the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The additionality problem of Ocean Alkalinity Enhancement
Lennart Thomas Bach
Abstract. Ocean Alkalinity Enhancement (OAE) is an emerging approach for atmospheric carbon dioxide removal (CDR). The net climatic benefit of OAE depends on how much it can increase carbon sequestration relative to a baseline state without OAE. This so-called ‘additionality’ can be calculated as:
Additionality = COAE - ∆Cbaseline
So far, feasibility studies on OAE have mainly focussed on enhancing alkalinity in the oceans (COAE) but not primarily how such anthropogenic alkalinity would modify the natural alkalinity cycle (∆Cbaseline). Here, I present incubation experiments where materials considered for OAE (sodium hydroxide, steel slag, olivine) are exposed to beach sand to investigate the influence of anthropogenic alkalinity on natural alkalinity sources and sinks. The experiments show that anthropogenic alkalinity can strongly reduce the generation of natural alkalinity, thereby reducing additionality. This is because the anthropogenic alkalinity increases the calcium carbonate saturation state, which reduces the dissolution of calcium carbonate from sand, a natural alkalinity source. I argue that this ‘additionality problem’ of OAE is potentially widespread and applies to many marine systems where OAE implementation is considered – far beyond the beach scenario investigated in this study. However, the problem can potentially be mitigated by dilute dosing of anthropogenic alkalinity into the ocean environment, especially at hotspots of natural alkalinity cycling such as in marine sediments. Understanding a potential slowdown of the natural alkalinity cycle through the introduction of an anthropogenic alkalinity cycle will be crucial for the assessment of OAE.
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Lennart Thomas Bach
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-122', Matthew Eisaman, 14 Aug 2023
Ocean Alkalinity Enhancement (OAE) is a promising approach to carbon dioxide removal (CDR) that is being investigated by many research labs and companies worldwide. To realize OAE’s full potential for CDR, careful measurement, reporting, and verification (MRV) must be employed to accurately determine the net CO2 removed. “The additionality problem of Ocean Alkalinity Enhancement” by L. Bach presents a well designed experimental study that, for the first time, quantifies an important response of the natural alkalinity cycle to OAE interventions. This work will be crucial to the accurate quantification of the net CO2 removed as the result of a given OAE intervention. My comments are included as comments in the attached pdf.
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RC2: 'Comment on bg-2023-122', Anonymous Referee #2, 28 Aug 2023
The manuscript by L. Bach ‘The additionality problem of Ocean Alkalinity Enhancement‘ deals with the question of how much of the natural marine benthic alkalinity release might be reduced due to the addition of alkaline minerals as carbon dioxide removal (CDR) measure. L. Bach found in an experimental investigation, that the natural alkalinity release might be significantly reduced due to the increase in the calcium carbonate saturation state.
The findings by Bach highlight an important aspect concerning the CDR measure of ocean alkalinity enhancement (OAE), which- to the best of my knowledge- has not been discussed in such detail to date. The experiments are thoroughly conducted and serve well to answer the research question. In general, I strongly support the publication of this study, though I have some points, where I would like to see a bit more detail in the discussion.
My biggest concern is the extremely short mention of the effect of ‘natural mineral grinding’, which is one of the largest arguments at the moment for beach applications in the context of OAE. This effect is only discussed in two sentences, in lines 546-550. I wonder, how much of the observed alkalinity increase is caused by this grinding effect and the continuous provision of reactive surfaces and how much by the changes in the carbonate saturation states. It would have been nice to conduct experiments without the shaking effect of the plankton wheel to study and quantify this grinding effect caused by the constant moving in. However, I am aware that such a comment in a review is not helpful when an experimental series is completed, but it might be worth to at least mention and suggest this additional kind of experiment.
Secondly, I am not a big fan of introducing another technical term to a topic where a lot of abbreviations and terms are already used and confused. I wonder, is it really necessary to call the observed effect ‘additionality’, when the term ‘net sequestration effect’ describes it perfectly. Same goes for ‘anthropogenic alkalinity’, whereby the latter is a bit more self-explanatory. I’m happy to wait and see if this term is accepted and used by the community, though I would urge the author to reconsider if it is really necessary to introduce this new term here!
