the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Apr 2025
A tracer study for the development of in-water monitoring, reporting, and verification (MRV) of ship-based ocean alkalinity enhancement
Abstract. Marine carbon dioxide removal (mCDR) is starting to supplement large-scale emissions reductions to meet internationally recognized climate targets. Ocean alkalinity enhancement (OAE) is an mCDR approach that relies on the addition of dispersed liquid or solid alkalinity into seawater to take up and neutralize carbon dioxide (CO2) from the atmosphere. Documenting the effectiveness of OAE for carbon removal requires research and development of measurement, reporting, and verification (MRV) frameworks. Specifically, direct observations of carbon uptake via OAE will be critical to constrain the total carbon dioxide removal (CDR), and to validate the model-based MRV approaches currently in use. In September 2023, we conducted a ship-based rhodamine water tracer (RT) release in federal waters south of Martha’s Vineyard, MA followed by a 36-hour tracking and monitoring campaign. We collected RT fluorescence data and a suite of physical and chemical parameters at the sea surface and through the upper water column using the ship's underway system, a CTD rosette, and Lagrangian drifters. We developed an MRV framework that explicitly references the OAE intervention and the resulting CDR to the baseline ocean state using these in situ observations. We evaluated the effectiveness of defining a "dynamic" baseline, in which the carbonate chemistry was continuously constrained spatially and temporally using the shipboard data outside of the tracer patch. This approach reduced the influence of baseline variability by 25 % for CO2 fugacity (fCO2) and 60 % for TA. We then constructed a hypothetical alkalinity release experiment using RT as a proxy for OAE. With appropriate sampling, and with suitable ocean conditions, OAE signals were predicted to be detectable in total alkalinity (TA >10 umol kg-1), pH (>0.01) and CO2 fugacity (fCO2 >10 μatm). Over 36 hours, the ensuing CDR signal, driven by the gradient in surface fCO2, grew to greater than 3 μatm in fCO2, 0.003 pH units, and 1.4 μmol kg-1 in dissolved inorganic carbon (DIC), translating to 8 % of the total potential CDR. This signal, and the CDR itself, would continue to grow as long as an fCO2 gradient persisted at the sea surface. Climatological results from a regional physical circulation model supported these findings and indicated that models and in-water measurements can be used in concert to develop a comprehensive MRV framework for OAE-based mCDR.
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RC1: 'Comment on egusphere-2025-1348', Anonymous Referee #1, 24 Apr 2025
- AC2: 'Reply on RC1', Adam Subhas, 11 Jun 2025
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RC2: 'Comment on egusphere-2025-1348', Anonymous Referee #2, 07 May 2025
I found this paper to be extremely well written, very nicely presenting the results of the field work tracking rhodamine tracer as a preliminary step to tracking a future OAE experiment via the dispersal of a NaOH solution. I commend the authors on stepping through every aspect of the experimental design, deployment, interpretation, and implications for the future OAE experiment in a highly readable and comprehensive way.
I have only one somewhat major suggestion, which doesn’t require new analysis but might trigger some careful thinking about reframing the motivation and recommendation of the paper. I think my suggestion is related to Reviewer #1’s question about “What is your MRV framework?” I see the work presented in the paper as foundational research that can inform an MRV framework, but not itself MRV nor a framework. The term MRV signals a set of processes used in a voluntary or regulated carbon marketplace, whereby carbon removal is monitored and reported to a third party who verifies that it met a given set of predefined standards. The language in the paper should make this distinction.
Further, I suggest the authors to reflect on how much of this kind of extraordinary research effort (deep domain expertise, research vessel and state-of-the-art laboratory infrastructure, a valuable equipment pool, and the funding to support the team) they would recommended for future MRV. At a minimum, the target audience for the work should be made explicit (my guess: researchers trying to design a comprehensive field experiment with relevance for tracking surface anomalies influencing air-sea gas exchange). It would be slightly more ambitious, but commensurately more valuable, for the authors to reflect on a trajectory for mCDR researchers and practitioners that uses the knowledge gained from this research to ultimately create a more parsimonious MRV framework for operational mCDR (or at least provide recommendations on how to bridge that gap).
