if Ocean Alkalinity Enhancement (OAE) is to become well established as an mCDR approach.

Thanks a lot for your dedication. We appreciate the opportunity to refine our work based on your feedback. In the following sections, we have addressed each of your comments comprehensively, aiming to clarify and enhance the quality of our work as per your suggestions.

The authors carried out a simple series of experiments to assess what happens after increasing seawater alkalinity. They studied the addition of two alternate solutions: one a simple strong alkali, the other a similar solution that had been equilibrated with CO2 at a partial pressure of ~420 µatm. They showed that large additions of alkalinity to a seawater could trigger the precipitation of calcium and magnesium carbonates, as well as magnesium hydroxide, thus reducing the alkalinity in the final solution. If however, the alkaline solution had been pre-equiilibrated with CO2, then higher alkalinity levels could be achieved without triggering the precipitation process. The experiments suggest that the kinetic process that is involved here is somewhat reproducible in its behavior (note: the experiments did not appear to be scrupulously replicated, rather they were repeated with differing initial solution compositions over a range of total alkalinity (and for the addition of pre-equiliibrated solution, also an associated range of total dissolved inorganic carbon).

That said, I did find the author's figures and text awkward to read and understand. The difficulty is that they are describing a time-dependent process, where an initial solution composition (formed as a mixture between seawater and a synthetic solution (either NaOH, or an Na2CO3/NaHCO3 mixture with a nomiinal p(CO2) of 420 µatm) changes as a result of precipitating inorganic solids: CaCO2, MgCO3, Mg(OH)2 . The changes are complex due to the equilibrium chemistry of CO2 in such systems, with not only the total alkalinity and total dissolved inorganic carbon changing, but also other compositional properties such as p(CO2), pH, and Ωaragonite.

The time-dependent process the authors describe is illustrated as a simple conceptual plot in Fig. 9, and also illustrated as alkalinity loss in Figs. 5 & 6.  Essentially the earlier figures (2,3,4) show the experimental measurements over the course of 20 or 25 days (depending on the experiments). It is these three figures that are hardest to follow (as in them the time dependence is not so clear, and the scales chosen seem somewhat arbitrary).

Thank you very much, the authors agree that Figures 2, 3 and 4 could be quite complex to read. From the standpoint of the authors Figures 2 and 3 provide a relatively space-efficient option to present the basic parameters. Time-dependent plots like Fig. 6 would double to triple the used space. Also, by omitting the time-dependencies, patterns emerge that might otherwise be difficult to distinguish. For example, the comparison between target and actual reached TA in figs. 2 and 3 demonstrates the consistency in adjusting the alkalinity to a specific level, as well as the divergence when immediate Mg(OH)₂ precipitation prevents further alkalinity increase within the examined ranges. The same applies to the uniform stabilization of TA, Ω_aragonite and pH, which occurred after the precipitation process significantly slowed down. These final configurations adhere to the rules of the underlying carbonate system, provided that the runaway processes are halted at experimentally specific, constant Ω_aragonite values.

Fig. 4 visualizes the dependency of the specified parameters on the carbonate formation (2:1 ΔTA: ΔDIC decline ratio). As one of the main objectives of this publication is to present repeating general patterns, comprehensibly following relatively simple effects of the carbonate/hydroxide formation, the temporal evolution was not the main focus of these diagrams.

The authors would like to provide possible measures to improve their readability:

1)            Additional time-dependent plots will be provided in the supplements

2)            Captions and the related text will be adjusted to improve clarity

3)            Design adjustments in Figs. 3 and 4, particularly for the non-CO₂-equilibrated abiotic approach (Fig 3 (d-f) and Fig 4(e)), such as reducing the number of highlighted plots or data points while fading out others. The comprehensive displayed diagrams would then be provided in the supplementary materials.

Nevertheless, I feel the authors are adequately clear in what they did, and in what they found and I believe the paper is a useful first step in this potentially important area.

That said, I have a significant number of small comments (some addressing typos) that  I feel the authors should consider changing.

The unit "µeq kgsw–1 " for alkalinity seems  both old-fashioned, and (sliightly) problematic. First, the use of equivalents has been deprecated in physical chemistry for many decades with moles being a preferred alternative.

Following your suggestion, “µeq” will be replaced by “µmol” throughout the text.

Second, as the experimental solutions are a mixture of natural seawater and another inorganic solution, referring to the amount content of a component as "per kilogram of seawater" seems misleading; strictly it is per kilogram of solution (viz amount content)

In fact, the experimental solution contained a certain degree of stock solution. For a total amount of 64 treatments, 6 (abiotic CO₂-equilibrated, ΔTA 5200-9200) surpassed 1 % of inorganic solution to seawater ratio. While “per kilogram of solution“ would be a technically correct description, the authors would like to suggest to use “per kilogram [µmol kg-1]  to minimize the potential for confusion by deviating from the common units used for carbonate chemistry parameters.

(line 37) "Once" not "Ones"

This will be corrected

(lines 46-48) This does not read right, perhaps words are missiing?

“Implementation of such identified stability and loss patterns into ocean biogeochemical models, capable of resolving mixing patterns of treated and untreated water parcels, would allow to predict, from the geochemical perspective, safe local application levels of TA, as well as the fate of added alkalinity, and therefore a more realistic carbon storage potential as if neglecting observed carbonate system response to OAE.” (L45-48)

Section was rewritten to:

“Incorporating such stability and loss patterns into ocean biogeochemical models, which are capable of resolving dilution processes of treated and untreated water parcels, would, from a geochemical perspective, facilitate the prediction of safe local application levels of OAE. This approach would also allow for an accurate determination of the fate of added alkalinity and a more realistic carbon storage potential estimation compared to the assessments that neglect carbonate system responses to OAE.”

(lines 58-59) I do not feel that the oceanic residence time for inorganic carbon is a meaningful concept when discussing OAE. If a parcel of seawater is taking up CO2 it is, by definition, at the surface not well-mixed around the ocean.

Thank you very much for the comment. The authors agree and understand that the sentence was misleading. This will be updated to: “Naturally, inorganic carbon is stored in the ocean over periods of time ranging from 10,000 to 100,000 years (Berner et al., 1983; Mackenzie & Garrels, 1966). This long-term carbon storage potential makes OAE a preferred option over other suggested marine CDR methods.”

(line 68) "ground" not "grinded"

This will be adjusted

(Fig. 1 legend). pCO2 is a partial pressure, and thus should be expressed in pressure units (e.g. 420 µatm) not as a mole fraction (420 ppm)

ppm will be replaced by µatm (Captions Fig. 1 and L106)

(lines 122-123)  What magnitude are "Minor shifts"?

In the following table we provide the shifts in the control treatments (ΔTA0), at the start and end (after 25 days) of each experiment:

The table will be added to the supplementary materials and a corresponding reference given in L122.

(lines 125-131) What are the measurement uncertainties of these techniques?

The uncertainties were set to: TA ±5 µmol kg^-1; pH ±0.02, based on frequent measurements of standards. The uncertainty for temperature, of ±0.1°C, was taken from the probe’s certified specifications. For salinity the uncertainty of ±0.1 was extracted from the calibration standard instructions. These numbers were also used for error propagations in CO2sys based on Orr et al. (2018). This information will be added within section 2.2, “Sampling and measurements”

(line 195) missing word?

A comma will be added in L194 and a “such” in L195

Will be adjusted to: “[…], indicating the formation of non-carbonate-bearing secondary phases such as Mg(OH)₂ (also see section 4.3).”