Further I wonder, how much the precipitation of secondary carbonates affects OAE in beach environments due to the quick dilution with seawater movement. This topic is shortly discussed in lines 680ff, were it is also pointed out that the batch experiments represent an extreme case which is likely not directly transferable to the open ocean environments. Here I’m missing a discussion and comparison with the beach transect results. Two beaches (Goats and Clifton North) show no difference between the swash zone and sampling stations with increasing distance to the beach. Also interesting is the difference between Clifton North and South, is there a difference between carbonate contents or is Clifton South a more sheltered environment with less extreme seawater movement and thus a higher chance of alkalinity accumulation? These environmental factors might be good to sum up as a sort of guidance for the site choice of OAE deployments as also highlighted in the conclusions.
Minor points:
Line 84: The official mineralogical abbreviation for aragonite is ‘Arg’. Please change throughout the manuscript.
Line 99-102: I would recommend to state more clearly, that carbonate dissolution as a natural alkalinity source was likely not considered in the context of CDR yet, but it is in general a well-known natural process (e.g. Torres et al., 2020 Earth-Science Reviews; Wallmann et al., 2022 Frontiers in Marine Sciences).
Line 103: Please give an amount or range and add a reference. Same for organic matter.
Line 131-134: I would suggest to start with the aim and then continue with the sampling details. Further I would recommend to move figure S1 to the main text, as it gives a nice impression of the setting. If possible, it might be good to indicate a transect in figureS1 C as an example of the sampling locations from the beach to off-shore in order to give this picture a bit more content.
Line 164: Grams as a unit for seawater? Further, ‘S’ for salinity was not introduced as abbreviation.
Line 262ff: There are several places of small grammar/spelling issues, for example line262: alkalinity needs a capital ‘A’; line 283: ‘a values’; citation styles; line 338: ‘we’, not ‘I’?; line 499: ‘carbonate sources’. Please check carefully throughout the manuscript.
Line 281: Strictly speaking, silicate is a solid, in solution it’s silicic acid.
Figure 7: in panel B, there is a large difference between the first and further data points for the sand only experiments. Is this correct or a calculation error? Any explanation for this difference?
Citation: https://doi.org/10.5194/bg-2023-122-RC2 -
RC3: 'Comment on bg-2023-122', Anonymous Referee #3, 29 Aug 2023
Ln 26: isnt it more useful to introduce alkalinity in are where low alkalinity is released until now?
Ln 69: the acid can also be used to acidify seawater and convert DIC to CO2 which is captured for storage.
Ln 107: At first CO2 is released into the surface water and reacts with acid-base system. Release into atmosphere is to my understanding rather minor except for the escape in gaseous forms (e.g. from oversaturation or vents)
Ln 133: I respect you paddle skills, but sampling surface water for sedimentary alkalinity fluxes is subject to variety of errors (residence time of the water, submarine groundwater discharge etc.), hence benthic flux chamber would have been a good accomplishment for better comparison of the different systems.
Ln 110-112: OAE is supposed to happen in the water column or sediment surface, whereas microbial respiration is mainly performed in the sediments. As soon as Alkalinity is lost to pore water and irrigation is not able to flush these sediments over saturation is occurring, with OAE source material or not. I think you should be more precise on how OAE is implemented and where natural processes and OAE application is supposed to react.
Ln 206: What about the anthropogenic CO2 that is sequesterd by natural TA emissions to the coastal waters such as in North Sea?
Ln115: Would over saturation at first place be beneficial for the ocean to buffer ocean acidification and further CaCO3 producers under the circumstances of ocean acidification and haven an impact on the biological carbon pump? Shouldn't this be integrated into OAE research and efficiency discussion?
Ln 262: Alkalinity
Ln: 283 a value, or value...
285 experiments when slag was incubated: not 100 % clear to me how you have assumed these values, consider some measured values/data for supplementary section
Ln: 430: How does this Experiment design help to understand the increase in Alkalinity under low-high DIC conditions?
Ln 432: Can you also add pHT plots to it. I assume that sudden DIC increase will lower the pH and causes acidification where as AT production and neutralization will react on different speeds (e.g. olivine lowest)
Ln 527: Could you prove that? This is an important aspect for “Leakage” in OAE and in your system precipitation could be investigated/monitored under controlled conditions (SEM or Ca concentrations). As for slag and olivine it wont make sense, since nuklides can be formed as well but did you test the nuclide hypothesis in your experiments by having NaOH + seawater only? Therefore interactions between alkalinity and dissolved ions can be investigated.