Minor comments:
The first line of abstract is deeply misleading: The solution to a 10^10th ton emissions problem is not meaningfully “supplemented” by processes that now collectively sum to 10^4th tons of removal. Please remove this sentence.
Line 30: Wording is a little confusing. I think you mean that “Over 36 hours, the ensuing anomalous carbon dioxide uptake by the ocean was driven by the enhanced air-sea gradient in fCO2. The calculated CDR signal was detectable as a 3 uatm surface ocean fCO2 increase, a pH decrease of 0.003 units, and…”. If this is an accurate interpretation of what is written, it would be more understandable to the reader to spell it all out.
51: typo: Researc
Figure 1 schematic is very helpful and well done.
Line 133 and throughout - specify air-sea gradient in fCO2 (or name a variable ΔfCO2) for clarity
357 - Why not make the gas exchange coefficient wind- or wind- and wave- dependent, as we know it is sensitive to wind speed and wave height. It would seem hard to justify using a constant, given the variability in Figure 2.
Figure 2: Specify that bathymetry is in m. Zooming closer to the ship track would be helpful, as would adding a circle of the size of the initial dispersal spiral.
Figure 3: If you had a drone photo of the patch taken at a later time than Figure 3a that showed the stretching and spreading of the dye, I think it would be super helpful for the reader.
Figure 6: consider adding salinity-normalized TA to panel d, and temperature and salinity-normalized fCO2 to panel e. The variability in T and S will be reflected in the carbon variables in a way that would be helpful to separate.
569 - DIC should also be sensitive to biology, no?
Figure 9 - I suggest using some colors! There are several solid black lines, so that it’s hard to track which one the caption refers to.
Figure 10 - I’m confused why there seems to be only 1 dot per hour, when the sampling frequency was 10-minute.
606 - yellow minus blue values — add note here (and/or in the caption) that the points are barely visible because of overlap in Figures 10 f and g
628 - 638 “MRV approaches that can accurately capture the entire patch budget … “ This is related to my “major suggestion" above: This statement makes it sound like MRV would be done in a way that mimics this experiment (with RT or similar tracer and a research vessel tracking it for days). But doing such hugely expensive field campaigns is a research-level activity that is likely beyond the scope of scalable MRV. In fact, I’d be shocked if 1 ton of CO2 wasn’t emitted by the MRV activities in their totality (4 days on the R/V Connecticut + chaser boat + all the instruments, supplies, and shipping), cancelling most/all of the mCDR here. Thus, I see this as a research activity, rather than an MRV approach, and I am very interested in the authors’ thoughts on the difference.
Citation: https://doi.org/10.5194/egusphere-2025-1348-RC2 - AC3: 'Reply on RC2', Adam Subhas, 11 Jun 2025
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RC3: 'Comment on egusphere-2025-1348', Anonymous Referee #3, 12 May 2025
- AC1: 'Reply on RC3', Adam Subhas, 11 Jun 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-1348', Anonymous Referee #1, 24 Apr 2025
- AC2: 'Reply on RC1', Adam Subhas, 11 Jun 2025
-
RC2: 'Comment on egusphere-2025-1348', Anonymous Referee #2, 07 May 2025
I found this paper to be extremely well written, very nicely presenting the results of the field work tracking rhodamine tracer as a preliminary step to tracking a future OAE experiment via the dispersal of a NaOH solution. I commend the authors on stepping through every aspect of the experimental design, deployment, interpretation, and implications for the future OAE experiment in a highly readable and comprehensive way.
