the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2024
Countering the effect of ocean acidification in coastal sediments through carbonate mineral additions
Abstract. Along with its impact on calcifying plankton, ocean acidification also affects benthic biogeochemistry and organisms. Compared to the overlying water, fluid composition in sediments is altered through the effect of the mineralization of organic matter, which can further lower both pH and the carbonate saturation state. This can potentially be counteracted by the addition of carbonate minerals to the sediment surface. To explore the biogeochemical effects of mineral additions to coastal sediments, we experimentally quantified carbonate mineral dissolution kinetics, and then integrated this data into a reactive transport model that represents early diagenetic cycling of C, O, N, S and Fe, and traces total alkalinity, pH and saturation state of CaCO3. Model simulations were carried out to delineate the impact of mineral type and amount added, porewater mixing and organic matter mineralization rates on sediment alkalinity and its flux to the overlying water. Model results showed that the added minerals undergo initial rapid dissolution and generate saturated conditions. Aragonite dissolution led to higher alkalinity concentrations than calcite. Simulations of carbonate mineral additions to sediment environments with low rates of organic matter mineralization exhibited a significant increase in mineral saturation state compared to sediments with high CO2 production rates, highlighting the environment-specific extent of the buffering effect. Our work indicates that carbonate additions have the potential to effectively buffer surficial sediments over multiple years, yielding biogeochemical conditions that counteract the detrimental effect of OA conditions on larval recruitment, and potentially increase benthic alkalinity fluxes to support marine carbon dioxide removal (mCDR) in the overlying water.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-796', Anonymous Referee #1, 26 Apr 2024
- AC1: 'Reply on RC1', Kadir Bice, 23 Aug 2024
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RC2: 'Comment on egusphere-2024-796', Anonymous Referee #2, 15 May 2024
Bice et al present a 1D reactive transport sediment diagenesis model, in which they test the effect of adding CaCO3 to sediments on a) porewater alkalinity and other relevant species, and b) the resulting net production (i.e., sediment flux) of alkalinity and DIC. They also describe lab-based CaCO3 dissolution experiments from which they derive the dissolution rate parameters employed in the model. While the porewater chemistry and related flux results are the primary focus of the study, they also use a box model to extend their modeled flux data to calculate the potential “marine CO2 uptake”. At the porewater scale of analysis, they find that CaCO3 addition to sediment results in little change in species affected by redox reactions, but pH and omega are increased significantly 2 years after mineral addition. They also conclude (via a CDR = ½ Alk flux relationship) that the CaCO3 application can result in significant atmospheric CO2 uptake.
This topic is timely and interesting, and I think the model study contributes meaningfully to the conversation. However, I find three major issues with the paper that need to be addressed before consideration for publication: 1) Unclear justification and aim (amelioration of benthic acidification seems to be the main point, but it’s not justified well; and mCDR via OAE is also discussed significantly); 2) the mCDR projections are coarse, misleading, and don’t fit well with the rest of the data; and 3) the kinetic parameterization used for CaCO3 dissolution needs to be improved or better justified (modeling CaCO3 dissolution is the meat of the paper, but the parameterization is quite simple, and doesn’t consider more thorough published data). Furthermore, the overall readability of the paper needs to be improved. A major issue on this point is that the lab methods are incompletely described, but reference an unpublished thesis. Despite these concerns, I think the topic is interesting enough that the study is valuable if these issues are addressed in a revised manuscript.
Major issues:
- Unclear aim: I found the aim of the paper unclear. Specifically, the focus on both mitigating the effects of OA on sediments, and mCDR (i.e., atmospheric CO2 uptake via OAE) is confusing. These are different enough topics to merit dedicated conversations and scales of analyses, yet neither are sufficiently explored. The jump between a porewater model and atmospheric CO2 uptake requires many intermediate assumptions, yet the assumptions made to calculate atmospheric CO2 uptake are coarse and inaccurate (described in subsequent point). I think this paper could be more valuable if it focused more on the sediment model, and limited discussion of atmospheric CO2 uptake. Second, the justification that sediment acidification (due to OA) is a problem, is not well established in the introduction. References to benthic organisms are scattered throughout the paper, and the Introduction does not convincingly establish that buffering porewater pH is indeed an important objective for improving the health of benthic organisms. (Indeed, I imagine many benthic organisms are evolved to tolerate low pH conditions.) Related to this, I found the paper poorly grounded in OA, OAE (specifically accelerated weathering of limestone and coastal enhanced weathering), and CaCO3 dissolution research, with foundational citations missing, or others mis-referenced (e.g., L 70: Montserrat et al, 2017, is not about open ocean CDR). Reframing the discussion, with better references, could help here.
- CaCO3 dissolution kinetics: The rate parameters used for CaCO3 dissolution, based on four experimental data points, need to be better justified. First, they use a linear dependency with omega, and assume a rate order of 1. Much existing research exists on this topic, and some of the most comprehensive studies (using both biotic and abiotic CaCO3) have shown non-linear trends (due to dissolution mechanism transitioning from step-edge retreat to etch pit formation), and reaction rate orders not equal to 1. See, for example, Adkins et al, 2021 (Annual Reviews in Marine Science); Subhas et al, 2018 (Marine Chemistry)). While the simplified dissolution rates used in this paper may be adequate as rough approximations for the RTM, I find it strange that these other papers aren’t considered, either using their published rate equations, or at least citing them. Second, aragonite dissolution has been shown to be slower than calcite dissolution (see same sources), but is comparable in the rate equations presented here. Furthermore, what are the dissolution constants used? They are not in Table 4, and only alluded to in Section 3.1.