Ln 547: Or movements of the sediment caused abrasion of smaller particles and edges, which might have dissolved a faster than more resistant material that remained afterwards. There is a new paper coming by Gunter et al 2023 (accepted for publication)
Ln 596: Hangx, Suzanne J. T., and Christopher J. Spiers. 2009. “Coastal Spreading of Olivine to Control Atmospheric CO2 Concentrations: A Critical Analysis of Viability.” International Journal of Greenhouse Gas Control 3 (6): 757–67.
Ln 699: In your Conclusion and Outlook you could give some recommendations of how we can tackle the additionality problem. Do we either look for areas in which low alkalinity turn-over is present and try to make them more productive or do we increase productivity of high productive system. What is the benefit of this paper to the discussion and where do you want it to guide us?
Citation: https://doi.org/10.5194/bg-2023-122-RC3 -
RC4: 'Comment on bg-2023-122', Adam Subhas, 30 Aug 2023
This review is for the manuscript submitted to Biogeosciences entitles “The additionality problem of Ocean Alkalinity Enhancement” by L.T. Bach. The manuscript describes incubation experiments conducted using beach sand collected from Tasmania, spiked with three different alkalinity sources. It then goes on to describe the results from these experiments in the context of the concept of additionality in CDR.
The manuscript as written provides much food for thought for scientists working on OAE, and how anthropogenic alkalinity will interact with natural alkalinity (and ultimately carbon) cycling. However, there are several places where more care could be taken in the framing of the problem, the description of the experiments, and the discussion of results. My largest question to the author is in the definition of “baseline” and whether ocean acidification (OA) and its modification of the ocean’s carbonate system should be included. Sedimentary CaCO3 dissolution will play a large role in neutralizing fossil fuel CO2 over the next ~10,000 years, with several indications that OA-induced dissolution is already occurring. Should this current scenario really be our baseline if it is already changing the natural alkalinity cycle? Does preventing OA-induced dissolution “count” in the author’s baseline framework? Or, do we reference the OAE “baseline” to a preindustrial state? These are larger questions that I am not sure can be answered with this manuscript and may need a much lengthier discussion, with a potentially completely new way to think about additionality for OAE.
In terms of the framing, a more precise description of additionality would be helpful to the reader, and the chemical equations presented could reflect this reference frame. In terms of the experiments, more detail could be given in several places (see specific comments below). In terms of the discussion, issues of kinetics and thermodynamics are at times conflated, with implications for how these experiments directly inform OAE deployments and/or carbon accounting. Please see the detailed comments below, and I hope these comments help to clarify and improve the manuscript on this important and emerging topic.
11-12: Is there also a climatic benefit in preventing ocean acidification (OA)-induced sedimentary CaCO3 dissolution?
14-17: I am not sure I understand this equation as written. Shouldn’t the left hand side be in units of carbon (moles) or DIC concentration? Is C(OAE) in units of carbon, if so, why is it referred to as alkalinity enhancement (units of alkalinity?) What is dC(baseline), and why is it a delta here? Why is it subtracted from C(OAE)? I worry that this description of additionality is too stripped down to be useful for the community at this stage, and raises the question of who the audience is for this manuscript.
47: On its face, I am not clear on how Eq. 2 describes the uptake of atmospheric CO2 into seawater and the resulting increase in DIC.
53: Perhaps “accomplished” instead of “generated”?
55-73: I found this section confusing because it conflates OAE and CDR in a single equation, whereas I think the point of the manuscript is to separate these two out and analyze the interaction of OAE with the natural alkalinity cycle, and the net effect on CDR. Could the author think about a way to write these alkalinity generating “half-reactions” and include the CDR component separately?
75: As written, Eq. (3) produces DIC and Alk in a 1:1 ratio, which will actually increase seawater pCO2 (most of the surface ocean has TA:DIC>1). By definition, this reaction can also not proceed if Omega > 1, so it cannot increase saturation states to drive excess precipitation.
84-85: The author should be careful when describing thermodynamic propensities to precipitate/dissolve in relation to kinetic effects. It is well-established that most of the surface ocean is supersaturated and does not precipitate CaCO3 spontaneously (with maybe a few exceptions…). This is brought up later in the context of previous OAE work, and could be considered and fleshed out here in the introduction.
93: Kinetic thresholds play a role here, too, not just thermodynamics.