I have only one somewhat major suggestion, which doesn’t require new analysis but might trigger some careful thinking about reframing the motivation and recommendation of the paper. I think my suggestion is related to Reviewer #1’s question about “What is your MRV framework?” I see the work presented in the paper as foundational research that can inform an MRV framework, but not itself MRV nor a framework. The term MRV signals a set of processes used in a voluntary or regulated carbon marketplace, whereby carbon removal is monitored and reported to a third party who verifies that it met a given set of predefined standards. The language in the paper should make this distinction.
Further, I suggest the authors to reflect on how much of this kind of extraordinary research effort (deep domain expertise, research vessel and state-of-the-art laboratory infrastructure, a valuable equipment pool, and the funding to support the team) they would recommended for future MRV. At a minimum, the target audience for the work should be made explicit (my guess: researchers trying to design a comprehensive field experiment with relevance for tracking surface anomalies influencing air-sea gas exchange). It would be slightly more ambitious, but commensurately more valuable, for the authors to reflect on a trajectory for mCDR researchers and practitioners that uses the knowledge gained from this research to ultimately create a more parsimonious MRV framework for operational mCDR (or at least provide recommendations on how to bridge that gap).
Minor comments:
The first line of abstract is deeply misleading: The solution to a 10^10th ton emissions problem is not meaningfully “supplemented” by processes that now collectively sum to 10^4th tons of removal. Please remove this sentence.
Line 30: Wording is a little confusing. I think you mean that “Over 36 hours, the ensuing anomalous carbon dioxide uptake by the ocean was driven by the enhanced air-sea gradient in fCO2. The calculated CDR signal was detectable as a 3 uatm surface ocean fCO2 increase, a pH decrease of 0.003 units, and…”. If this is an accurate interpretation of what is written, it would be more understandable to the reader to spell it all out.
51: typo: Researc
Figure 1 schematic is very helpful and well done.
Line 133 and throughout - specify air-sea gradient in fCO2 (or name a variable ΔfCO2) for clarity
357 - Why not make the gas exchange coefficient wind- or wind- and wave- dependent, as we know it is sensitive to wind speed and wave height. It would seem hard to justify using a constant, given the variability in Figure 2.
Figure 2: Specify that bathymetry is in m. Zooming closer to the ship track would be helpful, as would adding a circle of the size of the initial dispersal spiral.
Figure 3: If you had a drone photo of the patch taken at a later time than Figure 3a that showed the stretching and spreading of the dye, I think it would be super helpful for the reader.
Figure 6: consider adding salinity-normalized TA to panel d, and temperature and salinity-normalized fCO2 to panel e. The variability in T and S will be reflected in the carbon variables in a way that would be helpful to separate.
569 - DIC should also be sensitive to biology, no?
Figure 9 - I suggest using some colors! There are several solid black lines, so that it’s hard to track which one the caption refers to.
Figure 10 - I’m confused why there seems to be only 1 dot per hour, when the sampling frequency was 10-minute.
606 - yellow minus blue values — add note here (and/or in the caption) that the points are barely visible because of overlap in Figures 10 f and g
628 - 638 “MRV approaches that can accurately capture the entire patch budget … “ This is related to my “major suggestion" above: This statement makes it sound like MRV would be done in a way that mimics this experiment (with RT or similar tracer and a research vessel tracking it for days). But doing such hugely expensive field campaigns is a research-level activity that is likely beyond the scope of scalable MRV. In fact, I’d be shocked if 1 ton of CO2 wasn’t emitted by the MRV activities in their totality (4 days on the R/V Connecticut + chaser boat + all the instruments, supplies, and shipping), cancelling most/all of the mCDR here. Thus, I see this as a research activity, rather than an MRV approach, and I am very interested in the authors’ thoughts on the difference.
Citation: https://doi.org/10.5194/egusphere-2025-1348-RC2 - AC3: 'Reply on RC2', Adam Subhas, 11 Jun 2025
-
RC3: 'Comment on egusphere-2025-1348', Anonymous Referee #3, 12 May 2025
- AC1: 'Reply on RC3', Adam Subhas, 11 Jun 2025
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please see attached comments