- CDR estimates: The authors calculate atmospheric CO2 uptake as equivalent to ½ the sediment alkalinity flux resulting from CaCO3 dissolution. This is not accurate for several reasons, first, the overlying water will be different from porewater (e.g., temperature, pH, etc.), requiring further thermodynamic considerations to assess CO2 concentration in the water column. Second, a decrease in water column CO2 does not necessarily translate to a 1:1 decrease in atmospheric CO2, given air-sea flux time and potential subduction of water masses prior to equilibration. This is difficult to model. Third, the dependence on “unpublished data” to calculate CO2 uptake (L 222) is problematic. If these data are available from the author team, why not include them in the paper or supplement? Furthermore, there are no uncertainties or temporal variability given, making any extrapolation dubious. While the presented estimate is a nice back of the envelope calculation, it is insufficient to present as a significant part of the paper’s discussion, and should include many more caveats. Because of the public and commercial interest in CDR, studies like these should be particularly thoughtful and careful about making claims of CO2 capture rates.
- Other model concerns: the initial sediment conditions are not fully presented. Is there any initial CaCO3 in the sediment? Or CaCO3 deposition? Even a small amount of CaCO3 can have a significant buffering effect, and adding CaCO3 may not necessarily change alkalinity generation linearly. If Yaquina Bay sediments are 0% carbonate, that should be explained. Furthermore, seasonal variability is not considered. I imagine that changes in temperature / OM dissolution rates are important if considering year+ trends, but these are not considered or discussed.
- Readability: I found it difficult to follow both the study’s motivation and methods, given some writing issues. The paper needs some significant editing, in overall structure, sentence structure, and word choice. I recommend having the revised manuscript reviewed by someone with scientific editing skills.
Significant issues / clarifications needed:
- The combination of existing data from an unpublished thesis (Myers, 2022) and the new sediment model is confusing as presented. The conclusion starts with “Our study successfully combines lab experiments and modeling”. However, the lab data consist of only 4 data points (used to parameterize CaCO3 dissolution rates), which were not particularly critical to the model, given the availability of published equations for dissolution kinetics (e.g., Adkins et al, 2021). As written, the paper takes a middle ground where much of the methods from Myers, 2022, are presented as new, but important details are also skipped over with a reference (which made it seem as if Myers, 2022, was a published study). Specific examples:
- Description of the mineral dissolution kinetic experiments (L 85 to 106).
- L 183: boundary condition for TA flux. This is a very important value, and is mentioned casually without much explanation.
- L 184: Why is the remineralization rate altered to match the TA flux? What about mineral precipitation/dissolution, and oxidation of reduced species? Also, need to explain what the rates / rate ranges are, instead of just saying they they’re comparable to other studies. (E.g., Krumins 2013 provides values for both shallow banks/bays, and also deeper coastal sediments; why are you only comparing to the deeper sediments here?)
- L 235: “This [dissolution rate] is in line with previous reactive transport models focused on mineral dissolution.” I find it strange that you’re justifying your choice of dissolution rate equations by comparing to other RTM models (which surely cite other papers for their rate equations), instead of studies focused on CaCO3 dissolution. Furthermore, these references are all quite old, and progress has been made on constraining CaCO3 dissolution rates in the meantime.
- The different model runs are not clearly explained or outlined anywhere. Perhaps there should be a table of the initial parameters, as well as subsequent tests. Examples:
- L 202: I had no idea what the parameters used in the spin-up simulations were, and they’re only mentioned as an “(e.g., …)”. They should be explicitly listed in a table somewhere.
- L 217: surficial mixing was “varied” from 4 cm to 10 cm. Is this only two depths/tests? Or was a range of values tested?
- The different CaCO3 addition scenarios (8%, 16%) are not clearly discussed in the text. E.g., if 8% CaCO3 addition is the “baseline”, it should be identified as such earlier (L 200). Which scenario is represented in the Results, Discussion, and Figs?
- Presentation of both the experimental observations and model results is confusing. E.g., Section 3.1 presents measured data, but immediately follows the lengthy model methods description. You need to more clearly specify “experiment” and “model” results.
- Mixing discussion points about calcifying organisms into both the Methods and Results section is distracting. Example: L 288.
Minor comments:
- Throughout: abbreviations are not used consistently (e.g., “rate of OM mineralization” could be “R_C,” or included as a reminder, e.g., “rate of OM mineralization (R_C)” (L 210)).
- Not all variables are defined.
- L 149 (Equation 4): What is s?
- Subsequently R_C and R_C0 are used without definition.
- Table 3: What is RC? Is it supposed to be R_C?; What is R_M?
- Table 4: kfm and kdm are not defined
- L 108: the ‘volumetric dissolution rate’ is confusing following directly after the Equation 1. It should be a new paragraph.
- L 110: The jump from equation 2 to equation 3 is not obvious and needs to be explained better.
- L 127: What do you mean by “validation samples”? How were these used, and what were they compared to?
- L 180: “other acids/bases” needs to be explained
- L 184: RC0 is not yet defined.
- L 201: “respectively” does not make sense here; not needed
- L 267: aerobic respiration consumes alkalinity, so saying it produces “significantly less TA” is confusing.
- L 359: “reached similar values” is unclear. Similar to what?
- L 372: Specify that A is Omega_calcite. Also, caption should not say “2 years after” since data are for three timepoints
- L 415: “Loosing”
- L 428: Reword “… in the bay most.”
- L 430: “Increase of CO2 uptake” is not accurate. The sediment is still a net source of CO2; “decrease in CO2 flux” would be closer to accurate.
Citation: https://doi.org/10.5194/egusphere-2024-796-RC2 - AC2: 'Reply on RC2', Kadir Bice, 23 Aug 2024
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