100-101: As a rule, I try to avoid statements of primacy. This is new work, and everyone will recognize it as such!
108-111: Replacing CaCO3 dissolution with OAE through e.g. reactions 4-6 will not have a 1:1 impact on CDR. This is raised later on in the discussion, but could be introduced here to tee the reader up for this insight.
117: I am not sure that simplicity is warranted given the complexity of this topic. As stated above, this definition of additionality could use some more detail here, and be consistently applied throughout the text.
122: Mixed “three” and “3” in one sentence
131: What is the swash zone? It is used multiple times but never defined.
133: “The goal of these transects…”
152: Terminology for the alkaline feedstocks could be tightened up throughout the text. At times these are referred to as “weathering minerals”, “alkaline minerals”, “alkaline rocks or alkaline industrial products”…
159: Based on my calculations, this amount of NaOH should increase alkalinity by ~428 umol/kg. The targeted alkalinity enhancement would be useful to state here as a reference for later figures and results.
182: Are these HDPE bottles stable for DIC over the experiment duration? Polyethylene bottles typically have quite a high permeability to CO2 regardless of their density. This is especially relevant for interpreting the DIC enrichment experiments and the pH trends for all experiments.
191: How was this CO2-saturated water stored, and how was the DIC/CO2 saturation monitored?
196-197: “Filled with an increasing CO2 concentration” is unclear to me. What I think happened is that sequentially more volume of CO2-saturated seawater was mixed with natural seawater to create an array of DIC enrichment. However I don’t quite get that from this description.
215: Too many sig figs for the reported pH uncertainty of 0.015?
238: So, the grain size for the olivine and slag was 150-250 um?
240: What temperature did you dry the grains at?
255: How was the pH determination modified for beach samples in the field?
262: “Total alkalinity was determined…” What size of sample did you measure?
297: I am confident that this calculation goes back much further, no? To maybe a Humphries paper, or even earlier to other papers describing buffer factors? As described in this study, no component of mixing, advection, or isolation from the atmosphere is considered, which means that the calculation is not exactly analogous to the approach of Tyka et al., 2022.
313-314: I am not clear on this calculation. Can’t the buffer factor be calculated explicitly as an instantaneous change, using the latest CO2SYS packages?
322: I’m not sure I follow how eq. 10 is arrived at from eq. 9. Could you explain the individual terms, and how they relate to the terms in eq. 9?
329: Based on the discussion of these experiments, it does not appear that you can rule out CaCo3 precipitation in the olivine experiments, just that physical abrasion may have increased the dissolution rate of the olivine. Can you justify this assumption in another way?
330: Based on Table S2, these sands are only partially CaCO3. Couldn’t some other phase be dissolving to produce alkalinity, in which case some of the alkalinity change would be Alk(non-carbonate)?
332-334: I think this is a restatement of lines 330-331, for expt. 2. The DIC calculations (based on, I assume, TA and pH data) in these experiments is not presented in the main text or the supplemental, and could be useful as a figure to bolster this claim, especially given the low CaCO3 content of these sands.
341: I am not sure what operation the colon in this equation is performing.
343: and then delta-Alk(non-carbonate) makes up the difference? So, d(TA) = d(TA_carbonate) + d(TA_non-carbonate)? This would be helpful to state explicitly.
349: I’m not following this equation, or how it results from the previous equations 9-11.
358-360: Tenses changed from passive to active. Check for consistency throughout.
366-369: What is the purpose of fitting the data with quadratics? Is there a specific functional relationship that is described by a quadratic? It’s unclear how these fits are used, if at all, in the results and discussion, making it difficult to understand the choice of the function and its use in the interpretation. Perhaps the raw data could be used to evaluate trends, instead?
375: Is there a figure/table limit for biogeosciences? With only 2 additional tables and some consolidation of the supplemental figures, the author could consider eliminating the supplemental entirely, making for a smoother reading experience.
375-381: Could some of these alkalinity variations be explained by salinity-normalizing the data?
407: How does the change in alkalinity match up with the amount of NaOH you added? How about comparing to the amounts of olivine/slag and their stoichiometries, are you close to the maximum? Or, how much alkalinity yield was there over the 6.8 day period?
418-420: this claim could be confirmed by assessing the d(pH) data and CO2SYS calculations, assuming no CO2 gain/loss through the HDPE bottles.
439: Isn’t the increase quadratic, as described by your model, not exponential? What is the role of the quadratic model in fitting this data?
449: “The d(Alkalinity) increase in the unequilibrated treatment weakened along…”
461: How was the x-axis on this plot generated? I assume through measurements of TA and pH in the experimental bottles at time-zero of the experiment? It would help to clarify this point in the Methods section, and potentially here in the caption as well.
470: The manipulation of the carbonate chemistry in this fashion implies that the dependency could also be driven by pCO2, or total DIC, not necessarily pH.
471: How does fig. 4 demonstrate this two-way ANOVA result?
473-475: The second number in the range is the final pH in each experiment? I inferred this from the previous sentence, but would be good to state explicitly.
494-505: As discussed above, the other issue with carbonates is that they can only add alkalinity if the initial solution is undersaturated with respect to the mineral. This requires either respiration, or offline mixing of carbonates and a CO2 source.
519: Eq. 8 describes a change in the baseline, but nothing explicitly about the baseline state of the counterfactual. This is another example where some nuance in this framework would be useful further up front in the manuscript.
526-527: in the absence of CO2 gain/loss through the HDPE bottles, this hypothesis could be verified with your d(pH) measurements.
528-529: This statement is unclear. Are you referring to particles other than the slag itself? If so, where did these particles come from?
536-537: Could you also include the T and S conditions of these experiments? Or perhaps the saturation state, to be consistent with the following discussion?
537: “This suggests…”
540-542: Was the measured decrease in pH large enough to drive undersaturation for, e.g., aragonite? In general, this raises the question of how biological activity was controlled or monitored in these experiments.
548-550: In my opinion, it is not appropriate to cite work that has not made it through peer review. I defer to the editor on this point.
551-562: These experiments mix kinetic effects (dissolution rates) with thermodynamic drivers of CaCO3 precipitation. The dissolution rates of slag and olivine are different, whereas NaOH is essentially instantaneously added. These factors are important to consider when interpreting these results, as well as the effects of temperature, grain size, mixing/advection/diffusion in sediments…all of these complicate a purely thermodynamic interpretation of the results as is done in this paragraph.
557-558: Could the author provide a plot? Would it look similar to Fig. 6?
569-570: Could the increase in alkalinity also be due to the respiration of organic carbon and production of nitrate from fixed nitrogen, or the production of organic alkalinity? The sand does appear to have measurable POC. You could check the stoichiometry of dDIC:dTA and compare to, e.g., the stoichiometry for respiration of representative organic matter (i.e. C:N of 106:16).
582: Why just lines in 7A, but points in 7B? Could you not just show the data points instead of the fits?
590: subscript “non-carbonate”
593: The author could bring up the units (mol:mol) earlier in the manuscript when this term is introduced, and leave them off later on.
599: Do you mean high enough, not low enough? If, not, then this statement is confusing to me and could use some clarification.
605: Where does the respiratory CO2 come from? I’m not sure this sentence is necessary, or this effect could be described in a different way.
608: The DIC_OAE calculation could use a reference to the appropriate equation as well as the figure.
613: As defined, additionality is only about carbon and not explicitly about alkalinity at all. Thus, DIC_OAE is THE important parameter!
615-617: So, there is no additionality issue for slag or NaOH? But maybe one for olivine, although the expt. 1 results suggest a complicated story with respect to olivine dissolution, abrasion, and protection…?
620-623: This is an important point, and one that I believe people have made before, if not in the literature. I am glad it’s being made here.
626: Calculating DIC of the unequilibrated treatment using TA and pH data could help to assess ingassing/outgassing of CO2 through the HDPE bottles over the course of the experiment.
643: Are your mixing conditions on the plankton wheel comparable to the mixing conditions of a high energy wave impact zone?
647-648: not to mention the open ocean?
657: “were predicted to protect…” I am not sure this paper includes observations that directly verify the proposed hypothesis via the modeling results.
675-679: Organic alkalinity and sulfur (and other redox element) cycling complicates this as well.
683-684: These experiments take a snapshot along a kinetic curve, and how these dissolution/preicpitation kinetics play out in time will also be important to assess and verify.
696: But, as shown for equilibrated NaOH and slag, if that alkalinity release does not affect the total C uptake via OAE, then it doesn’t matter, correct?
Citation: https://doi.org/10.5194/bg-2023-122-RC4
Lennart Thomas Bach
Lennart Thomas Bach
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